Sunday, 21 July 2013

The 5-HT2C receptor

Interestingly one of the subtype of receptors that the neurotransmitter serotonin binds to and activates, the 5-HT2C receptor (which was once called the 5-HT1C receptor), is found on the X chromosome.1 What this means is that males have one copy of the gene encoding the 5-HT2C receptor, whereas women have two copies of this gene. Practically this means that if there is a defect in the gene that causes less of the 5-HT2C receptor protein (as you may or may not know all receptors are, in fact, proteins) to be synthesised and a man inherits it, he'll suffer more than a women that inherits one copy of the defective gene and one functional 5-HT2C receptor gene on their other X chromosome.

The role of the 5-HT2C receptor is complex. To the cell on which 5-HT2C receptors are expressed they are excitatory – that is, they increase the activity of the cells on which they are expressed. But because there are neurons (the electrically signalling cells of the brain, spinal cord and nerves) that, when excited, release inhibitory neurotransmitters like GABA, 5-HT2C receptors can, in some brain regions, have indirect (i.e. via these inhibitory GABAergic interneurons as they're called) inhibitory functions, that is they can reduce cellular activity in these regions.2

It appears to, via this indirect mechanism, to suppress the release of dopamine and norepinephrine in certain parts of the brain, particularly in the mesolimbic pathway.2-4 Which is a part of the brain from which the rewarding (or pleasurable) effects of recreational drugs, certain behaviours (like gambling) and even food originates from. It has hence been found that 5-HT2C antagonists (or blockers) facilitate the release of dopamine in the mesolimbic pathway induced by drugs of abuse like nicotine, cocaine, morphine and phencyclidine (PCP), hence potentially amplifying their rewarding or pleasurable effects. This is kind of ironic when you think about how patients with schizophrenia have a far higher incidence of substance abuse problems, including tobacco smoking addiction, seeing how many of the newer antipsychotics on the market that are frequently used to treat patients schizophrenia do, in fact, antagonise the 5-HT2C receptor. Albeit they also antagonise a few of the dopamine receptors that are involved in the pleasurable effects of these drugs but seeing how this effect is usually weaker than their inhibitory effects on the 5-HT2C receptor you could make the argument that they likely overall cause an increased liability to amplifying the addictive potential of recreational drugs.5,6

It has also been discovered that 5-HT2C receptors regulate feeding – 5-HT2C antagonists are known to cause an increase in appetite in humans and this can lead to obesity, as is exemplified by the fact that several of the newer antipsychotics like clozapine and olanzapine that antagonise the 5-HT2C receptors lead frequently to weight gain and obesity. Conversely 5-HT2C agonists are known to suppress appetite and the drug lorcaserin was recently approved by the US FDA for the treatment of obesity.

5-HT2C receptors also regulate mood and perception. For instance, the 5-HT2C agonist vabicaserin was being developed as a treatment for schizophrenia, until development was ceased in 2010 for a reason that escapes me. 5-HT2C antagonists display antidepressant activity, with several commercially (or clinically) available antidepressants such as fluoxetine (PROZAC), agomelatine (VALDOXAN), mirtazapine (AVANZA, REMERON), amitriptyline (ELAVIL, ENDEP) and mianserin (LUMIN, TOLVON) displaying clinically-significant affinity towards the receptor as an antagonist.7 It is possible that it is, in part, by antagonising the 5-HT2C receptor that atypical (newer) antipsychotics like olanzapine and aripiprazole that they manage to speed up and improve response rates to antidepressant therapy.7-10

Reference List:

  1. Milatovich A, Hsieh C-L, Bonaminio G, Tecott L, Francke U. Serotonin receptor 1c gene assigned to X chromosome in human (band q24) and mouse (bands D-F4). Hum Mol Genet [Internet]. 1992 Dec 1 [cited 2013 Jul 22];1(9):681–4. Available from: http://hmg.oxfordjournals.org/content/1/9/681
  2. Invernizzi RW, Pierucci M, Calcagno E, Di Giovanni G, Di Matteo V, Benigno A, et al. Selective activation of 5-HT2C receptors stimulates GABA-ergic function in the rat substantia nigra pars reticulata: A combined in vivo electrophysiological and neurochemical study. Neuroscience [Internet]. 2007 Feb 23 [cited 2013 Jul 22];144(4):1523–35. Available from: http://www.sciencedirect.com/science/article/pii/S0306452206015211
  3. Millan M, Dekeyne A, Gobert A. Serotonin (5-HT)2C receptors tonically inhibit dopamine (DA) and noradrenaline (NA), but not 5-HT, release in the frontal cortex in vivo. Neuropharmacology [Internet]. 1998 Jul [cited 2013 Jul 22];37(7):953–5. Available from: http://www.sciencedirect.com/science/article/pii/S0028390898000781
  4. Di Giovanni G, De Deurwaerdére P, Di Mascio M, Di Matteo V, Esposito E, Spampinato U. Selective blockade of serotonin-2C/2B receptors enhances mesolimbic and mesostriatal dopaminergic function: a combined in vivo electrophysiological and microdialysis study. Neuroscience [Internet]. 1999 Jun [cited 2013 Jul 22];91(2):587–97. Available from: http://www.sciencedirect.com/science/article/pii/S0306452298006551
  5. Porras G, Matteo VD, Fracasso C, Lucas G, Deurwaerdère PD, Caccia S, et al. 5-HT2A and 5-HT2C/2B Receptor Subtypes Modulate Dopamine Release Induced in Vivo by Amphetamine and Morphine in Both the Rat Nucleus Accumbens and Striatum. , Published online: 18 July 2001; | doi:101016/S0893-133X(01)00333-5 [Internet]. 2001 Jul 18 [cited 2013 Jul 22];26(3):311–24. Available from: http://www.nature.com/npp/journal/v26/n3/full/1395776a.html
  6. Navailles S, De Deurwaerdère P, Porras G, Spampinato U. In Vivo Evidence that 5-HT2C Receptor Antagonist but not Agonist Modulates Cocaine-Induced Dopamine Outflow in the Rat Nucleus Accumbens and Striatum. Neuropsychopharmacology [Internet]. 2003 Oct 15 [cited 2013 Jul 22];29(2):319–26. Available from: http://www.nature.com/npp/journal/v29/n2/full/1300329a.html
  7. Jenck F, Moreau J-L, Mutel V, Martin JR, Haefely WE. Evidence for a role of 5-HT1C receptors in the antiserotonergic properties of some antidepressant drugs. European Journal of Pharmacology [Internet]. 1993 Feb 9 [cited 2013 Jul 22];231(2):223–9. Available from: http://www.sciencedirect.com/science/article/pii/001429999390453O
  8. Dolder CR, Nelson M, Snider M. Agomelatine Treatment of Major Depressive Disorder. Annals of Pharmacotherapy. 2008 Nov 18;42(12):1822–31.
  9. Jenck F, Moreau J-L, Mutel V, Martin JR. Brain 5-HT1C receptors and antidepressants. Progress in Neuro-Psychopharmacology and Biological Psychiatry [Internet]. 1994 May [cited 2013 Jul 22];18(3):563–74. Available from: http://www.sciencedirect.com/science/article/pii/0278584694900132
  10. Jenck F, Bös M, Wichmann J, Stadler H, Martin J, Moreau J. The role of 5ht2c receptors in affective disorders. Expert Opinion on Investigational Drugs [Internet]. 1998 Oct [cited 2013 Jul 22];7(10):1587–99. Available from: http://informahealthcare.com/doi/abs/10.1517/13543784.7.10.1587


Saturday, 20 July 2013

Varenicline

Varenicline (CHANTIX, CHAMPIX) is the name of a popular, and, according to a few clinical trials, most of which were funded by Pfizer -- the pharmaceutical company sponsoring the development and profiting from the sales of the drug, an effective smoking cessation aid (i.e. a drug designed to help people quit smoking).1

A more impartial source (albeit since the bulk of the clinical trial data they would have analysed would be from Pfizer-funded trials it is impossible to say that it would get unbiased results), the Cochrane collaboration (which performs statistical analysis of available clinical trial data) demonstrated that it was more effective than a placebo and that it was more effective than the smoking cessation aid bupropion (WELLBUTRIN, ZYBAN), and while statistical significance wasn't reached there was a trend in this meta-analysis (statistical analysis) that favoured varenicline over nicotine replacement therapy (NRT; this is where attempts are made to replace the nicotine a smoker would normally get by smoking they get via less dangerous and less addicting ways like nicotine patches and gum). What this means is that the data we have comparing varenicline with NRT isn't strong enough to rule out the possibility of a coincidence occurring in the trials comparing the efficacy of NRT with that of varenicline that so happened to favour varenicline over NRT2

It works by serving as a partial agonist at the α4β2 subtype of nicotinic acetylcholine receptors (nAChR). To explain what this means I need to teach you a little bit about the nicotinic acetylcholine receptors.

nAChRs are receptors that are found throughout the central and peripheral nervous systems (explained in the sentence after next). The term acetylcholine refers to the fact that these receptors are activated by the body-synthesised neurotransmitter (chemical messenger between the electrically-signalling cells of the peripheral and central nervous systems), acetylcholine, which the body synthesises from choline – a member of the B group of essential vitamins.

In the central nervous system (CNS; brain and spinal cord) nAChRs are found in a number of places, particularly in the regions of the brain involved in reward (like where the rewarding, or pleasurable, effects of recreational drugs originate), wakefulness, sensory perception, memory, learning and mood. In the peripheral nervous system (PNS; the nerves we all use to sense pain, pressure, temperature, etc. and to control our voluntary and involuntary movements) they regulate muscular activity, both voluntary and involuntary, as is exemplified by the fact that the muscle relaxants often used to prevent twitching/shivering during surgeries and also in the lethal injection to stop breathing work by blocking certain peripheral nAChRs. Some nAChRs are also found on the vagus nerve (a nerve that goes directly from the brain down through much of the torso) where they appear to regulate immune function.3,4

Another fact about the nAChRs is that they are made out of subunits, which you could think of as the building blocks the receptor. These subunits are designated a Greek letter (e.g. α) and a number (e.g. 4), e.g. one subunit is called the α4 subunit based on this. Each receptor must contain a total of exactly five subunits in order for it to be functional (i.e. for it to actually work); for α4β2 nAChRs there are 2 α4 and 3 β2 subunits found in each receptor.

Varenicline binds to the α4β2 nAChRs and activates them, but it does so less “fully” or strongly than acetylcholine does, and this is why we call it a partial agonist at this receptor. α4β2 nAChRs are expressed solely in the CNS where they regulate the release of neurotransmitters including those involved in reward and hence the euphoric effects of many drugs (e.g. dopamine and β-endorphin would be examples of such neurotransmitters) and it is via this action that α4β2 nAChRs, when activated, produce euphoric (or rewarding) effects. It is believed that it is via these receptors that nicotine produces much of its rewarding and hence addictive effects.5 Varenicline, by serving as a partial agonist, it is able to antagonise (block or prevent) the increase in nAChR activity triggered by nicotine because it binds to the same site on the α4β2 nAChRs as nicotine hence preventing nicotine from binding to said site and hence preventing it from inducing many of its effects, including the rewarding effects it produces that leads to addiction. Simultaneously, via its ability to “partially” activate the α4β2 nAChRs it is able to reduce nicotine cravings by mimicking nicotine’s effects on dopamine and β-endorphin release, just to a lesser extent. Hence it is able to both block the rewarding effects of nicotine while simultaneously reducing cravings, and perhaps other symptoms characteristic of nicotine withdrawal.


For the past few years questions have been raised regarding the safety of varenicline, amidst reports of suicidal ideation (thinking about suicide) and cardiovascular (pertaining to the heart and blood vessels; e.g. myocardial infarctions [heart attacks]) adverse effects in patients receiving varenicline as a smoking cessation aid. The risks of both these types of adverse effects appears to be quite small, with mostly just those with pre-existing psychiatric or cardiovascular conditions, respectively, afflicted by such adverse effects.6 Despite these reports that varenicline can cause suicidality (even though depression does not strike me as a particularly unusual problem for people that by definition are trying to kick such an addictive habit) there is some evidence from tests on mice that suggests that it might have antidepressant effects, likewise, in mice, it was also found to potentiate the antidepressant effects of the popular, Therapeutic Goods Administration (TGA)-approved antidepressant, sertraline (ZOLOFT).7

Varenicline also serves as a full agonist at the α7 subtype of nAChRs, i.e. it activates the receptor to the same extent as acetylcholine, and since the α7 nAChRs appears to be heavily involved in memory and learning it may have some positive effects on the aforementioned functions. Varenicline also activates the 5-HT3 receptor,8 which might be responsible for its high propensity for causing nausea (approximately 50% of patients treated with varenicline experience nausea).6

References

  1. Jorenby DE, Hays J, Rigotti NA, et al. Efficacy of varenicline, an α4β2 nicotinic acetylcholine receptor partial agonist, vs placebo or sustained-release bupropion for smoking cessation: A randomized controlled trial. JAMA [Internet]. 2006 Jul 5 [cited 2013 Jul 21];296(1):56–63. Available from: http://dx.doi.org/10.1001/jama.296.1.56
  2. Cahill K, Stead LF, Lancaster T. Nicotine receptor partial agonists for smoking cessation. Cochrane Database of Systematic Reviews [Internet]. John Wiley & Sons, Ltd; 2013 [cited 2013 Jul 21]. Available from: http://onlinelibrary.wiley.com/doi/10.1002/14651858.CD006103.pub6/abstract
  3. De Jonge WJ, Ulloa L. The alpha7 nicotinic acetylcholine receptor as a pharmacological target for inflammation. Br J Pharmacol [Internet]. 2007 Aug [cited 2013 Jul 21];151(7):915–29. Available from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2042938/
  4. Van der Zanden EP, Snoek SA, Heinsbroek SE, Stanisor OI, Verseijden C, Boeckxstaens GE, et al. Vagus Nerve Activity Augments Intestinal Macrophage Phagocytosis via Nicotinic Acetylcholine Receptor α4β2. Gastroenterology [Internet]. 2009 Sep [cited 2013 Jul 21];137(3):1029–1039.e4. Available from: http://www.gastrojournal.org/article/S0016-5085(09)00737-9/abstract?referrer=http://www.ncbi.nlm.nih.gov/pubmed/19427310
  5. Albuquerque EX, Pereira EFR, Alkondon M, Rogers SW. Mammalian Nicotinic Acetylcholine Receptors: From Structure to Function. Physiol Rev [Internet]. 2009 Jan 1 [cited 2013 Jul 21];89(1):73–120. Available from: http://physrev.physiology.org/content/89/1/73.full.pdf
  6. Truven Health Analytics, Inc. DRUGDEX® System (Internet) [cited 2013 Jul 21]. Greenwood Village, CO: Thomsen Healthcare; 2013.
  7. Rollema H, Guanowsky V, Mineur YS, Shrikhande A, Coe JW, Seymour PA, et al. Varenicline has antidepressant-like activity in the forced swim test and augments sertraline’s effect. European Journal of Pharmacology [Internet]. 2009 Mar 1 [cited 2013 Jul 21];605(1–3):114–6. Available from: http://www.sciencedirect.com/science/article/pii/S0014299909000302
  8. Lummis SCR, Thompson AJ, Bencherif M, Lester HA. Varenicline Is a Potent Agonist of the Human 5-Hydroxytryptamine3 Receptor. J Pharmacol Exp Ther [Internet]. 2011 Oct [cited 2013 Jul 21];339(1):125–31. Available from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3186289/pdf/zpt125.pdf

Sunday, 14 July 2013

PDE4 inhibitors and their potential applications in the treatment of a variety of conditions

Note: Terms that are not explained in the body are explained in a Glossary near the end (before the references) of the post.

The enzyme phosphodiesterase 4 (PDE4) is an emerging drug target in the management of psychiatric, neurologic, immune and inflammatory disorders such as major depressive disorder1 (MDD), schizophrenia1, Alzheimer’s disease1 (AD), chronic obstructive pulmonary disease2 (COPD) and multiple sclerosis (MS). The drugs being developed to target PDE4 all inhibit the aforementioned enzyme – that is they reduce the activity of the PDE4 and this is how they elicit their therapeutic effects. These drugs are called the PDE4 inhibitors. In order to explain just how PDE4 inhibition leaves these drugs open to being used in the management of the aforementioned conditions I need to first explain the role of PDE4 in cells.

PDE4 belongs to a group of enzymes called the cyclic nucleotide phosphodiesterases which catalyse (or help) the reactions involved in the breakdown of cyclic nucleotides, which includes two chemical messengers that are of particular biologic significance – cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). These messengers are also referred to as second messengers. This is because when some G protein-coupled receptors (GPCRs; which are usually expressed on a cell’s surface) are activated these second messengers relay their respective signals to the inner components of the cell. Hence drugs that inhibit cyclic nucleotide phosphodiesterases are able to produce many of the same effects as drugs that directly interact with these G protein-coupled receptors on the cell surface. Not all GPCRs transmit their signals by increasing intracellular (inside the cell) concentrations of cyclic nucleotides, some, upon activation, decrease intracellular cyclic nucleotide concentrations and these receptors include, among numerous others, the dopamine, D2, family of receptors (D2, D3, D4).1

The different phosphodiesterases, however, do have different respective affinities towards the different cyclic nucleotides and hence the cyclic nucleotide(s) that PDE4 prefers to (or has a higher affinity to) degrade is relevant and is cAMP. Hence PDE4 inhibitors increase intracellular cAMP levels and since PDE4 is found in virtually every organ system of the body it increases intracellular cAMP levels throughout the body.1

The interesting thing about PDE4 inhibitors is that they also elicit very potent anti-inflammatory effects and they also elicit antidepressant effects, which would appear to be in agreement with the inflammatory hypothesis of depression. Although this could very easily a coincidence seeing how cAMP is a very common second messenger throughout the body and it is well known that serotonin is heavily involved in mood regulation, as is glutamate (I know this one is a lesser known neurotransmitter as far as its role in depression but ketamine, the anaesthetic turned rapid-acting and robust antidepressant acts on the glutamate receptors to elicit its effects and hence this is also a popular theory for the cause of depression) and norepinephrine (noradrenaline) and a few of the receptors for all three of these neurotransmitters use cAMP as their second messengers and hence by inhibiting PDE4 and raising cAMP levels it is entirely possible that it mimics the effects of those these neurotransmitters in the central nervous system (CNS).

Another thing that PDE4 inhibitors do that they share with all other antidepressants is that they promote neurogenesis and exert neuroprotective effects – that is, they induce the formation of new neurons and have protective effects on existing neurons, respectively. They also seem to induce long-term potentiation, the process by which the signalling between two neurons is enhanced in a long-lasting fashion.

PDE4 inhibitors also possess antipsychotic activity which may be related to the fact that the D2 family of receptors – which thing all antipsychotics in clinical use block – are G­i/o protein-coupled and hence inhibit the formation of cAMP. Hence by blocking them antipsychotics increase cAMP synthesis in cells that express the D2 family of receptors. Plus they also appear to interact with the metabotropic glutamate receptors which regulate the release of the neurotransmitter glutamate, which also appears to play a key role in schizophrenia. In fact until mid-last year it appeared that soon a drug that directly acted on two types of metabotropic glutamate receptor (mGluR 2 and 3) would soon be FDA approved for the treatment of schizophrenia as it showed promise in early clinical testing in the treatment of this indication. The serotonin 5-HT1A receptor has also been found to be involved in the antipsychotic action of some TGA and FDA approved antipsychotics and it also couples to a Gi/o protein.5-6

Currently there is only one PDE4 inhibitor that has received FDA and TGA approval – Roflumilast. It received TGA approval in November 2011 (see http://www.tga.gov.au/pdf/auspar/auspar-daxas.pdf for details) for the treatment of severe COPD, specifically chronic bronchitis. Roflumilast is, from what I have read on Micromedex, a rather well-tolerated drug, that is, its side effects are rather mild and rare, for the most part at least (it has been found to cause suicidal ideation – thinking about suicide – which is its only particularly worrying side effect). According to Micromedex aside from suicidal ideation it has the following side effects:7
  • weight decreased (7% to 20% ) 
  • Decrease in appetite (2.1% ) 
  • Diarrhoea (9.5% ) 
  • Nausea (4.7% ) 
  • Influenza (2.8% ) 
  • Backache (3.2% ) 
  • Dizziness (2.1% ) 
  • Headache (4.4% ) 
  • Insomnia (2.4% )
(with their respective incidence in brackets)
The side effect of influenza is particularly worrying to me, considering the fact that patients with COPD are particularly vulnerable to respiratory infections like influenza. It is also interesting because PDE4 inhibitors have also been found to reduce innate immunity – the first line of the body’s immune defences where the second-line is the more specific (to the microbe, that is) line of immunity, which also recognises prior bugs encountered by the body’s defenses8-10 which would explain an increased susceptibility of PDE4 inhibitor-treated individuals to common infections such as the flu.

Most of the original PDE4 inhibitors to be developed, such as rolipram, had one side effect in particular that limited their clinical utility – severe emesis. Despite this side effect it turns out that some South African individuals have been using a PDE4 inhibitor-containing plant for its antidepressant effects for centuries – the kanna plant. This plant contains mesembrenone, among other alkaloids which display PDE4 inhibitory effects.

Glossary

Central Nervous System (CNS) – the brain and spinal cord.
Chronic Obstructive Pulmonary Disease (COPD) – a collection of progressive, disabling, incurable and often, with time, fatal lung diseases that cause a progressive deterioration in lung function. They include emphysema, chronic bronchitis and even some cases of asthma. They are normally associated with smoking.  
Emesis – nausea and vomiting.
Endogenous Ligand – a ligand for a particular receptor
Enzyme – an enzyme is a chemical catalyst in reactions, that is, they serve to increase the rate of a chemical reaction despite not being directly involved in the reaction. By directly it is meant that the enzyme is not transformed by the chemical reaction, or used up by the reaction.
Extracellular – outside cells.
Intracellular – inside cells.
Ligand – a compound that binds to its respective receptor (for which it is a ligand). Agonists are a type of ligand that activate the receptor whereas antagonists do not activate the receptor but rather prevent agonists from activating the receptor.
Neuron – electrically-signalling cells of the brain, spinal cord and nerves.
Receptor – receptors are proteins that, upon the binding of an agonist, cause the modulation of cellular functions. G protein-coupled receptors (GPCR) are receptors that are bound to G proteins, which in turn, upon binding by an agonist to the receptor, set off a series of chemical reactions that alter the functioning of a cell. The Gi/o protein is a G protein that when the receptor it is coupled with is activated (and hence it is too as this is the purpose of coupling between receptor and G proteins) it inhibits adenylate cyclase – the enzyme responsible for the formation of cAMP, from the energy source of cells, adenosine triphosphate (ATP). Likewise the Gs protein is a G protein that enhances adenylate cyclase activity upon its activation.

Reference List

  1. Halene TB, Siegel SJ. PDE inhibitors in psychiatry – future options for dementia, depression and schizophrenia? Drug Discovery Today [Internet]. 2007 Oct [cited 2013 Jul 12];12(19–20):870–8. Available from: http://www.sciencedirect.com/science/article/pii/S1359644607003066
  2. Spina D. PDE4 inhibitors: current status. Br J Pharmacol [Internet]. 2008 Oct [cited 2013 Jul 12];155(3):308–15. Available from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2567892/
  3. Boswell-Smith V, Spina D, Page CP. Phosphodiesterase inhibitors. Br J Pharmacol [Internet]. 2006 Jan [cited 2013 Jul 12];147(Suppl 1):S252–S257. Available from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1760738/
  4. Barad M, Bourtchouladze R, Winder DG, Golan H, Kandel E. Rolipram, a type IV-specific phosphodiesterase inhibitor, facilitates the establishment of long-lasting long-term potentiation and improves memory. Proc Natl Acad Sci U S A [Internet]. 1998 Dec 8 [cited 2013 Jul 12];95(25):15020–5. Available from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC24568/
  5. Bantick RA, Deakin JFW, Grasby PM. The 5-HT1A receptor in schizophrenia: a promising target for novel atypical neuroleptics? J Psychopharmacol [Internet]. 2001 Jan 1 [cited 2013 Jul 14];15(1):37–46. Available from: http://jop.sagepub.com/content/15/1/37
  6. Ohno Y. Therapeutic Role of 5-HT1A Receptors in The Treatment of Schizophrenia and Parkinson’s Disease. CNS Neuroscience & Therapeutics [Internet]. 2011 [cited 2013 Jul 14];17(1):58–65. Available from: http://onlinelibrary.wiley.com/doi/10.1111/j.1755-5949.2010.00211.x/abstract
  7. Truven Health Analytics, Inc. DRUGDEX® System (Internet) [cited 2013 Jul 14]. Greenwood Village, CO: Thomsen Healthcare; 2013.
  8. Koo M-S, Manca C, Yang G, O’Brien P, Sung N, Tsenova L, et al. Phosphodiesterase 4 Inhibition Reduces Innate Immunity and Improves Isoniazid Clearance of Mycobacterium tuberculosis in the Lungs of Infected Mice. PLoS ONE [Internet]. 2011 Feb 25 [cited 2013 Jul 14];6(2):e17091. Available from: http://dx.doi.org/10.1371/journal.pone.0017091
  9. Sadrai Z, Stevenson W, Okanobo A, Chen Y, Dohlman TH, Hua J, et al. PDE4 Inhibition Suppresses IL-17–Associated Immunity in Dry Eye Disease. IOVS [Internet]. 2012 Jun 1 [cited 2013 Jul 14];53(7):3584–91. Available from: http://www.iovs.org/content/53/7/3584
  10. Serezani CH, Ballinger MN, Aronoff DM, Peters-Golden M. Cyclic AMP. Am J Respir Cell Mol Biol [Internet]. 2008 Aug [cited 2013 Jul 14];39(2):127–32. Available from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2720142/

Wednesday, 10 July 2013

The Obesity Pandemic: What can be done?

The statistics on obesity are just frightening... according to the World Health Organisation (WHO) in 2010 the following is true regarding individuals over the age of 15:

Country
% Population that are Overweight (BMI>25)
% Population that are Obese (BMI>30)

Females
Males
Females
Males
Australia
66.5
75.7
29.1
28.4
Japan
16.2
29.8
1.1
2.3
South Africa
68.5
41.3
36.8
7.6
United Kingdom
63.8
67.8
26.3
23.7
United States
76.7
80.5
48.3
44.2

A healthy BMI is between 18.5 and 25. A person is usually called overweight if their BMI is >25. Usually a BMI of >30 is considered obese. However, I should probably state that this model does not take into consideration their tissue type, i.e. whether their weight is mostly due to adipose tissue (i.e. fat) or muscle.
If you would like to see more stats on the subject you can go to https://apps.who.int/infobase/Comparisons.aspx.

These statistics are worrying considering the detrimental health effects of obesity. Obese individuals are approximately 40% more likely to develop cardiovascular disease, and substantially more likely to develop a variety of cancers including oesophageal, pancreatic, colorectal, breast, endometrial, kidney, thyroid and gallbladder cancers and substantially more likely to develop type II diabetes mellitus.1-4 In one study men with a BMI of >35 had a 42 fold higher risk of developing type II diabetes mellitus.5 These risks are not all those that have come to light in recent years with dementia and multiple sclerosis, among other conditions, being more common among the obese.4,6,7

I personally see obesity as an addiction, an addiction to simple sugars like fructose, glucose, sucrose and to unhealthy fats like saturated and transunsaturated fats. Treatments for this addiction include drugs that suppress appetite (e.g. bupropion [ZYBAN], atomoxetine [STRATTERA], modafinil [PROVIGIL], naltrexone [REVIA], lorcaserin [BELVIQ], sibutramine [MERIDIA; withdrawn from the market due to safety concerns], amfepramone [BONTRIL], topiramate [TOPAMAX], phentermine [ADIPEX-P], rimonabant [ACOMPLIA; now withdrawn from the market due to safety concerns], fenfluramine [along with phentermine it formed the famous dieting medication FEN-PHEN which has now been withdrawn from the market due to safety concerns] etc.) and drugs that prevent the metabolism of dietary fats by the body (e.g. orlistat [ALLI]). Plus there is substituting unhealthy flavourings (e.g. simple sugars) with healthy flavourings like artificial sweeteners or natural sweeteners.

Of course there is the age-old technique of dieting and exercise, but in my opinion these methods are rather ineffective in the long-run due to the fact that obesity is an addiction and as with people telling an addict to change it just does not work regardless of how much knowledge of the consequences they have. I do believe that people can overcome addictions cold turkey (or via dieting and exercising in the case of obesity) if they are given real motivation to do so, perhaps by their habit threatening to harm those they care about and hence this might give them sufficient motivation for dieting and exercise to work. Since I like to talk about drugs, however, I am going to devote a great deal of this post to talking about the drugs used to combat obesity.

Bupropion, atomoxetine and modafinil are sometimes used off-label (i.e. the US FDA and the Australian TGA has not approved either drug for this use) to treat obesity, probably due to their stimulant effects. See drugs that serve as stimulants (i.e. drugs that promote wakefulness, alertness and generally stimulate brain activity. This includes illicit stimulants like methamphetamine, amphetamine, methylphenidate [RITALIN], etc) nearly always have additional appetite-supressing effects and the same is true with these three drugs. Bupropion is traditionally used to treat depression or help people quit smoking and it works by raising the levels of norepinephrine (or noradrenaline) and dopamine in the body, this includes the hypothalamus which is a part of the brain that, among other things, regulates feeding behaviour. Likewise atomoxetine is normally used to treat attention deficit hyperactivity disorder (ADHD) but it also elevates central nervous system (CNS; brain and spinal cord) levels of norepinephrine (while having minimal effects on dopamine concentrations in the CNS) including in the hypothalamus. Modafinil on the other hand is normally used to treat sleep disorders like narcolepsy and works primarily by increasing CNS concentrations of dopamine while also altering the actions of other neurotransmitters in the brain and spinal cord. These three drugs are reuptake inhibitors of these two neurotransmitters which means in effect they prevent the “leakage” of these neurotransmitters from the gap between neurons (electrically signalling brain, spinal cord and nerve cells) which is also known as the synaptic cleft, across which they are used as chemical messengers between neurons (the electrically signalling cells of the brain, spinal cord and nerves).  

Amfepramone (or diethylpropion) is also a stimulant drug but is different in that it is converted by the body into ethcathinone which is a releasing agent of norepinephrine (in contrast to reuptake inhibitors releasing agents just cause neurons to release more of the respective neurotransmitter [in this case norepinephrine] into the synaptic cleft) which is something the stimulant amphetamine does, but unlike amphetamine it has minimal effects on dopamine and serotonin release, hence limiting its potential for causing addiction (since dopamine is associated with the euphoria recreational drugs create and hence are implicated in how they cause addiction). Amfepramone is also unique compared to the aforementioned three stimulants in that it is actually FDA and TGA approved for weight loss.

Sibutramine is similar to atomoxetine in that it increases brain concentrations of norepinephrine by inhibiting its reuptake. It has been withdrawn from the market, however, amidst concerns it can cause cardiovascular disease in susceptible individuals.8

The mechanism via which topiramate manages to promote weight loss is not unclear, but it is known that it promotes satiety (i.e. not eating) by supressing appetite.9

Phentermine and fenfluramine also work via serving as stimulants hence promoting satiety.

Naltrexone on the other hand is a drug used to treat opioid dependence (i.e. addiction to opioids like heroin, oxycodone, etc.). Naltrexone is currently being investigated, when combined with bupropion that is, for its ability to reduce body weight in obese patients. Naltrexone when used by itself has inconsistently (i.e. some clinical trials have failed to demonstrate any benefit of it in the indication of weight loss) been found to cause some weight loss with some evidence indicating that the weight-loss it encourages is more prominent in females. It is believed its beneficial effects on weight are due to its ability to block the opioid receptors of the central nervous system which are involved in reward, perhaps including the rewarding effects of eating.10

Rimonabant, however, was quite a promising appetite-suppressant until it was withdrawn from the market in late 2008. It worked by blocking the first cannabinoid receptor, which is the receptor that cannabis, by stimulating, manages to elicit its appetite-stimulating effects. Unfortunately, however, by blocking this receptor it caused a number of patients to become depressed even to the point of suicide.11,12 It was promising in a number of ways, including that it managed to improve insulin sensitivity, and hence might afford protection against the metabolic consequences of obesity.

Lorcaserin is a member of a new class of weight loss medications with, hopefully, a more favourable side effect profile. It works, or so we believe, by activating the 5-HT2C receptor which, among other things, appears to regulate feeding behaviour. The 5-HT2C receptor may also be a promising as a target for drug abuse disorders like cocaine dependence. This is due to the fact that it reduces the concentrations of dopamine in the mesolimbic pathway which is also known as the “reward pathway”. Increases in dopamine in the mesolimbic pathway are directly associated with the rewarding effect of drugs of abuse like cocaine,13 and interestingly, even fructose.14 Unfortunately, however, in diabetic individuals lorcaserin has been linked to hypoglycaemia – low blood sugar, which can be fatal if not promptly treated.15 Since over-functioning of the mesolimbic pathway has also been associated with schizophrenia it was hoped that another 5-HT2C agonist (activator) vabicaserin might possess antipsychotic effects, which thing was confirmed by one clinical trial (the reason why it is no longer being pursued as a treatment for schizophrenia is a mystery to me).16

Orlistat on the other hand is a drug that inhibits pancreatic lipases which are enzymes that the body uses to metabolise dietary fats into things it can use. By inhibiting pancreatic lipases the body is left with no choice but to leave dietary fats in the stool which is why a well-known side effect of orlistat is that it causes loose, oily and foul-smelling stools. It is the only pharmaceutical weight loss drug that I am aware of that is over the counter in many countries. Orlistat also appears capable of reducing the incidence of diabetes in the obese.17

Some have also suggested that banning processed sugars from our food would reduce the incidence of obesity and while it sounds perfect on paper in practice it is less so seeing how prohibition of drugs that the public are already addicted to or accustomed to normally results in nothing of any benefit considering the fact that when America tried this in the 20s and 30s with the prohibition of alcohol and while the availability of alcohol was unchanged because alcohol production and distribution had fallen into the black market organised crime was profiting from the law and this saw the uprising of organised crime in America. Plus I personally do not see any sizeable benefit that would come from banning sugar since as is seen in countries with a high religious anti-drug presence, such as Saudi Arabia, when one (or in the case of Saudi several) substance of abuse is stamped out what remains that has a potential for abuse (in the case of Saudi this is unhealthy food, seeing how obesity is very common there) is abused more than prior to the prohibition.18 People with an addiction are born, often, with an predisposition towards addiction, sort of like a gap if you would that they feel the need to fill with something, be that drugs, be that unhealthy foods, be that alcohol, etc. and if you take away one substance of abuse they will substitute it with something else.

Reference List

  1. Hubert HB, Feinleib M, McNamara PM, Castelli WP. Obesity as an independent risk factor for cardiovascular disease: a 26-year follow-up of participants in the Framingham Heart Study. Circulation [Internet]. 1983 May 1 [cited 2013 Jul 9];67(5):968–77. Available from: http://circ.ahajournals.org/content/67/5/968
  2. Kruijsdijk RCM van, Wall E van der, Visseren FLJ. Obesity and Cancer: The Role of Dysfunctional Adipose Tissue. Cancer Epidemiol Biomarkers Prev [Internet]. 2009 Oct 1 [cited 2013 Jul 9];18(10):2569–78. Available from: http://cebp.aacrjournals.org/content/18/10/2569
  3. Mokdad AH FE. Prevalence of obesity, diabetes, and obesity-related health risk factors, 2001. JAMA [Internet]. 2003 Jan 1 [cited 2013 Jul 9];289(1):76–9. Available from: http://dx.doi.org/10.1001/jama.289.1.76
  4. Haslam DW, James WPT. Obesity. The Lancet [Internet]. 1 [cited 2013 Jul 9];366(9492):1197–209. Available from: http://www.sciencedirect.com/science/article/pii/S0140673605674831
  5. Chan JM, Rimm EB, Colditz GA, Stampfer MJ, Willett WC. Obesity, Fat Distribution, and Weight Gain as Risk Factors for Clinical Diabetes in Men. Dia Care [Internet]. 1994 Sep 1 [cited 2013 Jul 9];17(9):961–9. Available from: http://care.diabetesjournals.org/content/17/9/961
  6. Munger KL, Chitnis T, Ascherio A. Body size and risk of MS in two cohorts of US women. Neurology [Internet]. 2009 Nov 10 [cited 2013 Jul 9];73(19):1543–50. Available from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2777074/
  7. Beydoun MA, Beydoun HA, Wang Y. Obesity and central obesity as risk factors for incident dementia and its subtypes: a systematic review and meta-analysis. Obesity Reviews [Internet]. 2008 [cited 2013 Jul 9];9(3):204–18. Available from: http://onlinelibrary.wiley.com/doi/10.1111/j.1467-789X.2008.00473.x/abstract
  8. Scheen AJ. Sibutramine on Cardiovascular Outcome. Diabetes Care [Internet]. 2011 Apr 27 [cited 2013 Jul 9];34(Supplement_2):S114–S119. Available from: http://care.diabetesjournals.org/content/34/Supplement_2/S114.full.pdf
  9. Verrotti A, Scaparrotta A, Agostinelli S, Di Pillo S, Chiarelli F, Grosso S. Topiramate-induced weight loss: A review. Epilepsy Research [Internet]. 2011 Aug [cited 2013 Jul 9];95(3):189–99. Available from: http://www.epires-journal.com/article/S0920-1211(11)00134-3/abstract
  10. Ornellas T, Chavez B. Naltrexone SR/Bupropion SR (Contrave). P T [Internet]. 2011 May [cited 2013 Jul 9];36(5):255–62. Available from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3138366/
  11. Stapleton JA. [Commentary] TRIAL COMES TOO LATE AS PSYCHIATRIC SIDE EFFECTS END HOPE FOR RIMONABANT. Addiction [Internet]. 2009 [cited 2013 Jul 9];104(2):277–8. Available from: http://onlinelibrary.wiley.com/doi/10.1111/j.1360-0443.2008.02487.x/abstract
  12. Topol EJ, Bousser M-G, Fox KA, Creager MA, Despres J-P, Easton JD, et al. Rimonabant for prevention of cardiovascular events (CRESCENDO): a randomised, multicentre, placebo-controlled trial. The Lancet [Internet]. 14 [cited 2013 Jul 9];376(9740):517–23. Available from: http://www.sciencedirect.com/science/article/pii/S014067361060935X
  13. Bubar M, Cunningham K. Serotonin 5-HT2A and 5-HT2C Receptors as Potential Targets for Modulation of Psychostimulant Use and Dependence. Current Topics in Medicinal Chemistry [Internet]. 2006 Sep 1 [cited 2013 Jul 9];6(18):1971–85. Available from: http://www.eurekaselect.com/77101/article
  14. Bernal SY, Dostova I, Kest A, Abayev Y, Kandova E, Touzani K, et al. Role of dopamine D1 and D2 receptors in the nucleus accumbens shell on the acquisition and expression of fructose-conditioned flavor–flavor preferences in rats. Behavioural Brain Research [Internet]. 2008 Jun 26 [cited 2013 Jul 9];190(1):59–66. Available from: http://www.sciencedirect.com/science/article/pii/S0166432808000788
  15. Truven Health Analytics, Inc. DRUGDEX® System (Internet) [cited 2013 Jul 10]. Greenwood Village, CO: Thomsen Healthcare; 2013.
  16. Shen JHQ, Zhao Y, Rosenzweig-Lipson S, Popp D, Williams JBW, Giller E, Detke MJ, Kane J. A 6-week Randomized, Double-Blind, Placebo-Controlled, Comparator Referenced, Multicenter Trial of Vabicaserin in Subjects with Acute Exacerbation of Schizophrenia. Neuropsychopharmacology [Internet]. 2011 Dec 5 [cited 2013 Jul 10];36(60):S75–S197. Available from: http://www.nature.com/npp/journal/v36/n1s/full/npp2011291a.html
  17. Torgerson JS, Hauptman J, Boldrin MN, Sjöström L. XENical in the Prevention of Diabetes in Obese Subjects (XENDOS) Study A randomized study of orlistat as an adjunct to lifestyle changes for the prevention of type 2 diabetes in obese patients. Dia Care [Internet]. 2004 Jan 1 [cited 2013 Jul 10];27(1):155–61. Available from: http://care.diabetesjournals.org/content/27/1/155.full.pdf
  18. Kim KH, Sobal J, Wethington E. Religion and body weight. Int J Obes Relat Metab Disord [Internet]. 2003 [cited 2013 Jul 9];27(4):469–77. Available from: http://www.nature.com/ijo/journal/v27/n4/full/0802220a.html

Cannabidiol, the novel natural and well-tolerated antipsychotic

It's intriguing that while we were searching for means to ameliorate the suffering of schizophrenic individuals with pharmaceutics, nature had already provided us with a means of alleviating their suffering from a most unlikely source: the cannabis sativa plant. The cannabis sativa plant, also known as the marijuana plant, contains a compound that has, of late, been found to possess prominent antipsychotic effects comparable to the antipsychotic drug amisulpride (if you want to put the efficacy of amisulpride into perspective since, of course, different drugs have different efficacy in treating the wide range of conditions out there, amisulpride has been found to be the 2nd most effective antipsychotic, behind the infamous antipsychotic clozapine, according to two meta analyses of clinical trial data) in patients with schizophrenia. This compound is known as cannabidiol, and unlike many of the other "active" constituents of cannabis its effects are in opposition with the psychoactive effects of infamous, chief psychoactive constituent of the cannabis plant, tetrahydrocannabinol (THC).1

Unlike amisulpride, however, which is notorious for its ability to prolong the QT interval (in other words alter the electrical activity of the heart in a way that can lead to potentially fatal irregularities in heart rate), elevate blood prolactin levels (which can lead to the enlargement of breast tissue in men, and even galactorrhoea [the release of milk] even in males, and can, after long-term elevation lead to osteoporosis [brittle bones] and sexual dysfunction in both sexes), cause Parkinson's disease-like side effects (i.e. tremor, trouble initiating and stopping movement, etc.), obesity and even cause type II diabetes mellitus and cardiovascular disease, cannabidiol in this trial seldom even caused any side effects and when it did they were mild. 

Cannabidiol also appears to have an added advantage over many antipsychotics in that it appears to have prominent antidepressant,2 anxiolytic (anti-anxiety)3 and antineoplastic (anti-cancer) effects4 which may be advantageous in schizophrenic patients seeing how they have a heightened risk of suicide, anxiety and premature death due to, among other things, cancer (interestingly approximately 80% of them smoke tobacco compared to around 20% of the general population). Cannabidiol is also known to possess some anticonvulsant (seizure preventing and suppressing) effects5 and hence may be advantageous when used to augment (improve responses to) existing regimens of antipsychotics which can, albeit rarely, cause seizures.

References

  1. Leweke FM, Piomelli D, Pahlisch F, Muhl D, Gerth CW, Hoyer C, et al. Cannabidiol enhances anandamide signaling and alleviates psychotic symptoms of schizophrenia. Transl Psychiatry [Internet]. 2012 Mar 20 [cited 2013 Jul 10];2(3):e94. Available from: http://www.nature.com/tp/journal/v2/n3/full/tp201215a.html
  2. Zanelati T, Biojone C, Moreira F, Guimaraes F, Joca S. Antidepressant-like effects of cannabidiol in mice: possible involvement of 5-HT1A receptors. Br J Pharmacol [Internet]. 2010 Jan [cited 2013 Jul 10];159(1):122–8. Available from:http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2823358/
  3. Schier AR de M, Ribeiro NP de O, Silva AC de O e, Hallak JEC, Crippa JAS, Nardi AE, et al. Cannabidiol, a Cannabis sativa constituent, as an anxiolytic drug. Revista Brasileira de Psiquiatria [Internet]. 2012 Jun [cited 2013 Jul 10];34:104–10. Available from: http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1516-44462012000500008&lng=en&nrm=iso&tlng=en
  4. Massi P, Solinas M, Cinquina V, Parolaro D. Cannabidiol as potential anticancer drug. British Journal of Clinical Pharmacology [Internet]. 2013 [cited 2013 Jul 10];75(2):303–12. Available from: http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2125.2012.04298.x/abstract
  5. Cunha JM, Carlini EA, Pereira AE, Ramos OL, Pimentel C, Gagliardi R, et al. Chronic Administration of Cannabidiol to Healthy Volunteers and Epileptic Patients. Pharmacology [Internet]. 1980 [cited 2013 Jul 10];21(3):175–85. Available from:http://www.karger.com/Article/Abstract/137430

Wednesday, 3 July 2013

A New Novel Class of painkillers, antidepressants, anxiety-relievers and nausea and vomiting relievers

Lately a new class of drugs have been pursued that might benefit virtually all of us due to the fact that it is being developed as a potentially safer alternative to the non-steroidal anti-inflammatory drugs (NSAIDs) which include aspirin, ibuprofen (NUROFEN), naproxen (NAPROSYN) and diclofenac (VOLTAREN) that we have grown so accustomed to popping when we suffer a headache, fever, cold*, flu*, etc. This class of drugs is so intriguing to me in particular because it takes an old herbal remedy that, despite its vital role in saving many and reducing the suffering of numerous others and has never claimed so much as a single life, has been criminalised without fair trial and deemed to serve no medical purpose, and turns it into a drug for which not only cannot kill but from what we can tell poses no potential for being used recreationally. The herbal remedy, of course, am referring to are the extracts of the cannabis sativa and cannabis indica (marijuana, hashish) plants and the new class of drugs are the fatty acid amide hydrolase (FAAH) inhibitors.

These drugs work by inhibiting the enzyme, fatty acid amide hydrolase (FAAH), which catalyses the degradation of a few biologically-active fatty substances the body synthesises known as fatty acid amides. The way how this relates to cannabis is that the fatty acid amides include the compound anandamide who’s roles in the body includes activating the cannabinoid CB1 receptor, which is the receptor the chief psychoactive constituent of cannabis, tetrahydrocannabinol (THC) activates in order to elicit its psychoactive effects. Interestingly, however, FAAH inhibitors, fail to produce many of the dose-limiting temporary side effects of cannabinoids (which are the compounds in, or related to those found in the cannabis sativa and indica plants) such as impairments in short-term memory, increased appetite which often can lead to obesity with chronic (long-term) use and motor deficits (trouble controlling voluntary movements with any degree of precision). To this end one phase I clinical trial (a clinical trial designed to establish the safety of a drug in healthy human volunteers and a safe, yet theoretically therapeutically optimal dosage range for the drug) which involved 32 adult volunteers randomised to receive the drug (and 8 randomised to receive placebo) demonstrated quite clearly that the first FAAH inhibitor to reach clinical testing, PF-04457845, was safe and well-tolerated (i.e. caused only mild and rare side effects).1

Unfortunately for a reason that completely baffles me in one randomised double-blind placebo-controlled phase II clinical trial involving patients with osteoarthritic pain in their knees PF-04457845 failed to produce a clinically-significant improvement in their pain compared to a placebo and naproxen-receiving groups that were also enrolled in this trial. I suspect, or hope rather, since this was such a promising potential target for new painkillers that were at least theoretically incapable of causing addiction and incapable of causing death on overdose or from rare side effects (I am basing this opinion on data that was obtained by testing the drug on laboratory animals, and what we know about it and related drugs based on how they interact with the molecular targets of the body), that either the dose was too low or it was being used against the wrong type of pain or there was some impurities in the drug they delivered to the patients that obscured the results.2


Another thing that gets me excited about FAAH inhibitors like PF-04457845 is that we have reason to believe that they would produce synergistic (additive) analgesic effects when used in combination with NSAIDs, paracetamol (acetaminophen in the US; PANADOL; TYLENOL in the US) and opioids like morphine, codeine, pethidine (meperidine in the US), oxycodone (OXYCONTIN), hydrocodone (VICODIN), methadone and fentanyl

* NOTE: GIVING CHILDREN OR YOUNG ADULTS (ANYONE UNDER 25 YEARS OF AGE IS COUNTED UNDER THIS) WITH A COLD, FLU OR OTHER VIRAL INFECTION ASPIRIN IS SOMETHING YOU SHOULD NEVER EVER DO AS IT CAN LEAD TO A DIFFICULT-TO-TREAT AND FREQUENTLY FATAL CONDITION KNOWN AS REYE'S SYNDROME. REYE’S SYNDROME IN ADULTS HAS BEEN REPORTED AND HENCE IT IS ADVISABLE THAT ADULTS ALSO AVOID ASPIRIN WHILE THEY HAVE A COLD/FLU.

Reference List

  1. Li GL, Winter H, Arends R, Jay GW, Le V, Young T, Huggins JP. Assessment of the pharmacology and tolerability of PF-04457845, an irreversible inhibitor of fatty acid amide hydrolase-1, in healthy subjects. Br J Clin Pharmacol. 2012 May [cited 2013 Jul 3];73(5):706–716. DOI: 10.1111/j.1365-2125.2011.04137.x
  2. Huggins JP, Smart TS, Langman S, Taylor L, Young T. An efficient randomised, placebo-controlled clinical trial with the irreversible fatty acid amide hydrolase-1 inhibitor PF-04457845, which modulates endocannabinoids but fails to induce effective analgesia in patients with pain due to osteoarthritis of the knee. PAIN [Internet]. 2012 Sep [cited 2013 Jul 3];153(9):1837–46. Available from: http://www.sciencedirect.com/science/article/pii/S0304395912002692
  3. Naidu PS, Booker L, Cravatt BF, Lichtman AH. Synergy between enzyme inhibitors of fatty acid amide hydrolase and cyclooxygenase in visceral nociception. J Pharmacol Exp Ther. 2009 Apr [cited 2013 July 3];329(1):48-56. DOI: 10.1124/jpet.108.143487


Tuesday, 2 July 2013

Porphyria: A Guide for the Laymen

Note: Some terms are not explained within the main text and are, instead, explained in the glossary which is immediately before the reference list, near the back of the post.


Introduction

The porphyrias are a collection of rare, often inherited, hematologic (blood) diseases that are often tricky for doctors to diagnose. They all stem from the body having a compromised ability to synthesise the biologically-significant compound, haem, from its chemical precursors, the porphyrins.1

Haem is so significant and important in the human body because it is used to create a class of proteins known as the haemoproteins which, in turn, play pivotal roles in the body – from carrying the oxygen stored within the red blood cells of the blood (this particular type of haemoproteins is known as haemoglobin), to the metabolism of vital bodily hormones and drugs (these haemoproteins are known as cytochromes). Haem is mostly synthesised in the bone marrow (85% of it is), probably due to its crucial role in the formation of haemoglobin, while the remaining 15% is mostly synthesised in the liver. It is synthesised via a very long process involving multiple enzymes (to those of you that do not understand much chemistry, enzymes are basically the helpers or accelerators of chemical reactions). For the details of this process see figure 1. In porphyria one of these enzymes is either faulty or found in substandard quantities usually due to mutations in the gene that regulates their formation.2


Figure 1: Haem Synthesis2
Enzymes are given in boxes, porphyrins are given outside of them (except glycine and succinyl-CoA are not porphyrins). Cytosol and mitochondrion are where, in cells, these reactions occur. This diagram may be helpful later when I go into detail about the different types of porphyria and the specific enzymes implicated in each type. You need not know anything about these porphyrins or the enzymes that catalyse their formation – just pay attention to their names because latter I will tie deficiencies of the specific enzymes involved in haem synthesis with the different types, or varieties, of porphyria.


Haem deficiency is not the only problem that leads to the symptoms and complications of porphyria, a major contributor to the manifestations of the disease are the porphyrins themselves. In unaffected individuals the amount of the different porphyrins in the body are regulated and limited by the enzymes involved in their transformation into haem. This is because the higher the concentrations of these porphyrins the more reactions will be underway transforming them into the next porphyrin in the production line (that is displayed in figure 1). In porphyric individuals, since they are deficient in one of the enzymes involved in haem synthesis, the porphyrin normally transformed by the deficient enzyme (into the next porphyrin, that is) accumulates, and since porphyrins can be toxic in high quantities this can lead to a number of problems, often neurologic (pertaining to nerves, the brain and/or the spinal cord) or cutaneous (pertaining to the skin) in nature.2

The Types of Porphyria and their Usual Manifestations

The porphyrias are commonly divided into two different groups based on their symptoms and pathophysiology (literally means the mechanism(s) of the disease), the acute porphyrias and the cutaneous porphyrias. All of which can potentially cause a, often very dark, red discoloration of the urine and there is some overlap between the two categories: some acute porphyrias can present with cutaneous symptoms (e.g. HCP, VP, ALAD deficiency porphyria, EHP).

Cutaneous Porphyrias

The cutaneous porphyrias present, usually, with almost solely cutaneous (skin) symptoms that occur upon sun exposure they include:3
  • Fragility of the skin
  • Skin blistering
  • Scaring
  • Opportunistic skin infections (i.e. infections that affect the skin when it is already compromised in its health) 
  • Changes in skin coloration
these symptoms persist throughout life in cases of genetic cutaneous porphyrias whenever patients are exposed to sunlight. Although the cutaneous porphyrias are usually limited, almost entirely, to the skin they can, on rare occasions cause significant liver damage and sometimes even liver failure severe enough to warrant a transplant. The cutaneous porphyrias are usually, at least in part, erythropoietic in nature. That is, they arise due to enzyme deficiencies in the bone marrow not the liver. One type is hepatoerythropoietic in nature, namely, hepatoerythropoietic porphyria, this means that it is due to enzyme deficiencies in both the bone marrow and the liver (hence the hepato prefix).

Table 1: Types of Cutaneous Porphyrias2-4

Porphyria Type
Relevant Enzyme
Notes
Porphyria cutanea tarda (PCT)
Uroporphyrinogen decarboxylase (UDOC)
PCT is usually (80-90% of cases are) sporadic (i.e. cases in which no mutation in the UDOC gene can be identified). Sporadic cases can be caused by a number of other conditions or even occur without a known cause. Known causes include hepatitis C and other viral infections including HIV, alcoholism, renal (kidney) dialysis, oestrogen supplements, excessive iron supplementation and hemochromatosis (a genetic condition that leads to the accumulation of iron in the body).2,5

10-20% of cases are due to a mutation in the UDOC gene that leads to insufficient quantities or qualities of the UDOC enzyme being present in the liver and bone marrow. These cases are often referred to as familial porphyria cutanea tarda. The UDOC gene is autosomal dominant in inheritance. That is the offspring of an affected individual will have a 50/50 chance of being susceptible to the condition.

PCT is the most common form of cutaneous porphyria and the most common form of porphyria overall, despite this it is still relatively rare and affects around 1/1000 people worldwide.
Hepatoerythropoietic porphyria (HEP)
Uroporphyrinogen decarboxylase (UDOC)
HEP is due to a mutation in the UDOC gene and is similar to cases of familiar porphyria cutanea tarda but more severe and it usually presents (i.e. displays symptoms) earlier on in life. HEP occurs in patients homozygous for subfunctional UDOC genes.4
Erythropoietic protoporphyria (EPP)
Ferrochelatase (FECH)
Usually presents in infancy to early childhood and it can follow both autosomal dominant and autosomal recessive modes of inheritance, although autosomal recessive inheritance is certainly its predominant mode of inheritance.6
Congenital erythropoietic porphyria (CEP)
Uroporphyrinogen III synthase (UROS)
It is incredibly rare (1/1,000,000 people have it) and often presents with other symptoms too like hair loss or excessive hair growth, vision loss, anaemia (low blood count), shortness of stature, abdominal pain, pink- or dark-stained urine, an enlarged spleen, inflammation of the outer layers of the eyes, fractures, abnormally-formed bones, gallstones, etc.3,5 The cutaneous manifestations of this type of porphyria is often more severe than in the other varieties of cutaneous porphyria and can even be triggered by surgical lights, not necessarily sunlight.4

Acute Porphyrias

Acute porphyrias, however, usually present with acute (short-term) bouts (or attacks) of gastrointestinal (digestive), neurologic, psychiatric, hepatic (liver), renal (kidney), cardiovascular (heart and blood vessel) and, depending on the deficient enzyme (and hence the type of acute porphyria since they are all categorised according to the deficient enzyme that causes them), cutaneous (which are not limited to occurring during attacks; they can occur whenever an affected person is exposed to sunlight) symptoms. The symptoms of an acute attack include:2-4,9
  • Severe abdominal pain (which is the single most common symptom of the condition, over 80% of individuals will suffer abdominal pain during attacks)
  • Autonomic dysregulation (Symptoms of autonomic dysregulation that are often seen in the acute porphyrias include: nausea, vomiting, constipation, ileus (paralysis of the bowels), tachycardia (high heart rate), hypertension (high blood pressure), sweating, urinary retention, urinary incontinence)
  • Seizures
  • Paralysis
  • Tremor
  • Restlessness
  • Fever
  • Elevated blood catecholamines (the catecholamines include the fight and flight hormones adrenaline and noradrenaline, hence when people have elevated blood catecholamines they get two previous mentioned symptoms– hypertension and tachycardia. These symptoms can also be due to the damage the porphyrias can do to nerves)
  • Mental changes (these occur in around 30% of cases and include anxiety, depression, hallucinations, delusions, insomnia, excessive drowsiness, irritability, problems with memory, learning, concentration, problem solving, etc. Rarely these might be the most prominent symptoms of an attack)
  • Hyponatraemia (low blood sodium, this often causes the seizures associated with the condition, it can also cause muscle cramping, spasms, loss of appetite, headache, etc.)
  • Opportunistic infections (infections that have just taken the opportunity of your ill-health to kick you while you are down)

due to the potential for acute attacks to cause comas, paralysis of the muscles involved in breathing, tachycardia, hypertension, depression and other potentially deadly complications acute attacks can be fatal. Rare complications that can also eventuate include chronic renal (kidney) failure, often due to the nephrotoxic (toxic to the kidneys) porphyrin metabolites and/or sustained hypertension and it has also been found that those with acute porphyrias are at a heightened risk of developing the, often fatal, liver cancer, hepatocellular carcinoma (HCC).2,

These acute attacks are typically triggered in genetically susceptible individuals by at least one of the following:2-4
  • Stress
  • Drugs (especially sulfa-containing antibiotics, barbiturates, synthetic oestrogens, certain antiepileptic drugs, alcohol and a few others. See http://www.drugs-porphyria.org/ for details regarding safe and unsafe drugs in acute porphyrias)
  • Dietary changes (high protein diets, low-carbohydrate diets and fasts are especially notable for causing acute attacks)
  • Endocrine factors (fluctuations in the levels of the different hormones present in the body; usually the sex hormones such as oestrogens and androgens [e.g. testosterone, dihydrotestosterone]. Acute attacks are often triggered by the hormonal changes that occur in the different stages of development, e.g. puberty, menopause, etc.)

Table 2: Types of Acute Porphyrias

Porphyria Type
Relevant Enzyme
Notes
Acute intermittent porphyria (AIP)
Porphobilinogen deaminase (PBG-D) also known as hydroxymethylbilane synthase (HMBS)
It is the second most common type of porphyria and hence the most common type of acute porphyria. Its incidence is estimated to be 5-10/100,000. It is inherited in an autosomal dominant fashion, yet 90% of patients do not display any symptoms despite being heterozygous for the mutated gene.6 It usually presents initially in the 30s and very rarely presents prior to puberty. It more frequently causes symptoms in women than in men, likely due to the fact that oestrogens are well-known triggers for acute attacks. Five cases of homozygous AIP have been described too, and they are usually very severe. It is a hepatic porphyria. Cutaneous symptoms are absent in this subtype of porphyria.2-4;8-9
Aminolevulinic acid dehydratase (ALAD) deficiency porphyria

Aminolevulinic acid dehydratase (ALAD)
This is the rarest form of porphyria with only six cases reported worldwide.2 Symptoms usually first appeared during adolescence in the six cases reported thus far.2 It follows the autosomal recessive inheritance pattern.10 It can cause cutaneous symptoms.4
Chester Porphyria
Porphobilinogen deaminase (PBG-D)
A very rare subtype of AIP. It shows characteristics of variegate and acute intermittent porphyrias yet recently it was demonstrated that it is, in fact, a rare subtype or variant of AIP.11
Erythropoietic harderoporphyria (EHP)
Coproporphyrinogen oxidase (CPOX)
Similar to HCP but more severe and due to more specific mutations in the CPOX gene. It normally occurs due to the affected being homozygous for a specific allele of the CPOX gene known as the K404E allele. Alternatively it can be due to null CPOX allele – that is, an allele that causes absolutely no CPOX to be formed in the body. This type of porphyria can also cause hematologic (blood-related) symptoms such as dyserythropoiesis (the abnormal formation of red blood cells) and iron overload. It can present with cutaneous symptoms.12
Hereditary Coproporphyria (HCP)
Coproporphyrinogen oxidase (CPOX)
Presents with acute attacks and cutaneous symptoms. It is autosomal dominant in inheritence.2
Variegate Porphyria (VP)
Protoporphyrinogen oxidase (PPOX)
Relatively common in the South African (1/300 people have it approximately) and Chilean populations.3,13 It presents with both acute and cutaneous signs.2

Diagnosis

The diagnosis of porphyrias is tricky: part of it is ruling out other conditions that could cause the currently manifest symptoms, part of it involves running tests that can confirm porphyria if it is currently active but cannot discount the possibility of the patient having a latent (i.e. one that is not currently causing symptoms) case of porphyria and part of it is genetic testing which requires several months of waiting for the patient to receive the results. Another aspect of AIP and VP, in particular, which makes them such a difficult diagnosis to make without genetic testing is that they been found to be associated chronic anxiety, even in patients that are asymptomatic (i.e. those that are currently not having an attack). In fact it seems like some cases of generalised anxiety disorder (GAD) are in fact due to asymptomatic cases of AIP.14,15

The usual tests that can be performed on symptomatic individuals during an attack to confirm the diagnosis (although they cannot rule it out as the cause of a patient’s symptoms altogether) include urine, faecal and blood tests that look for elevated levels of porphyrins in the aforementioned samples.2,5 These test usually take only a couple of hours but in Australia, of course, with our health care system these tests are likely to take far longer unless your current attack is severe enough to make you a high priority.
Conditions that can mimic the porphyrias vary according to the porphyria in question but the following is a small list of a few that can:2,5
  • Acute leukaemias (in the case of acute porphyrias)
  • Anaemia (acute porphyrias)
  • Bone marrow transplantation complications
  • Hepatic (liver) failure
  • Hepatocellular carcinoma
  • Hepatitis B
  • Hepatitis C
  • HIV infection
  • Lymphoma
  • Diabetes (particularly acute porphyrias)
  • Guillain-Barré syndrome (an autoimmune (the immune system attacks the body in these conditions) condition that causes the destruction of the nerves of the body, it presents with ascending (going upwards in the body, starts at the feet and works its way up) paralysis, peripheral neuropathy (e.g. tingling, numbness, pain, etc. in the parts of the body who’s nerves are affected), etc.)
  • Systemic lupus erythematosus (SLE), a systemic autoimmune condition16


Management

The management of porphyrias varies greatly between the two divisions and hence they shall be mentioned in this document separately.

Acute Porphyrias

Acute Attacks

During acute attacks the acute porphyrias are treated via a number of different ways, often in combination, that depend ultimately on the severity and symptoms of the attack. They are universally treated by identification and removal of the offending agent (i.e. the thing that triggered the attack), and, often, also with a glucose (sugar water) intravenous (into the vein via a needle) infusion. This is because porphyrin synthesis is inhibited by carbohydrates. If the attack is severe and life-threatening enough and it is worth the risk of adverse effects, sometimes patients are given intravenous haemin, which is basically a form of haem. Its purpose in the treatment of acute attacks is that it, among other things conducive to the treatment of acute attacks inhibits the enzyme ALA synthase hence preventing the formation of porphyrins right at their source.2

Symptomatic relief (i.e. treatment of the symptoms) is usually attempted with nausea being treated with the antiemetic drug ondansetron (ZOFRAN), metoclopramide (MAXOLON) and other “safe” antiemetics, pain is usually managed with opioids like morphine or pethidine, seizures are often managed with anticonvulsant drugs that are safe in acute porphyrias (e.g. lorazepam, diazepam [VALIUM] and gabapentin), hypertension and tachycardia being treated with beta blockers (drugs that interfere with noradrenaline binding to its receptors) such as propranolol and pindolol, constipation can be treated with laxatives, or, in laxative-resistant cases of suspected paralytic ileus with acetylcholinesterase inhibitors like neostigmine, pyridostigmine or donepezil due to their excitatory effects on the parasympathetic division of the autonomous nervous system. These medications may also help with the confusion that sometimes accompanies attacks.2

Chronic Management

To prevent future attacks most patients are advised to avoid known triggers, even triggers that have not triggered an attack for them yet. A high carbohydrate diet is often useful in the management of the porphyrias, and if weight loss is desired usually it is done under the supervision of a dietician in order to prevent the new diet from triggering acute attacks. Female patients with acute porphyrias sometimes develop attacks premenstrually and hence drugs that inhibit the menstrual cycle such as oral contraceptives may be helpful in these patients. Abstaining from alcohol is often recommended to prevent future attacks. The drug cimetidine, which is usually used in the management of stomach ulcer disease has been investigated as a potential preventative medication. Patients with acute porphyrias that are known to cause cutaneous manifestations are also advised to avoid sun exposure.2

It is also known that hepatic porphyrias can be cured with a liver transplant and since hepatocellular carcinoma is more common in acute porphyria patients this curative measure has been attempted in some instances. Doctors are unlikely to perform a transplant, however, unless it is the only viable option since liver transplantation in itself is life-threatening.2

Cutaneous Porphyrias

The treatment of cutaneous porphyria is mostly symptomatic and preventative. That is patients are recommended to avoid their trigger, that is, sunlight. Cutaneous porphyrias are often additionally treated with β-carotene, which is found in carrots its purpose is to absorb the free radicals generated when the porphyrins near their skin are exposed to light. Two drugs used in the treatment of malaria, chloroquine and hydroxychloroquine, can also be used to treat cutaneous porphyrias due to their ability to bind to hepatic porphyrins so that the body can excrete them in urine.5

Glossary

Acetylcholinesterase inhibitor – drugs that inhibit the enzyme acetylcholinesterase (AChE). AChE degrades (converts to biologically less active compounds) the neurotransmitter, acetylcholine, and hence AChE inhibitors elevate the concentrations of acetylcholine in the body, leading to heightened activity of the parasympathetic branch of the autonomic nervous system.
Allele – an alternate form of the same gene.

Autonomous nervous system –The autonomous nervous system controls the body’s involuntary movements like heart rate, the movement of food through the gastrointestinal (digestive) tract, the expulsion of sweat onto the surface of the skin, salivation, the contraction and dilation of the bronchial passages of the lungs, etc. It is further divided into the sympathetic (fight or flight) and parasympathetic divisions (or the rest and digest division).
The sympathetic nervous system involves things that occur pretty much instantly, like changes in heart rate, changes in blood pressure, and even the formation of goose bumps. The neurotransmitters (chemical messengers) of the sympathetic nervous system include adrenaline and noradrenaline. The parasympathetic branch, however, controls the more gradual involuntary movements like that of the gastrointestinal tract, urination, tears secretion, saliva secretion, defecation, etc. (I know that there can be confusion here seeing how urination and defecation is a voluntary action but there are also many involuntary components to it if you go into it). The neurotransmitter of the parasympathetic nervous system is acetylcholine.
The two branches of the autonomous nervous system are actually, usually, in opposition to each other. An increase in the activity of the parasympathetic nervous system leads to a decrease in the activity of the sympathetic nervous system.

Chromosome -- every animal has two copies of each chromosome (on which are contained each individual gene which regulates the formation of proteins like those that lead to the formation of skin), aside from the sex chromosomes (the X and Y chromosomes) of which females get two X’s and males get one X and one Y. These non-sex chromosomes are called autosomes. Sex chromosomes are also called allosomes. You get half your autosomes from your mum, half from your dad. You also get one X chromosome from your mum and if you are a female you also get one X chromosome from your dad and if you are male you get a Y chromosome from your dad.

Dominance – genes are either recessive or dominant. If an organism has even one copy of a particular dominant gene they will exhibit the characteristics that dominant gene confers. An example is black and blonde hair. The gene that encodes black hair is dominant over the gene that encodes blonde hair and hence if a blonde guy and a black haired-woman has a child the child will be black haired, assuming the woman is homozygous (i.e. has two copies of the gene) for the black hair gene. Someone with a copy of a recessive gene on the other hand will only exhibit the characteristics of the gene if they are homozygous for the aforementioned gene.

Emesis – nausea and vomiting. Hence antiemetics are nausea- and vomiting-treating agents
Generalised Anxiety Disorder (GAD) – an anxiety disorder characterised by excessive, disproportionate and irrational worry over everyday things.

Heterozygous – every animal has two copies of each autosome. If a creature is heterozygous for a particular gene that lies on one of these autosomes this means that only one of these two identical chromosomes has the aforementioned gene. Additionally, females can be heterozygous for a particular gene found on the X chromosome since they have two copies of the X chromosome.

Homozygous – this is when, instead of having one copy of a gene found on a particular chromosome one has two copies of the same gene from identical locations on the same chromosome.

Reference List

  1. Billoo AG, Lone SW. A family with acute intermittent porphyria. J Coll Physicians Surg Pak. 2008 May;18(5):316–8. Available from: http://ecommons.aku.edu/cgi/viewcontent.cgi?article=1011&context=pakistan_fhs_mc_women_childhealth_paediatr
  2. Frye RE, Coppes MJ, DeLoughery TG, et al. Acute Porphyria [Internet]. 2012 [cited 2013 Jul 1]. Available from: http://emedicine.medscape.com/article/957604
  3. Porphyria [Internet]. 2013 [cited 2013 Jul 1]. Available from: http://ghr.nlm.nih.gov/condition/porphyria
  4. Thadani H, Deacon A, Peters T. Diagnosis and management of porphyria. BMJ [Internet]. 2000 Jun 17 [cited 2013 Jul 1];320(7250):1647–51. Available from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1127427/
  5. Kanwar VS, Arceci RJ, DeLoughery TG, Frye RE, Adams DJ, Sarnaik SA, et al. Cutaneous Porphyria. 2012 Sep 28 [cited 2013 Jul 2]; Available from: http://emedicine.medscape.com/article/957765
  6. Rufenacht UB, Gouya L, Schneider-Yin X, Puy H, Schafer BW, Aquaron R, et al. Systematic analysis of molecular defects in the ferrochelatase gene from patients with erythropoietic protoporphyria. Am J Hum Genet [Internet]. 1998 Jun [cited 2013 Jul 1];62(6):1341–52. Available from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1377149/
  7. Reynolds NC, Berman SA, Neumeyer AM, et al. Diseases of Tetrapyrrole Metabolism - Refsum Disease and the Hepatic Porphyrias. 2012 Jun 17 [cited 2013 Jul 1]; Available from: http://emedicine.medscape.com/article/1148341-overview#a0199
  8. Solis C, Martinez-Bermejo A, Naidich TP, Kaufmann WE, Astrin KH, Bishop DF, et al. Acute intermittent porphyria: studies of the severe homozygous dominant disease provides insights into the neurologic attacks in acute porphyrias. Arch. Neurol. [Internet]. 2004 Nov;61(11):1764–70. Available from: http://dx.doi.org/10.1001/archneur.61.11.1764
  9. Whatley SD, Badminton MN. Acute Intermittent Porphyria. In: Pagon RA, Adam MP, Bird TD, Dolan CR, Fong C-T, Stephens K, editors. GeneReviews™ [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2013 [cited 2013 Jul 1]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK1193/
  10. Aminolevulinic Acid Dehydratase Porphyria (ADP) [Internet]. Rare Diseases Clinical Research Network. [cited 2013 Jul 1]. Available from: http://rarediseasesnetwork.epi.usf.edu/porphyrias/patients/ADP/
  11. Poblete-Gutiérrez P, Wiederholt T, Martinez-Mir A, Merk HF, Connor JM, Christiano AM, et al. Demystification of Chester porphyria: a nonsense mutation in the Porphobilinogen Deaminase gene. Physiol Res. 2006;55 Suppl 2:S137–144. Available from: http://www.biomed.cas.cz/physiolres/pdf/55%20Suppl%202/55_S137.pdf
  12. Schmitt C, Gouya L, Malonova E, Lamoril J, Camadro J-M, Flamme M, et al. Mutations in human CPO gene predict clinical expression of either hepatic hereditary coproporphyria or erythropoietic harderoporphyria. Hum. Mol. Genet. [Internet]. 2005 Oct 15 [cited 2013 Jul 1];14(20):3089–98. Available from: http://hmg.oxfordjournals.org/content/14/20/3089
  13. Poh-Fitzpatrick MB, Elston DM, Vinson RP, et al. Variegate Porphyria. 2013 Feb 13 [cited 2013 Jul 1]; Available from: http://emedicine.medscape.com/article/1103846-overview#showall
  14. Patience DA, Blackwood DHR, McColl KEL, Moore MR. Acute intermittent porphyria and mental illness – a family study. Acta Psychiatrica Scandinavica [Internet]. 1994 [cited 2013 Jul 1];89(4):262–7. Available from: http://onlinelibrary.wiley.com/doi/10.1111/j.1600-0447.1994.tb01511.x/abstract
  15. Millward LM, Kelly P, King A, Peters TJ. Anxiety and depression in the acute porphyrias. Journal of Inherited Metabolic Disease [Internet]. 2005 [cited 2013 Jul 2];28(6):1099–107. Available from: http://dx.doi.org/10.1007/s10545-005-4561-1
  16. Filiotou A, Vaiopoulos G, Capsimali V, Stavrianeas N, Kaloterakis A. Acute intermittent porphyria and systemic lupus erythematosus: report of a case and review of the literature. Lupus [Internet]. 2002 Mar 1 [cited 2013 Jul 2];11(3):190–2. Available from: http://lup.sagepub.com/content/11/3/190