Saturday, 26 April 2014

Opioids

Papaver somniferum.jpg
Figure 1: The opium poppy

The poppy

The opioids are a class of painkillers that are either derived from the opium poppy (Papaver somniferum; see figure 1) or have similar properties to those found in the poppy. The opium poppy has been used for countless centuries, with records of opium use dating back to at least 5000 BC. It’s been used for a wide range of different ailments, but today opioids are primarily used for their analgesic (painkilling), antidiarrhoeal and antitussive (cough-suppressing) properties. The poppy's resin is called opium and contains five major compounds that are useful to medicine (with their respective concentrations [weight-by-weight] in the poppy in brackets) ― morphine (10%), codeine (0.5%), thebaine (0.2%), papaverine (1%) and noscapine (6%).1-4

Noscapine and papaverine are devoid of opioid activity; although noscapine is sometimes used as an antitussive (although not available in Australia, the US or the UK) and papaverine is used for its vasodilatory (blood vessel-widening) and antispasmodic (spasm-relieving) effects.
Figure 2: Noscapine
Figure 3: Papaverine
Figure 4: Thebaine

Codeine and morphine (refer to table 2 for their structures) are the two analgesic constituents of opium; although codeine’s analgesic effects are less than one-tenth that of morphine on a dose-by-dose basis. Thebaine, on the other hand, is not therapeutically active in itself, but is closely chemically related to the opiates codeine and morphine and is usually used as a starting material for the synthesis of the semi-synthetic opioids (i.e. drugs that are synthetic derivatives of natural drugs, like thebaine).

Introduction to pharmacology

Before I go onto explain the effects of opioids on their respective receptors I need to introduce you to some fundamental pharmacology. Pharmacology can be further broken down into two areas: pharmacodynamics and pharmacokinetics. The area of pharmacodynamics is concerned with how the drug affects the body, whereas pharmacokinetics is concerned with how the body affects the drug.

Pharmacodynamics

The bits of pharmacodynamics I need you understand is that an agonist is an activator of a receptor and an antagonist is a blocker of the receptor in question. Both can be called a ligand. Both bind to the receptor in question; the agonist causes a change in the receptor's shape that leads to the physiologic reaction the body's own activator of receptor (the so called "endogenous ligand") causes; whereas an antagonist just binds to the receptor preventing any agonist (including the endogenous ligand) from binding to it and causes no physiologic response-causing changes in the receptor's shape. There are two types of agonist too — partial agonists and full agonists. Their distinction is that full agonists activate the receptor causing the same shape change as the endogenous ligand does, whereas partial agonists cause a shape change that is not the same as what the full agonists do and this difference causes them to produce a weaker corresponding physiologic response.

Two different partial agonists can produce differing degrees of physiologic response by causing different shape changes to the same receptor. The degree of receptor activation induced by a ligand is called its intrinsic activity (IA) — the higher the IA the greater the physiologic effect of the ligand binding to its receptor. The IA is given in units respective of the receptors' endogenous ligand, e.g. 10% IA indicates the ligand activates the receptor such that it produces 10% the physiologic effect of the endogenous ligand. Antagonist have an IA of 0%. Another important concept is called affinity which measures how strongly the ligand binds to its respective receptor, but a higher affinity does not necessarily mean a drug will produce a greater physiologic effect as antagonists also have an affinity towards their respective receptor. Potency is based on dose; a highly potent drug requires a low dose to produce the same effect as a less potent drug. Since partial agonists only partially activate their receptor, at a certain dose (which depends on the IA of the partial agonist), the effects reach a plateau at which no matter how much you increase the dose of the drug you see no increase in physiologic response.

Pharmacokinetics (PK)

Pharmacokinetics is less important here but if you want to grasp this blog better then I should explain a little about it. Pharmacokinetics really boils down to four things absorption, distribution, metabolism and excretion (often abbreviated ADME). Absorption is basically how the body absorbs the drug: from the digestive tract if it is taken orally (by mouth; usually in the form of a tablet, often given the abbreviation PO); from the mouth if it is given by a wafer that's held under the tongue (this is called sublingual administration (SL)); from the skin if applied to the skin in the form of a patch (like nicotine in a nicotine patch), this is called transdermal administration (TD); from muscles if it's injected into a muscle (called intramuscular administration (IM)); from just below the skin if it's given by subcutaneous injection (SC); injection into the spinal cord (intrathecal administration (IT)); injection near the spinal cord (epidural); from the rectum is given by suppository (this is called rectal administration), etc. The percentage of the drug that makes its way from the site of administration (e.g. the digestive tract if swallowed, injection site if given IM, SC; skin if given TD; etc.) into the bloodstream is called the bioavailability of the drug.

The distribution of the drug is basically how the drug distributes its way through the tissues of the body. I don't really mention it in this post. The metabolism of the drug is basically how the drug gets converted by the liver's enzymes or enzymes found in the rest of the body into other compounds, called the drug's metabolites. The body's aim in the metabolism process is basically to make the drug more water soluble as only then can it be excreted in the urine. These metabolites can have biologic effects and sometimes these biologic effects are far greater than the original drug (which can be referred to as the "parent compound").

One of the metabolising enzymes that's particularly relevant to the opioids is CYP2D6. Some people have a relatively low activity of this enzyme (~6-10% of Caucasians and 1-2% of Asians), whom are called poor metabolisers and others have an abnormally high activity of this enzyme (~10% of Caucasians; 1-2% of Asians and ~29% of Ethiopians), these people are called extensive metabolisers. Most opioids that depend on this enzyme for metabolism are metabolised into biologically active metabolites by said enzyme, hence poor metabolisers get weaker therapeutic effects, whereas extensive metabolisers get stronger effects, both therapeutic and undesirable.5

Excretion is basically how the body gets rid of the drug, which usually occurs via the faeces and urine. The combined process of metabolism and excretion is called elimination as either process gets rid of the original drug from the body; a measure of the rate of elimination is called the elimination half-life or half-life for short, which basically tells you how long it takes (usually in hours) for the body to eliminate half of the drug.


The opioid receptors

The effects of opioids are most often due to their ability to activate at least one of three opioid receptors (ORs) which are assigned the Greek letters μ (mu), κ (kappa) and δ (delta); which are designated MOR, KOR and DOR, respectively. These receptors are usually activated, in the body, by the so called endogenous opioids, which are commonly referred to as endorphins. These are peptides, which are basically a handful of amino acids (the base components of proteins also) bound together.4,6
Table 1: The roles of opioid receptors5
MOR Analgesia (seems to mediate these effects in the brain, spinal cord and nerves; seems to produce stronger analgesia than the other receptors), respiratory depression (causes you to breath less) pupil constriction (causes the pupils in your eyes to get smaller) constipation, euphoria (you get "high"), sedation and drug dependence.
KOR Analgesia (seems to mediate these effects in the spinal cord and nerves), pupil constriction, constipation (less than MOR/DOR), dysphoria (opposite of euphoria, including depression) hallucinations, delusions and sedation.
DOR Analgesia (spinal cord-mediated), respiratory depression and constipation.

Morphine binds predominantly to the MOR, although a weaker affinity towards the KOR is known to exist and its affinity towards the DOR is believed to be non-existent. The different roles of the different ORs is summarised in table. Morphine is a partial agonist at the MOR. Despite this most doctors and even pharmacists will tell you that it's a full agonist, as unlike the prototypical MOR partial agonist, buprenorphine, morphine can kill you in cases of overdose by means of respiratory depression which it mediates solely by means of its action at MOR. See buprenorphine is a lower IA partial agonist of the MOR than morphine and at clinically-utilised doses no plateau is seen for morphine as patients die from respiratory depression before such an effect can be seen. For buprenorpine the plateau is reached at a less than toxic (by means of respiratory depression) dose and hence it's impossible to OD on buprenorphine.4,6

Most opioids you know will probably act predominantly through the MOR.

Table 2: Comparison of opioids4-6
Drug
Morphine
Codeine
2D structure
Medical uses Moderate-severe pain, both acute (short-term) and chronic (long-term). Also used in general anaesthesia (like for surgery) on top of sedatives and for the relief of dyspnoea (shortness of breath). Can be used to treat cough, although it isn't usually used just for this.  Mild-moderate pain, cough and diarrhoea
Pharmacokinetics Bioavailability = 15-40% (PO)
Elimination half-life = 1.5-4.5 hours
Excretion = Urine, faeces.

Has an active metabolite called morphine-6-glucuronide which is actually significantly more active at the MOR but this metabolite has difficulty crossing the blood-brain barrier (BBB) which separates the blood in the body from the blood in the brain and spinal cord. Hence it produces most of its effects in the periphery (rest of the body).
Codeine is considered fairly inactive in itself but is metabolised (~10-20%) to morphine by CYP2D6
Routes of administration PO, IM, IV, SC, IT, rectal and epidural. PO, IM, IV, SC and rectal. 
Additional notes Mostly available as single-ingredient preparations. Some oral formulations containing naltrexone are available in the U.S., not Australia, though.  Combination with paracetamol, ibuprofen and aspirin are available in Australia. 
Side effects Nausea, vomiting, constipation, respiratory depression, euphoria, sedation, dizziness, itchiness, dry mouth, orthostatic hypotension (drops in blood pressure upon standing up which can lead to fainting) and addiction. Constipation, drowsiness, hypotension (low blood pressure), nausea, vomiting, false sense of well-being, abnormal heart rate, confusion, dizziness, headache, light-headedness, malaise, paradoxical CNS stimulation, restlessness, rash, urticaria, anorexia, dry mouth, reduced urination, blurred vision, dyspnoea and weakness. Rarely seizures, allergies and respiratory depression. 
Drug Oxycodone Hydrocodone
2D structure
Medical uses Moderate-severe pain, acute or chronic. Can be combined with naloxone so as to minimise constipation.  Chronic pain, cough, dyspnoea. 
Pharmacokinetics Bioavailability = 60-87% (PO)

Metabolism; CYP2D6-mediated metabolism to oxymorphone (minor pathway). Mostly metabolised via CYP3A4 to a fairly inactive metabolite. Biologically active in itself; metabolites only mildly (or even negligibly) contribute to its activity.7

Elimination half-life = 2-4 hours.

Excretion = Urine, faeces.
No data available on bioavailability.

Metabolism; CYP2D6-mediated activation to hydromorphone.

Elimination half-life = 3.8±0.3 hours.

Excretion = Urine, faeces. 
Routes of administration PO, IM, IV, SC, rectal. PO, IM, IV, SC. 
Additional notes Can be combined with naloxone in Australia; usually comes in free form, though. In the U.S. preparations containing paracetamol, ibuprofen and aspirin are available.  Not available in Australia; in the U.S. oral formulations also exist that also contain:
  • Ibuprofen
  • Paracetamol
  • Phenylephrine/guaifenesin
  • Phenylephrine/chlorphenamine
  • Pseudoephedrine/chlorphenamine
  • Pseudoephedrine
  • Guaifenesin
  • Homatropine.
† combination products containing hydrocodone with both of these two ingredients. 
Side effects As per morphine.  As per morphine.
Drug Oxymorphone Hydromorphone
2D structure
Medical uses Moderate-severe pain. Moderate-severe pain.
Pharmacokinetics Bioavailability = 10% (PO).

Metabolism; liver-mediated.

Elimination half-life = 7-9 hours

Excretion = Urine, faeces. 
Bioavailability = 50% (PO).

Metabolism; liver-mediated.

Elimination half-life = 2.5 hours

Excretion = Urine, faeces.
Routes of administration PO, IM, IV, SC.
Drug Dextropropoxyphene Fentanyl
2D structure


Reference list:


  1. Kamangar, F; Shakeri, R; Malekzadeh, R; Islami, F (February 2014). "Opium use: an emerging risk factor for cancer?". The Lancet Oncology 15 (2): e69–77. doi:10.1016/S1470-2045(13)70550-3. PMID 24480557
  2. Takahama, K; Shirasaki, T (July 2007). "Central and peripheral mechanisms of narcotic antitussives: codeine-sensitive and -resistant coughs." (PDF). Cough 3: 8. doi:10.1186/1745-9974-3-8. PMC 1950526. PMID 17620111
  3. Belvisi, MG; Hele, DJ (2009). "Cough Sensors. III. Opioid and Cannabinoid Receptors on Vagal Sensory Nerves". In Chung, KF; Widdicombe, J. Pharmacology and Therapeutics of Cough 187. Berlin: Springer. ISBN 978-3-540-79842-2
  4. Brunton, L; Chabner, B; Knollman, B (2010). "Chapter 18: Opioids, Analgesia, and Pain Management*". Goodman and Gilman's The Pharmacological Basis of Therapeutics (12th ed.). New York, USA: McGraw-Hill Professional. ISBN 978-0-07-162442-8.
  5. Rossi, S (ed.) (January 2014). Australian Medicines Handbook 2014. Adelaide, Australia: Australian Medicines Handbook Pty Ltd. ISBN 978-0-98-755011-8.
  6. Rang, HP; Dale, MM; Ritter, JM; Flower, RJ; Henderson, G (2011). "Chapter 41: Analgesic drugs". Rang and Dale's pharmacology (7th ed.). Edinburgh, UK: Churchill Livingstone. ISBN 978-0-70-203471-8.
  7. Lalovic, B; Kharasch, E; Hoffer, C; Risler, L; Liu-Chen, LY; Shen, DD (May 2006). "Pharmacokinetics and pharmacodynamics of oral oxycodone in healthy human subjects: role of circulating active metabolites.". Clinical Pharmacology and Therapeutics 79 (5): 461–79. doi:10.1016/j.clpt.2006.01.009. PMID 16678548.

* JCU login required for this link. 

Monday, 21 April 2014

Oh how I love the Germans and their ingenious organic chemists!

I've come across three painkillers that the Germans use that we don't (and by "we", I mean Aussy and other English-speaking countries); albeit these drugs are also used in other non-English speaking countries, I'm picking on Germany as these three drugs are all used in this country and are all prescription-only medicines there. They were also all first synthesised there. Plus Germany is, according to the United Nations, the 2nd most developed country with a population of over 50 million (1st is the US) in the world.1 If you'd like some more information about Germany I might as well give you some while I'm talking about it.

Some background on Germany

Germany has a population of 81 million (which is the highest population of any country in all the European Union) and its population has been fairly stable over the past few years. Its currency is the Euro (€), they drive on the right, they speak German (funny eh?; it belongs to the same family as English, Dutch and Afrikaans, that is, the West Germanic languages); its Government is similar to the U.S.'s in that they do have presidents (whom is presently, Joachim Gauck), although they also have chancellors; its capital and largest city is Berlin (~3.5 million people), 2nd largest is Hamburg (~1.8 million) and 3rd largest is Munich (~1.35 million). It has 16 states and has a free healthcare system and is also one of the places where my ancestors, the Ashkenazi Jews, lived for a significant period of time. In fact Ashkenazi is the Yiddish (a Jew language, a variant of Hebrew) word for "Germany".2
Figure 1: World Map with Germany highlighted in red
Figure 2: Albert Einstein
It's funny how we all tend to remember the worst people in Germany's past (such as Adolf Hitler, Joseph Mengele, etc.), yet they also gave us many of our greatest minds including: Albeit Einstein, Max Planck (a quantum physicist), Werner Heisenberg (a quantum physicist), Gabriel Daniel Fahrenheit (discoverer of the unit, Fahrenheit), Max Born (quantum physicist), Wilhelm Röntgen (the discovery of X rays), Otto Hahn (the discoverer of nuclear fission), Robert Koch and Ferdinand Cohn (microbiologists), Fritz Haber (a Nobel prize winning chemist), Carl Friedrich Gauss, David Hilbert, Bernhard Riemann and Gottfried Leibniz (a co-founder of calculus, along with Isaac Newton) who are all mathematicians, Friedrich Wegener (a doctor after whom Wegener's granulomatosis, a condition that causes one's immune system to attack healthy tissues, is named), Eduard Heinrich Henoch and Johann Lukas Schönlein, two famous doctors after whom the condition Henoch–Schönlein purpura is named.

Figure 3: Henoch–Schönlein purpura

Germany's healthcare system

In Germany drugs are regulated by the Federal Institute for Drugs and Medical Devices, the standard medical degree for doctors is the M.D. (Medicinae Doctor, Latin for "Teacher of Medicine" which is also the standard medical degree in the US and Canada; some Aussy universities offer it, although, most Aussy doctors have a M.B.B.S. [offered at JCU] or M.B.B.Ch. both of which are bachelor degrees that usually take 6 years to obtain; M.D. is a master's degree in effort; usually 8 years of university are required  4 years for a bachelor degree in biology/related fields that gets you into a M.D. program which is usually 4 years long) and German websites have the "top level domain" (TLD) of .de (i.e. their URL usually has .de in it). University education in Germany, is also free from what I can tell (my German friends can feel free to correct me in this or any other mistake I've made in this status). English is the most common second language in Germany. German isn't one of the six languages of the U.N.2

Figure 4:  Federal Institute for Drugs and Medical Devices logo
Figure 5:  Federal Institute for Drugs and Medical Devices

Germany's drugs

Now onto the drugs; the three painkillers that I've come across that's used in Germany but not in most English-speaking countries are metamizoletilidine and piritramide. All three of these drugs were also initially synthesised by Germans.

Metamizole (dipyrone is another name it sometimes goes by, especially in English-speaking countries), was once marketed in the U.S. and Australia, but has since been taken off the market amidst concerns that it causes potentially fatal blood disorders in between 1 in 100 and 1 in 10,000 patients (the estimate varies so much as there's a number of different variables that seem to influence this risk); it was taken off the market in English-speaking countries ~1970 but was before then (since 1922) a popular over-the-counter painkiller much like paracetamol, aspirin and ibuprofen (Nurofen). Metamizole is now a prescription-only medicine in Germany, most commonly used in the setting of severe post-surgery or internal organ-related pain. It possesses properties most similar to paracetamol; it relieves pains and fevers without much in the way of anti-inflammatory effects and also possesses some antispasmodic effects which means it is ideal for pain related to spasms (cramps) such as kidney colic.

Figure 6: Metamizole
Tilidine and piritramide are opioid painkillers (narcotic painkillers with similar properties to morphinecodeine and heroin) that are synthetic and less potent than morphine (i.e. a higher dose of these drugs is required to produce the same painkilling effect as morphine). Tilidine is a weak opioid (putting in the same class as codeine and tramadol) that is usually given orally (i.e. by mouth as a tablet/oral liquid), although rectal (i.e. suppositories) and injectable formulations are available in some countries (although not in Germany); often (especially so in Germany) in combination with naloxone (a drug that blocks the receptors the opioids use to produce their effects) in order to prevent people from abusing the drug. See naloxone is unable to enter the bloodstream to any significant degree when taken orally and hence if the drug is taken the way it is meant to be, that is, orally, it doesn't produce any significant inhibition of the opioid effects of tilidine, but if it is injected into a vein by addicts to get high it rapidly precipitates opioid withdrawal.3,4 Piritramide, on the other hand, is strong opioid that is solely given parenterally (i.e. by injection) for pain, especially for the rapid relief (within two minutes) of severe, cancer-related, operation-related or injury-related pain in adults or kids over the age of 2.5
Figure 7: Tilidine
Figure 8: Piritramide










Reference list (WP style):

  1. "Table 1: Human Development Index and its components". United Nations Development Programme. United Nations. 2012. Retrieved 22 April 2014.
  2. "The World Factbook". Central Intelligence Agency. United States Government. 11 April 2014. Retrieved 22 April 2014.
  3. "Tilidin N Sandoz® DP Lösung zum Einnehmen" (PDF). Google Drive. Wooden Churches: Sandoz Pharmaceuticals GmbH. December 2012. Retrieved 22 April 2014.
  4. Jage, J; Laufenberg-Feldmann, R; Heid, F (15 April 2008). "Medikamente zur postoperativen Schmerztherapie: Bewährtes und Neues". Der Anaesthesist (in German) (Springer) 57 (5): 491–498. doi:10.1007/s00101-008-1327-9. PMID 18409073.
  5. "FACHINFORMATION (Zusammenfassung der Merkmale des Arzneimittels)" (PDF). Janssen - Cilag Pharma GmbH (in German). November 2013. Retrieved 22 April 2014.

Sunday, 20 April 2014

Paracetamol (acetaminophen in the US/Canada/Japan)

Note: I italicise things that are important to note, whereas I bold things in this post that are particularly important. 

Paracetamol (brand names: Tylenol (US/Canada) and in Aussy: Panadol, Panamax, Paralgin, many others. When combined with codeine (usually at 8 mg) it goes by the brand names: PanadeineCodral PE Cold & Flu Day & Night (Day tablets)) is a drug that's more fascinating than you might think; partly because it's precise mechanism of action has remained elusive after more then one hundred years of clinical use. The mechanism of aspirin became apparent decades ago, but it's only been recently that we've figured out how paracetamol likely works.
Figure 1: Paracetamol's 2D molecular structure. 











1. The reason for the different generic names

This wouldn't be much of a blog entry if I didn't at least mention why paracetamol goes by two different generic names. See paracetamol is chemically known as para-acetylaminophenol; so to condense this the different countries decided to name it either paracetamol (para-acetylaminophenol) or acetaminophen (para-acetylaminophenol). The Americans and the Japanese decided that they preferred acetaminophen, whereas the rest of the world decided to call it paracetamol. This is also the name the World Health Organization (WHO) calls it by.

2. Some background biochemistry before we proceed

2.1 Enzymes

Enzymes are proteins (1) which are large molecules composed of the 21 (2) amino acids in a specific sequence which is unique to the protein in question and they are of biologic significance because they catalyse (3) chemical reactions, which is a fancy way of saying they speed them up usually be several hundred fold at least. (1 - I know this isn't 100% true, if you're one of my fellow healthcare students so dw I'm just simplifying it so the laymen can understand. 2 - I know most biochemists teach 20, but a 21st amino acid's been found in some proteins. 3 - which is by definition they wouldn't be enzymes if they didn't) It is important to note, however, that the enzyme doesn't directly undergo the catalysed reaction, that is, the reaction doesn't use up enzyme, the enzyme is still the way it was before the reaction, after the reaction. The substance they catalyse this reaction for is called the substrate of said enzyme. This sequence I was referring to is known as the protein's "primary structure". The primary structure of proteins is useless to the body unless it gains some additional structure, which is called secondary, tertiary and quaternary structure.2,3

This additional structure results from the weak chemical interactions between the different amino acids (which are not necessarily side-by-side as far as their primary sequence goes for this interaction to occur) combining to give the different proteins unique structures. The next image is a very good picture from Wikipedia I've come across on the 21 amino acids (note, my fellow pharm students, remember this is Wikipedia, the structures are sound but the pKa values do not seem to be accurate according to your lecture notes).1 It is their overall structure that enables to catalyse specific reactions as they serve as the key and the substrates are the locks — without the key the substrates are useless, but without the lock the keys can still function (as there's plenty of different locks they can open in this metaphor); albeit the converse can also be true. The site at which the substrate binds to is known as an active site whereas site(s) that can be bound by other molecules which influence the activity of the active site are known as allosteric site(s). It is the secondary, tertiary and quaternary structure that is broken by things like extremes of temperature (even slightly above our normal body temperature of 37℃ and you'll start to denature our proteins you'll begin to see this happening; this is why fevers can, if severe enough, cause permanent brain damage) and pH (acidity/basicity of our insides).2
Figure 2: The 21 amino acids found in proteins

2.2 Enzyme inhibitors

Additionally I need to mention some fundamentals on how drugs can inhibit certain enzymes. For instance some drugs can irreversibly inhibit an enzyme, while others do so reversibly and there is also some distinction between the different types of reversible inhibitors of an enzyme.2

The irreversible inhibitors do pretty much what their name suggests, they bind irreversibly, which means this cannot be reversed and hence any enzyme that's affected cannot ever regain its ability to catalyse chemical reactions so our bodies must replace said enzyme in order for the enzyme activity in our body to be regained. Examples of irreversible inhibitors, include aspirin (discussed later), most members of the class of antidepressants, the monoamine oxidase inhibitors (which includes phenelzine (Nardil), tranylcypromine (Parnate) and isocarboxazid (Marplan, no longer marketed in Australia or the UK); they're the most toxic class of antidepressants currently in clinical use and are known to cause people to die simply by eating fermented foods like cheese and vegemite) and various nerve gases/pesticides (including sarin) which all inhibit the enzyme acetylcholinesterase (AChE), an enzyme required for the breakdown of acetylcholine, a neurotransmitter that's used for pretty much everything in the brain, spinal cord and nerves, especially controlling our voluntary and involuntary movements like those in our limbs and bowels, respectively. Irreversible inhibitors are usually quite toxic as the host (i.e. who takes the drug/pesticide/nerve gas) must replace the lost enzyme before it regains the function the enzyme has.2

Reversible inhibitors include a number of different subtypes too as there are a few different ways a drug might achieve this; firstly, there's the so called "competitive inhibitors" like methotrexate and trimethoprim (which inhibit DHFR — a key enzyme involved in folic acid metabolism and hence required for the synthesis of the base components of DNA and RNA) which basically serve as "decoys" for the enzyme, by "looking like" the natural substrate(s) for that enzyme, hence reducing the amount of the natural substrate that can fit into the enzyme and hence have its corresponding chemical reaction catalysed. These drugs can also be thought of in a metaphorical framework; as an animal (the natural substrate) that eats a particular prey (the enzyme) when another predator is added to the equation (the drug) — the original predator gets out-competed for its grey, hence reducing the activity of the original predator, while making the prey more sparse. Most non-steroidal anti-inflammatory drugs (NSAIDs) fall into this category, as do the Alzheimer's drugs known by their class name the acetylcholinesterase inhibitors (this is the same enzyme as the pesticides/nerve gases irreversibly inhibit) which includes any drug the suffix, "stigmine".2

Then there's the non-competitive inhibitors which bind to an allosteric site. These inhibitors may also be called negative allosteric modulators (whereas substances that increase the activity of the active site are called positive allosteric modulators) and tend to be more powerful than competitive inhibitors as if you increase the substrate concentration the non-competitive inhibitors still have a significant effect on the substrate binding whereas if it's a competitive inhibitor we're talking about it's possible for the substrate to reach a concentration at which the inhibitor has no effect on the enzyme's ability to catalyse the reaction(s) in question.2

Other bits of relevant biochemistry/pharmacology

On top of these concepts I also need you know some basic bits of info regarding how the body deals with drugs; technically speaking I should call this pharmacology but as biochemistry and pharmacology are pretty similar I thought I might as well put these little points here.2

One bit of pharmacology I need you to know is the fact that the body metabolises drugs, most often with the help of enzymes found in the liver which allow chemical reactions to take place that make the drug more water-soluble and hence more easily disposable in the urine (as part of the filtration process of the kidneys urine passes by several parts of the kidney before being passed out in the urine, hence fat soluble drugs which can easily pass from the urine into the cells during this filtration process cannot be efficiently disposed off by the kidney. Urine is also mostly water and hence fat soluble drugs have difficulty dissolving in urine in the first place). Any drug the body metabolises a drug into is called a metabolite of said drug. If the metabolite has biologic activity it is often referred to as an active metabolite. Drugs that derive the bulk of their biologic action by means of their active metabolites (e.g. tamoxifen, a breast cancer drug) are called prodrugs as they provide a means of delivering the active metabolites into the body and hence produce the active metabolites in question.2

The time it takes for half of a drug to be eliminated by the body, by both metabolism and excretion by the kidneys is referred to as the elimination half-life. The half-life is usually the dosing interval we use for drugs; for instance, most antidepressants have a half-life of ~20-30 hours, hence once daily dosing is usually used for these drugs. Whereas the antidepressant, fluoxetine (Prozac), can be dosed once weekly (although once daily is more common) as it and its active metabolite, norfluoxetine, have half-lives that make once weekly dosing possible (overall their average is about a week).2


Figure 3: Fluoxetine and Norfluoxetine


The excretion of drugs usually occurs in either the urine or the faeces. For most NSAIDs this mostly occurs in the urine (~90% of it is excreted this way), although faecal excretion may also occur. Other forms of excretion include in breath (there's a drug called hydroxyurea that's used for certain blood disorders that is metabolised to CO2 and breathed out), sweat, tears and even breast milk (hence why breastfeeding women should always ask their doctor/pharmacist before taking a medication).2

Reuptake is also an important biochemical concept that I require you to understand in order for me to proceed. See reuptake is basically the process by which the body "reabsorbs" a neurotransmitter, hence (usually, but not always) terminating the activity of the neurotransmitter in the synaptic cleft, where it normally binds to receptors in order for it to relay chemical messages between neurons (electrically-active brain, spinal cord and nerve cells). Hence, reuptake inhibitors, potentiate the activity of the neurotransmitter whom's reuptake they inhibit. For example my previous example of fluoxetine/norfluoxetine was actually deliberate as fluoxetine and norfluoxetine are known to serve as selective serotonin reuptake inhibitors (SSRIs) which are drugs that specifically (i.e. selectively) inhibit the reuptake of the neurotransmitter, serotonin, hence potentiating the activity of serotonin in the body, especially the brain and spinal cord. As serotonin is believed to modulate mood SSRIs are known to produce antidepressant effects.2

3. NSAIDs

3.1 Introduction to the NSAIDs

Figure 4: Aspirin
Aspirin (see figure 4 for its structure), ibuprofen (Nurofen; see figure 5 for its structure), diclofenac (Voltaren; see figure 6 for its structure) and naproxen (Naprosyn; see figure 7 for its structure) all work by inhibiting an enzyme called cyclooxygenase (COX), which comes in two different "flavours", COX-1 and COX-2. It is based on this shared mechanism of action action they're called non-steroidal anti-inflammatory drugs (NSAIDs). They both produce a family of small fat-like molecules called prostaglandins which play crucial roles in inflammation, fever, pain and various other crucial biologic functions.2
Figure 5: Ibuprofen
Figure 6: Diclofenac


Figure 7: Naproxen

COX-1 is mostly important for the synthesis of prostaglandins that are used for some "house-keeping" functions like the maintenance of blood clotting and the protective mucous layer of the stomach (which protects it from the acid found inside the stomach). COX-1 is also believed to play a role in the lungs and is believed to be why aspirin and other NSAIDs can trigger asthma attacks and other respiratory (pertaining to breathing) complications. It's also believed to be responsible for aspirin's ability to prevent blood clots. Whereas COX-2 seems to be important in fever, inflammation and fever. Its substrates, regardless of flavour, is arachidonic acid (a long chain, polyunsaturated (omega-6) fatty acid that's found primarily in meat fats, although, in those sources it's fairly lacking in abundance. Most arachidonic acid is synthesised from omega-6 fatty acids in our diet which we generally get from plant oils).2

Figure 8: Arachidonic acid
With this knowledge a few COX-2 selective inhibitors (i.e. drugs with high COX-2 inhibitory activity but minimal effect on COX-1) or "coxibs" have been created with the hope of improving on the tolerability of NSAIDs, by reducing their potential for respiratory and gastrointestinal complications (e.g. stomach ulcers). Unfortunately, they had an unpredictably high propensity for causing renal (kidney) and cardiovascular complications like hypertension (high blood pressure), heart attacks and strokes. Hence all coxibs are now prescription-only and many of them were withdrawn from the market worldwide. It was later found that non-subtype selective NSAIDs also share these risks, except for aspirin which seems to prevent heart attacks and strokes. Of the non-subtype selective NSAIDs naproxen seems to be least associated with these renal/cardiovascular complications whereas diclofenac seems most strongly associated with these complications.3
Figure 9: COX-1
Figure 10: COX-2


3.2 Aspirin, a unique NSAID

Aspirin is the only NSAID that serves as an irreversible inhibitor of COX while this might sound like a bad thing it actually conveys to aspirin therapeutic benefit. See while aspirin's therapeutic efficacy against inflammation, pain and fever only last for a few hours (as aspirin itself has a half-life of 2-3 hours) as COX is regularly replaced in the cells of inflamed tissues its efficacy against blood clots last for a day or so.4,5  Aspirin achieves its irreversible inhibition of COX, if you're interested, by adding an acetyl (see figure 11 for what an acetyl group looks like) group to COX, hence irreversibly inactivating the enzyme.4,5
Figure 11: Acetyl


4. Paracetamol

Paracetamol is known to be unlike the NSAIDs in a multitude of different ways; for one, it produces minimal anti-inflammatory effects, which are, by definition, significant clinical effects with all NSAIDs; second, it, unlike the NSAIDs is comparatively safe for use in asthmatic individuals; thirdly, it tends to be less effective than the NSAIDs in relieving pain; forth, it produces minimal effects on blood pressure, risk for blood clots and one's stomach-lining integrity.6,7

4.1 A brief history of paracetamol

To understand have much of a miracle paracetamol really is we much take a journey back to the beginning, through its history. See paracetamol really was found to be an effective painkiller back in the 1880s and while it was shortly marketed for a little while after it was soon taken off the market when it was (erroneously) found to be unacceptably toxic. It was only taken back onto the market when it was found to be the active metabolite of two, once widely-used over-the-counter painkiller called phenacetin and acetanilide.7 Acetanilide was quickly taken off the market when it was found to be toxic in a number of patients, in the short-term. Later (circa. mid 1940s) phenacetin was found to cause lasting damage to the kidneys and increase one's risk of kidney and other urologic (pertaining to the urinary system) cancers and then the search for a safe alternative began. This alternative would end up being one of the common metabolites of these two drugs that was subsequently found to be comparatively non-toxic and equally effective, namely, paracetamol. Paracetamol slowly marched onto the international market, mostly, as a prescription-only medication; but soon after its superior safety compared to other anilide derivatives and comparable efficacy it became an international best-seller (circa. mid-late 1960s).7

4.2 The search for its mechanism of action

Paracetamol's mechanism of action was initially thought to be due to its purported (i.e. they hadn't proven this, they had just assumed this to be true) selectivity for inhibiting prostaglandin synthesis in the central nervous system (brain and spinal cord). Hence it would take a while before some real scientists began to look for a possible mechanism of action for paracetamol.6,7

Initial investigations proposed the existence of a third, "flavour" of COX, a so called, "COX-3" variant, but these theories were flaunted by negative test results; later, if it was found that paracetamol was a prodrug to an even more active metabolite, namely, N-arachidonylaminophenol (also known by the code, AM404; this name should give my fellow pharm students a hint as to what it's structure is going to be; the next image is a picture of it; see figure 12 for its structure) which among other things has been found to inhibit the reuptake of the neurotransmitter, anandamide (see figure 13 for its structure).6,7

Anandamide is a neurotransmitter that basically serves as the body's endogenous (body-synthesised) version of cannabis, as it activates the same receptors, especially the first cannabinoid receptor (CB1) which is responsible for the mind-altering effects of cannabis. Anandamide's name actually originates from the Sanskrit word, "Ananda" which means "Bliss". AM404 also activates the receptor that gives spicy food its spicy taste (TRPV1); this action may enable it to better relieve pain as long-term exposure (even a few hours can qualify) to TRPV1 agonists (activators) is known to relieve pain.5,6 Support for this hypothesis comes from the fact that CB1 antagonists (i.e. receptor blockers) seem to totally attenuate paracetamol's painkilling effects in mice.6,7
Figure 12: AM404
Figure 13: Anandamide

It also seems to have some affinity for the peroxidase domain of various enzymes, including COX; this domain is required for the synthesis of prostaglandins. This was originally theorised by using a bit of chemistry of paracetamol; see paracetamol contains a phenol group and phenols are known to be highly reactive with oxidising agents, that is, they're highly effective reducing agents. Phenol groups are a benzene ring (the six-member ring in the structure of paracetamol) connected to an alcohol (OH) group; they are different from ordinary alcohols (like ethanol, the one people get drunk on) in a number of different physicochemical ways and this so happens to be one way they are. AM404 is also formed due to a greater reactivity of the amine groups of aminophenols compared to the phenol component of the molecule; specifically, the active metabolite of paracetamol, 4-aminophenol (if you're a pharm student you'll recognise this name from the PC2002 practical in which we synthesised paracetamol; figure 14 is 4-aminophenol) and its high degree of reactivity with arachidonic acid, which is accelerated by an enzyme that breaks down anandamide, ironically, which is known as fatty acid amide hydrolase.6,7
Figure 14: 4-aminophenol
Another possible consequence of paracetamol's phenol chemistry is the fact it might scavenge for peroxides (e.g. peroxynitrate; see figure 15 for its structure) released as a result of tissue injury (which might mediate the inflammation usually seen from tissue injury), similarly to the endogenous compounds, uric acid (a by product of the breakdown of nucleic acids — the base components of DNA) and vitamin C. It also seems that these peroxidases released in sites of acute tissue injury might endow paracetamol with the ability to inhibit COX-2 selectively in these sites (which is supported by experiments in the laboratory). This also holds in that it would imply that COX-2 isn't inhibited in the kidneys, unless they're injured, hence sparing paracetamol of any significant ill effects on the kidneys or cardiovascular risk (i.e. odds of strokes/heart attacks). It has been found that in people with injured kidneys the drug does put strain on their kidneys.6,7

Figure 15: Peroxynitrate

It has also been proposed that paracetamol might (with some support from laboratory experiments) reduce the levels of nitric oxide synthase, an enzyme required for the activation of the NMDA receptors, which are play a crucial role in nociception (pain perception). It has also been proposed that serotonin might play a role in paracetamol's painkilling effects; in fact, studies in animals have revealed that paracetamol increases brain concentrations of serotonin in a number of different brain areas and when the levels of serotonin are suppressed in mice treated with paracetamol the painkilling effects of paracetamol are also suppressed. Serotonin is known to play a role in pain perception and in temperature regulation so it is a possible site of action, but there is evidence to the contrary as well. For instance, people on antidepressants, like myself, can safely take paracetamol without suffering from a state of excess serotonin, called a serotonin syndrome. In fact in the fifty years since paracetamol's re-introduction worldwide, during which there have been numerous available antidepressants, not one case of serotonin syndrome have been reported as resulting from such combinations.6,7

It is also possible that its painkilling effects result from a synergy of these actions.6,7


4.3 Other aspects of paracetamol's pharmacology

Paracetamol is also known to have favourable pharmacokinetics via the oral route (i.e. when taken as a tablet/liquid); that is, the drug reacts to the body in a favourable way; namely, it is well-absorbed by the stomach (about 88% reaches the bloodstream), with painkilling effects seen as early as 11 minutes of oral administration8,9 and its elimination half-life is about 1.25-3 hours.9 It is also minimally bound to the proteins in one's blood and hence dialysis can be used in cases of overdose to flush the drugs out; it has also been given intravenously for the relief of postoperative pain (i.e. pain after surgery).6,7

It is insanely toxic in cases of overdose and paracetamol overdose is the leading cause of liver failure in the developed world (fortunately most cases of overdose are not fatal if medically treated early on). This is also mediated by a metabolite of paracetamol, namely, N-acetyl-p-benzoquinoneimine (NAPQI; see figure 16 for its structure) which is highly toxic to hepatocytes (liver cells) by means of its ability to serve as an oxidising agent. Usually overdoses are treated with N-acetylcysteine (see figure 17 for its structure) which is an antioxidant and promotes the formation of the natural antioxidant, glutathione (see figure 18 for its structure). At therapeutic doses NAPQI production is minimal and hence no liver damage occurs.6

Figure 16: N-acetyl-p-benzoquinoneimine
Figure 17: N-acetylcysteine

Figure 18: Glutathione
Another property of paracetamol that's unique to it compared to the NSAIDs is the fact that it is generally accepted as being safe for use during pregnancy, there's only one possible adverse effect in the newborn that's been able to stick  it may cause asthma later on in life (ironic eh? As it doesn't do provoke attacks in adults/children, yet the NSAIDs can trigger an attack of asthma in adults/children).10,11 The NSAIDs when taken during pregnancy, on the other hand, are known to cause a potentially fatal heart defect, brain bleeds, kidney injury and numerous other ill effects.12


Reference list:

  1. Cojocari, D. (7 June 2011). File:Amino Acids.svg. Wikipedia, the Free Encyclopedia. The Wikimedia Foundation. Retrieved 20 April, 2014, from https://en.wikipedia.org/wiki/File:Amino_Acids.svg
  2. Brunton, L; Chabner, B; Knollman, B (2010). Goodman and Gilman's The Pharmacological Basis of Therapeutics (12th ed.). New York: McGraw-Hill Professional. ISBN 978-0-07-162442-8.
  3. Commission on Human Medicines (January 2010). "MHRA PUBLIC ASSESSMENT REPORT Non-steroidal anti-inflammatory drugs and cardiovascular risks in the general population" (PDF). MHRA.gov.uk. Medicines and Healthcare Products Regulatory Agency. Retrieved 21 April 2014.
  4. Simmons, DL; Botting, RM; Hla, T (September 2004). "Cyclooxygenase isozymes: the biology of prostaglandin synthesis and inhibition." (PDF). Pharmacological Reviews 56 (3): 387–437. doi:10.1124/pr.56.3.3. PMID 15317910.
  5. Hohlfeld, T; Saxena, A; Schrör, K (May 2013). "High on treatment platelet reactivity against aspirin by non-steroidal anti-inflammatory drugs--pharmacological mechanisms and clinical relevance." (PDF). Thrombosis and Haemostasis 109 (5): 825–33. doi:10.1160/TH12-07-0532PMID 23238666.
  6. Toussaint, K; Yang, XC; Zielinski, MA; Reigle, KL; Sacavage, SD; Nagar, S; Raffa, RB (December 2010). "What do we (not) know about how paracetamol (acetaminophen) works?"(PDF). Journal of Clinical Pharmacy and Therapeutics 35 (6): 617–38. doi:10.1111/j.1365-2710.2009.01143.x. PMID 21054454.
  7. Graham, GG; Davies, MJ; Day, RO; Mohamudally, A; Scott, KF (June 2013). "The modern pharmacology of paracetamol: therapeutic actions, mechanism of action, metabolism, toxicity and recent pharmacological findings.". Inflammopharmacology 21 (3): 201–32. doi:10.1007/s10787-013-0172-x. PMID 23719833.
  8. Moller, P; Sindet-Pedersen, S; Petersen, C; Juhl, G; Dillenschneider, A; Skoglund, L (2005). "Onset of acetaminophen analgesia: comparison of oral and intravenous routes after third molar surgery". British Journal of Anaesthesia 94 (5): 642–648. doi:10.1093/bja/aei109. PMID 15790675.
  9. "Tylenol, Tylenol Infants' Drops (acetaminophen) dosing, indications, interactions, adverse effects, and more". Medscape Reference. WebMD. Retrieved 21 April 2014.
  10. Eyers, S; Weatherall, M; Jefferies, S; Beasley, R (April 2011). "Paracetamol in pregnancy and the risk of wheezing in offspring: a systematic review and meta-analysis.". Clinical and Experimental Allergy 41 (4): 482–9. doi:10.1111/j.1365-2222.2010.03691.xPMID 21338428.
  11. Thiele, K; Kessler, T; Arck, P; Erhardt, A; Tiegs, G (March 2013). "Acetaminophen and pregnancy: short- and long-term consequences for mother and child.". Journal of Reproductive Immunology 97 (1): 128–39. doi:10.1016/j.jri.2012.10.014PMID 23432879.
  12. Bloor, M; Paech, M (May 2013). "Nonsteroidal anti-inflammatory drugs during pregnancy and the initiation of lactation.". Anesthesia and Analgesia 116 (5): 1063–75. doi:10.1213/ANE.0b013e31828a4b54. PMID 23558845.

Friday, 4 April 2014

Broccoli and its goodness

Broccoli is so good for you for a number of different reasons; firstly, it contains a number of different essential vitamins and minerals, with it containing about twice the recommended daily intake of vitamin C per cup. Other minerals and vitamins found in it are found at decent, but not spectacular levels. Secondly, it contains a number of indoles and isothiocyanates which are substances that have demonstrated some most remarkable properties in the lab and is perhaps why studies in humans have found that people that consume broccoli regularly live longer than those that don’t. Part of this is the fact they’re less likely to develop certain cancers, diabetes, dementia and heart disease. (Tarozzi, 2013; Bahadoran, 2013; Rogan, 2006; Bahadoran, 2013)

Of these active constituents in broccoli and related vegetables sulforaphaneindole-3-carbinol and diindolylmethane seem most important for the beneficial effects of broccoli on these different disease states. 

Sulforaphane works by increasing the body’s ability to eliminate carcinogens (cancer-causing substances) that we come into contact every day (examples of such carcinogens are benzene from fuel stations and tobacco smoke if you either smoke or hang around smokers). It also has antioxidant (which prevents damage to DNA mediated by these chemical species called, "free radicals" which occur spontaneously in the body; DNA damage in turn leads to mutations that lead to cancer) and anti-inflammatory effects. Sulforaphane also has the ability to inhibit the division of cancerous cells by activating the pathways that are inherently under-active in most cancer cells as these are the pathways that basically perform regular, "checks" on the cell to make sure that nothing has gone wrong in the cell that could lead to cancer if not taken care of, early on. Sulforaphane also has the ability to induce the death of cancer cells and prevent them from forming new blood vessels to feed the growth of the cancer. (Lenzi, 2014). 

On the dementia front sulforaphane's been found to protect brain cells from further damage and hence may slow down the progression of dementia. (Tarozzi, 2013). It has also been found to have positive effects on type II diabetes mellitus. (Bahadoran, 2013).
Figure 1: Sulforaphane's 2D structure. 










Indole-3-carbinol (ICN) and diindolylmethane (IDM) have been found to produce powerful preventative effects on cancer too and may also kill off cancer cells and prevent their dissemination (spread) through the body. (Banerjee, 2011). IDM is extensively converted, in the stomach, into ICN and hence very little actually reaches the bloodstream as unchanged IDM. (Banerjee, 2011; Weng, 2008).

Figure 2: Diindolylmethane










Figure 3: Indole-3-carbinol










Reference list:

  • Bahadoran, Z., Mirmiran, P., & Azizi, F. (2013). Potential efficacy of broccoli sprouts as a unique supplement for management of type 2 diabetes and its complications. Journal of Medicinal Food, 16(5), 375–382. doi:10.1089/jmf.2012.2559. PMID: 23631497.
  • Banerjee, S., Kong, D., Wang, Z., Bao, B., Hillman, G. G., & Sarkar, F. H. (2011). Attenuation of multi-targeted proliferation-linked signaling by 3,3’-diindolylmethane (DIM): from bench to clinic. Mutation Research, 728(1-2), 47–66. doi:10.1016/j.mrrev.2011.06.001. PMID: 21703360.
  • Clarke, J. D., Dashwood, R. H., & Ho, E. (2008). Multi-targeted prevention of cancer by sulforaphane. Cancer Letters, 269(2), 291–304. doi:10.1016/j.canlet.2008.04.018. PMID: 18504070. PMC: 2579766.
  • Lenzi, M., Fimognari, C., & Hrelia, P. (2014). Sulforaphane as a promising molecule for fighting cancer. Cancer Treatment and Research, 159, 207–223. doi:10.1007/978-3-642-38007-5_12. PMID: 24114482.
  • Rogan, E. G. (2006). The natural chemopreventive compound indole-3-carbinol: state of the science. In Vivo (Athens, Greece), 20(2), 221–228. Retrieved from http://iv.iiarjournals.org/content/20/2/221.full.pdf.
  • Tarozzi, A., Angeloni, C., Malaguti, M., Morroni, F., Hrelia, S., & Hrelia, P. (2013). Sulforaphane as a potential protective phytochemical against neurodegenerative diseases. Oxidative Medicine and Cellular Longevity, 2013, 415078. doi:10.1155/2013/415078. PMID: 23983898. PMC: 3745957.
  • Weng, J. R., Tsai, C. H., Kulp, S. K., & Chen, C. S. (2008). Indole-3-carbinol as a chemopreventive and anti-cancer agent. Cancer Letters, 262(2), 153–163. doi:10.1016/j.canlet.2008.01.033. PMID: 18314259. PMC: 2814317.