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


3.1 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
  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). 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.

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