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.




Monday, 23 September 2013

Huntington's Disease

Huntington's disease (HD) is a rare, hereditary, fatal (death usually occurs 10-25 years after the first symptoms appear) neurodegenerative disorder (i.e. a neurological [pertaining to the brain, spinal cord and/or nerve cells] yet progressive disorder) that can strike persons at any time in life. It causes a progressive decline in motor function (your ability to control your normally, voluntary movements), cognitive function (memory, learning, etc.), emotional stability, connection with reality (in the later stages people with the disorder can become psychotic with many of the classic symptoms of paranoid schizophrenia) and various other neurological functions. The first symptom is usually chorea (an abnormal involuntary movement disorder that often presents with involuntary flailing movements) hence the previous name of the condition, Huntington's chorea.1

It is due to the build-up of a faulty (or mutated) huntingtin protein (mHtt) which is due to a mutation in the huntingtin gene (HTT). This gene is used to create the huntingtin protein which plays a key role in various cellular processes in the brain and testes. The gene is inherited in an autosomal dominant fashion (i.e. it is inherited in a manner that is totally independent of the sex of the parent and the child. This is because we all get two autosomes [the type of chromosome on which the huntingtin gene is found; these are the non-sex chromosomes] of each variety [there's 21 different autosomes], one from our father, one from our mother. It takes just one autosome that contains a faulty huntingtin gene to cause Huntington's disease). The faulty huntingtin protein builds up and this leads to neuronal (electrically-signalling brain, spinal cord and nerve cell [in this case mostly just brain cell]) dysfunction and death (although we're not sure exactly how).1

At this point in time there are limited treatment options for people with Huntington's disease. Most treatments are aimed at making the patient as comfortable as possible as they die. Treatments that are designed to do this include neuroleptics (antidopaminergic drugs; drugs that counteract the actions of the neurotransmitter, dopamine, throughout the nervous system [brain, spinal cord and nerves] particularly in the basal ganglia -- the part of the brain that takes the biggest hit in Huntington's disease. It plays a key role in controlling our voluntary movements. Neuroleptics are basically the older antipsychotic medications [the so called "typical" antipsychotics] and some of the older antiemetics [anti-nausea and vomiting]), benzodiazepines (e.g. diazepam [VALIUM], clonazepam [RIVOTRIL]), dopamine-depleting agents (e.g. reserpine, tetrabenazine) and valproate (EPILIM, DEPAKENE, DEPAKOTE) which are designed to help with the choreas typical of HD. Likewise if symptoms include muscle rigidity and slowed movements then dopamine agonists (receptor activators; compounds that mimic dopamine's effects in the body) or levodopa (a compound the body uses to synthesise dopamine) are preferred. Depression is also common in patients with HD and is often treated standard antidepressants such as sertraline (ZOLOFT), fluoxetine (PROZAC), etc. None of these treatments are believed to be disease-modifying i.e. treatments that actually slow (or "modify") the course of the disease.1

I theorise that valproate may have disease-modifying effects in HD seeing how it is a known histone deacetylase (HDAC) inhibitor (which alter the generation of proteins from genes) and HDAC inhibitors are known to slow the progression of HD in animal models of the disease.2

Coenzyme Q10 is an enzyme that participates in the electron-transport chain that provides a significant portion of a cell's energy. Consequently it plays a pivotal role in mitochondrial (the "power house" of cells) function. When it's supplemented with dietary supplements it provides antioxidant, anti-inflammatory and neuroprotectant (neuron-protecting) effects. In animal models of HD it was found to possess diseases-modifying effects.3

Creatine, another naturally-occurring antioxidant, has been found to possess protective effects in animal models of HD.3,4


Reference List:

  1. Revilla FJ. Huntington Disease. 2013 May 14 [cited 2013 Sep 23]; Available from: http://emedicine.medscape.com/article/1150165-overview#showall
  2. Sadri-Vakili G, Cha J-H. Histone Deacetylase Inhibitors: A Novel Therapeutic Approach to Huntingtons Disease (Complex Mechanism of Neuronal Death). Current Alzheimer Research [Internet]. 2006 Sep 1 [cited 2013 Sep 23];3(4):403–8. Available from: http://www.eurekaselect.com/77042/article
  3. Yang L, Calingasan NY, Wille EJ, Cormier K, Smith K, Ferrante RJ, et al. Combination therapy with Coenzyme Q10 and creatine produces additive neuroprotective effects in models of Parkinson’s and Huntington’s Diseases. Journal of Neurochemistry [Internet]. 2009 [cited 2013 Sep 23];109(5):1427–39. Available from: http://onlinelibrary.wiley.com/doi/10.1111/j.1471-4159.2009.06074.x/abstract
  4. Dedeoglu A, Kubilus JK, Yang L, Ferrante KL, Hersch SM, Beal MF, et al. Creatine therapy provides neuroprotection after onset of clinical symptoms in Huntington’s disease transgenic mice. J Neurochem. 2003 Jun;85(6):1359–67. 

Monday, 9 September 2013

The Tuberous Sclerosis Complex

Tuberous sclerosis complex (TSC) is a disease that I learnt of today in my studies of the drug rapamycin (Sirolimus; RAPAMUNE) that's quite frankly rather sad. It is a genetic disease that causes benign (i.e. non-cancerous) tumours to arise in any and, often, every organ of the body. The way how this presents in terms of symptoms greatly depends, of course, on the location of the tumour(s). Epilepsy is common in patients with TSC and status epilepticus is not an uncommon cause of death in patients with TSC. Status epilepticus is basically either singular long, sustained seizures or a series of small seizures that occurs in short succession. Mental retardation and autism is not uncommon in patients with TSC which can result from episodes of status epilepticus or directly from the growth of tumours in the brain and the pressure this can exert on brain tissue. Many patients with TSC are also born with heart defects, many also have vision impairments.1

It is inherited in an autosomal dominant fashion, that is, there is no gender influence on how the gene is inherited and it takes just one copy (as opposed to two copies if the gene was recessive) of the gene to cause the disease. What this means is that if one parent has the disease there's a 1/4 chance one of their kids will develop the disease and if both parents have the disease then there's a 3/4 chance that their kids will inherit the disease and if one parent has two copies of the gene as opposed to the minimum of one copy that is needed to cause the disease then all their kids will have TSC.1

There are two distinct genes that, when mutated, can lead to TSC: TSC1 and TSC2. These genes are both what's known as tumour suppressor genes – (note: what follows is not the definition of a tumour suppressor gene, it's the term tumour suppressor gene applies to this case; in general it just means a gene that appears to severe as a safe-guard against cancer) they encode (used as instructions per say for the production of) proteins that in turn regulate the activity of the mammalian target of rapamycin (mTOR) and if the activity of these genes is impaired, by say a mutation (like is the case in TSC), mTOR activity increases leading to an increased propensity for uncontrolled cell growth like that seen in cancer. The place where rapamycin, the popular immunosuppressant (immune system-suppressing drug), comes in is that it inhibits mTOR, which is, as you can probably guess, the drug after which mTOR is named.1


Ironically not long ago I was reading up about the ketogenic diet (the diet that aims to induce ketogenesis [the formation of ketone bodies from dietary fats and their utilisation as an energy source of the body] by depriving the body of other energy sources like carbohydrates and, to a lesser extent, proteins) and I learnt that one of its potential mechanisms in the treatment of epilepsy (most often from non-TSC courses) is that it inhibits mTOR.2 mTOR appears to play a key role in longevity too and lately a novel class of drugs known as sirtuin activators have been in development and the way they work is by activating the sirtuin family of enzymes which includes SIRT1, an enzyme that in turn, by interacting with TSC1/2 inhibits mTOR.3 Resveratrol is a compound that occurs naturally in grapeskins and other plant sources that activates SIRT1. Hence it is conceivable that drugs like resveratrol may be of therapeutic benefit in patients with TSC. Resveratrol, however, is not exactly suited for this indication due to the fact that it has poor oral bioavailability, that is, very little of the original drug reaches circulation (in the blood) in the body after it is taken orally.3

Reference List:
  1. Franz DN. Tuberous Sclerosis [Internet]. 2013 [cited 2013 Sep 10]. Available from: http://emedicine.medscape.com/article/1177711-overview#showall
  2. Danial NN, Hartman AL, Stafstrom CE, Thio LL. How Does the Ketogenic Diet Work? Four Potential Mechanisms. J Child Neurol [Internet]. 2013 Aug 1 [cited 2013 Sep 10];28(8):1027–33. Available from: http://jcn.sagepub.com/content/28/8/1027
  3. Ghosh HS, McBurney M, Robbins PD. SIRT1 Negatively Regulates the Mammalian Target of Rapamycin. PLoS ONE [Internet]. 2010 Feb 15 [cited 2013 Sep 10];5(2):e9199. Available from: http://dx.doi.org/10.1371/journal.pone.0009199

Saturday, 24 August 2013

Valproic Acid

Valproic acid and its salts (e.g. sodium valproate [EPILIM]) are quite fascinating drugs when you get down to it. They're chemically quite simple (valproic acid's IUPAC name for those of you that understand what this means is just 2-propylpentanoic acid) were originally synthesised from a compound with a very humble source: the Valerian plant (this is because the Valerian contains valeric acid amongst hundreds of other compounds and valeric acid is also known as pentanoic acid which is quite similar to valproic acid). Valproic acid is also fascinating in that it has so many medical uses including:1
  • as an antimanic (a drug that treats the manic states often seen in patients with bipolar disorder and related psychiatric disorders)
  • as a mood stabiliser in the long-term treatment of bipolar disorder (unfortunately, however, it appears to have limited efficacy in preventing future depressive episodes in patients with bipolar disorder)
  • As an adjunctive (add-on) treatment to antipsychotic therapy in patients treatment-resistant schizophrenia2 and in the treatment of agitation in patients with schizophrenia3
  • Anticonvulsant (anti-seizure medication; this is, in fact, valproic acid's chief medical use. The great thing about valproic acid too is that it works against a broad spectrum of different seizure types. See in epileptic seizures can be divided into various different subtypes according to the parts of the brain that are affected and the symptoms of the seizure and valproic acid appears to work against all types of seizures which is not always the case with anticonvulsants)
  • As an anticancer agent (this use has only become apparent recently and isn't very well studied yet but in theory it definitely should have anticancer activity)4
  • In the prevention of migraines 
  • As an analgesic (painkiller) in patients with neuropathic (nerve injury-related) pain (ironically one type of neuropathic pain it has been tested in is neuropathic pain due to cancer and in this clinical trial it was found effective)
  • The treatment of hallucinations associated with alcohol abuse

It has also been tested as an adjunct (add-on treatment) in the treatment of HIV but in this indication it has been found to be ineffective.5

The mechanism by which valproic acid works these almost majestic effects is not entirely known but, as always, we do have our theories. Its anticonvulsant effects are believed to be mediated by its ability to do the following:
  • Block voltage-dependent sodium channels (in brain and other electrically excitable cells a message is propelled through the cell by means of ions like sodium ions. These sodium channels like other ion channels allows a particular ion [like sodium] into and out of the cell; ions are atoms that have either lost or gained an electron(s))6
  • Inhibit the activity of the enzyme GABA-T which catalyses the breakdown of the neurotransmitter, gamma-aminobutyric acid (GABA). This allows GABA levels in the brain and other tissues to rise and since seizures are due to pathological (usually pathologically excessive) electrical activity in the brain and GABA depresses neuronal [electrically-signalling cells of the brain, spinal cord and nerves] activity, this rise in GABA levels can reduce this pathologically excessive activity.7
  • Potentiating the activity of the enzyme glutamic acid decarboxylase (GAD) – an enzyme that catalyses the synthesis of GABA8

Its antimanic, anticancer and potentially its analgesic effects are probably, at least in part, due to its ability to inhibit the enzyme Histone Deacetylase (HDAC). Histone deacetylase enzymes catalyse the deacetylation of histones – proteins that package and order DNA in the nuclei (centre, almost like the "brain" of the cell) of cells and plays a key role in gene expression (i.e. the generation of proteins like receptors and enzymes from genes). By doing this HDAC inhibitors like valproic acid lead to a significant increase in the expression of certain genes and a reduction in the expression of others. In support of the role of HDAC inhibition in the antimanic effects of valproic acid the other HDAC inhibitor, butyric acid (which, interestingly, is something naturally found in our intestines as a by-product of the bacteria in our intestines metabolising dietary fibre), has been found to be efficacious in an animal model of mania.9 HDAC’s role in cancer cell proliferation (spread), differentiation, etc. is well established as is supported by the fact that the HDAC inhibitor, vorinostat, is Food and Drug Administration (FDA; the US Gov.’s regulatory administration on drugs and food) approved for certain types of cancer.

One protein that is upregulated (i.e. its gene expression is increased by valproic acid) by HDAC inhibitors that may be, in part, responsible for the antimanic and analgesic effects of valproic acid is the metabotropic glutamate receptor 2 (mGluR2).10,11 mGluR2 in turn regulates the release of the neurotransmitter glutamate in various brain and spinal cord areas including those involved in emotional processing and the perception of pain.12 It is also conceivable that mGluR2 receptors may play a role in the anticonvulsant effects of valproic acid since excess glutamatergic activity is implicated in epilepsy. Something that is often predictive of antimanic activity in animal models is antipsychotic activity (which in turn is determined in various different animal models of schizophrenia) and mGluR2 agonists (activators) and potentiators are known to possess significant antipsychotic activity, in fact, one mGluR2/3 receptor agonist (pomaglumetad methionil) was in clinical trials until recently as a potential treatment for schizophrenia. Unfortunately this drug failed phase III (the final phase of clinical testing prior to the approval of the drug) clinical testing despite displaying antipsychotic activity in phase II clinical trials. Another change in gene expression that might be involved in the antimanic effects of valproic acid is that of the brain-derived neurotrophic factor (BDNF) – a protein involved in the protection, reproduction and repair of neurons.13,14 It is also possible that the antimanic effects of valproic acid may be due to its inhibitory effects on glycogen synthase kinase 3β (GSK-3β) expression, which is supported by the fact that the antimanic agent, lithium, also inhibits GSK-3β, albeit via a different mechanism.15

How it works on cancer is very complex considering how many different genes are upregulated in cancer cells relative to their non-cancerous counterparts. The following genes are a few that may play a role in the anticancer effects of valproic acid and its salts:
  • Cyclin D216
  • Amyloid precursor protein17
  • Glycogen synthase kinase 3β (GSK-3β)15

Since the amyloid precursor protein is also involved in the pathogenesis (disease process) of Alzheimer’s disease it may be helpful there.18

However don't get me wrong valproic acid and its salts are definitely not very benign (i.e. has a limited potential to do harm) drugs and some of their side effects can leave one with permanent organ injuries and/or death. Potential life-threatening/debilitating side effects according to Micromedex includes:1
  • Hyperammonaemia (elevated blood levels of ammonia) – which can lead to permanent or temporary brain injury and coma 
  • Liver failure
  • Pancreatitis (inflammation of the pancreas)
  • Thrombocyotpaenia (reduction in the amount of platelets in your blood leading to an increased tendency to bleed) 
  • Palpitations (abnormally rapid, irregular or strong heart beat) 
  • Immune hypersensitivity reaction (a severe allergic reaction to the drug) 
  • Permanent Deafness 
  • Pleural effusion (a build-up of fluid around the lungs)

It has a number of less severe, common side effects but there’s so many that I deem it too tedious to write it out here.


Reference List:

  1. Truven Health Analytics, Inc. DRUGDEX® System (Internet) [cited 2013 Aug 24]. Greenwood Village, CO: Thomsen Healthcare; 2013.
  2. Suzuki T, Uchida H, Takeuchi H, Nakajima S, Nomura K, Tanabe A, et al. Augmentation of atypical antipsychotics with valproic acid. An open-label study for most difficult patients with schizophrenia. Human Psychopharmacology: Clinical and Experimental [Internet]. 2009 [cited 2013 Aug 24];24(8):628–38. Available from: http://onlinelibrary.wiley.com/doi/10.1002/hup.1073/abstract
  3. Yoshimura R, Shinkai K, Ueda N, Nakamura J. Valproic Acid improves Psychotic Agitation without Influencing Plasma Risperidone Levels in Schizophrenic Patients. Pharmacopsychiatry [Internet]. 2007 Jan [cited 2013 Aug 24];40(1):9–13. Available from: https://www.thieme-connect.com/DOI/DOI?10.1055/s-2007-958521
  4. Michaelis M, Doerr H, Cinatl Jr. J. Valproic Acid As Anti-Cancer Drug. Current Pharmaceutical Design [Internet]. 2007 Nov 1 [cited 2013 Aug 24];13(33):3378–93. Available from: http://www.eurekaselect.com/60121/article
  5. Routy J, Tremblay C, Angel J, Trottier B, Rouleau D, Baril J, et al. Valproic acid in association with highly active antiretroviral therapy for reducing systemic HIV-1 reservoirs: results from a multicentre randomized clinical study. HIV Medicine [Internet]. 2012 [cited 2013 Aug 24];13(5):291–6. Available from: http://onlinelibrary.wiley.com/doi/10.1111/j.1468-1293.2011.00975.x/abstract
  6. Antiepileptic drugs and agents that inhibit voltage-gated sodium channels prevent NMDA antagonist neurotoxicity. , Published online: 15 August 2002; | doi:101038/sj.mp4001087 [Internet]. 2002 Aug 15 [cited 2013 Aug 24];7(7). Available from: http://www.nature.com/mp/journal/v7/n7/full/4001087a.html
  7. Johannessen CU. Mechanisms of action of valproate: a commentatory. Neurochemistry International [Internet]. 2000 Aug 1 [cited 2013 Aug 24];37(2–3):103–10. Available from: http://www.sciencedirect.com/science/article/pii/S0197018600000139
  8. Wikinski SI, Acosta GB, Rubio MC. Valproic acid differs in its in vitro effect on glutamic acid decarboxylase activity in neonatal and adult rat brain. General Pharmacology: The Vascular System [Internet]. 1996 Jun [cited 2013 Aug 24];27(4):635–8. Available from: http://www.sciencedirect.com/science/article/pii/0306362395020926
  9. Steckert AV, Valvassori SS, Varela RB, Mina F, Resende WR, Bavaresco DV, et al. Effects of sodium butyrate on oxidative stress and behavioral changes induced by administration of d-AMPH. Neurochemistry International [Internet]. 2013 Mar [cited 2013 Aug 24];62(4):425–32. Available from: http://www.sciencedirect.com/science/article/pii/S0197018613000405
  10. Chiechio S, Zammataro M, Morales ME, Busceti CL, Drago F, Gereau RW, et al. Epigenetic Modulation of mGlu2 Receptors by Histone Deacetylase Inhibitors in the Treatment of Inflammatory Pain. Mol Pharmacol [Internet]. 2009 May 1 [cited 2013 Aug 24];75(5):1014–20. Available from: http://molpharm.aspetjournals.org/content/75/5/1014.full.pdf
  11. Chiechio S, Zammataro M, Morales ME, Busceti CL, Drago F, Gereau RW, et al. Epigenetic Modulation of mGlu2 Receptors by Histone Deacetylase Inhibitors in the Treatment of Inflammatory Pain. Mol Pharmacol [Internet]. 2009 May 1 [cited 2013 Aug 24];75(5):1014–20. Available from: http://molpharm.aspetjournals.org/content/75/5/1014.full.pdf
  12. Czapinski P, Blaszczyk B, Czuczwar S. Mechanisms of Action of Antiepileptic Drugs. Current Topics in Medicinal Chemistry [Internet]. 2005 Jan 1 [cited 2013 Aug 24];5(1):3–14. Available from: http://www.eurekaselect.com/79696/article
  13. Grande I, Fries GR, Kunz M, Kapczinski F. The Role of BDNF as a Mediator of Neuroplasticity in Bipolar Disorder. Psychiatry Investig [Internet]. 2010 Dec [cited 2013 Aug 24];7(4):243–50. Available from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3022310/
  14. Yasuda S, Liang M-H, Marinova Z, Yahyavi A, Chuang D-M. The mood stabilizers lithium and valproate selectively activate the promoter IV of brain-derived neurotrophic factor in neurons. Mol Psychiatry [Internet]. 2007 Oct 9 [cited 2013 Aug 24];14(1):51–9. Available from: http://www.nature.com/mp/journal/v14/n1/full/4002099a.html
  15. De Sarno P, Li X, Jope RS. Regulation of Akt and glycogen synthase kinase-3β phosphorylation by sodium valproate and lithium. Neuropharmacology [Internet]. 2002 Dec [cited 2013 Aug 24];43(7):1158–64. Available from: http://www.sciencedirect.com/science/article/pii/S0028390802002150
  16. Venkataramani V, Rossner C, Iffland L, Schweyer S, Tamboli IY, Walter J, et al. Histone Deacetylase Inhibitor Valproic Acid Inhibits Cancer Cell Proliferation via Down-regulation of the Alzheimer Amyloid Precursor Protein. J Biol Chem [Internet]. 2010 Apr 2 [cited 2013 Aug 24];285(14):10678–89. Available from: http://www.jbc.org/content/285/14/10678
  17. Venkataramani V, Rossner C, Iffland L, Schweyer S, Tamboli IY, Walter J, et al. Histone Deacetylase Inhibitor Valproic Acid Inhibits Cancer Cell Proliferation via Down-regulation of the Alzheimer Amyloid Precursor Protein. J Biol Chem [Internet]. 2010 Apr 2 [cited 2013 Aug 24];285(14):10678–89. Available from: http://www.jbc.org/content/285/14/10678
  18. Zhang X-Z, Li X-J, Zhang H-Y. Valproic acid as a promising agent to combat Alzheimer’s disease. Brain Research Bulletin [Internet]. 2010 Jan 15 [cited 2013 Aug 24];81(1):3–6. Available from: http://www.sciencedirect.com/science/article/pii/S0361923009002779

Wednesday, 21 August 2013

D-cycloserine -- An Old Drug Getting A New Life

D-cycloserine is a drug that was once frequently used in the treatment of tuberculosis, nowadays it is less frequently used, probably, in part, due to how rare tuberculosis is nowadays in developed countries like Australia. Nowadays D-cycloserine is generating some interest as a potential treatment for certain psychiatric disorders such as major depressive disorder (MDD) and schizophrenia.1,2 This is because D-cycloserine, much like the non-cyclic form of this amino acid, D-serine, (the related [sort of a mirror image of D-serine] amino acid, L-serine, which is also what the body synthesises D-serine from, is found in animal and vegetable proteins) has been found to bind to the so called glycine site on the NMDA glutamate receptor. The NMDA receptor is one of the receptors for the very important amino acid neurotransmitter, glutamate (also known as glutamic acid). The NMDA receptor primarily serves as an excitatory receptor, that is, it increases the activity of the cells on which it is expressed.

This receptor has been implicated in several of the major psychiatric disorders that have plagued our society for generations such as schizophrenia, bipolar disorder, major depressive disorder and anxiety disorders. In order to be activated, however, the NMDA receptor requires two events to occur simultaneously: the neurotransmitters, glutamate or aspartate (aspartic acid) must bind to the so called glutamate site on the receptor and the neurotransmitter glycine, or similar amino acid neurotransmitters such as D-alanine or D-serine need to bind to the so called glycine site on the receptor. What's so special about D-cycloserine, however, is that it serves as a partial agonist at the glycine site on the NMDA receptor. This means that when it binds to the glycine site, provided glutamate or aspartate is already bound to the glutamate site, a smaller response is seen than if D-serine, D-alanine or glycine had bound to the same glycine site. Hence because the glycine sites on the NMDA receptor are never completely occupied by the glycine, D-serine and the D-alanine obtained from the diet (after some chemical processing) [this is because there are so many NMDA receptors found in the body that it’s impossible for these neurotransmitters to occupy them all] at lower concentrations of D-cycloserine in the blood it (D-cycloserine) occupies the non-occupied glycine sites on the NMDA receptor and hence leads to an overall increase in the activity of the NMDA receptor. At higher concentrations, however, D-cycloserine competes with glycine, D-serine and D-alanine for the binding with the glycine site on the NMDA receptor and hence it displaces some of these amino acid neurotransmitters and hence since it produces a less full response than these amino acid neurotransmitters  it causes a net reduction in NMDA receptor activity.

The way how this relates to psychiatric illnesses like schizophrenia and major depressive disorder is that it is believed that in schizophrenia the NMDA receptor is underactive and hence by potentiating its activity it is hoped that low doses of D-cycloserine (which translates to low concentrations in the blood when the drug is absorbed by the body) may be of therapeutic benefit in patients with schizophrenia. Whereas in major depressive disorder the NMDA receptor is believed to be overactive and drugs that attenuate its activity have been shown to elicit rapid and robust antidepressant effects and hence it is hoped that high doses of D-cycloserine (or high concentrations) might likewise elicit rapid and robust antidepressant effects without some of the psychotomimetic (psychosis [a state characterised by hallucinations, delusions, etc.]-mimicking) effects of NMDA antagonists (blockers) like ketamine.1,2

Its efficacy in treating schizophrenia seems to be very limited, however, probably because the difference between doses that potentiate the activity of the NMDA receptor and doses that inhibit the activity of the NMDA receptor is rather small and the maximum potentiation of NMDA receptor that D-cycloserine is capable of is significantly less than that of glycine site full agonists (compounds that manage [with the help of glutamate/aspartate binding at the glutamate site] to produce full activation of the NMDA receptor) like glycine and hence it is easy to inadvertently give a patient a dose that overall inhibits NMDA activity hence exacerbating the symptoms of schizophrenia.1 

Whereas in treating major depressive disorder D-cycloserine appears to be rather effective, even in previously treatment-resistant cases probably because it’s been discovered that not only do NMDA antagonists produce antidepressant activity but so do NMDA agonists (activators) like glycine and glutamate.2

There is some evidence in rats to suggest that D-cycloserine might be helpful in the treatment of cocaine addiction.3


Reference List:

  1. Goff DC, Cather C, Gottlieb JD, Evins AE, Walsh J, Raeke L, et al. Once-Weekly D-Cycloserine Effects on Negative Symptoms and Cognition in Schizophrenia: An Exploratory Study. Schizophr Res [Internet]. 2008 Dec [cited 2013 Aug 22];106(2-3):320–7. Available from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2628436/
  2. Heresco-Levy U, Gelfin G, Bloch B, Levin R, Edelman S, Javitt DC, et al. A randomized add-on trial of high-dose d-cycloserine for treatment-resistant depression. The International Journal of Neuropsychopharmacology. 2013;16(03):501–6. Available from: http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=8852104
  3. Thanos PK, Bermeo C, Wang G-J, Volkow ND. D-cycloserine facilitates extinction of cocaine self-administration in rats. Synapse [Internet]. 2011 [cited 2013 Aug 22];65(9):938–44. Available from: http://onlinelibrary.wiley.com/doi/10.1002/syn.20922/abstract

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
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