ABSTRACT | PDF
SEMINAR
Cytochrome P450 and its relevance to psychiatry
Pranjal Sharma
Postgraduate Trainee of Psychiatry
Silchar Medical College and Hospital
History and background In 1955, Axelrod and Brodie et al., identified an enzyme system in the endoplasmic reticulum of the liver which was able to oxidize xenobiotic compounds. In 1958, Garfinkel and Klingenberg detected a carbon monoxide (CO) binding pigment in liver microsomes which had an absorption maximum at 450nm. This was demonstrated to be a haemoprotein. In 1964 this was named cytochrome P450 (CYP 450) after the strong feature in its absorption spectrum. In 1985 a full structure of (CYP101), a bacterial P450 from Pseudomonas putida, was obtained. Subsequently, crystal structures have been obtained for the enzymes as well as for substrate, inhibitors and substrate analogues.
Cytochromes P450 have been named on the basis of their cellular (cyto) location and spectrophotometric characteristics (chrome): when iron in the haeme is reduced and allowed to bind to CO (or oxygen), the complex absorbs light in the visual spectrum such that it becomes blue and violet colour at the wavelenght of maximum absorption 450nm. This peak of maximum absorption is called the Soret Peak or Soret Band named after its discoverer Jacques louis Soret.
Nomenclature Depending upon the extent of amino acid sequence homology, the cytochrome p450 isoenzymes are grouped in families designated by numericals (1,2,3…) each having several subfamilies designated by capital letters (A,B,C…) while individual isoenzymes are again alloted numericals (1,2,3…)
Root:- CYP
Family:- CYP 2
Subfamily:- CYP 2 D
Isoenzyme:- CYP 2 D 6
All isoenzymes in the same family have at least 40% structural similarities and those in the same subfamilies have 60% structural similarities. In human beings only a few members of three isoenzyme families namely CYP 1, 2 and 3 carry out metabolism of most of the drugs.
Substrate: The agent which is metabolized by an enzyme into a metabolic end product. Usually this result in eventual deactivation of the agent in preparation for elimination from the body. In rare case of "prodrug" these agents are initially inactive and rely on enzyme to be metabolized into active compounds.
Inhibitor: An agent that interfere with or inhibits the functioning of enzyme that metabolize a substrate. Enzyme inhibition are of two types - competitive and non-competitive.
Inducer: An agent that causes the target organ to produce more of an enzyme leading to increase metabolism of the substrate of the induced enzyme. The introduction of inducer leads to increase metabolite formation and rapid depletion of substrate. Induction of CYP450 generally takes two to three weeks to reach full effect.
Properties of human CYP 450 enzyme
1. All are haemoprotiens.
2. Liver contains the highest amount but found in other tissues including small intestine, kidney, adrenals, lungs.
3. Located in smooth endoplasmic reticulum or mitochondria.
4. They have a molecular mass of about 55KDa.
5. Basically they catalyze oxidation reaction involving introduction of one atom of oxygen into the substrate and one atom into water.
6. Involved in the phase I of the metabolism of innumerable xenobiotics including 50% of drugs administered.
7. Involved in the metabolism of many endogenous compounds.
8. Often exhibit broad substrate specificity thus acting on many compounds.
9. They are inducible, resulting in one cause of drug interaction.
10.Many are inhibited by various drugs or their metabolic products, providing another cause of interaction.
11.Some exhibit genetic polymorphism, which can result in atypical drug metabolism.
12.Also their activity is altered by diseased state affecting drug metabolism.
Enzyme mechanism CYP P450’s mechanism of action of enzyme activity can be explained in a series of simple chemical steps:
1) Substrate trapping begins to cytochrome ferric (Fe3 +).
2) An electron is transferred to the Fe atom, moving it to its ferrous state (Fe2 +), this transfer is often carried out by protein cytochrome b5.
3) The ferrous form binds to a molecule of O2.
4) A second reduction is done by adding an electron and a proton.
5) This intermediate loses a water molecule leaving a complex (FeO) 3 + that directly oxidizes the substrate.
The electrons involved in the oxidation of substrates in the hands of CYP P450 may come from NADPH (nicotinamide adenine dinucleotide phosphate) from the cytoplasm or cytochrome b5 which is located in the membrane.
Factors affecting biotransformation Biotransformation means a chemical alteration of the drug in the body which renders a nonpolar (lipid soluble) compound polar(water soluble) so that they are not reabsorbed in the renal tubules and are excreted. This is accomplished by two forms of reactions called the phase-I and phase-II reactions. Some of the factors which affects the process of biotransformation are -
i) Genetic polymorphism: Genetic variations are important contributors to the individual differences in drug biotransformation. These variations may be either mutation (rare variant often associated with disease) or polymorphism (much more common in a population) that may result from deletion or insertion of one or more nucleotide. These variations are identified either by genotyping or by phenotyping and on this basis three groups of individuals are made namely poor metabolizers, intermediate metabolizers and extensive metabolizers.
ii) Age: Hepatic microsomal enzymes and renal mechanisms are less functional at birth and develop more rapidly during the first four weeks of life. In elderly above 65 years the glomerular filtration rate (GFR) decrease by 30% and every following years it falls by one to two per cent as a result of cell loss and decrease renal blood flow. Besides this there is an age related decrease in liver mass, hepatic enzyme activity and hepatic blood flow.
iii) Disease: Disease state can impair liver functions including hepatitis, alcoholic liver diseases, billiary cirrhosis, hepatocellular carcinoma etc. Viral infections like influenza, herpes simples, infectious mononucleosis, adenovirus can also affect the biotransformation process.
iv) Drug interactions: The most common cause of altered drug biotransformation reaction are induction and inhibition of CYP 450 enzyme. Such drug interactions are especially important to take into account when using drugs of vital importance to the patient, drugs with important side effects and drugs with small therapeutic windows, but any drug may be subject to an altered plasma concentration due to altered drug metabolism. A classical example includes antiepileptic drugs. Phenytoin, for example, induces CYP1A2, CYP2C9, CYP2C19 and CYP3A4. Substrates for the latter may be drugs with critical dosage, like amiodarone or carbamazepine, whose blood plasma concentration may decrease because of enzyme induction.
CYP 450 enzyme and antidepressants The CYP 450 enzyme follows the principle of transforming substrate into products. The five most important enzyme for antidepressant and mood stabilizer drug metabolism are:
CYP 450 1A2, CYP 450 2C9, CYP 450 2C19, CYP 450 2D6, CYP450 3A4.
CYP 450 1A2: Substrates for this enzyme includes mainly the tricyclic antidepressants (TCAs) specially the tertiary amines like clomipramine and imipramine. The enzyme demethylates such TCAs but the products still are active.
Fluvoxamine is a selective serotonin reuptake inhibitor (SSRI) which is a substrate as well an inhibitor of the enzyme CYP 450 1A2, besides this other SSRIs like fluoxetine, paroxetine, sertraline,and other antidepressants like bupropion and venlafaxine are moderate to low inhibitors of this enzyme. Thus when these drugs are given concomitantly with other drugs that are metabolized by this enzyme, those drugs can no longer be metabolized efficiently and would lead to their accumulation in blood e.g. when fluvoxamine is given with theophylline or warfarin increase blood levels of these drugs may precipitate seizure or bleeding disorders.
CYP 450 2D6: Another important CYP 450 enzyme for antidepressant is 2D6. TCAs are substrate for 2D6 which hydroxylate them and thereby inactivates them. SSRIs on the other hand have wide range of potency for 2D6 inhibition with paroxetine and fluoxetine the most potent and fluvoxamine, sertraline the least potent. One of the most important drug interaction that SSRIs can cause through inhibition of 2D6 is to raise the plasma level of atypical antipsychotics and TCAs if given concomitantly.
CYP 450 3A4: A third important CYP450 enzyme for antidepressant and mood stabilizers is 3A4. Substrates for this enzyme includes benzodiazepines like alprazolam and triazolam, and other nonpsychotropic drugs like cisapride, terfenidine and astemizole. Some antidepressants like SSRIs fluoxetine, fluvoxamine and nefazodone are 3A4 inhibitors and concomitant use of these drugs can have adverse outcome.
CYP 450 inducers: Not only drugs be substrates or inhibitors for CYP 450 enzymes; they can also be inducers. One example is of the mood stabilizer and anticonvulsant carbamazepine which is both a substrate and an inducer of 3A4. Thus with chronic treatment with carbamazepine 3A4 is induced and carbamazepine blood levels will fall and failure to recognise this effect and to increase carbamazepine dosage to compensate for it may lead to failure of anticonvulsant or mood stabilizing efficacy of the drug. Another example is cigarette smoking which induces the enzyme 1A2.
CYP 450 enzyme and antipsychotics CYP 450 1A2: Three atypical antipsychotics are substrate for 1A2 namely olanzapine, clozapine and zotepine. When given with fluvoxamine (enzyme inhibitor) their levels will rise which may not be clinically much significant for olanzapine except for some increase sedation but in case of clozapine and zotepine the risk for siezures are increased. Again with people who are smokers use of these drugs may need higher doses as tobacco is an inducer and if not checked a relapse of symptoms may occur.
CYP 450 2C9: The new dopamine partial agonist (DPA) bifeprunox is a substrate of 2C9 and its levels are increased by coadministration of a 2C9 inhibitor like fluconazole, fluoxetine or amiodarone.
CYP 450 2D6: Atypical antipsychotics like resperidone, clozapine, olanzapine, aripiprazole are all substrate for this enzyme. Paliperidone a metabolite of resperidone is itself an atypical antipsychotic. It bypasses the 2D6 enzyme and therefore not affected by alteration of activity of the enzyme. Several antidepressants are inhibitors and their concomitant administration must have to be considered because of the possibility of developing extrapyramidal syndrome (EPS).
CYP 450 3A4: Substrates include clozapine, quetiapine, ziprasidone, sertindole, aripiprazole, zotepine and bifeprunox. Several psychotropics are weak inhibitors of this enzyme like fluoxetine, nefazodone. Several nonpsychotropic drugs are potent inhibitors of 3A4 like ketoconazole, protease inhibitors.
Barbiturates and CYP 450 enzyme system Best known effect of barbiturates on liver are those on microsomal drug metabolizing system. Chronic administration of barbiturates increases the protein and lipid content of hepatic endoplasmic reticulum and CYP 1A2, 2C9, 2C19, 3A4. Induction of these enzyme increases metabolism of many endogenous compounds, steroid hormones, cholesterol bile acids, Vit K and D. It also induces barbiturates metabolism (autoinduction) which accounts for tolerance of barbiturates.
Benzodiazepines and CYP 450 enzyme system Benzodiazepines (BZDs) are metabolised extensively by CYP 450 particularly CYP 3A4 and CYP 2C19. Some BZDs such as oxazepam are conjugated directly and are not metabolised by these enzymes. Erythromycin, clarithromycin, ritonavir, ketoconazole, itraconazole, grape fruit juice inhibits 3A4 and can affect BZD metabolism. The active metabolites of many BZDs are biotransformed more slowly then the parent compound and the duration of action of many BZDs bears little relationship to the half life of elimination of the drug that has been administered.
Clinical vignettes 1. A chronic schizophrenic who was on thioridazine 400mg for years was given paroxetine 20mg following a depressive episode. After one week patient complained of a new onset resting tremor, stiffness and racing heart?
2. A bipolar patient who was in acute manic state was kept for four weeks in hospital on olanzapine 20mg/day and was discharged with the same medications to continue. Patient was a habitual smoker and after three weeks of discharge he had a relapse and was readmitted?
3. A 55 years old women was given pimozide 4mg by her dermatologist for delusional parasitosis. One winter she caught cough and fever and was given clarithromycin 500mg BID for ten days. On day seven the patient went to sleep and never woke up. Postmortem finding led to the diagnosis of fatal arrhythmia?
References
1. Stephen M Stahl, Essential Psychopharmacology; 2nd and 3rd ed.
2. Harper's Illustrated Biochemistry; 26th ed.
3. Goodman & Gillman, Essential Pharmacology; 11th ed.
4. K.D Tripathi, Essentials of Medical Pharmacology; 6th ed.
5. Harrison’s Principles of Internal Medicine; 16th ed.
6. World wide web.
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