Acetylation phenotype, platelet monoamine oxidase inhibition, and the effectiveness of phenelzine in depression

The trace amine theory of spontaneous hypertension as induced by classic monoamine oxidase inhibitors

The classic monoamine oxidase inhibitors (MAOIs) tranylcypromine (TCP) and phenelzine (PLZ) are powerful antidepressants that come with an equally powerful stigma, and are thus rarely prescribed-despite their well-established effectiveness. Some of these preconceptions appear to stem from unclarity, as the etiology of a rare but important side effect, 'spontaneous hypertension' (SH)-a significant increase in blood pressure absent dietary tyramine ingestion-remains improperly elucidated. This paper aims at uprooting some of the stigma surrounding MAOIs by advancing the trace amine (TA) theory as the causative underpinning of SH. This theory posits that SH results from the considerable influx of TAs observed following TCP- or PLZ-administration. TAs are known, albeit at greatly supraphysiological levels, to raise blood pressure on account of their propensity to exert potent indirect sympathomimetic effects; additionally, some research posits that TAs may induce vasoconstrictive effects partly or wholly separate therefrom, which would then constitute a second hypertensive mechanism. TAs are endogenous to the human body in low quantities. Both TCP and PLZ cause marked elevations of 2-phenylethylamine (PEA), meta- and para-tyramine (m-/p-TYR), octopamine (OA), and tryptamine (TRYP), following both acute and (sub)chronic administration. This paper holds that TYR plays a pivotal role in causing SH, due to its strong pressor effect. Cautious treatment of SH is advised, given its typically self-limiting nature. The risk of hypotensive overshoots must be taken into account. For severe cases, this paper urges reconsideration, following suitable confirmation trials, of antipsychotics (notably risperidone) as these agents may reduce striatal p-TYR levels.

Pharmacogenetics, drug-metabolizing enzymes, and clinical practice

The application of pharmacogenetics holds great promise for individualized therapy. However, it has little clinical reality at present, despite many claims. The main problem is that the evidence base supporting genetic testing before therapy is weak. The pharmacology of the drugs subject to inherited variability in metabolism is often complex. Few have simple or single pathways of elimination. Some have active metabolites or enantiomers with different activities and pathways of elimination. Drug dosing is likely to be influenced only if the aggregate molar activity of all active moieties at the site of action is predictably affected by genotype or phenotype. Variation in drug concentration must be significant enough to provide "signal" over and above normal variation, and there must be a genuine concentration-effect relationship. The therapeutic index of the drug will also influence test utility. After considering all of these factors, the benefits of prospective testing need to be weighed against the costs and against other endpoints of effect. It is not surprising that few drugs satisfy these requirements. Drugs (and enzymes) for which there is a reasonable evidence base supporting genotyping or phenotyping include suxamethonium/mivacurium (butyrylcholinesterase), and azathioprine/6-mercaptopurine (thiopurine methyltransferase). Drugs for which there is a potential case for prospective testing include warfarin (CYP2C9), perhexiline (CYP2D6), and perhaps the proton pump inhibitors (CYP2C19). No other drugs have an evidence base that is sufficient to justify prospective testing at present, although some warrant further evaluation. In this review we summarize the current evidence base for pharmacogenetics in relation to drug-metabolizing enzymes.

The efficacy and tolerability of combined antidepressant treatment in different depressive subgroups

Eighty patients admitted to hospital with major depression were randomly allocated to six weeks of treatment with tranylcypromine, amitriptyline, or tranylcypromine and amitriptyline in combination, in a double-blind study. Scores on the HRSD improved significantly in all three groups, but there were no differences between the three groups. Patients on tranylcypromine and amitriptyline combined improved more according to their self-ratings after six weeks, and response was earlier as measured by a clinical global improvement scale. Those with endogenous depression improved more than those with neurotic depression, irrespective of treatment group. Combined treatment was less well tolerated than single treatments and gave rise to more side-effects, although there was no serious toxicity. Orthostatic hypotension was observed more frequently in patients on combined treatment. This group also experienced a significant increase in weight and prolongation of the P-R interval on ECG.

Efficacy and safety of moclobemide compared with imipramine in the treatment of major depressive disorder. Double-blind multicenter study, Austria

In a double-blind, 4-week, prospective, randomized multicenter (17 centers) study we checked on the efficacy, tolerability and safety of moclobemide (300-600 mg/d) compared to imipramine (100-200 mg/d) in parallel groups of patients with a Major Depressive Episode (DSM III). The mean % reduction of the HAMD at the end of treatment was 51.7 in the moclobemide group and 52.1 in the imipramine group. The percentage of patients in whom efficacy was globally judged as "good" or "very good" was 62% in the moclobemide group and 60% in the imipramine group. There was no statistically significant difference in the efficacy in both groups but in some factors there was a trend for a better amelioration favoring moclobemide. The final overall physician's judgement of tolerability was "good" or "very good" in 83% of moclobemide patients and in 74% of imipramine patients. Adverse events were reported or observed in 56% of moclobemide patients and in 69% of imipramine patients. The number of mild, moderate and severe adverse events was higher in the imipramine group with a total of 286 versus 189. There was a statistically high significant difference considering the tolerability favoring moclobemide again. In this project the basic goal to find a substance with at least the same efficacy but a much better tolerability for sure got fulfilled.

Platelet monoamine oxidase B: use and misuse

The human platelet in addition to having serotonin (5-HT) receptors, uptake carriers (receptor) and transmitter storage vesicles, primarily possesses mitochondrial monoamine oxidase (MAO) type B. Similar to the major form of MAO in the human brain, this enzyme actively oxidizes A-B and B substrates (tyramine, dopamine, phenylethylamine) as well as the novel secondary amine anticonvulsant, milacemide and dopaminergic neurotoxin, MPTP. 5-HT oxidation is hardly affected by the platelet enzyme and MAO inhibitors have no net effect on its accumulation. MAO-B is selectively inhibited by 1-deprenyl and thus the platelet enzyme may be useful to monitor the anti-Parkinson activity of such drugs, as related to their ability to inhibit brain MAO-B. The oxidation of the anticonvulsant, milacemide, to glycine in vitro and in vivo by MAO-B, may herald new prospects for the development of inert prodrugs capable of being metabolized to neuroactive substances by MAO-B. The plasma levels of their metabolites may be an index of MAO-B activity found in the platelet and brain.

Ethnicity and psychopharmacology

In clinical studies and cross-national surveys, cultural or ethnic factors have been found to be significant determinants in patients' responses to psychotropic medication. Dosage requirements and the potential for toxic reactions might differ amongst racial and ethnic groups. In this paper, the authors review the relevant literature and examine pharmacodynamic, pharmacokinetic and cultural-psychological factors, all of which could be responsible for such ethnic variations.

The present status of monoamine oxidase inhibitors

The present status of monoamine oxidase inhibitors in the treatment of depression is reviewed. With adequate doses they are effective antidepressants, but dosages have in the past been too low. Provided proper dietary precautions are taken, the incidence of fatality from dietary interactions is very small and should not deter doctors from using these drugs, especially in those depressed patients who do not respond to tricyclic-type antidepressants. The present status of combining monoamine oxidase inhibitors with tricyclics is discussed, as are the newer specific inhibitors particularly clorgyline and deprenyl.

Genetically determined variability in acetylation and oxidation. Therapeutic implications

The clinical significance of two separate genetic polymorphisms which alter drug metabolism, acetylation and oxidation is discussed, and methods of phenotyping for both acetylator and polymorphic oxidation status are reviewed. Particular reference is made to the dapsone method, which provides a simple means of distinguishing fast and slow - and possibly intermediate - acetylators, and to the sparteine method which allows a clear separation of oxidation phenotypes. Although acetylation polymorphism has been known for some time, definite indications for phenotyping are few. It is doubtful whether acetylator phenotype makes a significant difference to the outcome in most isoniazid treatment regimens, and peripheral neuropathy from isoniazid in slow acetylators is easily overcome by pyridoxine administration. However, in comparison with rapid acetylators, slow acetylators receiving isoniazid have an increased susceptibility to phenytoin toxicity, and perhaps also to carbamazepine toxicity. It is also possible that rapid acetylators receiving isoniazid attain higher serum fluoride concentrations from enflurane and similar anaesthetics than do similarly treated slow acetylators. Thus, when drug interactions of these types are suspected, phenotyping for acetylator status may be advisable. If routine monitoring of serum procainamide and N-acetylprocainamide concentrations is practised, phenotyping of subjects prior to therapy with these agents should not be necessary. Although acetylator phenotype influences serum concentrations of hydralazine, when this drug is given in combination with other drugs acetylator phenotype has not been shown to influence the therapeutic response. Slow acetylator phenotype along with female gender and the presence of HLA-DR antigens appear to be risk factors in the development of hydralazine-induced systemic lupus erythematosus (SLE). Determination of acetylator phenotype may therefore help determine susceptibility to this adverse reaction. In the case of sulphasalazine, adult slow acetylators require a lower daily dose of the drug than fast acetylators in order to maintain ulcerative colitis in remission without significant side effects. It is therefore advisable to determine acetylator phenotype prior to sulphasalazine therapy. Work on the association of acetylation polymorphism with various disease states is also reviewed. It is possible that a higher incidence of bladder cancer is associated with slow acetylation phenotype - especially in individuals exposed to high levels of arylamines. The question as to whether idiopathic SLE is more common in slow acetylators remains unresolved. There appears to be no difference between fa