Pharmacogenetics
From a toxicological perspective we are still at the early stages of understanding the impact of genetic polymorphisms in drug disposition. Nevertheless, we can anticipate that the impact of ‘toxicogenomics’ in toxicology will increase sharply over the coming years. Some of the known effects of pharmacogenetics on the toxicology of drugs are given below.
Cytochrome P450 is a family of mixed-function oxidases that participate actively in the disposition of drugs from the body. The large number of isoforms suggests that, in addition to participating in the metabolism of xenobiotics, physio logically they participate in the maintenance of homeostasis in the body. There are 39 functional human genes that encode isoforms of cytochrome P450. Three subfamilies (1, 2 and 3) include 19 of these isoforms, the most relevant in xenobiotics metabolism. Polymorphisms have been observed in several isoforms (CYP2D6, CYP2C9, CYP2C19, CYP2E1 and CYP3A4). Genetic polymorphisms, related in particular to CYP2D6, are of relevance from a pharmaco logical and toxicological point of view. This is because there are drugs for which disposition from the body is regulated by this enzyme (opiates, beta-blockers, anti-arrhythmics and antidepressants), and because of other drugs that act as inhibitors of this enzyme (i.e. methadone, dextropropoxyphene) and thus enhance the toxicity of drugs that are substrates. Patients with mutations in CYP2D6 have impaired metabolism of drugs if the metabolism co segregates with this enzyme; hence such drugs tend to accumulate in the body, with an enhanced risk of toxicity if the dose is not adjusted according to the genotype.
For example, the main metabolic pathway for codeine, dextromethorphan and ethylmorphine is the metabolism to morphine through O-dealkylation. Some 7% of caucasians are deficient in this enzyme and are unable to produce significant amounts of morphine. In these people, codeine and ethylmorphine appear to be far weaker analgesics than in those who are able to produce morphine. The same enzyme is involved in the bioconversion and activation of oxycodone to oxymorphone, hydrocodone to hydromorphone, risperidone to 9-hydroxyrisperidone and in the metabolism of olanzapine. The efficacy and toxicity of these drugs are therefore affected by this genetic difference.
P-Glycoprotein P-Glycoprotein (P-gp) is an adenosine triphos phate (ATP)-dependent transporter that participates in the active transport of drugs and their metabolites. P-gps are encoded by members of a gene family referred as the multidrug resistance (MDR) genes for their role in MDR in cancer chemotherapy. Physiologically, P-gp seems to act as a barrier to entry and as an efflux mechanism for xenobiotics and cellular metabolites. It is located in the liver, intestine, kidney, blood–brain barrier and other barrier-epithelial tissues. P-gp influences the oral bioavailability of drugs, and an overlap of substrates with CYP3A has been observed. This observation is of relevance because as many as 50% of drugs are metabolised through this enzyme. Hence, at the intestinal level, the co-ordinated activity of P-gp and CYP3A may condition the absorption and pre-systemic disposition of many drugs. From a toxicological point of view, drug–druginterac tions at the absorption level or at the biliary excretion levels may lead to accumulation of drugs and an enhanced toxicity.
Glucuronidation is an important conjugation reaction. It is considered as Phase II metabolism where metabolites from Phase I metabolism are conjugated. In the case of glucuronidation, metabolites are conjugated to glucuronic acid, making the resultant metabolite more water soluble. Two gene families encode for uridine diphosphate glucuronosyltransferase (UDPGT) isoforms: UGT1 and UGT2. Several polymorphic forms of isoforms encoded by UGT2 have been identified; none of them results in significant changes in the rate of glucuronidation of drugs and other xenobiotics. In UGT1, mutation of the isoform UGT1A1 leads to a partial or total impairment of bilirubin conjugation. This results in hyperbilirubinaemias associated with Gilbert syndrome. There is currently considerable interest in the direct detection of glucuronide metabolites because they typically have a longer excretion time than the Phase I metabolites and hence may be detected a longer time after administration. This can be particularly useful in cases of drug-facilitated sexual assault where victims often present sometime after the incident. When GC-MS was the primary tool for drugs analysis, glucuronide conjugates were hydro lysed back to the parent compound for analysis because glucuronides do not chromatograph well by GC. LC-MS enables direct analysis of glucuronides.
Glutathione S-transferases Glutathione S-transferases (GSTs) participate in the activation and detoxification of many drugs and xenobiotics. There is a high prevalence in humans of genetic polymorphisms for several GSTs. Several studies have tried to associate such polymorphisms with an increased risk for the development of cancer of environmental origin, with conflicting results. Paracetamol toxicity is associated at high doses with a depletion of hepatic glutathione stores. Other drugs using this metabolic pathway may result in an enhanced hepatotoxicity at relatively normal doses because of competition for the same detoxification mechanism.
N-Acetylation is the oldest and probably the best-known polymorphism of drug-metabolising enzymes. In addition to polymorphisms that give rise to slow and rapid acetylator phenotypes, there are high inter-ethnic variations in the prevalence of such phenotypes in the population. A cytosolic N-acetyltransferase (NAT-2) is the polymorphic enzyme. Acetylation polymorphism regulates the metabolism of drugs with arylamine (e.g. isoniazid) and hydrazine (e.g. hydralazine) chemical structures as well as pro mutagenic/mutagenic heterocyclic arylamines from dietary or environmental origin. Sulfation and methylation Sulfation and methylation are important path ways in the metabolism of many drugs and xenobiotics. Sulfotransferases (SULT, SULT1 and SULT2) and methyltransferases (methyl trans ferase (MT), catechol methyltransferase (COMT), thiopurine methyltransferase (TPMT) and thiol methyltransferase (TMT)) catalyse such reactions. Several polymorphisms have been identified for these enzymes but, to date, their clinical significance is unclear. The possible influence of genetic polymorphisms on drug metabolism should be taken into account in clinical treatment and interpret ation of toxicological analyses, although information on these polymorphisms may well not be available for many toxicological investigations.
