Forensic Pharmacogenetics

Pharmacogenetics and pharmacogenomics are gaining importance both in the clinical setting and in forensic pathology to investigate causes of death where no findings emerge from autopsy, and in the medical liability arena where scientific issues meet the justice system. Generally speaking, Pharmacogenetics is the study of how genetic variations give rise to differences in drug response, while pharmacogenomics (PGx) is the application of genomic technologies to the discovery of new therapeutic targets. Depending on the purpose, pharmacogenetics can be used to define applications of single gene sequences or a limited set of multiple gene sequences, but not gene expression or genome-wide scans, to study variations in DNA sequences related to drug action and disposition. Pharmacogenomics can be used to define applications of genome-wide single-nucleotide polymorphism (SNP) scans and genome-wide gene expression analyses to study variations influencing drug action. Pharmacogenetics is narrower in definition and refers to the study of inter-individual variations in DNA sequence related to drug absorption and disposition (pharmacokinetics) or drug action (pharmacodynamics), including polymorphic variation in genes encoding transporters, drug-metabolizing enzymes, receptors and other proteins. The best-known example of a genetic defect in drug biotransformation is the acetylation polymorphism in tuberculosis therapy with Isoniazid characterized by mutations in N-acetyltransferase-2 (NAT2) on chromosome 8. The reason for the exaggerated effect was found to be the lack of an enzyme almost exclusively responsible for the metabolic elimination of debrisoquine and the affected subjects were classified as poor metabolizers of debrisoquine.  The enzyme named “debrisoquine hydroxylase” is now known as CYP2D6. The oxidation of sparteine was found to be catalyzed by the same enzyme. Now it is well-established that the therapeutic failure of drugs, and adverse side-effects in individuals may also have a genetic component due to genetic variations in the receptors, ion channels, transporter, enzymes and regulatory proteins involved in drug metabolism that may influence pharmacodynamics (e.g. the binding and functional capacity of the receptor or regulatory proteins) and pharmacokinetics, consisting in drug bio-availability at the level of metabolic enzymes and transporters. Most studies have focused on single nucleotide polymorphisms (SNPs) in genes encoding important metabolizing enzymes, like the cytochrome P450 enzyme superfamily, revealing an association with clinical phenotypes of drug efficacy/toxicity. The primary site of drug metabolism is the liver, where enzymes chemically change drug components into substances known as metabolites that are then bound to other substances for excretion mainly through the kidneys, lungs or bodily fluids or by intestinal reabsorption. Some drugs do not change chemical structure and are removed from the body as such. Drug pharmacokinetics and pharmacodynamics are regulated by complex chemical reactions with the participation of numerous proteins encoded by different genes, deputies for the transport and metabolism of drugs, or involved in their mechanism of action. Two different types of metabolic reactions are involved: in phase 1 molecules are characterized by oxidation, reduction and hydrolysis reactions, in phase 2 drugs are conjugated with other compounds and then discarded. If two or more polymorphic genes regulate drug metabolism and transport inside a cell, the variability in the response to treatment depends on the interaction of these gene variants. The cytochrome P450 enzyme system plays a central role in phase I oxidative metabolism of the vast majority of prescribed drugs and also of endogenous substances. However, when an inactive “pro-drug” must be converted to the active metabolite (e.g. codeine and tamoxifen), the therapy will be ineffective in PM – poor metabolizers subjects and Ums – ultra rapid metabolizers will metabolize it quickly with accumulation of the metabolite and consequent toxicity. As a result, drug toxicity is related to metabolizer status. Moreover, the phenomenon of phenocopying must be taken into account where EM [extensive metabolizers] individuals turn into apparent PM or IM [intermediate metabolizers] phenotypes because of drug-drug interactions. Nevertheless, epigenetics, defined as heritable phenotypic changes not involving alteration in nuclear DNA, promises answer to interindividual variability in drug response not associated to genetic polymorphism. Multiple active gene copies are responsible for ultra-rapid metabolizer individuals.

Ethnic specificity has become an integral part of pharmacogenetics research but caution is required against the use of continental labels to lump together heterogeneous populations. The Asian category, for example, is applied to individuals of distinct ethnicity and/or living in different countries or regions of the vast continent of Asia. Not surprisingly, significant variation in the distribution of pharmacogenetics polymorphism is detected among Asians. Nevertheless, with increasing global migration, admixture gains relevance as anadditional challenge to the successful worldwide implementation of pharmacogenetics in clinical practice. The Brazilian population, with tri-hybrid ancestral roots in Amerindian, European and African groups and five centuries of extensive inter-ethnic mating, provides a valuable model for studying the impact of admixture on the conceptual development and clinical implementation of pharmacogenetics-informed prescription. Recognition of this fact is important in the design and interpretation of pharmacogenetics clinical trials in Brazilians, but does not imply that pharmacogenetics-informed drug prescription requires investigation of individual ancestry. Rather, individual genotyping should be directed to polymorphisms of proven clinical utility, irrespective of biogeographical ancestry. By investigating drug metabolism related to individual genetic polymorphism, pharmacogenetics has a significant impact on the clinical setting so that forensic implications may arise throughout the public health sector. The concept of “therapy with the right drug at the right dose in the right patient” was highlighted just about ten years ago. The development of pharmacogenetics has important implications in the medico-legal and forensic field because the classic topics of informed consent, shared genetic information, privacy and data base collection arise. It remains unclear whether personalized medicinenmor individualized drug therapy will ever be achievable by DNA testing alone. The unknown exact relationship between the genotype and phenotype, although in many cases the genotype explains most of the inter-individual variability, the lack of prospective clinical studies in large patient cohorts and no reliable data on the cost effectiveness of screening procedures explain the slow progress in clinical pharmacogenetics/pharmacogenomics. The expectation is that this research field will provide both the industry and clinicians with useful pharmacogenomic biomarkers that can aid in procedures for drug development and specific drug treatment in order to optimize the results and improve human health. The process is slow, and for a solid basis for decisions on mandatory biomarkers to be used further large prospective clinical studies are required. One fruitful manner in which this can be achieved is a closer collaboration between industry and academics. In the forensic context, pharmacogenetics can assist in the interpretation of drug-related deaths, especially accidental drug poisonings or cases of sudden death with “nearly normal autopsy called “white autopsy”. The ability to identify "invisible diseases" with post-mortem genetic testing has become a reality far more quickly than anyone had ever imagined. The role of pharmacogenetic analysis in forensic investigation has already been emphasized as the holistic approach of molecular analysis connected to macroscopic, microscopic and toxicological observations, constituting an integral part of modern medico-legal study of death. help resolve cases initially believed to be suicide or classified as sudden unexplained deaths especially in cases where poisoning, incapacitation, inebriation or certain diseases where pharmacotherapy is an essential treatment (such as epilepsy, depression, cardiac diseases or diabetes) are factors in the cause of death. An additional benefit is that pharmacogenetics analysis may provide health information (certainly only via proper ethical disclosure practices) to at-risk relatives.

Current pharmacogenetics research in the clinical and medico-legal settings provides new options for disease treatment and prevention of ADR avoiding correlated death. The ensuing information will be translated into routine clinical practice in the years to come benefitting millions of patients worldwide.  Indeed, epigenetics providing answers to interindividual variability in drug response not associated to genetic polymorphism, could represent the bridge that connects the environment to the genome.

Acknowledgements:

The Police Department;

www.politie.nl and a Chief Inspector – Mr. Erik Akerboom     ©

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