Drug misuse by teenagers in schools in Benin and genetic polymorphism of cytochrome p450: interest and perspectives of cytochrome p450 genotyping

INTRODUCTION

The management of medicines is a complex process that relies on the organisation of the medication circuit. Its objective is to ensure that the right patient receives the right medicine, in the right dosage, by the right mean, under the right conditions and at the best cost.¹ Human and organisational components, environmental factors, evaluation of professional practices and risk situations are the factors to be taken into account in the implementation of drug management.¹

In Benin, the school environment has become a place for the use and misuse of drugs and especially addictive substances with a propensity for abuse, coming from the formal and informal sectors. Stress, the quest for physical performance, the reinforcement of cognitive potential, the search for a social and generational identity are the main reasons leading school teenagers to engage in this practice.² The majority of these adolescents construct their own uses of different pharmaceutical classes of drugs for various purposes and through multiple consolidation networks. Their practice is based on self-medication which has progressively given rise to the phenomenon of diversion of medicines.² They are therefore exposed to poly-drug and psychotropic drug use which can lead to drug interactions. Drug interactions are a major source of accidents or therapeutic failures, particularly in the case of polymedication, especially when they are not detected.^3,4 They are responsible for one third of hospital admissions due to adverse events, 4% to 7% of emergency admissions and 1% of all hospital admissions.^5-7 Responses to drugs may differ from patient to patient in terms of both therapeutic efficacy and adverse effects. This variability may be a limitation to the use of certain drugs, as it is difficult to predict.^8-10 These variabilities are due to genetic, hormonal, physiological, pathophysiological and environmental factors as well as poor compliance and drug interactions.⁹ During the last decades, the genes responsible for drug metabolism and transport as well as their most common functional variants have been identified and characterised. Pharmacogenetics provides a better understanding of the genetic basis for the pharmacokinetic and pharmacodynamic interindividual variability of many analgesic drugs.¹¹ Enzymes responsible for drug metabolism, drug transporters or drug targets themselves are indeed subject to genetic polymorphism defined by differences in gene expression with a frequency greater than 1% of the population.¹² These enzymes responsible for drug metabolism form the cytochrome p450 superfamily. Polymorphisms in cytochrome p450 genes can lead to altered metabolic activity towards a plethora of clinically important drugs. Thus, nucleotide variants and copy number variations in cytochrome genes are major determinants of drug pharmacokinetics and toxicity and are pharmacogenetic biomarkers for drug dosage, efficacy and safety.¹³ This article aimed to show how cytochrome profiling would be an asset for treatment in high school and college students in Benin who already had a regular use of psychoactive substances and in whom the risk of becoming addicted to these substances was already high.

CYTOCHROMES P450

Cytochromes p450 are the most studied enzyme system not only because of their importance in drug metabolism and elimination, but also because of their ability to activate promutagenic and procarcinogenic substances.¹⁴ They are membrane enzymes located mainly in the endoplasmic reticulum membrane of hepatocytes. Other sites of expression are organs that are in contact with the external environment: lungs, intestine, skin and kidneys. They catalyse the oxidation of endogenous or exogenous lipophilic substances (drugs), transforming them into more polar products and facilitating their elimination.¹⁵

The CYP superfamily contains 74 gene families, 35 of which in humans are classified into 14 families.¹² Members of subfamilies representing a particular gene are then designated by a number according to the description of the subfamily.¹² Taking into account the clinical implications of pharmacogenetic polymorphisms and their role in the individual response of patients to drugs, treatments can be better explained both from the point of view of their toxicities and therapeutics, especially the failures that occur with standard doses.

DRUGS PRESENT IN BENINESE PHARMACIES AND METABOLISED BY CYP P450 2D6 AND 3A4

Cytochromes P450 (CYP) are now well known as an enzymatic system very much involved in drug metabolism.¹⁶ Of all the isoforms, CYP2D6 and CYP3A are most importantly involved in opioid metabolism.¹⁶ CYP2D6 is involved in the metabolism of about 25% to 30% of drugs while CYP3A is involved in the metabolism of about 50% of drugs.¹⁷ The drugs available in Beninese pharmacies and the cytochromes responsible for their metabolism are summarized in (Table 1).

GENETIC POLYMORPHISM OF CYP P450

According to the work of Sim et al there are 57 active CYP genes in the human genome (classified into 18 families), and variation in these genes results in several phenotypes defined as ultrafast, extensive, intermediate and slow metabolisers.¹⁹ In general, an ultra-rapid metaboliser carries duplicate or multiple gene copies of the same allele, while intermediate and slow metabolisers carry one and two defective alleles respectively (by gene inactivation or deletion).¹⁹ The black American population has fewer polymorphic variants than the Caucasian populations.²⁰ That is, genetic variation is greater in African populations than in Asian and Caucasian populations. Therefore, the African continent cannot be treated as a single entity in drug research and development, as is the African-American population, which is considered an adequate indicator of pharmacogenetic differences across Africa.²¹ There are very few primary studies on the frequencies of CYP 450 allelic variants in Africa, particularly in West Africa.²² Zhoul et al have found that the distribution of cytochrome alleles differs considerably between populations, which has important implications for drug therapy and personalised healthcare programmes.¹³ However, further research is needed to confirm these distributions and to identify populations at potentially higher risk of drug-induced adverse events or drug inefficacy.²¹ To provide a global distribution map of clinically relevant cytochrome alleles, Zhoul and colleagues had integrated whole genome and exome sequencing data from 56945 unrelated individuals from five major human populations (European, African, East Asian, South Asian, Mixed American). By combining this dataset with population-specific linkage information, they obtained frequencies of 176 cytochrome haplotypes, providing an extensive resource for key genetic determinants of drug metabolism.¹³ CYP p450 alleles were identified that may be unique to Africa or for which data on African populations are limited and regions where additional data need to be collected. It is likely that polymorphisms in other enzymes and transporters contribute to the diversity of drug response observed in Africa. Therefore, the involvement of clinicians in genomic research will facilitate this process of genomic knowledge transfer, helping to ensure that patients are treated with effective doses of therapeutic drugs.²¹

Considering patients' genotype as a complementary tool to therapeutic algorithms could potentially assist clinicians in not only evidence-based but also risk stratification decision making.²³ However, the performance of the suggested practical approach needs to be rigorously evaluated or validated in clinical studies, and the clinician's judgement remains paramount when making decisions.^24,25 The polymorphic nature of the cytochrome p450 gene largely affects an individual's response to the drug and adverse drug reactions.²⁶ With the decrease in sequencing costs and the next generation sequencing analyses can now be performed at the population level. This allows the whole landscape of pharmacogenetic variability to be mapped. Importantly, the functional interpretation of these rare variants cannot be based on experimental data and the development of tools to reliably translate these variants into clinical guidance is therefore an interesting frontier in contemporary pharmacogenomics.²⁷ Therefore, scientific evidence suggests that gene polymorphisms can explain differences in outcomes after drug administration and that pharmacogenomics-guided treatments are possible, but controlled clinical trials are needed to substantiate this claim.²³

ISSUES AND HOPES FOR INDIVIDUALISATION OF DOSING IN DRUG THERAPY

Published literature entitled drug interactions and cytochromes P450, shows us that several drugs that exist in pharmacies in Benin are metabolised by both cytochrome 2D6 and cytochrome 3A4.⁴ Also in this review, the authors showed that a drug can be metabolised mainly by one or both of these CYPs.⁴ The first questions that may arise from these different possibilities of metabolism are the following: in vivo, how would the metabolism of a drug metabolised predominantly by both CYP2D6 and CYP3A4 proceed? Would there be competition between these two cytochromes in the metabolism of this drug? If so, what are the parameters that will come into play to define the main metabolic pathway of this drug? What will happen in the event of enzymatic induction and/or inhibition of a cytochrome by this substrate? The work of Kpatchavi and Adounkpe in 2016 reveals that one of the most commonly used opioids in schools is Tramadol.² Between CYP2D6 and CYP3A4 which one would be responsible for the metabolisation of Tramadol in a Beninese school adolescent who experiments with this drug and who expresses these two cytochromes.

Systematic genotyping of the cytochromes in these adolescents and in general in subjects exposed to the use of psychoactive substances would be an asset in their therapeutic management and more specifically in the adaptation of dosages. The subject may be a slow metaboliser with one cytochrome and at the same time be a normal or fast metaboliser with another cytochrome. Indeed, opioids that require bioactivation by a cytochrome subject to genetic polymorphisms, such as 2D6, are not the analgesic treatments of choice in slow metabolizers for this enzyme.¹⁶ Thus a teenager who is a normal metaboliser of a drug or substrate may become a slow metaboliser when exposed to poly-drug use. The PharmGKB is a pharmacogenomics knowledge base. It is an NIH (National Institutes of Health) funded resource that provides information on how human genetic variation affects drug response. This knowledge base collects, organises and disseminates information on clinically relevant gene-drug associations and genotype-phenotype relationships. Prescribing information provided by PharmGKB includes clinical guidelines on how to adjust the treatment of certain drugs based on a person's genetic information. Several organisations around the world write these guidelines and PharmGKB provides annotations on its website.²⁸

CONCLUSION

For a better care and for dosage individualisation in therapy of subjects exposed to the misuse and use of psychoactive substances, the use of the PharmGKB knowledge base preceded by cytochrome genotyping in exposed subjects are assets that clinicians should make use of nowadays for disease treatment.

Funding: No funding sources

Conflict of interest: None declared

Ethical approval: Not required

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