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Author: Dr Matthew Howard, Clinic/Research Fellow, Victorian Melanoma Service, Melbourne, VIC, Australia. DermNet NZ Editor in Chief: Adjunct A/Prof Amanda Oakley, Dermatologist, Hamilton, New Zealand. Copy edited by Gus Mitchell. January 2020.
Pharmacogenomics is the study of how a patient’s whole genome influences their individual response to therapeutics. This field of research combines knowledge from the fields of:
Often pharmacogenetics is used interchangeably with pharmacogenomics. However, pharmacogenetics correlates drug efficacy and toxicity with interindividual variations in DNA sequence .
The interest in pharmacogenomics, pharmacogenetics, and personalised medicine has arisen because of the finding that only about half of the patients given recommended standard doses of many medications will receive the desired therapeutic benefit . Some of this variation in response to medications is attributed to genetic differences in the mechanisms responsible for the pharmacokinetics .
Much research has been devoted to the pharmacogenetics of phase 1 metabolism enzymes, particularly the cytochrome P450 (CYP450) system.
This variation in the nucleic acid base sequence of genes (the alleles) can alter the action and efficiency of the CYP450 enzymes.
Some medications are prodrugs; their active form is the metabolite of the administered compound. In this case:
Risks to the patient are heightened for medications with a narrow therapeutic index.
Pharmacogenetic testing to identify variant metabolic genes can:
More than half of the drugs that are commonly implicated in adverse effects are major substrates of enzymes that have a mutant form of the gene that poorly metabolises the medication .
Pharmacogenetic testing may also help the process of drug discovery if drug interactions and adverse effects can be predicted, limiting the use of a new drug to those who are low risk of such events .
Typically, a laboratory may only test a limited number of alleles and enzymes.
Drugs with a narrow therapeutic index used in dermatology include immunosuppressants such as tacrolimus, mycophenolate, and ciclosporin. Other examples of the application of pharmacogenomics to dermatology follow.
Effective targeted therapy of metastatic melanoma using BRAF inhibitors (eg, vemurafenib, dabrafenib) requires molecular pathology demonstrating the specific BRAF V600 mutation. BRAF inhibitors can actually enhance MAPK expression in those with wild type BRAF [13,14].
The metabolism of methotrexate is complex. Mutations in the key metabolic enzymes for methotrexate, thymidylate synthetase (TS) and methylenetetrahydrofolate reductase (MTHR), are associated with increased toxicity and decreased efficacy in rheumatological indications. Similar issues are likely in the dermatological use of methotrexate [15–17].
The current standard of care is to test patients for thiopurine methyltransferase (TPMT) activity prior to prescribing azathioprine. This is to evaluate the patient’s capacity to metabolise the drug. There are many alleles that cause low TPMT activity, and patients with low activity are at significantly increased risk for gastrointestinal toxicity or myelosuppression from azathioprine .
There is an increased risk of haemolytic anaemia and neurotoxicity from dapsone in slow acetylators .
The slow-acetylator genotype of NAT2 is a risk factor for sulphonamide-induced toxic epidermal necrolysis/Stevens-Johnson syndrome (TEN/SJS) .
Those who are heterozygous or homozygous for HLA-B*1502 and of Chinese or Asian ancestry are at risk of carbamazepine-induced TEN/SJS .
The HLA-A*31:01 allele has been associated (although less strongly) with severe cutaneous adverse reactions to carbamazepine in Caucasian populations .
The risk of severe cutaneous adverse reactions to allopurinol is greatly increased in Chinese and Thai persons, as well as Koreans with HLA-B*58:01 .
The sonic hedgehog (SHH) pathway inhibitor, vismodegib, is effective for basal cell carcinomas (BCCs) with mutations in the SHH pathway such as PTCH1 and SMO (which occur in 70 and 10–20% of BCCs respectively) .
Individuals with tumour necrosis factor-alpha (TNF-α) receptor or promoter mutations can have either an increased or decreased response to TNF-α inhibitors .
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