Pharmacogenetics/genomics has been around for a long time, and the impact of this has been largely hidden, but we are, in this century, beginning to see the effect pharmacogenomics can have on clinical practice. The first example of pharmacogenomics was when Pythagoras described the occurrence of haemolysis with the consumption of fava beans. We now know that this is due to a deficiency of glucose-6-phosphate dehydrogenase (G6PD), which is the most common human enzyme deficiency in the world. G6PD testing before prescribing some drugs, such as primaquine, is now standard practice in many parts of the world. Knowledge about genetic polymorphisms has also been included in drug labels for many decades, and this can have an impact on drug development, the avoidance of drug interactions, and adverse drug reactions. More recently, there has been greater emphasis on implementation of pharmacogenomic testing prior to the prescription of drugs (reactive testing). The best example of this with HLA-B*57:01 to prevent abacavir hypersensitivity and HLA-B*15:02 to prevent carbamazepine-induced Stevens-Johnson Syndrome. However, implementation of these tests, even with their strong evidence base, has not been straightforward. The Royal College of Physicians in London, together with the British Pharmacological Society, published a report on personalised prescribing, highlighting the need to implement pharmacogenomics into clinical practice. The approach favoured in this report was to move from a reactive to a pre-emptive approach – i.e. to have genotype data available in the electronic health record so that immediate decisions could be made about which drug (and dose) to prescribe. This requires the use of gene panel testing, where multiple genetic variants are tested at one time point, with the results being incorporated into the electronic health records. Indeed, the recent PREPARE study showed that using a 12-gene pharmacogenetic panel was able to reduce adverse drug reactions by 30%, a clinically impactful result. This implementation approach is being explored in many different parts of the world, but most frequently in the US. To date, no healthcare system in the world has implemented pharmacogenomics for the whole population. Barriers to implementation are no longer related to the availability of genotyping technologies or to the lack of evidence, but are due to difficulties in integrating the whole process into healthcare systems.
