Pharmacogenetics is a branch of pharmacology focusing on the influence of genetic variability on drug response. It correlates single nucleotide variations (SNVs) with drug efficacy and toxicity. Pharmacogenetics is becoming increasingly important in the field of personalised medicine, the right drug to the right person, cancer chemotherapy and HIV/TB treatments. In 1892, Sir William Osler made an observation that "If it were not for the great variability among individuals, medicine might as well be a science and not an art."
Because of a person’s genetic make-up, they can either be classified as slow drug metabolisers or fast metabolisers. Slow metabolisers have trouble processing the drugs, which could lead to accumulation of certain drugs in the body, causing toxicity. While fast metabolisers transform drugs at a faster pace, resulting in drug levels never becoming high enough to be effective. This is especially relevant to the cytochrome P450 (CYP) family of liver enzymes responsible for breaking down more than 30 different classes of drugs.
Another enzyme called TPMT (thiopurine methyltransferase), plays an important role in the chemotherapy treatment of a common childhood leukemia by breaking down thiopurines. A small percentage of Caucasians have genetic variants that prevent them from producing an active form of this protein. As a result, thiopurines elevate to toxic levels in the patient because the inactive form of TMPT is unable to break down the drug.
About 10% of African men and few African women have a deficiency of glucose-6-phosphate dehydrigenase (G6PD), which causes the red blood cells to be more fragile. Some drugs, such as chloroquine and primaquine – used for the treatment of malaria can cause haemolytic anaemia as they destroy the already fragile red blood cells.
Adverse drug reactions (ADRs) are an injury caused by taking a medication and they cause more than 100 000 deaths annually in the US. ADRs are reported in 14% of hospitalized patients with HIV/AIDs and TB. And approximately 45% of patient undergoing chemotherapy experience ADRs.
The aim of pharmacogenetics is to establish a rational means to optimize drug therapy in order to maximize efficacy and minimise the adverse effects. Pharmacogenomics combined with traditional pharmaceutical sciences could eventually lead to an overall decrease in the cost of health care because of decreases in:
- the number of adverse drug reactions;
- the number of failed drug trials;
- the time it takes to get a drug approved;
- the length of time patients are on medication;
- the number of medications patients must take to find an effective therapy;
- the effects of a disease on the body (through early detection);
- better vaccines.
Knowing your genetic risk may actually change how your doctor cares for you.