Despite its promise, PGx use has not become widespread. The reasons for this are many. One reason is that there are just too many variables at play with drug metabolism (diet, co-medications, age, organ function, etc.) to rely on a simple PGx result. With so many variables, the cost of obtaining a PGx genotype on a patient becomes hard to justify. As genetic test prices continue to fall this will present less of a barrier in the future. Another problem has been the lack of clinical studies that show that PGx improves morbidity and mortality for patients. That is, if PGx testing doesn't actually lead to better outcomes, then it is hard for clinicians and patients to justify its cost. There are some studies which suggest, for example, that PGx testing may reduce hospital admissions (for those on warfarin) but to-date there are few large, powerful studies which clearly demonstrate that PGx testing provides added benefit to patients.
The US Medicare/Medicaid system has also been slow to reimburse for PGx testing. Thus, it has not been adopted by clinical laboratories or health systems.
There are some drugs however for which PGx testing is becoming more popular. Some of these drugs have such a high potential for severe, lethal toxicity that PGx testing or enzyme testing for these drugs has become standard of care. Some examples of these drugs and the enzymes that are assessed before use include:
- Irinotecan (where UDP-glucuronosyl transferase 1A1 (UGT1A1) is assessed)
- 6-mercaptopurine (Thiopurine methyl transferase (TPMT) activity is assessed)
- 5-Fluorouricil (Dihydropyrimidine dehydrogenase (DPD) activity is assessed)
- HLA-5801: 6-7% of Asians have a mutation in the HLA 5801 gene which gives hypersensitivity to the drug allopurinol. Stevens-Johnson syndrome or toxic epidermal necrolysis can occur in susceptible patients. The presence of the HLA-B*58:01 allele is strongly associated with these adverse conditions.
- HLA allele B*1502: this is a marker for carbamazepine-induced Stevens-Johnson syndrome and toxic epidermal necrolysis. The FDA now recommends genotyping all Asians for the allele.
Clopidogrel (Plavix) is another drug in which PGx testing is getting more attention. For this anti-platelet drug, CYP2C19 or platelet function is often directly assessed to ensure that the patient is able to convert clopidogrel to its active metabolite to achieve the needed anti-platelet effect.