Abstract
The Dutch Pharmacogenetics Working Group (DPWG) describes gene-drug interactions to facilitate pharmacogenetic implementation in clinical practice. The current guideline describes the gene-drug interactions for TPMT, NUDT15 and thiopurines (azathioprine, 6-mercaptopurine and thioguanine). Evidence based recommendations were obtained via a literature review of published studies. This literature review showed that gene variants leading to a decreased activity of TPMT and/or NUDT15 result in higher risk of toxicities, especially bone-marrow depression. For intermediate metabolisers (IM) of TPMT or NUDT15, it is advised to start with 50% of the normal dose when treated with azathioprine or 6-mercaptopurine. For poor metabolisers (PM), it is advised to start with an alternative treatment or to reduce the dose to 10% of the normal dose. For thioguanine, a dose of 75% of the normal dose is advised for TPMT IM and 50% of the normal dose for NUDT15 IM. Alternative treatment or a start dose reduction is advised for PM. A start dose of 6-7% can be used for TPMT PM and of 10% for NUDT15 PM. For TPMT or NUDT15 IM treated for leukaemia, starting with the normal dose can be considered and then decrease the dose to the advised dose described above in case toxicities occur. For NUDT15 PM reduced starting dose is advised only if an alternative is not possible, due to a higher uncertainty in the calculated dose reduction for NUDT15 PM than for TPMT PM.DPWG classifies genotyping for TPMT and NUDT15 "essential" before thiopurine initiation.