Influence of Drug Absorption, Distribution, Metabolism, and Excretion (ADME) Variants On Sirolimus Blood Levels in Patients Following Allogeneic Hematopoietic Stem Cell Transplantation

Allelic variants implicated in drug absorption, distribution, metabolism, and excretion (ADME) affect drug pharmacokinetic variability and have been increasingly recognized as important factors in medical therapy. Our recently published study found an association of thrombotic microangiopathy (TMA) with high sirolimus serum levels in a cohort of 177 patients who received sirolimus and tacrolimus as GVHD prophylaxis following allogeneic transplantation. By multivariable analysis, increased risk of TMA was associated with day 14 serum sirolimus levels, prior aGVHD, and myeloablative conditioning. In the current study we explore the possible influence of ADME variants on sirolimus levels and development of TMA in the same patient cohort.

We obtained archived DNA samples from the patients on our TMA study and analyzed them using the iPLEX ADME PGx panel and the MassARRAY® Compact Analyzer. This panel is based on the PharmADME Working Group list and covers >99% of the Pharma ADME Core list; it interrogates 188 mutations and 12 copy number variation assays (in 36 pharmacogenetically relevant genes). Sirolimus levels were measured at least weekly until day 100 with dose adjustments made for target levels and/or clinical toxicity. Possible associations between early sirolimus serum levels (day +14) and assays were evaluated by the Kruskal-Wallis (non-parametric) test and the false discovery rate was used to control for multiple comparisons.

Using this Panel, 179 samples were genotyped, of which 173 showed high quality data. The average call rate for these samples was 98.85% over 200 assays, with a median call rate of 100%. Of these assays, 66 variants were identified that may be of relevance to sirolimus metabolism; other assays were excluded due to homzygosity or >10 % missing data. Using this panel, we found 3 assays showing an association with sirolimus drug level, rs1057910 (CYP2C9*3) at p=.04, rs1799931 (NAT2*7) at p=.03, and rs2032582 (ABCB1 2677G>A/T) at p=.007. In addition, the 0-copy haplotype of UGT2B17 also showed higher levels of sirolimus (p=.02). These assays were also tested for an association with TMA, showing a trend for increased TMA in the rs1799931 (NAT2*7) (p=0.08). However, after adjustment for multiple testing, only rs2032582 maintained statistical significance due to the small sample size.

In conclusion, this pilot study of the iPLEX ADME PGx panel showed feasiblity and provided high quality data. Despite the limitation of small sample size, several genetic variants were implicated in sirolimus levels and may warrant further investigation. Future analysis should focus on specific gene clusters or pathways and will require a large cohort to power validation and training sets.