Impact of Drug Transporters ABCB1 and ABCG2 and Regulators of Xenobiotic Transport and Metabolism Pxr and CAR Gene Polymorphisms on Clinical Efficacy of Imatinib in Chronic Myeloid Leukemia (CML)

Background: Inherited background in mechanisms regulating cellular uptake, elimination and metabolism of imatinib and other BCR-ABL tyrosine kinase inhibitors may influence treatment outcome in chronic myeloid leukemia (CML). Despite good efficacy of imatinib in CML therapy, a proportion of patients show suboptimal responses or even primary resistance. Imatinib is a well-established substrate of ABC-superfamily xenobiotic transporters ABCB1 (MDR1, Pg-P) and ABCG2 (BCRP). However, the results of published studies regarding impact of ABCB1 and ABCG2 single nucleotide polymorphisms (SNPs) on imatinib treatment outcome have shown discrepant results. Moreover, little is known on the potential role of polymorphisms of regulators of xenobiotic transport and metabolism, pregnane X receptor (PXR, NR1|2) and constitutive androstane receptor (CAR, NR1|3) in imatinib efficacy.

Aim: In this study we verified whether 61 tagging SNPs in drug transporters genes (ABCB1 and ABCG2) and regulators of xenobiotic transport and metabolism gene (PXR and CAR) genes affect imatinib therapy outcomes in CML.

Methods: Genotyping was performed in 256 patients of Polish Caucasian origin (141 men and 117 women) treated with imatinib as first line treatment (229; 89.5%) or after second-line after interferon therapy (27; 10.5%). Tagging SNP approach was used to select the genetic variants and resulted in a selection of 61 tagging SNPs including 26 SNPs for ABCB1, 17 SNPs for ABCG2, 11 SNPs for PXR and 7 SNPs for CAR gene. Complete clinical data on imatinib treatment outcome were assessable for analyses of associations with tested SNPs. The influence of SNPs on imatinib outcomes including achievement of complete cytogenetic response (CCyR) at 12 months, achievement of major molecular response (MMR) at 18 months, progression-free survival (PFS), time to treatment failure (TTF) and overall survival (OS) were tested in regard to established clinical and laboratory predictive factors using logistic regression and proportional-hazards regression models. Three inheritance models (co-dominant, dominant and recessive) were tested for all allelic variants.

Results: In this study we found that carriers of minor allele of CAR rs4073054A>C SNP significantly associated with shorter TTF on imatinib therapy (OR=2.42, 95%CI 1.41 - 4.17; p=0.0014; Bonferroni corrected p=0.048). In contrast, no impact of other tested allelic variants in ABCB1, ABCG2, PXR and CAR genes on proportion of patients who achieved CCyR at 12 months, MMR at 18 months as well as on probability of PFS, TTF and OS was observed.

Conclusion: Our data suggest that inherited variants in the regulator of xenobiotic transport and metabolism CAR gene may modify results of imatinib therapy in CML patients. In contrast, the results of this relatively large study indicate that polymorphic variants in ABCB1, ABCG2 and PXR genes do not seem to play major role as determinants of response to imatinib in CML.