01). Although these data are preliminary and require independent confirmation, it is possible that these polymorphisms could increase RAC1 expression enough in vivo to decrease efficacy of thiopurine therapy when administered at a standard dose. The authors reported a non-significant trend toward higher frequencies of the −289C and VNTR-3 alleles in IBD patients who did not develop leucopenia on azathioprine (P = 0.079, OR = 0.18, 95% CI 0.02–1.49).37

This observation arguably supports the hypothesis that these promoter polymorphisms do increase RAC1 expression in vivo and may influence the efficiency and toxicity of thiopurine therapy. Does an ABCC4 polymorphism account for enhanced thiopurine sensitivity?  Multi-drug resistance protein 4 (MRP4) is an ATP-dependent efflux pump that is able to transport 6-TGNs out of cells.38 Overexpression of this pump and the concurrent downregulation of influx transporters (plasma membrane Selleckchem GS1101 nucleoside transporters, NTs) have

been shown to confer resistance of human leukemic cell lines to thiopurine drugs.39 Analysis of the accumulation and efflux of radio-labeled 6-mercatopurine, revealed that the leukaemic cells that overexpressed MRP4 effluxed 72.3% of 6-mercaptopurine as 6TGNs into the culture medium within 1 h compared with 23.7% of 6-TGNs by the control cell line.39 Conversely, murine models have demonstrated that a deficiency in MRP4 expression results in accumulation of 6-TGNs HCS assay to toxic concentrations in myeloid progenitor cells. Krishnamurthy et al.40 tested the 6-mercaptopurine sensitivity of Mrp4+/+ and Mrp4−/− mice by administering intraperitoneal injections of this thiopurine to the mice each day for 15 days. By day 13 all Mrp4−/− medchemexpress mice were dead, whereas > 75% of wild type mice were alive at day 15. Bone marrow cell 6-TGN concentrations in Mrp4−/− mice

were 10 times higher than the concentrations found in Mrp4+/+ mice. Moreover myeloid progenitor cells after 5 days of treatment were reduced by 74% in Mrp4−/− mice but only by < 20% in Mrp4+/+ mice.40 The gene coding for human MRP4 (ABCC4) is highly polymorphic.41 At least one variant has been identified that significantly impairs the functioning of this pump and may explain why some IBD patients who have normal TPMT activity, still develop 6-TGN-induced myelotoxicity. The nonsynomous ABCC4 SNP 2269G>A (rs3765534, E857K) codes a variant MRP4 protein, which is unable to effectively localize to the plasma membrane.40 In HEK293 cells the 5-fold reduction in cell surface expression resulted in enhanced 6-mercaptopurine cytotoxicity, with an EC50 of 9.7 µmol/L versus 17.3 µmol/L in cells expressing the wild type (2269G) allele (P < 0.05).40 The frequency of the minor allele (2269A) is 15–22% in Japanese and 8.3% in Han Chinese, but less than 1% in Caucasians and Africans. Ban et al.