M. Li,1 E. L. Seiser,2 R. M. Baldwin,1 J. Ramirez,3 M. J. Ratain,3 F. Innocenti,2 D. L. Kroetz1; 1University of California, San Francisco, San Francisco, CA, 2University of North Carolina at Chapel Hill, Chapel Hill, NC, 3The University of Chicago, Chicago, IL

BACKGROUND: Neutropenia (NP) is a common toxicity associated with irinotecan (IRI) treatment. Up to 15% of NP risk outside of UGT1A1 genotype status remains unexplained. The objective of this study was to identify ABC transporter SNPs contributing to variability in IRI pharmacokinetics and NP.
METHODS: Seventy eight advanced cancer patients treated with IRI were genotyped for tag SNPs in ABCB1, ABCC1, and ABCG2 using the Illumina BeadXpress platform. A two-stage regression analysis identified SNPs significantly associated with neutrophil count nadir, IRI AUC, SN-38 AUC, and SN-38G/SN-38 AUC ratio. Genomic regions containing each of these SNPs were cloned into a luciferase reporter vector and tested for SNP effects on enhancer activity. Bioinformatic analyses were used to examine these regions for evidence of enhancer activity, differential expression, and altered transcription factor binding sites (TFBS).
RESULTS: The regression analysis identified five non-coding SNPs significantly associated (p<0.05) with changes in IRI-induced NP or drug/metabolite plasma levels. Eight additional SNPs in strong LD were selected for further examination. Three SNPs (rs12720066, rs17501331, rs5860117) reside in genomic regions with enhancer activity in vitro (p<0.05) but the SNPs do not affect this activity. Three SNPs (rs4148330, rs12505410, rs5860117) are located in regions with enhancer histone marks in HepG2 cells, and five SNPs (rs3784867, rs6498588, rs2133774, rs35237723, rs4148330) are predicted to alter TFBS. rs6498588 is associated with increased SN-38 AUC (p=0.0006) and increased ABCC1 expression in human liver (p=0.05).
CONCLUSION: Several SNPs may contribute to IRI-induced NP by affecting expression of ABC transporters that efflux IRI or its metabolites from the hepatocyte.