Polymorphisms in Exons 2 and 7 of the Human Organic Cation Transporter (hOCT1) (Solute Carrier Family 22, SLC22A1): Correlation with Imatinib Levels and Clinical Course in Chronic Myeloid Leukemia Patients,

Drug metabolism and disposition genes have a substantial impact on the pharmacology of many medications. For CML, the human organic cation transporter (hOCT1) actively transports imatinib mesylate (IM) into cells, thus is a crucial factor in the response to IM therapy, without a known effect on plasma levels. A number of single nucleotide polymorphisms (SNPs) are believed to affect the activity of hOCT1 and therefore may influence IM accumulation in cells. We therefore studied hOCT1 SNPs and correlated with IM plasma levels and clinical response to IM. We studied a SNP in exon 2 of hOCT1 (480C>G, Phe160Leu, rs683369), which, in the homozygous state (GG), has been associated with treatment failure on IM (DH Kim, Clin Cancer Res., 15:4750, 2009), and a SNP in exon 7 (1222A>G, Met408Val) which has been hypothesized as having increased activity (A Giannoudis, Haematologica 96(s2):87, 2011). Both of these are prevalent SNPs; the exon 2 SNP is found in 22% of Caucasians and the exon 7 SNP, in 60% of Caucasians (R Kerb, Cancer Letters 234:4, 2006).

We studied 48 patients with CML in chronic phase (M:F 21:27 aged 17–89 years. Time since CML diagnosis was between 10 months and 18 years. IM therapy was between 10–128 months' duration. Most of the patients were on a dose of 400 mg/once daily, with the exception of 11 patients who were on doses >400 mg/once daily (mean 609.1 mg/day +/− 70.1 mg), and 9 patients, whose dose was <400 mg/once daily (mean 281 mg/day +/− 35.4 mg). Trough IM levels were performed by Novartis Pharmaceuticals on samples taken 24 hours after last dose, reported in ng/ml. Compliance was verified by a patient diary during the trough drug testing period, but not throughout the entire period of IM therapy. Genotyping for hOCT1 SNPs was performed on 36 patients. The exon 2 SNP was detected by PCR amplification and Dde I restriction enzyme analysis. The exon 7 was detected by PCR and restriction with BmgB1. Detailed molecular data was available on 36 patients, and partial molecular data on 8 others. Complete molecular remission (CMR) was defined as PCR negativity using RT-PCR until 2004, and thereafter, using a highly sensitive real time PCR assay.

The mean IM trough level for all 48 patients was 1,153.0 (+/−493.1). There was much variation between IM levels in individual patients and levels also varied by dose. Patients who were on 400 mg/day had a mean IM level of 1064.1 (+/− 399.2). Patients on higher IM doses (>400 mg) had IM levels averaging 1397.2 (+/− 493.1). Patients on doses <400 mg/day had mean IM levels of 1057.3 (+/− 667.0), which was nearly identical to that of patients on 400 mg/day. Genotyping for the hOCT1 exon 2 SNP demonstrated that 66.7% of the 36 patients analyzed were homozygous for the wild type allele, 13.9% were heterozygous for the polymorphism and 19.4% were homozygous polymorphic. GG homozygotes had higher IM levels than CG/CC genotypes (1562.75 +/− 560 compared to 1310.6 +/− 470). This is because the patients were on doses greater than 400 mg/day (mean 483 mg/day compared to 405 mg/day). These patients took an average of 8.8 months to achieve a molecular CR and one transiently lost molecular CMR. Genotyping for the hOCT1 exon 7 SNP demonstrated that 47% of the 36 patients analyzed were heterozygous AG for the polymorphism and 53% of the patients were GG homozygotes. IM levels for the GG homozygotes were higher than for AG heterozygotes (1413 +/− 549.6 compared to 1254.0 +/− 393.9 ng/ml), despite the fact that they were on a slightly lower average dose (mean: 390.1 mg/day compared to 436.7 mg/day). For 13 of these GG homozygous patients, detailed molecular followup was available. Two of these patients never achieved CMR and 2 transiently lost CMR. The time to achieve complete molecular remission was generally long, 17.6 months (range 3–83 months). Importantly, 12 out of 13 of these patients with unsatisfactory clinical responses to IM were CC or CG at the exon 2 SNP, which are considered favorable genotypes for IM response (DH Kim, Clin Cancer Res., 15:4750, 2009).

These data may suggest that the GG genotype for exon 7 in hOCT1 (rs628031), independent of the exon 2 (rs683369) genotype, is an adverse prognostic parameter despite adequate IM levels. These patients may be candidates for an alternate tyrosine kinase inhibitor. Further studies are necessary on larger groups of patients to confirm these results, which are important in view of the high frequency of the polymorphic rs628031 allele.

Cohen:Novartis Pharmaceutical Company: Research Funding. Rund:Novartis Pharmaceutical Company: Honoraria, Research Funding.