Polymorphisms in the multidrug resistance gene MDR1 (ABCB1) predict for molecular resistance in patients with newly diagnosed chronic myeloid leukemia receiving high-dose imatinib

To the editor:

Recently, Dulucq and coworkers reported that multidrug resistance gene (MDR1) polymorphisms were associated with major molecular responses (MMRs) to standard-dose imatinib in chronic myeloid leukemia (CML).¹  Significantly more patients homozygous for allele 1236T achieved a MMR. Patients with this genotype had also higher imatinib concentrations. These findings suggested a better response in patients with the TT genotype.

We studied the same most prevalent ABCB1 gene single nucleotide polymorphisms (SNPs), C1236T, G2677T/A, and C3435T, with respect to molecular response in a cohort of 46 early chronic phase CML patients receiving high-dose imatinib (800 mg) according to the Hemato-Oncologie voor Volwassenen Nederland (HOVON)–51 protocol.^2,3  No significant differences in baseline characteristics including age, Sokal risk score, and dose of cytarabine were apparent among the different MDR1 genotypes. Genotyping was performed using Taqman allelic discrimination assays on an ABI Prism 7000 Sequence detection system (Applied Biosystems), with 2 allele-specific minor groove binding probes for each SNP. The distribution of the allelic variants of each of the 3 SNPs were in Hardy-Weinberg equilibrium. However, each combination of 2 SNPs was in strong linkage disequilibrium (P < .001). The overall incidences of a MMR and complete molecular response (CMR) were, respectively, 78% and 41% at 2 years from diagnosis, after a median follow-up of 46 months (range, 32-60 months). Among the patients homozygous for 1236C, a cumulative incidence of MMR of 92% after 1 year was observed, compared with 52% and 50% in patients with genotype CT or TT, respectively (P = .02; Table 1), which effect remained statistically significant in multivariate analysis. Hazard ratios to achieve a CMR for patients harboring the CT and TT alleles were, in multivariate analysis, respectively, 0.25 (0.10-0.63) and 0.27 (0.08-0.97), P = .01, indicating a 4-fold reduction of the probability to obtain a CMR. Also patients homozygous for 3435T and 2677T showed lower probabilities to obtain a MMR and CMR (Table 1). Summarizing, molecular response in CML patients receiving high-dose imatinib strongly depended on SNP-genotype, with the TT-genotype associated with worse response. These findings may be explained by enhanced clearance of imatinib by the 2677TT genotype, which leads to an amino acid substitution and thereby increased transport activity.⁴  Enhanced clearance significantly associated with the TT-genotype and less dose reduction was reported earlier by Gurney et al.⁵  In addition, recent clinical findings by Ni et al are also in agreement with these results, who studied cytogenetic resistance in patients treated with 400 mg imatinib.⁶  These observations are in contrast with the study by Dulucq et al.¹  Strikingly, responses in the French study were strongly related to imatinib plasma levels.¹  Therefore, pharmacokinetic resistance due to a variety of mechanisms affecting plasma levels rather than tumor cell resistance due to P-glycoprotein activity may be suggested as an alternative explanation for these results.

In conclusion, molecular resistance in CML patients receiving high-dose imatinib appeared strongly associated with the TT-genotype of ABCB1, suggesting a role for P-glycoprotein–mediated drug efflux in residual malignant hematopoietic progenitor cells that may possibly account for persistent molecular residual disease.