ABCB1 Haplotypes Are Associated With Lower P-Gp Activity and Major Molecular Response But Do Not Affect Imatinib Plasma Levels In Chronic Myeloid Leukemia Patients

Despite the high efficacy of IM treatment in chronic myeloid leukemia (CML), some patients fail to achieve optimal response. Several studies demonstrated that IM is a substrate of membrane transporters, such as ABCB1 (P-gp, MDR1) and variations in protein expression or activity could affect the pharmacokinetics of IM by reducing or increasing its bioavailability. These alterations could be related with single nucleotide polymorphisms in ABCB1 gene. Previous data confirms that haplotypes containing the mutated alleles for ABCB1 c.1236C>T, c.3435C>T and c.2677G>T showed major structural modifications that result in changes in the conformation of the binding sites of P-gp. These modifications could affect the pharmacokinetics of IM.

The aim of this study was to evaluate the influence of the different haplotypes for ABCB1 c.1236C>T, c.3435C>T and c.2677TG>T polymorphisms in IM plasma concentration, P-gp activity and IM response from CML patients treated with standard dose of IM (400 mg/day).

Twenty eight patients in chronic phase of CML were selected according to the haplotypes for ABCB1 c.1236C>T, c.3435C>T and c.2677G>T polymorphisms at two health centers in São Paulo, Brazil. Ten patients with ABCB1 1236CC/3435CC/2677GG haplotype comprised the wild-type group and 18 carriers of haplotypes with at least one mutated allele in each genotype for three ABCB1 polymorphisms (10 patients with 1236CT/3435CT/2677GT and 8 with 1236TT/3435TT/2677TT) comprised the mutated group. Patients were matched for IM time of use. All patients were in chronic phase of CML, treated with a standard dose of IM (400 mg/day) for a median time of 63.5±12.6 months and with complete cytogenetic response (CCyR). Major molecular response (MMR) was defined as a reduction of BCR-ABL1 transcripts levels to ≤ 0.1% in the peripheral blood standardized on the International scale. Complete molecular response (CMR) was defined as a reduction ≤0.0032% of BCR-ABL1 transcripts levels. Real-Time PCR was performed to evaluate ABCB1 mRNA expression to control gene GAPDH. P-gp functional activity was determinated by rhodamine123 efflux assay. Analysis of P-gp expression and functional activity were performed by flow cytometry. The determination of plasma concentration of IM was performed by capillary electrophoresis.

Patients without MMR had lower plasma concentration of IM when compared to those that achieved this response (0.51 µg/mL vs. 1.42 µg/mL, P=0.001) but no association was found between the different haplotypes and IM plasma levels or ABCB1 mRNA/P-gp expression. The median of Rh123 efflux in wild-type and mutated groups was 59.1 (54.8 - 69.5) and 38.3 (27.4 - 47.9) (P<0.05), respectively. Patients who did not achieve MMR showed a higher rate of efflux mediated by P-gp compared to individuals who did not achieve this response (64.7% vs. 45.7%, P =0.001). All patients who did not achieve MMR showed efflux above 60%.There was a strong and positive correlation between ABCB1 mRNA expression and P-gp expression (r=0.747, P=0.001). P-gp activity was positive and moderate correlated with BCR-ABL1 transcripts (r=0.570; P=0.001).

ABCB1 1236CT/3435CT/2677GT and 1236TT/3435TT/2677TT haplotypes are associated with lower P-gp activity and higher frequency of MMR but not with IM plasma concentration in chronic phase CML patients treated with standard-dose of IM (400 mg/day)

No relevant conflicts of interest to declare.