Gender Is A Major Determinant of Cytidine Analogue Metabolism and May Contribute to Differences in Treatment Outcomes

Abstract 1434

Cytidine analogues are mainstays in hematologic malignancy therapy. One documented limitation of therapy is interindividual variability in cytidine analogue pharmacokinetics that compromises predictability, safety and efficacy. Causes of interindividual variability include single nucleotide polymorphisms (SNPs) in cytidine metabolizing enzymes, particularly cytidine deaminase (CDA): the CDA SNP A79C (rs2072671) is present in >70% of Caucasians and decreases CDA enzyme activity. Gender is another known genetic determinant of cytidine analogue clinical activity, although the pathway is unknown (Dimopoulou et al, Ann Oncol 2004). With high dose cytarabine therapy, CDA may be saturated, dampening the clinical impact of CDA SNPs. However, the recently approved cytidine analogues 5-azacytidine and decitabine are used as single agents at relatively low doses, and interindividual variability could have greater clinical significance, since drug levels may be close to minimum thresholds required for efficacy. To evaluate this possibility, the impact of A79C and gender on treatment outcomes was examined in a cohort of patients with myelodysplastic syndrome (MDS, n=127) and acute myeloid leukemia (AML, n=74) classified by WHO criteria, initiated on treatment between 1/2002 and 12/2007, with IRB approved tissue-banked bone marrow samples available for analysis for the A79C SNP by sequencing and SNP array, and with verifiable follow-up and survival annotation. Other variables analyzed were those known to have major prognostic importance in patients with MDS and AML (bone marrow myeloblast percentage, karyotype, age). Patients were analyzed in three treatment groups: (i) did not receive cytidine analogues (n=35), (ii) treated with cytarabine (n=76), (iii) treated with 5-azacytidine or decitabine (n=90). In all three treatment groups, heterozygosity or homozygosity for A79C was not associated with significant differences in overall survival (OS). Similarly, there was no significant difference in overall survival in females versus males who were treated with cytarabine or who did not receive cytidine analogues. However, in the 5-azacytidine/decitabine treatment group, females had significantly longer overall survival than males (median 1033 v 563 days, Log Rank p=0.014). In multivariate analysis incorporating age, karyotype, bone marrow blast percentage and A79C, only age, gender and karyotype were significantly (Cox Proportional Hazards p<0.01) associated with survival, with the hazard ratio for female gender 0.356 (95%CI 0.165–0.766). An important caveat is that MDS in females has a less aggressive natural history (Nosslinger et al, Ann Oncol 2010), therefore, it is possible that gender differences in cytidine analogue metabolism do not underlie the difference in overall survival. Nonetheless, in AML myeloblasts (GSE15434) and normal liver tissue (measured by microarray), CDA mRNA expression in females was significantly lower than in males (Wilcoxon test p=0.005 and p=0.025 respectively). To measure functional CDA activity, uridine conversion from cytidine by plasma was measured by uHPLC (Dionex). As expected, there was a significant 2-fold decrease in mean enzyme activity in individuals homozygous for the A79C variant (CC, n=32) compared to individuals homozygous for the ancestral allele (AA, n=32, t-test p=0.02). However, the decrease in mean enzyme activity in females (n=48) compared to males (n=48) was even greater (3-fold, t-test p<0.001). In an analysis of decitabine pharmacokinetics after administration of oral decitabine 1 mg/kg to female (n=18) and male (n=18) adult mice, plasma levels of decitabine measured by LCMSMS were consistently higher in females at all time-points (1.169-4.375 fold at the different time points, paired t-test p=0.001). In conclusion, decreased cytidine analogue conversion to uridine counterparts in females, most likely from differences in CDA expression, could be relevant to gender differences in treatment outcomes with 5-azacytidine or decitabine.