MDR1 C3435T Polymorphism Is Highly Correlated with Early Response to Therapy and Outcome in Childhood Acute Lymphoblastic Leukaemia: Malaysia-Singapore 2003 ALL Study.

Early response to therapy as determined by Day 8 prednisolone response (PR) and minimal residual disease (MRD) are highly predictive of risk of relapse in childhood ALL. They are currently employed in risk-stratification in modern chemotherapeutic regimens. Leukemia biological factors like hyperdiploidy, oncogene fusions like TEL-AML1, MLL rearrangements and BCR-ABL are widely used for risk stratification but patient’s pharmacogenetic profile has thus far not been utilised for tailoring therapy because of conflicting or equivocal results. We investigated potential candidate genes involved in drug transport and metabolism of steroids, vincristine, L-asparaginase and anthracyclines used in the BFM induction backbone to exploit the plausible pathways of early chemoresistance. We studied 282 children with ALL enrolled in the Malaysia-Singapore 2003 ALL trial. Poor prednisolone response (PPR) is defined as absolute blast count ≥1,000/uL after 7 days of prednisolone and 1 IT MTX. MRD quantification at day 33 induction using IgH and TCR markers based on European MRD Study Group standards. Four potential candidate genes MDR1 3435C>T, GSTT1, GSTM1, NQO1 609C>T were studied using Polymerase Chain Reaction and Restriction Fragment Length Polymorphism. Statistical analysis was carried out using multinomial regression model. We had 241 good prednisolone responders (GPR), and 34 poor prednisolone responders (PPR). There were 112 patients with negative MRD (<10⁻⁴) while 129 had positive MRD ≥10⁻⁴ (35 had high MRD > 5 × 10⁻³). Only MDR1 3435TT genotype was strongly associated with poor PR (p=0.011; OR=2.8; 95%CI=1.3–6.4), MRD positive disease ≥10⁻⁴ (p=0.011, OR=2.8, 95%CI= 1.3–5.9) and high levels of MRD >5×10⁻³ as compared to MRD <10⁻⁴ (p<0.0001; OR=5.9; 95%CI=2.3–15.1). Interestingly, MDR1 3435CT heterozygosity is significantly associated good early response: with low levels of <MRD10⁻⁴ as compared to high MRD levels >5×10⁻³ (p=0.022; OR=0.4; 95%CI=0.2–0.9). Oral prednisolone and dexamethasone are the most powerful drugs in the treatment of childhood ALL. MDR1 3435C>T although a synonymous SNP, is associated with lower P-glycoprotein because of reduced MDR1 3435T mRNA stability. MDR1 3435 TT homozygote has 4 fold reduced levels of P-glycoprotein in the duodenum and is associated with reduced drug absorption like digoxin. We postulate that TT homozygote; there is reduced absorption of oral steroids in the gut, resulting in low oral bioavailability and poor early response as determined by poor prednisolone response and high MRD. On the other hand, in the CT heterozygote, drug oral bioavailability is not the drug-limiting step. The predominant effect of reduced P-glycoprotein expression in the cells are associated with reduced efflux of drugs, hence higher levels of intracellular drug resulting in good treatment response. MDR1 C3435T polymorphism is highly correlated with early response to therapy in childhood ALL as determined by prednisolone response and MRD levels at the end of induction.