Antimetabolite dose intensity and adverse outcomes in children with acute lymphoblastic leukemia: a COG-AALL03N1 report.

The association between antimetabolite dose intensity (DI) and adverse events among children receiving maintenance therapy for acute lymphoblastic leukemia (ALL) remains unclear, especially in the context of antimetabolite adherence. Using Children's Oncology Group AALL03N1 data, we examined the association between high DI during the first 4 study months and (i) treatment-related toxicities during the subsequent 2 study months; and (ii) relapse risk. Patients were classified into a high DI phenotype (either 6-mercaptopurine [6-MP] or methotrexate [MTX] DI ≥110% during the first 4 study months, or 6-MPDI or MTXDI 100%-110% at study enrollment and ≥25% increase over the 4 study months) and normal DI phenotype (all others). Only patients with wild-type TPMT and NUDT15 were included. 6-MP adherence data were available for 63.7% of study participants and used to stratify as adherent (median adherence ≥85%) and nonadherent (median adherence <85%) participants. Multivariable analyses were adjusted for sociodemographic and clinical prognosticators. Of the 644 patients, 29.3% were exposed to high DI. High DI was associated with a 2.1-fold greater odds of hematologic toxicity (95% confidence interval [CI] = 1.4-3.1; reference: normal DI) in the entire cohort and 2.9-fold higher among adherers (95% CI = 1.6-5.1); odds were comparable among nonadherers (2.1-fold; 95% CI = 0.4-10.1). Although high DI was not associated with relapse in the entire cohort (adjusted hazard ratio [aHR] = 1.4; 95% CI = 0.8-2.4), it was associated with a greater hazard of relapse among adherent participants (aHR = 2.4; 95% CI = 1.0-5.5) but not among nonadherent participants (aHR = 0.9; 95% CI = 0.2-3.8). Dose escalation above protocol doses during maintenance therapy for ALL should be done cautiously after assessing adherence to prescribed therapy.