(ChemotherapyAdvisor) – The first study to demonstrate conclusively that adherence to oral mercaptopurine influences risk of relapse in children with acute lymphoblastic leukemia (ALL) also found an ethnic difference in relapse risk that differed by level of adherence, according to a study published in the Journal of Clinical Oncology online May 14.

“Hispanics are projected to account for 29% of the U.S. population by 2050,” the investigators wrote. “According to the Institute of Medicine, Hispanics are vulnerable to adverse outcomes because of socioeconomic and cultural issues that create barriers to healthcare access and influence the understanding of prescribed treatment. This study describes for the first time to our knowledge differences in adherence to oral mercaptopurine by ethnicity and the contribution of adherence to ethnic differences in ALL outcome.”

Participants included 327 with ALL, 169 of whom were Hispanic and 158, non-Hispanic white. The data and time of oral mercaptopurine bottle openings were recorded using medication event-monitoring system caps. Adherence was defined as the ratio of days of oral mercaptopurine bottle openings to prescribed days and calculated monthly.

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“After 53,394 person-days of monitoring, adherence declined from 94.7% (month 1) to 90.2% (month 6; P<0.001). Mean adherence over 6 months was significantly lower among Hispanics (88.4% vs. 94.8%; P<0.001), patients age ≥12 years (85.8% vs. 93.1%; P<0.001), and patients from single-mother households (80.6% vs. 93.1%; P=0.001),” the investigators wrote.

A progressive increase in relapse was observed with decreasing adherence. Cumulative incidence of relapse was found to be higher among Hispanics (16.5% vs. 6.3%; P=0.02). Association between Hispanic ethnicity and relapse (HR, 2.6; P=0.02) became nonsignificant (HR, 1.8; P=0.26) after adjusting for adherence and socioeconomic status. At adherence rates ≥90%, Hispanics continued to demonstrate higher relapse, whereas at rates <90%, relapse risk was comparable to that of non-Hispanic whites.

The investigators are currently examining why ethnic difference in relapse risk may differ by level of adherence.