ATLANTA—In patients with newly diagnosed chronic phase chronic myeloid leukemia (CP-CML) treated with bosutinib, a Bcr-Abl/Abl ratio of ≤10% (vs >10%) at 3, 6, and 9 months was associated with higher rates of major molecular response (MMR) and complete cytogenetic response (CCyR) by later time points, according to data presented at the 54th American Society of Hematology Annual Meeting and Exposition.

“Overall, this study suggests that patients with early reduction in Bcr-Abl/Abl ratio during bosutinib or imatinib therapy have a higher likelihood of experiencing better long-term outcomes,” said Tim H. Brümmendorf, of Universitätsklinikum Aachen, Germany. “However, because of the limited number of outcome events and the relatively few patients with a Bcr-Abl/Abl ratio >10%, longer follow-up in the BELA trial is needed to fully characterize the predictive properties of early reduction in Bcr-Abl/Abl ratio on long-term out outcomes.”

In the phase 3 randomized trial, bosutinib 500mg/day demonstrated good clinical activity and manageable toxicity distinct from that of imatinib 400mg/day, with bosutinib associated with deeper cytogenetic and molecular responses.

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This analysis found MMR, defined as Bcr-Abl/Abl ratio ≤0.1% on the International Scale, to be higher at all time points for bosutinib, an oral, dual Src/Abl kinase inhibitor, versus imatinib, “with a significantly shorter median time to MMR for bosutinib (48 weeks) vs imatinib (73 weeks; P<0.001).”

Among patients with molecular assessment at each time point, the rate of Bcr-Abl/Abl ratio ≤10% on the International Scale was significantly higher with bosutinib vs imatinib: 86% vs 65% at month 3 (P<0.001); 94% vs 82% at Month 6 (P<0.001), and 97% vs 90% at Month 9 (P=0.01). A similar trend was observed for those with a Bcr-Abl/Abl ratio ≤1% at each time point.

In both arms, a Bcr-Abl/Abl ratio ≤10% vs >10% at Months 3, 6, and 9 was predictive of significantly higher cumulative rates of MMR and CCyR by both 12 and 24 months.

Rates of MMR by 12 and 24 months, “were generally highest among patients with a Bcr-Abl/Abl ratio ≤1% at Month 3, 6, and 9 in both arms,” Dr. Brümmendorf reported.

A reduction in Bcr-Abl/Abl ratio was also generally predictive of event-free survival after 24 months of follow-up except for imatinib at Month 3 and bosutinib at Month 6.