Pfizer's (PFE) Bosutinib Misses Endpoint in Phase 3 Versus Imatinib

December 6, 2010 8:07 AM EST
Pfizer Inc. (NYSE: PFE) announced the following data from a Phase 3 of bosutinib in patients with chronic myeloid leukemia. The study did not meet its primary endpoint of superior complete cytogenetic response (CCyR) rate at one year versus imatinib (70 percent vs. 68 percent, respectively, [p=0.601]), in the intent-to-treat population.

From the release:

"Preliminary data show that fewer patients who took bosutinib progressed to an advanced phase of the disease (n=4, 1.6 percent) compared to patients treated with imatinib (n= 10, 4.0 percent), and there were fewer deaths in the bosutinib arm (n=4, 1.2 percent) than in the imatinib arm (n= 10, 3.2 percent). Patients responding to bosutinib achieved CCyR faster than those responding to imatinib (13 weeks vs. 25 weeks, p<0.001).

A pre-specified exploratory analysis also showed that bosutinib produced a higher rate of CCyR at one year compared to imatinib when CCyR was assessed only in the evaluable patient population, 78 percent with bosutinib (n=219) compared to 69 percent with imatinib (n=241). The evaluable population was different from the ITT population in that it included only those patients who received follow-up assessments for efficacy.

The most frequently reported all-grade drug-related adverse events with bosutinib were diarrhea (66 percent), nausea (27 percent), vomiting (25 percent), and rash (18 percent). The most frequent grade 3/4 adverse events with bosutinib included diarrhea (8 percent) and rash (2 percent), although no patients on the bosutinib arm discontinued therapy due to diarrhea. Gastrointestinal events associated with bosutinib had an early onset and usually subsided within the first four weeks of treatment. Most frequent grade 3/4 laboratory abnormalities with bosutinib included elevated ALT (21 percent), elevated AST (10 percent), and thrombocytopenia (7 percent).

More patients on bosutinib experienced serious adverse events (25.4 percent vs. 13.5 percent) and adverse events leading to discontinuation (19.4 percent vs. 5.6 percent) than on imatinib. Adverse events leading to discontinuation were most frequently due to liver enzyme elevations in the bosutinib arm and neutropenia in the imatinib arm. There were no deaths in the study due to treatment-related adverse events. The majority of patients on both treatment arms continued on study treatment after a median follow-up of 14 months."

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