Assessment Of Early Cytogenetic Response By 3 Months Versus >3-6 Months Or No Response By 6 Months As a Predictor Of Long-Term Clinical Outcomes In a Phase 1/2 Study Of Bosutinib In Chronic Phase CML

Bosutinib (BOS), an oral dual Src/Abl tyrosine kinase inhibitor (TKI), showed clinical activity and manageable toxicity in an open-label, phase 1/2 trial in patients (pts) with chronic phase (CP) chronic myeloid leukemia (CML) following resistance (R)/intolerance (I) to imatinib (IM) only (2^nd line; CP2L) or to IM plus dasatinib (D) and/or nilotinib (N) (3^rd/4^th line; CP3L). In this retrospective analysis, a major cytogenetic response (MCyR) by 3 or by 6 mo (but not by 3 mo) was assessed as a predictor of long-term outcomes in CP2L or CP3L pts receiving BOS.

CP-CML pts aged ≥18 y received BOS starting at 500 mg/d. MCyR and complete cytogenetic response (CCyR) rates and maintenance of MCyR were assessed in CP2L pts at 4 y and CP3L pts at 3 y. Pts with MCyR newly attained or maintained from baseline by 3 mo, by >3 to ≤6 mo (but not by 3 mo), or no MCyR by 6 mo were assessed for overall survival (OS) at 2 y (all pts) and cumulative incidence of progression (including lack of efficacy)/death at 4 y (CP2L) or 3 y (CP3L), adjusted for competing risks (see Table). OS rates were limited to 2 y (pts were followed for only 2 y from BOS discontinuation). P values were based on Gray's test for comparison of cumulative incidence distributions and log-rank test for OS distributions (no adjustment for multiple comparisons).

CP2L pts (n=286 [IM-R, n=196; IM-I, n=90]) had a median (range) age of 53 (18–91) y. CP3L pts (n=118 with prior IM failure [D-R, n=38; D-I, n=50; N-R, n=26; N-I or D-R/I + N-R/I, n=4]) had a median (range) age of 56 (20–79) y. Median time from CML diagnosis was 3.7 (0.1–15.1) and 6.6 (0.6–18.3) y for CP2L and CP3L pts, respectively; BOS treatment duration was 24.8 (0.2–83.4) and 8.5 (0.2–78.1) mo; follow-up duration was 47.3 (0.6–90.6) and 33.1 (0.3–84.8) mo. Time from last enrolled pt's first dose to database snapshot was ≥48 mo for CP2L and ≥36 mo for CP3L.

Of 264 CP2L pts with a valid baseline assessment, 107/183 (58%) IM-R and 49/81 (60%) IM-I pts attained/maintained a MCyR; 88/183 (48%) and 42/81 (52%) pts had CCyR. The Kaplan-Meier (KM) probability of maintaining MCyR at 4 y was 69% for IM-R and 86% for IM-I pts. Of 110 CP3L pts with valid assessment, overall MCyR and CCyR rates were 40% and 32%; the KM probability of maintaining MCyR at 3 y was 65%.

There was no significant difference in OS or cumulative incidence of progression/death distribution between CP2L pts with MCyR by 3 mo vs by >3 to ≤6 mo; however, OS (P=0.0004) and cumulative incidence of progression/death (P=0.0002) distributions were significantly better for CP2L pts with MCyR by 3 mo vs no MCyR by 6 mo (Table). For CP3L pts, there was no significant difference in long-term outcomes between early response groups.

In conclusion, in CP-CML pts receiving BOS as 2^nd-line therapy following IM failure (CP2L), pts who attained/maintained a MCyR by 3 mo had better OS and a lower progression/death distribution vs pts without MCyR by 6 mo; no significant differences were observed between pts achieving MCyR by 3 mo vs by >3 to ≤6 mo, although pt number was low for late responders. There were no significant differences in outcomes for CP-CML pts receiving BOS as 3^rd/4^th-line therapy, regardless of when or if MCyR was achieved, although the number of pts with MCyR was low. These results suggest that pts not achieving a MCyR by 6 mo, particularly in the 2nd-line setting, may require alternative therapies if options with better likelihood of response are available.