ABT-199, a new Bcl-2-specific BH3 mimetic, has in vivo efficacy against aggressive Myc-driven mouse lymphomas without provoking thrombocytopenia

Abstract

BH3-only proteins trigger the stress apoptosis pathway and chemical mimetics have great potential for cancer therapy. BH3-only proteins inhibit antiapoptotic members of the Bcl-2 family. Promising BH3 mimetic ABT-737 and the related orally available compound ABT-263 (navitoclax) bind avidly to antiapoptotic Bcl-2, Bcl-xL, and Bcl-w. However, their interaction with Bcl-xL provokes thrombocytopenia, which has proven to be the dose-limiting toxicity. We have tested the efficacy of ABT-199, a new Bcl-2-specific BH3 mimetic, against aggressive progenitor cell lymphomas derived from bitransgenic myc/bcl-2 mice. As a single agent, ABT-199 was as effective as ABT-737 in prolonging survival of immunocompetent tumor-bearing mice without causing thrombocytopenia. Both drugs acted rapidly but, contrary to prevailing models, their apoptotic activity did not rely upon the BH3-only protein Bim. When ABT-737 was combined with the proteosome inhibitor bortezomib or CDK inhibitor purvalanol, many treated animals achieved long-term remission.

Figure 1

Comparison of responsiveness to ABT-737 and ABT-199. (A-B) WBC counts, (C-D) platelet counts, and (E-F) Kaplan-Meier survival curves of mice transplanted with 4 bim^+/+ (nos. 9, 12, 16, and 47) and 3 bim^−/− (nos. 58, 96, and 324) myc/bcl-2 lymphomas and treated with ABT-737, ABT-199, or the respective vehicles (3 mice per treatment arm for each independent tumor). Blood analyses were performed at 0 hours, 3 hours, and 6 days after starting treatment (day 11 after transplantation), using an ADVIA 2120 hematology analyzer (Siemens Australia New Zealand, VIC, Australia). Bars represent mean + SEM; significant changes observed at 3 hours are indicated: *P < .05, **P < .01, ***P < .001, Student t test. The x-axis in panels E and F indicates days elapsed since start of treatment; the bar indicates the duration of treatment (10 days). Significance for Kaplan-Meier survival curves was determined using the log-rank (Mantel-Cox) test. The median survival of bim^+/+ lymphomas treated with ABT-737 was 21.5 days vs 14 days for vehicle (P = .0088), and 23.5 days with ABT-199 vs 13 days for vehicle (P = .0003). The median survival of bim^−/− lymphomas treated with ABT-737 was 20.5 days vs 14 days for vehicle (P < .0001), and 19.5 days with ABT-199 vs 13 days for vehicle (P < .0001). One bim^−/− lymphoma (no. 324) rebounded early following treatment; unusually, it comprised 50% progenitor (B220⁺CD4⁺) and 50% B-lymphoid (B220⁺CD4⁻) cells, the latter having higher levels of Mcl-1 (see supplemental Figure 1A), which would confer greater resistance.

Figure 2

Combination therapy with ABT-737 extends survival. Safe regimens were established from previously published data^,, and our own dose-finding studies (data not shown). Nonirradiated C57BL/6 recipients were transplanted with 3 independent bim^+/+ myc/bcl-2 lymphomas (nos. 9, 12, 16) (3 × 10⁶ tumor cells per mouse) 4 days prior to commencement of treatment. ABT-737 was administered intraperitoneally at 75 mg/kg per day for 10 days (bar), starting on day 1; cyclophosphamide (CTX; 50 mg/kg) intraperitoneally on days 3, 8; bortezomib (0.5 mg/kg) intraperitoneally on days 1, 3, 5, 8, 10; purvalanol A (20 mg/kg) intraperitoneally on days 1, 3, 5, 8, 10, alone or in combination with ABT-737, with at least 6 mice per indicated treatment arm (supplemental Tables 1-2). With tumor 16, all mice treated with ABT-737 alone remained healthy until the experiment was terminated at day 150 (supplemental Table 2 and data not shown; controls treated with vehicle were all dead by day 35).