Synergistic effect of AT-101, an orally active pan Bcl-2 family proteins inhibitor in combination with dual-Raf/MEK and VEGFR kinase inhibitor Sorafenib (Nexavar)

AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA 1452 The Raf /MEK kinase cascade are pivotal regulators of cellular proliferation. The majority of human cancers overexpress Bcl-2, Bcl-xL and/or Mcl-1, the important negative regulators of apoptosis. Recent studies suggest that the Bcl-2 and Raf/MEK pathways together confer an aggressive, apoptosis-resistant phenotype. Sorafenib (Nexavar) is the first oral multikinase inhibitor that targets Raf/MEK and cell proliferation and angiogenesis, and is approved for treating renal cancer. However, expression of Bcl-2 or Mcl-1 could protect cells from Sorafenib induced apoptosis. AT-101 is a potent small molecule pan-Bcl-2 (Bcl-2, Bcl-xL and Mcl-1) inhibitor currently in Phase II clinical trials. In this study we investigated whether the simultaneous inhibition of Bcl-2 by AT-101 and Raf/MEK function by Sorafenib could result in enhanced antitumor activity. Methods: Cell growth inhibition was measured using WST-based assays. The CalcuSyn program was used to assess drug interaction by calculating the Combination Index (CI) value. In vivo , the ability of AT-101 to potentiate the anticancer effect of Sorafenib in xenograft models was evaluated. Summary: Three cell lines, breast cancer MDA-231, NSCLC H460 and A549, with overexpression of Bcl-2, Bcl-xL and Mcl-1 proteins, were treated with Sorafenib and AT-101 as a single agent and in combination. Sorafenib inhibited the growth of three cell lines with IC50 at 3-8 uM, whereas AT-101 inhibited the growth with IC50 at 3-6 uM. When treating cells simultaneously with both agents, AT-101 demonstrated strong synergy with Sorafenib in A549 (CI value=0.38, average of the CI values obtained at the ED50, ED75 and ED90), H460 (CI values avg.=0.67), and MDA-231 (CI values avg.=0.72). In the MDA-231 xenograft model, AT-101 demonstrated dose-dependent anti-tumor activity. At a dose of 7.5 mg/kg (p.o., qd x 28 days), AT-101 alone resulted in limited tumor growth inhibition (T/C=69%), and at a dose of 30 mg/kg, AT-101 caused tumor growth inhibition with a T/C=33%. However, the combination effect of both compounds was more than additive, resulting in a T/C of 8%, as compared to Sorafenib alone (20mg/kg, p.o, qd x 18 days, T/C=18%) (P<0.0001). In the same model, a combination of high dose AT-101 with a lower dose of Sorafenib (7.5 mg/kg) was also more effective than either agent alone (T/C value of 17%, versus T/C=33% for AT-101 and 30 % for Sorafenib, respectively) (P=0.0026). Combination therapy also achieved significant tumor growth delay (T-C = 35 or 46 days) as compared with either agent alone.Further studies with combination therapy in NSCLC models are currently underway. Conclusion: In summary, these data suggest that targeting both cell survival and cell growth pathways may offer significant synergy over targeting cell growth signaling alone.