Co-inhibition of BCL-XL and MCL-1 with selective BCL-2 family inhibitors enhances cytotoxicity of cervical cancer cell lines

Development of resistance to chemo- and radiotherapy in patients suffering from advanced cervical Cancer narrows the therapeutic window for conventional therapies. Previously we reported that a combination of the selective Bcl-2 Family inhibitors ABT-263 and A-1210477 decreased cell proliferation in C33A, SiHa and CaSki human cervical Cancer cell lines. As ABT-263 binds to both Bcl-2 and Bcl-xL with high affinity, it was unclear whether the synergism of the drug combination was driven either by singly inhibiting Bcl-2 or Bcl-xL, or inhibition of both. In this present study, we used the Bcl-2 selective inhibitor ABT-199 and the Bcl-xL selective inhibitor A1331852 to resolve the individual antitumor activities of ABT-263 into Bcl-2 and Bcl-xL dependent mechanisms. A-1210477 was substituted for the orally bioavailable S63845. Four cervical Cancer cell lines were treated with the selective Bcl-2 Family inhibitors ABT-199, A1331852 and S63845 alone and in combination using 2-dimensional (2D) and 3-dimensional (3D) Cell Culture models. The SiHa, C33A and CaSki cell lines were resistant to single agent treatment of all three drugs, suggesting that none of the Bcl-2 Family of proteins mediate survival of the cells in isolation. HeLa cells were resistant to single agent treatment of ABT-199 and A1331852 but were sensitive to S63845 indicating that they depend on Mcl-1 for survival. Co-inhibition of Bcl-2 and Mcl-1 with ABT-199 and S63845, inhibited cell proliferation of all Cancer cell lines, except SiHa. However, the effect of the combination was not as pronounced as combination of A1331852 and S63845. Co-inhibition of Bcl-xL and Mcl-1 with A1331852 and S63845 significantly inhibited cell proliferation of all four cell lines. Similar data were obtained with 3-dimensional spheroid Cell Culture models generated from two cervical Cancer cell lines in vitro. Treatment with a combination of A1331852 and S63845 resulted in inhibition of growth and invasion of the 3D spheroids. Collectively, our data demonstrate that the combination of MCL-1-selective inhibitors with either selective inhibitors of either Bcl-xL or Bcl-2 may be potentially useful as treatment strategies for the management of cervical Cancer.