Targeting Bcl2 in cancer

The decision phase of apoptosis is mainly regulated by the Bcl-2 family, which renders these proteins potential targets for cancer therapy through apoptotic mechanisms. Bcl2 family members have homology clustered within four conserved Bcl2 homology (BH) domains (BH1, BH2, BH3 and BH4). Only the antiapoptotic proteins, such as Bcl2, Bcl-XL, Bcl-w and A1, bear the NH 2-terminal BH4 domain [1]. Mcl-1 does not have a typical BH4 domain but has a helical BH4-like domain which is located between the PEST region and the BH3 domain. The BH1, BH2 and BH3 domains form the surface binding pocket of Bcl2 which mediates protein-protein interactions involving Bcl2 family members. Mutations in this hydrophobic surface binding pocket abolish the antiapoptotic function of Bcl2, demonstrating its requirement in the biological function of Bcl2. To interfere with Bcl2 function, it is necessary to impede binding to the hydrophobic cleft. Synthetic peptides that bind this surface pocket of Bcl-XL and Bcl2 have been shown to induce apoptosis in vitro. The hydrocarbon-stapled BH3 peptide not only induces apoptosis but also possesses potent antitumor activity in vivo. Recently, an approach called " structure-activity relationship by nuclear magnetic resonance (SAR by NMR) " has been used for the discovery of novel Bcl2/ Bcl-XL inhibitors [2]. In this approach, the relatively large site to be targeted is divided into two smaller half-sites that are individually targeted by small molecules. The two lead molecules are then chemically linked to improve the binding affinity. Subsequent iterative chemical manipulations to improve affinity and decrease binding to human serum albumin, guided by NMR and three-dimensional structures of antiapoptotic proteins, yielded the small molecules ABT-737 and ABT-263, which bind to the hydrophobic pocket of Bcl2 or Bcl-XL with high-affinity and subsequently disrupt the antiapoptotic function of Bcl2 and Bcl-XL with potent anti-tumor effect [2]. Thus, such small molecules that directly target Bcl2 and/ or the Bcl2-like proteins by mimicking the BH3 domain should be highly effective anticancer drugs. However, ABT-737 and ABT-263 can induce thrombocytopenia due to their inhibitory effects on both Bcl2 and Bcl-XL [3]. To generate a more selective Bcl2 inhibitor, a tethered indole was incorporated into ABT-263 to fill the P4 hot spot, which specifically interacts with aspartic acid (Asp 103) of Bcl2 but not Glu96 of Bcl-XL, leading to the generation of the Bcl2-selective inhibitor ABT-199 [4]. Since ABT-199 did not cause thrombocytopenia in vivo [4], this suggests that selective inhibition of …