Abstract 2782: Bcl-2 selective antagonists show antitumor activity without dose limiting platelet toxicity
Authors: Olivier Geneste, James Murray, Thierry Le Diguarher, Jérome Starck, Patrick Casara, Guillaume De Nanteuil, James Davidson, Claire Nunns, Chris Graham, Natalia Matassova, Ijen Chen, Neil Whitehead, Pawel Dokurno, Alain Bruno, Alain Pierré, John. A. Hickman, Mike Wood, Francisco Cruzalegui, Stéphane Depil
Journal: Cancer Research (2012)

Abstract Proteins of the Bcl-2 family are central regulators of programmed cell death. Pro-survival Bcl-2 proteins, such as Bcl-2, Bcl-xL and Mcl-1 are often over-expressed in human tumours and participate in tumour initiation, progression and chemo-resistance. Therefore drugs targeting these pro-survival Bcl-2 proteins represent a promising therapeutic approach for cancer treatment. The most advanced drug targeting this protein family is ABT-263, a potent Bcl-2 and Bcl-xL inhibitor showing anti-tumour efficacy in preclinical models of leukaemia and small cell lung carcinoma. Survival of circulating platelets has been shown to be highly Bcl-xL dependent; consequently the dose-limiting toxicity of ABT-263 is an on-target peripheral thrombocytopenia. We have used a range of biophysical methods to guide the structure-based generation of a significant number of small molecules,* which bind with high affinity (MW &amp;lt; 780; KD &amp;lt; 1 nM) to the BH3 binding groove of Bcl-2, and with high selectivity versus other members of the Bcl-2 family (selectivity &amp;gt; 100 fold). In cellular assays, our lead compounds efficiently displace Bax from Bcl-2 with near complete inhibition of Bcl-2 / Bax co-immunoprecipitation at 100 nM. These compounds are strong inducers of cell death in Bcl-2 dependent cellular models such as the acute myeloblastic leukaemia (AML) cell line RS4;11, affording sub-10 nM IC50's for the most potent compounds. In vivo, in agreement with their mechanism of action, these Bcl-2 selective inhibitors, given either intravenously or orally, elicit a rapid (30 min iv, and 2 hours po) and strong apoptotic response in mouse xenografts of the RS4:11 cell line. When the most potent compounds are given orally to RS4;11 xenograft-bearing mice, apoptosis in tumor cells is induced more than 15 fold (at 25 mg/kg) and more than 20 fold (at 50 mg/kg) compared to untreated mice. Importantly, in agreement with the selectivity of the compounds for Bcl-2 versus Bcl-xL, no platelet loss was observed in mice treated with our compounds, in sharp contrast to ABT-263. Finally, we observe very robust anti-tumor activity when a lead compound is given orally at 50 mg/kg and 100 mg/kg (with complete regression at 100 mg/kg) in an RS4;11 mouse xenograft model. This anti-tumor activity was similar whether the compound was dosed daily or twice a week over two weeks. Altogether our data demonstrate that highly Bcl-2 selective antagonists show anti-tumor activity and no platelet toxicity, in contrast to Bcl-2 / Bcl-xL dual inhibitors. Such compounds represent promising drug candidates for the treatment of Bcl-2 dependent malignancies such as chronic lymphocytic leukaemia (CLL) and other leukaemias and lymphomas. * Chemical structures of compounds will not be disclosed. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2782. doi:1538-7445.AM2012-2782