Preclinical studies of orally active, pan Bcl-2 small molecule inhibitor AT-101 in small cell lung cancer.

A51 Small cell lung cancer (SCLC) is an aggressive form of lung cancer accounting for approximately 18% of all cases of lung cancer. Overexpression of the anti-apoptotic protein Bcl-2 is seen in approximately 80% of (SCLC) and has been associated with resistance to chemotherapy and poor prognosis. AT-101 is a pan-Bcl-2 family inhibitor which binds to Bcl-2, Mcl-1 and also Bcl-xL. AT-101 is an orally active agent currently in clinical development in several phase II trials, including in SCLC. Here, we report pre-clinical efficacy of AT-101, in a number of SCLC cell lines, as a single agent and combined with chemotherapy.
 Methods: Cell growth inhibition was measured using WST-based assays. The CalculSyn program was used to assess drug interaction by calculating the combination Index (CI) value. In vivo , NCI-H446 human SCLC xenograft was used to investigate tumor inhibition induced by AT-101 as well as pharmacodynamic parameters (TUNEL assay, CD 31 expression, Protein expresssion).
 Results: Using a panel of SCLC cell lines, growth inhibition was demonstrated with AT-101 (H82 IC 50 =1.94 uM, DMS79 IC 50 =3.13 uM, H417 IC 50 =3.98 uM, H69 IC 50 = 6.84 uM). Anti-proliferation assays in vitro were conducted in a second panel of SCLC cell lines (NCI-H446, DMS53, H1688 and H69AR) to compare AT-101 with the Bcl-2 binding small molecule ABT-737 (Abbott, Inc.); (NCI-H446 (AT-101 IC 50= 0.229uM, ABT-737 IC 50 =9.26 uM), H69AR (AT-101 IC 50= 1.5uM, ABT-737 IC 50 >10uM); DMS53 (AT-101 IC 50 =1.005uM, ABT-737 IC 50 =0.041uM); H1688 (AT-101 IC 50 =2.096uM; ABT-737 IC 50 =1.266 uM).
 ABT- 737 was reported to bind Bcl-2 and Bcl-xl but not Mcl-1 (Oltersdorf et al , Nature 2005 2;435 (7042); 677-8). The response of these cell lines to ABT-737 is consistent with the reported data and might be associated with lack of binding to Mcl-1.
 Strong synergy in vitro was observed with the combination of AT-101 and chemotherapeutic agents in NCI-H446 (topotecan CI value ave. =0.437, etoposide CI value ave.=0.6, carboplatin a CI value ave.=0.8). In vivo, combined treatment with AT-101 and topotecan synergistically suppressed subcutaneous NCI-H446 tumor growth compared with treatment with either agent alone. The combination of AT-101 (25mg/kg, p.o., daily) and topotecan (2 mg/kg, p.o, 5 days a week) showed anti-tumor efficacy with a T/C value of 0.118 whereas the single agent treatment was less effective (T/C value =0.906, 0.261 for AT-101 and topotecan, respectively) (P in vivo by AT-101 alone or in combo with topotecan have been quantified and the result shows that the combination treatment is significantly more active that single agent alone. Taken together, these observations demonstrate the superior efficacy of AT-101 and support the planned or ongoing clinical trials of AT-101 as single agent or combined with topotecan in SCLC.