Abstract Background In order to survive bladder cancer (BCa) cells deregulate key pathways - including the inhibition of apoptosis - while developing “oncogene addiction” to molecular chaperones such as Heat Shock Protein 90 (HSP90). While successful in vitro, monotherapies targeting such pathways have failed in clinical trials. We aim to show the superiority of combination therapy using proteostatic and targeted cytotoxic agents. We hypothesize that these combinations will synergize to increase cytotoxicity (CT) in BCa by amplifying the apoptosis via simultaneous activation of both intrinsic and extrinsic apoptotic pathways. Methods The proteostatic agents tested were the HSP90 inhibitor STA9090 (9090) and the proteasome inhibitor Marizomib (NPI). The cytotoxic agent used was a synthetic TRAIL ligand. These agents were tested in multiple ATCC BCa cell lines and the NCI patient tumor-derived BCa cell line UOBL 101 (UOBL). CT was determined via MTT assay. Mechanism of cytotoxic response was investigated using a series of immunoblots targeting the components of intrinsic and extrinsic apoptotic pathways and nuclear degradation proteins. All the assays were performed in triplicates for validation. Results 9090 (IC50 7.4-10 nM) was effective in all cell lines, as was NPI (IC50 10-20 nM). TRAIL caused 100% CT in UOBL (IC50 2.5 ng/mL, 95% CI 1.0, 7.0) and a maximal 50% CT in T24, however the cytotoxicity of individual agents was not sustained. Combination effects of 9090 + TRAIL and NPI + TRAIL demonstrated an increase from 50% CT to 100% CT at the IC50 of both 9090 and NPI, and this effect was sustained over the course of the experiment. The increased cytotoxicity of combination therapy had corresponded to rapid cytoplasmic release of cytochrome c, additive increase in cleaved caspase 3 and caspase 8, with the corresponding increase in cleaved PARP and γ-H2ax. Conclusions Combination of a pro-apoptotic agent with a molecular chaperone or proteasome inhibitor holds promise in BCa therapyby affecting intracellular anti-apoptotic signaling and compromising efficient proteostasis in cancer cells lines, and should be investigated further. Citation Format: Raju Chelluri, Piyush K. Agarwal, Leonard M. Neckers, Armine K. Smith. Synergistic effect of targeted combination therapy in bladder cancer model using HSP90 inhibitors. abstract. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3796. doi:10.1158/1538-7445.AM2015-3796