Abstract 3055: In vitro evaluation of combination chemotherapy/targeted radiotherapy for translation to an in vivo model of prostate cancer
Authors: Rold Tammy L. , Bernskoetter Nicole E. , Hoffman Timothy J. 
Published: July 2016

Introduction: We have previously shown that radiosensitizing chemotherapy when used in combination with targeted radiotherapy agents can result in improved tumor growth inhibition in preclinical xenograft mouse models of prostate cancer (PC). The challenge is determining optimum drug combinations to achieve therapeutic synergy. Herein we performed in vitro studies to determine if synergism existed and then optimize effective drug combination levels to maintain synergism for the treatment of PC.

Methods: The Lu-177 radiolabeled peptide (DOTA-4-amino-1-carboxymethyl-piperidine-D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2) was prepared using an Eckert & Ziegler automated system. Cultured PC-3 cells were allowed to adhere overnight in a standard tissue culture environment after plating. Docetaxel chemotherapy was prepared in culture media without supplements and added to wells containing fresh growth media to achieve a final concentration of docetaxel of 0, 1.25, 2.5, and 5 nM. After 24 hrs, all media was removed and replaced with fresh growth media (non-chemo containing) prior to treatment with 0, 5, 10, 20, or 40 uCi/well of Lu-177 labeled peptide receptor targeted radiotherapy (TRT) to each well. At 72hrs post TRT addition, all media was removed and fresh growth media was added. MTT reagent was added to each well at pre-determined time points. Plates were incubated in a standard tissue culture environment for 4 hours prior to removal of all liquid from the wells followed by addition of 200uL of DMSO to each well. Plates were read at 540nm and data was normalized to non-treated controls. Calcusyn software was used to calculate CI values to determine if synergy was achieved. Cell cycle experiments were performed after docetaxel treatment at 0, 2.5, 5, 10, 20, 40, and 80 nM. After 6, 24, 48, and 72 hrs of treatment, cells were collected from each well, fixed in 70% ethanol, and stained with propidium iodide prior to cell cycle analysis (5,000 events).

Results: Cell cycle analysis confirmed a sustained shift (&gt; 50%) of docetaxel treated cells to the radio-sensitive G2/M phase of the cell cycle after 24, 48, and 72hrs of treatment. Synergy was observed when docetaxel at 1.25 and 2.5 nM was combined with 20 and 40 uCi of Lu-177 TRT. At concentrations of docetaxel &gt;2.5nM only additive effects of combination therapy were observed. At concentrations of Lu-177 TRT &lt;10uCi/well in combination with docetaxel only slight synergy or antagonism was observed. Conclusions: Effectively optimizing combination chemotherapy and Lu-177 TRT in an in vivo model of prostate cancer remains challenging. These initial results demonstrate a synergistic window where chemotherapy and Lu-177 TRT can potentially yield optimum results however achieving synergy requires further detailed drug combination optimization both in vitro and in vivo.

Citation Format: Tammy L. Rold, Nicole E. Bernskoetter, Timothy J. Hoffman. In vitro evaluation of combination chemotherapy/targeted radiotherapy for translation to an in vivo model of prostate cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3055.