Polymerase theta inhibitors combinations with approved and investigational agents in patient-derived tumor multi-cell type (mct) spheroids

The potential of novobiocin, recently identified to be a DNA POLtheta inhibitor, to augment cancer chemotherapy was explored in the late 1980s and early 1990s in tumor cells, tumor-bearing mice and in Phase 1 clinical trial in combination with cyclophosphamide or cisplatin. Genetic alterations which may increase or decrease POLtheta inhibitor effects have been elucidated. Thirty patient-derived tumor cell lines with known BRCA, ATM, ATR, POL?, XRCC1, PALB2, PARP1, LIG3 alterations as well as know gLOH% and MSI status were screened in a mct-spheroid assay (tumor cells, endothelial cells, mesenchymal stem cells) with a POLtheta inhibitor, novobiocin, ART-558, and RP6685, alone or in simultaneous combination with a FDA-approved or investigational anticancer small molecule with a 7-day exposure and a CellTiter-Glo 3D luminescence endpoint. As single agents, the POL? inhibitors had little or no cytotoxicity. In simultaneous combination with ART-558, talazoparib produced greater-than-additive cytotoxicity at the highest concentrations of the POL theta inhibitors in the 922,993-354-T-J3-PDC endometrial serous carcinoma mct-spheroids. Activity of the Chk1/2 inhibitor prexasertib was potentiated by either ART-558 or RP6685 in the 922,993-354-T-J3 mct-spheroids. The combination of POL? inhibitors ART-558 and RP6685, and the Chk1/2 inhibitor prexasertib produced up to 1 log increase in cytotoxicity in the 922,993-354-T-J3 mct-spheroids. Regions of potentiation were evident in the 922,993-354-T-J3-PDC endometrial carcinoma survival surface plots at the highest concentration of paclitaxel tested, while regions of potentiation were evident in the paclitaxel mid-concentrations of the 299,254-011-R-J1-PDC melanoma mct-spheroids survival surface plots as determined by the Bliss independence calculation. DNA POLtheta is recruited to DNA double-strand breaks as a component of repair. POLtheta allosteric inhibitors, novobiocin, ART558 and RP-6685, have entered clinical trial. The current study explores the cytotoxicity of POLtheta inhibitors in combination with anticancer drugs and investigational agents in patient-derived cell lines grown as mct-spheroids.

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