PARP and ABCB1 (MDR1) inhibitor treatment of ovarian cancer cell lines and PDX models demonstrate no synergistic effect (244)

Objectives: PARP inhibition is an important and effective treatment modality in high-grade serous ovarian cancer (HGSOC); however resistance to therapy can develop by various mechanisms. One of the proposed mechanisms is through excretion of the PARP inhibitor (PARPi) via the MDR1 pump, an ATP-dependent efflux pump located in the cell membrane. Both PARPis and taxanes have been previously shown to be substrates for the MDR1 pump. Preclinical studies have demonstrated a synergistic effect when treating cell lines with paclitaxel and the MDR1 pump inhibitor, SPL-108, and clinical trials in humans are underway with this compound. We propose that PARPi resistance can similarly be overcome by concurrent treatment with SPL-108 and that combination therapy will result in greater toxicity to ovarian cancer cells compared to PARPi treatment alone. Methods: Four ovarian cancer cell lines (OVCAR8, OVCAR4, CAOV3, and COV318) were treated with the PARPis talazoparib and olaparib at a range of concentrations with and without SPL-108. Cell viability after five days of treatment was assessed using PrestoBlue. The IC50 was calculated for all cell lines with and without the addition of SPL-108. HGSOC patient-derived xenograft (PDX) models were generated and treated with talazoparib (0.33mg/kg daily) with and without SPL-108 until tumors reached the humane endpoint of 2000mm3. The mean number of days from treatment start to a humane endpoint for each model was evaluated and compared. Results: The calculated IC50 for talazoparib, with and without SPL-108 treatments were: 15.8nM and 20.0nM for OVCAR8; 6.3nM and 5.0nM for OVCAR4; 6.3nM and 15.8nM for CAOV3; and 10nM and 10nM for COV318. For olaparib, the IC50 for OVCAR4 was 63.1nM and 15.8nM; for OVCAR8 was 630nM and 1,259nM; and for CAOV3 was 3162nM and 6309nM, with and without SPL108, respectively. Combination treatment with PARPis and SPL-108 demonstrated no synergistic effect in any of the four cell lines when compared to treatment with PARPis alone. Additionally, no differences were observed in the HGSOC PDX mouse models treated with talazoparib with or without SPL-108. Combination treatment with PARPis and SPL-108 demonstrated no synergistic effect in any of the four cell lines when compared to treatment with PARPis alone. Additionally, no differences were observed in the HGSOC PDX mouse models treated with talazoparib with or without SPL-108. Conclusions: In previous studies, the MDR1 inhibitor, SPL-108, has been shown to increase the efficacy of taxanes in ovarian cancer cell lines and is well tolerated in humans in phase I studies. Despite promising a mechanism to reverse PARPi resistance, there does not seem to be any synergy when combining SPL-108 with PARPi in ovarian cancer cell lines or HGSOC PDX mouse models. Objectives: PARP inhibition is an important and effective treatment modality in high-grade serous ovarian cancer (HGSOC); however resistance to therapy can develop by various mechanisms. One of the proposed mechanisms is through excretion of the PARP inhibitor (PARPi) via the MDR1 pump, an ATP-dependent efflux pump located in the cell membrane. Both PARPis and taxanes have been previously shown to be substrates for the MDR1 pump. Preclinical studies have demonstrated a synergistic effect when treating cell lines with paclitaxel and the MDR1 pump inhibitor, SPL-108, and clinical trials in humans are underway with this compound. We propose that PARPi resistance can similarly be overcome by concurrent treatment with SPL-108 and that combination therapy will result in greater toxicity to ovarian cancer cells compared to PARPi treatment alone. Methods: Four ovarian cancer cell lines (OVCAR8, OVCAR4, CAOV3, and COV318) were treated with the PARPis talazoparib and olaparib at a range of concentrations with and without SPL-108. Cell viability after five days of treatment was assessed using PrestoBlue. The IC50 was calculated for all cell lines with and without the addition of SPL-108. HGSOC patient-derived xenograft (PDX) models were generated and treated with talazoparib (0.33mg/kg daily) with and without SPL-108 until tumors reached the humane endpoint of 2000mm3. The mean number of days from treatment start to a humane endpoint for each model was evaluated and compared. Results: The calculated IC50 for talazoparib, with and without SPL-108 treatments were: 15.8nM and 20.0nM for OVCAR8; 6.3nM and 5.0nM for OVCAR4; 6.3nM and 15.8nM for CAOV3; and 10nM and 10nM for COV318. For olaparib, the IC50 for OVCAR4 was 63.1nM and 15.8nM; for OVCAR8 was 630nM and 1,259nM; and for CAOV3 was 3162nM and 6309nM, with and without SPL108, respectively. Combination treatment with PARPis and SPL-108 demonstrated no synergistic effect in any of the four cell lines when compared to treatment with PARPis alone. Additionally, no differences were observed in the HGSOC PDX mouse models treated with talazoparib with or without SPL-108. Combination treatment with PARPis and SPL-108 demonstrated no synergistic effect in any of the four cell lines when compared to treatment with PARPis alone. Additionally, no differences were observed in the HGSOC PDX mouse models treated with talazoparib with or without SPL-108. Conclusions: In previous studies, the MDR1 inhibitor, SPL-108, has been shown to increase the efficacy of taxanes in ovarian cancer cell lines and is well tolerated in humans in phase I studies. Despite promising a mechanism to reverse PARPi resistance, there does not seem to be any synergy when combining SPL-108 with PARPi in ovarian cancer cell lines or HGSOC PDX mouse models.