Evaluating pegylated SN38 (PLX38A) and rucaparib in <i>TP53</i> wild-type endometrial cancer.

e15125 Background: Endometrial cancer (EC) is the most common gynecologic malignancy, with endometrioid being the most prevalent histology. Approximately 75% of high-grade endometrioid tumors are TP53 wild-type. During studies testing a novel combination of the PARP inhibitor rucaparib and TOP1 poison SN38 in high-risk primary patient serous and serous-like ECs, a subset of non-serous ECs with normal p53 expression showed unexpected synergy. This suggests broad efficacy in EC, warranting in vivo studies to assess tolerability and efficacy. Given potential overlapping toxicity, a novel formulation, pegylated SN38 (PLX038A), was tested. Methods: Primary patient tumors (n=4) were treated with rucaparib (0µM-25 µM) and SN38 (0µM-0.5 µM) in 3D cell culture experiments. RealTime Glo assessed viability after 3-5 days of drug exposure. Combination indices were determined using the Chou-Talalay method. Tolerability and efficacy were assessed in female SCID-beige mice (n=45) injected with intraperitoneal EC PDX tumors. Mice were randomized to controls, PLX038A (15 mmol/µmol IP, every 14 days), rucaparib (150 mg/kg daily gavage), or combination (same dose and schedule) for 8 weeks. Tolerability was assessed by daily animal weight and total condition score (TCS) from 1 (emaciated, non-ambulatory) to 13 (obese). Weight change more than -20% or TCS &lt; 5 defined moribund criteria. Weekly ultrasound measurements assessed tumor response. Results: All four primary patient tumors exhibited synergy, with a combination index less than 1. Moribund criteria were not met in any mice across all cohorts. The average percent weight change and lowest TCS across cohorts were: rucaparib (-8.51%, min/max -19 to +14%; TCS 6.25, range 5-8, n=8), PLX038A (-3.97%, min/max -19 to +5%; TCS 6.8, range 6-9, n=10), the combination (-5.52%, min/max -15 to +1.8%; TCS 7.18, range 6-8, n=11), and control (+1.61%, range -6 to +8%; TCS 6.5, range 6-7, n=4). Scores were similar between groups, but the slightly lower TCS in the rucaparib cohort was associated with higher tumor burden from ineffective therapy. Tumors in the rucaparib monotherapy group grew similarly to controls. In contrast, the combination therapy cohort exhibited significant regression below baseline during treatment. Conclusions: These findings suggest that the combination of rucaparib and SN38 (PLX038A) is effective in EC primary patient tumor organoids and PDXs with wild-type TP53. No significant negative physiological effects on animal TCS and weight change were observed. No drug-related toxicity was observed in mice receiving combination treatment. Mice receiving combination treatment displayed significant tumor regression below baseline compared to both single agents and control. These data support future exploration of this combination therapy in ECs.