110 PROMPT: phase II trial of maintenance pembrolizumab following weekly paclitaxel for recurrent ovarian, fallopian tube or peritoneal cancer

Introduction/Background Most women with advanced ovarian cancer eventually develop platinum-resistant disease which has a poor outcome. Median progression-free survival (PFS) is around 3–4 months and overall survival (OS) approximately 12 months. We explored the addition of maintenance pembrolizumab to extend PFS in patients who do not progress on weekly paclitaxel. Methodology Patients were enrolled in a phase II single-arm study with response or stable disease after at least 4 cycles of paclitaxel. They were treated with pembrolizumab 200mg IV every 21 days until progression, unacceptable toxicity, patient or clinician decision. Primary endpoint was 6-month PFS rate with a target rate of 65%; Results Twenty evaluable patients with high grade serous carcinoma were enrolled. Median age 61 years (41–78), ECOG 0/1 50%/50%. 85% of patients had stage III/IV at diagnosis and had received a median of 5.5 prior cycles (4–17) weekly paclitaxel, with 35% achieving partial response and 65% stable disease. A median of 3.5 cycles (2–18) of pembrolizumab were given; 19 patients (95%) stopped due to progression and 1 discontinued due to a treatment-related adverse event (TRAE) of hepatitis. Two (10%) patients had 3 grade 3 TRAEs: rash; hepatitis and diarrhoea. There were no grade 4/5 TRAEs. After a median follow-up of 16.5 months, median PFS from the start of pembrolizumab was 2.0 months (95%CI: 1.8–3.6) and median OS was 9.8 months (95%CI: 6.2–20.7). As the 6-month PFS rate was 5.0% (95%CI: 0.3–20.5) the trial stopped early due to futility. Conclusion Maintenance pembrolizumab did not improve PFS in patients with platinum-resistant ovarian cancer with non-progressive disease after paclitaxel. Planned translational research on diagnostic, tissue biopsies and blood samples is crucial to understand why most patients with ovarian cancer do not benefit from immune checkpoint inhibitors. Disclosures Please see attached signed COI forms.