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Summary Background PD-1 and PD-L1 inhibitors are active in metastatic urothelial carcinoma, but positive randomised data supporting their use as a first-line treatment are lacking. In this study we assessed outcomes with first-line pembrolizumab alone or combined with chemotherapy versus chemotherapy for patients with previously untreated advanced urothelial carcinoma. Methods KEYNOTE-361 is a randomised, open-label, phase 3 trial of patients aged at least 18 years, with untreated, locally advanced, unresectable, or metastatic urothelial carcinoma, with an Eastern Cooperative Oncology Group performance status of up to 2. Eligible patients were enrolled from 201 medical centres in 21 countries and randomly allocated (1:1:1) via an interactive voice-web response system to intravenous pembrolizumab 200 mg every 3 weeks for a maximum of 35 cycles plus intravenous chemotherapy (gemcitabine [1000 mg/m2] on days 1 and 8 and investigator's choice of cisplatin [70 mg/m2] or carboplatin [area under the curve 5] on day 1 of every 3-week cycle) for a maximum of six cycles, pembrolizumab alone, or chemotherapy alone, stratified by choice of platinum therapy and PD-L1 combined positive score (CPS). Neither patients nor investigators were masked to the treatment assignment or CPS. At protocol-specified final analysis, sequential hypothesis testing began with superiority of pembrolizumab plus chemotherapy versus chemotherapy alone in the total population (all patients randomly allocated to a treatment) for the dual primary endpoints of progression-free survival (p value boundary 0·0019), assessed by masked, independent central review, and overall survival (p value boundary 0·0142), followed by non-inferiority and superiority of overall survival for pembrolizumab versus chemotherapy in the patient population with CPS of at least 10 and in the total population (also a primary endpoint). Safety was assessed in the as-treated population (all patients who received at least one dose of study treatment). This study is completed and is no longer enrolling patients, and is registered at ClinicalTrials.gov , number NCT02853305 . Findings Between Oct 19, 2016 and June 29, 2018, 1010 patients were enrolled and allocated to receive pembrolizumab plus chemotherapy (n=351), pembrolizumab monotherapy (n=307), or chemotherapy alone (n=352). Median follow-up was 31·7 months (IQR 27·7–36·0). Pembrolizumab plus chemotherapy versus chemotherapy did not significantly improve progression-free survival, with a median progression-free survival of 8·3 months (95% CI 7·5–8·5) in the pembrolizumab plus chemotherapy group versus 7·1 months (6·4–7·9) in the chemotherapy group (hazard ratio [HR] 0·78, 95% CI 0·65–0·93; p=0·0033), or overall survival, with a median overall survival of 17·0 months (14·5–19·5) in the pembrolizumab plus chemotherapy group versus 14·3 months (12·3–16·7) in the chemotherapy group (0·86, 0·72–1·02; p=0·0407). No further formal statistical hypothesis testing was done. In analyses of overall survival with pembrolizumab versus chemotherapy (now exploratory based on hierarchical statistical testing), overall survival was similar between these treatment groups, both in the total population (15·6 months [95% CI 12·1–17·9] with pembrolizumab vs 14·3 months [12·3–16·7] with chemotherapy; HR 0·92, 95% CI 0·77–1·11) and the population with CPS of at least 10 (16·1 months [13·6–19·9] with pembrolizumab vs 15·2 months [11·6–23·3] with chemotherapy; 1·01, 0·77–1·32). The most common grade 3 or 4 adverse event attributed to study treatment was anaemia with pembrolizumab plus chemotherapy (104 [30%] of 349 patients) or chemotherapy alone (112 [33%] of 342 patients), and diarrhoea, fatigue, and hyponatraemia (each affecting four [1%] of 302 patients) with pembrolizumab alone. Six (1%) of 1010 patients died due to an adverse event attributed to study treatment; two patients in each treatment group. One each occurred due to cardiac arrest and device-related sepsis in the pembrolizumab plus chemotherapy group, one each due to cardiac failure and malignant neoplasm progression in the pembrolizumab group, and one each due to myocardial infarction and ischaemic colitis in the chemotherapy group. Interpretation The addition of pembrolizumab to first-line platinum-based chemotherapy did not significantly improve efficacy and should not be widely adopted for treatment of advanced urothelial carcinoma. Funding Merck Sharp and Dohme, a subsidiary of Merck, Kenilworth, NJ, USA.

420 Background: RX-3117 is an oral small molecule antimetabolite, cyclopentyl pyrimidyl nucleoside that is activated by cancer-enriched uridine cytidine kinase 2. Single agent RX-3117 has demonstrated efficacy in a phase III single agent clinical study of RX-3117 in met-PC and bladder cancer. RX-3117 in combination with nab-pac is being evaluated as first line treatment of met-PC cancer. Methods: This is a multicenter, open label phase I/II study (NCT03189914). Eligible subjects (aged ≥ 18 years) have histologically or radiologically proven met-PC with no prior therapies for metastatic disease, ECOG PS 0-1, and normal lab values. phase I identified the MTD dose that is being further evaluated in the phase II: RX-3117 (700 mg administered orally once-daily for 5 consecutive days with 2 days off per week) and nab-pac (125 mg/m2 administered once weekly) for 3 weeks with 1 week off per 4-week cycle. The Safety Committee reviewed data from phase I before moving to phase II. The primary endpoint of phase II (dose expansion) is an adequate number of responders based on PFS at 4 months and/or objective clinical response per RECIST v1.1. Results: As of September 21, 2018, 8 phase I subjects and 13 phase II subjects were enrolled and treated (9 males and 12 females, median age 67 years). The most common (≥ 15%) related adverse events were nausea, diarrhea, fatigue, alopecia, decreased appetite, rash, vomiting, and anemia. Fourteen subjects had at least one on-study scan (after 2 cycles). One subject experienced a complete response (CR) after 6 cycles of therapy with normalization of CA19.9 (-76%). Three subjects exhibited a partial response (PR): two after 2 cycles (39-47%) and one after 4 cycles of therapy (36%). Eight subjects had stable disease for at least 2 months, and 4 subjects had PFS of at least 4 months. The disease control rate (CR+PR+SD) was 86% in evaluable subjects while the overall response rate (CR+PR) was 29%. Conclusions: RX-3117 in combination with nab-pac appears to be safe and well tolerated in subjects with met-PC. Antitumor activity per RECIST was observed in 12 subjects. Phase II of the clinical study is currently ongoing. Clinical trial information: NCT03189914.