Background: Patients (pts) with unresectable biliary tract cancer (BTC) have a poor prognosis. In preclinical models of cholangiocarcinoma (CCA) telotristat ethyl (TE), a tryptophan hydroxylase inhibitor that reduces production of serotonin, enhanced the antiproliferative effects of gemcitabine and cisplatin (GC) chemotherapy. This trial assessed the safety and efficacy of adding TE to GC chemotherapy in pts with advanced BTC. Methods: This was a Phase 2 open-label study. Eligible pts had unresectable, locally advanced, or metastatic BTC, were treatment naïve and had plans to initiate GC chemotherapy. On Day 1, pts received Gem 1000mg/m2 and Cis 25mg/m2 with TE 250 mg orally three times daily (TID) for 7 days. Thereafter, pts received TE 500 mg TID continuously plus GC therapy on days 1 and 8 of each 21-day cycle. The study comprised of 2 stages: stage 1 would enroll 20 patients with a safety run-in cohort in the first 6 pts to monitor safety and tolerability of TE + GC treatment for 21 days. Stage 2 would enroll 33 patients if no significant/unresolved grade 3 or higher toxicities related to study drug occurred in stage 1. The primary endpoint was 6-month (mo) progression free survival (PFS) rate according to RECIST 1.1. If 60% of pts (≥34) in the safety population were alive and progression free at 6-mos, the study was considered successful. Secondary endpoints included overall response rate (ORR) and disease control rate (DCR). Restaging occurred every 9 weeks using RECIST 1.1 and toxicities were graded using CTCAE v 5.0. Results: Between 2019 and 2021, fifty-three pts were enrolled across 12 study sites and comprised the safety population. The majority of pts were female (62.3%) and median age was 66 yrs (range 33 to 79). Baseline mean plasma 5-HIAA was 11.1 µg/L (ULN defined as >10.8 µg/L). Thirty-one pts (58.5%) had plasma 5-HIAA levels ≤ULN and 22/53 (41.5%) had 5-HIAA levels >ULN. Majority of pts had metastatic BTC (85%) and liver was the most common primary tumor site (66%). The 6-month PFS rate was 22.6% (12/53). Based on blinded independent central review, the ORR was 13.2% and DCR was 62.3%. Mean reduction in plasma 5-HIAA at 6 mos was -1.735 µg/L (SD 8.69) in 24 patients with available data. There was no difference in 6-mo PFS rate based on pts baseline 5-HIAA status; 58% (14/24) pts had ≥30% decrease from baseline in 5-HIAA, however there was no correlation with 6-month PFS. The most common TE related TEAEs were constipation (43.4%), nausea (28.3%) and fatigue (28.3%). Conclusions: The study did not achieve its primary endpoint of 60% of patients on TE plus GC chemotherapy being alive and progression free at 6 months. TE did reduce 5-HIAA levels in a subset of pts; however, this did not correlate to improved clinical outcomes. No significant safety issues noted with TE in combination with GC chemotherapy. Clinical trial information: NCT03790111.