Background: FGFR2 promotes gastric cancer progression, suggesting that inhibition of FGFR2 may be an important therapeutic strategy. Alofanib (RPT835) is a small molecule, allosteric inhibitor that binds to the non-active extracellular site of IIIc and IIIb FGFR2 isoforms with IC50 < 10 nM. Methods: RPT835GC1B is a Phase 1b open-label study evaluating the safety and preliminary efficacy of alofanib in patients with metastatic gastric adenocarcinoma pretreated with ≥ 1 previous lines of therapy. The standard dose-escalation part (design 3+3) aimed to establish the maximum tolerated dose (MTD) or recommended phase 2 dose (RP2D) as a primary endpoint. Secondary endpoints included pharmacokinetic (PK) parameters, rate of adverse events, progression-free survival (PFS), overall survival (OS), and objective response rate (ORR). Alofanib was administered daily intravenously for 5-days followed by a 2-day interval (rest). There were five dose levels: 50, 100, 165, 250, and 350 mg/m2. All patients received alofanib until disease progression or unacceptable toxicity. Results: As of 20 September 2021, 21 patients have been enrolled in the study. Patients were Caucasian (100%), predominantly male (71%), 67% had 2 and more metastatic sites, including liver metastases (43%), 14% had ECOG PS 2, and were heavily pretreated (86% had previous 3 and more lines of therapy). All enrolled patients have been studied for safety and have not experienced any dose-limiting toxicities (DLTs) within the 28-day DLT-assessment window. 17 (81%) patients had at least one treatment-related adverse event (trAE). Grade 3 or higher trAEs (5/23.8%) have included raised ALT/AST at 50 mg/m2, neutropenia at 50 mg/m2, diarrhea at 165 mg/m2, headache at 165 mg/m2, increased serum amylase at 350 mg/m2, and reactions immediately after intravenous injections (facial flushing, dizziness, weakness, sweating, and sinus tachycardia) at 350 mg/m2. Three (14.3%) patients discontinued treatment due to trAEs. Most common Grade 1-2 adverse events included reactions immediately after intravenous injections, diarrhea, thrombocytopenia, arthralgia, and headache. Grade 1 hyperphosphatemia was found in 1 (4.8%) patient. Table summarizes efficacy data. Conclusions: The MTD has not been reached and dose of 350 mg/m2 has been declared as RP2D. Alofanib showed acceptable tolerability and preliminary signs of clinical activity in the late-line treatment of metastatic gastric cancer. Clinical trial information: NCT04071184.