Background: Fibroblast activation protein alpha (FAP) is a 170 kDa transmembrane glycoprotein expressed during development, active tissue remodeling, and cancer. Overexpression on cancer-associated fibroblasts in the tumor microenvironment, and on some cancer cells of mesenchymal origin, leads to the presence of FAP in > 90% of epithelial tumors, making FAP a compelling anti-cancer target. PNT6555, a potent FAP-targeting compound with rapid clearance from normal tissues and prolonged tumor retention in preclinical models¹, is a radioligand consisting of a DOTA chelator (1,4,7,10-tetraazacyclododecane-1,4,7,10- tetraacetic acid) and a FAP-targeting moiety (Bz-D-Ala-boroPro) connected via an aminomethyl linker. The corresponding gallium-68 and lutetium-177 chelates constitute the imaging and therapeutic theranostic pair of PNT6555 under evaluation in this phase I study. Methods: FRONTIER (NCT05432193) is a multicenter, open-label, multi-dose phase I study of PNT6555 in participants with pancreatic ductal adenocarcinoma, esophageal cancer, colorectal cancer, melanoma skin cancer, cholangiocarcinoma, or soft tissue sarcoma solid tumors with FAP over-expression. The primary aim is to evaluate the safety and tolerability of [Lu-177]-PNT6555 in order to determine a recommended phase II dose (RP2D). Dosimetry, pharmacokinetics, and tumor and immune responses will also be assessed for [Lu-177]-PNT6555, as well as safety, tolerability, and diagnostic performance of [Ga-68]-PNT6555 PET/CT. Key eligibility criteria include being refractory to prior treatment with no alternative available therapeutic options, having adequate organ function, and being FAP-positive on [Ga-68]-PNT6555 PET/CT scan, defined as ≥50% of lesions with an SUV_max of ≥1.5x the liver SUV_mean. [Ga-68]-PNT6555 will be dosed at 120 - 220 MBq (3.2 - 5.9 mCi), with PET/CT initiated 90 (±30) min later. FAP-positive participants will receive [Lu-177]-PNT6555 once every 6 weeks for up to 6 cycles. Starting at 4 GBq (108 mCi) ±10%, [Lu-177]-PNT6555 may be escalated by 4 GBq, with a maximum planned dose of 12 GBq. De-escalation by 2 GBq may also occur. Dose finding will be guided by the modified toxicity probability interval (mTPI-2) statistical design with a targeted toxicity rate of 30%. The planned total sample size is 30, with targets of 3-12 DLT evaluable patients per dose level. Cohort 1 has been completed without a DLT. Enrollment to cohort 2 began in January 2023. 1. Hallet R, et al. Pre-clinical characterization of the novel fibroblast activation protein (FAP) targeting ligand PNT6555 for the imaging and therapy of cancer. Presented at SNMMI Annual Meeting April 2022; Vancouver, BC, Canada.Clinical trial information: NCT05432193.