Background: Type I insulin-like growth factor receptor (IGF-1R) is a transmembrane protein which is overexpressed in solid tumors including non–small cell lung, prostate, and breast cancers. [225Ac]-FPI-1434 is a radioimmunoconjugate consisting of a humanized monoclonal antibody that binds to the external domain of IGF-1R, a proprietary bifunctional chelate, and an alpha-emitting radionuclide actinium-225 (Ac-225), which binds to the external domain of IGF-1R. Internalization of the conjugate and decay of Ac-225 causes tumor cell death primarily through double stranded DNA breaks. The indium-111 analog, [111In]-FPI-1547, with the identical antibody and bifunctional chelate is used for patient selection, in-vivo imaging, and quantification of IGF-1R targets prior to therapy. Based on anti-tumor activity of [225Ac]-FPI-1434 in preclinical models, favorable toxicology studies in cynomolgus monkeys, and prior human experience with the unconjugated antibody, the first in human clinical evaluation was initiated. Methods: This open-label multi-center phase I study (NCT03746431) follows a modified 3+3 dose-escalation design to characterize the safety profile, determine a maximum tolerated dose (MTD), evaluate dose-limiting toxicities (DLT), describe pharmacokinetics, derive radiation dose estimates to normal organs, and evaluate the objective response rate of [225Ac]-FPI-1434 therapy in patients with IGF-1R expressing solid tumors. Eligibility requirements for therapy include: presence of at least one measurable lesion as determined by sufficient tumor uptake using SPECT/CT of an imaging analog [111In]-FPI-1547; radiation dose estimates of the planned therapeutic activity within prespecified limits; and adequate bone marrow reserves, hepatic, and renal function. Dose cohorts begin with 10 kBq [225Ac]-FPI-1434 per kilogram (kg) patient weight and successively increase to 20, 40, 80, and 120 kBq/kg as a single intravenous injection per patient followed by an 8-week DLT evaluation period. This trial is currently enrolling patients. Clinical trial information: NCT03746431.