Phase I Study of GS-3583, an FMS-like Tyrosine Kinase 3 Agonist Fc Fusion Protein, in Patients with Advanced Solid Tumors

Purpose: GS-3583, an FMS-like tyrosine kinase 3 (FLT3) agonist Fc fusion protein, expanded conventional dendritic cells (cDC) in the periphery of healthy volunteers, suggesting potential for GS-3583 to increase cDCs in the tumor microenvironment and promote T cell- mediated antitumor activity in cancer patients. This phase Ib openlabel study assessed GS-3583 in adults with advanced solid tumors. Patients and Methods: Multiple escalating doses of GS-3583 (standard 3+3 design) were administered intravenously on days 1 and 15 of cycle 1 and day 1 of each subsequent 28-day cycle for up to 52 weeks. Dose-limiting toxicity (DLT) was evaluated during the first 28 days of GS-3583 at each dose level. Results: Thirteen participants enrolled in four dose-escalation cohorts, after which the study was terminated following safety review. Median (range) age was 71 (44-79), and 7 (54%) participants were male. There were no DLTs. Seven participants had grade ≥3 AEs; 2 participants had grade 5 AEs, including a second primary malignancy (acute myeloid leukemia) considered treatment-related. Dose-dependent increase in GS- 3583 serum exposure was observed in the dose range of 2- 20 mg with GS-3583 accumulation at higher dose levels. Expansions of cDCs occurred at all four doses with a dosedependent trend in the durability of the cDC expansion. Conclusions: GS-3583 was relatively well tolerated and induced dose-dependent expansion of cDCs in the periphery of patients with advanced solid tumors. However, development of a second primary malignancy provides a cautionary tale for the FLT3 agonist mechanism.