Phase 1b dose escalation study to evaluate the safety, tolerability, pharmacokinetics, and preliminary efficacy of GS-3583, a FLT3 agonist Fc fusion protein, in patients with advanced solid tumors.

Abstract

TPS3147 Background: Productive antitumor immune responses in nonclinical models depend on a type of dendritic cell (DC), conventional DC subtype 1 (cDC1), which in the context of cancer, primes tumor-reactive T cells through presentation of tumor-derived antigens. FMS-related tyrosine kinase 3 ligand (FLT3L) is a hematopoietic growth factor that binds to and activates FLT3 on terminally differentiated DCs. Activated FLT3 promotes proliferation, inhibits cell death, and is required for the differentiation, expansion, and maintenance of DCs in peripheral and lymphoid organs. GS-3583 is a fusion protein composed of the extracellular domain of recombinant human FLT3L fused to an engineered fragment crystallizable (Fc) region of human immunoglobulin G4. GS-3583 has PK properties that support sustained cDC in patients and potential combination with established immunotherapies. This phase 1b, open-label, multicenter, dose-finding study will evaluate safety, tolerability, PK, and preliminary efficacy of GS-3583 monotherapy in patients with advanced solid tumors (NCT04747470). Methods: Approximately 33 adults aged ≥18 years with a histologically or cytologically confirmed locally advanced or metastatic malignant solid tumor that is refractory to or intolerant of standard therapy or for which no standard therapy is available will be enrolled. The study employs a 3+3 dose escalation design in which GS-3583 is administered intravenously for up to 52 weeks or until progressive disease or unacceptable toxicity. Up to five dose escalation cohorts have been planned. The maximum tolerated dose is the highest dose with incidence of DLT in < 33% of 6 or more patients in the first 28 days of GS-3583 dosing; recommended phase 2 dose will be determined. Assessments include safety, PK, pharmacodynamics including cDCs, immunogenicity, and efficacy by RECIST 1.1 in CT/MRI imaging conducted every 8 weeks. Accrual at approximately 3-4 centers in the US is ongoing. Clinical trial information: NCT04747470.