STRIvE-02: A First-in-Human Phase I Study of Systemically Administered B7-H3 Chimeric Antigen Receptor T Cells for...

B7-H3 is an immunoregulatory protein overexpressed by many pediatric solid tumors with limited expression on critical organs, making it an attractive immunotherapy target. We present a first-in-human phase I clinical trial systemically administered B7-H3 chimeric antigen receptor (CAR) T cells for young patients with relapsed or refractory solid tumors.

Patients were enrolled onto a phase I trial to examine the safety of B7-H3-specific CARs at various dose levels (DLs) using a standard 3 + 3 dose escalation design.

Sixteen patients (range, 11-24 years; median, 18.5 years) were enrolled, and nine were treated at DL1 (0.5 × 10⁶ CAR T cells/kg; n = 3) or DL2 (1 × 10⁶ CAR T cells/kg; n = 6). There were no first infusion dose-limiting toxicities. Maximum first-infusion circulating CAR T cells detected in the peripheral blood were 4.98 cells/μL (range, 0-4.98 cells/μL) with detection of CAR T cells colocalizing with tumor cells at the site of metastatic disease in one patient. Patients were eligible for subsequent infusions. An objective partial response by PERCIST criteria was observed 28 days after a second CAR T cell infusion in a patient who did not have an objective response after the first infusion. The second infusion demonstrated marked enhancement of CAR T cell expansion to 1,590 cells/μL and was accompanied by cytokine release syndrome and dose-limiting transaminitis. Detailed peripheral blood cytokine profiling revealed elevated IL-21 levels preinfusion 2 compared with infusion 1.

B7-H3 CAR T cells are tolerable and demonstrate limited antitumor activity without acute on-target, off-tumor toxicity. High levels of CAR T cell expansion may be necessary to achieve objective responses, but undefined host and tumor microenvironment factors appear to be critical (ClinicalTrials.gov identifier: NCT04483778).