Tri-Specific Humabody CB307 Targeting CD137, Prostate-Specific Membrane Antigen, and Human Serum Albumin Whole-Body Biodistribution Measured with 89Zr-CB307 PET

This study investigated (pre)clinically the biodistribution of the tri-specific Humabody CB307, composed of three heavy variable chain domains (VH) targeting prostate-specific membrane antigen (PSMA), CD137, and human serum albumin. CB307 internalization was assessed in PSMA- and CD137-positive tumor and T cells. In xenograft mouse models bearing PSMA-positive and PSMA-negative tumors, the biodistribution of 99mTc-PSMA-targeting VHs bound/unbound to mouse serum albumin (MSA) and full-length 99mTc-PSMA monoclonal antibody J591 was assessed by single-photon emission computed tomography. Five patients with PSMA-positive metastatic castration-resistant prostate cancer (mCRPC) underwent baseline 18F-PSMA-positron emission tomography (PET) scans to define PSMA-positive lesions, followed by 89Zr-CB307 (37 MBq/5 mg) administration and escalating doses of unlabeled CB307, up to 100 mg. PET was performed at multiple time points. Spherical volumes of interest were manually drawn around tumors and organs to calculate standardized uptake values (SUV). Additionally, tumor biopsies were assessed for PSMA expression and autoradiography. CB307 internalized into PSMA- and CD137-positive cells. PSMA-VH x MSA exhibited superior xenograft penetration compared with unbound PSMA-VH and full-length PSMA-directed antibodies. In patients, no 89Zr-CB307-related adverse events occurred. In the three patients receiving the highest protein dose (5 mg 89Zr-CB307 with 100 mg unlabeled CB307), the geometric mean 89Zr-CB307-SUVmax in all 18F-PSMA-positive lesions increased from 3.53 (day 2) to 4.12 (day 7). The highest uptake was observed in the liver [SUVmean 13.06 ± 2.68 (day 7)], with no significant uptake in the spleen, normal lymph nodes, bone marrow, or salivary glands. Tumor biopsies confirmed radioactivity. 89Zr-CB307 selectively accumulated in PSMA-positive tumor lesions without specific uptake in PSMA-positive normal or lymphoid tissues in patients with mCRPC.