Head-to-head evaluation of DFO and 3,4,3-LI(1,2-HOPO) chelators through site-specific conjugation of Anti-STEAP1 antibody V6-19-D11 for 89Zr-immuno-PET imaging.

Six-transmembrane epithelial antigen of the prostate-1 (STEAP1) is continuously overexpressed in PCa and represents a promising target for immuno-PET imaging. This study aimed to evaluate the in vitro and in vivo performance of two 89Zr-labeled anti-STEAP1 immunoconjugates, 89Zr-DFO-V6-19-D11 and 89Zr-HOPO-V6-19-D11, in terms of tumor targeting, stability, and biodistribution with Standardized Uptake Values (SUVs) in LNCaP xenograft models. Site-specifically conjugated humanized anti-STEAP1 monoclonal antibody V6-19-D11 with p-SCN-Bn-DFO or 3,4,3-LI(1,2-HOPO)-maleimide and 89Zr radiolabeling was assessed for radiochemical purity, serum stability, and immunoreactivity. All of the tracers were administered intravenously to LNCaP tumor-bearing nude mice, and biodistribution was quantitated at 24, 48, 72, and 96 h post-injection by γ-counting and SUV. Both radioconjugates were >98 % radiochemically pure, exhibited strong STEAP1 binding (KD ≈ 2.2 nM), and retained good immunoreactivity (>87 %). The HOPO-conjugate also demonstrated improved in vitro stability in serum (88.6 % intact at 96 h vs. 82.0 % for DFO). Tumor accumulation of 89Zr-HOPO-V6-19-D11 was comparable or superior at all-time points (15.3 ± 1.5 %ID/g at 24 h, maximum 17.1 ± 1.2 %ID/g at 48 h and 15.9 ± 1.2 %ID/g at 96 h) with significantly reduced off-target uptake, notably in bone (96 h femur: 1.8 ± 0.3 %ID/g vs. 4.5 ± 0.4 %ID/g for DFO; p < 0.01). Tumor-to-background ratios, notably tumor-to-bone (8.8 vs. 3.4), favored the HOPO-based tracer. The 89Zr-HOPO-V6-19-D11 immunoconjugate has better in vivo stability, reduced nonspecific bone uptake, and excellent tumor targeting, making it an outstanding candidate for quantitative immuno-PET imaging of STEAP1-positive prostate cancer. This study is indicative of the translational value of HOPO chelation chemistry for enhancing performance of zirconium-89-based antibody tracers for non-invasive cancer characterization.