Novel peptide radioligands targeting CAIX for theranostic applications in clear cell renal cell carcinoma (ccRCC)

Carbonic anhydrase IX (CAIX) is a hypoxia-inducible transmembrane enzyme highly expressed in clear cell renal cell carcinoma (ccRCC) and several other solid tumors, making it an attractive target for radiotheranostic development.Using PepLib's peptide discovery and optimization platform, we identified a monocyclic peptide ligand, 4B043 (19 amino acids, one disulfide bond) as a high-affinity CAIX binder. 4B043 exhibited sub-nanomolar affinity to human CAIX (K D ≈ 86 pM) and strong cross-species binding to canine, mouse, and cynomolgus monkey CAIX. In Balb/c nude mice bearing VMRC-RCW tumor models, ⁶⁸Ga-4B043 showed high tumor uptake (~55 %ID/g) with a tumor-to-kidney (T/K) ratio ≈ 0.5 at 4 h post-injection. After structural optimization, ⁶⁸Ga-2B002 maintained high tumor uptake while significantly reducing kidney accumulation (T/K ≈ 2 at 4 h). Encouraged by these preclinical data, ⁶⁸Ga-4B043 was advanced into a human IIT. In ccRCC patients, ⁶⁸Ga-4B043 demonstrated high uptake in renal tumor lesions. Due to CAXI expression in normal gastric and small intestinal tissues, ⁶⁸Ga‑4B043 also showed notable uptake in these organs in patients. Notably, kidney uptake was remarkably low in patients, with a tumor-to-kidney ratio > 8 at 2 h, in clear contrast to the CDX model.4B043 and its optimized derivative 2B002 represent a novel class of CAIX-targeted peptide radioligands with excellent tumor uptake, favorable biodistribution, and promising translational potential. These data support their further development as theranostic agents for imaging and targeted radioligand therapy in CAIX-expressing tumors, including ccRCC.