Abstract 7201: Preclinical evaluation of click-cleavable radioimmunoconjugates for enhanced treatment efficacy in radioimmunotherapy

Abstract Targeted radiotherapy (TRT) with radiolabeled antibodies is hampered by long circulation times, resulting in dose-limiting toxicity. Tagworks developed a novel TRT approach based on its Click-to-Release platform, comprising an internalizing mAb functionalized with a chelator via a click-cleavable trans-cyclooctene (TCO) linker, and a small-molecule trigger (tetrazine). The radiolabeled mAb is administered and allowed to internalize into tumor cells. Then, a non-cell permeable trigger is injected that selectively reacts with the residual circulating mAb, releasing a small fast-clearing radioactive fragment. This approach is envisioned to unlock the treatment benefits of TRT with long-lived potent beta- (177Lu) and alpha (225Ac) radionuclides, minimizing toxicity to the patient’s marrow and kidney while delivering a high therapeutic radioactive dose to the tumor. Tagworks’ TGW211 program in Phase 0/1 aims at increasing the therapeutic index of radiolabeled trastuzumab (TGW211-DC), targeting HER2+ tumors. In addition, this approach is being applied to mAbs directed to various other targets in a preclinical setting. Chelate-conjugated mAbs were radiolabeled with 111In or 177Lu and release of the radiolabeled-DOTA was measured upon reaction with trigger. Binding and internalization were characterized using BT-474 cells. Pharmacokinetics and biodistribution were determined in nude mice +/- BT-474 xenografts. Patient dosimetry with different radionuclides was modelled using clinical data for [89Zr]Zr-trastuzumab using IDAC-Dose 2.1 and OLINDA. The trigger rapidly released 111In/177Lu-labeled DOTA in PBS and plasma (e.g. 91.8% in 5 min for [111In]In-TGW211-DC). [111In]In-TGW211-DC demonstrated high-affinity in vitro (KD = 8.4 nM), and efficient internalization. In healthy mice, [111In]In-TGW211-DC was highly stable with >20d TCO isomerization half-life, while trigger effectively released the fast-clearing 111In-fragment in blood. In tumor bearing mice, radioactivity was largely retained inside tumor cells, but cleared from blood by the trigger, reaching maximum tumor-to-blood-ratios (T/B) of 16.3, 24h p.i. and 34.8, 72h p.i. Similar results were obtained for [177Lu]Lu-TGW211-DC. Dosimetry projections indicate maximal healthy tissue dose decrease when trigger is administered 24 hrs post TGW211-DC, reaching up to 92% for 225Ac and 88% for 177Lu in marrow. In kidney this resulted in 89% dose decrease for 225Ac. Finally, this click-cleavable TRT approach could be effectively applied to other internalizing mAb-target combinations. In conclusion, the click-cleavable TRT approach has demonstrated strong platform potential and markedly improved the T/B in mice for TGW211. Moreover, beneficial dosimetry of click-cleavable TRT was projected for TGW211. A First-in-human Phase 0/1 study with HER2+ cancer patients has recently been initiated. Citation Format: Marleen H. van Stevendaal, Kim E. de Roode, Luc H. Zijlmans, Jenne T. Meinema, Hendrina M. de Wert-Wattimury, Renate C. van Daalen-Bruens, Bianca Matthee, Mark W. Konijnenberg, James Nagarajah, Raffaella Rossin, Marc S. Robillard. Preclinical evaluation of click-cleavable radioimmunoconjugates for enhanced treatment efficacy in radioimmunotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 7201.