Bispecific antibody pretargeting of tumor neovasculature for improved systemic radiotherapy of solid tumors

Purpose: Extra domain B (ED-B) fibronectin is a specific tumor matrix marker for targeting angiogenesis in solid tumors. In this study, the radiotherapeutic potential of the directly radioiodinated divalent anti-ED-B antibody fragment, L19 small immunoprotein (L19-SIP; 75,000 Da), was compared with a pretargeting approach using the bispecific antibody AP39xm679 (bsMAb; 75,000 Da). Experimental Design: The bsMAb was prepared by coupling an anti-ED-B single-chain Fv (AP39) to the Fab' of the murine antibody m679, which binds to the small peptidic hapten histamine-succinyl-glycine (HSG). As an effector molecule for the pretargeting approach, the In-111-labeled HSG-DOTA complex was injected 25 or 41 hours after the bsMAb. The kinetics of both the iodinated bsMAb and the pretargeted In-111-labeled HSG hapten were investigated! in mice bearing human glioblastoma xenografts (U251) and compared with the kinetics and tumor accumulation of radioiodinated L19-SIP. In-111 and I-125 were used as surrogate marker for the therapeutic radioisotopes Y-90/Lu-177 and I-131, respectively. Results: Tumor uptake of the pretargeted In-111-labeled peptide was significantly higher than I-125-L19-SIP over 7 days. At the calculated maximally tolerated dose for each agent (with the kidney being the dose-limiting organ for pretargeting and the bone marrow for direct targeting), a mouse tumor dose of 146 Gy could be given by pretargeting versus 45 Gy delivered by the direct approach, Conclusions: These data suggest that pretargeting of ED-B with AP39xm679 and subsequent injection of the Y-90-hapten-peptide would improve the therapeutic efficacy in solid tumors by >3-fold compared with directly radiolabeled I-131-L19-SIP.