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 111 In-labeled HSG-DOTA complex was injected 25 or 41 hours after the bsMAb. The kinetics of both the iodinated bsMAb and the pretargeted 111 In-labeled HSG hapten were investigated in mice bearing human glioblastoma xenografts (U251) and compared with the kinetics and tumor accumulation of radioiodinated L19-SIP. 111 In and 125 I were used as surrogate marker for the therapeutic radioisotopes 90 Y/ 177 Lu and 131 I, respectively. Results: Tumor uptake of the pretargeted 111 In-labeled peptide was significantly higher than 125 I-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 90 Y-hapten-peptide would improve the therapeutic efficacy in solid tumors by >3-fold compared with directly radiolabeled 131 I-L19-SIP.