Pretargeted Radioimmunotherapy of Mesothelin- Expressing Cancer Using a Tetravalent Single- Chain Fv-Streptavidin Fusion Protein

Mesothelin is a glycoprotein that is overexpressed in several human tumors, including mesotheliomas and ovarian cancers, and has been identified as a potential target for therapy. We evaluated the biodistribution and tumor-targeting ability of an antimesothelin tetravalent single-chain Fv-streptavidin fusion protein (SS1scFvSA) in mice. Methods: SS1scFvSA was la-beled with 125I or 111In for evaluation of internalization in vitro and for optimization of its biodistribution. The A431-K5 mesothelin transfected cell line was used as the target. We used a 3-step pretargeting approach consisting of injections of (i) SS1scFvSA, followed 20 h later by (ii) a synthetic clearing agent, and (iii) 4 h later, radiolabeled (111In, 88Y/90Y, or 177Lu) 1,4,7,10-tetraazacy-clododecane-N,N,N,N-tetraacetic acid (DOTA)-biotin. To op-timize the tumor uptake, the effect of the specific activity of 111In-DOTA-biotin was evaluated. Results: Approximately 60