Biodistribution of anti-PECAM scFv-lmw scuPA and lmw-scuPA in vivo. Ten micrograms fusion protein or equal molar lmw-scuPA was mixed with 0.2 μg radiolabeled tracer protein and injected intravenously into wild-type or PECAM null mice, respectively. One hour later, tissue uptake was measured. (A) Percentage of injected dose per gram tissue (%ID/g). Note that scFv-uPA, but not scuPA, shows preferential uptake in the lungs and other vascularized organs in wild-type (WT), but not in PECAM KO mice. (B) Organ-to-blood ratio for various organs. Broken line indicates blood level; ratio equal to 1.0. (C) Immunospecificity index (ISI), calculated as ratio of organ-to-blood ratios of targeted and untargeted counterpart. The interrupted line shows an ISI of 1.0, reflecting equal tissue levels of targeted and untargeted counterparts. Error bars indicate SEM.