OPTIMIZATION OF IN-VIVO TUMOR TARGETING IN SCID MICE WITH DIVALENT FORMS OF 741F8 ANTI-C-ERBB-2 SINGLE-CHAIN FV - EFFECTS OF DOSE-ESCALATION AND REPEATED IV ADMINISTRATION

Single-chain Fv molecules in monovalent (sFv) and divalent [(sFv')2] f orms exhibit highly specific tumor targeting in mice as a result of th eir small size and rapid systemic clearance. As a consequence, there i s a rapid reversal of the sFv blood/tumor gradient, resulting in dimin ished retention of sFv species in tumors. In this report we investigat e two distinct strategies, dose escalation and repetitive intravenous (i.v.) dosing, aiming to increase the absolute selective retention of radiolabeled anti-c-erbB-2 I-125-741F8 (sFv')(2) in c-erbB-2-overexpre ssing SK-OV-3 tumors in mice with severe combined immunodeficiency (SC ID). A dose-escalation strategy was applied to single i.v. injections of I-125-741F8 (sFv')(2). Doses from 50 mu g to 1000 mu g were adminis tered without a significant decrease in tumor targeting or specificity . High doses resulted in large increases in the absolute retention of I-125-741F8 (sFv')(2). For example, raising the administered dose from 50 mu g to 1000 mu g increased the tumor retention 24 h after injecti on from 0.46 mu g/g to 9.5 mu g/g, and resulted in a net increase of g reater than 9 mu g/g. Over the same dose range, the liver retention ro se from 0.06 mu g/g to 1 mu g/g, and resulted in a net increase of les s than 1 mu g/g. The retention of 9.5 mu g/g in tumor 24 h following t he 1000-mu g dose of (sFv')(2) was comparable to that seen 24 h after a 50-mu g dose of I-125-741F8 IgG, indicating that the use of large do ses of (sFv')(2) may partially offset their rapid clearance. When two doses were administered by i.v. injection 24 h apart, the specificity of delivery to tumor observed after the first dose was maintained foll owing the second injection. Tumor retention of I-125-741F8 (sFv')2 was 0.32 mu g/g at 24 h and 0.22 mu g/g at 48 h following a single inject ion of 20 mu g, while 0.04 mu g/ml and 0.03 mu g/ml were retained in b lood at the same assay times. After a second 20-mu g injection at the 24-h assay time, tumor retention increased to 0.49 mu g/g, and blood r etention was 0.06 mu g/ml, at the 48-h point. These results suggest th at multiple high-dose administrations of radiolabeled 741F8 (sFv')(2) map lead to the selective tumor localization of therapeutic radiation doses.