Tumor targeting in a murine tumor xenograft model with the (sFv′)2 divalent form of anti-c-erbB-2 single-chain Fv

This investigation has utilized novel forms of the single-chain Fv (sFv), wherein a cysteine-containing peptide has been fused to the sFv carboxyl terminus to facilitate disulfide bonding or specific crosslinking of this sFv′ to make divalent (sFv′)2. The 741F8 anti-c-erbB-2 monoclonal antibody was used as the basis for construction of 741F8 sFv, from which the sFv′ and (sFv′)2 derivatives were prepared. Recombinant c-erbB-2 extracellular domain (ECD) was prepared in CHO cells and the bivalency of 741F8 (sFv′)2 demonstrated by its complex formation with ECD. The tumor binding properties of125I-labeled anti-c-erbB-2 741F8 sFv, sFv′, and (sFv′)2 were compared with radiolabeled antidigoxin 26-10 sFv′ and (sFv′)2 controls. Following intravenous administration of radiolabeled species to severe combined immune-deficient (SCID) mice bearing SK-OV-3 tumors (which overexpress c-erbB-2), blood and organ samples were obtained as a function of time over 24 h. Comparative analysis of biodistribution and tumor-to-organ ratios demonstrated the 741F8 sFv, sFv′, and (sFv′)2 had excellent specificity for tumors, which improved with time after injection. This contrasted with nonspecific interstitial pooling in tumors observed with the 26-10 sFv, sFv′, and (sFv′)2, which decreased with time after administration. Tumor localization was significantly better for disulfide or peptide crosslinked 741F8 (sFv′)2 having Gly4Cys tails than for monovalent 741F8 sFv′ or Fab. The superior properties of the 741F8 (sFv′)2 in targeting SK-OV-3 tumors in SCID mice suggests the importance of further investigations of divalent sFv analogs for immunotargeting.