Pharmacokinetics and biodistribution of engineered single-chain antibody constructs of MAb CC49 in colon carcinoma xenografts.

UNLABELLED: Monoclonal antibodies (MAbs) have been proven useful in clinical studies for both diagnostic and therapeutic applications. The single-chain Fv (scFv) construct made from MAbs has potential applications for improved cancer diagnosis and therapy. A new CC49 scFv construct recognizing a tumor-associated mucin, TAG-72, was engineered and evaluated by immunological, pharmacokinetic and biodistribution analysis. METHODS: The CC49 scFv construct was generated in which the V(L) and V(H) variable region genes were joined together with a 25-amino acid helical linker (205C). The new CC49 scFv(205C) was expressed as a monomer as well as a stable noncovalent dimer ([scFv]2). The pharmacokinetic, biodistribution and tumor targeting characteristics of radiolabeled CC49 scFv were compared with CC49 IgG and enzymatically derived fragments F(ab')2 and Fab', using the athymic mice bearing human colon cancer xenografts. RESULTS: The association constant (K(A)) for the intact CC49, dimeric scFv (scFv)2 and monomeric scFv were 1.7 x 10(9), 1.99 x 10(9) and 0.52 x 10(9) M(-1) by Scatchard analysis and 1.14 x 10(8), 4.46 x 10(7) and 1.5 x 10(7) M(-1), respectively, by BIAcore analysis. Pharmacokinetic studies showed that more than 50% of monomeric scFv (approximately 27 kDa) was cleared from the blood in less than 10 min. The CC49 Fab' generated enzymatically from the parent murine Mab' (50 kDa) had a blood clearance that was faster than that of the (scFv)2 (60 kDa) with half of the activity cleared from the serum within 30 and 50 min, respectively. The CC49 dimeric scFv(205C) showed a two-fold higher tumor uptake (than scFv or Fab') reaching 10 %ID/g at 60 min after injection. The scFv dimer also showed an excellent stability and increased avidity in vivo compared with the monomer, as demonstrated by the longer retention in tumor with 3%ID/g remaining at 48 h. CONCLUSION: The rapid clearance from the blood, higher tumor uptake and longer retention of the stable dimer of CC49 scFv make it an important agent for potential imaging and therapeutic applications.