BACTERIAL EXPRESSION OF A KEMPTIDE FUSION PROTEIN FACILITATES P-32 LABELING OF A HUMANIZED, ANTICARCINOEMBRYONIC ANTIGEN (HMN-14) ANTIBODY FRAGMENT

Despite the potential advantages of P-32 over Other isotopes for radio immunotherapy, its development as a therapeutic has been hindered by t he difficulty of the labeling chemistry. Recently, a heptapeptide [Kem ptide (KPT)] has been chemically conjugated to antibodies, and the con jugates have successfully been labeled with P-32 enzymatically by usin g bovine protein kinase. By using genetic engineering, we have produce d a chimera (Pab.KPT) consisting of the Fab' moiety of the complementa rity-determining region-grafted anti-carcinoembryonic antigen-monoclon al antibody, MN14, and a heptapeptide derivative of KPT (Trp-Arg-Arg-A la-Ser-Leu-Gly). The recombinant protein was expressed in Escherichia coli as a soluble secretory product, The presence of the KPT derivativ e downstream of the COOH terminus of the hinge region did not impair t he binding affinity of the antibody fragment, The Fab,KPT was enzymati cally phosphorylated with P-32 by bovine protein kinase, without signi ficant effect on the resultant immunoreactivity; 100% of the P-32-labe led Fab.KPT was complexed with liquid carcinoembryonic antigen, The P- 32-labeled humanized MN-14 Fab.KPT is expected to have longer blood ci rculation half-life, allowing for an improved therapeutic efficacy in radioimmunotherapy.