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We present a system for cancer targeting based on single–chain Fv (scFv) antibodies selected from combinatorial libraries, produced in bacteria and purified by using an engineered tag. Combinatorial libraries of scFv genes contain great diversity, and scFv antibodies with characteristics optimized for a particular task can be selected from them using filamentous bacteriophage. We illustrate the benefits of this system by imaging patients with carcinoembryonic antigen (CEA)–producing cancers using an iodine–123 labeled scFv anti–CEA selected for high affinity. All known tumor deposits were located, and advantages over current imaging technology are illustrated. ScFvs are produced in a cloned form and can be readily engineered to have localizing and therapeutic functions that will be applicable in cancer and other diseases.

Insertion of antibody genes into filamentous bacteriophage makes it possible to generate and screen libraries of 107 or more antibodies. Each phage expresses an antibody on its surface and contains the corresponding antibody gene. Genes that encode antibodies with desired characteristics are readily selected and their antibodies expressed as soluble proteins in Escherichia coli. We used this system to produce an antibody to carcinoembryonic antigen with higher affinity and better tumour specificity than antibodies currently in use.