Designer genes: recombinant antibody fragments for biological imaging

Monoclonal antibodies (MAbs), with high specificy and high affinity for the ir target antigens, can be utilized for delivery of agents such as radionuc lides, enzymes, drugs, or toxins in vivo. However, the implementation of ra dio-labeled antibodies as "magic bullets" for detection and treatment of di seases such as cancer has required addressing several shortcomings of murin e MAbs. These include their immunogenicity, sub-optimal targeting and pharm acokinetic properties, and practical issues of production and radiolabeling . Genetic engineering provides a powerful approach for redesigning antibodi es for use in oncologic applications in vivo. Recombinant fragments have be en produced that retain high affinity for target antigens, and display a co mbination of rapid, high-level tumor targeting with concomitant clearance f rom normal tissues and the circulation in animal models. An important first step was cloning and engineering of antibody heavy and light chain variabl e domains into single-chain Fvs (molecular weight, 25-27 kDa), in which the variable regions are joined via a synthetic Linker peptide sequence. Altho ugh scFvs themselves showed limited tumor uptake in preclinical and clinica l studies, they provide a useful building block for intermediate-sized reco mbinant fragments. Covalently linked dimers or non-covalent dimers of scFvs (also known as diabodies) show improved targeting and clearance properties due to their higher molecular weight (55 kDa) and increased avidity. Furth er gains can be made by generation of larger recombinant fragments, such as the minibody,an scFv-C(H)3 fusion protein that self-assembles into a bival ent dimer of 80 kDa. A systematic evaluation of scFv, diabody, minibody, an d intact antibody (based on comparison of tumor uptakes, tumor:blood activi ty ratios, and calculation of an Imaging Figure of Merit) can form the basi s for selection of combinations of recombinant fragments and radionuclides for imaging applications. Ease of engineering and expression, combined with novel specificities that will arise from advances in genomic and combinato rial approaches to target discovery, will usher in a new era of recombinant antibodies for biological imaging.