An scFv has been engineered to bind carcinoembryonic antigen (CEA) with a dissociation half-time >4 days at 37 degrees C. Two mutations responsible for this affinity increase were isolated by screening yeast surface-displayed mutant libraries by flow cytometry. Soluble expression of the mutant scFv in a yeast secretion system was increased 100-fold by screening mutant libraries for improved yeast surface display level. This scFv will be useful as a limiting case for evaluating the significance of affinity in tumor targeting to non-internalizing antigens.

A nonimmune library of 10(9) human antibody scFv fragments has been cloned and expressed on the surface of yeast, and nanomolar-affinity scFvs routinely obtained by magnetic bead screening and flow-cytometric sorting. The yeast library can be amplified 10(10)-fold without measurable loss of clonal diversity, allowing its effectively indefinite expansion. The expression, stability, and antigen-binding properties of >50 isolated scFv clones were assessed directly on the yeast cell surface by immunofluorescent labeling and flow cytometry, obviating separate subcloning, expression, and purification steps and thereby expediting the isolation of novel affinity reagents. The ability to use multiplex library screening demonstrates the usefulness of this approach for high-throughput antibody isolation for proteomics applications.

The interplay among antibody/antigen binding kinetics, antibody diffusion, and antigen metabolic turnover together determines the depth of penetration of antitumor antibodies into prevascular tumor spheroid cell clumps. A sharp boundary between an outer shell of bound high-affinity antibody and an inner antibody-free core has been previously observed and mathematically modeled and was termed the "binding site barrier." We show here that this process is well described by a simplified shrinking core model wherein binding equilibration is much more rapid than diffusion. This analysis provides the following experimentally testable predictions: (a) the binding site barrier is a moving boundary whose velocity is proportional to the time integral of antibody concentration at the spheroid surface (i.e. plasma antibody AUC); (b) the velocity of this moving boundary is independent of binding affinity, if the affinity is sufficiently high to strongly favor antibody/antigen complex formation at prevailing antibody concentrations; and (c) maximum tumor retention is achieved when the antibody/antigen dissociation rate approaches the rate of antigen metabolic turnover. The consistency of these predictions with published experimental results is demonstrated. The shrinking core model provides a simple analytic relationship predicting the effects of altered antibody pharmacokinetics, antibody molecular weight, antigen turnover rate, antigen expression level, and micrometastasis size on antibody penetration and retention. For example, a formula is provided for predicting the bolus dose necessary to accomplish tumor saturation as a function of antibody and tumor properties. Furthermore, this analysis indicates certain attributes necessary for an optimal tumor targeting agent.