Engineering antibodies for improved targeting of solid tumors

Abstract

Monoclonal antibodies have emerged as an important class of cancer therapeutics due to their ability to specifically bind tumor-expressed antigens. Unfortunately, attempts to treat solid tumors with these drugs are often limited by an inability of the antibodies to fully penetrate the tumor tissue, leaving large regions of untargeted and viable cells. The goal of this thesis is to understand the transport phenomena that contribute to poor antibody distribution in tumors, and engineer novel antibody variants with improved targeting properties. Previous studies identified a core set of parameters that impact tumor uptake including antibody size, binding affinity, plasma clearance rate, and cellular catabolism. Here we probe each of these parameters and its effect on tumor penetration using a combination of computational modeling and protein engineering. In the first part of this thesis, we characterize the cellular internalization kinetics of a series of anti-carcinoembryonic antigen (CEA) antibodies and antibody fragments. We demonstrate that internalization is independent of antibody affinity, stability, and valency, and that the measured rates can be used to mathematically predict antibody penetration distance in tumor spheroids. Next, we examine the effect of antibody size and affinity by developing a computational model of in vivo tumor targeting that incorporates size-dependent trends for capillary permeability, interstitial diffusion, available volume fraction, and plasma clearance. The model predicts that intermediate size antibody fragments (MW ~30 kDa) have the lowest tumor uptake with greater accumulation of small and large proteins. To probe size effects experimentally, we engineered a novel 79 kDa ds(Fv)-Fc antibody fragment that is approximately half the size of an IgG but retains its binding and Fc salvage activity. In mice, the ds(Fv)-Fc fragments are cleared from the plasma more rapidly than IgGs but have similar tumor uptake levels at 24 hours, likely due to higher capillary permeability. In the last section, we develop a series of matrix metalloproteinase (MMP) activatable antibody fragments that bind their target antigen up to 300 times faster following cleavage by the tumor expressed protease MMP-2. We believe that MMP dependent binding should prevent targeting of antigen depots in healthy tissues and further improve tumor specificity.