Quantitative analysis of perivascular antibody distribution in solid tumors

Abstract

Monoclonal antibodies and proteins derived from them are an emerging class of anticancer therapeutics that have shown efficacy in a range of blood and solid tumors. Antibodies targeting solid tumors face considerable transport barriers in vivo, including blood clearance, extravasation, diffusion within the tumor interstitium, binding to antigen, endocytosis, and degradation. The unique pathology of the blood supply to solid tumors only serves to exacerbate these problems. A consequence of poor delivery of antibodies to solid tumors is a characteristic perivascular distribution of antibodies around tumor blood vessels. Often, antibodies bind only cells within a few cell layers of blood vessels, leaving large areas of tumor cells farther from perfused vessels completely untargeted. This phenomenon has been observed in multiple studies involving different antibodies, antigens, and tumor types, both in animal models and in clinical settings. In this thesis, the perivascular localization of antibodies is explored as a function of quantitative parameters of the antibody and associated antigen. A novel experimental system to quantitatively determine bound antibody levels, antigen levels, and blood vessel localization on a microscopic scale throughout entire tumor cross sections has been developed. This system has been used to quantitatively measure antibody and antigen distribution in tumor tissue under a variety of conditions. Effects of varying antibody dose, antibody affinity, and tumor type and site have been explored and quantitated using this model. To guide experimental design, we have developed a simplified mathematical model of the tumor vasculature. This model offers insights into the effects of antigen and antibody parameters, including dose, affinity, antigen density, and endocytosis rates, which are measurable in vivo and affect antibody penetration into tumor tissue. A simple scaling analysis further allows the quantitative determination of the minimum antibody dose required to saturate a tumor given the antigen turnover rate and density. Together, the mathematical model and quantitative experimental analysis allow conclusions to be made regarding antibody design and antigen selection for improved tumor penetration of therapeutic antibodies.