Larissa Wenning, PhD 1997

Thesis Title: Quantitative analysis of immunotoxin effect on tumor spheroids

Immunotoxins have the potential to be powerful tools for selective cell killing. Made by linking an extremely lethal toxin to a tumor-specific targeting agent, immunotoxins can overcome non-specific toxicity problems encountered with traditional cancer chemotherapies and radiation treatment. This thesis describes the development of an experimentally verified mathematical model capable of predicting immunotoxin effect on a solid tumor-like structure, and use of this model to determine which molecular, cellular, or tissue properties are most important in determining immunotoxin efficacy in solid tumors. A mathematical model linking cellular processing of anti-transferrin receptor immunotoxins made with the diverse toxins gelonin and CRM107 with cytotoxicity on the level of a single cell was first developed and experimentally verified. This single-cell cytotoxicity model was combined with a model describing immunotoxin diffusion in spheres. The resulting model was successfully used to predict protein synthesis inhibition kinetics of anti-transferrin receptor immunotoxins made with both gelonin and CRM107 in multicell tumor spheroids, which are similar to small solid tumors. The experimentally verified multicell tumor spheroid cytotoxicity model was used to explore the conditions under which immunotoxin fails to penetrate to the center of a spheroid, resulting in attenuation of toxicity in spheroids as compared to monolayers. A competition between diffusion and cellular processing, especially binding, under some conditions results in segregation of immunotoxin to the outer layer of a spheroid. There were found to be optimum values of affinity, receptor density, and, under some conditions, internalization and recycling rate constants for immunotoxins in spheroids. Receptor saturation was also found to be an important factor in determining the magnitude of penetration barriers, with barriers increasing at concentrations farther from saturation. Understanding the interactions between diffusion and processing which retard immunotoxin penetration and attenuate toxicity will thus become increasingly important as immunotoxins are made ever more effective. The multicell tumor spheroid cytotoxicity model developed in this thesis provides a useful framework for exploring these interactions, and is a valuable tool for immunotoxin design.