(114f) Engineered Strategies to Overcome Multidrug Resistance in Solid Tumors

Characterization of the microenvironments in tumors is an essential step in the development of therapies to overcome multidrug resistance. The heterogeneity of microenvironments within human tumors causes significant resistance to standard cancer therapies. Poor nutrient supply creates regions of quiescent cells that are resistant to most chemotherapeutics. Regions of low oxygen concentration are unresponsive to radiation therapy. Often these hard-to-treat regions overlap, further limiting effective therapy. Fortunately, the harsh microenvironments found in these regions are unique to tumors and are a distinguishing attribute that can be specifically targeted. We have created a model of the cell populations in tumors based on nutrient diffusion and intracellular metabolism diffusion to predict the therapeutic susceptibility of tumors based on individual cell physiology. We have used isotopic labeling, NMR spectroscopy, and metabolic flux analysis to quantify the intracellular mechanisms that control cell survival and death in tumors. Together these experimental and theoretical approaches have shown how cell survival is affected by nutrient depletion and hypoxia. In addition, we are developing bacterial vectors that are specifically targeted to quiescent cells and the harsher microenvironments of tumors. The lack of functional vasculature in these regions renders them difficult to target with passive blood-borne compounds. Bacteria, specifically Salmonella typhimurium, are motile and can actively penetrate tissue beyond the limits of passive diffusion. To quantify bacterial accumulation in tumors we have developed cylindroids, an in vitro culture technique that mimics the microenvironment gradients in tumors and can be observed microscopically. Current results indicate that 1) the number of quiescent cells in tumors is more dependent on oxygen than glucose availability, 2) quiescent cells in spheroids have reduced flux through the pentose phosphate pathway, pyruvate carboxylase and an increased anaerobic/aerobic ratio, 3) knocking out HIF1-α has minimal effect on cellular metabolism in three-dimensional culture, 4) oxygen sensing by HIF1-α does not promote cell survival in tumors, 5) Salmonella typhimurium are specifically attracted to compounds released by quiescent cells in tumors, and 6) both chemotaxis and preferential growth are critical for bacterial accumulation in tumors. The results from this work will enable the generation of diagnostic tools that can predict the effectiveness of current cancer therapies based on standard imaging techniques. By better characterizing the microenvironments of tumors we will develop specifically targeted therapeutics, including bacterial vectors, that can overcome multidrug resistance.