In our research we use experimental and computational methods to understand the cellular mechanisms that give rise to drug resistance in tumors and we use engineering methods to design therapeutic strategies to overcome resistance in tumors. These goals are divided into five main projects. A synopsis of each is presented below.
To date the major advances of our group have been 1) determination of the mechanisms that control the localization of therapeutic bacteria in tumors; 2) development of therapeutic bacteria that secrete an anti-cancer protein and dramatically increase survival in mice; 3) quantification of the effects of spatial heterogeneity on tumor metabolism, cell survival, and cell cycle progression; 4) development of computation tools to analyze the interactions of therapeutics with tumors; and 5) demonstration that the properties of nanoparticles can be tuned to enhance tumor targeting.
1) Mechanisms of Bacterial Accumulation in Tumors
Our laboratory is developing motile, nonpathogenic bacteria to overcome drug resistance in tumors. Drug resistance greatly reduces the efficacy of most conventional cancer therapeutics and is a considerable cause of patient mortality. Passive drug molecules delivered in the blood have limited ability to penetrate tumor tissue and are ineffective at killing quiescent cells far from tumor vasculature. Our research is based on our theory that motile bacteria could overcome these therapeutic limitations because they can actively penetrate tumor tissue. However, the motility of bacteria must be carefully controlled in order for them to be effective therapies. Prior to our research efforts, the mechanisms that control bacterial motility in tumors were poorly understood.
We have shown that the accumulation of S. typhimurium in tumors is controlled by two mechanisms: 1) chemotaxis towards compounds produced by quiescent cancer cells and 2) preferential growth within tumor tissue. We have also shown that individual chemoreceptors target S. typhimurium to specific regions of tumors by controlling their chemotaxis towards specific tumor microenvironments. This improved understanding of the mechanisms that control Salmonella migration in tumors will enable us to develop bacterial therapies with improved targeting to therapeutically inaccessible regions of tumors.
2) Quantification of Tumor Metabolism
One of the primary research interests of our group is metabolic heterogeneity in tumors and how it affects cancer therapy. Because of diffusion limitations some regions of tumors receive limited amounts of nutrients and oxygen. These quiescent regions do not grow and are therefore unresponsive to standard therapies, which target rapidly growing cells. To better understand how oxygen availability affects cell behavior in tumors we investigated the effects of the transcription factor hypoxia-inducible-factor-1a (HIF-1a), which responds to low oxygen environments by upregulating genes for cell survival and metabolism. Our research efforts were the first to quantify the metabolic effects of HIF-1a in three-dimensional tissue.
We have shown that HIF-1a does not affect cell survival and metabolism in the center of spheroids. This discovery was surprising because HIF-1a affects many aspects of cell behavior including, survival, apoptosis, and many metabolic enzymes. We had a clue, however, that HIF-1a would not affect cell behavior in tumor tissue because preliminary experiments had shown that HIF-1a action is dependent on glucose availability. Many investigations have shown that regions of spheroids and tumors that have low oxygen concentrations also have low glucose concentrations.
Using metabolic flux analysis we determined that cell survival and intracellular metabolism were not different between wild-type and HIF-1a-null tissues. We also determined that small spheroids, which contain less quiescent cells and are less nutritionally limited, have increased carbon flux through the biosynthetic pentose phosphate and pyruvate carboxylase pathways. These results show how nutrient gradients affect cell growth and metabolism in spheroids and suggest that metabolic microenvironment should be taken into account when developing HIF-1a-based therapies.
3) Development of In Vitro Tumor Models
A connective theme that runs through all of the research in my group is metabolic heterogeneity in tumors and how it affects cancer therapy. To understand the role of metabolism and diffusion in tumors, we have created numerous methods to analyze and control the conditions in the in vitro tumor models we have developed.
4) Mathematical Model of Tumor Metabolism and Therapeutic Efficacy
The interactions of intracellular metabolism, cell growth and death, and the action of cancer therapeutics are a complicated system. Understanding this system clearly and accurately will have many ramifications on how cancer is treated. We have formulated a mathematical model that describes the dynamic interactions of the elements of this system. Our model incorporates a description of intracellular energy metabolism within reaction-diffusion equations to predict local glucose, oxygen, and lactate concentrations. It also incorporates transitions between cell-cycle phases, drug penetration, and drug pharmacokinetics. The model is based on the premise that cellular growth and death are controlled by intracellular ATP production and energy metabolism.
We have used the model to determine the extent of quiescence in tumors with different cellular characteristics and to determine the critical cell survival parameters that have the greatest impact on overall spheroid physiology. We have shown with the model that 1) oxygen transport has a greater effect than glucose transport on the distribution of quiescent cells, 2) drugs with intermediate diffusion coefficients are more effective at reducing spheroid volume than fast or slow diffusing drugs, and 3) tumors with less proliferating cells are more responsive to chemotherapeutics than tumors with more proliferating cells because of cell re-population.
5) Nanoparticles for targeted cancer therapy
Gold nanoparticles have great potential as drug delivery moieties because of their small size and the simplicity of modifying their surface chemistry. Modularity makes them excellent therapies to overcome diffusion limitations and be targeted intratumoral delivery agents. We have shown that fluorescent dye molecules can be bound to gold cores and specifically released intracellularly. We have also investigated particle transport properties in cylindroids.