Application of chemical engineering principles to the study of tumor formation and treatment is fertile new ground for research and is necessary for the advancement of cancer therapy. Over the last century, researchers have discovered many of the genetic causes of cancer and yet nearly 46,000 people will die this year from cancer in the United States alone. Standard cancer therapies often fail because of spatial heterogeneity of nutrients, wastes, and therapeutics. Transport barriers prevent therapeutic agents from reaching effective concentrations throughout tumors. Models based on mass balance, transport phenomena and reaction kinetics are powerful tools able clarify the connection between the genetic aberrations and the compositional heterogeneity of tumors. Therapeutic strategies designed using engineering principles will be able to overcome transport barriers and create more effective therapies.
Single treatments of radiation or chemotherapy often do not kill all cancer cells within a tumor. Between treatments, surviving cells proliferate and regrow as tumors and metastases, eventually leading to death. Chaotic and irregular tumor vasculature and high interstitial pressure prevent blood-borne chemotherapeutics from diffusing equally throughout all tumor regions. Additionally, non-proliferating cells, distant from vasculature, survive chemotherapeutic therapies specifically targeted to proliferating cells. Radiation therapy is also less effective in the low oxygen regions distant from vasculature because it depends on the formation of oxygen radicals. Furthermore, low oxygen environments have been shown to select for more aggressive and metastatic cells.
In my laboratory we characterize and utilize tumor heterogeneity to develop novel cancer therapies that overcome the limitations of current cancer therapies. Unique three-dimensional tumor models are designed to mimic the metabolic variations observed in tumors in vivo. The metabolic profiles of the tumor models, both in vitro and in vivo, are characterized using fluorescence microscopy, nuclear magnetic resonance spectroscopy and metabolic flux analysis.
Targeted Bacteriolytic Therapy
To specifically target tumors we are investigating motile, facultative anaerobic bacteria that specifically target and accumulate within the therapeutically inaccessible regions of tumors. Over the past 50 years numerous strains of bacteria have been shown to localize and cause lysis in transplanted mouse tumors, but their application has had minimal success in the clinic. By specifically targeting bacteria to specific sub-regions of tumors we hope to dramatically increase their affectivity.
Bacteria selected by these strategies have numerous uses. Administered alone, selected bacteria will compete with the tumor for nutrients, killing cells in inaccessible tumor regions. A combination of bacteriolytic therapy and standard anti-proliferative chemotherapy, which selectively kills proliferating cells growing close to vasculature, will attack tumors from both the inside and out. Additionally, selected bacteria could deliver therapeutic agents or amplification agents (e.g. toxins, prodrug cleaving enzymes or anti-angiogenic factors), or could express markers detectable by MRI, PET, or another imaging device.
Localized Quantification of Tumor Metabolism
In addition to diffusion, cells with different metabolic profiles cause component gradients and distinct regions of proliferation in tumors. Most people over 50 have pre-malignant lesions throughout their breasts and prostates. These lesions are kept small (<1mm) by a balance of cell proliferation and death. Cells at the centers die because of nutrient depletion and waste toxicity. Once the lesions develop vasculature, proliferation exceeds death and the tumor grows. Thus, both metabolism and diffusion play key roles in the early formation of tumors.
In my laboratory we quantify the metabolic state of different tumor regions in order to map nutrient gradients and explain mechanistically why cell growth diminishes away from vasculature. Metabolic flux analysis determines the flow of carbon through the pathways of primary and secondary metabolism using nuclear magnetic resonance spectra of extracted cellular cytoplasm. Whole cell models of metabolism are capable of i) quantifying metabolite transport across biological membranes, ii) quantifying changes in enzyme activity due to extracellular signals and iii) detect inactive pathways. In addition to explaining cancer cell proliferation, metabolic flux analysis will identify enzymatic targets for inhibitors of metabolism and proliferation in the different tumor regions.
Bacterial Migration and Segregation in Solid Tumors
In collaboration with Baystate Medical Center we are investigating how bacteria segregate in tumors. The ability of bacteria to migrate within a tumor defines their usefulness for drug delivery. We use surgical, histological and standard staining techniques to measure the rate of bacterial spread throughout subcutaneous tumors in mice following systemic injection. These experiments were designed to determine the mechanism of specific bacterial localization in tumors.