Bacterial therapies: completing the cancer treatment toolbox

Abstract

Current cancer therapies have limited efficacy because they are highly toxic, ineffectively target tumors, and poorly penetrate tumor tissue. Engineered bacteria have the unique potential to overcome these limitations by actively targeting all tumor regions and delivering therapeutic payloads. Examples of transport mechanisms include specific chemotaxis, preferred growth, and hypoxic germination. Deleting the ribose/galactose chemoreceptor has been shown to cause bacterial accumulation in therapeutically resistant tumor regions. Recent advances in engineered therapeutic delivery include temporal control of cytotoxin release, enzymatic activation of pro-drugs, and secretion of physiologically active biomolecules. Bacteria have been engineered to express tumor-necrosis-factor-alpha, hypoxia-inducible-factor-1-alpha antibodies, interleukin-2, and cytosine deaminase. Combining these emerging targeting and therapeutic delivery mechanisms will yield a complete treatment toolbox and increase patient survival.

Figure 1. Tumor targeting mechanisms of obligate and facultative anaerobic bacteria

The three modes of tumor targeting are specific chemotaxis, preferential growth, and hypoxic germination. Top) Motile facultative anaerobes direct their movements by chemotaxis, , sense nutrients or chemicals (blue ellipses and purple squares) with specific chemoreceptors (complementary y-shapes), extravasate from blood vessels (red regions), and target the tumor compartments secreting these compounds (quiescent pink region and necrotic tan region). Middle) Facultative anaerobes, Salmonella and Escherichia, are administered in active/motile form, migrate inside tumors and recognize specific regions as favorable places to proliferate. Preferential mitosis is indicated by splitting cells. Bottom) Intravenously injected spores (stars) of obligate anaerobes, such as Clostridium, target avascular/necrotic regions (tan) in tumors by spore diffusion and germination in only the hypoxic environment.

Figure 2. Microenvironment Gradients Present in Tumors

Multiple microenvironments form in tumors because of concentration gradients around sparse blood vessels. Nutrients and oxygen extravasate from the blood lumen (red region), through the endothelial vessel lining (purple), and diffuse into the interstitial tissue. Close to vessels, cells are viable and proliferating. Far from vessels, hypoxia and nutrient depletion create regions of necrosis. Large inter-vessel distances also reduce the concentration of blood-borne chemotherapeutics in distal tissue.

Figure 3. Mechanisms of Controlled Therapeutic Delivery

Three mechanisms used to control therapeutic delivery include temporally controlled cytotoxin release, enzyme drug activation, and biomolecule secretion. Top) Bacteria located within a radiation field (yellow curves) express and secrete (dashed arrows) a cytotoxic compound (purple shapes), while non-irradiated bacteria do not, illustrating how irradiation can be used to spatially and temporally control delivery. Middle) Bacteria expressing (dashed arrows) an enzyme gene (dark blue shapes) are used for drug activation. The pro-drugs (light blue diamonds) enter tumors (tan) via the bloodstream (red region) and are subsequently converted to the active drug (purple triangles). Bottom) Many biologically active compounds (blue squares), including antibodies (blue “Y” figures) and DNA (black curves) can be engineered for bacterial expression and secretion (dashed arrows).