Engineering the perfect (bacterial) cancer therapy

Abstract

Bacterial therapies possess many unique mechanisms for treating cancer that are unachievable with standard methods. Bacteria can specifically target tumours, actively penetrate tissue, are easily detected and can controllably induce cytotoxicity. Over the past decade, Salmonella, Clostridium and other genera have been shown to control tumour growth and promote survival in animal models. In this Innovation article I propose that synthetic biology techniques can be used to solve many of the key challenges that are associated with bacterial therapies, such as toxicity, stability and efficiency, and can be used to tune their beneficial features, allowing the engineering of 'perfect' cancer therapies.

Figure 1. Bacteria are the optimal robot factory cancer therapies

A) The perfect cancer therapy would be able to perform six important functions: target tumors, produce cytotoxic molecules, self-propel, respond to triggering signals, sense the local environment and produce externally detectable signals. B) Bacteria have biological mechanisms to perform these functions: gene translation machinery to produce anticancer proteins (green); flagella to chemotax,;specific gene promoter regions to respond to molecular signals (purple cubes);chemotaxis receptors (orange);and 5) machinery to produce detectable molecules (red). C) The number of papers describing bacterial anti-cancer therapies has grown exponentially (black line) since the mid-1990s.

Figure 2. The transport properties of bacterial therapies produce preferable drug concentration profiles

When injected systemically, bacteria (red syringe, green organisms), specifically accumulate in tumors and migrate to distal regions far from vasculature (brown cells). These distal regions are typically hypoxic and hypoglycemic and contain quiescent and necrotic cells. Once triggered (small red arrows), bacteria begin to produce therapeutic molecules (red ovoids) that diffuse (large red arrows) into viable tissue (clear cells). Systemically injected (small blue arrows), passive chemotherapeutic molecules (blue cubes) diffuse into tumor tissue from blood vessels (large blue arrows). The concentration of bacterially produced molecules (red lines) is greatest in distal tumor regions and would remain constant as long as expression of these proteins continues. The concentration of chemotherapeutic molecules is greatest in systemic blood and drops as it is cleared by the liver or kidneys. Based on these profiles, bacterially produced molecules will be more cytotoxic (dotted line) in the distal regions of tumors and less systemically toxic. The profile of passive molecules is less favorable, with more systemic toxicity and less efficacy deep in tissue.

Figure 3. Gene triggering systems

A) The pBAD system, which responds to extracellular l-arabinose, contains two components: the arabinose sensitive protein AraC and the pBAD promoter. Constitutively expressed regulator AraC induces transcription by binding to the pBAD promoter. AraC is a positive and negative regulator of pBAD: it activates transcription in the presence of arabinose and represses transcription in its absence. B) The salicylate cascade system utilized a two salicylate-sensitive regulator proteins, nahR and xylS2 to maintain tight regulation. In the presence of salicylate, nahR activates transcription from the promoter Psal, leading to the expression of XylS2. XylS2, which is also sensitive to salicylate, activates transcription from the promoter PmC) The RecA system senses γ-irradiation, which causes DNA damage. This damage activates RecA, which induces autoproteolysis of LexA. Transcription is induced when LexA, a repressor of the recA promoter, releases from DNA. Feed-forward regulation increases the RecA concentration when the system is active. D) The FNR system turns on in hypoxic environments. The absence of oxygen promotes dimerization of FNR, which induces transcription. Multiple promoters bind FNR, including FF+20*, HIP-1, pflE and ansB.