Potent and tumor specific: arming bacteria with therapeutic proteins

Abstract

Bacteria are perfect vessels for targeted cancer therapy. Conventional chemotherapy is limited by passive diffusion, and systemic administration causes severe side effects. Bacteria can overcome these obstacles by delivering therapeutic proteins specifically to tumors. Bacteria have been modified to produce proteins that directly kill cells, induce apoptosis via signaling pathways, and stimulate the immune system. These three modes of bacterial treatment have all been shown to reduce tumor growth in animal models. Bacteria have also been designed to convert nontoxic prodrugs to active therapeutic compounds. The ease of genetic manipulation enables creation of arrays of bacteria that release many new protein drugs. This versatility will allow targeting of multiple cancer pathways and will establish a platform for individualized cancer medicine.

Figure 1. Modes of bacterial protein delivery

Bacteria can produce and release protein drugs (A–C) and convert prodrugs into active compounds (D). (A) Bacterial toxins (orange hexagons) can be expressed to kill cancer cells (blue). (B) Bacteria can produce eukaryotic proteins (yellow diamonds) that interact with receptors on the surface of cancer cells. These proteins often target cell signaling pathways that induce apoptosis. (C) Bacteria can also produce cytokines (pink circles) that attract granulocytes and dendritic cells (red amorphous). These cells use dead tumor tissue for antigen uptake and induce the innate and adaptive immune systems to kill cancer cells, often by phagocytosis. (D) Bacteria can express enzymatic catalysts (yellow) that convert nontoxic prodrugs (orange stars with purple squares) into toxic therapeutics (orange stars). BDEPT: Bacterial-directed enzyme prodrug therapy. For color images please see online www.future-science.com/doi/full/10.4155/TDE.14.113

Figure 2. Amplification system based on cell-cell communication

The system is based on the quorum-sensing machinery of Vibrio fischeri. Communication is initiated by addition of an external molecule, l-arabinose (green squares), which triggers production of an intercellular autoinducer molecule, AI-1 (blue triangles). In this system, transcription of luxI is controlled by the P_BAD promoter. Arabinose addition induces luxI transcription, which enzymatically produces AI-1. The rise in AI-1 concentration activates P_luxI which produces LuxR (orange boxes) and the desired protein drug (orange circle). Diffusion of AI-1 into inactivated bacteria activates P_luxI thus communicating the activation with surrounding bacteria. For color images please see online www.future-science.com/doi/full/10.4155/TDE.14.113

Figure 3. Bacterial membrane vesicles

(A) Bacterial ghosts are cell membrane shells that can be used to delivery toxic compounds to tumors. The membrane vesicle is created by controlled expression of a phage lysis gene (here lysis protein E; blue ovals). Induction of the lysis gene creates pores in the bacterial membrane and releases the cytoplasm. The remaining membrane vesicle is loaded with doxycycline (brown ovals) and administered locally to cancer cells, where the membrane releases the drug. (B) Salmonella with a minCDE-chromosomal deletion generate 400 nm bacterial minicells. Bicistronic antibodies that recognize bacterial membrane (blue) and mammalian cell membrane (red) target bacterial minicells to cancer cells. Via endocytosis, minicells are taken up by cancer cells and the drug cargo is released in the cell cytoplasm. (C) Bacterial OMVs are discharged from Gram-negative bacteria (20–250 nm). Affibodies, expressed on the bacterial membrane, are enriched in OMVs and target the vesicles to cancer cells. Endocytosis of OMVs leads to release of the vesicle content into the cancer cell cytoplasm which results in cell death. OMV: Outer membrane vesicle. For color images please see online www.future-science.com/doi/full/10.4155/TDE.14.113