Tumor Targeting and Drug Delivery by Anthrax Toxin.

Christopher Bachran,Stephen Leppla

doi:10.3390/toxins8070197

Abstract

Anthrax toxin is a potent tripartite protein toxin from Bacillus anthracis. It is one of the two virulence factors and causes the disease anthrax. The receptor-binding component of the toxin, protective antigen, needs to be cleaved by furin-like proteases to be activated and to deliver the enzymatic moieties lethal factor and edema factor to the cytosol of cells. Alteration of the protease cleavage site allows the activation of the toxin selectively in response to the presence of tumor-associated proteases. This initial idea of re-targeting anthrax toxin to tumor cells was further elaborated in recent years and resulted in the design of many modifications of anthrax toxin, which resulted in successful tumor therapy in animal models. These modifications include the combination of different toxin variants that require activation by two different tumor-associated proteases for increased specificity of toxin activation. The anthrax toxin system has proved to be a versatile system for drug delivery of several enzymatic moieties into cells. This highly efficient delivery system has recently been further modified by introducing ubiquitin as a cytosolic cleavage site into lethal factor fusion proteins. This review article describes the latest developments in this field of tumor targeting and drug delivery.

Highlights

Bacillus anthracis is a Gram-positive, endospore-forming, rod-shaped bacterium which is the causative agent of the disease anthrax
protective antigen antigen (PA), lethal factor (LF), and edema factor (EF) have been studied in great detail and this has resulted in detailed knowledge of their molecular mechanisms of action
In addition to the well-studied and well-described PEIII, some other protein toxins have been used in combination with LFn to be delivered to tumor cells. Both LFn-DTA and LFn-ricin toxin A chain were combined with a mutant PA unable to bind to CMG2 or TEM8, and fused to an affibody against human HER2 for targeting of HER2-positive human cancer cell lines [78]

Summary

Introduction

Bacillus anthracis is a Gram-positive, endospore-forming, rod-shaped bacterium which is the causative agent of the disease anthrax. The protease sensitivity was changed for urokinase plasminogen activator (uPa)-activation [39] These PA variants greatly increased the specificity for certain tumor cells lines and allowed for subsequent preclinical testing of modified anthrax toxin in the context of tumor therapies. A further improvement of specificity of modified anthrax toxin was achieved by designing PA variants that required simultaneous activation by both uPa and MMPs to form functional oligomers [42]. The details on these modified anthrax toxins are reviewed in detail by other authors in this volume and we refer you to the review by Liu et al for the details on protease-specific activation of anthrax toxin This idea of increased target specificity was further improved by engineering PA variants that can only form octamers after activation by both of the tumor-selective proteases, uPa and MMPs and, achieved a safe dual-activity dependent delivery system [43]. It should be possible to use the gained knowledge on PA octamers and protease-specific activation to design safer variants of tumor-specific AT for future preclinical and clinical testing

Cellular Delivery of Fusion Proteins

Retargeting of PA

Conclusions

Findings

Anthrax can Figure