Vaccines and Therapeutics for Anthrax

National Institute Of Allergy And Infectious Diseases

Investigators

Linked publications, trials & patents

Abstract

Mutations that activate the MEK proteins are a frequent cause of the uncontrolled growth that defines malignancy, so the MEK pathways have long been a target of drug development. While FDA-approved small molecule MEK inhibitors have demonstrated some level of efficacy in patients with metastatic melanomas, these "targeted" therapeutics typically have a very low therapeutic index since these agents affect normal cells, causing undesirable, even fatal, side effects. The lethal factor (LF) component of anthrax toxin is a protease which selectively cleaves members of the MEK family of kinases. For this reason, LF was early on recognized as an alternative, potentially useful MEK inhibitor having therapeutic potential. Our continuing work has sought to add additional factors to increase the specificity of LF-based agents for use in tumor treatment. During FY2022, we reported on the creation of an agent that combines three separate features that synergistically combine to achieve very high specificity. These features are (1) introducing sequences requiring proteolytic activation by both of two tumor-associated cell surface proteases, urokinase plasminogen activator and matrix metalloproteases, (2) altering the protective antigen (PA) delivery protein to have specific binding to the preferred toxin receptor CMG2 and not to the alternative receptor TEM8, and (3) altering the LF protein to limit its activity to MEKs in the ERK pathway. Each of these features independently decreases off-target effects, so the resulting agent was shown to have an extremely high therapeutic index, >15, in several mouse tumor models. As seen previously, these agents were active on many different solid tumors in mice. Together, these data show that engineered bacterial toxins can be modified to have significant in vitro and in vivo therapeutic effects with high therapeutic index. In other work reported during FY2022, a separate group of collaborators presented another demonstration of the potential of anthrax-toxin based agents in cancer therapy, in this case for treatment of ovarian cancer. Treatments for advanced and recurrent ovarian cancer remain a challenge due to a lack of potent, selective, and effective therapeutics. A new approach to this challenge implemented the concept of making the toxic agent dependent on activation by the catalytic power of zymogen-activating proteases on the surface of malignant tumor cells. Activation of a class of membrane-anchored serine proteases (MASPs) is correlated with advanced stages of ovarian cancer. Proteases in this group include prostasin and testisin. As in the work described above, the PA protein was altered to contain a sequence cleaved by prostatin. When combined with an LF-based toxic effector, this agent was highly cytotoxic to ovarian tumor cell lines and ovarian tumor spheroids derived from patient ascites. Preclinical studies demonstrate that toxin treatment induced tumor regression in several in vivo ovarian cancer models, including patient-derived xenografts, without adverse side effects, supportive of progression toward clinical evaluation. These data lay the groundwork for developing therapeutics for treating women with late-stage and recurrent ovarian cancers, utilizing a mechanism distinct from current anticancer therapies.