Combination Therapy to Enhance the Efficacy of Recombinant Immunotoxins: Evaluating the Role of Mitochondrial Translation in Toxin‐Mediated Cell Death

Abstract

Recombinant immunotoxins (RITs) based on Pseudomonas exotoxin A (PE) are targeted cytotoxic proteins currently in clinical trials for the treatment of cancer. PE covalently modifies and inhibits elongation factor 2 (EF2), which halts protein synthesis within the cytosol and ultimately induces apoptotic cell death. Although PE inhibits translation in the cytoplasm, eukaryotes also have a translation system in mitochondria that is similar to the translation system in bacteria. Recent findings have highlighted the importance of mitochondrial stability and function in tumorigenesis, sustaining cancer cell proliferation, and maintaining cancer cell survival. By inhibiting protein synthesis within the mitochondria through the usage of antibiotics, mitochondria may become unable to properly function, resulting in the inability to meet the energy requirements of the cell and the activation of apoptotic cell death. I proposed that combination treatment with both PE‐based RITs and antibiotics that inhibit mitochondrial protein synthesis will reduce cancer cell survival compared to independent treatments of either therapeutic alone. To test this hypothesis, combination treatment of RITs and seven different antibiotics (tetracycline, chloramphenicol, streptomycin, linezolid, fusidic acid, doxycycline, and kanamycin) have been applied to CA46 and HEK293 cell lines followed by the assessment of cell viability through WST‐8 assay. No statistically relevant effects were observed for combination treatments in HEK293 cells, but the addition of a nontoxic level of chloramphenicol to RIT‐treated CA46 cells showed a ~50% enhancement in cell killing compared to RIT treatment alone (p=.05). In response, we are continuing to explore the usage of chloramphenicol in combination therapies with RIT. Though we conclude that targeting mitochondrial translation with antibiotics is not a general strategy for RIT combination therapy, combination therapy may be applicable in specific instances.Support or Funding InformationThis project was supported by funds from Towson University's Office of Undergraduate Research and the Fisher College of Science and Mathematics, as well as an undergraduate research award from the American Society for Biochemistry and Molecular Biology.