A Tale of Two Poisons: Overcoming deleterious anticancer side effects and antibacterial resistance
Oviatt, Alexandria Argelia
The type II topoisomerases are the targets of both anticancer and antibacterial therapies. These enzymes perform critical actions in the cell, including unwinding, unknotting, and untangling the genome, through the introduction of a transient break in a segment of DNA. Many anticancer and antibacterial drugs act as “poisons,” stabilizing DNA breaks mediated by the enzymes and overwhelming DNA repair pathways to kill cells. Unfortunately, the field of chemotherapy is plagued by off-target effects that kill “healthy” cells, while rising rates of antimicrobial resistance are threatening the clinical use of all antibacterial classes, including fluoroquinolones, which target the bacterial type II topoisomerases. The first part of this dissertation addresses the need for more targeted chemotherapy. Etoposide is a successful anticancer therapeutic; however, it suffers from cardiotoxicity and secondary leukemogenesis. Replacement of the sugar substituent at C4 of etoposide with a polyamine allows preferential uptake of the compound by cells expressing the polyamine transport system, including many cancer types. The effects of six different etoposide-polyamine derivatives on human topoisomerase IIα and IIβ were examined. All the compounds increased enzyme-mediated DNA cleavage to levels comparable to or greater than that of the parent drug etoposide. The compounds were more active against the β isoform. Modeling studies suggest this specificity is a result of stabilizing drug-protein interactions with a glutamine residue in human topoisomerase IIβ that corresponds to a methionine in topoisomerase IIα. The second part of this dissertation addresses fluoroquinolone resistance. The novel bacterial topoisomerase inhibitors (NBTIs) are a promising new class of antibacterials that also target the bacterial type II topoisomerases gyrase and topoisomerase IV. However, they interact with the proteins differently than fluoroquinolones. The examination of two NBTIs, GSK126 and gepotidacin, revealed that these compounds enhance enzyme-mediated single-stranded DNA cleavage and inhibit overall enzymatic activity of gram-positive and gram-negative gyrase and topoisomerase IV. Additionally, the compounds can overcome some fluoroquinolone resistance mutations. Importantly, this work has confirmed that NBTI resistance mutations in Escherichia coli confer resistance at the enzyme level. Finally, this work makes clear that NBTI actions differ across different compound and species combinations.