Evaluation of topoisomers of snake venom-derived peptides as potential antimicrobial and antitumor agents

This thesis reports an investigation of the effect of topoisomeric modification on the activity of two peptides: crotalicidin (Ctn), a cathelicidin from the venom of a South American rattlesnake, and Ctn[15-34], a Ctn segment with interesting antimicrobial and antitumoral properties, as well as excellent serum stability. Topoisomeric refers to composition-identical (hence isomeric) yet three-dimensionally distinct (hence topo-) variants of a biomolecule where the structural maneuvers bring about potentially useful physiological effects. For peptides, the topoisomer term is often reserved to describe all-D-amino acid versions with sequences identical [i.e., enantio (e) form] or fully reversed [i.e., retroenantio (re)] relative to the native primary structure. To comprehensively assess the properties and activities of Ctn and Ctn[15-34] e and re topoisomers, I have compared them with both the parent peptide and its sequence-inverted variant [retro (r) form, also treated as a topoisomer]. While all topoisomers were active against gram-negative bacteria and tumor cells, those featuring D-amino acids were more cytotoxic than their L- counterparts towards normal cells. Even so, the higher stability of the e and re versions in human fluids prompted further exploration. Thus Ctn[15-34], Ctn re and Ctn[15-34] re were selected for an in vivo efficacy evaluation of their anti-infective potential in a murine model of Acinetobacter baumannii systemic infection. Following an in vivo toxicology study in CD1 mice, Ctn[15-34] re was deemed too toxic at the examined doses and excluded. For the remaining two topoisomers, Ctn[15-34] and Ctn re, a single intraperitoneal dose did not ensure mice survival. For Ctn[15-34] in particular, reasonably good results warranted further investigation despite some animals' deaths. Unfortunately, a three-day regimen of Ctn[15-34] intraperitoneal injections was lethal for all animals after the first day. Based on these results, this thesis is a sobering reminder that topoisomeric modification of bioactive peptides, which has proven quite succesful in several other cases, cannot be taken for granted to provide viable antimicrobial leads and must be therefore pursued with caution until confirmed by in vivo evidence. Aside from the unexpectedly adverse outcomes, the thesis illustrates a cogent approach for evaluating antimicrobial peptide topoisomeric leads from bench to in vivo infection model stages.