Development of iridoid based arthropod repellents through stability assessment, chemical synthesis, and human metabolism
Creators: name: Lockhart, Anthony
Publisher: name: No Publisher Supplied
Year: 2024

description: Naturally occurring compounds derived from plants have been a cornerstone of human health for hundreds of years. Although it’s mainly known for its euphoric effects on felines, catnip (Nepeta cataria), has seen use as a botanical treatment for fever, diarrhea, insomnia, and adjustment of menstruation cycles, while also being efficacious in traditional medicine based treatments of colic, gastrointestinal, and respiratory issues. Today, the research pursuits of catnip products are focused on using its bioactive compounds for improving human health or on developing arthropod repellent products to reduce infectious diseases to humans and animals because of their iridoid compounds. These compounds are unique bicyclic monoterpene lactones found in very few species of plants. Nepetalactone (NL) is the most recognized iridoid from catnip as it is a pleasant smelling chemical which acts as both a feline attractant and arthropod repellent. Other iridoids have been unearthed in catnip, yet their relevance as bioactive analytes remain unclear. The focus of this dissertation centered on nepetalactone and the other similarly structured iridoids: nepetalic acid (NA), dihydronepetalactone (DHNL), and nepetalactam (NT) as potential arthropod repellents. Arthropods are known vectors of serious diseases such as malaria, Dengue fever, Lyme disease, Rocky Mountain spotted fever, among others. Ticks (Ixodes scapularis and Haemaphysalis longicornis) and bed bugs (Cimex lectularius) were the research organisms for these studies considering their well-documented impacts on human health and wellness. Novel catnip cultivars, CR3 and CR9, were processed into extracts and their ability to be highly repellent towards the aforementioned tick species was examined which resulted in a product that was competitive with the gold standard repellent, DEET. By removing the chemosensory Haller’s organ from the ticks, a general mechanism of action for these extracts was developed, with additional insight into their potential arachnicidal properties. While efficacious, there are documented doubts about the long-term efficacy of catnip and other phytochemically based repellents. A two-year stability study measuring the concentration of the iridoids in catnip was completed to discern if these claims are warranted. We found a distinct difference between the stability of the two identified NL isomers, Z, E-NL and E, Z-NL. The knowledge of which is impactful for those that cultivate, harvest, and process catnip materials which contain more of one isomer over another. In addition, the conversion of NL to NA was observed which substantiated anecdotal claims of degradation reactions. Dihydronepetalactone was also quantified over the same timeframe, and we found this compound to be stable as there was no decline in concentration. Catnip iridoids were isolated and synthesized to provide a direct comparison of their ability to repel arthropods. Only NA was ineffective in this endeavor, providing important structure activity relationships (SAR) for iridoid repellents. Using NL and NT as pharmacophores, a series of synthetic analogues were then designed and produced for further mechanistic exposition and to develop a novel class of compounds. Alpha-substituted analogues of NL were powerful and long-lasting (&gt;24 hours) repellents of Ixodes scapularis ticks and Cimex lectularius bed bugs that improved upon the short acting repellency of nepetalactone and were comparable to DEET. The final endeavor in this dissertation was to determine the human liver metabolites of the naturally occurring iridoids as a preliminary assessment of their ADME properties should products containing such chemicals be applied to people. Each compound was digested by enzymatic preparations and the resulting metabolites were structurally elucidated by HRMS. A total of 30 metabolites were identified through these means, some specific to CYP3A4 enzymes, and others found in full microsomal incubations.