Structure-Activity Relationship Analysis of Potential New Vapor-Active Insect Repellents

Repellent activity of essential oils to the Lone Star tick, Amblyomma americanum

BACKGROUND

The Lone Star tick, Amblyomma americanum is important to human health because of a variety of pathogenic organisms transmitted to humans during feeding events, which underscores the need to identify novel approaches to prevent tick bites. Thus, the goal of this study was to test natural and synthetic molecules for repellent activity against ticks in spatial, contact and human fingertip bioassays.

METHODS

The efficacy of essential oils and naturally derived compounds as repellents to Am. americanum nymphs was compared in three different bioassays: contact, spatial and fingertip repellent bioassays.

RESULTS

Concentration response curves after contact exposure to 1R-trans-chrysanthemic acid (TCA) indicated a 5.6 μg/cm2 concentration required to repel 50% of ticks (RC50), which was five- and sevenfold more active than DEET and nootkatone, respectively. For contact repellency, the rank order of repellency at 50 μg/cm2 for natural oils was clove > geranium > oregano > cedarwood > thyme > amyris > patchouli > citronella > juniper berry > peppermint > cassia. For spatial bioassays, TCA was approximately twofold more active than DEET and nootkatone at 50 μg/cm2 but was not significantly different at 10 μg/cm2. In spatial assays, thyme and cassia were the most active compounds tested with 100% and 80% ticks repelled within 15 min of exposure respectively and was approximately twofold more effective than DEET at the same concentration. To translate these non-host assays to efficacy when used on the human host, we quantified repellency using a finger-climbing assay. TCA, nootkatone and DEET were equally effective in the fingertip assay, and patchouli oil was the only natural oil that significantly repelled ticks.

CONCLUSIONS

The differences in repellent potency based on the assay type suggests that the ability to discover active tick repellents suitable for development may be more complicated than with other arthropod species; furthermore, the field delivery mechanism must be considered early in development to ensure translation to field efficacy. TCA, which is naturally derived, is a promising candidate for a tick repellent that has comparable repellency to commercialized tick repellents.

Repellency and Toxicity of Vapor-Active Benzaldehydes against Aedes aegypti

Chemical screening efforts recently found that 3-phenoxybenzaldehyde, a breakdown product of alpha-cyano pyrethroids, was a potent spatial repellent against Aedes aegypti mosquitoes in a glass tube repellency assay. In order to characterize this molecule further and identify structure-activity relationships, a set of 12 benzaldehyde analogues were screened for their repellency and toxicity in vapor phase exposures at 100 μg/cm2. Dose-response analyses were performed for the most active compounds in order to better characterize their repellent potency and toxicity compared to those of other commercially available toxicants. The three most toxic compounds (LC50 values) were 3-chlorobenzaldehyde (CBA) (37 μg/cm2), biphenyl-3-carboxaldehyde (BCA) (48 μg/cm2), and 3-vinylbenzaldehyde (66 μg/cm2), which makes them less toxic than bioallethrin (6.1 μg/cm2) but more toxic than sandalwood oil (77 μg/cm2), a repellent/toxic plant essential oil. The most repellent analogues with EC50 values below 30 μg/cm2 were 3-phenoxybenzaldehyde (6.3 μg/cm2), isophthalaldehyde (23 μg/cm2), BCA (17 μg/cm2), and CBA (22 μg/cm2), which makes them about as active as N,N-diethyl-3-methylbenzamide (25.4 μg/cm2). We further investigated the activity of a select group of these benzaldehydes to block the firing of the central nervous system of A. aegypti larvae. Compounds most capable of repelling and killing mosquitoes in the vapor phase were also those most capable of blocking nerve firing in the larval mosquito nervous system. The results demonstrate that benzaldehyde analogues are viable candidate repellent and insecticidal molecules and may lead to the development of future repellent and vapor toxic vector control tools.

Structural Exploration of Novel Pyrethroid Esters and Amides for Repellent and Insecticidal Activity against Mosquitoes

The emergence of pyrethroid-resistant mosquitoes is a worldwide problem that necessitates further research into the development of new repellents and insecticides. This study explored the modification of existing pyrethroid acids to identify structural motifs that might not be affected by kdr active site mutations that elicit pyrethroid resistance. Because synthetic pyrethroids almost always contain activity-dependent chiral centers, we chose to focus our efforts on exploring alkoxy moieties of esters obtained with 1R-trans-permethrinic and related acids, which we showed in previous studies to have repellent and/or repellent synergistic properties. To this end, compounds were synthesized and screened for spatially acting repellency and insecticidal activity against the susceptible, Orlando, and pyrethroid-resistant, Puerto Rico, strains of Aedes aegypti mosquito. Screening utilized a high-throughput benchtop glass tube assay, and the compounds screened included a mixture of branched, unbranched, aliphatic, halogenated, cyclic, non-cyclic, and heteroatom-containing esters. Structure-activity relationships indicate that n-propyl, n-butyl, n-pentyl, cyclobutyl, and cyclopentyl substituents exhibited the most promising repellent activity with minimal kdr cross resistance. Preliminary testing showed that these small alcohol esters can be synergistic with phenyl amides and pyrethroid acids. Further derivatization of pyrethroid acids offer an interesting route to future active compounds, and while mosquitoes were the focus of this work, pyrethroid acids and esters have potential for use in reducing pest populations and damage in cropping systems as well.

Resistance-Breaking Insecticidal Activity of New Spatial Insecticides against Aedes aegypti

The use of N-aryl amide derivatives as spatially acting insecticides remains relatively unexplored. To expand this knowledge, we synthesized eighty-nine N-aryl amide analogues and screened them for mortality against an insecticide-susceptible strain of Aedes aegypti mosquitoes, Orlando (OR), using a vapor exposure glass tube assay. Of the screened compounds, twenty-two produced >92% mortality at 24 h and warranted further investigation to determine LC₅₀ values. Fifteen of these analogues had LC₅₀ values within 2 orders of magnitude of transfluthrin, and of significant interest, N-(2,6-dichloro-4-(trifluoromethyl)phenyl)-2,2,3,3,3-pentafluoropropanamide (compound 70) was nearly as potent as transfluthrin and exhibited greater toxicity than metofluthrin when screened against OR A. aegypti. Compounds exhibiting potent toxicity against OR A. aegypti or whose structure-activity relationship potentially offered beneficial insights into structure optimization were screened against the insecticide-resistant, Puerto Rico (PR), strain of A. Aegypti, and it was discovered that not only did these N-arylamides typically show little resistance, some such as N-(2,6-dichloropyridin-4-yl)-2,2,3,3,4,4,4-heptafluorobutanamide (compound 36) and 2,2,3,3,4,4,4-heptafluoro-N-(3,4,5-trifluorophenyl)butanamide (compound 40) were actually more potent against the PR mosquitoes. Due to this promising insecticidal activity, five compounds were administered orally to mice to determine acute oral rodent toxicity. All five compounds were found to have mouse oral toxicity LD₅₀ values well above the minimum safe level as set by the Innovative Vector Control Consortium (50 mg/kg). In addition to the promising biological activity documented here, we report the structure-activity relationship analysis used to guide the derivatization approach taken and to further inform future efforts in the development of N-arylamides as potential resistance-breaking, spatially acting insecticides.

Vapor phase repellency and insecticidal activity of pyridinyl amides against anopheline mosquitoes

It is important to identify repellents that can provide reliable protection from arthropod biting and prevent arthropod-borne diseases, such as malaria. In the present study, the spatial repellent activity and toxicity of two novel pyridinyl amides (1 and 2) were evaluated against Anopheles albimanus, Anopheles quadrimaculatus, and Anopheles gambiae. In vapor repellency bioassays, compound 2 was generally more effective than DEET and 2-undecanone, while compound 1 was about as active as these standards. Overall, transfluthrin was the most active compound for inducing anopheline mosquito repellency, knockdown, and lethality. Although they were not the most active repellents, the two experimental amides produced the largest electroantennographic responses in female antennae. They also displayed modest toxicity to anopheline mosquitoes. Significant synergism of repellency was observed for the mixture of a pyrethroid-derived acid and the repellent 2-undecanone against anopheline mosquitoes, similar to that observed previously in Aedes aegypti. Overall, this study provides insight for further synthesis of alternative amide compounds for use as spatial treatments.

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Two experimental pyridyl amides were synthesized.

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They were more repellent than DEET, equal to 2-undecanone and less than transfluthrin.

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They were about as toxic as DEET and 2-undecanone, but less than transfluthrin.

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Experimental amides performed about the same across all anopheline species.