BRIEFING DOCUMENT: Novel Approach to Malaria Control Targeting Mosquito-Stage Plasmodium Parasites
Date: Received - 29 March 2025 | Accepted - 17 April 2025 | Published - 21 May 2025
Subject: Development and testing of novel antiparasitic compounds for incorporation into mosquito bed nets to combat insecticide resistance and reduce malaria transmission.
Summary:
This research presents a promising new strategy for malaria control by targeting the Plasmodium falciparum parasite directly within its mosquito vector (Anopheles species). Recognizing the growing challenge of insecticide resistance in mosquitoes, the study explores the potential of incorporating antiparasitic compounds into long-lasting insecticide-treated nets (LLINs). The authors performed an in vivo screen of 81 compounds, identifying 22 active against mosquito-stage parasites. Notably, endochin-like quinolones (ELQs) targeting the parasite's cytochrome bc1 complex (CytB) showed high potency and were further optimized through medicinal chemistry. Two lead ELQ compounds, ELQ-453 and ELQ-613, demonstrated potent, long-lasting activity when incorporated into bed net-like materials, including in insecticide-resistant mosquitoes. The study also highlights the potential of a dual-target strategy using a combination of Qo-site and Qi-site ELQ inhibitors to reduce the risk of resistance, as CytB mutants show impaired development in mosquitoes. This approach offers a complementary tool to existing malaria control strategies, particularly in areas with high insecticide resistance.
Key Themes and Important Ideas/Facts:
- Malaria Burden and the Challenge of Insecticide Resistance:
- Malaria deaths have stalled in recent years, with an estimated 263 million cases and 597,000 deaths in 2023.
- Vector control, particularly LLINs, has been crucial in reducing malaria prevalence, but widespread insecticide resistance in Anopheles vectors is jeopardizing their effectiveness.
- "The decline in malaria deaths has recently stalled owing to several factors, including the widespread resistance of Anopheles vectors to the insecticides used in long-lasting insecticide-treated nets (LLINs)..."
- Targeting Mosquito-Stage Parasites as a Mitigation Strategy:
- Directly killing parasites during their mosquito-stage development by incorporating antiparasitic compounds into LLINs can prevent onward transmission, even if insecticides lose efficacy.
- This strategy avoids conferring fitness costs or selective pressure on the mosquito, thus preventing vector resistance to the antiparasitic compound.
- "Interventions that directly target parasites in the mosquito represent a promising approach to disrupt parasite transmission and to reduce malaria burden..."
- "Of note, the use of a Plasmodium-specific compound would not confer any fitness cost or selective pressure to the anopheline mosquito, which therefore avoids potential development of resistance by the vector."
- In Vivo Compound Screening and Identification of Key Targets:
- An in vivo screen of 81 antiplasmodial compounds with diverse modes of action was performed in Anopheles gambiae mosquitoes.
- 22 compounds spanning seven distinct P. falciparum targets significantly reduced parasite infection (oocyst prevalence) in the mosquito midgut.
- Key identified targets included the ubiquinol oxidation (Qo) and ubiquinone reduction (Qi) sites of the P. falciparum cytochrome bc1 complex (CytB), the sodium-proton antiporter P-type ATPase 4 (ATP4), and eukaryotic elongation factor 2 (EF2).
- "This screen ultimately identified 22 compounds that significantly reduced parasite infection, and these compounds spanned seven P. falciparum targets..."
- Endochin-Like Quinolones (ELQs) as Lead Compounds:
- Endochin-like quinolones (ELQs) targeting CytB were among the most effective compounds in the topical screen.
- ELQ-456 (targeting the CytB Qo-site) completely inhibited infection in topical applications.
- ELQ-331 (targeting the CytB Qi-site) strongly reduced infection prevalence.
- Medicinal chemistry was used to improve the antiparasitic activity of ELQ hits, particularly for uptake via tarsal contact (mosquito legs).
- Tarsal Contact Activity and the Importance of Compound Structure:
- Tarsal contact assays, mimicking mosquito interaction with treated surfaces like bed nets, showed that most compounds active in topical application were inactive.
- Only ELQ-456 initially showed significant activity in tarsal contact assays.
- Modifications to the ELQ structure significantly enhanced tarsal-based efficacy. ELQ-453 (Qo-site inhibitor) and ELQ-613 (Qi-site inhibitor), with specific alkyl chain lengths, demonstrated potent activity after tarsal contact.
- "Of the 13 compounds we tested in tarsal-contact assays, only our most potent hit in the topical screen, ELQ-456 (CytB Qo-site inhibitor) reduced infection (69.5% reduction in oocyst prevalence)."
- "These results highlight that uptake through mosquito tarsi is strongly affected by the structure of the compound and is key to its activity against P. falciparum."
- ELQ Combination (ELQ-453 + ELQ-613) Enhances Potency and Longevity:
- A combination of ELQ-453 and ELQ-613 showed increased potency compared to either compound alone in tarsal contact assays.
- This combination significantly reduced oocyst size and sporozoite prevalence even when mosquitoes were exposed to the compounds after establishing infection (3 days post-infection), suggesting an extension of the extrinsic incubation period (EIP).
- "These data demonstrate that a single, short period of mosquito contact with surfaces treated with the ELQ combination can both prevent establishment of infection and slow down ongoing infections, thereby extending the parasite developmental time in the mosquito..."
- Activity in Bed Net-Like Materials and Insecticide Resistance:
- ELQ-453 and ELQ-613, alone and in combination, maintained their activity when incorporated into low-density polyethylene (LDPE) and high-density polyethylene (HDPE) films, mimicking bed net materials.
- These films retained complete antiplasmodial activity for over a year when stored at room temperature.
- Crucially, the ELQ combination was fully effective in insecticide-resistant A. gambiae strains, demonstrating its potential to circumvent insecticide resistance.
- "Crucially, these films maintained their antiplasmodial activity when tested 1 year later after being stored at room temperature with light exposure and when tested in an insecticide-resistant A. gambiae strain..."
- Resistance Considerations and CytB Mutants:
- Targeting the parasite during the mosquito stage (a bottleneck with lower parasite numbers) is likely to reduce the propensity for de novo resistance mutations.
- A combination therapy targeting different sites in CytB (Qo and Qi) further mitigates the risk of resistance emergence and spread.
- Drug selection experiments showed no cross-resistance between Qo-site and Qi-site ELQ inhibitors in asexual blood stages.
- Notably, CytB mutants with resistance mutations (particularly Qo-site mutations) showed severely impaired sporogony and transmissibility in mosquitoes.
- "...if resistance were to arise, mutants would most probably exhibit severe impairments in development at the mosquito stage."
- "...even if CytB mutants emerged under double Qo–Qi-site selection, they would not be easily transmissible because of severe fitness costs that the parasites face during the mosquito stage of development."
- Translational Potential and Future Directions:
- The ELQ compounds can be synthesized through a relatively simple and scalable three-step process, suggesting promising cost-effectiveness.
- Strategically incorporating ELQs into bed nets, such as in roof panels, could minimize additional costs while maintaining efficacy.
- Future studies should prioritize identifying compounds active against mosquito stages that do not share targets with clinically used antimalarials to prevent cross-resistance issues between human treatment and mosquito-targeted interventions.
- "This straightforward synthesis is a promising indication of cost-effectiveness, and we anticipate that with additional process chemistry optimization, manufacturing-scale synthesis and bulk net production and procurement, these compounds will be an affordable and effective addition to bed nets..."
- "Such compounds would be well suited to reduce malaria transmission and extend the efficacy of existing bed nets and prevent the transmission of resistance against classical curative antimalarials."
Conclusion:
This research successfully identifies and validates novel antiparasitic compounds, particularly ELQs targeting P. falciparum CytB, that are highly effective against mosquito-stage malaria parasites. The ability of these compounds, especially the ELQ-453 and ELQ-613 combination, to be taken up via tarsal contact, retain activity in bed net materials over time, and work effectively in insecticide-resistant mosquitoes, offers a powerful new tool in the fight against malaria. The finding that parasite resistance mutations come with significant fitness costs during mosquito development further strengthens the potential of this approach to complement existing vector control strategies and combat the growing challenge of insecticide resistance.