Selection of a Lead Long-Acting Formulation of Ivermectin to Target Major Malaria Vectors in Western Africa: Evaluation of Pharmacokinetics and Mosquitocidal Efficacy in Cattle under Laboratory Conditions

Background Ivermectin, a semisynthetic endectocide, is widely used against parasitic nematodes in humans and animals. Its lethality to Anopheles mosquitoes after feeding on treated hosts represents a promising malaria control strategy, particularly against outdoor transmission. However, standard oral formulations for use in humans produce short-lived mosquitocidal blood concentrations, limiting epidemiological impact. To meet WHO Preferred Product Characteristics (PPC) for endectocides against malaria vectors (Hazard Ratios >4 for at least one month), three long-acting injectable ivermectin formulations (LAIFs) based on BEPO® depot technology were developed and compared in cattle to identify the most suitable candidate for future human use. Methods A cattle– Anopheles model was used under laboratory conditions in Bobo-Dioulasso, Burkina Faso. Three LAIF candidates (mdc-STM-001, mdc-STM-002, mdc-STM-003) were injected to calves (n=5 per formulation) at 0.6 mg/kg, with untreated calves as controls (n=5). Plasma ivermectin concentrations were measured over 130 days and analyzed using non-compartmental pharmacokinetics. Direct skin feeding assays were conducted at 15 timepoints (days 2–126 post-injection) using insecticide-susceptible (KIS) and wild-derived resistant (VK5) Anopheles colonies. Efficacy was assessed through 10-day cumulative mortalities, hazard ratios, 50% lethal concentrations (LC50), and duration of exposure above the 10-day LC50, accounting for the extrinsic incubation period of Plasmodium falciparum . Results All formulations were well tolerated. Mdc-STM-001 showed the most favorable pharmacokinetic profile, with a controlled peak concentration (Cmax = 34.5 ± 12.7 ng/mL) and the lowest inter-individual variability (12%). Ten-day hazard ratios exceeded 4 and cumulative mortalities were >50% for at least 60 days in both strains. Median mosquito lifespan remained below 10 days for at least 90 days post-injection. The 10-day LC50 for resistant mosquitoes (3.66 [2.69–4.97] ng/mL) was maintained for ≥126 days. Conclusion The Mdc-STM-001 was identified as the optimal candidate. A single injection induced sustained mosquitocidal efficacy for at least two months, achieving HR >4 against both susceptible and resistant Anopheles populations and meeting WHO PPC for malaria endectocides. Although extrapolation from cattle to humans requires caution, the favorable pharmacokinetic profile and robust entomological outcomes support progression to Phase 1 clinical trials. Ivermectin’s established safety record further strengthens the rationale for clinical development.