Aspartyl protease inhibition interferes with Plasmodium falciparum asexual blood-stage and early gametocyte development.

Background: Plasmodium falciparum is the most influential species of malaria parasites, capable of causing severe illness and mortality, especially in pregnant women and children under the age of 5. Global distribution of disease impacted on billions of endemic people and travellers. Asexual stage and gametocyte cause harmful manifestations, impacting the patients and contributing to the spread of the disease in the community, respectively. Moreover, most recent therapeutic drugs did not affect the gametocyte. The discovery of a new drug with dual actions on both stages could elucidate a cost-effective way to combat malaria. Within a human host, the parasite possesses many activities for its survival, such as invasion, egress, haemoglobin degradation, and protein trafficking, many of which are related to aspartyl protease, revealing the potential for antimalarial drug targets.

Methods: Pepstatin A, the representative of the board-spectrum aspartyl protease inhibitor, was utilized to investigate the effects of aspartyl protease inhibition on parasite development. The experiments were separately performed in vitro for different developmental stages of parasites, including the asexual blood-stage, early gametocytes, late gametocytes, and gamete. To demonstrate the effects of pepstatin A, the number of intact parasites and their stage distribution were counted under the microscope and calculated as a percentage of inhibition compared to the control. Additionally, the morphology of pepstatin A-treated parasites was observed to identify cellular alterations in the parasites.

Results: Pepstatin A at 100 µM inhibited the asexual stage and early-stage gametocyte development by 47% and 73%, respectively. They exhibited morphological defects, including chromatin condensation, vacuolization and haemozoin clumping in both asexual blood-stage and early-stage gametocyte. However, it could not influence the late-stage gametocyte development and gamete formation.

Conclusion: The inhibition of aspartyl protease by pepstatin A moderately affected both asexual blood-stage and early-stage gametocyte development. Morphological changes on treated parasites implied the effect of pepstatin A on haemoglobin degradation process, suggesting its potential for reducing the severity of the disease and minimizing malaria transmission. However, further research and development are required to use aspartyl protease as a drug target, focusing on identifying and modifying the drug to be more sensitive and effective.