Functional Effects of Cyclization on Li1 Peptide Activity against Metacyclic Leishmania amazonensis Internalization

Area of Science:

• Parasitology
• Medicinal Chemistry
• Immunology

Background:

• Leishmaniases are protozoan diseases transmitted by sandflies.
• Metacyclic promastigotes of *Leishmania* initiate infection in mammalian hosts.
• Previous studies explored peptides against *Leishmania infantum*.

Purpose of the Study:

• To evaluate the efficacy of the cyclic peptide Li1 and its linear analogue Li1nc against *Leishmania amazonensis* infection.
• To compare the structure-activity relationship between Li1 and Li1nc.
• To assess the potential of Li1 as a prophylactic agent.

Main Methods:

• Solid-phase peptide synthesis (SPPS-Fmoc) for peptide preparation.
• Pre-exposure of *Leishmania* metacyclic promastigotes to peptides before macrophage infection (murine peritoneal and THP-1 derived).
• In vitro cytotoxicity assays (CC50) and antipromastigote activity assays (IC50).
• Confocal microscopy for peptide-parasite binding analysis.
• In vivo efficacy assessment in a murine model of cutaneous leishmaniasis.
• Evaluation of systemic toxicity via hepatic and renal biochemical parameters.

Main Results:

• Pre-exposure to Li1 significantly reduced *L. amazonensis* infection rates and indices in macrophages.
• Li1nc showed moderate inhibition, indicating structure-dependent activity.
• Neither peptide exhibited cytotoxicity (CC50 > 0.5 mg mL-1) or direct antipromastigote activity (IC50 > 0.5 mg mL-1).
• Confocal microscopy revealed stronger Li1 binding to the parasite surface, especially the flagellum.
• In vivo, Li1 reduced parasite burden by 35.2% and Li1nc by 20.4% in a murine model.
• No systemic toxicity was observed in mice.
• No activity was detected under a postinfection treatment regimen.

Conclusions:

• Li1 interferes with early host-parasite interactions, reducing *Leishmania* infectivity.
• The cyclic structure of Li1 is crucial for its enhanced activity compared to Li1nc.
• Li1 demonstrates a favorable safety profile and translational potential for prophylactic or paratransgenic strategies against leishmaniasis transmission.