Protozoan Neglected Tropical Diseases (NTDs) Target Inhibition of Alkaloids from Croton linearis Jacq Leaves: A Molecular Docking and ADMET Approach

Background/Objectives: Neglected tropical diseases (NTDs) caused by protozoan parasites such as Trypanosoma cruzi, Trypanosoma brucei, Leishmania spp., and Plasmodium falciparum remain a global health challenge due to limited therapies and increasing drug resistance. Natural products provide diverse scaffolds for antiparasitic drug discovery. This study aimed to investigate the multitarget inhibitory potential of alkaloids isolated from Croton linearis Jacq. against validated protozoan enzymes. Methods: Eighteen alkaloids were virtually screened against 17 molecular targets relevant to protozoan parasites. Protein–ligand docking simulations were performed using crystallographic structures of enzymes, including Cyp51, DHFR-TS, PTR1, AD-kinase, and DHODH. Predicted interactions were analyzed to identify hydrogen bonds, hydrophobic contacts, and π–π stacking with key residues in the active sites. Results: Several alkaloids exhibited high binding affinities, in some cases surpassing co-crystallized ligands. Reticuline, norsalutaridine, laudanosine, and jacularine consistently showed the strongest activity, with docking scores ranging from −8.0 to −9.3 kcal/mol across multiple targets. Notably, norsalutaridine displayed the highest predicted affinity for L. infantum Cyp51, while reticuline showed strong binding to T. cruzi DHFR-TS and L. major PTR1. Conclusions: The study highlights the potential of C. linearis alkaloids as multitarget inhibitors against protozoan parasites. These compounds represent promising lead candidates for the development of antiparasitic agents, while emphasizing the value of natural product scaffolds for neglected disease drug discovery. The findings also support the future exploration of semisynthetic derivatives to optimize activity and selectivity.