Higher-order multicomponent crystals as a strategy to decrease the IC50 parameter: the case of praziquantel, niclosamide and acetic acid

We successfully assembled via mechanochemistry 3 anthelmintic molecules in a single stable solid, namely praziquantel (PZQ), niclosamide (NCM), and acetic acid (AA). We obtained a cocrystal solvate with largely superior anthelmintic activity against in vitro Schistosoma mansoni adults and, notably, against Newly Transformed Schistosomula compared to pure individual drugs (i.e., PZQ and NCM) and to its binary counterparts (i.e., PZQ-NCM cocrystal and PZQ-AA monosolvate). We also demonstrated 5 different strategies for synthesizing the ternary cocrystal not only starting from individual coformers but also by combining different building blocks (i.e. preformed binary solids). The new phase was only obtainable through mechanochemistry as comparative slurry experiments were unsuccessful. Even though the ternary solid was crystallized through all five investigated routes, a pure phase was obtained by milling the preformed praziquantel-acetic acid monosolvate and raw niclosamide in an equimolar ratio for 60 min in the presence of 160 μL of acetic acid. Acetic acid acted both as a solvate-forming molecule and a liquid additive. The purity of this new solid phase was confirmed by SS-NMR spectrum, also suggesting the presence of one independent molecule of PZQ, one of NCM and one of AA, as confirmed by 1H NMR. The cocrystal structure was solved from the Synchrotron powder X-ray pattern and optimized via DFT calculations. Crystallizing in the triclinic P-1 space group, the solid comprises one PZQ, one NCM, and one AA molecule linked via hydrogen bonds, as demonstrated by FT-IR analyses. The solid phase exhibits small plate agglomerates, as observed through SEM analysis, a desolvation event at ∼107 °C (TGA weight loss: 9.77 %), and physical stability over 24 months at room temperature. The drastic reduction in IC50 (0.01 μM against Newly Transformed Schistosomula and Schistosoma mansoni adults) of the new solid fully justifies the ambitious challenge of incorporating more than two components into a single crystalline phase, underscoring the pivotal role of the ternary system in enhancing bioactivity. This finding highlights the need to tackle the next challenge: identifying the most suitable oral dosage form for a cocrystal solvate, a requirement that remains unmet in the pilot studies conducted in this work.