Drug Property Optimization: Design, Synthesis, and Characterization of Novel Pharmaceutical Salts and Cocrystal-Salt of Lumefantrine

Lumefantrine (LMF) is a low-solubility antimalarial drug that cures acute, uncomplicated malaria. It exerts its pharmacological effects against erythrocytic stages of Plasmodium spp. and prevents malaria pathogens from producing nucleic acid and protein, thereby eliminating the parasites. Modifying the structure of a drug through the formation of a pharmaceutical cocrystal or salt presents an avenue through which its physicochemical properties can be optimized. In this work, we report the design/synthesis and solid-state characterization of four new salts and cocrystal-salt forms of LMF; an LMF-ADP salt, monoclinic space group P2₁/n; an LMF-FUM cocrystal-salt, monoclinic space group P2₁/c; an LMF-TAR solvate salt, monoclinic space group P2₁/n; and an LMF-SUC salt, triclinic, space group P1̅ (ADP, dianion of adipic acid; FUM, monoanion of fumaric acid; TAR, dianion of tartaric acid; SUC, dianion of succinic acid). These salts can be obtained by solution as well as by mechanochemical cocrystallization methods. The multicomponent systems gain their stability from hydrogen and partial ionic bonding interactions (N-H···O, O-H···O, N⁺-H···O^-, and O-H⁺···O^-) originating from both the dibutyl ammonium (N⁺-H) site and the alcohol hydroxyl (-OH) site of LMF toward the carboxylate (-C(O^-)═O) functional groups of the coformer anions. SCXRD indicates for LMF-ADP, LMF-TAR, and LMF-SUC complete transfer of all carboxylic acid protons (H⁺) toward the LMF nitrogen, while for LMF-FUM, one of the protons is transferred (leaving a hydrofumarate monoanion). Using salicylic and acetylsalicylic acids as coformers yielded coamorphous solids. Solid-state characterization using powder X-ray diffraction (XRD) and thermal techniques (DSC and TGA) support and confirm the structures obtained from single-crystal XRD. LMF-ADP and LMF-FUM present superior stability under standard conditions (40 ± 2 °C, 75 ± 5% RH, and 3 months) compared to the amorphous samples and the other two salts. LMF-SUC showed poor thermal stability by DSC/TGA, and powder XRD patterns for LMF-TAR showed substantial change after the 3-month stability test. Finally, the calculated equilibrium solubilities for the cocrystal salts indicate an increase of more than twofold compared to LMF's solubility.