Host–guest complexes of cucurbit[6]uril with the trypanocide drug diminazene and its degradation product 4-aminobenzamidine†

African trypanosomiasis is a parasitic disease of humans and animals that plagues sub-Saharan Africa and is transmitted by tsetse flies. The infection is known as sleeping sickness in humans and is lethal if left untreated. However, the choice of chemotherapeutic agents is very limited, and approved drugs have many side effects. Diminazene (berenil) is used in the treatment of domestic animals and is quite effective against trypanosomiasis with a short treatment period and low cost. Diminazene formulated as a diminazene aceturate salt is registered as a veterinary drug but is not currently approved for use in humans. Diminazene is a bis-benzamidine compound composed of two benzamidine moieties linked with a triazene group (Fig. 1). The anti-trypanosomial activity of diminazene is due to its binding to the parasitic kinetoplast DNA via hydrogen bonds. The drug is administered by intramuscular injection, and its main side effect is a burning sensation at the injection site. It can potentially be administered orally; however, the drug molecule undergoes acid-catalyzed decomposition in the human gastrointestinal tract. Specifically, the triazene linker is susceptible to hydrolysis with formation of 4-aminobenzamidine and diazonium salt. The limitations of diminazene could potentially be overcome by developing coated formulations, for example nanoparticles or host– guest complexes. Here, we would like to present our results on the cocrystallization attempts for the drug diminazene with the macrocyclic host cucurbit[6]uril (CB6) (Fig. 1). Such complexation through co-crystallization can improve the solubility, bioavailability and stability of pharmaceutically active molecules. Cucurbit[n]urils are pumpkin-shaped supramolecular hosts which have attracted much attention owing to their excellent ability to bind various inorganic, organic and biological molecules and ions both in solution and in the solid state. The highly polar carbonyl rims of CB6 seem to be good candidates for multi-point interactions with two amidinium groups of diminazene. Besides favorable cation– dipole interactions between the cationic guest and the host, the CB6 carbonyl groups can potentially serve as acceptors for numerous hydrogen bonds in the amidinium groups of diminazene. The initial experiments on the co-crystallization of the drug with CB6 revealed that diminazene is susceptible to decomposition even in the presence of the macrocyclic host. The hydrolysis of the drug is induced by the remnants of the crystallization acid in CB6. The cleavage reaction is additionally promoted by CB6 through electrostatic enhancement of diminazene basicity. As a result, crystallization of the ternary complex of CB6 with diminazene and its degradation product 4-aminobenzamidine was observed. Interestingly, the crystals were unstable and slowly dissolved in the mother solution, which was followed by the crystallization of a new phase that turned out to be the host–guest complex of CB6 and 4-aminobenzamidine with simultaneous coordination of magnesium ions (magnesium chloride was used to improve the solubility of CB6). When calcium chloride was