Enhanced cytotoxicity of benzimidazole carbamate derivatives and solubilisation by encapsulation in cucurbit[n]uril

The albendazole derivatives (2-methoxyethyl) 5-propylthio-1H-benzimidazole-2-yl carbamate (MEABZ), N1-(2-methoxyethoxycarbonyl)-2-amino-5-propylthiobenzimidazole and N1-(2-methoxyethoxycarbonyl)-2-amino-6-propylthiobenzimidazole (MEABZ isomers A and B) and (2-hydroxyethyl) 5-propylthio-1H-benzimidazole-2-yl carbamate (HEABZ) have been synthesised. The cytotoxicity of these compounds was evaluated against a human colorectal cancer cell line (HT-29) and a human prostate cancer cell line (PC-3). The results demonstrate MEABZ, a new benzimidazole, is up to ten times more cytotoxic than the parent drug albendazole, whereas the MEABZ isomers A and B and HEABZ show no activity. A comparison of the cytotoxicity of these compounds, relative to ABZ, provides structure–activity data for this important class of anticancer agents. The aqueous solubilities of MEABZ encapsulated in Q[n] have been determined by 1H NMR spectroscopy. The aqueous solubility of MEABZ at a physiologically relevant pH increased by 1200-fold by encapsulation in Q[8], from 8 μM to 9.4 mM, while Q[6,7] encapsulation substantially increased the solubility to more than 2 mM. Encapsulation in Q[7] and Q[8] induced significant upfield shifts for the MEABZ propyl and benzimidazole resonances. The upfield shifts indicate that the propyl and benzimidazole protons are located within the Q[7] and Q[8] cavity upon encapsulation. By contrast, encapsulation in Q[6] induced large upfield shifts for the 1H resonances from the carbamate functional group, indicating that MEABZ associates with Q[6] at its portals, with only the carbamate group binding within the cavity.