Self-Association and Solubility Behaviors of a Novel Anticancer Agent, Brequinar Sodium

To aid in the selection of appropriate excipients to formulate brequinar sodium [6-fluoro-2-(2′-fluoro-1,1′-biphenyl-4-yl)-3-methyl-4-quinolinecarboxylic acid sodium salt; DuP 785], studies were initiated to characterize thoroughly its solubility behavior. The measured solubilities at RT (~23°C) agreed with the theoretical values in the pH range from 0.5 to 7.2, but became significantly greater than theoretical values at pH values above 7.2. This deviation was likely due to the vertical stacking-type self-association between brequinar molecules in water. The NMR and pH methods determined a critical association concentration of 15 mg/mL. Sodium salicylate, which has been proven to interfere with molecular self-association, reduced drug solubility from 116 to 10 mg/mL. But urea, another deaggregative agent, gave about a twofold increase rather than a decrease in solubility. Addition of sodium chloride caused a 226-fold decrease in solubility. The apparent solubility product did not remain constant but decreased as sodium chloride concentration increased, suggesting that the added salt decreased the degree of self-association between brequinar molecules. Among four surfactants examined (a bile salt with a rigid fused ring versus three ordinary surfactants with a flexible chain structure), only sodium cholate significantly increased the aqueous solubility of brequinar sodium.