Natural Amino Acid-Bearing Carbamate Prodrugs of Daidzein Increase Water Solubility and Improve Phase II Metabolic Stability for Enhanced Oral Bioavailability

Daidzein (DAN) is an isoflavone, and it is often found in its natural form in soybean and food supplements. DAN has poor bioavailability owing to its extremely low water solubility and first-pass metabolism. Herein, we hypothesized that a bioactivatable natural amino acid-bearing carbamate prodrug strategy could increase the water solubility and metabolic stability of DAN. To test our hypothesis, nine amino acid prodrugs of DAN were designed and synthesized. Compared with DAN, the optimal prodrug (daidzein-4′-O-CO-N-isoleucine, D-4′-I) demonstrated enhanced water solubility and improved phase II metabolic stability and activation to DAN in plasma. In addition, unlike the passive transport of DAN, D-4′-I maintained high permeability via organic anion-transporting polypeptide 2B1 (OATP2B1)-mediated transport. Importantly, D-4′-I increased the oral bioavailability by 15.5-fold, reduced the gender difference, and extended the linear absorption capacity in the pharmacokinetics of DAN in rats. Furthermore, D-4′-I exhibited dose-dependent protection against liver injury. Thus, the natural amino acid-bearing carbamate prodrug strategy shows potential in increasing water solubility and improving phase II metabolic stability to enhance the oral bioavailability of DAN.