The pH of Piperazine Derivative Solutions Predicts Their Utility as Transepithelial Permeation Enhancers

The oral delivery of macromolecular drugs, including proteins and nucleic acids, is one of the greatest unmet needs in modern biomedicine. Although engineering solutions have been used to overcome enzymatic degradation and the low pH in the stomach, poor absorption across the intestinal epithelium into the bloodstream continues to pose the most significant challenge to clinical translation. One common approach to increase the flux of macromolecules across the intestinal epithelium is the use of chemical permeation enhancers. Unfortunately, the vast majority of effective enhancers have been thwarted by toxicity, and the structural and molecular parameters that contribute to this behavior are poorly understood. Previous work has shown that select piperazine-derived molecules favorably affect transepithelial and intracellular delivery outcomes, suggesting that piperazine-derived molecules interface uniquely with cellular barriers. To gain a better understanding of piperazine-mediated permeation enhancement, this work examined piperazine and 13 of its simple, hydrocarbon-substituted derivatives using Caco-2 monolayers as a model of the intestinal epithelium. After evaluating each piperazine for permeation enhancement efficacy and cytotoxicity at three concentrations, it became clear that piperazine derivatives consistently enhance permeability with each derivative resulting in noncytotoxic permeation enhancement at one or more concentrations. In attempting to identify structure–function relationships for the piperazine derivatives, it was found that treatment concentration, structural characteristics, and molecular pK_a were not reliable indicators of permeation potential. Interestingly, the pH of the enhancer solution was identified as a controlling parameter even when accounting for the effects from pH change alone. Specifically, piperazine treatments with a pH between 9.2 and 9.6 guaranteed noncytotoxic efficacy. Furthermore, all effective treatments resulted in pH values between 8.7 and 9.6, behavior that was not shared by the other small, noncyclic amines studied. These data have important implications in the design of oral biologic delivery systems that employ permeation enhancers and underscore the need to carefully control the final treatment pH of the local intestinal epithelial environment.