Structure-Function Analysis of Phenylpiperazine Derivatives as Intestinal Permeation Enhancers

Katherine C. Fein, Nicholas G. Lamson, Kathryn A. Whitehead

A major obstacle preventing oral administration of macromolecular therapeutics is poor absorption across the intestinal epithelium into the bloodstream. One strategy to improve transport across this barrier is the use of chemical permeation enhancers. Several molecular families with permeation enhancing potential have been identified previously, including piperazines. In particular, 1-phenylpiperazine has been shown to enhance transepithelial transport with minimal cytotoxicity compared to similarly effective molecules. To better understand how the chemistry of 1-phenylpiperazine affects its utility as an intestinal permeation enhancer, this study examined a small library of 13 derivatives of 1-phenylpiperazine. The efficacy and cytotoxicity of 13 phenylpiperazine compounds were assessed in a Caco-2 model of the intestinal epithelium. Efficacy was measured using the paracellular diffusion marker calcein as well as by immunostaining and confocal imaging of Caco-2 monolayers. Of the 13 derivatives, two enhanced the permeability of the fluorescent marker calcein over 100-fold. It was found that hydroxyl or primary amine substitutions on the phenyl ring significantly increased toxicity, while aliphatic substitutions resulted in efficacy and toxicity profiles comparable to 1-phenylpiperazine. Several potent derivatives, including 1-methyl-4-phenylpiperazine and 1-(4-methylphenyl)piperazine, displayed lower toxicity than 1-phenylpiperazine, suggesting promise in future applications.