Partially unfolded proteins efficiently penetrate cell membranes--implications for oral drug delivery.

We have previously reported on the biological activity of members of a library of low molecular weight compounds (carriers) that enable the oral delivery of proteins (Milstein, Proceedings of the 1995 Miami Bio/Technology Winter Symposium on Protein Engineering and Structural Biology, IRL Press at Oxford University Press, 1995, p. 13; Leone-Bay et al., J. Med. Chem. 38 (1995) 4263-4269; Leone-Bay et al., J. Med. Chem. 39 (1996) 2571-2578; [1-3]). When rats or primates are orally administered a solution of carrier and either recombinant human alpha-interferon (rhIFN), insulin or recombinant human growth hormone (rhGH) significant serum concentrations of the proteins are detectable. The transport activity of these compounds is positively correlated with their structural effects on the protein molecules. Direct measurement of the interaction of these carrier molecules with the proteins indicates that they reversibly destabilize the native state of the molecule favoring a partially unfolded conformation. Apparently these intermediate protein conformations are transport competent and are able to be absorbed through the intestinal tissue and into the bloodstream. Since the measured binding of the carriers to the partially unfolded proteins is relatively weak (Kb = 100 M(-1)) and the systemic activity of the proteins appears to be unaffected, the changes in the structure of the proteins are manifestly reversible.