Oral peptide drug delivery: polymer-inhibitor conjugates protecting insulin from enzymatic degradation in vitro.

A drug-carrier matrix has been developed which protects embedded insulin from degradation by the luminally secreted serine-proteases trypsin (EC 3.4.21.4), chymotrypsin (EC 3.4.21.1) and elastase (EC 3.4.21.36) in vitro. Increasing amounts of the Bowman-Birk inhibitor (BBI) and elastatinal, respectively, were thereby covalently bound to the mucoadhesive polymer sodium carboxymethylcellulose (Na-CMC). The inhibitory efficacy of resulting polymers was evaluated. On the one hand, all polymer-BBI conjugates showed a strong inhibitory activity towards trypsin and chymotrypsin whereas it was markedly lower towards elastase. The polymer-elastatinal conjugates, on the other hand, displayed a comparatively higher inhibitory activity towards elastase. In an artificial intestinal fluid containing trypsin, chymotrypsin and elastase in physiological concentrations insulin, being incorporated in unmodified Na-CMC, was rapidly degraded at 37 degrees C. Within 1 h 98.7 +/- 0.4% (mean +/- SD, n = 3) of the peptide drug were thereby metabolized. On the contrary, the incorporation of insulin in a mixture of the two polymer-inhibitor conjugates CMC-BBI (40%; w/w) and CMC-elastatinal conjugate (60%; w/w) led to a peptide degradation of 22.3 +/- 2.5% (mean +/- SD, n = 3) within the same time period. Even after 4 h of incubation, 33.6 +/- 3.2% (mean +/- SD, n = 3) of the therapeutic agent remained stable towards enzymatic attack. Hence, the polymer-inhibitor conjugates described in this study seem to be a useful tool in overcoming the luminal enzymatic barrier in peroral insulin delivery.