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°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.