Welling, Søren H3; Hubálek, František3; Jacobsen, Jette4; Brayden, David J3; Rahbek, Ulrik L5; Buckley, Stephen T3
1 Pharmaceutical Design and Drug Delivery, Department of Pharmacy, Faculty of Health and Medical Sciences, Københavns Universitet2 Department of Drug Design and Pharmacology, Department of Drug Design and Pharmacology, Faculty of Health and Medical Sciences, Københavns Universitet3 unknown4 Pharmaceutical Design and Drug Delivery, Department of Pharmacy, Faculty of Health and Medical Sciences, Københavns Universitet5 Department of Drug Design and Pharmacology, Department of Drug Design and Pharmacology, Faculty of Health and Medical Sciences, Københavns Universitet
relationship between calcium chelation and proteolysis inhibition
The excipient citric acid (CA) has been reported to improve oral absorption of peptides by different mechanisms. The balance between its related properties of calcium chelation and permeation enhancement compared to a proteolysis inhibition was examined. A predictive model of CA's calcium chelation activity was developed and verified experimentally using an ion-selective electrode. The effects of CA, its salt (citrate, Cit) and the established permeation enhancer, lauroyl carnitine chloride (LCC) were compared by measuring transepithelial electrical resistance (TEER) and permeability of insulin and FD4 across Caco-2 monolayers and rat small intestinal mucosae mounted in Ussing chambers. Proteolytic degradation of insulin was determined in rat luminal extracts across a range of pH values in the presence of CA. CA's capacity to chelate calcium decreased ~10-fold for each pH unit moving from pH 6 to pH 3. CA was an inferior weak permeation enhancer compared to LCC in both in vitro models using physiological buffers. At pH 4.5 however, degradation of insulin in rat luminal extracts was significantly inhibited in the presence of 10mM CA. The capacity of CA to chelate luminal calcium does not occur significantly at the acidic pH values where it effectively inhibits proteolysis, which is its dominant action in oral peptide formulations. On account of insulin's low basal permeability, inclusion of alternative permeation enhancers is likely to be necessary to achieve sufficient oral bioavailability since this is a weak property of CA.
European Journal of Pharmaceutics and Biopharmaceutics, 2014, Vol 86, Issue 3, p. 544-51