Biopharmaceutical approaches for developing and assessing oral peptide delivery strategies and systems: in vitro permeability and in vivo oral absorption of salmon calcitonin (sCT).

PURPOSE: To evaluate a biopharmaceutical approach for selecting formulation additives and establishing the performance specifications of an oral peptide delivery system using sCT as a model peptide. METHODS: The effect of formulation additives on sCT effective permeability and transepithelial electrical resistance (TEER) was evaluated in side-by-side diffusion chambers using rat intestinal segments. Baseline regional oral absorption of sCT was evaluated in an Intestinal and Vascular Access Port (IVAP) dog model by administration directly into the duodenum, ileum, and colon by means of surgically implanted, chronic catheters. The effect of varying the input rate and volume of the administered solution on the extent of sCT absorption was also evaluated. Citric acid (CA) was utilized in all studies to cause a transient reduction in local pH. In vitro samples and plasma samples were analyzed by radioimmunoassay (RIA). Two oral delivery systems were prepared based on the results of the in vitro and IVAP studies, and evaluated in normal dogs. RESULTS: Maximal permeability enhancement of sCT was observed using taurodeoxycholate (TDC) or lauroyl carnitine (LC) in vitro. Ileal absorption of sCT was higher than in other regions of the intestine. Low volume and bolus input of solution formulations was selected as the optimal condition for the IVAP studies since larger volumes or slower input rates resulted in significantly lower sCT bioavailability (BA). Much lower BA of sCT was observed when CA was not used in the formulation. The absolute oral bioavailability (mean+/-SD) in dogs for the control (sCT + CA) and two proprietary sCT delivery systems was 0.30%+/-0.05%, 1.10+/-0.18%, and 1.31+/-0.56%, respectively. CONCLUSIONS: These studies demonstrate the utility of in vitro evaluation and controlled in vivo studies for developing oral peptide delivery strategies. Formulation additives were selected, the optimal intestinal region for delivery identified, and the optimal release kinetics of additives and actives from the delivery system were characterized. These methods were successfully used for devising delivery strategies and fabricating and evaluating oral sCT delivery systems in animals. Based on these studies, sCT delivery systems have been fabricated and tested in humans with favorable results.

Utility of pharmacodynamic measures for assessing the oral bioavailability of peptides. 1. Administration of recombinant salmon calcitonin in rats.

Salmon calcitonin (sCT) is a therapeutic peptide used in the treatment of Paget's Disease, postmenopausal osteoporosis, and hypercalcemia due to malignancy. In this study, recombinant sCT (rsCT) was administered intravenously (iv), subcutaneously (sc), and intraduodenally (id.) in rats to evaluate pharmacodynamic (PD) response as a measure of rsCT bioavailability (F) and to test the feasibility of delivering rsCT orally. rsCT pharmacokinetics were linear throughout the range of iv and sc doses studied. Following sc administration, F ranged from 11.2% to 23.1% and was linear. The absorption of rsCT after id. administration was low (0.022%); however, a significant lowering of serum calcium concentrations was observed. Serum calcium lowering was nonlinear and saturable after sc administration with the minimum dose required for maximum calcium lowering (Dmin/max) equal to 10.2 ng and a maximal response of 426.8 mg min/dL. Using Dmin/max as the reference dose, absolute Fs were recalculated using PD response after id. administration of 1 and 2 mg of rsCT and were 0.040% and 0.029%, respectively. Substantial overestimates of F were obtained when the reference dose was not properly selected. While the absorption of rsCT was low, the significant lowering of serum calcium levels suggests that oral delivery of sCT is feasible. The results of these studies also suggest that PD response is useful in assessing the oral bioavailability of peptides; however, when PD response is saturable, as is the case for rsCT, the reference dose should be carefully selected in order to avoid overestimates of F.