Hydrolysis of peptides in the nasal cavity of humans

Intranasal delivery--modification of drug metabolism and brain disposition

Intranasal route continues to be one of the main focuses of drug delivery research. Although it is generally perceived that the nasal route could avoid the first-pass metabolism in liver and gastrointestinal tract, the role of metabolic conversions in systemic and brain-targeted deliveries of the parent compounds and their metabolites should not be underestimated. In this commentary, metabolite formations after intranasal and other routes of administration are compared. Also, the disposition of metabolites in plasma and brain after nasal administrations of parent drugs, prodrugs and preformed metabolites will be discussed. The importance and implications of metabolism for future nasal drug development are highlighted.

Estimation of Bioavailability of Salmon Calcitonin from the Hypocalcemic Effect in Rats (I): Pharmacokinetic-Pharmacodynamic Modeling Based on the Endogenous Ca Regulatory System

The hypocalcemic effect of salmon calcitonin (sCT) after intravenous administration was explained on the basis of an integrated pharmacokinetic-pharmacodynamic (PK-PD) model with the endogenous Ca regulatory system in the rat. The pharmacokinetics of sCT described by a conventional two-compartment model showed the extremely rapid elimination of sCT from plasma (MRT; 6.86 min). The hypocalcemic effect of sCT reached a peak from 0.5 to 1.5 hrs after administration, and the peak time tended to prolong with increasing doses. This delay in pharmacological effect of sCT against plasma concentration may be a result of a summation of multiple actions of the endogenous Ca regulatory system including feedback control. The plasma Ca regulation system in the rat was investigated by i.v. bolus administration of calcium gluconate and/or endogenous (rat) calcitonin (rCT). Since non-linearity in the relationship between Ca and rCT concentrations in plasma was observed, we assumed that rCT was secreted in accordance with the plasma Ca level via an exponential function. The pharmacokinetics of rCT was represented as a linear one-compartment model. To link the rCT level with plasma Ca level, an additional effect compartment was required to explain the delay in onset and decline of the pharmacological effect. This Ca regulation model explained the observed Ca and rCT profiles in plasma after administration of Ca and/or rCT. The plasma Ca levels after administration of sCT could be well described by the present integrated model. This suggested the potential for prediction of plasma sCT concentration only from the hypocalcemic effect after extravascular administration of sCT, using this PK-PD model.

Characterization of human nasal primary culture systems to investigate peptide metabolism

The objectives of this study were to validate and compare the suitability of different primary cell culture systems as models to investigate peptide enzymatic stability following nasal administration. The degradation kinetics of a model peptide, leucine enkephalin (Tyr-Gly-Gly-Phe-Leu, Leu-Enk), was determined in four nasal cell culture systems: immersion, air-liquid interface, sequential monolayer-suspension, floating collagen. The influence of enzyme inhibitors (bestatin, puromycin) and Leu-Enk metabolite analogs (Tyr-Gly, Phe-Leu, Tyr-Gly-Gly, Gly-Phe-Leu) on the Leu-Enk degradation profile was also investigated. The disappearance of Leu-Enk in all the cell culture systems followed first order kinetics. The specific activity in the cell culture systems followed the rank: sequential monolayer-suspension (32.60 microM min(-1) mg(-1)) >air-liquid interface (15.19 microM min(-1) mg(-1)) >immersion (11.49 microM min(-1) mg(-1)) >floating collagen (4.57 microM min(-1) mg(-1)). At equimolar concentration, bestatin had a higher inhibitory effect than puromycin. The rate of hydrolysis of Leu-Enk was reduced significantly by co-incubation with Leu-Enk metabolite analogs. This study showed that immersion, sequential monolayer-suspension and air-liquid interface culture systems may be potentially suitable for further studies on peptide enzymatic stability following nasal administration.

Nasal epithelial permeation of thymotrinan (TP3) versus thymocartin (TP4): competitive metabolism and self-enhancement

PURPOSE

To investigate concentration dependent permeabilities and metabolism kinetics of thymotrinan (TP3) versus thymocartin (TP4) in nasal epithelium in vitro.

METHODS

Excised bovine nasal mucosa was used as an in vitro model. Permeabilities were studied in a diffusion chamber, metabolism kinetics in a reflection kinetics set-up. Studies were performed at various TP3 and TP4 concentrations. The 3H-mannitol flux was measured to monitor junctional permeability. Potential Ca(2+)-complexation was investigated using a Ca(2+)-selective electrode.

RESULTS

Permeability of TP3 was negligible at 0.1 and 0.2 mM and increased drastically above 0.4 mM up to -2 X 10(-5) cm s(-1). In the presence of 2 mM TP4 the TP3 permeabilites were significantly above (approximately 4 x 10(-5) cm s(-1)) the level of TP3 without TP4, and TP3 metabolism was totally inhibited. TP3 and TP4 showed a significant concentration dependent effect on the permeability of 3H-mannitol. A hyperosmolarity effect of the peptide solutions was excluded. Transepithelial electrical resistance (TEER; approximately 30 ohms cm2) was unchanged by either TP3 or TP4. At 1 mM TP3 the mucosal-to-serosal permeability was four times higher than serosal-to-mucosal, indicating enzyme polarization. In reflection kinetics studies, TP3 degradation was slightly higher on the mucosal than on the serosal side. TP3 and TP4 followed the same non-linear metabolism kinetics.

CONCLUSIONS

Increase in permeability at high TP concentrations involves competitive enzyme saturation combined with self-enhanced paracellular permeation.

Intranasal insulin delivery and therapy

Intranasal insulin delivery has been widely investigated as an alternative to subcutaneous injection for the treatment of diabetes. The pharmacokinetic profile of intranasal insulin is similar to that obtained by intravenous injection and, in contrast to subcutaneous insulin delivery, bears close resemblance to the 'pulsatile' pattern of endogenous insulin secretion during meal-times. The literature suggests that intranasal insulin therapy has considerable potential for controlling post-prandial hyperglycaemia in the treatment of both IDDM and NIDDM. However, effective insulin absorption via the nasal route is unlikely without employing the help of absorption enhancers or promoters which are able to modulate nasal epithelial permeability to insulin and/or prolong the residence time of the drug formulation in the nasal cavity. This article discusses the structure and function of the nasal cavity, the barriers which prevent nasal insulin absorption and through the use of absorption enhancers or promoters methods by which these barriers may be overcome.

The effect of cyclodextrins on the stability of peptides in nasal enzymic systems

Leucine enkephalin (YGGFL) undergoes rapid degradation in sheep nasal mucosa to yield GGFL which is further degraded to FL. The activity of the nasal mucosal homogenate against YGGFL and GGFL (t1/2 12 and 7 min) was significantly greater than that observed with a nasal wash fluid (t1/2 40 and 13 min). The effect of cyclodextrins on the rate of degradation of FGG and YGGFL by leucine aminopeptidase (LAP) and of GGF by carboxypeptidase A (CPA) was monitored. Little effect was observed with FGG (with LAP) but the half-life of YGGFL (with LAP) was extended from approximately 44 min to approximately 75 min in the presence of a 25-fold excess of beta-cyclodextrin. The stability of GGF (with CPA) was also enhanced; an effect was observable with a 5-fold excess of cyclodextrin and the half-life could be extended by 40-75%. An equation is presented which allows the estimation of the concentration of free peptide in the peptide-cyclodextrin solutions.