Improvement of subcutaneous bioavailability of insulin by sulphobutyl ether beta-cyclodextrin in rats.

The objective of this study was to examine and compare how hydrophilic beta-cyclodextrin derivatives (beta-CyDs) improve the bioavailability of insulin following subcutaneous injection of insulin solution in rats. When insulin solutions in the absence of beta-CyDs were injected into the dorsal subcutaneous tissues of rats, the absolute bioavailability of insulin calculated from plasma immunoreactive insulin (IRI) levels was approximately 50%. When maltosyl-beta-cyclodextrin was added to the solutions, there was no change in the plasma IRI levels and hypoglycaemia compared with those of the insulin-alone solution. Dimethyl-beta-cyclodextrin decreased the bioavailability of insulin, although it increased the maximal concentration of IRI in plasma and the capillary permeability of the fluorescein isothiocyanatedextran 40, a non-degraded permeation marker. When insulin solutions containing sulphobutyl ether-beta-cyclodextrin with a degree of substitution of the sulphobutyl group of 3.9 (SBE4-beta-CyD) were injected, the IRI level rapidly increased and maintained higher IRI levels for at least 8 h. The bioavailability of the insulin/SBE4-beta-CyD system was about twice that of insulin alone and approached 96%. The enhancing effects of SBE4-beta-CyD may be in part due to the inhibitory effects of SBE4-beta-CyDs on the enzymatic degradation and/or the adsorption of insulin onto the subcutaneous tissue at the injection site, although this does not apparently facilitate capillary permeability. These results suggest that SBE4-beta-CyD in aqueous insulin injection for subcutaneous administration is useful for improving the bioavailability and the hence the pharmacological effects of insulin.

Varying effects of cyclodextrin derivatives on aggregation and thermal behavior of insulin in aqueous solution.

Maltosyl-beta-cyclodextrin (G2-beta-CyD) suppressed the aggregation of insulin in neutral solution, while the sulfate of beta-CyD (S-beta-CyD) accelerated the aggregation. On the other hand, the sulfobutyl ether of beta-CyD (SBE-beta-CyD) showed varying effects on insulin aggregation, depending on the degree of substitution of the sulfobutyl group: i.e., the inhibition at relatively low substitution and acceleration at higher substitution. Differential scanning calorimetric studies indicate that the self-association of insulin stabilized the native conformation of the peptide, as indicated by an increase in the mean unfolding temperature (Tm). G2-beta-CyD and SBE-beta-CyD decreased the Tm value of insulin oligomers, while S-beta-CyD increased the Tm value. 1H-Nuclear magnetic resonance spectroscopic studies suggest that G2-beta-CyD includes accessible hydrophobic side chains of insulin within the CyD cavity, and hence perturbs the intermolecular hydrophobic contacts between aromatic side chains across the monomer-monomer interfaces. By contrast, the electrostatic interaction between the positive charges of insulin and the concentrated negative charges of the sulfate and sulfonate groups of the anionic beta-CyDs seems to be more of a factor than the inclusion effects. These results suggest proper use of the CyD derivatives could be effective in designing rapid or long-acting insulin preparations.