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The fibrinogen receptor antagonist SK&F 106760 is a water-soluble tetrapeptide (Mr634) with very low oil/water partitioning and membrane permeability. Thus, the intraduodenal bioavailability of this peptide in the rat from a saline formulation was found to be less than 1%. Upon formulation, however, in w/o microemulsions of different composition and particle size, the intraduodenal bioavailability of SK&F 106760 was increased up to 29% depending on the microemulsion composition. There was no apparent correlation between the particle size of microemulsions (mean droplet diameter of 0.010–1.0 μm) and enhanced absorption. None of the investigated microemulsions induced gross changes in gastrointestinal mucosa at a dosing volume of 3.3 ml/kg. Lipids containing medium-chain fatty acids play a major role on the observed absorption enhancement. These findings further support the use of microemulsion formulations for oral drug/peptide delivery, provided however, that development issues with these systems are properly addressed.

We developed self-emulsifying water-in-oil (w/o) microemulsions incorporating medium-chain glycerides and measured their conductance, viscosity, refractive index and particle size. Formulation of Calcein (a water-soluble marker molecule, MW = 623), or SK&F 106760 (a water-soluble RGD peptide, MW = 634) in a w/o microemulsion having a composition of Captex 355/Capmul MCM/Tween 80/Aqueous (65/22/10/3, % w/w), resulted in significant bioavailability enhancement in rats relative to their aqueous formulations. Upon intraduodenal administration the bioavailability was enhanced from 2% for Calcein in isotonic Tris, pH 7.4 to 45% in the microemulsion and from 0.5% for SK&F 106760 in physiological saline to 27% in the microemulsion formulation. The microemulsion did not induce gross changes in GI mucosa at a dosing volume of 3.3 ml/kg. These results suggest that water-in-oil microemulsion systems may be utilized for enhancement of intestinal drug absorption.