Development of therapeutic contact lenses using a supercritical solvent impregnation method

We present some selected results indicating the feasibility of preparing therapeutic finished ophthalmic articles, namely commercially available soft contact lenses, using a supercritical solvent impregnation (SSI) technique. Several commercial soft contact lenses were tested and, among these, four lenses were selected for more complete studies: Nelfilcon A (FocusDailies ®, CIBA Vision), Omafilcon A (Proclear ® Compatibles, CooperVision), Methafilcon A (Frequency ® 55, CooperVision) and Hilafilcon B (SofLens ® 59 Comfort, Bausch & Lomb). Supercritical carbon dioxide (scCO 2) was the chosen supercritical fluid and two ophthalmic drugs were tested: flurbiprofen (a NSAID, hydrophobic) and timolol maleate (an anti-glaucoma drug, hydrophilic). The effects of operational pressure, of impregnation duration and of the addition of a cosolvent (ethanol) were studied on the overall drug loading yields. Depending on the experiment, we employed pressures from 9 up to 16 MPa and impregnation times from 30 up to 180 min. Temperature was kept constant and equal to 313 K. The employed depressurization rates were kept low and between 0.1 and 0.2 MPa/min. Results are discussed in terms of the employed operational conditions and taking in consideration all the possible interactions between supercritical fluids, drugs, cosolvents and the polymers which compose the employed hydrogel contact lenses. In vitro drug release experiments were carried out in order to evaluate the resulting drug release profiles. Obtained results were also compared with drug-loaded contact lenses obtained by conventional drug “soaking” in aqueous solutions. Results also proved that SSI can be considered as a viable, efficient and safe alternative for the impregnation of drugs, including those of hydrophobic character or presenting low aqueous solubility, into commercial soft contact lenses. SSI proved to be a “tunable” process since the variation of the employed operational conditions indicated that it is possible to control the amount of impregnated drug. In the end, the ophthalmic articles were recovered undamaged and without the presence of harmful solvent residues. This method also permits to process already prepared commercial contact lenses, without interfering with their manufacture methods and, after processing, store them for future use.