Preparation of niosomes as an ocular delivery system for naltrexone hydrochloride: physicochemical characterization

Recent reports have demonstrated that topical and systemic application of naltrexone markedly improves the characteristic signs of diabetic keratopathy; most notably, impaired corneal sensation and delayed wound repair. The aim of this study was to prepare and characterise non-ionic surfactant vesicles (niosomes) for the ocular drug delivery of naltrexone hydrochloride. The niosomes were prepared using the thin film hydration method and characterised using polarized light microscopy, cryo-scanning electron microscopy (Cryo-SEM), percent drug entrapment efficiency (EE %), laser light diffraction and differential scanning calorimetry (DSC). Two classes of non-ionic surfactants (sorbitan esters and polyoxyethylene alkyl ethers) were investigated. The results revealed that tuning of cholesterol concentrations can significantly alter the niosome's physical properties including sizes, EE% and membrane fluidity (thermo-responsiveness). The prepared vesicles were in the range of 7.0 +/- 1.0 to 14.6 +/- 0.8 microm in size. The prepared niosomes showed different abilities to accommodate cholesterol. This was highly dependent on the structure and continuity of the hydrophobic chains of the used surfactants. Span 60-based vesicles containing 30% mol/mol of cholesterol showed the highest EE%. The microstructure and lamellarity of the niosomes were studied using Cryo-SEM. Typical concentric multilayered structures (onion or rose-like) were seen suggesting the formation of multilamellar vesicles. DSC-studies conducted on Span 60-based niosomes containing 30% mol/mol cholesterol revealed liquid-gel transition (T(m) and entropy of 43.5 degrees C and 0.82 kcal/mol, respectively). Such transition reflects potential thermo-responsive properties, which is desirable for ocular delivery.