Abstract
Sildenafil citrate is used to treat mild to moderate erectile dysfunction and premature ejaculation. However, it has low oral bioavailability, numerous adverse effects, and delayed onset of action. These problems may be resolved by transdermal delivery to the penis. Hence, sildenafil citrate was formulated as a microemulsion system using isopropyl myristate, Tween 80, PEG400, and water (30:20:40:10). The hydrogel used in the microemulsion was 2% w/w poloxamer 188. The sildenafil microemulsion-loaded hydrogels were characterised for their appearance, particle size, pH, spreadability, swelling index, viscosity, sildenafil drug content, membrane permeation, epithelial cell cytotoxicity, and in vitro drug metabolism. The optimised formulated microemulsion showed the lowest droplet size and highest solubility of sildenafil citrate. The in vitro skin permeation of the sildenafil citrate microemulsion-loaded hydrogel was significantly higher than that of the sildenafil suspension, with a 1.97-fold enhancement ratio. The formulated microemulsion exhibited a 100% cell viability, indicating its safety for skin epithelial cells. The major metabolic pathway of sildenafil citrate loaded in the microemulsion formulation was hydroxylation. Furthermore, loading sildenafil in the microemulsion reduced the drug metabolite by approximately 50% compared to the sildenafil in aqueous suspension. The sildenafil citrate-loaded isopropyl myristate-based microemulsion hydrogels were physically and chemically stable over 6 months of storage. The sildenafil citrate microemulsion-loaded hydrogel showed in vitro results suitable for used as a transdermal drug delivery system.
Highlights
Sildenafil citrate is a selective phosphodiesterase type 5 (PDE5) inhibitor
A previous study prepared sildenafil microemulsions, and the results revealed that the solubility of sildenafil was considerably higher in oleic acid (75.5 ± 11.3 mg/mL) than that of sildenafil citrate in the present study
Sildenafil citrate exhibits a higher water solubility compared to the sildenafil base; sildenafil citrate requires oil to improve its permeability through the skin
Summary
Sildenafil citrate is a selective phosphodiesterase type 5 (PDE5) inhibitor. The mechanism of action of sildenafil involves the specific degradation of cyclic guanosine monophosphate (cGMP), resulting in smooth muscle relaxation via the nitric oxide (NO) pathway [1]. Sildenafil citrate is an effective oral treatment for erectile dysfunction and premature ejaculation [2,3]. It is available only in the form of tablets. Some studies have shown that sildenafil citrate or PDE inhibitor drugs can be administered topically [4,5,6,7]. Patients with mild and moderate erectile dysfunction and premature ejaculation may benefit from the accepted efficacy of topical therapy without systemic adverse effects. Results from randomised controlled trials on the efficacy of topical sildenafil in men with mild to moderate erectile dysfunction associated with premature ejaculation are limited and contradictory [2,6,8]. The contents of each vial were filtered through a 0.45-μm nylon membrane filter, and the supernatant was assayed for drug content using high-performance liquid chromatography (HPLC), according to the method described
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