Transdermal delivery of risedronate via spanlastics for osteoporosis: formulation optimization, skin deposition, and pharmacokinetic assessment.

Risedronate is widely used for osteoporosis management, but its oral administration has limitations like poor bioavailability and gastrointestinal side effects. We developed and optimized a transdermal spanlastic formulation of risedronate to improve bioavailability. Risedronate-loaded spanlastics were formulated by ethanol injection and optimized through a Box-Behnken design. The effects of Span 60:Tween 40 ratio, stirring speed, and sonication time on vesicle size, polydispersity index (PDI), zeta potential, entrapment efficiency, and drug release were evaluated. The optimized spanlastics were incorporated into a Carbopol 934 gel for transdermal application and further studied for physicochemical parameters and pharmacokinetic evaluation in a rat model. The optimized vesicles exhibited a mean size of 191.80 ± 1.22 nm, PDI of 0.44 ± 0.06, zeta potential of - 27.2 ± 1.65 mV, entrapment efficiency of 90.49 ± 0.15%, and cumulative drug release of 95.16 ± 1.18% after 24 h. FTIR studies confirmed the absence of drug-excipient interactions, while TEM revealed spherical vesicles. Ex vivo studies demonstrated a twofold increase in skin permeation with spanlastics gel than plain drug gel. Skin irritation studies confirmed the biocompatibility of the optimized formulation. The pharmacokinetic parameters showed risedronate bioavailability improved by 1.84-fold versus plain drug and 3.90-fold versus a marketed formulation. The formulation was found to be stable for 6 months after the stability study. Risedronate-loaded spanlastics represent a promising transdermal delivery system that overcomes oral limitations by improving bioavailability and skin permeation. Still, further investigation of risedronate-loaded spanlastics gel in in vivo pharmacodynamics studies is necessary to confirm its potential in the long-term treatment of osteoporosis.