Silk fibroin hydrogels as carriers for sustained release of tetracycline-class antibiotics: an in vitro and in silico evaluation

The high oral bioavailability of tetracycline (TC) family antibiotics facilitates systemic absorption but limits sustained local drug concentrations at the infection site. Silk fibroin (SF) hydrogels offer a promising system for controlled local drug delivery. In this study, doxycycline hyclate (DH), minocycline (MN), and TC-loaded SF hydrogels were prepared using a freeze-drying method and comparatively evaluated. Swelling studies, scanning electron microscopy, Fourier transform infrared spectroscopy, drug entrapment efficiency, drug release, and antibacterial assays were performed. Molecular docking, 100 ns molecular dynamics (MD) simulations, and MM-PBSA binding free-energy calculations were performed to evaluate the molecular basis of antibiotic retention within the SF matrix. All three hydrogels exhibited interconnected porous structures and high drug entrapment efficiencies (>97%), with drug release following Korsmeyer-Peppas model kinetics with an initial burst followed by sustained release. SF-TC exhibited the highest cumulative release, reaching 83.89% at pH 6.4 and 81.1% at pH 7.2, followed by SF-DH (68.32% and 66.11%) and SF-MN (63.26% and 62.46%). Antibiotic-loaded SF hydrogels exhibited smaller zones of inhibition against Staphylococcus aureus and Streptococcus mutans than the corresponding free antibiotics. Molecular docking and MD simulations revealed that SF-antibiotic binding was primarily driven by van der Waals and electrostatic interactions, along with hydrogen bonding. MD simulations demonstrated greater structural stability of the SF-DH and SF-MN complexes than the SF-TC complex, consistent with their more sustained experimental release profiles. Overall, SF hydrogels show potential as carriers for localised and sustained delivery of TC-class antibiotics and provide molecular-level insights into controlled drug release.