Synthesis and Characterization of Thiol-Ene Microparticles for Doxycycline Delivery

Osteomyelitis is a significant clinical challenge that leads to bone destruction and loss. Systemic antibiotic treatments do not reach infected sites at therapeutic levels, necessitating high dosages, which in turn can lead to toxicity and promote the emergence of antibiotic-resistant bacteria. To overcome these challenges, our objective was to synthesize a biocompatible drug-delivery system that has the potential to achieve localized antibiotic delivery to completely eradicate bacteria. We synthesized four formulations of microparticles using thiol-ene click chemistry by varying the thiol monomer-to-crosslinker (2-thiol:4-thiol) ratio and the ene monomer-to-surfactant (2-ene:Hitenol) ratio. The microparticles were loaded with doxycycline by soaking them in 20 or 50 mg/mL doxycycline (D20 or D50) over 24 h. The microparticles exhibited a payload-dependent release, with the D20 groups releasing approximately 11%-22% of their total doxycycline content, while the D50 groups released less than 10% of the total doxycycline content by 96 h. All doxycycline-loaded microparticle formulations showed antibacterial activity against Methicillin-resistant Staphylococcus aureus (MRSA) over a 7-day period, with the formulation with a 2-thiol:4-thiol ratio of 0.1:0.9 and a 2-ene:Hitenol ratio of 0.8:0.2 showing complete eradication of bacteria by Day 7. The MTT and Live/Dead assays showed that doses of 12 mg or less of this formulation were not cytotoxic to mouse pre-osteoblast cells. Taken together, we successfully synthesized a biocompatible microparticle-based drug delivery system that can be loaded with doxycycline and that exhibits drug release under physiological conditions.