High Stability and Low Irritation of Enrofloxacin-Colistin Combination Injection Through a Tripartite Strategy

Area of Science:

• Pharmaceutical Chemistry
• Veterinary Medicine
• Drug Delivery

Background:

• Bacterial resistance and limited new antimicrobial development necessitate combination therapies.
• Formulation challenges, particularly drug solubility and stability, hinder effective combination drug development.
• Enrofloxacin's low solubility impedes its co-formulation with other agents.

Purpose of the Study:

• To develop a stable enrofloxacin-colistin combination injection using a novel tripartite strategy.
• To investigate the molecular mechanisms underlying the formulation process.
• To evaluate the in vitro and in vivo safety and efficacy of the combination injection.

Main Methods:

• A tripartite strategy involving enrofloxacin salt formation, 1,2-propanediol addition, and pH adjustment was employed.
• Independent gradient model based on Hirshfeld surface (IGMH), NMR, and UV-vis spectroscopy were used for mechanistic studies.
• In vitro and in vivo evaluations assessed irritancy, toxicity, and therapeutic efficacy against *Pasteurella multocida*.

Main Results:

• The tripartite strategy increased enrofloxacin solubility by 1500-fold, ensuring stability for 6 months.
• 1,2-Propanediol improved hydrogen bonding and inhibited enrofloxacin self-aggregation.
• The combination injection showed no significant liver/kidney toxicity and achieved a 62.5% survival rate in *P. multocida* pneumonia models.

Conclusions:

• The tripartite strategy effectively overcomes enrofloxacin precipitation and pH limitations in complex formulations.
• This approach enhances solubility, stability, and therapeutic efficacy of enrofloxacin-colistin combinations.
• The findings offer a novel solution for developing complex antibacterial formulations, particularly for veterinary applications.