In vitro performances of novel co-spray-dried azithromycin/rifampicin microparticles for Rhodococcus equi disease treatment

This work was aimed at providing clues on the in vitro performances of novel azithromycin/rifampicin combinations, in the form of co-spray-dried microparticles (AZM/RIF MP), against Rhodococcus equi, an animal and emerging human pathogen found responsible for worrying zoonosis. Various AZM/RIF combinations were spray-dried and characterized for their morphology and size. Susceptibility studies included determination of MIC, MBC, Fractional Inhibitory/Bactericidal Concentration Indexes and intracellular activity in R. equi-infected THP-1 cells. Cytotoxicity was tested on BEAS-2B cells through MTT assay and combination index assessment for drug interaction. Spray-dried MP were collapsed and 3–10 times smaller than commercial powders. Drug combinations showed an enhancement of in vitro antibacterial activity with a remarkable synergistic bactericidal effect. Azithromycin MP and AZM/RIF MP 2:1 led to a CFU reduction of >90% up to 4 days after treatment at all tested concentrations (p = 0.001) but AZM/RIF MP 2:1 were at least four-fold more potent than AZM MP alone. IC50 values of >100 mg/L supported low cytotoxicity of drug combinations and the combination index suggested an antagonistic toxic effect. Co-spray-drying enhanced powder dispersibility and solubility, which may improve bioavailability as well as provide administration alternatives. The novel AZM/RIF MP combinations could result a valid platform to develop new treatment strategies against R. equi infections in animals and humans.