A review of the in-vitro activity of quinupristin/dalfopristin against intracellular pathogens and mycoplasmas

Quinupristin/dalfopristin displays in-vitro bacteriostatic activity against all Legionella spp. (MICs = 0.06-2 mg/L), with Legionella pneumophila usually being at least two-fold more sensitive to quinupristin/dalfopristin than Legionella bozemanii, Legionella dumoffii, Legionella gormanii and Legionella micdadei (MIG = 0.06-2 vs 1-2 mg/L, respectively). Against Legionella spp., quinupristin/dalfopristin was at least as active as erythromycin. Quinupristin/dalfopristin was active in vitro against all Mycoplasma spp. tested (MIG = 0.05-2 mg/L), with Mycoplasma hominis being less susceptible than other species. Quinupristin/dalfopristin was active against erythromycin-resistant strains of Mycoplasma fermentans and M. hominis (MIC50 = 0.5 and 2 mg/L, respectively), and doxycycline-resistant strains of Ureaplasma urealyticum (MIC90 = 1 mg/L). The in-vitro bacteriostatic activity against Mycoplasma pneumoniae and Mycoplasma genitalium (MIC90 = 0.1 and 0.05 mg/L, respectively) was similar to that of erythromycin and doxycycline. Quinupristin/dalfopristin was actively taken up by murine macrophages, and incubation of the drug (2.5 mg/L) with macrophages containing ingested Staphylococcus aureus resulted in the death of 70% of intracellular bacteria within 120 min. Intracellular concentrations of quinupristin/dalfopristin reached 50 and 30 times the extracellular concentration, respectively, showing that these compounds readily penetrate into cells. The intracellular activity of quinupristin/dalfopristin may make it suitable for use in some, presently difficult-to-treat, infections caused by intracellular organisms.