In vitro: Quinupristin can bind to sequential sites located on the 50s subunit of the bacterial ribosome. Dalfopristin binding causes a conformational change in the ribosome that subsequently increases the binding of quinupristin. The combined actions of the two agents create a stable drug-ribosome complex causing inhibition of protein synthesis by prevention of peptide-chain formation, blockade of extrusion of newly formed peptide chains, and bacterial cell death [1].

In vivo: The combination of quinupristin-dalfopristin (Q-D) and gentamicin was tested against two strains of gentamicin- and dalfopristin-susceptible methicillin-resistant Staphylococcus aureus (MRSA). A rabbit endocarditis model simulated the pharmacokinetics achieved in humans receiving intravenous injections of Q-D and gentamicin. For the MLSB-susceptible strain, a 4-day regimen reduced mean bacterial titers (MBT) in vegetations from 8.5 ± 0.8 logs CFU/g (control group) to 3.0 ± 0.9 (Q-D) and 2.6 ± 0.5 logs CFU/g (Q-D plus gentamicin). For the strain constitutively resistant to MLSB, a 4-day regimen reduced MBT in vegetations from 8.7 ± 0.9 logs CFU/g (control group) 5.2 ± 2.2 (Q-D) and 5.1 ± 2.4 log CFU/g (Q-D plus gentamicin). The differences between control and treatment groups were significant for both strains, although there was no significant difference between treatment groups [2].