ABSTRACT:GSK1322322 is a novel inhibitor of peptide deformylase (PDF) with goodin vitroactivity against bacteria associated with community-acquired pneumonia and skin infections. We have characterized thein vivopharmacodynamics (PD) of GSK1322322 in immunocompetent animal models of infection withStreptococcus pneumoniaeandHaemophilus influenzae(mouse lung model) and withStaphylococcus aureus(rat abscess model) and determined the pharmacokinetic (PK)/PD index that best correlates with efficacy and its magnitude. Oral PK studies with both models showed slightly higher-than-dose-proportional exposure, with 3-fold increases in area under the concentration-time curve (AUC) with doubling doses. GSK1322322 exhibited dose-dependentin vivoefficacy against multiple isolates ofS. pneumoniae,H. influenzae, andS. aureus. Dose fractionation studies with twoS. pneumoniaeandS. aureusisolates showed that therapeutic outcome correlated best with the free AUC/MIC (fAUC/MIC) index inS. pneumoniae(R2, 0.83), whereasfAUC/MIC and free maximum drug concentration (fCmax)/MIC were the best efficacy predictors forS. aureus(R2, 0.9 and 0.91, respectively). Median dailyfAUC/MIC values required for stasis and for a 1-log10reduction in bacterial burden were 8.1 and 14.4 for 11S. pneumoniaeisolates (R2, 0.62) and 7.2 and 13.0 for fiveH. influenzaeisolates (R2, 0.93). The data showed that for eightS. aureusisolates,fAUC correlated better with efficacy thanfAUC/MIC (R2, 0.91 and 0.76, respectively), as efficacious AUCs were similar for all isolates, independent of their GSK1322322 MIC (range, 0.5 to 4 μg/ml). MedianfAUCs of 2.1 and 6.3 μg · h/ml were associated with stasis and 1-log10reductions, respectively, forS. aureus.