The basic construction of the HFS was similar to the adult HFS model for extracellular tuberculosis, which was recently qualified by the European Medicines Agency and endorsed by the US Food and Drug Administration given its high forecasting accuracy [16 (link)–18 (link)]. We have adapted this to a HFS model of intracellular tuberculosis, by changing the media to RPMI/FBS and inoculating HFS with infected THP-1 cells, as recently described in detail [9 (link)]. We inoculated 20 mL of Mtb-infected THP-1 cells into the peripheral compartment of each of 16 HFSs that had been preconditioned with RPMI/FBS and maintained in incubators at 37°C for at least 72 hours. Linezolid was administered by computer-programmed syringe pumps via an infusion port into the central compartment. A combined exposure-effect and dose-scheduling study was performed wherein 9 of the 16 HFSs were dosed with linezolid to achieve AUC0–24 exposure of 0, 3, 6,12, 24, 31, 48, 77.5, and 118 mg × hour/L, and the remaining 7 systems were dosed with linezolid twice daily to achieve AUC 0–24 exposures that are twice the once-daily schedule, but at the same peak concentrations. This allows the breaking of the co-linearity of pharmacokinetic/pharmacodynamic (PK/PD) indices that would otherwise occur with dose changes. PK/PD indices under study included the AUC0–24/MIC ratio, peak concentration to MIC ratio (peak/MIC), and percentage of time concentration persists above MIC (%TMIC). The duration of therapy was 28 days. Fresh media were pumped into and out of the HFSs at predefined rates to mimic the linezolid half-life of 3 hours encountered in infants [3 (link)]. Concentration-time profiles of linezolid achieved in the HFS were substantiated by sampling the central compartment of each HFS at 7 time points over a 24-hour period post–drug infusion based on optimal sampling theory [19 (link)]. The peripheral compartments were sampled on days 0, 7, 14, 21, and 28 for quantification of the Mtb population and THP-1 cells. Samples were also cultured on agar supplemented with 3 times the linezolid MIC to capture the linezolid-resistant subpopulation at each time point.