Front-loaded linezolid regimens result in increased killing and suppression of the accessory gene regulator system of Staphylococcus aureus

Abstract

Front loading is a strategy used to optimize the pharmacodynamic profile of an antibiotic through the administration of high doses early in therapy for a short duration. Our aims were to evaluate the impact of front loading of linezolid regimens on bacterial killing and suppression of resistance and on RNAIII, the effector molecule of the accessory gene regulator system (encoded by agr) in methicillin-resistant Staphylococcus aureus (MRSA). Time-killing experiments over 48 h were utilized for linezolid against four strains of MRSA: USA100, USA300, USA400, and ATCC 29213. A hollow-fiber infection model simulated traditional and front-loaded human therapeutic regimens of linezolid versus USA300 at 10(6) CFU/ml over 240 h. Over 48 h in time-kill experiments, linezolid displayed bacteriostatic activity, with reductions of >1 log(10) CFU/ml for all strains. Front-loaded regimens that were administered over 5 days, 1,200 mg every 12 h (q12h) (total, 10 doses) and 2,400 mg q12h (total, 10 doses) followed by 300 mg q12h thereafter, resulted in sustained bactericidal activity, with reductions of the area under the CFU curve of -6.15 and -6.03, respectively, reaching undetectable limits at the 10-day study endpoint. All regimens displayed a reduction in RNAIII relative expression at 24 h and 240 h compared with that of the growth control. Monte Carlo simulations predicted a <1.27× increase in the fractional decreases in platelets for all front-loaded regimens versus the 600 mg q12h regimen, except for the highest-dose front-loaded regimen. Front-loading strategies for linezolid are promising and may be of utility in severe MRSA infections, where early aggressive therapy is necessary.

Fig 1

Time-kill experiments for linezolid versus ATCC 29213 (A), MRSA USA100 (B), MRSA USA300 (C), or MRSA USA400 (D). MICs to linezolid for all strains were 2.0 mg/liter.

Fig 2

Hollow-fiber infection model experiments simulating linezolid traditional regimen (A) and front-loaded regimens (B to E) versus MRSA USA300. (A) Traditional regimen: 600 mg every 12 h. (B) Front-loaded regimen: 1,200 mg every 12 h (total, 2 doses) on days 0 to 1 followed by 600 mg every 12 h (total, 18 doses) on days 1 to 10. (C) Regimen of 2,400 mg every 12 h (total, 2 doses) on days 0 to 1 followed by 600 mg every 12 h (total, 18 doses) on days 1 to 10. (D) regimen of 1,200 mg every 12 h (total, 10 doses) on days 0 to 5 followed by 300 mg every 12 h (total, 10 doses) on days 5 to 1. (E) Front-loaded regimen: 2,400 mg (total, 10 doses) on days 0 to 5 followed by 300 mg every 12 h (total, 10 doses) on days 5 to 10.

Fig 3

Relative expression of RNAIII, the primary transcript of the agr quorum-sensing system, in response to front-loaded and traditional dosing regimens of linezolid.

Fig 4

(A) Mechanism-based based pharmacodynamic mathematical modeling. The final model contained two subpopulations with different susceptibilities. (B) Observed versus population-predicted plot for all time-kill experiments for linezolid versus ATCC 29213, MRSA USA100, USA300, and USA400.

Fig 5

Dose-response relationship for all linezolid regimens. The y axis in panel A represents the log ratio area-under-the-curve approach as a measure of response, while panel B represents the point estimates comparing the bacterial reductions at 240 h to levels at 0 h. The x axis represents the derived mean 24-h fAUC, which was averaged over the 240 h. “R²” represents the coefficient of determination and values of model-fitted parameter estimates from equation 2.

Fig 6

Monte Carlo simulations displaying the distribution of fAUCs based on previously published population pharmacokinetic analysis from the linezolid compassionate-use trials (20). All patients in the previously published study received 600 mg q12h. A protein binding level of 31% was utilized to calculate the 24-h free-drug AUC from the total AUC in the population PK study. The simulated doses studied in the in vitro hollow-fiber model are within this distribution and are overlaid as symbols.