Activities of ceftobiprole and other cephalosporins against extracellular and intracellular (THP-1 macrophages and keratinocytes) forms of methicillin-susceptible and methicillin-resistant Staphylococcus aureus

Lemaire, Sandrine [UCL] Glupczynski, Gerald [UCL] Duval, Valérie Joris, Bernard Tulkens, Paul M. [UCL] Van Bambeke, Françoise [UCL]

S. aureus is an opportunistic intracellular organism. Although poorly accumulating in eukaryotic cells, beta-lactams show activity against intracellular MSSA if exposure times and drug concentrations are sufficient. Intraphagocytic MRSA are susceptible to penicillins and carbapenems because acidic pH favors the acylation of PBP 2a by these beta-lactams through pH-induced conformational change. Ceftobiprole, showing almost similar in vitro activities against MRSA and MSSA in broth, was examined for intracellular activity (THP-1 macrophages, keratinocytes) against a panel of hospital-acquired and community-acquired MRSA (MICs: 0.5-2.0 mg/L at pH 7.4 and 0.25-1.0 mg/L at pH 5.5) vs. MSSA isolates, with measurement of the key pharmacological descriptors of relative efficacy (Emax), relative potency (E50), and static concentration (Cs). All strains showed sigmoidal dose-responses with Emax at about 1 log10 CFU decrease from post-phagocytosis inoculum, and E50 and Cs at 0.2 to 0.3 x and 0.6 to 0.9 x the MIC, respectively. Ceftobiprole effectively competed with Bocillin FL (a fluorescent derivative of penicillin V) for binding to PBP 2a at both pH 5.5 and 7.4. In contrast, cephalexin, cefuroxime, cefoxitin, or ceftriaxone (i) were less potent in PBP 2a competitive binding assays; (ii) showed only partial restoration of activity towards MRSA in broth at acidic pH; (iii) were collectively less effective against MRSA in THP-1 macrophages and ineffective in keratinocytes. The improved activity of ceftobiprole towards intracellular MRSA vs. conventional cephalosporins, can be explained, at least in part, by its greater ability to bind to PBP 2a not only at neutral but also at acidic pH.