SAR studies in a series of related 3-(heteroarylthio)cephems determined that a relatively high chemical reactivity of the beta-lactam ring, modulated by electronic effects of substituents at C-3 and C-7, is necessary to achieve high in vitro activity against methicillin-resistant Staphylococcus aureus (MRSA). Such high reactivity results in lowered hydrolytic stability and concomitantly increases susceptibility to beta-lactam ring opening mediated by serum enzymes. Therefore, optimization of anti-MRSA activity versus stability toward serum-mediated degradation required a fine balance of substituent effects. Serum stability studies (measured as percentage of parent drug degraded after 60 min incubation) revealed up to 80-fold difference in degradation rate in a series of closely related (3-heteroarylthio)cephems. Of the compounds evaluated, RWJ-333441 (MC-04,546) possessed the best balance of serum stability (6% degradation after 60 min incubation) and in vitro activity versus MRSA (S. aureus COL MIC=1 microgram/mL). Accordingly, RWJ-333441 displayed excellent in vivo efficacy versus methicillin-susceptible Staphylococcus aureus (MSSA, ED(50)=0.39 mg/kg in mouse sepsis model with S. aureus Smith) and good pharmacokinetic properties in the rat (Cl(total)=0.39 L/h/kg).