N-Thiolated -Lactams: Novel Antibacterial Agents for Methicillin-Resistant Staphylococcus aureus Edward Turos, a, * Timothy E. Long, a Monika I. Konaklieva, a Cristina Coates, a Jeung-Yeop Shim, a Sonja Dickey, b Daniel V. Lim b and Andrew Cannons b a Department of Chemistry, 4202 E. Fowler Avenue, University of South Florida, Tampa, FL 33620, USA b Department of Biology, University of South Florida, Tampa, FL 33620, USA Received 10 January 2002; accepted 2 April 2002 Abstract—In this report we describe a new family of N-thiolated b-lactams that have antibacterial activity against methicillin- resistant Staphylococcus aureus (MRSA). The compounds show unprecedented structure–activity features and an unusual mode of action for a b-lactam antibiotic. # 2002 Elsevier Science Ltd. All rights reserved. Infections caused by methicillin-resistant Staphylo- coccus aureus (MRSA) are becoming extremely difficult to treat with conventional antibiotics, leading to a sharp rise in clinical complications. 1,2 The need for new anti- bacterial agents and protocols for treating MRSA infections is serious. In this report we describe a novel family of lipophilic b-lactam antibacterials that are effective against MRSA, and whose mode of action and structure–activity profiles differ dramatically from those of traditional b-lactam drugs. 3 In addition, their selec- tivity for Staphylococcus bacteria over most other com- mon microorganisms, and their stability to b-lactamase proteins, make these b-lactam molecules interesting leads for further investigation. In this study, a selection of N-methylthio-substituted b-lactams (1–9, Fig. 1) were examined for antimicrobial activity by the Kirby–Bauer disk diffusion method on agar plates. A variety of common Gram-positive and Gram-negative bacteria were tested, including clinical isolates of methicillin-resistant S. aureus (MRSA). Table 1 gives the zones of growth inhibition that were observed after 24 h. The data indicate that lactams 1–9 are most active against Staphylococcus and Micrococcus bacteria, but act only weakly against Neisseria gonor- rhoeae, Bacteroides fragalis, and Haemophilus influen- zae. The compounds have no activity against other common microorganisms we examined, including Kleb- siella pneumoniae, Listeria monocytogenes, Vibrio cho- lorae, Streptococcus pyrogenes (GAS), Streptococcus 0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0960-894X(02)00343-8 Bioorganic & Medicinal Chemistry Letters 12 (2002) 2229–2231 Table 1. Compound susceptibility measurements obtained from agar disk diffusion experiments using 6-mm air-dried disks impregnated with 20 mg of the test compound Bacterial strain 123456789PenG S. aureus (ATCC 25923) 27 18 25 26 15 12 14 23 27 34 MRSA USF652 31 19 30 30 17 12 14 23 28 8 MRSA USF653 30 23 30 30 22 14 16 27 28 16 MRSA USF654 28 16 26 26 16 12 12 23 27 10 MRSA USF655 29 17 25 26 14 14 12 23 29 14 MRSA USF656 30 19 28 28 18 11 13 25 28 12 MRSA USF657 30 17 27 28 18 12 10 23 26 12 MRSA USF658 27 16 26 27 17 12 12 22 26 19 MRSA USF659 25 15 24 24 11 13 12 20 24 15 S. epidermidis 30 23 31 29 20 12 20 25 28 50 S. simulans (ATCC 11631) 21 11 14 16 13 0 0 14 0 13 S. saprophyticus (ATCC 3552) 22 12 22 20 15 8 14 15 20 30 M. luteus 23 20 21 24 22 20 21 21 15 40 N. gonorrhoeae 13 11 14 19 12 11 12 12 11 0 The values correspond to the average diameters in mm (triplicate experiments) for the zone of growth inhibition observed after 24h. S. aureus (ATCC 25923) and b-lactamase-producing strains of methi- cillin-resistant S. aureus (labeled as MRSA USF652–659) were obtained from Lakeland Regional Medical Center, Lakeland, FL. S. epidermidis, S. simulans, S. saprophyticus, and M. luteus are clinical isolates from University of South Florida Medical Clinic. N. gonnor- hoeae (b-lactamase positive) was obtained from the Tampa Branch State Laboratory. PenG is penicillin G (potassium salt). Small zones ( < 15mm) were observed against B. fragalis and H. influ- enzae 561. No zones were observed against K. pneumoniae 512, L. monocytogenes, V. cholerae 1018 (CDC E5906, toxin +), V. cholerae 1019 (CDC 1074–78, toxin-), S. pyrogenes (GAS), S. agalactiae (GBS), S. marcessens 519 (ATCC 29634), S. typhimurium 515, P. aeruginosa (ATCC 15442), P. mirabilis, M. smegmatis, E. cloacae, or E. coli (ATCC 23590). *Corresponding author. Tel.: +1-813-974-7312; fax: +1-813-974- 1733; e-mail: eturos@chuma.cas.usf.edu