N-Thiolated b-lactam antibacterials: Effects of the N-organothio substituent on anti-MRSA activity Bart Heldreth, a Timothy E. Long, a Seyoung Jang, a G. Suresh Kumar Reddy, a Edward Turos, a, * Sonja Dickey b and Daniel V. Lim b a Department of Chemistry, 4202 East Fowler Avenue, SCA 400, University of South Florida, Tampa, FL 33620, USA b Department of Biology, University of South Florida, Tampa, FL 33620, USA Received 19 October 2005; revised 13 January 2006; accepted 16 January 2006 Available online 15 February 2006 Abstract—A study on the structure–activity profiles of N-thiolated b-lactams 1 is reported which demonstrates the impor- tance of the N-organothio moiety on antibacterial activity. Our results indicate that elongation of the N-alkylthio residue beyond two carbons, or extensive branching within the organothio substituent, diminishes antibacterial effects. Of the deriv- atives we examined, the N-sec-butylthio b-lactam derivative 5g possesses the strongest growth inhibitory activity against methicillin-resistant Staphylococcus aureus strains. Sulfur oxidation state is important, as the N-sulfenyl and N-sulfinyl groups provide for the best antibacterial activity, while lactams bearing the N-sulfonyl or N-sulfonic acid functionalities have much weaker or no anti-MRSA properties. Stereochemistry within the organothio chain does not seem to be a significant factor, although for N-sec-butylthio b-lactams 15a–d, the 3R,4S-lactams 15c, d are more active than the 3S,4R-stereoisomers 15a, b in agar diffusion experiments. The N-methylthio lactams are the most sensitive to the presence of glutathione, followed by N-ethylthio and N-sec-butylthio lactams, which indicates that bioactivity and perhaps bacterial selectivity of the lactams may be related to the amount of organothiols in the bacterial cell. These results support the empirical model for the mechanism of action of the compounds in which the lactam transverses the bacterial membrane to deliver the organothio moiety to its cellular target. Ó 2006 Elsevier Ltd. All rights reserved. 1. Introduction N-Thiolated b-lactams 1 are a new family of antibac- terial agents active against Staphylococcus bacteria, including methicillin-resistant strains of Staphylococcus aureus (MRSA). 1 Although these compounds exhibit antigrowth properties against other Staphylococcus species, including S. epidermidis, S. simulans, S. sap- rophyticus, and a few other genera such as Micrococ- cus luteus and Neisseria gonorrhoeae, a wide range of other Gram-positive and Gram-negative microbes appear to be unaffected. 1b This observed selectivity for certain bacteria is rather interesting and undoubt- edly relates to the mode of action, which has not yet been completely defined. Moreover, the ability of lac- tams 1 to retain their full antibacterial activity against drug-resistant microbes such as MRSA is attributable to the stability these compounds have toward bacterial penicillinases. 1b The compounds are inert to hydrolytic degradation by these enzymes, a property highly desir- able for a b-lactam drug, and do not inhibit the hydrolytic cleavage of penicillin G by b-lactamases. Additionally, the lactams possess promising anticancer properties, yet are not cytotoxic to normal mammalian (human fibroblast) cells at concentrations well beyond that needed to inhibit bacterial growth. 2 Our studies also indicate that these N-thiolated lactams display different structure–activity profiles to those of tradi- tional b-lactam antibacterials and exert their growth inhibitory effects in a manner which is unique to those of other b-lactams. For instance, the C 3 and C 4 ring substituents (R 1 –R 4 ) have only a marginal effect on anti-MRSA activity, and neither relative nor absolute chirality at these two centers seems to significantly perturb activity. 3 These preliminary structure–activity studies also indicate that the N-organothio group may play a more important role, however, since 0968-0896/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.bmc.2006.01.029 Keywords: N-Thiolated b-lactams; MRSA; SAR; Antibiotics. * Corresponding author. Tel.: +1 813 974 7312; fax: +1 813 974 1733; e-mail: eturos@shell.cas.usf.edu Bioorganic & Medicinal Chemistry 14 (2006) 3775–3784