Journal zyxwvutsrqponm of Applied Bacteriology zyxwvutsrq 1995, zyxwvutsr 70, 373-377 Antibiotic activity of isoxazolylnaphthoquinone imines on mice infected with Staphylococcus aureus I. Albesa, P. Bogdanov, A. Eraso, N.R. Sperandeo and M.M. de Bertorello Departamento de Farmacia, Facultad de Ciencias et Quimicas, Universidad Nacional de Cordoba, Cordoba, Argentina 4949/06/94: received 10 June 1994 and accepted 17 November 1994 zyxwvu I. zyxwvutsrqponmlkjihg ALBESA, P. BOGDANOV, A. ERASO, N.R. SPERANDEO AND M.M. DE BERTORELLO. 1995. The antibiotic activity of new synthetic zyxwvut isoxazolylnaphthoquinone imines was studied. zyxw Pseudomonas aeruginosa ATCC 27853 and Escherichia coli ATCC 25922 were resistant to the four compounds studied (MIC > 128 ,ug ml-'), but Staphylococcus aureus ATCC 25923, ATCC 29213 and 30 clinical isolates of Staph. aureus were inhibited by 2-hydroxy-N-(3,4- dimethyl-5-isoxazoly1)-1,4-naphthoquinone-4-imine (I). This compound diminished bloodstream infection of mice injected i.m. with Staph. aureus ; septicaemia decayed significantly when I was applied at the beginning of the infection while when I was given 3 d after bacterial challenge, a significant protection was afforded. Bactericidal activity in serum increased during the 5 h after I was administered i.p. The acetyl derivative of I had a high MIC but when inoculated orally in mice decreased the Staph. aureus counts in circulation. This protection occurred only when the schedule of administration started close to the bacterial challenge. Antibiotic activity in vivo may be associated with in vitro effects. INTRODUCTION Naphthoquinones have been extensively studied for their broad spectrum of biological activities which ranges from antimalarials (Martin et al. 1973), antiprotozoals (Arraujo et al. 1992; Hudson et al. 1985; Hughes et al. 1991) as well as antineoplastics Ueng Lin and Sartorelli 1976). Their antibacterial activity (Muthusubramanian and Misra 1986) was also investigated and it was observed that, among others, lapachol and related heterocyclic compounds were inhibitory in vitro against Staphylococcus aureus and Escherichia coli, the concentrations required to reduce microbial growth by 50% (ID5,,) being 78-100 pg ml-' (Cortes et a/. 1983). Because of the relationship of isoxazolylnaphthoquinones and naphthoquinones the biological activity of some members of this family against the causative agent of Chagas disease was also studied (Amuchastegui et al. 1990; Tarlovsky et al. 1990). Based on these findings and the continuing need to find new active compounds against Gram-positive bacteria, par- ticularly against P-lactamase-positive staphylococci, the Correspondenceto : Professor Dr Inhs .4lbesa, Departamento de Farmacra, Facultad de Gencras et Quimrcas, Unruersrdad Nacronal de CQrdoba, C.C. 61, Suc. Ih. 5016 Chrdoba, Arxentma. antibacterial activity of isoxazolylnaphthoquinones was pre- liminarily investigated (Bogdanov et zyxw al. 1993). Encouraged by those results the antibiotic activity of 2-hydroxy-N-(3,4- dimethyl-5-isoxazoly1)- 1,Cnaphtho- quinone-4-imine (I, Fig. 1) was studied in vitro and in vivo and compared with three of its derivatives (11-IV, Fig. 1) to discover the effects of different substituents on its anti- microbial activity. MATERIALS AND METHODS Organisms Bacteria used included Pseudomonas aeruginosa ATCC 27853, E. coli ATCC 25923, Staph. aureus ATCC 25923 and 29213, 30 clinical isolates of p-lactamase positive Staph. aureus and six clinical isolates of E. coli. lsoxazolylnaphthoquinone imines The compounds 2-hydroxy-N-(3,4-dimethyl-5-isoxazolyl)- 1,4-naphthoquinone-4-imine( I), 2-acetoxy-N-(3,4dimethyl- 5-isoxazolyl)-l,4-naphthoquinone-4-imine (11), 2-hydroxy- N-( 3,4-dimethyl-5-isoxazolyl)- 1,4-naphthoquinone-4-imine-