['Novel synthetic polymyxin derivatives including NAB737 and NAB739 are as effective as polymyxin B in vitro against the common opportunistic pathogen Escherichia coli. Another derivative, NAB7061, lacks direct antibacterial action but sensitizes E. coli to several other antibacterial agents including macrolides. The renal handling of NAB739 and NAB7061 in rats differs from that of polymyxin B. Furthermore, the affinities of NAB739 and NAB7061 for isolated rat kidney brush border membrane are significantly lower than that of polymyxin B. Here we investigate the in vivo antibacterial effect of these compounds.', 'The polymyxin derivatives were evaluated in an experimental murine peritonitis model. Immunocompetent mice were infected intraperitoneally with E. coli IH3080 and were subcutaneously treated with NAB737, NAB739 or NAB7061.', 'A >4.0 log(10) reduction in bacterial load compared with saline control was achieved 6 h after initiation of treatment with 1 mg/kg of NAB739 twice or 4 mg/kg of NAB737 twice. Combination therapy with NAB7061 (5 mg/kg) twice and erythromycin (10 mg/kg) resulted during the same time course in a >2.0 log(10) reduction in bacterial load compared with saline control. Neither NAB7061 nor erythromycin was effective as monotherapy. Together with the ability to reduce bacterial load, the NAB compounds also improved the clinical status of the mice.'] Are novel polymyxin derivatives effective in treating experimental Escherichia coli peritoneal infection in mice?