Mammalian
cathelicidins are a class of innate
antimicrobial peptides isolated from leukocytes and epithelial cells that aid host defense against
bacterial infections. Synthetic analogs of
cathelicidins offer the promise of potent broad-spectrum antimicrobial efficacy. We developed a combined lung
infection and ex vivo whole-blood assay model to characterize the toxicity and efficacy of synthetic
cathelicidin-derived
peptides. Male C57BL/6 mice were administered saline or Klebsiella pneumoniae by intratracheal instillation. Five hours later, the Klebsiella-infected mice were instilled with saline,
tobramycin (1 mg/kg of
body weight or 10 mg/kg),
novispirin G10 (0.4 mg/kg), or a combination of
tobramycin (1 mg/kg) and G10 (0.4 mg/kg). At 24 h, bronchoalveolar lavage fluid (BAL) was collected for analysis of culturable bacteria and for markers of
inflammation and lung toxicity. Blood samples were analyzed for circulating
cytokines. Recovery of Klebsiella from the lung, recruitment of neutrophils, and production of
interleukin-6 (IL-6) in BAL samples were highly correlated (r=0.68 and 0.84, respectively; P<0.01). Animals treated with G10 or G10 plus
tobramycin had increased
hemoglobin (P<0.001) and
protein (P<0.001) levels compared to those for Klebsiella-infected or
tobramycin-alone-treated animals. The levels of circulating
IL-6 in mice infected with Klebsiella were 1000- to 10,000-fold higher than in the noninfected controls. The highest levels of
IL-6 were measured in mice given G10 alone or in combination with
tobramycin. These studies demonstrated that G10 was relatively nontoxic in saline-treated mice but was highly toxic in mice infected with Klebsiella. This finding establishes the importance of investigating candidate
antimicrobial agents in an in vivo
infection model.