Comparison of 2 antibiotics that inhibit protein synthesis for the treatment of infection with Yersinia pestis delivered by aerosol in a mouse model of pneumonic plague

Introduction. Intentional release of Yersinia pestis will likely be propagated by aerosol exposure. We explored the effects of neutropenia on the outcome of doxycycline and gentamicin therapy. Methods. Female BALB/c mice were exposed to 20 LD50 of Y. pestis CO92 by aerosol. Treatments were saline (negative control), levofloxacin at 15 mg/kg every 12 h (positive control), doxycycline at 40 mg/kg every 6 h, and gentamicin at 12 mg/kg every 6 h, 24 mg/kg every 12 h, and 48 mg/kg every 24 h in cohorts of normal and neutropenic mice for 5 days. Results. Control mice died. Positive control mice ( levofloxacin) had 100% survivorship in both neutropenic and nonneutropenic groups. Doxycycline treatment in the presence of granulocytes yielded 90% survivorship; all neutropenic mice died after the termination of treatment (P << .001). For gentamicin, survivorship of mice receiving drug every 24, 12, and 6 h was, respectively, 80%, 80%, and 90% for normal mice and 80%, 100%, and 70% for neutropenic mice. No significant differences were seen in the neutropenia versus normal mouse comparison or by schedule. Conclusions. Doxycycline behaves in vivo as a bacteriostatic drug, requiring an intact immune system for clearance of the infection after aerosol challenge with Y. pestis. Gentamicin is bactericidal, even when given on a daily schedule. Neutropenia did not significantly affect survivorship.