Ceftolozane-Tazobactam in the Treatment of Experimental Pseudomonas aeruginosa Pneumonia in Persistently Neutropenic Rabbits: Impact on Strains with Genetically Defined Mechanisms of Resistance.

Ceftolozane-tazobactam (C/T) is a novel cephalosporin with in vitro activity against Pseudomonas aeruginosa that is resistant to extended-spectrum penicillins and antipseudomonal cephalosporins. In order to assess the antimicrobial effect of C/T in treatment of Pseudomonas pneumonia, we investigated the pharmacokinetics and efficacy of C/T in persistently neutropenic rabbits. Pseudomonas pneumonia was established by direct endotracheal inoculation. Treatment groups consisted of C/T, ceftazidime (CAZ), piperacillin-tazobactam (TZP), and untreated controls (UC). Rabbits received a dosage of C/T of 80 mg/kg every 4 h (q4h) intravenously (i.v.) (53 mg/kg ceftolozane/26 mg/kg tazobactam) to match the free drug time above the MIC as well as a comparable plasma area under the concentration-time curve (AUC) (humanized doses of ceftolozane-tazobactam of 3 g [2 g/1 g]) q8h, due to the more rapid elimination of ceftolozane in rabbits (0.75 h) than in humans (2.5 h). Four molecularly characterized clinical P. aeruginosa isolates from patients with pneumonia were studied, including one isolate from each classification group: pan-susceptible (PS), outer membrane porin D (OPRD) porin loss (OPRDPL), efflux pump expression (EPE), and AmpC hyperexpression (ACHE). Treatment was continued for 12 days. Treatment with ceftolozane-tazobactam resulted in a ≥105 reduction in residual pulmonary and bronchoalveolar lavage (BAL) fluid bacterial burdens caused by all 4 strains (P ≤ 0.01). This antibacterial activity coincided with reduction of lung weight (an organism-mediated pulmonary injury marker) (P < 0.05). CAZ was less active in ACHE-infected rabbits, and TZP had less activity against EPE, ACHE, and OPRDPL strains. Survival was prolonged in the C/T and CAZ treatment groups in comparison to the TZP and UC groups (P < 0.001). Ceftolozane-tazobactam is highly active in treatment of experimental P. aeruginosa pneumonia in persistently neutropenic rabbits, including infections caused by strains with the most common resistance mechanisms.