In vivo pharmacodynamic characterization of a novel plectasin antibiotic, NZ2114, in a murine infection model

Abstract

NZ2114 is a novel plectasin derivative with potent activity against gram-positive bacteria, including multiply drug-resistant strains. We used the neutropenic murine thigh infection model to characterize the time course of antimicrobial activity of NZ2114 and determine which pharmacokinetic/pharmacodynamic (PK/PD) index and magnitude best correlated with efficacy. Serum drug levels following administration of three fourfold-escalating single-dose levels of NZ2114 were measured by microbiologic assay. Single-dose time-kill studies following doses of 10, 40, and 160 mg/kg of body weight demonstrated concentration-dependent killing over the dose range (0.5 to 3.7 log(10) CFU/thigh) and prolonged postantibiotic effects (3 to 15 h) against both Staphylococcus aureus and Streptococcus pneumoniae. Mice had 10(6.3) to 10(6.8) CFU/thigh of strains of S. pneumoniae or S. aureus at the start of therapy when treated for 24 h with 0.625 to 160 mg/kg/day of NZ2114 fractionated for 4-, 6-, 12-, and 24-h dosing regimens. Nonlinear regression analysis was used to determine which PK/PD index best correlated with microbiologic efficacy. Efficacies of NZ2114 were similar among the dosing intervals (P = 0.99 to 1.0), and regression with the 24-h area under the concentration-time curve (AUC)/MIC index was strong (R(2), 0.90) for both S. aureus and S. pneumoniae. The maximum concentration of drug in serum/MIC index regression was also strong for S. pneumoniae (R(2), 0.96). Studies to identify the PD target for NZ2114 utilized eight S. pneumoniae and six S. aureus isolates and an every-6-h regimen of drug (0.156 to 160 mg/kg/day). Treatment against S. pneumoniae required approximately twofold-less drug for efficacy in relationship to the MIC than did treatment against S. aureus. The free drug 24-h AUCs/MICs necessary to produce a stasis effect were 12.3 +/- 6.7 and 28.5 +/- 11.1 for S. pneumoniae and S. aureus, respectively. The 24-h AUC/MIC associated with a 1-log killing endpoint was only 1.6-fold greater than that needed for stasis. Resistance to other antimicrobial classes did not impact the magnitude of the PD target required for efficacy. The PD target in this model should be considered in the design of clinical trials with this novel antibiotic.

FIG. 1.

Serum pharmacokinetics of plectasin derivative NZ2114 following single subcutaneous doses. Each symbol represents the mean concentration from three mice. The error bars represent the standard deviations. Three dose levels (fourfold escalating) of NZ2114 were studied. The measured C_max and calculated elimination half-life and AUC (zero to infinity) are shown in the table. Protein binding was determined by ultrafiltration using concentrations of 25 and 100 μg/ml. A microbiologic assay using S. aureus 6538p was used for determination of all NZ2114 concentrations.

FIG. 2.

Impact of plectasin derivative NZ2114 dose escalation on burden of either S. pneumoniae or S. aureus in the thighs of neutropenic mice over time. Each symbol represents the mean CFU/thigh from two mice (four thighs). The error bars represent the standard deviations. Solid symbols represent growth of organisms in control mice over time. Open symbols represent the burden of organisms in NZ2114-treated mice. The horizontal boxes represent the duration of time that free drug NZ2114 serum concentrations remained above the MIC of the infecting organism. The PAE is expressed in hours.

FIG. 3.

Impact of plectasin derivative NZ2114 dosing interval on efficacy against a strain of S. aureus and a strain of S. pneumoniae in the neutropenic murine thigh infection model. Five total (mg/kg/24-h) doses were fractionated into one, two, four, or six doses over the 24-h treatment period (q24 h, q12 h, q6 h, and q4 h, respectively). Total escalating doses varied fourfold. Efficacy is expressed as change in CFU/thigh compared to organism burden at the start of therapy. Each symbol represents the mean CFU/thigh from two mice (four thighs). The error bars represent the standard deviations. The horizontal dashed line represents the burden of organisms at the start of therapy.

FIG. 4.

Relationship between the NZ2114 pharmacodynamic index (24-h AUC/MIC, C_max/MIC, and percent time above MIC) and efficacy over 24 h against S. pneumoniae 10813 in a neutropenic murine thigh infection model. Unbound (free drug) concentrations were used for index calculations. Efficacy is expressed as change in CFU/thigh compared to organism burden at the start of therapy. Each symbol represents the mean CFU/thigh from two mice (four thighs). The sigmoid line represents the best fit using the sigmoid E_max model. R² is the coefficient of determination.

FIG. 5.

Relationship between the NZ2114 pharmacodynamic index (24-h AUC/MIC, C_max/MIC, and percent time above MIC) and efficacy over 24 h against S. aureus 25923 in a neutropenic murine thigh infection model. Unbound (free drug) concentrations were used for index calculations. Efficacy is expressed as change in CFU/thigh compared to organism burden at the start of therapy. Each symbol represents the mean CFU/thigh from two mice (four thighs). The sigmoid line represents the best fit using the sigmoid E_max model. R² is the coefficient of determination.

FIG. 6.

Relationship between NZ2114 free drug 24-h AUC/MIC and efficacy against eight S. pneumoniae (left) and six S. aureus (right) strains. Each symbol represents the mean CFU/thigh from two mice (four thighs). Efficacy on the y axis is expressed as the change in CFU/thigh compared to the burden of organisms at the start of therapy. The dashed horizontal line represents the burden of organisms in thighs at the start of therapy. The sigmoid line represents the best-fit curve using the sigmoid E_max model. R² is the coefficient of determination.