Pharmacodynamic effects of simulated standard doses of antifungal drugs against Aspergillus species in a new in vitro pharmacokinetic/pharmacodynamic model

Construction of an in vitro simulated one compartment extravascular administration model and its comparison with classic in vitro administration model in copper chloride induced HepG2 cell death

In this study, we designed an in vitro administration device based on compartment model theory and utilized it to construct an in vitro simulated one compartment extravascular administration model of copper chloride. Within the Cmax range of 3.91-1000.00 μM, the measured concentration-time curves of the simulated one compartment extravascular administration model almost coincide with the corresponding theoretical curves. The measured values of toxicokinetic parameters, including ke, T1/2, ka, T1/2a, Tmax, Cmax, CL, and AUC0-∞ are close to the corresponding theoretical values. The fitting coefficients are >0.9990. In simulated one compartment extravascular administration and classic in vitro administration, copper chloride dose-dependently induced HepG2 cell death. When Cmax/administration concentration is equal, classic in vitro administration induces a higher cell death rate than simulated one compartment extravascular administration. However, there is no significant difference in inducing cell death between the two administration models when area under the curve is equal. In conclusion, the device designed in this study can be used to in vitro simulate one compartment extravascular administration, making in vitro toxicity testing more similar to in vivo scenarios. There are differences in copper chloride induced HepG2 cell death between simulated one compartment extravascular administration and classic in vitro administration.

Assessment of Antifungal Pharmacodynamics

Pharmacokinetic-pharmacodynamic (PK-PD) analysis is of central importance to the progress of an antifungal agent into clinical use. It is crucial to ensure that preclinical studies give the best possible prediction of the way drugs are likely to behave in a clinical setting. This review details the last 30 years of progress in terms of disease model design, efficacy outcome selection and translational modelling in antifungal PK-PD studies. The principles of how PK-PD parameters inform current clinical practice are also discussed, including a review of how these apply to existing and novel agents.

Pharmacodynamics of colistin resistance in carbapenemase-producing Klebsiella pneumoniae: the double-edged sword of heteroresistance and adaptive resistance

Introduction. The presence of heteroresistant subpopulations and the development of resistance during drug exposure (adaptive resistance) limits colistin's efficacy against carbapenemase-producing Klebsiella pneumoniae (CP-Kp) isolates.Hypothesis/Gap statement. The pharmacokinetic/pharmacodynamic (PK/PD) characteristics of both types of colistin resistance against CP-Kp are unknown.Aim. We therefore studied the PK/PD characteristics of colistin resistance in an in vitro PK/PD model simulating clinical colistin exposures.Methods. Two K. pneumoniae clinical isolates, one non-CP-Kp and one CP-Kp, with colistin MICs of 0.5-1 mg l^-1 at a final inoculum of 10⁷ c.f.u. ml^-1 were used in an in vitro PK/PD dialysis/diffusion closed model simulating 4.5 MU q12h and 3 MU q8h clinical dosing regimens. Heteroresistant (HRS, bacteria with stable high-level resistance present before drug exposure) and adaptive resistant (ARS, bacteria with reversible low-level resistance emerging after drug exposure) subpopulations were measured and optimal PK/PD targets for reducing both ARS and HRS were determined. Cumulative fractional response (CFR) was calculated with Monte Carlo simulation for 9 MU q24h, 4.5 MU q12h and 3 MU q8h clinical dosing regimens.Results. A 2-5 log₁₀c.f.u. ml^-1 decrease of the total bacterial population was observed within the first 2 h of exposure, followed by regrowth at 12 h. Colistin exposure was positively and negatively correlated with HRS and ARS 24-0 h c.f.u. ml^-1 changes, respectively. An optimal PK/PD (~0.5log₁₀ increase) target of 35 fAUC/MIC (the ratio of the area under the unbound concentration-time curve to the MIC) was found for reducing both HRS and ARS of high-level resistance (MIC >16 mg l^-1). The 4.5 MU q12h regimen had slightly higher CFR (74 %) compared to the other dosing regimens.Conclusions. High colistin exposures reduced high-level adaptive resistance at the expense of selection of heteroresistant subpopulations.

The Application of Hollow Fiber Cartridge in Biomedicine

The hollow fiber cartridge has the advantages of good semi-permeability, high surface area to volume ratio, convenient operation, and so on. Its application in chemical analysis, drug in vitro experiment, hemodialysis, and other fields has been deeply studied. This paper introduces the basic structure of hollow fiber cartridge, compares the advantages and disadvantages of a hollow fiber infection model constructed by a hollow fiber cartridge with traditional static model and animal infection model and introduces its application in drug effects, mechanism of drug resistance, and evaluation of combined drug regimen. The principle and application of hollow fiber bioreactors for cell culture and hollow fiber dialyzer for dialysis and filtration were discussed. The hollow fiber cartridge, whether used in drug experiments, artificial liver, artificial kidney, etc., has achieved controllable experimental operation and efficient and accurate experimental results, and will provide more convenience and support for drug development and clinical research in the future.

Pharmacodynamic Parameters of Pharmacokinetic/Pharmacodynamic (PK/PD) Integration Models

Pharmacokinetic/pharmacodynamic (PK/PD) integration models are used to investigate the antimicrobial activity characteristics of drugs targeting pathogenic bacteria through comprehensive analysis of the interactions between PK and PD parameters. PK/PD models have been widely applied in the development of new drugs, optimization of the dosage regimen, and prevention and treatment of drug-resistant bacteria. In PK/PD analysis, minimal inhibitory concentration (MIC) is the most commonly applied PD parameter. However, accurately determining MIC is challenging and this can influence the therapeutic effect. Therefore, it is necessary to optimize PD indices to generate more rational results. Researchers have attempted to optimize PD parameters using mutant prevention concentration (MPC)-based PK/PD models, multiple PD parameter-based PK/PD models, kill rate-based PK/PD models, and others. In this review, we discuss progress on PD parameters for PK/PD models to provide a valuable reference for drug development, determining the dosage regimen, and preventing drug-resistant mutations.

Voriconazole efficacy against Candida glabrata and Candida krusei: preclinical data using a validated in vitro pharmacokinetic/pharmacodynamic model

BACKGROUND

Voriconazole exhibits in vitro activity against Candida glabrata and Candida krusei (EUCAST/CLSI epidemiological cut-off values 1/0.25 and 1/0.5 mg/L, respectively). Yet, EUCAST found insufficient evidence to set breakpoints for these species. We explored voriconazole pharmacodynamics (PD) in an in vitro dynamic model simulating human pharmacokinetics (PK).

METHODS

Four C. glabrata and three C. krusei isolates (voriconazole EUCAST and CLSI MICs of 0.03-2 mg/L) were tested in the PK/PD model simulating voriconazole exposures (t½ ∼6 h q12h dosing for 3 days). PK/PD breakpoints were determined calculating the PTA for exposure indices fAUC0-24/MIC associated with half-maximal activity (EI50) using Monte Carlo simulation analysis.

RESULTS

Fungal load increased from 3.60±0.35 to 8.41±0.24 log10 cfu/mL in the drug-free control, with a maximum effect of ∼1 log10 kill of C. glabrata and C. krusei isolates with MICs of 0.06 and 0.25 mg/L, respectively, at high drug exposures. The 72 h log10 cfu/mL change versus fAUC0-24/MIC relationship followed a sigmoid curve for C. glabrata (R2=0.85-0.87) and C. krusei (R2=0.56-0.76) with EI50 of 49 (32-76) and 52 (33-78) fAUC/MIC for EUCAST and 55 (31-96) and 80 (42-152) fAUC/MIC for CLSI, respectively. The PTAs for C. glabrata and C. krusei isolates with EUCAST/CLSI MICs ≤0.125/≤0.06 mg/L were >95%. Isolates with EUCAST/CLSI MICs of 0.25-1/0.125-0.5 would require trough levels 1-4 mg/L; isolates with higher MICs would not attain the corresponding PK/PD targets without reaching toxicity.

CONCLUSIONS

The in vitro PK/PD breakpoints for C. glabrata and C. krusei for EUCAST (0.125 mg/L) and CLSI (0.06 mg/L) bisected the WT populations. Trough levels of >4 mg/L, which are not clinically feasible, are necessary for efficacy against WT isolates.

The Role of New Posaconazole Formulations in the Treatment of Candida albicans Infections: Data from an In Vitro Pharmacokinetic-Pharmacodynamic Model

Posaconazole is more active than fluconazole against Candida albicans in vitro and is approved for the treatment of oropharyngeal candidiasis but not for that of invasive candidiasis (IC). Here, we explored the efficacy of posaconazole against C. albicans in an in vitro pharmacokinetic/pharmacodynamic (PK/PD) model of IC and determined the probability of pharmacodynamic target attainment for the oral solution and intravenous (i.v.)/tablet formulations. Three clinical C. albicans isolates (posaconazole MICs, 0.008 to 0.25 mg/liter) were studied in the in vitro PK/PD dilution model simulating steady-state posaconazole PK. The in vitro exposure-effect relationship, area under the 24-h free drug concentration curve (fAUC_0-24)/MIC, was described and compared with in vivo outcome in animals with IC. PK/PD susceptibility breakpoints and trough levels required for optimal treatment were determined for EUCAST and CLSI 24-h/48-h (CLSI24h/CLSI48h) methods using the fAUC_0-24/MIC associated with half-maximal activity (EI₅₀) and Monte Carlo simulation analysis for oral solution (400 mg every 12 hours [q12h]) and i.v./tablet formulations (300 mg q24h). The in vitro mean (95% confidence interval [CI]) EI₅₀ was 330 (183 to 597) fAUC_0-24/MIC for CLSI24h and 169 (92 to 310) for EUCAST/CLSI48h methods, which are close to the near-stasis in vivo effect. The probability of target attainment for EI₅₀ was estimated; for the wild-type isolates (MIC ≤ 0.06 mg/liter), it was low for the oral solution and higher than 95% for the i.v./tablet formulations for the EUCAST/CLSI48h methods but not for the CLSI 24-h method. Non-wild-type isolates with EUCAST/CLSI48h MICs of 0.125 and 0.25 mg/liter would require trough levels of >1.2 and >2.4 mg/liter, respectively. Posaconazole i.v./tablet formulations may have a role in the therapy of invasive infections by wild-type C. albicans isolates, provided that a steady state is reached quickly. A PK/PD susceptibility breakpoint at the epidemiological cutoff (ECV/ECOFF) of 0.06 mg/liter was determined.

Modeling Invasive Aspergillosis: How Close Are Predicted Antifungal Targets?

Animal model systems are a critical component of the process of discovery and development of new antifungal agents for treatment and prevention of invasive aspergillosis. The persistently neutropenic rabbit model of invasive pulmonary aspergillosis (IPA) has been a highly predictive system in identifying new antifungal agents for treatment and prevention of this frequently lethal infection. Since its initial development, the persistently neutropenic rabbit model of IPA has established a strong preclinical foundation for dosages, drug disposition, pharmacokinetics, safety, tolerability, and efficacy for deoxycholate amphotericin B, liposomal amphotericin B, amphotericin B lipid complex, amphotericin B colloidal dispersion, caspofungin, micafungin, anidulafungin, voriconazole, posaconazole, isavuconazole, and ibrexafungerp in treatment of patients with invasive aspergillosis. The findings of combination therapy with a mould-active triazole and an echinocandin in this rabbit model also predicted the outcome of the clinical trial for voriconazole plus anidulafungin for treatment of IPA. The plasma pharmacokinetic parameters and tissue disposition for most antifungal agents approximate those of humans in persistently neutropenic rabbits. Safety, particularly nephrotoxicity, has also been highly predictive in the rabbit model, as exemplified by the differential glomerular filtration rates observed in animals treated with deoxycholate amphotericin B, liposomal amphotericin B, amphotericin B lipid complex, and amphotericin B colloidal dispersion. A panel of validated outcome variables measures therapeutic outcome in the rabbit model: residual fungal burden, markers of organism-mediated pulmonary injury (lung weights and infarct scores), survival, and serum biomarkers. In selected antifungal studies, thoracic computerized tomography (CT) is also used with diagnostic imaging algorithms to measure therapeutic response of pulmonary infiltrates, which exhibit characteristic radiographic patterns, including nodules and halo signs. Further strengthening the predictive properties of the model, therapeutic response to successfully developed antifungal agents for treatment of IPA has been demonstrated over the past two decades by biomarkers of serum galactomannan and (1→3)-β-D-glucan with patterns of resolution, that closely mirror those documented responses in patients with IPA. The decision to move from laboratory to clinical trials should be predicated upon a portfolio of complementary and mutually validating preclinical laboratory animal models studies. Other model systems, including those in mice, rats, and guinea pigs, are also valuable tools in developing clinical protocols. Meticulous preclinical investigation of a candidate antifungal compound in a robust series of complementary laboratory animal models will optimize study design, de-risk clinical trials, and ensure tangible benefit to our most vulnerable immunocompromised patients with invasive aspergillosis.

Toward Harmonization of Voriconazole CLSI and EUCAST Breakpoints for Candida albicans Using a Validated In Vitro Pharmacokinetic/Pharmacodynamic Model

CLSI and EUCAST susceptibility breakpoints for voriconazole and Candida albicans differ by one dilution (≤0.125 and ≤0.06 mg/liter, respectively) whereas the epidemiological cutoff values for EUCAST (ECOFF) and CLSI (ECV) are the same (0.03 mg/liter). We therefore determined the pharmacokinetic/pharmacodynamic (PK/PD) breakpoints of voriconazole against C. albicans for both methodologies with an in vitro PK/PD model, which was validated using existing animal PK/PD data. Four clinical wild-type and non-wild-type C. albicans isolates (voriconazole MICs, 0.008 to 0.125 mg/liter) were tested in an in vitro PK/PD model. For validation purposes, mouse PK were simulated and in vitro PD were compared with in vivo outcomes. Human PK were simulated, and the exposure-effect relationship area under the concentration-time curve for the free, unbound fraction of a drug from 0 to 24 h (fAUC_0-24)/MIC was described for EUCAST and CLSI 24/48-h methods. PK/PD breakpoints were determined using the fAUC_0-24/MIC associated with half-maximal activity (EI₅₀) and Monte Carlo simulation analysis. The in vitro 24-h PD EI₅₀ values of voriconazole against C. albicans were 2.5 to 5 (1.5 to 17) fAUC/MIC. However, the 72-h PD were higher at 133 (51 to 347) fAUC/MIC for EUCAST and 94 (35 to 252) fAUC/MIC for CLSI. The mean (95% confidence interval) probability of target attainment (PTA) was 100% (95 to 100%), 97% (72 to 100%), 83% (35 to 99%), and 49% (8 to 91%) for EUCAST and 100% (97 to 100%), 99% (85 to 100%), 91% (52 to 100%), and 68% (17 to 96%) for CLSI for MICs of 0.03, 0.06, 0.125, and 0.25 mg/liter, respectively. Significantly, >95% PTA values were found for EUCAST/CLSI MICs of ≤0.03 mg/liter. For MICs of 0.06 to 0.125 mg/liter, trough levels 1 to 4 mg/liter would be required to attain the PK/PD target. A PK/PD breakpoint of C. albicans voriconazole at the ECOFF/ECV of 0.03 mg/liter was determined for both the EUCAST and CLSI methods, indicating the need for breakpoint harmonization for the reference methodologies.

Pharmacokinetic/Pharmacodynamic Integration of Doxycycline Against Mycoplasma hyopneumoniae in an In Vitro Model

Doxycycline is a broad-spectrum antibacterial drug. It is used widely to treat diseases caused by Mycoplasma species. We investigated the antibacterial activity of doxycycline against the Mycoplasma hyopneumoniae strain ATCC25934. The minimum inhibitory concentration (MIC) of doxycycline against M. hyopneumoniae determined by a microdilution method was 0.125 μg/ml. Static time-kill curves with constant drug concentrations (0-64 MIC) showed that a bacteriostatic effect occurred if the doxycycline concentration reached 4 MIC. Doxycycline produced a maximum antimycoplasmal effect (reduction of 2.76 log₁₀CFU/ml) at 64 MIC within 48 h. The effect of doxycycline against M. hyopneumoniae was analyzed by a sigmoid E _max model, and there was high correlation between the kill rate and doxycycline concentration (R ² = 0.986). A one-compartment open model with first-order absorption was adopted and was used to simulate doxycycline pharmacokinetics in porcine plasma. The dynamic time-concentration curve showed that the area under the curve at 24 h (AUC_{24 h}) and C _max (peak concentration) after each drug administration was 1.78-48.4 μg h/ml and 0.16-3.41 μg/ml, respectively. The reduction of M. hyopneumoniae (log₁₀CFU/ml) for 1, 2.5, 5, 7.5, 10, 15, 20, and 30 mg/kg body weight was 0.16, 1.29, 1.75, 2.94, 3.35, 3.91, 4.35, and 5.77, respectively, during the entire experiment, respectively. When the dose was >10 mg/kg body weight, continuous administration for 3 days could achieve a bactericidal effect. The correlation coefficient of AUC_{24 h}/MIC, C _max/MIC, and %T > MIC (the cumulative percentage of time over a 24-h period that the drug concentration exceeds the MIC) with antibacterial effect was 0.917, 0.923, and 0.823, respectively. Doxycycline showed concentration-dependent activity, and the value of AUC_{24 h}/MIC and C _max/MIC required to produce a drop of 1 log₁₀CFU/ml was 164 h and 9.89, respectively.

Pharmacokinetic and Pharmacodynamic Integration and Resistance Analysis of Tilmicosin Against Mycoplasma gallisepticum in an In Vitro Dynamic Model

Mycoplasma gallisepticum is the major pathogen causing chronic respiratory disease in chickens. In the present study, we successfully established a one-compartment open model with first-order absorption to determine the relationship between tilmicosin pharmacokinetic and pharmacodynamic (PK/PD) indices and M. gallisepticum in in vitro. The aim was to simulate the PK/PD of tilmicosin against M. gallisepticum in lung tissues. The results of static time-killing curves at constant drug concentrations [0–64 minimum inhibitory concentration (MIC)] showed that the amount of M. gallisepticum was reduced to the limit of detection after 36 h when the drug concentration exceeded 1 MIC, with a maximum kill rate of 0.53 h^-1. In dynamic time-killing studies, tilmicosin produced a maximum antimycoplasmal effect of 6.38 Log₁₀ CFU/ml reduction over 120 h. The area under the concentration–time curve over 24 h divided by the MIC (AUC_24h/MIC) was the best PK/PD parameter to predict the antimicrobial activity of tilmicosin against M. gallisepticum [R² = 0.87, compared with 0.49 for the cumulative time that the concentration exceeds the MIC (%T > MIC)]. Therefore, tilmicosin showed concentration-dependent activity. Seven M. gallisepticum strains (M1–M7) with decreased susceptibility to tilmicosin were isolated from seven dose groups. These strains of M. gallisepticum had acquired resistance to erythromycin as well as to tylosin. However, no change in susceptibility to amikacin and doxycycline was observed in these strains. Gene mutation analysis was performed on the basis of annotated single nucleotide polymorphisms using the genome of strain S6 as the reference. For strain M5, a G495T mutation occurred in domain II of the 23S rrnA gene. In strain M3, resistance was associated with a T854A mutation in domain II of the 23S rrnB gene and a G2799A mutation in domain V of 23S rrnB. To the best of our knowledge, these tilmicosin resistance-associated mutations in M. gallisepticum have not been reported. In conclusion, tilmicosin shows excellent effectiveness and concentration-dependent characteristics against M. gallisepticum strain S6 in vitro. Additionally, these results will be used to provide a reference to design the optimal dosage regimen for tilmicosin in M. gallisepticum infection and to minimize the emergence of resistant bacteria.

Exploring colistin pharmacodynamics against Klebsiella pneumoniae: a need to revise current susceptibility breakpoints

Objectives

Because the pharmacokinetic/pharmacodynamic (PK/PD) characteristics of colistin against Enterobacteriaceae are not well explored, we studied the activity of colistin against K. pneumoniae in an in vitro PK/PD model simulating different dosing regimens.

Methods

Three clinical isolates of K. pneumoniae with MICs of 0.5, 1 and 4 mg/L were tested in an in vitro PK/PD model following a dose-fractionation design over a period of 24 h. A high and low inoculum of 107 and 104 cfu/mL with and without a heteroresistant subpopulation, respectively, were used. PK/PD indices associated with colistin activity were explored and Monte Carlo analysis was performed in order to determine the PTA for achieving a bactericidal effect (2 log kill).

Results

The fAUC/MIC (R2 = 0.64-0.68) followed by fCmax/MIC (R2 = 0.55-0.63) best described colistin's 24 h log10 cfu/mL reduction for both low and high inocula. Dosing regimens with fCmax/MIC ≥6 were always associated with a bactericidal effect (P = 0.0025). However, at clinically achievable concentrations, usually below fCmax/MIC = 6, an fAUC/MIC ≥25 was more predictive of a bactericidal effect. Using a dosing regimen of 9 MU/day, the PTA for this pharmacodynamic target was 100%, 5%-70% and 0%, for isolates with MICs of ≤0.5, 1 and ≥2 mg/L, respectively. Dosing regimens that aim for a trough level of 1 mg/L achieve coverage of strains up to 0.5 mg/L (target trough/MIC = 2 mg/L).

Conclusions

Characterization of the pharmacodynamics of colistin against Enterobacteriaceae in an in vitro model of infection indicates that a revision of current susceptibility breakpoints is needed. Therapeutic drug monitoring of colistin to achieve pharmacodynamic targets in individual patients is highly recommended.

Impact of unresolved neutropenia in patients with neutropenia and invasive aspergillosis: a post hoc analysis of the SECURE trial

Background

Historically, baseline neutropenia and lack of neutrophil recovery have been associated with poor outcomes in invasive aspergillosis (IA). It is unclear how treatment with the new Aspergillus-active triazoles isavuconazole and voriconazole affects outcomes in neutropenic patients with IA.

Methods

A post hoc analysis of the Phase 3 SECURE trial assessed patients with neutropenia (neutrophil count <0.5 × 109/L for >10 days at baseline) with IA (proven/probable) who had received either isavuconazole or voriconazole. The primary endpoint was all-cause mortality (ACM) through day 42. ACM in patients with resolved versus unresolved neutropenia at day 7 and overall success at end of treatment (EOT) were also assessed.

Results

One hundred and forty-two patients with neutropenia and IA were included (isavuconazole n = 78, voriconazole n = 64). ACM through day 42 (primary endpoint), day 7 and EOT were higher for patients with unresolved versus resolved neutropenia at each timepoint (day 42, unresolved: 45.0% isavuconazole, 45.2% voriconazole; resolved: 5.0% isavuconazole, 5.9% voriconazole; day 7, unresolved: 31.0% isavuconazole, 29.8% voriconazole; resolved: 5.0% isavuconazole, 5.9% voriconazole; EOT, unresolved: 48.6% isavuconazole, 36.4% voriconazole; resolved: 5.0% isavuconazole, 14.3% voriconazole). ACM was significantly higher for isavuconazole-treated patients with unresolved versus resolved neutropenia (day 7, P = 0.031; day 42, P < 0.001; EOT, P < 0.001). In voriconazole-treated patients, ACM was significantly higher among patients with unresolved versus resolved neutropenia at day 42 (P = 0.002) and numerically higher at day 7 and EOT (P > 0.05 for both).

Conclusions

Isavuconazole had comparable efficacy and safety to voriconazole in neutropenic patients with IA. Resolution of neutropenia was associated with improved outcomes.

Resistant cutoff values and optimal scheme establishments for florfenicol against Escherichia coli with PK-PD modeling analysis in pigs

Florfenicol, a structural analog of thiamphenicol, has broad-spectrum antibacterial activity against gram-negative and gram-positive bacteria. This study was conducted to investigate the epidemiological, pharmacokinetic-pharmacodynamic cutoff, and the optimal scheme of florfenicol against Escherichia coli (E. coli) with PK-PD integrated model in the target infectious tissue. 220 E. coli strains were selected to detect the susceptibility to florfenicol, and a virulent strain P190, whose minimum inhibitory concentration (MIC) was similar to the MIC₅₀ (8 μg/ml), was analyzed for PD study in LB and ileum fluid. The MIC of P190 in the ileum fluid was 0.25 times lower than LB. The ratios of MBC/MIC were four both in the ileum and LB. The characteristics of time-killing curves also coincided with the MBC determination. The recommended dosages (30 mg/kg·body weight) were orally administrated in healthy pigs, and both plasma and ileum fluid were collected for PK study. The main pharmacokinetics (PK) parameters including AUC_24 hr , AUC_0-∞ , T_max , T_1/2 , C_max , CL_b, and K_e were 49.83, 52.33 μg*h/ml, 1.32, 10.58 hr, 9.12 μg/ml, 0.50 L/hr*kg, 0.24 hr^-1 and 134.45, 138.71 μg*hr/ml, 2.05, 13.01 hr, 16.57 μg/ml, 0.18 L/hr*kg, 0.14 hr^-1 in the serum and ileum fluid, respectively. The optimum doses for bacteriostatic, bactericidal, and elimination activities were 29.81, 34.88, and 36.52 mg/kg for 50% target and 33.95, 39.79, and 42.55 mg/kg for 90% target, respectively. The final sensitive breakpoint was defined as 16 μg/ml. The current data presented provide the optimal regimens (39.79 mg/kg) and susceptible breakpoint (16 μg/ml) for clinical use, but these predicted data should be validated in the clinical practice.

Relationship between danofloxacin PK/PD parameters and emergence and mechanism of resistance of Mycoplasma gallisepticum in In Vitro model

Mycoplasma gallisepticum is a serious pathogen for poultry that causes chronic respiratory disease in chickens. Increased embryonic mortality, as well as reduced weight gain and egg production have been found in infected chickens, which can lead to considerable economic losses in poultry production. Increased antibiotic resistance compromises the use of tetracyclines, macrolides and quinolones in the farm environment. In the present study, danofloxacin concentrations were simulated below the MIC₉₉, between the MIC₉₉ and MPC (the mutant prevention concentration), and above the MPC in an in vitro dynamic model against M. gallisepticum. The relationship between the simulated danofloxacin pharmacokinetics, pharmacodynamics (PK/PD) parameters and development of resistance for M. gallisepticum was explored based on the available data obtained from various dosing regimens in the in vitro model. Danofloxacin concentration, counts of viable cell and susceptibility were determined during the experiment. The mutations in gyrA, gyrB, parC and parE as well as efflux pumps were examined. The MIC of danofloxacin against M. gallisepticum was increased when drug concentrations were between the lower and upper boundaries of the mutant selection window. The upper boundary of the selection window in vitro was estimated as a C_max/MPC value of 1. The lower boundary was estimated as C_max/MPC value of 0.05. Both in terms of the MIC and resistance frequency, M. gallisepticum resistance was developed when danofloxacin concentrations fell inside the mutant selection window (ratios of C_max to MPC between 0.05 and 1). The single mutation in gyrA (Ser-83→Arg) was found in all mutants, while double mutations in gyrA and parC (Ala-64→Ser) were observed only in the mutant with the highest MIC. In addition, no change of susceptibility in the mutants was observed in the presence of reserpine and carbonyl cyanide 3-chlorophenylhydrazone (CCCP). This suggested that ATP-binding cassette superfamily (ABC transporter) and major facilitator superfamily (MFS transporter) did not play a role in danofloxacin efflux.

A New Marker of Echinocandin Activity in an In Vitro Pharmacokinetic/Pharmacodynamic Model Correlates with an Animal Model of Aspergillus fumigatus Infection

The lack of a quantifiable marker for echinocandin activity hinders in vitro pharmacokinetic/pharmacodynamic (PK/PD) studies for Aspergillus spp. We developed an in vitro PK/PD model simulating the pharmacokinetics of anidulafungin and assessing its pharmacodynamics against Aspergillus fumigatus with a new, easily quantifiable, sensitive, and reproducible marker. Two clinical A. fumigatus isolates previously used in animals (AZN8196 and V52-35) with identical anidulafungin EUCAST (0.03 μg/ml) and CLSI (0.015 μg/ml) minimal effective concentrations (MEC) and one isolate (strain AFU79728) with an MEC of >16 μg/ml were tested in a two-compartment PK/PD dialysis/diffusion closed model containing a dialysis membrane (DM) tube inoculated with 10³ CFU/ml. During anidulafungin exposure, two types of fungal forms were observed inside the DM tube: floating conidia that were quantified by cultures and aberrant mycelia that were quantified by the vertical height of the mycelia attached on the DM tube. No aberrant mycelia were found for the resistant isolate or in the drug-free controls. An in vitro exposure-effect relationship was similar to that found in animals using survival as an endpoint, with a free-drug area under the concentration-time curve from 0 to 24 h (fAUC_0-24) associated with 50% of maximal activity of 2.21 (range, 1.81 to 2.71) mg · h/liter in vitro versus 2.62 (range, 1.88 to 3.65) mg · h/liter in vivo (P = 0.41). The hillslopes were also similar, with 1.96 versus 1.34 (P = 0.29). Analysis of each isolate separately showed increased antifungal susceptibility between AZN8196 and V52-35 (P < 0.001) even though they have the same CLSI and EUCAST MECs, but the strains have two 2-fold dilutions lower MICs using Etest and the XTT {2,3-bis (2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino) carbonyl]-2H-tetrazolium hydroxide} method. Dose fractionation studies with all three echinocandins showed that their activities are best described by fAUC and not the maximum concentration of free drug (fC_max). The new marker correlated with in vivo outcome and can be used for in vitro PK/PD studies exploring the pharmacodynamics of echinocandins against Aspergillus spp.

The pharmacokinetic-pharmacodynamic modeling and cut-off values of tildipirosin against Haemophilus parasuis

The goal of this study was to establish the epidemiological, pharmacodynamic cut-off values, optimal dose regimens for tildipirosin against Haemophilus parasuis. The minimum inhibitory concentrations (MIC) of 164 HPS isolates were determined and SH0165 whose MIC (2 μg/ml ) were selected for PD analysis. The ex vivo MIC in plasma of SH0165 was 0.25 μg/ml which was 8 times lower than that in TSB. The bacteriostatic, bactericidal and elimination activity (AUC_24h/MIC) in serum were 26.35, 52.27 and 73.29 h based on the inhibitory sigmoid E_max modeling. The present study demonstrates that 97.9% of the wild-type (WT) isolates were covered when the epidemiological cut-off value (ECV) was set at 8 μg/ml. The parameters including AUC_24h, AUC, T1/2, C_max, CL_b and MRT in PELF were 19.56, 60.41, 2.32, 4.02, 56.6, and 2.63 times than those in plasma, respectively. Regarding the Monte Carlo simulation, the CO_PD was defined as 0.5 μg/ml in vitro, and the optimal doses to achieve bacteriostatic, bactericidal and elimination effect were 1.85, 3.67 and 5.16 mg/kg for 50% target, respectively, and 2.07, 4.17 and 5.78 mg/kg for 90% target, respectively. The results of this study offer a more optimised alternative for clinical use and demonstrated that 4.17 mg/kg of tildipirosin by intramuscular injection could have an effect on bactericidal activity against HPS. These values are of great significance for the effective treatment of HPS infections, but it also be deserved to be validated in clinical practice in the future research.

Pharmacokinetics of antifungal drugs: practical implications for optimized treatment of patients

Introduction

Because of the high mortality of invasive fungal infections (IFIs), appropriate exposure to antifungals appears to be crucial for therapeutic efficacy and safety.

Materials and methods

This review summarises published pharmacokinetic data on systemically administered antifungals focusing on co-morbidities, target-site penetration, and combination antifungal therapy.

Conclusions and discussion

Amphotericin B is eliminated unchanged via urine and faeces. Flucytosine and fluconazole display low protein binding and are eliminated by the kidney. Itraconazole, voriconazole, posaconazole and isavuconazole are metabolised in the liver. Azoles are substrates and inhibitors of cytochrome P450 (CYP) isoenzymes and are therefore involved in numerous drug–drug interactions. Anidulafungin is spontaneously degraded in the plasma. Caspofungin and micafungin undergo enzymatic metabolism in the liver, which is independent of CYP. Although several drug–drug interactions occur during caspofungin and micafungin treatment, echinocandins display a lower potential for drug–drug interactions. Flucytosine and azoles penetrate into most of relevant tissues. Amphotericin B accumulates in the liver and in the spleen. Its concentrations in lung and kidney are intermediate and relatively low myocardium and brain. Tissue distribution of echinocandins is similar to that of amphotericin. Combination antifungal therapy is established for cryptococcosis but controversial in other IFIs such as invasive aspergillosis and mucormycosis.

Prognostic value of galactomannan: current evidence for monitoring response to antifungal therapy in patients with invasive aspergillosis

Galactomannan (GM) is a polysaccharide present in the cell wall of Aspergillus spp. that is released during growth of the organism. It has been successfully used to aide in the diagnosis of invasive aspergillosis allowing for earlier recognition of disease compared to conventional methods. Since its implementation in the clinic as a diagnostic tool, GM has been used in experimental models to measure therapeutic response. Several clinical studies describe the prognostic value of GM. Herein, we review the evidence supporting the utilization of GM antigen as a biomarker to measure response to systemic antifungal therapy.

Comparison of Short Versus Prolonged Infusion of Standard Dose of Meropenem Against Carbapenemase-Producing Klebsiella pneumoniae Isolates in Different Patient Groups: A Pharmacokinetic-Pharmacodynamic Approach

Dose optimization is required to increase carbapenem's efficacy against carbapenemase-producing isolates. Four clinical Klebsiella pneumoniae isolates were used: one susceptible to meropenem with minimum inhibitory concentration (MIC) 0.031 mg/L and 3 verona integron-borne metallo bete-lactamase-1-producing isolates with MICs 8, 16, and 128 mg/L. The human pharmacokinetics of short (0.5-h) and prolonged (3-h) infusion regimens of 1 g meropenem every 8 h were simulated in an in vitro pharmacokinetic-pharmacodynamic model. Time-kill curves were constructed for each isolate and dosing regimen, and the %T > MIC associated with maximal bactericidal activity was estimated. The percentage of pharmacodynamic target attainment for isolates with different MICs was calculated for 350 ICU, surgical, and internal medicine patients. The isolates with MIC ≤8 mg/L were killed with both dosing regimens. The %T > MIC corresponding to maximal bactericidal activity was ∼40%. The percentages of target attainment were >90%, 61%-83%, 23%-33%, and <3% with the short infusion regimen and >90%, 98%-99%, 55%-79%, and <5% with the prolonged infusion regimen for isolates with MIC ≤2, 4, 8, and ≥16 mg/L, respectively. The lowest target attainment rates were observed for the ICU patients and the highest for internal medicine patients. The prolonged infusion regimen was more effective than the short infusion regimen against isolates with MIC 4-8 mg/L.

Impact of bacterial load on pharmacodynamics and susceptibility breakpoints for tigecycline and Klebsiella pneumoniae

OBJECTIVES

In the absence of other therapeutic options, tigecycline is used to treat bloodstream infections and pneumonia caused by carbapenemase-producing Klebsiella pneumoniae (CP-Kp). In this study, the standard and high tigecycline dosing regimens were simulated and tested against different inocula of CP-Kp isolates in an in vitro pharmacokinetic (PK)/pharmacodynamic (PD) model.

METHODS

Four susceptible isolates (EUCAST MICs of 0.125-1 mg/L) and two intermediately susceptible CP-Kp clinical isolates (MICs of 2 mg/L) were tested at three different inocula (10⁷, 10⁵ and 10³ cfu/mL), simulating tigecycline serum and lung fC_max concentrations of 0.15 and 1.5 mg/L, respectively, of 50 mg tigecycline every 12 h for 48 h. The exposure-effect relationships were described and the probability of target attainment was calculated for each inoculum in order to determine PK/PD susceptibility breakpoints.

RESULTS

No cfu reduction was observed at serum concentrations. At lung concentrations and low inocula, a bacteriostatic and killing effect was found for isolates with MICs of 0.25 and 0.125 mg/L, respectively. The fAUC_0-24/MIC (tAUC_0-24/MIC) associated with half-maximal activity was 16 (150) with 10³ cfu/mL, 28 (239) with 10⁵ cfu/mL and 79 (590) with 10⁷ cfu/mL. A PK/PD susceptibility breakpoint of ≤0.06 and ≤0.125 mg/L for bacteraemia with ≤10¹ cfu/mL and ≤0.25 and ≤0.5 mg/L for pneumonia with ≤10³ cfu/g was determined for the standard tigecycline dose of 50 mg and the higher dose of 100 mg, respectively.

CONCLUSIONS

Tigecycline monotherapy with either 50 or 100 mg would not be sufficient for most patients with bacteraemia, though the higher dose of 100 mg could be effective for patients with pneumonia with low bacterial load.

Pharmacodynamics for antifungal drug development: an approach for acceleration, risk minimization and demonstration of causality

The treatment of invasive fungal diseases constitutes a significant unmet medical need. There are relatively few antifungal agents in clinical development and a paucity of novel targets. Morbidity and mortality remain high and clinical outcomes are compromised by submaximal efficacy, emergence of drug resistance and drug-related toxicity. Thus, new antifungal agents are urgently required. A deep understanding of exposure-response relationships underpins the development of safe and effective clinical regimens of any therapeutic agent. Pharmacokinetics (PK) and pharmacodynamics (PD) is increasingly recognized as a vital tool in the development of new antimicrobial agents and maximizes the probability that the right dose will be studied the first time. There is currently no information or agreement as to what constitutes an adequate PK/PD package for the development of a new antifungal agent. This review provides a summary of the achievements of antifungal PK/PD for the treatment of invasive candidiasis, invasive aspergillosis and cryptococcal meningoencephalitis, and outlines the necessary components of a PK/PD package for a new antifungal agent. Such information is critical for the accelerated and efficient development of new agents and enables improved clinical outcomes to be secured.

Dose optimization of voriconazole/anidulafungin combination against Aspergillus fumigatus using an in vitro pharmacokinetic/pharmacodynamic model and response surface analysis: clinical implications for azole-resistant aspergillosis

BACKGROUND

Combination therapy of voriconazole with an echinocandin is often employed in order to increase the efficacy of voriconazole monotherapy.

METHODS

Four clinical Aspergillus fumigatus isolates with different in vitro susceptibilities to voriconazole (MIC 0.125-2 mg/L) and anidulafungin (MEC 0.008-0.016 mg/L) were tested in an in vitro pharmacokinetic/pharmacodynamic model simulating human serum concentrations of standard dosages of voriconazole and anidulafungin. Fungal growth was assessed using galactomannan production and quantitative PCR. Drug concentrations were determined with bioassays. Pharmacodynamic interactions were assessed using Bliss independence analysis (BI) and Loewe additivity-based canonical mixture response-surface non-linear regression analysis (LA). Probability of target attainment (PTA) was estimated with Monte Carlo analysis for different doses of anidulafungin (25, 50 and 100 mg) and azole resistance rates (5%-25%).

RESULTS

Synergy [BI 51% (8%-80%), LA 0.63 (0.38-0.79)] was found at low anidulafungin (fC_max/MEC <10) and voriconazole (fAUC/MIC <10) exposures, whereas antagonism [BI 12% (5%-18%, LA 1.12 (1.04-4.6)] was found at higher drug exposures. The largest increase in PTA was found with 25 mg of anidulafungin and voriconazole MIC distributions with high (>10%) resistance rates. PTAs for isolates with voriconazole MICs of 1, 2 and 4 mg/L was 78%, 12% and 0% with voriconazole monotherapy and 96%-100%, 68%-82% and 9%-20% with combination therapy, respectively. Optimal activity was associated with a voriconazole tC_min/MIC ratio of 1.5 for monotherapy and 0.75 for combination therapy.

CONCLUSIONS

The present study indicated that the combination of voriconazole with low-dose anidulafungin may increase the efficacy and reduce the cost and potential toxicity of antifungal therapy, particularly against azole-resistant A. fumigatus isolates and in patients with subtherapeutic serum levels. This hypothesis warrants further in vivo verification.

The PK/PD Interactions of Doxycycline against Mycoplasma gallisepticum

Mycoplasma gallisepticum is one of the most important pathogens that cause chronic respiratory disease in chicken. This study investigated the antibacterial activity of doxycycline against M. gallisepticum strain S6. In static time–killing studies with constant antibiotic concentrations [0–64 minimum inhibitory concentration (MIC)], M. gallisepticum colonies were quantified and kill rates were calculated to estimate the drug effect. The half-life of doxycycline in chicken was 6.51 ± 0.63 h. An in vitro dynamic model (the drug concentrations are fluctuant) was also established and two half-lives of 6.51 and 12 h were simulated. The samples were collected for drug concentration determination and viable counting of M. gallisepticum. In static time–killing studies, doxycycline produced a maximum antimycoplasmal effect of 5.62log₁₀ (CFU/mL) reduction and the maximum kill rate was 0.11 h⁻¹. In the in vitro dynamic model, doxycycline had a mycoplasmacidal activity in the two regimens, and the maximum antimycoplasmal effects were 4.1 and 4.75log₁₀ (CFU/mL) reduction, respectively. Furthermore, the cumulative percentage of time over a 48-h period that the drug concentration exceeds the MIC (%T > MIC) was the pharmacokinetic–pharmacodynamic index that best correlated with antimicrobial efficacy (R² = 0.986, compared with 0.897 for the peak level divided by the MIC and 0.953 for the area under the concentration–time curve over 48 h divided by the MIC). The estimated %T > MIC values for 0log₁₀ (CFU/mL) reduction, 2log10 (CFU/mL) reduction and 3log₁₀ (CFU/mL) reduction were 32.48, 45.68, and 54.36%, respectively, during 48 h treatment period of doxycycline. In conclusion, doxycycline shows excellent effectiveness and time-dependent characteristics against M. gallisepticum strain S6 in vitro. Additionally, these results will guide optimal dosing strategies of doxycycline in M. gallisepticum infection.

Pharmacokinetic-pharmacodynamic modelling of meropenem against VIM-producing Klebsiella pneumoniae isolates: clinical implications

VIM-producing Klebsiella pneumoniae isolates are usually associated with high MICs to carbapenems. Preclinical studies investigating the pharmacokinetic-pharmacodynamic (PK-PD) characteristics of carbapenems against these isolates are lacking. The in vitro antibacterial activity of meropenem against one WT and three VIM-producing K. pneumoniae clinical isolates (median MICs 0.031, 8, 16 and 128 mg l- 1) was studied in a dialysis-diffusion PK-PD model and verified in a thigh infection neutropenic animal model by testing selected strains and exposures. The in vitro PK-PD target associated with bactericidal activity was estimated and the target attainment for different dosing regimens was calculated with Monte Carlo analysis. The in vitro model was correlated with the in vivo data, with log10CFU/ml reduction of < 1 for the VIM-producing (MIC 16 mg l- 1) and >2 for the WT (MIC 0.031 mg l- 1) isolates, with %f T >MIC 25 and 100%, respectively. The in vitro bactericidal activity for all isolates was associated with 40 % f T>MIC and attained in >90% of cases with the standard 1 g q8 0.5 h infusion dosing regimen only for isolates with MICs up to 1 mg l- 1. For isolates with MICs of 2-8 mg l- 1, prolonged infusion regimens (4 h infusion q8 or 2 h infusion q4) of standard (1 g) and higher (2 g) doses or continuous infusion regimens (3-6 g) are required. For isolates with a MIC of 16 mg l- 1 the unconventional dosing regimen of 2 g as 2 h infusion q4 or 12 g continuous infusion will be required. Prolonged and continuous infusion regimens of meropenem may increase efficacy against VIM-producing K. pneumoniae isolates.

Optimization of polyene-azole combination therapy against aspergillosis using an in vitro pharmacokinetic-pharmacodynamic model

Although amphotericin B-azole combination therapy has traditionally been questioned due to potential antagonistic interactions, it is often used successfully to treat refractory invasive aspergillosis. So far, pharmacodynamic (PD) interactions have been assessed with conventional in vitro tests, which do not mimic human serum concentrations and animal models using limited doses. We therefore simulated the human serum concentration profiles of amphotericin B and voriconazole in an in vitro dialysis/diffusion closed pharmacokinetic-pharmacodynamic (PK-PD) model and studied the pharmacodynamic interactions against an azole-resistant and an azole-susceptible Aspergillus fumigatus isolate, using Bliss independence and canonical mixture response surface analyses. Amphotericin B dosing regimens with the drug administered every 24 h (q24h) were combined with voriconazole q12h dosing regimens. In vitro PK-PD combination data were then combined with human PK data by using Monte Carlo analysis. The target attainment rate and the serum concentration/MIC ratio were calculated for isolates with different MICs. Synergy (20 to 31%) was observed at low amphotericin B-high voriconazole exposures, whereas antagonism (-6 to -16%) was found at high amphotericin B-low voriconazole exposures for both isolates. Combination therapy resulted in 17 to 48% higher target attainment rates than those of monotherapy regimens for isolates with voriconazole/amphotericin B MICs of 1 to 4 mg/liter. Optimal activity was found for combination regimens with a 1.1 total minimum concentration of drug in serum (tCmin)/MIC ratio for voriconazole and a 0.5 total maximum concentration of drug in serum (tCmax)/MIC ratio for amphotericin B, whereas the equally effective monotherapy regimens required a voriconazole tCmin/MIC ratio of 1.8 and an amphotericin B tCmax/MIC ratio of 2.8. Amphotericin B-voriconazole combination regimens were more effective than monotherapy regimens. Therapeutic drug monitoring can be employed to optimize antifungal combination therapy with low-dose (≤0.6 mg/kg) amphotericin B-based combination regimens against resistant isolates for minimal toxicity.

Treatment of Aspergillus terreus infections: a clinical problem not yet resolved

Despite the use of recommended therapies, invasive infections by Aspergillus terreus show a poor response. For years, investigative studies on the failure of therapy of fungal infections have focused on in vitro susceptibility data. However, it is well known that low minimum inhibitory concentrations (MICs) are not always predictive of response to therapy despite a correct dosage schedule. Many experimental and clinical studies have tried to establish a relationship between MICs and outcome in serious fungal infections but have come to contradictory and even surprising conclusions. The success or failure of treatment is determined by many factors, including the in vitro susceptibility of the causative fungal isolate, the pharmacokinetics/pharmacodynamics of the drug used for treatment, pharmacokinetic variability in the population, and the underlying disease that patients suffer. To try to understand this poor response to treatment, available data on the in vitro susceptibility of A. terreus, the experimental and clinical response to amphotericin B, triazoles and echinocandins, and the pharmacokinetics/pharmacodynamics of these antifungals have been reviewed. Of special interest are the fungistatic activites of these drugs against A. terreus and the high interpatient variability of serum drug levels observed in therapy based on triazoles, which make monitoring of infected patients necessary.

Susceptibility breakpoints for amphotericin B and Aspergillus species in an in vitro pharmacokinetic-pharmacodynamic model simulating free-drug concentrations in human serum

Although conventional amphotericin B was for many years the drug of choice and remains an important agent against invasive aspergillosis, reliable susceptibility breakpoints are lacking. Three clinical Aspergillus isolates (Aspergillus fumigatus, Aspergillus flavus, and Aspergillus terreus) were tested in an in vitro pharmacokinetic-pharmacodynamic model simulating the biphasic 24-h time-concentration profile of free amphotericin B concentrations in human serum with free peak concentrations (fCmax) of 0.1, 0.3, 0.6, 1.2, and 2.4 mg/liter administered once daily. Drug concentrations were measured with a bioassay, and fungal growth was monitored for 72 h with galactomannan production. The fCmax/MIC corresponding to half-maximal activity (P50) was determined for each species, and the percentage of target attainment was calculated for different MICs for the standard (1 mg/kg of body weight) and a lower (0.6-mg/kg) dose of amphotericin B with Monte Carlo simulation analysis. The fCmax/MICs (95% confidence intervals) corresponding to P50 were 0.145 (0.133 to 0.158), 0.371 (0.283 to 0.486), and 0.41 (0.292 to 0.522) for A. fumigatus, A. flavus, and A. terreus, respectively. The median percentages of P50 attainment were ≥88%, 47%, and 0% for A. fumigatus isolates with MICs of ≤0.5, 1, and ≥2 mg/liter, respectively, and ≥81%, 24%, and 0% and ≥75%, 15%, and 0% for A. flavus and A. terreus isolates with MICs of ≤0.25, 0.5, and ≥1 mg/liter, respectively. The lower dose of 0.6 mg/kg would retain efficacy for A. fumigatus, A. flavus, and A. terreus isolates with MICs of ≤0.25, ≤0.125, and ≤0.125 mg/liter, respectively. The susceptibility, intermediate susceptibility, and resistance breakpoints of ≤0.5, 1, and ≥2 mg/liter for A. fumigatus and ≤0.25, 0.5, and ≥1 mg/liter for A. flavus and A. terreus were determined for conventional amphotericin B with a pharmacokinetic-pharmacodynamic model simulating free-drug serum concentrations.

Single-dose pharmacodynamics of amphotericin B against Aspergillus species in an in vitro pharmacokinetic/pharmacodynamic model

Conventional MIC testing of amphotericin B results in narrow MIC ranges challenging the detection of resistant strains. In order to discern amphotericin B pharmacodynamics, the in vitro activity of amphotericin B was studied against Aspergillus isolates with the same MICs by using a new in vitro pharmacokinetic/pharmacodynamic (PK/PD) model that simulates amphotericin B human plasma levels. Clinical isolates of Aspergillus fumigatus, A. terreus, and A. flavus with the same Clinical and Laboratory Standards Institute modal MICs of 1 mg/liter were exposed to amphotericin B concentrations following the plasma concentration-time profile after single-bolus administration with C(max) values of 0.6, 1.2, 2.4, and 4.8 mg/liter. Fungal growth was monitored for up to 72 h based on galactomannan production. Complete growth inhibition was observed only against A. fumigatus with amphotericin B with a Cmax of ≥ 2.4 mg/liter. At the lower C(max) values 0.6 and 1.2 mg/liter, significant growth delays of 34 and 52 h were observed, respectively (P < 0.001). For A. flavus, there was no complete inhibition but a progressive growth delay of 1 to 50 h at an amphotericin B C(max) of 0.6 to 4.8 mg/liter (P < 0.001). For A. terreus, the growth delay was modest (up to 8 h) at all C(max)s (P < 0.05). The C(max) (95% confidence interval) associated with 50% activity for A. fumigatus was 0.60 (0.49 to 0.72) mg/liter, which was significantly lower than for A. flavus 3.06 (2.46 to 3.80) mg/liter and for A. terreus 7.90 (5.20 to 12.29) mg/liter (P < 0.001). A differential in vitro activity of amphotericin B was found among Aspergillus species despite the same MIC in the order A. fumigatus > A. flavus > A. terreus in the in vitro PK/PD model, possibly reflecting the different concentration- and time-dependent inhibitory/killing activities amphotericin B exerted against these species.

Evaluation of antimicrobial activity against Mycoplasma mycoides subsp. mycoides Small Colony using an in vitro dynamic dilution pharmacokinetic/pharmacodynamic model

The objectives of this study were to assess the activity of oxytetracycline (OTC), danofloxacin and tulathromycin against Mycoplasma mycoides subsp. mycoides Small Colony, the causative agent of contagious bovine pleuropneumonia, in an in vitro dynamic concentration model and to determine the concentration and/or time dependence of such activity. Time-kill assays that simulated elimination of antimicrobials from the body were performed. Initial antimicrobial concentrations corresponded to various multiples of the MIC and cultures were diluted in a stepwise fashion with either drug-free or drug-containing artificial medium to mimic administration by single-release bolus or infusion, respectively. Where appropriate, data were fitted to sigmoidal E(max) models. OTC produced no change in mycoplasma titre from the initial inoculum size, regardless of the concentration or means of drug exposure. Both danofloxacin and tulathromycin resulted in a decrease in mycoplasma titre but neither was bactericidal (99.9 % kill) over 12 h. A greater antimycoplasmal effect, defined as the change in log(10) (c.f.u. ml(-1)) over 12 h, was achieved when danofloxacin was administered as a single-release bolus, suggesting concentration-dependent activity, whereas the antimycoplasmal effect of tulathromycin was comparable following administration by single-release bolus or infusion, owing to its long half-life.

In vitro pharmacokinetic/pharmacodynamic modeling of voriconazole activity against Aspergillus species in a new in vitro dynamic model

The pharmacodynamics (PD) of voriconazole activity against Aspergillus spp. were studied using a new in vitro dynamic model simulating voriconazole human pharmacokinetics (PK), and the PK-PD data were bridged with human drug exposure to assess the percent target (near-maximum activity) attainment of different voriconazole dosages. Three Aspergillus clinical isolates (1 A. fumigatus, 1 A. flavus, and 1 A. terreus isolate) with CLSI MICs of 0.5 mg/liter were tested in an in vitro model simulating voriconazole PK in human plasma with C(max) values of 7, 3.5, and 1.75 mg/liter and a t(1/2) of 6 h. The area under the galactomannan index-time curve (AUC(GI)) was used as the PD parameter. In vitro PK-PD data were bridged with population human PK of voriconazole exposure, and the percent target attainment was calculated. The in vitro PK-PD relationship of fAUC(0-24)-AUC(GI) followed a sigmoid pattern (global R(2) = 0.97), with near-maximum activities (10% fungal growth) observed at an fAUC(0-24) (95% confidence interval [CI]) of 18.9 (14.4 to 23.1) mg · h/liter against A. fumigatus, 26.6 (21.1 to 32.9) mg · h/liter against A. flavus, and 36.2 (27.8 to 45.7) mg · h/liter against A. terreus (F test; P < 0.0001). Target attainment for 3, 4, and 5 mg/kg-of-body-weight voriconazole dosages was 24% (11 to 45%), 80% (32 to 97%), and 93% (86 to 97%) for A. fumigatus, 12% (5 to 26%), 63% (17 to 93%), and 86% (73 to 94%) for A. flavus, and 4% (2 to 11%), 36% (6 to 83%), and 68% (47 to 83%) for A. terreus. Based on the in vitro exposure-effect relationships, a standard dosage of voriconazole may be adequate for most patients with A. fumigatus but not A. flavus and A. terreus infections, for which a higher drug exposure may be required. This could be achieved using a higher voriconazole dosage, thus highlighting the usefulness of therapeutic drug monitoring in patients receiving a standard dosage.