Pharmacodynamics of Ceftaroline against Staphylococcus aureus Studied in an In Vitro Pharmacokinetic Model of Infection

An in vitro single-compartment dilutional pharmacokinetic model was used to study the pharmacodynamics of ceftaroline against Staphylococcus aureus (both methicillin-susceptible S. aureus [MSSA] and methicillin-resistant S. aureus [MRSA]). Mean serum free concentrations of ceftaroline (the active metabolite of the prodrug ceftaroline fosamil) dosed in humans at 600 mg every 12 h (q12h) were simulated, and activities against 12 S. aureus strains (3 MSSA strains and 9 MRSA strains, 3 of which had a vancomycin-intermediate phenotype) were determined. Ceftaroline produced 2.5- to 4.0-log₁₀-unit reductions in viable counts by 24 h with all strains and a 0.5- to 4.0-log-unit drop in counts at 96 h. The antibacterial effect could not be related to the strain MIC across the ceftaroline MIC range from 0.12 to 2.0 μg/ml. In dose-ranging studies, the cumulative percentage of a 24-h period that the free drug concentration exceeded the MIC under steady-state pharmacokinetic conditions (fT_MIC) of 24.5% ± 8.9% was associated with a 24-h bacteriostatic effect, one of 27.8% ± 9.5% was associated with a −1-log-unit drop, and one of 32.1% ± 8.1% was associated with a −2-log-unit drop. The MSSA and MRSA strains had similar fT_MIC values. fT_MIC values increased with increasing duration of exposure up to 96 h. Changes in ceftaroline population analysis profiles were related to fT_MIC. fT_MICs of <50% were associated with growth on 4× MIC recovery plates at 96 h of drug exposure. These data support the use of ceftaroline fosamil at doses of 600 mg q12h to treat S. aureus strains with MICs of ≤2 μg/ml. An fT_MIC of 25 to 30% would make a suitable pharmacodynamic index target, but fT_MIC values of ≥50% are needed to suppress the emergence of resistance and require clinical evaluation.     

Pharmacokinetics and Pharmacodynamics of Nemonoxacin against Streptococcus pneumoniae in an In Vitro Infection Model

The aim of this paper was to investigate the pharmacokinetics (PK) and pharmacodynamics (PD) of nemonoxacin, a novel nonfluorinated quinolone, against Streptococcus pneumoniae in vitro. A modified infection model was used to simulate the pharmacokinetics of nemonoxacin following scaling of single oral doses and multiple oral dosing. Four S. pneumoniae strains with different penicillin sensitivities were selected, and the drug efficacy was quantified by the change in log colony counts within 24 h. A sigmoid maximum-effect (E_max) model was used to analyze the relationship between PK/PD parameters and drug effect. Analysis indicated that the killing pattern of nemonoxacin shows a dualism which is mainly concentration dependent when the MIC is low and that the better PK/PD index should be the area under the concentration-time curve for the free, unbound fraction of the drug divided by the MIC (fAUC_0–24/MIC), which means that giving the total daily amount of drug as one dose is appropriate under those conditions. When the MIC is high, the time (T) dependency is important and the valid PK/PD index should be the cumulative percentage of a 24-h period in which the drug concentration exceeds the MIC under steady-state pharmacokinetic conditions (f%T>MIC), which means that to split the maximum daily dose into several separate doses will benefit the eradication of the bacteria. To obtain a 3-log₁₀-unit decrease, the target values of fAUC_0–24/MIC and f%T>MIC are 47.05 and 53.4%, respectively.     

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 C_max 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_maxs (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.     

Pharmacodynamics of a Simulated Single 1,200-Milligram Dose of Oritavancin in an In Vitro Pharmacokinetic/Pharmacodynamic Model of Methicillin-Resistant Staphylococcus aureus Infection

The safety and efficacy of a single 1,200-mg dose of the lipoglycopeptide oritavancin are currently being investigated in two global phase 3 studies of acute bacterial skin and skin structure infections. In this study, an in vitro pharmacokinetic/pharmacodynamic model was established to compare the free-drug pharmacodynamics associated with a single 1,200-mg dose of oritavancin to once-daily dosing with daptomycin at 6 mg/kg of body weight and twice-daily dosing with vancomycin at 1,000 mg against three methicillin-resistant Staphylococcus aureus (MRSA) strains over 72 h. The area under the bacterial-kill curve (AUBKC) was used to assess the antibacterial effect of each dosing regimen at 24 h (AUBKC_0-24), 48 h (AUBKC_0-48), and 72 h (AUBKC_0-72). The rapid bactericidal activities of oritavancin and daptomycin contributed to lower AUBKC_0-24s for the three MRSA strains than with vancomycin (P < 0.05, as determined by analysis of variance [ANOVA]). Oritavancin exposure also resulted in a lower AUBKC_0-48 and AUBKC_0-72 against one MRSA strain and a lower AUBKC_0-48 for another strain than did vancomycin exposure (P < 0.05). Furthermore, daptomycin exposure resulted in a lower AUBKC_0-48 and AUBKC_0-72 for one of the MRSA isolates than did vancomycin exposure (P < 0.05). Lower AUBKC_0-24s for two of the MRSA strains (P < 0.05) were obtained with oritavancin exposure than with daptomycin. Thus, the antibacterial effect from the single-dose regimen of oritavancin is as effective as that from either once-daily dosing with daptomycin or twice-daily dosing with vancomycin against the MRSA isolates tested in an in vitro pharmacokinetic/pharmacodynamic model over 72 h. These results provide further justification to assess the single 1,200-mg dose of oritavancin for treatment of acute bacterial skin and skin structure infections.     

Pharmacodynamics of Ampicillin-Sulbactam in an In Vitro Infection Model against Escherichia coli Strains with Various Levels of Resistance

The activity of ampicillin-sulbactam against β-lactamase-producing Escherichia coli has been questioned. Therefore, in this study, the killing activity of ampicillin-sulbactam was investigated in an in vitro infection model which simulates human pharmacokinetics. One ampicillin-sensitive strain (E. coli ATCC 25922, ampicillin-sulbactam MIC = 4/2 μg/ml) and three ampicillin-resistant TEM-1-producing strains with various levels of ampicillin-sulbactam resistance (EC11, MIC = 4/2 μg/ml; TIM2, MIC = 12/6 μg/ml; and GB85, MIC > 128/64 μg/ml) were studied. The E. coli strains were exposed to ampicillin-sulbactam at a starting inoculum of 6 to 7 log₁₀ CFU/ml. Ampicillin-sulbactam was infused over 30 min to simulate doses of 3 and 1.5 g every 6 h for 24 h. The 3-g ampicillin-sulbactam dose was bactericidal against E. coli ATCC 25922, EC11, and TIM2. The 1.5-g dose displayed bactericidal activity against ATCC 25922 and EC11 similar to that of the higher dose but failed to kill TIM2 due to inadequate time above the MIC and increased MICs over 24 h. GB85 was highly resistant and grew similarly to controls. Despite an MIC at 10⁷ CFU/ml indicating resistance (20/10 μg/ml), TIM2 was killed by the 3-g dose of ampicillin-sulbactam. Current MIC breakpoints may not adequately portray the activity of ampicillin-sulbactam, considering both the activity in in vitro infection models and clinical data.     

Pharmacodynamics of Trovafloxacin and Levofloxacin against Bacteroides fragilis in an In Vitro Pharmacodynamic Model

An in vitro pharmacodynamic investigation was conducted to explore whether the area under the concentration time curve from 0 to 24 h (AUC(0-24))/MIC ratio could predict fluoroquinolone performance against Bacteroides fragilis. An in vitro model was used to generate kill curves for trovafloxacin (TVA) and levofloxacin (LVX) at AUC(0-24)/MIC ratios of 1 to 406 against three strains of B. fragilis (ATCC 25285, ATCC 23745, and clinical isolate M97-117). TVA and LVX were bolused prior to the start of experiments to achieve the corresponding AUC(0-24)/MIC ratio. Experiments were performed in duplicate over 24 h and in an anaerobic environment. Analyses of antimicrobial performance were conducted by comparing the rates of bacterial kill (K) using nonlinear regression analysis with 95% confidence intervals. Statistical significance was defined as a lack of overlap in the 95% confidence limits generated from the slope of each kill curve. For both TVA and LVX, K was maximized once an AUC(0-24)/MIC ratio of > or =40 was achieved and was not further increased despite a 10-fold increase in AUC(0-24)/MIC from approximately 40 to 400 against all three strains of B. fragilis. No significant differences were found in K between AUC(0-24)/MIC ratios of approximately 40 to 200. In experiments where AUC(0-24)/MIC ratios that were > or = 5 and < or = 44 were conducted, 64% demonstrated regrowth at 24 h. Resistant strains were selected in 50% of those experiments, demonstrating regrowth, which resulted in increased MICs of two- to 16-fold for both TVA and LVX. Regrowth did not occur, nor were resistant strains selected in any studies with an AUC/MIC that was > 44. Our findings suggest that fluoroquinolones provide antibacterial effects against B. fragilis in a concentration-independent manner associated with an AUC(0-24)/MIC ratio of > or =40. Also, the potential for the selection of resistant strains of B. fragilis may increase with an AUC(0-24)/MIC ratio of < or =44.     

Pharmacokinetic Determinants of Virological Response to Raltegravir in the In Vitro Pharmacodynamic Hollow-Fiber Infection Model System

Daily administration (q24h) of raltegravir has been shown to be as efficacious as twice-daily administration (q12h) in the hollow-fiber infection model (HFIM) system. However, q24h regimens were not noninferior to q12h dosing in a clinical trial. We hypothesized that between-patient variability in raltegravir pharmacokinetics (PK) was responsible for the discordance in conclusions between the in vitro and in vivo studies. Hollow-fiber cartridges were inoculated with HIV-infected H9 cells and uninfected CEM-SS cells. Four cartridges received the total daily exposure (800 mg) q24h and four received half the daily exposure (400 mg) q12h. PK profiles with half-lives of 8, 4, 3, and 2 h were simulated for each dosing interval. Cell-to-cell viral spread was assessed by flow cytometry. Viral inhibition was similar between q24h and q12h dosing at the 8- and 4-h half-lives. The q24h dosing was not as efficacious as the q12h dosing when faster half-lives were simulated; a lack of viral suppression was observed at days 3 and 4 for the 2- and 3-h half-lives, respectively. The discrepancy in conclusions between the in vitro HFIM system studies and clinical trials is likely due to the large interindividual variation in raltegravir PK.     

Pharmacodynamics of the Antibacterial Effect of and Emergence of Resistance to Doripenem in Pseudomonas aeruginosa and Acinetobacter baumannii in an In Vitro Pharmacokinetic Model

An in vitro dilutional pharmacokinetic model of infection was used to study the pharmacodynamics of doripenem in terms of the ability to kill Pseudomonas aeruginosa or Acinetobacter baumannii and also changes in their population profiles. In dose-ranging studies, the cumulative percentages of a 24-h period that the drug concentration exceeds the MIC under steady-state pharmacokinetic conditions (T(MIC)s) required for doripenem to produce a 24-h bacteriostatic effect and a -2-log-unit reduction in viable count were 25% ± 11% and 35% ± 13%, respectively, for P. aeruginosa (MIC range, 0.24 to 3 mg/liter) and 20% ± 11% and 33% ± 12%, respectively, for Acinetobacter spp. (MIC range, 0.45 to 3.0 mg/liter). A T(MIC) of >40 to 50% produced a maximum response with both species at 24 h or 48 h of exposure. After 24 h of exposure to doripenem at a T(MIC) in the range of 12.5 to 37.5%, P. aeruginosa and A. baumannii population profiles revealed mutants able to grow on 4× MIC-containing medium; such changes were further amplified by 48 h of exposure. Dose-fractionation experiments targeting T(MIC)s of 12.5%, 25%, or 37.5% as six exposures, two exposures, or a single exposure over 48 h with a single strain of P. aeruginosa indicated that changes in population profiles were greatest with multiple exposures at T(MIC) targets of 12.5 or 25%. In contrast, multiple exposures at 37.5% T(MIC) most effectively suppressed total bacterial counts and changes in population profiles. Simulations of human doses of doripenem of 500 mg, 1,000 mg, 2,000 mg, and 3,000 mg every 8 h over 96 h showed marked initial killing up to 6 h but growback thereafter. Changes in population profiles occurred only in the regimen of 500 mg every 8 h against P. aeruginosa but occurred with all dose regimens for A. baumannii strains. A doripenem T(MIC) of ≥40 to 50% is maximally effective in killing P. aeruginosa or A. baumannii and suppressing changes in population profiles in short-term experiments for up to 48 h; however, a T(MIC) of 12.5 to 25% amplifies population changes, especially with exposures every 8 h. In longer-term experiments, up to 96 h, even doripenem doses of 4 to 6 times those used in human studies proved incapable of pathogen eradication and prevention of changes in population profiles. The association of a T(MIC) of 25 to 37.5% with changes in population profiles has implications in terms of future clinical breakpoint setting.     

Pharmacodynamics of Gatifloxacin against Streptococcus pneumoniae in an In Vitro Pharmacokinetic Model: Impact of Area under the Curve/MIC Ratios on Eradication

Previous studies have demonstrated that fluoroquinolone area under the curve (AUC)/MIC ratios of 30 to 50 are sufficient to eradicate pneumococci from in vitro pharmacokinetic models (IVPMs). However, more systematic studies of the impact of AUC/MIC ratios on the antipneumococcal activities of fluoroquinolones are needed. In the present study, a two-compartment IVPM was used to evaluate the impact of AUC/MIC ratios on the pharmacodynamics of gatifloxacin against four strains of Streptococcus pneumoniae. Gatifloxacin MICs were 0.4 to 1 microg/ml, whereas levofloxacin MICs were 1.8 to 3.2 microg/ml. Since both peak concentration/MIC (peak/MIC) and AUC/MIC ratios affect fluoroquinolone pharmacodynamics, logarithmic-phase cultures (5 x 10(7) CFU/ml) were exposed to gatifloxacin at constant peak/MIC ratios of 2:1 to 3:1 at 0 and 24 h, elimination half-lives were varied to provide a range of AUC/MIC ratios, and changes in viable counts were measured over 30 h. As a comparison, levofloxacin was evaluated at similar peak/MIC ratios and at AUC/MIC ratios of 30 to 38. For each strain, killing rates through 4 to 8 h were similar since peak/MIC ratios were kept constant. However, continued killing and eradication were observed only when gatifloxacin AUC/MIC ratios were 27 to 48. Levofloxacin also provided eradication. In contrast, substantial regrowth was observed in most experiments when gatifloxacin AUC/MIC ratios were 9 to 24. These data provide further support that fluoroquinolone AUC/MIC ratios of approximately 30 or higher can be sufficient for eradication of pneumococci from IVPMs. Furthermore, the overall impact of the AUC/MIC ratio was not influenced by the strain evaluated or its susceptibility to gatifloxacin. Further studies with other fluoroquinolones and pneumococci that exhibit wider ranges of susceptibilities are warranted. In addition, similar studies with higher peak/MIC ratios are needed to better define the impact of AUC/MIC ratios and peak/MIC ratios on the antipneumococcal pharmacodynamics of fluoroquinolones.     

Pharmacokinetic/Pharmacodynamic Investigation of Colistin against Pseudomonas aeruginosa Using an In Vitro Model

Colistin plays a key role in treatment of serious infections by Pseudomonas aeruginosa. The aims of this study were to (i) identify the pharmacokinetic/pharmacodynamic (PK/PD) index (i.e., the area under the unbound concentration-time curve to MIC ratio [ƒAUC/MIC], the unbound maximal concentration to MIC ratio [ƒC_max/MIC], or the cumulative percentage of a 24-h period that unbound concentrations exceed the MIC [ƒT_>MIC]) that best predicts colistin efficacy and (ii) determine the values for the predictive PK/PD index required to achieve various magnitudes of killing effect. Studies were conducted in a one-compartment in vitro PK/PD model for 24 h using P. aeruginosa ATCC 27853, PAO1, and the multidrug-resistant mucoid clinical isolate 19056 muc. Six intermittent dosing intervals, with a range of ƒC_max colistin concentrations, and two continuous infusion regimens were examined. PK/PD indices varied from 0.06 to 18 for targeted ƒC_max/MIC, 0.36 to 312 for ƒAUC/MIC, and 0 to 100% for ƒT_>MIC. A Hill-type model was fit to killing effect data, which were expressed as the log₁₀ ratio of the area under the CFU/ml curve for treated regimens versus control. With ƒC_max values equal to or above the MIC, rapid killing was observed following the first dose; substantial regrowth occurred by 24 h with most regimens. The overall killing effect was best correlated with ƒAUC/MIC (R² = 0.931) compared to ƒC_max/MIC (R² = 0.868) and ƒT_>MIC (R² = 0.785). The magnitudes of ƒAUC/MIC required for 1- and 2-log₁₀ reductions in the area under the CFU/ml curve relative to growth control were 22.6 and 30.4, 27.1 and 35.7, and 5.04 and 6.81 for ATCC 27853, PAO1, and 19056 muc, respectively. The PK/PD targets identified will assist in designing optimal dosing strategies for colistin.Globally there is a growing threat from the emergence of multidrug-resistant (MDR) microorganisms (38), especially among a number of important Gram-negative bacterial pathogens (16, 29, 38). Colistin (polymyxin E) still retains significant activity against many of these MDR Gram-negative pathogens, including Pseudomonas aeruginosa, Acinetobacter baumannii, and Klebsiella pneumoniae, which often leaves it as the only therapeutic option available (19, 26). With very few new chemical entities against Gram-negative infections in the drug development pipeline (29, 30, 38), particularly against P. aeruginosa (38), the use of colistin, a once-neglected antibiotic, has increased dramatically over the last 5 years (11, 26).Colistin is available commercially as colistin sulfate (hereafter referred to as colistin) and sodium colistin methanesulfonate (CMS), which is administered parenterally. CMS is an inactive prodrug of colistin (3) and, after parenteral administration, colistin is formed in vivo (21, 27, 33). Despite its newfound importance in therapy, there is a dearth of information on the pharmacokinetic (PK) and pharmacodynamic (PD) properties of colistin, a situation of significant concern given that resistance to colistin is beginning to emerge (1, 15, 18, 26, 28). Thus, the aims of the present study were to utilize an in vitro PK/PD model to (i) identify the PK/PD index (i.e., the area under the unbound concentration-time curve to MIC ratio [ƒAUC/MIC], the unbound maximal concentration to MIC ratio [ƒC_max/MIC], or the cumulative percentage of a 24-h period that unbound concentrations exceed the MIC [ƒT_>MIC]) that best predicts colistin efficacy and (ii) determine the magnitude of the predictive PK/PD index required to achieve various magnitudes of killing effect.(Parts of the present study were presented at the 48th Annual Interscience Conference on Antimicrobial Agents and Chemotherapy [ICAAC], Washington, DC, 25 to 28 October 2008, and at the Second American Conference on Pharmacometrics, Mashantucket, CT, 4 to 7 October 2009.)     

Pharmacodynamics of Isavuconazole in an Aspergillus fumigatus Mouse Infection Model

Azole resistance is an emerging problem in Aspergillus fumigatus which translates into treatment failure. Alternative treatments with new azoles may improve therapeutic outcome in invasive aspergillosis (IA) even for strains with decreased susceptibility to current azoles. The in vivo efficacy of 0.25, 1, 4, 16, 64, 128, 256, and 512 mg/kg of body weight/day prodrug isavuconazonium sulfate (BAL8557) (isavuconazole [ISA]-equivalent doses of 0.12, 0.48, 1.92, 7.68, 30.7, 61.4, 122.9, and 245.8 mg/kg/day, respectively) administered by oral gavage was assessed in an immunocompetent murine model of IA against four clinical A. fumigatus isolates: a wild-type isolate (ISA MIC_EUCAST, 0.5 mg/liter) and three azole-resistant isolates harboring substitutions in the cyp51A gene: G54W (ISA MIC_EUCAST, 0.5 mg/liter), M220I (ISA MIC_EUCAST, 4 mg/liter), and TR₃₄/L98H (ISA MIC_EUCAST, 8 mg/liter). The maximum effect (100% survival) was reached at a prodrug isavuconazonium sulfate dose of 64 mg/kg for the wild-type isolate, 128 mg/kg for the G54W mutant, and 256 mg/kg two times per day (q12) for the M220I mutant. A maximum response was not achieved with the TR₃₄/L98H isolates with the highest dose of prodrug isavuconazonium sulfate (256 mg/kg q12). For a survival rate of 50%, the effective AUC_0–24/MIC_EUCAST ratio for ISA total drug was 24.73 (95% confidence interval, 22.50 to 27.18). The efficacy of isavuconazole depended on both the drug exposure and the isavuconazole MIC of the isolates. The quantitative relationship between exposure and effect (AUC_0–24/MIC) can be used to optimize the treatment of human infections by A. fumigatus, including strains with decreased susceptibility.     

Ceftaroline plus Avibactam Demonstrates Bactericidal Activity against Pathogenic Anaerobic Bacteria in a One-Compartment In Vitro Pharmacokinetic/Pharmacodynamic Model

Anaerobic pathogens are often associated with polymicrobial infections, such as diabetic foot infections. Patients with these infections are often treated with broad-spectrum, multidrug therapies targeting resistant Gram-positive bacteria, such as methicillin-resistant Staphylococcus aureus, as well as Gram-negative bacteria and anaerobes. The broad-spectrum, non-beta-lactam, beta-lactamase inhibitor avibactam has been combined with ceftaroline and may provide a single-product alternative for complicated polymicrobial infections. We compared the activity of ceftaroline-avibactam (CPA) to that of ertapenem (ERT) against common anaerobic pathogens in an in vitro pharmacokinetic/pharmacodynamic (PK/PD) model. Simulations of doses of ceftaroline-fosamil at 600 mg every 8 h (q8h) (maximum free drug concentration [fC_max], 17.04 mg/liter, and half-life [t_1/2], 2.66 h) plus avibactam at 600 mg q8h (fC_max, 11.72 mg/liter, and t_1/2, 1.8 h) and of ertapenem at 1 g q24h (fC_max, 13 mg/liter, and t_1/2, 4 h) were evaluated against two strains of Bacteroides fragilis, one strain of Prevotella bivia, and one strain of Finegoldia magna in an anaerobic one-compartment in vitro PK/PD model over 72 h with a starting inoculum of ∼8 log₁₀ CFU/ml. Bactericidal activity was defined as a reduction of ≥3 log₁₀ CFU/ml from the starting inoculum. Both CPA and ERT were bactericidal against all four strains. CPA demonstrated improved activity against Bacteroides strains compared to that of ERT but had similar activity against Finegoldia magna and P. bivia, although modest regrowth was observed with CPA against P. bivia. No resistance emerged from any of the models. The pharmacokinetics achieved were 92 to 105% of the targets. CPA has potent in vitro activity against common anaerobic pathogens at clinically relevant drug exposures and may be a suitable single product for the management of complicated polymicrobial infections.     

Pharmacodynamics of Isavuconazole in a Dynamic In Vitro Model of Invasive Pulmonary Aspergillosis

Isavuconazonium sulfate is a novel triazole prodrug that has been recently approved for the treatment of invasive aspergillosis by the FDA. The active moiety (isavuconazole) has a broad spectrum of activity against many pathogenic fungi. This study utilized a dynamic in vitro model of the human alveolus to describe the pharmacodynamics of isavuconazole against two wild-type and two previously defined azole-resistant isolates of Aspergillus fumigatus. A human-like concentration-time profile for isavuconazole was generated. MICs were determined using CLSI and EUCAST methodologies. Galactomannan was used as a measure of fungal burden. Target values for the area under the concentration-time curve (AUC)/MIC were calculated using a population pharmacokinetics-pharmacodynamics (PK-PD) mathematical model. Isolates with higher MICs required higher AUCs in order to achieve maximal suppression of galactomannan. The AUC/MIC targets necessary to achieve 90% probability of galactomannan suppression of <1 were 11.40 and 11.20 for EUCAST and CLSI, respectively.     

In Vivo Pharmacokinetic/Pharmacodynamic Profiles of Valnemulin in an Experimental Intratracheal Mycoplasma gallisepticum Infection Model

Valnemulin, a semisynthetic pleuromutilin antibiotic derivative, is greatly active against Mycoplasma. The objective of our study was to evaluate the effectiveness of valnemulin against Mycoplasma gallisepticum in a neutropenic intratracheal model in chickens using a pharmacokinetic/pharmacodynamic (PK-PD) method. The PK of valnemulin after intramuscular (i.m.) administration at doses of 1, 10, and 20 mg/kg of body weight in M. gallisepticum-infected neutropenic chickens was evaluated by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Real-time PCR (RT-PCR) was used for quantitative detection of M. gallisepticum. The ratio of the 24-h area under the concentration-time curve divided by the MIC (AUC₂₄/MIC) correlated well with the in vivo antibacterial effectiveness of valnemulin (R² = 0.9669). The AUC₂₄/MIC ratios for mycoplasmastasis (a reduction of 0 log₁₀ color-changing unit [CCU] equivalents/ml), a reduction of 1 log₁₀ CCU equivalents/ml, and a reduction of 2.5 log₁₀ CCU equivalents/ml are 28,820, 38,030, and 56,256, respectively. In addition, we demonstrated that valnemulin at a dose of 6.5 mg/kg resulted in a reduction of 2.5 log₁₀ CCU equivalents/ml. These investigations provide a solid foundation for the usage of valnemulin in poultry with M. gallisepticum infections.     

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.     

Exploration of the Pharmacokinetic-Pharmacodynamic Relationships for Fosfomycin Efficacy Using an In Vitro Infection Model

Fosfomycin, a phosphonic class antibiotic with a broad spectrum of antibacterial activity, has been used outside the United States since the early 1970s for the treatment of a variety of infections. In the United States, an oral (tromethamine salt) formulation is used for uncomplicated urinary tract infections. Recently, there has been interest in the use of an intravenous solution (ZTI-01) for the treatment of a broad range of infections associated with multidrug-resistant bacteria. In this era of multidrug-resistant bacteria with few treatment options, it is critical to understand the pharmacokinetic-pharmacodynamic (PK-PD) determinants for fosfomycin efficacy. Since such data are limited, a one-compartment in vitro infection model was used to determine the PK-PD index associated with efficacy and the magnitude of this measure necessary for various levels of effect. One challenge isolate (Escherichia coli ATCC 25922, for which the fosfomycin agar MIC is 0.5 mg/liter and the broth microdilution MIC is 1 mg/liter) was evaluated in the dose fractionation studies, and two additional clinical E. coli isolates were evaluated in the dose-ranging studies. Mutation frequency studies indicated the presence of an inherently fosfomycin resistant E. coli subpopulation (agar MIC = 32 to 64 mg/liter) within the standard starting inoculum of a susceptibility test. Due to the presence of this resistant subpopulation, we identified the percentage of the dosing interval that drug concentrations were above the inherent resistance inhibitory concentration found at baseline to be the PK-PD index associated with efficacy (r² = 0.777). The magnitudes of this PK-PD index associated with net bacterial stasis and 1- and 2-log₁₀ CFU/ml reductions from baseline at 24 h were 11.9, 20.9, and 32.8, respectively. These data provide useful information for modernizing and optimizing ZTI-01 dosing regimens for further study.