Limited Sampling Strategy to Predict AUC of the CYP3A Phenotyping Probe Midazolam in Adults: Application to Various Assay Techniques

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/97211/1/00912700222011418.pd

Pharmacodynamic Modeling of Anti-Cancer Activity of Tetraiodothyroacetic Acid in a Perfused Cell Culture System

Unmodified or as a poly[lactide-co-glycolide] nanoparticle, tetraiodothyroacetic acid (tetrac) acts at the integrin αvβ3 receptor on human cancer cells to inhibit tumor cell proliferation and xenograft growth. To study in vitro the pharmacodynamics of tetrac formulations in the absence of and in conjunction with other chemotherapeutic agents, we developed a perfusion bellows cell culture system. Cells were grown on polymer flakes and exposed to various concentrations of tetrac, nano-tetrac, resveratrol, cetuximab, or a combination for up to 18 days. Cells were harvested and counted every one or two days. Both NONMEM VI and the exact Monte Carlo parametric expectation maximization algorithm in S-ADAPT were utilized for mathematical modeling. Unmodified tetrac inhibited the proliferation of cancer cells and did so with differing potency in different cell lines. The developed mechanism-based model included two effects of tetrac on different parts of the cell cycle which could be distinguished. For human breast cancer cells, modeling suggested a higher sensitivity (lower IC50) to the effect on success rate of replication than the effect on rate of growth, whereas the capacity (Imax) was larger for the effect on growth rate. Nanoparticulate tetrac (nano-tetrac), which does not enter into cells, had a higher potency and a larger anti-proliferative effect than unmodified tetrac. Fluorescence-activated cell sorting analysis of harvested cells revealed tetrac and nano-tetrac induced concentration-dependent apoptosis that was correlated with expression of pro-apoptotic proteins, such as p53, p21, PIG3 and BAD for nano-tetrac, while unmodified tetrac showed a different profile. Approximately additive anti-proliferative effects were found for the combinations of tetrac and resveratrol, tetrac and cetuximab (Erbitux), and nano-tetrac and cetuximab. Our in vitro perfusion cancer cell system together with mathematical modeling successfully described the anti-proliferative effects over time of tetrac and nano-tetrac and may be useful for dose-finding and studying the pharmacodynamics of other chemotherapeutic agents or their combinations

In Vivo Pharmacodynamics of Torezolid Phosphate (TR-701), a New Oxazolidinone Antibiotic, against Methicillin-Susceptible and Methicillin-Resistant Staphylococcus aureus Strains in a Mouse Thigh Infection Model ▿

Torezolid phosphate (TR-701) is the phosphate monoester prodrug of the oxazolidinone TR-700 which demonstrates potent in vitro activity against Gram-positive bacteria, including methicillin-susceptible Staphylococcus aureus (MSSA) and methicillin-resistant S. aureus (MRSA). The pharmacodynamics of TR-701 or TR-700 (TR-701/700) against S. aureus is incompletely defined. Single-dose pharmacokinetic studies were conducted in mice for TR-701/700. Forty-eight-hour dose range and 24-hour dose fractionation studies were conducted in a neutropenic mouse thigh model of S. aureus infection using MRSA ATCC 33591 to identify the dose and schedule of administration of TR-701/700 that was linked with optimized antimicrobial effect. Additional dose range studies compared the efficacies of TR-701/700 and linezolid for one MSSA strain and one community-associated MRSA strain. In dose range studies, TR-701/700 was equally bactericidal against MSSA and MRSA. Mean doses of 37.6 and 66.9 mg/kg of body weight/day of TR-701/700 resulted in stasis and 1 log CFU/g decreases in bacterial densities, respectively, at 24 h, and mean doses of 35.3, 46.6, and 71.1 mg/kg/day resulted in stasis and 1 and 2 log CFU/g reductions, respectively, at 48 h. Linezolid administered at doses as high as 150 mg/kg/day did not achieve stasis at either time point. Dose fractionation studies demonstrated that the area under the concentration-time curve over 24 h in the steady state divided by the MIC (AUC/MIC ratio) was the pharmacodynamic index for TR-701/700 that was linked with efficacy. TR-701/700 was highly active against MSSA and MRSA, in vivo, and was substantially more efficacious than linezolid, although linezolid's top exposure has half the human exposure. Dose fractionation studies showed that AUC/MIC was the pharmacodynamic index linked with efficacy, indicating that once-daily dosing in humans is feasible

Pharmacodynamics of Levofloxacin in a Murine Pneumonia Model of Pseudomonas aeruginosa Infection: Determination of Epithelial Lining Fluid Targets▿

The dose choice for Pseudomonas aeruginosa remains a matter of debate. The actual exposure targets required for multilog killing of organisms at the primary infection site have not been delineated. We studied Pseudomonas aeruginosa PAO1 using a murine model of pneumonia. We employed a large mathematical model to fit all the concentration-time data in plasma and epithelial lining fluid (ELF) as well as colony counts in lung simultaneously for all drug doses. Penetration into ELF was calculated to be approximately 77.7%, as indexed to the ratio of the area under the concentration-time curve for ELF (AUCELF) to the AUCplasma. We determined the ELF concentration-time profile required to drive a stasis response as well as 1-, 2-, or 3-log10(CFU/g) kill. AUC/MIC ratios of 12.4, 31.2, 62.8, and 127.6 were required to drive these bacterial responses. Emergence of resistance was seen only at the two lowest doses (three of five animals at 50 mg/kg [body weight] and one of five animals at 100 mg/kg). The low exposure targets were likely driven by a low mutational frequency to resistance. Bridging to humans was performed using Monte Carlo simulation. With a 750-mg levofloxacin dose, target attainment rates fell below 90% at 4 mg/liter, 1 mg/liter, and 0.5 mg/liter for 1-, 2-, and 3-log kills, respectively. Given the low exposure targets seen with this strain, we conclude that levofloxacin at a 750-mg dose is not adequate for serious Pseudomonas aeruginosa pneumonia as a single agent. More isolates need to be studied to make these observations more robust

Once-Weekly Micafungin Therapy Is as Effective as Daily Therapy for Disseminated Candidiasis in Mice with Persistent Neutropenia

The effect of micafungin dose scheduling on the treatment of candidemia is unknown. Neutropenic mice with disseminated Candida glabrata infection were treated with single intraperitoneal micafungin doses of 0 to 100 mg/kg of body weight and sacrificed 7 days later. The maximal decline in kidney fungal burden was 5.8 log(10) CFU/g. A 1-week pharmacokinetic-pharmacodynamic study revealed a micafungin serum half-life of 6.13 h. In mice treated with ≥50 mg/kg, there was maximal fungal decline without regrowth during the 1-week dosing interval. Next, doses associated with 34% (34% effective dose [ED(34)]) and 50% (ED(50)) of maximal kill were administered at one of three dose schedules: a single dose at t = 0, two equal doses at t = 0 and t = 3.5 days, and 7 equal doses daily. Some mice received a single dose of 100 mg/kg. Fungal burden was examined on days 1, 5, and 7. In mice treated with the ED(34), microbial kill with the daily therapy initially lagged behind the intermittent doses but exceeded it by day 7. In mice treated with the ED(50), daily and intermittent doses had equivalent day 7 effects. In mice treated with 100 mg/kg, there was no regrowth. The relative likelihoods that the area under the concentration-time curve/MIC ratio was linked to microbial kill versus peak concentration/MIC ratio or time above the MIC was 10.3 and 10,161.2, respectively. In all the experiments, no paradoxical increase in fungal burden was observed with high micafungin doses. However, only a single Candida isolate was tested. Regimens that simulated micafungin concentration-time profiles in patients treated with a single micafungin dose of 1,400 mg once a week demonstrated maximal fungal decline. Once-weekly micafungin therapy is as efficacious as daily therapy in a murine model of disseminated candidiasis

In Vitro Infection Model Characterizing the Effect of Efflux Pump Inhibition on Prevention of Resistance to Levofloxacin and Ciprofloxacin in Streptococcus pneumoniae▿

The prevalence of fluoroquinolone-resistant Streptococcus pneumoniae is slowly rising as a consequence of the increased use of fluoroquinolone antibiotics to treat community-acquired pneumonia. We tested the hypothesis that increased efflux pump (EP) expression by S. pneumoniae may facilitate the emergence of fluoroquinolone resistance. By using an in vitro pharmacodynamic infection system, a wild-type S. pneumoniae strain (Spn-058) and an isogenic strain with EP overexpression (Spn-RC2) were treated for 10 days with ciprofloxacin or levofloxacin in the presence or absence of the EP inhibitor reserpine to evaluate the effect of EP inhibition on the emergence of resistance. Cultures of Spn-058 and Spn-RC2 were exposed to concentration-time profiles simulating those in humans treated with a regimen of ciprofloxacin at 750 mg orally once every 12 h and with regimens of levofloxacin at 500 and 750 mg orally once daily (QD; with or without continuous infusions of 20 μg of reserpine/ml). The MICs of ciprofloxacin and levofloxacin for Spn-058 were both 1 μg/ml when susceptibility testing was conducted with each antibiotic alone and with each antibiotic in the presence of reserpine. For Spn-RC2, the MIC of levofloxacin alone and with reserpine was also 1 μg/ml; the MICs of ciprofloxacin were 2 and 1 μg/ml, respectively, when determined with ciprofloxacin alone and in combination with reserpine. Reserpine, alone, had no effect on the growth of Spn-058 and Spn-RC2. For Spn-058, simulated regimens of ciprofloxacin at 750 mg every 12 h or levofloxacin at 500 mg QD were associated with the emergence of fluoroquinolone resistance. However, the use of ciprofloxacin at 750 mg every 12 h and levofloxacin at 500 mg QD in combination with reserpine rapidly killed Spn-058 and prevented the emergence of resistance. For Spn-RC2, levofloxacin at 500 mg QD was associated with the emergence of resistance, but again, the resistance was prevented when this levofloxacin regimen was combined with reserpine. Ciprofloxacin at 750 mg every 12 h also rapidly selected for ciprofloxacin-resistant mutants of Spn-RC2. However, the addition of reserpine to ciprofloxacin therapy only delayed the emergence of resistance. Levofloxacin at 750 mg QD, with and without reserpine, effectively eradicated Spn-058 and Spn-RC2 without selecting for fluoroquinolone resistance. Ethidium bromide uptake and efflux studies demonstrated that, at the baseline, Spn-RC2 had greater EP expression than Spn-058. These studies also showed that ciprofloxacin was a better inducer of EP expression than levofloxacin in both Spn-058 and Spn-RC2. However, in these isolates, the increase in EP expression by short-term exposure to ciprofloxacin and levofloxacin was transient. Mutants of Spn-058 and Spn-RC2 that emerged under suboptimal antibiotic regimens had a stable increase in EP expression. Levofloxacin at 500 mg QD in combination with reserpine, an EP inhibitor, or at 750 mg QD alone killed wild-type S. pneumoniae and strains that overexpressed reserpine-inhibitable EPs and was highly effective in preventing the emergence of fluoroquinolone resistance in S. pneumoniae during therapy. Ciprofloxacin at 750 mg every 12 h, as monotherapy, was ineffective for the treatment of Spn-058 and Spn-RC2. Ciprofloxacin in combination with reserpine prevented the emergence of resistance in Spn-058 but not in Spn-RC2, the EP-overexpressing strain

Zanamivir, at 600 Milligrams Twice Daily, Inhibits Oseltamivir-Resistant 2009 Pandemic H1N1 Influenza Virus in an In Vitro Hollow-Fiber Infection Model System▿

In 2009, a novel H1N1 influenza A virus emerged and spread worldwide, initiating a pandemic. Various isolates obtained from disparate parts of the world were shown to be uniformly resistant to the adamantanes but sensitive to the neuraminidase inhibitors oseltamivir and zanamivir. Over time, resistance to oseltamivir became more prevalent among pandemic H1N1 virus isolates, while most remained susceptible to zanamivir. The government has proposed the use of intravenous (i.v.) zanamivir to treat serious influenza virus infections among hospitalized patients. To use zanamivir effectively for patients with severe influenza, it is necessary to know the optimal dose and schedule of administration of zanamivir that will inhibit the replication of oseltamivir-sensitive and -resistant influenza viruses. Therefore, we performed studies using the in vitro hollow-fiber infection model system to predict optimal dosing regimens for zanamivir against an oseltamivir-sensitive and an oseltamivir-resistant virus. Our results demonstrated that zanamivir, at a dose of 600 mg given twice a day (Q12h), inhibited the replication of oseltamivir-sensitive and oseltamivir-resistant influenza viruses throughout the course of the experiment. Thus, our findings suggest that intravenous zanamivir, at a dose of 600 mg Q12h, could be used to treat hospitalized patients suffering from serious infections with oseltamivir-sensitive or -resistant influenza viruses

Effect of Half-Life on the Pharmacodynamic Index of Zanamivir against Influenza Virus Delineated by a Mathematical Model▿

Intravenous zanamivir is recommended for the treatment of hospitalized patients with complicated oseltamivir-resistant influenza virus infections. In a companion paper, we show that the time above the 50% effective concentration (time>EC50) is the pharmacodynamic (PD) index predicting the inhibition of viral replication by intravenous zanamivir. However, for other neuraminidase inhibitors, the ratio of the area under the concentration-time curve to the EC50 (AUC/EC50) is the most predictive index. Our objectives are (i) to explain the dynamically linked variable of intravenous zanamivir by using different half-lives and (ii) to develop a new, mechanism-based population pharmacokinetic (PK)/PD model for the time course of viral load. We conducted dose fractionation studies in the hollow-fiber infection model (HFIM) system with zanamivir against an oseltamivir-resistant influenza virus. A clinical 2.5-h half-life and an artificially prolonged 8-h half-life were simulated for zanamivir. The values for the AUC from 0 to 24 h (AUC0-24) of zanamivir were equivalent for the two half-lives. Viral loads and zanamivir pharmacokinetics were comodeled using data from the present study and a previous dose range experiment via population PK/PD modeling in S-ADAPT. Dosing every 8 h (Q8h) suppressed the viral load better than dosing Q12h or Q24h at the 2.5-h half-life, whereas all regimens suppressed viral growth similarly at the 8-h half-life. The model provided unbiased and precise individual (Bayesian) (r2, >0.96) and population (pre-Bayesian) (r2, >0.87) fits for log10 viral load. Zanamivir inhibited viral release (50% inhibitory concentration [IC50], 0.0168 mg/liter; maximum extent of inhibition, 0.990). We identified AUC/EC50 as the pharmacodynamic index for zanamivir at the 8-h half-life, whereas time>EC50 best predicted viral suppression at the 2.5-h half-life, since the trough concentrations approached the IC50 for the 2.5-h but not for the 8-h half-life. The model explained data at both half-lives and holds promise for optimizing clinical zanamivir dosage regimens