Pharmacokinetics and Pharmacodynamics of Mesna-Mediated Plasma Cysteine Depletion

Cellular glutathione (GSH) levels are related to the resistance of tumor cells to platinum and alkylating agents, and depletion of GSH may enhance the activity of these drugs. The pharmacodynamic effects of mesna on depleting plasma cysteine, a GSH precursor, were evaluated in 22 patients as part of a Phase I study. Escalating doses of ifosfamide and mesna were administered; carboplatin was administered to achieve an AUC of 4 mg x min/mL. Plasma samples were collected and assayed by reverse-phase high-performance liquid chromatography (HPLC) for total mesna and total cysteine concentrations at 0, 1, 3, 6, 24, 25, 28, and 48 hours. A one-compartment pharmacokinetic model was fit to the mesna plasma concentrations, using M.A.P. Bayesian estimation (ADAPT II). Pharmacodynamics were evaluated by fitting an inhibitory Emax model to the cysteine concentration data. Both the pharmacokinetic (median R2 = 0.95; range = 0.85-0.98) and pharmacodynamic (median R2 = 0.96; range = 0.74-1.0) models fit the data well. Mean (coefficient of variation [CV%]) mesna pharmacokinetic parameter estimates were as follows: Vss of 15.3 (29) L/m2, CL of 4.6 (29) L/h/m2, and half-life of 2.2 (37) hours. Mean (CV%) pharmacodynamic parameter estimates were as follows: Emax of 31.7 (19) microg/mL and EC50 of 10.3 (52) microg/mL. Mesna produced a rapid, concentration-dependent reduction in plasma cysteine concentrations that could be adequately characterized by an inhibitory Emax pharmacodynamic model. The depletion of plasma cysteine was facilitated by ifosfamide, suggesting a pharmacodynamic interaction between these two agents. Further increases in mesna doses beyond those administered in this study would be unlikely to provide additional benefit.

Pharmacodynamics of cefprozil against Haemophilus influenzae in an in vitro pharmacodynamic model

An in vitro pharmacodynamic model was used to determine the pharmacokinetic-pharmacodynamic (PK-PD) measure and magnitude most strongly related to cefprozil activity against Haemophilus influenzae. Using 3 clinical isolates of H. influenzae, a series of dose-fractionation studies were conducted, simulating cefprozil pediatric pharmacokinetics. The studies were designed to deliver a range of free cefprozil AUC(24) given once daily, twice daily, and by continuous infusion (CI). Drug effect, characterized by computing the log(10) ratio of the area under the 24-h bacterial colony-forming unit (CFU) (AUC(CFU)) curve to drug-free control, was fit to a Hill-type model for 3 PK-PD measures of activity: AUC(24)/MIC, C(max)/MIC, and %T > MIC. Once daily regimens provided much less activity than twice daily or CI regimens. AUC(24)/MIC and %T > MIC characterized the data well, whereas C(max)/MIC did not. Based on the PK-PD model results, for cefprozil twice daily, 50% and 80% of maximum drug effect (E(max)) was achieved at a %T > MIC of approximately 52% and 75%, respectively. A 2-log(10) reduction in log(10) ratio would require free drug %T > MIC of 58% or AUC(24)/MIC of 86. Bacteriostasis was achieved at a %T > MIC and an AUC(24)/MIC of approximately 25% and 30%, respectively. An in vitro pharmacodynamic model was able to characterize the PK-PD of cefprozil against H. influenzae. Consistent with limited clinical data, a minimum %T > MIC of 40% to 50% would be suggested to achieve in vivo activity in otitis media, with maximal activity at approximately 70%T > MIC.

Development of a Pharmacokinetic and Bayesian Optimal Sampling Model for Individualization of Oral Busulfan in Hematopoietic Stem Cell Transplantation

Therapeutic drug monitoring is used to minimize toxicity and maximize the therapeutic efficacy of busulfan, which shows high intra- and interpatient pharmacokinetic variability and erratic oral absorption. This study was designed to develop a pharmacokinetic model that could accommodate the erratic oral absorption of busulfan and to use this model to develop an optimal sparse pharmacokinetic sampling strategy to improve the precision and efficiency of therapeutic drug monitoring. Twenty-one pharmacokinetic profiles were collected from 12 patients receiving oral busulfan before hematopoietic stem cell transplantation. Each pharmacokinetic profile was defined by 5 to 9 plasma concentrations. Candidate pharmacokinetic models were initially fit to the data by maximum likelihood, with model discrimination by Akaike's Information Criterion. Maximum likelihood results were used to derive Bayesian previous parameter estimates, and D-optimal design was used to determine optimal sparse sampling strategies. Each candidate sampling strategy was tested in each patient by comparing the resultant Css obtained from the sparse strategy to the actual Css derived from each patient's full pharmacokinetic dataset. The final model was a 1-compartment model, with oral busulfan absorbed in 1 to 3 phases, and fit the data well. All limited sampling models tested were unbiased in their results, and a 4-sample scheme proved to adequately characterize busulfan pharmacokinetics, and should allow for a reduced sampling frequency for therapeutic drug monitoring.

Cytochrome P450 mRNA expression in peripheral blood lymphocytes as a predictor of enzyme induction

OBJECTIVE

Previous reports have supported the concept that messenger ribonucleic acid (mRNA) concentrations for cytochrome P450 (CYP) enzymes in peripheral blood mononuclear cells may be predictive of systemic enzyme activity. We investigated whether changes in mRNA expression for CYP1A2,CYP2C19, CYP2D6 and CYP3A4 in peripheral blood lymphocytes (PBLs) may serve as surrogate markers for changes in CYP enzyme activity following the administration of rifampin.

METHODS

On day 1 and day 9 of the study, 12 healthy volunteers were administered caffeine 100 mg, debrisoquine 10 mg and omeprazole 40 mg orally, along with midazolam 0.025 mg/kg intravenously. Blood samples and urine were collected for 8 h after drug administration. The subjects took rifampin 300 mg (n = 6) or 600 mg (n = 6) daily on days 2-8. Total RNA was isolated from PBLs on day 1 and day 9, and mRNA expression for the CYP enzymes and hGAPDH were determined by means of quantitative, real-time, reverse-transcriptase polymerase chain reaction. CYP1A2 activity was estimated by calculating the plasma paraxanthine to caffeine AUC ratio (caffeine metabolic ratio; CMR), CYP2C19 activity by the 2-h omeprazole hydroxylation index (HI), CYP2D6 activity by the urinary debrisoquine recovery ratio (DBRR) and CYP3A4 activity by midazolam clearance.

RESULTS

Median midazolam clearance (0.362 to 0.740 l/kg/h), omeprazole HI (0.752 to 0.214), CMR (0.365 to 0.450) and DBRR (0.406 to 0.479) all changed significantly following rifampin, consistent with the expected enzyme induction. CYP1A2,CYP2D6 and CYP3A4 mRNA content were measurable in all samples. CYP2C19 mRNA was inconsistently detectable. There were no significant correlations between changes in enzyme activity and mRNA expression by Spearman's rank order correlation.

CONCLUSION

The results do not support the use of mRNA expression assays for CYP1A2, CYP2C19, CYP2D6 and CYP3A4 enzymes in PBLs as surrogates for quantifying changes in systemic enzyme activity in the setting of enzyme induction.

Application of an in vitro infection model and simulation for reevaluation of fluoroquinolone breakpoints for Salmonella enterica serotype typhi

Salmonella enterica serotype Typhi and nontyphoidal Salmonella remain major causes of morbidity and mortality worldwide. Ampicillin, trimethoprim-sulfamethoxazole, and chloramphenicol no longer provide reliable coverage of Salmonella, and fluoroquinoloes have emerged as first-line treatment options. Due to mounting evidence of decreased in vitro susceptibility and diminished clinical response to fluoroquinolone therapy, it has been suggested that the NCCLS breakpoints for the salmonellae be reevaluated. We utilized an in vitro infection model to determine which pharmacokinetic-pharmacodynamic (PK-PD) measure was most closely linked to fluoroquinolone activity against salmonellae and the magnitude that was predictive of efficacy. Monte Carlo simulation was utilized to determine the probability of attaining potential susceptibility breakpoints for three fluoroquinolones. The free-drug area under the concentration-time curve from 0 to 24 h/MIC ratio was the PK-PD measure most predictive of efficacy, and a ratio of 105 corresponded to 90% of maximal activity. Simulation results suggested susceptible breakpoints of 0.12 microg/ml for ciprofloxacin and gatifloxacin and 0.25 microg/ml for levofloxacin. These proposed breakpoints correspond to the MIC separating the wild-type susceptible organism population from those strains possessing single-step mutations in the quinolone resistance-determining region. These results that integrate PK-PD measures and fluoroquinolone MIC distributions in the genetic context of examined Salmonella isolates clearly demonstrate that the prudent use of a lower susceptibility breakpoint minimizes the probability of clinical failure or delayed response in fluoroquinolone-treated patients.

Comparative in vitro activities of daptomycin, linezolid, and quinupristin/dalfopristin against Gram-positive bacterial isolates from a large cancer center

Our objective was to evaluate and compare the in vitro activity of daptomycin, linezolid, and quinupristin/dalfopristin against clinical bloodstream isolates of Gram-positive pathogens from a large cancer center in the Northeastern United States. Minimum inhibitory concentrations (MICs) were determined for daptomycin, quinupristin/dalfopristin, and linezolid against 258 isolates; bactericidal activity was evaluated using time-kill experiments against 14 representative pathogens. Vancomycin-resistant enterococci represented the largest proportion of bacteria tested (32% of the isolates), followed by methicillin-resistant coagulase-negative staphylococci (23%), and vancomycin sensitive enterococci (14%). Against staphylococci, the MIC90 was 1 microg/mL for both daptomycin and quinupristin/dalfopristin and 4 microg/mL for linezolid. Against enterococci, the MIC90 for both daptomycin and linezolid was 4 microg/mL and was 16 microg/mL for quinupristin/dalfopristin. The quinupristin/dalfopristin MIC90 for Enterococcus faecium was 2 microg/mL. Two enterococci were linezolid resistant and remained susceptible to daptomycin. In vitro time-kill studies found daptomycin to be rapidly bactericidal against the majority of organisms tested, killing 99.9% of bacteria within 6 h. Quinupristin/dalfopristin was bactericidal against staphylococci and bacteriostatic against most enterococci. Linezolid was bacteriostatic against all organisms evaluated. Daptomycin, quinupristin/dalfopristin, and linezolid each demonstrated in vitro activity against this collection of organisms. Future clinical studies to evaluate a potential role for these agents in the management of infections in cancer patients, including the treatment of febrile neutropenia, appear warranted.

The Influence of St. John’s Wort on the Pharmacokinetics and Protein Binding of Imatinib Mesylate

STUDY OBJECTIVE

To determine the effect of St. John's wort on the pharmacokinetics of imatinib mesylate.

DESIGN

Open-label, complete crossover, fixed-sequence, pharmacokinetic study.

SETTING

Clinical research center.

SUBJECTS

Ten healthy adult volunteers.

INTERVENTION

Single 400-mg oral doses of imatinib were administered before and after 2 weeks of treatment with St. John's wort 300 mg 3 times/day.

MEASUREMENTS AND MAIN RESULTS

The pharmacokinetics of imatinib were significantly altered by St. John's wort, with reductions of 32% in the median area under the concentration-time curve from time zero to infinity (p=0.0001), 29% in maximum observed concentration (p=0.005), and 21% in half-life (p=0.0001). Protein binding ranged from 97.7-90.3% (mean 94.9%), was concentration independent, and was not altered by St. John's wort. Therapeutic outcomes of imatinib have been shown to correlate with both dose and drug concentrations.

CONCLUSION

Coadministration of imatinib with St. John's wort may compromise imatinib's clinical efficacy.

Effect of Tenofovir Disoproxil Fumarate on the Pharmacokinetics and Pharmacodynamics of Total, R-, and S-Methadone

STUDY OBJECTIVE

To evaluate the potential effect of tenofovir disoproxil fumarate (DF) on the pharmacokinetics of methadone.

DESIGN

Phase I, open-label, fixed-sequence, pharmacokinetic drug-drug interaction study.

SETTING

Clinical research center.

SUBJECTS

Fourteen volunteers receiving stable methadone maintenance therapy who were not infected with the human immunodeficiency virus.

INTERVENTION

Tenofovir DF was added to the subjects' methadone regimens.

MEASUREMENTS AND MAIN RESULTS

The pharmacokinetics of total, R-, and S-methadone were evaluated at baseline and after 2 weeks of daily tenofovir DF coadministration with a light meal. Steady-state tenofovir DF pharmacokinetics were evaluated at day 15. Bioequivalence testing was conducted of total, R-, and S-methadone area under the serum or plasma concentration-time curve during the 24-hour dosing interval at steady state (AUCss) and maximum concentration in serum or plasma (Cmax). Subjects were evaluated for changes in methadone pharmacodynamics by the Short Opiate Withdrawal Scale (SOWS) and pupillary diameter measurements at frequent intervals. Coadministration with tenofovir DF did not affect the pharmacokinetics of methadone. Geometric mean R-methadone systemic exposures, AUCss and Cmax, differed by 5% or less when methadone was dosed with tenofovir DF. Similar results were observed for S-methadone and for total methadone. Both AUCss and Cmax met the strict criteria for bioequivalence between the two study periods for total, R-, and S-methadone, indicating a lack of drug interaction when tenofovir DF was coadministered with methadone. No significant changes in SOWS scores or pupillary diameter measurements occurred, and no notable clinical adverse events were reported.

CONCLUSION

Tenofovir DF pharmacokinetics were comparable to previously reported values of tenofovir DF in HIV-infected patients. Coadministration of methadone with tenofovir DF did not alter the pharmacokinetics or pharmacodynamics of total, R-, or S-methadone. Tenofovir DF may be given as part of a once-daily antiretroviral regimen in patients receiving methadone maintenance therapy.

Pharmacokinetic and pharmacodynamic interactions between diltiazem and methylprednisolone in healthy volunteers

OBJECTIVES

The pharmacokinetics and pharmacodynamics after administration of methylprednisolone alone, diltiazem alone, and both drugs jointly were assessed in healthy volunteers.

METHODS

An unblinded, controlled, fixed-sequence, 2-period study was carried out in 5 healthy white men who received a single dose of intravenous methylprednisolone, 0.3 mg/kg, on day 2, followed by diltiazem alone, 180 mg, on days 5, 6, and 7, with joint dosing of both drugs on day 8. Methylprednisolone and diltiazem disposition was assessed from plasma concentrations. Pharmacodynamic factors were assessed by plasma cortisol and T-helper and T-suppressor lymphocytes by means of extended indirect response models.

RESULTS

The clearance of methylprednisolone was significantly reduced in the presence of diltiazem (25.2 L/h versus 16.8 L/h), resulting in a longer half-life (2.28 hours versus 3.12 hours) and increased area under the plasma concentration-time curve (AUC) (871 ng x h/mL versus 1299 ng x h/mL). The AUC of diltiazem was unchanged in the presence of methylprednisolone. No significant intrinsic pharmacodynamic differences were observed for methylprednisolone versus methylprednisolone-diltiazem. The 50% inhibitory concentration values were 0.446 ng/mL versus 0.780 ng/mL for cortisol, 9.20 ng/mL versus 10.7 ng/mL for T-helper cells, and 18.5 ng/mL versus 20.9 ng/mL for T-suppressor cells (P >.05). Greater net suppression, as indicated by the area between the effect curve and suppression ratios, was observed for the methylprednisolone-diltiazem combination versus methylprednisolone alone, which was attributed to reduced elimination of methylprednisolone.

CONCLUSIONS

Controlled-delivery diltiazem, 180 mg, significantly increased methylprednisolone AUC and half-life and reduced clearance, lending to greater systemic exposure to the steroid. However, significant differences between 50% inhibitory concentration values for methylprednisolone when given alone and for methylprednisolone in combination with diltiazem were not seen, which implies no change in cortisol or cell-trafficking sensitivity in the presence of diltiazem.

Pharmacokinetics and pharmacodynamics of aztreonam and tobramycin in hospitalized patients

BACKGROUND

Pharmacokinetic/pharmacodynamic (PK/PD) optimization of antibiotic therapy has been shown to improve outcomes in several antibiotic classes. Despite the frequent use of beta-lactams, clinical data in humans remain limited.

OBJECTIVE

This study evaluated the relationship between serum pharmacokinetics, pharmacodynamics, pathogen susceptibility, and clinical outcomes in patients receiving aztreonam or tobramycin monotherapy.

METHODS

The case-report forms of hospitalized patients who received either aztreonam or tobramycin for a bacterial infection in 3 clinical trials conducted between 1982 and 1984 were reviewed for the present study. A pathogen was identified for all included patients, and susceptibility testing was performed to determine the minimum inhibitory concentration (MIC) for each agent. Pharmacokinetic parameters for each antibiotic were determined using population modeling, and variables potentially related to outcomes were evaluated using tree-based modeling, logistic regression, and nonlinear regression methods.

RESULTS

Data from 91 patients were analyzed, 68 treated with aztreonam monotherapy and 23 treated with tobramycin monotherapy. Of the types of infections treated, 39 were intra-abdominal, 42 involved the lower respiratory tract, and 10 involved the skin and skin structures. The pharmacodynamic ratio of the 24-hour area under the curve (AUC24) to the MIC was associated with clinical outcome for both antibiotics: aztreonam and to-bramycin patients with ratios meeting or exceeding the respective 24-hour inverse serum inhibitory titer breakpoints of 184 and 110 were significantly more likely to achieve a successful outcome than were those with ratios not meeting these values (P < 0.01). The probabilities of clinical success in patients at or above and below the AUC24/MIC breakpoints were a respective 85% and 53% for aztreonam and 80% and 47% for tobramycin (both, P < 0.01). When all patients were considered, the likelihood of achieving cure was 5.1 times greater in patients exceeding the target ratios (P < 0.01).

CONCLUSION

PK/PD optimization of both aztreonam and tobramycin is associated with improved patient outcomes.