Statistical considerations in pharmacokinetic study design

The pharmacokinetics of cefazolin in patients undergoing elective & semi-elective abdominal aortic aneurysm open repair surgery

Background

Surgical site infections are common, so effective antibiotic concentrations at the sites of infection are required. Surgery can lead to physiological changes influencing the pharmacokinetics of antibiotics. The aim of the study is to evaluate contemporary peri-operative prophylactic dosing of cefazolin by determining plasma and subcutaneous interstitial fluid concentrations in patients undergoing elective of semi-elective abdominal aortic aneurysm (AAA) open repair surgery.

Methods/Design

This is an observational pharmacokinetic study of patients undergoing AAA open repair surgery at the Royal Brisbane and Women's Hospital. All patients will be administered 2-g cefazolin by intravenous injection within 30-minutes of the procedure. Participants will have samples from blood and urine, collected at different intervals. Patients will also have a microdialysis catheter inserted into subcutaneous tissue to measure interstitial fluid penetration by cefazolin. Participants will be administered indocyanine green and sodium bromide as well as have cardiac output monitoring performed and tetrapolar bioimpedance to determine physiological changes occurring during surgery. Analysis of samples will be performed using validated liquid chromatography tandem mass-spectrometry. Pharmacokinetic analysis will be performed using non-linear mixed effects modeling to determine individual and population pharmacokinetic parameters and the effect of peri-operative physiological changes on cefazolin disposition.

Discussion

The study will describe cefazolin levels in plasma and the interstitial fluid of tissues during AAA open repair surgery. The effect of physiological changes to the patient mediated by surgery will also be determined. The results of this study will guide clinicians and pharmacists to effectively dose cefazolin in order to maximize the concentration of antibiotics in the tissues which are the most common site of surgical site infections.

Oral olanzapine disposition in adolescents with schizophrenia or bipolar I disorder: a population pharmacokinetic model

BACKGROUND

Olanzapine is an atypical antipsychotic approved for the treatment of adults and adolescents (aged 13 -17 years) with schizophrenia or bipolar I disorder (manic or mixed episodes).

OBJECTIVES

To characterize the pharmacokinetics of olanzapine in adolescents, to estimate the sources of variability, and to identify significant co-variates. In addition, olanzapine pharmacokinetic parameters in adolescents were compared with those in adults to guide appropriate dosing recommendations for adolescent patients.

METHODS

A population pharmacokinetic modeling study was performed. The majority of pharmacokinetic data for the model came from a multicenter, open-label study in which 4.5 weeks of oral olanzapine 2.5-20 mg once daily was administered to a total of 105 patients aged 13-17 years (41.1-148 kg) who had a diagnosis of schizophrenia or bipolar I disorder. Four blood samples at steady state were obtained from each patient. Olanzapine concentrations in plasma were determined using a validated high-performance liquid chromatography method with electrochemical detection. Similar data from 11 adolescents from three previous studies were also included. A pharmacokinetic model was developed and the potential effects of patient characteristics (sex, bodyweight, age, ethnic origin) were investigated using a nonlinear mixed effects modeling program. The distributions of pharmacokinetic parameters for olanzapine in adolescents were compared with those previously reported in adults (n = 912, diagnosis of schizophrenia, olanzapine 5-20 mg/day) using the Kolmogorov-Smirnov 2-sample test. A visual predictive check was performed using Monte Carlo simulations on an external validation dataset.

RESULTS

The pharmacokinetics of oral olanzapine in adolescent patients were described by a one-compartment pharmacokinetic model. The typical model estimates were 13.6 L/h (70 kg female patient) for oral clearance (CL/F) and 899 L for oral volume of distribution (V/F). Interpatient variability (40.5% for CL/F, 65.4% for V/F) and residual error (27%) were moderate. Bodyweight and sex had a significant influence on CL/F, which was lower in patients with lower weights and approximately 30% higher in males than females. Olanzapine exposure was typically 27% higher in adolescents versus adults. Approximately 77% of adolescents and adults had comparable CL/F values and 69% had comparable V/F values.

CONCLUSIONS

The pharmacokinetics of oral olanzapine in adolescent patients are similar to those in adults, and are linear in the dosage range of 2.5-20 mg/day. Given the small magnitude of co-variate effects and the interpatient variability, dose adjustments based on bodyweight or sex are not necessary in adolescents.

Optimising intraperitoneal gentamicin dosing in peritoneal dialysis patients with peritonitis (GIPD) study

Background

Antibiotics are preferentially delivered via the peritoneal route to treat peritonitis, a major complication of peritoneal dialysis (PD), so that maximal concentrations are delivered at the site of infection. However, drugs administered intraperitoneally can be absorbed into the systemic circulation. Drugs excreted by the kidneys accumulate in PD patients, increasing the risk of toxicity. The aim of this study is to examine a model of gentamicin pharmacokinetics and to develop an intraperitoneal drug dosing regime that maximises bacterial killing and minimises toxicity.

Methods/Design

This is an observational pharmacokinetic study of consecutive PD patients presenting to the Royal Brisbane and Women's Hospital with PD peritonitis and who meet the inclusion criteria. Participants will be allocated to either group 1, if anuric as defined by urine output less than 100 ml/day, or group 2: if non-anuric, as defined by urine output more than 100 ml/day. Recruitment will be limited to 15 participants in each group. Gentamicin dosing will be based on the present Royal Brisbane & Women's Hospital guidelines, which reflect the current International Society for Peritoneal Dialysis Peritonitis Treatment Recommendations. The primary endpoint is to describe the pharmacokinetics of gentamicin administered intraperitoneally in PD patients with peritonitis based on serial blood and dialysate drug levels.

Discussion

The study will develop improved dosing recommendations for intraperitoneally administered gentamicin in PD patients with peritonitis. This will guide clinicians and pharmacists in selecting the most appropriate dosing regime of intraperitoneal gentamicin to treat peritonitis.

Trial Registration

ACTRN12609000446268

Opsonized erythrocyte ghosts for liver-targeted delivery of antisense oligodeoxynucleotides

The use of antisense oligodeoxynucleotides (AS-ODNs) in therapeutic applications requires the development of appropriate analysis and delivery systems. Here, we report a quantitation method and a carrier-mediated AS-ODN delivery system. AS-ODN levels were quantitated using an enzyme-linked immunosorbent assay (ELISA) in which biotinylated AS-ODNs bound to streptavidin-coated plates were detected by binding of a complementary, dinitrophenol-labeled detector ODN. The ELISA-based assay could detect AS-ODNs at the femtomole level. AS-ODN delivery systems based on opsinized erythrocyte ghosts (EGs) were developed using various combinations of hypotonic solution and resealing buffer to optimize AS-ODN encapsulation efficiencies. AS-ODN and polyethyleneimine (PEI) complex formation did not affect encapsulation into EGs. The ELISA-based assay showed that the pharmacokinetics of AS-ODNs differed significantly among the various delivery methods. Opsonized EG-encapsulated AS-ODNs exhibited a mean residence time (MRT) significantly shorter than AS-ODN encapsulated in EGs. The biodistribution of EG-loaded AS-ODNs depended on opsonization, with opsonized EG carriers producing 4.5-fold higher levels of AS-ODN in the liver compared with unopsonized EGs. These results indicate that opsonized EGs can be used for liver-targeted delivery of AS-ODN and suggest that an ELISA-based method may be useful for studying the in vivo fate of AS-ODNs.

Neuroendocrine response to intravenous citalopram in healthy control subjects: pharmacokinetic influences

RATIONALE

The neuroendocrine response to intravenous citalopram may provide an acute, functional, in vivo measure of the neural serotonin (5-HT) system.

OBJECTIVE

To refine the quantification of acute neuroendocrine responses following intravenous citalopram in studies of 5-HT function.

METHODS

In 75 adult healthy subjects taking part in four similar protocols, we measured plasma prolactin and cortisol, as well as serial citalopram concentrations following intravenous citalopram (10 mg, 20 mg, 40 mg, 0.33 mg/kg) and placebo. The relationship between the AUC for intravenous citalopram during the first 150 min (AUC(150)) and the magnitude of the neuroendocrine response was determined. The role of pharmacokinetic (PK) parameters, as well as sensitivity to placebo injections, in influencing the neuroendocrine response to citalopram was then evaluated.

RESULTS

Citalopram produced a dose-dependent increase in cortisol and prolactin. The maximal increase from baseline correlated significantly but modestly with citalopram's AUC(150) (prolactin r(2)=0.23, P<0.0001; cortisol r(2)=0.3, P<0.0001). Additionally, citalopram's AUC(150) was affected by between-subject differences in both the peripheral and central volume of distribution. However, the neuroendocrine responses to citalopram did not correlate with the responses to placebo.

CONCLUSIONS

The parenteral citalopram challenge test is characterized by a modest concentration-response relationship, with concentration influenced by variable PK factors. Accounting for individual differences in drug distribution may improve the power of the citalopram challenge test, when used as an in vivo measure of central 5-HT function.

Montelukast dose selection in children ages 2 to 5 years: comparison of population pharmacokinetics between children and adults

Montelukast, a leukotriene receptor antagonist, has demonstrated efficacy and tolerability in the treatment of asthma in patients age 6 years and older. The purpose of this open, one-period, multicenter population pharmacokinetic study was to identify a chewable tablet (CT) dose of montelukast for administration to children ages 2 to 5 years with asthma, yielding a single-dose pharmacokinetic profile (area under the plasma concentration-time curve [AUC]) comparable to that of the 10 mg film-coated tablet (FCT) dose in adults. Because patient numbers were small and the volume of blood that could be collected from individual 2- to 5-year-old patients was limited, a population pharmacokinetic approach was used to estimate population AUC (AUCpop). The 4 mg CT dose of montelukast was well tolerated and yielded an AUCpop (2721 ng.h/mL) similar to that of the adult AUCpop (2595 ng.h/mL) observed after a 10 mg FCT dose. These results support the selection of a 4 mg once-daily CT dose of montelukast for future efficacy and safety studies in children ages 2 to 5 years with asthma.

Toxicokinetics of ethylene glycol during fomepizole therapy: implications for management. For the Methylpyrazole for Toxic Alcohols Study Group

STUDY OBJECTIVE

The elimination kinetics of ethylene glycol (EG) in human subjects treated with fomepizole (4-methylpyrazole) were analyzed to establish the efficacy of alcohol dehydrogenase (ADH) inhibition and to characterize elimination pathways.

METHODS

Drug concentration data from patients enrolled in the EG arm of the Methylpyrazole for Toxic Alcohols trial, a prospective, multicenter, open-label trial of fomepizole, were analyzed and compared with published estimates.

RESULTS

In 19 patients analyzed (EG concentrations of 3.5 to 211 mg/dL), elimination was first order during fomepizole monotherapy (half-life of 19.7+/-1.3 hours) and was not affected by the presence of ethanol. The elimination rate was significantly faster (half-life of <8.6+/-1.1 hours, P <.001) in the absence of fomepizole and ethanol. EG elimination by the kidneys was directly proportional to remaining renal function as estimated by creatinine clearance, with a fractional excretion of 25.5%+/-9.4%. Renal elimination and hemodialysis were the only significant routes of EG elimination as long as fomepizole concentrations were maintained well above 10 micromol/L (EG/fomepizole molar ratio, <100:1). All patients with normal serum creatinine concentrations at the initiation of fomepizole treatment had rapid rates of renal elimination (half-life of 16.8+/-0.8 hours).

CONCLUSION

At doses used, fomepizole effectively inhibits ADH-mediated metabolism of EG. Serum creatinine concentration at presentation and creatinine clearance can be used to predict EG elimination during fomepizole therapy and can help determine which patients will require hemodialysis to expedite EG elimination. An absolute EG concentration above 50 mg/dL should no longer be used as an independent criterion for hemodialysis in patients treated with fomepizole.

Optimal statistical design for toxicokinetic studies

Optimal statistical design strategies are applied to toxicokinetic experiments, for determining proper allocations of subjects and/or spacings of sampling times under a variety of nonlinear concentration-time models. The strategies include: (i) optimal allocations of subjects assuming the placement of time points is fixed, (ii) optimal spacing of design time points while assuming an equal allocation of subjects per time points and (iii) allocations/time-point spacings optimized jointly. Emphasis is placed on the first case, where a variance-minimization method is illustrated for optimizing the allocations when estimating specific toxicokinetic parameters. Appeals to forms of D-optimality are also considered, for cases when no specific toxicokinetic parameter is of specialized interest.

Some approaches to the pharmacology of multisubstrate enzyme systems

Analytical and exploratory in vitro, in situ and in vivo, physio-pharmacotoxicology, from enzymology to population epidemiology, now embraces those approaches that correlate complex dynamic multisubstrate kinetics through conventional and more recent non-invasive quantitative methodologies. Basically, substrates may be classed as pertaining to fundamental energy turnovers (first-order cellular metabolic pathways or networks) and to iso- vs allosteric modulator systems (second-order metabolic control network). Pairs of substrates and cofactors set-up the third-order multienzyme-receptor patterns, which in intact, native in vivo structures establish and maintain the compartmentalized, dynamically superimposed overall coordination of local redox and phosphate potentials. Perturbations of the various levels of the metabolic hierarchy induced by drugs, as well their relaxations, can be readily submitted to non-invasive kinetic analysis. Both indirect and direct titrations of substrate levels, their modelling and statistical ad hoc evaluations of their interrelations can lead to the identification of the multiple sites involved in drug effects as structured at the different orders/levels of concomitant functional variations. Fractal geometries contribute towards defining the space- and time-related events.