A standard approach to compiling clinical pharmacokinetic data

Pharmacokinetics of Rosuvastatin: A Systematic Review of Randomised Controlled Trials in Healthy Adults

BACKGROUND

Rosuvastatin is a lipid-lowering drug that works by inhibiting 3-hydroxy-3-methylglutaryl coenzyme A reductase, the rate-limiting enzyme responsible for producing cholesterol in humans. The pharmacokinetic data of rosuvastatin are considerably variable across studies.

OBJECTIVE

To review the pharmacokinetics of rosuvastatin from randomised controlled trials (RCTs) in healthy adults.

METHODS

A review of the pharmacokinetics of rosuvastatin was performed using systematic search strategies. The Sheiner method was used to summarise the pharmacokinetics of the drug.

RESULTS

Randomised controlled studies (n = 70) involving healthy subjects (n = 2355) that examined the pharmacokinetics of rosuvastatin following single and multiple doses were included in the review. Rosuvastatin is given once daily in the dose range of 5-80 mg, with 40 mg being the maximum approved daily dose. Rosuvastatin achieves maximum plasma concentration at a median of 5 h (range: 0.5-6 h) under fasting conditions following single and multiple doses. Following single doses, rosuvastatin has a mean absolute oral availability of 20%, an overall mean total clearance of 28.3 L/h and an average terminal elimination half-life of approximately 20 h. The overall mean total clearance of the drug in Caucasian subjects was 1.7-fold higher than that in healthy Chinese subjects. The systemic exposure of rosuvastatin is characterised by a large coefficient of variation (48%.) There is a small accumulation with repeated dosing. The interaction of rosuvastatin with darunavir/ritonavir was considered statistically and clinically relevant. Interactions of rosuvastatin single doses with erythromycin, fluconazole, itraconazole and antacid were statistically significant.

DISCUSSION AND CONCLUSIONS

There is considerable variation in the pharmacokinetics of rosuvastatin between races. The clinical relevance of the statistically significant drug interactions is yet to be investigated following repeated co-administration for at least 15 days, consistent with a half-life of low-density lipoprotein of 3 days.

Clinical Pharmacokinetics and Pharmacodynamics of Febuxostat

Febuxostat is a xanthine oxidoreductase inhibitor that has been developed to treat chronic gout. In healthy subjects, the pharmacokinetic parameters of febuxostat after multiple oral dose administration include an oral availability of about 85 %, an apparent oral clearance (CL/F) of 10.5 ± 3.4 L/h and an apparent volume of distribution at steady state (V _ss/F) of 48 ± 23 L. The time course of plasma concentrations follows a two-compartment model. The initial half-life (t _½) is approximately 2 h and the terminal t _½ determined at daily doses of 40 mg or more is 9.4 ± 4.9 h. Febuxostat is administered once daily. The maximum (peak) plasma concentrations are approximately 100-fold greater than the trough concentrations. Consequently, there is no significant accumulation of the drug during multiple dose administration. There are few data on the pharmacokinetics of febuxostat in patients with gout. While the pharmacokinetic parameters are not affected by mild to moderate hepatic impairment, there is no consensus on whether renal impairment has any effect on the pharmacokinetics of febuxostat. Febuxostat is extensively metabolised by oxidation (approximately 35 %) and acyl glucuronidation (up to 40 %); febuxostat acyl glucuronides are cleared by the kidney. In healthy subjects treated with multiple doses of febuxostat 10-240 mg, the concentrations of serum urate are reduced by a maximum of about 80 %. The percentage reduction in the concentrations of serum urate is slightly less in gouty patients than in healthy subjects.

Clinical pharmacokinetics of metformin

Metformin is widely used for the treatment of type 2 diabetes mellitus. It is a biguanide developed from galegine, a guanidine derivative found in Galega officinalis (French lilac). Chemically, it is a hydrophilic base which exists at physiological pH as the cationic species (>99.9%). Consequently, its passive diffusion through cell membranes should be very limited. The mean ± SD fractional oral bioavailability (F) of metformin is 55 ± 16%. It is absorbed predominately from the small intestine. Metformin is excreted unchanged in urine. The elimination half-life (t(½)) of metformin during multiple dosages in patients with good renal function is approximately 5 hours. From published data on the pharmacokinetics of metformin, the population mean of its clearances were calculated. The population mean renal clearance (CL(R)) and apparent total clearance after oral administration (CL/F) of metformin were estimated to be 510 ± 130 mL/min and 1140 ± 330 mL/min, respectively, in healthy subjects and diabetic patients with good renal function. Over a range of renal function, the population mean values of CL(R) and CL/F of metformin are 4.3 ± 1.5 and 10.7 ± 3.5 times as great, respectively, as the clearance of creatinine (CL(CR)). As the CL(R) and CL/F decrease approximately in proportion to CL(CR), the dosage of metformin should be reduced in patients with renal impairment in proportion to the reduced CL(CR). The oral absorption, hepatic uptake and renal excretion of metformin are mediated very largely by organic cation transporters (OCTs). An intron variant of OCT1 (single nucleotide polymorphism [SNP] rs622342) has been associated with a decreased effect on blood glucose in heterozygotes and a lack of effect of metformin on plasma glucose in homozygotes. An intron variant of multidrug and toxin extrusion transporter [MATE1] (G>A, SNP rs2289669) has also been associated with a small increase in antihyperglycaemic effect of metformin. Overall, the effect of structural variants of OCTs and other cation transporters on the pharmacokinetics of metformin appears small and the subsequent effects on clinical response are also limited. However, intersubject differences in the levels of expression of OCT1 and OCT3 in the liver are very large and may contribute more to the variations in the hepatic uptake and clinical effect of metformin. Lactic acidosis is the feared adverse effect of the biguanide drugs but its incidence is very low in patients treated with metformin. We suggest that the mean plasma concentrations of metformin over a dosage interval be maintained below 2.5 mg/L in order to minimize the development of this adverse effect.

Pediatric clinical pharmacology studies in Chagas disease: focus on Argentina

Chagas disease is a neglected parasitic disease endemic in the Americas. It mainly affects impoverished populations and the acute phase of the infection mostly affects children. Many cases have also been detected in nonendemic countries as a result of recent migratory trends. The chronic phase is relatively asymptomatic, but 30% of patients with chronic infection eventually develop cardiac and digestive complications that commonly lead to death or disability. Only two drugs are available for the treatment of Chagas disease, benznidazole and nifurtimox. These drugs have been shown to be effective in the treatment of both acute and early chronic phases in children, but the pharmacokinetics of these drugs have never been studied in this population. We have set out to conduct a pharmacokinetics study of benznidazole in a pediatric population with Chagas disease. The results of this study are expected to allow better estimation of the optimal doses and schedule of pharmacotherapy for Chagas disease in children.

Clinical pharmacokinetics and pharmacodynamics of allopurinol and oxypurinol

Allopurinol is the drug most widely used to lower the blood concentrations of urate and, therefore, to decrease the number of repeated attacks of gout. Allopurinol is rapidly and extensively metabolised to oxypurinol (oxipurinol), and the hypouricaemic efficacy of allopurinol is due very largely to this metabolite. The pharmacokinetic parameters of allopurinol after oral dosage include oral bioavailability of 79 +/- 20% (mean +/- SD), an elimination half-life (t((1/2))) of 1.2 +/- 0.3 hours, apparent oral clearance (CL/F) of 15.8 +/- 5.2 mL/min/kg and an apparent volume of distribution after oral administration (V(d)/F) of 1.31 +/- 0.41 L/kg. Assuming that 90 mg of oxypurinol is formed from every 100mg of allopurinol, the pharmacokinetic parameters of oxypurinol in subjects with normal renal function are a t((1/2)) of 23.3 +/- 6.0 hours, CL/F of 0.31 +/- 0.07 mL/min/kg, V(d)/F of 0.59 +/- 0.16 L/kg, and renal clearance (CL(R)) relative to creatinine clearance of 0.19 +/- 0.06. Oxypurinol is cleared almost entirely by urinary excretion and, for many years, it has been recommended that the dosage of allopurinol should be reduced in renal impairment. A reduced initial target dosage in renal impairment is still reasonable, but recent data on the toxicity of allopurinol indicate that the dosage may be increased above the present guidelines if the reduction in plasma urate concentrations is inadequate. Measurement of plasma concentrations of oxypurinol in selected patients, particularly those with renal impairment, may help to decrease the risk of toxicity and improve the hypouricaemic response. Monitoring of plasma concentrations of oxypurinol should also help to identify patients with poor adherence. Uricosuric drugs, such as probenecid, have potentially opposing effects on the hypouricaemic efficacy of allopurinol. Their uricosuric effect lowers the plasma concentrations of urate; however, they increase the CL(R) of oxypurinol, thus potentially decreasing the influence of allopurinol. The net effect is an increased degree of hypouricaemia, but the interaction is probably limited to patients with normal renal function or only moderate impairment.

Population pharmacokinetics of digoxin in Japanese patients: a 2-compartment pharmacokinetic model

OBJECTIVE

To clarify the observed variability of digoxin disposition by performing a population pharmacokinetic analysis in a Japanese population.

DESIGN

Retrospective analysis of clinical pharmacokinetic data.

PATIENTS AND PARTICIPANTS

Data were obtained from 106 patients with heart failure and atrial fibrillation (43 males and 63 females).

METHODS

Digoxin concentrations in serum were measured by fluorescence polarisation immunoassay. Population pharmacokinetic analysis was performed using a 2-compartment open pharmacokinetic model with the computer program NONMEM.

RESULTS

246 serum concentrations were obtained. Final pharmacokinetic parameters were: CL (L/h) = (0.036 x TBW + 0.112 x CL(CR)) x 0.77SPI x 0.784CCB, V1 = 1.83 L/kg, V2 = 22.6 L/kg and Q = 0.629 L/h/kg, where CL is total body clearance, V1 and V2 are the apparent volumes of distribution in the central and peripheral compartments, Q is intercompartmental clearance, TBW is total bodyweight (in kg), CL(CR) is creatinine clearance (in ml/min), SPI = 1 for concomitant administration of spironolactone (and zero otherwise) and CCB = 1 for concomitant administration of calcium antagonists (and zero otherwise). Concomitant administration of digoxin and spironolactone resulted in a 23% decrease in digoxin clearance. Concomitant administration of digoxin and calcium antagonists (diltiazem, nicardipine, nifedipine or verapamil) resulted in a 21.6% decrease in digoxin clearance.

CONCLUSIONS

The estimated population parameter values may assist clinicians in the individualisation of digoxin dosage regimens.

Effect of itraconazole on the pharmacokinetics of digoxin in guinea pigs

The effect of itraconazole (ITZ) on the pharmacokinetics of digoxin (DGX) was investigated in guinea pigs. The plasma concentrations of DGX in guinea pigs following treatment with ITZ (20 mg/kg intraperitoneally (i.p.)) after intravenous (i.v.) administration of DGX (0.125 mg/kg) were significantly higher than in the controls. The percentage cumulative excretion (6.9%) of DGX in bile of the ITZ-treated group up to 6 h after administration of DGX was significantly reduced (p < 0.05) compared with 10.5% in the control. The percentage cumulative excretion (3.1%) of DGX in urine of the ITZ-treated group up to 6 h after administration was also one third that in the controls. The total, biliary, renal and metabolic clearances for DGX in the ITZ-treated group were significantly reduced, while the V(dss) was unaffected. The plasma unbound fraction of DGX (ranged 47-58%) in the ITZ-treated group was generally similar to that in the controls (50-57%). The blood-to-plasma concentration ratio of DGX (range 1.28-1.42) in the absence of ITZ did not change in the presence of ITZ. Based on these results, the pharmacokinetic interaction between DGX and ITZ may be due not only to a reduction in the renal clearance but also to a reduction in the metabolic clearance of DGX by ITZ.

Mean time parameters in pharmacokinetics. Definition, computation and clinical implications (Part II)

Part I of this article, which appeared in the previous issue of the Journal, covered the following topics: fundamental definitions, general mean time parameter relationships and mean time parameters of classical compartmental systems. It also offered a number of examples to clarify and illustrate the various concepts.

Quantification of dideoxycytidine in human plasma by gas chromatography/mass spectrometry

A gas chromatographic/mass spectrometric procedure has been developed for the quantification in plasma of dideoxycytidine (DDC), a candidate anti-AIDS drug. The assay uses an extraction with ethyl acetate containing 10% methanol followed by three derivatization steps: (i) reaction with t-butyl dimethyl chlorosilane to silylate the 5'-hydroxyl group; (ii) pentafluorobenzoylation of the amino group with pentafluorobenzoyl chloride; (iii) methylation of the ring nitrogen adjacent to the amino group with diazomethane. The resulting derivative is quantified using stable isotope dilution, selective ion monitoring and methane negative chemical ionization mass spectrometry. Plasma concentrations of DDC were measured over a range of 2-200 ng ml-1 using 1 ml plasma for extraction.

Dynamical dosage regimen calculations in linear pharmacokinetics

The objective of this analysis of a linear compartment system is to compute drug input functions that are optimal in producing nontoxic pharmacological responses of maximal therapeutic efficacy. Pharmacokinetics should underlie the rational use of drugs and when a therapeutic range is known, the achievement of safe and effective target concentrations may be assured by a dosage regimen computed for a given administration schedule. The method developed herein is based on linearity and superimposition principles applicable to the class of systems considered. This method requires estimated values of model individual parameters and computes optimum dosage regimens in an iterative scheme, corresponding to a real time dynamical context. An interactive computer program has been developed to perform dosage regimen calculations.

Nonparametric maximum likelihood estimation for population pharmacokinetics, with application to cyclosporine

A new method, nonparametric maximum likelihood (NPML), for statistical analysis of population kinetic data is proposed. NPML provides a discrete estimate of the whole probability density function of the pharmacokinetic parameters. This permits a straightforward derivation of usual population characteristics. To illustrate the application of the NPML method, a population analysis of cyclosporine RIA measured plasma levels in 188 bone marrow transplant patients after intravenous infusion, is presented. The capability of NPML to extract population information from sparse individual data is also outlined.

Determination of 6-(2'-chlorophenyl)-4-hydroxy-4H-imidazo[1,5-a] [1,4]benzodiazepine-3-carboxamide, a mixed agonist-antagonist anxiolytic agent, in human plasma and urine by gas chromatography-negative-ion chemical-ionization mass spectrometry

A simple, sensitive and specific assay was developed for the determination in plasma and urine of 6-(2'-chlorophenyl)-4-hydroxy-4H-imidazo[1,5-alpha] [1,4]benzodiazepine- 3-carboxamide, compound I, a mixed agonist-antagonist anxiolytic agent. A hexadeuterated analogue of the compound was added to plasma or urine as the reference standard. The titled compound was extracted with benzene at pH 11. Following evaporation of the solvent, the residue was reacted with pentafluoropropionic anhydride in the presence of triethylamine. The derivatizing reagents were evaporated, and the carbonitrile derivative of the analyte was extracted into ethyl acetate at pH 11. The residue remaining after removal of the ethyl acetate was silylated with bis(trimethylsilyl)trifluoroacetamide, and a portion of this solution was analyzed by gas chromatography-negative-ion chemical-ionization mass spectrometry. The mass spectrometer was set to monitor, in the gas chromatographic effluent, the M-. ion of the titled compound and its hexadeuterated reference standard. The ratio of these two ions was calculated and converted to a concentration of analyte using a calibration curve that was generated from the analyses of control plasma fortified with various amounts of analyte and a fixed amount of the hexadeuterated reference standard. The limit of quantitation of the assay was 1 ng/ml for plasma and urine.

Intramuscular tobramycin pharmacokinetics in geriatric patients

Twelve elderly institutionalized patients (mean age of 81 years) received intramuscular injections of tobramycin for infections at regular dosage intervals. The subjects had stable renal function but had limited mobility. Periodic blood specimens were collected for eight hours and analyzed for tobramycin by an immunofluorescence technique. The procedure was repeated in each patient one day later to detect day-to-day variability in the absorption kinetics. The absorption of tobramycin occurred primarily within two hours, although peak concentrations in some patients were indicative of a sustained plateau and exhibited intrapatient variability from one dose to the next. The time to peak and slowly declining concentrations suggest that the absorption process is extended in this population as compared to younger adults. Our data suggest that peak tobramycin concentrations following intramuscular injections to geriatric patients may be slightly delayed, though the extent of absorption appears to be consistent.

Importance of analytical methods in pharmacokinetic and drug metabolism studies

Pharmacokinetic and drug metabolism studies first request that good analytical data are available. The various methods that permit unchanged drugs and their metabolites to be separated, identified and quantitatively assayed are briefly reviewed. The present importance of gas chromatography/mass spectrometry is emphasized, as well as the limits of immunological assays. The sensitivity of the analytical assay has a direct impact on the validity of the pharmacokinetic model which is built up from plasma concentration data. The precision and accuracy of the assay is also critical, and it is not always easily estimated. A new significant parameter is the speed of analysis, and the resulting massive production of analytical data. New drugs coming from biotechnology, and new dosage forms, like targeted drugs, will create new analytical problems in the future. They will probably call for the development of new biological or pharmacological assay procedures, in addition to the physicochemical means of analysis.

Steady-state dosage regimen calculations in linear pharmacokinetics

This paper describes a general approach to compute steady-state dosage regimens. The method deals with individual linear compartmental mammillary pharmacokinetic models and requires estimated values of model parameters. It is based on the linearity principle and computes dosage regimens to achieve and maintain specified steady-state plasma concentrations, e.g. mean value or minimum effective and maximum safe levels. An interactive computer program performing the calculations has been developed.

Mean time and first-pass metabolism

The theoretical principles are outlined for estimating the fraction of a drug undergoing first-pass metabolism using only the plasma levels found after a single oral dose. Data for 3 drugs are used to illustrate the method. It involves analysis of the parent drug and the metabolite formed during the first passage through the gut wall and liver and evaluation of their total mean times. The mean time characteristics of molsidomine, nortriptyline and propranolol are considered and they confirm the theoretically deduced dependency of the mean time of the parent drug and the metabolite. Whether the results are more precise than those obtained from comparison of areas after oral and intravenous administration is discussed. From the data presented it is clear that the mean time method depends on the scatter inherent in the data. In order to estimate the true first-pass effect, greater scatter requires an increased number of data pairs, i.e. subjects. If intravenous data are not available, however, the method described provides a rough but worthwhile estimate of the first pass effect.

A pocket calculator program for pharmacokinetic dosing of drugs exhibiting single compartment first-order elimination and zero-order or first-order absorption

A pocket computer program has been designed to assist clinicians in the appropriate administration of most drugs exhibiting single compartment first-order elimination and either zero-order (intravenous infusions) or first-order (intramuscular and oral) absorption. The program utilizes well-established pharmacokinetic parameters to calculate an optimal drug dose and dosing interval based upon a patient's demographic characteristics and the drug half-life, volume of distribution, and absorption rate constant or infusion time. It also allows the user to estimate peak and trough drug serum concentrations. When measured serum concentrations are available during steady state, it is possible to determine individual patient drug half-life and volume of distribution for more accurate adjustments in dose and/or dosing interval. Usage of this program should enable clinicians to better select effective but safe drug dosing regimens based on individual patient needs and characteristics.

Model-independent assessment of accumulation kinetics based on moments of drug disposition curves

The bounds of the accumulation profile can be predicted on the basis of the mean disposition residence time (MDRT) of a drug. The time to reach 90% of the plateau level (t0.9) is less than 3.7 MDRT. This prediction can be improved if, in addition, the variance of disposition residence time (VDRT, CV2D = VDRT/MDRT2), or the terminal exponential coefficient (lambda), is known. For CV2D----1 or lambda MDRT----1, the time to reach steady state (t0.9) approaches 2.3 MDRT (limiting case of monoexponential drug disposition curve). Conditions are stated under which lambda can be regarded as the principal determinant of the accumulation rate.

Rapid achievement of a serum concentration plateau of digoxin through controlled infusion

Using a volume-controlled infusion pump, a mean serum plateau level of digoxin of 4-5 ng/ml was rapidly achieved and maintained in 6 healthy volunteers. The infusion scheme was calculated on the basis of data published on the pharmacokinetics and pharmacodynamics of digoxin following bolus intravenous injection. The magnitude of the response (change in electromechanical systole) at the end of the plateau phase was comparable to that observed with the concentration in the therapeutic range at steady state.