Population analysis of the pharmacokinetic variability of high-dose metoclopramide in cancer patients

Metoclopramide infusions are used to prevent nausea and vomiting in cancer patients during chemotherapy. 47 patients received metoclopramide during 109 chemotherapeutic treatments as a loading (dose range = 0.55 to 4.5 mg/kg over 15 minutes) and maintenance (dose range = 0.57 to 4.8 mg/kg over 8 hours) infusion. During and up to 24 hours after the end of the maintenance infusion between 4 and 10 blood samples were collected per treatment. Metoclopramide was analysed in plasma by liquid chromatography. Pharmacokinetic and demographic data of 83 treatments were analysed by the NONMEM program using a linear 2-compartment model. It was found that bodyweight and serum alkaline phosphatase activity explain some of the interindividual variability in clearance (CL). The typical pharmacokinetic parameters for an average individual (70kg, alkaline phosphatase = 100 IU/L) were: CL = 20 L/h; volume of distribution at steady state (Vdss) = 190L; terminal half-life = 8h. The interindividual variabilities in clearance, volume of central compartment and Vdss were 50%, 35% and 35%, respectively. The residual variability in plasma concentrations was estimated as 13%.

Changes in lignocaine disposition during long-term infusion in patients with acute ventricular arrhythmias

Lignocaine disposition was studied in 30 patients with acute ventricular arrhythmias. Serum concentrations of lignocaine, its metabolites Monoethylglycine xylidide (MEGX) and glycine xylidide (GX), and alpha 1-acid glycoprotein (AAG) were analyzed during and after a 48-h lignocaine infusion. AAG concentrations tended to rise in patients with acute myocardial infarction (AMI), leading to binding of the drug in plasma. Lignocaine clearance was estimated at various times during the infusion using a Bayesian parameter estimation program and was found to decline over the course of the infusion. There was a significant reduction in clearance based on estimates obtained at the end of the infusion compared with estimates obtained during the first 0-5 h. Clearance was reduced both in patients who had an AMI and those who did not. Multiple linear regression analysis of the clearance data revealed that these changes could be described by a linear function of time and AAG concentration. These findings suggest that other factors in addition to protein binding changes may influence lignocaine disposition during long-term infusion.

Who needs molar units for drugs?

The standardisation of units for drug concentration measurement in clinical medicine is an urgent necessity. The obvious choice is mass units based on the litre. A change to molar units for drug concentrations would make no sense unless drugs were also prescribed in moles, which would cause disruption and inconvenience. There would be considerable danger to patients and the change would be expensive. Most importantly, molar units for drugs will not benefit doctors or their patients. Mass units should be retained and proposals for the adoption of molar units should be abandoned.

The pharmacokinetics and pharmacodynamics of lignocaine and MEGX in healthy subjects

Lignocaine clearance declines during continuous intravenous infusion in man and in vitro studies suggest that this may partly be due to inhibition by MEGX, a metabolite of lignocaine. MEGX is pharmacologically active in animals, but this is not yet proven in man. This study examined the pharmacokinetics and pharmacodynamics of lignocaine and MEGX in eight healthy male volunteers given lignocaine HCl 120 mg, MEGX HCl 120 mg, lignocaine HCl 120 mg + MEGX HCl 120 mg, and placebo, administered according to a randomized double-blind protocol. One-, two-, or three-compartment models were fitted to drug and metabolite blood concentration-time profiles and clearance, volume (Vss), and half-life values were calculated and compared by paired t-test. Systolic time intervals and QT interval were recorded and compared by repeated measures ANOVA. When administered in combination with MEGX, lignocaine clearance was significantly reduced from 58 +/- 18 to 48 +/- 13 L hr-1 (p less than 0.02). The Vss was unchanged and there was a trend toward an increase in terminal half-life. Lignocaine, MEGX, and the combination significantly reduced QT interval up to 30 min after injection and this was maintained to 2 hr with the lignocaine and the combination. Transient side effects were experienced with all active treatments, but were most pronounced with the combination. Thus, lignocaine clearance was inhibited by MEGX, which was pharmacologically active in man.

A clinically significant interaction between ciprofloxacin and theophylline

We report a case of theophylline toxicity following the co-administration of ciprofloxacin. Total theophylline clearance fell from 2.3 l.h-1 to 0.8 l.h-1 when ciprofloxacin was added to the treatment regimen and returned to 2.1 l.h-1 after ciprofloxacin was discontinued.

Population pharmacokinetics of lisinopril in hypertensive patients

If optimal dosage administration schedules are to be defined, the factors that influence the disposition of a drug in the population of patients who are likely to receive it should first be determined. Traditionally, separate groups of patients, each with different underlying disease, are given the drug and then detailed studies are performed to assess which groups have significantly different pharmacokinetic parameters and therefore require an adjustment in dose. An alternative approach, utilizing large groups of patients studied less intensively, will be described here. The analysis used the results of two multicenter trials of lisinopril: one in elderly (greater than 65 years) hypertensive patients and one in a group of hypertensive patients with impaired renal function (creatinine clearance rate less than 60 ml/min). Both protocols allowed for a stepwise increase in the dose of lisinopril until optimum control of BP was achieved. Required dosages ranged from 2.5 to 40 mg/day. During the course of therapy, regular trough concentrations were analyzed to assess compliance and a steady-state profile was determined during one dosage interval. Only patients who appeared to be compliant on the basis of multiple trough concentrations were used in the analysis. Data from a total of 53 patients were used, of whom 35 took part in the elderly study, and 18 in the renal study. There were 25 men and 28 women in this combined study group with ages ranging from 21 to 85 years (mean 65 years) and weights from 51 to 115 kg (mean 72 kg).(ABSTRACT TRUNCATED AT 250 WORDS)

The prophylactic use of phenytoin during iopamidol contrast studies of the subarachnoid space

Contrast examinations of the subarachnoid space are associated with side effects including convulsions. Attention has been given to the prophylactic use of anticonvulsants. We describe a simple oral regimen using the established anticonvulsant phenytoin that can be administered to short-stay patients and that achieves effective serum and CSF concentrations. A preliminary account of this work was presented to the British Pharmacological Society in January 1984 in London.

Population pharmacokinetics. Theory and clinical application

Good therapeutic practice should always be based on an understanding of pharmacokinetic variability. This ensures that dosage adjustments can be made to accommodate differences in pharmacokinetics due to genetic, environmental, physiological or pathological factors. The identification of the circumstances in which these factors play a significant role depends on the conduct of pharmacokinetic studies throughout all stages of drug development. Advances in pharmacokinetic data analysis in the last 10 years have opened up a more comprehensive approach to this subject: early traditional small group studies may now be complemented by later population-based studies. This change in emphasis has been largely brought about by the development of appropriate computer software (NONMEM: Nonlinear Mixed Effects Model) and its successful application to the retrospective analysis of clinical data of a number of commonly used drugs, e.g. digoxin, phenytoin, gentamicin, procainamide, mexiletine and lignocaine (lidocaine). Success has been measured in terms of the provision of information which leads to increased efficiency in dosage adjustment, usually based on a subsequent Bayesian feedback procedure. The application of NONMEM to new drugs, however, raises a number of interesting questions, e.g. 'what experimental design strategies should be employed?' and 'can kinetic parameter distributions other than those which are unimodal and normal be identified?' An answer to the later question may be provided by an alternative non-parametric maximum likelihood (NPML) approach. Population kinetic studies generate a considerable amount of demographic and concentration-time data; the effort involved may be wasted unless sufficient attention is paid to the organisation and storage of such information. This is greatly facilitated by the creation of specially designed clinical pharmacokinetic data bases, conveniently stored on microcomputers. A move towards the adoption of population pharmacokinetics as a routine procedure during drug development should now be encouraged. A number of studies have shown that it is possible to organise existing, routine data in such a way that valuable information on pharmacokinetic variability can be obtained. It should be relatively easy to organise similar studies prospectively during drug development and, where appropriate, proceed to the establishment of control systems based on Bayesian feedback.

Quantitation of dose and concentration-effect relationships for fenclofenac in rheumatoid arthritis

Response to non-steroidal anti-inflammatory drugs (NSAIDs) is not usually assessed on the basis of concentration measurements: identification of a concentration-effect relationship has proved difficult to achieve. Dose and concentration-effect relationships of fenclofenac have been determined in a group of 18 patients with rheumatoid arthritis at three dose levels (600, 1200 and 1800 mg day-1). The study was double-blind and treatments were randomised according to a Latin square design. A multiple linear regression technique (GLIM) was used in the analysis. The best model to describe the change in effect in terms of dose and concentration incorporated an average slope and an individual subject intercept for each effect measurement. On average, an improvement in grip strength of 20 mm Hg could be obtained with an increase in fenclofenac (trough) concentration of 100 micrograms ml-1.

The pharmacokinetics of R- and S-tocainide in patients with acute ventricular arrhythmias

The pharmacokinetics of R(-) and S(+)- tocainide were studied in twelve patients requiring intravenous tocainide. In all patients, a progressive increase in the S(+):R(-) ratio was observed during the infusion. Mean +/- s.d. ratios increased from 1.03 +/- 0.05 at 2 min to 1.76 +/- 0.35 at 48.5 h. Data from eight patients were fitted to a two-compartment model and there was a significant difference (Wilcoxon matched-pairs test P less than 0.01) in the clearance estimates for the two enantiomers. The median values were: S(+)-tocainide = 6.25 l h-1 and R(-)-tocainide = 9.31 l h-1. There was no differences in V1 or Vss.

Evaluation of nonlinear regression with extended least squares: simulation study

A new approach to nonlinear least-squares regression analysis using extended least squares (ELS) was compared with three conventional methods: ordinary least squares (OLS); weighted least squares 1/C (WLS-1) and weighted least squares 1/C2 (WLS-2). With Monte Carlo simulation techniques, 3 X 200 data sets were constructed with constant proportional error (5, 10, and 15% error) and 3 X 200 with constant additive error (0.05, 0.10, and 0.15 g/mL) from an initial (perfect) data set based on known parameters. Two sampling strategies were employed: one with 17 time points and one with 10 time points. All data sets were fitted by each of the four methods, and parameter estimation bias was assessed by comparing the mean parameter estimate with the known value. The relative precision of each method was investigated by examination of the absolute deviations of each individual parameter estimate from the known value. ELS performed as well as the appropriate weighting scheme (WLS-2 for constant proportional error sets and OLS for constant additive error sets) and was superior with regard to both bias and precision to less appropriate methods.

The pharmacokinetics of high dose metoclopramide in patients with neoplastic disease

High dose metoclopramide infusions (10 mg/kg) were administered to nineteen patients with bronchial carcinoma who were receiving intravenous cyclophosphamide as single agent chemotherapy. Considerable interindividual variability in metoclopramide disposition was observed. Mean clearance was 0.33 +/- 0.13 (s.d.) l h-1 kg-1, mean volume of distribution at steady state was 3.8 +/- 1.2 (s.d.) l/kg and mean elimination half-life was 8.3 +/- 4.4 (s.d.) h. These results were significantly different from mean values previously reported for young healthy volunteers given conventional doses (0.70 l h-1 kg-1, 2.2 l/kg and 2.6 h respectively). Significant correlations were found between serum urea, serum creatinine and metoclopramide clearance. The metoclopramide regimens were well tolerated and, with the exception of two patients, were completely effective in the prevention of nausea and vomiting. To achieve and maintain target serum metoclopramide concentrations of 1 microgram/ml, we now administer a loading infusion of 3.61 mg/kg over 30 min followed by a maintenance infusion of 0.36 mg kg-1 h-1 for 10 h. Cyclophosphamide is normally administered concurrently with the second infusion. For patients with evidence of mild renal impairment, the maintenance infusion rate of metoclopramide hydrochloride should be adjusted according to the predicted individual clearance value; CL (l h-1 kg-1) = 0.57 - [0.036 X urea (mmol/l)].

Estimation of gentamicin clearance and volume of distribution in neonates and young children

Gentamicin therapy should be guided by serum level monitoring in all age groups, dosage adjustments depending on age related changes in pharmacokinetics. Population data analysed from two centres (43 infants from Glasgow and 100 infants and children from Manchester) by the computer program NONMEM showed that volume of distribution was related to body weight by a proportionality factor that decreased from the region of 0.41-0.46 l/kg in children less than 3 months to 0.25-0.32 l/kg in older children, a value which merges with that accepted for adults (0.25 l/kg). In both young and older children, clearance was also found to be dependent on body weight. Renal function (creatinine concentrations) provided no further explanatory power. When these results were used prospectively to forecast gentamicin concentrations with a Bayesian kinetic parameter estimation program, trough concentrations were more precisely predicted than peaks when a single concentration measurement was used. In clinical practice, however, two concentration measurements are usually routinely available and these should lead to greater precision of both peak and trough predictions. These results have been incorporated into a simple nomogram which can be used to determine a dose of gentamicin which will achieve target peak concentrations in infants, assuming that troughs should not exceed 2 micrograms/ml.

Clinical pharmacokinetics: a comprehensive system for therapeutic drug monitoring and prescribing

Clinical pharmacokinetics is an expanding scientific discipline which can make an impact on treatment in coronary care, intensive care, paediatrics, general medicine and surgery, and general practice. The aim of this study was to establish a rapid system of drug assay, to report the result, to assess the influence of pathological and clinical factors on the pharmacokinetics of certain drugs, and to use a computer to determine the optimum dosage of drugs. The clinical pharmacokinetics laboratory in Stobhill is available to all clinical departments and to general practitioners in the area. Digoxin, theophylline, and phenytoin have been assessed. Initial samples of these drugs showed that only about a third were in the therapeutic range; samples obtained after the issue of the laboratory report showed an improvement. The predictive performance of the computer program improved with feedback of one or two drug concentrations. Dosages of drugs chosen on an empirical basis may not lead to optimum treatment, and by testing samples early the dosage of the drug can be adjusted. It is hoped that the results achieved will encourage other clinical, pharmaceutical, and scientific colleagues to develop laboratories along similar lines.

Binding of [3H]mianserin to bovine serum albumin, human serum albumin and alpha 1-acid glycoprotein

Scatchard plots which were curvilinear with negative slopes were obtained when the binding of [3H]mianserin to bovine serum albumin (BSA), human serum albumin (HSA), defatted human serum albumin (D-HSA) and alpha 1-acid glycoprotein (alpha 1-AGP) was studied with equilibrium dialysis with constant protein concentrations and various ligand concentrations. Binding parameters were estimated graphically and with a non-linear least-squares computer program, assuming two classes of independent binding sites. alpha 1-AGP had the highest binding affinity (K) and binding capacity (nK). The binding parameters, n and K were not independent of protein concentration when the BSA concentration was varied. Linear atypical Scatchard plots with positive slopes were obtained when the protein concentration was varied for BSA, HSA and D-HSA, at a fixed ligand concentration.

Prediction of response to theophylline in chronic bronchitis

In chronic bronchitis, intersubject variability in both theophylline pharmacokinetics and pharmacodynamics must be taken into account if the drug is to be used to its best advantage. Both kinds of variability can be integrated into a model which relates the steady state concentration of theophylline to simultaneously measured ventilatory response (most conveniently, the FVC). In a group of 56 patients with chronic bronchitis, the mean +/- s.d. linear response to increasing steady state concentrations of theophylline was 0.04 +/- 0.012 1 microgram-1 ml, starting from a mean +/- s.d. pretreatment FVC of 1.58 +/- 0.791. Using these population parameter values, with or without a pretreatment FVC and/or one steady state concentration -FVC observation, it was possible to predict the degree of response which would be achieved by a smaller group of 20 similar patients. These estimates were obtained using a mathematical procedure based on Bayesian Probability Theory and Maximum Likelihood Estimation. Estimates of the overall response in individual patients allowed prediction of the response at any steady state concentration. These estimates were unbiased and accurate enough for clinical use when they were based on a pretreatment FVC and/or one paired steady state concentration -FVC observation.

Kinetic predictive techniques applied to lignocaine therapeutic drug monitoring

As lignocaine clearance is influenced by factors such as cardiac failure and liver impairment, clinical pharmacokinetic principles should be used to account for kinetic variability so that target concentrations are achieved consistently throughout the course of intravenous therapy. Two groups of patients with ischaemic heart disease, who received lignocaine, were studied: a control group with no feedback or intervention from therapeutic drug monitoring, and an intervention group in which strict guidelines for lignocaine administration were introduced. Lignocaine plasma concentrations were measured by EMIT (Syva), and rapid feedback of concentration data in the intervention group allowed adjustment of infusion rates using the Chiou equation. The mean concentration in the intervention group remained within the therapeutic range (2-5 micrograms/ml) at all times, whereas it exceeded 5 micrograms/ml after the first 7 h in the control group. The distribution of concentrations in the intervention group was always narrower than that in the control group. The study also included a comparison of the ability of the Chiou equation and a Bayesian optimisation procedure to estimate pharmacokinetic parameters and to forecast lignocaine concentrations over various periods of time. There was no significant difference between prediction errors determined by the two methods at various points throughout a 32-h period; both methods were associated with a negative prediction bias beyond the first 12 h of infusion. It is likely that this reflects assumptions made about lignocaine clearance and indicates the need for more sophisticated kinetic models.

Assessment of the interaction between mianserin and centrally-acting antihypertensive drugs

1 The interaction between mianserin and centrally-acting antihypertensive drugs was evaluated in normal volunteers and in patients with essential hypertension receiving either clonidine or methyldopa. 2 The administration of the first dose of 20 mg mianserin to the normal volunteers was associated with a significant sedative effect and transient postural hypotension. 3 In the normal volunteers, the blood pressure responses to a single oral dose of 300 micrograms clonidine were not modified by pretreatment with mianserin. The bradycardia associated with clonidine alone, however, was significantly attenuated. 4 In the patient study, no significant changes in blood pressure control were observed, either after the first dose of 30 mg mianserin or after one and two weeks' continued treatment with mianserin. 5 There is no evidence from these studies that the addition of mianserin therapy results in a clinically significant impairment of the antihypertensive effects of clonidine or methyldopa.

The pharmacokinetics of mianserin

1 The pharmacokinetics of mianserin hydrochloride have been determined in eight normal healthy volunteers, mean age 27, and 14 elderly patients, mean age 76. 2 Mianserin was administered to volunteers by intravenous infusion (0.011 mg/kg/min for 15 min) and, on another occasion, by mouth, in a single dose of 30 mg. Elderly patients received a single oral dose of 40-60 mg. 3 The terminal elimination half-life was significantly prolonged in the elderly. In young subjects it was 9.6 +/- 1.9 (s.d.) h. In the elderly it was 27 +/- 13.1 (s.d.) h. 4 Apparent oral clearance was significantly reduced in the elderly. In young subjects it was 87.1 +/- 32 (s.d.) h. In the elderly, it was 38.1 +/- 14.8 (s.d.) h. 5 These kinetic differences may have an important bearing on the sedative effects of mianserin.

The effect of glyceryl trinitrate ointment on heart rate--corrected systolic time intervals in angina pectoris

1 Individual systolic time intervals (STI) v heart rate regression equations were determined using small doses of intravenous atropine in seven patients with angina pectoris. 2 Regression lines of the form LVET = A + B.HR and QS2 = C + D.HR were calculated. There was no significant correlation between PEP and HR. The values of parameters A C and D were significantly different from a group of previously studied normal volunteers. 3 STI changes were recorded for 5 h after the application of GTN or placebo ointment. The active drug caused shortening of LVET and QS2 with the peak effect occurring between 105 and 180 min. After 180 min neither LVET nor QS2 were significantly shortened. There was no significant effect on PEP or heart rate.

OPT: a package of computer programs for parameter optimisation in clinical pharmacokinetics

1 OPT is a series of computer programs designed to assist dose optimisation for individual patients. It is based on Bayesian Statistical Theory and Maximum Likelihood Estimation. 2 OPT uses prior information on the distribution of population pharmacokinetic parameters and plasma drug concentration measurements to obtain the "most likely' set of parameters for the individual. 3 Complex dosage regimes and non-steady state conditions can be handled. 4 OPT is designed for use in a Clinical Pharmacokinetics Laboratory where informed interpretation of results is essential. 5 The drugs for which the system is currently available include theophylline, digoxin, lignocaine, disopyramide, gentamicin and phenytoin (steady state data only).

Comparison of a high pressure liquid chromatographic analysis and an enzyme immunoassay technique for quantitation of disopyramide in serum or plasma

High pressure liquid chromatographic (HPLC) and enzyme immunoassay (Emit) methods for measuring disopyramide concentrations in plasma and serum were compared. The precision of both methods was satisfactory, with all coefficients of variation in the range 1.3--6.5%. Quantitation was comparable, with a correlation coefficient of 0.991 (n = 96). There was no interference in either method from lignocaine, digoxin, propranolol, procainamide or N-monodealkylated disopyramide. HPLC was superior in terms of lower cost, the ability to quantitate metabolite concentrations, lack of interference by lipaemia or haemolysis and slightly better within-run precision. However, Emit was considered the method of choice for routine therapeutic drug monitoring because of its relative simplicity and speed of performance.

Parameter optimisation in clinical pharmacokinetics

This paper describes a package of computer programs designed to be used in Clinical Pharmacokinetics or Clinical Chemistry Laboratories to assist in the interpretation of plasma drug concentration measurements. A simple pharmacokinetic model is utilised, and values of the necessary parameters for the general population determined using standard nomograms. Parameter estimates for individual patients are obtained by a feedback process using Bayes' theorem and the principle of maximum likelihood. Thus optimal dosage regimes can be obtained for individual patients. The package can be used with a series of drugs.

Haemodynamic effects of BM 10.188, a new orally active inotropic agent, in healthy volunteers

1 BM 10.188 (Doxaminol, Boehringer Mannheim, GmbH) is a recently developed beta-sympathomimetic agent which has shown promising positive inotropic activity in experimental animal models. It is a dibenzoxepine derivative. 2 The effects of 20 mg oral BM 10.188 on systolic time intervals and standard echocardiographic parameters have been studied in six normal healthy male volunteers. 3 When assessed by repeated measured analysis of variance, QS2 and LVET shortened significantly (P less than 0.005) in the 8 h period following BM 10.188. Mean maximum shortening values were: QS2, 25.7 +/- 25.6 (s.d.) ms at 155 min and LVET, 10.7 +/- 9.5 (s.d.) ms at 155 min. There was a corresponding small but significant increase of 4.2 +/- 3.9 (s.d.) ml in stroke volume at 6 h (P less than 0.025). 4 These results indicate that in normal volunteers, BM 10.188 exhibits effects on noninvasive cardiological indices similar to those observed after cardiac glycosides.

Oral disopyramide dosage regimes in ischaemic heart disease

The serum concentration/time profiles resulting from two oral disopyramide dosage regimes were studied in ten patients with ischaemic heart disease. Conventional dosing on a 6 r 8 hourly basis consistently achieved disopyramide concentrations within the accepted therapeutic range of 2-7 micrograms/ml. In contrast, a twice daily regime was associated in some patients with trough levels below the minimum effective concentration. The mean elimination half-life was 5.8 h; this does not substantiate previous reports of significantly prolonged disopyramide half-lives in patients with ischaemic heart disease. Unless significant renal impairment or cardiac failure is present, or a sustained release preparation is used, the dosage interval for oral disopyramide should not exceed 8 h.

Disopyramide pharmacokinetics during recovery from myocardial infarction

1 Previous pharmacokinetics studies of disopyramide in patients with ischaemic heart disease include unexplained reports of poor bioavailability and extremely long elimination half-lives which undermine accepted dosage recommendations. 2 Disopyramide pharmacokinetics were investigated after intravenous and oral administration to nine such patients. 3 Mean elimination half-life (6.82 h) and bioavailability (79.8%) were consistent with findings from a previous study in young healthy volunteers. 4 Volume of distribution was reduced by 25%: the mean +/- s.d. value was 0.61 +/- 0.17 l/kg. Total body clearance was significantly reduced: the mean +/- s.d. value was 1.02 +/- 0.16 ml min-1 kg-1. 5 These figures indicate that, in this patient group, if renal function is not significantly impaired, a standard loading dose of 2 mg/kg should be followed by the appropriate maintenance dose administered three or four times daily.

The influence of theophylline on maximal response to salbutamol in severe chronic obstructive pulmonary disease

We have previously shown that inhaled salbutamol further increases the bronchodilator response after the maximum effect of theophylline has been obtained in patients with severe chronic bronchitis. We now report the results of adding maximally effective doses of theophylline to the maximum response obtainable from salbutamol in ten of these patients. We constructed dose response curves to ensure maximum possible effect from salbutamol. Response plateaus (in nine out of ten patients) were achieved with cumulative doses of between 200 micrograms and 3,000 micrograms salbutamol and there was a significant response (p less than 0.05) in every subject: the mean FVC response was 1.11 (ranging from 0.5 to 1.81) and the mean FEV1 response was 0.41 (ranging from 0.1 to 0.81). Theophylline, in their previously determined maximally effective doses, produced statistically significant (p less than 0.05) small further increases in both FVC (0.2 to 0.61) and FEV (0.1 to 0.61) in four patients only. The other six did not respond. In patients classified as chronic bronchitics there is clearly a wide variation in response to bronchodilators and a surprising degree of reversibility can be achieved. But because of this variation in response, conventional drug doses may be too small in some cases. Ideally, each bronchodilator should be prescribed after some form of individual dose response studies. Although this acute study shows little or no benefit in the height of the bronchodilator response the usefulness of this combination can only really be decided after similar studies including the duration of effect in long term administration.

Modelling theophylline response in individual patients with chronic bronchitis

1 In six patients with chronic bronchitis, serial changes in both ventilatory function and plasma theophylline concentrations were measured simultaneously for 8 h following 500 mg aminophylline intravenously. 2 Using empirical mathematical models which can integrate this data, parameters were estimated which can express response as a linear function of steady state plasma theophylline levels. 3 Taking Forced Vital Capacity (FVC) as the target response, the mean (+/- s.d.) increase in FVC was 0.06 +/- 0.02 l/micrograms ml-1, starting with pretreatment values in the range 1-21. 4 This analytical approach could be used to determine whether or not a patient with chronic bronchitis could obtain a satisfactory response to theophylline at plasma levels normally encountered in clinical practice.

Systolic time interval v heart rate regression equations using atropine: reproducibility studies

1. Systolic time intervals (STI) were recorded in six normal male subjects over a period of 3 weeks. On one day per week, each subject received incremental doses of atropine intravenously to increase heart rate, allowing the determination of individual STI v HR regression equations. On the other days STI were recorded with the subjects resting, in the supine position. 2. There were highly significant regression relationships between heart rate and both LVET and QS2, but not between heart rate and PEP. 3. The regression relationships showed little intra-subject variability, but a large degree of inter-subject variability: they proved adequate to correct the STI for the daily fluctuations in heart rate. 4. Administration of small doses of atropine intravenously provides a satisfactory and convenient method of deriving individual STI v HR regression equations which can be applied over a period of weeks.

The prediction of individual systolic time interval v heart rate regression equations

1. Twenty-eight sets of systolic time interval (STI) and heart rate (HR) data were available from studies in which small bolus doses of atropine had been given to alter heart rate. 2. Regression lines of the form LVET = A+B.HR and QS2 = C+D.HR were calculated. There was no significant relationship between PEP and HR. The values of the parameters A-D were normally distributed. 3. The maximum likelihood Estimation was used to obtain the most likely values of the parameters A-D for individual subjects. 4. The technique proved to be highly satisfactory and was subsequently validated with a further six sets of data.

The pharmacokinetics of meclofenamic acid in the horse

The pharmacokinetics of meclofenamic acid were studied in Thoroughbred horses and in ponies. After intravenous (i.v.) administration of either 2 mg/kg or 4 mg/kg sodium meclofenamate the elimination half-life was of the order of 0.9 h while the volume of distribution was found to be 0.128 litre/kg. Elimination was in accordance with a one-compartment model. Following oral administration of either meclofenamic acid (4 mg/kg) or sodium meclofenamate (4 mg/kg) a much longer terminal half-life than that calculated for Kel from i.v. data was found. This anomaly indicated that the 'flip-flop' phenomenon was present, i.e. Ka exceeded Kel. More rapid and higher peak levels occurred following sodium meclofenamate than meclofenamic acid, although total bioavailability was similar. Studies in ponies with meclofenamic acid showed a lower absorption than that found in Thoroughbreds. Overnight fasting before meclofenamic acid administration did not alter the rate or extent of absorption. Intramuscular administration of sodium meclofenamate resulted in low plasma concentrations and after 25 h only 46% of the drug had been absorbed.

The changing pattern of toxicity of digoxin

Toxicity to digoxin was monitored in 437 consecutive recipients in a comprehensive drug surveillance programme, Adverse reactions developed in 19.5% and, in contrast to previous reports, were generally of a relatively benign nature. There were no drug-related deaths, but patients with adverse reactions spent longer in hospital. Low body weight, impaired renal function, old age and concurrent use of diuretics individually did not increase the risk of toxicity. This was attributed to improvements in the prescribing of digoxin. There was a highly significant excess of gastrointestinal reactions in women, which tended to occur early in the course of therapy. This susceptibility is not widely recognized. The use of loading doses may have caused many early reactions and it is suggested that this practice cold be abandoned in all but the most urgent cases.

Effect of naproxen on glucose metabolism and tolbutamide kinetics and dynamics in maturity onset diabetics

1 The influence of the nonsteroidal anti-inflammatory drug naproxen on glucose metabolism and on tolbutamide pharmacokinetics and pharmacodynamics has been studied in ten maturity-onset diabetics. 2 Comparison of both plasma glucose decay curves and insulin responses during an intravenous glucose tolerance test before and after eight 12 hourly doses of naproxen revealed that naproxen had no significant influence on fasting glucose levels or on rates of glucose elimination. 3 When the subjects were given a combination of naproxen and tolbutamide for 3 days naproxen had no influence on tolbutamide absorption, protein binding, disposition or pharmacological effect. 4 Treatment with tolbutamide in maturity-onset diabetics need not be modified if concurrent administration of naproxen is contemplated.

Theophylline-salbutamol interaction: bronchodilator response to salbutamol at maximally effective plasma theophylline concentrations

1 The effect of inhaled salbutamol following a maximally effective dose of theophylline given by intravenous infusion was determined in 12 patients with chronic bronchitis. 2 An initial single intravenous dose study was performed to estimate each patient's theophylline kinetics and to identify those patients who would respond to theophylline. 3 Pulmonary function was assessed at hourly intervals during four to five incremental steady state theophylline infusions over the concentration range 5-25 mg/l. 4 Inhaled salbutamol (400 micrograms) was administered after the maximum effect from theophylline had been achieved or when theophylline concentrations reached 25 mg/l without maximum effect: pulmonary function was again assessed. 5 Ten patients achieved a further significant improvement in pulmonary function after salbutamol: in five, predicted values for FVC were exceeded. 6 Patients with chronic bronchitis may benefit from the combination of theophylline and salbutamol if steady state theophylline concentrations of 15-20 mg/l are achieved.

Comparative pharmacokinetics and pharmacodynamics of cardiac glycosides

1 The pharmacokinetics and pharmacodynamics of ouabain, digoxinn and beta-methyl digoxin (medigoxin) have been investigated in a crossover study in four normal healthy volunteers. 2 Pharmacokinetics were studied using [3H]-labelled glycosides and the shortening of the left ventricular ejection time (LVET) was used as a measure of the effect of the drugs. A graded exercise protocol was used to correct for the effects of heart rate on LVET. 3 In three of the four subjects, both digoxin and beta-methyl digoxin produced a shortening in the LVET, but no such change could be detected with ouabain in any of the four subjects. 4 There was a good linear correlation between the shortening of the LVET and the amounts of digoxin or beta-methyl digoxin present in the body tissues. 5 One subject who showed no drug-related LVET shortening had greatly enhanced clearances of all three drugs studied.

Dose dependent methotrexate elimination following bolus intravenous injection

The pharmacokinetics of methotrexate have been assessed at two dose levels in six patients receiving the drug for treatment of malignant disease. Each patient received bolus intravenous doses of 25 mg and 100 mg given at least one week apart, the order of administration being random. Blood and urine were collected until 48 h for methotrexate analysis by radioimmunoassay and data analysed by a model-independent pharmacokinetic approach. In each patient area under the methotrexate serum concentration-time curve (o to chi) increased out of proportion to the increase in methotrexate dose. This was reflected in a mean clearance value after the 100 mg dose of 31 +/- 16 (SD ml x min(-1) compared with a mean clearance of 62 +/- 19 ml x min(-1) following injection of 25 mg methotrexate. Renal clearance of methotrexate was markedly lower following the 100 mg dose (18 +/- 6 ml x min(-1) than after 25 mg (53 +/- 19 ml x min(-1). Saturation of the proximal tubular organic acid transport system is the likely cause of methotrexate's capacity limited elimination.

Modeling of drug response in individual subjects

Pharmacokinetic and drug response data from individual subjects are analyzed empirically by two different mathematical techniques. The drugs involved are the antiarrhythmic agent disopyramide, whose kinetics can be described by a two-compartment model, and two cardiac glycosides, digoxin and beta-methyl digoxin, for which three-compartment models are appropriate. The first analytical approach uses multiple linear regression to describe response in terms of the amount of drug in several kinetic compartments. The second approach describes response in terms of the drug concentration in an "effect" compartment. Both approaches describe the data equally well and require the same number of parameters for model specificaton.