Quantitative analysis of the disopyramide concentration-effect relationship

1. A combined pharmacokinetic-pharmacodynamic model has been used to analyse the relationship between QT prolongation and changes in plasma concentration which occurred after disopyramide was given intravenously and orally to eight healthy subjects. 2. The pharmacokinetic models appropriate to intravenous and oral disopyramide have been extended by an 'effect compartment' which has no influence on the predetermined mass of drug in the body. 3. The model incorporates an adjustment for lag of effect behind any rapid changes in plasma concentration such as occur in the early distributive phase following intravenous administration. This permits calculation of the proportionality constant relating plasma concentration to effect. 4. Irrespective of the route of administration the mean (+/- s.d.) prolongation of the QT interval was 14.5 +/- 6.5 ms/micrograms ml-1. 5. There was no evidence that metabolite produced during first pass after oral administration made any significant contribution to effect. 6. This modelling technique should be applicable to the study of the concentration-effect relationship of a number of other drugs, both in health and in disease.

Bioavailability of methotrexate: implications for clinical use

The absorption of oral methotrexate in syrup form has been compared in six patients with that of an identical IV dose (50 mg/m2). There was variable absorption amongst the group with respect to maximum levels achieved and the time taken to reach those levels. The area under the time-concentration curve was always smaller when the drug was given orally than after IV administration. A total of 33 patients receiving methotrexate for a variety of tumour types were followed for response to treatment and toxicity. A significantly longer methotrexate half-life (t1/2) was found in nine partial responders (9.2 +/- 1.6 h) than in the nonresponders (3.8 +/- 0.7 h). Severe methotrexate toxicity was not seen though occasional mucositis, conjunctivitis, and diarrhoea occurred in seven patients. The side effects could not be predicted from the dose, the bioavailability data, or the serum creatinine. Measurements of serum and urine methotrexate levels are useful in the assessment of absorption and bioavailability of the drug the prediction of tumour response.

Enhancement of methotrexate absorption by subdivision of dose

A comparison was made in fasting patients between a single 100 mg oral dose of methotrexate formulated as its sodium salt in a palatable syrup and the same total quantity of drug administered in four divided doses of 25 mg taken at 2-h intervals. Allocation to the order of these treatment schedules was on a random basis. The area under the serum methotrexate concentration-time curve until 50 h was found to be considerably greater after the divided dose regimen, the mean ratio AUC 25 mg x 4/AUC 100 mg being 1.86 (+/- 0.90). There was no significant difference in peak serum methotrexate concentrations or methotrexate half-life estimates between the two regimens, however. The results of this study are consistent with saturation of an intestinal transport process when methotrexate is administered orally in a single large dose.

The renal clearance of disopyramide after bolus intravenous injection

Following bolus intravenous injection of disopyramide in eight normal volunteers the renal clearance of the drug appeared to fall with time. In the first two hours after injection renal clearance had a mean value of 89.0 ml min-1 and fell to 29.4 ml min-1 between 48 and 72 h. In a separate study disopyramide was given by continuous intravenous (i.v.) infusion for 8 h following a loading dose of the drug. Renal clearance of disopyramide was thus estimated hourly over three narrow serum concentration ranges in a single volunteer. The estimate of renal clearance of the drug over the first hour following the start of these infusions was considerably in excess of values obtained later in the experiments. The change in disopyramide renal clearance following bolus injection is partially time-dependent. There are, however, fallacies in calculating short-term clearance values after bolus drug injection from the venous concentration-time curve and these may partially explain the change in renal clearance of disopyramide with time.

Correlation of ethanol concentrations in blood and saliva

1. The concentrations of ethanol in blood, mixed saliva obtained before and after rinsing and drying the mouth and parotid saliva have been monitored in 12 healthy subjects after the ingestion of alcohol. 2. A highly significant linear correlation was found between blood and the three types of saliva examined from 20 min after completion of drinking. 3. Blood and mixed saliva samples were obtained from 20 patients attending the Casualty Department with evidence of ethanol intoxication. A similar correlation was obtained. 4. These results show that salivary ethanol may be used as an index of blood ethanol concentrations, provided that the salivary sample is not obtained within 20 min of the ingestion of alcohol.

Alterations of phenytoin protein binding with in vivo haemodialysis in dialysis encephalopathy

Protein binding of phenytoin was assesed in one patient with dialysis encephalopathy before and after haemodialysis. Phenytoin concentrations were measured by radioimmunoassay and continuous ultrafiltration was used to assess phenytoin binding. At a serum concentration of 60 micromol.1-1 the percentage of phenytoin bound to serum albumin was considerably lower in the patient serum (79.95% predialysis; 92.09% postdialysis) than that in three normal sera (97.90 +/- 0.17%). Analysis of Scatchard plots indicated two classes of binding sites. In class I both the affinity and capacity for binding phenytoin were reduced in the pre and post-dialysis serum, whereas in class II the capacity of the uraemic serum was increased although the intrinsic association constant was greatly reduced. It was concluded that in vivo haemodialysis is associated with large fluctuations in the protein binding of phenytoin, in which the concentration of endogenous dialysible metabolites are strongly implicated.

Serum, tear and salivary concentrations of methotrexate in man

1. Methotrexate handling has been studied in four patients with psoriasis and eleven patients with neoplastic disease. 2. Methotrexate levels in serum, tears and saliva were measured by radioimmunoassay while protein binding studies were performed by continuous ultrafiltration. 3. There was a close correlation between methotrexate concentrations in tears and serum (P less than 0.001, r = 0.714). Parotid salivary and serum methotrexate levels were similarly correlated (P less than 0.001, r = 0.557) but not mixed salivary and serum levels (P greater than 0.1, r = 0.232). 4. The mean protein binding was 95.11% +/- 2.26 (s.d.) while the ratio of methotrexate levels in tears to the free serum methotrexate level was 1:1.04. The corresponding parotid salivary level: free serum level ratio was 1:18.11. 5. No relationship could be determined between the methotrexate levels in tears and conjunctivitis observed in some of the patients under study.

Effect of acute alcohol intoxication on the metabolism and plasma kinetics of chlordiazepoxide

1. The metabolism and plasma kinetics of chlordiazepoxide have been determined in a group of volunteers and in a group of patients with acute alcohol intoxication. 2. Using the SAAM 26 non-linear least squares fitting programme, all chlordiazepoxide plasma concentration v time data following oral administration could be analysed in terms of a one-compartment open model with metabolic conversion of chlordiazepoxide to desmethylchlordiazepoxide. 3. Acutely intoxicated patients showed a prolonged elimination of chlordiazepoxide and a reduced clearance when compared with alcohol-free volunteers. The elimination of desmethylchlordiazepoxide, on the other hand, appeared to be faster in the alcoholics. 4. Alcohol exerts significant effects on the metabolism of chlordiazepoxide in acutely intoxicated patients.

Disopyramide serum and pharmacologic effect kinetics applied to the assessment of bioavailability

1 Serum, urine and pharmacologic effect (prolongation of the QT interval) kinetics of the antiarrhythmic disopyramide have been investigated in eight volunteers after intravenous administration (2 mg/kg) and oral administration (300 mg) of the two commercially available preparations, Rythmodan (Roussel Laboratories) and Norpace (Searle Laboratories). 2 An open one compartment body model adequately described the kinetics of disopyramide in serum and urine. 3 After intravenous administration, the following average pharmacokinetic parameters were found: biological half-life, 7.8 h; total clearance, 95 ml/min; renal clearance, 54 ml/min; apparent volume of distribution, 60 litres. 4 After oral Rythmodan and Norpace, serum concentration profiles and urinary excretion data revealed significant differences in rates of absorption, times required to achieve peak serum concentrations and biological half-lives. These differences were largely due to the relatively slow absorption characteristics of Norpace. 5 The absence of hysteresis in plots of QT prolongation against disopyramide serum concentration after oral administration indicated that serum and pharmacologic effect kinetics were indistinguishable within a kinetically equivalent compartment. 6 Analysis of both serum and urine data showed that while Norpace had a significantly higher degree of bioavailability (P less than 0.005), the 5--15% difference between the two formulations should not normally be of any clinical significance.

The application of HPLC to the determination of some 1,4 benzodiazepines and their metabolites in plasma

1 A high pressure liquid chromatographic (HPLC) assay has been developed for the measurement of chlordiazepoxide and its metabolites, N-desmethylchlordiazepoxide and demoxepam in microgram concentrations in human plasma. 2 Another metabolite was detected in appreciable amounts in the plasma from patients receiving high doses of chlordiazepoxide after the first 3 days of treatment. 3 Preliminary studies would indicate that this metabolite is N-desmethyldiazepam. 4 A modified HPLC system, using a microparticulate column was developed to separate N-desmethyldiazepam from chlordiazepoxide and its other metabolites in plasma.

Digoxin therapy: a clinical pharmacokinetic approach

Within certain limitations, digoxin can be prescribed for individual patients on a sound scientific basis. The use of pharmacokinetic principles and digoxin radioimmunoassay can improve the overall quality of treatment with this drug and reduce the incidence of adverse reactions. Variations in bioavailability and intestinal absorption are important factors in the determination of dosage and should be reduced to a minimum by improved pharmaceutical formulations.

Computer-assisted review of digoxin therapy in the elderly

Current practice with digoxin was assessed in a group of 42 elderly patients by comparing plasma digoxin concentrations attained on previously established maintenance doses with those generated by a computer programme designed to calculate dosage schedules to suit individual patients. Discrepancies between measured and computed plasma levels and between established and computed doses dictated withdrawal of the drug or revision of dosage in 26 patients (62%), with obvious clinical benefit. An important determinant of dosage was renal function; reduction in creatinine clearance provided good evidence for the loss of ability of the elderly kidney to eliminate digoxin. Simple bedside methods are available which permit a reliable estimate of creatinine clearance without a 24-hour urine collection, provoding a rational basis for the choice of digoxin dosage in the elderly.

Phenytoin concentrations in mixed, parotid and submandibular saliva and serum measured by radioimmunoassay

1 Concentrations of phenytoin in mixed, parotid and submandibular saliva and serum were determined in normal subjects after an oral dose, using a specific double antibody radioimmunoassay which requires only 20 micronl fluid. 2 Semi-log concentration-time plots of phenytoin concentration in mixed saliva and serum gave good parallelism after the initial 14 h post-administration period. 3 The mean ratio of the mixed saliva: serum phenytoin concentration was 10.3% +/- 1.5 (s.d.) in seven normal subjects. 4 Phenytoin concentrations found in separate parotid and submandibular salivary fractions did not differ but were significantly greater (P less than 0.001) than those found in mixed saliva. 5 Phenytoin concentrations in all salivary fractions were independent of the volume of fluid produced and the degree of stimulation. 6 The rate of phenytoin secretion in the parotid and submandibular fluid was proportional to the salivary flow rate. 7 These data suggest that mixed saliva may be a suitable medium for the monitoring of phenytoin concentrations and may provide a non-invasive alternative to the direct determination of phenytoin in serum.

Factors affecting drug binding in plasma of elderly patients

The binding of salicylate, sulphadiazine and phenylbutazone to plasma proteins has been studied in young and elderly subjects. Elderly patients had significantly reduced concentrations of plasma albumin, compared with subjects under 40 years of age. Sifnificant increases in free levels of all three drugs were found in elderly patients receiving multiple drug therapy, and a correlation obtained with the number of drugs being taken. It is suggested that because of their low albumin levels, elderly patients may be more susceptible to the effects of multiple drug therapy on drug binding. The clinical implications of these observations are discussed.

An assessment of digoxin radio-immunoassay

Using a commercially available digoxin radio-immunoassay kit, an accuracy of the order of ±10 per cent has been achieved. Precision could be improved by increasing the ratio (volume of plasma/volume of radioactive digoxin). The assay permitted separation of clinically toxic and non-toxic patients with plasma digoxin values of 3.5±0.6 (S.D.) ng. per ml. and 1.4±0.6 ng. per ml. respectively (p <0.001). A correlation between blood urea and plasma digoxin concentration was demonstrated (r=0.57, p <0.001).

The assay was particularly useful in the confirmation and follow-up of suspected digoxin poisoning in attempted suicide, and in renal failure to arrive at a modified digoxin dosage schedule.

Treatment of hypertension with clonidine

Clonidine, in a daily dosage of 0.15-4.8 mg., effectively lowered systolic and diastolic pressures in 26 out of 28 impatients with moderate to severe hypertension, including five with primary renal disease. The action of the drug did not depend on posture and was not associated with reduction in renal function. Side-effects were not severe, but mental changes occurred in four patients.Clonidine is a useful alternative to currently available antihypertensive drugs, but further evaluation of its longterm efficacy is required.