Cytoprotective agents and ulcer relapse

This study establishes that real differences between relapse rates exist, and confirmed previous observations by Pounder et al. that relapse rates influence clinical outcomes in ulcer populations.8 The results make it clear that the way in which a drug influences relapse is an important determinant of therapeutic efficiency, additional to the requirement for healing efficiency. Cytoprotective agents as a group appear to be associated with lower relapse rates than cimetidine. It si not known whether the differences are peculiar to cimetidine, specific to the blockade of H2-histamine receptors, or whether they relate to the process of inhibition of secretory processes generally, further comparisons are needed; however, conventional, clinical trials are not designed to provide the necessary information. For the healing of peptic ulcers, lower relapse rates appear to provide therapeutic advantage to members of the cytoprotective group when compared with agents acting via anti-secretory mechanisms, in addition to that associated with the local (non-systemic) mode of action where applicable.

Variation in hepatic extraction ratio with unbound drug fraction: discrimination between models of hepatic drug elimination

Systematic examination of model-dependent predictions of changes in the hepatic extraction ratio (E), following alteration in the unbound fraction of drug in plasma (fub), should allow sensitive discrimination between the venous equilibrium model (model I) and the sinusoidal perfusion model (model II) of hepatic sinusoidal function if drugs which show high clearance of free drug are used. Analysis of experimental data from the literature confirmed the utility of this approach. Specifically, data related to diazepam (E = 0.95 at fub = 1) clearly conformed to the predictions of the sinusoidal perfusion model and differed markedly from those of the venous equilibrium model. Conversely, data for phenytoin (E = 0.69 at fub = 1) failed to discriminate between models, as predicted by our analysis. We identify a sensitive, convenient method for discrimination between current models of hepatic sinusoidal function and establish for the first time that a drug substrate (diazepam) conforms closely to the predictions of the sinusoidal perfusion model.

Current clinical applications and dose regimens of metronidazole and related nitroimidazoles

Metronidazole, in particular, and the other nitroimidazoles (tinidazole, ornidazole) available in Australia are well established drugs for the treatment of protozoal (trichomonal or amoebic) infections; recent data testify to their efficacy in the prevention and therapy of anaerobic infections. Administration by oral and rectal routes is indicated rather than by the intravenous route on the basis of efficacy, safety and cost; this recommendation is applicable to both loading and maintenance dosing. Intravenous administration should be restricted to emergency preoperative loading (single 500-mg dose); to patients with proven anaerobic infections; patients with serious sepsis associated with an unidentified organism; patients who are unable to take medication by mouth and those without a functional rectum or with diarrhoea; and patients with leukaemia who are vomiting. These drugs are remarkably safe under conditions of acute use if the intravenous route is avoided. However, extreme caution in their long-term use and use in obstetric and paediatric patients should be exercised until toxicological issues are resolved. It is concluded that the nitroimidazoles are effective, cheap and safe drugs for the short-term treatment of protozoal and bacterial (anaerobic) infections.

Influence of chronic dosing on theophylline clearance

Theophylline clearance was studied in normal healthy volunteers before and after chronic oral dosing. Oral theophylline clearance showed a significant decrease (P less than 0.001) from 59.3 +/- 6.48 ml/min (mean +/- s.e. mean, n = 10) to 48.0 +/- 6.4 ml/min. Steady-state intravenous clearance decreased by 26 +/- 2% from 55.1 +/- 7.7 ml/min to 41.0 +/- 5.96 ml/min (mean +/- s.e. mean, n = 6; P less than 0.001) at constant steady-state intravenous test concentrations (mean 15.0 +/- 1.6 micrograms/ml after chronic oral dosing), excluding significant concentration dependence. Extrapolation of acute clearance findings to maintenance requirement may not be valid even in the same individual, with theophylline therapy requiring on-going review beyond acute stabilisation.

Interaction between oral hydralazine and propranolol. I. Changes in absorption, presystemic clearance and splanchnic blood flow

A study was undertaken of propranolol pharmacokinetics in dogs before and after oral coadministration of hydralazine to determine whether interactions described in humans could be reproduced in an animal model. Additionally, physiological parameters considered to be relevant to the pharmacokinetic handling (absorption rate and splanchnic hemodynamics) were studied. Coadministration of oral hydralazine and propranolol in conscious dogs caused an increase in peak plasma concentration ( Cpmax ) and area under the oral plasma concentration-time curve (AUC) of propranolol ( Cpmax = 19.2 +/- 5.8 ng/ml, control; Cpmax = 91.5 +/- 12.8 ng/ml, posthydralazine : AUC = 65.7 +/- 14.6 ng X hr/ml, control; AUC = 152.4 +/- 23.9 ng X hr/ml, posthydralazine : mean +/- S.E.M., n = 5; P less than .01 and P less than .01), without change either in the peak plasma level, time to peak or plasma AUC of [14C] propranolol and metabolites (P greater than .70, P greater than .90 and P greater than .60, respectively) or in urinary recovery (urinary recovery = 39.7 +/- 4.3% dose, control; urinary recovery = 41.8 +/- 6.2% dose, posthydralazine ). When propranolol was administered i.v., hydralazine caused a small (42.3 +/- 18%), but significant (P less than .025), increase in systemic clearance. Oral bioavailability increased from 7.3 +/- 2.1 to 23.6 +/- 5.1% (mean +/- S.E.M., n = 5, P less than .025), hepatic extraction showed correspondingly inverse changes and estimated hepatic blood flow increased from 34.9 +/- 3.8 to 63.3 +/- 10.8 ml/min/kg (P less than .025).(ABSTRACT TRUNCATED AT 250 WORDS)

Metronidazole pharmacokinetics in patients with acute renal failure

The pharmacokinetics and metabolism of intravenous metronidazole were studied in six patients with acute renal failure. In two of the patients a single dose (500 mg) of metronidazole was administered, whereas in four patients the steady-state pharmacokinetics were studied after four days therapy of 500 mg twice daily. Plasma concentrations of metronidazole and its hydroxy and acetic acid metabolites were measured by a specific and sensitive HPLC method. The volume of distribution was 0.65 +/- 0.13 l/kg (mean +/- S.D.), elimination half-life was 9.9 +/- 2.5 h and total plasma clearance was 55.5 +/- 17.7 ml/min. Renal clearance was almost non-existent (1.4 +/- 1.4 ml/min), whereas non-renal clearance was 54.0 +/- 18.2 ml/min. Steady-state plasma concentrations of metronidazole were 15.3 +/- 3.8 mg/l, the hydroxy metabolite were 17.4 +/- 2.0 mg/l and the acetic acid metabolite were 1.2 +/- 0.8 mg/l. In the patients studied, a dosing regimen of 500 mg twice daily resulted in therapeutically adequate blood levels of metronidazole.

Stable oral availability of sustained release propranolol when co-administered with hydralazine or food: evidence implicating substrate delivery rate as a determinant of presystemic drug interactions

A study was made of the influence of hydralazine on the oral availability of a sustained release formulation of propranolol (Inderal LA). Sustained release propranolol 160 mg was given orally either alone or in combination with oral hydralazine 25 mg on separate occasions to six healthy volunteers. Blood and urine samples were collected post-dosing over 34 h. Peak concentrations of propranolol, time to peak and area under the plasma concentration-time curve (AUC) were not altered by co-administration of hydralazine with sustained release propranolol. Similarly, there was no change in recovery of 11C-labelled propranolol and metabolites in those individuals to whom tracer label was given. These results contrast with previous reports of marked interaction between the conventional formulation of propranolol and hydralazine or food. Interactions were confirmed between hydralazine and conventional propranolol in three subjects who had been studied previously with sustained release propranolol. Analysis of metabolite profiles in one of these subjects established that the major metabolites do change under hydralazine stimulus. These results indicate that substrate delivery rates may determine presystemic drug interactions, suggesting capacity limitations of hydroxylation processes or short-term flow redistribution following hydralazine, resulting in functional shunting past the hydroxylation enzymes. These results exclude global or lasting enzyme inhibition by hydralazine or simple flow-sensitivity of presystemic clearance.

Computer-based prescription audit as a research, educational and management tool

An existing dedicated pharmacy computer system (MIDAS system, Alfred Hospital and Health Computing Services, Victoria) used primarily for pharmacy labelling and dispensing, has been rewritten for an in-house minicomputer, with enhancements that allow it to be used in budgetary predictions, drug use monitoring, therapeutic audit and as a research tool. An automatically updated hard copy record of individual patient prescription profiles is generated and represents a valuable addition to the patient medical record. Allied to automatic "sorting" and retrieval techniques, all patients on nominated drugs can be identified and their records examined for particular combination use. Current application to assessment of nifedipine- beta blocker use will be presented, as will an application to cimetidine audit. Prescriptions can be scanned and segregated in terms of numbers of prescriptions in addition to segregation in terms of a particular therapeutic agent. Initial application to audit has detected an unacceptable level of polypharmacy (more than 26% of all prescriptions involve greater than or equal to 5 items and more than 3% involved greater than or equal to 10 items). Statistical information on the volume and unit cost of outpatient prescribing allows a precise estimate of ongoing budgetary commitments, while trends in discharge medication profiles allow a basis for prediction of budgetary trends, particularly with respect to the use of recent additions to the formulary. The system allows for expansion at minimal cost with potentially revolutionary applications, for example, interactive prescriber education if and when "on-line" capabilities are developed. Cross-linking with relevant data-bases will allow a major expansion of applied research into therapeutic drug use and its consequences.

Interaction of alpha-methyldopa with autonomic reflex tachycardia in man

Effects of alpha-methyldopa on cardiovascular reflexes were studied in normal healthy volunteers, and comparisons made with clonidine. Graded bicycle ergometer exercise and cumulative nitroprusside infusions were used as stimuli to autonomic reflexes. alpha-Methyldopa produced reduction in resting heart rate and a parallel shift in the stimulus-heart rate response relationship to nitroprusside and exercise 5 h after bolus intravenous dosing with 1,500 mg, but no effects at 0.5, 1.75, and 3 h during cumulative intravenous dosing with 1,875 mg. Clonidine produced a similar pattern of change in nitroprusside responses as early as 0.5 h after an intravenous dose of 300 micrograms. After atropine, alpha-methyldopa still produced a parallel shift in the nitroprusside stimulus-heart rate response relationship; however, clonidine reduced resting heart rate and abolished all increments in heart rate to nitroprusside infusion. Quantitative and qualitative differences in the effects of alpha-methyldopa and clonidine on autonomic reflexes in humans indicate differences in sites or mechanisms of action within the autonomic nervous system.

Disposition and removal of metronidazole in patients undergoing haemodialysis

The pharmacokinetics and haemodialysis clearance of metronidazole were investigated in four renal failure patients after a single 500 mg intravenous dose and in two renal failure patients on continuous treatment with metronidazole. During dialysis, the volume of distribution of metronidazole was 0.60 +/- 0.04 l/kg, total clearance was 196.0 +/- 60.6 ml/min and the elimination half-life had an harmonic mean of 2.14 h. Extraction across the dialyser was 51.5 +/- 7.8% and was limited to the distribution of drug in plasma water. Dialysis clearance was 125.0 +/- 32.7 ml/min, which represented 62 +/- 6% of total clearance and was 1.75 times the sum of the other clearance mechanisms. The hydroxy metabolite was extracted and cleared by the dialyser to the same degree as metronidazole itself. During the 4 h-dialysis 44.9 +/- 2.6% of the dose was removed by the dialyser in the four patients administered a single dose. Metronidazole is efficiently cleared and extensively removed by dialysis, and therefore dosage adjustments and alterations in the timing of dosage administration are essential in patients undergoing haemodialysis.

No stereoselective first-pass hepatic extraction of propranolol

The plasma level: time profile for l-propranolol and total propranolol concentrations were examined in normotensive subjects after intravenous and oral dl-propranolol. l-Propranolol concentrations in plasma accounted for about 60% of total propranolol. This was attributed to lower volume of distribution for the isomer. Mean plasma clearance of 1-propranolol was similarly affected while apparent plasma half-life for the l-isomer and total propranolol were of the same order. Oral bioavailability of 1- and total propranolol averaged 40.7 +/- 8.5% and 42.4 +/- 12.9%. Food and hydralazine increased oral bioavailability of total and l-propranolol by similar magnitudes. We conclude that difference in the kinetics of l- and total propranolol concentrations in plasma are small and probably of no clinical significance. Presystemic clearance of propranolol in man does not appear to be stereospecific.

Tocainide kinetics and metabolism: effects of phenobarbital and substrates of glucuronyl transferase

Tocainide, a lidocaine congener with low hepatic clearance, is eliminated predominantly by formation of a novel glucuronide conjugate. This suggested the possibility of metabolic interactions with enzyme inducers or competitive substrates for glucuronyl transferase. The time course of tocainide blood concentration as well as the urinary excretion-time profiles of drug and principal metabolite (a glucuronide of tocainide carbaminic acid, TOCG) were examined in six subjects before and after 15 days on phenobarbital (100 mg/day). In another study, the effect of salicylamide and clofibrate on the time courses of tocainide and TOCG urinary excretion were examined in four of the same six subjects. After 600 mg tocainide HCl by mouth, the area under the tocainide blood concentration-time curve was 48.2 +/- 11.9 hr micrograms/ml for the control dose and 49.6 +/- 4.2 hr micrograms/ml (mean = SD) after phenobarbital. Percent of dose excreted unchanged in urine (46.0 +/- 4.9 and 43.4 +/- 5.6) and percent of dose excreted as TOCG (30.6 +/0 3.3 and 27.7 +/- 7.2) were not affected by phenobarbital (data presented as control and after phenobarbital). Because salicylamide has been reported to be a potent inhibitor of the glucuronidation of some drugs and because clofibrate yields metabolites that may be competitive inhibitors of tocainide conjugation, the two were given together with tocainide. Average percent of dose recovered in urine as unchanged tocainide in 24 hr was 26.8%, 28.3%, and 29.7% in the control, salicylamide, and clofibrate studies. The urinary excretion of TOCG was also not affected. It is concluded that under the conditions of our investigation, the principal urinary metabolite of tocainide, a glucuronide of tocainide carbaminic acid, is formed by a mechanism not subject to induction by phenobarbital or competitive inhibition by salicylamide or clofibrate.

Nonlinear pharmacokinetics of indocyanine green in the rabbit and rat

The pharmacokinetic behavior of indocyanine green (ICG) in the rabbit can be described by a two-compartment open model, allowing for saturable transport of drug to the peripheral compartment and for its first-order elimination from the peripheral compartment. Use of this model led to the prediction of the accumulation of ICG in the plasma of a rabbit following the administration of repeated i.v. injections. Furthermore, studies conducted in the rat were also consistent with this model. One characteristic of the model is that above certain dose levels, the accumulation of ICG in the liver (i.e., the peripheral compartment) should reach a maximum independent of dose during certain time periods. This prediction was confirmed in a series of studies in the rat. The findings presented in this report provide evidence that a single model may be capable of explaining the variety of pharmacokinetic characteristics which have been reported for ICG, at least in the dose range studied.

Interaction between oral propranolol and hydralazine

Seven healthy subjects were given oral propranolol (1 mg/kg) alone or in combination with hydralazine 25, 50, or 100 mg on separate occasions. Hydralazine induced variable increases in the peak concentrations (p less than 0.05) and in the area under the propranolol concentration: time curves (p less than 0.02) without change in the recovery of 14 C-propranolol/metabolites in urine or in the systemic clearance of propranolol; i.e., oral hydralazine enhanced the systemic availability of propranolol by alteration of "first-pass" (hepatic) clearance. The results indicate the possibility of change in presystemic clearance without reciprocal change in systemic clearance and also suggest that propranolol or any other high-clearance drug should be administered in fixed relationship to hydralazine and other drugs capable of altering "first-pass" hepatic extraction (either due to alteration in splanchnic blood flow or competitive inhibition of metabolism) if large variations in plasma concentrations are to be avoided.

Dose-dependence of the apparent half-life of phenytoin in the rat

The effect of dose on phenytoin pharmacokinetics was examined in the rat. In a randomized study on nine animals (not crossed over) it was observed that the apparent half-life ((t 1/2) of this drug increased four-fold as dose was increased from 10 to 50 mg/kg (6 1/2 - 64 +/- 42 min vs. 267 +/- 118 min, respectively; mean +/- S.D.). In a crossover study on three animals, an even larger dose-dependent increase in t 1/2 was observed (37.8 vs. 271 min). Finally, six animals received the 10 and 50 mg/kg doses in a randomized crossover fashion and urine was collected for 72 hours. A dose-dependent decrease in the fraction of the dose excreted as conjugates of 5-(p-hydroxyphenyl)-5-phenylhydantoin (59 +/- 20 vs. 39 +/- 12%) was found. It is concluded that phenytoin pharmacokinetics in the rat are non-linear but that a simple one compartment Michaelis-Menten model cannot account for the observed behavior.