Critical compilation of terminal half-lives, percent excreted unchanged, and changes of half-life in renal and hepatic dysfunction for studies in humans with references

Pharmacokinetic considerations in the eradication of Helicobacter pylori

As Helicobacter pylori plays an important role in the aetiopathogenesis of peptic ulcer, therapeutic strategies aimed at maintaining long term remission have shifted from the control of intragastric pH to targeting H. pylori. According to recent international guidelines the clinical goals--rapid ulcer healing and prevention of relapse--can be best accomplished by combination therapy consisting of an antisecretory drug (proton pump inhibitor or ranitidine) and 2 antimicrobial agents (preferable amoxicillin, clarithromycin or metronidazole). When applying such multidrug regimens, possible synergy between the agents suggests that pharmacokinetic considerations might help to improve H. pylori eradication rates, which should be above 85 to 90% on an intention-to-treat basis. The present review summarises the pharmacokinetic properties and interaction potential of all drugs presently used in the various H. pylori eradication regimens, with emphasis on particular patient populations such as the elderly and those with renal impairment. The drugs considered are omeprazole, lansoprazole, pantoprazole, rabeprazole, ranitidine and ranitidine bismutrex, bismuth salts, amoxicillin, clarithromycin, azithromycin, roxithromycin, metronidazole, tinidazole and tetracycline. When addressing the clinically important questions of the efficacy, safety and costs of the recommended regimens, the impact of drug disposition on H. pylori eradication should not be neglected.

Pharmacokinetics of terfenadine in healthy elderly subjects

The pharmacokinetics of the terfenadine active metabolite, metabolite I, was examined in ten healthy elderly adults and ten younger adults after single-dose oral administration of 120-mg terfenadine. All subjects successfully completed the study without reporting sedation or other adverse events. Absorption was rapid in both the young and elderly. The mean Cmax was the same for both groups, 501 ng/mL, and occurred at 2.3 hours in the young subjects and 2.5 hours in elderly subjects. However, the apparent clearance was reduced by about 25% in the elderly. After correcting clearance for bodyweight, this difference was not statistically significant.

Experience with brulamycin therapy in dialysed patients

Forty-two patients with chronic renal insufficiency were subjected to Brulamycin therapy to combat complications such as sepsis, infection of the lower respiratory tract (bronchopneumonia) or of the urinary tract, Cimino fistula inflammation or peritonitis when the pathogenic agent was sensitive to nothing but aminoglycosides. The proper Brulamycin dose was selected under close clinical and laboratory control so as to fit the dialysed patient and his state of impaired renal function. The various forms of dialysis have been studied to determine their Brulamycin blood level diminishing effects. Familiarity with these effects is as important as with the patient's age, sex, body weight and actual state of renal function. The results confirm the experience gained by other authors that Brulamycin treatment--in curative non-toxic doses as indicated by the blood level index--is useful against severe infectious complications in anuric patients.

Effects of nephrotoxic compounds on active uptake of drugs in isolated renal tubules in rabbits

The uptake of sulfonamides and phenolsulfonphthalein (PSP) was examined in vitro using isolated renal proximal tubule suspension, and the effects of nephrotoxic compounds on the uptake of sulfamethizole (SMZ) were studied. The uptake of SMZ and PSP was energy dependent and was inhibited competitively by iodopyracet (IP), which is transported actively by the p-aminohippurate mechanism. The uptake of sulfamethoxazole was also reduced by IP but that of sulfanilamide was negligible. The present results correspond well with those of in vitro experiments reported previously. Nephrotoxic compounds, mercuric chloride, neomycin, viomycin and kanamycin, decreased the uptake of SMZ non-competitively. The inhibitory action of the three antibiotics corresponds with in vivo potency, suggesting that this renal tubule preparation may provide a simple method for predicting the nephrotoxicity of drugs.

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.

A collection of therapeutic, toxic and fatal blood drug concentrations in man

In order to assess the significance of drug concentrations measured in clinical and toxicological investigations, it is essential that good collections of data are readily available. As a guide to interpreting findings, the present work provides a compilation of therapeutic, toxic and fatal blood concentration ranges of 298 drugs of interest to clinical pharmacologists, clinical toxicologists, and forensic toxicologists. Wherever possible, ranges are expressed concisely in terms of the maximum blood concentrations which account for 10, 50 and 90% of the data collected. They provide easy access to the most reliable information which relates the blood drug concentration to the biological response it produces. Where appropriate, the different toxic effects of a drug and/or the different degrees of severity of toxic symptoms associated with different drug levels are clearly defined. The original sources of all data used are provided to allow the analyst to obtain further analytical, pharmacokinetic and toxicological information should this be necessary. Those factors (e.g. age, capacity for drug metabolism, drug interactions, etc) which can modify the relationship between a drug concentration and the response it produces are briefly discussed.

Acute phenylbutazone poisoning in a child

Accidental acute intoxication with phenylbutazone in a 2 1/2-year-old child produced an acute picture of coma, convulsions, diarrhoea, and of cholestatic jaundice which evolved over the succeeding 10 days. Transient, unexplained hyperglycaemia occurred during the first few hours of the illness. Recovery was complete within three weeks after the poisoning. Her clinical progress was monitored with the aid of regular estimations of plasma phenylbutazone levels.

The pharmacokinetics of furazlocillin in healthy humans

The pharmacokinetics of the novel acylureidopenicillin furazlocillin, 6-[D-2-(3-furfurylidenamino-2-oxo-imidazolidine-1-carboxamido)-2 -(4-hydroxyphenyl)-acetamido]-penicillanic acid and of its penicilloic acid derivative were investigated in five healthy male volunteers after intravenous administration of 2 and 4 g dosages. The volunteers were either in a lying or sitting position throughout the duration of the studies. The concentrations of the drug in plasma and urine were measured by two different methods in parallel: a microbiological assay and a newly developed high pressure liquid chromatography method. The latter method was also applicable for quantitation of the penicilloic acid derivative in these biological fluids. The drug's plasma protein binding (66%) and apparent red cell-plasma partition coefficient (0.055) were concentration independent. The pharmacokinetics of the drug were first order only at the lower dose level. The apparent half lives of three distinguishable phases were, respectively, 4(t1/21), 18 (t1/22), and 64 (t1/2z) min. The total and renal clearances of the drug were, respectively, 303 and 79 ml/min. The latter value implied tubular secretion of the drug. Graphical and digital computer analyses of the data were performed with a linear three compartment body model. Small but consistent deviations from linear kinetics caused by the nonrenal elimination route were observed after administration of the higher dose (4 g). In contrast, renal elimination showed no such dose dependency and was first order. The disposition kinetics of furazlocillin were body position independent. The penicilloic acid derivative of furazlocillin was found in plasma and urine in all the five subjects tested. The percentage of the dose excreted renally as the derivative amounted, respectively, to 5.2 and 7.0% after the lower and higher dosage of furazlocillin, with significant inter- and intrasubject variability. The renal clearance of the derivative was 41 ml/min.

Bioavailability of bemetizide and triamterene from a combination formulation

The bioavailability of the thiazide diuretic bemetizide from a tablet containing 25 mg of this drug and 50 mg of the chemically unrelated diuretic triamterene was lower than, and significantly different (p less than 0.01) from that from a tablet containing 25 mg bemetizide alone. The mean peak plasma level of bemetizide after administration of the combination tablet (68.3 ng ml-1) was lower than that after administration of bemetizide alone (87.9 ng ml-1), although the times of occurrence of the peak levels were similar. The bioavailability of triamterene from the combination tablet was greater than, but not significantly different from that after administration of a capsule containing 50 mg triamterene alone. The mean peak plasma level of triamterene after administration of the combination tablet (44.6 ng ml-1) was higher than and significantly different (p less than 0.001) from that after administration of triamterene alone (15.7 ng ml-1). Although bemetizide is unstable in urine, measurement of the apparent excretion of unchanged drug in the 24 h post-dose urine (less than 4 per cent of the dose) agreed with the estimate of drug bioavailability from the plasma level data. Less than 2 per cent of the dose of triamterene was excreted unchanged in the 24 h post-dose urine, but the urinary excretion data also agreed with the bioavailability estimates from the plasma level data. The results of this study and those reported in the literature suggest that because of their physicochemical properties, the bioavailability of some thiazides and triamterene needs to be evaluated when new formulations of these drugs are produced. However, with respect to the combination formulation reported in this paper, the difference in bioavailability of the thiazide component did not detectably effect the diuretic activity of the formulation.

Pharmacokinetic studies of flumecinol in man and dog

The pharmacokinetics of flumecinol (Zixoryn) a new hepatic enzyme inducer has been studied in four beagle dogs and six healthy volunteers. The beagle dogs and the volunteers received the drug orally in a dose of 40 mg/kg of body weight and of 100 mg single dose respectively. Flumecinol was extracted from plasma with diethyl ether and analysed by gas-liquid chromatography using a flame ionisation detector (FID). The pharmacokinetic parameters of flumecinol were determined by computer evaluation of the plasma concentration-time curves. The peak plasma concentrations were found to be 5.3 and 2.1 hours in dogs and humans, respectively. Flumecinol is eliminated from the plasma of dogs and humans with half-lives of 38.95 and 17.16 hours, corresponding to a clearance of 53.2 litres/hour and 94.0 litres/hour, respectively.

Hemodialysis of theophylline in uremic patients

Hemodialysis of theophylline was studied in three uremic patients. The dialysis clearance ranged from 75.6 to 97.9 ml/min and averaged 88.1 ml/min. A much smaller value of 32.8 ml/min was reported by Levy and associates. The difference may be attributed to the two monitoring factors during hemodialysis, namely, blood and dialyzate flow rates. Both were higher in our study. Analysis of the semilogarithmic plots of the arterial plasma concentration versus time over a 3-hour period gave apparent half-lives of 3.15, 2.04, and 2.73 hours, respectively, for the three patients. Half-life of theophylline in normal subjects ranged from 4 to 6 hours or even longer. A prolonged half-life of theophylline in uremia could be expected. Our kinetic study indicated an approximately 50 per cent reduction in terminal half-life during hemodialysis. Hourly dialyzate was collected from one patient to account for drug recovery in the dialysis fluid. Forty per cent of the administered dose was recovered in the dialyzate during a 3-hour dialysis period, indicating effective removal. Dialysis clearance for creatinine was calculated by arterial-venous difference and correlated with that of theophylline. We found that theophylline was cleared by the dialyzer at a rate approximating 63 per cent of creatinine removal.

Rapid estimation of volume of distribution after a short intravenous infusion and its application to dosing adjustments

A simple and a rapid method to estimate the apparent volume of distribution of drug after single or during multiple short-term intravenous infusion is proposed. This is based on the back extrapolation to the midpoint of infusion. An equation simpler than one previously reported in the literature is also derived to calculate the maintenance dose for multiple short-term intravenous infusion. In addition, an equation to estimate the "priming dose" for infusion during multiple infusion regimen is also derived. The derivations of the equations are based on a linear one-compartment open model for drug disposition in patients. The prposed method is thought to be adequate for the purpose of rapid individualization of dosage regimens. The simplicity of the method, in particular, the solution by the graphic method for estimation of the apparent volume of distribution, might be specially useful for clinicians not well versed in mathematics in applying clinical pharmacokinetics to drug therapy.

Pharmacokinetics of trimethoprim (TMP) in normal and febrile rabbits

The pharmacokinetics of trimethoprim (TMP) were investigated in normal and febrile rabbits. The half-life of TMP in rabbits is about 40 minutes and no differences were found between the half-life in normal and febrile rabbits. A significantly greater volume of distribution of TMP, however, was found in the febrile rabbits. Analysis according to the two-compartment model showed that the differences are due to a change in the distribution of TMP towards the peripheral compartments.

Active drug metabolites and renal failure

Drugs that are administered to man may be biotransformed to yield metabolites that are pharmacologically active. These metabolites may accumulate in patients with end-stage renal disease if renal excretion is a major elimination pathway for the metabolite. This is true even if the active metabolite is a minor metabolite of the parent drug as long as the minor metabolite is not further biotransformed but is mainly excreted in the urine. Minor metabolite accumulation may also occur if it is further biotransformed by a pathway that is inhibited in uremia. Some clinical consequences of accumulation of the active drug metabolites of procainamide, meperidine, clofibrate, allopurinol, sulfadiazine and nitrofurantoin in patients with renal failure are discussed. The high incidence of adverse drug reactions seen in renal failure may be explained, in part, by the accumulation of active drug metabolites. Examples of active drug metabolites that do not accumulate in patients with renal failure because of further biotransformations are also included.

Pharmacologically active drug metabolites: therapeutic and toxic activities, plasma and urine data in man, accumulation in renal failure

Drugs that are administered to man may be biotransformed to yield metabolites that are pharmacologically active. The therapeutic and toxic activities of drug metabolites and the species in which this activity was demonstrated are compiled for the metabolites of 58 drugs. The metabolite to parent drug ratio in the plasma of non-uraemic man and the percentage urinary excretion of the metabolite in non-uraemic man are also tabulated. Those active metabolites with significant pharmacological activity and high plasma levels, both relative to that of the parent drug, will probably contribute substantially to the pharmacological effect ascribed to the parent drug. Active metabolites may accumulate in patients with end stage renal disease if renal excretion is a major elimination pathway for the metabolite. This is true even if the active metabolite is a minor metabolite of the parent drug, as long as the minor metabolite is not further biotransformed and is mainly excreted in the urine. Minor metabolite accumulation may also occur if it is further biotransformed by a pathway inhibited in uraemia. Some clinical examples of the accumulation of active drug metabolites in patients with renal failure are: (a) The abolition of premature ventricular contractions and prevention of paroxysmal atrial tachycardia in some cardiac patients with poor renal function treated with procainamide are associated with high levels of N-acetylprocainamide. (b) The severe irritability and twitching seen in a uraemic patient treated with pethidine (meperidine) are associated with high levels of norpethidine. (c) The severe muscle weakness and tenderness seen in patients with renal failure receiving clofibrate are associated with excessive accumulation of the free acid metabolite of clofibrate. (d) Patients with severe renal insufficiency taking allopurinol appear to experience a higher incidence of side reactions, possibly due to the accumulation of oxipurinol. (e) Accumulation of free and acetylated sulphonamides in patients with renal failure is associated with an increase in toxic side-effects (severe nausea and vomiting, evanescent macular rash). (f) Peripheral neuritis seen after nitrofurantoin therapy in patients with impaired renal function is thought to be due to accumulation of a toxic metabolite. The high incidence of adverse drug reactions seen in patients with renal failure may for some drugs be explained in part, as the above examples illustrate, by the accumulation of active drug metabolites. Monitoring plasma levels of drugs can be an important guide to therapy. However, if a drug has an active metabolite, determination of parent drug alone may cause misleading interpretations of blood level measurements. The plasma level of the active metabolite should also be determined and its time-action characteristics taken into account in any clinical decisions based on drug level monitoring.