Kinetics and disposition in toxicology

Pharmacokinetic analysis of the behaviour of xenobiotics in living organisms is the basis for the understanding of dose (concentration-) response relationships. Risk estimates may lead to erroneous results if pharmacokinetic principles are neglected. The pharmacokinetic behaviour of a xenobiotic should be known before planning long-term experiments on toxicity and carcinogenicity. This is demonstrated using ethylene as an example.On the basis of the pharmacokinetics in rats of ethylene and ethylene oxide, its carcinogenic metabolite, we estimated the theoretical risk for the carcinogenic potential of ethylene. Our results demonstrate that exposure of rats to ethylene concentrations higher than 1000 ppm correspond to a (theoretical) exposure to 5.6 ppm ethylene oxide: exposures to ethylene at 40 ppm are equivalent to ethylene oxide exposures of 1 ppm, which is the (new) TLV for ethylene oxide.Dedicated to Professor Dr. med. Herbert Remmer on the occasion of his 65th birthday     

Metabolism and disposition of sulfinalol in laboratory animals

Peak levels of radioactivity in blood occurred 1.0 hr after oral administration of 3H-sulfinalol hydrochloride to rats, dogs, and monkeys. The plasma decay curve for intact sulfinalol in the dog was biphasic, with apparent first-order half-lives of 0.55 and 6.2 hr. Rats excreted 42.5% of the dose in the urine and 31.8% in the feces after 24 hr. Urinary and fecal recovery were 53.8% and 41.2%, respectively, after 10 days for dogs and 57.8% and 38.0%, respectively, after 9 days for monkeys. Free sulfinalol (11.8% of the dose) was the major component in dog feces with lesser amounts of the sulfide and sulfone metabolites, also in the unconjugated form. All metabolites in dog urine were conjugated with glucuronic acid, with sulfinalol (28.5%) and desmethylsulfinalol (8.5%) representing the major constituents, whereas the sulfone and sulfide metabolites were minor ones. Monkey feces contained primarily unconjugated forms of the desmethyl sulfide metabolite (17.0%) and sulfinalol (7.5%); lesser amounts of desmethylsulfinalol and the sulfone metabolite were present. Desmethylsulfinalol (8.7%) and its sulfate (7.0%) and glucuronide (4.0%) conjugates were the major urinary metabolites in the monkey; sulfinalol (1.4%), its glucuronide conjugate (5.1%), the desmethyl sulfide metabolite (and its sulfate conjugate), and the sulfone metabolite were also present.     

Metabolic disposition of Rolziracetam in laboratory animals

The metabolic disposition of [14C]-labeled Rolziracetam (CI-911) was studied in rhesus monkeys, beagle dogs, and Wistar rats after both p.o. and i.v. doses. Intravenously administered CI-911 was rapidly eliminated from systemic circulation with an apparent elimination half-life of less than 25 min. Plasma radioactivity was 10-20 times higher than that of parent drug and persisted much longer. After oral doses, only traces of intact drug were detected in plasma, whereas total radioactivity reached peak concentrations within 0.5-1 h indicating rapid absorption. The extent of absorption determined from plasma radioactivity and urinary excretion data was 90% or better. Tissue distribution of radioactivity in rats showed the highest concentrations in the liver and kidneys (12-30 times greater than plasma levels) with decreasing levels in the lungs, gonads, heart, spleen, muscle, and brain. Urinary excretion accounted for nearly 90% of the administered dose while fecal recovery was less than 5%. The sole metabolite present in plasma, tissues, and urine was identified as 5-oxo-2-pyrrolidinepropanoic acid (PD 106,687).     

Triprolidine radioimmunoassay: disposition in animals and humans

A hapten derivative of triprolidine, bearing an acrylic acid side chain ortho to the pyridine ring nitrogen atom, was synthesized and coupled to bovine serum albumin. Immunization of New Zealand White rabbits with the resulting drug-protein conjugate resulted in the production of antisera capable of binding a radioiodinated tyramine conjugate of the triprolidine hapten derivative at high antiserum dilutions (1:70,000-1:150,000). These antisera were used to develop a radioimmunoassay (RIA) for triprolidine in human plasma with a sensitivity limit of 0.1 ng/mL (0.01 ng of actual mass). The known hydroxymethyl and carboxyl metabolites of triprolidine cross-reacted weakly (less than 2 and less than 0.05%, respectively) with this antiserum. The RIA could be used for the direct analysis of triprolidine in human and rabbit plasma, but not for rat or dog plasma, presumably due to the presence of other interfering substances (possibly metabolites). The validity of the RIA procedure in human plasma was demonstrated by comparative analysis of a number of samples by quantitative TLC (r = 0.985, slope = 1.076). The assay was employed to describe the pharmacokinetics of triprolidine in the rabbit (t 1/2, beta = 1.7 h). The assay had adequate sensitivity to detect low circulating drug concentrations in humans after therapeutic oral doses and also substantiated previous disposition experiments with triprolidine in humans (t 1/2, beta = 2.27 h). TLC analysis demonstrated that the absolute oral bioavailability of triprolidine (1-mg/kg dose) in the dog was low (4%). A comparison of triprolidine pharmacokinetic parameters in dogs, rabbits, rats, and humans revealed considerable similarity in elimination characteristics in these species.     

The disposition of saccharin in animals and man--a review

Recent studies on saccharin in animals and man have allowed a detailed understanding of its fate in the body. Saccharin is slowly absorbed from the gut but rapidly eliminated in the urine, largely by renal tubular secretion. Saccharin does not undergo detectable metabolism in either animals or man. Tissue specific accumulation in the urinary bladder, suggested by single and multiple dose studies in rats, was not found during chronic administration in the diet. The bladder tissue is part of the central, rapidly equilibrating, compartment. The sex- and generation-specificity of the tumorigenic effect is not due to unique accumulation in the urinary bladder of F1 males. Saturation of renal tubular secretion, which occurs in rats fed 5% saccharin or more in the diet, was not found in human volunteers given large oral doses (2 g).     

Kinetics of oral trifluoperazine disposition in man

The disposition of trifluoperazine (TFP) was studied in five healthy volunteers following oral administration of a 5 mg tablet. Using a very sensitive GC-MS technique plasma TFP concentrations were measured up until 24 h following drug ingestion. Peak plasma concentrations varied widely (range 0.53-3.09 ng ml-1) and were reached 2.8 +/- 0.5 h following ingestion of the TFP tablet. The apparent terminal elimination half-life of TFP was 12.5 +/- 1.4 h. The area under the plasma concentration-time curve differed widely between subjects (range: 5.9-17.6 ng ml-1 h) suggesting large individual differences in the extent of presystemic TFP elimination.     

Kinetics of d-tubocurarine disposition and pharmacologic response in rats

After bolus intravenous dosing of d-tubocurarine (d-TC) to rats, the twitch heights of the tibialis anterior muscle indirectly stimulated were followed, and its decrease was defined as pharmacologic response of d-TC. The relation between dose and response intensity was found to be well described with Hill's equation. According to a theory proposed by Smolen, Hill's equation was also applicable to the biophase d-TC concentration-response relation; the time courses of the relative biophase d-TC concentration indicated linear kinetics with dose levels 0.15 mg/kg and the occurrence of dose-dependent disposition with 0.30 mg/kg. After bolus i.v. dosing of³H-d-TC, plasma d-TC concentration obeyed a dose-independent two compartment model with doses 0.15mg/kg, but not with 0.30 mg/kg. This finding matched the above estimated with pharmacologic data. The active metabolite was not found in plasma and urine. The extent of d-TC plasma protein binding was independent of the wide range of plasma levels and its mean (±SD) value was 30.5 (±3.8). Plasma d-TC levels and pharmacologie response intensity were well correlated by Hill's equation and a three compartment model (the general two and the biophase compartments) in the dose range 0.15 mg/kg.This work was presented at the First Japanese-American Symposium on Pharmacokinetics and Biopharmaceutics, Tokyo, July 1981, which was held in memory of Dr. Sidney Riegelman.     

Kinetics of ajmaline disposition and pharrnacologic response in beagle dogs

Pharmacokinetics and pharmacodynamics of ajmaline were studied in four healthy dogs after intravenous administration of the drug at the infusion rate of 1.0 mg/min for 45 min. Ajmaline exhibited a saturable binding to plasma protein. One kind of binding site was found in the range of observed drug concentrations and its binding capacity showed nearly threefold interindividual difference. The time course of ajmaline concentration in whole blood Cbcould be described by the two-compartment open model and the unbound concentration of ajmaline in plasma Pf wasestimated from Cbby using the hematocrit value and the parameters of plasma protein binding and erythrocyte partitioning. The pharmacologic responses to ajmaline were assessed by recording ECG, and the changes in PQ and QRS interval were studied in relation to ajmaline disposition. When ECG changes were related to the ajmaline concentration, a significant degree of hysteresis was observed. The relationship between the unbound drug concentration and the pharmacologic effect was analyzed by a combined pharmacokinetic-pharmacodynamic model, where the hypothetical effect compartment is connected to the Pfin the central compartment by a first-order process. This model allows estimation of the changes in PQ and QRS intervals after intravenous administration of ajmaline. By comparing the drug effect on PQ and QRS intervals, it was suggested that ajmaline distributes to the atrial and the ventricular tissue in a similar degree and causes a reduction in the conduction rate in both sites with similar activity.     

Kinetics of ajmaline disposition and pharmacologic response in beagle dogs

Pharmacokinetics and pharmacodynamics of ajmaline were studied in four healthy dogs after intravenous administration of the drug at the infusion rate of 1.0 mg/min for 45 min. Ajmaline exhibited a saturable binding to plasma protein. One kind of binding site was found in the range of observed drug concentrations and its binding capacity showed nearly threefold interindividual difference. The time course of ajmaline concentration in whole blood Cb could be described by the two-compartment open model and the unbound concentration of ajmaline in plasma Pf was estimated from Cb by using the hematocrit value and the parameters of plasma protein binding and erythrocyte partitioning. The pharmacologic responses to ajmaline were assessed by recording ECG, and the changes in PQ and QRS interval were studied in relation to ajmaline disposition. When ECG changes were related to the ajmaline concentration, a significant degree of hysteresis was observed. The relationship between the unbound drug concentration and the pharmacologic effect was analyzed by a combined pharmacokinetic-pharmacodynamic model, where the hypothetical effect compartment is connected to the Pf in the central compartment by a first-order process. This model allows estimation of the changes in PQ and QRS intervals after intravenous administration of ajmaline. By comparing the drug effect on PQ and QRS intervals, it was suggested that ajmaline distributes to the atrial and the ventricular tissue in a similar degree and causes a reduction in the conduction rate in both sites with similar activity.     

Kinetics and disposition of fluorescein-labelled liposomes in healthy human subjects

Summary The in vivo kinetics and organ uptake of multilamellar liposomes have been studied in healthy volunteers. Sodium fluorescein-containing liposomes composed of equimolar amounts of egg phosphatidylocholine and cholesterol were injected into a peripheral vein in 4 healthy subjects. Blood samples collected from the femoral artery, hepatic vein and pulmonary artery, were analysed for liposomal dye content.The results, showing involvement of the reticuloendothelial system (RES) in the removal of liposomes, confirmed those previously obtained with radiolabelled preparations.Use of an innocuous liposomal marker (sodium fluorescein) and conventional vascular catheterization techniques, as employed here, may provide a reliable and clinically acceptable approach to establishing disease-induced changes in the kinetics of uptake of drug-containing liposomes by the RES, and thus help in the design of protocols for effective treatment.     

Kinetics of oral fluphenazine disposition in humans by GC-MS

The disposition of fluphenazine was investigated in six healthy volunteers following oral administration (5 mg). Using a sensitive and specific GC-MS procedure plasma fluphenazine concentrations were measured up until 32 h after drug administration. Peak plasma concentrations varied widely (range: 0.26-1.06 ng/ml) and were observed at 2.8 +/- 0.5 h following fluphenazine administration. The apparent terminal elimination half-life of fluphenazine was 33.1 +/- 8.1 h. The area under the plasma concentration-time curve differed widely between subjects (range: 7.1-28.6 ng/ml h) suggesting large interindividual differences in the extent of fluphenazine presystemic elimination.     

Studies on the disposition of a 5-nitroimidazole in laboratory animals

The disposition and metabolism of a 5-nitroimidazole compound (SC 28538) was investigated in the rat, beagle dog and rhesus monkey. The absorption of [14C]-SC 28538 was rapid and essentially complete after oral dosage in male animals, and also after intravaginal dosage in the female rat. Peak plasma levels of radioactivity occurred within 2 h of dosage. In the rat and dog the radioactivity was excreted predominantly in the faeces (greater than 60%) but in the monkey more than 60% was excreted in the urine. In both the male and pregnant female rat radioactivity was concentrated in the gastro-intestinal tract, liver and harderian gland and the concentrations of radioactivity in other tissues was generally lower than in plasma. Radioactivity was cleared more rapidly from plasma than from the majority of tissues. SC 28538 was extensively metabolised to form glucuronide and amino acid conjugates. The half-life of SC 28538 was of the order of 1 h in the dog and 3.7 h in the monkey.     

Mass spectrometric studies of cocaine disposition in animals and humans using stable isotope-labeled analogues

Ion cluster technique in conjunction with gas chromatography-mass spectrometry (GC-MS) was used for the identification and quantitation of major metabolites of cocaine (1a) in rat and humans. In a typical experiment, a female rat weighing 250 gm was intraperitoneally administered a 20-mg/kg mixture of 1a, NCD3-cocaine (1b), OCD3-cocaine (1c), and 4T2-cocaine (1d). The urine was collected, extracted with organic solvents, and separated into several fractions using TLC and HPLC techniques. Tritium radioactivity in a metabolically stable position in 1d was useful in the separation of metabolites, while the deuterium labeled 1(b + c), creating an artificial isotopic cluster, provided specific identification of metabolites by mass spectrometric interpretation. Norcocaine (2), benzoylnorecgonine (3), N-hydroxynorcocaine (4), methylecgonidine (5), benzoylecgonine (11), ecgonine methyl ester (9), hydroxycocaine (7), hydroxymethoxycocaine (10), and dimethoxyhydroxycocaine (6) were found to be the major metabolites of 1a in the rat urine as well as in plasma. The whole brain analysis showed significant amounts of unmetabolized 1a and 2, and minor concentrations of 9, 5, 7, and 10, and traces of 6. Some of these metabolites have been reported earlier by us as well as other investigators and are unequivocally confirmed in this work. Unmetabolized 1a, its pharmacologically active metabolite 2, and other major metabolites were quantitated in the rat brain, plasma, and urine using stable isotope-labeled analogues as internal standards and selected ion monitoring (SIM) mass spectrometry. The pharmacokinetic profiles of 1a and 2 indicate half-lives of less than 20 min in the brain and plasma. These data are in good agreement with widely reported short-lived behavioral effects of cocaine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Physiological disposition of alendronate, a potent anti-osteolytic bisphosphonate, in laboratory animals.

Alendronate (4-amino-1-hydroxybutylidene-1,1-bisphosphonate) is currently under investigation as an anti-osteolytic agent in the treatment of a broad range of bone disorders. This study describes the absorption and disposition of the drug in laboratory animals. Following iv administration, alendronate was rapidly cleared from plasma, either taken up and sequestered in the bone or excreted by the kidney. About 30 to 40% of the dose was excreted in the urine in 24 hr, with most of the drug being excreted in the first 3 to 4 hr. There was little or no accumulation of the drug in noncalcified tissues and only a very small fraction of the dose was excreted in the bile. Most of the dose was rapidly taken up by bone tissues: 30% in 5 min, 60% in 1 hr. Absorption of alendronate was very poor. Based on the ratios in bone of the labels from the 14C-labeled oral dose and the 3H-labeled iv dose, absorption was estimated to be about 0.9% for the rat, 1.8% for the dog, and 1.7% for the monkey. Comparison of the concentrations of alendronate in bones of the same rats in fasted (3H-labeled) and fed (14C-labeled) states indicated that food caused a substantial decrease in absorption, by about 6- to 7-fold. The terminal half-life of alendronate in bone was about 200 days for the rat. Based on urinary excretion, the terminal half-life was estimated to be about 1000 days for the dog. The long persistence of alendronate in bone was likely due to its slow dissolution rate from bone tissues.     

Kinetics and disposition of hexarelin, a peptidic growth hormone secretagogue, in rats.

To document the disposition of hexarelin, a peptidyl growth hormone secretagogue, male Sprague-Dawley rats received a 5-microg/kg bolus i.v. dose or three single s.c. doses of 5, 10, and 50 microg/kg. To assess hexarelin tissue distribution and excretion, rats were given 1 microg/kg of [(3)H]hexarelin (9.4 Ci/mmol). Metabolism of [(3)H]hexarelin was assessed in bile duct-exteriorized rats given 50 microg/kg where radiolabeled hexarelin biliary and urinary excretion was quantified. After its i.v. injection, hexarelin displayed a half-life of 75.9 +/- 9.3 min, a systemic clearance of 7.6 +/- 0.7 ml/min/kg, and a volume of distribution at steady state of 744 +/- 81 ml/kg. After s.c. administration, the area under the curve (477-3826 pmol.min/ml) estimated with increasing doses confirmed the absence of hexarelin accumulation. Clearance/F (12-15 ml/min/kg) and volume of distribution/F (1208-1222 ml/kg) were dose independent. Hexarelin bioavailability given s.c. was 64%. The highest radioactivity levels were detected in the kidney, liver, and duodenum. The pattern of hexarelin excretion was similar after i.v. or s.c. administrations. Total radioactivity in bile, urine, and feces corresponded to 60, 22, and 10% of the dose, respectively. Of the radioactivity excreted in bile and urine, 90 and 71% was unchanged hexarelin, respectively. These results suggest that: 1) the kinetics of hexarelin appear to be first order up to 50 microg/kg; 2) hexarelin is rapidly absorbed after s.c. administration; 3) biliary excretion is the primary route of hexarelin elimination; and 4) the high recovery of unchanged peptide in bile and urine demonstrates hexarelin stability toward proteolytic enzymes.     

Plasma disposition and tissue depletion of chlortetracycline in the food producing animals,chickens for fattening

Chickens were used to investigate plasma disposition of chlortetracycline after single IV (15 mg/kg) and multiple oral administration (60 mg/kg, 5 days) and residue depletion of chlortetracycline after multiple oral doses (60 mg/kg, 5 days). Plasma and tissue samples were analyzed by HPLC. Mean elimination half-lives in plasma were 7.96 and 13.15 h after IV and multiple oral administration. Maximum plasma concentration was 4.33 μg/ml and the interval from oral administration until maximal concentration was 1.79 h. Oral bioavailability was 17.76%. After multiple oral dose, mean kidney, liver and muscle tissue concentrations of chlortetracycline + 4-epi-chlortetracycline of 835.3, 192.7, and 126.3 μg/kg, respectively, were measured 1 day after administration of the final dose of chlortetracycline. Chlortetracycline residues were detected in kidney and liver (205.4 and 81.7 μg/kg, respectively), but not in muscle, 3 days after the end of chlortetracycline treatment. The mean chlortetracycline + 4-epi-chlortetracycline concentrations were below LOQ at 3 and 5 days after cessation of medication in muscle and liver, respectively. A withdrawal time of 3 days was necessary to ensure that the chlortetracycline residues were less than the maximal residue limits (MRLs) established by the European Union (100, 300, and 600 μg/kg in muscle, liver, and kidney, respectively).     

Possibility for induction of convulsion by fleroxacin and its disposition in the central nervous system in animals

Fleroxacin and its metabolites were examined for their possibility of the induction of convulsion in mice, inhibitory activity on GABA receptor binding, and disposition in the central nervous system of mice, rats and dogs. No convulsion was evoked in mice at high oral combination doses of fleroxacin and fenbufen. Brain-to-serum concentration ratios of fleroxacin after oral dose were 0.13 in mice, 0.19 in rats and 0.28 in dogs. Rats showed no accumulation of fleroxacin in brain and similar elimination from brain to that from serum after the oral dose. Dogs exhibited the similar distribution of fleroxacin into various brain regions. The inhibitory effect of fleroxacin on GABA receptor binding was relatively weak and slightly potentiated in the presence of 4-biphenylacetic acid. Demethyl fleroxacin showed more potent inhibitory activity on GABA receptor binding and potentiation with 4-biphenylacetic acid. The combination of intravenous demethyl fleroxacin and oral fenbufen showed no convulsion in mice. Fleroxacin N-oxide showed only slight inhibitory activity on GABA receptor binding, being not influenced with 4-biphenylacetic acid.