Zolantidine (SK&F 95282) is a potent selective brain-penetrating histamine H2-receptor antagonist

1. The novel benzthiazole derivative zolantidine (SK&F 95282) is a potent antagonist of histamine at H2-receptors in guinea-pig atrium and rat uterus. Only apparent pA2 values of 7.46 and 7.26 respectively could be calculated since the slopes of the Schild plots were significantly less than unity. 2. Zolantidine is equally potent as an antagonist at histamine H2-receptors in guinea-pig brain. The compound inhibited histamine stimulated adenylate cyclase (pKi 7.3) and dimaprit stimulated adenosine 3':5'-cyclic monophosphate (cyclic AMP) accumulation (approx pA2 7.63), and competed with [3H]-tiotidine binding (pKi 7.17). 3. Zolantidine is at least 30 fold more potent at H2-receptors than at other peripheral and central receptors investigated. 4. Infusion of zolantidine into rats produces a brain concentration greater than the plateau blood concentration (brain/blood ratio 1.45). 5. Zolantidine is thus characterized as a potent selective brain-penetrating H2-receptor antagonist, and will be a valuable pharmacological tool for investigating possible physiological and pathological roles for histamine in the central nervous system.

Pharmacokinetics of famotidine in man

Famotidine (F) is an effective new H2-receptor antagonist. Knowledge of its pharmacokinetic properties and metabolism is scanty. Therefore, we investigated the disposition of F in 6 healthy male volunteers following a single oral (40 mg) and intravenous (20 mg) dose. F and a metabolite were monitored in plasma or urine by a HPLC method. After intravenous administration plasma levels declined biexponentially with an initial half-life (t1/2) of 0.5 h and a terminal t1/2 of 4.0 h. F was slightly bound to plasma proteins (less than 1 to 15%) and its distribution volume averaged 1.13 l/kg. About 72% of the dose could be recovered as unchanged F in urine. Thus, hepatic clearance contributes to the total plasma Cl of 309 ml/min only 88 ml/min. Consequently, a high hepatic first-pass effect can be excluded. Following oral administration maximum plasma concentrations of 104 +/- 39 ng/ml (mean +/- SD) were observed after 2.3 +/- 1 h. F was eliminated with a t1/2 of 3.6 +/- 1.1 h and its absolute bioavailability ranged from 20 to 66%. In urine an oxidized metabolite could be identified which accounts to about 2% of the given dose. In conclusion, F is rapidly eliminated mainly by the renal route and its t1/2 is slightly longer than those of other available H2-receptor antagonists.

Urinary excretion kinetics of famotidine in rats

Famotidine is a new histamine H2-receptor antagonist which has been demonstrated to be more potent than cimetidine and ranitidine in inhibiting gastric acid secretion. Nine groups of adult male Sprague-Dawley rats received an ia injection of various loading doses of famotidine followed immediately by a constant infusion of the drug at different rates for 6 hr. When steady state famotidine concentrations in plasma were low, renal clearance of the drug (CLR) was greater than glomerular filtration (GFR), and the ratio CLR/GFR was about 4.5 at plasma concentrations of 0.2-1.8 micrograms/ml, suggesting that famotidine was actively secreted by the renal tubules. The CLR decreased as famotidine concentration in plasma increased, and the ratio CLR/GFR approached 1 in the concentration range of 25-76 micrograms/ml, thus providing evidence for saturation of the secretory mechanism. The maximum rate of secretory transport (Tm) of famotidine averaged 180 micrograms/min/kg. On average, some 50-70% of an ia bolus dose was excreted in the urine as unchanged drug within 24 hr of administration. Over the dose range of 0.3-30 mg/kg famotidine, there was no dose-dependent effect on total or renal clearance. Since the lowest dose level, 0.3 mg/kg, is below the recommended human therapeutic dose for famotidine (0.6 mg/kg), the saturation of the renal excretion process observed here in rats is not likely to be of clinical significance.

Interaction of diazepam with famotidine and cimetidine, two H2-receptor antagonists

Famotidine is currently under investigation as an H2-receptor antagonist. Eleven healthy male volunteers received a single 10 mg intravenous dose of diazepam on three occasions: once during coadministration of famotidine 40 mg bid, once during coadministration of cimetidine 300 mg qid, and once without other drug treatment (control). Multiple blood samples were drawn during the seven days after each diazepam dose. Diazepam and desmethyldiazepam plasma concentrations were measured by electron capture gas chromatography. There were no significant differences among the three treatment conditions in diazepam central compartment volume or total volume of distribution. During the cimetidine as compared with the control treatment, diazepam elimination half-life was significantly increased (72 vs 55 hr, P less than .05), total area under the curve (AUC) increased (11.8 vs 9.8 hr-micrograms/mL, P less than .05), and total clearance reduced (0.20 vs 0.28 mL/min/kg, P less than .05). Seven-day AUC for desmethyldiazepam also increased (4.6 vs 3.8 hr-micrograms/mL, P less than .05). However, there were no significant differences between famotidine and control treatment conditions in diazepam elimination half-life (53 vs 55 hr), total AUC (9.5 vs 9.8 hr-micrograms/mL), or total clearance (0.28 vs 0.28 mL/min/kg) or in seven-day AUC for desmethyldiazepam (3.9 vs 3.8 hr-micrograms/mL). Thus, therapeutic doses of cimetidine significantly impair the clearance of diazepam and desmethyldiazepam. Therapeutic doses of famotidine do not impair diazepam and desmethyldiazepam kinetics, suggesting that there is no significant kinetic interaction when diazepam and famotidine are administered concurrently in clinical practice.

Development of enzyme immunoassay for Platonin (NK 19)

A sensitive and specific enzyme immunoassay for an immunomodulator PLATONIN (NK 19) has been developed and applied to the evaluation of pharmacokinetics in human and rats. NK 19 analog, succinyl NK 19 has been synthesized and coupled to bovine serum albumin. This conjugate is immunogenic and when injected into rabbits antibodies of high affinity for NK 19 were obtained. Some light decomposed products of Platonin were of lower cross-reactivity with these antibodies. For competitive reactions, these antibodies were incubated with a mixture of succinyl NK 2900-beta-D-galactosidase conjugate and standard NK 19 or sample. Then, antibody-bound enzyme-hapten were separated from free one using anti-rabbit IgG immobilized on polystyrene balls. Activity of the enzyme on the solid phase was fluorometrically determined. The present immunoassay allows detection of as low as 10 pg of NK 19 in the biological fluids using 50 microliters of the samples. The accuracy and reproducibility were also quite satisfactory. By use of this assay, plasma levels of NK 19 after a single oral administration of 5 mg or 10 mg/body to human volunteers or intravenous injection of 2 micrograms/kg to rats could be determined.

Nizatidine, an H2-blocker. Its metabolism and disposition in man

The disposition of a single oral dose of about 150 mg of nizatidine, an H2-blocker, was studied in five men. Plasma levels of both parent drug and radioactivity peaked in 1-3 hr. Nizatidine accounted for about 60% of the plasma radioactivity. The t1/2 of nizatidine was 1.6 hr. About 35% of nizatidine became bound to plasma proteins in vitro, particularly to alpha-1-glycoprotein. Warfarin, acetaminophen, phenobarbital, propantheline, diazepam, and propranolol did not notably affect the amount of nizatidine bound. Two to 3 times more radioactivity was in plasma than in blood cells or saliva. Greater than 90% of the dose of nizatidine was excreted in urine, probably by glomerular filtration and active tubular secretion. Nizatidine accounted for about 65% of the urinary radioactivity. The major metabolite of nizatidine was N2-monodesmethylnizatidine; it represented about 7% of the nizatidine dosage. Another metabolite, constituting about 5% of the dose, is proposed to be nizatidine N2-oxide. Nizatidine sulfoxide also may be a minor metabolite of nizatidine.

Avoidable accumulation of potentially toxic levels of benzothiazoles in babies receiving intravenous therapy

Large amounts of a compound of exogenous origin were present in the serum of premature babies receiving prolonged intravenous therapy. This compound, identified as 2-(carboxymethylthio)benzothiazole (CMB) is derived from oxidation of 2-(hydroxyethylthio)benzothiazole which is leached out of rubber components of intravenous administration sets and syringes. Serum concentrations of CMB after prolonged exposure can exceed 500 mumol/l; concentrations over 35 mumol/l were observed in 91 babies over a period of twenty-seven months. This potentially toxic accumulation can be largely avoided by judicious choice of equipment.

Nizatidine, a new histamine H2-receptor antagonist, and hepatic oxidative drug metabolism in the rat: a comparison with structurally related compounds

The effects of nizatidine (a new H2-receptor antagonist) and of related compounds were studied on oxidative drug metabolism in the rat both in vivo and in vitro. Nizatidine is a structural analog of the H2-receptor antagonists ICI 125,211 (Tiotidine) and ranitidine (Zantac). Nizatidine (120 mg/kg, ip) had no effect on the [14C]aminopyrine (ABT) or [14C]caffeine breath (CBT) tests, nor on the clearance from plasma of aminopyrine despite high tissue and plasma concentrations of nizatidine. Binding of nizatidine (1 mM) to rat hepatic microsomal P-450 determined by spectral analysis was not observed. In vitro aminopyrine demethylation was inhibited by nizatidine only at high concentrations (Ki = 92 mM). Cimetidine, ICI 125,211, and imidazole bind avidly to rat hepatic microsomal cytochrome P-450 and are potent inhibitors of aminopyrine demethylation in vitro. Imidazole inhibited the aminopyrine breath test, while imidazole, ranitidine, and ICI 125,211 inhibited the caffeine breath in vivo. These data indicate that nizatidine has no acute inhibitory effect on hepatic oxidative drug metabolism in the rat, both in vitro and in vivo. The composite structural-activity data suggest that inhibition of in vivo oxidative drug metabolism by H2-antagonists may not depend primarily on either the imidazole ring side chain or the thiazole ring per se. Furthermore, the in vivo inhibition may not correlate with in vitro data.

Bioavailability studies with ciglitazone in beagles. I. Effect of a meal on the bioavailability of three ciglitazone dosage forms

Three separate Latin square crossover studies were conducted in beagles to examine the effect of a meal on the bioavailability of a ciglitazone tablet, suspension, and solution. In these studies, drug was administered to fasted animals with either 50 ml water or with 180 g Purina Dog Chow and 20 g butter. The data indicated that the meal significantly increased the AUC by about 40 per cent for both the tablet and the suspension but had no significant effect on the solution treatment. Comparisons across studies indicated low bioavailability in fasted animals from either the tablet or suspension relative to the solution. When drug was co-administered with a meal, however, bioavailability appeared to be independent of dosage form.

Bioavailability studies with ciglitazone in beagles. II. Effect of propantheline bromide and metoclopramide HCL on bioavailability of a tablet

Bioavailability studies in fasted dogs with ciglitazone (CGZ), an oral hypoglycemic agent, suggested that an absorption window could contribute to the poor oral availability of CGZ. If so, propantheline bromide (PPB) could increase the residence time of CGZ at absorption sites and increase its bioavailability. Using this rationale, a Latin square study was conducted with CGZ in fasted dogs (n = 10) using treatments of a single 125 mg tablet with and without 1.2 mg kg-1 i.m. PPB. PPB was given in a single dose 1 h prior to administration of CGZ. Plasma concentrations of CGZ were assayed by HPLC. PPB significantly increased the AUC of CGZ by a ratio of 1.2:1 (p less than 0.01). PPB also increased Tmax from 2-8 h (p less than 0.001), and appeared to produce first order absorption of CGZ. In a separate CGZ study using fasted dogs (n = 10), a single 125 mg tablet was administered with and without i.v. metoclopramide HC1 (MCP). A 10 mg dose of MCP was given 15 min prior to dosing with CGZ and repeated 1 h after dosing. MCP increases GI motility and was expected to decrease residence time of CGZ. MCP had no effect on Tmax, but significantly decreased AUC by 8 per cent (p = 0.05). MCP also reduced Cmax by 16 per cent (p = 0.06). Taken as a whole, these data suggest that the effect of meals to increase bioavailability of CGZ could be mediated at least in part, through an increase in GI residence time.

Isomodal column switching high-performance liquid chromatographic technique for the analysis of ciglitazone and its metabolites in human serum

The application of isomodal column switching high-performance liquid chromatography as an alternative to gradient elution was investigated for the analysis of ciglitazone , a potential oral antidiabetic agent, and its monohydroxyl metabolites in human serum. A high-performance liquid chromatographic apparatus was designed to perform on-line fractionation of the serum extract into non-polar (drug) and polar (metabolite) fractions which were then automatically routed into individually optimized, isocratic, reversed-phase high-performance liquid chromatographic systems for simultaneous analysis. Sample fractionation was performed with a reversed-phase guard column, and solvent routing was accomplished with microprocessor-controlled switching valves. Serum was extracted for analysis by a one-step mode sequencing procedure using disposable bonded-phase columns, and quantitation was accomplished with spiked serum standards. Performance specifications of the method were defined for precision, accuracy, linearity, and sensitivity. The column switching method was found to be both expedient and reliable, and it may have general utility for the routine, quantitative analysis of drug/metabolite mixtures that cannot be assayed by simple isocratic elution methods.

Inhibition of gastric secretion by a new H2-antagonist, YM-11170 in healthy subjects

Oral administration of YM-11170 (5-20 mg) inhibited both basal and tetragastrin-induced gastric secretion of acid and pepsin in healthy volunteers. YM-11170 was at least 20 times more potent than cimetidine in inhibiting stimulated acid secretion. The area under the plasma concentration of YM-11170 vs time curve correlated positively to both dose and percent inhibition of acid output in response to tetragastrin. YM-11170 significantly inhibited basal and stimulated acid secretion even 10 h after a 20-mg dose. A plasma level of YM-11170 required for 50% inhibition of stimulated acid secretion was found to be 13 ng/ml. These results indicate that YM-11170 is a very potent inhibitor of gastric acid secretion and that twice daily medication of 20 mg YM-11170 is recommendable for further antisecretory studies with ulcer patients.

Rat-plasma metabolites of ciglitazone, a new antidiabetic agent

The rat-plasma metabolites of ciglitazone, a new antidiabetic agent, were characterized by g.l.c.-mass spectrometry as follows: the 2'-, cis-3'-, trans-3'-, cis-4'- and trans-4'-hydroxycyclohexyl derivatives, and 3'- and 4'-oxocyclohexyl derivatives. The 2'-hydroxycyclohexyl derivative contains cis- and/or trans-isomers (unresolved). Three other metabolites, which were postulated to be dihydroxy derivatives of ciglitazone, were also detected in the plasma. All monohydroxy and monoketo metabolites showed hypoglycaemic and hypotriglyceridemic activities in genetically obese-diabetic mice. These results suggest that the pharmacological activities of ciglitazone are due, at least partly, to the metabolites.

Disposition in rats and dogs of ciglitazone, a new antidiabetic agent

Oral 14C-ciglitazone was well absorbed by rats to give a maximum plasma level at two hours and an apparent half-life of 4.9 h. In dogs, the plasma level of the compound, after oral administration, reached a plateau at one hour, persisted till ten hours and then declined with a half-life of 23.5 h. In rats, plasma levels of metabolites were higher than those of unchanged ciglitazone, whereas the reverse was noted in dogs. Plasma metabolites were the monohydroxycyclohexyl derivatives (mono-ol) and monoketocyclohexyl derivatives (mono-oxo), together with other components consisting largely of dihydroxycyclohexyl derivatives (di-ol) and unknown polar metabolites. Metabolites found in rats were pharmacologically active trans-4'-ol, 3'-ol, 4'-oxo, cis-4'-ol, 3'-oxo and 2'-ol in the decreasing order listed, and those in dogs were 3'- and/or 4'-ols. Ciglitazone was highly bound to plasma protein of both animals. After oral administration of 14C-ciglitazone to rats, 14C was widely distributed in tissues, with the highest concn. in the gastrointestinal tract, followed by liver, adipose, plasma, adrenal gland, kidney, pancreas, spinal cord, heart and lung, and the lowest in the brain. The concn. of 14C in erythrocytes of rats and dogs was very low, as was the level of 14C in rat fetuses. Elimination of 14C-ciglitazone was complete within 96 h in rats and 144 h in dogs. In both animals, the dosed 14C was excreted largely in faeces as metabolites, with the remainder appearing in urine. Biliary excretion and reabsorption of 14C were obvious in rats. In both rats and dogs, the major metabolites found in faeces were 3'- and/or 4'-ols and other components derived from bile, and those in urine were other components. On repeated oral administration of 14C-ciglitazone to rats for seven days, no accumulation of 14C occurred in plasma and tissues, and 97.5% of the dose was eliminated from the body within 96 h after the last administration.

Determination of thioproline in plasma using high performance liquid chromatography

The determination of thioproline in plasma of cancer patients, using high performance liquid chromatography (HPLC) is reported. As column support, a silica-bonded cation exchanger was used. Detection was performed at 205 nm. The detection limit of the method was 5 X 10(-6) M and the linear dynamic range was over 500. No sample clean-up procedure was necessary other than deproteinization of the plasma. The method was applied to the measurement of plasma drug levels in 3 patients, part of a clinical trial testing the effectiveness of thioproline as an anti-cancer agent.

Tiotidine, a new long-acting histamine H2-receptor antagonist: comparison with cimetidine

The effects of tiotidine, a new histamine H2-receptor antagonist, and cimetidine on food-stimulated gastric acid secretion were evaluated in duodenal ulcer patients. Homogenized steak meals were infused immediately after, 1 h after, 5 h after, and 10 h after an oral dose of medication, and food-stimulated acid secretion was measured by in vivo intragastric titration. Tiotidine and cimetidine had a similar onset of action; however, tiotidine was more potent and had a longer duration of effect. Increased potency was demonstrated by the fact that from 1 to 2 h after medication 150 mg tiotidine inhibited acid secretion to approximately the same extent as did 300 mg cimetidine, and by the fact that for a given percent inhibition of acid secretion, plasma tiotidine concentration was eight to nine times lower than plasma cimetidine concentration. Longer duration of effect was demonstrated by the fact that from 5 to 7 h after medication, acid secretion was inhibited by 80% and 97% with 150 and 300 mg tiotidine, respectively, whereas 300 mg cimetidine inhibited acid secretion by only 22%. Also, 10-12 h after medication, 150 and 300 mg tiotidine inhibited acid secretion by 22% and 53%, respectively, while 300 mg cimetidine had no inhibitory effect. The long duration of effect was due in part to increased potency and in part to a plateau in plasma concentration of tiotidine, which was maintained from 2 to 6 h after medication. Neither tiotidine nor cimetidine had a significant effect on food-stimulated gastrin release or gastric emptying of a nonabsorbable marker.

The metabolic fate of tizoprolique acid in baboons

1. After oral administration of the anti-lipolytic drug [14C]tizoprolique acid (2-propyl-5-carboxyl[4-14C]thiazole), to baboons (60 mg/kg), the radioactivity was well absorbed and rapidly excreted. During 6 and 24 h respectively, 60 +/- 25% (S.D.) and 90 +/- 2% were excreted in the urine. 2. Plasma concn. of 14C reached a max. (182 +/- 65, range 85-221 micrograms equiv./ml) at 1-1.5 h after an oral dose, and declined rapidly with an apparent half-life of about 0.5 h. A mean of 77 +/- 7% of the 14C in peak plasma samples was bound to plasma proteins, somewhat less than that of [14C]tizoprolique acid (84 +/- 5%). 3. Tissue concn. of 14 C were highest in an animal killed at 0.5 h after an oral dose, but were lower than those in plasma in all tissues examined except the kidneys. 4. The major metabolite of tizoprolique acid was its glycine conjugate, which accounted for about 80% dose excreted in the 24 h urine. About 2% dose was excreted as unchanged drug. About 70% and 20% respectively of plasma 14C were associated with the unchanged drug and its glycine conjugate during the period 15 min to 4 h after dosing.

Quantitative high-performance liquid chromatography determination of ICI 125,211 in plasma

A rapid, specific and quantitative high-performance liquid chromatography assay procedure is described for the determination of ICI 125, 211, (I), a guanidino-thiazole H2-receptor antagonist, in small samples of animal plasma. A 0.2 mL aliquot of plasma is extracted at pH 9 with ethyl acetate. After reconstitution in mobile phase, the sample is run on a silica column using ultraviolet detection at 280 nm. The procedure utilizes ICI 125, 253, (II), an analogous compound as the internal standard. The assay is linear (r2 = 0.99) and the recovery is 97.6 +/- 6.38% over a concentration range of 0.050-5.00 microgram/mL. Detection of levels as low as 0.025 microgram/mL has been documented in some animal study samples. Typical day-to-day precision is 3-9%, relative, for triplicate determinations of four concentrations in the 0.050-0.500 microgram/mL concentration range. Compound (I) is stable in whole blood for at least two hours at room temperature. Plasma concentrations remain constant for at least 48 hours at room temperature and six months at -5 degrees C. The whole blood/plasma ratio averages 0.97 +/- .073 for dog blood (N = 5). Pharmacokinetic data gathered for two dogs dosed orally with an HPMC-TWEEN 80 suspension of (I) at 40 mg/kg gave elimination half-life values of 4.3 and 7.4 hours. The specificity of the HPLC procedure has been confirmed by mass spectrometry.

Pharmacokinetics of clomethiazole in healthy adults

The systemic availability of clomethiazole was assessed by comparing blood levels after intravenous and oral administration. Clomethiazole was rapidly absorbed after oral administration to volunteers, particularly when administered as syrup. The fraction of the given dose that reached the systemic circulation after 1 capsule of clomethiazole (192 mg clomethiazole) was 0.25 +/- 0.18, after 2 capsules (384 mg clomethiazole) 0.38 +/- 0.18, and after 15 ml syrup (480 mg clomethiazole) 0.42 +/- 0.20. The time-blood concentration profiles were consistent with a two-compartment open model and the mean elimination half-lives of 3.6--5.0 hrs. were found for the different formulations and administration routes. Elimination half-lives showed little variation and a mean systemic clearance of 49 ml/min./kg was found for clomethiazole after intravenous administration. Clomethiazole is bound to human plasma proteins (63.4 +/- 1.6%, 37 degrees), a binding which is not affected by Vacutainer sample tubes. The blood/plasma distribution of clomethiazole was 0.76 +/- 0.02 at 37 degrees. A sensitive mass fragmentographic assay for the determination of clomethiazole in blood/plasma down to levels of 1 ng/ml (6.2 nmol/l) is described.

Thiamine whole blood pharmackinetics in rats using both a specific S-thiamine liquid scintillation assay and the thiochrome fluorescence assay

The study of factors altering the CNS and GI absorption of thiamine in rats required the development of a specific assay for thiamine from 100-microliter samples of blood and plasma and small quantities of tissue. The specific thiochrome fluorescence assay for thiamine was modified to handle microsamples and to use S-thiamine. This sensitive and specific radioassay using S-thiamine gave pharmacokinetic parameters for 4-mg/kg iv doses of thiamine in rats equivalent to those using the less sensitive thiochrome fluorescence assay. The new assay, because of its lower limit of detection, allowed the study of the time profile of thiamine after a 1-mg/kg iv dose in rats. Such a time profile could not have been followed using the standard thiochrome fluorescence assay.

The isolation and identification of 2-methyl-2,4-thiazolidine dicarboxylate as a by-product in the conversion of cysteine to glucose in the perfused rat liver

Cysteine is one of the more toxic amino acids, however the toxic agent associated with cysteine toxicity has not been identified. Recently it was shown that 2-methyl-2,4-thiazolidine dicarboxylate (MTD) was formed fromcy steine by rat liver and would be toxic to the rat. This suggested that MTD formed by the rat liver can be produced both enzymatically and by chemical interaction between cysteine and another compound, possibly pyruvate. When MTD (1 mmol/rat) was injected into 6 rats, two of them died. Only 70-80% of the MTD was excreted within 24 h, suggesting an accumulation of MTD. It is possible the MTD could accumulate in the tissues until it reached a toxic concentration. Whether this could account for the death associated with cysteine toxicity is unknown.