Effect of aging on hepatic elimination of cimetidine and subsequent interaction of aging and cimetidine on aminopyrine metabolism

Aging and cimetidine may each impair hepatic microsomal drug metabolism. To test if and by what mechanisms advanced age may increase sensitivity to the inhibitory effects of cimetidine, the interaction of these two factors with aminopyrine metabolism in the rat was studied using a correlative approach. Initial studies using the aminopyrine breath test indicated that a 40 mg/kg dose of cimetidine, i.p., impaired the 14CO2 exhaled by up to 76% more in aged (26-month) than in young (3- to 4-month-old) rats. Using an isolated liver perfusion to dissect out hepatic components of this phenomenon, it was found that various doses of cimetidine impaired aminopyrine clearance to a greater degree (P less than 0.05) in aged than in young livers. However, cimetidine metabolism in this system ranged from 36 to 78% less in aged versus young livers (P less than 0.05). Subsequent in vitro studies indicated that microsomes isolated from aged livers also averaged a 76% lower rate of cimetidine metabolism (P less than 0.05). A fixed cimetidine concentration, however, inhibited aminopyrine demethylation to the same degree in aged versus young rats (P less than 0.05). In vivo pharmacokinetics showed an age-related decrease in both aminopyrine and cimetidine systemic clearance. In the young rat the liver contributed about 30% to total systemic clearance of cimetidine. In the aged rat, all clearance was renal. Despite a decrease in glomerular filtration rate, net tubular cimetidine secretion was well-maintained. Despite this, absence of the hepatic component resulted in decreased overall systemic clearance of the drug in aged rats. It is concluded that (1) the aged rat liver exhibits impaired cimetidine metabolism, resulting in decreased overall systemic clearance of the drug despite normal net renal tubular secretion, (2) there is no age-related enhanced sensitivity to cimetidine of the hepatic microsomal oxidizing system using aminopyrine as the probe drug, and (3) the larger inhibition of aminopyrine metabolism in aged rats following various doses of cimetidine is due to decreased overall cimetidine clearance, resulting in higher concentrations of the inhibitor in the liver of aged rats.

Pentopril-cimetidine interaction caused by a reduction in hepatic blood flow

The interactive effects of the coadministration of steady-state cimetidine and single-dose pentopril, an angiotensin converting enzyme inhibitor, on the pharmacokinetic disposition of each other were studied in humans. Cimetidine reduced the clearance of pentopril by 11 to 14%. This reduction in clearance was shown to be caused by a reduction in liver blood flow probably mediated through H2 receptor blockade. Meanwhile pentopril induced the oral clearance of cimetidine by 21%, presumably by a reduction in the bioavailable fraction of cimetidine. The mechanism of this interaction is unknown.

The role of iron chelates on the selectivity of Fenton reagent in hydroxylation, N-demethylation, and sulfoxidation of cimetidine: a novel biomimetic model for the regioselectivity of cytochrome P-450

The effect of iron chelates on the reaction of cimetidine with Fenton reagent [Fe(II)/H2O2] has been investigated. Iron chelates with high affinity to ferrous ions inhibited this reaction. However, iron chelates with high affinity to ferric ions selectively promote either hydroxylation, N-demethylation, or sulfoxidation of cimetidine. These results indicate that the oxidation of cimetidine with hydrogen peroxide activated by various chelated ferrous ions serves as a biomimetic model for the regioselectivity of multiple forms of cytochrome P-450 in the metabolism of cimetidine.

Selective inhibition of drug oxidation after simultaneous administration of two probe drugs, antipyrine and tolbutamide

The effects of sulphaphenazole, cimetidine and primaquine on the disposition of antipyrine and tolbutamide in healthy volunteers have been investigated. The model substrates were administered simultaneously in order more clearly to define any selective effects of the potential inhibitors. Sulphaphenazole produced a significant increase in the half-life of tolbutamide (7.10 to 21.50 h) and a corresponding decrease in its clearance (0.260 to 0.084 ml.min-1.kg-1). Clearance to hydroxytolbutamide (OHTOL) and carboxytolbutamide (COOHTOL) was also significantly decreased. In contrast, sulphaphenazole had no effect on the disposition of antipyrine. Administration of cimetidine did not significantly alter the disposition of either model drug. However, a 1.6-times higher dose of cimetidine did increase the half lives both of tolbutamide and antipyrine (6.21 to 9.04 h and 14.2 to 19.2 h, respectively) and decrease their clearance (0.226 to 0.148 and 0.50 to 0.31 ml.min-1 kg-1, respectively). Clearance to OHTOL and hydroxymethylantipyrine (HMA) was reduced. A single dose of primaquine had no demonstrable effect on tolbutamide disposition whereas the half-life of antipyrine was increased (12.1 to 15.0 h) and its clearance decreased (0.63 to 0.38 ml.min-1.kg-1). The partial clearance to HMA, 4-hydroxyantipyrine (OHA) and norantipyrine (NORA) was also significantly reduced. The two main inferences are first, that tolbutamide and antipyrine are metabolised by different forms of cytochrome P-450, and second that a battery of model substrates is needed to investigate the inhibitory effects of a drug in man.

Identification of structural characteristics of some potential H2-receptor antagonists that determine the interaction with rat hepatic P-450

Several potential H2-receptor antagonists have been tested in vitro, using liver microsomal preparations from untreated rats, in order to study their interaction with P-450. The aim of this investigation was to establish structure-activity relationships for the P-450-inhibition developed by cimetidine and related drugs. Most of the compounds tested demonstrate an inhibitory activity and a binding ability to P-450, via type II (ligand type) binding. Our results strongly indicate that the cyano-guanidine moiety is an essential structural feature for both the inhibition of a ferrocytochrome P-450-metabolic intermediate complex formation occurring during the metabolism of tofenacine, and the binding of the compounds to the heme iron of P-450. The presence of an imidazole group is not necessary for these activities. Furthermore, it is pointed out that the lipophilic character of the cyano-guanidine side chain contributes to the interaction of the test compounds with P-450, since a trend for a parabolic relationship between lipophilicity and inhibitory activity or binding ability is observed. Finally, under the experimental conditions used, no increase of the inhibitory activity of cimetidine on the metabolism of tofenacine and 7-ethylresorufin is observed after preincubation of rat liver microsomes with cimetidine, confirming earlier results in similar studies.

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.

Evidence for cytosolic glutathione transferase-mediated denitrosation of nitrosocimetidine and 1-methyl-2-nitro-1-nitrosoguanidine

Nitrosocimetidine (NC) and 1-methyl-2-nitro-1-nitrosoguanidine (MNNG) are closely related N-nitrosamidines. NC is the nitrosated derivative of cimetidine (Tagamet), an orally administered compound used extensively in the treatment of gastric ulcers. MNNG is a potent carcinogen capable of initiating tumors close to the site of administration and used experimentally to produce stomach cancer. It has become evident that the primary metabolic fate of both of these agents is denitrosation. We have discovered an activity in the cytosol fraction of hamster liver which is capable of denitrosating these nitrosamidines with an efficiency approaching 100%. The activity is heat sensitive and requires reduced glutathione as a cofactor. Inhibition of the denitrosating activity with compounds which inhibit in parallel the conjugation of glutathione with 1-chloro-2,4-dinitrobenzene (CDNB) provides evidence that the activity is glutathione transferase. One molecule of reduced glutathione is consumed in each denitrosation event. Nitrite is formed as denitrosation proceeds with a yield equivalent to 25-50% of the denitrosated product produced. Glutathione disulfide is a minor reaction product, representing 3% of the denitrosation product yield in the MNNG case and 12% in the NC case. Thus far in our survey of N-nitrosamines, N-nitrosamides and N-nitrosamidines, only the nitrosamidines appear to be vulnerable to the cytosolic denitrosating activity. In an attempt to evaluate the importance of the glutathione-dependent reaction in the intact hamster, we have depleted glutathione by pretreatment with the commonly used agents diethyl maleate (DEM) and L-buthionine-S,R-sulfoximine (L-BSO). Nitroso compound was administered i.v. and the circulating blood levels of intact and denitrosated compound 5 min after dosing quantified. NC- and MNNG-derived methylation of organ DNA was also monitored. Pretreatment had no effect on the cytosolic denitrosating or CDNB-conjugating activities. L-BSO pretreatment had no apparent effect on the denitrosative metabolism of NC or MNNG. With DEM pretreatment we obtained clear indications of a decreased rate of denitrosation and observed a 10-fold increase in MNNG-derived liver DNA methylation. The differential effects of these pretreatments are taken as an indication that DEM-sensitive processes other than those requiring glutathione dominate N-nitrosamidine denitrosation in the hamster.

Interactions of the histamine H2-receptor antagonist etintidine with rat liver cytochrome P-450: a comparison with cimetidine

The two imidazole histamine H2-receptor antagonists etintidine and cimetidine interact with the rat liver microsomal cytochrome P-450. From type II spectral changes follows that the affinity of rat liver microsomal preparations for etintidine is about 5 times as high as for cimetidine when comparing both high and low affinity binding sites. After pretreatment with phenobarbital etintidine inhibited benzphetamine N-demethylation competitively (app. Ki: 4.0 mmol/l). Cimetidine inhibited benzphetamine N-demethylation in the same range. After pretreatment with phenobarbital both drugs inhibited the oxidation of benzo(a)pyrene for which etintidine showed a higher inhibitory potency than cimetidine. However, this oxidation could not be inhibited when microsomes of 5,6-benzoflavone pretreated rats were used. After pretreatment with 5,6-benzoflavone only etintidine but not cimetidine inhibited the O-deethylation of ethoxyresorufin competitively (app. Ki: 0.2 mmol/l). Etintidine and cimetidine were metabolized by rat liver microsomes to their corresponding sulphoxides. In conclusion, etintidine may cause mainly the same drug interactions as cimetidine but seems to be a more potent inhibitor.

Effects of H2-receptor antagonists on 3H-cimetidine binding and histamine-stimulation of cellular cAMP in isolated guinea pig gastric glands

3H-Cimetidine binding to plasma membranes of isolated guinea pig gastric glands was investigated, and the effects of five H2-receptor antagonists on 3H-cimetidine binding and histamine stimulation of cellular cAMP were compared. Of the five cations tested, Cu++ markedly increased specific 3H-cimetidine binding. 3H-Cimetidine had high affinity (Kd = 0.41 x 10(-6) M) and low affinity (Kd = 12.8 x 10(-6) M) binding sites. Cimetidine and etintidine were potent inhibitors of 3H-cimetidine binding, while famotidine, ranitidine and TZU-0460 were not. Histamine stimulation of cellular cAMP was competitively inhibited by H2-receptor antagonists yielding pA2 values of 6.41 for cimetidine, 6.82 for etintidine, 6.87 for ranitidine, 6.94 for TZU-0460 and 7.60 for famotidine. Because the KB value (log KB = -pA2) of 0.39 x 10(-6) M for cimetidine is close to the Kd value for the high affinity 3H-cimetidine binding site, it is presumed to represent a part of the H2-receptor, and the relative potency of etintidine against cimetidine in inhibiting 3H-cimetidine binding is similar to that in inhibiting histamine stimulation of cellular cAMP. These results suggest that imidazole-derived H2-receptor antagonists (cimetidine and etintidine) and non-imidazole H2-receptor antagonists (famotidine, ranitidine and TZU-0460) compete with histamine at different sites on the H2-receptor of the gastric glands.

Cimetidine does not impair lung host defense in experimental pneumococcal pneumonia

Normal CD-1 mice were administered cimetidine (400 mg/kg/24 h) or control solution by subcutaneous injection and inoculated intratracheally with type S Streptococcus pneumoniae in order to evaluate the effect of the histamine H2-receptor antagonist on pulmonary antibacterial mechanisms. Therapy with cimetidine did not influence overall survival rates. After challenge with 1 X 10(6) colony-forming units (cfu), the eradication of viable pneumococci from the lungs (pulmonary clearance), the generation of a local inflammatory response, and the prevalence of bacteremia were similar in both study groups. Cimetidine also failed to influence pulmonary antimicrobial systems after challenge with lower bacterial inocula (5 X 10(4) and 5 X 10(3) cfu). Further, treatment with cimetidine had no effect on the in vivo phagocytic or bactericidal activity of resident murine alveolar macrophages. Thus, in this animal model, cimetidine does not disrupt host defenses of the lower respiratory tract.

Cimetidine transport in isolated luminal membrane vesicles from rabbit kidney

Experiments were conducted to study the transport of the histamine H2-receptor antagonist, cimetidine, in luminal membrane vesicles prepared from rabbit renal cortex. Cimetidine accumulated in the vesicles with time. Cimetidine uptake was sensitive to changes in vesicle size, suggesting that the compound is transported into an osmotically reactive intravesicular space. Its rate of uptake could be described by both a saturable and a nonsaturable process. The Km was 4.6 +/- 4.0 microM and the Vmax was 6.8 +/- 2.3 pmol X s-1 X mg protein-1 (mean +/- SD, n = 4). N1-methylnicotinamide (NMN), cimetidine, cimetidine sulfoxide, and ranitidine inhibited the uptake of cimetidine. Cimetidine uptake in the presence of an outwardly directed proton gradient was enhanced in vesicles preloaded with a higher concentration of unlabeled cimetidine (2.4 X 10(-4) M). An outwardly directed proton gradient enhanced the uptake of cimetidine to values exceeding its equilibrium accumulation. Uptake stimulated in this way could be inhibited by the cation, NMN, the bases, ranitidine, and cimetidine sulfoxide, and interestingly, by the anion, probenecid. The effect of probenecid did not appear to be due to nonspecific effects on membrane binding, membrane potential, or vesicle size. These data are consistent with data obtained in isolated perfused proximal tubules, demonstrating that probenecid inhibits cimetidine transport. The data in this study suggest that the effect of probenecid on cimetidine transport specifically involves the transporter in the luminal membrane.

Cimetidine pharmacokinetics in patients with Zollinger-Ellison syndrome

Although most patients with Zollinger-Ellison syndrome can be effectively treated with histamine H2-receptor antagonists, many patients require large doses of drug to inhibit gastric acid secretion adequately. The purpose of the present study was to compare the pharmacokinetics of a 1200-mg oral dose of cimetidine in 9 patients with Zollinger-Ellison syndrome requiring more than 2.4 g/day of cimetidine with 5 age-matched normal volunteers receiving intravenous pentagastrin infusions. Poor responsiveness to cimetidine in patients with Zollinger-Ellison syndrome has several different causes. The concentration of cimetidine in the blood required to inhibit gastric acid secretion by 50% was markedly increased in 3 of the patients with Zollinger-Ellison syndrome, suggesting parietal cell resistance. One patient showed a substantial decrease in cimetidine absorption and 4 patients had delayed cimetidine absorption. Thus 7 of the 9 patients with Zollinger-Ellison syndrome who required more than 2.4 g/day of cimetidine to inhibit gastric acid secretion had abnormal cimetidine pharmacokinetics.

Relation between pharmacological response and receptor binding with histamine blocking drugs. Irreversible antagonism of three analogues of mifentidine on right atrium and cerebral cortex of the guinea-pig

The effects of the H2-receptor antagonists cimetidine, ranitidine, mifentidine and three analogues of mifentidine, were studied on the spontaneously beating right atrium (H2-antagonism) and membranes of the cerebral cortex (displacement of 3H-tiotidine), both obtained from the male guinea-pig. The choice of these compounds was based on preliminary experiments in which some mifentidine analogues were shown to displace tiotidine from the H2-receptor in a deviant manner. In the present study we investigated the relation between pharmacological response and receptor binding, also testing the degree of irreversible antagonism of these compounds in the atrium (functional) and cerebral cortex (binding) model. Our data indicate that a relation between the two different approaches for measuring the effect on the H2-receptor can be found, although some differences emerged as well.

Pharmacokinetic interactions of cimetidine 1987

The number of studies on drug interactions with cimetidine has increased at a rapid rate over the past 5 years, with many of the interactions being solely pharmacokinetic in origin. Very few studies have investigated the clinical relevance of such pharmacokinetic interactions by measuring pharmacodynamic responses or clinical endpoints. Apart from pharmacokinetic studies, invariably conducted in young, healthy subjects, there have been a large number of in vitro and in vivo animal studies, case reports, clinical observations and general reviews on the subject, which is tending to develop an industry of its own accord. Nevertheless, where specific mechanisms have been considered, these have undoubtedly increased our knowledge on the way in which humans eliminate xenobiotics. There is now sufficient information to predict the likelihood of a pharmacokinetic drug-drug interaction with cimetidine and to make specific clinical recommendations. Pharmacokinetic drug interactions with cimetidine occur at the sites of gastrointestinal absorption and elimination including metabolism and excretion. Cimetidine has been found to reduce the plasma concentrations of ketoconazole, indomethacin and chlorpromazine by reducing their absorption. In the case of ketoconazole the interaction was clinically important. Cimetidine does not inhibit conjugation mechanisms including glucuronidation, sulphation and acetylation, or deacetylation or ethanol dehydrogenation. It binds to the haem portion of cytochrome P-450 and is thus an inhibitor of phase I drug metabolism (i.e. hydroxylation, dealkylation). Although generally recognised as a nonspecific inhibitor of this type of metabolism, cimetidine does demonstrate some degree of specificity. To date, theophylline 8-oxidation, tolbutamide hydroxylation, ibuprofen hydroxylation, misonidazole demethylation, carbamazepine epoxidation, mexiletine oxidation and steroid hydroxylation have not been shown to be inhibited by cimetidine in humans but the metabolism of at least 30 other drugs is affected. Recent evidence indicates negligible effects of cimetidine on liver blood flow. Cimetidine reduces the renal clearance of drugs which are organic cations, by competing for active tubular secretion in the proximal tubule of the kidney, reducing the renal clearances of procainamide, ranitidine, triamterene, metformin, flecainide and the active metabolite N-acetylprocainamide. This previously unrecognised form of drug interaction with cimetidine may be clinically important for both parent drug, and metabolites which may be active.(ABSTRACT TRUNCATED AT 400 WORDS)

Dose-dependent pharmacokinetics of cimetidine in the rat

The pharmacokinetics of cimetidine were studied in the rat after intraperitoneal administration of 10, 40 and 100 mg/kg. The area under the plasma concentration-time curve increased more than proportionately with dose. The total plasma clearance of cimetidine decreased as the dose increased (4.11 to 2.21 l/h per kg) with a consequent increase in half-life (24.0 to 37.9 min) but no change in volume of distribution (mean 2.31 l/kg). The fraction of dose excreted unchanged increased slightly with dose (0.37 to 0.45), whereas the fraction excreted as the sulphoxide metabolite decreased significantly with increased dose (0.35 to 0.14). Both the renal clearance (1.52 to 0.99 l/h per kg) and the formation clearance of the sulphoxide metabolite (1.45 to 0.30 l/h per kg) decreased with increasing dose. Residual clearance, calculated as the difference between total clearance and the sum of renal and metabolic clearance, did not change with dose (mean 1.08 l/h per kg). The formation clearance of the sulphoxide metabolite became saturated at a lower cimetidine concentration than the renal clearance.

Cimetidine transport in rabbit renal cortical brush-border membrane vesicles

Cimetidine is an organic cation and commonly prescribed drug that is eliminated primarily by proximal renal tubular secretion. The present studies evaluated cimetidine transport in rabbit renal cortical brush-border membrane vesicles (BBMV). [3H]Cimetidine uptake varied inversely with media osmolarity and was stimulated with uphill transport above equilibrium values (overshoot) produced by an initial proton gradient directed from the vesicle interior outwardly. Uphill transport occurred earlier and was of greater magnitude at 25 degrees C than at 5 degrees C. pH-stimulated [3H]cimetidine uptake was inhibited by excess nonradiolabeled cimetidine, quinidine, and procainamide but was affected little by probenecid. Tetraethylammonium inhibited cimetidine uptake in the presence and absence of an initial proton gradient, indicating that nonionic diffusion and simple diffusion cannot totally account for cimetidine transport in BBMV. The protonophore carbonyl cyanide trifluoromethoxyphenylhydrazone (FCCP) inhibited pH-stimulated cimetidine uptake but had no effect on uptake occurring in the absence of an initial pH gradient. Preloading BBMV with an excess of procainamide enhanced cimetidine uptake. However, in the presence of FCCP, the combination of FCCP and valinomycin, or nigericin the effect of preloading with procainamide was diminished, suggesting that the apparent countertransport of cimetidine produced by procainamide was indirect and due to generation of a transvesicular proton gradient. These results are consistent with the hypothesis that cimetidine is transported across BBMV by organic cation-proton exchange.

Relation between transport maxima and inhibition of organic cation excretion in the chicken kidney

The ability of several organic cations to inhibit differentially the renal excretion of two prototypical organic cations, tetraethylammonium (TEA) and N1-methylnicotinamide (NMN), was investigated using the Sperber technique in chickens. TEA and NMN excretion were inhibited by the following organic cations in order of decreasing potency: quinine, TEA and NMN. The respective competitive potency of these substances was related inversely to their maximum tubular transport rates (Tm). Regardless of inhibitor used (quinine, TEA or NMN), NMN excretion was always inhibited more easily than TEA excretion. In addition, TEA excretion was suppressed more easily than cimetidine excretion by the competitive inhibitor quinine. The Tm of cimetidine was determined to be less than the Tm of TEA, which in turn is less than that of NMN. These results indicate that the Tm of an organic cation is related inversely to its inhibitory potency and related directly to its susceptibility to inhibition, reflecting different affinities of organic cations for the same carrier-mediated transport system.

Species differences in blood-mediated nitrosocimetidine denitrosation

Nitrosocimetidine (NC) is the nitrosated derivative of cimetidine (Tagamet), a p.o. administered drug used widely in the treatment of stomach ulcers. NC is capable of methylating DNA in vitro and in cultured cells in a manner similar to that of the laboratory carcinogens 1-methyl-2-nitro-1-nitrosoguanidine and methylnitrosourea (MNU) and gives positive indications in short-term in vitro tests for genotoxicity, generally held to be prognostic of compound carcinogenic potential. Nevertheless NC has been found to be a weak or non-carcinogen in the rat and mouse model systems and to produce minimal levels of tissue DNA alkylation when dosed p.o. or i.v. to rats. The results from our earlier experiments (D. E. Jensen, Cancer Res., 43: 5258-5267, 1983) indicated that compound denitrosation is the primary fate of NC in the rat and suggested that denitrosation is the blood, mediated by hemoglobin sulfhydryl residues, is perhaps the major detoxification mechanism. We now report that whole blood and hemoglobin isolated from various mammalian species differ in their capacity for NC degradation rate enhancement and for compound denitrosation. The observed whole blood activity in the degradation reaction (rat greater than mouse/guinea pig greater than human/hamster) paralleled the hemoglobin activity. The NC half-life in isolated rat blood, 37 degrees C, was found to be about 2 min and in hamster or human blood 27 min. For reference, the MNU half-life in isolated blood is 8 min. Compound denitrosation accounted for at least 75% of the degradation in rat blood and 40 to 55% in human and hamster blood. Parallel NC denitrosation activity was found in the various hemoglobin preparations. The NC degradation rates in the presence of the several hemoglobin species were roughly proportional to the number of sulfhydryls on the hemoglobin tetramers available for reaction with p-chloromercuribenzoate and approximated the rates observed in solutions containing equivalent concentrations of L-cysteine. The percentage of total decomposition due to compound denitrosation in the presence of rat hemoglobin, 95%, was found to be unique relative to the L-cysteine-mediated reactions (about 35%) and the reactions studied over the pH range 6 through 10, the denitrosation process never accounted for more than 50% of the total degradation. Chemically blocking the sulfhydryls on human hemoglobin using iodoacetamide deleted the NC degradation rate enhancement. We found no evidence for nitrosylhemoglobin formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Approaches to the problem of individual doxorubicin dosing schedules

Doxorubicin is one of the most commonly used antineoplastic agents today. Dosing schedules are inexact and there remains a need to develop predictive models for its administration. Data from prior work in humans is difficult to interpret because of poor patient selection, poor drug assays, lack of knowledge of metabolite toxicity, concurrent treatment with other hepatically metabolized drugs, and individual pharmacogenetics which are poorly described. We have developed a rabbit model of in vivo drug pharmacokinetics in the setting of enzyme inhibition and sublethal hepatocellular necrosis. Our data suggest that the rabbit may be used as a model of hepatic drug metabolism and that changes in drug pharmacokinetics and pharmacodynamics in isolated hepatic disease may be simulated in the rabbit. The results obtained may be applied in more directed and controlled studies in humans.

Cimetidine inhibits cerebral and hepatic mitochondrial respiration in rat

The presence of cimetidine in the incubation medium of rat brain mitochondria caused decreased oxygen uptake, especially during oxidative phosphorylation (state 3). This inhibition of the respiratory control and of ATP synthesis was dose-dependent. The same observations were made for hepatic mitochondria. The significance of these results is discussed in terms of both the neurological side-effects of cimetidine and its effect on regulatory mechanisms of cerebral or hepatic blood flow.

Transport of cimetidine by the rat choroid plexus in vitro

To characterize the transport system of cimetidine, an organic cation, in the blood-cerebrospinal fluid barrier, the accumulation of cimetidine by the isolated rat choroid plexus was examined. Accumulation of cimetidine was against a concentration gradient via a saturable process (Km = 53 microM, Vmax = 12 nmol/ml/min) that was inhibited by sulfhydryl reagents (p-hydroxymercuribenzoate), metabolic inhibitors (KCN and 2,4-dinitrophenol) and hypothermia (Q10 = 4.5), but did not require inward Na+ gradient. Organic cations such as 1N-methylnicotinamide, tetraethylammonium, choline, histamine and creatinine did not affect the accumulation of cimetidine at the concentration of 1 mM. Cimetidine did not affect the accumulation of tetraethylammonium. More lipophilic cations such as quinidine and quinine inhibited not only the accumulation of cimetidine but also that of an organic anion, benzylpenicillin, although the inhibitory mechanisms are not known. One millimolar of organic anions, such as 5-hydroxyindoleacetic acid, p-aminohippuric acid, homovanillic acid, salicylic acid and benzylpenicillin, inhibited the accumulation of cimetidine. Furthermore, the accumulation of organic anions (benzylpenicillin and salicylic acid) showed saturability and was inhibited by cimetidine. Cimetidine and the organic anions thus showed a mutual inhibition. Oligopeptides also inhibited the accumulation of cimetidine. These findings suggested that cimetidine transport in the choroid plexus is via carrier-mediated active transport process, but does not require inward Na+ gradient. This transport is inhibited by several compounds with different properties like oligopeptides, lipophilic cations and organic anions, although the inhibitory mechanism is not known.

3H-tiotidine binding to guinea-pig cortical and striatal membranes

3H-Tiotidine has been identified as a suitable radioligand for the H2-receptor. We have confirmed and extended structure-binding affinity studies in the guinea-pig cortex, and established a structure-binding affinity relationship consistent with the H2-receptor in guinea-pig striatum. Cimetidine-displaceable 3H-tiotidine binding was observed also in the nucleus accumbens.