Active Pharmaceutical Ingredient Uptake by Zebrafish (Danio rerio) Oct2 (slc22a2) Transporter Expressed in Xenopus laevis Oocytes

Uptake of active pharmaceutical ingredients (APIs) across the gill epithelium of fish is via either a passive or facilitated transport process, with the latter being more important at the lower concentrations more readily observed in the environment. The solute carrier (SLC) 22A family, which includes the organic cation transporter OCT2 (SLC22A2), has been shown in mammals to transport several endogenous chemicals and APIs. Zebrafish oct2 was expressed in Xenopus oocytes and the uptake of ranitidine, propranolol, and tetraethylammonium characterized. Uptake of ranitidine and propranolol was time- and concentration-dependent with a k_m and V_max for ranitidine of 246 µM and 45 pmol/(oocyte × min) and for propranolol of 409 µM and 190 pmol/(oocyte × min), respectively. Uptake of tetraethylammonium (TEA) was inhibited by propranolol, amantadine, and cimetidine, known to be human OCT2 substrates, but not quinidine or ranitidine. At external media pH 7 and 8 propranolol uptake was 100-fold greater than at pH 6; pH did not affect ranitidine or TEA uptake. It is likely that cation uptake is driven by the electrochemical gradient across the oocyte. Uptake kinetics parameters, such as those derived in the present study, coupled with knowledge of transporter localization and abundance and API metabolism, can help derive pharmacokinetic models. Environ Toxicol Chem 2022;41:2993-2998. © 2022 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Uptake and effects of a mixture of widely used therapeutic drugs in Eruca sativa L. and Zea mays L. plants

Pharmaceutically active compounds (PACs) are continuously dispersed into the environment due to human and veterinary use, giving rise to their potential accumulation in edible plants. In this study, Eruca sativa L. and Zea mays L. were selected to determine the potential uptake and accumulation of eight different PACs (Salbutamol, Atenolol, Lincomycin, Cyclophosphamide, Carbamazepine, Bezafibrate, Ofloxacin and Ranitidine) designed for human use. To mimic environmental conditions, the plants were grown in pots and irrigated with water spiked with a mixture of PACs at concentrations found in Italian wastewaters and rivers. Moreover, 10× and 100× concentrations of these pharmaceuticals were also tested. The presence of the pharmaceuticals was tested in the edible parts of the plants, namely leaves for E. sativa and grains for Z. mays. Quantification was performed by liquid chromatography mass spectroscopy (LC/MS/MS). In the grains of 100× treated Z. mays, only atenolol, lincomycin and carbamazepine were above the limit of detection (LOD). At the same concentration in E. sativa plants the uptake of all PACs was >LOD. Lincomycin and oflaxacin were above the limit of quantitation in all conditions tested in E. sativa. The results suggest that uptake of some pharmaceuticals from the soil may indeed be a potential transport route to plants and that these environmental pollutants can reach different edible parts of the selected crops. Measurements of the concentrations of these pharmaceuticals in plant materials were used to model potential adult human exposure to these compounds. The results indicate that under the current experimental conditions, crops exposed to the selected pharmaceutical mixture would not have any negative effects on human health. Moreover, no significant differences in the growth of E. sativa or Z. mays plants irrigated with PAC-spiked vs. non-spiked water were observed.

Experimental and modeling study on removal of pharmaceutically active compounds in rotating biological contactors

The aim of this work was to study the biological removal of pharmaceutical compounds in rotating biological contactors (RBCs) under continuous operation. A two-stage RBC was used, providing a total surface area of 1.41 m(2). Four pharmaceuticals of different therapeutic classes; caffeine, sulfamethoxazole, ranitidine and carbamazepine, were studied. Six experimental scenarios were applied to the RBC-system by varying substrates' loadings (12-54 gCOD/d), volumetric flow rate (2-5L/d), and pharmaceuticals' concentration (20-50 μg/L). The different conditions resulted to different solid retention times (SRT: 7-21 d) in each scenario. The increase of SRT due to variations of the operating conditions seemed to have a positive effect on pharmaceuticals' removal. Likewise, a negative correlation was observed between substrates' loading and pharmaceuticals' removal. An increase of initial pharmaceuticals' concentration resulted to decrease of SRT and pharmaceuticals' removal, suggesting a toxic effect to the biofilm. The maximum removals achieved were greater than 85% for all pharmaceuticals. Finally, a mathematical model which includes biofilm growth, substrates' utilization and pharmaceuticals' elimination was developed. The model predicts the contribution of sorption and biodegradation on pharmaceuticals' elimination taking into account the diffusion of pharmaceuticals inside biofilm.

Controlled delivery of ranitidine in the stomach using magnetic field

An attempt has been made to localize ranitidine loaded microspheres in the stomach by magnetic means. Since ranitidine undergoes metabolism by microbial enzymes in the intestine, it is ideal to localize the controlled drug delivery system within the stomach to get uniform release and absorption of the drug for the desired period. Gelatin magnetic microspheres loaded with 9.1, 17.9, 26.3 and 33.3% w/w of ranitidine hydrochloride were prepared by emulsification-cross linking technique. The formulated microspheres were characterized by magnetite content, particle size and in vitro drug release. The efficiency of microspheres to be localized in the stomach is tested in vivo in rats. The prepared microspheres were spherical and had a size distribution from 10 to 105 microm. The in vitro study revealed the capability of microspheres to release the drug over a period of 8 to 12 hours, depending on drug loading. The release was found to be diffusion controlled and followed fickian diffusion principle. The in vivo study showed the efficiency of microspheres to be retained in the stomach over a period of 8 hours.

Spectrophotometric determination of H(2)-receptor antagonists via their oxidation with cerium(IV)

A simple, accurate and sensitive spectrophotometric method has been developed and validated for determination of H(2)-receptor antagonists: cimetidine, famotidine, nizatidine and ranitidine hydrochloride. The method was based on the oxidation of these drugs with cerium(IV) in presence of perchloric acid and subsequent measurement of the excess Ce(IV) by its reaction with p-dimethylaminobenzaldehyde to give a red colored product (lambda(max) at 464nm). The decrease in the absorption intensity of the colored product (DeltaA), due to the presence of the drug was correlated with its concentration in the sample solution. Different variables affecting the reaction were carefully studied and optimized. Under the optimum conditions, linear relationships with good correlation coefficients (0.9990-0.9994) were found between DeltaA values and the concentrations of the drugs in a concentration range of 1-20microgml(-1). The assay limits of detection and quantitation were 0.18-0.60 and 0.54-1.53microgml(-1), respectively. The method was validated, in terms of accuracy, precision, ruggedness and robustness; the results were satisfactory. The proposed method was successfully applied to the determination of the investigated drugs in pure and pharmaceutical dosage forms (recovery was 98.3-102.6+/-0.57-1.90%) without interference from the common excipients. The results obtained by the proposed method were comparable with those obtained by the official methods.

In vitro and in vivo evaluation of a novel oral insulin formulation

AIM

To develop a stable self-emulsifying formulation for oral delivery of insulin.

METHODS

Caco-2 cell line and diabetic beagles were used as in vitro and in vivo models to study the absorption mechanism and the hypoglycemic efficacy of the formulation. In addition, various physicochemical parameters of the formulation such as droplet size, insulin encapsulation efficiency and stability were evaluated.

RESULTS

This formulation enabled changes in barrier properties of Caco-2 monolayers, as referred by transepithelial electrical resistance (TEER) and apparent permeability coefficients (P(app)) of the paracellular marker ranitidine (20-fold greater than control) but not transcellular marker propranolol, suggesting that the opening of tight junctions was involved. In diabetic beagle dogs, the bioavailability of this formulation was up to 15.2% at a dose of 2.5 IU/kg in comparison with the hypoglycemic effect of native insulin (0.5 IU/kg) delivered by subcutaneous injection.

CONCLUSION

This formulation, recently approved by the China State Food and Drug Administration to enter clinical trials, was stable, degradation-protected and absorption-enhanced, and provided a promising formulation for oral insulin delivery.

Saturable Absorptive Transport of the Hydrophilic Organic Cation Ranitidine in Caco-2 Cells: Role of pH-Dependent Organic Cation Uptake System and P-Glycoprotein

PURPOSE

The purpose of this work was to investigate the involvement of carrier-mediated apical (AP) uptake and efflux mechanisms in the absorptive intestinal transport of the hydrophilic cationic drug ranitidine in Caco-2 cells.

METHODS

Absorptive transport and AP uptake of ranitidine were determined in Caco-2 cells as a function of concentration. Permeability of ranitidine in the absorptive and secretory directions was assessed in the absence or presence of the P-glycoprotein (P-gp) inhibitor, GW918. Characterization of the uptake mechanism was performed with respect to inhibitor specificity, pH, energy, membrane potential, and Na+ dependence. Efflux from preloaded monolayers was evaluated over a range of concentrations and in the absence or presence of high extracellular ranitidine concentrations.

RESULTS

Saturable absorptive transport and AP uptake of ranitidine were observed with Km values of 0.27 and 0.45 mM, respectively. The ranitidine absorptive permeability increased and secretory permeability decreased upon inhibition of P-gp. AP ranitidine uptake was inhibited in a concentration-dependent fashion by a diverse set of organic cations including tetraethylammonium, 1-methyl-4-phenylpyridinium, famotidine, and quinidine. AP ranitidine uptake was pH and membrane potential dependent and reduced under conditions that deplete metabolic energy. Efflux of [3H]ranitidine across the basolateral membrane was neither saturable as a function of concentration nor trans stimulated by unlabeled ranitidine.

CONCLUSIONS

Saturable absorptive transport of ranitidine in Caco-2 cells is partially mediated via a pH-dependent uptake transporter for organic cations and is subject to attenuation by P-gp. Inhibition and driving force studies suggest the uptake carrier exhibits similar properties to cloned human organic cation transporters. The results also imply ranitidine transport is not solely restricted to the paracellular space.

Intestinal absorptive transport of the hydrophilic cation ranitidine: a kinetic modeling approach to elucidate the role of uptake and efflux transporters and paracellular vs. transcellular transport in Caco-2 cells

PURPOSE

The mechanism of intestinal drug transport for hydrophilic cations such as ranitidine is complex, and evidence suggests a role for carrier-mediated apical (AP) uptake and saturable paracellular mechanisms in their overall absorptive transport. The purpose of this study was to develop a model capable of describing the kinetics of cellular accumulation and transport of ranitidine in Caco-2 cells, and to assess the relative contribution of the transcellular and paracellular routes toward overall ranitidine transport.

METHODS

Cellular accumulation and absorptive transport of ranitidine were determined in the absence or presence of uptake and efflux inhibitors and as a function of concentration over 60 min in Caco-2 cells. A three-compartment model was developed, and parameter estimates were utilized to assess the expected relative contribution from transcellular and paracellular transport.

RESULTS

Under all conditions, ranitidine absorptive transport consisted of significant transcellular and paracellular components. Inhibition of P-glycoprotein decreased the AP efflux rate constant (k21) and increased the relative contribution of the transcellular transport pathway. In the presence of quinidine, both the AP uptake rate constant (k12) and k21 decreased, resulting in a predominantly paracellular contribution to ranitidine transport. Increasing the ranitidine donor concentration decreased k12 and the paracellular rate constant (k13). No significant changes were observed in the relative contribution of the paracellular and transcellular routes as a function of ranitidine concentration.

CONCLUSIONS

These results suggest the importance of uptake and efflux transporters as determinants of the relative contribution of transcellular and paracellular transport for ranitidine, and provide evidence supporting a concentration-dependent paracellular transport mechanism. The modeling approach developed here may also be useful in estimating the relative contribution of paracellular and transcellular transport for a wide array of drugs expected to utilize both pathways.

Biodegradability and toxicity of pharmaceuticals in biological wastewater treatment plants

In this experimental study both biological treatability of pharmaceuticals and their potential toxic effect in biological processes were evaluated. The pharmaceuticals were selected among those that are present at higher concentration in the Italian wastewater treatment plant effluents and widely used as antiulcer (ranitidine), beta-blocker (atenolol) and antibiotic (lincomycin). The present paper is the continuation of a work already presented,[1] which used a synthetic wastewater fed to laboratory scale SBR (Sequencing Batch Reactor) operated with different sludge ages (8 and 14 days), different biochemical conditions (aerobic or anoxic-aerobic mode) and several influent drug concentrations (2, 3 and 5 mg/L). In this case a real municipal wastewater was used as influent to the SBR. In parallel, batch tests were conducted to determine the removal kinetics of drugs and nitrogen. Toxicity tests using a titrimetric biosensor to verify possible inhibition on microorganisms were also performed. Finally, the possible adsorption of the pharmaceuticals on activated sludge was evaluated. The drugs under investigation showed different behaviours in terms of both biodegradability and toxicity effect on nitrifiers. Ranitidine showed generally low removal efficiencies (17-26%) and a chronic inhibition on nitrification. Atenolol showed generally higher removal efficiencies than ranitidine, even if the fairly good efficiency obtained in the previous experimentation with synthetic wastewater (up to 90%) was not attained with real wastewater (36%). No inhibition on nitrification was observed on both acclimated and non acclimated microorganisms with a high nitrification activity, whilst it was present with activated sludge characterised by a lower nitrification activity. Consistently with his pharmaceutical properties, lincomycin showed significant inhibition on nitrification activity.

Flavin-containing monooxygenase activity can be inhibited by nitric oxide-mediated S-nitrosylation

Nitric oxide (NO) modifies the functions of a variety of proteins containing cysteine thiols or transition-metal centers, particularly by S-nitrosylation. In inflamed liver, NO is overproduced and hepatic drug-metabolizing enzymes, the flavin-containing monooxygenases (FMOs) and cytochrome P450s (CYPs), are suppressed. However, the NO-related mechanisms underlying the loss of these activities are not well understood, particularly for FMOs. In this study, we suggest that FMO3, the major FMO in human liver, is modified post-translationally by NO. This hypothesis is based on the imbalance observed between the decrease in FMO3 expression (40.7% of controls) and FMO3-specific ranitidine N-oxidation activity (15.1%), and on the partial or complete reversibility of FMO inhibition by sulfhydryl-reducing regents such as DTT (effective on both S-S and S-NO adducts) and ascorbate (effective on S-NO only). Furthermore, NO donors (SNP, SNAP, and Sin-1), including the pure NO donor DEA/NO, directly suppressed in vitro FMO activity (N- or S-oxidation of ranitidine, trimethylamine, and thiobenzamide) in human liver microsomal proteins and recombinant human FMO3. These activities were restored completely after treatment with DTT or ascorbate. These results suggest that NO-mediated S-nitrosylation is involved in the rigorous inhibition of FMO activity in vitro and in vivo, resulting in the suppression of FMO-based drug metabolism or detoxification.

Carrier-mediated uptake of H2-receptor antagonists by the rat choroid plexus: involvement of rat organic anion transporter 3

The choroid plexus (CP) acts as a site for the elimination of xenobiotic organic compounds from the cerebrospinal fluid (CSF). The purpose of the present study is to investigate the role of rat organic anion transporter 3 (rOat3; Slc22a8) in the uptake of H(2)-receptor antagonists (cimetidine, ranitidine, and famotidine) by the isolated rat CP. Saturable uptake of cimetidine and ranitidine was observed in rOat3-LLC with K(m) values of 80 and 120 microM, respectively, whereas famotidine was found to be a poor substrate. The steady-state concentration of the H(2)-receptor antagonists in the CSF was significantly increased by simultaneously administered probenecid, although it did not affect their brain and plasma concentrations. Saturable uptake of cimetidine and ranitidine was observed in the isolated rat CP with K(m) values of 93 and 170 microM, respectively, whereas 50% of the uptake of famotidine remained at the highest concentration examined (1 mM). The K(i) value of ranitidine for the uptake of cimetidine by the isolated CP (50 microM) was similar to its own K(m) value, suggesting that they share the same transporter for their uptake. The inhibition potency of organic anions such as benzylpenicillin, estradiol 17beta-glucuronide, p-aminohippurate, and estrone sulfate for the uptake of cimetidine by the isolated rat CP was similar to that for benzylpenicillin, the uptake of which has been hypothesized to be mediated by rOat3, whereas a minimal effect by tetraethylammonium excludes involvement of organic cation transporter(s). These results suggest that rOat3 is the most likely candidate transporter involved in regulating the CSF concentration of H(2)-receptor antagonists at the CP.

Secretory transport of ranitidine and famotidine across Caco-2 cell monolayers

The secretory transport of the H(2)-antagonists, ranitidine and famotidine, across Caco-2 cell monolayers was found to be a saturable process. Both drugs exhibited greater permeability in the basolateral (BL) to apical (AP) direction than in the AP to BL direction, indicating apically directed secretion; BL to AP transport was inhibited by P-glycoprotein (P-gp) inhibitors verapamil and cyclosporin A. The cellular uptake of ranitidine across the BL membrane was saturable and temperature dependent, indicative of carrier-mediated transport. The K(m) and V(max) for the uptake process were estimated to be 66.9 mM and 20.9 nmol/mg of protein/min, respectively. The uptake of [(14)C]ranitidine across the BL membrane was inhibited by unlabeled ranitidine and structurally diverse organic cations. The tetraethylammonium (TEA)-sensitive organic cation transporters are not involved in the uptake of ranitidine and famotidine across the BL membrane of Caco-2. This conclusion was based on the evidence that functionally active TEA-sensitive organic cation transporters did not exist in the BL membranes of the Caco-2 cells, whereas the functionally active TEA-sensitive organic cation transporter(s) in LLC-PK(1) cells did not contribute to the transport of ranitidine or famotidine across the cell monolayers. Thus, we conclude that the secretory transport of ranitidine and famotidine across Caco-2 cell monolayers is mediated by 1) a carrier in the BL membrane that is distinct from the TEA-sensitive organic cation transporter(s) and 2) P-gp in the apical membrane.

Colonic metabolism of ranitidine: implications for its delivery and absorption

The aim of this study was to assess the in vitro stability of ranitidine to colonic bacteria by utilising a batch culture fermentation system to simulate the conditions of the colon. Three quantities of ranitidine, 100, 200 and 500 mg, in the form of the hydrochloride salt, were introduced into individual 100 ml fermenters consisting of buffer medium inoculated with freshly voided human faeces (10% w/v). Control experiments were also run in parallel using equivalent drug quantities in buffer medium without the presence of faeces. Samples were removed at pre-determined time intervals over a 24 h period and were subsequently analysed by high-performance liquid chromatography (HPLC) for drug concentration. A selection of the samples removed from the fermenters was also analysed by conventional UV spectroscopy and mass spectrometry. Subsequent to an initial dissolution phase in the fermentation system, a marked decline in ranitidine concentration was noted over time, thereby suggesting degradation and metabolism of the drug by colonic bacteria. No such decline in concentration was noted in the control buffer systems. The rate and extent of metabolism was rapid and complete within 12 and 24 h for the 100 mg and 200 mg samples, respectively, although the largest sample size, 500 mg, was only partly metabolised over the course of the experiment. UV and mass spectrometry analysis indicated that metabolism occurred via cleavage of an N-oxide bond within the molecule with the resultant loss of an oxygen atom, although further metabolic reactions are possible. Such metabolism may in part be responsible for the poor bioavailability of ranitidine from the colon.

Oxidation of ranitidine by isozymes of flavin-containing monooxygenase and cytochrome P450

Rat and human liver microsomes oxidized ranitidine to its N-oxide (66-76%) and S-oxide (13-18%) and desmethylranitidine (12-16%). N- and S-oxidations of ranitidine were inhibited by metimazole [flavin-containing monooxygenase (FMO) inhibitor] to 96-97% and 71-85%, respectively, and desmethylation of ranitidine was inhibited by SKF525A [cytochrome P450 (CYP) inhibitor] by 71-95%. Recombinant FMO isozymes like FMO1, FMO2, FMO3 and FMO5 produced 39, 79, 2180 and 4 ranitinine N-oxide and 45, 0, 580 and 280 ranitinine S-oxide pmol x min(-1) x nmol(-1) FMO, respectively. Desmethyranitinine was not produced by recombinant FMOs. Production of desmethylranitidine by rat and human liver microsomes was inhibited by tranylcypromine, a-naphthoflavon and quinidine, which are known to inhibit CYP2C19, 1A2 and 2D6, repectively. FMO3, the major form in adult liver, produced both ranitidine N- and S-oxides at a 4 to 1 ratio. FMO1, expressed primarily in human kidney, was 55- and 13-fold less efficient than the hepatic FMO3 in producing ranitidine N- and S-oxides, respectively. FMO2 and FMO5, although expressed slightly in human liver, kidney and lung, were not efficient producers of ranitidine N- and S-oxides. Thus, urinary contents of ranitidine N-oxide can be used as the in vivo probe to determine the hepatic FMO3 activity.

Prediction of dissolution-absorption relationships from a continuous dissolution/Caco-2 system

The objectives were 1) to design a continuous dissolution/Caco-2 system to predict the dissolution-absorption relationships for fast and slow dissolving formulations of piroxicam, metoprolol tartrate, and ranitidine HCl, and compare the predicted relationships with observed relationships from clinical studies; 2) to estimate the effect of croscarmellose sodium on ranitidine dissolution-absorption relationships; and 3) to estimate the effect of solubilizing agents on piroxicam dissolution-absorption relationships. A continuous dissolution/Caco-2 system was constructed from a dissolution apparatus and a diffusion cell, such that drug dissolution and permeation across a Caco-2 monolayer would occur sequentially and simultaneously. The continuous system generally matched observed dissolution-absorption relationships from clinical studies. For example, the system successfully predicted the slow metoprolol and slow ranitidiine formulations to be permeation-rate-limited. The system predicted the slow piroxicam formulation to be dissolution-rate-limited, and the fast piroxicam formulation to be permeation-rate-limited, in spite of piroxicam's high permeability and low solubility. Additionally, the system indicated croscarmellose sodium enhanced ranitidine permeability and predicted solubilizing agents to not modulate permeability. These results suggest a dissolution/Caco-2 system to be an experimentally based tool that may predict dissolution-absorption relationships from oral solid dosage forms, and hence the relative contributions of dissolution and permeation to oral drug absorption kinetics.

Heterologous expression of rat epitope-tagged histamine H2 receptors in insect Sf9 cells

1. Rat histamine H2 receptors were epitope-tagged with six histidine residues at the C-terminus to allow immunological detection of the receptor. Recombinant baculoviruses containing the epitope-tagged H2 receptor were prepared and were used to infect insect Sf9 cells. 2. The His-tagged H2 receptors expressed in insect Sf9 cells showed typical H2 receptor characteristics as determined with [125I]-aminopotentidine (APT) binding studies. 3. In Sf9 cells expressing the His-tagged H2 receptor histamine was able to stimulate cyclic AMP production 9 fold (EC50=2.1+/-0.1 microM) by use of the endogenous signalling pathway. The classical antagonists cimetidine, ranitidine and tiotidine inhibited histamine induced cyclic AMP production with Ki values of 0.60+/-0.43 microM, 0.25+/-0.15 microM and 28+/-7 nM, respectively (mean+/-s.e.mean, n=3). 4. The expression of the His-tagged H2 receptors in infected Sf9 cells reached functional levels of 6.6+/-0.6 pmol mg(-1) protein (mean+/-s.e.mean, n=3) after 3 days of infection. This represents about 2 x 10(6) copies of receptor/cell. Preincubation of the cells with 0.03 mM cholesterol-beta-cyclodextrin complex resulted in an increase of [125I]-APT binding up to 169+/-5% (mean+/-s.e.mean, n=3). 5. The addition of 0.03 mM cholesterol-beta-cyclodextrin complex did not affect histamine-induced cyclic AMP production. The EC50 value of histamine was 3.1+/-1.7 microM in the absence of cholesterol-beta-cyclodextrin complex and 11.1+/-5.5 microM in the presence of cholesterol-beta-cyclodextrin complex (mean+/-s.e.mean, n=3). Also, the amount of cyclic AMP produced in the presence of 100 microM histamine was identical, 85+/-18 pmol/10(6) cells in the absence and 81+/-11 pmol/10(6) cells in the presence of 0.03 mM cholesterol-beta-cyclodextrin complex (mean+/-s.e.mean, n=3). 6. Immunofluorescence studies with an antibody against the His-tag revealed that the majority of the His-tagged H2 receptors was localized inside the insect Sf9 cells, although plasma membrane labelling could be identified as well. 7. These experiments demonstrate the successful expression of His-tagged histamine H2 receptors in insect Sf9 cells. The H2 receptors couple functionally to the insect cell adenylate cyclase. However, our studies with cholesterol complementation and with immunofluorescent detection of the His-tag reveal that only a limited amount of H2 receptor protein is functional. These functional receptors are targeted to the plasma membrane.

Quantitation and kinetic properties of hepatic microsomal and recombinant flavin-containing monooxygenases 3 and 5 from humans

Variable amounts of flavin-containing monooxygenase isoforms 3 and 5 (FMO3 and FMO5) are present in microsomal preparations from adult, male, human liver. Quantitation with monospecific antibodies and recombinant isoforms as standards showed levels of FMO3 and of FMO5 that ranged from 12.5 to 117 and 3.5 to 34 pmol/mg microsomal protein, respectively. The concentration of FMO3 was greater than that of FMO5 in all samples, but the ratio of FMO3 to FMO5 varied from 2:1 to 10:1. Human hepatic microsomal samples also showed variable activities for the S-oxidation of methimazole. This activity was associated totally with FMO3; no participation of FMO5 was apparent. This conclusion was supported by several lines of evidence: first, the catalytic efficiency of FMO3 with methimazole was found to be approximately 5000 times greater than that of FMO5; second, the rate of metabolism showed a direct, quantitative relationship with FMO3 content; third, the plot of the relationship between metabolism and FMO3 content extrapolated close to the origin. A second reaction, the N-oxidation of ranitidine, exhibited a much higher Km with recombinant FMO3 than did methimazole (2 mM vs. 35 microM). However, a direct relationship between this reaction and FMO3 content in human hepatic microsomal preparations was also apparent. This result shows that even with a high Km substrate, FMO3-catalyzed metabolism can account for the majority of the product formation with some drugs. Our findings demonstrate that the contribution of FMO isoforms to human hepatic drug metabolism can be assessed quantitatively on the basis of the characteristics of the enzymes expressed in Escherichia coli.

Neurotoxic convulsions induced by histamine H2 receptor antagonists in mice

Convulsive potency was evaluated to investigate the mechanism of neurotoxic convulsion induced by histamine H2 receptor antagonists (H2 blockers). Four H2 blockers, cimetidine (721-1236 nmol), ranitidine (477-954 nmol), famotidine (7.4-44 nmol), and nizatidine (226-603 nmol) were administered intracerebrally (i.c.) to mice. Dose dependency of clonic and/or tonic convulsion was observed, and the ED50 values of convulsive occurrence for cimetidine, ranitidine, famotidine, and nizatidine were 997, 662, 23.4, and 404 nmol, respectively. Intraperitoneal pretreatment of muscimol, aminooxy acetic acid, diazepam, (+/-)2-amino-7-phosphonoheptanoic acid (APH), or (+)MK801 suppressed the tonic convulsion after i.c. administration of ranitidine, but had no effect on clonic convulsion. Furthermore, the convulsive threshold concentration in the brain determined by constant rate infusion of ranitidine was not affected by the pretreatment of muscimol, diazepam, APH, and MK801. Ed50 values for convulsive occurrence after i.c. administration of four H2 blockers correlated well with the EC50 values for gastric acid secretion inhibition. The convulsive threshold concentrations of cimetidine and ranitidine in the brain were 11 and 2.5 microM, respectively, which were similar to the dissociation constants determined from the inhibition of gastric acid output in mice. From these results, tonic convulsion induced by H2 blockers can be suppressed by GABAergic or glutamatergic anticonvulsants, while clonic convulsion induced by H2 blockers may be associated with the blockade of H2 receptor in the brain and not be directly associated with the GABA and glutamate-mediated neurotransmission.

Gastric persorption of bismuth from ranitidine bismuth citrate

AIM

To determine whether bismuth penetrates the gastric mucosa after dosing with ranitidine bismuth citrate.

METHODS

Twelve patients presenting with dyspepsia were randomized to receive either ranitidine bismuth citrate or placebo, 20-40 min prior to endoscopy. Biopsies were taken from four sites during endoscopy: the first and second parts of the duodenum, the antrum, and the body of the stomach. Biopsies were analysed by electron microscopy and X-ray microanalysis.

RESULTS

Bismuth particles were found to be interposed between epithelial cells in the antral mucosa of three of eight patients who were dosed with ranitidine bismuth citrate. Columns of bismuth particles could be tracked down the lamina propria and were seen to be surrounding blood vessels. Bismuth particles were observed in the inter- and intra-cellular channels of the endothelial cells of the blood vessels in the lamina propria and also close to the luminal surface of the endothelial cell. This process of persorption was similar to that described in a previous report of electron microscopy appearances of the gastric antrum after dosing with tripotassium dicitrato bismuthate, but was quantifiably smaller and not observed in all the patients dosed with ranitidine bismuth citrate. No penetration of the mucosa by bismuth particles was seen in the body of the stomach or the duodenum.

CONCLUSION

Penetration of bismuth particles into the gastric mucosa may occur after oral dosing with ranitidine bismuth citrate.

Theophylline-ranitidine interaction in elderly COPD patients

Most controlled studies in humans indicate that ranitidine does not alter theophylline metabolism, even at high doses. However, there have been several case reports published recently which demonstrate the development of theophylline toxicity mostly in older patients receiving stable oral doses of this drug when ranitidine was administered simultaneously. We studied eleven elderly (mean age, 69.0 +/- 6.2 years) patients with chronic obstructive pulmonary disease (COPD). During one week the patients took slow-release theophylline, 200 mg every 12 h, followed by one week intake of the same dose of theophylline plus ranitidine tablets, 150 mg every 12 h. At the end of each period, blood samples were obtained 0, 1, 2, 3, 4, 5, 6, 7, 8 and 12 h after the morning dose for the determination of serum theophylline levels. The peak theophylline concentration (Tmax) was achieved after 4.1 +/- 0.9 h while the patients were taking theophylline, and after 2.9 +/- 1.4 h with the combined regimen. This difference was statistically significant (P < 0.01). In only 3/11 subjects did Tmax remain unchanged during both phases of the study. The mean theophylline clearance rates while the patients were receiving theophylline alone (39.58 +/- 19.89 ml/min) and when they were receiving both medications (34.42 +/- 10.55 ml/min) were similar. The mean serum levels while the patients were receiving theophylline alone were slightly higher but not statistically different. These results suggest that the reported increases in serum theophylline levels in older patients receiving theophylline and ranitidine cannot be ascribed to slower theophylline metabolism in the geriatric patients with COPD who is also given ranitidine.

Sulphoxidation and sulphation capacity in patients with primary biliary cirrhosis

We have previously reported an association of impaired S-oxidation with primary biliary cirrhosis. In order to confirm and further define this relationship, we retested S-oxidation capacity via three metabolic pathways and sulphation capacity via a fourth pathway. Metabolism of S-carboxymethyl-L-cysteine is polymorphic -20% of healthy individuals being poor S-oxidisers. We found 26% with primary biliary cirrhosis were poor S-oxidisers, compared with 36% with other liver disease and 25% of healthy controls. Differences were not statistically significant. S-oxidation of ranitidine is dependent upon flavin mono-oxygenases. We showed a non-significant trend toward less S-oxide in primary biliary cirrhosis and other liver disease, compared with healthy controls, with no significant difference between disease groups. Conversion of cysteine to sulphate depends predominantly on cysteine dioxygenase. Impaired activity may be reflected by decreased plasma sulphate and elevated cysteine. We found that the plasma cysteine: sulphate ratio was significantly elevated not only in primary biliary cirrhosis (p < 0.0001), but also in other liver disease (p < 0.0001), compared with healthy individuals. Sulphation capacity was studied by analysing paracetamol metabolism. Paracetamol sulphate and sulphate: glucuronide ratio were reduced in primary biliary cirrhosis compared with normal individuals, (p < 0.05). A trend towards less sulphate in primary biliary cirrhosis compared other liver disease was not significant (p = 0.42). We conclude that although sulphation and some sulphoxidation pathways are impaired in primary biliary cirrhosis, we can currently find no evidence to substantiate the hypothesis that primary biliary cirrhosis is a disease specifically associated with poor S-oxidation, as assessed via these metabolic pathways.

Kinetics of ranitidine metabolism in dog and rat isolated hepatocytes

1. Freshly isolated hepatocytes from rat and dog have been evaluated as a model for the metabolism of ranitidine in vivo. 2. Isolated hepatocytes from the male and female dog and male Wistar and random hooded rat metabolized ranitidine to ranitidine N-oxide, ranitidine S-oxide, desmethylranitidine and two unidentified minor metabolites. The furoic acid metabolite of ranitidine, previously reported to be a minor metabolite in vivo in rat and dog, was not detected in hepatocytes from either species. 3. The kinetics for ranitidine metabolism in hepatocytes were monophasic for the formation of the three major metabolites in dog and Wistar rat and for N-demethylation of ranitidine in the random hooded rat, but biphasic in this latter strain for the N- and S-oxidation of ranitidine. 4. Ranitidine N-oxide was reduced to ranitidine by Wistar rat hepatocytes but not by hepatocytes from the random hooded rat or dog. Ranitidine S-oxide was metabolized by hepatocytes from both species to one of the unidentified metabolites but was not reduced to ranitidine in either species. Desmethylranitidine was not a substrate for metabolism in hepatocytes from either species. 5. The relative quantitative importance of ranitidine N-oxide, ranitidine S-oxide and desmethylranitidine produced by the hepatocytes was consistent with the profiles of these three metabolites in vivo in rat and dog. The results confirm the value of isolated hepatocytes as a predictive model for in vivo drug metabolism.

H2-receptor antagonists are scavengers of oxygen radicals

Potential oxygen radical scavenging properties of the H2-receptor antagonists cimetidine, ranitidine and famotidine were investigated. These drugs, although ineffective against superoxide anion and hydrogen peroxide, can scavenge hydroxyl radical (OH.) with a very high rate constant, which is about tenfold higher than that of the specific scavenger mannitol for famotidine (1.7 x 10(10) mol-1 s-1) and cimetidine (1.6 x 10(10) mol-1 s-1), ranitidine displaying a rate constant of 7.5 x 10(9) mol-1 s-1. These OH. savenging effects are significant beginning from 10, 28 and 100 mumol l-1 concentration for famotidine, cimetidine and ranitidine, respectively, thus suggesting that the drugs may effectively act as OH. scavengers in vivo especially in the gastric lumen. Only cimetidine can apparently bind and inactivate iron, which further emphasizes its antioxidant capacity. Moreover, all drugs, even at 10 mumol l-1 concentration, show powerful scavenging effects on hypochlorous acid and monochloramine, which are cytotoxic oxidants arising from inflammatory cells, such as neutrophils. These data suggest that some therapeutical effects of H2-receptor antagonists in peptic ulcer may also be related to their antiradical-antioxidant capacity, and that these drugs could potentially be used in other disease entities characterized by free radical-mediated oxidative stress in vivo.

Structural characteristics of histamine H2 receptor antagonists that scavenge hypochlorous acid

Gastric and duodenal ulcers are characterized by hypersecretion of gastric acid and pepsin, inflammation of the mucosa and an influx of neutrophils. These cells can produce reactive oxygen species after stimulation. Particularly hydroxyl radicals and hypochlorous acid (HOCl) can be cytotoxic and damage the gastroduodenal mucosa. It has been shown that histamine H2 receptor antagonists such as cimetidine, ranitidine and famotidine are good hydroxyl radical scavengers. This study was undertaken to investigate whether these agents were able to scavenge the cytotoxic HOCl, and if so, which part of the molecule could be responsible for this action. It appears that the sulfur atom in the compound is of importance for scavenging HOCl. This finding should be taken into consideration in the development of new anti ulcer drugs, as HOCl is a detrimental factor in the pathophysiology of gastroduodenal ulcers.

Effect of cyclosporine on colchicine secretion by the kidney multidrug transporter studied in vivo

The multidrug resistance (MDR) transport protein is a normal constituent of proximal renal tubules although its function has not been defined in vivo. We find that colchicine, an MDR substrate, is secreted into urine by a process which is distinct from the organic cation transporter responsible for tetraethylammonium and N-methylnicotinamide secretion. Cyclosporine (CsA), which reverses MDR in vitro presumably by inhibiting the MDR transporter, inhibits colchicine renal secretion but does not inhibit the net secretion of the organic cation, ranitidine, or the organic anion, p-aminohippurate. After CsA (2 mg/kg i.v.), colchicine renal clearance decreased from 6.23 +/- 0.46 to 3.58 +/- 0.31 ml/min.kg (P less than .05), glomerular filtration rate was unchanged (4.21 +/- 0.08 and 3.88 +/- 0.17 ml/min.kg, before and after CsA, respectively; P = .09) and colchicine secretory ratio decreased from 1.48 +/- 0.11 to 0.92 +/- 0.07 (P less than .05). Cremophor (CsA vehicle) increased colchicine renal clearance (6.77 +/- 0.29 to 7.7 +/- 0.3 ml/min.kg, P less than .05) and colchicine secretory ratio (1.425 +/- 0.071 to 1.621 +/- 0.061, P less than .05). The inhibition of colchicine secretion was long-lived lasting at least 30 hr after CsA. Thus, colchicine is actively secreted into urine by the multidrug transporter in vivo. CsA profoundly inhibits colchicine secretion into urine while having no effect on the secretion of the organic cation ranitidine or the organic anion p-aminohippurate.

Chromatographic retention relationships between aliphatic tertiary amines and their putative N-oxide metabolites--preliminary results

The chromatography of a series of tertiary amines and the corresponding N-oxides have been studied and the Functional Group Contribution Approach used to describe retention correlations between them. Good correlations are observed between the behaviour of compounds on HPLC and with the exception of some alicyclic nitrogen compounds also on thin layer chromatography. The models produced are used to predict k' and Rf values for ranitidine N-oxide and tamoxifen N-oxide based upon those for the parent molecule. The deviation between actual and predicted values was larger than expected, presumably due to the structural or physico-chemical differences of ranitidine and tamoxifen compared to the model compounds.

A comparison of the cholinergic activity of selected H2-antagonists and sulfoxide metabolites

Famotidine and selected H2-antagonists were evaluated with respect to toxicity and selected pharmacological activities. When administered intraperitoneally to mice at a dose equivalent to 10 times their respective H2-antagonist ED50 values, no deaths were observed. Similarly, no alteration in brain ACh concentrations or overt pharmacological effects were noted. However, at 400 mg/kg, ranitidine produced 89% lethality, followed by cimetidine (11%) and famotidine. Only cimetidine and famotidine at this dose significantly elevated brain acetylcholine levels. These results do not correlate with the in vitro data, where ORF-17578 and ranitidine were the most potent entities with respect to acetylcholinesterase inhibition (approximately 1-2 X 10(-6) M), followed by nizatidine greater than cimetidine greater than famotidine. The sulfoxide metabolites of ranitidine and cimetidine were approximately one-tenth as potent as their parent compounds with respect to inhibition of acetylcholinesterase. Direct muscarinic stimulation or potentiation of acetylcholine-induced contraction in ileal tissue was not observed for any of the H2-antagonists.

The effects of cimetidine, ranitidine and famotidine on rat hepatic microsomal cytochrome P-450 activities

It has been questioned whether the interaction of H2-antagonists with cytochrome P-450 that is observed in vitro is also relevant for the in vivo situation. Until now the possibility that cytochrome P-450 may function with different modes of action has been neglected in this respect. We studied the effect of cimetidine, ranitidine and famotidine on the monoxygenase, the oxidase and the peroxidase action of cytochrome P-450. Biotransformation catalyzed by the monoxygenase and oxidase action of cytochrome P-450 was affected by cimetidine (probably via its ligand interaction with cytochrome P-450), whereas metabolism by the peroxidase mode of action of cytochrome P-450 was hardly influenced. Ranitidine and famotidine (both pharmacodynamically more potent than cimetidine) only slightly affected cytochrome P-450 activities.

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.

Kinetics of rate controlled rectally administered ranitidine to male volunteers

Ranitidine hydrochloride solution was given rectally to six healthy volunteers by means of an osmotic pump (Osmet) at a zero-order rate for 8 h. Quite constant steady-state drug concentrations were achieved in the range of 64-123 ng/ml plasma (mean Css: 100.6 ng/ml). The mean absorption time was 0.45 h (range 0-I.02 h), indicating that absorption was not always instantaneous. It is concluded that the Osmet system together with the rectal route offer the possibility for achieving steady-state concentrations of ranitidine, enabling the determination of pharmacodynamic effects. In addition, the rectal route can be considered as an alternative to oral and intravenous administration.

Ranitidine disposition in severe hepatic cirrhosis

The effect of severe liver disease on ranitidine disposition was evaluated by comparing its kinetics in 5 healthy subjects and 11 patients with alcoholic cirrhosis. Cirrhotic patients had severe liver disease as evidenced by the presence of ascites, hepatic encephalopathy, jaundice, muscle wasting, and low serum albumin, but creatinine clearance did not differ significantly between controls and cirrhosis. Following intravenous administration of ranitidine, systemic clearance was decreased in cirrhosis. These decrease may be associated with changes in renal function, and decrease in hepatic metabolism, usually present in patients with severe hepatic failure. The distribution volume of ranitidine was also decreased in cirrhotics, but the difference between patients and controls was not significant. Biological half-life was significantly longer in cirrhotic patients than volunteers. This difference may be due to decrease in total body clearance found in cirrhotic patients. It is concluded that patients with severe liver cirrhosis could have elevated plasma level of ranitidine and that a reduction of ranitidine dosage is warranted in these patients.

Effect of aluminum phosphate on the bioavailability of ranitidine

The effect of aluminum phosphate on the bioavailability of ranitidine has been investigated in 10 young, healthy volunteers. Following a random cross over design, each subject took at a 1 week interval 150 mg ranitidine alone or with 11 g aluminum phosphate. Plasma and urine ranitidine levels were measured by HPLC. The antacid reduced both the maximum plasma ranitidine concentration by 40% and the area under the curve by 30%. Elimination of ranitidine was not changed. The results indicate that aluminum phosphate significantly diminished the bioavailability of ranitidine.

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.

Lack of clinically significant in vitro and in vivo interactions between ranitidine and sucralfate

The hypothesis that the cytoprotective agent sucralfate (sucrose octakis(hydrogen sulfate)-aluminum complex) interacts with the H2-antagonist ranitidine (N-[2-[[dimethylamino)methyl]furfuryl]- thio]ethyl]-N'-methyl-2-nitro-1,1-ethenediamine) by decreasing ranitidine absorption was tested in vitro and in vivo. The in vitro results show that ranitidine may bind to a small extent (approximately 10%) to sucralfate paste in the gastrointestinal fluids. The in vivo ineraction of 150 mg of ranitidine and 1 g of sucralfate was evaluated in a crossover study in six healthy volunteers. The results indicate no significant difference in pharmacokinetic parameters when ranitidine was given alone and in combination with sucralfate. Thus, ranitidine bioavailability is not diminished by sucralfate and the two drugs can be given concomitantly. For the determination of bioavailability, it has to be taken into account that renal clearance of ranitidine is lower after oral than after intravenous administration, and that enterohepatic recirculation of the drug is likely.

No interaction between H2 blockers and isoniazid

Antacids and anticholinergic drugs may delay gastric emptying and cause reduced absorption of isoniazid, but the influence of reduced gastric acidity has not been determined. In the present investigation, the influence of cimetidine and ranitidine on uptake and elimination of isoniazid was determined in six male and seven female healthy volunteers. The group comprised five rapid and eight slow acetylators, similar to the proportions in the general population. H2 blockers did not delay or reduce uptake of isoniazid, which thus seemed to be pH independent. Neither the acetylation of isoniazid nor the oxidative metabolism of acetylated isoniazid appeared to be affected by cimetidine or ranitidine.

The effect of age on ranitidine pharmacokinetics

Oral ranitidine was given to 68 healthy subjects between 18 and 75 years old at a dosage of 150 mg twice a day for seven doses. Fifteen subjects were 18 to 35 years old (group I), 19 subjects were 36 to 50 years old (group II), 19 subjects were 51 to 65 years old (group III), and 15 subjects were 66 to 75 years old (group IV). Venous blood samples were drawn and the AUC from 0 to 12 hours, the maximum plasma concentration, the time of the maximum plasma concentration, the minimum plasma concentration, and the elimination t1/2 were determined. When groups III and IV were compared with groups I or II, significant (P less than 0.05) increases were seen in the AUC(0-12) (42% and 50%), the maximum plasma concentration (36% and 41%), the minimum plasma concentration (91% and 85%), and the elimination t1/2 (29% and 33%). Positive linear correlations were found when the AUC(0-12) (r = 0.68; P less than 0.01), the maximum plasma concentration (r = 0.34; P less than 0.01), the minimum plasma concentration (r = 0.55; P less than 0.01), and the elimination t1/2 (r = 0.46; P less than 0.01) were regressed with age. Our results suggest that it may be appropriate to consider dosage adjustments for patients over 50 years of age who take ranitidine.

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.

Pharmacokinetics and bioavailability of ranitidine in normal subjects and cirrhotic patients

Pharmacokinetic parameters of ranitidine were determined comparatively in nine cirrhotic patients and eight healthy volunteers after administration of a single oral (150 mg) or intravenous dose (50 mg). Bioavailability was virtually the same in patients and in normal subjects, and there was no significant difference between the two groups for total plasma clearance, intravenous elimination half-life, or volume of distribution. The mean maximum plasma concentration was 43% higher in patients than in volunteers, but the difference was not significant. The values for the kinetic parameters did not significantly differ between patients with and without ascites or with and without hepatocellular insufficiency. These results indicate that ranitidine kinetics are not appreciably altered in cirrhotic patients and that reduction of dosage is not required in such patients unless their renal function is impaired.

Selectivity of the cimetidine-induced alterations in the renal handling of organic substrates in humans. Studies with anionic, cationic and zwitterionic drugs

Cimetidine reduces the renal clearances of the organic cations procainamide and n-acetylprocainamide in humans and in vitro preparations by inhibition of renal cationic proximal tubular secretion. The aim of this study was to investigate in humans the selectivity of cimetidine in altering the renal handling of three different organic ions as drug substances: anion (cephalothin), cation (ranitidine) and zwitterion (cephalexin). The study was conducted in six healthy subjects who received the above drugs as single doses with and without chronic cimetidine administration. Cimetidine had no statistically significant (P greater than .05) effect on cephalothin disposition including renal clearance, but significantly reduced the renal clearance of ranitidine by over 40% between 4 and 12 hr after administration, with a concomitant increase in ranitidine plasma concentrations and elimination half-life prolongation. In addition, the renal clearance of cephalexin was reduced by cimetidine by 27% between 1 and 2 hr, but there was no change in cephalexin plasma concentrations and elimination half-life. These findings confirmed the hypothesis that cimetidine-mediated inhibition of renal drug clearance in humans is selective for a common cationic secretory transport mechanism in the proximal tubule of the kidney, rather than a nonspecific action on renal function.

Pharmacokinetics of ranitidine in acute upper gastrointestinal haemorrhage

A pharmacokinetic study of ranitidine was performed in 14 patients with haematemesis divided into two groups according to the severity of blood loss. Pharmacokinetic values were calculated from plasma concentrations after the first of three daily injections (100 mg) and compared with those obtained in five healthy volunteers (50 mg i.v.). There were no significant differences between patients in the two haemorrhage groups and controls. The low, or even questionable, effectiveness of histamine H2-receptor antagonists in the treatment of upper gastrointestinal haemorrhage does not seem to be due to pharmacokinetic factors.

Pharmacokinetics of ranitidine following oral administration with ascending doses and with multiple-fixed doses

The aim of these studies was to further delineate pharmacokinetic characteristics of ranitidine, a new histamine H2-receptor antagonist. In one study, ranitidine was administered orally to six normal men in increasing doses of 100 mg, 150 mg, 250 mg, and 400 mg weekly over a four-week period. The peak serum concentrations increased with the corresponding increases in dose but the time needed to reach peak serum concentration did not vary significantly with increased doses. The pharmacokinetic parameters were calculated for each subject at each of the four dose levels. The total area under the curve (AUC) at the four different doses was linearly related to the dose for each individual subject; and a plot of AUC versus dose had a correlation coefficient of .886 (P less than .001). The apparent plasma clearance did not vary with the increase in dose; and the average corrected clearance values ranged between 6.7 and 10 mL/(min X kg). Elimination half-life was between 2.6 and 3.0 hours; and the volume of distribution (Vd area) was between 1.6 and 2.4 L/kg. About 35% of the ranitidine dose was excreted in the urine in the unchanged form over a 12-hour excretion interval. In the second study, ranitidine was administered orally to 12 normal subjects in doses of 150 mg and 200 mg twice daily for 28 days. The pharmacokinetic parameters for ranitidine with multiple-dose treatment were similar to those obtained with single-dose administration. Predose ranitidine concentrations (trough levels) did not increase with multiple dose administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetic determination of ranitidine pharmacodynamics in pediatric ulcer disease

The pharmacokinetics and pharmacodynamics of ranitidine were evaluated during three methods of administration in 12 children ranging in age from 3.5 to 16 years with documented gastric or duodenal ulcer disease. First, a continuous intravenous infusion of ranitidine was administered to determine the serum concentration necessary to suppress gastric acid secretion by at least 90%. From these data a therapeutic dose of ranitidine was calculated and administered on separate days via the intravenous bolus and oral routes. Half-life, volume of distribution, and clearance values for ranitidine were the same after intravenous bolus and oral doses (1.8 vs 2.0 hours, 2.3 vs 2.5 L/kg, and 794.7 vs 788.0 ml/min/1.73 m2, respectively). The bioavailability of ranitidine given orally averaged 48%. Serum ranitidine concentrations necessary to inhibit gastric acid secretion by at least 90% ranged between 40 and 60 ng/ml for all children studied. No adverse clinical or biochemical effects were observed in any child during the 6 weeks of orally administered treatment. Endoscopic reevaluation after 6 weeks indicated complete healing of initial ulcers.