Modeling uptake of selected pharmaceuticals and personal care products into food crops from biosolids-amended soil

Biosolids contain a variety of pharmaceuticals and personal care products (PPCPs). Studies have observed the uptake of PPCPs into plants grown in biosolids-amended soils. This study examined the ability of Dynamic Plant Uptake (DPU) model and Biosolids-amended Soil Level IV (BASL4) model to predict the concentration of eight PPCPs in the tissue of plants grown in biosolids-amended soil under a number of exposure scenarios. Concentrations in edible tissue predicted by the models were compared to concentrations reported in the literature by calculating estimated human daily intake values for both sets of data and comparing them to an acceptable daily intake value. The equilibrium partitioning (EqP) portion of BASL4 overpredicted the concentrations of triclosan, triclocarban, and miconazole in root and shoot tissue by two to three orders of magnitude, while the dynamic carrot root (DCR) portion overpredicted by a single order of magnitude. DPU predicted concentrations of triclosan, triclocarban, miconazole, carbamazepine, and diphenhydramine in plant tissues that were within an order of magnitude of concentrations reported in the literature. The study also found that more empirical data are needed on the uptake of cimetidine, fluoxetine, and gemfibrozil, and other ionizable PPCPs, to confirm the utility of both models. All hazard quotient values calculated from literature data were below 1, with 95.7% of hazard quotient values being below 0.1, indicating that consumption of the chosen PPCPs in plant tissue poses de minimus risk to human health.

Segmental-dependent membrane permeability along the intestine following oral drug administration: Evaluation of a triple single-pass intestinal perfusion (TSPIP) approach in the rat

In this paper we evaluate a modified approach to the traditional single-pass intestinal perfusion (SPIP) rat model in investigating segmental-dependent permeability along the intestine following oral drug administration. Whereas in the traditional model one single segment of the intestine is perfused, we have simultaneously perfused three individual segments of each rat intestine: proximal jejunum, mid-small intestine and distal ileum, enabling to obtain tripled data from each rat compared to the traditional model. Three drugs, with different permeabilities, were utilized to evaluate the model: metoprolol, propranolol and cimetidine. Data was evaluated in comparison to the traditional method. Metoprolol and propranolol showed similar P(eff) values in the modified model in all segments. Segmental-dependent permeability was obtained for cimetidine, with lower P(eff) in the distal parts. Similar P(eff) values for all drugs were obtained in the traditional method, illustrating that the modified model is as accurate as the traditional, throughout a wide range of permeability characteristics, whether the permeability is constant or segment-dependent along the intestine. Three-fold higher statistical power to detect segmental-dependency was obtained in the modified approach, as each subject serves as his own control. In conclusion, the Triple SPIP model can reduce the number of animals utilized in segmental-dependent permeability research without compromising the quality of the data obtained.

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.

Impaired expression and function of breast cancer resistance protein (Bcrp) in brain cortex of streptozocin-induced diabetic rats

The aim of this study was to investigate whether diabetes mellitus (DM) affected breast cancer resistance protein (Bcrp) function and expression in rat brain. 5-week and 8-week diabetic rats were induced by streptozocin (STZ). Bcrp expression and function in brain cortex were assessed by western blot and measuring the brain-to-plasma concentration ratios of two typical substrates prazosin and cimetidine, respectively. The diabetic rats were treated with three different agents insulin, aminoguanidine (AG) and metformin (MET). It was found that the brain-to-plasma ratios of prazosin and cimetidine in diabetic rats were significantly higher than those of control rats, which were dependent on duration of diabetes. Lower levels of Bcrp were found in brain cortex of diabetic rats, which were in parallel with increase of brain-to-plasma ratios. Insulin treatment may attenuate the impairment of Bcrp expression and function induced by diabetes. Aminoguanidine and metformin treatment did not prevent the impairment of Bcrp function and expression in brain cortex of diabetic rats. All results gave a conclusion that STZ-induced DM may induce the impairment of function and expression of Bcrp in brain cortex, and lower levels of insulin may mainly contribute to Bcrp dysfunction in brain.

Effect of sodium ozagrel on the activity of rat CYP2D6

The aim of the study was to investigate the influence of sodium ozagrel on CYP2D6 (cytochromeP450 2D6) activity. The studies were performed with rat urine and liver microsomes and chemical inhibitors. The metabolism of dextromethorphan (dextrophan/dextromethorphan, dextrophan is a metabolite of dextromethorphan) and phenacetin (paracetamol/phenacetin, paracetamol is a metabolites of phenacetin) was used as probe to measure CYP2D6 and CYP1A2 (cytochromeP450 1A2) activity, respectively, determined by high-performance liquid chromatography (HPLC). The results showed that the metabolism of dextrophan/dextromethorphan in the sodium ozagrel-treated group (37 mg/kg) was higher than that of the control (P<0.05/6) in both in vivo and in vitro studies (r=0.9811). The rate of dextromethorphan metabolism was inhibited by sodium ozagrel and cimetidine in rat liver microsomes prepared from sodium ozagrel-treated rats and control rats group (sodium ozagrel IC(50)=26.5 microM, cimetidine IC(50)=86.3 microM in sodium ozagrel-treated group; sodium ozagrel IC(50)=13.9 microM, cimetidine IC(50)=24.8 microM in control group). The inhibitory effect of sodium ozagrel on CYP2D6 activity was noncompetitive with dextromethorphan with a K(i) of 324.94 microM. Kinetic parameters of the reactions were established by using Lineweaver-Burk with K(m)=0.67 mM and V(max)=2.13 pm/min/mg protein for the sodium ozagrel-treated group and K(m)=0.29 mM, and V(max)=0.91 pm/min/mg protein for the control group, respectively. The expression of CYP2D6 protein in the treated group was higher than that of the control group, as determined by Western blotting. The activity and expression of CYP1A2 did not show obvious differences in the control group and sodium ozagrel treated group. In conclusion, sodium ozagrel metabolism in rats is mediated primarily through CYP2D6, and sodium ozagrel can induce CYP2D6 activity.

Functional involvement of organic cation transporter1 (OCT1/Oct1) in the hepatic uptake of organic cations in humans and rats

The contribution of organic cation transporters to the saturable component in the hepatic uptake of 1-methyl-4-phenylpyridinium (MPP), tetraethylammonium (TEA), cimetidine, and metformin was examined by the use of human/rat organic cation transporter (hOCT1/rOct1)-expressing cells and human/rat hepatocytes. Transfection of rOct1 resulted in a considerable increase in the uptake of metformin, whereas that of hOCT1 resulted in only a slight increase. All test compounds (MPP, TEA, cimetidine, and metformin) accumulated in human and rat hepatocytes in a carrier-mediated manner. The Km values for the uptake of MPP, TEA, cimetidine, and metformin into human and rat hepatocytes were comparable with those into hOCT1 and rOct1-expressing cells, respectively. In addition, the relative uptake activities, which were obtained by normalizing the intrinsic uptake clearances of TEA, cimetidine, and metformin against those values of MPP in human and rat hepatocytes, were similar with the uptake activities in hOCT1 and rOct1, respectively. These results suggest that the saturable component in the hepatic uptake of these cationic compounds may be mediated mainly by hOCT1/rOct1; therefore, it is meaningful to evaluate the saturable uptake profile of cationic compounds by the liver using both hOCT1/rOct1-expressing cells and human/rat hepatocytes.

Thiolated chitosan: development and in vitro evaluation of an oral delivery system for acyclovir

The aim of the study was to develop a novel oral delivery system for the efflux pump substrate acyclovir (ACY) utilizing thiolated chitosan as excipient which is capable of inhibiting P-glycoprotein (P-gp). Three chitosan-4-thiobutylamidine (Chito-TBA) conjugates with increasing molecular mass (Chito-9.4kDa-TBA, Chito-150kDa-TBA and Chito-600kDa-TBA) were synthesized and permeation studies on rat intestinal mucosa and Caco-2 monolayers were performed. Additionally, tablets comprising the conjugates and ACY were tested towards their drug release behaviour. The efflux ratio (secretory P(app)/absorptive P(app)) of ACY across Caco-2 monolayers was determined to be 2.5 and in presence of 100microM verapamil 1.1 which indicates ACY as P-gp substrate. In comparison to buffer only, the transport of ACY in presence of 0.5% (m/v) unmodified chitosan, 0.5% (m/v) Chito-150kDa-TBA and 0.5% (m/v) Chito-150kDa-TBA with 0.5% (m/v) reduced glutathione (GSH), was 1.3-, 1.6- and 2.1-fold improved, respectively. Transport studies across Caco-2 monolayers showed that P-gp inhibition is dependent on the average molecular mass of thiolated chitosan showing following rank order: 0.5% (m/v) Chito-150kDa-TBA/GSH>0.5% (m/v) Chito-9.4kDa-TBA/GSH>0.5% (m/v) Chito-600kDa-TBA/GSH. The higher the molecular mass of Chito-TBA was, the more sustained was the release of ACY. Chito-150kDa-TBA/GSH might be an appropriate sustained release drug delivery system for ACY, which is able to enhance ACY transport due to efflux pump inhibition.

Molecular identification and functional characterization of rabbit MATE1 and MATE2-K

An electroneutral organic cation (OC)/proton exchanger in the apical membrane of proximal tubules mediates the final step of renal OC excretion. Two members of the multidrug and toxin extrusion family, MATE1 and MATE2-K, were recently identified in human and rodent kidney and proposed to be the molecular basis of renal OC/H+ exchange. To take advantage of the comparative value of the large database on the kinetic and selectivity characteristics of OC/H+ exchange that exists for rabbit kidney, we cloned rbMATE1 and rbMATE2-K. The rabbit homologs have 75% (MATE1) and 74% (MATE2-K) amino acid identity to their human counterparts (and 51% identity with each other). rbMATE1 and rbMATE2-K exhibited H+ gradient-dependent uptake and efflux of tetraethylammonium (TEA) when expressed in Chinese hamster ovary cells. Both transporters displayed similar affinities for selected compounds [IC50 values within 2-fold for TEA, 1-methyl-4-phenylpyridinium, and quinidine] and very different affinities for others (IC50 values differing by 8- to 80-fold for choline and cimetidine, respectively). These results indicate that rbMATE1 and rbMATE2-K are multispecific OC/H+ exchangers with similar, but distinct, functional characteristics. Overall, the selectivity of MATE1 and MATE2-K correlated closely with that observed in rabbit renal brush-border membrane vesicles.

CC12, a high-affinity ligand for [3H]cimetidine binding, is an improgan antagonist

Improgan, a chemical congener of cimetidine, is a highly effective non-opioid analgesic when injected into the CNS. Despite extensive characterization, neither the improgan receptor, nor a pharmacological antagonist of improgan has been previously described. Presently, the specific binding of [(3)H]cimetidine (3HCIM) in brain fractions was used to discover 4(5)-((4-iodobenzyl)thiomethyl)-1H-imidazole, which behaved in vivo as the first improgan antagonist. The synthesis and pharmacological properties of this drug (named CC12) are described herein. In rats, CC12 (50-500nmol, i.c.v.) produced dose-dependent inhibition of improgan (200-400nmol) antinociception on the tail flick and hot plate tests. When given alone to rats, CC12 had no effects on nociceptive latencies, or on other observable behavioral or motor functions. Maximal inhibitory effects of CC12 (500nmol) were fully surmounted with a large i.c.v. dose of improgan (800nmol), demonstrating competitive antagonism. In mice, CC12 (200-400nmol, i.c.v.) behaved as a partial agonist, producing incomplete improgan antagonism, but also limited antinociception when given alone. Radioligand binding, receptor autoradiography, and electrophysiology experiments showed that CC12's antagonist properties are not explained by activity at 25 sites relevant to analgesia, including known receptors for cannabinoids, opioids or histamine. The use of CC12 as an improgan antagonist will facilitate the characterization of improgan analgesia. Furthermore, because CC12 was also found presently to inhibit opioid and cannabinoid antinociception, it is suggested that this drug modifies a biochemical mechanism shared by several classes of analgesics. Elucidation of this mechanism will enhance understanding of the biochemistry of pain relief.

Characterization of the transportation of berberine inCoptidis rhizoma extract through rat primary cultured cortical neurons

The aim of this study was to investigate the transport behavior and efflux of berberine through the primary culture cortical neurons. High-performance liquid chromatography coupled with an UV-vis detector at 347 nm was applied. The mobile phase was 0.05 m potassium dihydrogen phosphate solution (containing 0.5% triethylamine, pH 3.0)-acetonitrile (73:27, v/v). Neurons were incubated with Coptidis rhizoma extract 6.5 microg/mL (containing 1.91 microg/mL berberine) and verapamil, KCN or cimetidine for 2 h, and then lysed in methanol to extract intracellular berberine. A 20 microL aliquot of sample was injected into the HPLC system to determine berberine concentration. The results showed that metabolic inhibitor KCN and P-glycoprotein inhibitor verapamil could increase berberine concentration within the neurons, indicating that efflux of berberine was energy-dependent and P-glycoprotein was likely to be involved. Moreover, the organic cation transporter inhibitor cimetidine could decrease berberine concentration within the neurons, suggesting that the organic cation transporter might be involved in the berberine transport process.

A study of the in vitro interaction between lidocaine and premedications using human liver microsomes*

OBJECTIVE

To investigate potential interactions between lidocaine (lignocaine) metabolism and premedication drugs, i.e. psychotropic and antianxiety agents (diazepam, midazolam), hypnotics (pentobarbital, thiamylal), depolarizing neuromuscular blocking agents (vecuronium, pancuronium and suxamethonium), an antihypertensive agent (clonidine) and an H2-receptor blocking agent (cimetidine) using human liver microsomes in vitro.

METHODS

The interaction effects between lidocaine and premedication were examined using human liver microsomal preparations and monitored for enzyme activity. The lidocaine and its main metabolite (monoethylglycinexylide) were measured by HPLC/UV.

RESULTS

Lidocaine metabolism was non-competitively inhibited by midazolam (Ki = 77.6 microM). Thiamylal was a competitive inhibitor of lidocaine metabolism (Ki = 885 microM). Cimethidine, pancuronium and vecuronium weakly inhibited lidocaine metabolism in a concentration-depend manner over the therapeutic range in human liver microsomes. On the contrary, suxamethonium, pentobarbital and clonidine did not inhibit lidocaine metabolism over the therapeutic range in human liver microsomes.

CONCLUSION

These results show that the interactions between lidocaine and midazolam and thiamylal are of potential toxicological and clinical significance.

Relative contribution of OAT and OCT transporters to organic electrolyte transport in rabbit proximal tubule

We compared the characteristics of several cloned rabbit organic electrolyte (OE) transporters expressed in cultured cells with their behavior in intact rabbit renal proximal tubules (RPT) to determine the contribution of each to basolateral uptake of the weak acid ochratoxin A (OTA) and the weak base cimetidine (CIM). The activity of organic anion transporters OAT1 and OAT3 proved to be distinguishable because OAT1 had a high affinity for PAH ( Ktof 20 μM) and did not support estrone sulfate (ES) transport, whereas OAT3 had a high affinity for ES ( Ktof 4.5 μM) and a weak interaction with PAH (IC50> 1 mM). In contrast, both transporters robustly accumulated OTA. Intact RPT also accumulated OTA, with OAT1 and OAT3 each responsible for ∼50%: ES and PAH each reduced uptake by ∼50%, and the combination of the two eliminated mediated OTA uptake. The weak base CIM was transported by OAT3 ( Ktof 80 μM) and OCT2 ( Ktof 2 μM); OCT1 had a comparatively low affinity for CIM, and CIM uptake by OAT1 was equivocal. Intact RPT accumulated CIM, with TEA and ES reducing CIM uptake by 20 and 75%, respectively, suggesting that OAT3 plays a quantitatively more significant role in CIM uptake in the early proximal tubule than OCT1/2. In single S2 segments of RPT, ES and TEA each blocked ∼50% of CIM uptake. Thus the fractional contribution of different OE transporters to renal secretion is influenced by their affinity for substrate and relative expression level in RPT.

Different transport properties between famotidine and cimetidine by human renal organic ion transporters (SLC22A)

Histamine H2 receptor antagonist famotidine and cimetidine are commonly used for treatment of gastrointestinal ulcer diseases. Inasmuch as these drugs are mainly secreted by renal tubules, dosages have been adjusted according to renal function. Although many studies have been performed on the molecular mechanisms of renal handling of cimetidine, little is known about that of famotidine. In this study, to examine the recognition and transport of famotidine by human organic anion transporters (OATs; hOAT1, hOAT3) and human organic cation transporter (OCT; hOCT2), the uptake studies using Xenopus laevis oocytes were performed in comparison with cimetidine. The half-maximal inhibitory concentrations of famotidine for [3H]estrone sulfate transport by hOAT3 and [14C]tetraethylammonium transport by hOCT2 (300 microM and 1.8 mM, respectively) were higher than those of cimetidine (53 and 67 microM, respectively). While cimetidine inhibited p-[14C]aminohippurate transport by hOAT1 in a concentration dependent manner, famotidine did not affect it at 5 mM. In addition, hOAT3 mediated famotidine uptake, but hOAT1 and hOCT2 did not show famotidine transport. These results indicate that there are marked differences between famotidine and cimetidine in the recognition and transport by organic ion transporters and that hOAT3 contributes to the renal tubular secretion of famotidine. Present findings should be useful information to understand the renal handling of famotidine and cimetidine.

Cardiovascular effects of the toxin(s) of the Australian paralysis tick, Ixodes holocyclus, in the rat

An extract of toxin(s) from the Australian paralysis tick, Ixodes holocyclus, produced positive inotropic responses in rat left ventricular papillary muscles and positive contractile responses in rat thoracic aortic rings. There was no measurable chronotropic response in rat right atria, but positive inotropic concentrations in papillary muscles produced arrhythmias in right atria. Positive inotropic responses were attenuated by verapamil, but unaffected by metoprolol, cimetidine, pyrilamine, tetrodotoxin and pinacidil. Microelectrode studies on isolated left ventricular papillary muscles demonstrated that the extract prolonged action potential duration at 20, 50 and 90% of repolarisation and delayed ventricular papillary muscle relaxation. Cardiovascular tissues isolated from rats with experimentally induced tick paralysis showed no myocardial damage as identified by histological and ultrastructural examination. The basal rate and force of contraction of isolated cardiac tissues were lower from tick-paralysed than normal rats. Concentration-response curves to dobutamine and calcium chloride were similar between tissues from tick-paralysed and normal rats. Thus, the Australian paralysis tick, I. holocyclus, produces one or more toxins with direct cardiovascular effects which mimic the effects produced by direct blockade of cardiac and vascular K+ channels.

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.

Phenobarbital modifies antitumor effect of cyclophosphamide depending on the type of tumor cell death caused by it

Antitumor effect of cyclophosphamide on LS and P388 tumors is realized via apoptosis and on HA-1 and Krebs-2 tumors resistant to apoptosis via necrosis of tumor cells. Phenobarbital induction of cyclophosphamide-metabolizing enzymes decreases and cimetidine inhibition potentiates the effect of cyclophosphamide on LS and P388 cells and does not modulate the effect on HA-1 and Krebs-2 cells. Presumably, apoptosis and necrosis of tumor cell are induced by different cyclophosphamide metabolites.

Influence of lysine on cimetidine uptake and on excretion of cimetidine by the rat mammary gland

Cimetidine is actively transported into human and rat milk. However, the transporters involved have not been characterized. It is possible that xenobiotics may be actively transported into milk by an amino acid transport system. The objective of these studies was to determine the influence of lysine on the uptake of cimetidine into rat mammary explants (study 1), and on the excretion of cimetidine into rat milk (study 2). In study 1, excised lactating rat mammary epithelial tissue fragments were exposed to 3H-cimetidine and 14C-lysine in the presence of 10 microM, 1 mM, or 1 M cold lysine, and the uptake of 3H-cimetidine and 14C-lysine were measured by liquid scintillation counting after 5 or 20 minutes of incubation. After 5 minutes of incubation, 1 M lysine inhibited 3H-cimetidine uptake by 47.7% (SD +/- 6.5%), compared with 10 microM lysine (P < 0.05), and 14C-lysine uptake was also inhibited by 54.1% (SD +/- 6.4%) (P < 0.05). Similar results were seen after 20 minutes of incubation. In a randomized crossover study (study 2), 6 lactating female rats were infused to steady state with cimetidine (0.5mg/h) in the presence or absence of lysine (360mg/h). Cimetidine concentrations in serum and milk were determined by high-performance liquid chromatography. Cimetidine systemic clearance (28.6+/-15.0mL/kg/min vs. 38.9+/-3.9 mL/kg/min, mean +/- SD) and milk to serum cimetidine ratio (M/S) (28.0+/-16.1 vs. 28.9+/-6.7), respectively, were not significantly altered by the presence or absence of lysine. Although 1 M lysine inhibited uptake of cimetidine in rat mammary explants, the concentrations of lysine used in this study, which approached toxicity in vivo, produced no significant effects on cimetidine transport into milk or the systemic clearance of cimetidine.

Effects of N-alpha-methyl-histamine on human H(2) receptors expressed in CHO cells

BACKGROUND

Production of N-alpha-methyl-histamine (NAMH), a histamine H(3) receptor (H3R) agonist, is reportedly promoted in Helicobacter pylori infected human gastric mucosa. NAMH was suggested to act directly on histamine H(2) receptors (H2Rs) in animals to stimulate acid secretion and to be a H2R agonist. As H2Rs and H3Rs play different roles in gastric acid secretion, it is very important to verify that NAMH is a H2R agonist.

AIMS

To determine whether NAMH is a H2R agonist, as well as a H3R agonist.

METHODS

We used a Chinese hamster ovary (CHO) cell line expressing human H2Rs (CHO-H2R) and control CHO cells. Expression of human H2Rs was confirmed by tiotidine binding. cAMP production in CHO-H2R and control cells in response to histamine or NAMH was measured. cAMP production in response to 10(-7) M NAMH was also measured in the presence or absence of the H2R antagonist famotidine and the H3R antagonist thioperamide.

RESULTS

NAMH dose dependently stimulated cAMP productions in CHO-H2R cells. This production was inhibited by famotidine but not by thioperamide. Control CHO cells were unresponsive to either histamine or NAMH. In addition, the effect of NAMH, in terms of cAMP production in CHO-H2R cells, was more potent than that of histamine-that is, with a lower EC(50) concentration and higher maximal cAMP production. Both NAMH and histamine, but not R-alpha-methyl-histamine, effectively inhibited [(3)H] tiotidine binding to CHO-H2R cells.

CONCLUSIONS

NAMH, which is produced in the gastric mucosa by H pylori, is a potent H2R agonist as well as a H3R agonist.

Pd(II)- and Pt(II)-cimetidine complexes. Crystal structure of trans-[Pt(N,S-cimetidine)(2)]Cl(2)(*)12H(2)O

The influence of cimetidine on patients under cisplatin treatment for cancer is controversial. It has moderate or no effects on several types of cancer and its effects on the nephrotoxicity induced by cisplatin are uncertain. To examine the binding properties and antiproliferative effects of the known anticancer noble metals, cimetidine (cim) was complexed to platinum(II) and palladium(II). The crystal structure of the Pt-cim compound shows two molecules of cimetidine coordinated to the metal through thioether sulfur and imidazolic nitrogen whereas spectroscopic studies in solution for Pd-cim reveal that the ratio of the metal to cimetidine is 1:1 with identical coordination environments. To determine the antitumor activity of the drugs, the interaction of the metallic complexes and free cimetidine with DNA was assessed. Their cytotoxic activity was compared with that of cisplatin.

Susceptibility of the H2-receptor antagonists cimetidine, famotidine and nizatidine, to metabolism by the gastrointestinal microflora

The H(2)-receptor antagonist ranitidine has previously been shown to be a substrate for colonic bacterial metabolism. The objective of the present study was to assess the in vitro stability of the other H(2)-receptor antagonists, cimetidine, famotidine and nizatidine, to colonic bacteria. One hundred milligrams of each drug were introduced into individual batch culture fermenters (100 ml) consisting of buffer medium inoculated with freshly voided human faeces (10% w/v). Control experiments, equivalent drug quantities in buffer medium without the presence of faeces, were also run in parallel. Samples were removed at set time intervals over a 24 h period and were subsequently analysed by HPLC. A selection of the samples removed from the fermenters was also subjected to analysis by UV spectroscopy and mass spectrometry. Following an initial dissolution phase in the fermentation system, a marked decline in nizatidine concentration was noted over time with virtually no drug remaining after 12 h, thereby suggesting degradation and metabolism of the drug by colonic bacteria. No such decline in concentration was noted for cimetidine or famotidine or for any of the drugs in the control buffer systems. The metabolic reaction pathway for nizatidine was complex, although UV and mass spectrometry analysis indicated that metabolism was initiated via cleavage of an N-oxide bond within the molecule. These results in combination with those obtained from a previous study indicate that of the four commercially available H(2)-receptor antagonists, nizatidine and ranitidine are susceptible to metabolism by colonic bacteria, which in turn has ramifications for drug delivery and absorption.

G649, an allelic variant of the human H2 receptor with low basal activity, is resistant to upregulation upon antagonist exposure

Orange et al reported an allelic variant of the human histamine H2 receptor, in which adenine 649 was replaced with guanine, to be more frequent in the schizophrenic population than controls in British Caucasians. The A649 to G change causes an Asn to Asp transition at amino acid position 217 in the third intracellular region, which is postulated to be important for receptor function. Herein, we analyzed the functional significance of this variant using wild-type and variant receptors expressed in Chinese hamster ovary cells. The variant receptor was associated with markedly lower basal cAMP productions than the wild-type receptor. Histamine-dependent cAMP productions via the variant receptor were lower as well. Treatment of cells expressing variant receptors with 10(-5) M ranitidine for 24 h resulted in a reduced degree of receptor upregulation as compared with the wild-type receptor. Thus, this is the first report of an allelic variant of the human H2 receptor which confers altered receptor function. To analyze gastric acid secretion in individuals with this variant, we examined 100 Japanese control subjects. However, neither heterozygotes nor homozygotes were found, suggesting that this variant, if present, is uncommon in the Japanese population.

Characterization of histamine H(2)-like receptors in duck cerebral cortical membranes by [(3)H]tiotidine binding

A selective (according to mammalian criteria) histamine (HA) H(2)-receptor radioligand [(3)H]tiotidine ([(3)H]TIOT) was used to characterize HA receptors in duck cerebral cortex by an in vitro binding technique. The specific binding of [(3)H]TIOT to duck cerebral cortical membranes was found to be rapid, stable, saturable, reversible, and of high affinity. Saturation analysis resulted in a linear Scatchard plot suggesting binding to a single class of receptor binding sites with high affinity (K(d)=19.5 nM) and high capacity (B(max)=356 fmol/mg protein). Competition studies showed the following relative rank order of potency of various HA receptor ligands to inhibit the [(3)H]TIOT binding: antagonists, tiotidine>>ranitidine=zolantidine>or=cimetidine>>mepyramine>thioperamide; agonists, HA>or=4-methylHA>2-methylHA>dimaprit>>R(alpha)-methylHA. The biphasic nature of the displacement curve for HA and the effect of 5'-guanylimidodiphosphate indicate the coupling of the studied receptor to G-protein. It is suggested that HA receptors in the duck cerebral cortex labelled with [(3)H]TIOT represent either avian-specific H(2)-like HA receptors or a novel subtype of HA receptors, coupled to a signalling pathway other than the adenylyl cyclase/cyclic adenosine monophosphate one.

Interaction of cimetidine with P450 in a mouse model of hepatocarcinogenesis initiation

Many drugs and xenobiotics are lipophilic and they should be transformed into more polar water soluble compounds to be excreted. Cimetidine inhibits cytochrome P450. The aim of this study was to investigate the preventive and/or reversal action of cimetidine on cytochrome P450 induction and other metabolic alterations provoked by the carcinogen p-dimethylaminoazobenzene. A group of male CF1 mice received a standard laboratory diet and another group was placed on dietary p-dimethylaminoazobenzene (0.5% w w(-1). After 40 days of treatment, animals of both groups received p-dimethylaminoazobenzene and two weekly doses of cimetidine (120 mg kg(-1), i.p.) during a following period of 35 days. Cimetidine prevented and reversed delta-aminolevulinate synthetase induction and cytochrome P450 enhancement provoked by p-dimethylaminoazobenzene. However, cimetidine did not restore haem oxygenase activity decreased by p-dimethylaminoazobenzene. Enhancement in glutathione S-transferase activity provoked by p-dimethylaminoazobenzene, persisted in those animals then treated with cimetidine. This drug did not modify either increased lipid peroxidation or diminution of the natural antioxidant defence system (inferred by catalase activity) induced by p-dimethylaminoazobenzene. In conclusion, although cimetidine treatment partially prevented and reversed cytochrome P450 induction, and alteration on haem metabolism provoked by p-dimethylaminoazobenzene AB, it did not reverse liver damage or lipid peroxidation. These results further support our hypothesis on the necessary existence of a multiple biochemical pathway disturbance for the onset of hepatocarcinogenesis initiation.

Potent and long-lasting action of lafutidine on the human histamine H(2) receptor

BACKGROUND/AIM

Based on animal models, lafutidine, a novel histamine H(2) receptor (H(2)R) antagonist, is reported to show potent and long-lasting antagonisms of histamine H(2)R-mediated effects. However, no reports have been published concerning its direct interaction with the human H(2)R. This study aims at characterizing its interaction with human H(2)R.

METHODS

Chinese hamster ovary cell lines stably expressing human H(2)Rs were obtained. The dose-dependent effects of lafutidine and famotidine on [(3)H]tiotidine binding and histamine-stimulated cAMP production were analyzed. The effects of preincubation with 2.78 x 10(-7) M of lafutidine or famotidine for 30 min on histamine-dependent cAMP production and [(3)H]tiotidine binding were also examined after 0, 1, 2, 4, and 12 h. This concentration is below the C(max) of lafutidine (10 mg p.o.) and above the C(max) of famotidine (20 mg p.o.).

RESULTS

Lafutidine inhibited [(3)H]tiotidine binding and histamine-stimulated cAMP production as or more potently than famotidine. At higher concentrations lafutidine was more potent than famotidine. In addition, preincubation with 2.78 x 10(-7) M lafutidine, but not with 10(-5) M famotidine, had marked inhibitory effects which persisted as long as after extensive washing.

CONCLUSION

Lafutidine shows a potent and long-lasting antagonism on the human H(2)R.

Distinct interaction of human and guinea pig histamine H2-receptor with guanidine-type agonists

It is unknown why the potencies and efficacies of long-chained guanidine-type histamine H2-receptor (H2R) agonists are lower at the H2R of human neutrophils than at the H2R of the guinea pig atrium. To elucidate these differences, we analyzed fusion proteins of the human H2R (hH2R) and guinea pig H2R (gpH2R), respectively, and the short splice variant of Gsalpha (GsalphaS) expressed in Sf9 cells. The potencies and efficacies of small H2R agonists in the GTPase assay and the potencies of antagonists at inhibiting histamine-stimulated GTP hydrolysis by hH2R-GsalphaS and gpH2R-GsalphaS were similar. In contrast, the potencies and efficacies of guanidines were lower at hH2R-GsalphaS than at gpH2R-G(salphaS). Guanidines bound to hH2R-GsalphaS with lower affinity than to gpH2R-GsalphaS, and high-affinity binding of guanidines at gpH2R-GsalphaS was more resistant to disruption by GTPgammaS than binding at hH2R-GsalphaS. Molecular modeling suggested that the nonconserved Asp-271 in transmembrane domain 7 of gpH2R (Ala-271 in hH2R) confers high potency to guanidines. This hypothesis was confirmed by Ala-271-->Asp-271 mutation in hH2R-GsalphaS. Intriguingly, the efficacies of guanidines at the Ala-271-->Asp-271 mutant and at hH2R/gpH2R chimeras were lower than at gpH2R. Our model suggests that a Tyr-17/Asp-271 H-bond, present only in gpH2R-GsalphaS but not the other constructs studied, stabilizes the active guanidine-H2R state. Collectively, our data show 1) distinct interaction of H2R species isoforms with guanidines, 2) that a single amino acid in transmembrane domain 7 critically determines guanidine potency, and 3) that an interaction between transmembrane domains 1 and 7 is important for guanidine efficacy.

Histamine deficiency induces tissue-specific down-regulation of histamine H2 receptor expression in histidine decarboxylase knockout mice

Histidine decarboxylase (HDC) is the single enzyme responsible for histamine synthesis. HDC-deficient mice (HDC(-/-)) have no histamine in their tissues when kept on a histamine-free diet. Therefore, the HDC(-/-) mice provide a suitable model to investigate the involvement of histamine in the regulation of histamine receptor expression. Gene expression of H1 and H2 histamine receptors was studied in several organs of HDC(-/-) mice and compared to standard (HDC(+/+)) mice. In many tissues, prolonged absence of histamine induced down-regulation of the H2 receptor subtype. The expression of the H1 receptor was less sensitive to histamine deficiency. Exogenous histamine present in the diet abolished the differences observed in H2 receptor expression. These results suggest that the expression of mouse H2 receptor is under the control of histamine in a tissue-specific manner.

Arginine 454 and lysine 370 are essential for the anion specificity of the organic anion transporter, rOAT3

Organic anion transporters (OATs) and organic cation transporters (OCTs) mediate the flux of xenobiotics across the plasma membranes of epithelia. Substrates of OATs generally carry negative charge(s) whereas substrates of OCTs are cations. The goal of this study was to determine the domains and amino acid residues essential for recognition and transport of organic anions by the rat organic anion transporter, rOAT3. An rOAT3/rOCT1 chimera containing transmembrane domains 1-5 of rOAT3 and 6-12 of rOCT1 retained the specificity of rOCT1, suggesting that residues involved in substrate recognition reside within the carboxyl-terminal half of these transporters. Mutagenesis of a conserved basic amino acid residue, arginine 454 to aspartic acid (R454D), revealed that this amino acid is required for organic anion transport. The uptakes of p-aminohippurate (PAH), estrone sulfate, and ochratoxin A were approximately 10-, approximately 48-, and approximately 32-fold enhanced in oocytes expressing rOAT3 and were only approximately 2-, approximately 6-, and approximately 5-fold enhanced for R454D. Similarly, mutagenesis of the conserved lysine 370 to alanine (K370A) suggested that K370 is important for organic anion transport. Interestingly, the charge specificity of the double mutant, R454DK370A, was reversed in comparison to rOAT3-R454DK370A preferentially transported the organic cation, MPP(+), in comparison to PAH (MPP(+) uptake/PAH uptake = 3.21 for the double mutant vs 0.037 for rOAT3). These data indicate that arginine 454 and lysine 370 are essential for the anion specificity of rOAT3. The studies provide the first insights into the molecular determinants that are critical for recognition and translocation of organic anions by a member of the organic anion transporter family.

Mutagenicity of cimetidine in nitrite-enriched human gastric juice

A mutagenic response was obtained in the Salmonella/microsome reversion test by preincubating sodium nitrite and cimetidine in human gastric juice from untreated individuals, or even by adding nitrite to gastric juice samples from patients receiving cimetidine. Both base-pair substitutions (strains TA1535 and TA100) and, though very weakly, also frameshift errors (TA1537, TA1538 and TA98) were induced by such reaction. Mutagenicity was not affected by S-9 mix containing rat liver homogenates, neither in the sense of activation nor of deactivation. The optimal reaction occurred at high equimolar concentrations of the two precursor compounds, under physiological pH and temperature conditions and within a short time of contact. Ascorbic acid was efficient in preventing the formation of mutagenic nitrosoderivative(s).

Analysis of drug transport and metabolism in cell monolayer systems that have been modified by cytochrome P4503A4 cDNA-expression

Human CYP3A4, the major human, intestinal, drug metabolizing cytochrome P450, has been introduced into three mammalian cell lines (Caco-2, MDCK and LLC-PK1) suitable for making drug permeability measurements. The levels and stability of expression were analyzed by enzyme assays (testosterone 6beta-hydroxylase and nifedipine oxidase). Long term, stable CYP3A4 expression/cell growth rate was obtained in MDCK cells. In the LLC-PK1 system, shorter term, stable expression was achieved. However, in Caco-2 cells, derivatives with better properties than those previously reported could not be obtained. The highest level of CYP3A4 catalytic activity was obtained in LLC-PK1 cells. In this system, CYP3A4 activity levels appeared comparable to median level human intestinal microsomes. Metabolite formation and inhibition kinetics were examined in cell monolayers. Nifedipine was found to be extensively metabolized (19%) during passage across cell monolayers. In general, affinity related parameters (apparent Km and apparent Ki) were 1.5- to three-fold higher under conditions of flux through the monolayers relative to steady-state conditions. These systems should be useful for examining the role of intestinal CYP3A4 in first-pass metabolism and drug-drug interactions.

Molecular properties of central and peripheral histamine H1 and H2 receptors

We identified and characterised histamine H1 and H2 receptor subtypes on rat cortical astrocytes in primary culture with radioligand binding studies and compared their molecular properties with peripheral (bovine vascular smooth muscle and endothelial cells) histamine H1 and H2 receptors. Our results showed the existence of a homogenous population of high affinity binding sites for 3H-mepyramine (Bmax = 281 fmol/mg protein, K(D) = 3.5 +/- 0.7 nM) and 3H-tiotidine (Bmax = 59 fmol/mg protein, K(D) = 1.9 +/- 0.7 nM) on astrocytes, which was further confirmed by competition binding studies using various H1 and H2 specific agonists/antagonists. We showed differences in the density of receptors and differences in the affinities of competing drugs for the same histamine receptor subtype (H1 and H2) between the tissues used, which indicate different molecular properties of the central and peripheral histamine receptors.

Regulation of paracellular absorption of cimetidine and 5-aminosalicylate in rat intestine

PURPOSE

Isolating the relative contributions of parallel transcellular and paracellular transport to the intestinal absorption of small hydrophilic molecules has proven experimentally challenging. In this report, lumenal appearance of drug metabolite is utilized as a tool to assess the contribution of paracellular transport to the absorption of cimetidine and 5-aminosalicylate (5ASA) in rat small intestine.

METHODS

Steady-state intestinal absorption and elimination of cimetidine and 5ASA were studied in single-pass intestinal perfusions in rats.

RESULTS

Both drugs were metabolized in intestinal epithelia with subsequent metabolite secretion into the intestinal lumen. Jejunal cimetidine absorption decreased with increasing perfusion concentration while the ratio of lumenal metabolite to lumenal drug loss increased. Cimetidine uptake at perfusion concentrations above 0.4 mM resulted in over 80% drug elimination into the jejunal lumen. Inhibition of intracellular metabolism of cimetidine by methimazole did not alter epithelial uptake but totally abolished transepithelial cimetidine flux indicating an elevation of intracellular cimetidine. Similarly, co-perfusion of 5ASA with cimetidine and methimazole totally abolished 5ASA absorption but increased lumenal levels of N-acetyl 5ASA indicating an increase in intracellular uptake of 5ASA.

CONCLUSIONS

Cimetidine and 5ASA absorption across rat jejunal epithelia are exclusively paracellular. Elevation of intracellular cimetidine, inferred from mass balance considerations, restricts paracellular transport of both drugs.

Cimetidine transport in brush-border membrane vesicles from rat small intestine

In previous studies, sulfoxide metabolite was observed in animal and human intestinal perfusions of cimetidine and other H2-antagonists. A sequence of follow-up studies is ongoing to assess the intestinal contributions of drug metabolism and drug and metabolite transport to variable drug absorption. An evaluation of these contributions to absorption variability is carried out in isolated fractions of the absorptive cells to uncouple the processes involved. In this report, data is presented on the drug entry step from a study on [3H]cimetidine uptake into isolated brush-border membrane vesicles from rat small intestine. A saturable component for cimetidine uptake was characterized with a Vmax and Km (mean +/- S.E.M.) of 6.1 +/- 1.5 nmol/30s/mg protein and 8.4 +/- 2.0 mM, respectively. Initial binding, and possibly intravesicular uptake, was inhibited by other cationic compounds including ranitidine, procainamide, imipramine, erythromycin, and cysteamine but not by TEA or by the organic anion, probenecid. Initial uptake was not inhibited by amino acids methionine, cysteine, or histidine, by the metabolite cimetidine sulfoxide, or by inhibitors of cimetidine sulfoxidation, methimazole, and diisothiocyanostilbene-2,2'-disulfonic acid. Equilibrium uptake was inhibited by ranitidine, procainamide, and cysteamine but not by erythromycin or imipramine. Initial cimetidine uptake was stimulated by an outwardly directed H+ gradient, and efflux was enhanced by an inwardly directed H+ gradient. Collapse of the H+ gradient as well as voltage-clamping potential difference to zero significantly reduced initial cimetidine uptake. The data is supportive of both a cimetidine/H+ exchange mechanism and a driving-force contribution from an inside negative proton or cation diffusion potential.

Roles of histamine receptor in a guinea pig asthma model

Histamine plays an important role in bronchoconstriction mediated by histamine receptors which provoke bronchial asthma attack. In this study, we measured H1 and H2 receptors in the guinea pig lung membrane fraction and obtained the following results. The maximum binding (Bmax) of H1 receptors in the guinea pig lung membrane fraction was significantly higher in the OA-sensitized group than that in the non-sensitized group, but affinity (Kd) did not differ between the groups. Otherwise, the maximum binding (Bmax) of H2 receptors in the guinea pig lung membrane fraction was significantly lower in the OA-sensitized group than that in the non-sensitized group. But affinity (Kd) did not differ between the groups. These findings suggest a close association of Histamine receptors both H1 and H2 in the pathology of asthma.

SAR studies on H2 antagonists containing alkylamino substituted 1,2, 5-thiadiazole 1-oxide moieties

A number of ranitidine analogues in which the diamino-1,2,5-thiadiazole 1-oxide substructure bearing alkyl chains of different length is present as the urea equivalent group, were synthesised and studied for their lipophilic and H2 antagonist properties. Derivatives which displayed a logP < or = 3 behaved as competitive antagonists of histamine at H2 receptors present on guinea pig right atrium. The remaining more lipophilic members of the series showed an insurmountable antagonism not completely reversible after prolonged washing. A binding study suggested that an increase in the length of alkyl chain gave rise to hydrophobic interactions with the receptor which were responsible for the apparent irreversible H2 antagonism shown by the higher homologues of the series.

Characterization of the histamine H2 receptor structural components involved in dual signaling

We previously demonstrated that the histamine H2 receptor can activate both adenylate cyclase (AC) and phospholipase C (PLC) signaling pathways via separate GTP-dependent mechanisms. We examined whether H2 receptor-specific peptides corresponding to the amino (N) or carboxyl terminus (C) of the second (2i) or third (3i) intracytoplasmic loops or the carboxyl terminal tail (P4iN) could effect histamine- stimulated AC and PLC activity in cell membranes prepared from HEPA cells stably transfected to express the canine H2 histamine receptor cDNA. Tiotidine binding and basal signaling were not altered by the synthetic peptides. H2P2iN, H2P2iC, H2P3iN and H2P4iN did not effect histamine stimulated AC activity although H2P3iC (10(-4) M) significantly inhibited this parameter (65.6 +/- 7.2% of maximal stimulation) (n = 6). Combination of the five peptides (H2P2iN, H2P2iC, H2P3iN, H2P3iC and H2P4iN) abolished histamine stimulated AC activity. Although all of the peptides inhibited histamine-stimulated PLC activity to a moderate degree individually, H2P3iC (10(-4) M) had the greatest effect, decreasing PLC activation to 20.8 +/- 6.3% of maximal stimulation (IC50 = 7.5 X 10(-7) M) (n = 6). H2P3iC and the peptide combination did not alter, forskolin, GTP gamma s or epinephrine-stimulated AC activity nor GTP gamma s and vasopressin-stimulated PLC. These studies demonstrate that both the second and third intracytoplasmic loops of the histamine H2 receptor are linked to separate signaling pathways in a differential manner.

Substituted pyrido[3,2-b]oxazin-3(4H)-ones: synthesis and evaluation of antinociceptive activity

A new series of N-substituted pyrido[3,2-b]oxazinones has been synthesized, pharmacologically evaluated, and compared with acetyl salicylic acid. The compound with the maximal combination of safety and analgesic efficacy was 4-¿3-[4-(4-fluorophenyl-1-piperazinyl)propyl]¿-2H-pyrido[3,2-b]-1, 4-oxazin-3(4H)-one (6c) with ED50 values of 12.5 mg/kg po (mouse: phenylquinone writhing test) and 27.8 mg/kg po (rat: acetic acid writhing test), respectively. Compound 6c proved to be more active than aspirin with a safety index of 5.1.

Prediction of in vivo drug interaction from in vitro systems exemplified by interaction between verapamil and cimetidine using human liver microsomes and primary hepatocytes

Emphasis on drug safety is increasing as newly developed drugs become more potent. Interest in the prediction and description of drug interactions is growing accordingly. The study of potential interactions at a very early stage of drug development requires suitable in vitro models that describe drug interactions both qualitatively and quantitatively. The purpose of the work described here was to help assess the predictive value of in vitro drug interaction tests with liver microsomes and hepatocytes by means of the interaction between verapamil and cimetidine. The in vitro inhibition of verapamil metabolism by cimetidine observed during the studies was quantitatively similar to the results reported in published clinical studies after intravenous application. Studies using liver microsome fractions showed that the intrinsic clearances for the formation of various metabolites could be used to predict drug interactions. In addition, work with hepatocyte cultures revealed that an in vitro system covering both phase I and phase II reactions should be included in such studies to permit quantitative prediction of the various metabolic pathways. Both human hepatocyte cultures and human microsomes offer certain advantages for predicting the degree of drug metabolism and interactions in humans at the biotransformation level. Therefore, it seems likely that the simultaneous application of both systems will yield conclusions that most closely approximate the situation in humans.

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.

Role of cytochrome P450 enzymes in drug-drug interactions

Many adverse drug-drug interactions are attributable to pharmacokinetic problems and can be understood in terms of alterations of P450-catalyzed reactions. Much is now known about the human P450 enzymes and what they do, and it has been possible to apply this information to issues related to practical problems. A relatively small subset of the total number of human P450s appears to be responsible for a large fraction of the oxidation of drugs. The three major reasons for drug-drug interactions involving the P450s are induction, inhibition, and possibly stimulation, with inhibition appearing to be the most important in terms of known clinical problems. With the available knowledge of human P450s and reagents, it is possible to do in vitro experiments with drugs and make useful predictions. The results can be tested in vivo, again using assays based on our knowledge of human P450s. This approach has the capability of not only improving predictions about which drugs might show serious interaction problems, but also decreasing the number of in vivo interaction studies that must be performed. These approaches should improve with further refinement and technical advances.

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.

Atypical association of H1 and H2 histamine receptors with signal transduction pathways during multistage mouse skin carcinogenesis

OBJECTIVE

In the present work we studied the association of histamine receptors with second messengers during multistage carcinogenesis in Sencar mice skin.

METHODS

96 Sencar female mouse, divided into six groups were used. Tumors appeared only in the 7, 12-dimethylbenz[a]anthracene-initiated and 12-O-tetradecanoylphorbol-13-acetate-promoted group. Control groups received only TPA, or acetone or no treatment at all. Periodically during the promotion period, cAMP and inositol phosphate production were measured after stimulation with H1 or H2 agonists in samples from all groups.

RESULTS

In non-treated skin, H1 receptors were coupled to phosphatidylinositol hydrolysis and H2 receptors mediated cAMP production. Conversely, in tumors H2 receptors were associated with phosphatidylinositol hydrolysis and H1 mediated a rise in cAMP levels. The skin among tumors and the skin from all control groups maintained the same coupling as non-treated skin. An increase in mast cell number, with a homogeneous subepithelial distribution and marked phenotypic changes, was also observed in promoted skin.

CONCLUSIONS

These findings indicate an atypical association of histamine receptors with second messengers that could be a critical feature for the postulated action of histamine in tumor growth.

Role of the C terminus in histamine H2 receptor signaling, desensitization, and agonist-induced internalization

To evaluate the role of the histamine H2 receptor C terminus in signaling, desensitization, and agonist-induced internalization, canine H2 receptors with truncated C termini were generated. Wild-type (WT) and truncated receptors were tagged at their N termini with a hemagglutinin (HA) epitope and expressed in COS7 cells. Most of the C-terminal intracellular tail could be truncated (51 of 70 residues, termed T308 mutant) without loss of functions: cAMP production, tiotidine binding, and plasma membrane targeting. In fact, the T308 mutant produced more cAMP than the WT when cell-surface expression per cell was equivalent. Pretreatment of cells with 10(-5) M histamine desensitized cAMP productions via WT and T308 receptors to similar extents. Incubation of cells expressing WT receptors with 10(-5) M histamine reduced cell-surface anti-HA antibody binding by approximately 30% (by 30 min, t1/2 approximately 15 min), but did not affect the Bmax of tiotidine in membrane fractions, which represents total receptor amounts, suggesting that WT receptors were internalized from the cell surface. In contrast, no internalization was observed with T308 receptors following histamine treatment. A mutant with a deletion of the 30 C-terminal amino acids, termed T329, was functional but was as potent as the WT in terms of cAMP production. Apart from being desensitized by histamine, the internalization of the receptor was indistinguishable from that of the WT. Internalization was observed in the T320 but not in T313 mutant, narrowing the region involved in internalization to that between Glu314 and Asn320 (ETSLRSN). Of these seven residues, either Thr315, Ser316, or both, were replaced with Ala. Thr315 and Ser316 are conserved among species. The mutation at Thr315 (but not that at Ser316) abolished internalization. Taken together, these results demonstrate that Thr315 is involved in agonist-induced internalization. Furthermore, the finding that T308 receptors were desensitized in the absence of internalization suggests that internalization and desensitization are meditated by independent mechanisms.

Action of ebrotidine, ranitidine and cimetidine on the specific binding to histamine H1- and H2-receptors

Ebrotidine (N-[(E)-[[2-[[[2-[(diaminomethylene)-4-thiazolyl]methyl]thio] ethyl]amino]methylene]-4-bromo-benzenesulfonamide, CAS 100981-43-9, FI-3542), a selective H2-receptor antagonist, has proved to competitively inhibit the positive chronotropism induced by histamine in isolated guinea pig atrium. The affinity of ebrotidine to histamine H1- and H2-receptors through the displacement of 3H-pyrilamine and 3H-thiotidine binding to guinea pig cerebellum and brain cortex membranes was investigated. Ebrotidine displaced 3H-thiotidine specific binding to histamine H2-receptors (Ki: 127.5 nmol/l), showing a higher affinity (p < 0.05) than ranitidine (Ki: 190.0 nmol/l) and cimetidine (Ki: 246.1 nmol/l). None of the three substances displaced 3H-pyrilamine binding to H1-receptors (Ki: > 5000 nmol/l). The results showed that ebrotidine is a drug with a high affinity for H2 receptors, higher than cimetidine and ranitidine.

Mediation of cimetidine secretion by P-glycoprotein and a novel H(+)-coupled mechanism in cultured renal epithelial monolayers of LLC-PK1 cells

1. Previous studies have shown that the weak base, cimetidine, is actively secreted by the renal proximal tubule. In this study we have examined the transport of cimetidine by renal LLC-PK1 epithelial cell monolayers. 2. In LLC-PK1 cell monolayers the basal-to-apical flux of cimetidine was significantly greater than the apical-to basal flux, consistent with net secretion of cimetidine in a basal-to-apical direction. 3. Net secretion of cimetidine was significantly (70%) reduced by the addition of either 100 microM verapamil or 100 microM nifedipine to the apical membrane. The reduction in net secretion was the result of an inhibition of basal-to-apical flux; these agents had no effect upon flux in the apical-to-basal direction. These results suggest that cimetidine secretion is mediated primarily by P-glycoprotein located in the apical membrane. In addition we found no evidence of a role for organic cation antiport in the secretion of cimetidine. 4. In the presence of an inwardly directed proton gradient across the apical membrane (pH 6.0), cimetidine secretion was significantly reduced compared to that measured at an apical pH of 7.4. The reduction in net secretion at pH 6.0 was the result of a stimulation of cimetidine uptake across the apical membrane. This pH-dependent uptake mechanism was sensitive to inhibition by DIDS (100 microM). 5. Experiments with BCECF (2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein) loaded monolayers demonstrated that cimetidine influx across the apical membrane was associated with proton flow into the cell and was sensitive to inhibition by DIDS. 6. These results suggest that net secretion of cimetidine across the apical membrane is a function of the relative magnitudes of cimetidine secretion mediated by P-glycoprotein and cimetidine absorption mediated by a novel proton-coupled, DIDS-sensitive transport mechanism.

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.