Pharmacokinetic analysis of the FAK scaffold inhibitor C4 in dogs

Inhibition of focal adhesion kinase-vascular endothelial growth factor receptor 3 complex by C4 was previously shown to reduce tumor growth alone and synergistically with other chemotherapeutic agents in animal tumor models. Single and multiple dose IV and oral dosing studies were performed in dogs to determine C4 pharmacokinetics. C4 was administered to 4 dogs at 1.25 or 2.50 mg/kg IV, or 7.50 mg/kg oral gavage. Single- (IV and oral) and multiple- (IV) dose pharmacokinetic samples were collected on days 1 and 3 at pre-dose and 0.5, 1, 2, 4, 8, 24, 120, 144, and 168 h post-dose. C4 concentrations were determined using liquid chromatography with tandem mass spectral detection with a limit of quantitation of 2.50 pg/mL. Pharmacokinetics of C4 was characterized by a 3-compartment model with linear distributional and elimination clearances using Phoenix 64 WinNonlin 6.3. Mean C4 plasma concentration-time profiles revealed a triexponential decline following either IV or oral administration, independent of dose with no accumulation. For the 2.5 mg/kg dose, the median half-life was ~21 h. Median C max and area under the curve (AUC0-24) were similar for days 1 and 3. Oral bioavailability for formulations of PBS, TPGS, Maalox(®), and Pepcid(®) was greatest with TPGS (45 %), followed by Maalox(®) (42 %), Pepcid(®) (37 %), and PBS (30 %). The pharmacokinetic study revealed that C4 has linear pharmacokinetics and does not accumulate following multiple-dose administration. Characterization of C4 pharmacokinetics provides a better understanding of the novel targeted agent, which will help facilitate further development of C4.

Peripheral and central H1 histamine receptor occupancy by levocetirizine, a non-sedating antihistamine; a time course study in the guinea pig

BACKGROUND AND PURPOSE

The H(1) receptor occupancy (H1RO) in brain is an indicator of central side effects of antihistamines. Here, we determined the kinetics of central and peripheral H1RO by levocetirizine in relation to its brain and plasma concentration, and investigated the role of the blood-brain barrier in any delay in brain H1RO.

EXPERIMENTAL APPROACH

Concentration-time profiles in plasma and brain were obtained after 0.1 and 1 mg kg(-1) oral doses of levocetirizine in guinea pigs. H1RO in brain was measured ex vivo using [3H]-mepyramine and, in the periphery, by measuring the degree of inhibition of histamine-induced contractions of isolated guinea pig ileum.

KEY RESULTS

The concentration-time profile of levocetirizine indicated lower levels (partition coefficient, K(p)=0.06-0.08), higher t(max) (2-4 h vs 1-1.5 h) and longer terminal half-life (4-5.6 h vs 2.1-2.8 h) in brain than plasma. The H1RO at 0.1 and 1 mg kg(-1) were 75% and 97%, respectively, at 1 hr in the periphery and, in the brain, were <20% and 28-67% respectively, at all time points studied. Brain H1RO vs plasma concentrations profile showed a delay, but not when compared to brain concentrations.

CONCLUSIONS AND IMPLICATIONS

This study demonstrates an effective peripheral antihistamine effect of levocetirizine without central adverse effects at the dose close to human therapeutic dose. The slow increase in H1RO in the brain with time was caused by slow blood-brain barrier transport of levocetirizine. This demonstrates the importance of measuring time course of brain H1RO in relation to brain concentrations of drugs.

Mutation in an Adaptor Protein PDZKI Affects Transport Activity of Organic Cation Transporter OCTNs and Oligopeptide Transporter PEPT2

Genetic polymorphisms in xenobiotic transporters have recently been clarified to be associated with change in drug distribution and disposition. To expand on recent identification of direct interaction and functional regulation of several transporters by a PDZ (PSD95, Dlg and ZO1) domain containing protein PDZK1, the effect of mutation in PDZK1 on transport activity and subcellular localization of organic cation/carnitine transporters OCTN1 and OCTN2, and oligopeptide transporter PEPT2 was examined in the present study. HEK293 cells stably expressing a mutant transcript PDZK1-E195K (HEK293/PDZK1-E195K) were constructed, followed by transient transfection of cDNA for each transporter. Uptake of tetraethylammonium by OCTN1 was much higher in HEK293/PDZK1 cells, compared with that in the parent HEK293 cells, the uptake in HEK293/PDZK1-E195K cells showing middle range between the two values. Such difference in transport activity was accounted for the difference in transport capacity, with minimal change in affinity of OCTN1 to the substrate or other compounds. The similar difference among HEK293/PDZK1, HEK293/PDZK1-E195K and HEK293 cells was also observed in transport property of OCTN2 and PEPT2, whereas the difference was not so remarkable in each transporter with the last four amino acids deleted, that has much lower interaction potential with PDZK1. Immunohistochemical analysis indicated that OCTN1 was colocalized with PDZK1 on cell-surface, whereas colocalization with PDZK1-E195K was partially observed in cytoplasmic region. These results suggest a novel hypothesis that mutation in PDZK1 potentially changes transport property of various types of xenobiotic transporters by affecting their subcellular localization, possibly leading to change in disposition of various types of substrate drugs.

Influence of histamine, cimetidine and pyrilamine on naloxone-induced jumping in morphine-dependent mice

In the present study, the effects of histamine on naloxone-induced jumping in the presence or absence of adrenoceptor or acetylcholine receptor antagonists in morphine-dependent mice were examined. In these experiments, the drugs were used before s.c. injection of naloxone (2 mg/kg), to test their effects on the expression of jumping. The i.c.v. administration of histamine (5-20 microg/mouse) 15 min before naloxone injection decreased the number of jumps in mice. When the histamine H(2) receptor antagonist, cimetidine (5-20 mg/kg), and the histamine H(1) receptor antagonist, pyrilamine (5-20 mg/kg), were administered i.p. to morphine-dependent mice, only cimetidine enhanced the jumping behaviour. Administration of cimetidine (20 mg/kg, i.p.), 30 min, of the beta-adrenoceptor antagonist, propranolol (2.5-10 mg/kg, i.p.), 15 min but not of pyrilamine (20 mg/kg, i.p.), 30 min before naloxone injection, decreased the histamine effect. The i.p. administration of an acetylcholine receptor antagonist, atropine (5 and 10 mg/kg, i.p.), the alpha(1)-adrenoceptor antagonist, prazosin (0.5, 1 and 2 mg/kg, i.p.), and alpha(2)-adrenoceptor antagonist, yohimbine (0.5, 1 and 2 mg/kg, i.p.), 15 min before naloxone injection, had no effect on the histamine response. Single administration of propranolol, atropine or prazosin decreased, while yohimbine increased the naloxone-induced jumping. It is concluded that the histamine H(2) receptor mechanism may be involved in the influence of histamine on the expression of naloxone-induced jumping in morphine-dependent mice.

Molecular characterization of specific H1-receptor agonists histaprodifen and its Nalpha-substituted analogues on bovine aortic H1-receptors

We determined the molecular properties of the selective and potent H(1)-receptor agonist histaprodifen and its N(alpha) substituted analogues: methyl-, dimethyl-, and imidazolylethyl-histaprodifen (suprahistaprodifen). All derivatives show high affinity for (3)H-mepyramine labeled bovine aortic H(1)-receptor binding sites with the following order of potency: suprahistaprodifen > dimethylhistaprodifen > methylhistaprodifen > histaprodifen > histamine. Suprahistaprodifen and dimethylhistaprodifen were the most potent displacers of (3)H-mepyramine binding (K(i)=4.3 and 4.9 nM, respectively). Histaprodifen, methylhistaprodifen and suprahistaprodifen binding was differentially influenced by GTP, whereas dimethylhistaprodifen was not affected. All drugs, except dimethylhistaprodifen, were activators of G-proteins. Their order of potency was suprahistaprodifen > histamine > histaprodifen > methylhistaprodifen. Their effect on G-protein activation was abolished by the addition of the H(1)-receptor antagonist triprolidine (10 microM), which given alone did not activate G-proteins. Our data suggest that histaprodifens are potent but heterogeneous H(1)-receptor ligands with diverse effects on the molecular level in our model system. While the histaprodifen, methylhistaprodifen and suprahistaprodifen data are in agreement with their agonistic nature, as shown in the functional studies performed on different species (rat and guinea pig H(1)-receptor), dimethylhistaprodifen behaved as an antagonist in our study.

Multiple Cedar Pollen Challenge Diminishes Involvement of Histamine in Allergic Conjunctivitis of Guinea Pigs

It has been reported that antihistamines do not fully modify symptoms of allergic conjunctivitis in clinical settings, suggesting that histamine is not the only contributor to symptom generation in the disease. However, in the majority of experimental allergic conjunctivitis models, antihistamines are very effective in the reduction of symptoms. In the present study, we used our recently developed guinea pig model of allergic conjunctivitis and evaluated whether involvement of histamine in the induction of symptoms of allergic conjunctivitis is altered by multiple antigen challenges. Guinea pigs were sensitized by intraperitoneal injection of Japanese cedar pollen extracts adsorbed on aluminum hydroxide gel, and then challenged by dropping a pollen suspension without the adjuvant on each eye once a week until the 15th challenge. The magnitude of the conjunctivitis intensity score (CIS), itch-associated scratching response and albumin leakage were found to increase with repeated challenges. At the 1st-3rd challenges, histamine H(1) receptor antagonist, mepyramine (10 mg/kg, p.o.), strongly reduced all these symptoms. However, symptoms at the 5th-15th challenges were not inhibited by mepyramine. On the other hand, a nitric oxide synthase (NOS) inhibitor, N(omega)-nitro-L-arginine methyl ester (10 mg/kg, i.v.), potently inhibited the increase of CIS and albumin leakage at the 15th challenge. In conclusion, histamine involvement in the induction of conjunctivitis symptoms in our model was diminished by multiple antigen challenges. The allergic conjunctivitis at the chronic stage is partly mediated by nitric oxide (NO) derived from NOSs that may be activated by mediators other than histamine. The histamine-independent allergic conjunctivitis may be useful for analyzing mechanisms underlying chronic conjunctivitis.

Putative sigma(3) sites in mammalian brain have histamine H(1) receptor properties: evidence from ligand binding and distribution studies with the novel H(1) radioligand [(3)H]-(-)-trans-1-phenyl-3-aminotetralin

A novel phenylaminotetralin (PAT) radioligand, [(3)H]-(1R, 3S)-(-)-trans-1-phenyl-3-dimethylamino-1,2,3,4-tetrahydronaphthalene ([(3)H]-[-]-trans-H(2)-PAT), is shown here to label a saturable (B(max)=39+/-6 fmol/mg protein) population of sites with high affinity (K(d)=0.13+/-0.03 nM) in guinea pig brain. Consistent with previous studies which showed that PATs stimulate catecholamine (dopamine) synthesis in rat striatum, autoradiographic brain receptor mapping studies here indicate that [(3)H]-(-)-trans-H(2)-PAT-labeled sites are highly localized in catecholaminergic nerve terminal fields in hippocampus, nucleus accumbens, and striatum in guinea pig brain. Competition binding studies with a broad range of CNS receptor-active ligands and CNS radioreceptor screening assays indicate that the pharmacological binding profile of brain [(3)H]-(-)-trans-H(2)-PAT sites closely resembles histamine H(1)-type receptors. Comparative studies using the histamine H(1) antagonist radioligand, [(3)H]mepyramine, indicate that the H(1) ligand binding profile and guinea pig brain distribution of H(1) receptors and [(3)H]-(-)-trans-H(2)-PAT sites are nearly identical; moreover, both sites have about 40-fold stereoselective affinity for (-)- over (+)-trans-H(2)-PAT. These results are discussed in light of previous studies which suggested that PATs stimulate dopamine synthesis through interaction with a novel sigma-type (sigma(3)) receptor in rodent brain; it now appears instead that PATs represent a new class of ligands for brain histamine H(1) receptors that can be stereoselectively labeled with [(3)H]-(-)-trans-H(2)-PAT.

Mechanisms of non-drowsiness after oral administration of TMK688, a novel antiallergic drug

The mechanisms of non-drowsiness after oral administration of TMK688 (1- [[5'-(3"-methoxy-4"-ethoxycarbonyloxyphenyl)-2',4'-pentadienoyl ] aminoethyl]-4-diphenylmethoxypiperidine, CAS 110501-66-1) were investigated using mice. TMK688 inhibited the histamine-induced vascular permeability at oral doses of 3.2-10 mg/kg with an ID50 value of 5.4 mg/kg. More than 100 times higher doses were needed to prolong the hexobarbital-induced sleeping. Pyrilamine, a typical antihistamine agent, showed little difference among these doses and antiallergic drugs having antihistamine activity, i.e., terfenadine, azelastine and ketotifen, had effects between TMK688 and pyrilamine. The inhibitory activity of orally administered TMK688 against ex vivo [3H]-pyrilamine binding to mouse cerebral histamine receptors appeared at the same doses as its potentiating activity against hexobarbital-induced sleeping. When given orally, TMK688 was hydrolyzed to TMK777 (CAS 101619-11-8), then conjugated with glucuronic acid to TMK777-glucuronide. No TMK688 was detected in the blood. The main metabolite TMK777-glucuronide could hardly penetrate the blood-brain barrier because of its polarity. Although the plasma concentrations of TMK777 were far lower than those of TMK777-glucuronide, TMK777 was penetrable into the brain and the cerebral concentrations of TMK777 increased in parallel with the plasma concentrations of the drug. Since intracerebroventricularly-injected TMK777 prolonged the sleeping time, and since the threshold concentration of TMK777 in the cerebral cortex to potentiate the hexobarbital-induced sleeping was consistent despite different administration routes, the drowsiness elicited by markedly high doses of TMK688 is though to be caused by intracerebral TMK777. In other words, TMK688 does not seem to cause drowsiness at effective doses because of the poor prenetrability of its main metabolites into the brain.

Analysis of plasma metabolites during human PET-studies with three receptor ligands, [11C]YM-09151-2, [11C]doxepin and [11C]pyrilamine

Carbon-11 labeled metabolites in human plasma were analyzed by high-performance liquid chromatography during positron emission tomography (PET) studies using the dopamine D2 ligand [11C]YM-09151-2 as well as the histamine H1 ligands [11C]doxepin and [11C]pyrilamine. For all the three tracers, blood clearance of the radioactivity was extremely rapid after an i.v. injection. The plasma protein-binding of [11C]YM-09151-2 and [11C]doxepin had protective effects upon the metabolic alteration of the ligands, whereas [11C]pyrilamine was free from the protein-binding and immediately degraded. The degradation of [11C]doxepin was more rapid in epileptic patients on medication than in normal subjects. These results indicate that analysis of metabolites in the plasma is necessary to determine the accurate arterial input function for quantitative PET measurement.

Carrier-mediated transport of H1-antagonist at the blood-brain barrier: a common transport system of H1-antagonists and lipophilic basic drugs

The blood-brain barrier (BBB) transport system for H1-antagonists was studied using primary cultured bovine brain capillary endothelial cells (BCEC). The uptake of [3H]mepyramine was inhibited by various H1-antagonists. Ketotifen competitively inhibited [3H]mepyramine uptake with an inhibition constant (Ki) of 46.8 microM. Lipophilic basic drugs such as propranolol, lidocaine and imipramine significantly inhibited [3H]mepyramine uptake. In particular, propranolol inhibited [3H]mepyramine uptake competitively at an inhibition constant (Ki) of 51.1 microM. Moreover, in ATP-depleted BCEC, [3H]mepyramine uptake was stimulated by preloading with H1-antagonists and lipophilic basic drugs. These results indicated that H1-antagonists are transported across the BBB via a carrier-mediated transport system common to lipophilic basic drugs.

Antiallergic effect of ZCR-2060: antihistaminic action

The antihistaminic effect of 2-[2-[4-(diphenylmethyl)-1-piperadinyl]ethoxy] benzoic acid maleate (ZCR-2060), a newly synthesized antiallergic agent, was investigated in both in vitro and in vivo studies. ZCR-2060 clearly antagonized histamine-induced contraction of isolated guinea pig ileum and trachea. In contrast, carbachol-, BaCl2- and 5-hydroxytryptamine-induced contractions of isolated guinea pig ileum were slightly inhibited by higher concentrations of ZCR-2060. 3H-Mepyramine specific binding to membranes from guinea pig lung and brain were markedly inhibited by ZCR-2060 in a concentration-dependent fashion. In the in vitro studies, the antihistaminic effect of ZCR-2060 was greater than those of cetirizine and terfenadine, but was less than that of ketotifen. In the in vivo studies, ZCR-2060 significantly inhibited the histamine-induced cutaneous reaction in rats, when administered orally 1 hr before the histamine injection. Moreover, ZCR-2060 has a long-lasting antihistaminic effect. In the in vivo studies, the antihistaminic effect of ZCR-2060 was found to be greater than that of cetirizine and terfenadine, and it was the same as that of ketotifen. Thiopental-induced sleep and spontaneous ambulatory activity in mice, however, were unaffected by ZCR-2060 at higher doses. These results indicate that ZCR-2060 has a potent, selective and long acting histamine H1-receptor antagonistic action without causing any unwanted CNS side effect.

Carrier-mediated transport of H1-antagonist at the blood-brain barrier: mepyramine uptake into bovine brain capillary endothelial cells in primary monolayer cultures

The transport mechanism of the H1-antagonist mepyramine at the blood-brain barrier (BBB) was studied by using primary cultured monolayers of bovine brain capillary endothelial cells (BCEC). The initial uptake of [3H]mepyramine into the BCEC showed strong temperature and concentration dependency, indicating that it involves both saturable and nonsaturable processes. Transport at the luminal membrane may be the rate-limiting process in the transcellular transport, since the values of the uptake coefficient of [3H]mepyramine at the luminal membrane (609 microliters/mg protein/min) and the transcellular permeability coefficient (488 microliters/mg protein/min) are very similar. The initial uptake of [3H]mepyramine was not affected by metabolic inhibitors, but was stimulated by preloading with the drug. Mepyramine appears to be transported into the BCEC by a carrier-mediated transport system which does not require metabolic energy, probably via a facilitated diffusion mechanism.

Transport mechanism of an H1-antagonist at the blood-brain barrier: transport mechanism of mepyramine using the carotid injection technique

The blood-brain barrier (BBB) permeability of mepyramine was measured by the carotid injection technique to elucidate the transport mechanism of an H1-antagonist in the central nervous system. Mepyramine was found to enter the brain by saturable and carrier-mediated transport. The in vivo kinetic parameters were estimated as follows: the maximum uptake rate (Jmax) was 7.12 +/- 1.37 mumol/min/g of brain, the Michaelis constant (Kt) was 4.40 +/- 2.00 mM, and the nonsaturable first order rate (Kd) was 0.28 +/- 0.02 ml/min/g of brain. The mepyramine transport was not inhibited either by nutrients or by choline, hemicholinium-3, though it was inhibited by the classical H1-antagonists such as diphenhydramine, diphenylpyraline, and also by propranolol. The above inhibitory effects suggest that a transport system different from the amine transport system exists for the BBB transport of mepyramine, and that this transporter is common not only for H1-antagonists but also for basic drugs.

Calcium-dependence of histamine- and carbachol-induced inositol phosphate formation in human U373 MG astrocytoma cells: comparison with HeLa cells and brain slices

1. Histamine (1 mM) induced an accumulation of inositol monophosphate ([3H]-IP1) in the U373 MG human astrocytoma cell line which increased with time in the presence of 30 mM Li+. After a 30 min incubation period with 1 mM histamine [3H]-IP1 was the major product detected (84 +/- 1% of total [3H]-IPx) and was present at a level 11 (+/- 1) fold of basal accumulation. 2. Concentration-response curves for histamine-induced [3H]-IP1 accumulation in U373 MG cells (EC50 5.4 +/- 0.5 microM) were shifted to the right in a parallel fashion by mepyramine (slope of a Schild plot 0.99 +/- 0.08), yielding a Kd for mepyramine of 3.5 +/- 0.3 nM, consistent with the involvement of histamine H1-receptors. 3. The temelastine-sensitive binding of [3H]-mepyramine to a membrane fraction from U373 MG cells was hyperbolic and had a mean Kd of 2.5 +/- 1.0 nM. The maximum amount of temelastine-sensitive binding was 86 +/- 19 pmol g-1 membrane protein. 4. Carbachol also induced [3H]-IP1 accumulation in U373 MG cells, 2.8 (+/- 0.1) fold of basal with 1 mM carbachol, with an EC50 of 48 +/- 8 microM. Pirenzepine shifted carbachol concentration-response curves to the right (slope of Schild plot 0.89 +/- 0.07) giving a Kd for pirenzepine of 0.10 +/- 0.01 microM, suggesting that phosphoinositide hydrolysis in U373 MG cells is mediated by the M3-, rather than the M1-, muscarinic receptor subtype. 5. [3H]-IP1 accumulation induced by both 1 mM histamine and by 1 mM carbachol increased when the Ca2+ concentration of the medium was increased from 'zero' (no added Ca2+) to 0.3 mM. Histamine-stimulated [3H]-IP1 accumulation was further increased, although not so markedly, as the Ca2+ was raised to 4 mM. The same pattern was apparent with histamine-induced accumulations of [3H]-IP2 and [3H]-IP3. In contrast, [3H]-IPx accumulation in response to carbachol increased between 0.3 and 1.3 mM, but thereafter remained unchanged ([3H]-IP1) or declined ([3H]-IP2 and [3H]-IP3). 6. In HeLa cells, [3H]-IP1 accumulations induced by 1 mM histamine and 1 mM carbachol showed the same pattern of Ca2+ dependence and were independent of extracellular Ca2+ above 0.3 mM (histamine) or 1.3 mM (carbachol). The response to carbachol appeared to be mediated by an M3-muscarinic receptor (apparent Kd for pirenzepine 0.09 microM). 7. In cross-chopped slices of guinea-pig cerebral cortex and guinea-pig cerebellum, [3H]-IPI accumulation induced by 1 mM histamine in the presence of 10 mM Li+ increased as the extracellular Ca2+ was increased from 0.3 to 2.5 mM, but a further increase to 4 mM had no further effect. In contrast the response to histamine in rat cerebral cortex increased markedly between 1.3 and 4 mM Ca2+. Accumulations of [3H]-IP1 induced by carbachol in guinea-pig or rat cerebral cortical slices were not increased as extracellular Ca2+ was raised from 0.3 to 4 mM.8. Nimodipine (100 nM) and w-conotoxin (3 microM) had no significant effect on histamine-induced [3H]-IP1accumulation in rat cerebral cortical slices or in U373 MG cells. 9. We conclude that histamine-induced [3H]-IP1 accumulation in U373 MG cells does appear to have a component dependent on the extracellular Ca2+ concentration. The degree of Ca2+-dependence approaches that observed in guinea-pig cerebral cortex but is much less than in rat cerebral cortex.Whether U373 MG cells will be of use as a model system for the apparent Ca2+-entry component observed in guinea-pig or rat brain slices remains to be established.

Noncompartmental and compartmental modeling of the kinetics of carbon-11 labeled pyrilamine in the human brain

The kinetic pattern of a 11C-labeled histamine H1 receptor antagonist, [11C]pyrilamine, was investigated in the human brain by factor analysis of dynamic PET studies. Tissue time activity curves were also processed by compartment model curve fitting preceded by deconvolution analysis. Factor analysis revealed two statistically significant and physiologically meaningful kinetic patterns: one for specific and another for nonspecific binding of the radioligand. From these two factors a compartment model containing two tissue compartments (one for specific binding and another for nonspecific binding and free ligand) was constructed. The two-compartment model was also supported by the impulse response function, which was obtained by deconvolution and showed two components. The factor image constructed from factor two demonstrated a distribution pattern characteristic for brain regions rich (frontal, parietal, and temporal lobes) or poor (occipital lobe and cerebellum) in H1 receptors. Blockade of H1 receptors with unlabeled pyrilamine, diphenhydramine, or hydroxyzine caused a significant reduction of this factor. Blockade produced no significant changes in factor one representing nonspecific binding. We conclude that the kinetics of [11C]pyrilamine in the brain can be described by two tissue compartments, one related to the distribution of the H1 receptors. Factor analysis of dynamic studies can be used to locally separate these two compartments, for identification of regions rich and poor in H1 receptors and for noninvasive quantitative investigation of the effects of H1 receptor blockers such as pyrilamine, diphenhydramine, or hydroxyzine.

Desensitization of histamine H1 receptor-mediated inositol phosphate accumulation in guinea pig cerebral cortex slices

1. Histamine stimulated the production of [3H]-inositol phosphates in untreated (control) guinea-pig cerebral cortex slices with a best-fit EC50 of 17 +/- 4 microM, and a best-fit maximum response of 385 +/- 23% over basal accumulation. 2. Histamine pretreatment desensitized guinea-pig cortex slices to a subsequent challenge with histamine, which was observed as a reduction in the best-fit maximum response to 182 +/- 32% over basal accumulation. 3. The time-course for the histamine-induced production of [3H]-inositol phosphates was approximately linear over 90 min of stimulation in both control and histamine pretreated slices. The rate of production in pretreated slices was significantly slowed compared to control, such that by 90 min of histamine stimulation the desensitized slices produced 2.8 times the basal [3H]-inositol phosphate accumulation compared to 5.3 fold the basal [3H]-inositol phosphate accumulation in the control slices. 4. Displacement of [3H]-mepyramine binding to homogenates of guinea-pig cerebral cortex by mepyramine and histamine revealed that histamine pretreatment did not alter the apparent affinity of the H1 receptor for histamine (control Kd = 6.3 +/- 0.7 microM, desensitized Kd = 7.9 +/- 1.6 microM) or mepyramine (control Kd = 3.4 +/- 0.8 nM, desensitized Kd = 3.4 +/- 1.3 nM), nor was there any reduction in the calculated maximum number of [3H]-mepyramine binding sites (control Bmax = 192 +/- 31 fmol mg-1 protein, desensitized Bmax = 220 +/- 50 fmol mg-1 protein). 5. The histamine-mediated desensitization of response in guinea-pig slices was mediated by the HI receptor subtype, since the attentuated maximum histamine-stimulated [3H]-inositol phosphate accumulation could not be prevented by inclusion of an H2- (ranitidine) and an H3- (thioperamide) receptor antagonist during the pretreatment period.6. The desensitized histamine-stimulated [3H]-inositol phosphate accumulation recovered to 90% of control levels over a period of 150 min after the removal of the conditioning dose of histamine, with a half-time of recovery of about 95 min.

Denervation hypersensitivity of histamine H1-receptors in rat brain cortex

We have studied the effects of the unilateral electrolytic lesion of the medial forebrain bundle at the level of the lateral hypothalamus on the density and functionality of histamine H1 receptors in rat brain cortex. The treatment resulted, after two and four weeks, in an increase in the maximal phosphoinositide breakdown induced by histamine, which can be accounted for by the appearance of a higher potency component for the response. On the other hand, the density of cortical histamine H1 receptors, determined by the specific binding of [3H]mepyramine to membranes, remained unchanged two weeks after the lesion but after four weeks a small but significant increase was also found. These results suggest that the denervation hypersensitivity developed may initially be the result of a more efficient coupling of the H1 receptors to the effector system prior to the subsequent increase in receptor numbers.

Is there a difference in the affinity of histamine H1 receptor antagonists for CNS and peripheral receptors? An in vitro study

An extended series of structurally different histamine H1 receptor antagonists was investigated for binding at central and peripheral histamine H1 receptors in vitro. Antagonist affinities were measured by displacements of [3H]mepyramine from both guinea-pig cerebellum and lung membrane suspensions. Single [3H]mepyramine binding sites with identical affinity for [3H]mepyramine were found in both tissues; however, the H1 receptor density was 6-fold lower in lungs than in cerebellum. None of the antagonists tested showed substantial preference for either of the receptors. It is concluded from the displacement data that there is no difference between the antagonist binding sites of cerebellum and lung H1 receptors.

[The effect of antihistaminic preparations on the binding of labelled mepyramine, ketanserin and quinuclidinyl benzilate in the rat brain]

The effects of some antiallergic drugs on H1-histamine, 5-HT2-serotonin, and M-cholinoreceptors ligand binding in the rat brain were studied in vitro. Dimedrol, dimebon, and phencarol bonded to H1-receptors: IC50 were 76 +/- 10, 153 +/- 15, 320 +/- 60 nM, respectively. Diazoline and dimebon had some affinity for 5-HT2-receptors, its IC50 was 880 +/- 90 nM. Dimedrol, phencarol and diazoline were found to be active against M-cholinoceptors, but when given in the maximal concentration (10 microM) it acted nonspecifically. In contrast to the other drugs, bicarphen had no effects on the binding of [3H]-mepyramine, [3H]-ketanserine, and [3H]-quinuclidinyl benzylate in the rat brain.

Histamine H1 receptors in human brain visualized in vivo by [11C]doxepin and positron emission tomography

Histamine H1 receptors in the living human brain were visualized by positron emission tomography (PET) using [N-11C-methyl]-(E)-doxepin ([11C]doxepin). The regional distribution of the carbon-11-labeled compound in the brain corresponded well with that of the histamine H1 receptors measured in vitro using [3H]pyrilamine. The radioactivity in the brain was significantly reduced by intravenous pretreatment with d-chlorpheniramine (5 mg), a histamine H1 antagonist. The regional distribution of [11C]doxepin in the brain 45-90 min after its injection was almost the same as that of [11C]pyrilamine in the brain. These results indicate that [11C]doxepin is useful for measuring histamine H1 receptors in human brain by PET.

Biodistribution and radiation absorbed dose of (N-methyl[11C])pyrilamine: a histamine H-1 receptor radiotracer

The in vivo biodistribution for (N-methyl[11C])pyrilamine in mice is reported for various times after i.v. bolus injection, together with estimates of the radiation absorbed dose for the same radiotracer in man. More than 60% of the injected dose was excreted via the kidneys in urine after 90 min post injection. The brain regional distribution in mice showed a favorable hypothalamus-to-cerebellum ratio, indicative of H-1 receptor binding, encouraging in vivo histamine H-1 receptor imaging studies in man.

(N-methyl-[11C])pyrilamine, a radiotracer for histamine H-1 receptors: radiochemical synthesis and biodistribution study in mice

The histamine H-1 receptor antagonist, pyrilamine (N-((4-methoxyphenyl)methyl)-N',N'-dimethyl-N-2-pyridinyl-1,2-ethaned i ami ne) was labeled with carbon-11 by N-alkylation of desmethylpyrilamine with [11C]iodomethane, and purified by preparative high performance liquid chromatography. The chemically and radiochemically pure labeled pyrilamine was obtained with specific activity of approximately 2500 mCi/mumol (EOS). In vivo distribution studies in mice suggest that the distribution of this compound parallels the known histamine H-1 receptor density in the brain.