Latent-transforming growth factor beta-binding protein-2 (LTBP-2) is required for longevity but not for development of zonular fibers

Latent-transforming growth factor beta-binding protein 2 (LTBP-2) is a major component of arterial and lung tissue and of the ciliary zonule, the system of extracellular fibers that centers and suspends the lens in the eye. LTBP-2 has been implicated previously in the development of extracellular microfibrils, although its exact role remains unclear. Here, we analyzed the three-dimensional structure of the ciliary zonule in wild type mice and used a knockout model to test the contribution of LTBP-2 to zonule structure and mechanical properties. In wild types, zonular fibers had diameters of 0.5-1.0 micrometers, with an outer layer of fibrillin-1-rich microfibrils and a core of fibrillin-2-rich microfibrils. LTBP-2 was present in both layers. The absence of LTBP-2 did not affect the number of fibers, their diameters, nor their coaxial organization. However, by two months of age, LTBP-2-depleted fibers began to rupture, and by six months, a fully penetrant ectopia lentis phenotype was present, as confirmed by in vivo imaging. To determine whether the seemingly normal fibers of young mice were compromised mechanically, we compared zonule stress/strain relationships of wild type and LTBP-2-deficient mice and developed a quasi-linear viscoelastic engineering model to analyze the resulting data. In the absence of LTBP-2, the ultimate tensile strength of the zonule was reduced by about 50%, and the viscoelastic behavior of the fibers was altered significantly. We developed a harmonic oscillator model to calculate the forces generated during saccadic eye movement. Model simulations suggested that mutant fibers are prone to failure during rapid rotation of the eyeball. Together, these data indicate that LTBP-2 is necessary for the strength and longevity of zonular fibers, but not necessarily for their formation.

Correction to Identification of Quinoline-Based RIP2 Kinase Inhibitors with an Improved Therapeutic Index to the hERG Ion Channel

[This corrects the article DOI: 10.1021/acsmedchemlett.8b00344.].

Extended pharmacodynamic responses observed upon PROTAC-mediated degradation of RIPK2

Proteolysis-Targeting Chimeras (PROTACs) are heterobifunctional small-molecules that can promote the rapid and selective proteasome-mediated degradation of intracellular proteins through the recruitment of E3 ligase complexes to non-native protein substrates. The catalytic mechanism of action of PROTACs represents an exciting new modality in drug discovery that offers several potential advantages over traditional small-molecule inhibitors, including the potential to deliver pharmacodynamic (PD) efficacy which extends beyond the detectable pharmacokinetic (PK) presence of the PROTAC, driven by the synthesis rate of the protein. Herein we report the identification and development of PROTACs that selectively degrade Receptor-Interacting Serine/Threonine Protein Kinase 2 (RIPK2) and demonstrate in vivo degradation of endogenous RIPK2 in rats at low doses and extended PD that persists in the absence of detectable compound. This disconnect between PK and PD, when coupled with low nanomolar potency, offers the potential for low human doses and infrequent dosing regimens with PROTAC medicines.

Mares et al. develop Proteolysis-Targeting Chimeras (PROTACs) that degrade its target RIPK2 in vivo at low doses for a prolonged period of time. This study suggests that PROTAC has a therapeutic potential that is superior to traditional RIPK2 small-molecule inhibitors.

Discovery of a First-in-Class Receptor Interacting Protein 2 (RIP2) Kinase Specific Clinical Candidate, 2-((4-(Benzo[d]thiazol-5-ylamino)-6-(tert-butylsulfonyl)quinazolin-7-yl)oxy)ethyl Dihydrogen Phosphate, for the Treatment of Inflammatory Diseases

RIP2 kinase has been identified as a key signal transduction partner in the NOD2 pathway contributing to a variety of human pathologies, including immune-mediated inflammatory diseases. Small-molecule inhibitors of RIP2 kinase or its signaling partners on the NOD2 pathway that are suitable for advancement into the clinic have yet to be described. Herein, we report our discovery and profile of the prodrug clinical compound, inhibitor 3, currently in phase 1 clinical studies. Compound 3 potently binds to RIP2 kinase with good kinase specificity and has excellent activity in blocking many proinflammatory cytokine responses in vivo and in human IBD explant samples. The highly favorable physicochemical and ADMET properties of 3 combined with high potency led to a predicted low oral dose in humans.

Identification of a RIP1 Kinase Inhibitor Clinical Candidate (GSK3145095) for the Treatment of Pancreatic Cancer

RIP1 regulates cell death and inflammation and is believed to play an important role in contributing to a variety of human pathologies, including immune-mediated inflammatory diseases and cancer. While small-molecule inhibitors of RIP1 kinase have been advanced to the clinic for inflammatory diseases and CNS indications, RIP1 inhibitors for oncology indications have yet to be described. Herein we report on the discovery and profile of GSK3145095 (compound 6). Compound 6 potently binds to RIP1 with exquisite kinase specificity and has excellent activity in blocking RIP1 kinase-dependent cellular responses. Highlighting its potential as a novel cancer therapy, the inhibitor was also able to promote a tumor suppressive T cell phenotype in pancreatic adenocarcinoma organ cultures. Compound 6 is currently in phase 1 clinical studies for pancreatic adenocarcinoma and other selected solid tumors.

Author Correction: Design of amidobenzimidazole STING receptor agonists with systemic activity

Change history: In this Letter, author Ana Puhl was inadvertently omitted; this error has been corrected online.An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Identification of Quinoline-Based RIP2 Kinase Inhibitors with an Improved Therapeutic Index to the hERG Ion Channel

RIP2 kinase was recently identified as a therapeutic target for a variety of autoimmune diseases. We have reported previously a selective 4-aminoquinoline-based RIP2 inhibitor GSK583 and demonstrated its effectiveness in blocking downstream NOD2 signaling in cellular models, rodent in vivo models, and human ex vivo disease models. While this tool compound was valuable in validating the biological pathway, it suffered from activity at the hERG ion channel and a poor PK/PD profile thereby limiting progression of this analog. Herein, we detail our efforts to improve both this off-target liability as well as the PK/PD profile of this series of inhibitors through modulation of lipophilicity and strengthening hinge binding ability. These efforts have led to inhibitor 7, which possesses high binding affinity for the ATP pocket of RIP2 (IC₅₀ = 1 nM) and inhibition of downstream cytokine production in human whole blood (IC₅₀ = 10 nM) with reduced hERG activity (14 μM).

Use of RIP1 Kinase Small-Molecule Inhibitors in Studying Necroptosis

RIP1 kinase plays a key role in regulating signaling pathways downstream of a number of innate immune receptors such as TNFRI and TLRs. The discovery of Necrostatin-1 (Nec-1) as a small-molecule inhibitor of RIP1 kinase has been very instrumental in defining the necroptotic and other signalling pathways regulated by RIP1, but certain characteristics of Nec-1 limits its utility in experimental systems. Next generation RIP1 kinase inhibitors have been identified and the use of these tool inhibitors along with Nec-1 has revealed that RIP1 is emerging as a key driver of inflammation and tissue injury in the pathogenesis of various diseases. Further studying the role of RIP1 to carefully unravel the complex biology requires the selection of the correct tool small-molecule inhibitors. In addition, it is important to consider the proper application of current tool inhibitors and understand the current limitiations. Here we will discuss key parameters that need to be considered when selecting and applying tool inhibitors to novel biological assays and systems. General protocols to explore the in vitro and in vivo potency, cellular selectivity, and pharmacokinetic properties of current small-molecule inhibitors of RIP1 kinase are provided.

Discovery of a First-in-Class Receptor Interacting Protein 1 (RIP1) Kinase Specific Clinical Candidate (GSK2982772) for the Treatment of Inflammatory Diseases

RIP1 regulates necroptosis and inflammation and may play an important role in contributing to a variety of human pathologies, including immune-mediated inflammatory diseases. Small-molecule inhibitors of RIP1 kinase that are suitable for advancement into the clinic have yet to be described. Herein, we report our lead optimization of a benzoxazepinone hit from a DNA-encoded library and the discovery and profile of clinical candidate GSK2982772 (compound 5), currently in phase 2a clinical studies for psoriasis, rheumatoid arthritis, and ulcerative colitis. Compound 5 potently binds to RIP1 with exquisite kinase specificity and has excellent activity in blocking many TNF-dependent cellular responses. Highlighting its potential as a novel anti-inflammatory agent, the inhibitor was also able to reduce spontaneous production of cytokines from human ulcerative colitis explants. The highly favorable physicochemical and ADMET properties of 5, combined with high potency, led to a predicted low oral dose in humans.

The Identification and Pharmacological Characterization of 6-(tert-Butylsulfonyl)-N-(5-fluoro-1H-indazol-3-yl)quinolin-4-amine (GSK583), a Highly Potent and Selective Inhibitor of RIP2 Kinase

RIP2 kinase is a central component of the innate immune system and enables downstream signaling following activation of the pattern recognition receptors NOD1 and NOD2, leading to the production of inflammatory cytokines. Recently, several inhibitors of RIP2 kinase have been disclosed that have contributed to the fundamental understanding of the role of RIP2 in this pathway. However, because they lack either broad kinase selectivity or strong affinity for RIP2, these tools have only limited utility to assess the role of RIP2 in complex environments. We present, herein, the discovery and pharmacological characterization of GSK583, a next-generation RIP2 inhibitor possessing exquisite selectivity and potency. Having demonstrated the pharmacological precision of this tool compound, we report its use in elucidating the role of RIP2 kinase in a variety of in vitro, in vivo, and ex vivo experiments, further clarifying our understanding of the role of RIP2 in NOD1 and NOD2 mediated disease pathogenesis.

The novel gamma secretase inhibitor N-[cis-4-[(4-chlorophenyl)sulfonyl]-4-(2,5-difluorophenyl)cyclohexyl]-1,1,1-trifluoromethanesulfonamide (MRK-560) reduces amyloid plaque deposition without evidence of notch-related pathology in the Tg2576 mouse

There is a substantial body of evidence indicating that beta-amyloid peptides (Abeta) are critical factors in the onset and development of Alzheimer's disease (AD). One strategy for combating AD is to reduce or eliminate the production of Abeta through inhibition of the gamma-secretase enzyme, which cleaves Abeta from the amyloid precursor protein (APP). We demonstrate here that chronic treatment for 3 months with 3 mg/kg of the potent, orally bioavailable and brain-penetrant gamma-secretase inhibitor N-[cis-4-[(4-chlorophenyl)-sulfonyl]-4-(2,5-difluorophenyl)cyclohexyl]-1,1,1-trifluoromethanesulfonamide (MRK-560) attenuates the appearance of amyloid plaques in the Tg2576 mouse. These reductions in plaques were also accompanied by a decrease in the level of reactive gliosis. The morphometric and histological measures agreed with biochemical analysis of Abeta(40) and Abeta(42) in the cortex. Interestingly, the volume of the plaques across treatment groups did not change, indicating that reducing Abeta levels does not significantly alter deposit growth once initiated. Furthermore, we demonstrate that these beneficial effects can be achieved without causing histopathological changes in the ileum, spleen, or thymus as a consequence of blockade of the processing of alternative substrates, such as the Notch family of receptors. This indicates that in vivo a therapeutic window between these substrates seems possible--a key concern in the development of this approach to AD. An understanding of the mechanisms whereby MRK-560 shows differentiation between the APP and Notch proteolytic pathway of gamma-secretase should provide the basis for the next generation of gamma-secretase inhibitors.

Opposite effects of lithium on proximal and distal caspases of immature and mature primary neurons correlate with earlier paradoxical actions on viability

To provide an explanation for earlier paradoxical findings of lithium on survival of mature and immature neurons, this study monitors changes in cytosolic caspases in rat cerebellar granule cells (CGC) grown 2-7 days in vitro (DIV), or in murine E-17 cortical neurons. Data show Li+ protects mature 7-DIV CGC parallel to a decrease in proximal and distal caspases but increases levels for immature 2-DIV-CGC or E-17 cortical neurons. Caspases mirror viability based on morphological analyses (dye uptake, phase-contrast, DNA fragmentation), and suggest protection occurs by suppressing activation of a cascade resulting in distal effectors that destroy proteins essential for neuronal survival. Protection was dose-dependent with EC50 3.0 mM and extended to 64 h in K+-serum deprived apoptotic media. Neuronal extracts contain a spectrum of proximal (-2, -8, -9) and distal (-3, -6) caspases sensitive to Li+ on assay with preferred peptide substrates and by immunoblotting. The lack of direct effect on activated cytosols indicates Li+ acts upstream only on intact cells, at sites for recruitment of pivotal procaspases. Alterations of procaspase-9 p46 and membrane-bound cytochrome c (Apaf-1) point to interaction with an intrinsic Mt-mediated pathway as one of the targets. The opposite effects on caspases and viability of immature or embryological neurons point to existence of alternative pathways that alter during neurite outgrowth suggesting the use of Li+ as a probe to unravel events relevant to neurogenesis.

Three-dimensional echocardiography improves accuracy and compensates for sonographer inexperience in assessment of left ventricular ejection fraction

This study was performed to determine whether 3-dimensional echocardiography (3DE) with a magnetic tracking system for image plane localization, which unlike standard 2-dimensional echocardiography (2DE), does not require acquisition of specific image planes or "standard views" for quantitative measurement of left ventricular volume and ejection fraction (EF), could compensate for sonographer inexperience. Eight adults underwent magnetic resonance imaging (MRI) scanning; they also had 2DE and 3DE performed by 2 experienced and 3 novice sonographers. Data were analyzed by a single expert reader blinded to patient and sonographer identity. Linear regression of MRI EF (reference standard) against echocardiographic EF yielded the following results, where RD indicates the residual difference between measured MRI values and those predicted using echocardiographic results: expert 3DE: r = 0.97, RD = 2.4%, and r = 0.96, RD = 2.8%; novice 3DE: r = 0. 83, RD = 5.1%, to r = 0.95, RD = 4.8%; expert 2DE: r = 0.85, RD = 4. 8%, and r = 0.86, RD = 4.9%; and novice 2DE: r = 0.34, RD = 11.7%, to r = 0.69, RD = 6.6%. Comparison of error variances indicated that novices who used 3DE equaled the performance of experts who used 2DE, although experts were always more accurate than novices when both used the same echocardiographic method (3DE vs 3DE, 2DE vs 2DE). In a comparison of methods, 3DE was always superior to 2DE, regardless of sonographer experience. Three-dimensional echocardiography allows even novice sonographers to obtain diagnostic-quality data sets, which they were unable to accomplish with 2DE. These results suggest that scanning with 3DE, combined with remote expert interpretation, may be useful in providing echocardiographic services in regions where they are presently unavailable.

Phenylalanine kinetics are associated with tardive dyskinesia in men but not in women

RATIONALE

An association between tardive dyskinesia (TD) and severely impaired metabolism of the large neutral amino acid (LNAA), phenylalanine (Phe) was defined in a group of mentally retarded patients. Subsequently, an altered kinetics of Phe was associated with TD in men with schizophrenia based on plasma analyses subsequent to the ingestion of a protein meal.

METHODS

In the present study, a standardized oral challenge of pure Phe (100 mg/kg in 170 ml orange juice) was administered to psychiatric patients of both sexes (n = 312), with and without TD after an overnight fast. Plasma LNAA levels were assayed both fasting and 2 h subsequent to the ingestion of the challenge. The extent of the increase in plasma Phe levels 2 h following a standardized challenge is determined by the sum of the kinetic processes of plasma absorption, tissue distribution, metabolism and elimination.

RESULTS

The study hypothesis, that TD would be associated with significantly higher post-challenge plasma Phe indices of an absolute plasma Phe level and plasma Phe/LNAA ratio (a brain availability measure), was verified for the study men (n = 209), but not for the study women (n = 103).

CONCLUSIONS

The demonstrated altered kinetics of Phe in men with TD indicates a greater availability of Phe to the brain in these men. We suggest that the disorder may be related to the effects of this greater availability. Such effects could be the direct neurotoxic effects of Phe and its metabolites and/or the modulating effects of these compounds on the synthesis of the monoamine neurotransmitters. The fact that TD (Yes/No) group differences in post-challenge plasma Phe indices were not seen for the study women suggests the possibility of a sex difference in the biology of TD that we propose may be reflective of the young age of the study sample.

Glutamatergic receptor kinetics are not altered by perinatal exposure to aspartame

Observation of reduced levels of glutamic acid and aspartic acid in brain of weanling rats exposed perinatally to aspartame prompted a study of the effect of this food additive on glutamatergic receptor kinetics. Aspartame 500 mg/kg/day in drinking water was administered to Sprague-Dawley rats throughout gestation and lactation. Brain was excised from weanlings 20-22 days old, and kinetics of the N-methyl-D-aspartate receptor and total glutamatergic binding in cerebral cortex and hippocampus were found to be unaffected by perinatal exposure to high levels of aspartame. Glutamic acid was decreased in both brain regions studied, and aspartic acid was decreased in hippocampus following perinatal aspartame exposure. These changes were reversible when aspartame administration was terminated. It is concluded that perinatal exposure to high doses of aspartame does not alter glutamatergic neurotransmission in cerebral cortex or hippocampus from weanling rats.

The characterization of the metabolites of ranolazine in man by liquid chromatography mass spectrometry

The metabolism of ranolazine (RS-43285) or (+)N-(2,6-dimethylphenyl)-4[2-hydroxy-3-(2-methoxyphenoxy)-propyl]-1- piperazine acetamide dihydrochloride was investigated in man using plasma samples obtained from four different clinical studies. The metabolite profiles following single and multiple doses of 342 mg instant release (IR) ranolazine, following multiple doses of 1000 mg sustained release (SR) ranolazine and following dosing with both ranolazine (IR) and a potentially co-administered drug, diltiazem, were compared. Metabolism of ranolazine in man was shown by LC/MS analysis to be extensive with up to seven primary routes of metabolism identified. N-dealkylation by hydrolysis at the piperazine ring produced three metabolites whilst O-demethylation and O-dearylation at the methoxyphenoxy moiety produced a further two compounds. Additionally, hydrolysis of the amide group formed one other species. Oxygenation at various points in the molecule produced a further four metabolites. Direct conjugation of ranolazine with glucuronic acid and with an uncharacterized adduct were also identified as a route of elimination. Ten other biotransformation products were formed as a result of multiple metabolic steps. Conjugation was also associated with the desmethyl metabolite (glucuronide and unidentified conjugates) of hydroxylated ranolazine. In a previous publication (Journal of Chromatography, 1995, accepted for publication) semi-quantitative analyses of pooled plasma from the study where ranolazine was dosed at 1000 mg twice daily showed that of the twelve metabolites studied only four accounted for AUC's in excess of 10% of the ranolazine AUC.

Chronic nicotine administration increases binding of [3H]domperidone in rat nucleus accumbens

An apparent inverse relationship between smoking and Parkinson's disease prompted an investigation of the effect of chronic nicotine administration on dopaminergic and serotonergic receptors in rat brain. Nicotine, 0.8 mg/kg, was injected once daily, five times per week, for 6 weeks. In nucleus accumbens the Kd for [3H]domperidone was increased 2-4-fold, and the Bmax was increased 1.5-2-fold. No changes were observed in the binding of [3H]domperidone in caudate-putamen or in that of [3H]ketanserin in frontal cortex. It is concluded that chronic nicotine administration may have a suppressant effect on central nervous system release of dopamine that in pre-parkinsonian persons causes an aversion to the effects of smoking.

Intraneuronal dopaminergic action of cocaine and some of its metabolites and analogs

The present study investigated the actions of cocaine and some of its metabolites and analogs upon the synaptosomal (P2) synthesis and release of dopamine appearing from [14C]phenylalanine. Also examined was the influence of reserpine upon these actions of cocaine. The P2 preparations from the rat caudate nucleus were incubated with the drugs for [14C]phenylalanine and, after filtration, the particulates and medium fractions were analyzed for [14C]dopamine and [14C]phenylalanine. Labelled dopamine in the medium was taken as a measure of its release by any addition of drug. Cocaine, norcocaine and the synthetic cocaine analogs WIN 35428 and WIN 35-065(2) each stimulated both the total (medium plus particulates) formation and the release of dopamine, while benzoylecgonine and ecgonine were without effect. Reserpine inhibited the synthesis and enhanced the release of dopamine. An addition of reserpine blocked the stimulating effect of cocaine, norcocaine, WIN 35428 and WIN 35-065(2) on synthesis and furthermore, these drugs had, in the same experiments, inhibitory effects on the synthesis, additive to the reserpine-induced inhibition. Benzoylecgonine and ecgonine were only weakly inhibitory in the presence of reserpine. Stimulation, rather than additive inhibition, by cocaine was observed in the presence of exogenous dopamine, and amphetamine increased the synthesis in the presence of either reserpine or added dopamine. Pretreatment of rats with reserpine also blocked the stimulation of synthesis by cocaine and WIN 35428. The uptake of labelled substrate was not affected by addition of drug, or by the pretreatment.

Influence of chronic lithium administration on binding to benzodiazepine- and histamine H1-receptors in rat brain

In rat, chronic lithium treatment lowered the Kd of [3H]-mepyramine in midbrain, and reduced the Bmax in midbrain and pons-medulla. Binding of [3H]flunitrazepam in cerebellum and midbrain was not altered. The suggestion that some actions of lithium may occur by way of central nervous system receptors is further supported by these observations that lithium's effects on neurotransmitter systems are regional and specific.

Suppression of histamine-induced adrenocorticotropic hormone release by antihistamines and antidepressants

The antihistaminic activity of many antidepressant drugs is well documented in vitro but has not been investigated as thoroughly in vivo. In the course of an investigation of the roles of H1 and H2 receptors in histamine-induced adrenocorticotropic hormone (ACTH) release in rats, it was observed that several antidepressants were potent inhibitors of this response. Male Sprague-Dawley rats were injected with test drugs and then with histamine or histamine agonists. Serum ACTH concentrations were determined by radioimmunoassay. ACTH secretion was induced by both H1 and H2 receptor stimulation. Histamine-induced ACTH release was markedly attenuated by several H1 antihistamines, whereas the H2 antagonists were not as effective. The antidepressants imipramine, doxepin, mianserin, desipramine and amitriptyline suppressed histamine-induced ACTH release. However, iprindole and the antianxiety agent diazepam were without effect. ACTH release induced by histamine agonists was also diminished by pretreatment with some of these blocking agents.

Effects of various S-adenosylmethionine preparations on histamine methylation in vitro and in vivo

To test possible enhancement of in vivo methylation of histamine, mice were injected with S-adenosylmethionine (SAM) of approximately 96% purity. Instead of the expected enhancement, very strong inhibition of methylation was observed. Tests indicated that S-adenosylhomocysteine (SAH) probably was not the inhibitor. Pure SAM, from another source, showed no inhibition of methylation in rats and guinea pigs, and only slight inhibition in mice. Pure SAM was much more effective than the 96% SAM for histamine methylation in vitro. It seems either that injected SAM cannot enter cells containing the histamine methylating enzyme (HME), or that the endogenous supply of SAM is optimal. The inhibitor may be structurally related to SAM, possibly a by-product of SAM synthesis. As it is very effective even in the presence of large amounts of SAM, in pure form it would probably be the most potent inhibitor of histamine methylation available.

Effect of histamine methylation inhibitors on entry of 14C-histamine into brain

Pretreatment of mice, rats and guinea pigs with inhibitors of histamine methylation, followed by systemic injection of 14C-histamine, results in a significant increase in brain 14C-histamine relative to controls. It is suggested that the histamine methylating enzyme participates in the blood-brain barrier to histamine.

Methyl derivatives of histamine; interaction with histamine metabolism

Several histamine analogs, with ring or side chain methyl groups, were tested for interaction with certain aspects of 14C-histamine metabolism. The physiological catabolite, denoted as 3-methylhistamine under the nomenclature we have adopted, is known to inhibit histamine methylation in vitro and in vivo. At low concentrations, all analogs inhibited histamine methylation by mouse brain homogenates; however 3-methylhistamine was completely ineffective against the highly active enzyme from rat didney, while 2-methylhistamine was most effective. None of the drugs showed a significant effect on in vivo formation of 14C-histamine by mouse stomach. All analogs had a definite effect on the distribution and fate of intravenously injected 14C-histamine. The 2-methyl analog was the strongest in vivo inhibitor of histamine methylation, and 3-methylhistamine next. The side chain N-methylated histamies altered uptake of 14C-histamine in some tissues but inhibited methylation weakly, if at all. Data on blood kidney and urine for 4-methylhistamine were unique, and suggested some effect on tubular reabsorption. 2-methylhistamine may prove to be the inhibitor of choice for in vivo studies on histamine methylation.

Activity changes in mouse brain histidine decarboxylase

Mice were tested for effects of various factors on histidine decarboxylase of whole brain, or major brain regions. Factors included age, sex, fasting and feeding, season of year, time of day, freezing for various time intervals, and intracerebral injection of protein synthesis inhibitors daily for 2-5 days. In the latter test, 5 day treatment with acetoxycycloheximide reduced brain histidine decarboxylase activity. In one experiment, in which a marked activity increase was observed, brain infection seemed to be the stimulus. Brain enzyme activity in normal mice showed large fluctuations over periods of several months to one year; presumably this is of some physiological significance but the causes are obscure.

Comparison of in vitro and in vivo histamine methylation by tissues

A method for determining the histamine-methylating enzyme (HME) using crude enzyme, and minute quantities of the substrate, was applied to tissues of mice, guinea-pigs and rats. Since high levels of endogenous histamine can affect the results, tissue homogenates were dialyzed prior to incubation. Findings were compared with in vivo data on methylating ability of individual tissues; most of this in vivo data is published but a new test of guinea-pig tissues was made using amodiaquine as an inhibitor. The correlation was good, better than that obtained by other procedures. It was observed that dialysis caused an increase in HME for some guinea-pig tissues, but a loss for some mouse tissues. Possible explanations are considered. Quinacrine N-mustard, a derivative of a known HME inhibitor, was tested in mice; it altered the distribution of injected 14C-histamine but showed no evidence of HME inhibition.

Effect of glucocorticoids on histamine metabolism in mice

1. Histamine formation and catabolism were studied in vivo in standardized experiments; for the former, animals pretreated with inhibitors of histamine catabolism were killed 10 min after intravenous injection of (14)C-L-histidine; for the latter, no inhibitors were used and animals were killed 2.5 min after intravenous injection of (14)C-histamine. In vitro assays of histidine decarboxylase activity were also made.2. Pretreatment of adrenalectomized mice with cortisol for one day caused a major alteration in histamine formation; stomach and intestine were abnormally high in the (14)C-histamine:total (14)C ratio, while most other tested tissues were lower. Brain and thymus were unaffected. Cortisol treatment caused a marked increase in urinary excretion of both (14)C-histamine and total (14)C.3. Following injection of a rapidly absorbed corticoid, significant effects on urine and stomach were demonstrable at 30 min and 4 h, respectively.4. In cortisol-treated adrenalectomized mice histidine decarboxylase activity, relative to controls, was decreased in lung but virtually unaffected in kidney, heart, muscle and liver. In stomach, activity was increased but the statistical significance was low.5. In mice injected with (14)C-histamine, cortisol pretreatment caused a small drop in tissue levels of (14)C-histamine, (14)-methylhistamine and total (14)C, but increased levels in urine. In all cases, however, the ratio of the two amines to total (14)C was not significantly different from controls.6. From the foregoing experiments it was concluded that the results in (2) could be largely attributed to entry into urine of (14)C-histamine and total (14)C, thus reducing the availability of these substances in blood for extraction by tissues. In stomach, the cortisol-induced increase in histamine formation may involve some process other than increased histidine decarboxylase activity.7. The activation of histidine decarboxylase in liver and lung of intact mice by Freund's adjuvant, or by endotoxin, was reduced by corticoid treatment.8. In a single short experiment on the effect of cortisol on histamine formation in adrenalectomized rats, results from stomach, lung and heart, the only tissues assayed, were similar to those from the mouse experiment.

Further studies on histamine catabolism in vivo

1. Histamine catabolism in vivo was studied in mice and rats; tissues from animals killed 2.5 min after intravenous injection of (14)C-histamine were assayed for (14)C-histamine and total (14)C. Aminoguanidine, a diamine oxidase inhibitor, and methylhistamine, an inhibitor of the histamine methylating enzyme, were used to evaluate the roles of these enzymes in individual tissues.2. Mouse liver and lung appeared to catabolize exogenous histamine rapidly and completely by methylation. In vivo histamine methylating activity was also found in mouse muscle, heart, kidney and lymph node, but not in stomach or intestine.3. In rats (14)C-histamine was inactivated more slowly than in mice. Catabolism was most rapid in intestine where both diamine oxidase and methylating activities were found. Liver had only diamine oxidase activity. Heart showed no catabolism but had extraordinary ability to extract (14)C-histamine from blood.4. The in vivo evaluation of histamine methylation by individual mouse and rat tissues agrees closely with in vitro findings by another laboratory.5. Aminoguanidine reduced uptake of blood (14)C-histamine by some tissues presumably by occupying sites where histamine is normally bound.6. The marked differences between tissues of mice and rats in destroying (14)C-histamine support earlier evidence that there is no apparent relationship between histamine catabolism and function.7. Urine from mice with both major catabolic pathways blocked showed evidence of an abnormal excretory product of (14)C-histamine. Paper chromatograms of urine of male and female mice showed no evidence of a sex difference in catabolism of injected (14)C-histamine.8. A procedure which may permit evaluating contributions of individual tissues to histamine formation in vivo is presented.9. Pretreatment of mice with antihistamines, or with a histamine analogue, betahistine, did not significantly affect the rate of in vivo(14)C-histamine inactivation.

Techniques for measring histamine formation in mice

1. Formation of (14)C-histamine from (14)C-L-histidine was studied in mice using various inhibitors of histamine catabolism; these included aminoguanidine, pargyline and methylhistamine, inhibitors of diamine oxidase, monoamine oxidase, and the histamine-methylating enzyme, respectively.2. Four general approaches were used: inhibiting diamine oxidase and the histamine-methylating enzyme and measuring (14)C-histamine in tissues or urine, or inhibiting diamine oxidase and monoamine oxidase and measuring (14)C-methylhistamine in tissues or urine. In some tests mice with normal concentrations of histidine decarboxylase were used; in others the enzyme was activated by pretreating mice with Freund's adjuvant.3. Methylhistamine pretreatment increased (14)C-histamine in several tissues of mice but aminoguanidine had no significant effect; it was concluded that endogenously formed histamine is inactivated almost entirely by methylation.4. There was no evidence of parallelism between the ability of tissues to form histamine and to inactivate endogenous histamine.5. Effects of Freund's adjuvant on tissue concentrations of (14)C-histamine were tested in mice with or without inhibitors of histamine catabolism. Results were essentially parallel in both cases but higher in the former.6. The method of choice is measurement of (14)C-histamine in tissues of mice given aminoguanidine and methylhistamine, followed by (14)C-L-histidine.7. Other approaches listed above may be useful but require improvement, for example, a more specific assay for (14)C-methylhistamine and a stronger, longer-lasting inhibitor of histamine-methylation.

In vivo studies on histamine catabolism and its inhibition

1. Histamine catabolism in vivo was studied in mice subjected to various forms of pretreatment; tissues from mice killed 2.5 min after intravenous injection of (14)C-histamine were assayed for (14)C-histamine, (14)C-methylhistamine and total (14)C.2. Pretreatment of mice with aminoguanidine, an inhibitor of diamine oxidase, strongly increased levels of (14)C-histamine in intestine; pretreatment with aminoguanidine plus a monoamine oxidase inhibitor strongly increased levels of (14)C-methylhistamine in liver. Effects in other tissues are reported and discussed.3. Pretreatment of mice with non-isotopic methylhistamine increased levels of (14)C-histamine in liver. Methylhistamine is the first known inhibitor of histamine-methylation in vivo.4. Pretreatment of mice with inhibitors of protein synthesis, drugs which reduce the basal activity of histidine decarboxylase and which block its activation, failed to affect histamine catabolism.5. Pretreatment of mice with endotoxin or with Freund's adjuvant, irritants known to cause activation of histidine decarboxylase, failed to affect histamine catabolism.6. There was no evidence of parallelism between the histamine-destroying enzymes and the histamine-forming enzyme, histidine decarboxylase, either in distribution or ability to undergo changes in activity. No support was obtained for the view that histamine-catabolizing enzymes play a role in the local control of responses to newly formed histamine.

Enhancement of inflammation and histamine formation by actinomycin D

1. An inflammation model developed in other laboratories was used in this study. Mice given endotoxin intranasally developed lung inflammation which progresses in intensity for several days. Lung weight is a satisfactory measure of inflammation.2. In lungs of mice treated intranasally with endotoxin, histidine decarboxylase was activated within 6 hr, before lung weight had increased substantially. Enzyme activity was near maximal at 24 hr but had dropped by 48 hr, at which time inflammation was increasing. The data are consistent with an early role for histamine in mediating inflammation, but not with an essential role in the later stages.3. When actinomycin D, an inhibitor of RNA synthesis, was mixed with endotoxin solution and given to mice intranasally, both lung inflammation and histidine decarboxylase activation were markedly enhanced at 24 and 48 hr, relative to effects produced by endotoxin alone.4. Evidence is presented that intranasal instillation of endotoxin into mice increases histamine formation in lung in vivo.5. We previously found protein synthesis inhibitors, the only drugs shown capable of blocking histidine decarboxylase activation, to be powerful anti-inflammatory agents. Now we have found that actinomycin D, the only drug shown capable of enhancing histidine decarboxylase activation, to be strongly pro-inflammatory. These observations support a causative role for histamine in slowly-developing inflammation, and also provide a rigorous test for the participation of other mediator candidates.

Further studies on the histidine-histamine relationship in vivo: effects of endotoxin and of histidine decarboxylase inhibitors

1. Mice were injected with ((14)C)-L-histidine, killed at various intervals, and tissues assayed for ((14)C)-histamine. In some cases free ((14)C)-L-histidine and total ((14)C) were also determined.2. Removal of stomach, the most active histamine-forming tissue, failed to reduce the ((14)C)-histamine content of any tested tissue; ((14)C)-histamine concentrations in lung and muscle of gastrectomized mice were higher than in shamoperated controls.3. In mice pretreated with endotoxin and subsequently injected with ((14)C)-L-histidine, the ((14)C)-histamine content of liver, lung and muscle was markedly higher than in controls. The increased concentrations of ((14)C)-histamine in endotoxin-pretreated mice seemed to reflect a greater rate of formation; they could be attributed neither to changes in tissue concentration of ((14)C)-L-histidine, to increased uptake from other tissues, nor to impaired ability to inactivate histamine.4. Results of studies on in vivo effectiveness of several histidine decarboxylase inhibitors are reported.5. The following conclusions are supported by the evidence presented: (a) in stressed mice, those tissues which show activation of histidine decarboxylase also show increased ability to form histamine in vivo; (b) tissue histamine is largely formed locally; (c) histidine decarboxylase inhibitors are highly effective in blocking histamine formation in mast cells and in stomach, but do not normally reach the locus of an inducible form of histidine decarboxylase; (d) the inducible form of histidine decarboxylase in liver may be located in phagocytic microvascular endothelial cells; (e) in conditions favouring near-maximal activation of histidine decarboxylase, the histamine-methylating enzyme of liver and diamine oxidase of intestine showed no inducible characteristics; (f) blood histamine concentrations do not accurately reflect changes in tissue histamine formation.