The effect of Ca2+ on the pharmacological specificity of [3H]glutamate binding sites in rat hippocampus

The inhibition of [3H]glutamate binding to rat hippocampal membranes by various compounds was measured. L-Glutamic, L-2-amino-4-phosphonobutyric and 4-fluoroglutamic acids had similar IC50 values (2.12 microM, 4.98 microM and 3.90 microM respectively). D-Glutamate and D-2-amino-4-phosphonobutyrate were less potent than L-glutamate (IC50S 18.9 microM and 46.5 microM respectively) and L-glutamate diethylester (IC50 519 microM) was the weakest inhibitor tested. Addition of 10 mM CaCl2 caused a 2.27-fold increase in displaceable binding but did not alter the IC50 values of any of the inhibitors tested.     

Effect of drugs inhibiting spermidine biosynthesis and metabolism on the in vitro development of Plasmodium falciparum

Treatment of Plasmodium falciparum with the potent inhibitor dicyclohexylamine completely arrests in vitro cell proliferation of the chloroquine-susceptible P. falciparum strain NF54 and the R strain, which shows less sensivity to chloroquine. The average inhibitory concentration (IC50) values determined for both strains revealed different inhibition profiles. The IC50 value for the chloroquine-sensitive NF54 strain was 97 microM and 501 microM for the R strain. Monitoring polyamine pools after treatment with dicyclohexylamine leads to a significant decrease in the intracellular spermidine content, which was nearly reversed by supplementation with spermidine. Since spermidine is an important precursor for the biosynthesis of hypusine and homospermidine in eukaryotes, we studied the developmental effect on both P. falciparum strains of 1,7-diaminoheptane as an inhibitor of deoxyhypusine synthase (EC 1.1.1.249) in mammalian cells, and agmatine as a moderate inhibitor of homospermidine synthase (EC 2.5.1.44). Inhibition profiles with 1,7-diaminoheptane resulted in an IC50 value of 466 microM for the NF54 strain and 319 microM for the R strain. Spermidine pools changed significantly. Inhibition with agmatine caused a strong decrease in parasitemia for the chloroquine-susceptible NF54 strain, with a determined IC50 value of 431 microM and an IC50 value of 340 microM for the less chloroquine-susceptible R strain. Spermidine was not detectable after inhibition. The uncommon triamine homospermidine occurred in both P. falciparum strains. To our knowledge this is the first evidence of homospermidine in P. falciparum. The use of specific inhibitors of spermidine metabolism might be a novel strategy for the design of new antimalarials, and suggests the occurrence of both enzymes in the parasite.     

Synthesis and biological evaluation of 2-, 3-, and 4-acylaminocinnamyl-N-hydroxyamides as novel synthetic HDAC inhibitors

A new series of 2-, 3-, and 4-acylaminocinnamyl-N-hydroxyamides 1-3 have been prepared, and their anti-HDAC (against maize HD2, HD1-B, and HD1-A enzymes) activities have been assessed. Cinnamyl-hydroxyamides bearing acylamino substituents at the C2 position of the benzene ring (compounds 1a-g) showed very low HDAC inhibiting activities, with IC(50) values in the high micromolar range. By shifting the same acylamino groups from C2 to C3 (compounds 2a-g) as well as C4 (compounds 3a-f) position of the benzene ring, a number of highly potent HDAC inhibitors have been obtained. In the anti-HD2 assay 3c (IC(50) = 11 nM) was the most potent compound, being >11600-, 4.5-, and 10-fold more potent than sodium valproate, SAHA, and HC-toxin, respectively, and showing the same activity as trapoxin. HD1-B and HD1-A assays have been performed to screen the inhibitory action of 1-3 against mammalian class I (HD1-B) and class II (HD1-A) HDAC homologous enzymes. From the corresponding IC(50) data, a selectivity ratio has been calculated. In general, compounds 1-3 showed no or little selectivity towards the class II homologue HD1-A, the most selective being 2a with class II selectivity ratio = 4.3. About the inhibitory potency, the 4-(2-naphthoylamino)cinnamyl-N-hydroxyamide 3f showed the highest inhibiting effect against the two enzymes (IC(50-HD1-B) = 36 nM; IC(50-HD1-A) = 42 nM). Selected 2 and 3 compounds will be evaluated to determine their antiproliferative and cyto differentiating activities on HL-60 cells.     

Development of an in-vitro test system for the evaluation of cyclooxygenase-2 inhibitors

OBJECTIVE AND DESIGN: The aim of our study was to establish an in-vitro test system, capable of fast and efficient screening of Cyclooxygenase-2 (COX-2) inhibitors. MATERIALS: Mononuclear cells were isolated out of human whole blood, in a one-step centrifugation procedure. TREATMENT AND METHODS: The time- and concentration-dependent induction of COX-2 expression in the blood monocytes (1 x 10(6) cells/ml) was evaluated by a kinetic profile. The optimal test conditions were fixed at an LPS concentration of 10 micrograms/ml and a 5 hour incubation time. The test compounds (10(-5) to 10(-8) mol/l) were set at t = 0 into the assay and were co-incubated for the whole period of COX-2 expression (5 hr). RESULTS: The following are representative examples of inhibitors with different distinct selectivity for COX-1/2. Indomethacin as a COX-1 selective compound inhibited PGHS-1 (IC50: 0.002 microM) 200 times stronger than PGHS-2 (IC50: 0.43 microM). Diclofenac had an almost equipotent efficacy on PGHS-1 (IC50: 0.05 microM) and PGHS-2 (IC50: 0.03 microM). NS-398 inhibited highly selective COX-2 (IC50 PGHS-1: 10.75 microM vs IC50 PGHS-2: 0.16 microM). CONCLUSIONS: The model reached the set targets with regard to the differentiation of COX-2 selective compounds, the reproducibility of results and practicability of the assay. In contrast to previous propounded theories, we could demonstrate, that mononuclear cells are not unusually sensitive to NSAIDs and apparently possess no further COX isoforms.     

Antiproliferative and leishmanicidal effect of ajoene on various Leishmania species: ultrastructural study

Ajoene [(E,Z)-4,5,9 trithiadodeca 1,6,11 triene 9-oxide], the major bioactive compound derived from garlic, shows a potent trypanolytic and antimicotic activity. In this paper we evaluate its effect on Leishmania mexicana(Lm:MHOM/VE/80/NR), L eishmania mexicana venezuelensis (Lmv: MHOM/VE/80/H16), L eishmania mexicana amazonensis (Lma: M112, IFLA/BR/67/PH8) and L eishmania donovani chagasi (Ldch: MHOM/BR/74/PP75). Ajoene showed a potent leishmanicidal activity in vitro against all species studied. Concentrations higher than 0.3 microM led to total inhibition of growth, and 10 microM induced 100% lysis of Leishmaniaafter 96 h of incubation in a chemically defined culture medium. The 50% inhibitory concentration (IC(50)) for lysis, for all species, was about to 2 microM. The effect was dose-dependent and a threefold increase in concentration (30 microM) produced 100% lysis of cultured forms after 72 h. Ultrastructural studies showed a time- and dose-dependent morphological alteration of the mitochondrial membrane and nuclear envelope, as well as the formation of large autophagic vacuoles.     

Design, synthesis, and preliminary evaluation of new pyrrolidine derivatives as neuraminidase inhibitors

A series of pyrrolidine derivatives were designed and synthesized in good yields starting from commercially available 4-hydroxy-L-proline using a suitable synthetic strategy. And their ability to inhibit neuraminidase was evaluated. These compounds showed potent inhibitory activity against influenza A (H3N2) neuraminidase. Within this series, four compounds, 6e, 9c, 9f and 10e, have the good potency (IC(50)=1.56 approximately 2.40microM) which is compared to the NA inhibitor oseltamivir (IC(50)=1.06microM), and could be used as lead compound in the future.     

Correlation of the efficiency of fatty acid derivatives in suppressing Plasmodium falciparum growth in culture with their inhibitory effect on acyl-CoA synthetase activity

The intraerythrocytic malaria parasite depends on the surrounding medium for a supply of phospholipid precursors. Efficient inhibition (IC50 7-90 microM) of Plasmodium falciparum growth in vitro was achieved using modified fatty acids. The fatty acid analogues most effective in suppressing P. falciparum growth in culture were also the most active inhibitors of acyl-CoA synthetase from the monkey parasite P. knowlesi.     

Cytotoxicity of ester and ether lysophospholipids on Leishmania donovani promastigotes

The cytotoxic activity of four ester lysophospholipids, three ether lysophospholipids, and two radylglycerols on Leishmania donovani promastigotes was determined by measuring the inhibition of cell growth. The 1-acyl lysophospholipids reduced cell growth to 50% of controls at concentrations of 6.4-10.9 microM. In contrast, 1-O-alkenyl-sn-glycero-3-phosphoethanolamine, 1-O-hexadecyl-sn-glycero-3-phosphocholine, and 1-O-hexadecyl-sn-glycerol already showed a 50% inhibition of growth at concentrations between 2.1 and 2.8 microM. Moreover, the unnatural alkyl lysophospholipid analogue 1-O-octadecyl-2-methoxy-sn-glycero-3-phosphocholine was even 10-fold more toxic. Incubations of L. donovani promastigotes with radioactively labelled ether lysophospholipids revealed a rapid uptake of these compounds and their incorporation into cellular lipids at a non-toxic concentration of 1.0 microM. An accumulation of the lysophospholipids in the cell due to insufficient metabolism may be the cause of its cytotoxic effect. The sensitivity of L. donovani cells towards ether lysophospholipids was found to be similar to that reported for tumor cells.     

Tea polyphenols inhibit acetyl-CoA:1-alkyl-sn-glycero-3-phosphocholine acetyltransferase (a key enzyme in platelet-activating factor biosynthesis) and platelet-activating factor-induced platelet aggregation

BACKGROUND: Tea extracts have antiallergic and anti-inflammatory actions in rats and mice. However the mechanism through which tea polyphenols act in vivo are still incompletely understood. We found inhibitory effects of black tea extracts on an fMLP-induced aggregating response in a rabbit platelet-polymorphonuclear leukocyte (PMN) system. METHOD: To elucidate whether 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine (PAF) production in PMNs and/or PAF-stimulated platelet activation were inhibited, the effects of tea polyphenols were investigated on the enzyme activity of acetyl-CoA:1-alkyl-sn-glycero-3-phosphocholine acetyltransferase (EC 2.3.1.67), PAF biosynthesis in A23187-activated rabbit PMNs, and rabbit platelet aggregation. By comparing the inhibitory effects of 31 galloyl esters and gallic acid, the structure-inhibitory activity relationship was characterized. RESULTS: Theaflavin and its galloyl esters and pentagalloyl glucose were found to be potent inhibitors of the acetyltransferase (IC(50) = 28-58 microM) and the PAF biosynthesis as well as (-)-epicatechin-3-O-gallate (IC(50) = 72 +/- 13 microM) and (-)-epigallocatechin-3-O-gallate (IC(50) = 46 +/- 6 microM). On the other hand, flavan-3-ols without galloyl group at C-3 and gallic acid did not show significant enzyme inhibition. In addition, theaflavin and its galloyl esters (IC(50) = 32-77 microM) and geranyl gallate, farnesyl gallate and geranylgeranyl gallate (IC(50) = 6.4-7.6 microM) were found to be potent inhibitors of PAF- and TPA-induced rabbit platelet aggregation but not A23187-induced aggregation. CONCLUSIONS: Theaflavin and its galloyl esters in black tea extract, and isoprenyl gallates were potent inhibitors of PAF synthesis and platelet aggregation and these activities may be relevant to the claimed therapeutic effects of tea extracts.     

Antiplasmodial activity of alkaloid extracts from Pavetta crassipes (K. Schum) and Acanthospermum hispidum (DC), two plants used in traditional medicine in Burkina Faso

In the course of the search for new antimalarial compounds, a study of plants traditionally used against malaria in Burkina Faso was made. An ethnobotanical study permitted the identification of plants currently used by the traditional healers and herbalists. Two plants among them were selected for further study: Pavetta crassipes (K. Schum) and Acanthospermum hispidum (DC). Alkaloid extracts of these plants were tested in vitro against two reference clones of Plasmodium falciparum: the W2 chloroquine-resistant and the D6 chloroquine-sensitive strains. Significant inhibitory activity was observed with Pavetta crassipes (IC(50)=1.23 microg/ml) and A. hispidum (IC(50)=5.02 microg/ml). Antiplasmodial activity was also evaluated against six Plasmodium falciparum isolates from children between 4 and 10 years old. The IC(50) values for the alkaloid extracts were in the range 25-670 ng/ml. These results indicated that P. falciparum wild strains were more sensitive to the alkaloid extracts than strains maintained in continuous culture. Moreover, the alkaloid extracts exhibit good in vitro antimalarial activity and weak cytotoxicity against three human cell lines (THP1, normal melanocytes, HTB-66). Isolation and structural determination are now necessary in order to precisely determine the active compounds.     

Anti-HIV-1 activity of antiviral compounds, as quantitated by a focal immunoassay in CD4+ HeLa cells and a plaque assay in MT-4 cells

The inhibitory effects of a series of antiviral compounds on human immunodeficiency virus type 1 (HIV-1) were evaluated in a plaque assay (PA) in MT-4 cells and a focal immunoassay (FIA) in CD4+ HeLa cells. Similar 50% inhibitory concentrations (IC50) were obtained for the sulfated polysaccharides when measured by PA or FIA: the IC50 values of dextran sulfate and pentosan polysulfate were 0.8 microgram/ml and 0.35 microgram/ml, respectively. Also, comparable IC50 values (ranging from 1.42 to 2.71 microM) were obtained for purine 2',3'-dideoxyribosides (i.e. DDA, DDI and DDG) when evaluated by PA or FIA. In contrast, the IC50 values of pyrimidine 2',3'-dideoxyribosides were invariably 4- to 10-fold lower when monitored by PA than FIA: the IC50s of AZT, D4T and DDC in the PA were 0.015, 0.094 and 0.038 microM, respectively, and in the FIA were 0.062 microM, 0.29 microM and 0.46 microM, respectively. The differential anti-HIV-1 activities found with AZT, D4T and DDC in the PA and FIA systems may at least be related in part to differences in the metabolism of the compounds (i.e. phosphorylation by thymidine kinase or 2'-deoxycytidine kinase) between MT-4 and CD4+ HeLa cells. The novel anti-HIV-1 compounds tetrahydro-imidazo[4,5,1-jk][1,4]-benzodiazepin-2(1H)-thione (TIBO) derivatives, R82150 and R82913, and the acyclouridine derivative 1-[(2-hydroxyethoxy)methyl]-6-phenylthiothymine (HEPT) were also more inhibitory to HIV-1 in the PA than FIA system. The IC50 values of R82150, R82913 and HEPT, as based on PA, were 0.005, 0.003 and 0.79 microM, respectively. Their IC50 values, as based on FIA, were 0.020 microM, 0.015 microM and 3.77 microM, respectively. The TIBO derivatives emerged as the most effective HIV-1 inhibitors of the compounds tested whether assayed by PA or FIA.     

Evaluation of the activity of a novel metabotropic glutamate receptor antagonist (+/-)-2-amino-2-(3-cis and trans-carboxycyclobutyl-3-(9-thioxanthyl)propionic acid) in the in vitro neonatal spinal cord and in an in vivo pain model

The cyclobutylglycine (+/-)-2-amino-2-(3-cis and trans-carboxycyclobutyl-3-(9-thioxanthyl)propionic acid) (LY393053) has been identified as a functionally potent metabotropic glutamate receptor antagonist. It is most potent on the two group I metabotropic glutamate receptors, 1alpha and 5alpha, with IC50 values of 1.0+/-0.4 microM and 1.6+/-1.4 microM, respectively. In this study, LY393053 has also been evaluated electrophysiologically on native group I metabotropic glutamate receptors in an in vitro spinal cord preparation as well as behaviourally, in a mouse model of visceral pain. LY393053 dose-dependently antagonised group I agonist, (RS)-3, 5-dihydroxyphenylglycine, or a broad-spectrum agonist (1S,3R)-amino-1,3-cyclopentanedicarboxylic acid-induced depolarisation of spinal motoneurons. The apparent Kd values were estimated to be 0.3 microM against (RS)-3, 5-dihydroxyphenylglycine-induced depolarisation and 0.5 microM against (1S,3R)-amino-1,3-cyclopentanedicarboxylic acid-induced depolarisation, respectively. On the other hand, the dorsal root-ventral root potential elicited at 8 x threshold was depressed by LY393053 with IC50 values of 9.0+/-0.7 microM and 12.7+/-1.7 microM on monosynaptic and polysynaptic responses, respectively. When investigated using the mouse acetic acid writhing test, LY393053 showed significant analgesic effects at doses of 1-10 mg/kg intraperitoneally. An ED50 value of 6.0 mg/kg was obtained in this test. By revealing a potent effect of LY393053 in antagonising the native group I metabotropic receptor-mediated responses in the spinal cord in rodents, and an antinociceptive efficacy in a mouse visceral pain model, these results, therefore, provide additional evidence in support of the analgesic potential of metabotropic glutamate receptor antagonists.     

Syntheses, urease inhibition, and antimicrobial studies of some chiral 3-substituted-4-amino-5-thioxo-1H,4H-1,2,4-triazoles

Chiral 3-substituted-4-amino-5-thioxo-1H,4H-1,2,4-triazoles (5a-i) were synthesized. The target molecules were prepared by cyclization of the corresponding dithiocarbazinic acids, obtained from hydrazides, in the presence of hydrazine hydrate. The chiral hydrazides were in turn synthesized form L-amino acids. The structures of all the compounds were confirmed by modern spectroscopic techniques and purity ascertained by elemental analysis. The synthesized compounds 5a-i were evaluated for urease inhibition and found to exhibit varying degrees of urease inhibition activity showing IC(50) values ranging from 22.0 +/- 0.5 to 43.8 +/- 0.3 microM. Compound 5b was found to be the most active, exhibiting IC(50) = 22.0 +/- 0.5 microM comparable to the standard, thiourea (IC(50) = 21.0 +/- 0.1 microM). Triazoles 5a-i were also screened for their antimicrobial properties and promising antibacterial activities were observed against five pathogenic bacteria. However, all the compounds were devoid of any antifungal activity.     

Antiplasmodial and cytotoxic activity of coumarin derivatives from dried roots of Angelica gigas Nakai in vitro

The butanol-soluble fraction of the dried root of Angelica gigas exhibited significant protection against chloroquine-sensitive strains of Plasmodium falciparum using the parasite lactate dehydrogenase assay method. Using antiplasmodial activity-guided fractionation, five coumarins, marmesinin (1), nodakenin (2), skimmin (3), apiosylskimmin (4), and magnolioside (5), were isolated and evaluated for in vitro antiplasmodial activity, as well as for their cytotoxic potential on SK-OV-3 cancer cell lines. Compounds 1 and 5 showed notable growth inhibitory activity against chloroquine-sensitive strains of P. falciparum with IC(50) values of 5.3 and 8.2 μM. The compounds showed no significant cytotoxicity (IC(50) > 100 μM) toward the SK-OV-3 cancer cell line. This is the first report on the antiplasmodial activity of these coumarin derivatives from the dried root of A. gigas.     

Synthesis and in vitro evaluation of the anti-viral activity of N-[4-(1H(2H)-benzotriazol-1(2)-yl)phenyl]alkylcarboxamides

A series N-[4-(1H(2H)-benzotriazol-1(2)-yl)phenyl]alkylcarboxamides (8e-k, 9e-i, k, l) and their parent amines (5a-c and 6a-d) were prepared according to Schemes (1 and 2). Compounds were evaluated in vitro for cytotoxicity and antiviral activity against a wide spectrum of RNA (positive- and negative-sense) viruses, like [Bovine Viral Diarrhea Virus (BVDV), Yellow Fever Virus (YFV), Coxsackie Virus B (CVB-2), Polio Virus (Sb-1), Human Immunodeficiency Virus (HIV-1), Respiratory Syncytial Virus (RSV)] or double-stranded (dsRNA) virus, like Reoviridae (Reo-1). The Entero (CVB-2 and Sb-1) were the only viruses inhibited by title compounds. In particular, two of them emerged for their selective, although not very potent, antiviral activity: 8i, which was the most active against CVB-2 (CC50 >100 microM; EC50 = 10 microM) and 9l, which was the most active against Sb-1 (CC50 90 microM; EC50 = 30 microm). Title compounds were evaluated in silico against the Sb-1 helicase, as the crystal structure of this enzyme was not available, the corresponding 3D model was obtained by homology techniques (see Fig. 2).     

Inhibition of baclofen binding to rat cerebellar membranes by phaclofen, saclofen, 3-aminopropylphosphonic acid and related GABAB receptor antagonists

The inhibition of the binding of the GABAB agonist [3H](-)-baclofen to rat cerebellar membranes by some sulfonic and phosphonic acid analogues of GABA has been studied. These analogues have been shown to act as GABAB antagonists in the rat cortical wedge and the guinea-pig isolated ileum preparations. The order of potency of phaclofen (IC50 118 microM), 2-hydroxysaclofen (IC50 5.1 microM) and saclofen (IC50 7.8 microM) as inhibitors of [3H](-)-baclofen binding was similar to the order of potency of these compounds as GABAB antagonists, whereas 3-aminopropylphosphonic acid (IC50 1.5 microM) and 4-aminobutyl-phosphonic acid (IC50 3.9 microM) were much more potent than anticipated from their relatively weak GABAB antagonist actions. These results indicate that inhibition of [3H](-)-baclofen binding to rat cerebellar membranes does not reflect antagonist activity at GABAB receptors seen in the rat cortical wedge preparation or the guinea-pig isolated ileum preparation. This may indicate a heterogeneity of GABAB binding and receptor sites.     

Novobiocin induces apoptosis-like cell death in topoisomerase II over-expressing arsenite resistant Leishmania donovani

Leishmaniasis affects millions of people worldwide every year. Lack of effective vaccination, co-infection with other dreaded diseases like AIDS and generation of drug resistant strains demand immediate attention into this neglected area of research. The sodium m-arsenite (NaAsO2) resistant Leishmania donovani used in this study is resistant to 20 microM NaAsO2, which shows a 13-fold increase in resistance compared with wild type. Here we report that the arsenite resistant strain of L. donovani promastigotes shows cross-resistance to novobiocin, a catalytic inhibitor of topoisomerase II, with IC50 value of 320 microg ml-1 as compared with 242 microg ml-1 for wild type L. donovani. Leishmanicidal action of novobiocin induces dose- and time-dependent increase in cell death. Treatment with IC50 of novobiocin caused morphological and biochemical changes which lead to induction of cell death exhibiting characteristic features of metazoan apoptosis. Phosphatidylserine externalization, cytochrome C release to cytoplasm, activation of caspases, oligonucleosomal DNA fragmentation and in situ labelling of condensed and fragmented nuclei in both wild type and arsenite resistant L. donovani promastigotes strongly suggest the apoptosis-like mode of cell death. Cross-resistance to novobiocin in arsenite resistant strain has been correlated to over-expression of topoisomerase II and substantiated by differential inhibition of enzyme activity in wild type and arsenite resistant L. donovani.     

The naphthalenesulphonamide calmodulin antagonist W7 and its 5-iodo-1-C8 analogue inhibit potassium and calcium currents in NG108-15 neuroblastoma x glioma cells in a manner possibly unrelated to their antagonism of calmodulin.

Patch clamp techniques were used to record voltage-sensitive calcium and potassium currents from NG108-15 cells. N-(6-aminohexyl)-5-chloro-1-naphthalene- sulphonamide (W7), a calmodulin (CaM) antagonist and its more potent (10 times) 5-iodo-1-C8 analogue (J8) inhibited these currents in a dose-dependent manner. The inhibition was not dependent on internal or external Ca2+. W7 was about four times more potent as an inhibitor of the transient potassium current (IC50 = 8 microM) than of the M-current or of the calcium current. J8 was also selective for the potassium currents (IC50 values: transient current 4 microM, M-current 11 microM) compared to the calcium current (IC50 36 microM). It is suggested that the inhibition does not result from an anti-CaM action of the compounds.     

Antiallergic activity profile in vitro of RHC 2963 and related compounds

RHC 2963 (7-methyl-pyrido (3',2':4,5)-thieno (3,2-d)-1,2,3 triazine-4(3H)-one and 20 related compounds have been investigated for their antiallergic activities in 3 in vitro models of anaphylaxis and for their effects on cyclic nucleotide phosphodiesterases (cNUC-PDE) from purified rat mast cells (RMC). Nine compounds were potent (I50 less than or equal to 80 microM) inhibitors of antigen-induced release of histamine (AIR) from RMC, 2 compounds inhibited anti-IgE-induced release of histamine from human basophils (I50 less than or equal to 60 microM) and one compound inhibited AIR from guinea pig lung slices (I50 = 55 microM). RHC 2963 was 18 times more potent than disodium cromoglycate (DSCG) as inhibitor of AIR from RMC and had an activity profile identical to that of DSCG in the following respects: loss of inhibitory activity with increasing preincubation time, tachyphylactic properties and inability to inhibit non-immunologic release of histamine induced by compound 48/80. Neither RHC 2963 nor DSCG had any effect on anti-IgE-induced release of histamine from human basophils or IgG1-mediated release of histamine from guinea pig lung. Twelve of the compounds in this chemical series were more potent than theophylline as inhibitors of cyclic AMP and/or cyclic GMP phosphodiesterase (PDE) from RMC. Paired regression analysis of the I50 values for inhibition of AIR and cNUC-PDE from RMC revealed no statistically significant correlation between the inhibition of AIR and inhibition of cAMP- or cGMP-PDE. We conclude: (1) RHC 2963 and some of the related compounds are potent inhibitors of immunologic release of histamine from RMC with a mechanism of action similar to that of DSCG, and (2) inhibition of cAMP- or cGMP-PDE by these compounds is not the biochemical mechanism by which they inhibit AIR from RMC.