Comparative action of carbocyclic thromboxane A(2) stereoisomers on platelets.

Stereospecific requirements for the interaction of the thromboxane A(2) carbocyclic mimetic CTA(2) 1 with the human platelet PGH(2)/TXA(2) receptor have been explored. The two pairs of trans-1,2 and cis-3,4 side chain diastereoisomers were synthesised and evaluated for agonist and antagonist activity in human platelet rich plasma. Interestingly, the natural and unnatural trans diastereoisomers, both possessed potent aggregatory activity and equipotently inhibited platelet responses to subsequent addition of agonists, whereas, the respective unnatural cis isomers proved only weakly active or inert.

Kinetic characteristics of ZENECA ZD5522, a potent inhibitor of human and bovine lens aldose reductase.

Aldose reductase (aldehyde reductase 2) catalyses the conversion of glucose to sorbitol, and methylglyoxal to acetol. Treatment with aldose reductase inhibitors (ARIs) is a potential approach to decrease the development of diabetic complications. The sulphonylnitromethanes are a recently discovered class of aldose reductase inhibitors, first exemplified by ICI215918. We now describe enzyme kinetic characterization of a second sulphonylnitromethane, 3',5'-dimethyl-4'-nitromethylsulphonyl-2-(2-tolyl)acetanilide (ZD5522), which is at least 10-fold more potent against bovine lens aldose reductase in vitro and which also has a greater efficacy for reduction of rat nerve sorbitol levels in vivo (ED95 = 2.8 mg kg-1 for ZD5522 and 20 mg kg-1 for ICI 215918). ZD5522 follows pure noncompetitive kinetics against bovine lens aldose reductase when either glucose or methylglyoxal is varied (K(is) = K(ii) = 7.2 and 4.3 nM, respectively). This contrasts with ICI 215918 which is an uncompetitive inhibitor (K(ii) = 100 nM) of bovine lens aldose reductase when glucose is varied. Against human recombinant aldose reductase, ZD5522 displays mixed noncompetitive kinetics with respect to both substrates (K(is) = 41 nM, K(ii) = 8 nM with glucose and K(is) = 52 nM, K(ii) = 3.8 nM with methylglyoxal). This is the first report of the effects of a sulphonylnitromethane on either human aldose reductase or utilization of methylglyoxal. These results are discussed with reference to a Di Iso Ordered Bi Bi mechanism for aldose reductase, where the inhibitors compete with binding of both the aldehyde substrate and alcohol product. This model may explain why aldose reductase inhibitors follow noncompetitive or uncompetitive kinetics with respect to aldehyde substrates, and X-ray crystallography paradoxically locates an ARI within the substrate binding site. Aldehyde reductase (aldehyde reductase 1) is closely related to aldose reductase. Inhibition of bovine kidney aldehyde reductase by ZD5522 follows uncompetitive kinetics with respect to glucuronate (K(ii) = 39 nM), indicating a selectivity greater than 5-fold for bovine aldose reductase relative to aldehyde reductase.

The effect of interleukin-4 on tumour necrosis factor-alpha induced expression of tissue factor and plasminogen activator inhibitor-1 in human umbilical vein endothelial cells.

The pro-inflammatory cytokine tumour necrosis factor-alpha (TNF-alpha) is able to alter the haemostatic balance of human umbilical vein endothelial cells (HUVECs) towards that of a procoagulant and anti-fibrinolytic state. Treatment of HUVECs in culture with human recombinant TNF-alpha (0.5-50 U/ml; 6 h) significantly increased total cell expression of tissue factor (TF) 10-fold from 40 mU/well to 400-500 mU/well. Levels of plasminogen activator inhibitor-1 (PAI-1) antigen secreted from HUVECs also increased up to 2-fold in concentration-dependent fashion following addition of TNF-alpha (10-100 U/ml; 24 h). TNF-alpha induced total and cell surface expression of TF on HUVECs was significantly inhibited when the cells were pre-incubated with interleukin-4 (IL-4; p < 0.001). This effect was time and concentration dependent. Pretreatment of HUVECs with IL-4 for 4 h had no significant effect, but increasing inhibition of total TF expression occurred after 8 and 16 h pre-incubations. Treatment with IL-4 at 20 and 200 U/ml significantly inhibited cell surface TF responses induced by TNF-alpha, whereas a low concentration (0.2 U/ml) was without effect. In contrast, the production of PAI-1 from HUVECs stimulated by TNF-alpha (50 U/ml) was unaffected by the presence and/or prior incubation with 200 U/ml IL-4. Thus, IL-4 may regulate the pro-coagulant but not the antifibrinolytic effects of TNF-alpha at sites of vascular inflammation.

Inhibition of aldose reductase by (2,6-dimethylphenylsulphonyl)nitromethane: possible implications for the nature of an inhibitor binding site and a cause of biphasic kinetics.

Aldose reductase (aldehyde reductase 2, ALR2) is often isolated as a mixture of two forms which are sensitive (ALR2S), or insensitive (ALR2I), to inhibitors. We show that ICI 215918 ((2-6-dimethylphenylsulphonyl)-nitromethane) follows either noncompetitive, or uncompetitive kinetics with respect to aldehyde for ALR2S, or the closely related enzyme, aldehyde reductase (aldehyde reductase 1, ALR1). Similar behaviour is exhibited by two other structural types of aldose reductase inhibitor (ARI), spirohydantoins and acetic acids, when either aldehyde, or NADPH is varied. For ALR2S, we have demonstrated kinetic competition between a sulphonylnitromethane, an acetic acid and a spirohydantoin. Thus, different ARIs probably have overlapping binding sites. Published studies imply that ALR2 follows an ordered mechanism where coenzyme binds first and induces a reversible conformation change (E.NADPH-->E*.NADPH). Reduction of aldehyde appears rate-limited by the step E*.NADP+-->E.NADP+. Spontaneous activation converts ALR2S into ALR2I and increases kcat. This must be associated with acceleration of the rate-determining step. We now propose the following hypothesis to explain characteristics of ARIs. (1) Inhibitors preferentially bind to the E* conformation. (2) The ARI binding site contains residues in common with that for aldehyde substrates. When aldehyde is varied, uncompetitive inhibition arises from association at the site for alcohol product in the E*.NADP+ complex which has little affinity for the substrate. Any competitive inhibition arises from use of the aldehyde site in the E*.NADPH complex. (3) Acceleration of the E*.NADP+-->E.NADP+ step upon activation of ALR2 reduces steady state levels of E* and so decreases sensitivity to ARIs.

5-[6-1 -(Cyclohexyl-1 H-tetrazol-5-YL)hexyl]-1,8-naphthyridin-2-(1H)-one, SC-44368, a Potent Anti-aggregatory Agent which Selectively Inhibits Platelet Cyclic AMP Phosphodiesterase.

SC-44368 (5-[6-(1-cyclohexyl-1H-tetrazol-5-y)hexyl]-1,8-naphthyridin-2(1H)-one) is a potent and selective competitive inhibitor of platelet cyclic AMP-dependent phosphodiesterase (cAMP-PDE) (Ki: 1.65 µM). For the phosphodiesterase isoenzyms from human platelets SC-44368 shows a 26-fold selectivity (IC50 ratio) for the inhibition of the cAMP-PDE over the cyclic GMP-dependent phosphodiesterase (cGMP-PDE). By comparison, 3-isobutyl-1-methyl-xanthine (IBMX) inhibited the cAMP-PDE and cGMP-PDE from human platelets with approximately equal efficacy. Broad inhibitory activity was evident against human platelet aggregatory responses in vitro. IC50 values of 18.1 ± 5.3 µM (25 nM platelet activating factor, PAF), 17.3 ± 3.0 µM (1.0 µg/ml collagen) and 24.2 ± 10.3 µM (1µM ADP) were obtained against maximum increases in platelet-rich plasma (PRP) light transmission achieved by each agonist. SC-44368 potentiated the prostacyclin-induced increase of intra-platelet cAMP levels but did not potentiate the sodium nitroprusside-induced increase of intraplatelet cGMP levels. In an ex vivo model of platelet aggregation SC-44368 (3 mg/kg, i.v.) produced a potent inhibition of collagen-induced platelet aggregation. SC-44368 produced only weak hypotensive activity in the rat. Thus, SC-44368 is a novel cAMP-PDE inhibitor which possesses potent, broad spectrum anti-aggregatory properties.

(2,6-Dimethylphenylsulphonyl)nitromethane: a new structural type of aldose reductase inhibitor which follows biphasic kinetics and uses an allosteric binding site.

Many of the complications of diabetes seem to be due to aldose reductase (aldehyde reductase 2, ALR2) catalysing the increased conversion of glucose to sorbitol. Therapy with aldose reductase inhibitors (ARIs) could, therefore, decrease the development of diabetic complications. (2,6-Dimethylphenylsulphonyl)nitromethane (ICI 215918) is an example from a newly discovered class of ARIs, and we here describe its kinetic properties. Preparations of bovine lens ALR2 exhibit biphasic kinetics with respect to glucose and various inhibitors including ICI 215918. The inhibitor sensitive form (ALR2S) has a higher affinity for glucose than does the inhibitor insensitive form (ALR2I). Only ALR2S was characterized in detail because ALR2I activity is very low at physiological levels of glucose and is difficult to measure with accuracy. Aldehyde reductase (ALR1) is the most closely related enzyme to ALR2. Inhibition of ALR1 was, therefore, investigated in order to assess the specificity of ICI 215918. The values of Ki and Kies (dissociation constants for inhibitor from enzyme-inhibitor and enzyme-inhibitor-substrate complexes, respectively) for ICI 215918 with bovine kidney ALR1 and bovine lens ALR2S have been determined. When glucose is varied, the compound is an uncompetitive inhibitor of ALR2S (Kies = 0.10 microM and Ki is much greater than Kies), indicating that ICI 215918 associates with an allosteric site on the enzyme. These kinetic characteristics would cause a decrease in the concentration required to give 50% inhibition when glucose levels rise during hyperglycaemia. ICI 215918 is a mixed noncompetitive inhibitor of ALR1 (Ki = 10 microM and Kies = 1.8 microM) when glucuronate is varied. Thus, the compound has up to 100-fold specificity in favour of ALR2S relative to ALR1. Therapeutic interest has now centred upon at least three distinct structural types of ARIs: spirohydantoins, acetic acids and sulphonylnitromethanes. Using one representative of each type, we have demonstrated kinetic competition for inhibition of ALR2S. This observation strongly suggests that the different inhibitors use overlapping binding sites.

Thrombin receptor antagonists. Structure-activity relationships for the platelet thrombin receptor and effects on prostacyclin synthesis by human umbilical vein endothelial cells.

Structure-activity studies on a series of analogues of N-(3-methyl-S-(1-pyrrolidinyl carbonyl) butyl)-D-alanine ethyl ester hydrochloride (SC42619) have defined the features of this dipeptide analogue required for observation of thrombin receptor antagonist activity on the human platelet. The affinity for SC42619, and for its structural analogue SC43583 is enhanced by pretreatment of the platelets with chymotrypsin. Endothelial cell prostacyclin (PGI2) synthesis induced by thrombin and trypsin is selectively inhibited by SC42619 provided that prolonged exposure to this antagonist is avoided. However inhibition of PGI2 synthesis by SC42619 is not overcome by increasing the thrombin concentration. The data provide further support for identification of SC42619 and certain of its analogues as selective antagonists at the platelet thrombin receptor but suggest that these compounds may have more complex, and possibly non-selective effects on the endothelial cell.

Ponalrestat: a potent and specific inhibitor of aldose reductase.

Many of the complications of diabetes appear to be closely linked to increased conversion of tissue glucose to sorbitol which is catalysed by aldose reductase (aldehyde reductase 2, ALR2). Inhibition of ALR2 could, therefore, lead to a reduction in the development of diabetic complications. Ponalrestat ["Statil" (a trademark, the property of Imperical Chemical Industries PLC), "Prodiax" (a trademark, the property of Merck, Sharp and Dohme), ICI 128436, MK538] inhibits ALR2 from a number of sources. Until now, the mechanism of this inhibition has not been fully elucidated. In this paper, we present a detailed mechanism for inhibition of bovine lens ALR2 by ponalrestat. Treatment of humans with some ALR2 inhibitors leads to side-effects, some of which may result from interactions with other enzymes. Aldehyde reductase (ALR1) is probably the most closely related enzyme to ALR2. Inhibition of ALR1 from bovine kidney was, therefore, investigated in order to assess the specificity of ponalrestat. The values of Ki and Kies (apparent dissociation constants for inhibitor from enzyme-inhibitor and enzyme-inhibitor-substrate complexes, respectively) for the interactions of ponalrestat with ALR1 and ALR2 has been calculated by non-linear fitting of kinetic data. These values indicate that ponalrestat does not compete with binding of glucose of NADPH to ALR2, nor with binding of glucuronate or NADPH to ALR1. Lack of competition and the structural dissimilarity of substrates and inhibitor make it unlikely that ponalrestat will utilize substrate binding sites on other enzymes, and so produce undesirable side-effects via such a mechanism. Ponalrestat is a potent inhibitor (Ki = Kies = 7.7 nM) of ALR2 and follows a pure noncompetitive mechanism with respect to glucose. Efficacy, therefore, will not be decreased by development of hyperglycaemia. The compound is a mixed noncompetitive inhibitor of ALR1 when glucuronate is varied. The values of Ki and Kies are 60 microM and 3 microM, respectively, so that inhibition tends towards uncompetitive. The selectivity of ponalrestat in favour of ALR2, therefore, lies in the range 390 to 7,800-fold, being higher at lower concentrations of glucuronate. The high selectivity of ponalrestat in favour of ALR2 rather than ALR1 suggests that the compound is unlikely to inhibit other enzymes which have less homology with ALR2.

Inhibitory effect of a selective thromboxane A2 receptor antagonist, EP 092, on platelet aggregation in whole blood ex vivo and in vivo.

1. The inhibitory effect of a selective prostaglandin H2 (PGH2)/thromboxane A2 receptor antagonist, EP 092, on platelet aggregatory responses in whole blood ex vivo (guinea-pig: Rhesus monkey) and intravascular aggregation in vivo (rabbit) has been investigated. 2. Collagen (0.1-10.0 micrograms ml-1) caused a concentration-dependent decrease in single platelet count in samples of both guinea-pig and Rhesus monkey citrated whole blood incubated ex vivo. EP 092 administered to guinea-pigs by intravenous (0.1-3.0 mg kg-1) or oral (1.0-10.0 mg kg-1) routes significantly inhibited the platelet responses to collagen (ED50 values 1.3 +/- 0.2 and 1.4 +/- 0.2 mg kg-1 respectively). Similar potency against collagen-induced whole blood aggregation was observed in Rhesus monkey blood samples following EP 092 given orally (ED50 0.9 +/- 0.3 mg kg-1). 3. The duration of action of EP 092 against collagen aggregatory responses ex vivo in both guinea-pigs and Rhesus monkeys was between 3 and 6 h following oral administration at 3.0 mg kg-1. 4. The inhibitory activity demonstrated by EP 092 against collagen-induced aggregation of Rhesus monkey whole blood ex vivo was not accompanied by any significant reduction in thromboxane A2 formation except at the highest dose tested (10 mg kg-1). 5. The intravascular aggregatory response induced by collagen or thrombin in the anaesthetized rabbit was significantly inhibited by an intravenous infusion of EP 092 (10 mg kg-1). EP 092 appeared less potent and its effect was of shorter duration in this preparation compared with its inhibitory effect on ex vivo aggregation, being evident immediately after infusion of drug but not after a further 30 min. 6. It is concluded that collagen-induced platelet aggregatory response in guinea-pig and Rhesus monkey whole blood ex vivo and rabbit in vivo exhibit a thromboxane-dependent component which can be inhibited in a dose-related fashion by pretreatment with the thromboxane antagonist EP 092. In the rabbit, moreover, the data support the possibility of a role for thromboxane in the intravascular aggregatory response to thrombin.

Identification of small peptide analogues having agonist and antagonist activity at the platelet thrombin receptor.

Two tripeptide analogues (N-[3-methyl-1-S[[2-S [(methyl-amino)carbonyl]-1-pyrrolidinyl] carbonyl]butyl-D-analine) (SC40476) and N-[3-methyl-S-(1-pyrrolidinylcarbonyl)butyl]-D-alanine, ethyl ester, hydrochloride (SC42619], inhibit aggregation of, and secretion from, human platelets induced by thrombin but cause no significant inhibition of esterolysis or fibrin formation catalysed by this enzyme. Inhibition by SC40476 of the aggregatory response induced by thrombin is incomplete. Neither peptide analogue inhibits aggregation induced by ADP, collagen, vasopressin or 11,9-epoxymethanoprostaglandin H2 (U-46619). Enhancement of the response is observed when nonsaturating concentrations of these agonists are employed. SC42619 causes a parallel shift to the right in the concentration-response curve describing aggregation induced by thrombin. The Schild plot of these data has a slope of 1.05 and the pA2 is 2.9 +/- 0.1. Both SC40476 and SC42619 induced a small but significant decrease in the single platelet content of platelet suspensions. Neither peptide analogue increases platelet cytosolic [Ca2+] measured using quin 2 or Fura 2. Both analogues cause inhibition of the increase in cytosolic [Ca2+] induced by thrombin. Inhibition by SC42619 is competitive with respect to thrombin when the extracellular [Ca2+] is reduced to less than 0.1 microM but is non-competitive in the presence of 1 mM Ca2+. SC42619 also inhibits the increase in cytosolic [Ca2+]induced by ADP in the presence of 1 mM Ca2+ but not the smaller increase caused by this agonist when the medium contains less than 0.1 microM Ca2+. SC42619 inhibits Mn2+ influx induced by thrombin and ADP. SC40476 and SC42619 inhibit the enhanced incorporation of [32P] into phosphatidic acid observed on stimulation by thrombin of platelets pre-labelled with [32P]-phosphate. Addition of the peptide analogues alone fails to increase significantly the 32P content of phosphatidate, phosphatidylcholine, phosphatidylserine or phosphatidylethanolamine. SC40476 causes no detectable hydrolysis of glycoprotein V as detected by release of the proteolytic product (glycoprotein VFR). The results indicate that SC40476 and SC42619 interact selectively with the platelet thrombin receptor. Both peptide analogues act as effective antagonists for this receptor but also possess weak agonist activity which may also result from interaction with the thrombin receptor. The molecular basis for this latter activity has not been defined. SC42619 non-selectively inhibits Ca2+ influx induced by several agonists but this effect does not appear to contribute to the observed inhibition of the aggregatory and secretory responses.

Effect of SC 38249, a novel substituted imidazole, on platelet aggregation in vitro and in vivo.

SC 38249 [RS)-1-(2,3-bis-[(4-methoxyphenyl)methoxy]propyl)-1H-imidazole) caused dose-related inhibition of collagen-induced thromboxane A2 formation in human platelet rich plasma (IC50: 9.9 +/- 1.0 microM) accompanied by a dose-dependent increase in plasma PGE2. Broad inhibitory activity was evident against human platelet aggregatory and secretory responses in vitro. IC50 values of 11.9 +/- 1.9 microM (0.64 mM arachidonic acid), 18.3 +/- 3.8 microM (0.5 microgram ml-1 collagen) and 37.6 +/- 6.1 microM (25 nM Paf-acether) were obtained against maximum increase in PRP light transmission achieved by each agonist. Although less potent, SC 38249 retained significant inhibitory activity against PRP responses induced by a higher (3.0 micrograms ml-1) concentration of collagen (IC50: 272.5 +/- 24.6 microM), and against Paf-acether-induced responses in PRP pre-treated with 10 microM indomethacin (I.C.50: 192.0 +/- 16.1 microM). Experimental animal studies confirmed the in vitro anti-aggregatory efficacy of SC 38249, since significant inhibitory activity was observed against Paf-acether and ADP-induced responses in dog PRP ex vivo, anti-Forssman antibody-induced thrombocytopenia in anaesthetized guinea pigs, and collagen-induced intravascular aggregation in anaesthetized rabbits. Thus, SC 38249 is a novel thromboxane synthase inhibitor which possesses interesting anti-aggregatory properties which cannot wholly be attributed to prevention of platelet thromboxane A2 formation.

Thromboxane synthase inhibitors. Synthesis and pharmacological activity of (R)-, (S)-, and (+/-)-2,2-dimethyl-6-[2-(1H-imidazol-1-yl)-1-[[(4-methoxyphenyl)- methoxy]methyl]ethoxy]hexanoic acids.

A series of substituted omega-[2-(1H-imidazol-1-yl)ethoxy]alkanoic acid derivatives were synthesized and evaluated for their ability to inhibit thromboxane synthase both in vitro and in vivo. Compound 13 was identified as a potent and selective competitive inhibitor of human platelet thromboxane synthase having a Ki value of 9.6 X 10(-8) M. In collagen-treated human whole blood, 13 potentiated levels of 6-keto PGF1 alpha. Enantiospecific syntheses afforded the R and S enantiomers of 13, of which the S enantiomer 13b was the more potent. Compounds 13 and 13b were potent in vivo inhibitors of thromboxane synthase with good oral activity and duration of action.

Structure-activity relationships in an imidazole-based series of thromboxane synthase inhibitors.

Analogues of 4-[[2-(1H-imidazol-1-yl)-1-[[(4-methoxyphenyl)methoxy]methyl] ethoxy]methyl]benzoic acid (5m) were prepared and evaluated as thromboxane synthase inhibitors. A series of esters of 5m showed a parabolic relationship between lipophilicity and inhibition of TxB2 generation in intact platelets, with activities up to 50 times greater than that of dazoxiben. However, on administration to rabbits the ethyl ester 5d had a short duration of action, due to rapid metabolism and excretion via deesterification and beta-glucuronidation. Attempts at replacing the carboxylate group with other potential pharmacophores were unsuccessful.

The effect of thromboxane A2 synthesis inhibitors on platelet aggregation in whole blood.

Aggregatory responses to arachidonic acid and collagen in vitro were compared in blood (single platelet counting) and PRP (light aggregometry) from four species. Sensitivity of mouse, rat and rabbit PRP to these agonists was not predictive of respective potency in whole blood, whereas for human platelets, responses in blood did not differ significantly from PRP. Indomethacin (3-300 microM) inhibited arachidonic acid-induced aggregation in each species, and collagen responses in all except mouse. In contrast, the thromboxane synthase inhibitors dazoxiben (3.7-372 microM) and SC 38249 (2.6-260 microM) demonstrated activity only in rabbit and human blood. However, since in many experiments drug efficacy decreased significantly in blood compared to corresponding PRP, the concentrations of each agent necessary to inhibit responses were above those at which selectivity has previously been demonstrated against isolated enzyme preparations or in PRP. A fundamental reappraisal of both the potency and selectivity of these inhibitors in whole blood appears essential before their mechanism of action can be firmly established.

Effect of indomethacin and dazoxiben on intravascular platelet aggregation in the anaesthetized rabbit.

Collagen (10-40 micrograms kg-1), thrombin (1-10 units kg-1), adenosine diphosphate (ADP; 3-300 micrograms kg-1), 1-0-hexadecyl Paf-acether and 1-0-octadecyl Paf-acether (1-300 ng kg-1) administered by bolus intravenous injection each caused dose-dependent thrombocytopoenia accompanied by marked hypotension in anaesthetized rabbits. Responses to ADP and the Paf-acether derivatives were transient in nature (3-8 min) whereas those induced by collagen and thrombin were always of longer duration (5-20 min) and frequently fatal at high doses. Responses to collagen, thrombin, and the Paf-acether derivatives were invariably accompanied by substantial, dose-related increases in plasma levels of thromboxane B2 in samples obtained 30 s after agonist administration, whereas following ADP, no change in plasma thromboxane B2 was detected at any dose level. Indomethacin (3.0 mg kg-1 by infusion) had no effect on responses to thrombin or Paf-acether, partially inhibited collagen-induced thrombocytopenia, and potentiated responses to ADP. In contrast, dazoxiben (10 mg kg-1 by infusion) partially but significantly inhibited responses to thrombin, whereas those induced by collagen, Paf-acether or ADP were unchanged. These results indicate that in this model of intravascular aggregation, whilst platelet responses to collagen and thrombin appear partially dependent on intact cyclic endoperoxide and thromboxane A2 synthetic capacity respectively, responses to ADP and Paf-acether are independent of arachidonate metabolism via cyclo-oxygenase despite measurably increased TXB2 formation in the latter case.

Effect of calcium and calcium antagonists on [3H]-Paf-acether binding to washed human platelets.

[3H]-Platelet activating factor (Paf-acether, 1-O-octadecyl-2-acetyl-sn-glycero-3-phosphocholine) binds to washed human platelets in a specific, dose-dependent, and saturable manner. Scatchard analysis reveals a high affinity site with a KD value of 0.25 +/- 0.033 nM (245 +/- 30 sites per platelet), and a second low affinity site with a KD value of 9.22 +/- 1.17 nM (1616 +/- 165 sites per platelet). Binding to the high affinity site is independent of buffer calcium concentration, inhibited on an equimolar basis by unlabelled 1-O-octadecyl-Paf-acether, but remains unchanged in the presence of 1-O-octadecyl-lyso-Paf-acether. The relative inhibitory effect of four calcium antagonists on [3H]-Paf-acether high affinity binding correlates closely with their respective anti-aggregatory activity against Paf-acether induced responses in human PRP; order of potency being (+)-cis diltiazem greater than (+/-)-verapamil greater than (-)-cis diltiazem greater than nifedipine. In the case of (+)-cis diltiazem, the effect is competitive, stereo-specific and progressively reversed by addition of calcium (1.0 mM and 5.0 mM). A close spatial relationship may thus exist between the Paf-acether receptor and membrane calcium channels in the human platelet.

Pharmacological effects of (+/-)-11-deoxy, 16-phenoxy-prostaglandin E1 derivatives in the cardiovascular system.

M&B 28,767 [(+/-)-11-deoxy-16-phenoxy-17,18,19,20-tetranor prostaglandin E1] and a series of close analogues have been compared with U-46619 [(15S)-hydroxy-11 alpha, 9 alpha-(epoxymethano)-prosta-(5Z,13E)-dienoic acid] for prostaglandin endoperoxide-like pharmacological actions in vitro and in vivo. M&B 28,767 caused powerful dose-related contraction of rabbit aorta (EC50: 2.0 microM) and mesenteric artery (EC50: 0.2 microM) strips in vitro, but was less active than U-46619 and/or noradrenaline. M&B 28,767 induced rapid and irreversible aggregation of rat (0.9 times potency of U-46619) and human (0.25 times potency of U-46619) platelets in platelet-rich plasma (PRP) in vitro. Intravenous administration of M&B 28,767 to urethane-allobarbitone anaesthetized rats produced immediate and dose-related thrombocytopoenia (equipotent with U-46619), accompanied in some animals by transient small pressor effects at low doses (1-2 micrograms kg-1) which were not statistically significant and invariably by sharp depressor effects at higher doses (3-10 micrograms kg-1). U-46619 caused moderate, but not dose-related, pressor effects at all doses tested. Considerable variation in potency occurred amongst the thirteen structural analogues of M&B 28,767. Platelet aggregatory activity for those members of the 11-deoxy 16-phenoxy-PGE1 series tested in rat PRP in vitro demonstrated a positive and significant correlation with pro-aggregatory activity in vivo and agonist potency on rabbit aortic strip in vitro.

Specific binding of [3H]-1-O-octadecyl Paf-acether to washed human platelets.

[3H]-Paf-acether binds to washed human platelets in a dose-dependent manner. Scatchard analysis reveals two distinct binding sites; a high affinity site with a KD value of 0.259 +/- 0.33 nM (245 +/- 30 sites per platelet) and a lower affinity site with a KD value of 9.22 +/- 1.17 nM (1616 +/- 165 sites per platelet). Association of 3H-Paf-acether to the high affinity receptor is rapid, being maximal within two minutes and remaining constant for at least twenty minutes. Dissociation from the low affinity receptor is also rapid (t1/2: less than 10s) whereas dissociation from the high affinity site is significantly slower (t1/2 : approximately 70s). [3H]-Paf-acether binding is inhibited by unlabelled (R)-C16-Paf (IC50: 0.08 +/- 0.01 nM) greater than (R)-C18-Paf (0.48 +/- 0.03 nM) greater than (RS)-C18-Paf (1.06 +/- 0.19 nM), but remains unchanged in the presence of lyso-C18-Paf at 3.0-300 nM. [3H]-Paf-acether binding and its inhibition by unlabelled (R)-C18-Paf-acether is independent of buffer Ca2+ within the range 0-5.0 mM.