Discovery and recognition of purine receptor subtypes on platelets

TRimetazidine as an Agent to affeCt clopidogrEl Response: The TRACER Study

INTRODUCTION

This prospective study aimed to determine whether trimetazidine (TMZ) alters the pharmacodynamic (PD) effects of clopidogrel.

METHODS

Patients with stable coronary artery disease (SCAD) (n = 24) who were actively treated with dual antiplatelet therapy (DAPT) of aspirin 81 mg daily and clopidogrel 75 mg daily were recruited. Platelet function was measured with the VerifyNow P2Y12 assay (Accriva Diagnostics, San Diego, CA, USA) and assessed before the initiation of and after 14 days of treatment with TMZ. Results were compared using a paired t test.

RESULTS

Almost 80% of the study population were of South Asian descent and had diabetes mellitus (DM). P2Y12 reaction units (PRUs) were higher in patients on TMZ (204 ± 56 compared with 174 ± 71 before TMZ, p = 0.005). The average increase in PRU score was 29 (95% confidence interval 8.8-49.7). Before TMZ, the proportion of patients with high on-treatment platelet reactivity (PRU > 208 units) was 25%, which increased to 42% for patients on TMZ.

CONCLUSION

Higher platelet reactivity was seen in patients on TMZ, suggesting that TMZ attenuated the PD effects of clopidogrel in this study of a predominantly South Asian diabetic subpopulation. Alternative therapies should be considered and further research is warranted.

TRIAL REGISTRATION

ClinicalTrials.gov number, NCT03603249.

Inhibitory Effects of Methyl Brevifolincarboxylate Isolated from Phyllanthus niruri L. on Platelet Aggregation

A platelet-aggregatory inhibitor was isolated from the 50% MeOH extract of Phyllanthus niruri L. leaf. Its structure was determined to be methyl brevifolincarboxylate on the basis of the 1H-, 13C-NMR, and high-resolution mass spectral data. We compared the antiplatelet aggregatory effects of the constituent with adenosine, a well-known inhibitor of platelet aggregation. Platelet aggregation was induced by collagen or adenosine 5'-diphosphate as an activating agent; the extent of inhibition was monitored with a platelet aggregometer employing a laser-scattering method. The inhibitory effects of methyl brevifolincarboxylate were found to be as potent as adenosine that is known to act on an A2A subtype receptor.

Leukocyte count and leukocyte ecto-nucleotidase are major determinants of the effects of adenosine triphosphate and adenosine diphosphate on platelet aggregation in human blood

ADP induces platelet aggregation in human whole blood and platelet-rich plasma (PRP). ATP induces aggregation in whole blood only; this involves leukocytes and is mediated by ADP. Here we studied ATP- and ADP-induced aggregation in patients with raised leukocyte counts (mean 46.2x10(3) leukocytes/microl). Platelet aggregation was measured by platelet counting. ATP, ADP and metabolites were measured by HPLC. Aggregation to ADP (1-10 microM) and ATP (10-100 microM) was markedly reduced, but to ATP (1000 microM) was enhanced (all p<0.001). Aggregation to ADP in PRP was normal. Increasing the leukocyte count in normal blood reproduced the findings in the patients. Adding leukocytes (either MNLs or PMNLs) to normal PRP enabled a response to ATP and caused marked inhibition of ADP-induced aggregation. Breakdown of ATP or ADP to AMP and adenosine in leukocyte-rich plasma was rapid (t1/2=4 min) and far higher than in cell-free plasma or PRP. With ATP there was also formation of ADP, maximal at 4 min. The presence of the ectonucleotidase NTPDase1 (CD39) was demonstrated on MNLs (all of the monocytes and a proportion of the lymphocytes) and all PMNLs by flow cytometry. We conclude that leukocytes provide a means of dephosphorylating ATP which enables ATP-induced aggregation via conversion to ADP, but also convert ADP to AMP and adenosine. Platelet aggregation extent is a balance between these activities, and high white cell counts influence this balance.

Adenine nucleotide metabolism in human blood--important roles for leukocytes and erythrocytes

Adenosine diphosphate (ADP) released into blood induces platelet aggregation and contributes to hemostasis and thrombosis. Released ATP can also induce platelet aggregation and there is evidence that blood leukocytes and also erythrocytes play important roles in this. Rapid metabolism of ADP and ATP by endothelial cells is important in protecting platelets from their effects. Here we have performed a systematic investigation of adenine nucleotide metabolism in human blood and the involvement of blood cells. Conversion of ATP to ADP in blood was due almost exclusively to the presence of leukocytes; plasma, platelets and erythrocytes made little or no contribution. Mononuclear leukocytes (MNLs) and polymorphonuclear leukocytes (PMNLs) were equally effective. Conversion of ADP to AMP was also promoted by leukocytes, with no involvement of platelets or erythrocytes. Some ADP was also converted to ATP in blood, apparently via an enzyme present in plasma, but ATP was then rapidly removed by the leukocytes. Conversion of AMP to adenosine occurred via a plasma enzyme with little or no contribution from any cellular element. As expected, in blood the adenosine produced was removed very rapidly by erythrocytes and then converted to inosine and then hypoxanthine. In the absence of erythrocytes plasma supported only a slow conversion of adenosine to inosine and hypoxanthine, which was not influenced by platelets or leukocytes. This study has demonstrated that leukocytes and erythrocytes play a major role in adenine nucleotide metabolism in blood and that these cells, as well as endothelial cells, may be important determinants of the effects of ATP and ADP on platelets.

The novel suramin analogue NF864 selectively blocks P2X1 receptors in human platelets with potency in the low nanomolar range

The role of ATP-stimulated P2X1 receptors in human platelets is still unclear. They may act alone or in synergy with other pathways, such as P2Y1 or P2Y12 receptors, to accelerate and enhance calcium mobilisation, shape change and aggregation. To date very few pharmacological means of selectively inhibiting platelet P2X1 receptors have been described, although recent work has shown that suramin is a useful lead compound for the development of high-affinity P2X1 antagonists. We therefore investigated the effects of a series of bivalent and tetravalent suramin analogues on alphabeta meATP (P2X1 receptors)-induced or ADP (P2Y1 receptors)-induced intracellular calcium increases and shape change, as well as on ADP-induced aggregation (P2Y1 & P2Y12 receptors) in human platelets. Changes in intracellular calcium were measured using standard fluorescence techniques, while shape change and aggregation were determined by turbidimetry. The novel tetravalent compound NF864 (8,8',8'',8'''-(carbonylbis(imino-5,1,3-benzenetriyl-bis(carbonylimino)))tetrakis-naphthalene-1,3,5-trisulfonic acid-dodecasodium salt) proved to be the most potent platelet P2X1 antagonist reported to date, blocking alphabeta meATP-induced Ca2+ increases and shape change in a concentration-dependent manner, with a pA2 of 8.17 and 8.49, respectively. The ability to inhibit the platelet P2X1 receptor displayed the following order : NF864 > NF449 > or = NF110 > NF023 = MK-HU1 = suramin. A different antagonistic profile was observed for ADP-induced Ca2+ increases, shape change and aggregation; however, overall four compounds showed sufficient ability to selectively inhibit P2X1 responses, with the order NF110 > NF449 > or = NF864 > or = MK-HU1. Therefore, these compounds should prove useful tools for investigating the functional significance of platelet P2X1 receptors in thrombosis and haemostasis, NF864 being the most promising compound.

Mechanisms involved in adenosine triphosphate--induced platelet aggregation in whole blood

OBJECTIVE

Effects on platelet aggregation of adenosine triphosphate (ATP) released from damaged cells and from platelets undergoing exocytosis have not been clearly established. In this study we report on the effects of ATP on platelet aggregation in whole blood.

METHODS AND RESULTS

Aggregation, measured using a platelet-counting technique, occurred in response to ATP and was maximal at 10 to 100 micromol/L. It was abolished by MRS2179, AR-C69931, and creatine phosphate/creatine phosphokinase, implying that conversion to adenosine diphosphate (ADP) is required. ATP did not induce aggregation in platelet-rich plasma, but aggregation did occur when apyrase or hexokinase was added. Aggregation also occurred after addition of leukocytes to platelet-rich plasma (as a source of ecto-ATPase), and this was potentiated on removal of adenosine by adenosine deaminase, indicating that adenosine production modulates the response. Dipyridamole, which inhibits adenosine uptake into erythrocytes, inhibited aggregation induced by ATP in whole blood, and adenosine deaminase reversed this. DN9693 and forskolin synergized with dipyridamole to inhibit ATP-induced aggregation.

CONCLUSIONS

ATP induces aggregation in whole blood via conversion of ATP to ADP by ecto-ATPases on leukocytes. This is inhibited by agents that prevent adenosine removal. Reduced aggregation at high concentrations of ATP (>100 micromol/L) may be a consequence of inhibition by ATP of ADP action at ADP receptors.

Adenine, an inhibitor of platelet aggregation, from the leaves of Cassia alata

Adenine was isolated as a platelet aggregating inhibitor from the leaves of Cassia alata by HPLC using a triacontylsilyl silica (C(30)) column. The inhibitory effects of adenine and adenosine (positive control) on the platelet aggregation induced by collagen or adenosine 5'-diphosphate (ADP) as an aggregating agent was evaluated with a platelet aggregometer using a laser-scattering method. As a result, the inhibitory effect of adenine was observed in the platelet aggregation induced by collagen (1.0 microg/ml as the final concentration), but little inhibitory effect was noted in the aggregation induced by ADP (5.0 microM as the final concentration), whereas adenosine exhibited potent inhibitory effects on platelet aggregation induced both by collagen and ADP under the same experimental conditions.