ADP receptor antagonists as antiplatelet therapeutics

P2Y12 expression and function in alternatively activated human microglia

Objective:

To investigate and measure the functional significance of altered P2Y12 expression in the context of human microglia activation.

Methods:

We performed in vitro and in situ experiments to measure how P2Y12 expression can influence disease-relevant functional properties of classically activated (M1) and alternatively activated (M2) human microglia in the inflamed brain.

Results:

We demonstrated that compared to resting and classically activated (M1) human microglia, P2Y12 expression is increased under alternatively activated (M2) conditions. In response to ADP, the endogenous ligand of P2Y12, M2 microglia have increased ligand-mediated calcium responses, which are blocked by selective P2Y12 antagonism. P2Y12 antagonism was also shown to decrease migratory and inflammatory responses in human microglia upon exposure to nucleotides that are released during CNS injury; no effects were observed in human monocytes or macrophages. In situ experiments confirm that P2Y12 is selectively expressed on human microglia and elevated under neuropathologic conditions that promote T_h2 responses, such as parasitic CNS infection.

Conclusion:

These findings provide insight into the roles of M2 microglia in the context of neuroinflammation and suggest a mechanism to selectively target a functionally unique population of myeloid cells in the CNS.

Thienopyrimidine-based P2Y12 platelet aggregation inhibitors

Herein we describe the design and synthesis of a novel series of potent thienopyrimidine P2Y12 inhibitors and the negative impact protein binding has on the inhibition of platelet aggregation.

P2Y1 and P2Y2 receptor distribution varies along the human placental vascular tree: role of nucleotides in vascular tone regulation

The expression of purinergic P2Y receptors (P2YRs) along the cord, superficial chorionic vessels and cotyledons of the human placenta was analysed and functional assays were performed to determine their vasomotor activity. Immunoblots for the P2Y(1)R and P2Y(2)R revealed a 6- to 8-fold increase in receptor expression from the cord to the chorionic or cotyledon vessels. In the cord and chorionic vessels the receptor distribution was mainly in the smooth muscle, whereas in the cotyledon vessels these receptors were equally distributed between the endothelium and smooth muscle cells. An exception was the P2Y(2)R at the umbilical artery, which was distributed as in the cotyledon. mRNA coding for the P2Y(1)R and P2Y(2)R were detected by RT-PCR and the mRNA coding for the P2Y(4)R, P2Y(6)R and P2Y(11)R was also identified. Application of 2-MeSADP and uridine triphosphate (UTP), preferential P2Y(1)R and P2Y(2)R ligands, respectively, resulted in contraction of isolated rings from umbilical and chorionic vessels. The vasoconstriction was blocked in a concentration-dependent manner by 10-100 nm indomethacin or 10 nm GR32191, suggesting the involvement of thromboxane receptors. MRS 2179, a selective P2Y(1)R antagonist, reduced the 2-MeSADP- but not the UTP-evoked contractions. Perfusion of cotyledons with 2-MeSADP or UTP evoked concentration-dependent reductions in perfusion pressure mediated by the NO-cGMP pathway. Blockade of NO synthase abolished the vasodilatation and the rise in luminal NO elicited by either agonist. MRS 2179 antagonized the dilatation and rise in luminal NO evoked by 2-MeSADP but not by UTP. In summary, P2Y(1)R and P2Y(2)R are unevenly distributed along the human placental vascular tree; both receptors are coupled to different signalling pathways in the cord/chorionic vessels versus the cotyledon leading to opposing vasomotor responses.

Molecular aspects of thrombosis and antithrombotic drugs

There have been major advances in our understanding of thrombosis and antithrombotic drugs. This review focuses on the molecular aspects of thrombus formation and antithrombotic therapy. Molecules involved in arterial thrombosis are derived from inflammatory cells in the atherosclerotic plaque and blood platelets. These molecules work in concert to promote plaque instability and thrombogenicity. Thrombus formation on the ruptured plaque is mediated by platelet and coagulation activation. By contrast, molecules involved in venous thrombosis are derived from the activated coagulation cascade. Platelets appear to play a secondary role. The antithrombotic drugs are classified according to their targeted constituents: antiplatelet agents and anticoagulants; the latter are further divided into non-specific anticoagulants, such as vitamin K antagonists and heparin, and direct thrombin inhibitors, including hirudin and argatroban. Currently available antiplatelet agents target glycoprotein IIbIIIa (abciximab, tirofiban, eptifibatide), cyclooxygenase-1 (aspirin) or adenosine diphosphate receptor, P2Y12 (clopidogrel).

The platelet P2 receptors as molecular targets for old and new antiplatelet drugs

Platelet activation by ADP and ATP plays a crucial role in haemostasis and thrombosis, and their so-called P2 receptors are potential targets for antithrombotic drugs. The ATP-gated channel P2X1 and the 2 G protein-coupled P2Y1 and P2Y12 ADP receptors selectively contribute to platelet aggregation. The P2Y1 receptor is responsible for ADP-induced shape change and weak and transient aggregation, while the P2Y12 receptor is responsible for the completion and amplification of the response to ADP and to all platelet agonists, including thromboxane A2 (TXA2), thrombin, and collagen. The P2X1 receptor is involved in platelet shape change and in activation by collagen under shear conditions. Due to its central role in the formation and stabilization of a thrombus, the P2Y12 receptor is a well-established target of antithrombotic drugs like ticlopidine or clopidogrel, which have proved efficacy in many clinical trials and experimental models of thrombosis. Competitive P2Y12 antagonists have also been shown to be effective in experimental thrombosis as well as in several clinical trials. Studies in P2Y1 and P2X1 knockout mice and experimental thrombosis models using selective P2Y1 and P2X1 antagonists have shown that, depending on the conditions, these receptors could also be potential targets for new antithrombotic drugs.