This article is dedicated to late Professor María Teresa Miras-Portugal. She was a leading scientist in the field of purinergic signaling and P2 receptors for extracellular nucleotides [111,112,113,114]. She contributed to many important studies, advisory and editorial boards, the Purine club and the IUPHAR sub-committee for the nomenclature of P2Y receptors. She was the expert in the field of biochemistry, physiology and pharmacology of dinucleoside polyphosphates.

There are eight human subtypes of G-protein-coupled receptors (GPCRs) for extracellular nucleotides, P2Y receptors [23, 89]. They belong to the delta-subgroup of class A GPCRs [49]. The P2Y receptor family can be divided into two subfamilies [2, 156]. The first subfamily consists of the P2Y1, P2Y2, P2Y4, P2Y6 and P2Y11 receptors. The receptors couple via Gq-proteins to stimulation of phospholipase C (references in Tables 1, 2, 3, 4, and 5). P2Y11-receptors couple in addition through Gs and increases in adenylate cyclase activity (references in Table 5). In contrast, the P2Y12 receptor subfamily (P2Y12, P2Y13, and P2Y14 receptors) signals through activation of Gi-proteins (references in Tables 6, 7, and 8; see also [83, 158, 160]. P2Y receptors play important roles in physiology and pathophysiology [2, 22, 24, 120, 138, 139, 156, 158, 159]. Genetic knockout models can be used to identify the subtypes involved in responses to nucelotides. A fast alternative is the use of subtype specific ligands modifying cellular or tissue responses. Due to the efforts of medicinal chemistry novel compounds have been developed [10, 80,81,82, 118, 119, 121, 135, 143]. This article discusses pharmacological tools used to characterize P2Y receptor subtypes.

Table 1 Selected ligands acting at the human P2Y1-receptor
Table 2 Selected ligands acting at the human P2Y2-receptor
Table 3 Selected ligands acting at the human P2Y4-receptor
Table 4 Selected ligands acting at the human P2Y6-receptor
Table 5 Selected ligands acting at the human P2Y11-receptor
Table 6 Selected ligands acting at the human P2Y12-receptor
Table 7 Selected ligands acting at the human P2Y13-receptor
Table 8 Selected ligands acting at the human P2Y14-receptor

When using nucleotides such as ATP as agonists an interaction with ecto-nucleotidases [172] may be important. Hence, ATP may be degraded to ADP and further to adenosine which may activate adenosine receptors [74].

FormalPara P2Y1 receptor ligands

The P2Y1 receptor plays important roles in physiology including vasodilation, platelet aggregation, pain sensation, and astroglial signaling [2, 12, 48, 78, 124]. ADP is the principal agonist acting at the human P2Y1 receptor Table 1, [7, 84, 98]. Application of ATP may induce responses mediated by the P2Y1 receptor due to the fast breakdown of ATP to ADP [172]. When studied in detail with purified nucleotides, ATP has also been shown to act as a partial agonist at the receptor [64, 161]. 2-Methylthio-ADP activates the receptor (Table 1). The Northern (N-methanocarba analogue of 2-methylthio-ADP MRS2365; Table 1; [30] is a very potent and selective agonist that is inactive at the other two ADP receptors, P2Y12 and P2Y13.

Bisphosphate analogues including MRS2279 (2-chloro-N6-methyl-(N)-methanocarba-2´-deoxyadenosine 3´,5´-bisphosphate; [162] and MRS2500 (2-iodo-N6-methyl-(N)-methanocarba-2´-deoxyadenosine 3´,5´-bisphosphate; [91] act as antagonists. MRS2500 is a potent and selective antagonist Table 1, [91]. 2,2´-Pyridylisatogen tosylate [54] and BPTU (N-[2-[2-(1,1-dimethylethylphenoxy]-3-pyridinyl]-N'-[4-(trifluoromethoxyphenyl]urea; [28, 171] act as allosteric inhibitors. Interestingly, MRS2500 and BPTU have been used to grow crystals for the crystallography of the human P2Y1 receptor protein [171]. The data of that study clearly show two disparate ligand-binding sites at the receptor protein [171]. MRS2500 binds within the seven transmembrane helixes and BPTU binds to an allosteric pocket on the receptor interface with the cell membrane [171].

([32P]-labeled and [125I]-labeled) analogues of MRS2500 can be used for binding studies [73, 124], Table 1. A [18F]PET tracer ([18F]1-{2-[2-(tert-butyl)phenoxy]pyridin-3-yl}-3-[4-(2-fluoroethyl)phenyl]urea) is available for imaging of the tissue distribution of P2Y1 receptors [116].

FormalPara P2Y2 receptor ligands

The P2Y2 receptor plays roles in ion transport, regulation of epithelial cells, migration, vasodilatation and immune responses [31, 44, 97, 133, 140, 141]. Both ATP and UTP activate the receptor Table 2, [45]. Diadenosine-tetraphosphate (Ap4A), Up4U (diquafosol, INS365 [96, 131], and P1-(uridine 5´)-P4-(2´-deoxycytidine-5´)tetraphosphate (INS37217, denufosol; [83] also act as agonists. In fact, diquafosol is used in Japan and South Korea for the treatment of the dry eye syndrome [153, 165]. The 2-thio-analogues of UTP MRS2698 and PSB-1114 are more selective P2Y2 receptor agonists Table 2, [43, 77].

AR-C118925 is a potent and selective non-nucleotide antagonist at the P2Y2 receptor Table 2; [87, 92, 136, 137]. A fluorescent analogue of AR-C118925 can be used for labeling of the receptor at the cell membrane [37].

FormalPara P2Y4 receptor ligands

The P2Y4 receptor controls cellular responses including ion transport, growth and migration [71, 142]. The human P2Y4 receptor is activated by UTP and blocked by ATP Table 3, [32, 88, 123]. In contrast, both UTP and ATP act as full agonists at the rat P2Y4 receptor [21]. CTP analogues including MRS4062, and N4-(phenylethoxy)-CTP activate the human P2Y4 receptor with a preference over P2Y2 and P2Y6 receptors Table 3, [108].

PSB-1699 and PSB-16133 inhibit P2Y4 receptor activation with a clear selectivity versus other P2Y receptor subtypes [137]. They may act as allosteric inhibitors [137].

FormalPara P2Y6 receptor ligands

The human P2Y6 receptor plays roles vasoconstriction, ion secretion, migration, and inflammation [58, 60, 95, 97, 100, 103, 115, 134]. UDP is the endogenous agonist activating the receptor Table 4, [33, 123]. Synthetic agonists include PSB-0474 [43], MRS2693 [17] and the R(p) isomer of 5-OMe-UDP(α-B) Table 4, [59, 79].

MRS2578 blocks the P2Y6 receptor in a non-surmountable manner Table 4; [104]. MRS2578 may exert off target effects inhibiting cell migration [145]. TIM-38 (3-nitro-2-(trifluoromethyl)-2H-chromene) is a novel antagonist inhibiting P2Y6 receptor activation with an IC50 value of 4.3 μM and selectivity versus other P2Y receptor subtypes [76].

FormalPara P2Y11 receptor ligands

The P2Y11 receptor plays roles in granulocyte differentiation, dendritic cell maturation, chemotactic responses of neutrophils and neuropathic pain [164]. The human P2Y11 receptor is activated by ATP Table 5, [34]. In contrast, ADP is the principal agonist activating the canine P2Y11 receptor [166]. Synthetic agonists include the P2Y12 receptor antagonist 2-propylthio-β,γ-dichloromethylene-D-ATP (AR-C67085) and the suramin analogue NF546 Table 5, [109].

Suramin and the suramin analogues NF157 and NF340 act as antagonists at the human P2Y11 receptor Table 5; [109, 154]. NF340 is selective and slightly more potent Table 5, [109].

FormalPara P2Y12 receptor ligands

The P2Y12 receptor plays a crucial role in ADP-induced platelet aggregation [26, 63, 70, 152, 168]. Receptor activation is also involved in vasoconstriction, control of microglia, immune responses and control of bone mass [5, 16, 62, 72, 105, 115, 148, 151, 163]. As mentioned above, ADP is the endogenous agonist acting at the P2Y12 receptor Table 6, [30, 70]. ATP has antagonistic properties [86]. 2-Methylthio-ADP is a more potent agonist (Table 6).

Reduction of platelet aggregation is an important strategy in the prevention or therapy of cardiovascular events such as myocardial infarction. A number of potent antagonists have been developed (Table 6). AR-C67085 and cangrelor (AR-C69931MX; [75] are surmountable antagonists. Cangrelor is used to reduce platelet aggregation by intravenous application [99]. Cangrelor also blocks the P2Y13 receptor in a non-competitive manner see below Table 7, [106].

The radiolabeled nucleotide derivative [3H]PSB-0413 has a nanomolar affinity at the P2Y12 receptor Table 6; [42, 125]. The nucleoside analogue ticagrelor (AZD6140) is orally available and used for the reduction of platelet aggregation in patients after myocardial infarction [126, 155]. Ticagrelor appears to act in a surmountable manner at the human P2Y12 receptor Table 6, [68], see also [6, 57, 129]. In addition to the P2Y12 receptor, ticagrelor also blocks the P2Y13 receptor [19] and the equilibrative nucleoside transporter 1 [6]. Non-nucleotide antagonists include the analogue of reactive blue 2, PSB-0739 Table 6, [11, 67], 6-amino-2-thio-3H-pyrimidin-4-one derivatives [38], ethyl 6-aminonicotinate acyl sulfonamides [8], flavonolignans [18], morpholine derivatives [3], piperazinyl glutamates [130, 167], as well as salvianolic acids [101]. Selatogrel (ACT-246475) has a nanomolar affinity at the human P2Y12 receptor [9] and can be applied by subcutaneous administration [14, 147]. Selatogrel may act as inverse agonist at the human P2Y12 receptor [132].

Thienopyridine compounds are used for decades to reduce platelet aggregation in patients with cardiovascular diseases [15, 26]. The active metabolites of clopidogrel Table 6, [144], and prasugrel [149, 150] interact in an irreversible manner with Cys973.25 of the receptor protein (see red arrow in Fig. 1, [4, 41, 144]. Treatment with prasugrel is more potent when compared to ticlopidine or clopidogrel [15].

Fig. 1
figure 1

Predicted two-dimensional structure of the human P2Y12 receptor. TM transmembrane region, EL extracellular loop. Modified from [156]. The red arrow indicates Cys97 that is important for the interaction with the active metabolites of clopidogrel and prasugrel [4, 41, 144] and plays a role in receptor activation [169, 170]. The roles of the amino acid residues marked in red have been analyzed by Hoffmann et al. [69]. Arg256 and Lys280 are important for ligand recognition.

The crystallography of P2Y12 receptor proteins bound to agonists and antagonists will further facilitate the development of novel P2Y12 receptor ligands [169, 170].

FormalPara P2Y13 receptor ligands

The P2Y13 receptor is involved in degranulation of mast cells, metabolic effects and neuroprotection [20, 46, 55, 127, 128]. ADP is the principal agonist activating the P2Y13 receptor Table 7, [36]. ATP may act as a partial agonist [106].

Analogues of PPADS (pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid) including MRS2211 and MRS2603 inhibit the activation of the human P2Y13 receptor Table 7; [90]. Cangrelor blocks both P2Y12 receptor and the P2Y13 receptor [52, 106].

FormalPara P2Y14 receptor ligands

The P2Y14 receptor is involved in inflammation, pain, control of mast cells and microglia, insulin release, and vasoconstriction [1, 13, 39, 56, 110, 117]. UDP and UDP-sugars such as UDP-galactose and UDP-glucose activate the receptor Table 8, [25, 27, 50]. UDP analogues such as 5-iodo-UDP (MRS2690) and α,β-methylene-2-thio-UDP (MRS2905) are potent and selective agonists activating the P2Y14 receptor Table 8, [25, 40, 94].

Potent and selective antagonists include MRS4608 Table 8; [85] and PPTN (4-[4-(4-piperidinyl)phenyl]-7-[4-(trifluoromethyl)phenyl]-2-naphthalenecarboxylic acid; Table 8; [13]. MRS4174 is a fluorescent antagonist with an affinity in the nanomolar range Table 8, [93].