Adenosine diphosphate (ADP) receptor inhibitors are a drug class of antiplatelet agents, used in the treatment of acute coronary syndrome (ACS) or in preventive treatment for patients who are in risk of thromboembolism, myocardial infarction or a stroke. These drugs antagonize the P2Y12 platelet receptors and therefore prevent the binding of ADP to the P2Y12 receptor. This leads to a decrease in aggregation of platelets, prohibiting thrombus formation. The P2Y12 receptor is a surface bound protein found on blood platelets. They belong to G protein-coupled purinergic receptors (GPCR) and are chemoreceptors for ADP.
The first drug introduced in this class was ticlopidine but due to adverse effects it is not much used today. Ticlopidine, clopidogrel and prasugrel (Efient) are all thienopyridines that cause irreversible inhibition of P2Y12 receptor. They are all prodrugs which need to be converted to an active metabolite in-vivo to inhibit the P2Y12 receptor. On the other hand, novel drugs like ticagrelor (Brilinta®) and cangrelor (Kengrexal®) are non-thienopyridines and reversibly inhibit P2Y12 meaning they act directly on the receptor without the requirement of metabolic activation and display faster onset and offset of action.
These drugs are frequently administered in combination with aspirin (acetylsalicylic acid) to enhance platelet inhibition especially in patients with ACS or undergoing percutaneous coronary intervention (PCI).
History Before the time of ADP inhibitors the only antiplatelet agent on the market to treat antithrombotic events was aspirin. However, because of recurrent ischemic events in high risk patients there was a basis for development of antiplatelet drugs to target other important signaling pathways.
The history of ADP inhibitors started in the year 1972 when researchers were searching for drugs similar to the anti-inflammatory agent tinoridine, a thienopyridine with anti-inflammatory and analgesic effects that had been published two years before. Based on the knowledge of thienopyridine's chemistry a significant number of derivatives of thienopyridines were synthesized. The derivatives were tested in-vivo and ex-vivo in mice and rats but the results of the tests didn't demonstrate any anti-inflammatory or analgesic effects at all but instead they displayed unexpected antiplatelet and antithrombotic effects. At that time it was very uncommon to screen for new antiplatelet agents, as the connection between platelet aggregation, thrombosis and cardiovascular incidents was disputed. However the most active derivative, ticlopidine, was selected for further development. Ticlopidine was the first-generation thienopyridine that enhanced platelet inhibition and thus used for treating in actue coronary syndrome and other cardiovascular diseases. Due to reported severe adverse effects of ticlopidine second and third-thienopyridines, clopidogrel and prasugrel, were developed.
When ticlopidine and clopidogrel were first brought to the market, ticlodipine in 1978 and clopidogrel in 1998, the mechanism of action of these two major antithrombotic drugs was not fully understood. What had been shown was that they were potent inhibitors of ADP-induced platelet aggregation, but the P2Y12 receptor had not been identified. Furthermore, it was clear that ticlopidine and clopidogrel were prodrugs which means they are inactive in-vitro and therefore need metabolism to be activated in-vivo. It wasn't until the year 2000 when the active metabolites of these drugs were isolated and characterized. About one year later the P2Y12 platelet receptor for ADP was identified. Following these discoveries the active metabolites and the enzymes responsible for their formation were progressively identified. Thus it was more than 30 years later after the discovery of ticlopidine, and more than 10 years later after the discovery of clopidogrel the mechanism of action of these two drugs was explained.
Development
Thienopyridines
Ticlopidine The first P2Y12 inhibitors were of the thienopyridine family. They are indirect antagonists, which block the ADP-induced platelet aggregation and activation. The first drug of this class was ticlopidine and was discovered in 1972 at Porcor (now Sanofi). It was discovered while screening for a new anti-inflammatory drug based on tinoridine. It was screened with a phenotypic screening approach, tested both in vivo and ex vivo rodent models. It showed a high antiplatelet activity.
Ticlopidine had good promises and was selected for clinical trials. It was marketed in France in 1978 and went global 1991 when it reached US market for the primary and secondary prevention of stroke.
The search for another thienopyridine analog with a better activity/toxicity ratio in animals started as soon as ticlopidine went into preclinical trials. It became more urgent to find a new analog after reports of patients having severe hematological disorders due to ticlopidine.
Clopidogrel Clopidogrel, a second generation thienopyridine, started in preclinical trials in 1987 and reached global market in 1998. Its mechanism of action and of its precursor ticlopidine was still unknown. The only things that were known were that they were prodrugs as they didn't show any activity in vitro, that they affect platelets irreversibly because of their long duration of action and the active metabolite was chemically and biologically unstable. It wasn't until 2000 that the active metabolite was discovered and its platelet target was discovered one year later, the P2Y12 receptor of ADP.
Although clopidogrel had better activity/toxicity ratio than ticlopidine there were still problems with its activity as 30% of patients have clopidogrel resistance. The major factor in clopidogrel resistance is CYP2C19 polymorphism, which occurs in 30-55% patients. This led to loss of functions of the enzyme which led to poor conversion of clopidogrel into its active metabolite. The metabolite itself is very unstable and can therefore not be stored as a part of a chemical library. This led to the development of a compound which relies less on CYP-mediated metabolism, prasugrel.
Prasugrel Prasugrel, third generation thienopyridine was brought to the market in 2009 by the pharmaceutical companies, Daiichi Sankyo/Eli Lilly. Prasugrel, like its precursors is a pro-drug but its metabolism starts in the intestines where it is metabolized by esterase into a thiolactone, this inactive intermediate then undergoes CYP-mediated ring opening, mainly by CYP3A4 and CYP2B6 to the active metabolite. Thus, prasugrel is not subjected to clopidogrel resistance.
New generation ADP receptor inhibitors
Ticagrelor The focus went towards finding a P2Y12 inhibitor that is not a metabolite and with faster onset of action. It was known that ATP competitively antagonizes ADP-induced platelet aggregation, however ATP is very unstable. The attention went to create ATP analogues with higher potency and stability. These analogues had very short half-life due to retention of the triphosphate groups and thus needed to be given IV. Modification of these analogues led to the discovery of ticagrelor, a selective and stable non-phosphate P2Y12 receptor antagonist. Ticagrelor belongs to the class of cyclopentyl-triazolopyrimidine (CPTP). Ticagrelor came to the market in 2010 in Europe, and 2011 in USA.
Cangrelor Cangrelor another ATP analogue like ticagrelor, is stable to enzymatic degradation. It has a fast onset of action as it is not broken down into an active metabolite like thienopyridines. AstraZeneca got exclusive license for cangrelor in December 2003. In 2009 the sponsor for the phase 3 trial pulled out, where cangrelor was being tested against placebo. Cangrelor development was halted for a time when the sponsor pulled out after the interim analysis review committee (IARC) decided that the trial wouldn't show the "persuasive" clinical efficacy that is need for regulatory approval. However Champion phoenix trial (sponsored by the biopharmaceutical company, The Medicines Company), a double-dummy, double-blind placebo-controlled trial where 11,145 patients who were undergoing rather urgent or elective PCI were randomly assigned to receive cangrelor or clopidogrel before PCI showed that cangrelor significantly reduced the rate of ischemic events during PCI. Cangrelor lowered stent thrombosis development more than clopidogrel. These findings were published in 2013. Cangrelor got FDA approval in June 2015 as an antiplatelet drug for intravenous application.
Mechanism of action The molecular target of the active metabolite of ADP receptor inhibitors is the P2Y12 receptor. P2Y12 receptor is a G-coupled receptor and is activated by adenosine diphosphate. ADP binds to the P2Y12 receptor that leads to inhibition of adenyl cyclase and thereby decreases the intracellular levels of cAMP. This reduction of cAMP reduces phosphorylation of vasodilator stimulated phosphoprotein that leads to the activation of the glycoprotein IIb/IIIa receptors. Activation of the glycoprotein IIb/IIIa receptors increases thromboxane production and prolonged platelet aggregation. Ticlopidine, clopidogrel and prasugrel are thienopyridine prodrugs that are irreversible platelet inhibitors of the P2Y12 receptor. Cangrelor and ticagrelor are direct –acting P2Y12 inhibitors that change the conformation of the P2Y12 receptor and therefore, results in reversible platelet inhibition of the receptor. Thienopyridines are metabolized in the liver and the intestinal to active metabolites.
Metabolism Ticlopidine is a prodrug and is metabolized by at least five main pathways. There is one active metabolite that has been identified and shown to have antiplatelet activity. This active metabolite is formed by a CYP-dependent pathway. CYP2C19 and CYP2B6 are enzymes suggested to contribute to the metabolic transformation of ticlopidine to the thiolactone intermediate, 2-oxo-ticlopidine in the liver. The thiolactone intermediate is then converted to ticlopidine active metabolite via CYP oxidation where oxidation activation occurs. However the CYP enzymes that are involved in this pathway are unknown. In the formation of the active metabolite additional metabolites have been identified and they are dihydrothienopyridinium (M5) and thienodihydropyridinium metabolites (M6). These metabolites may be responsible for the toxic side effects of ticlopidine.