High bleeding risk (HBR) patients, comprising up to 50% of those presenting with acute coronary syndrome (ACS), are a high-risk group that is increasing in size due to an aging population. The optimal selection of the potency and duration of antiplatelet therapy to reduce the risk of recurrent ischemic and bleeding events in HBR patients is still a matter of debate. Multiple strategies to reduce bleeding during secondary prevention, such as reducing the duration of dual antiplatelet therapy, using single antiplatelet therapy with a P2Y12 inhibitor, or de-escalating to a lower potency or lower-dose P2Y12 inhibitor, have been proposed. De-escalation to a lower potency or lower-dose P2Y12 inhibitor is particularly attractive because it maintains efficient pharmacological inhibition of multiple platelet pathways while potentially reducing bleeding through less aggressive activity. Yet, there has been no study comparing the effects of different de-escalation strategies with the standard potent P2Y12 inhibitors in HBR patients. The aim of the DESC-HBR study is to assess the impact of de-escalating P2Y12 inhibitor to clopidogrel 75mg, prasugrel 5mg or ticagrelor 60mg bid in HBR patients, in comparison with full-dose potent P2Y12 inhibitors, on the proportion of patients with optimal platelet reactivity (OPR). Secondary objectives involve exploring the effect of de-escalation on clinical events and patients' quality of life.

The aim of the DESC-HBR trial is to compare the impact of de-escalating P2Y12 inhibitor to clopidogrel 75mg, prasugrel 5mg or ticagrelor 60mg bid, with full-dose potent P2Y12 inhibitors, on the proportion of patients with optimal platelet reactivity (OPR). The secondary objective is to explore the effect of de-escalating P2Y12 inhibitor therapy on clinical events and patients' quality of life. The primary outcome is the proportion of patients in the OPR range measured through the VerifyNow system at peak level after drug maintenance dose (MD) at 14±2 days. OPR is defined as a platelet reactive unit (PRU) between 85 and 208 reactivity units based on international consensus. A key secondary outcome will be major, minor and nuisance bleeding according to the bleeding academic research consortium (BARC) definition up to 5 months. Secondary pharmacodynamic outcomes include platelet reactivity with the VerifyNow and T-TAS system at different timepoints (baseline, 2h after first dose, through levels before MD and peak levels after MD at 14±2 days). Further pharmacodynamic assessment at 1 and 2 weeks after P2Y12 inhibitor discontinuation will be performed in the subset of patients discontinuing P2Y12 inhibitor at study conclusion or in case of P2Y12 discontinuation at anytime during the study.Other secondary endpoints will be explored including all-cause death, major adverse cardiac and cerebrovascular events (MACCE), a composite of cardiac death, myocardial infarction (MI), target vessel revascularization (TVR) and stroke, net adverse clinical events (NACE), a composite of MACCE and BARC 2-5 bleeding, and each individual endpoint singularly appraised. Quality of life will be evaluated with health mobility and performance scales (i.e. EQ-5D-5L, SF-12), perceived stress scale (i.e. PPS).Cost-effectives analysis will be also carried out by inputting direct and indirect costs in relation to outcomes. Study visits are scheduled at baseline,14±2 days, 3 and 5 months after randomization.

Participants fulfilling all the following inclusion criteria are eligible for the study:

• Informed Consent signed and dated.
• Patients deemed at HBR according to standard definitions (i.e. PRECISE-DAPT ≥25 or HBR-ARC with at least 1 major or 2 minor criteria).
• Treated with PCI due to a recent ACS (i.e. unstable angina, non-ST segment elevated myocardial infarction or ST segment elevated myocardial infarction) 30 ±7 days earlier.
• Treated with DAPT with full-dose potent P2Y12 inhibitors (e.g. prasugrel 10mg or ticagrelor 90mg bid) according to international guidelines recommendations.

The presence of anyone of the following exclusion criteria will lead to exclusion of the participant:

• Age < 18 years
• Known intolerance, hypersensitivity or contraindication (including active bleeding) to aspirin, clopidogrel, prasugrel, ticagrelor or to any of the excipients
• Indication to oral anticoagulation
• Indication to prolonged treatment with full-dose potent P2Y12 inhibitors (e.g. previous stent thrombosis, stenting of last remaining vessel, stent with indication for longer-term DAPT, perceived very high coronary ischemic risk)
• Any planned major surgery or interventional procedure requiring treatment modification
• Prior transient ischemic attack, ischemic or haemorrhagic stroke
• Severe hepatic insufficiency (Child-Pugh class C)
• Ongoing therapy with strong CYP3A inducers or strong CYP3A inhibitors (e.g. ketoconazole, clarithromycin, nefazodone, ritonavir, atazanavir etc.)
• Women who are pregnant, breast feeding or of childbearing potential (i.e. fertile, following menarche and who are not surgically sterile, including hysterectomy, bilateral salpingectomy, bilateral oophorectomy, or post-menopausal defined as no menses for 12 months without an alternative medical cause); Participation in another study with investigational drug within the 30 days, or 5 half-lives of the study drug whichever is longer, preceding and during the present study
• Enrolment of the investigator, his/her family members, employees
• Inability to follow the procedures of the study (language problems, mental disorders, dementia) or comorbidities associated with less than 12 months-life expectation (active malignancies drug or alcohol abuse, etc.) or other conditions that might result in protocol non-compliance.