Dual antiplatelet therapy

# Overview
Dual antiplatelet therapy (or DAPT) refers to the combination of aspirin and a P2Y12 receptor antagonist. DAPT is approved for SIHD and interventions for ACS, such as stent placement following PCI or CABG. The duration of treatment with DAPT for each of these categories differs and guidelines for treatment have been updated in the 2016 ACC/AHA Guideline Focused Update on Duration of Dual Antiplatelet Therapy in Patients With Coronary Artery Disease. Much of the studies done on DAPT compared the use of different types of P2Y12 receptor antagonists, the dosage of drugs, as well as the duration of treatment. While the use of DAPT is associated with decreased risk of stent thrombosis, the benefits of treatment should be weighed against the increased risk of major bleeding.

# Superiority of Dual Antiplatelet Therapy (Thienopyridine Plus Aspirin) Over Coumadin Plus Aspirin in PCI Patients
Several studies during the early stent era demonstrated the superiority of the combination of ticlopidine plus aspirin over coumadin plus aspirin. has deployment. While progress was made over coumadin, Ticlopidine itself was associated with side effects and complications which included neutropenia in > 1%, thrombotic thrombocytopenia purpura in 0.2%, rash, nausea  and diarrhea. Given the improved side effect profile and the results of the CLASSICS study, clopidogrel has replaced ticlopidine as the thienopyridine of choice.

# Data Indicating that the Addition of Aspirin to a Thienopyridine Improves Clinical Outcomes in Patients with Acute Coronary Syndromes
There is surrogate marker data indicating that the level of platelet inhibition is greater among patients in whom aspirin is added to a thienopyridine. In one study, aspirin added to clopidogrel was associated with a greater degree of inhibition of collagen-induced aggregation. The level of platelet aggregation for the combination was only 16.4 +/- 2.4%, which is less than that for aspirin alone (36.5 +/- 4.2%) or clopidogrel alone (59.3 +/- 5.1%, 3 way p < 0.001).  Further, aspirin added to clopidogrel was more effective than either aspirin or clopidogrel alone after activation with low dose thrombin (p < 0.05).  In rabbit models of stent thrombosis, aspirin potentiated the antithrombotic activity of clopidogrel in the following models: 1)thrombosis induced by a silk thread; 2) thrombosis in stents placed in an arteriovenous shunt; 3) thrombus formation following electrical stimulation of the rabbit carotid artery; and 4) 111 In-labeled platelet deposition on a stent implanted in an arteriovenous shunt. The clinical benefit of adding aspirin to clopidogrel is also demonstrated indirectly by the following observation: among patients on clopidogrel, patients with stent thrombosis were more often resistant to aspirin.

# DAPT in Aspirin Intolerant Patients
One question that arises among patients who have aspirin hypersensitivity is the safety and efficacy of thienopyridine monotherapy in the mangement of the PCI patient including those who have been stented.  There is one single center, small randomized trial purporting to compare the safety and efficacy of thienopyridine monotherapy to that of thienopyridine plus aspirin. 378 stents were placed in 243 patients who were randomly assigned to treatment with either 2 x 250 mg of ticlopidine (n=121) or the combination of 2 x 250 mg ticlopidine + 100 mg aspirin (122 patients) daily. All patients received 500 mg of intravenous aspirin at the time of the procedure.  Two hundred and thirty-seven patients (97.5%) were free from the primary endpoint of death, cardiac events and vascular access-site complications through three months with no differences between treatment groups. Although 2 stent thromboses were observed in the combined treatment group, none were observed in the ticlopidine monotherapy group. There are several important limitations to this study. One is the fact that all patients received a high (500 mg) intravenous aspirin during the PCI which would have led to significant levels of platelet inhibition over the next week due to irreversible acetylation and inhibition of prostaglandin H-synthase/cyclooxygenase. This is a period of high vulnerability to stent thrombosis and ischemic complications. Thus, this was not truly a study of thienopyridine monotherapy as all patients received intravenous aspirin. The study by Machraoui is also limited by its small sample size.  Finally, the study administered ticlopidine, which is not a pro-drug and may be associated with a lower rate of hyporesponsiveness than clopidogrel. In patients with aspirin hypersensitivity or intolerance, aspirin desensitization may be done. There are currently no guidelines or recommendations for the use of dual P2Y12 inhibitors as an alternative for DAPT in patients with aspirin hypersensitivity or intolerance.

# Types and Dosage of Drugs

## Aspirin
Aspirin 81 mg once daily (range 75-100 mg) is used in all patients with Stable Ischemic Heart Disease (SIHD), stent placement following PCI or CABG. The use of aspirin should be continued indefinitely.

## P2Y12 Inhibitors
There are several P2Y12 inhibitors currently on the market and they are given in the following doses:
- Clopidogrel: 75 mg once daily
- Ticagrelor: 90 mg once daily
- Prasugrel: 10 mg once daily
The drug of choice and duration of treatment depends on the medical condition and current recommendations.

# Recommendations
The 2016 ACC/AHA Guideline Focused Update on Duration of Dual Antiplatelet Therapy in Patients With Coronary Artery Disease includes recommendations for ACS treated with medical therapy and/or PCI, ACS treated with CABG, as well as stable ischemic heart disease:
- Unstable Angina/ NSTEMI Treated with Medical Therapy Alone
- STEMI Treated with Medical Therapy Alone
- Following PCI
- CABG for NSTEMI
- CABG for STEMI
- Stable Ischemic Heart Disease

# The use of DAPT in Stroke
The 2014 AHA/ASA Guidelines for the Prevention of Stroke in Patients With Stroke and Transient Ischemic Attack included the following updated recommendations for the use of DAPT in stroke patients:

# Pre-Procedural Use of DAPT

## Unstable Angina/ NSTEMI
In patients with unstable angina/ NSTEMI undergoing PCI, the following are the guidlelines for the use of DAPT:

## STEMI
In patients with STEMI undergoing PCI, the following are the guidelines for the use of DAPT:

# Post-Procedural Duration of DAPT

## Short-Term Use of DAPT
The risk of thrombotic events is highest during the first few weeks to months after PCI. In addition, premature cessation of DAPT is an independent risk factor for stent thrombosis, with the risk being highest in the first 6 months following PCI. While the optimal duration after PCI and DES implantation remains unclear, the decision to continue therapy beyond standard recommendations should be tailored according to patient's risk factors for bleeding, as well as type of stent used. A shorter duration of DAPT treatment means less bleeding. However, no significant difference in major adverse cardiac events (MACE) exists between short (3-6 months) and long (12 months) term duration of DAPT. An ST segment elevation MI following DES implantation beyond 6 months may be a complex and multifactorial phenomenon, which may not be solely related to the discontinuation of thienopyridine therapy.

## The Leaders Free Trial
The Leaders Free Trial looks at the use of a very short duration of DAPT in patients with high bleeding risk following PCI and the insertion of a stent. Patients with a high bleeding risk are often times excluded from clinical trials on PCI and anti-thrombotic therapy. In this trial, high bleeding risk patients were given 1 month of DAPT following PCI and the insertion of either a bare metal stent (BMS) or a drug-coated stent (DCS) and were followed up for a period of two years. While it was initially thought that bare metal stents are superior to other types of stents in patients with high bleeding risk, the results of the Leaders Free Trial suggest that the incidence of coronary thrombotic events in patients who received a drug-coated stent were lower than those who received a bare metal stent, with no significant difference in the incidence of major bleeding between the two groups.

## Long-Term Use of DAPT

### The DAPT score
The DAPT score is a risk score derived from the DAPT Trial. It has been designed as a helpful tool for the continuation of dual antiplatelet therapy following PCI and the insertion of a drug-eluting stent (DES).