A 68-year-old man presents to the emergency department with 2 hours of central chest pain radiating to his left arm. ECG shows 1 mm ST depression in leads V4-V6 and high-sensitivity troponin is elevated. He has already received 300 mg chewable aspirin from paramedics. He has no history of stroke or intracranial haemorrhage and is planned for early coronary angiography. Which additional antiplatelet agent should be given now?