Ticagrelor is associated with more prompt and potent antiplatelet effects compared with clopidogrel, leading to better clinical outcomes, including reduced cardiovascular mortality, across the spectrum of patients with acute coronary syndrome, including those with ST-elevation myocardial infarction (STEMI) undergoing primary percutaneous coronary intervention (PCI). However, in this latter setting a delay in the onset of its antiplatelet effects has been shown. Morphine has been identified as a cause of delayed P2Y12 inhibition in patients with STEMI. Methylnaltrexone is a parenteral peripheral opioid receptor antagonist which has the potential to prevent or reverse opioid-induced peripherally mediated side effects without affecting analgesia. However, whether the use of intravenous methylnaltrexone may overcome the effects of morphine administration on the pharmacokinetic (PK) and pharmacodynamics (PD) profiles of ticagrelor has not been investigated yet. The proposed investigation will include patients with coronary artery disease and will have a prospective, randomized, cross-over design.