Clopidogrel, in combination with aspirin, is currently the recommended treatment for secondary prevention of ischemic events in high-risk patients and for prevention of coronary artery stent thrombosis. Patients receiving aspirin and clopidogrel are frequently treated with proton pump inhibitors, such as omeprazole or pantoprazole, in order to prevent the risk of gastrointestinal bleeding, accorded to guidelines. An interaction between proton pump inhibitors and clopidogrel has been suggested, which may lead to a decrease of clopidogrel effects. It remains unclear whether this interaction between PPIs and clopidogrel might be a class effect or if this may be affected by timing regimen.

The objectives of this two-phase investigation are:

1. to compare clopidogrel platelet inhibitory effects when taken at the same time versus separated at least 8 hours from omeprazole administration.
2. to compare clopidogrel-induced inhibitory effects when taken at the same time versus staggered at least 8 hours from pantoprazole administration.

Clopidogrel, in combination with aspirin, is currently the recommended treatment for secondary prevention of ischemic events in high-risk patients and for prevention of coronary artery stent thrombosis. Patients receiving aspirin and clopidogrel are frequently treated with proton pump inhibitors, such as omeprazole or pantoprazole, in order to prevent the risk of gastrointestinal bleeding, accorded to guidelines. An interaction between proton pump inhibitors and clopidogrel has been suggested, which may lead to a decrease of clopidogrel effects. It remains unclear whether this interaction between PPIs and clopidogrel might be a class effect or if this may be affected by timing regimen. The objectives of this two-phase investigation are: 1. to compare clopidogrel platelet inhibitory effects when taken at the same time versus separated at least 8 hours from omeprazole administration. 2. to compare clopidogrel-induced inhibitory effects when taken at the same time versus staggered at least 8 hours from pantoprazole administration. The clopidogrel dose will be a 600mg loading dose followed by a 75mg daily maintenance dose, starting the next day for 7 days. Omeprazole will be used at a daily dose of 40mg and pantoprazole at 80mg.

The proposed study will have a prospective, randomized, cross-over design. Subjects are randomized in a 1:1 fashion to take PPI concomitantly (CONC regimen) or staggered by 8-12 hours (STAG regimen) for one-week on a background of clopidogrel therapy. In particular, in the CONC regimen both drugs were taken in the morning, while in the STAG regimen clopidogrel was taken in the morning and omeprazole in the evening. After a 2-4 week washout period, subjects crossed-over treatment regimen. After completing these two treatment phases, subjects underwent another washout period of 2-4 weeks and were treated for 1 week with clopidogrel alone, without receiving omeprazole therapy (CLOP regimen). The sequence with the PPI pantoprazole will have the same prospective, randomized, cross-over design as the omeprazole sequence. A CLOP regimen in the absence of pantoprazole will be collected before entering randomization phase with adequate wash-out period.

Blood sampling for platelet function assessments were performed at all three phases of the study at the following time points: a) baseline, b) 24 hours after LD (before intake of study medication), and c) 7 days (24 hours after the last MD).

Inclusion Criteria:

• Healthy volunteers aged between 18 and 75 years

Exclusion Criteria:

1. Known allergies to clopidogrel or omeprazole.
2. Blood dyscrasia or bleeding diathesis.
3. Recent antiplatelet treatment (< 30 days) with a glycoprotein IIb/IIIa antagonist, thienopyridine (ticlopidine, clopidogrel), cilostazol or dipyridamole.
4. Treatment with other medications that may interfere with the CYP system (ketoconazole, itraconazole, diltiazem, erythromycin, clarithromycin, fluvoxamine, fluoxetine, nefazodone, or sertraline).
5. Platelet count <100x106/microL.
6. Diabetes mellitus
7. History of coronary artery disease, gastrointestinal bleed, gastroesophageal reflux disease (GERD), cerebrovascular event or any active malignancy.
8. Active bleeding or hemodynamic instability.
9. Serum creatinine >2mg/dL.
10. Baseline ALT >2.5 times the upper limit of normal.
11. Pregnant females.
12. Patients taking omeprazole or any H2 antagonist or proton pump inhibitors