Anti-platelet Drugs after Coronary Intervention

The type and duration of anti-platelet therapy after cardiac catheterization and stent placement (also called percutaneous coronary intervention or PCI) is an ongoing debate.

There are two main groups of drugs. One is aspirin which is under use for over 100 years and we know a lot about it. The other group is P2Y12 blockers. P2Y12 is a receptor on the surface of platelets and these drugs block this receptor. The most well-known drug in this group is clopidogrel or Plavix.

A Japanese study examined the role of aspirin + clopidogrel for one month followed by 12-months of clopidogrel versus aspirin + clopidogrel for 12 months showed that the earlier strategy was superior. In contrast, a Korean study, also published in JAMA, found that there is no statistically significant difference between the two.

The results may appear different and the reason my be that the two studies were somewhat different. Below are some of the differences:

1. Korean study allowed the use of other P2Y12 inhibitors/blockers such as prasugrel or ticagrelor while Japanese study allowed on clopidogrel.

2. Korean study had lower adherence to drug therapy in the P2Y12 group as compared to combined group (79% vs. 95%).

3. P2Y12 inhibitors were given for 3-months in the first group in Korean study while Japanese study had for 1-month only.

Despite these somewhat difference results, the overall message is the same. Patients may have similar outcomes when given clopidogrel (or other P2Y12 inhibitors) alone after an initial period of combined therapy with aspirin. This may be a particularly good news for those patients who are at higher risk of bleeding.

Anti-platelet Therapy after Cardiac Stent Placement

The type and duration of anti-platelet therapy after cardiac catheterization and stent placement (also called percutaneous coronary intervention or PCI) is an ongoing debate. Aspirin is under use for over 100 years and we know a lot about it. Plus, it is a cheaper drug. Clopidogrel, better known by its market name Plavix, is now in marker for many years and its generic versions are available. Almost all studies have examined the use of additional anti-platelet drugs after PCI on top of aspirin use. What if we examine the use of anti-platelet drugs on top of clopidogrel use?

A study published in JAMA examined this question in a multi-center, open-label, randomized clinical trial conducted in Japan. They enrolled 3045 patients to either 1 month of aspirin + clopidogrel therapy followed by clopidogrel therapy alone versus 12 months of aspirin + clopidogrel therapy. The outcome they were looking at was a combined outcome of cardiovascular death, myocardial infarction, stroke (ischemic or hemorrhagic), stent thrombosis, or bleeding.

I am certain that authors were hoping for finding that the two treatments were similar. To their surprise, not only 1-month of aspirin + clopidogrel followed by clopidogrel alone was similar in efficacy but was superior to the 12-months of aspirin + clopidogrel (2.35% vs. 3.70%; P<0.001). In other words, a shorter duration of combined therapy followed by clopidogrel therapy is as effective, if not better, than 12-months of combined therapy. These findings are important especially for patients who are at high risk of bleeding.

Aspirin and Body Weight

Low dose aspirin has been shown to reduce cardiovascular events but the observed effect may vary by weight. In fact, in one of our studies, we have shown that not only obese individuals had greater baseline platelet reactivity, they also had greater residual platelet reactivity after low-dose aspirin therapy. Comparing obese and nonobese individuals after aspirin therapy, results for aggregometry to collagen were 6.7 vs 6.1 ohms, P=.008; aggregometry to adenosine diphosphate were 13.1 vs 11.8 ohms,P<.0001; aggregometry to arachidonic acid (AA) were 4.9% vs 8.3% nonzero aggregation, P=.002; urinary excretion of 11-dehydro-thromboxane B2 (Tx-M) were 4.9% vs 8.3% nonzero aggregation, P=.002; and aspirin resistance were 26.% vs 20.5%, P=.002; respectively.

Now an individual-patient meta-analysis of primary prevention randomized controlled trials with low-dose aspirin have shown that clinical outcomes are also different in patients with weight>70Kg than in patients with weight<70Kg. On the other hand, when a higher dose of aspirin was examined, individuals with weight>70Kg obtained clinical benefits. Clinical benefits were not limited to cardiovascular outcomes but also included colorectal outcomes.

An obvious implication of such studies is that individuals with weight>70Kg should take perhaps more than one baby aspirin to receive benefit from aspirin.

Updated Guidelines - Diabetics and Cardiovascular Disease

American Heart Association and American Diabetes Association has recently published an update to the guidelines for the prevention of cardiovascular disease in adults with type 2 diabetes mellitus. There are certain interesting aspects to it.

1. Use of Hemoglobin A_1C for the diagnosis of diabetes mellitus
 Pre-diabetes = A_1C between 5.7% and 6.4%
Diabetes Mellitus = A_1C >/= 6.5%

2. Lifestyle Management of Diabetes
Physical Activity
Nutrition
Weight Reduction (through physical activity, nutrition, weight-loss drugs, and/or bariatric surgery)

3. Cardiovascular risk reduction
Control of blood glucose (A_1C <7%)
Control of blood pressure (< 140/90)
Control of Cholesterol (statins)
Aspirin for moderate 10-year CVD risk (5-10%)

4. Screening for cardiovascular diseases in asymptomatic patients
Paucity of data suggesting any specific benefits of invasive interventions over medical therapy alone makes any CAD screening in the asymptomatic patient with diabetes mellitus highly controversial.

Platelet Glycoprotein IIIa and Aspirin Resistance

Aspirin is the mainstay of treatment for the prevention of cardiovascular disease. It acts by irreversibly inhibiting COX1 enzymes and hence blocking arachidonic acid-thromboxane pathway. Due to lack of a nucleus, platelets cannot generate new COX1 and hence COX1 is inhibited for the lifetime of platelets (8 days). Aspirin treatment results in marked decrease in the excretion of urinary metabolites of thromboxane; the residual excretion is thought to be of endothelial origin where presence of nucleus results in formation of new COX1.

Despite adequate aspirin therapy, a significant number of individuals continue to have higher platelet reactivity and incomplete inhibition of platelet function. Such individuals are also at increased risk of future cardiovascular events. However, the underlying mechanisms that result in higher residual platelet reactivity after aspirin treatment are unclear and are being extensively explored by several researchers.

To identify mechanism of aspirin resistance, this study examined the differences in proteome of 2 aspirin resistant and 4 aspirin sensitive individuals and found that the levels of glycoprotein IIIa were higher in aspirin resistant individuals than in those without aspirin resistance.

Due to small sample size, it is difficult to rule-out the possibility of a type I error, however, considering the role of glycoprotein IIIa in platelet biology, it is conceivable that protein may play a role in aspirin resistant although exact mechanism remains unknown.

Benefits of Aspirin Use in the General Population

Aspirin is perhaps the most commonly used drug world-wide for various ailments. Its prophylactic use in secondary prevention of cardiovascular diseases is well-established, however its use for cardiovascular disease prophylaxis in primary prevention has not been as clear. In particular, some recent meta-analysis has raised concern that aspirin use may not be beneficial for primary prevention of cardiovascular diseases. In support of a role of aspirin use in general population, a systematic review recently concluded:

“For average-risk individuals aged 50–65 years taking aspirin for 10 years, there would be a relative reduction of between 7% (women) and 9% (men) in the number of cancer, myocardial infarction or stroke events over a 15-year period and an overall 4% relative reduction in all deaths over a 20-year period”

In other words, aspirin use is beneficial for both cardiovascular and cancer standpoint, its takes three years to see a beneficial effect, and effect lasts long after aspirin use has been discontinued.

Greater Collagen-Induced Platelet Aggregation Following Cyclooxygenase 1 Inhibition Predicts Incident Acute Coronary Syndromes

Platelets have several pathways that initiate aggregation such as thrombin-, collagen-, thromboxane-, and ADP-mediated pathways. Individuals vary widely in their ability to have platelet aggregation through each of these pathways. Furthermore, this variability is not correlated. In other words, an individual may have low aggregation though one pathway but may have high aggregation through another pathway. This phenomenon may be more important when we try to measure platelet aggregation the laboratory to assess whether there is increased risk of in vivo platelet aggregation (and hence cardiovascular events).

Collagen is one of the first agonists that initiates platelet aggregation at the site of vessel wall injury. Therefore, examining an association of collagen-mediated platelet aggregation with subsequent cardiovascular events makes sense. Moreover, if we can decrease variability in platelet aggregation by blocking one or another pathway, we may be better able to assess activation through collagen pathway.

Aspirin is commonly used for prevention of cardiovascular disease and works by inhibiting thromboxane-pathway an dis very effective in completely inhibiting activity through this pathway. Thus aspirin can be used to inhibit variability through one pathway and effect of collagen can be studied with fewer interactions. We followed the same logic in this paper where we examined platelet aggregation after 2-week aspirin therapy in healthy individuals. Most of these individuals did not use aspirin after the 2-week study period. During follow-up increased collagen-mediated platelet aggregation was significantly associated with acute coronary syndrome.

“After COX1 pathway inhibition, collagen-induced aggregation was significantly greater in participants with ACS compared with those without (29.0 vs. 23.6 ohms, p < 0.001). In Cox proportional hazards models, this association remained significant after adjusting for traditional cardiovascular risk factors (HR = 1.10, 95%CI = 1.06-1.15; p < 0.001).”