DR. JENNINGS: The purpose of this roundtable discussion is to reach out to clinical cardiologists and translational investigators to discuss the role of platelet function testing in improving clinical outcomes. We are interested in discussing the implications of platelet function testing in terms of assessing the risk for thrombosis, or for bleeding, and where it may play a role in improving clinical outcomes for patients, particularly acute coronary syndrome (ACS) patients.
Thank you very much for attending. We will begin with some introductions. I am Dr. Lisa Jennings, Clinical Professor at the University of Tennessee Health Science Center. I am also Founder of CirQuest Labs, a specialty platelet function and coagulation laboratory. My area of expertise is hemostasis and thrombosis.
DR. SCHNEIDER: I am Dr. David Schneider, a Professor of Medicine and Director of Cardiovascular Services at the University of Vermont Health Network. I have a long-standing research interest in platelet function assessment.
DR. LEVY: I am Dr. Jerrold Levy, an intensivist and a cardiac anesthesiologist. I am Co-Director of a 32-bed cardiothoracic intensive care unit at Duke, where we manage patients following major cardiovascular surgery including ventricular-assist devices, and extracorporeal membrane oxygenation (ECMO).
I have a long-standing interest in this topic. I am interested in both inhibiting and activating thrombin generation, and developing purified and recombinant strategies for treating life-threatening hemorrhage as well as anticoagulation for extracorporeal circulation. Thank you for the opportunity to participate.
DR. JENNINGS: Thank you. DR. ANGIOLILLO?
DR. ANGIOLILLO: Thank you for having me. I am Dr. Dominick Angiolillo, Professor of Medicine at the University of Florida in Jacksonville, where I practice as an interventional cardiologist.
I am also the Director of Cardiovascular Research and Program Director for the Interventional Cardiology Fellowship Program, and my interest for the past two decades has been on pharmacodynamic testing, and I have been involved with the development of a series of antiplatelet agents.
DR. JENNINGS: Thank you all for participating as faculty in this important discussion. For the purpose of this discussion, we will address if, or when, platelet function testing may serve a role in patient evaluation, and aid in improving clinical outcomes. Hopefully today we will touch on the many factors that play into evaluating platelet function in the setting of ACS, and post-treatment.
Let’s participate in an open-ended discussion. I can start by posing some questions, and I think we all, with our vast experience, will add some insight or identify some areas where platelet function testing may serve a purpose, and where some limitations still exist.
There has been much discussion about inadequate antiplatelet drug responsiveness, and so the question is really, what contributes to this? Is it an inadequate loading or maintenance dose of an antiplatelet therapy? Is there a need to consider other antiplatelet agents in the treatment of patients? How personalized should we be, in terms of treating our patients, to improve critical outcomes?
We have talked about drug resistance, or perhaps low levels of platelet inhibition that may lead to increased risk of thrombosis. So, I would appreciate anyone’s viewpoint in terms of antiplatelet drug responsiveness?
DR. SCHNEIDER: I think that it is a complicated situation. Individuals have varying platelet function, even over the course of a given day, so when we look at the response to an antiplatelet drug, we start from a background of variation in platelet function between individuals, and even within an individual over a period of time.
Then, as you move beyond the inter-individual variability, you add, on top of that—particularly with some of the drugs, such as clopidogrel—differences in absorption of the drug, differences in the metabolism of the drug, and then, ultimately, differences in its ability to achieve its end-organ effect, which ultimately can lead to rather substantial variability in the antiplatelet effect.
I think the field has begun to move away from looking at percent inhibition with respect to a given drug, and toward looking at the residual platelet reactivity during treatment, and how we can associate that with outcomes in patients. Certainly, there have been ample studies demonstrating that high on-treatment platelet reactivity is associated with a greater risk of subsequent cardiovascular events.1–3
DR. ANGIOLILLO: I wanted to echo what David just said. Also, based on your question, it is obviously a complex topic. It is not a matter of the loading dose or the maintenance dose. The loading dose is a one-time thing. We really define resistance or impaired response once the patient theoretically should have received the full therapeutic effect of a drug, which we evaluate during maintenance dosing.
In addition to that, not only can we speak about impaired response, but sometimes there is excessive response, where there have been studies in which patients with very low levels of platelet reactivity have been associated with an increased risk of bleeding, although they are not as consistent as observed for patients with impaired response.
DR. LEVY: I think these are interesting points. The other important point is, we are focusing on platelet reactivity in the milieu of a complexity of various degrees of vascular-endothelial activation, along with other factors that affect hemostasis.
In critically ill patients, there are multiple events that can occur to influence coagulation. Heparin causes microparticalization, and all the things that in the milieu in which we function, we focus on platelet reactivity, a critically important factor, but there are other obviously modulating factors that we do not routinely measure or really think about.
I think we need to better focus on platelets, but platelet function in bleeding and critically ill patients may be difficult to evaluate, and there may also be important influencing factors that we do not routinely measure.
DR. JENNINGS: Those are all relevant points. As we think about platelet function testing, Dominick mentioned that not only do we have issues around thrombotic complications—e.g., post-ACS—we also have bleeding risk, and there has been a continual search for tests that might assess bleeding risk, as well as patients that are at risk for thrombosis. We will try to touch on assessment of bleeding risk later in the discussion.
We still have a considerable amount of work to do. We can identify many platelet function tests (Table).4,5 Briefly, there are light transmission aggregometry (LTA); lumi-aggregometry that combines platelet aggregation testing with measuring platelet secretion; impedance aggregometry that utilizes whole blood; and there is thromboelastography (TEG) or rotational thromboelastometry (ROTEM). There are platelet-activation tests such as flow cytometric tests, particularly for P2Y12 receptor signaling, using the vasodilator-stimulated phosphoprotein phosphorylation (VASP) assay and flow cytometric tests for platelet activation markers and for platelet signaling. There are also the point-of-care tests such as the PFA-100 and the VerifyNow® System (Accriva Diagnostics, San Diego, CA). The multi-plate electrode aggregometry might be considered an intermediate point-of-care test. I do not think many studies have proven that the correlation between these assays is high—in fact, at best, correlation can be modest to dismal and data from one test cannot be translated to another type of test. We have considerable work to do to define these thresholds of platelet reactivity for each available test that are associated with adverse events, whether it be thrombosis or bleeding.
For example, in VerifyNow®, a point-of-care instrument, there have been various thresholds recommended for residual high-platelet reactivity. We have not readily identified acceptable reactivity thresholds for LTA.
I am interested in your viewpoints as to when we talk about the limitations of platelet function testing, whether it is because we really do not know the values to associate with risk of adverse events, or are there other limitations that remain to be addressed?
DR. LEVY: I have a question. When you are doing P2Y12 testing, my understanding is that there is quite a variability in the concentration of adenosine diphosphate (ADP) that is used. We don’t even know the EC50 for clopidogrel, and there are other people who isolated the active metabolite.
Not to diverge, but a partial thromboplastin time (PTT) on bivalirudin, a PTT on argatroban, and a PTT on heparin of the same number may be totally different in terms of thrombin generation.
Have we standardized P2Y12 testing to the actual concentration of ADP? My gestalt is, it is variable from 5, 10, 20 µmol, and I think that is just one of the many different reasons—just a small part of it. Can you answer that?
DR. JENNINGS: Point well taken. My thinking is that when we talk about ADP testing, for example, with LTA, we have both the flexibility and the conundrum of what dose of ADP we might use, whereas some of the point-of-care tests—basically within a cartridge, so to speak—the concentration of ADP or combination of reagents that may initiate a platelet activation or aggregation is fairly set, which may provide a consistent test, but may not be the ideal concentration for a particular drug combination therapy or a particular drug itself.
The thing that I wrestle with often in platelet function testing is that we know that ADP comes from the dense granule of the platelet, and so the platelet must be activated for ADP to be released in the first place, and then of course it binds to the P2Y1 and P2Y12 receptors.