The Effect of Antiplatelet Medications on Innate Immune Activation

Platelets are the main type of blood cell involved in the formation of blood clots that cause heart attacks. We give antiplatelet drugs (aspirin, for example) to reduce the risk of another clot forming and causing another heart attack. Platelets are also known to have a role in inflammation and infection. In a recent large clinical trial of patients with heart attacks, known as the PLATO study, it was shown that patients treated with a new antiplatelet medication (ticagrelor) developed fewer lung infections, as well as fewer heart attacks, compared to the previous standard treatment (clopidogrel). There was evidence that this might partly explain the reduced risk of death after a heart attack when ticagrelor is used instead of clopidogrel.

I therefore hypothesise that ticagrelor and clopidogrel have different effects on the immune response, which is, in part, due to their differing effect on platelet receptors and, in part, due to mechanisms unrelated to platelet receptors. I therefore plan to study the effect of ticagrelor and other antiplatelet agents on the immune response. This may help us to further improve the treatment of patients after a heart attack with the prospect of developing new drugs that have a better effect on the immune response. To date, no studies have been published that directly investigate the effect of ticagrelor on the immune response.

In the first study, I will take blood from healthy volunteers and examine the effects of antiplatelet agents on white blood cells in detail, as they are the main type of cell involved in the immune response. I will study whether adding these antiplatelet agents to the blood affects white blood cell surface markers of activation and their ability to localise towards areas of infection. I will also study whether antiplatelet agents affect the ability of white blood cells to consume and kill bacteria. I will assess whether or not the antiplatelet agents interfere with platelets interactions with white blood cells. I will determine whether ticagrelor affects white blood cells through its known mechanisms of action by comparing its effects to other agents that also affect the same pathways. By performing the experiments in the presence and absence of platelets, I will also determine whether ticagrelor's effects are due to its effects on platelets and their subsequent interaction with white blood cells or whether it acts directly on white blood cells.

In the second study, I will investigate the effect of antiplatelet medications on the immune response of healthy volunteers. Thirty healthy volunteers will be randomised to take ticagrelor, clopidogrel or no antiplatelet medication for one week (ten participants in each group). They will then attend the Sheffield Clinical Research Facility where I will use the safe, well-established method of injection of a very low dose of endotoxin (a component of bacteria rather than actual bacteria) into the bloodstream to stimulate an immune response. I will study whether ticagrelor or clopidogrel affect this immune response by measuring their effect on inflammatory markers, white blood cell function and the interaction of platelets with white blood cells. I will use the findings from my first study to guide which aspects of white blood cell function to examine in detail.

The third study will involve patients with coronary artery disease who are due to undergo a stenting procedure to treat their narrowed coronary arteries. These patients are often started on clopidogrel prior to the procedure. I will take blood before and one week after the initiation of clopidogrel. I will study whether initiating clopidogrel causes any changes in patients' immune response, particularly the function of white blood cells and their interactions with platelets. I will also explore effects on white blood cell function in detail, guided by the findings from my first study.

Background
Platelets have a central role in acute coronary syndromes (ACS). Ticagrelor is a novel antiplatelet drug that binds to platelet P2Y12 ADP receptors and also inhibits adenosine uptake, both of which have been shown to affect immune response. The Platelet Inhibition and Patient Outcomes (PLATO) study showed that ticagrelor reduces the incidence of pulmonary infections and has a differential effect on inflammatory markers compared to clopidogrel in patients with ACS.

Hypothesis
Ticagrelor and clopidogrel differ in their effect on innate immune activation, due to P2Y12 and non-P2Y12 dependent mechanisms.

Objectives
1.) Determine the in-vitro effects of ticagrelor on innate immune activation compared to other P2Y12 antagonists and inhibitors of adenosine reuptake.
2.) Determine the in-vivo effect of ticagrelor and clopidogrel on innate immune activation during experimental human endotoxaemia.
3.) Determine the effect of clopidogrel therapy on ex-vivo innate immune activation.

Design/Methodology
I will compare the effects of ticagrelor and other antiplatelet agents (including P2Y12 antagonists and inhibitors of adenosine reuptake) on innate immune activation in vitro and also in vivo using a safe, well-established model of experimental human endotoxaemia. I will study the effect of these compounds on key cellular functional pathways, including leukocyte expression of surface markers of activation, cytokine production, phagocytosis, pneumococcal killing, apoptosis and interaction with platelets. I will also investigate the effect of clopidogrel therapy on these pathways in patients with stable coronary artery disease ex vivo.

Scientific and Medical Opportunities
This will provide insights into the effects of ticagrelor and clopidogrel on innate immune responses and identify their effects on cellular pathways that may guide the development of novel antiplatelet strategies.

1. Patients with Coronary Artery Disease
Coronary artery disease is one of the leading causes of morbidity and mortality worldwide. Antiplatelet medications are key in the treatment of acute coronary syndromes (ACS) and are, therefore, amongst the most commonly prescribed medications worldwide. The number of patients taking the new antiplatelet medication, ticagrelor, will likely increase dramatically now that the European Society of Cardiology recommends it as first line antiplatelet therapy in moderate-to-high risk patients with ACS. Patients who are treated with ticagrelor will benefit from my research due to an increased understanding of the mechanisms by which this drug provides benefits compared to clopidogrel. The findings of my research could allow optimisation of antiplatelet treatment regimens for patients with coronary artery disease, particularly in the context of infection and sepsis, within the next 5 years. Despite reducing mortality compared to clopidogrel, ticagrelor also increases the incidence of spontaneous bleeding. My research will dissect some of the cellular mechanisms contributing to the mortality benefit of ticagrelor, which could guide the development of novel antiplatelet strategies. This would have the potential to benefit patients with coronary artery disease within the next 5-10 years.

3. Patients with Sepsis
Sepsis continues to have a high morbidity and mortality. It is known that prolonged activation of the innate immune system contributes to the mortality associated with sepsis, and there is scope for the development of medications that modulate these pathways. My research will add to the knowledge base regarding on effect of adenosine and adenosine reuptake inhibitors on innate immune activation. If it was found to be beneficial, modulation of theses pathways could have a therapeutic impact within the next 5-10 years.

4. Patients Treated with Clopidogrel
A possible explanation for the finding that ticagrelor is associated with fewer lung infections, and subsequent sepsis and mortality, compared to clopidogrel, could be related to an immunosuppressant effect of clopidogrel rather than a beneficial effect of ticagrelor. Clopidogrel is a very commonly prescribed medication in patients with coronary artery disease, in part due to the fact that it is now out of patent and therefore has a low cost. The finding that clopidogrel was immunosuppressant would have far-reaching implications and this knowledge would benefit patients receiving, or planned to receive, clopidogrel. My research could, for example, help decision making in clopidogrel-treated patients with severe infections and those who are immunosuppressed for other reasons.

My overall goal is to elucidate the beneficial mechanisms of action of antiplatelet medications so that these can be translated into novel treatments for the benefit of patients, including those with cardiovascular disease in particular.