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Circulation on the Run

Each monthly episode will discuss recent publications in the fields of genomics and precision medicine of cardiovascular disease.
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Sep 24, 2018

Dr Carolyn Lam:                Welcome to Circulation on the Run, your weekly podcast summary and backstage pass to the journal and its editors. I'm Dr Carolyn Lam, Associate Editor from the National Heart Center and Duke National University of Singapore.

                                                Ticagrelor has shown superior efficacy to clopidogrel in the management of acute coronary syndromes. But what about in patients undergoing PCI for stable coronary artery disease? Well, our feature paper this week gives us answers to this question but you're going to have to wait to the feature discussion to hear these answers. That's coming up right after these summaries.

                                                Our first original paper this week shows that RBM20 mutation carriers have an increased risk of arrhythmias. You may recognize RBM20 as that splicing factor which targets multiple pivotal cardiac genes such as Titin and Calcium/Calmodulin-Dependent Kinase 2 Delta or CAMK2D. In today's paper first author Dr van den Hoogenhof and co-corresponding authors Dr Pinto and Creemers from Academic Medical Center Amsterdam, compared the clinical characteristics of RBM20 and Titin mutation carriers and used RBM20 knock out mice to investigate the downstream effects of RBM20 dependent splicing. They showed that loss of RBM20 disturbed calcium handling and led to more pro-arrhythmic calcium releases from the sarcoplasmic reticulum. Patients that carried a pathogenic RBM20 mutation had more ventricular arrhythmias despite a similarly depressed left ventricular function compared to patients with a Titin mutation.

                                                Targets of RBM20 splicing were enriched for calcium and ion handling genes, most notably CAMK2D and type 2 Ryanodine receptor. Loss of RMB20 induced an increased L-Type Calcium current density, intracellular calcium overload, increased sarcoplasmic reticulum calcium content and increased spontaneous calcium releases which all could be attenuated with treatment with an L-type calcium channel blocker. Furthermore, these results suggest that RBM20 mutation carriers should be closely monitored for potential electrical disturbances and cardiac arrhythmias even in the early stages of disease.

                                                Echocardiographic quantitation of degenerated mitral regurgitation is recommended in clinical guidelines but is it really scalable to routine clinical practice? First author Antoine, corresponding author Sorano from Mayo Clinic Rochester Minnesota and their colleagues looked at more than 3900 patients diagnosed with isolated mitral valve prolapse between 2003 and 2011 and to any degree of mitral regurgitation quantified by any physician or sonographer in routine clinical practice. They found that in multi-variable analysis routinely measured effective regurgitant orifice area was associated with mortality independent of left ventricular ejection fraction and systolic diameter symptoms or age and comorbidities. Furthermore, compared with general population mortality excess mortality appeared for moderate mitral regurgitation with an effective regurgitant orifice area above 20 squared millimeters and became notable with an effective regurgitant orifice area above 30 squared millimeters which then steadily increased with even higher levels of above 40. Thus, quantitation of degenerative mitral regurgitation is scalable to routine clinical practice with strong independent prognostic power when performed routinely by multiple practitioners.

                                                The next study identifies a novel mechanism of lipid homeostasis that is linked to a pseudo gene associated with the recently discovered apolipoprotein known as APOO. Co-first authors Montasser and O'Hare, corresponding author Dr Mitchell from University of Maryland School of Medicine in Baltimore, performed an array based association analysis in more than 1100 Amish subjects and identified a variant strongly associated with LDL cholesterol levels. They identified a founder haplotype on chromosome 5 which was associated with a 15 mg/dl increase in LDL cholesterol after recombination mapping, the associated region contained eight candidate genes. Using a zebra fish model to evaluate the relevance of these genes to cholesterol metabolism they found that the expression of the transcribed pseudo gene APOOP1 increased LDL cholesterol and vascular plaque formation. Thus, based on these data the authors proposed that APOOP1  regulates levels of LDL cholesterol in humans and represents a novel mechanism of lipid homeostasis.

                                                The Orion-1 trial demonstrated that inclisiran which is a small interfering RNA therapeutic that targets PCSK9 MRNA with [inaudible 00:05:42] produces significant LDL reduction. In today's study from Dr Ray from Imperial College London and colleagues, the authors described in detail the effect of inclisiran on prespecified secondary lipid and lipoprotein outcomes over time for up to 210 days and also described the individual variation and response in these measures. They found that a single 300 milligram dose of inclisiran  lowered non-HDL cholesterol at day 180 by 35% and a second dose at day 90 resulted in a 46% reduction at day 180. Similarly a single dose of 300 milligrams of inclisiran  reduced apolipoprotein B by 31% at day 180 and a second dose of 300 milligrams administered in day 90 reduced apolipoprotein B by 41%. Significant reductions in all atherogenic lipoproteins measured were sustained through today 210. Furthermore, every individual had a reduction of apolipoprotein B and non-HDL cholesterol at 180 days with the 300 milligram two-dose regimen of inclisiran. Thus, inhibiting the synthesis of PCSK9 through small interfering RNA may be a viable alternative to monoclonal antibodies with respect to effects on atherogenic lipoproteins and that brings us to the end of our summaries. Now for our feature discussion.

                                                Ticagrelor has superior efficacy to clopidogrel in the management of acute coronary syndrome but it has not really been assessed in patients undergoing PCI for stable coronary artery disease. For our feature paper today it's going to shed some light and help us with this question and these are the results of the STEEL-PC trial. I'm so pleased to have with me right now the corresponding author Dr Robert Storey from University of Sheffield in the UK as well as our associate editor who managed this none other than Dr Stefan James from Uppsala University. Thank you.

                                                Rob, could you tell us what is the issue you tried to address and because your study is not that simple, we're not used to thinking about these pharmacodynamic and kinetic studies so could you explain a bit of what you did?

Rob Storey:                        Well it's quite a few concepts that we assessed in this study. We've got data from a number of studies showing that Ticagrelor both at doses of 90 mg twice daily and 60 mg twice daily is more reliable and superior P2Y12 inhibitor compared to clopidogrel. We've got this issue of very variable response to clopidogrel with some poor responders and some high responders and a range in between. That's fairly well established and part of this study was to get more data on the 60 mg dose of Ticagrelor in these stable CAD patients undergoing PCI and get some pilot data on clinical efficacy obviously this study was not part of clinical outcomes.

                                                But, there's another issue in terms of adenosine uptake so Ticagrelor has a relatively weak effect on adenosine uptake into red cells and other cells and this may or may not explain some of its clinical effects including some adverse effects such as dyspnea. We wanted to get a better idea of the impact of Ticagrelor at both these doses on adenosine uptake.

Dr Carolyn Lam:                Could I ask ... Okay this may be naïve. I'm not an interventional cardiologist but why would you expect something different in an acute coronary syndrome compared to stable coronary artery disease? Is there an underlying hypothesis there?

Rob Storey:                        Well there can be changes to their differences in platelet reactivity although those aren't particularly great and overwhelmed really by P2Y12 inhibitor like Ticagrelor which gives such reliable inhibition of the P2Y12 receptor. But, there have been a limited number of groups that have looked at adenosine uptake and so we wanted to get independent confirmation or not of whether Ticagrelor therapeutic concentrations impacting on adenosine uptake and get some ideas of whether it's affecting circulating adenosine levels. That's an important question in terms of understanding the mechanisms and actions of Ticagrelor.

Dr Carolyn Lam:                Got it. Thanks for breaking it down so nicely. So what did you find?

Rob Storey:                        What we found was surprisingly that we saw no impact of Ticagrelor at either dose and at any time point within a month after PCI on adenosine uptake. That is the circulating levels of adenosine and the rate at which adenosine is taken up by cells in the blood mainly red blood cells. The explanation for that really is that the therapeutic levels of Ticagrelor that you see are not sufficient to impact on adenosine uptake because it's a very weak inhibitor of the adenosine uptake pathway known as the MT1. The therapeutic levels are just not getting up to a high enough concentration to have a significant impact on that.

Dr Carolyn Lam:                Stefan, you've thought a lot about this. What did you think of the findings?

Stefan James:                    I think it's very interesting. Of course, the pharmacodynamic effects that you can measure by pretty simple means, the level of platelet inhibition, it should be similar in ACS and stable coronary artery disease and I think it's sort of confirming what Rob has been showing in other populations with ACS ... we have been very interested in trying to understand the additional mechanisms of action of Ticagrelor... try to understand the mortality rate without the benefit for Plato, for example. Was it only -- platelet  inhibition or were there other mechanisms? And, there is a specific Ticagrelor related side effect, dyspnea, which we would have been interested in understanding... is this a  mechanism of action? We can't really explain that.  There are other mechanisms and other effects that we have seen can also be explained by adenosine, so I thought it was very interesting and important to understand more about these mechanisms.  

Dr Carolyn Lam:                Yeah.

Stefan James:                    But I would like to ask you, Rob. Do you think this adenosine hypothesis now, is dead, or should we still try to explore this?

Rob Storey:                        Well of course in this study what we didn't look at was the adenosine kinetics in the tissue level which is where we hypothesize the dyspnea may arise from stimulation of C5 is in the lung tissue so we're missing that piece of information. It's still conceivable that very weak levels of ENT-1 inhibition may impact from adenosine levels in the tissue. We're not seeing a strong ENT-1 inhibition sufficient to raise circulating levels or something that we can pick up on this in vitro assay.

                                                I think it still remains an open question. We've got this sort of contradictory information from drugs like cangrelor and other drugs in development like Elinogrel  where we don't see an impact on adenosine but they still may cause dyspnea.  So I think it's a very open question still.

Stefan James:                    Do you think that your paper gives us additional strength to the hypothesis that the mortality benefit for ticagrelor as seen in Plato is explained by the platelet inhibition and the balance between the reduction in ...

Rob Storey:                        Well I think what we see really in all these studies is that Ticagrelor is a fantastically effective PTY12 inhibitor. It gives you the best level of platelet inhibition during maintenance therapy out of all the available PTY12 inhibitors. And clearly having such more reliable PTY12 inhibition than clopidogrel could still be driving a mortality benefit in high risk patients so we can't exclude the adenosine pathway contributing to some of the clinical effects but I think this sways me a little bit more to the position of thinking this is most of the benefits through platelet inhibition.

Dr Carolyn Lam:                Interesting. So you're on the cutting edge of this. What's the next step then?

Rob Storey:                        Clearly we can see that very effective and reliable P2Y12 inhibition is important and leads to clinical benefits and I think we need to implement that wherever we're using P2Y12 inhibitors. We need to take that message and use a more consistent therapy rather among those with associated with variable response which doesn't seem to make sense. I think this stable PCI population, their risk has fallen. And we see that in this study, quite a number of patients report a response to clopidogrel but no stent thrombosis.

                                                That really reflects, I think improvements in stent design and implantation techniques, so the implication is that maybe aspirin alone is enough to prevent stent thrombosis with modern techniques if you get a good result but in the higher risk patients particularly the ACS patients it's likely you need much more reliable platelet inhibition and that's why Ticagrelor really provides this security.

Dr Carolyn Lam:                So, Rob there is one thing you tested two doses and they seemed to be equivalent at least in antiplatelet inhibition, right? So what does this mean? Should we maybe preferentially use the lower dose from now on, is there still room for the higher dose? Could you share some insights there?

Rob Storey:                        Well I think one has to be cautious in not jumping to adopt a dose just on the basis of pharmacodynamic data but clearly what we show is that the 60 mg dose of Ticagrelor offers a very reliable and consistent level of PTY12 inhibition and that's likely to be very effective in preventing stent thrombosis in combination with aspirin. We also show signals that were also shown in the Pegasus study that the 60 mg dose may be better tolerated such as with lower levels of dyspnea.

                                                So, there is the option for off label use of the lower dose of Ticagrelor in those who cannot tolerate the high dose due to dyspnea because certainly they'll have better platelet inhibition down titrating from 90 to 60 and if they were to switch to Clopidogrel. So I think our study offers some comfort in terms of that aspect. The only caveat is that you have to be careful not to use strong CYP3A inducers such as some epilepsy drugs with Ticagrelor cause that can increase the metabolism and we did have one case of high platelet reactivity with strong CYP3A inducers so using a higher dose initially I think is a good idea. The label says 90 mg for 1 year following ACS and the 6 is licensed beyond one year as a down titration predominantly.

                                                Our study certainly gives some comfort that down titrating earlier if a patient can't tolerate the 90 for whatever reason, seems to be a justifiable thing. And the other thing is the European guidelines support the use of Ticagrelor off label in elective PCI and our study certainly gives some comfort that off label use and the low risk elective PCI patients of the 60 mg dose can be justified at least from a pharmacodynamic point of view.

Dr Carolyn Lam:                Well, thank you because that's exactly what our audience is loving to hear. How do these findings translate into the clinical practice - Would you have any other take home messages for the clinicians listening in?

Rob Storey:                        Well I think one thing we looked at also was troponin release which is very common after PCI. We didn't see an impact of PTY12 inhibition high levels on troponin  release and I think that sort of caveat in terms of that's not going to be the best measure in terms of surrogate for efficacy in the PCI population. The other question really is, how much of the platelet inhibition and how much of the adenosine effects of Ticagrelor influence the clinical outcomes and clearly the studies sways towards the platelet inhibition very consistent high level of platelet inhibition explaining most of the benefits.

Carolyn Lam:                      You've been listening to circulation on the run, don't forget to tune in again next week.

 

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