1. Platelet Function Testing: Is GRAVITAS the Last Word?
Michael A. Gaglia, Jr., MD, MSc
Center for Platelet Studies
Washington Hospital Center/Medstar Research Institute
February 27, 2011

2. Michael A. Gaglia, MD
I have no real or apparent conflicts of interest to report.

3. Overview Platelet agonists and receptors Platelet function tests
Light transmission aggregometry (LTA)
Vasodilator stimulated phosphoprotein phosphorylation (VASP)
VerifyNow P2Y12
High on-treatment platelet reactivity and clinical events
Recent and future clinical trials
Optimal testing strategy for the cath lab

4. Major platelet agonists and receptors involved in thrombus formation
Major platelet agonists and receptors involved in thrombus formation. Disruption of the endothelium exposes TF and collagen, liberating thrombin and initiating platelet adhesion via the GP Ib/Ix/V and GP VI platelet receptors. Adhesion also initiates the release of platelet agonists, including ADP and TXA2, which thereby activate other platelets. Activated platelets are then cross-linked by fibrinogen, leading to platelet thrombus formation.
Gaglia et al. AHJ 2010;160:595

5. Light Transmission Aggregometry
“Gold standard” (misnomer)
Various agonists (e.g. ADP, AA) induce platelet aggregation in PRP
Strength: most data with clinical outcomes
Weakness: variable reproducibility, expensive, time consuming, lots of blood
i.e. not practical

6. Vasodilator Stimulated Phosphoprotein Phosphoryation
Flow cytometery-based
Measures level of phosphorylated VASP protein; inversely correlated with P2Y12 activity
Strength: specific to P2Y12 inhibition; one (small) study proving “tailoring” of therapy
Weakness: expensive, time-consuming
i.e. not practical

7. VerifyNow P2Y12
Light transmittance of platelets and fibrinogen-coated beads
ADP as agonist (and PGE1 to eliminate P2Y1)
Strength: bedside, whole blood, clinical data, FDA approved
Weakness: percent inhibition not reliable; can’t test on IIb/IIIa

8. Verify Now P2Y12 Concept
Concept: agonist (ADP or arachidonic acid) used to induce platelet aggregation. Beads coated with fibrinogen.
Inhibited platelets (i.e. responders to drug) do not aggregate and produce low level of light transmittance
Normal platelets aggregate more rapidly and produce higher level of transmittance (platelets occupy less space because of binding)

9. Others Dade PFA Collagen/ADP Innovance PFA P2Y Impact-R
Thromboelastography (TEG)
Clot strength
Multiple electrode aggregometry
Multiplate analyzer
Plateletworks
Adaptation of LTA
Not useful

10. High On-Treatment Platelet Reactivity and Adverse Events post-PCI
Initial studies examined the difference between pre- and post-clopidogrel platelet reactivity
Nearly 30 studies linking high on-treatment platelet reactivity and ischemic events
Most data for LTA, VASP, and VerifyNow P2Y12

11. Thresholds for High On-Treatment Platelet Reactivity
Study n
Assay
Endpoint Sn Sp
NPV
Threshold
Breet 2010
1069
VerifyNow
Plateletworks
LTA 5 μmol
1 y death, MI, ST, CVA
63%
60%
59%
94%
PRU 236
81%
43%
Marcucci 2009
683
1 y CV death, MI
61%
70%
96%
PRU 240
Price 2008
380
6 mo CV death, MI, ST
78%
99%
PRU 235
Frere 2007
195
VASP
30d recurrent ischemia
93%
50%
PRI 53%
Bonello 2007
144
6 mo CV death, CVA, TVR
100%
25% NR
PRU 50%
Sibbing 2009
1608
Multiplate analyzer
30d stent thrombosis
84%
468 AUmin
Cuisset 2009
598
LTA 10 μmol
68%
67%

12. JACC “White Paper”
Key concept is high on-treatment platelet reactivity, not “responsiveness” to clopidogrel (i.e. the delta).
Proposed thresholds for high on-treatment platelet reactivity:
LTA 5 μmol ADP: >46%
Multiplate: 468 AU/min
VASP: PRI >50%
VerifyNow P2Y12: PRU >235 to 240
Bonello et al 2010;56:919

13. GRAVITAS
Hypothesis: High-dose clopidogrel for 6 months is superior to standard clopidogrel therapy for the prevention of adverse CV events after PCI in patients with high residual reactivity.

14. GRAVITAS Study Design R Elective or Urgent PCI with DES*
VerifyNow P2Y12 Test hours post-PCI
PRU ≥ 230 R
High-Dose Clopidogrel†
clopidogrel 600-mg, then
clopidogrel 150-mg daily X 6 months
Standard-Dose Clopidogrel†
clopidogrel 75-mg daily X 6 months
GRAVITAS tested the clinical efficacy of a treatment strategy of prolonged high-dose clopidogrel based upon the results of the VerifyNow P2Y12 test after PCI.
The trial enrolled patients undergoing elective or urgent PCI who received a peri-procedural clopidogrel regimen designed to ensure a steady-state level 12 to 24 hours after the procedure.
At that time, platelet function was assessed. The VerifyNow P2Y12 test provides a result is in terms of PRU – the higher the PRU, the greater the level of residual reactivity on clopidogrel.
In GRAVITAS, high reactivity was defined as a PRU greater than or equal to 230, a cutoff similar to what has been suggested by previous studies to provide the maximal sensitivity and specificity for subsequent cardiac events.
Patients with high reactivity were randomized in a double-blind fashion to either 6 months of high dose clopidogrel – that is, an additional loading dose followed by 150-mg a day – or standard dose clopidogrel – that is, 75-mg a day. Platelet reactivity was re-assessed at 30-days and 6 months in a blinded fashion, but no treatment decisions were made based upon the results of these tests. The primary efficacy endpoint was a composite of cardiovascular death, non-fatal MI, or stent thrombosis at 6-months. The Key safety endpoint was GUSTO severe or moderate bleeding.
Primary Efficacy Endpoint: CV Death, Non-Fatal MI, Stent Thrombosis at 6 mo
Key Safety Endpoint: GUSTO Moderate or Severe Bleeding at 6 mo
Pharmacodynamics: Repeat VerifyNow P2Y12 at 1 and 6 months

15. Primary Endpoint: CV Death, MI, Stent Thrombosis
At 6-months, the observed rate of the primary endpoint was 2.3% in the high-dose group compared with 2.3% in the standard dose group, resulting in a hazard ratio of 1.01 and a p value of There were a total of 50 events which was lower than the 68 that we had anticipated.

16. Secondary Comparison: High vs. Not High Reactivity
Here are the results of this non-randomized compariion of patients with and without high reactivity treated with clopidogrel 75-mg a day.
At 6 months, the observed rate of the composite endpoint was 2.3% in patients with high reactivity compared with 1.4% in patients without high reactivity, representing a hazard ratio of 1.7.
The difference in event rates did not reach statistical significance; note that the lower boundary of the 95% CI is less than one.
The point estimate of this large hazard ratio appears consistent with previous studies documenting a significant association between residual reactivity and cardiovascular events after PCI.

17. End of story, right?

18. Prasugrel vs. Clopidogrel
Brandt et al. AHJ 2007;153:66

19. Ticagrelor vs. Clopidogrel
Storey et al. JACC 2007;50:1852

20. Ongoing Studies ARCTIC TARGET-PCI TRIGGER-PCI
PRU ≥230: tailoring of clopidogrel dose
TARGET-PCI
Clopidogrel naïve: CYP 2C19
Chronic clopidogrel: PRU ≥230
TRIGGER-PCI
PRU ≥208: Prasugrel vs. clopidogrel

21. WHC Testing Protocol
Clopidogrel naïve patients: VerifyNow P2Y12 between 4-24 hrs post clopidogrel load and PCI
Chronic clopidogrel patients (at least 5 days): VerifyNow P2Y12 just before cath
PRU ≥235: recommend change to prasugrel
No longer recommending double dose clopidogrel
No opinion regarding cilostazol

22. Practical Advice
Conservative: don’t change anything and wait for ticagrelor
Less conservative: prasugrel for high-risk patients (e.g. diabetics, Hx stent thrombosis, left main stent)
Tailored option: consider prasugrel for patients with PRU ≥235 (or PRI ≥50% if you are from France)
Must be mindful of many exclusions (age, weight, CVA, bleeding history)