Determination of clopidogrel resistance by whole blood platelet aggregometry and inhibitors of the P2Y12 receptor.
BACKGROUND
Inhibition of platelet aggregation by clopidogrel may be insufficient in up to 30% of users. These nonresponders carry an increased risk of cardiovascular events. We reported here a simple assay to study clopidogrel responsiveness.


METHODS
Electrical impedance aggregometry was performed in diluted whole blood in the presence of 5 and 20 micromol/L ADP. Some samples were incubated with 0.1 mmol/L methyl-S-adenosine monophosphate (MeSAMP), a P2Y12 receptor blocker, to maximize inhibition of aggregation before aggregometry. To validate the assay, we analyzed 6-min impedance in 21 healthy probands and 244 patients with coronary artery disease (CAD).


RESULTS
At 5 micromol/L ADP, the imprecision of the assay was 11%. Mean (SD) impedance of the healthy cohort was 12.2 (2.2) Omega. The mean - 3 SD was used to define the cutoff for clopidogrel responsiveness: responders and nonresponders exhibited a 6-min impedance < or =5 Omega and >5 Omega, respectively. Samples from nonresponders were incubated with MeSAMP and analyzed again to distinguish pharmacokinetic and pharmacodynamic types of resistance. Sixteen percent of CAD patients were classified as nonresponders (38 and 2 cases of pharmacokinetic and pharmacodynamic resistance, respectively). Female sex was strongly associated with clopidogrel resistance (P = 0.0002, Fisher exact test). A higher clopidogrel loading dose (P = 0.0353, Mann-Whitney U-test) was given to responders (median, 450 mg) than nonresponders (median, 300 mg). Age and cardiovascular diagnosis showed no significant associations.


CONCLUSIONS
Impedance aggregometry using 5 mumol/L ADP is a useful tool for studying clopidogrel responsiveness. MeSAMP allows characterization of responsiveness "on treatment" and may be useful for optimizing clopidogrel dosing.
Ivandic
Boris T
BT
Department of Medicine III, University of Heidelberg, Heidelberg, Germany. Boris.Ivandic@med.uni-heidelberg.de
Schlick
Philipp
P
Staritz
Peter
P
Kurz
Kerstin
K
Katus
Hugo A
HA
Giannitsis
Evangelos
E
D000293:Adolescent; D000328:Adult; D001792:Blood Platelets / Q000187:drug effects*; D000077144:Clopidogrel; D003327:Coronary Disease / Q000097:blood / Q000188:drug therapy; D004351:Drug Resistance; D017097:Electric Impedance; D005260:Female; D006801:Humans; D008297:Male; D008565:Membrane Proteins / Q000037:antagonists & inhibitors*; D008875:Middle Aged; D010975:Platelet Aggregation Inhibitors / Q000493:pharmacokinetics / Q000494:pharmacology* / Q000627:therapeutic use; D058919:Purinergic P2 Receptor Antagonists; D058925:Receptors, Purinergic P2Y12; D013988:Ticlopidine / Q000031:analogs & derivatives* / Q000493:pharmacokinetics / Q000494:pharmacology / Q000627:therapeutic use
D008565:Membrane Proteins; C548576:P2RY12 protein, human; D010975:Platelet Aggregation Inhibitors; D058919:Purinergic P2 Receptor Antagonists; D058925:Receptors, Purinergic P2Y12; D000077144:Clopidogrel; D013988:Ticlopidine