Is light transmittance aggregometry still a useful tool to assess pharmacodynamic effects of antiplatelet therapy?

• Antiplatelet therapy
• light transmittance aggregometry
• platelet function testing

To the editor,

Laboratory evaluation of platelet function by a reproducible in vitro method is crucial to patients and to better understand the role of platelet establish the presence of pharmacodynamic effect of antiplatelet agents in high risk function in thrombotic event occurrence. The first in vitro quantitative determination of platelet aggregation was reported in 1962 by Born with light transmittance aggregometry (LTA) using platelet-rich plasma (PRP) [1]. In addition, Born was the first to highlight the important role of adenosine diphosphate (ADP) in platelet aggregation. LTA, also known as turbidimetric aggregation, optical aggregometry, and conventional aggregometry, remains the “gold standard” platelet function assay in the opinion of some investigators. LTA has been extensively used to evaluate the effects of novel agents on platelet function. The fundamental observation that clopidogrel reduced ex vivo platelet function determined by LTA was the key stimulus for the advancement of this drug to large scale clinical trials. Furthermore, the greater ex vivo antiplatelet properties of the new P2Y₁₂ receptor blockers compared to clopidogrel by LTA stimulated Phase II and III trials that in turn, demonstrated their greater clinical efficacy. The greater clinical efficacy of the new P2Y₁₂ receptor blockers has been attributed to their “greater inhibition of ADP-induced platelet aggregation” compared to clopidogrel [2].

Since LTA is a functional assay, unlike other biomarker assays widely used in cardiovascular medicine, it must be performed immediately. It is a time consuming, labor intensive assay that may be associated with more chance of laboratory artifact introduction. Many of the earlier studies that attempted to link ex vivo measurement of platelet reactivity to clinical ischemic event occurrence using LTA were criticized for wide intra-assay variability [3]. The development of user-friendly point-of-care (POC) methods to assess platelet function in response to ADP addressed some of the concerns with LTA and allowed the study of thousands of patients. Recent large studies by POC have re-established the strong relation between platelet function and clinical event occurrence suggested by much smaller studies that used LTA [4]. The VerifyNow P2Y12 assay is a FDA approved POC assay with a reported 7.5 % coefficient of variation (package insert, www.accumetrics.com). However, the LTA coefficient of variation has never been reported from a large dataset (Table I).

Table I. Coefficient of variation from duplicate measurements.

		Maximum		Final
	N	Mean ± SD	CV	Mean ± SD	CV
Overall	1390	51 ± 3.6	7.02	36 ± 3.8	10.37

In the current analysis, data were evaluated from patients enrolled at the Sinai Center for Thrombosis Research in the RESPOND (NCT00642811) and ONSET OFFSET (NCT00528411) studies that compared the pharmacodynamic effects of clopidogrel and ticagrelor. Detailed information regarding inclusion and exclusion criteria, treatment strategies and blood sample collections have been described [4, 5]. LTA (20 uM ADP) was assessed in duplicate with a Chronolog Optical Aggregometer (model 490–4D) at various times pre- and post-ticagrelor or clopidogrel administration on a background of aspirin therapy. Aggregation was expressed as the maximum and final (6 minutes) percent change in light transmittance from baseline after the addition of an agonist with platelet-poor plasma as a reference. Samples (n = 1390) were run in duplicate. LTA was performed by two technicians over the duration of the study. Five thousand five hundred and sixty aggregation values were interpreted [maximum (n = 2780); final (n = 2780)]. The overall mean ± SD for maximum and final aggregation were 51 ± 4% and 36 ± 4%, respectively. The coefficient of variation for maximum and final platelet aggregation was 7.0% and 10.4%, respectively. Maximum aggregation was less variable than final and was comparable to the variability reported for the most widely used POC analyzer, the VerifyNow P2Y12 assay. Thus, in an established translational research laboratory, LTA can be successfully implemented with acceptable intra-assay variability to monitor the antiplatelet properties of clopidogrel as well as a new antiplatelet agent, ticagrelor. We did not perform duplicate measurements of the VerifyNow P2Y12 assay, so we do not have a direct comparison of intra-assay CV between the two assays for the same samples. Finally, to our knowledge, this is the largest study of the intra-assay variability of LTA. These data demonstrate that LTA is a valuable method to assess the pharmacodynamic effects of antiplatelet therapy and can be utilized in translational research as well as routine antiplatelet therapy monitoring.

Declaration of interest

Dr. Gurbel serves as a consultant for Daiichi Sankyo, Sankyo, Lilly, Bayer, AstraZeneca, Accumetrics, Merck, Medtronic, CSL, and Haemonetics; he received grants from the National Institutes of Health, Daiichi Sankyo, Lilly, CSL, AstraZeneca, Harvard Clinical Research Institute, Haemonetics and Duke Clinical Research Institute; he received payment for lectures, including service on speakers’ bureaus, from Lilly, Daiichi Sankyo, and Merck; received payment for development of educational presentations from Merck, the Discovery Channel, and Pri-Med; Dr. Gurbel holds stock or stock options in Merck, Medtronic, and Pfizer; and holds Patents in the area of Personalized Antiplatelet Therapy and Interventional Cardiology. Other authors report no conflict of interest.