Biomedical Engineering Reference
In-Depth Information
an increased concentration of the active metabolite. Sibbing et al. showed that
individuals with the
*17
polymorphism were associated with an increased risk of
bleeding [
22
]. Although not specifically mentioned in the FDA Black Box warning,
testing for this variant is also indicated. Clinicians should be cautioned by clinical
laboratorians who only test for the CYP
*2
and
*3
polymorphism, as the wild
type (
*1
) will be erroneously inferred in the absence of direct testing for the
*17
variant.
2.2.2
Nongenetic Factors and Pharmacodynamics
Individuals with genetic variances to hepatic enzymes such as CYP 2C19 represent
a pharmacokinetic mechanism towards drug resistance, i.e., insufficient concentra-
tions of the active metabolite. These are a subset of individuals who have clopi-
dogrel resistant as measured by functional testing. There are several methods to
measure platelet function as an assessment of the pharmacodynamics of clopidogrel,
including as light transmittance aggregometry, impedance measurement of whole
blood aggregometry, vasodilator-stimulated phosphoprotein analysis (VASP) as
measured by flow cytometric analysis, and various commercial assays and plat-
forms such as the PFA100 (Siemens Healthcare, Deerfield, IL), VerifyNow
(Accumetrics, Carlsbad, CA), and Impact-R (Cresier, Switzerland) [
23
] . Individuals
can be resistant to clopidogrel even if they are wild type for CYP 2C19. The mecha-
nisms include polymorphism in other metabolic enzymes such as CYP 2C9, drug-
drug interactions, variable drug absorption or clearance, P2Y12 receptor variability
such as an increase in the number of receptors or upregulation of alternate platelet
activation pathways [
24
]. A combination of testing for the pharmacokinetic (CYP
2C19) and pharmacodynamic (platelet function testing) provides the most insight
for a particular patient.
2.2.3
Analytical Assays and TDM Testing
The absence of a specific therapeutic algorithm based on pharmacogenomic and
platelet function testing that has been endorsed by international cardiology societies
has slowed the adoption of testing into routine clinical practice. An important
parameter that would add to the understanding of clopidogrel resistance in a particu-
lar patient would be therapeutic drug monitoring for the active metabolite. While
LC tandem MS assays have been described for the carboxylic acid metabolite [
25,
26
], this product is inactive and will not likely be clinically useful. More recently,
an LC-MS assay has been developed for the active metabolite [
27
] . Accurate analy-
sis requires production of the thiol group with alkylating agents such as
N
-ethylmaleimide added to the blood sample within a few minutes of collection.
Measurement of the concentration of the active metabolite can be used to assess
clopidogrel efficacy for patients who have a pharmacokinetic mechanism for plate-
let resistance. Mega et al. showed that patients who are intermediate or slow metabolizers
