Clopidogril as aprodrug

1. University of Suleimani
Collage of pharmacy
Subject: Clopidogril as prodrug
Prepared by : Hardi Sdiq

2. Clopidogrel
Introduction
Clopidogrel, 2-(2-chlorophenyl)-2-(2, 4, 5, 6, 7, 7a hexahydrothieno [3, 2c]
pyridine-5yl-acetic acid methylester, is a thienopyridine prodrug used
clinically to inhibit ADP-induced platelet aggregation .The drug, usually in
form of hydrogen sulfate, was launched on the market following a successful
clinical evaluation and demonstration of superior efficacy versus aspirin in
preventing thrombotic events in high risk patients .
In 1997, it was approved by the United States Food and Drug Administration
(FDA) for the reduction of myocardial infarction, stroke, and vascular death in
patients with recent stroke recent myocardial infarction, or established
peripheral arterial disease. Clopidogrel is a prescription with proven efficacy
and a well-established safety profile in a broad range of patients, and it should
be noted that this drug, as a prodrug, shows no appreciable activity in vitro and
it requires hepatic biotransformation catalyzed by cytochrome P450 isozymes to
display its antiaggregation property .
As well known, prodrug has lots of advantages. For example, it can be
designed to overcome pharmaceutical, pharmacokinetic, or pharmacodynamic
barriers such as insufficient chemical stability, poor solubility, unacceptable
taste or odor, irritation or pain, insufficient oral absorption, inadequate blood-
brain barrier permeability, marked presystemic metabolism, and toxicity.
However, it is worth recalling that many successful prodrugs in current use are
in fact accidental prodrugs .

3. As for clopidogrel, its variability in individual responsiveness becomes an
emerging clinical problem, currently drawing a hot global attention. Despite
use of clopidogrel, a considerable number of patients continue to have
cardiovascular events and notably, there is a growing degree of evidence that
recurrence of ischemic complications may be attributed to poor response to
clopidogrel . Quite recently, on basis of four large trials, which reported that
cytochrome P450 2C19 (CYP2C19) were important risk factors predicting
apparent failure of clopidogrel efficacy, clopidogrel has been regarded as a
drug with high-risk pharmacokinetics, defined as a risk of serious drug toxicity
when drug concentrations depend on variable activity of a single metabolic
pathway. In this paper, the panorama advancement in pharmacological
research on clopidogrel was reviewed in aspects of its anti-platelet mechanism,
pharmacokinetic studies, drug resistance mainly focusing on drug-drug
interactions and genetic polymorphisms At last we emphasized the importance
of detecting the patients’ platelet aggregation during use of clopidogrel, aiming
to provide reference for reasonable drug applications in clinic .
Antiplatelet mechanism
Clopidogrel has an absolute S configuration at carbon 7. and the corresponding
R enantiomer is totally devoid of anti-platelet aggregation, thus indicating the
necessity of the configuration of this asymmetric carbon for the biological
activity . It is an orally administered prescription and requires biotransformation
in vivo by hepatic cytochrome P450 isoenzymes to an active metabolite (Act-
Met) containing a free thiol group. The highly unstable active metabolite
presumably forms a disulphide bridge between a reactive thiol group and a
cysteine residue of the P2Y12 platelet receptor. The P2Y12 receptors play an
important role in the activation of platelets, and coactivation of this receptor is
essential for ADP-induced activation of platelet fibrinogen receptor. Activation
of the P2Y12 receptor leads to a complex series of intracellular signaling events
that result in activation of the glycoprotein (GP) platelet aggregation, and
stabilization of the platelet aggregate. The P2Y12 receptor has 2 free cysteines
IIb/IIIa receptor, granule release, amplification of in its extracellular domains
(Cys17 and Cys270), both of which are targets of thiol reagents. Ding et al
speculated on that the active metabolite of clopidogrel formed disulfide bridges
with both Cys17 and Cys270 in the P2Y12 receptor, and thereby inactivated the
receptor. But Savi et al . disagreed with that hypothesis. They further
deciphered the mechanism of action of clopidogrel and of its active metabolite
on P2Y12 receptors . They pointed out the involvement of cysteine 97 within
the first extracellular loop of P2Y12 in the mechanism of action of Act-Met and
demonstrated that P2Y12 receptors preferentially associated as functional

4. oligomeric complexes within microdomains at the cell surface, and the active
metabolite of clopidogrel coupled through a disulfide bridge to the P2Y12
receptor, presumably to the cysteine residue in the first extracellular loop and
this resulted in oligomers dissociating into dimeric receptors that are partitioned
out of lipid rafts, thereby losing the ability to bind their endogenous ligands.
Another study showed that clopidogrel inhibited ADP-induced platelet
aggregation and also inhibited collagen-, and thrombin- induced aggregation,
however the inhibitory effect on collagen- and thrombin-induced aggregation
could be overcome by increased concentrations of these agonists, suggesting
that clopidogrel indirectly inhibits the effect of these agonists via the attenuation
of ADP-mediated amplification of the platelet response.

5. Pharmacokinetic studies
Clopidogrel requires oxidation by cytochrome P450 enzymes (CYPs) to
generate a metabolite that is an active inhibitor of ADP induced platelet
aggregation. However, only a small proportion of administered clopidogrel is
metabolized by P450. The majority of clopidogrel is hydrolyzed by esterases to
an inactive carboxylic acid derivative that accounts for 85% of the clopidogrel
related compounds circulating in plasma . The active metabolite of clopidogrel
is formed via two sequential CYP-dependent steps , involving CYP1A2,
CYP2B6, CYP2C9, CYP2C19, and CYP3A4.
Pharmacokinetic applications require highly selective assays with high sample
throughput capacity. So initially, the easier strategy employed for the
determination of clopidogrel was to merely measure the inactive moiety because
it was found in abundance following in vivo dosing either in clinical samples or
in preclinical species. The measurement of the inactive carboxylic acid
metabolite served the purpose of documenting the pharmacokinetic profile of
clopidogrel in an indirect manner. It has been well known that the actual plasma
concentrations of parent drug and/or active metabolite(s) are of major interest in
pharmacokinetic studies. However, the plasma concentrations of the parent drug
clopidogrel are very low (pg·mL-1 levels) due to its extensive metabolism
following oral administration in humans, and although pharmacodynamic
activity is produced by the active moiety in vivo, it was elusive for
quantification in any of the body fluids owing to its labile nature until recently,
therefore the bioanalysis of the parent clopidogrel itself and its active metabolite
(AM) pose immense challenges. Thanks to the advancement in the LC-MS/MS
technology and availability of plethora of options for optimization and enhanced
detection, the way has been well paved for the better understanding of the
pharmacokinetics of clopidogrel as well as its AM. Nowadays, a number of
analytical methods have been published that have the ability to measure the
parent drug, clopidogrel, and while some of them have the ability to even
measure the active moiety, a thiol compound, at extremely low concentrations .
Nirogi et al developed and validated a simple, sensitive and rapid high-
performance liquid chromatography/positive electrospray ionization tandem
mass spectrometry method for the assay of clopidogrel in human plasma. The
LLOQ of this method was determined as much low as 5 pg·mL-1. Takahashia et
al used the alkylating reagent2-bromo-3’-methoxyacetophenone(MP) to
stabilize clopidogrel-AM in blood and then extracted the samples by solid-phase
with a C2 disk plate. The calibration curve ranged from 0.5 to 250 ng·mL-1
with good linearity. In the intra- and inter-assay reproducibility tests, the
accuracy and precision were within 12% relative error and 6% coefficient of
variation, respectively. The derivatized MP-AM was stable in human plasma for
4 months at -80 . The validated method was also successfully used to analyze
clinical samples and determine the pharmacokinetics of clopidogrel AM.

6. What should I avoid while taking Plavix
While you are taking Plavix, do not take aspirin or other NSAIDs (non-steroidal
anti-inflammatory drugs) without your doctor's advice. NSAIDs include
ibuprofen (Advil, Motrin), naproxen (Aleve, Naprosyn, Naprelan, Treximet),
celecoxib (Celebrex), diclofenac (Cataflam, Voltaren), indomethacin (Indocin),
meloxicam (Mobic), and others
Avoid activities that may increase your risk of bleeding or injury. Use extra care
to prevent bleeding while shaving or brushing your teeth
Avoid drinking alcohol. It may increase your risk of bleeding in your stomach
or intestines.
Conclusion
Atherothrombosis is important and affects different parts of our circulation, not
only coronary circulation but also cerebral and peripheral circulation, and
platelets play an important role in thrombus formation in response to vascular
injury. Therefore, use of anti platelet agents has been thought of as the
cornerstone of treatment for patients with thrombosis diseases. Clopidogrel, a
second-generation thienopyridine, inhibits platelet aggregation by blocking the
adenosine diphosphate (ADP)-dependent activation of platelets mediated by the
ADP P2Y12 receptor on platelets.
Although the clinical benefits conferred by clopidogrel have been well
demonstrated, there are still a number of clinical limitations including delayed
platelet inhibition, variability in response, drug resistance, and a prolonged time
to recovery of platelet function. Now we can sense that the concerns about this
drug have been transferred from its efficacy to these limitations, which
stimulate interests in studying the reasons and resolvents of its disadvantages as
well as in developing new agents with more potent and consistent inhibition of
platelet function. . So far, the mechanisms underlying interindividual variability
in response to clopidogrel have not been fully elucidated but are probably
multifactorial, and even patients with a high body mass index (BMI) exhibited a
suboptimal platelet response , thus managing an inadequate response to
antiplatelet therapy still represents a major challenge. Clopidogrel is a prodrug

7. with high-risk pharmacokinetics, on the other hand, clopidogrel
nonresponsiveness is prevalent , so we agree with such standpoint .that
clinically useful risk stratification requires an easily performed and reproducible
measure of platelet aggregation, and standardized definitions of response that
correlate with clinical outcomes and point-of-care assays of platelet function
may ultimately improve the ability of clinicians to modify therapy on the basis
of response .
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ISSN 1538-3679. PMID 20421557