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CLOPIDOGREL RESISTANCE AND ITS
SUBLINGUAL FILM FORMULATION: A
COMPREHENSIVE REVIEW
Swati Pandey, Ashutosh Badola, Meenu Chaudhary
Student, associate professor, professor
School of Pharmaceutical Sciences, Shri Guru Ram Rai University, Patel Nagar Dehradun, Uttarakhand,
248001, India
Abstract
Clopidogrel, a second-generation thienopyridine drug, is extensively utilized for inhibiting platelet aggregation
in the management of coronary artery disease (CAD) and percutaneous coronary intervention (PCI). Inspite
extensive use of clopidogrel its resistance poses a significant challenge, impacting its therapeutic efficacy. The
mechanisms of underlying clopidogrel resistance, encompassing both pharmacokinetic and pharmacodynamic
factors, and it explores various platelet function tests utilized for evaluating clopidogrel responsiveness, which
includes Verify Now, VASP, and TEG assays. TEG is common application used in clinical applications.
Additionally introduction of innovative drug delivery system—sublingual film formulation. The sublingual
film's rapid dissolution and oromucosal absorption offer a promising alternative route for administering
antiplatelet therapy giving patients choice with of more compliance. Paper discusses about future directions,
pre-emptive genetic testing and artificial intelligence various approaches, aimed at optimizing antiplatelet
therapy selection and improving clinical outcomes with CAD and PCI patients. This extensive review provides
insight into addressing clopidogrel resistance its definition and improving the effect of antiplatelet therapy
through innovative drug delivery strategies and personalized medicine approaches. Sublingual film that are
available in the market with respective manufacturer, brand name and its indications. Factors such as pH, flow
of saliva, residence time and drug absorption that are require for formulations stability and potency.
Keywords: clopidogrel, clopidogrel resistance, platelet function test, sublingual film, pharmacogenetic testing,
artificial intelligence.
Introduction
second-generation thienopyridine drug clopidogrel is used for inhibiting platelet aggregation, for effective
medication of CAD and percutaneous coronary intervention1. It is widely used for lower cost and rate of
bleeding than third generation p2y12 inhibitors2. It was introduced in 1998 in the United States. Clopidogrel is
a prodrug taken orally, which requires hepatic bioactivation to generate the active metabolite responsible for
platelet inhibition3,4. Injury in CVD is triggered with platelet activation or abnormal vascular endothelium,
cause platelet aggregation, subsequent pathologic thrombus formulation and ischemic events so antiplatelet
therapy is the mainline and preventive treatment of CVD5. Clopidogrel has superseded the first generation
thienopyridine ticlopidine in both better tolerability and safety. In addition, none of them requires for routine
monitoring, and they have an equivalent efficacy6. The common side effects including bleeding, gastrointestinal
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disorder and rash, hepatotoxicity and thrombotic thrombocytopenic purpura are also possible, but rare. As a
result, patients tolerate them extremely well7.
Mechanism of action
Hepatic cytochrome (CYP) P450 enzymes including CYP3A4, CYP3A5, and CYP2C19 convert, intestinally
absorbed inactive clopidogrel bisulphate prodrug to active Clopidogrel thiol metabolite (CTM)8. CTM consists
of four isomers; H1-H4, where H3 (inactive form) and H4 (active circulating form) are mainly considered in
monitoring the action of clopidogrel9. These isoenzymes are commonly found in hepatocytes, but also in other
tissues like the gut and skin10. CTM irreversibly bind to form a disulfide bridge with two cysteine residues
(cys17 and cys270) present in the extracellular domain of the (ADP) p2Y12 receptor11. Thus, it inhibits platelet
dense granule secretion, leading to a reduction in arachidonic acid (AA), collagen, and thrombin-induced
platelet activation observed in clopidogrel. This effect causes the activation of intracellular pathways and
inhibits the conformational change of platelet Glycoprotein (GP) IIb/IIIa receptors necessary for fibrinogen
crosslinking and platelet activation. Additionally, clopidogrel exhibits anti-inflammatory effects by reducing
CRP, platelet leukocyte aggregation, p-selectin, and CD40L levels. It may also impact the enzymatic
components of coagulation, lowering speed of thrombin formation. The cumulative outcome of these processes
is the activation of intracellular pathways and inhibit the conformational change of platelet Glycoprotein (GP)
IIb/IIIa receptors crucial for fibrinogen cross-linking and platelet activation11.
Clopidogrel resistance
While there is currently no universally agreed-upon definition for this occurrence, there exists a commonly
accepted description that the prolonged activity for clopidogrel target, namely the P2Y12 receptors on platelets,
persists despite an appropriate antiplatelet regimen12.Clopidogrel resistance platelet function testing can be
defined in two main ways. Firstly, it involves a inadequate reaction to clopidogrel therapy, evaluated by the
change in ADP-induced platelet reactivity compared to the baseline. Secondly, it can be identified as high
"ontreatment" platelet reactivity, aligning with the assessment of other drug responses, by applying international
normalized ratio for warfarin13.The primary association with clopidogrel resistance is linked to the loss-of-
function allele CYP2C19*2 genotype. Additionally, genetic variations impact active metabolite conversion with
CYP P450, which contribute significantly to resistance. Studies indicate that prevalent clopidogrel resistance in
the population ranges from 4% to 30%, with variations allotted by utilization of diverse platelet function
studies14.
Clopidogrel Resistance Mechanisms
Pharmacokinetic Factors
Firstly, intestinal absorption can be hindered, potentially because of genetic variations in ABCB1 gene, which
encodes a P-glycoprotein efflux transporter. The nucleotide polymorphism of ABCB1 gene C3435T reduces
clopidogrel absorption, dose ranging from 300 mg or 600 mg LD, regardless of whether it is in homozygous or
heterozygous form. Significantly, in studies individuals with the homozygous form dose were more likely to
experience adverse events such as death, nonfatal MI, stroke. Secondly, there exists significant interindividual
change in the hepatic CYP P450 enzymes crucial for transformation of clopidogrel into active metabolite.
Pharmacogenetic factors are increasingly recognized in this domain, with CYP3A4, CYP3A5, and CYP2C19 all
linked to clopidogrel activation15. Suh et al. demonstrated that patients with the non-expressor genotype for
CYP3A5 exhibited both reduced clopidogrel responsiveness and poorer outcomes following stent implantation,
although conflicting studies exist. CYP2C19 may hold particular importance, responsible for clopidogrel
activation. Various clopidogrel therapy revealed that individuals carrying CYP2C19 loss of function alleles
experienced an increased chance of cardiovascular events, especially among PCI patients. Among these alleles,
CYP2C19∗2 increases risk of cardiovascular events16.
Pharmacodynamic
Obese associated with high weight referred from the BMI index or patients having diabetes
exhibit an increased tendency for "resistance" and heightened sensitivity to ADP concerning
both platelet adhesion and aggregation 17. The genetic factors also influence clopidogrel's
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efficacy involving P2RY12 and ITGB3 genes, although investigations have not consistently
linked genes encoding the P2Y12 receptor with clopidogrel responsiveness. While some
indicate a correlation between genetic change in the GPIIb/IIIa receptor and clopidogrel
response diversity. Additionally, polymorphisms in platelet membrane receptors like GP1a,
crucial for the aggregatory response, have been reported. Poor responders may exhibit
upregulation in intracellular P2Y12-dependent and -independent pathways, including those
reliant on the P2Y1 pathway or other platelet agonist as AA, thrombin, and collagen 18,19.
Platelet function test
The Clopidogrel PFT is used to assess an individual's reaction to antiplatelet medication. It helps identify on-
treatment high platelet reactivity (HPR), correlated with a rise in events of cardiovascular, and low platelet
reactivity (LPR), linked to an increased bleeding risk13.Clopidogrel and its effect is often assessed through
platelet function tests. Firstly, light transmission aggregometry with ADP stimulation is considered gold
standard, but point-of-care tests like Verify Now and specific assays like VASP platelet reactivity index provide
alternative for it. The VASP platelet reactivity index is a specific assay for P2Y12 receptor blockade. It
measures levels of the protein VASP, which is phosphorylated in the presence of P2Y12 stimulation, using flow
cytometry. VASP has shown correlation with optical aggregation and has been used to guide treatment
modifications in clopidogrel poor responders, potentially leading to improved clinical outcomes secondly,
Thromboelastography (TEG) assay, particularly its modified version (mTEG), offers a more comprehensive
assessment of clopidogrel's impact on blood clotting by evaluating enzymatic coagulation, overall clotting
tendency, and platelet activation which is induced by arachidonic acid. Clopidogrel plays a crucial role in
managing cardiovascular conditions, and platelet function tests helps in evaluating its efficacy in individual
patients. TEG shows a good correlation with optical aggregation in detecting the effects of clopidogrel, and
results can be obtained in 15 minutes. This makes it appropriate for common clinical application, offering a
more global perspective on the impact of clopidogrel on various aspects of blood clotting20,21.
Sublingual film
Mouth dissolving films or strips represent an innovational drug delivery system designed for oral
administration, leveraging the technology utilized in transdermal patches. These films consist of ultra-thin strips
that are effortlessly kept on the patient's tongue or any oral mucosal tissue. Upon exposure to saliva, the film
rapidly hydrates and adheres to the site of application. Subsequently, it undergoes swift disintegration and
dissolution, facilitating the delivery of medication for oromucosal absorption. Moreover, through formula
modifications, these strips can maintain their quick-dissolving properties, enabling gastrointestinal absorption
upon swallowing. Sublingual strips share similarities with tablets, as they readily melt and dissolve rapidly in
mouth, exemplified by medications like Suboxone22. This innovative concept introduces a novel sublingual
film design utilizing a blend of components, including a water-soluble carrier encased with fine particles of
active substances and a bioadhesive polymer. This formulation facilitates rapid dissolution while concurrently
reducing drug dispersion within the oral cavity through bioadhesion23,24. The drug absorption through the
sublingual region is significantly higher, ranging from 5 to 10 times greater compared to other delivery systems,
making it a preferable option over hypodermic injection. Additionally, the minimal saliva present in this area
reduces chance of tablet breakage. Moreover, the veins returning from this region directly enter systemic
circulation, bypassing presystemic drug elimination mechanisms. Various drug properties, such as solubility,
crystal morphology, particle size, hygroscopicity, compressibility, and bulk density, serve important roles in
assessing the effectiveness of sublingual medications24.
Physiological factors influencing sublingual drug delivery
Consideration of these factors is crucial for optimizing sublingual drug delivery, ensuring effective and safe
medication administration.
1. Residence Time of Formulation:
Absorption depends on how long the drug stays in sublingual areas.
Formulations include tablets, films, wafers, or sprays.
Patients should avoid eating, drinking, or swallowing to prevent decreased
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effectiveness.
2. Drug Absorption:
Drug needs balance between hydrophilic and lipophilic properties.
Should be soluble in buccal fluids and have high lipid solubility for
membrane crossing.
Suitable for low to medium molecular weight drugs.
Open sores or inflammation can affect absorption.
Smoking can decrease absorption due to vasoconstriction.
3. pH of Saliva:
Affects drug absorption by influencing ionization state.
Passive absorption pathways depend on physicochemical characteristics.
Favorable when drug is non-ionized and lipophilic.
High pKa drug values are preferred.
Saliva pH can be altered by various factors, affecting absorption.
4. Flow of Saliva:
Influences drug delivery by affecting formulation disintegration and
dissolution.
Dry mouth can hinder absorption, while excessive saliva flow can lead to
swallowing before absorption.
Saliva flow can be influenced by age, medications, and medical conditions
like Sjögren’s syndrome or dysphagia25.
Table 1: Commercialized marketed products for sublingual film
drug
indications
brand name
manufacturer
references
apomorphine
hydrochloride
sublingual film
parkinson’s
disease
kynmobi®
sunovion medical.
(25)
dexmedetomidine
(sublingual film)
schizophrenia
igalmi ®
bioxcel
therapeutics
(25)
buprenorphine
hydrochloride +
naloxone
narcotic opioid
analgesic
subuzone ®
reckitt-benckiser
pharmaceutical
(26)
simethicone
(sublingual film)
used to treat
stomach bloating
gas-x thin strips®
novartis consumer
healthcare
(27)
Genetic testing and artificial intelligence
The text discusses the potential benefits of pre-emptive pharmacogenetic testing in addressing the challenge of
rapidly obtaining genetic test results, particularly in the context of coronary artery disease (CAD), stroke, and
percutaneous coronary intervention (PCI). This approach involves incorporating CYP2C19 genotype data into
electronic medical records, enabling alerts whenever clopidogrel is prescribed to individuals with a CYP2C19
LOF allele. The increasing popularity of direct-to-consumer genetic testing is noted, with results becoming
available to healthcare providers. Given the low adverse event rate in PCI patients, the focus is on identifying
high- or low-risk individuals through genetic testing, potentially guiding the adjustment of antiplatelet therapy.
Scores combining CYP2C19 genetic testing and clinical factors have been effectively created for this objective.
Additionally, approaches such as machine learning are being employed to integrate pharmacogenomic and
clinical data, aiming to identify personalized risk profiles using high-resolution profiling to select antiplatelet
therapy in stroke and CAD patients28.
Conclusion
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In conclusion, clopidogrel, a second-generation thienopyridine drug, plays a important part in inhibiting platelet
aggregation for therapy of CAD and percutaneous coronary intervention. Overcoming the resistance of
Clopidogrel, genetic variations, and other factors contribute to various challenges and reduced efficacy.
Understanding both pharmacokinetic and pharmacodynamic factors also patient-specific factors, that contribute
to resistance. Platelet function tests, including Clopidogrel Platelet Function Test (PFT), are employed to assess
individual responses to antiplatelet therapy, aiding in identifying high platelet reactivity or resistance. To
enhance the effects of clopidogrel, its sublingual film formulations offer an advance drug delivery system with
possible benefits. This approach leverages rapid dissolution and oro-mucosal absorption, providing an
alternative to traditional oral administration. Looking toward the future, pre-emptive genetic testing and
artificial intelligence approaches hold promise in addressing clopidogrel resistance. Incorporating CYP2C19
genotype data into electronic medical records, combine with machine learning approaches, allows for
personalized risk profiling and the optimization of antiplatelet therapy, particularly in the context of CAD,
stroke, and PCI.
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