A Study On Adverse Drug Reactions Biology Essay


The purpose of writing this paper is to review the polymorphism of human CYP2C19 and its clinical impact. CYP2C19 enzyme metabolises many commonly prescribed drugs. It is encoded by CYP2C19 gene that is highly polymorphic, which give rise to different phenotypes that respond differently toward drug therapy, and result in adverse drug reactions such as drug resistance and toxicity.

As of year 2008, adverse drug reactions were the fifth leading cause of death in the United States. 7% of hospitalization cases were due to adverse drug reactions, and the frequency increased among the elderly up to 30%. [1]

Increased knowledge in CYP2C19 polymorphism and its clinical impact helps to overcome adverse drug reactions through implementation of effective drug therapy and reduction of drug toxicity.

Literature Review

2.1 Cytochrome P450 (CYP)

CYP genes are located on the long arm of chromosome 10 (10q24). [2] As of year 2006, 115 human CYP genes were discovered, with 57 being functional genes, and 58 being pseudogenes. [3] Functional CYP genes encode for CYP enzymes, which are found primarily in the microsomes of hepatocytes.

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CYP enzymes are phase I metabolism enzymes, which make endogenous and exogenous compounds more hydrophilic to be eliminated by the kidneys. Examples of endogenous compounds are steroid hormones and prostaglandins; examples of exogenous compounds are drugs, toxins and carcinogens. [4]

There are two classes of CYP enzymes. Class I CYP enzymes such as CYP1A1, CYP1A2, CYP2E1 and CYP3A4 do not have functional polymorphism, and metabolises procarcinogens and drugs. Class II CYP enzymes such as CYP2A6, CYP2B6, CYP2C9, CYP2C19 and CYP2D6 are encoded by highly polymorphic genes, and metabolises drugs but not procarcinogens. [1]

CYP enzyme system consists of a haem domain and an NADPH-P450 reductase. [5] The mechanism of CYP enzymatic action begins when substrate binds to haem iron (III) at the active site. Firstly, reductase transfers an electron from NADPH to reduce iron (III). Secondly, molecular oxygen binds to the reduced iron (II). Thirdly, protons (derived from NADPH) split the molecular oxygen to produce a single oxygen atom, which is added to substrate. [6]

The current nomenclature system uses CYP, followed by a number (family), then a letter (subfamily), and another number (protein). For instance, CYP2C19 is the nineteenth protein in family 2, subfamily C. [3] As of year 2008, 18 families and 57 subfamilies of human CYP enzymes are identified and grouped based on genetic sequence similarity. [7]

2.2 Cytochrome P450, Family 2, Subfamily C, Polypeptide 19 (CYP2C19)

CYP2C19 is a highly polymorphic gene located within a cluster of CYP genes on chromosome 10q24. [8]

Polymorphism is defined as genetic variation that occurs in at least 1% of human population. In CYP2C19 gene, single-nucleotide polymorphisms (SNPs) occur in which one nucleotide base pair replaces another, resulting in two types of alleles namely wild type and variant (inactive or increased activity) alleles that are shown in Table 1. [9]

CYP2C19 gene encodes for CYP2C19 enzyme, which is present primarily in the liver. [3] CYP2C19 enzyme metabolises and activates many commonly prescribed drugs and prodrugs, [10] though it only plays a minor role in metabolisation of endogenous substrates. [11] The common substrates (or drugs) of CYP2C19 enzyme is shown in Table 4.

SNPs of CYP2C19 gene alter the function or amount of enzyme, which affects the metabolisation and activation of drugs and prodrugs, thus predisposes patients to adverse drug reactions such as drug resistance and toxicity. [9]

2.2.1 CYP2C19 Alleles

Table 1 shows the alleles of CYP2C19 gene and their enzymatic activities. Among the inactive alleles, CYP2C19*2 and CYP2C19*3 are more common in the population, while CYP2C19*4 to CYP2C19*8 are less common. [10]

Table 1: CYP2C19 alleles and their enzymatic activities.

CYP2C19 allele

Enzymatic activity


Normal (Wild type)

















[10] Source: Genelex. http://genelex.com/2C19tech.pdf (accessed 31 August 2012).

SNPs in the coding region of CYP2C19 gene include aberrant splice site which results in CYP2C19*2, and premature stop codon which results in CYP2C19*3. CYP2C19*2 encodes a defective enzyme that catalyses drug molecules at an alternative site instead of the active site, while CYP3C19*3 encodes a defective enzyme with abnormally short polypeptide chain. [9]

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CYP2C19*17 is a result of SNP that occurs in the 5'-flanking region of CYP2C19 gene, which leads to increased gene transcription, increased enzyme translation and increased metabolic activity. [12]

Table 2 shows various genotypes that give rise to their respective phenotypes, and the metabolic activities of these phenotypes.

Table 2: CYP2C19 genotypes, phenotypes and their metabolic activities.


Extensive metaboliser


Intermediate metaboliser


Poor metaboliser






2 wild type alleles

1 active allele

1 inactive allele

2 inactive alleles

1 or 2 increased activity alleles

Metabolic activity




[10] Source: Genelex. http://genelex.com/2C19tech.pdf (accessed 31 August 2012).

Table 3 shows the prevalence of CYP2C19 polymorphism in various ethnicities. Although EM phenotype (given by two wild type alleles) is not stated in Table 3, it is the most common phenotype in all ethnicities. [9]

Table 3: Prevalence of CYP2C19 polymorphism in various ethnicities.


Prevalence (%)

Poor metaboliser


13 - 19% in Asian population

10 - 20% in African population

2 - 6% in Caucasian population

Intermediate metaboliser


24% - 36% in U.S. population

Rapid metaboliser


Approximately 5% in U.S. population

[13] Source: Desta et al, Clinical Pharmacokinetics 2002.

2.2.2 Common Substrates of CYP2C19

Table 4 shows the common substrates of CYP2C19 enzyme.

Table 4: Common substrates of CYP2C19.

Drug classes





Proton pump inhibitor










Clopidogrel (anti-platelet drug)

Cyclophosphamide (anti-cancer drug)

Leflunomide (treats rheumatoid arthritis)

Proguanil (anti-malarial drug)

[10] Source: Genelex. http://genelex.com/2C19tech.pdf (accessed 31 August 20)

2.3 Clinical impact of CYP2C19 Polymorphism

Therapeutic Failure of Clopidogrel

Clopidogrel and low-dose aspirin are prescribed as a combination anti-platelet therapy to prevent recurrent cardiovascular events in existing patients. [14]

Clopidogrel is a prodrug that requires activation by CYP2C19 enzyme to form an active metabolite. The active metabolite inhibits platelet aggregation and prevents blood clot formation, hence reduces the risk of recurrent cardiovascular events. [15]

IMs and PMs have reduced clopidogrel activation, hence they are exposed to less active metabolite, which results in less platelet inhibition and higher risk of recurrent cardiovascular events.

A research conducted by Mega et al, 2009 showed that in comparison to EMs, the plasma concentration of active metabolite in IMs and PMs was 32.4% lower, and the platelet aggregation time in IMs and PMs reduced by 9% indicating there was less platelet inhibition. As a result, the risk of recurrent cardiovascular events (myocardial infaction or stroke) increased by 53%, and the risk of stent thrombosis increased by a factor of 3. [16]

However, since IMs and PMs have less platelet inhibition, they have lower risk of bleeding. [17]

Helicobacter pylori (H. pylori) eradication

H. pylori infection is treated with triple therapy as first-line regimen, and quadruple therapy as second-line regimen. Both triple and quadruple therapies consist of a proton pump inhibitor (PIP) and two types of antibiotics. An addition, bismuth is prescribed for quadruple therapy. [18]

PPI function by inhibiting acid production in stomach in order to create a less acidic intragastric pH. Antibiotics against H. pylori have greater stability and bioavailability under a less acidic condition, which increases the eradication rate of H. pylori. [19]

PPI (such as lansoprazole, omeprazole or rabeprazole) is metabolised by CYP2C19 enzyme. IMs and PMs have reduced PPI metabolism that prolongs the half-life of PPI, which results in higher H. pylori eradication rate.

A research conducted by Schwab et al, 2004 showed that serum level of lansoprazole and H. pylori eradication rate were correlated. They were highest in PMs (753 ng/ml and 100%), intermediate in IMs (59 ng/ml and 97.8%) and lowest in EMs (21 ng/ml and 21%). [20]

In order to increase H. pylori eradication rate in EMs, physicians can either increase PIP dosage, or prescribe rabeprazole. A research conducted by Kuo et al, 2010 showed that usage of rabeprazole in second-line regimen results in higher H. pylori eradication rate among EMs. [21]

2.3.3 Toxicity

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CYP2C19 enzyme is involved in drug metabolism and elimination. In IMs and PMs, both metabolic and elimination rate of drugs are reduced, making them prone to toxicity. Table 5 shows the metabolic and elimination rate of drugs in IMs and PMs. Table 6 shows the types of toxicity caused by certain drugs.

Table 5: Metabolic and elimination rate of drugs in IMs and PMs.


Metabolic / Elimination rate in IMs and PMs


(anti-retroviral drug)

Metabolic rate reduced by 50% [22]


(anti-cancer drug)

Elimination rate reduced by 50% [23]


(anti-fungal drug)

Elimination rate reduced by 49% [24]


[22] Hirt et al, British Journal of Clinical Pharmacology 2008.

[23] Anthe et al, Clinical Cancer Research 2007.

[24] Weiss et al, The Journal of Clinical Pharmacology 2009.

Table 6: Types of toxicity caused by certain drugs.


Types of toxicity



Neurological toxicity [25]


(anti-cancer drug)

Hematological toxicity [23]


(anti-cancer drug)

Ovarian toxicity [26]


(treats rheumatoid arthritis)

Hepatoxicity [27]


[23] Anthe et al, Clinical Cancer Research 2007.

[25] Dorado et al, The Pharmacogenomics Journal 2012.

[26] Singh et al, The Journal of Rheumatology 2007.

[27] Wiese et al, Arthritis Research and Therapy 2012.

Ethical Issues

Firstly, researchers must obtain informed consent from human subjects who participate in clinical studies. The risks associated with studies must be disclosed, as subjects often suffer from serious medical complications such as drug toxicity, myocardial infarction, stroke, and stent thrombosis.

Secondly, research institutions should provide proper medical care for subjects who suffer from medical complications.

Research Plan

Researches on discovery of more clinically important CYP2C19 alleles, solutions to overcome drug resistance and toxicity caused by CYP2C19 polymorphism, and alternative treatments that are not affected by CYP2C19 polymorphism should be conducted. [28]

In addition, a more affordable CYP2C29 genotyping assay should be discovered so that treatment regimen can be personalised to increase success rate.


CYP2C19 polymorphism affects the metabolisation of drugs and activation of prodrugs, which leads to drug resistance and toxicity. Drug dosage adaptation is the current solution to overcome this problem. In future, drugs that are not affected by CYP2C19 polymorphism should be discovered, so that they can be administered in all ethnicities with better therapeutic outcomes.