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Munir Pirmohamed Department of Pharmacology
and Therapeutics, University of Liverpool, Box 147, Liverpool L69 3BX
Correspondence to: Dr Pirmohamed munirp{at}liv.ac.uk
An adverse drug reaction is any undesirable effect of a
drug beyond its anticipated therapeutic effects occurring during
clinical use. In contrast, an adverse drug event is an untoward
occurrence after exposure to a drug that is not necessarily caused by
the drug.1
When a drug is marketed little is known about its safety in
clinical use because only about 1500 patients are likely to have been
exposed to it.
1 2
Thus drug safety assessment should be
considered an integral part of everyday clinical practice since detection and diagnosis often depend on clinical acumen.
In this article we review the current status of adverse drug reactions,
briefly describing the complexity of the more bizarre reactions and
outlining a strategy to eliminate serious adverse drug reactions.
We conducted a search on the BIDS ISI database between 1981 and
1997 using key words such as toxicity and hypersensitivity combined
with drug. The references most relevant to this review were then
scanned together with any other relevant references cited within the
articles. We also continuously review the literature because of our
research interests.
Adverse drug reactions are a major clinical problem, accounting
for 2-6% of all hospital admissions (box).3-6 Recent
surveys in the United States have indicated that adverse drug events
increase the length of hospital stay and costs.
5 6
Adverse drug reactions are type A (pharmacological) or type B
(idiosyncratic).7 Type A reactions represent an
augmentation of the pharmacological actions of a drug. They are
dose-dependent and are therefore readily reversible on reducing the
dose or withdrawing the drug. In contrast, type B adverse reactions are
bizarre and cannot be predicted from the known pharmacology of the
drug.
Pharmacological adverse drug reactions
Adverse drug reactions:
Summary points
Adverse drug reactions are a common clinical problem
They are diagnosed on clinical grounds from the temporal relation
between the start and finish of drug treatment and the onset and offset
of the reaction
Pharmacological adverse reactions are generally dose-dependent, related
to the pharmacokinetic properties of the drug, and resolve when the
dose is reduced
Idiosyncratic adverse reactions are not related to the known
pharmacology of the drug, do not show any simple dose-response
relation, and resolve only when treatment is discontinued
Vigilance by clinicians in detecting, diagnosing, and reporting adverse
reactions is important for continued drug safety monitoring
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Methods
Top
Methods
Conclusion
References
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Importance of adverse drug reactions
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Types of adverse drug reactions
Type A adverse drug reactions are more common than type B
reactions,3 accounting for over 80% of all reactions.
They can be divided into those due to the primary pharmacology of the
drug
that is, augmentation of the drug's therapeutic actions
and
those due to the secondary pharmacology of the drug
that is, an action
different from the drug's therapeutic actions but still rationalisable
from the known pharmacology of the drug.
Importance of adverse drug reactions
blockers, bradycardia and heart block are primary
pharmacological adverse effects while bronchospasm is a secondary pharmacological adverse effect. More emphasis is now placed on the
secondary pharmacology of new drugs during preclinical evaluation to
anticipate problems that might arise once the drug is given to humans.
Recent experience with fialuridine, an experimental drug for hepatitis
B, highlights the need for continued development of appropriate in vivo
and bridging in vitro test systems to predict secondary pharmacological
adverse effects in humans. In June 1993, during phase II trials, 5 out
of 15 patients died while two others required emergency liver
transplantation for liver and kidney failure8; this effect
had not been observed in four animal species. On the basis of results
from in vitro studies in cultured hepatoblasts, the toxicity may be due
to inhibition of mitochondrial DNA polymerase
by fialuridine and
its metabolites.9
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Mechanisms of idiosyncratic adverse drug reactions
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for example, if five
drugs are given simultaneously the chance of an adverse interaction
occurring is 50%.10 To date, this has largely been a
problem in elderly people but it is becoming increasingly common in
younger patients with chronic diseases such as AIDS, who may be taking
6-10 different drugs.11
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Idiosyncratic adverse drug reactions
Idiosyncratic adverse reactions are less common than
pharmacological adverse reactions, but they are as important because they are often serious and account for many deaths. Mechanisms of
idiosyncratic adverse effects12 are listed in the box.
a process termed
bioactivation.
12 13 16
Such metabolites may be toxic. In
most people the formation of chemically reactive metabolites is
counterbalanced by detoxification mechanisms
a process termed
bioinactivation. In susceptible people the usually favourable balance
between bioactivation and bioinactivation may be perturbed by either
genetic or host factors such as age, enzyme induction, and disease, all
of which allow the toxic metabolites to escape detoxification. Under
these circumstances, the toxic metabolites may bind covalently to
various cellular macromolecules and cause toxicity. With most drugs,
however, the factors which cause this imbalance are unknown, which
explains why such reactions continue to occur.
In some cases chemically reactive metabolites will be formed
irrespective of the dose.16 At therapeutic doses any toxic metabolite formed will be detoxified by cellular defence mechanisms, but an imbalance between bioactivation and bioinactivation may result
after overdoses. This will lead to the formation of large amounts of
chemically reactive metabolite, which will overwhelm cellular
detoxification capacity and lead to cell damage.
The clearest example of this occurs in paracetamol overdose, which
causes hepatotoxicity and kills about 160 people each year in the
United Kingdom.17 Paracetamol hepatotoxicity should not be
classed as an adverse reaction since the hepatic injury occurs when the
drug is used inappropriately. However, the occurrence and severity of
liver damage with paracetamol is a function not only of the dose but
also of various host factors.13 Indeed, paracetamol
hepatotoxicity has been reported with therapeutic drug use. For
example, a recent study in 67 alcoholic patients with paracetamol
hepatotoxicity showed that 40% had taken less than 4 g/day (the
recommended therapeutic dose) and 20% had taken 4-6 g/day (a
non-toxic dose).18 Paracetamol is largely metabolised by
phase II processes (glucuronidation and sulphation) to stable metabolites, with 5-10% undergoing P-450 metabolism to
the toxic quinoneimine metabolite.19 This is detoxified by
cellular glutathione. At overdose, saturation of the phase II pathways
results in a greater proportion of the drug undergoing bioactivation.
This leads to glutathione depletion and allows the toxic metabolite to
bind to proteins, resulting in hepatocellular damage.19
The use of N-acetylcysteine to treat paracetamol
overdose shows that elucidation of the mechanism of drug toxicity can
lead to the development of rational treatments that will prevent
toxicity. Alcoholic patients show increased susceptibility to
paracetamol because excess alcohol consumption depletes
glutathione20 and induces the CYP2E1 isoform of cytochrome
P-450,21 the primary enzyme concerned with
paracetamol bioactivation.22
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Importance of the immune system |
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Many idiosyncratic adverse reactions are thought to be
mediated by the immune system on the basis of clinical
criteria.
12 13 23
The mechanism by which a drug leads to
an immune mediated adverse reaction is explained by the hapten
hypothesis.24 Central to the hypothesis is the assumption
that small molecules such as drugs can be recognised as
immunogens
that is, a substance capable of eliciting a specific immune
response
only when they become covalently bound to macromolecules such
as proteins (to form haptens).24 The type of
hypersensitivity is partly determined by the nature of the immune
response and the site of antigen formation. The best understood
reactions are the type I hypersensitivity reactions induced by
penicillins and mediated by IgE antibodies directed against a drug
hapten conjugated to protein.
13 25
Severe anaphylactic reactions occur in only 1 in 2000 patients; the genetic basis of the
IgE response to penicillins remains unclear.
Less well understood are the immunological mechanisms underlying severe
reactions such as the Stevens-Johnson syndrome and immunoallergic
hepatitis. In vitro studies have shown that drugs causing these
reactions undergo oxidative metabolism to chemically reactive
metabolites that can form haptens with proteins.26 Both
humoral and cell mediated responses directed against drug induced
antigen have been detected in patients
for example, in halothane
hepatitis.27 With some compounds the immune response is
directed predominantly towards an autoantigen. For example, in
hepatitis induced by tienilic acid patients have circulating autoantibodies directed against the P-450 isoform
(CYP2C9) that is responsible for bioactivation of the
drug.28 However, whether such autoantibodies are
pathogenic or represent an epiphenomenon (their appearance is
secondary) needs further study. The role of T cells in drug induced
tissue injury is also poorly understood, although recent
immunohistochemical studies, particularly of skin reactions, suggests
that they subserve a pathogenic role.29
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Host factors and adverse drug reactions |
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Genetically determined alterations in drug metabolising enzymes can predispose to both pharmacological and idiosyncratic toxicity.26 Single gene defects account for only a minority of adverse drug reactions. For most adverse reactions, particularly the idiosyncratic drug reactions, predisposition seems to be multifactorial, involving not only defects at multiple gene loci but also environmental factors such as concomitant infection. 13 26 Most work has focused on enzyme polymorphisms in drug oxidation and conjugation as risk factors for drug toxicity, but this search for genes affecting susceptibility needs to be extended to include cell repair mechanisms, elaboration of cytokines, and immune responsiveness. Such investigations may in the future provide us with the capability to predict a person's susceptibility to the different forms of drug toxicity.
Concomitant host disease may also influence susceptibility to adverse reactions. The best recent example is HIV disease, which increases the frequency of idiosyncratic toxicity with anti-infective drugs such as co-trimoxazole.30 Around 50% of patients receiving high doses of co-trimoxazole for Pneumocystis carinii pneumonia and 30% receiving prophylactic doses develop skin rashes.31 This contrasts with a frequency of 3% in people who are negative for HIV infection.31 Glutathione deficiency has been suggested by some 32 33 but not all 34 35 investigators to be responsible for the increased frequency of reactions. 30 31 The reasons are likely to be more complex and to include not only changes in drug metabolising capacity (bioactivation and bioinactivation) but also immune dysregulation.
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Spontaneous reporting schemes |
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The exposure of 1500 patients to a drug by the time of
licensing
1 2
will allow the more common adverse reactions
to be detected but not necessarily characterised. At least 30 000
people need to be treated with a drug to discover
with a power of 0.95
at least one patient with an adverse reaction which has an
incidence of 1 in 10 000.36 Thus, postmarketing
surveillance is important to permit detection of less common adverse
effects.
Spontaneous adverse drug reaction reporting schemes, as exemplified by
the yellow card system in the United Kingdom, form the cornerstone of
postmarketing drug safety surveillance. Indeed, spontaneous reporting
schemes remain the only way of monitoring the safety of a drug
throughout its marketed life. The yellow card scheme is important in
identifying previously undetected adverse reactions37 and
over the years has provided many early warnings of drug safety
hazards
for example, remoxipride and aplastic anaemia
to allow
appropriate drug regulatory action to be taken. A problem with
spontaneous reporting is that less than 10% of all serious and 2-4%
of non-serious adverse reactions are reported.
2 38
All
doctors need to be aware that adverse drug reaction reporting is part
of overall patient care and is not simply an afterthought. Since 1964 reporting in the United Kingdom has been restricted to doctors,
dentists, and coroners, although more recently a reporting scheme for
pharmacists has been introduced. In some European countries all
healthcare professionals are allowed to report adverse drug reactions,
while in the United States patients can also report through the
MEDWatch scheme.39
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Conclusion |
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The importance of adverse drug reactions is often underestimated. They are common and can be life threatening and unnecessarily expensive. The measures outlined in the box above are important to improve the benefit to risk ratio of drug treatment by reducing the burden of drug toxicity. Because of the wide range of drugs available, the manifestations of toxicity may vary and affect any organ system. In fact, adverse reactions have taken over from syphilis and tuberculosis as the great mimics of other diseases. The pattern of toxicity is likely to change with the introduction of new biotechnology products. It is therefore important for prescribing clinicians to be aware of the toxic profile of drugs they prescribe and to be ever vigilant for the occurrence of unexpected adverse reactions.
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Strategy to improve drug safety
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References |
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implications for prevention.
JAMA
1995;
274:
29-34[Abstract].
allergic or toxic.
AIDS
1995;
9:
217-222[Medline].
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