Munir Pirmohamed and Kevin Park. Department of Pharmacology ... PirnlOhamed & Park has been shown to ..... The mode of action of clozapine has not been.
ADVERSE EFFECTS
eNS Drugs 1997 Feb: 7 (2) 139-158 1172-7047/97/0002-0139/$10.00/0 © Adis Internotionalumlted. All rights reserved.
Mechanism of Clozapine-Induced Agranulocytosis
Current Status of Research and Implications for Drug Development Munir Pirmohamed and Kevin Park
Department of Pharmacology and Therapeutics, The University of Liverpool, Liverpool, England
Contents Summary ..................... . 1. General Overview of Adverse Drug Reactions . 1.1 Types of Adverse Drug Reactions . . . . . . 1.2 Mechanisms of Idiosyncratic Adverse Drug Reactions 2. Characteristics of Clozapine-Induced Agranulocytosis. 2.1 Syndromes.. 2.2 Epidemiology 2.3 Risk Factors . 2.4 Predictors .. 3. Other Haematological Abnormalities Induced by Clozapine 4. Mechanisms of Clozapine-Induced Agranulocytosis . . . . . 4.1 Site of Toxicity - Central or Peripheral? . . . . . . . . . . 4.2 Is Toxicity Due to Clozapine Itself and Is It Related to the Pharmacology of the Drug? . . . . . . . . . . . . . . . . 4.3 Is Toxicity Due to Stable or Chemically Reactive Metabolites of Clozapine? . 4.4 Is Toxicity Direct or Immune Mediated? . . . . . . . . 4.5 What Determines Individual Susceptibility? . . . . . . 5. Comparison with Amodiaquine-Induced Agranulocytosis 6. Areas for Further Research . . . . . . . . 7. Prospects for Future Drug Development 8. Conclusion . . . . . . . . . . . . . . . . .
Summary
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Clozapine is an atypical antipsychotic agent that has several advantages over conventional anti psychotics, not least of which is its superior efficacy. However, the high risk of agranulocytosis (0.8% of patients) associated with c10zapine therapy has resulted in restricted indications for its use. The mechanism of c1ozapine-induced agranulocytosis is not clear. The target cells affected are the myeloid precursors, although the mature neutrophil may also be targeted simultaneously. There is no convincing evidence of direct toxicity of the parent compound or its stable metabolites (demethyl-c1ozapine and c1ozapine N-oxide). Clozapine is also metabolised by liver microsomes, peripheral blood neutrophils and their bone marrow precursors to a chemically reactive intermediate that has been postulated to be a nitrenium ion. This toxic metabolite
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has been shown to covalently bind to neutrophil proteins, suggesting that it may be involved in the pathogenesis of the toxicity. However, it is not clear how toxicity is mediated. The nitrenium ion may bind to essential cellular proteins and disrupt neutrophil function or, alternatively, it may act as a hapten and initiate an immune reaction resulting in immune-mediated destruction of the neutrophil. Indirect evidence exists to support both mechanisms, although clear direct evidence is still lacking. The role of cytokines and apoptosis in the pathogenesis of the agranulocytosis is unclear. The reason why only approximately I % of individuals who are treated with clozapine are affected by agranulocytosis has not been elucidated. Evidence exists to implicate both the major histocompatibility complex antigens and heat shock protein variants in determining individual susceptibility, although more patients of different ethnic backgrounds need to be studied. The ultimate aim of research into clozapine-induced agranulocytosis should be to either prospectively predict which individuals are going to develop agranulocytosis and/or to develop analogues that retain efficacy but are not toxic. The former is complicated by the fact that predisposition may be multifactorial, and thus prediction may require multiple tests that may be of statistical but not absolute validity. The latter depends on identifying the mechanism of toxicity and the chemical characteristics of clozapine that are responsible for the toxicity. This knowledge may allow rational design of new analogues that do not cause agranulocytosis. Clozapine, a dibenzodiazepine compound, is an atypical antipsychotic that has a unique history (table 1). In the mid 1970s, it was either withdrawn or its use restricted in most countries as a result of the high risk of agranulocytosis associated with its use. By the late 1980s, largely because of the unique pharmacological profile of the drug, the situation had changed - its use had greatly increased, but was accompanied by careful patient selection and/or haematological monitoring. Clozapine shows superior efficacy when compared with conventional anti psychotics such as chlorpromazine,1 I I and is now indicated primarily in patients who have schizophrenia that is resistant to conventional agents. Additional major advantages of clozapine over conventional antipsychotics are that it: • has a lower incidence of extrapyramidal adverse effects including tardive dyskinesia;121 • has a beneficial effect on the negative symptoms of schizophrenia;13 1 • reduces suicidality.13 1 These advantages, however, are mitigated by its propensity to cause agranulocytosis.1 41 ~)
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In order to prevent fatalities from clozapine-induced agranulocytosis, in most countries, all patients being treated with clozapine are required to have haematological monitoring. ISI The extent of this monitoring differs in different countries. For example, in the UK, white blood cell counts are monitored weekly for the first 18 weeks of treatment, fortnightly from 18 to 52 weeks, and monthly thereafter. In the US, in contrast, the monitoring requirements are more stringent: patients are monitored weekly for the entire duration of treatment. Since its reintroduction, a great deal of research has been undertaken in an attempt to elucidate the mechanism(s) of clozapine-induced agranulocytosis. The purpose of this review is to critically evaluate what has been learnt from this research, what still needs to be determined and how this may help in future drug development. Clearly, the ultimate aim of this research is to improve the benefit-risk ratio associated with clozapine therapy, either by developing a congener that has similar pharmacological properties but does not cause agranulocytoeNS Drugs 1997 Feb: 7 (2)
Mechanism of Clozapine-Induced Agranulocytosis
Table I. The regulatory and clinical history of clozapine Year
Event
1960
Developed by Hunziker and colleagues[11
1972
Marketed in Europe
1975
8 deaths from agranulocytosis (of 16 cases) reported from Finland
1976
Clozapine either withdrawn or use restricted in most countries
Mid-1980s
Clozapine trials started as a result of pressure from psychiatrists in the absence of other compounds effective in treatment-resistant schizophrenia
1989
Clozapine approved by the US Food and Drug Administration and the UK Committee on Safety of Medicines
1990
Reintroduced for patients with schizophrenia that is resistant to conventional therapy. Weekly monitoring of white blood cell count mandatory (Clozaril@ Patient Monitoring Scheme)
sis, or to prospectively identify at-risk patients and thus avoid the toxicity.
1. General Overview of Adverse Drug Reactions Before discussing cIozapine-induced agranulocytosis, a brief overview of the different types of adverse drug reactions is given together with a discussion of the mechanisms of idiosyncratic drug reactions. 1.1 Types of Adverse Drug Reactions
In general, adverse drug reactions can be divided into 2 types, A and B.l 6,71 Type A reactions are an exaggeration of the normal pharmacological effects of the drug and thus can be predicted from the known pharmacology of the drug. These reactions are usually dose-dependent and can be prevented by dose reduction. They are common, usually not severe and are detected early during the development of the drug. Agranulocytosis and bone marrow depression occurring with cytotoxic chemotherapeutic agents usually fall into this category. Type B reactions are also known as idiosyncratic reactions. They cannot be predicted from the © Adis International Umited. All rights reserved.
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pharmacology of the drug, are dose-independent, and tend to be influenced by host factors (which may be genetic or environmental). They are less common than type A reactions, but tend to be more severe and occasionally result in fatalities. Because they are uncommon, they are usually not detected during the early phase of drug development. From the evidence available, cIozapine-induced agranulocytosis falls into this category. 1.2 Mechanisms of Idiosyncratic Adverse Drug Reactions
Many different mechanisms have been postulated as being responsible for the idiosyncratic toxicity caused by drugs, and these have been reviewed elsewhere ,171 The parent compound, or more usually its metabolite(s), have often been implicated in the mechanism of drug-induced white blood cell toxicity. Of particular importance with regard to the metabolism of the drug is the formation of chemically reactive intermediates, a process termed 'bioactivation'17-1I1 (fig. 1). These are unstable, highly reactive species that can bind covalently to cellular macromolecules and cause toxicity by 2 mechanisms (fig. 1),17.9 1 First, they may interfere with essential functions of the cell (i.e. direct cell toxicity) and cause cell death. The typical example of this is hepatic necrosis resulting from paracetamol (acetaminophen) overdosage.l12-151 The mechanism by which cell death occurs is not fully understood. Secondly, the reactive species may act as a hapten and initiate an immune reaction (i.e. indirect or immune-mediated toxicity)p,9,16 1The immune reaction may be directed against: (i) the drug antigen; (ii) a neoantigen created by the interaction of the metabolite with cellular protein; (iii) an autoantigen; or (iv) towards more than one of these antigens. I 171 The immune reaction may be characterised by antibody production (humoral immunity), drug-specific T lymphocytes (cellular immunity) or a combination of the two. In the majority of individuals, bioactivation is counter-balanced by the detoxification mechanisms that are present in most cell types. lg ,9 1 eNS Drugs 1997 Feb: 7 (2)
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Neutrophil
Liver
CytotoxiCity
Sensitisation
Necrosis! apoptosis
Fig. 1. The postulated role of drug metabolism in the pathogenesis of agranulocytosis. Drugs may be metabolised by either the liver or neutrophils (or their precursors) to chemically reactive intermediates, a process that is termed 'bioactivation'. Bioactivation usually represents a minor metabolic pathway, the conversion of drugs to stable metabolites being by far the most common biotransformation. If the chemically reactive metabolite is not bioinactivated, it may bind to neutrophil proteins and cause either direct toxicity resulting in cell death (i.e. cytotoxicity) or act as a hapten and initiate immune-mediated toxicity (i.e. hypersensitivity). With compounds other than clozapine, both mechanisms have been shown to result in agranulocytosis.
Clearly, those individuals who have an imbalance between bioactivation and detoxification are most likely to develop idiosyncratic toxicity. Of relevance here is the site of metabolism of the drug. Different tissues in the body will have different complements of drug activation and drug detoxification enzymes, and this may be one factor determining site-specific toxicity. I'll The liver is obviously the commonest site of metaboli sm in the body.!lgl Thus, metabolism within the liver leading to hepatic injury, as occurs in paracetamol overdosage, is easy to understand since metabolism and toxicity are occurring within the same organ. With regard to extra-hepatic toxicity, it is difficult to envisage how a chemically reactive interme© Ads Internatio nal Limited . All rights reserved.
diate (often with a half-life of less than I minute) can be formed within the liver and then travel in the circulation to its site of toxicity.!9 1 Therefore, it is likely that metabolism and bioactivation also occurs in extra-hepatic tissues (fig . I). With regard to drugs causing agranulocytosis, there is evidence to support metabolism within white blood cells as being responsible for the toxicity.IID,II ,19 1 An alternative mechanism may involve the hepatic metabolism of the parent drug to a stable intermediate which then passes to its site of toxicity where it undergoes bioactivation to the ultimate toxic metabolite. Such a mechanism has been postulated for the haematological toxicity associated with benzeneI2D-22I and chloramphenicol.123I e NS Drugs 1997 Fe b : 7 2) (
Mechanism of Clozapine-Induced Agranulocytosis
Even when an imbalance exists between drug bioactivation and detoxification, this does not necessarily mean that the patient will go on to develop toxicity.18 1 For example, the damage caused by a chemically reactive metabolite may be nullified by other cellular repair mechanisms, such as DNA repair enzymes in the case of potentially carcinogenic drugs. For drugs causing immune-mediated toxicity, the formation of a drug antigen may not result in an immune response as the patient may not be able to mount such a response,l24 1 Even when an immune response is mounted, a hypersensitivity reaction does not necessarily ensue. With penicillin, for example, there is little interindividual variation in the circulating levels of the penicilloyl hapten,125 1yet over 60% of patients fail to mount a serological response.[24 1Similarly, with halothane it is thought that all individuals can generate the antigen but susceptibility to hepatitis is dependent on immune responsiveness to the antigen.[261 It can be seen from this overview that the mechanism(s) of idiosyncratic drug reactions are complex and multifactorial, susceptibility being determined by individual variability at different levels. In fact, with most drugs, multifactorial predisposition is likely to be the rule rather than the exception. 1271 This is also likely to be the case with clozapine-induced agranulocytosis, as discussed in section 4.
2. Characteristics of ClozapineInduced Agranulocytosis In this section, the clinical characteristics of clozapine-induced agranulocytosis, as well as the effects of the drug on other haematological parameters, are reviewed. This information is relevant, as it may throw light on the possible mechanisms of the toxicity. 2.1 Syndromes
Clozapine-induced suppression of the granulocyte series can result in 3 contiguous syndromes: 12H1 © Adis International Lirnited. All rights reserved.
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• leucopenia - a white blood cell count of less than 3500 cells/mm 3 with granulocytes above 1500 cells/mm3; • neutropenia - a granulocyte count of less than 1500 cells/mm 3 but more than 500 cells/mm3; • agranulocytosis - a granulocyte count of less than 500 cells/mm 3 . 2.2 Epidemiology
Agranulocytosis is obviously the most severe form and has resulted in fatalities. An epidemiological study in the US covering the period between February 1990 and April 1991 showed that the incidence of agranulocytosis after treatment with c10zapine for I year was 0.8%.[29,301 This contrasts with an incidence of 1.5 to 2.0% for c1ozapine-induced neutropenia.J3 11 The US study also showed that in 24 of 73 patients who developed agranulocytosis, the white cell count did not fall below 3500 cells/mm 3 , and in 16 of these patients, the count had remained above 3500 cells/mm 3 within 8 days before the occurrence of agranulocytosis.[ 30I In the UK, of 6316 patients who recei ved c10zapine from January 1990 to July 1994, 2.9% developed neutropenia and 0.8% developed agranulocytosis.1 321 The risk of both agranulocytosis and neutropenia decreased with time (fig. 2). More recently, an analysis of 99 502 US patients who received c10zapine in the 5-year period starting from 1990 revealed 2931 cases of leucopenia (2.95%), 382 cases of agranulocytosis (0.38%) and 12 deaths (0.012%).1331 The rate of agranulocytosis is less than half of that which would have been predicted, suggesting that the stringent criteria used for haematological monitoring of clozapine therapy has reduced the incidence of agranulocytosis by early detection of susceptible patients. The risk of death associated with the occurrence of agranulocytosis is 3 to 4% in the US;1 33 1 this again is a marked improvement on a predicted death rate of 15% and the observed rate of 50% in Finland in 1975 when the drug was first introduced.J341 eNS Drugs 1997 Feb; 7 (2)
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ceptible. Conversely, it has been suggested that the Chinese may be less susceptible, since no cases of agranulocytosis were reported among 290 Chinese patients treated with c1ozapine. i361 However, this is based on small numbers of patients, and since there is no mandatory monitoring system in China it is difficult to confirm. Concomitant treatment with other drugs known to induce agranulocytosis such as carbamazepine may increase the risk of agranulocytosis.[37,38 1 It is important to note that c10zapine dosage and the baseline white blood cell count do not act as risk factors for agranulocytosis . 12~ ,30.3 21
Neutropenia
D Agranulocytosis
2.4 Predictors
1st year
No of patients
receiving
6316
3rd year 2nd year Year of use
2858
1625
4th year
661
clozaplne
Fig. 2. The incidence of clozapine-induced agranulocytosis and neutropenia in patients who received clozapine for 1, 2, 3or 4 years in the UK and Ireland between 1990 and 1994. The error bars represent the upper 95% confidence interval. The risk of agranulocytosis and neutropenia is highest during the first year of use, with the risk decreasing thereafter. The lower part of the figure shows the number of patients receiving clozapine and the duration of use. Data kindly provided by Dr Karen Atkin (Sandoz Pharmaceuticals, UK) [adapted from Atkin et a1. 132) I.
About 75 % of the cases of c1ozapine-induced agranulocytosis occur within the first 24 weeks of treatment, with 95 % occurring within 6 months. Although the ri sk of agranulocytosis decreases with time, it does not reach zero, with some cases being reported after 2 years or more of continued therapy. 12~ I
2.3 Risk Factors Older age and female gender are considered to be risk factors for c1ozapine-induced agranulocytosis.1 301 Certain ethnic groups such as Scandinavians l341 and Ashkenazic Jews l3 )1 may be more sus
