The dose-independent side effects are less frequent. They include skin rashes and abnormal organ function, mostly seen in the liver [294, 295]. Mild skin rashes are the most common form of hypersensitivity reactions and can be observed in up to 10% of patients treated with CBZ [296]. These can develop into severe CBZ-induced hypersensitivity reactions. The most severe forms comprise SJS, TEN and DRESS as described under the clinical manifestations of cutaneous drug reactions (see section 1.2.2). DRESS associated with rash, fever, eosinophilia and internal organ involvement, generally occurs during the first eight weeks of therapy with an estimated frequency of 4 in 10 000 [80, 297]. The most severe skin reactions SJS and TEN occur in Caucasians rarely with an estimated frequency of 1 in 10 000 [298]. In Han Chinese, the occurrence of SJS and TEN is roughly ten-fold higher [299]. These severe DHRs often cannot be predicted and require close monitoring of the patient.
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1.7.3.1 Pharmacogenetics
CBZ-induced hypersensitivity reactions were thought to have a genetic origin. This theory was supported by reports of hypersensitivity reactions observed in members of the same family and in monozygotic twins [300]. These studies have suggested that reactive metabolites of CBZ, such as arene oxides, cause cytotoxicity and thus trigger an immune response. Case reports supported that the DHRs of CBZ have a genetic association and that genetic polymorphisms in metabolising enzymes might be involved [300, 301]. For example, a case report demonstrated that a patient that had developed DHRs during treatment with CBZ possessed a less active form of an epoxide hydroxylase which is necessary for the metabolism of CBZ epoxide to the CBZ diol [302]. Therefore studies have focused on genetic differences in metabolising enzymes, particularly enzymes involved in the detoxification of CBZ such as microsomal epoxide hydroxylase, glutathione transferase, catechol-O-methyl transferase, and quinone reductase [300]. However, the results failed to show any association [303, 304]. Furthermore, antibodies against CYP isoforms involved in the metabolism of CBZ have been detected in patients with DHRs [305]. This observation led to the suggestion that reactive metabolites of CBZ bind to CYP enzymes, act as haptens and activate the adaptive immune system as per the hapten hypothesis. However, a genetic association between isoforms of CYP and CBZ-induced hypersensitivity reactions has not been found [44, 306].
The first evidence of an association between genetic polymorphism in genes with immune function in epileptic patients on CBZ was found by Pirmohamed and colleagues [307]. They showed that a polymorphism in TNF α at position 308 together with HLA-DR3 and HLA-DQ2 increased the risk of Caucasian patients developing DRESS but not of any other forms of DHRs.
A major breakthrough was the identification of an association between particular HLA alleles and specific clinical phenotypes observed in patients suffering from CBZ-induced hypersensitivity reactions. In 2004, a strong association between CBZ-induced SJS/TEN and HLA-B*15:02 in Han Chinese has been reported [44]. All patients, which had suffered from SJS or
55 TEN were carriers of the HLA allele while it was only present in 3% of CBZ tolerant patients and in 8.6% of healthy volunteers [44]. The association was also found in other Southeast Asian populations, including Indians, Malaysians, and Thai [45, 308-310]. Other studies showed that HLA-B*15:02 is only associated with CBZ-induced SJS/TEN and not with other forms of CBZ-induced hypersensitivity reactions, including MPE and DRESS [306, 311]. The association between HLA-B*15:02 and DHRs was not found in Europeans, possibly due to the low frequency of the HLA-B*15:02 allele in Caucasians [312]. Due to the evidence for a strong association between HLA-B*15:02 and SJS/TEN, genetic testing for this allele is recommended by the regulatory committees in patients with Asian ancestry before the start of CBZ therapy [313].
Recently an association was discovered between HLA-A*31:01 and all clinical phenotypes of CBZ-induced hypersensitivity reaction. This association was found by two independent research groups in Japan and Europe [46, 314]. In Europeans, the presence of HLA-A*31:01 increased the risk of developing ADRs from 5 to 26%, while the absence decreased the risk from 5 to 3.8%. In Japanese patients, HLA-A*31:01 was found in 60% of the patients who had suffered from DHRs. It has been reported that this HLA allele is also associated with CBZ-induced MPE in Han Chinese [306]. Based on the evidence, the association of HLA-A*31:01 and CBZ-induced hypersensitivity reactions is included in the label of the drug [315].
1.7.3.2 Functional studies
The clinical phenotypes of CBZ-induced hypersensitivity reactions led to the assumption that these DHRs are immune-mediated. The detection of CBZ- specific T cells in patients with CBZ-induced hypersensitivity reactions supported the hypothesis [152, 316]. The discovery of the above mentioned genetic association suggests a direct involvement of specific HLA alleles in presenting CBZ to cytotoxic T cells, leading to a T cell mediated immune response.
56 The role of HLA-B*15:02 in the role of CBZ-induced SJS in patients has been well studied and described: the immune reaction is mediated by cytotoxic T cells, which interact with HLA-B*15:02 [317, 318]. One in vitro study showed that CBZ activated drug-specific T cells directly, without the need of drug metabolism [164]. It was also shown that the parent drug CBZ does not modify peptides presented by the HLA [319]. Furthermore, T cell stimulation by CBZ in the presence of chemically fixed APCs, incapable of antigen processing, has been demonstrated [316]. All these observations led to the assumption that CBZ interacts with HLA or the TCR, thus activating the cytotoxic T cells, as described in the p-i concept. Recently, it has been shown that CBZ binds directly but non-covalently to HLA-B*15:02 [317]. Computer modelling revealed that the 5-carboxamide group on the tricyclic ring of CBZ is responsible for the interaction and identified three possible sites for interaction in the binding grove of HLA-B*15:02. These findings are supported by another study in which the non-covalent binding of CBZ to HLA-B*15:02 was also observed [140]. Furthermore, the analysis of the peptides mounted on HLA-B*15:02 in the presence of CBZ showed a change towards peptides with increased hydrophobicity. This shift in preferred self- peptides in the presence of CBZ implies that drug-specific T cell stimulation occurs according to the altered self-peptide hypothesis.
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