title: Lack of cross-toxicity between isoniazid and ethionamide in severe cutaneous adverse drug reactions: a series of 25 consecutive confirmed cases.
abstract: BACKGROUND
  Isoniazid and ethionamide are important first- and second-line anti-TB drugs (FLDs and SLDs), respectively. Ethionamide is a structural analogue of isoniazid and the two drugs share other similarities, including their metabolism, therapeutic targets, hepato-toxicity patterns and drug resistance. As a result, there has always been concern about possible cross-reactivity between them.
  
  
  METHODS
  Among 69 patients with drug rash with eosinophilia and systemic symptoms (DRESS), Stevens-Johnson syndrome (SJS) or toxic epidermal necrolysis (TEN) to FLDs, FLDs were stopped and SLDs added when the skin and laboratory parameters had settled. This was followed by sequential and additive rechallenge with FLDs. We report 25 consecutive cases that developed confirmed cutaneous adverse drug reactions (CADRs) to isoniazid or ethionamide used as FLD and SLD, respectively.
  
  
  RESULTS
  Sixty-nine participants who developed CADRs on FLDs were enrolled in the study. Twenty developed a rechallenge reaction to isoniazid and five reacted to ethionamide. Four of the 20 isoniazid cases were patch test positive, 3/20 were skin prick test positive and 13/20 reacted to oral rechallenge. All seven cases that were patch and skin prick test positive were associated with systemic reactions. Twenty of the 25 cases had DRESS and 5 had SJS/TEN. Twenty-three of the 25 cases with rechallenge reactions were HIV infected. Importantly, none of the cases that reacted to ethionamide during the rechallenge reacted to isoniazid and none who subsequently reacted to isoniazid reacted to ethionamide.
  
  
  CONCLUSIONS
  Our findings strongly suggest that the risk of cross-reactivity of isoniazid and ethionamide in DRESS syndrome and SJS/TEN is low. These findings have implications for clinical management.
authors.affiliation: Division of Dermatology, Department of Medicine, University of Cape Town, Cape Town, South Africa Lung Infection and Immunity Unit, Department of Medicine, University of Cape Town, Cape Town, South Africa.
authors.forename: Rannakoe J
authors.initials: RJ
authors.lastname: Lehloenya
authors.affiliation: Department of Paediatrics and Child Health, University of Cape Town, Cape Town, South Africa.
authors.forename: Rudzani
authors.initials: R
authors.lastname: Muloiwa
authors.affiliation: Division of Infectious Disease and HIV Medicine, Department of Medicine, University of Cape Town, Cape Town, South Africa.
authors.forename: Sipho
authors.initials: S
authors.lastname: Dlamini
authors.affiliation: Division of Dermatology, Department of Medicine, University of Cape Town, Cape Town, South Africa.
authors.forename: Nomphelo
authors.initials: N
authors.lastname: Gantsho
authors.affiliation: Division of Dermatology, Department of Medicine, University of Cape Town, Cape Town, South Africa.
authors.forename: Gail
authors.initials: G
authors.lastname: Todd
authors.affiliation: Lung Infection and Immunity Unit, Department of Medicine, University of Cape Town, Cape Town, South Africa keertan.dheda@uct.ac.za.
authors.forename: Keertan
authors.initials: K
authors.lastname: Dheda
mesh_terms: D000293:Adolescent; D000328:Adult; D000995:Antitubercular Agents / Q000009:adverse effects*; D004347:Drug Interactions; D064420:Drug-Related Side Effects and Adverse Reactions / Q000473:pathology; D005000:Ethionamide / Q000009:adverse effects*; D005260:Female; D006801:Humans; D007538:Isoniazid / Q000009:adverse effects*; D008297:Male; D012871:Skin Diseases / Q000139:chemically induced / Q000473:pathology; D055815:Young Adult
chemical_list: D000995:Antitubercular Agents; D005000:Ethionamide; D007538:Isoniazid