Thiodiglycolic acid as a possible causative agent of fixed drug eruption provoked only after continuous administration of S-carboxymethyl-l-cysteine: case report and review of reported cases

Abstract

Conflict of interest: None declared Sir, A provocation test of a fixed drug eruption (FDE) usually induces an erythematous response at the previously affected sites following a single, normal dose and the response is typically observed within 24 h.1, 2 We recently experienced two cases of FDE induced by S‐carboxymethyl‐L‐cysteine (SCMC) and found that in both cases it required the intake of the agent for more than two consecutive days to elicit the eruption. Similar observations have been described in the Japanese literature, although the reason is so far unknown.3, 4 We therefore characterized this feature of SCMC‐induced FDE and we discuss its possible aetiology. Two Japanese females (patient 1: 32 years old, patient 2: 8 years old) who had histories of a fixed drug eruption two days after consecutive ingestions of SCMC, were examined by oral challenge tests and patch tests. Oral challenge tests were performed with the consent of the patients and the parents. In both cases, skin eruptions were not induced by a single challenge of the usual dose of SCMC when it was given either in the morning or in the evening and the skin observed for 1 week. However, if SCMC was consecutively taken at night and three times on the following day, erythema developed by the morning of the third day. We performed patch tests with SCMC and its metabolites, S‐methyl‐L‐cysteine (SMC) (Sigma Chemical Co, Poole, Dorset, U.K.) and thiodiglycolic acid (TDA) (Sigma Chemical Co.). Both 1% and 10% concentrations of TDA showed positive patch tests at the affected skin sites in both patients. Neither SCMC nor SMC elicited any positive findings. Five healthy volunteers did not show a positive response to any of the three chemicals. Lymphocyte transformation tests for SCMC, SMC and TDA were negative in both patients.