Pharmacovigilance data of eperisone were collected from the Korea Institute of Drug Safety-Korea Adverse Event Reporting System (KIDS-KAERS) database of the Korea Institute of Drug Safety and Risk Management (Ministry of Food and Drug Safety). Data on ADRs nationwide reported by doctors, nurses, pharmacists, pharmaceutical employers and patients are all collected in this system.8 The KIDS-KAERS database provides information on age, sex, reporting personnel, generic name of the drug, administration day/dose/route, clinical manifestations/severity, re-challenge and causality criteria. It uses the Anatomical Therapeutic Chemical Classification System for drug name, World Health Organization (WHO)-Adverse Reaction Terminology for clinical manifestations and World Health Organization-Uppsala Monitoring Center (WHO-UMC) causality criteria for assessment.9 ADR reports from January 2010 to December 2015 were analyzed. From this database, cases in which eperisone was reported only as a suspected drug and not as concomitant medication were used for the analysis. Cases in which causal relationship was assessed as certain or probable/likely were selected.

For a more accurate analysis of hypersensitivity, all reports were reviewed and revalidated if they were indeed anaphylaxis according to the WHO diagnostic criteria.10 Reviews were doctors who certified with a subspecialty in allergy. They had a minimum of 7 years of experience in the pharmacovigilance field. To overcome the limited medical information of anonymously converted computerized database, Severance Hospital (Seoul, Korea) pharmacovigilance data from January 2010 to December 2017 were analyzed for a more detailed investigation. This tertiary teaching hospital in Yonsei University operates a well-established pharmacovigilance system, and drug allergy tests (e.g., skin test, patch test, serum-specific immunoglobulin E (IgE) test, tryptase, provocation test and basophil activation test [BAT]) are available. Past medical history, drug history and underlying conditions for the use of eperisone were thoroughly reviewed. The study design schematic is shown in Fig. 1. This study was approved by the Institutional Review Board of the Yonsei University Health System (No. 2018-0770-001).

Eperisone (Exoperin^®; Hanmi Pharmaceutical, Seoul, Korea) dissolved in normal saline to a concentration of 10 mg/mL was used for SPT. Normal saline with 0.3% phenol and 50% glycerol was used as the negative control, and 0.1% histamine (Allergy Therapeutics, Worthing, UK) was used as the positive control. All participants discontinued medications that might influence the test result. Wheal and erythema sizes were measured after 15 minutes, and wheal sizes greater than 3 mm were considered positive reactions. As eperisone is only available in oral tablet form in Korea, intradermal test was not performed in this study considering the patients' safety.

Patients requiring oral challenge were admitted to the hospital for close monitoring of adverse reactions. Open challenge OPT was performed using medications suspected to induce anaphylaxis. Eperisone provocation test was started with 5 mg (1/10 of once dose) or 10 mg (1/5 of once dose) according to previous anaphylactic reactions, increased to 10, 25 and 50 mg per hour. OPT was conducted under the supervision of an allergy specialist, and the test was stopped immediately if an allergic reaction occurred. The tryptase level was checked using fluoroenzymeimmunoassay kits (Thermo Fisher Scientific Inc., Waltham, MA, USA) in cases of anaphylaxis.

Fresh whole blood from patients allergic to eperisone was collected in EDTA vacutainer tubes (BD Biosciences, San José, CA, USA), and BAT was performed with the FlowCAST^® kit (BÜHLMANN, Schönenbuch, Switzerland) according to the manufacturer's protocol. Briefly, 50 μL of the patient's whole blood was treated with various concentrations of eperisone (1, 0.1, 0.01 and 0.001 mg/mL),11 positive controls (FcεRI Ab, fMLP), and background control in 100 μL of stimulation buffer and staining reagent for 15 minutes at 37°C. For the cellular lysis, 2 mL of lysing reagent was incubated for 15 minutes at 37°C. Then, samples were centrifuged, the supernatant was decanted, and the cell pellet was resuspended with 300 μL of wash buffer. Flow cytometric analysis was performed via FACS Verse (BD Biosciences). Data were analyzed using FlowJo software (TreeStar, Ashland, OR, USA). Stimulation index (SI) was calculated as follows:

SI = % of basophil activated by the drug/% of activated basophils in the negative control

If SI was 2 or more and activated basophils were increased by more than 5%, the basophil test was regarded as positive.

There were 2,213 patients who had eperisone-induced ADRs. Among them, 207 whose ADRs were considered to have causal relationships (certain or probable/likely relationship based on the WHO-UMC criteria) were selected for analysis (Supplementary Table S1). The most common ADRs were cutaneous hypersensitive reactions (30.4%) such as urticaria, itchiness and angioedema. Anaphylaxis, a systemic hypersensitive reaction, developed in 16.9% of the patients (n = 35, Supplementary Table S2).

Information about the patients with eperisone anaphylaxis is shown in Table 1. The mean age was 50.6 years, and majority was women. Doctors reported the most number of anaphylaxis cases (48.6%) than other medical personnel. A total of 70% of enrolled cases were rated as “certain” in the causal relationship evaluation. Re-challenge information was available in 20 patients, and 70% of them re-experienced ADRs.

Among the 35 patients with anaphylaxis, 26 (74.3%) were originally reported to have anaphylaxis and 9 were newly re-assessed to have anaphylaxis based on the WHO criteria. Respiratory (e.g., dyspnea and wheezing) and cardiovascular compromises were observed in 45.7% and 20% of the patients, respectively. Meanwhile, 2.7% of the patients experienced altered mental status.

The baseline characteristics of patients with eperisone-induced ADR at a single pharmacovigilance center are shown in Supplementary Table S3 (n = 37). The mean age was 56.5 years, and majority was women (64.9%). In this center, cutaneous manifestations (29.7%) and anaphylaxis (29.7%) were the most common ADRs. There were 11 patients who experienced eperisone-induced anaphylaxis. The demographics, co-administered medications and results of the allergy diagnostic study are shown in Table 2. The average age was 53.9 years, and women were 2.7 times more likely to develop ADRs than men. Most patients were prescribed with eperisone due to muscle cramping (63.6%), and there were no patients who had any other allergic diseases. Eperisone was co-administered with NSAIDs, tramadol, acetaminophen or gastrointestinal protective agents.

Allergy screening test to determine the cause was performed using SPT, BAT and open challenge test. All the patients with anaphylaxis showed a positive reaction to OPT using eperisone. SPT of eperisone was performed in 4 patients, and all results were negative. BAT data of the 4 patients are shown in Supplementary Fig. S1A. None of the patients met the BAT positivity criteria described earlier. Tryptase levels were checked in 3 patients after anaphylaxis developed. One had elevated absolute levels (15.5 μg/L, reference range: 0-13.5 μg/L), the other had a 7.7-fold elevation baseline tryptase (from 1.11 to 8.53 μg/L, measured after provocation test), while the other had no changes in tryptase levels. During OPT, 5 patients experienced hypotension, and 7 patients showed urticaria and/or angioedema (Supplementary Fig. S1B). Cumulative dose for positive reaction was 5 to 50 mg of eperisone. On average, 59 minutes were taken for a positive reaction.

As other pain controllers such as NSAIDs are co-administered with eperisone, and they are more commonly known as the cause of drug allergy than eperisone, open challenge test using pain controllers were performed initially. However, all OPTs were negative for pain controllers. Therefore, eperisone was suspected to be the cause of anaphylaxis and OPT was performed.