Background and Importance A 55 year old male patient, developed a DRESS (drug rash with eosinophilia and systemic symptoms) reaction after starting first-line tuberculosis treatment with rifampicine, ethambutol, isoniazid and pyrazinamide. To assess the responsability of a suspected drug in a DRESS reaction and posterior safe reintroduction of therapy, patch tests (PT) are the most useful tool. For the purpose, the Hospital Pharmacy was asked to develop magistral preparations of ethambutol, isoniazid and pyrazinamide. The PT were performed with each tuberculostatic drug diluted in 4 IQ Ultra Chambers, applied on the patient’s skin at the back and kept in occlusion for 48 hours. The readings were performed at day 2 and day 3. Only erythema, infiltration, papules or vesicles were considered positive reactions.

Aim and Objectives Development and validation of magistral formulas for topical application to accomplish patch tests of ethambutol 10% (w/w), isoniazid 15% (w/w) and pyrazinamide 25% (w/w).

Scarce bibliographic information Review article published by the French Society of Dermatology, in which the concentrations of the active ingredient to be used in each PT are established.

Application of the general rules of Good Handling Practices, according to the Portuguese Galenic Formulary.

Results The pastes used in the PT were obtained by geometric dilution of pulverised ethambutol 400 mg , isoniazid 300 mg and pyrazinamide 500 mg tablets in white petrolatum.

After quality-control tests that includes colour, homogeneity and mass verification assays, the pastes were placed in a syringe for an easier application in the skin. It was given 30 days of stability at room temperature.

Conclusion and Relevance This preparation made possible to develop PT for the study of a delayed hypersensibility reaction to tuberculostatic drugs, that was not available before in the market, allowing a safer reintroduction of the treatment.

Although the PTs were negative in this patient, it was possible to develop and validate three compounding formulas with an adequate safety profile and low cost. This accomplishment will be useful in further cases.

References and/or Acknowledgements 1. Ingen-Housz-OroS, Assier H, et al.Hypersensibilité retardée aux traitements antituberculeux. Proposition d’une conduite à tenir pratique devant un exanthème: quand arrêter,quelles explorations allergologiques et comment réintroduire le traitement. Annales de Dermatologie et de Vénéréologie

2. Portuguese Galenic Formulary 2001

Conflict of Interest No conflict of interest