Approximately three million Canadians carry the label of ‘ß-lactam allergy’. Patients so labeled are prescribed less effective, or potentially more toxic, or costly alternative drugs leading to increased health care costs and an increased risk for both morbidity and mortality. Incorrect labeling of patients also contributes to antimicrobial resistance. More pointedly, several studies have suggested that 9 out of every 10 patients labelled as allergic are not and can in fact tolerate ß-lactam antibiotics. While there are well established protocols for immediate hypersensitivity, approximately 50% of patients cannot be de-labeled using these protocols. We have demonstrated that in vitro assays can be used to successfully identity those at risk and those who can in fact tolerate ß-lactam antibiotics and have demonstrated that these assays can be done on patients from hospitals across Ontario, having tested patients from Windsor, London, Hamilton, Toronto and Ottawa.

The objective of this project was to explore the use of in vitro assays to determine which patients are at risk for serious adverse reactions to beta-lactam antibiotics. We undertook this work by recruiting patients with histories of serious adverse events associated with beta-lactam therapy and obtaining blood samples from them as well as from healthy control volunteers to conduct in vitro toxicity studies. We isolated white blood cells and platelets and then incubated them with amoxicillin or cephalexin over a concentration range of 0 to 250 µM as well as with a microsomal activating system to simulate in vivo drug metabolism. After overnight incubation cell viability was determined using a tetrazolium-based viability assay read spectrophotometrically. We demonstrated that the cells of patients with serious delayed adverse drug events such as DRESS and Stevens-Johnson Syndrome had a significantly decreased viability compared to the cells of controls (p<0.05). We also demonstrated that these assays could be conducted remotely, as we studied patients from a number of centres across Ontario. This work has provided insights into the role of reactive drug metabolites in the pathogenesis of delayed drug hypersensitivity as well as providing potential approaches for the diagnosis and potentially de-labeling of patients with serious delayed adverse events associated with beta-lactam antibiotic therapy. While this is a promising direction moving forward, the lymphocyte toxicity assay that we used was technically challenging and in its current form required a considerable amount of technician time, presenting cost challenges. Moving forward we used this data to obtain additional external grant funding to continue exploring the potential use of in vitro approaches for de-labeling of patients. These studies include the use of platelets as in vitro models of drug hypersensitivity and studies to correlate in vitro findings with pharmacogenomic testing.

Wong T, Atkison A, Rieder MJ, Chen E t’Jong G, Abrams E. Beta lactam allergy in the paediatric population. Paeds Child Health. 2020;25:62-63.
Elzagallaai AA, Rieder MJ. Model based evaluation of hypersensitivity adverse drug reactions for antimicrobial agents in children. Front Pharmacol 2021 Apr 30;12:638881.

Elzagallaai AA, Rieder MJ. Genetic markers of drug hypersensitivity in pediatrics: current state and promise. Expert Rev Clin Pharmacol 2022;15:715-728.
Abuzgaia AM, Elzagallaai AA, Mullowney T, Rieder MJ. Drug Hypersensitivity Reactions in Patients with Cystic Fibrosis: Potential Value of the Lymphocyte Toxicity Assay to Assess Risk. Mol Diagn Ther 2023;27(3):395-403.

Elzagallaai AA, Rieder MJ. Pathogenesis of Drug Hypersensitivity. Exp Opin Clin Pharmacol (in-press).