INTRODUCTION
Penicillin “allergy” is the most common drug-class allergy identified in electronic medical records of large healthcare systems.1-3 Specifically, penicillin allergy was reported by 12.8% of the entire patient population (n = 1,766,328) of a study by Zhou et al.2 Urticaria (hives) and rash are the reactions most commonly reported by patients presenting for a penicillin allergy evaluation.4,5 In fact, natural penicillins (i.e., penicillin G and penicillin V) and aminopenicillins (i.e., ampicillin and amoxicillin) were the causative drugs counting for 100,943 (37.5%) of 269,493 “rash/dermatitis” cases and for 61,457 (40.8%) of 150,450 “hives/urticaria” instances listed in a US-based electronic-health-record analysis.6
Based on their chronology, hypersensitivity reactions to penicillins are classifiable as immediate or nonimmediate (also called delayed).7,8 The former occurs within 6 hours after the last drug administration, though typically within one hour after the first dose of a new treatment course.7,8 They usually manifest as isolated symptoms, such as urticaria, angioedema, bronchospasm, and hypotension, or as anaphylaxis, and are mostly associated with an IgE-mediated pathogenic mechanism. Nonimmediate reactions occur more than one hour after the initial drug administration, commonly after many days of treatment7-9, are more frequent than immediate reactions6 and are characterized by a wide range of clinical manifestations, including severe ones such as Stevens-Johnson syndrome/toxic epidermal necrolysis, drug reaction (or rash) with eosinophilia and systemic symptoms (DRESS), and acute generalized exanthematous pustulosis (AGEP). Maculopapular exanthema (MPE) and delayed-appearing urticaria are the most common clinical presentations of nonimmediate reactions.7,8 In some nonimmediate reactions, especially DRESS, AGEP, and MPE, a T-cell-mediated pathogenic mechanism has been demonstrated on the basis of positive responses to patch tests and/or delayed-reading intradermal tests.8,10
Despite a high prevalence of nonimmediate reactions to penicillins, little is known about the influence of genetic determinants, in contrast to the pharmacogenetic data published on immediate reactions.11-14 An Italian case-control study by Romano et al. 15 assessed the HLA-A2 and HLA-DRw52 alleles in 24 patients with MPE caused by delayed hypersensitivity to aminopenicillins compared to 522 subjects from the general population and found that they were significantly more prevalent among the former. However, it is noteworthy that no genome-wide association study (GWAS) or HLA genotyping with next-generation sequencing (NGS) has been performed to evaluate the contribution of the genetic determinants.14,15 To address this issue, we evaluated the influence of HLA genetic determinants on the risk of nonimmediate reactions to penicillins by performing HLA NGS genotyping/ NGS-based HLA typing in 24 patients with delayed hypersensitivity and 20 patients with immediate hypersensitivity to penicillins recruited in Italy. Then we performed NGS-based HLA typing for a case-control study to estimate the association between the potentially enriched HLA alleles obtained and the risk of nonimmediate hypersensitivity to penicillins. We subsequently replicated the results using in silico data from the Illumina Immunochip genotyping array that covered the HLA loci in 98 Spanish patients with delayed hypersensitivity compared to 1,308 controls.