Irreversible myeloperoxidase inhibitor mitiperstat and mitiperstat metabolites display immunogenicity in humans

Myeloperoxidase (MPO) dysregulation contributes to cardiovascular disease, making MPO a promising therapeutic target. Mitiperstat, an irreversible 2-thioxanthine MPO inhibitor in clinical development, has been linked to delayed-onset maculopapular rash. This study investigated whether mitiperstat and its metabolites (M7, M9, M10, M12) activate human T-cells in vitro. De novo T-cell responses from healthy donors were assessed using a dendritic cell:T-cell co-culture assay, and T-cell clones (TCCs) were characterised for compound specificity, phenotype, function and cross-reactivity. Proliferation was measured by [3H]-thymidine incorporation, and cytokine/cytolytic molecule release (IFN-γ, IL-5, IL-13, granzyme-B) by ELISpot. Twenty-nine predominantly CD4+ TCCs were specific to mitiperstat, while forty-seven CD4+ and CD8+ TCCs responded to its metabolites. All drug-specific TCCs proliferated dose-dependently and secreted a heterogeneous Th1/Th2 cytokine and cytolytic profile. CD4+ and CD8+ TCC activation was attenuated by MHC-class II and MHC-class I blockade, respectively. Despite the irreversible on-target pharmacology of mitiperstat, nearly all responsive TCCs were activated via direct pharmacological interactions with proliferation occurring with drug or respective metabolite in the presence of glutaraldehyde-fixed antigen-presenting cells. In contrast, one TCC was activated with drug-pulsed antigen presenting cells. TCCs showed high specificity for their cognate compounds with no cross-reactivity. These findings demonstrate the intrinsic immunogenicity of mitiperstat and its metabolites and establish a framework for assessing and comparing T-cell reactivity of future MPO-targeting therapies.