Identifying and targeting pathogenic PI3K/AKT/mTOR signaling in IL-6 blockade-refractory idiopathic multicentric Castleman disease

BACKGROUND. Idiopathic multicentric Castleman disease (iMCD) is a hematologic illness involving cytokine-induced lymphoproliferation, systemic inflammation, cytopenias, and life-threatening multi-organ dysfunction. The molecular underpinnings of interleukin-6 (IL-6) blockade-refractory patients remain unknown; no targeted therapies exist. In this study, we searched for therapeutic targets in IL-6 blockade-refractory iMCD patients with the thrombocytopenia, anasarca, fever/elevated C-reactive protein, reticulin myelofibrosis, renal dysfunction, organomegaly (TAFRO) clinical subtype. METHODS. We analyzed tissues and blood samples from 3 IL-6 blockade-refractory iMCD-TAFRO patients. Cytokine panels, quantitative serum proteomics, flow cytometry of PBMCs, and pathway analyses were employed to identify novel therapeutic targets. To confirm elevated mTOR signaling, a candidate therapeutic target from the above assays, immunohistochemistry was performed for phosphorylated S6, a read-out of mTOR activation, in 3 iMCD lymph node tissue samples and controls. Proteomic, immunophenotypic, and clinical response assessments were performed to quantify the effects of administration of the mTOR inhibitor sirolimus. RESULTS. Studies of 3 IL-6 blockade-refractory iMCD cases revealed increased CD8(+) T cell activation, VEGF-A, and PI3K/Akt/mTOR pathway activity. Administration of sirolimus substantially attenuated CD8(+) T cell activation and decreased VEGF-A levels. Sirolimus induced clinical benefit responses in all 3 patients with durable and ongoing remissions of 66, 19, and 19 months. CONCLUSION. This precision medicine approach identifies PI3K/Akt/mTOR signaling as the first pharmacologically targetable pathogenic process in IL-6 blockade-refractory iMCD. Prospective evaluation of sirolimus in treatment-refractory iMCD is planned (NCT03933904).