JAK inhibition for murine HLH requires complete blockade of IFN-γ signaling and is limited by toxicity of JAK2 inhibition

Hemophagocytic lymphohistiocytosis (HLH) is an inflammatory disorder in which numerous cytokines are elevated, though interferon-γ (IFN-γ) is central to disease pathogenesis and a key therapeutic target. Experimental and early clinical reports have shown that ruxolitinib, a small molecule inhibitor of Janus kinases (JAKs), which are essential for cytokine signaling, may be therapeutic in HLH. In contrast, we found that intermittently administered ruxolitinib at various dose levels failed to prevent HLH development or treat established murine HLH. High doses of ruxolitinib blocked IFN-γ signaling only transiently after administration, consistent with human pharmacokinetics, and only continuously administered drug could prevent HLH development or treat established HLH. Continuously administered ruxolitinib was therapeutic in only a narrow dose range and intermittently dosed ruxolitinib worsened survival and decreased bone marrow cellularity of animals concurrently treated with anti-IFN-γ antibody, indicating a narrow therapeutic window and potential toxicity. Because JAK2 is essential for hematopoietic cytokine signaling, we also tested a JAK1-selective inhibitor and observed therapeutic benefit without apparent toxicity, though it did not improve survival when combined with anti-IFN-γ. We conclude that continuous blockade of IFN-γ signaling is necessary for optimal control of HLH and that JAK2 inhibition may be toxic in this disorder.