Patients With Refractory iMCD May Respond to a Second IL-6 Inhibitor

Idiopathic multicentric Castleman disease (iMCD) is a rare hematologic disorder characterized by lymphadenopathy, distinctive histopathology, and systemic inflammation capable of progressing to multiorgan failure. Siltuximab, an anti-interleukin-6 (IL-6) monoclonal antibody, is the sole FDA-approved first-line therapy for iMCD. Tocilizumab, an IL-6 receptor antagonist approved for iMCD in Japan and utilized off-label elsewhere, has frequently been regarded as clinically interchangeable with siltuximab because of shared pathway inhibition. However, treatment alternatives remain scarce for patients refractory to IL-6 blockade.

An analysis across two independent iMCD patient registries evaluated individuals who received both siltuximab and tocilizumab at distinct time periods with assessable responses to each agent. Among 14 identified patients, 8 were refractory to their first IL-6 inhibitor but subsequently responded to the second inhibitor. The remaining 6 patients were refractory to both agents. Responses achieved with the second agent remained durable over the available follow-up period and were accompanied by significant improvements in inflammatory markers and disease severity scores.

All patients met refractoriness criteria after receiving at least three doses of the initial IL-6 inhibitor, though the authors note that improved responses in a subset might relate to shorter treatment durations or concurrent medications. These observations suggest differing IL-6-related mechanisms among individuals with iMCD and indicate that selected patients failing an initial IL-6 inhibitor should be considered for a trial of an alternative IL-6 inhibitor before abandoning the pathway.