Background: Rituximab induced acute thrombocytopenia (RIAT) is a rare side effect characterised by severe thrombocytopenia within hours of first episode drug exposure. The mechanism remains unclear. We hypothesise that RIAT is immune-mediated secondary to drug-dependent auto-anti-platelet antibodies.

Aims: To clarify the mechanism of rituximab induced acute thrombocytopenia.

Methods: Serum was analysed from three patients with severe first dose rituximab infusion reaction who developed thrombocytopenia within 24 hours post infusion. Serum samples taken post rituximab exposure or control serum(1:25) were incubated with washed donor platelets in the absence and in the presence of 600µg/ml of rituximab and analysed for anti-platelet antibody binding by flow cytometry using anti-human IgG-AF647. Control serum was pre-exposure serum when available or normal serum. Patients provided written consent and the study was approved by the local ethics committee.

Results: All patients had marginal zone lymphoma with bone marrow infiltration and splenomegaly. Thrombocytopenia nadir ranged from 5-60 x 10⁹/L. Rituximab-dependent antibody binding on platelets was not seen with control serum. In the post-rituximab exposure serum samples, antibody binding was detected only in the presence of added rituximab.

Conclusions: Rituximab-dependent anti-platelet antibodies were identified in three patients with acute thrombocytopenia following first dose of rituximab. The rapidity of onset suggests peripheral destruction and the testing is consistent with immune mediated platelet clearance due to drug-dependent recognition of platelet antigen. To our knowledge, this is the first demonstration of rituximab dependent anti-platelet anti-bodies and suggests that the cause of the rapid thrombocytopenia in these cases was immune mediated.

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ISTH Congress Abstracts - https://abstracts.isth.org/abstract/first-report-of-rituximab-dependent-anti-platelet-antibodies-as-cause-of-acute-thrombocytopenia/