Miriam M. Moser ¹ Christa Firbas ^1* Ulla Derhaschnig ^1* Renate Thalhammer ² Christian Sillaber ^3* Ulrich Jäger ³ Bernd Jilma ^1* Christian Schoergenhofer ¹

• ¹ Department of Clinical Pharmacology, Medical University of Vienna, Vienna, Vienna, Austria
• ² Clinical Institute for Laboratory Medicine, Medical University of Vienna, Vienna, Vienna, Austria
• ³ Clinical Department of Hematology and Hemostaseology, University Clinic for Internal Medicine I, Medical University of Vienna, Vienna, Vienna, Austria

Although rituximab is approved for several autoimmune diseases, no formal dose finding studies have been conducted. The amount of CD20+ cells differs significantly between autoimmune and B-cell malignancies. Hence, dose requirements of anti-CD20 therapies may differ accordingly.We conducted a phase II pilot trial investigating the effects and safety of very low doses of rituximab, i.e. 5mg/m 2 every three weeks, 20mg every four weeks, 50mg every three months and 100mg every three months in patients with autoimmune hemolytic anemia (AIHA) to effectively suppress plasma CD20 + cell counts. The minimum sample size was n=3 per dose group. Doses were increased if circulating CD20 + cell depletion was insufficient (i.e. <95% reduction from baseline) in a dose group. Plasma rituximab concentrations were quantified by enzyme-linked immunosorbent assay, CD20 + cell counts were determined by flow cytometry.Ten patients were included in the final analysis (7 with cold agglutinin disease, 2 with warm AIHA, 1 with mixed-type AIHA). The first infusion depleted >95% of CD20 + cells in all but one of the included patients. However, the dosing regimens were found ineffective, because a sustained CD20 + cell depletion was not achieved, and CD20 + cells recovered with a high interindividual variability. CD20 + lymphocytes were below the detection limit if rituximab plasma concentrations exceeded 0.4µg/mL. Rituximab doses as low as 5mg/m 2 transiently depleted CD20 + cells in almost all patients, but the tested low-dose regimens failed to permanently suppress CD20 + cells. The empirically identified EC95% of 0.4µg/mL rituximab may guide future studies using low-doses of rituximab.