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Purpose SBI-087 is a Small Modular Immunopharmaceutical Protein™(SMIP™) drug that binds to CD20 and has been reported to deplete B cells in murine/primate studies. The safety, tolerability and pharmacokinetic/pharmacodynamic properties of SBI-087 were evaluated in patients with rheumatoid arthritis (RA) or systemic lupus erythematosus (SLE). Methods Single-dose SBI-087 was evaluated in 2 Phase I, open-label, escalating-dose studies in patients with RA or SLE. The studies included 6 IV/4 SC escalating doses (RA) and 1 IV/4 SC escalating doses (SLE). Escalation was determined by tolerability/rate of B-cell depletion. Serum was collected for analyses of pharmacokinetic and pharmacodynamic (CD19 + B cells) properties and immunogenicity. Patients were followed until B-cell counts were normalized or stabilized. Safety, tolerability was evaluated from adverse events, physical examinations, vital sign measurements, ECG, and clinical laboratory results. Findings Sixty patients with RA (IV, 28; SC, 32) and 30 patients with SLE (6 per cohort) were enrolled. Mild to moderate infusion reactions occurred in several patients at the top doses in the RA study despite a pretreatment regimen of IV doses. Unanticipated reactions after SC administration of SBI-087 included fever, chills, and malaise, seen on the day of dosing in the lowest-dose cohorts in both studies. These events were abrogated in subsequent cohorts by a pre/postdose treatment regimen consisting of oral corticosteroids, acetaminophen, and an antihistamine. SBI-087 clearance (IV) ranged from 22 to 229 mL/h; volume of distribution at steady state ranged from 5 to 12 L. Apparent clearance (SC) ranged from 44.7 to 105 mL/h; volume of distribution ranged from 14.3 to 32.1 L. Overall, PK properties were similar at equivalent doses between IV/SC administrations in patients with RA/SLE. Mean t ½ (IV) ranged from 2.1 to 10.7 days (less at lower doses). SBI-087 concentration and B-cell depletion were generally dose proportional across IV and SC cohorts. However, the extent of B-cell depletion was less, and rate of repletion was faster, in patients with SLE versus RA. In both studies, B-cell repletion to baseline did not occur in the majority of patients by the end of the observation period. Overall, the prevalence and type of adverse events were similar to those seen with other anti-CD20–depleting agents. Implications In patients with mild RA/SLE, SBI-087 was well tolerated when administered intravenously or subcutaneously with pre- and posttreatment regimens. B-cell depletion is long lasting, and the duration and extent of depletion may be greater in RA compared with SLE. SBI-087 exhibited slow elimination and low distribution in both populations. Clinicaltrials.gov identifiers: NCT00641225 (RA) and NCT00714116 (SLE). [ABSTRACT FROM AUTHOR]