3055 Background: rIL-21 is a pleiotropic class I cytokine that activates CD8+ T cells and NK cells. The safety and pharmacologic profile of rIL-21 was characterized in two phase 1 dose escalation studies including patients with MM, performed in Australia and the US, respectively. Two dosing schedules were tested: “5+9” (5 days of dosing followed by 9 days of rest) and “3/wk” (dosing 3 times per week for 6 weeks). The maximum tolerated dose (MTD) was determined to be 30 μg/kg for both dosing regimens and two complete remissions (at the 30 μg/kg dose level) were observed in the MM patients. A phase 2a study was initiated to estimate the preliminary efficacy of rIL-21 in patients with advanced MM. Methods: The phase 2a study design is an open-label, two-stage trial. Primary objective: antitumor efficacy as determined by response rate. Secondary objectives: safety, effects on blood biomarkers, and measurement of anti-rIL-21 antibodies. Eligible patients had unresectable MM with measurable disease, no prior systemic therapy (adjuvant interferon was permitted), adequate major organ function, good performance status, no brain metastases, and no evidence of significant autoimmune disease. rIL-21 was administered by i.v. bolus injection using the “5+9” regimen for 6 weeks (= three cycles) at 30 μg/kg dose level. Results: At the time of writing (January 2007), all 14 patients have entered the first stage of the phase 2a study and currently seven patients are evaluable for response after completion of 3 treatment cycles (6 weeks). One patient had a complete remission, five patients had stable disease, and one patient had progressive disease. So far, six patients have gone on to receive further treatment with rIL-21. Similar to the phase 1 experience, treatment with rIL-21demonstrated an acceptable safety profile. Updated interim study results, including response data, will be presented. Conclusions: rIL-21 administered at 30 μg/kg/day using the “5+9” regimen is well tolerated by patients with MM. Preliminary evidence of clinical response has been observed and the second stage of the two-stage phase 2a study has opened for accrual. [Table: see text]