Systemic lupus erythematosus (SLE) is characterized by the production of pathogenic autoantibodies to nucleoprotein antigens, including double-stranded DNA (dsDNA). The deposition of IgG dsDNA immune complexes in glomeruli is thought to be crucial for disease pathogenesis and complement activation. rhDNase catalyzes the hydrolysis of extracellular DNA and has been shown to delay the development of dsDNA antibodies, reduce proteinuria, and delay mortality in a lupus-prone murine model. We conducted a 40 d, phase Ib, randomized, double-masked, placebo-controlled trial to determine the safety and pharmacokinetics of rhDNase, and to measure any changes in markers of disease activity in 17 patients with lupus nephritis. Patients were assigned to receive either: (1) 25 mg/kg rhDNase (n=8); (2) 125 mg/kg rhDNase (n=6); or (3) placebo (n=3) initial single intravenous (IV) dose followed by 10 subcutaneous (SC) doses. Skin biopsies performed on nine patients pre-and post-treatment were studied for immune complex deposition by immunofiuorescence. Serum cytokine levels (sIL2-R, IL-6, IL-10, and TNF-a) were analyzed by ELISA. Cytokine secretion and antibody production were measured by ELISPOT analysis and ELISA. Serum hydrolytic activity of rhDNase was achieved after IV administration at 25 and 125 mg/kg, but not after SC administration at either dose. A t¹₂ of 3–4 h was estimated from serum concentration profiles following IV administration. Serum dsDNA antibodies were unchanged (mean values: 33 IU/mL vs 39 IU/mL [pre-and post-treatment] for the 25 mg/kg group, and 74 IU/mL vs 74 IU/mL for the 125 mg/kg group, and 14 IU/mL vs 20 IU/mL for the placebo group). Complement levels (C3 and C4) and circulating immune complexes did not change appreciably during the treatment period for any of the groups. Serum cytokine profiles by ELISA revealed no changes in sIL-2 receptor, IL-6, IL-10, or TNF-a. There was no change in the number of cells secreting either Th1 or Th2 specific cytokines, nor in the number of cells secreting dsDNA antibodies. Neutralizing antibodies to rhDNase were not detected in serum at any time during the study. Immune complex deposition was unchanged in pre-and post-treatment in skin biopsies in both dose groups. rhDnase was well tolerated without significant adverse events following administration, and treatment was not associated with the development of neutralizing antibodies to rhDNase. Serum rhDNase concentrations capable of hydrolytic activity of rhDNase were achieved for a few hours following IV, but not SC administration. Serum markers of disease activity were unchanged during the study period.