Drugs that induce systemic lupus erythematosus inhibit complement component C4.

The syndrome resembling systemic lupus erythematosus (SLE) associated with long-term treatment with hydralazine and isoniazid seems to be due to the drugs themselves rather than their metabolites. This syndrome is associated with deposition of immune complexes; and in the complement system inhibition of C4 is likely to increase deposition of immune complexes. In vitro, hydralazine and isoniazid inhibited 50% of the binding of C4 at 840 mumol/l and 1.05 mmol/l, respectively--ie, within the concentration ranges that have been used in therapy. Acetylated metabolites were not inhibitory at the maximum concentrations tested; and iproniazid, which does not cause SLE, gave 50% inhibition only at a concentration far exceeding that used in therapy.