OBJECTIVES: I. Determine whether marimastat prolongs progression-free survival in women with metastatic breast cancer who have responding or stable disease after receiving standard systemic chemotherapy. II. Determine the toxic effects of marimastat compared with placebo in patients with metastatic breast cancer who have responding or stable disease after receiving standard systemic chemotherapy. III. Determine whether there is an association between trough marimastat concentration and time to disease progression and toxicity.