We hope that this test-case may shed light to future developments of this sideline approach in figuring out adverse events of biological therapies. The matrix metalloproteinases are a family of 23 zincdependent endopeptidases with important CE3F4 functions in tissue morphogenesis, wound healing, and other physiological processes that require remodeling of the extracellular matrix. MMP activity is regulated in vivo by a family of four endogenous protein protease inhibitors, the tissue CNQX inhibitors of metalloproteinases, that bind to MMPs in 1:1 stoichiometry and block the protease active site. Disruption of the balance between MMPs and TIMPs is evidenced under many pathological conditions, and excess MMP activity has long been recognized for important contributions to the development and progression of many diseases including cardiovascular, vascular, and pulmonary diseases, arthritis, multiple sclerosis, and cancer. Diverse roles in disease development and progression have led MMPs to be regarded as promising therapeutic targets, resulting in development of many small-molecule MMP inhibitors, but clinical trials of early-generation MMP inhibitors in cancer and arthritis proved disappointing. Broad-spectrum MMP inhibitors produced serious dose-limiting musculoskeletal toxicity, failed to reach therapeutic plasma levels, and failed to extend progression-free survival in cancer trials ; these disappointing outcomes have been attributed both to the toxicity and off-target effects of the drugs and to inadequate specificity for target MMPs. A less toxic alternative to synthetic MMP inhibitors might be offered by TIMPs. Studies using many preclinical cancer models have shown that overexpression of natural TIMPs in tumors often leads to reduced tumor growth and metastasis. Systemic gene transfer of TIMPs in animal models of cancer has likewise produced antitumor effects, with minimal toxicity. In a handful of studies investigating the suppressive effect of TIMP-1 on tumor cell proliferation and metastasis, mice have been treated with recombinant human TIMP-1 protein at doses of 2�C50 mg/kg with no reported toxicity. Recombinant human TIMPs -1 and -2 have also been investigated as inhibitors of airway inflammation in a murine model of asthma, via intranasal instillation, with promising results. For many applications, one barrier that will likely need to be addressed for TIMPs to enter the clinic as recombinant therapeutics is the short half-life in circulation of these small proteins. Persistence in the circulation is desirable because protein therapeutics generally cannot be administered orally and typically are administered by subcutaneous, intramuscular, or intravenous injection or infusion. Animal studies using recombinant TIMPs have thus far been limited in part by rapid clearance of the protein; the plasma clearance of murine TIMP-1 in rats was reported to occur within minutes, and the blood elimination half-life of human TIMP-1 in mice was reported to be,4 hours.