Antitumor activity of sequential treatment with topotecan and anti-epidermal growth factor receptor monoclonal antibody C225

Epidermal growth factor (EGF)-related proteins such as transforming growth factor alpha (TGF-alpha) control cancer cell growth through autocrine and p aracrine pathways. Overexpression of TGF-alpha and/or its receptor (EGFR) h as been associated with a more aggressive disease and a poor prognosis. The blockade of EGFR activation has been proposed as a target for anticancer t herapy. Monoclonal antibody (MAb) C225 is an anti-EGFR humanized chimeric m ouse MAb that is presently in Phase II clinical trials in cancer patients. Previous studies have suggested the potentiation of the antitumor activity of certain cytotoxic drugs, such as cisplatin and doxorubicin, in human can cer cell lines by treatment with anti-EGFR antibodies. We have evaluated in human ovarian, breast, and colon cancer cell lines, which express function al EGFR, the antiproliferative activity of MAb C225 in combination with top otecan, a cytotoxic drug that specifically inhibits topoisomerase I and tha t has shown antitumor activity in these malignancies. A dose-dependent supr aadditive increase of growth inhibition in vitro was observed when cancer c ells were treated with topotecan and MAb C225 in a sequential schedule. In this respect, the cooperativity quotient, defined as the ratio between the actual growth inhibition obtained by treatment with topotecan followed by M Ab C225 and the sum of the growth inhibition achieved by each agent, ranged from 1.2 to 3, depending on drug concentration and cancer cell line. Treat ment with MAb C225 also markedly enhanced apoptotic cell death induced by t opotecan. For example, in GEO colon cancer cells, 5 nM topotecan, followed by 0.5 mu g/ml MAb C225, induced apoptosis in 45% cells as compared with un treated cells (6%) or to 5 nM topotecan-treated cells (22%). Treatment of m ice bearing established human GEO colon cancer xenografts with topotecan or with MAb C225 determined a transient inhibition of tumor growth because GE O tumors resumed the growth rate of untreated tumors at the end of the trea tment period. In contrast, an almost complete tumor regression was observed in all mice treated with the two agents in combination. This determined a prolonged life span of the mice that was significantly different as compare d with controls (P < 0.001), to MAb C225-treated group (P < 0.001), or to t he topotecan-treated group (P < 0.001), All mice of the topotecan plus MAb C225 group were the only animals alive 14 weeks after tumor cell injection. Furthermore, 20% of mice in this group were still alive after 19 weeks. Th e combined treatment with MAb C225 and topotecan was well tolerated by mice with no signs of acute or delayed toxicity. These results provide a ration ale for the evaluation of the anticancer activity of the combination of top oisomerase I inhibitors and anti-EGFR blocking MAbs in clinical trials.