Hydrogen peroxide degradation and selective carbidopa-induced cytotoxicity against human tumor lines

The carcinoid tumor, an uncommon neuroendocrine neoplasm, is associated with serotonin overproduction as is more common small cell lung carcinoma (SCLC). alpha-Methyl-dopahydrazine (carbidopa), an inhibitor of the serotonin synthetic enzyme aromatic-L-amino acid decarboxylase, proved lethal to NCI-H727 lung carcinoid cells as well as NCI-H146 and NCI-H209 SCLC cells, but not to five other human tumor cell lines of differing origins [Gilbert JA, Frederick LM, Ames MM. The aromatic-L-amino acid decarboxylase inhibitor carbidopa is selectively cytotoxic to human pulmonary carcinoid and small cell lung carcinoma cells. Clin. Cancer Res. 2000;6:4365-72]. The mechanism of carbidopa cytotoxicity remained an unanswered question. We present data here that incubation of the catechol carbidopa (100 microM) in RPMI and DMEM culture media yielded molar equivalents of hydrogen peroxide (H2O2) within 2-4 h. Alkaline elution studies revealed carbidopa-dependent single-strand DNA breaks in sensitive carcinoid cells comparable to those induced by similar concentrations of H2O2. Neither compound induced significant DNA damage in carbidopa-resistant NCI-H460 large cell lung carcinoma cells. Furthermore, when carbidopa was incubated with a variety of tumor cell types, not only were decreased media H2O2 concentrations detected in the presence of cells, but cell lines least sensitive to carbidopa degraded exogenous H2O2 more rapidly than did sensitive cells. Implicated in these studies, pyruvate degraded H2O2 in RPMI in a dose- and time-dependent manner and reversed carbidopa-induced cytotoxicity to carcinoid cells. Extracellular pyruvate levels produced per h by resistant large cell lung carcinoma cells averaged four-fold that of sensitive carcinoid cells plated at equal density (24 h time course). Finally, carbidopa exposure (100 microM, 24 h) depleted extracellular pyruvate from sensitive carcinoid cells, but reduced pyruvate levels from resistant NCI-H460 cells less than 17%.