Metabolic Activation and Cytotoxicity of 4-ipomeanol in Human Non-Small Cell Lung Cancer Lines

In the normal lungs of many animal species, 4-ipomeanol is transformed to a highly reactive metabolite preferentially in pulmonary bronchiolar Clara cells and to a lesser extent in alveolar type II cells, potentially leading to damage or destruction of these cell types. Since Clara cells and type II cells are suspected sites of origin of certain “non-small cell” lung cancers, the metabolic activation of 4-ipomeanol (measured by the metabolism-dependent covalent binding of 4-ipomeanol to cellular macromolecules) was compared in two human non-small cell carcinoma derived cell lines (NCI-H322 and NCI-H358) and two human small cell carcinoma derived cell lines (NCI-H128 and NCI-H69). Metabolic activation of 4-ipomeanol was evident in the non-small cell lines; the production of covalently bound metabolite was somewhat greater in NCI-H322 (morphology related to Clara cells) compared to NCI-H358 (morphology related to alveolar type II cells), but was entirely undetectable in the small cell lines. The activation pathway was concentration (4-ipomeanol) and time dependent and followed Michaelis-Menten kinetics. Metabolism to the reactive intermediate required oxygen and was strongly inhibited by carbon monoxide. Covalent binding was enhanced in the non-small cell lines by prior incubation with 0-naphthoflavone and by supplementation of the incubate with exogenous reduced nicotinamide adenine dinucleotide phosphate. 4-Ipomeanol was more cytotoxic to the non-small cell lines than to the small cell lines under the in vitro growth conditions used. These studies indicate that certain human non-small cell lung cancers have metabolic characteristics of normal bronchiolar Clara cells and alveolar type II cells; these results would therefore be consistent with an origin of these tumors from Clara cells or type II cells, respectively. The present studies indicate that the further preclinical testing and development of 4-ipomeanol is warranted, with a view toward possible clinical evaluation against human lung cancers.