Isolation, Structure Elucidation, and Antitumor Activity of Spirohexenolides A and B

In this report, we describe the discovery of a pair of bioactive spirotetronates, spirohexenolides A (1) and B (2), that arose from the application of mutagenesis, clonal selection techniques, and media optimization to strains of Streptomyces platensis. The structures of spirohexenolides A (1) and B (2) were elucidated through X-ray crystallography and confirmed by 1D and 2D NMR Studies. Under all examined culture conditions, spirohexenolide A (1) was the major metabolite with traces of spirohexenolide B (2) arising in cultures containing increased loads of adsorbent resins. Spirohexenolide A (1) inhibited tumor cell growth with GI(50) values spanning from 0.1 to 17 mu M across the NO 60 cell line panel. An increased activity was observed in leukemia (GI(50) value of 254 nM in RPMI-8226 cells), lung cancer (GI(50) value of 191 nM in HOP-92 cells), and colon cancer (GI(50) value of 565 nM in SW-620 cells) tumor cells. Metabolite 1 was fluorescent and could be examined on it confocal fluorescent microscope with conventional laser excitation and filter sets. Time lapse imaging studies indicated that spirohexenolide A (1) was readily taken up by tumor cells, appearing through the cell immediately after closing and subcellularly localizing in the lysosomes. This activity, combined with a unique selectivity in NCI 60 cancer cell line screening, indicates that 1 warrants further chemotherapeutic evaluation.