A new chlorinated metabolite designated chlokamycin (1), was isolated along with ikarugamycin (2) from the culture broth of the marine-derived Streptomyces sp. MA2-12. The structure of 1 was elucidated based on spectroscopic analyses (1D and 2D NMR data and ROESY correlations). Chlokamycin moderately inhibited the growth of Jurkat cells and HCT116 cells with IC 50 values of 24.7 and 33.5 M, respectively.

A new cytotoxic agent designated isomethoxyneihumicin (1 and 2), a mixture of lactam-lactim tautomers, was isolated along with methoxyneihumicin (3) from the culture broth of the marine Nocardiopsis alba KM6-1. The structures of 1 and 2 were elucidated in spectroscopic analyses (1D and 2D NMR data, and ROESY correlations). Isomethoxyneihumicin (15.0 μM) and 3 (15.0 μM) arrested the cell cycle of Jurkat cells at the G2/M phase (66 and 67%) in 12 h. Isomethoxyneihumicin and 3 exhibited cytotoxicity against Jurkat cells with IC 50 values of 6.98 and 30.5 μM in 20 h, respectively. These results strongly suggest that isomethoxyneihumicin and 3 exhibit cytotoxicity against Jurkat cells by inhibiting the cell cycle at the G2/M phase.