Amamistatins A and B are natural products that were isolated from the actinomycete Nocardia asteroides after their identification during a screen for growth inhibitors of human tumor cells. Amamistatin A was found to have anti-proliferative effects against MCF-7 breast, A549 lung, and MKN45 stomach tumor cell lines (IC50 0.48, 0.56, 0.24 ?M, respectively). Both compounds were also cytotoxic to P388 mouse lymphocytic leukemia cells (IC50 15 and 16 nM). Amamistatins share important structural features with iron-binding mycobacterial siderophores (mycobactins and carboxymycobactins) and retrohydroxamate HDAC inhibitors. Based on this structural similarity, two possible mechanisms of tumor cell inhibition were proposed, both involving metal binding.
In order to probe the biological activity of the amamistatin family, synthetic compounds with specific structural variations were needed. During the course of this work, the total synthesis of amamistatin B was completed, along with the syntheses of a diastereomer of amamistatin B and a structural variant lacking two metal-binding functionalities. All three compounds, along with synthetic intermediates, were screened in several assays to study their effect on the growth of tumor cells and M. tuberculosis, as well as their ability to inhibit HDAC.
Amamisatin B strongly inhibited the growth of M. tuberculosis H37Rv at high concentrations, but acted as a growth promoter of M. smegmatis and other bacteria. Synthetic analogs and intermediates were largely ineffective at inhibiting mycobacterial growth. Smaller amamistatin precursors displayed varying levels of inhibition against both the HDAC enzyme and two human tumor cell lines (MCF-7 and PC-3), but HDAC activity was not retained by the larger amamistatin compounds. Amamistatin B was, however, an extremely effective growth inhibitor of MCF-7 breast cancer cells, with an IC50 of 120 nM. The lower level of activity displayed by the synthetic analog with reduced metal-binding ability supports the hypothesis that the anti-tumor activity of amamistatin B is related to iron chelation.
