Inhibition of tumor xenograft growth in mice without systemic toxicity

The management of breast cancer involves various therapeutic approaches. However, many current anticancer treatments indiscriminately damage both cancerous and healthy cells. In cancer treatment, Streptomycetes have already been proven as the main resources producing anti-cancer drugs. Streptomyces coelicolor is a representative of the genus Streptomyces. The current study aimed to explore the effect of a yellow pigment (OR3), from a new isolate of Streptomyces coelicolor JUACT03 on metastatic breast cancer.

In this study, the anticancer mechanism was analyzed in MDA-MB-231 cells using MTT, lactate dehydrogenase, caspase, DNA fragmentation, clonogenic, flow cytometry, Western blot, and scratch assays. The effects of OR3 on xenograft mouse models were evaluated by tumor volume measurement, hematological analysis, and histopathological observation. The characterization of OR3 was also performed using gas chromatograohy-mass spectrometry and nuclear magnetic resonance spectroscopy.
OR3 exhibited potent cytotoxicity against MDA-MB-231 cells, with no observed effects on HEK-293 cells. Caspase-9 activation was detected in OR3-treated MDA-MB-231 cells.

Flow cytometry showed a dose-dependent induction of apoptosis and cell cycle arrest at the sub-G₁ and S phases. Furthermore, OR3 completely inhibited MDA-MB-231 cell migration and demonstrated anti-proliferative effects by downregulating the protein expression of KPNA2, XPO1, RAB5B, and p38 MAPK. In in vivo studies, OR3 was non-toxic to mice, inhibited tumor xenograft growth, and maintained normal hematological parameters and tissue architecture. Nuclear magnetic resonance spectroscopy demonstrated the presence of a prodigiosin-like compound, while gas chromatography-mass spectrometry analysis identified additional compounds in OR3.
Our findings validate OR3 pigment as a promising compound for the treatment of metastatic breast cancer, warranting further studies.