**4. Conclusions**

Isoquinoline-quinones from marine invertebrates and associated streptomycetes attracted scientific attention due to their strong anticancer activities [1–6]. From marinederived *Streptomyces* spp., we isolated recently five isoquinoline-quinone derivatives, the mansouramycins A-D and 3-methyl-7-(methylamino)-5,8-isoquinolinedione (**4a**), which showed significant cytotoxicity in a panel of up to 36 tumor cell lines, with pronounced selectivity for non-small cell lung cancer, breast cancer, melanoma, and prostate cancer cells [10]. After a culture optimization, we succeeded now to isolate three further mansouramycins E–G (**1a–3a**) from the same marine *Streptomyces* sp. strain B1848, used optimized culture conditions. Their structures were elucidated by computer-assisted interpretation of 1D and 2D NMR spectra, high resolution mass spectrometry, by comparison with *ab initio*-calculated NMR data and by comparison with related compounds. Cytotoxicity profiling of the mansouramycins in a panel of up to 36 tumor cell lines indicated only a moderate cytotoxicity and tumor selectivity for the new quinones E (**1a**) and F (**2a**). The novel azaphenanthrene-diquinone mansouramycin G (**3a**) was not tested due to insufficient material.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/md19120715/s1, NMR spectra and other supplementary data.

**Author Contributions:** Conceptualization, M.S., K.A.S. and H.L.; methodology, M.S., K.A.S. and G.K.; validation, M.S., K.A.S., G.K., H.H.F. and H.L.; formal analysis, M.S., K.A.S., G.K., H.H.F. and H.L.; investigation, M.S. and K.A.S.; resources, H.L.; data curation, M.S., K.A.S. and H.L.; writing—Original draft preparation, M.S. and K.A.S.; writing—Review and editing, M.S., K.A.S., G.K., H.H.F. and H.L.; visualization, M.S., K.A.S., G.K., H.H.F. and H.L.; supervision, H.L.; project administration, H.L.; funding acquisition, H.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** The financial support of this work was funded by a gran<sup>t</sup> from the Bundesministerium für Bildung und Forschung (BMBF, gran<sup>t</sup> 03F0415A).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Further data are available on request from the corresponding author.

**Acknowledgments:** We thank R. Machinek for the NMR measurements, H. Frauendorf for the mass spectra, and F. Lissy and A. Kohl for technical assistance.

**Conflicts of Interest:** The authors have no competing interest to declare.
