*Article* **Mansouramycins E–G, Cytotoxic Isoquinolinequinones from Marine Streptomycetes**

**Mohamed Shaaban 1,2,†, Khaled A. Shaaban 1,†,‡, Gerhard Kelter 3, Heinz Herbert Fiebig 4 and Hartmut Laatsch 1,\***


**Abstract:** Chemical investigation of the ethyl acetate extract from the marine-derived *Streptomyces* sp. isolate B1848 resulted in three new isoquinolinequinone derivatives, the mansouramycins E–G (**1a**–**3a**), in addition to the previously reported mansouramycins A (**5**) and D (**6**). Their structures were elucidated by computer-assisted interpretation of 1D and 2D NMR spectra, high-resolution mass spectrometry, and by comparison with related compounds. Cytotoxicity profiling of the mansouramycins in a panel of up to 36 tumor cell lines indicated a significant cytotoxicity and good tumor selectivity for mansouramycin F (**2a**), while the activity profile of E (**1a**) was less attractive.

**Keywords:** mansouramycins; isoquinolinequinones; marine-derived *Streptomyces* sp.; cytotoxicity
