Halogenation‐Guided Chemical Screening Provides Insight into Tjipanazole Biosynthesis by the Cyanobacterium Fischerella ambigua

Halogenated natural products (HNPs) show a wide range of interesting biological activities. Chemistry‐guided screening with a software tool dedicated to identifying halogenated compounds in HPLC‐MS data indicated the presence of several uncharacterised HNPs in an extract of the cyanobacterium Fischerella ambigua (Näg.) Gomont 108b. Three new natural products, tjipanazoles K, L, and M, were isolated from this strain together with the known tjipanazoles D and I. Taking into account the structures of all tjipanazole derivatives detected in this strain, reanalysis of the tjipanazole biosynthetic gene cluster allowed us to propose a biosynthetic pathway for the tjipanazoles. As the isolated tjipanazoles show structural similarity to arcyriaflavin A, an inhibitor of the clinically relevant multidrug‐transporter ABCG2 overexpressed by different cancer cell lines, the isolated compounds were tested for ABCG2 inhibitory activity. Only tjipanazole K showed appreciable transporter inhibition, whereas the compounds lacking the pyrrolo[3,4‐c] ring or featuring additional chloro substituents were found to be much less active. Seeking biosynthesis intermediates: The software tool HaloSeeker enabled us to detect halogenated tjipanazole derivatives in the complex HRMS data of an extract from the cyanobacterium F. ambigua. In conjunction with genomic data, this allowed us to propose a biosynthetic pathway for the tjipanazoles. Five isolated derivatives were tested for inhibition of the multidrug resistance transporter ABCG2, revealing structure‐activity relationships.