Antifungal actinomycete metabolites discovered in a differential cell-based screening using a recombinant TOPO1 deletion mutant strain.

In the course of a natural product screening for inhibitors of fungal topoisomerase 1 (TOPO 1), extracts from the actinomycete strains WS 1410 and BS 1465 exhibited promising activities. Bioguided fractionation of the culture broth by preparative HPLC methods yielded the collismycins A (1) and B (2) as active principles of strain WS 1410. Out of the mycelial extracts of strain BS 1465 the bioactive new natural products, cyclo-homononactic acid (3) and cyclo-nonactic acid (5) and the structurally related but inactive homononactic acid (4), were isolated. Both collismycin isomers inhibited the recombinant yeast strains ScAL 141 and ScAL 143 (TOPO 1 deletion mutant) in a non-specific manner with an MIC in the range of 2 micrograms/ml. The novel cyclo-homononactic acid (3) and cyclo-nonactic acid (5) showed higher selectivity towards the wild type strain (MIC = 2 micrograms/ml as compared to 10 micrograms/ml for the deletion mutant). All compounds obviously address a target other than TOPO 1 since they do not exhibit activities in a concurrent TOPO 1 enzyme assay.