iso-Migrastatin and related glutarimide-containing polyketides are potent inhibitors of tumor cell migration and their implied potential as antimetastatic agents for human cancers has garnered significant attention. Genome scanning of Streptomyces platensis NRRL 18993 unveiled two candidate gene clusters (088D and mgs); each encodes acyltransferase-less type I polyketide synthases commensurate with iso-migrastatin biosynthesis. Both clusters were inactivated by -RED-mediated PCR-targeting mutagenesis in S. platensis; iso-migrastatin production was completely abolished in the ⌬mgsF mutant SB11012 strain, whereas inactivation of 088D-orf7 yielded the SB11006 strain that exhibited no discernible change in iso-migrastatin biosynthesis. These data indicate that iso-migrastatin production is governed by the mgs cluster. Systematic gene inactivation allowed determination of the precise boundaries of the mgs cluster and the essentiality of the genes within the mgs cluster in iso-migrastatin production. The mgs cluster consists of 11 open reading frames that encode three acyltransferase-less type I polyketide synthases (MgsEFG), one discrete acyltransferase (MgsH), a type II thioesterase (MgsB), three post-PKS tailoring enzymes (MgsIJK), two glutarimide biosynthesis enzymes (MgsCD), and one regulatory protein (MgsA). A model for isomigrastatin biosynthesis is proposed based on functional assignments derived from bioinformatics and is further supported by the results of in vivo gene inactivation experiments.Cell migration is essential for invasion of the extracellular matrix and for cell dissemination during tumor metastasis (1). The glutarimide-containing polyketides iso-migrastatin (iso-MGS), 2 migrastatin (MGS), the dorrigocins (DGNs), and lactimidomycin (LTM) (Fig. 1) are potent inhibitors of human tumor cell migration and thus represent novel leads for anticancer drug discovery (2-5). Synthetic analogs of these natural products have also been investigated and found to retain potent activity despite significant structural truncation (6 -8). Retention of activity by such analogs supports the effectiveness of the privileged scaffolds highlighted by iso-MGS, MGS, LTM, and related natural products. Complementary to organic synthesis, combinatorial biosynthesis offers an alternative means of accessing natural product structural diversity. We have previously studied the biosynthetic pathway of these polyketides and shown that MGS and the DGNs are shunt metabolites of iso-MGS (9). As has been demonstrated in Streptomyces platensis NRRL 18993, one of the known iso-MGS producers, iso-MGS, MGS, and the DGNs are produced by a single biosynthetic machinery and iso-MGS undergoes H 2 O-mediated, non-enzymatic ring-expansion, and ring-opening rearrangements to afford MGS and the DGNs as shunt metabolites. We have also isolated iso-MGS congeners as minor fermentation products, produced a small library of glutarimide-containing polyketides featuring the iso-MGS, LTM, MGS, and DGN scaffolds, and found selected analogs with biological a...

Iso-Migrastatin (10) has been shown to be the main natural product of Streptomyces platensis, which undergoes a facile, H2O-mediated rearrangement into dorrigocin A (2), 13-epi-dorrigocin A (11), dorrigocin B (3), and migrastatin (1). Eight new congeners (12-19) of 10 were characterized. They can undergo the same H2O-mediated rearrangement into the corresponding 1, 2, 3, and 11 analogues (20-43) or 1,4-Michael addition with cysteine to afford the corresponding analogues (44-51) of NK30424 A and B (5, 6). This study generated a 47-member library of glutarimide polyketides, setting the stage to investigate the SAR for this family of natural products. These results also established the absolute stereochemistry of 5 and 6 and shed new light into the post-polyketide synthase steps for 10 biosynthesis.

Fermentation of Streptomyces platensis NRRL18993 typically accumulated migrastation (1), dorrigocin A (2) and B (3), and 13-epi-dorrigocin A (5). Supplement of XAD-16 resin to the fermentation, in contrast, resulted in exclusive production of iso-migrastatin (4). In vitro studies showed that 1, 2, 3, and 5 are stable in aqueous solution but 4 undergoes rapid conversion into 1, 2, 3, and 5 under the same condition. These results revealed that 4 is the only bona fide natural product biosynthesized by S. platensis, and 1, 2, 3, and 5 are shunt metabolites of 4. This study also established the stereochemistry of 2-5 with the exception of C-11 for 3 and 4. A mechanism for H2O-mediated regio- and stereospecific rearrangement of 4 to 1, 2, 3, and 5 is proposed and supported by incorporation of 18O from H218O.

Migrastatin (1), iso-migrastatin (5) and lactimidomycin (7) are all glutarimide-containing polyketides known for their unique structures and cytotoxic activities against human cancer cell lines. Migrastatin, a strong inhibitor of tumor cell migration, has been an important lead in the development of antimetastatic agents. Yet studies of the related 12-membered macrolides iso-migrastatin, lactimidomycin and related analogs have been hampered by their limited availability. We report here the production, isolation, structural characterization and biological activities of iso-migrastatin, lactimidomycin, and 23 related congeners. Our studies showed that, as a family, the glutarimide-containing 12-membered macrolides are extremely potent cell migration inhibitors with some members displaying activity on par or superior to that of migrastatin as exemplified by compounds 5, 7, and 9–12. On the basis of these findings, the structures and activity of this family of compounds as cell migration inhibitors are discussed.