Long-range goals: Leptomycin B (LMB) offers the opportunity to develop a polyketide natural product into a cancer drug with a novel mechanism of action. This microbial metabolite inhibits protein export from the cell nucleus by binding tightly to CRM1 (syn. exportin 1) and inhibiting its ability to shuttle protein complexes across the nuclear membrane. Numerous cell signalling processes can be affected, including ones essential to specific steps in the onset and development of cancer as well as viral replication. The ability of LMB to synergize with and overcome acquired resistance to imatinib (Gleevec), a protein tyrosine kinase inhibitor used to treat chronic myelogenous leukemia (CML), has been demonstrated in vitro. This suggests that LMB could be combined effectively with other drugs that act by cancer cell specific mechanisms. Preclinical studies have been hampered by variability of the quality of LMB production lots, which were probably due to impurities or minor congeners. This may have also compromised the performance of LMB in a Phase I clinical trial carried out in 1994-95. To overcome this limitation, we propose (i) to develop a reliable fermentation process for producing highly pure LMB and (ii) to use Kosan's proprietary polyketide synthase (PKS) gene manipulation technology to express the LMB biosynthesis genes, or a suitably engineered form, in a heterologous host so as to produce only LMB. Collaborators at the University of San Diego and the National Cancer Institute will assess the efficacy of co-administration of Gleevec with purified LMB in an animal model of CML, and conduct preclinical pharmacokinetic and toxicology studies of LMB in a suitable animal system.
Specific aims of Phase I research: (1) Produce LMB of at least 95% chemical purity by an optimized fermentation process with Streptomyces sp ATCC 39366. (2) Express the native LMB biosynthetic genes, or an engineered form designed to enhance the selective formation of LMB, in Streptomyces coelicolor so as to produce LMB exclusively. (3) Provide purified LMB to UCSD and NCI collaborators for biological evaluation. If we can produce highly pure LMB in consistent quality in the native producer or in S. coelicolor, we will proceed with preclinical development of LMB or the discovery and development of an LMB analog in Phase II research.
