BAL27862: A Unique Microtubule Destabilizer Active Against Chemorefractory Breast Cancers

Background: BAL27862, a novel, synthetic, small molecule, is a potent inhibitor of tubulin polymerization that induces cancer cell death. BAL27862 elicits a unique microtubule (MT) phenotype, distinct from paclitaxel, vinblastine and colchicine, has broad in vitro anti-proliferative activity against a diverse range of human tumor lines (low nM IC50s) and induces significant antitumor responses in a range of animal models of human cancer when administered orally (p.o.) or intravenously (i.v.). In this study, BAL27862 activity in a panel of experimental breast cancer models was assessed.Materials and Methods: Anti-proliferative activity was analyzed using a monolayer (crystal violet) or soft agar (clonogenic) assay. Effects on MT phenotypes were assessed by immunofluorescence for α-tubulin. Efficacy was assessed in mouse xenograft models bearing chemosensitive and multidrug resistant human breast tumors.Results: The unique BAL27862-associated MT phenotype in interphase cells consisted of a partially collapsed MT network without peripheral MTs. In dividing cells, tiny MT asters were found scattered within the nuclear region. Potent anti-proliferative activity was demonstrated against 8 breast cancer cell lines (crystal violet assay IC50 range: 6.5 – 22 nM for the SKBR3, MCF7, BT474, T47D, BT549, MDA-MB231, MDA-MB453 and MDA-MB468 lines), although one line (HCC1937) appeared relatively insensitive (IC50: >1000 nM). Interestingly, two breast cancer lines were sensitive to BAL27862 treatment in a clonogenic assay (IC50/IC70: MAXF 401 = 13/18 nM; MAXF MX1 = 22/46 nM), despite one being relatively resistant to paclitaxel treatment (MAXF 401 = 11/48 nM; MAXF MX1 = 127/>3500 nM). Moreover, using monolayer assays, BAL27862 activity was retained against five tumor lines overexpressing the Pgp efflux pump (including MT-3/ADR mammary adenocarcinoma cells), which were up to several thousand-fold resistant to paclitaxel and vinblastine. BAL27862 showed little activity against human stem cells or peripheral blood mononucleocytes.When administered p.o or i.v. to mice at well tolerated doses, BAL27862 treatment elicited statistically significant antitumor activity (p≤0.05) in three chemosensitive human breast tumor xenograft models (including MAXF 401, MaCa 4049 and MT-3); resulting in a final %T/C (ratio of mean tumor volume of treated and control group x 100) equivalent to that observed with comparator cytotoxics using MTD schedules (e.g. final %T/C in MT-3 model: 36% BAL27862, 35% paclitaxel, 50% doxorubicin). Strikingly, significant antitumor activity was maintained in the Pgp-overexpressing MT-3/ADR xenograft model, where paclitaxel and doxorubicin were ineffective (final %T/C: 37% BAL27862, 112% paclitaxel, 108% doxorubicin).Conclusions: BAL27862 is a new tubulin-interacting agent with an apparently novel mechanism of action. A potent antitumor activity in experimental models of breast cancer, including chemorefractory models, strongly support further development of BAL27862 as a novel breast cancer treatment modality with a possibility for both i.v. and p.o. administration.

Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 2093.

Analysis of a C-methyltransferase gene (aviG1) involved in avilamycin biosynthesis in Streptomyces viridochromogenes Tü57 and complementation of a Saccharopolyspora erythraea eryBIII mutant by aviG1

Streptomyces viridochromogenes Tü57 is the principal producer of avilamycin A. aviG1, a putative methyltransferase gene, was detected in the avilamycin biosynthetic gene cluster. To determine the function of aviG1, a targeted gene inactivation experiment was performed. The resulting chromosomal mutant, carrying an in-frame deletion in aviG1, was deficient in avilamycin production. aviG1 was used to complement an eryBIII mutant of the erythromycin A producer Saccharopolyspora erythraea [Gaisser, S., Bohm, G. A., Doumith, M., Raynal, M. C., Dhillon, N., Cortes, J. & Leadlay, P. F. (1998). Mol Gen Genet 258, 78-88]. The presence of erythromycin A in the culture supernatant of the complemented mutant indicated that L-mycarose biosynthesis could be restored and that AviG1 could take over the function of the C-methyltransferase EryBIII.

A defined system for hybrid macrolide biosynthesis in Saccharopolyspora erythraea

The biological activity of polyketide antibiotics is often strongly dependent on the presence and type of deoxysugar residues attached to the aglycone core. A system is described here, based on the erythromycin-producing strain of Saccharopolyspora erythraea, for detection of hybrid glycoside formation, and this system has been used to demonstrate that an amino sugar characteristic of 14-membered macrolides (D-desosamine) can be efficiently attached to a 16-membered aglycone substrate. First, the S. erythraea mutant strain DM was created by deletion of both eryBV and eryCIII genes encoding the respective ery glycosyltransferase genes. The glycosyltransferase OleG2 from Streptomyces antibioticus, which transfers L-oleandrose, has recently been shown to transfer rhamnose to the oxygen at C-3 of erythronolide B and 6-deoxyerythronolide B. In full accordance with this finding, when oleG2 was expressed in S. erythraea DM, 3-O-rhamnosyl-erythronolide B and 3-O-rhamnosyl-6-deoxyerythronolide B were produced. Having thus validated the expression system, endogenous aglycone production was prevented by deletion of the polyketide synthase (eryA) genes from S. erythraea DM, creating the triple mutant SGT2. To examine the ability of the mycaminosyltransferase TylM2 from Streptomyces fradiae to utilise a different amino sugar, tylM2 was integrated into S. erythraea SGT2, and the resulting strain was fed with the 16-membered aglycone tylactone, the normal TylM2 substrate. A new hybrid glycoside was isolated in good yield and characterized as 5-O-desosaminyl-tylactone, indicating that TylM2 may be a useful glycosyltransferase for combinatorial biosynthesis. 5-O-glucosyl-tylactone was also obtained, showing that endogenous activated sugars and glycosyltransferases compete for aglycone in these cells.

Knowledge-based design of bimodular and trimodular polyketide synthases based on domain and module swaps: a route to simple statin analogues

BACKGROUND

Polyketides are structurally diverse natural products that have a range of medically useful activities. Nonaromatic bacterial polyketides are synthesised on modular polyketide synthase (PKS) multienzymes, in which each cycle of chain extension requires a different 'module' of enzymatic activities. Attempts to design and construct modular PKSs that synthesise specified novel polyketides provide a particularly stringent test of our understanding of PKS structure and function.

RESULTS

We have constructed bimodular and trimodular PKSs based on DEBS1-TE, a derivative of the erythromycin PKS that contains only modules 1 and 2 and a thioesterase (TE), by substituting multiple domains with appropriate counterparts derived from the rapamycin PKS. Hybrid PKSs were obtained that synthesised the predicted target triketide lactones, which are simple analogues of cholesterol-lowering statins. In constructing intermodular fusions, whether between modules in the same or in different proteins, it was found advantageous to preserve intact the acyl carrier protein-ketosynthase (ACP-KS) didomain that spans the junction between successive modules.

CONCLUSIONS

Relatively simple considerations govern the construction of functional hybrid PKSs. Fusion sites should be chosen either in the surface-accessible linker regions between enzymatic domains, as previously revealed, or just inside the conserved margins of domains. The interaction of an ACP domain with the adjacent KS domain, whether on the same polyketide or not, is of particular importance, both through conservation of appropriate protein-protein interactions, and through optimising molecular recognition of the altered polyketide chain in the key transfer of the acyl chain from the ACP of one module to the KS of the downstream module.