Candidaantarctica Lipase B mediated kinetic resolution: A sustainable method for chiral synthesis of antiproliferative ß-lactams.

Biocatalysis is a valuable industrial approach in active pharmaceutical ingredient (API) manufacturing for asymmetric induction and synthesis of chiral APIs. Herein, we investigated synthesis of a panel of microtubule-destabilising antiproliferative ß-lactam enantiomers employing a commercially available immobilised Candida antarctica lipase B enzyme together with methanol and MTBE. The ß-lactam ring remained intact during chiral kinetic resolution reactions, plausibly due to a bulky N-1 phenyl substituent on the ß-lactam ring substrate. The predominant reaction mediated by CAL-B was methanol catalysed conversion of the ß-lactam 3-acetoxy substituent to a 3-hydroxyl group, with preferential methanolysis of the 3S, 4S enantiomer. The unreacted substrate underwent progressive enantioenrichment to the 3R, 4R enantiomer. Substitution patterns on the B ring C3 meta position of the ß-lactam scaffold greatly affected the rate of reaction. Halo substituents (fluoro-, chloro- and bromo-) reduced the rate of conversion compared to unsubstituted analogues, which in turn increased enantiomeric excess (ee). Ee values up to 86 % for the 3S, 4S 3-hydroxyl enantiomer were achieved. A double resolution approach for unreacted substrate yielded high ee values (>99 %) for the 3R, 4R 3-acetoxy enantiomer. CAL-B mediated methanolysis is a more sustainable method for resolution of racemic antiproliferative ß-lactams compared to a previous technique of chiral diastereomeric resolution. Yields of ß-lactams obtained using CAL-B are far superior than previously described, which will facilitate progression toward pre-clinical and clinical development. Biocatalysis is a useful tool in the toolbox of the medicinal chemist.

Synthesis by diastereomeric resolution, biochemical evaluation and molecular modelling of chiral 3-hydroxyl b-lactam microtubule-targeting agents for the treatment of triple negative breast and chemoresistant colorectal cancers.

The synthesis and biochemical activity of a series of chiral trans 3-hydroxyl ß-lactams targeting tubulin is described. Synthesis of the series of enantiopure ß-lactams was achieved using chiral derivatising reagent N-Boc-l-proline. The absolute configuration was determined as 3S,4S for (+) enantiomer 4EN1 and 3R,4R for (-) enantiomer 4EN2. Antiproliferative studies identified chiral 3S,4S b-lactams with subnanomolar IC50 values across a range of cancer cell lines, improving potency with respect to the corresponding racemates. Fluoro-substituted (+)-(3S,4S)-4-(3-fluoro-4-methoxyphenyl)-3-hydroxy-1-(3,4,5-trimethoxyphenyl)azetidin-2-one (27EN1) was determined as the lead eutomer with dual antiproliferative activity in triple negative breast cancer cells (TNBC), and combretastatin A-4 resistant HT-29 colorectal cancer cells. IC50 values were in the range of 0.26-0.7 nM across four cell lines. Tubulin polymerisation assays, confocal microscopy and molecular modelling studies indicated that 3S,4S eutomers are microtubule destabilisers, while 3R,4R distomers have lower potency as microtubule destabilisers. 27EN1 demonstrated anti-mitotic and pro-apoptotic activity in MDA-MB-231 and HT-29 cells in addition to selective toxicity toward MCF-7 breast cancer versus non-tumorigenic MCF-10-2A cells. The related 3S,4S ß-lactam eutomer 4EN1 downregulated expression of key cell survival anti-apoptotic proteins Bcl-2 and Mcl-1 in MDA-MB-231 cells while 27EN1 downregulated Mcl-1 in HT-29 cells. Chiral ß-lactam 27EN1 will be further developed for treatment of TNBC and CA-4 resistant colorectal cancers.

Application of 2D EXSY and qNMR Spectroscopy for Diastereomeric Excess Determination Following Chiral Resolution of ß-Lactams.

Trans-ß-lactam isomers have garnered much attention as anti-cancer microtubule targeting agents. Currently available synthetic methods are available for the preparation of enantiopure ß-lactams and favour isomeric cis/trans ß-lactam mixtures. Indirect chiral resolution offers the opportunity for isolation of exclusively enantiopure trans-ß-lactams. In this study, liquid chromatography chiral resolution of ß-lactams derivatized as diastereomer mixtures with a panel of N-protected amino acids is explored, where N-(Boc)-L-proline served as the optimal chiral derivatising reagent. High-performance liquid chromatography failed to adequately determine diastereomeric excess (de) of resolved diastereomers. Variable temperature, 1 H NMR and 2D EXSY spectroscopic analyses of proline-derivatised diastereomers were successfully employed to characterise equilibrating rotamers of resolved diastereomers and determine their de. Integration of resolved resonances corresponding to H3 and H4 of the ß-lactam ring served as a quantitative qNMR tool for the calculation of de following resolution.

Synthesis, Characterisation and Mechanism of Action of Anticancer 3-Fluoroazetidin-2-ones.

The stilbene combretastatin A-4 (CA-4) is a potent microtubule-disrupting agent interacting at the colchicine-binding site of tubulin. In the present work, the synthesis, characterisation and mechanism of action of a series of 3-fluoro and 3,3-difluoro substituted ß-lactams as analogues of the tubulin-targeting agent CA-4 are described. The synthesis was achieved by a convenient microwave-assisted Reformatsky reaction and is the first report of 3-fluoro and 3,3-difluoro ß-lactams as CA-4 analogues. The ß-lactam compounds 3-fluoro-4-(3-hydroxy-4-methoxyphenyl)-1-(3,4,5-trimethoxy phenyl)azetidin-2-one 32 and 3-fluoro-4-(3-fluoro-4-methoxyphenyl)-1-(3,4,5-trimethoxyphenyl)azetidin-2-one) 33 exhibited potent activity in MCF-7 human breast cancer cells with IC50 values of 0.075 µM and 0.095 µM, respectively, and demonstrated low toxicity in non-cancerous cells. Compound 32 also demonstrated significant antiproliferative activity at nanomolar concentrations in the triple-negative breast cancer cell line Hs578T (IC50 0.033 µM), together with potency in the invasive isogenic subclone Hs578Ts(i)8 (IC50 = 0.065 µM), while 33 was also effective in MDA-MB-231 cells (IC50 0.620 µM). Mechanistic studies demonstrated that 33 inhibited tubulin polymerisation, induced apoptosis in MCF-7 cells, and induced a downregulation in the expression of anti-apoptotic Bcl2 and survivin with corresponding upregulation in the expression of pro-apoptotic Bax. In silico studies indicated the interaction of the compounds with the colchicine-binding site, demonstrating the potential for further developing novel cancer therapeutics as microtubule-targeting agents.

Correction: McLoughlin, E.C.; O'Boyle, N.M. Colchicine-Binding Site Inhibitors from Chemistry to Clinic: A Review. Pharmaceuticals 2020, 13, 8.

We, the authors, wish to make the following corrections to our paper[...].

Colchicine-Binding Site Inhibitors from Chemistry to Clinic: A Review.

It is over 50 years since the discovery of microtubules, and they have become one of the most important drug targets for anti-cancer therapies. Microtubules are predominantly composed of the protein tubulin, which contains a number of different binding sites for small-molecule drugs. There is continued interest in drug development for compounds targeting the colchicine-binding site of tubulin, termed colchicine-binding site inhibitors (CBSIs). This review highlights CBSIs discovered through diverse sources: from natural compounds, rational design, serendipitously and via high-throughput screening. We provide an update on CBSIs reported in the past three years and discuss the clinical status of CBSIs. It is likely that efforts will continue to develop CBSIs for a diverse set of cancers, and this review provides a timely update on recent developments.