Exploring of drug leads from diversity-oriented Michael-acceptor library derived from natural products

A potential strategy for drug lead identification and in-active natural products re-discovery is elaborated. Starting from fifteen structurally diverse natural products, a focused library featured by Michael acceptors is constructed with IBX mediated oxidation. Biological assay on five tumor cell lines indicates that four Michael acceptors, 8a, 11a, 12a, 14a, are with improved cytotoxicity (3–10 folds more potent than the parent compounds), which merit further investigations. Further thiol-sensitive assay of the active hit 8a revealed that it was an irreversible Michael acceptor. The results suggest that the strategy is not only effective and relatively high discovery rate (28%), but also resource saving.


Introduction
Drug-lead identification is the first step of drug discovery. A good start may set a success keynote for the whole process. So it is worthwhile to pay more attention on drug lead identification. Though plenty methods, including random screening, high-throughput screening from database, fragment based lead design and computer aided lead identification, are available for drug lead discovery 1,2 , some challenges remain un-addressed. The discovery efficiency and structure diversity are often low, and the leads are always hampered on the way to drug candidates. In one word, the lead quality is frequently unsatisfactory 3,4 .
On the other hand, the abundant structure and function diversity fueled by natural products has provided a steady flow for the novel drugs. Many drugs used in clinic are natural products or compounds derived from natural products [5][6][7] . It is particularly true in anti-cancer and anti-infectious therapy field. We would like to dig out something valuable from this underestimated repository.
What's more, in the drug lead identification process, the major attentions are focused on the active hits, leaving vast amount of counterparts under-valued. And what shocks us most is that plenty of the "in-active" natural products are abundant in plants. So how to make sustainable use of these "in-active" natural products becomes a highly valuable project. We wonder that if we can turn the "trash" to the "treasure" by incorporation of pharmaceutically privileged structure moiety through some simple chemical transformations.
Based on the reasons mentioned above, we would like to combine the diverse natural products and the pharmaceutically privileged moiety together so as to identify novel drug leads. So what moiety will be chosen? The α,β-unsaturated ketone moiety came to our sight and became our right choice.
It is generally believed that Michael acceptors form covalent bond with the active site cysteine of the proteases to solicit a biological effect [8][9][10][11] . The Cysteine proteases, including papains, cathepsins, calpains, caspases and lugumain, represent a biologically important clan of protein, which are important therapeutic target of tumor, inflammation and auto-immune disease [12][13][14] . So it is no surprise that Michael acceptors have the potential of anti-tumor activity. In 2005, a Michael acceptor derived from oleanic acid, TP235, was approved as an orphan drug for treatment of pancreatic cancer and also used as an agent for treatment of diabetes associated chronic kidney disease in late clinical trial 15,16 . This discovery has provoked the keen interests in Michael acceptors. Since then, more and more successful examples in drug discovery spring up. A cruzain inhibitor, CRA-3386, was approved into clinical trial for treatment of Chagas disease 17 . Another Michael acceptor, AG7088, entered into clinical trial Ⅱ for therapy of rhinovirus infection 18 . Schust J. et al. identified a novel Michael acceptor, static, as a selective STAT3 activation inhibitor from a compound library, and proved that the Michael acceptor moiety was its key pharmacophore ( Figure 1) 19 . The pioneer work leads us to an idea that *To whom correspondence should be addressed. E-mail: qinshizhao@mail.kib.ac.cn (Q.S. Zhao); liyanb@mail.kib.ac.cn (Y. Li) identification of novel drug leads from structurally diverse and functionally under-mined natural products by introduction Michael acceptor moiety.
With the idea in mind, we then started the execution of the plan. Though plenty of methods available in our chemical tool box, including Pd(OAc) 2 catalyzed Saegusa oxidation 20 , benzeneselenenylation-oxidative elimination sequence 21 , both the methods require trivial operations and relative harsh reaction conditions. Fortunately, we have a newly invented method namely IBX mediated oxidation available 22 , which features with higher efficiency and simpler operations. The power of the method is not fully exemplified, especially in complex natural products.
Herein, we reported the construction of a small diversityoriented combinatorial library derived from natural products, which featured with α,β-unsaturated carbonyl moiety using IBX mediated oxidation.

Results and Discussion
Starting from structurally diverse natural products, we prepared fourteen Michael acceptors efficiently ( Table 1).
As summarized in Table 2, four Michael acceptors, 8a, 11a and 12a, 14a, are found to be with significantly improved antitumor activity. Especially, compounds 8a and 12a exhibit single digit micro-molar inhibitory activity against HL-60, SK-BR-3 and PANC-1 cell lines, which are up to 10 times more potent than the corresponding parent compounds. It's also pleased to know that the parent compounds 8 and 12 are relatively abundant in plants, which are worthy of our further attentions.
Moreover, we also investigated the possible action mode of the active hit 8a in a thiol-sensitive assay using an NMR method, which was developed by Appendino G. et al 23 . The method was proved effective for identifying Michael acceptors, and sorting them into reversible and irreversible thiol sinks. Upon treatment of two equivalents of cysteamine, a known biologically relevant model thiol 24 , in deuterated DMSO, a Michael adduct of 8b was formed instantaneously (Figure 2), and the adduct was irreversible by diluting with CDCl 3 . The results reveal that the Michael acceptor moiety is the key pharmacophore of the active hit 8a, and 8a is classified as the irreversible Michael acceptor accordingly.
In conclusion, a potential strategy to identify drug leads and re-discover the "trash" natural products is exemplified. The possible action mode of typical active hit 8a was also investigated. It was confirmed that the Michael acceptor moiety was the key pharmacophore and 8a was an irreversible thiol sink. And the superiority of the strategy is apparent, including resource saving, increased discovery rate, higher efficiency. Optimization process can be furthered based on the leads identified. Moreover, the strategy also provides an alternative view to evaluate the "trash" natural products, and may reward us serendipity.

Entry
Substrates Products Yields   Experimental Section Material and Methods. Reagents and solvents were used as commercial grade. Toluene and dimethyl sulfoxide (DMSO) were treated as anhydrous solvents prior to use. Chromatographies were performed with 300-400 mesh silica gels. Thin layer chromatographies were carried out on Merck silica plates (0.25 mm layer thickness). ESIMS and HRESIMS were taken on a VG Auto Spec-3000 or on a Finnigan MAT 90 instrument. Optical rotations were measured with a Horiba SEPA-300 polarimeter. 1 H and 13 C NMR experiments were performed on a Bruker AM-300, AM-400 and DRX-500 NMR spectrometer at ambient temperature. And chemical shifts were given in δ with TMS as internal reference.
General Procedure for Preparation of Michael Acceptors 1a-14a 16 . To a solution of the substrate (1.0 mmol) in DMSO/toluene (0.8 mL/0.2 mL) was added IBX (3.0 mmol) in one portion. The mixture was heated to 85-95 °C, and was followed by TLC until no starting material was detected. Then the mixture was cooled to room temperature and diluted with H 2 O. The aqueous layer was extracted with Et 2 O for three times. The combined organic layer was washed with 5% NaHCO 3 (3 × 10 mL), H 2 O (1 × 10 mL), and dried (MgSO 4 ), followed by removal of solvent in vacuo, leading to crude compounds which was purified using flash column chromatography on silica gel.