Yuka Unno

S-benzylisothiourea derivatives as small-molecule inhibitors of indoleamine-2,3-dioxygenase.

S-benzylisothiourea 3a was discovered by its ability to inhibit indoleamine-2,3-dioxygenase (IDO) in our screening program. Subsequent optimization of the initial hit 3a lead to the identification of sub-muM inhibitors 3r and 10h, both of which suppressed kynurenine production in A431 cells. Synthesis and structure-activity relationship of S-benzylisothiourea analogues as small-molecule inhibitors of IDO are described.

Potent Proteasome Inhibitors Derived from the Unnatural cis-Cyclopropane Isomer of Belactosin A: Synthesis, Biological Activity, and Mode of Action

The natural product belactosin A (1) with a trans-cyclopropane structure is a useful prototype compound for developing potent proteasome (core particle, CP) inhibitors. To date, 1 and its analogues are the only CP ligands that bind to both the nonprimed S1 pocket as well as the primed substrate binding channel; however, these molecules harbor a high IC50 value of more than 1 μM. We have performed structure-activity relationship studies, thereby elucidating unnatural cis-cyclopropane derivatives of 1 that exhibit high potency to primarily block the chymotrypsin-like active site of the human constitutive (cCP) and immunoproteasome (iCP). The most active compound 3e reversibly inhibits cCP and iCP similarly with an IC50 of 5.7 nM. X-ray crystallographic analysis of the yeast proteasome in complex with 3e revealed that the ligand is accommodated predominantly into the primed substrate binding channel and covalently binds to the active site threonine residue via its β-lactone ring-opening.

Synthesis of 2,3- and 3,4-methanoamino acid equivalents with stereochemical diversity and their conversion into the tripeptide proteasome inhibitor belactosin a and its highly potent cis-cyclopropane stereoisomer.

A series of chiral 2,3- and 3,4-methanoamino acid equivalents of stereochemical diversity were designed and synthesized from our chiral cyclopropane units, using a diastereoselective Grignard addition with ( R)- or ( S)- t-butanesulfinyl imines as the key step. These equivalents were converted into the proteasome inhibitor belactosin A and its cis-cyclopropane stereoisomer. The unnatural cis-isomer was shown to be more than twice as potent as belactosin A as a proteasome inhibitor.

Identification of a small-molecule inhibitor of the interaction between Survivin and Smac/DIABLO

The protein Survivin is selectively overexpressed in a variety of cancers, but not in normal tissues. It has been reported to be involved in cell survival and cell division. However, the molecular mechanisms involved in its function are not clear, although several binding partner proteins have been proposed to date. Here, we report the identification of a novel small molecule Survivin antagonist, which disrupts the Survivin-Smac/DIABLO interaction in cells. In order to identify Survivin-directed antagonists, we developed a high-throughput screening system based on AlphaScreen technology, which allows the identification of small molecules with the ability to inhibit the interaction of Survivin with Smac/DIABLO or INCENP in vitro. We screened chemical libraries, generated in-house, using this system and identified a 5-deazaflavin analog (compound 1) as a hit compound that selectively inhibited the interaction of Survivin with Smac/DIABLO but not INCENP. In cultured cells, compound 1 abrogated the formation of the complex between Survivin and Smac/DIABLO. In addition, this compound was able to sensitize cultured cells to doxorubicin-mediated DNA damage stress and synergistically enhance apoptotic cell death. Thus, the small-molecule inhibitor described here may serve as a proof-of-principle agent for discriminating between the multiple functions of Survivin.

Investigation of the noncovalent binding mode of covalent proteasome inhibitors around the transition state by combined use of cyclopropylic strain-based conformational restriction and computational modeling.

To develop potent covalent inhibitors, the noncovalent interactions around the transition state to form covalent bonding should be optimized because the potency of the inhibitor can be depending on the energy of the transition state. Here, we report an efficient analysis of the noncovalent binding mode of a potent covalent proteasome inhibitor 3a around the transition state by a combined use of the chemical approach, i.e., the cyclopropylic strain-based conformational restriction, and the computational docking approach. Furthermore, we calculated the binding energy of a series of salinosporamide derivatives in the predicted noncovalent complex around the transition state with the simulation model of proteasome constructed in this study, which was well correlated to their pIC50. Thus, the proposed docking methods to predict the noncovalent binding mode around the transition state of covalent inhibitors will be helpful toward the development of covalent inhibitors.

S-trityl-L-cysteine derivative induces caspase-independent cell death in K562 human chronic myeloid leukemia cell line.

Effect of CF(3)-STLC, a potent kinesin spindle protein (KSP) inhibitor, on K562 human CML cell line was investigated. Treatment with CF(3)-STLC induced mitotic arrest of the cell cycle with the appearance of characteristic monoastral spindles, subsequent apoptotic cell death and cleavage of PARP-1, caspase-3, and 4E-BP1. The wide ranging caspase inhibitor z-VAD fmk prevented the cleavage of caspase-3 and 4E-BP1, but failed to attenuate PARP-1 cleavage or cell death triggered by CF(3)-STLC. These results suggest that CF(3)-STLC can induce apoptotic cell death in a caspase-independent manner, and may work effectively as an anti-cancer agent for hematological malignancies.

Structurally Novel Highly Potent Proteasome Inhibitors Created by the Structure-Based Hybridization of Nonpeptidic Belactosin Derivatives and Peptide Boronates

We previously developed highly potent proteasome inhibitor 1 (IC50 = 5.7 nM) and its nonpeptide derivative 2 (IC50 = 29 nM) by systematic structure-activity relationship studies of the peptidic natural product belactosin A and subsequent rational topology-based scaffold hopping, respectively. Their cell growth inhibitory activities, however, were only moderate (IC50 = 1.8 μM (1) and >10 μM (2)). We therefore planned to replace the unstable β-lactone warhead with a more stable boronic acid warhead. Importantly, belactosin derivatives bind mainly to the proteasome binding site, which is different from that occupied by known peptide boronate proteasome inhibitors such as bortezomib, suggesting that their hybridization might lead to the development of novel potent inhibitors. Here we describe design, synthesis, and biological activities of the newly developed potent hybrid proteasome inhibitors. Interestingly, these hybrids, unlike bortezomib, were highly selective for proteasomes and have long residence times despite having the same boronic acid warhead.

Design and synthesis of the stabilized analogs of belactosin A with the unnatural cis-cyclopropane structure.

The belactosin A analog 2a, having the unnatural cis-cyclopropane structure instead of the trans-cyclopropane structure in belactosin A, is a much more potent proteasome inhibitor than belactosin A. However, its cell growth inhibitory effect is rather lower than that expected from its remarkable proteasome inhibitory effect, probably due to its instability under cellular conditions. We hypothesized that the instability of 2a was due to chemical and enzymatic hydrolysis of the strained β-lactone moiety. Thus, to increase the stability of 2a by chemical modification, its analogs with a sterically more hindered β-lactone moiety and/or cyclopropylic strain-based conformational restriction were designed and synthesized, resulting in the identification of a stabilized analog 6a as a proteasome inhibitor with cell growth inhibitory effects. Our findings suggest that the chemical and biological stability of 2a is significantly affected by the steric hindrance around its β-lactone carbonyl moiety and the conformational flexibility of the molecule.

Rational hopping of a peptidic scaffold into non-peptidic scaffolds: structurally novel potent proteasome inhibitors derived from a natural product, belactosin A.

Rational scaffold hopping of a natural product belactosin A derivative was successfully achieved based on the pharmacophore model constructed. The peptidic scaffold was replaced by significantly simplified non-peptidic scaffolds, by which weak belactosin A (IC50 = 1440 nM) was converted into highly potent non-peptidic inhibitors (IC50 = 26-393 nM).

Identification of novel kynurenine production-inhibiting benzenesulfonamide derivatives in cancer cells

Kynurenine (Kyn), a metabolite of tryptophan (Trp), is known to be a key regulator of human immune responses including cancer immune tolerance. Therefore, abrogation of Kyn production from cancer cells by small molecules may be a promising approach to anticancer therapy. Indeed, several small molecule inhibitors of indoleamine 2,3-dioxygenase (IDO), a rate-limiting enzyme in the catabolism of Trp to Kyn, exert antitumor effects in animal models. We screened our chemical libraries using a cell-based Kyn production assay to identify a new type of small molecules that regulate Kyn production, and for the first time identified a benzenesulfonamide derivative (compound 1) as a hit with the ability to inhibit Kyn production in interferon-γ (IFN-γ)-stimulated A431 and HeLa cells. Unlike the previously identified S-benzylisothiourea derivative, compound 2, compound 1 had little effect on the enzymatic activity of recombinant human IDO in vitro but suppressed the expression of IDO at the mRNA level in cells. Furthermore, compound 1 suppressed STAT1-dependent transcriptional activity and DNA binding, whereas no decrement in either the expression or phosphorylation level of STAT1 was observed. The inhibition of IDO expression by several benzenesulfonamide derivatives is associated with the suppression of STAT1. Thus, compound 1 and its analogs might be useful for analyzing the regulation of IDO activation, and STAT1-targeting could be an alternative to the IDO-directed approach for the regulation of Kyn levels by small molecules in the tumor microenvironment.