Rationally Designed Polypharmacology: α-Helix Mimetics as Dual Inhibitors of the Oncoproteins Mcl-1 and HDM2

Augmenting the Anti-Leukemic Activity of the BCL-2 Inhibitor Venetoclax Through Its Transformation Into Polypharmacologic Dual BCL-2/HDAC1 and Dual BCL-2/HDAC6 Inhibitors

Motivated by the anti-leukemic synergy between histone deacetylase (HDAC) inhibitors and the FDA-approved BCL-2 inhibitor venetoclax, coupled with our interests in polypharmacology, we sought to bolster the anti-leukemic efficacy of the clinical drug by grafting HDAC1-selective or HDAC6-selective inhibitor motifs onto a solvent-accessible domain of venetoclax. We discovered multiple polypharmacological agents that both retained the potent BCL-2 inhibitory activity of venetoclax and effectively inhibited either HDAC1 or HDAC6 with excellent (up to 80-fold) selectivities for the desired HDAC isoform. In addition, relative to parental venetoclax, two of our lead compounds, BD-4-213 and AMC-4-154, exhibited superior activities against the acute myeloid leukemia cell line MV4;11 and an MV4;11 cell line engineered to overexpress BCL-2. Annexin-V assay results confirmed an on-target mechanism of apoptosis for these novel chimeric molecules. Efforts to further boost the HDAC1 or HDAC6 binding affinities and/or selectivities proved unsuccessful due to synthetic chemistry challenges and solubility problems, which may underscore the difficulties of polypharmacology approaches involving a large inhibitor, such as venetoclax.

Preassembly-Controlled Radical Recombination at Bismuth: Decarboxylative C─N Coupling with Sulfonamides

Persistent transition metal radicals form the foundation for many metallaphotoredox protocols. Their ability to efficiently trap organic radicals and convert them into various coupling products has inspired the exploration of selective radical reactions even beyond the d-block. Radical processes involving bismuth hold great potential, but innovative strategies are required to control the reactivity of bismuth intermediates. Herein, we report preassembly as a powerful strategy to enforce a selective recombination of a bismuth(II) radical and an organic radical. As a result, an inner-sphere pathway is accessed, enabling the formation of C─N coupling products.

The polypharmacy combination of the BCL-2 inhibitor venetoclax (VEN) and the FLT3 inhibitor gilteritinib (GIL) is more active in acute myeloid leukemia cells than novel polypharmacologic BCL-2/FLT3 VEN-GIL hybrid single-molecule inhibitors

Current treatments for acute myeloid leukemias (AMLs) cure fewer than 30 % of patients. This low efficacy is due, in part, to the inter-patient and intra-patient heterogeneity of AMLs; accordingly, all current AML treatment regimens involve drug combinations (polypharmacy). A recently-completed clinical trial in relapsed/refractory AML using a combination of two newer targeted antileukemics, the BCL-2 inhibitor venetoclax (VEN) plus the FLT3 inhibitor gilteritinib (GIL), yielded highly promising results for this two-drug polypharmacy combination. Polypharmacology - wherein a single drug molecule that inhibits two or more biological targets is created - has been proposed to offer superior therapeutic results, as compared to the corresponding polypharmacy approach. Herein, we designed and synthesized several polypharmacologic dual BCL-2/FLT3 hybrid single-molecule inhibitors by tethering VEN to GIL, through their solvent-exposed domains. While the in vitro antileukemic activity of the two-drug VEN + GIL polypharmacy combination proved superior to our focused library of VEN-GIL hybrids, alternative grafting points on GIL may yield improved results for future hybrid compounds.

Discovery of N-sulfonylated aminosalicylic acids as dual MCL-1/BCL-xL inhibitors

The anti-apoptotic protein MCL-1, which is overexpressed in multiple cancers, is presently a focus for the development of targeted drugs in oncology. We previously discovered inhibitors of MCL-1 based on 1-sulfonylated 1,2,3,4-tetrahydroquinoline-6-carboxylic acids ("1,6-THQs"). However, with the nitrogen atom constrained in the bicyclic ring, we were unable to modify the alkyl portion of the tertiary sulfonamide functionality. Moreover, the introduction of additional functional groups onto the benzene ring portion of the THQ bicycle would not be trivial. Therefore, we elected to deconstruct the piperidine-type ring of the 6-carboxy-THQ lead to create a new 4-aminobenzoic acid scaffold. Given its simplicity, this permitted us to introduce diversity at the sulfonamide nitrogen, as well as vary the positions and substituents of the benzene ring. One of our most potent MCL-1 inhibitors, 6e-OH, exhibited a K i of 0.778 μM. Heteronuclear single quantum coherence experiments suggested 6e-OH bound in the canonical BH3-binding groove, with significant perturbations of R263, which forms a salt bridge with MCL-1's pro-apoptotic binding partners, as well as residues in the p2 pocket. Selectivity studies indicated that our compounds are dual inhibitors of MCL-1 and BCL-xL, with 17cd the most potent dual inhibitor: K i = 0.629 μM (MCL-1), 1.67 μM (BCL-xL). Whilst selective inhibitors may be more desirable in certain instances, polypharmacological agents whose additional target(s) address other pathways associated with the disease state, or serve to counter resistance mechanisms to the primary target, may prove particularly effective therapeutics. Since selective MCL-1 inhibition may be thwarted by overexpression of sister anti-apoptotic proteins, including BCL-xL and BCL-2, we believe our work lays a solid foundation towards the development of multi-targeting anti-cancer drugs.

1-Sulfonylated 1,2,3,4-tetrahydroquinoline-6-carboxylic acids as simple, readily-accessible MCL-1 inhibitors

MCL-1 is a member of the BCL-2 family of proteins that regulates the mitochondrial pathway of apoptosis. Overexpression of MCL-1 is associated with the development and progression of a range of human cancers, and is also responsible for the onset of resistance to conventional chemotherapies. Although several MCL-1 inhibitors have now advanced to clinical trials, recent suspensions and terminations reveal the urgency with which new inhibitor chemotypes must be discovered. Building on our previous studies of a chiral, isomeric lead, we report the discovery of a new chemotype to inhibit MCL-1: 1-sulfonylated 1,2,3,4-tetrahydroquinoline-6-carboxylic acid. The nature of the sulfonyl moiety contributed significantly to the resulting inhibitory ability. For example, transforming a phenylsulfonyl group into a 4-chloro-3,5-dimethylphenoxy)phenyl)sulfonyl moiety elicited more than a 73-fold enhancement in inhibiton of MCL-1, possibly through targeting the p2 pocket in the BH3-binding groove, and so it is anticipated that further structure-activity studies here will lead to continued improvements in binding. It should be underscored that this class of MCL-1 inhibitors is readily accessible in four simple steps, is achiral and offers many avenues for optimization, all factors that are welcomed in the search for safe and effective inhibitors of this driver of cancer cell survival.

Recent applications of covalent chemistries in protein-protein interaction inhibitors

Protein-protein interactions (PPIs) are large, often featureless domains whose modulations by small-molecules are challenging. Whilst there are some notable successes, such as the BCL-2 inhibitor venetoclax, the requirement for larger ligands to achieve the desired level of potency and selectivity may result in poor "drug-like" properties. Covalent chemistry is presently enjoying a renaissance. In particular, targeted covalent inhibition (TCI), in which a weakly electrophilic "warhead" is installed onto a protein ligand scaffold, is a powerful strategy to develop potent inhibitors of PPIs that are smaller/more drug-like yet have enhanced affinities by virtue of the reinforcing effect on the existing non-covalent interactions by the resulting protein-ligand covalent bond. Furthermore, the covalent bond delivers sustained inhibition, which may translate into significantly reduced therapeutic dosing. Herein, we discuss recent applications of a spectrum of TCIs, as well as covalent screening strategies, in the discovery of more effective inhibitors of PPIs using the HDM2 and BCL-2 protein families as case studies.

Scaffold hopping from indoles to indazoles yields dual MCL-1/BCL-2 inhibitors from MCL-1 selective leads

Overexpression of the anti-apoptotic BCL-2 proteins is associated with the development and progression of a range of cancers. Venetoclax, an FDA-approved BCL-2 inhibitor, is fast becoming the standard-of-care for acute myeloid leukemia and chronic lymphocytic leukemia. However, the median survival offered by venetoclax is only 18 months (as part of a combination therapy regimen), and one of the primary culprits for this is the concomitant upregulation of sister anti-apoptotic proteins, in particular MCL-1 (and BCL-xL), which provides an escape route that manifests as venetoclax resistance. Since inhibition of BCL-xL leads to thrombocytopenia, we believe that a dual MCL-1/BCL-2 inhibitor may provide an enhanced therapeutic effect relative to a selective BCL-2 inhibitor. Beginning with a carboxylic acid-containing literature compound that is a potent inhibitor of MCL-1 and a moderate inhibitor of BCL-2, we herein describe our efforts to develop dual inhibitors of MCL-1 and BCL-2 by scaffold hopping from an indole core to an indazole framework. Subsequently, further elaboration of our novel N2-substituted, indazole-3-carboxylic acid lead into a family of indazole-3-acylsulfonamides resulted in improved inhibition of both MCL-1 and BCL-2, possibly through occupation of the p4 pocket, with minimal or no inhibition of BCL-xL.

Discovery of spirooxindole-derived small-molecule compounds as novel HDAC/MDM2 dual inhibitors and investigation of their anticancer activity

Simultaneous inhibition of more than one target is considered to be a novel strategy in cancer therapy. Owing to the importance of histone deacetylases (HDACs) and p53-murine double minute 2 (MDM2) interaction in tumor development and their synergistic effects, a series of MDM2/HDAC bifunctional small-molecule inhibitors were rationally designed and synthesized by incorporating an HDAC pharmacophore into spirooxindole skeletons. These compounds exhibited good inhibitory activities against both targets. In particular, compound 11b was demonstrated to be most potent for MDM2 and HDAC, reaching the enzyme inhibition of 68% and 79%, respectively. Compound 11b also showed efficient antiproliferative activity towards MCF-7 cells with better potency than the reference drug SAHA and Nutlin-3. Furthermore, western blot analysis revealed that compound 11b increased the expression of p53 and Ac-H4 in MCF-7 cells in a dose-dependent manner. Our results indicate that dual inhibition of HDAC and MDM2 may provide a novel and efficient strategy for the discovery of antitumor drug in the future.

A potential inhibitor of MDM2 by restoring the native conformation of the p53 α-helical peptide on gold nanoparticles

Many efforts have been made to develop inhibitors of MDM2 as potential drugs for cancer therapy. In this work, we use our previous developed conformational engineering technique to stabilize the binding conformation of the p53 transcription activation domain (TAD) peptide on gold NPs (AuNPs), and create an AuNP-based anti-MDM2 artificial antibody, denoted as Goldbody, that specifically binds MDM2. Though the free TAD peptide is unstructured, circular dichroism spectra confirm that its α-helical conformation in the original p53 protein is restored on the anti-MDM2 Goldbody, and surface plasmon resonance (SPR) experiments confirm that there is strong specific interaction between the anti-MDM2 Goldbody and MDM2, demonstrating the anti-MDM2 Goldbody as a potential inhibitor of MDM2. This work demonstrates that the conformational engineering technique is not limited to the antigen-antibody systems, but can also be applied more widely in other protein-protein interfaces to create more and more artificial proteins for various biomedical applications.