Sustained mTORC1 activity during palbociclib-induced growth arrest triggers senescence in ER+ breast cancer cells.

Palbociclib, a selective CDK4/6 kinase inhibitor, is approved in combination with endocrine therapies for the treatment of advanced estrogen receptor positive (ER+) breast cancer. In pre-clinical cancer models, CDK4/6 inhibitors act primarily as cytostatic agents. In two commonly studied ER+ breast cancer cell lines (MCF7 and T47D), CDK4/6 inhibition drives G1-phase arrest and the acquisition of a senescent-like phenotype, both of which are reversible upon palbociclib withdrawal (incomplete senescence). Here we identify an ER+ breast cancer cell line, CAMA1, in which palbociclib treatment induces irreversible cell cycle arrest and senescence (complete senescence). In stark contrast to T47D and MCF7 cells, mTORC1 activity is not stably suppressed in CAMA1 cells during palbociclib treatment. Importantly, inhibition of mTORC1 signaling either by the mTORC1 inhibitor rapamycin or by knockdown of Raptor, a unique component of mTORC1, during palbociclib treatment of CAMA1 cells blocks the induction of complete senescence. These results indicate that sustained mTORC1 activity promotes complete senescence in ER+ breast cancer cells during CDK4/6 inhibitor-induced cell cycle arrest. Consistent with this mechanism, genetic depletion of TSC2, a negative regulator of mTORC1, in MCF7 cells resulted in sustained mTORC1 activity during palbociclib treatment and evoked a complete senescence response. These findings demonstrate that persistent mTORC1 signaling during palbociclib-induced G1 arrest is a potential liability for ER+ breast cancer cells, and suggest a strategy for novel drug combinations with palbociclib.

Purine Nucleotide Availability Regulates mTORC1 Activity through the Rheb GTPase.

Pharmacologic agents that interfere with nucleotide metabolism constitute an important class of anticancer agents. Recent studies have demonstrated that mTOR complex 1 (mTORC1) inhibitors suppress de novo biosynthesis of pyrimidine and purine nucleotides. Here, we demonstrate that mTORC1 itself is suppressed by drugs that reduce intracellular purine nucleotide pools. Cellular treatment with AG2037, an inhibitor of the purine biosynthetic enzyme GARFT, profoundly inhibits mTORC1 activity via a reduction in the level of GTP-bound Rheb, an obligate upstream activator of mTORC1, because of a reduction in intracellular guanine nucleotides. AG2037 treatment provokes both mTORC1 inhibition and robust tumor growth suppression in mice bearing non-small-cell lung cancer (NSCLC) xenografts. These results indicate that alterations in purine nucleotide availability affect mTORC1 activity and suggest that inhibition of mTORC1 contributes to the therapeutic effects of purine biosynthesis inhibitors.

Tumor cells chronically treated with a trastuzumab-maytansinoid antibody-drug conjugate develop varied resistance mechanisms but respond to alternate treatments.

Antibody-drug conjugates (ADC) are emerging as clinically effective therapy. We hypothesized that cancers treated with ADCs would acquire resistance mechanisms unique to immunoconjugate therapy and that changing ADC components may overcome resistance. Breast cancer cell lines were exposed to multiple cycles of anti-Her2 trastuzumab-maytansinoid ADC (TM-ADC) at IC80 concentrations followed by recovery. The resistant cells, 361-TM and JIMT1-TM, were characterized by cytotoxicity, proteomic, transcriptional, and other profiling. Approximately 250-fold resistance to TM-ADC developed in 361-TM cells, and cross-resistance was observed to other non-cleavable-linked ADCs. Strikingly, these 361-TM cells retained sensitivity to ADCs containing cleavable mcValCitPABC-linked auristatins. In JIMT1-TM cells, 16-fold resistance to TM-ADC developed, with cross-resistance to other trastuzumab-ADCs. Both 361-TM and JIMT1-TM cells showed minimal resistance to unconjugated mertansine (DM1) and other chemotherapeutics. Proteomics and immunoblots detected increased ABCC1 (MRP1) drug efflux protein in 361-TM cells, and decreased Her2 (ErbB2) in JIMT1-TM cells. Proteomics also showed alterations in various pathways upon chronic exposure to the drug in both cell models. Tumors derived from 361-TM cells grew in mice and were refractory to TM-ADC compared with parental cells. Hence, acquired resistance to trastuzumab-maytansinoid ADC was generated in cultured cancer cells by chronic drug treatment, and either increased ABCC1 protein or reduced Her2 antigen were primary mediators of resistance. These ADC-resistant cell models retain sensitivity to other ADCs or standard-of-care chemotherapeutics, suggesting that alternate therapies may overcome acquired ADC resistance. Mol Cancer Ther; 14(4); 952-63. ©2015 AACR.

Fibrosterol sulfates from the Philippine sponge Lissodendoryx (Acanthodoryx) fibrosa: sterol dimers that inhibit PKCzeta.

Three new sulfated sterol dimers, fibrosterol sulfates A-C (1-3), have been isolated from the sponge Lissodendoryx (Acanthodoryx) fibrosa, collected in the Philippines. The structures were assigned on the basis of extensive 1D and 2D NMR studies as well as analysis by HRESIMS. Compounds 1 and 2 inhibited PKCzeta with IC(50) values of 16.4 and 5.6 microM, respectively.

Acylguanidine inhibitors of beta-secretase: optimization of the pyrrole ring substituents extending into the S1' substrate binding pocket.

The proteolytic enzyme beta-secretase (BACE-1) produces amyloid beta (Abeta) peptide, the primary constituent of neurofibrillary plaques, implicated in Alzheimer's disease, by cleavage of the amyloid precursor protein. A small molecule inhibitor of BACE-1, (diaminomethylene)-2,5-diphenyl-1H-pyrrole-1-acetamide (1, BACE-1 IC(50)=3.7 microM), was recently described, representing a new small molecule lead. Initial SAR investigation demonstrated the potential of accessing the nearby S(3) and S(1)(') substrate binding pockets of the BACE-1 enzyme by building substituents off one of the phenyl substituents and guanidinyl functional group. We report here the optimization of guanidinyl functional group substituents on 1, leading to potent submicromolar BACE-1 inhibitors.

Acylguanidine inhibitors of beta-secretase: optimization of the pyrrole ring substituents extending into the S1 and S3 substrate binding pockets.

Proteolytic cleavage of amyloid precursor protein by beta-secretase (BACE-1) and gamma-secretase leads to formation of beta-amyloid (A beta) a key component of amyloid plaques, which are considered the hallmark of Alzheimer's disease. Small molecule inhibitors of BACE-1 may reduce levels of A beta and thus have therapeutic potential for treating Alzheimer's disease. We recently reported the identification of a novel small molecule BACE-1 inhibitor N-[2-(2,5-diphenyl-pyrrol-1-yl)-acetyl]guanidine (3.a.1). We report here the initial hit-to-lead optimization of this hit and the SAR around the aryl groups occupying the S(1) and S(2') pockets leading to submicromolar BACE-1 inhibitors.

Development and comparison of nonradioactive in vitro kinase assays for NIMA-related kinase 2.

NIMA (never in mitosis arrest)-related kinase 2 (Nek2) is a serine/threonine kinase required for centrosome splitting and bipolar spindle formation during mitosis. Currently, two in vitro kinase assays are commercially available: (i) a radioactive assay from Upstate Biotechnology and (ii) a nonradioactive fluorescence resonance energy transfer (FRET) assay from Invitrogen. However, due to several limitations such as radioactive waste management and lower sensitivity, a need for more robust nonradioactive assays would be ideal. Accordingly, we have developed four quantitative and sensitive nonradioactive Nek2 in vitro kinase assays: (i) a dissociation-enhanced lanthanide fluorescence immunoassay (DELFIA) using peptides identified from a physiologically relevant protein substrate, (ii) DELFIA using Nek2 itself, (iii) a homogeneous time-resolved FRET assay termed LANCE, and (iv) A method of detecting phosphorylated products by HPLC. The DELFIA and LANCE assays are robust in that they generated more than 10-fold and 20-fold increases in signal-to-noise ratios, respectively, and are amenable to robotic high-throughput screening platforms. Validation of all four assays was confirmed by identifying a panel of small molecule ATP competitive inhibitors from an internal corporate library. The most potent compounds consistently demonstrated less than 100 nM activity regardless of the assay format and therefore were complementary. In summary, the Nek2 in vitro time-resolved FRET kinase assays reported are sensitive, quantitative, reproducible and amenable to high-throughput screening with improved waste management over radioactive assays.

A novel approach to identifying beta-secretase inhibitors: bis-statine peptide mimetics discovered using structure and spot synthesis.

Beta-secretase 1 (BACE1) is an aspartic protease believed to play a critical role in Alzheimer's disease. Inhibitors of this enzyme have been designed by incorporating the non-cleavable hydroxyethylene and statine isosteres into peptides corresponding to BACE1 substrate sequences. We sought to develop new methods to quickly characterize and optimize inhibitors based on the statine core. Minimal sequence requirements for binding were first established using both crystallography and peptide spot synthesis. These shortened peptide inhibitors were then optimized by using spot synthesis to perform iterative cycles of substitution and deletion. The present study resulted in the identification of novel "bis-statine" inhibitors shown by crystallography to have a unique binding mode. Our results demonstrate the application of peptide spot synthesis as an effective method for enhancing peptidomimetic drug discovery.

Synthesis and SAR of diazepine and thiazepine TACE and MMP inhibitors.

Potent and selective TACE and MMP inhibitors utilizing the diazepine and thiazepine ring systems were synthesized and evaluated for biological activity in in vitro and in vivo models of TNF-alpha release. Oral activity in the mouse LPS model of TNF-alpha release was seen. Efficacy in the mouse collagen induced arthritis model was achieved with diazepine 20.

Characterization of (2R, 3S)-2-([[4-(2-butynyloxy)phenyl]sulfonyl]amino)-N,3-dihydroxybutanamide, a potent and selective inhibitor of TNF-alpha converting enzyme.

TNF-alpha converting enzyme (TACE) is a validated therapeutic target for the development of oral tumor necrosis factor-alpha (TNF-alpha) inhibitors. Here we report the pre-clinical results and characterization of a selective and potent TACE inhibitor, (2R, 3S)-2-([[4-(2-butynyloxy)phenyl]sulfonyl]amino)-N,3-dihydroxybutanamide (TMI-2), in various in vitro and in vivo assays. TMI-2 is a potent TACE inhibitor in an enzymatic FRET assay (IC50=2 nM). It is more than 250-fold selective over MMP-1, -7, -9, -14, and ADAM-10 in vitro. In cell-based assays and human whole blood, TMI-2 inhibits lipopolysaccharide (LPS)-induced TNF secretion with IC50s<1 uM. Importantly, TMI-2 inhibits the spontaneous release of TNF-alpha in human synovium tissue explants of rheumatoid arthritis patients with an IC50 of 0.8 microM. In vivo, TMI-2 potently inhibits LPS-induced TNF-alpha production in mice (ED50=3 mg/kg). In the adjuvant-induced arthritis (AIA) model in rats, treatment with TMI-2 at 30 mg/kg and 100 mg/kg p.o. b.i.d. was highly effective in reducing joint arthritis scores. In a semi-therapeutic collagen-induced arthritis (CIA) model in mice, TMI-2 is highly effective in reducing disease severity scores after oral treatment at 100 mg/kg twice per day. In summary, TMI-2 is a potent and selective TACE inhibitor that inhibits TNF-alpha production and reduces the arthritis scores in pre-clinical models. TMI-2 represents a novel class of TACE inhibitors that may be effective and beneficial in the treatment of rheumatoid arthritis as well as other TNF-mediated inflammatory autoimmune diseases.

Synthesis and structure-activity relationships of 4-alkynyloxy phenyl sulfanyl, sulfinyl, and sulfonyl alkyl hydroxamates as tumor necrosis factor-alpha converting enzyme and matrix metalloproteinase inhibitors.

A series of 4-alkynyloxy phenyl sulfanyl, sulfinyl and sulfony alkyl and piperidine-4-carboxylic acid hydroxamides were synthesized. Their structure-activity relationships, against tumor necrosis factor-alpha (TACE) and matrix metalloproteinase (MMP) inhibitor activities, are presented by investigating the oxidation state on sulfur and altering the P1' substituent. The sulfonyl derivatives 20-24 carrying a 4-butynyloxy moiety were selective TACE inhibitors over the MMPs tested. The sulfinyl derivatives showed a preference for a specific oxidation on sulfur as in compounds 25-28. The selectivity over MMPs was also demonstrated in the sulfonyl series. The enhanced cellular activity was achieved upon incorporating a butynyloxy substituent in the piperidene series. Compounds 64 and 65 were potent inhibitors of TNF-alpha release in the mouse at 100 mg/kg po.

Evaluation of fluorescence-based thermal shift assays for hit identification in drug discovery.

The fluorescence-based thermal shift assay is a general method for identification of inhibitors of target proteins from compound libraries. Using an environmentally sensitive fluorescent dye to monitor protein thermal unfolding, the ligand-binding affinity can be assessed from the shift of the unfolding temperature (Delta Tm) obtained in the presence of ligands relative to that obtained in the absence of ligands. In this article, we report that the thermal shift assay can be conducted in an inexpensive, commercially available device for temperature control and fluorescence detection. The binding affinities obtained from thermal shift assays are compared with the binding affinities measured by isothermal titration calorimetry and with the IC(50) values from enzymatic assays. The potential pitfalls in the data analysis of thermal shift assays are also discussed.

Benzodiazepine inhibitors of the MMPs and TACE. Part 2.

A series of benzodiazepine MMP/TACE inhibitors bearing polar moieties has been synthesized in an effort to optimize inhibitory activity against LPS-stimulated TNF production in human monocytes and oral activity in a murine LPS model.

Substrate specificity and inducibility of TACE (tumour necrosis factor alpha-converting enzyme) revisited: the Ala-Val preference, and induced intrinsic activity.

Tumour necrosis factor alpha (TNF alpha)-converting enzyme (TACE/ADAM-17, where ADAM stands for a disintegrin and metalloproteinase) releases from the cell surface the extracellular domains of TNF and several other proteins. Previous studies have found that, while purified TACE preferentially cleaves peptides representing the processing sites in TNF and transforming growth factor alpha, the cellular enzyme nonetheless also sheds proteins with divergent cleavage sites very efficiently. More recent work, identifying the cleavage site in the p75 TNF receptor, quantifying the susceptibility of additional peptides to cleavage by TACE and identifying additional protein substrates, underlines the complexity of TACE-substrate interactions. In addition to substrate specificity, the mechanism underlying the increased rate of shedding caused by agents that activate cells remains poorly understood. Recent work in this area, utilizing a peptide substrate as a probe for cellular TACE activity, indicates that the intrinsic activity of the enzyme is somehow increased.

Synthesis and structure-activity relationship of alpha-sulfonylhydroxamic acids as novel, orally active matrix metalloproteinase inhibitors for the treatment of osteoarthritis.

The matrix metalloproteinases (MMPs) are a family of zinc-containing endopeptidases that play a key role in both physiological and pathological tissue degradation. These enzymes are strictly regulated by endogenous inhibitors such as tissue inhibitors of MMPs and alpha(2)-macroglobulins. Overexpression of these enzymes has been implicated in various pathological disorders such as arthritis, tumor metastasis, cardiovascular diseases, and multiple sclerosis. Developing effective small-molecule inhibitors to modulate MMP activity is one approach to treat these degenerative diseases. The present work focuses on the discovery and SAR of novel N-hydroxy-alpha-phenylsulfonylacetamide derivatives, which are potent, selective, and orally active MMP inhibitors.

Synthesis and structure-activity relationship of N-substituted 4-arylsulfonylpiperidine-4-hydroxamic acids as novel, orally active matrix metalloproteinase inhibitors for the treatment of osteoarthritis.

The matrix metalloproteinases (MMPs) are a family of zinc-containing endopeptidases that play a key role in both physiological and pathological tissue degradation. In our preceding paper, we have reported on a series of novel and orally active N-hydroxy-alpha-phenylsulfonylacetamide derivatives. However, these compounds had two drawbacks (moderate selectivity and chirality issues). To circumvent these two problems, a series of novel and orally active N-substituted 4-benzenesulfonylpiperidine-4-carboxylic acid hydroxyamide derivatives have been synthesized. The present paper deals with the synthesis and SAR of these compounds. Among the several compounds synthesized, derivative 55 turned out to be a potent, selective, and an orally active MMP inhibitor in the clinically relevant advanced rabbit osteoarthritis model. Detailed pharmacokinetics and metabolism data are described.

Benzodiazepine inhibitors of the MMPs and TACE.

A series of benzodiazepine inhibitors of the MMPs and TACE has been developed. These compounds display an interesting selectivity profile and should be useful tools for exploring the biological relevance of such selectivity.

Anthranilate sulfonamide hydroxamate TACE inhibitors. Part 1: Structure-based design of novel acetylenic P1' groups.

The structure-based design of potent sulfonamide hydroxamate TACE inhibitors bearing novel acetylenic P1' groups has led to compounds with excellent in vitro potency against TACE and selectivity over MMP-1.

A continuous fluorimetric assay for tumor necrosis factor-alpha converting enzyme.

Fluorogenic peptide substrates with fluorophore/quencher-capped ends have found extensive use in monitoring protease activity in the screening of small-molecule libraries for protease inhibitors. We report here the identification and characterization of a fluorogenic substrate for tumor necrosis factor-alpha converting enzyme (TACE). This substrate is a 10-amino-acid peptide (LAQAVRSSSR) capped with an o-aminobenzoyl group on the N-terminal end and with a 3-(2,4-dinitrophenyl)-L-2,3-diaminopropionic amide group on the C-terminal end. Exhaustive enzymatic conversion of the substrate to products resulted in a fluorescence enhancement of -11-fold. A single cleavage occurred at the A-V scissile bond of the peptide. The validity of this fluorimetric assay for TACE was corroborated by an independent HPLC method. Interestingly, the hydrolysis of the substrate displayed positive cooperativity with a Hill coefficient of 1.5, while the hydrolysis of the corresponding uncapped peptide displayed Michaelis-Menten kinetics. A k(cat) value of 21.6 s(-1) and an S(0.5) value of 342 microM were obtained for the fluorogenic substrate. The addition of the two capping groups on the two ends of the peptide enhanced the k(cat) value by 64-fold. Nine additional decapeptides that contained the same capping groups on the two ends and substitutions at the P1 and P1' sites were also tested. TACE appears to slightly prefer the A-V scissile bond. The enzyme also cleaves scissile bonds such as F-V, A-I, and A-L efficiently.