Continuous treatment with abemaciclib leads to sustained and efficient inhibition of breast cancer cell proliferation

Abemaciclib is an oral, selective cyclin-dependent kinase 4 & 6 inhibitor (CDK4 & 6i), approved for hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2–) advanced breast cancer (ABC) as monotherapy for endocrine refractory disease, and with endocrine therapy (ET) for initial treatment and after progression on ET. Abemaciclib has also shown clinical activity in combination with ET in patients with high risk early BC (EBC). Here, we examined the preclinical attributes of abemaciclib and other CDK4 & 6i using biochemical and cell-based assays. In vitro, abemaciclib preferentially inhibited CDK4 kinase activity versus CDK6, resulting in inhibition of cell proliferation in a panel of BC cell lines with higher average potency than palbociclib or ribociclib. Abemaciclib showed activity regardless of HER2 amplification and phosphatidylinositol 3-kinase (PI3KCA) gene mutation status. In human bone marrow progenitor cells, abemaciclib showed lower impact on myeloid maturation than other CDK4 & 6i when tested at unbound concentrations similar to those observed in clinical trials. Continuous abemaciclib treatment provided profound inhibition of cell proliferation, and triggered senescence and apoptosis. These preclinical results support the unique efficacy and safety profile of abemaciclib observed in clinical trials.

Abstract 1236: Preclinical characterization of LY3484356, a novel, potent and orally bioavailable selective estrogen receptor degrader (SERD)

Nearly 70% of newly diagnosed breast cancers are estrogen receptor alpha (ERα) positive, for which endocrine therapy is a primary treatment. However, approximately 40% of those patients on endocrine therapy develop resistance, which includes mutations in ERα (ESR1) that drive constitutive activation of the receptor. One approach to overcome this resistance is to develop a potent degrader and pure antagonist of the estrogen receptor, which can effectively suppress estrogen receptor signaling. Fulvestrant, a pure antagonist and selective estrogen receptor degrader (SERD) is approved for the treatment of HR+/HER2- metastatic breast cancer. However, fulvestrant is limited by poor pharmacokinetics properties, limited exposure and sub-optimal in vivo estrogen receptor degradation. Here we describe the preclinical profile of LY3484356, an oral SERD and pure estrogen receptor antagonist, with potent activity against the wild type and mutant estrogen receptor. LY3484356 has Ki values of 0.64 nM and 2.8 nM against wild type ERα and Y537S mutant ERα proteins, respectively. It is a potent and highly efficient degrader of wild type ERα and Y537N mutant ERα proteins in cells, with IC50 values 3.0 nM and 9.6 nM, respectively. LY3484356 is also a potent inhibitor of ERα-mediated transcription in vitro and in vivo. It inhibits cell proliferation in wild type ERα and ESR1 Y537N mutant breast cancer cell lines, with average IC50 values of 3 nM and 17 nM, respectively. In a panel of breast cancer cell lines, 11 out of 12 ER+ breast cancer cell lines were sensitive to LY3484356 (IC50 less than 100 nM), whereas all ER- cell lines tested were insensitive. LY3484356 has demonstrated sustained and prolonged target inhibition (>75% inhibition of PGR transcription up to 96h after last dose) in ESR1 wild type (MCF7) and ESR1 Y537S mutant (ST941/C) xenograft tumors. Consistent with its profile as a pure antagonist, immature rat studies demonstrated no significant effect on uterine wet weight. LY3484356 demonstrated significant tumor growth inhibition and tumor regressions in wild type ESR1 breast cancer xenograft models such as MCF7, T47D and ZR-75-1, as well as ESR1 mutant breast cancer PDX models. LY3484356 has shown synergy or additivity in combination with CDK4/6 inhibitor abemaciclib, mTOR inhibitor everolimus and PIK3CA inhibitor alpelisib in inhibiting cell proliferation in ER+ breast cancer cell lines in vitro and tumor growth inhibition in relevant xenograft or PDX models in vivo. The first-in-human Phase 1/2 clinical trial of LY3484356 (EMBER, ClinicalTrials.gov NCT04188548) is currently ongoing and a window-of-opportunity study evaluating the pharmacodynamic effects of LY3484356 in early stage breast cancer is expected to begin early 2021.

Citation Format: Shripad V. Bhagwat, Baohui Zhao, Weihua Shen, Cecilia Mur, Robert Barr, Lisa J. Kindler, Almudena Rubio, Jolie A. Bastian, Jeffrey D. Cohen, Brian E. Mattioni, Eunice Yuen, Thomas K. Baker, Mark A. Castanares, Dongling Fei, Jason R. Manro, Maria Jose Lallena, Sheng-Bin Peng, Alfonso de Dios. Preclinical characterization of LY3484356, a novel, potent and orally bioavailable selective estrogen receptor degrader (SERD) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1236.

Abemaciclib, a CDK4 and 6 inhibitor with unique pharmacological properties for breast cancer therapy

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Background: Breast cancer is the second most common cancer worldwide. Pharmacologically targeting cyclin-dependent kinases 4 and 6 (CDK4 & 6) has proven to be a successful therapeutic approach in patients with estrogen receptor positive (ER+) breast cancer. Differences in both efficacy and toxicity among the available CDK4 & 6 inhibitors has generated interest in a biological explanation. Abemaciclib is an adenosine triphosphate-competitive, reversible, selective inhibitor of CDK4 & 6 that has shown antitumor activity as a single agent and in combination with standard endocrine therapy (ET), in hormone receptor positive (HR+) metastatic breast cancer patients including those with ET resistance, and in combination with ET in high-risk early breast cancer patients. This study examines attributes of abemaciclib and other CDK4 & 6 inhibitors. Methods: The potency of abemaciclib for CDK4 was evaluated using biochemicals and breast cancer cell-based assays. Additionally, different combinations with an anti-estrogen therapy (e.g., tamoxifen, fulvestrant) were analyzed in an in vitro palbociclib (CDK4 & 6 inhibitor)-resistant breast cancer cell model, as well as in a set of CDK4 & 6 sensitive breast cancer cell models. Using cell-free assays, high content imaging and flow cytometry approaches, a subset of markers were monitored to characterize the phenotype of sensitive cell lines in a continuous dose schedule. Results: In in vitro, cell-free assays, abemaciclib shows selectivity for CDK4 over CDK6, and in cell-based assays, abemaciclib preferentially inhibits the proliferation of cells dependent on the presence of CDK4, not CDK6. Abemaciclib inhibits cell proliferation in a wide range of breast cancer cell lines, showing activity regardless of human epidermal growth factor receptor 2 (HER2) and PI3KCA gene mutation status. Furthermore, in a cell line resistant to palbociclib, abemaciclib in combination with fulvestrant (ET) restores CDK4 & 6 sensitivity, leading to cell senescence and cell death. Finally, in human bone marrow progenitor cells, abemaciclib shows a lesser impact on myeloid maturation than other CDK4 & 6 inhibitors, palbociclib and ribociclib, allowing for continuous dosing. Conclusions: In pre-clinical experiments, abemaciclib is a potent cell growth inhibitor, inhibiting preferentially the CDK4/CyclinD1 complex, leading to cell senescence and cell death. These pre-clinical results support the differentiated safety and efficacy profile of abemaciclib observed in clinical trials.

Cyclin dependent kinase (CDK) inhibitors as anticancer drugs: Recent advances (2015-2019)

Sustained proliferative capacity and gene dysregulation are hallmarks of cancer. In mammalian cells, cyclin-dependent kinases (CDKs) control critical cell cycle checkpoints and key transcriptional events in response to extracellular and intracellular signals leading to proliferation. Significant clinical activity for the treatment of hormone receptor positive metastatic breast cancer has been demonstrated by palbociclib, ribociclib and abemaciclib, dual CDK4/6 inhibitors recently FDA-approved. SY-1365, a CDK7 inhibitor has shown initial encouraging data in phase I for solid tumors treatment. These results have rejuvenated the CDKs research field. This review provides an overview of relevant advances on CDK inhibitor research since 2015 to 2019, with special emphasis on transcriptional CDK inhibitors, new emerging strategies such as target protein degradation and compounds under clinical evaluation.

Preclinical characterization of abemaciclib in hormone receptor positive breast cancer

Abemaciclib is an ATP-competitive, reversible kinase inhibitor selective for CDK4 and CDK6 that has shown antitumor activity as a single agent in hormone receptor positive (HR+) metastatic breast cancer in clinical trials. Here, we examined the mechanistic effects of abemaciclib treatment using in vitro and in vivo breast cancer models. Treatment of estrogen receptor positive (ER+) breast cancer cells with abemaciclib alone led to a decrease in phosphorylation of Rb, arrest at G1, and a decrease in cell proliferation. Moreover, abemaciclib exposure led to durable inhibition of pRb, TopoIIα expression and DNA synthesis, which were maintained after drug removal. Treatment of ER+ breast cancer cells also led to a senescence response as indicated by accumulation of β-galactosidase, formation of senescence-associated heterochromatin foci, and a decrease in FOXM1 positive cells. Continuous exposure to abemaciclib altered breast cancer cell metabolism and induced apoptosis. In a xenograft model of ER+ breast cancer, abemaciclib monotherapy caused regression of tumor growth. Overall these data indicate that abemaciclib is a CDK4 and CDK6 inhibitor that, as a single agent, blocks breast cancer cell progression, and upon longer treatment can lead to sustained antitumor effects through the induction of senescence, apoptosis, and alteration of cellular metabolism.

Preclinical characterization of the CDK4/6 inhibitor LY2835219: in-vivo cell cycle-dependent/independent anti-tumor activities alone/in combination with gemcitabine

The G1 restriction point is critical for regulating the cell cycle and is controlled by the Rb pathway (CDK4/6-cyclin D1-Rb-p16/ink4a). This pathway is important because of its inactivation in a majority of human tumors. Transition through the restriction point requires phosphorylation of retinoblastoma protein (Rb) by CDK4/6, which are highly validated cancer drug targets. We present the identification and characterization of a potent CDK4/6 inhibitor, LY2835219. LY2835219 inhibits CDK4 and CDK6 with low nanomolar potency, inhibits Rb phosphorylation resulting in a G1 arrest and inhibition of proliferation, and its activity is specific for Rb-proficient cells. In vivo target inhibition studies show LY2835219 is a potent inhibitor of Rb phosphorylation, induces a complete cell cycle arrest and suppresses expression of several Rb-E2F-regulated proteins 24 hours after a single dose. Oral administration of LY2835219 inhibits tumor growth in human tumor xenografts representing different histologies in tumor-bearing mice. LY2835219 is effective and well tolerated when administered up to 56 days in immunodeficient mice without significant loss of body weight or tumor outgrowth. In calu-6 xenografts, LY2835219 in combination with gemcitabine enhanced in vivo antitumor activity without a G1 cell cycle arrest, but was associated with a reduction of ribonucleotide reductase expression. These results suggest LY2835219 may be used alone or in combination with standard-of-care cytotoxic therapy. In summary, we have identified a potent, orally active small-molecule inhibitor of CDK4/6 that is active in xenograft tumors. LY2835219 is currently in clinical development.

A novel CDK9 inhibitor shows potent antitumor efficacy in preclinical hematologic tumor models

DNA-dependent RNA polymerase II (RNAP II) largest subunit RPB1 C-terminal domain (CTD) kinases, including CDK9, are serine/threonine kinases known to regulate transcriptional initiation and elongation by phosphorylating Ser 2, 5, and 7 residues on CTD. Given the reported dysregulation of these kinases in some cancers, we asked whether inhibiting CDK9 may induce stress response and preferentially kill tumor cells. Herein, we describe a potent CDK9 inhibitor, LY2857785, that significantly reduces RNAP II CTD phosphorylation and dramatically decreases MCL1 protein levels to result in apoptosis in a variety of leukemia and solid tumor cell lines. This molecule inhibits the growth of a broad panel of cancer cell lines, and is particularly efficacious in leukemia cells, including orthotopic leukemia preclinical models as well as in ex vivo acute myeloid leukemia and chronic lymphocytic leukemia patient tumor samples. Thus, inhibition of CDK9 may represent an interesting approach as a cancer therapeutic target, especially in hematologic malignancies.

Dipeptides as effective prodrugs of the unnatural amino acid (+)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (LY354740), a selective group II metabotropic glutamate receptor agonist

(+)-2-Aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (1), also known as LY354740, is a highly potent and selective agonist for group II metabotropic glutamate receptors (mGlu receptors 2 and 3) tested in clinical trials. It has been shown to block anxiety in the fear-potentiated startle model. Its relatively low bioavailability in different animal species drove the need for an effective prodrug form that would produce a therapeutic response at lower doses for the treatment of anxiety disorders. We have investigated the increase of intestinal absorption of this compound by targeting the human peptide transporter hPepT1 for active transport of di- and tripeptides derived from 1. We have found that oral administration of an N dipeptide derivative of 1 (12a) in rats shows up to an 8-fold increase in drug absorption and a 300-fold increase in potency in the fear-potentiated startle model in rats when compared with the parent drug 1.

Design of potent and selective 2-aminobenzimidazole-based p38alpha MAP kinase inhibitors with excellent in vivo efficacy

We report the design and discovery of a 2-aminobenzimidazole-based series of potent and highly selective p38alphainhibitors. The lead compound 1 had low-nanomolar activity in both ATP competitive enzyme binding and inhibition of TNFalpha release in macrophages. Compound 18 showed excellent pharmacokinetics properties and oral activity in the rat collagen induced arthritis model compared with other p38 reference compounds. A SAR strategy to address CyP3A4 liability is also described.

Total synthesis of (+)-aspidospermidine: a new strategy for the enantiospecific synthesis of aspidosperma alkaloids

A new strategy was developed for the enantiospecific synthesis of aspidosperma alkaloids. The key steps involve a novel ketene-lactonization reaction of a chiral vinyl sulfoxide to efficiently set up the quaternary carbon center, and a tandem Michael addition-alkylation reaction sequence to form the polycyclic core structure. This new strategy was employed in the total synthesis of natural product (+)-aspidospermidine.

Structural and conformational studies of 5-(1H-pyrrol-2-ylmethylene)-substituted imidazolidine-2,4-diones and thiazolidine-2,4-diones

Some imidazolidine-2,4-dione (hydantoin) and thiazolidine-2,4-dione (TZD) derivatives with a 1H-pyrrol-2-ylmethylene substituent at the 5-position (1–8) have been synthesized via an aldol condensation reaction. A mixture of Z- and E- stereoisomers was obtained, as confirmed by HPLC and NMR studies. Assignment of the stereochemistry was achieved through chemical shift knowledge, NOE, and 3JH,C data. The conformation of the molecules depends on the configuration at the double bond. While the (NH,C cis) form is the most stable conformer for the E-isomer, the (NH,C trans) form is the preferred conformer for the Z-isomer. The temperature coefficients of the NH pyrrole protons reveal the existence of an intramolecular hydrogen bond for the E-isomers.Key words: hydantoin, TZD, NMR spectroscopy, conformational analysis, temperature coefficient.

4-Substituted D-glutamic acid analogues: the first potent inhibitors of glutamate racemase (MurI) enzyme with antibacterial activity

The first potent inhibitors of glutamate racemase (MurI) enzyme that show whole cell antibacterial activity are described. Optically pure 4-substituted D-glutamic acid analogues with (2R,4S) stereochemistry and bearing aryl-, heteroaryl-, cinnamyl-, or biaryl-methyl substituents represent a novel class of glutamate racemase inhibitors. Exploration of the D-Glu core led to the identification of lead compounds (-)-8 and 10. 2-Naphthylmethyl derivative 10 was found to be a potent competitive inhibitor of glutamate racemase activity (K(i) = 16 nM, circular dichroism assay; IC(50) = 0.1 microg/mL high-performance liquid chromatography (HPLC) assay). Thorough structure-activity relationship (SAR) studies led to benzothienyl derivatives such as 69 and 74 with increased potency (IC(50) = 0.036 and 0.01 microg/mL, respectively, HPLC assay). These compounds showed potent whole cell antibacterial activity against S. pneumoniae PN-R6, and good correlation with the enzyme assay. Compounds 69, 74 and biaryl derivative 52 showed efficacy in an in vivo murine thigh infection model against Streptococcus pneumoniae. Data described herein suggest that glutamate racemase may be a viable target for developing new antibacterial agents.