SummaryThe 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.Electronic supplementary materialThe online version of this article (doi:10.1007/s10637-014-0120-7) contains supplementary material, which is available to authorized users.

Effective treatments for primary brain tumors and brain metastases represent a major unmet medical need. Targeting the CDK4/CDK6-cyclin D1-Rb-p16/ink4a pathway using a potent CDK4 and CDK6 kinase inhibitor has potential for treating primary central nervous system tumors such as glioblastoma and some peripheral tumors with high incidence of brain metastases. We compared central nervous system exposures of two orally bioavailable CDK4 and CDK6 inhibitors: abemaciclib, which is currently in advanced clinical development, and palbociclib (IBRANCE; Pfizer), which was recently approved by the U.S. Food and Drug Administration. Abemaciclib antitumor activity was assessed in subcutaneous and orthotopic glioma models alone and in combination with standard of care temozolomide (TMZ). Both inhibitors were substrates for xenobiotic efflux transporters P-glycoprotein and breast cancer resistant protein expressed at the blood-brain barrier. Brain K p,uu values were less than 0.2 after an equimolar intravenous dose indicative of active efflux but were approximately 10-fold greater for abemaciclib than palbociclib. K p,uu increased 2.8-and 21-fold, respectively, when similarly dosed in P-gp-deficient mice. Abemaciclib had brain area under the curve (0-24 hours) K p,uu values of 0.03 in mice and 0.11 in rats after a 30 mg/kg p.o. dose. Orally dosed abemaciclib significantly increased survival in a rat orthotopic U87MG xenograft model compared with vehicle-treated animals, and efficacy coincided with a dose-dependent increase in unbound plasma and brain exposures in excess of the CDK4 and CDK6 K i values. Abemaciclib increased survival time of intracranial U87MG tumor-bearing rats similar to TMZ, and the combination of abemaciclib and TMZ was additive or greater than additive. These data show that abemaciclib crosses the blood-brain barrier and confirm that both CDK4 and CDK6 inhibitors reach unbound brain levels in rodents that are expected to produce enzyme inhibition; however, abemaciclib brain levels are reached more efficiently at presumably lower doses than palbociclib and are potentially on target for a longer period of time.

SummaryThe HGF/MET signaling pathway regulates a wide variety of normal cellular functions that can be subverted to support neoplasia, including cell proliferation, survival, apoptosis, scattering and motility, invasion, and angiogenesis. MET over-expression (with or without gene amplification), aberrant autocrine or paracrine ligand production, and missense MET mutations are mechanisms that lead to activation of the MET pathway in tumors and are associated with poor prognostic outcome. We report here preclinical development of a potent, orally bioavailable, small-molecule inhibitor LY2801653 targeting MET kinase. LY2801653 is a type-II ATP competitive, slow-off inhibitor of MET tyrosine kinase with a dissociation constant (Ki) of 2 nM, a pharmacodynamic residence time (Koff) of 0.00132 min−1 and t1/2 of 525 min. LY2801653 demonstrated in vitro effects on MET pathway-dependent cell scattering and cell proliferation; in vivo anti-tumor effects in MET amplified (MKN45), MET autocrine (U-87MG, and KP4) and MET over-expressed (H441) xenograft models; and in vivo vessel normalization effects. LY2801653 also maintained potency against 13 MET variants, each bearing a single-point mutation. In subsequent nonclinical characterization, LY2801653 was found to have potent activity against several other receptor tyrosine oncokinases including MST1R, FLT3, AXL, MERTK, TEK, ROS1, DDR1/2 and against the serine/threonine kinases MKNK1/2. The potential value of MET and other inhibited targets within a number of malignancies (such as colon, bile ducts, and lung) is discussed. LY2801653 is currently in phase 1 clinical testing in patients with advanced cancer (trial I3O-MC-JSBA, NCT01285037).Electronic supplementary materialThe online version of this article (doi:10.1007/s10637-012-9912-9) contains supplementary material, which is available to authorized users.

Purpose: Selective inhibition of cyclin-dependent kinases 4 and 6 (CDK4/6) represents a promising therapeutic strategy. However, despite documented evidence of clinical activity, limited information is available on the optimal dosing strategy of CDK4/6 inhibitors. Here, we present an integrated semimechanistic pharmacokinetic/pharmacodynamic model to characterize the quantitative pharmacology of LY2835219, a CDK4/6 inhibitor, in xenograft tumors.Experimental Design: LY2835219 plasma concentrations were connected to CDK4/6 inhibition and cell-cycle arrest in colo-205 human colorectal xenografts by incorporating the biomarkers, phospho-(ser780)-Rb, topoisomerase II a, and phosphohistone H3, into a precursor-dependent transit compartment model. This biomarker model was then connected to tumor growth inhibition (TGI) by: (i) relating the rate of tumor growth to mitotic cell density, and (ii) incorporating a concentration-dependent mixed cytostatic/ cytotoxic effect driving quiescence and cell death at high doses. Model validation was evaluated by predicting LY2835219-mediated antitumor effect in A375 human melanoma xenografts.Results: The model successfully described LY2835219-mediated CDK4/6 inhibition, cell-cycle arrest, and TGI in colo-205, and was validated in A375. The model also demonstrated that a chronic dosing strategy achieving minimum steady-state trough plasma concentrations of 200 ng/mL is required to maintain durable cell-cycle arrest. Quiescence and cell death can be induced by further increasing LY2835219 plasma concentrations.Conclusions: Our model provides mechanistic insight into the quantitative pharmacology of LY2835219 and supports the therapeutic dose and chronic dosing strategy currently adopted in clinical studies.

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.

The PI3K/AKT/mTOR pathway is among the most frequently altered pathways in cancer cell growth and survival.

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Effective treatments for primary brain tumors and brain metastases represent a major unmet medical need. The blood-brain barrier (BBB) arises from both a structural barrier and drug efflux transporters that prevent most anti-cancer drugs from efficiently reaching brain tumors or metastases. The CDK4/6 pathway (CDK4/6-cyclin D1-Rb-CDKN2) plays a key role in regulating cellular proliferation. The importance of this pathway is highlighted by its inactivation in a majority of human tumors including glioblastoma multiforme. We have identified and characterized a potent and selective dual cdk4/6 inhibitor, LY2835219. Preclinical characterization was performed with the monomesylate salt (LY2835219-MsOH), which inhibits these kinases with IC50 values of 2 and 10 nM for CDK4 and CDK6, respectively. LY2835219-MsOH is a potent inhibitor of Rb phosphorylation in vitro and in vivo that induces G1 specific arrest and inhibition of tumor growth. To determine the potential of LY2835219-MsOH for the treatment of brain tumors and metastases, we assessed the ability of LY2835219-MsOH to cross the BBB and its interaction with the P-gp and BCRP efflux pumps that are expressed at the BBB. Using MDCK cells over-expressing either human ABCB1 (P-gp) or mouse abcg2 (Bcrp), LY2835219-MsOH and a second CDK4/6 inhibitor (PD0332991) are substrates for these two pumps, but each cross the murine BBB in vivo to a different degree. Unlike PD0332991, LY2835219-MsOH saturates BBB efflux with an unbound plasma IC50 of ∼95 nM (1.8 uM total plasma). The percent of dose in brain for LY2835219-MsOH is 0.5–3.9% and is comparable to that for temozolomide (1.9% plasma). In both a subcutaneous and intracranial human glioblastoma model (U87MG), LY2835219-MsOH suppressed tumor growth in a dose-dependent manner both as a single agent, and in combination with temozolomide. In summary, LY2835219-MsOH is a potent and selective oral CDK4/6 inhibitor that crosses the BBB and inhibits the growth of intracranial tumors alone or in combination with other agents. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr B234.