Brain Exposure of Two Selective Dual CDK4 and CDK6 Inhibitors and the Antitumor Activity of CDK4 and CDK6 Inhibition in Combination with Temozolomide in an Intracranial Glioblastoma Xenograft.

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 Kp,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. Kp,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) Kp,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 Ki 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.

Raub TJ ，Wishart GN ，Kulanthaivel P ，Staton BA ，Ajamie RT ，Sawada GA ，Gelbert LM ，Shannon HE ，Sanchez-Martinez C ，De Dios A ... -  《-》

A highly potent CDK4/6 inhibitor was rationally designed to overcome blood brain barrier in gliobastoma therapy.

Glioblastoma multiforme (GBM) is the most common and deadliest of malignant brain tumors in adults. Disease development is associated with dysregulation of the cyclin D-CDK4/6-INK4-Rb pathway, resulting in increased proliferation; thus, CDK4/6 kinase inhibitors are promising candidates for GBM treatment. The recently developed CDK4/6 inhibitors, palbociclib, ribociclib, and abemaciclib, are effective in subcutaneous glioma models, but their blood-brain barrier (BBB) permeability is poor, limiting drug delivery to the central nervous system. Here, we designed and synthesized a series of novel CDK4/6 inhibitors with favorable BBB permeability for the treatment of GBM. Compound 11 exhibited a favorable pharmacological profile and significant penetration of the BBB with the Kp value of 4.10 and the Kp,uu value of 0.23 in mice after an oral dose of 10 mg/kg. IC50 values for CDK4/cyclin D1 and CDK6/cyclin D3 were 3 nM and 1 nM, respectively. In vivo studies with an orthotopic xenograft mouse model of GBM showed that 11 had tumor growth inhibition values ranging from 62% to 99% for doses ranging from 3.125 to 50 mg/kg, and no significant body weight loss was observed. The increase in life span based on the median survival time of vehicle-treated animals in mice administered a dose of 50 mg/kg was significant at 162% (p < 0.0001). These results suggest that compound 11 is a promising candidate for further investigation as an effective drug for the treatment of GBM.

Yin L ，Li H ，Liu W ，Yao Z ，Cheng Z ，Zhang H ，Zou H ... -  《-》

Biological specificity of CDK4/6 inhibitors: dose response relationship, in vivo signaling, and composite response signature.

Knudsen ES ，Hutcheson J ，Vail P ，Witkiewicz AK ... -  《Oncotarget》

Design of a brain-penetrant CDK4/6 inhibitor for glioblastoma.

CDK4 and CDK6 are kinases with similar sequences that regulate cell cycle progression and are validated targets in the treatment of cancer. Glioblastoma is characterized by a high frequency of CDKN2A/CCND2/CDK4/CDK6 pathway dysregulation, making dual inhibition of CDK4 and CDK6 an attractive therapeutic approach for this disease. Abemaciclib, ribociclib, and palbociclib are approved CDK4/6 inhibitors for the treatment of HR+/HER2- breast cancer, but these drugs are not expected to show strong activity in brain tumors due to poor blood brain barrier penetration. Herein, we report the identification of a brain-penetrant CDK4/6 inhibitor derived from a literature molecule with low molecular weight and topological polar surface area (MW = 285 and TPSA = 66 Å2), but lacking the CDK2/1 selectivity profile due to the absence of a basic amine. Removal of a hydrogen bond donor via cyclization of the pyrazole allowed for the introduction of basic and semi-basic amines, while maintaining in many cases efflux ratios reasonable for a CNS program. Ultimately, a basic spiroazetidine (cpKa = 8.8) was identified that afforded acceptable selectivity over anti-target CDK1 while maintaining brain-penetration in vivo (mouse Kp,uu = 0.20-0.59). To probe the potency and selectivity, our lead compound was evaluated in a panel of glioblastoma cell lines. Potency comparable to abemaciclib was observed in Rb-wild type lines U87MG, DBTRG-05MG, A172, and T98G, while Rb-deficient cell lines SF539 and M059J exhibited a lack of sensitivity.

Bronner SM ，Merrick KA ，Murray J ，Salphati L ，Moffat JG ，Pang J ，Sneeringer CJ ，Dompe N ，Cyr P ，Purkey H ，Boenig GL ，Li J ，Kolesnikov A ，Larouche-Gauthier R ，Lai KW ，Shen X ，Aubert-Nicol S ，Chen YC ，Cheong J ，Crawford JJ ，Hafner M ，Haghshenas P ，Jakalian A ，Leclerc JP ，Lim NK ，O'Brien T ，Plise EG ，Shalan H ，Sturino C ，Wai J ，Xiao Y ，Yin J ，Zhao L ，Gould S ，Olivero A ，Heffron TP ... -  《-》

Physiologically Based Pharmacokinetic Modeling of Central Nervous System Pharmacokinetics of CDK4/6 Inhibitors to Guide Selection of Drug and Dosing Regimen for Brain Cancer Treatment.

A better understanding of the human central nervous system (CNS) pharmacokinetics is critical to the selection of the right drug and refinement of dosing regimen for more effective treatment of primary and metastatic brain cancer. Using the physiologically-based pharmacokinetic (PBPK) modeling approach, we systematically compared the CNS pharmacokinetics of three cyclin D-cyclin dependent kinase 4 and 6 (CDK4/6) inhibitors (ribociclib, palbociclib, and abemaciclib) in patients with cancer. A PBPK model platform was developed and verified for predicting plasma and CNS pharmacokinetics. Target engagement ratio (TER), defined as the ratio of the average steady-state unbound drug brain concentration to the in vitro half-maximal inhibitory concentration (IC50 ) for CDK4/6 inhibition, was used as a crude predictor of efficacy. As compared with ribociclib and palbociclib, abemaciclib penetrated into the human brain to a larger extent, but at a slower rate, and was retained in the brain longer. Following the standard dosing regimens, the predicted CDK4/6 TERs were 26/5.2 for abemaciclib, 2.4/0.62 for ribociclib, and 0.36/0.27 for palbociclib. Simulations suggested that abemaciclib achieved comparable TERs following twice daily or daily dosing; ribociclib may sufficiently inhibit both CDK4 and CDK6 at the maximum tolerated dose; whereas, palbociclib achieved TERs < 0.5 even at a dose 50% higher than the standard dose. In conclusion, the PBPK modeling, supported by available preclinical and clinical evidence, suggests that abemaciclib is the best CDK4/6 inhibitor for brain cancer treatment, whereas palbociclib is not recommended. The model refined dosing regimen is 300 mg daily on a 4-weeks-on schedule for abemaciclib, and 900 mg daily on a 3-weeks-on/1-week-off schedule for ribociclib.

Li J ，Jiang J ，Wu J ，Bao X ，Sanai N ... -  《-》