CDK8 inhibitor KY-065 rescues skeletal abnormalities in achondroplasia model mice

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A new study highlights KY-065, a CDK8 inhibitor, as a potential breakthrough treatment for achondroplasia (ACH). ACH, the most common form of genetic dwarfism, is caused by a mutation in FGFR3, leading to disrupted bone growth via the MAPK and STAT1 pathways.

So, what’s new?

• KY-065 targets the STAT1 pathway, which remains untapped by current treatments like vosoritide (focused on MAPK).
• In a mouse model of ACH, KY-065 restored normal chondrocyte differentiation, improved growth plate structure, and significantly elongated long bones.
• This was achieved by blocking CDK8 activity and reducing STAT1 phosphorylation without interfering with MAPK signaling.

These findings suggest CDK8 could be a promising therapeutic target, with KY-065 leading the way as a potential treatment for children with achondroplasia.

highlights

• KY-065 inhibits CDK8 with high affinity in a ATP-competitive manner by binding preferentially to the DMG-out conformation.
• KY-065 blocks the STAT1^Ser727 phosphorylation in chondrocytes of a mouse model of achondroplasia.
• KY-065 rescues impaired chondrogenesis and stunted long bone growth in a mouse model of achondroplasia in vitro and in vivo.

abstract

Cyclin-dependent kinase 8 (CDK8) is a transcription-related CDK family member implicated in the regulation of bone homeostasis, and we recently demonstrated that our internally developed CDK8 inhibitor KY-065 can prevent postmenopausal osteoporosis in a mouse model. Achondroplasia (ACH), the most common form of genetic dwarfism in humans, is caused by a gain-of-function mutation in fibroblast growth factor receptor 3 (FGFR3), a receptor tyrosine kinase that activates downstream mitogen-activated protein kinase (MAPK) and signal transducer and activator of transcription (STAT) signaling pathways. The first precision drug approved for the treatment of ACH in children, the C-type natriuretic peptide analog vosoritide, antagonizes the MAPK pathway, while there are currently no effective and safe medications targeting the STAT1 pathway. Here, we demonstrate that KY-065 rescues impaired chondrogenesis and stunted long bone growth in the Fgfr3^Ach mouse model of ACH. KY-065 inhibited CDK8 with high affinity in vitro by competing with ATP. The CDK8 expression and STAT1^Ser727 phosphorylation were upregulated in chondrocytes isolated from ACH model mice, and KY-065 repressed its phosphorylation and restored normal chondrogenic differentiation without affecting MAPK activation. Moreover, daily administration of 10 mg/kg KY-065 to Fgfr3^Ach mice (yielding a peak concentration of 22.0 ± 1.47 ?M in plasma) resulted in significant elongation of long bone and improved growth plate cytoarchitecture. Collectively, these findings identify the CDK8 in chondrocytes as a potential therapeutic target for ACH and KY-065 as a promising candidate drug treatment for this debilitating skeletal disease.

introduction

The cyclin-dependent kinases (CDKs) are a family of 20 serine/threonine kinases that regulate a multitude of downstream processes including the cell cycle by binding to specific cyclin (Cyc) protein partners [1,2]. They are divided into two major classes: the cell cycle-associated CDKs CDK1, CDK2, CDK4, and CDK6, and the transcription-associated CDKs CDK7, CDK8, CDK9, CDK12, CDK13, and CDK19. Cell cycle-associated CDKs are regarded as promising anti-cancer targets, and a number of pharmacological inhibitors are in various stages of development as chemotherapeutics, including CDK4/6 inhibitors approved for metastatic breast cancer [3,4]. In contrast, there has been limited development of inhibitors targeting transcription-associated CDKs for therapeutic applications. CDK8 and its closely related paralog CDK19 form the CDK module of a complex that regulates the transcription of multiple gene pathways through distinct mechanisms [5]. We recently developed a CDK8 inhibitor, KY-065, and demonstrated that it suppresses both stemness and tumorigenicity of glioma stem cells, suggesting potential efficacy for the treatment of glioblastoma [6]. Moreover, we have also shown that CDK8 controls osteoclastogenesis and bone homeostasis through expression in mesenchymal stem cells, and that KY-065 prevents postmenopausal osteoporosis, the most common skeletal disease globally, in a mouse model [7].

Achondroplasia (ACH) is the most common form of genetic dwarfism in humans, with over 250,000 afflicted individuals worldwide [8,9]. Cardinal features of ACH include disproportionally shorter proximal skeletal segments (rhizomelia), larger head size (macrocephaly) with frontal bossing and midface hypoplasia, thoracolumbar kyphosis, and lumbar lordosis [10]. ACH is associated with increased mortality in early childhood and greater risk of obesity in adults [11,12]. The condition arises from a gain-of-function mutation in fibroblast growth factor receptor 3 (FGFR3), a receptor tyrosine kinase that among other functions regulates endochondral bone growth through multiple downstream intracellular signaling pathways, including signal transducer and activator of transcription (STAT) and mitogen-activated protein kinase (MAPK) pathways [13,14]. The vast majority of ACH cases (>98 %) are associated with a glycine-to-arginine substitution (G380R) in the transmembrane domain of FGFR3, resulting in its stabilization at the cell membrane and a ligand-independent increase in signaling [[15], [16], [17]]. In chondrocytes, this constitutive activation impairs normal chondrogenic properties through sustained STAT and MAPK pathway signaling [18,19].

At one time there were few treatment options for ACH. In Japan, pediatric cases were treated with growth hormone, but with limited success [20]. The C-type natriuretic peptide (CNP) analog vosoritide is the first precision drug approved for the treatment of ACH in children (by once-daily subcutaneous injection) [21,22]. Vosoritide binds natriuretic peptide receptor type B and antagonizes FGFR3 signaling by inhibiting the MAPK pathway [23]. On the contrary, there are no effective, safe, and convenient therapies targeting the FGFR3–STAT axis so far. Further, the development of novel drugs that can be administered orally would reduce the burden on pediatric patients. Thus, in this study, we investigated the potential efficacy of KY-065 for reversing the effects of the Fgfr3^G380R mutation on chondrocyte differentiation in vitro and on skeletal abnormalities in the Fgfr3^G380R mouse model of ACH.

citation

Sadamori K, Kubo T, Yoshida T, Yamamoto M, Shibata Y, Fukasawa K, Tokumura K, Horie T, Kadota T, Yamakawa R, Hojo H, Tanaka N, Kitao T, Shirahase H, Hinoi E. CDK8 inhibitor KY-065 rescues skeletal abnormalities in achondroplasia model mice. Biochim Biophys Acta Mol Basis Dis. 2024 Dec 12:167626. doi: 10.1016/j.bbadis.2024.167626. Epub ahead of print. PMID: 39674288.