Abstract P051: Identification of an orally bioavailable dual Cyclin K glue degrader - CDK12/13 inhibitor
Authors: Ainscow Edward K. , Campbell Adrian , Cripps Michael , Workman Robert , Thomson Stuart , Chohan Kam , Crepin Damien , Sunose Mahiro , Patient Jamie , Kendrew Jane , Bahl Ash 
Published: December 2021

Abstract
CDK12 and CDK13 regulate gene expression through translation and splicing of mRNA. Impaired activity of CDK12 has been shown to reduce the expression of a range of genes involved in DNA homologous repair leading to the induction of a ‘BRCAness’ phenotype in cells. CDK12 inhibitors have the potential in many different indications, including overcoming emergent resistance to PARP inhibition, enhancing the efficacy of PARP inhibition in patients with BRCA1/2 wt status [1] and extending the efficacy of immune checkpoint inhibition through immunogenic cell death [2]. Recently, certain classes of CDK12 inhibitors have also demonstrated ‘glue degrader’ properties [1]: causing proteosomal degradation of the obligate co-factor for CDK12 and 13, Cyclin K. Such glue-degraders have the capability of enhancing cellular potency and specificity over classical kinase inhibitors. Taking as a starting point the selective CDK7 inhibitor samuraciclib, we have taken a rational design approach to develop inhibitors that preferentially inhibit CDK12. By utilising differences in the active site between CDK7 and CDK12 we were able to increase potency against CDK12 in biochemical assays by &gt; 100 fold, with no significant inhibition of kinases outside the CDK class. Cellular potency of these molecules was evaluated in the Ewing’s sarcoma cell line A673, which is known to be sensitive to CDK12 inhibition [4]. Remarkably the potency of the molecules to induce cell death was greater than their biochemical potency against CDK12 by &gt; 10 fold. Analysis of Cyclin K levels in cells demonstrated that the molecules lead to Cyclin K degradation. Furthermore, cellular potency was reduced by the presence of the neddylation inhibitor MLN4924 confirming that the compounds enhanced potency was mediated by glue degradation of Cyclin K. The molecules were found to cause potent inhibition of DNA homologous repair genes including BRCA1, FANCF and ERCC4 and lead to the accumulation of DNA strand breaks as assessed by the marker γ-H2AX. In the BRCA wt ovarian cancer cell line, OV-90, strong synergy was observed in the ability to induce cell death with a range of PARP inhibitors. In A673 tumour bearing mice, dosing with the compounds was found to cause significant reduction in the levels of cyclin K in tumours, which was sustained for 24 hr. Additionally, assessment of CDK12 protein in these tumours also showed a reduction, suggesting greater turnover of CDK12 protein in the absence of Cyclin K. Repeat dose studies at doses that lead to Cyclin K degradation in tumours demonstrated that the compounds were well tolerated, with no weight loss in mice. Two molecules, CT7439 and CT7510, were found to have oral bioavailability in mice that indicate that they would be suitable for once-daily oral administration in humans and are being progressed through regulatory toxicity studies. [1] Johnson, et al, (2016) Cell Rep. 17, 2367 [2] Li, et al, (2020) Cancer Lett. 495, 12 [3] Słabicki, et al, (2020) Nature 585, 293 [4] Iniguez, et al, (2018) Cancer Cell 33, 202
Citation Format: Edward K. Ainscow, Adrian Campbell, Michael Cripps, Robert Workman, Stuart Thomson, Kam Chohan, Damien Crepin, Mahiro Sunose, Jamie Patient, Jane Kendrew, Ash Bahl. Identification of an orally bioavailable dual Cyclin K glue degrader - CDK12/13 inhibitor [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P051.