Entwicklung hochselektiver kovalenter Inhibitoren für die Ribosomale Protein S6 Kinase Beta 2 (S6K2)

01.11.2022 bis 31.10.2025

The family of p70 ribosomal protein S6 kinases comprises the two isoforms S6K1 (p70S6K) and S6K2 (p70S6Kβ), which act downstream of the PI3K-AKT-mTOR pathway. While S6K1 has been thoroughly investigated as an anticancer target, S6K2 has remained poorly studied. S6K2 has been associated with poor prognosis and therapy resistance in breast and prostate cancers. It has also been shown that survival of NRAS-mutant melanoma cells resistant to mitogen-activated protein kinase (MAPK) inhibitors crucially depends on S6K2, suggesting S6K2 inhibition as a treatment strategy. Several lines of evidence indicate that concomitant S6K1 inhibition abrogates the benefits of S6K2 inhibition. Thus, detailed pharmacological studies of S6K2 biology will require highly isoformselektive chemical probes.

Due to the almost identical ATP binding sites of S6K1 and S6K2, design of selective inhibitors is challenging. We have recently developed the first isoform-selective S6K2 inhibitor. Covalent targeting of a poorly conserved cysteine, which is absent in S6K1, via nucleophilic aromatic substitution (SNAr) chemistry was key to success. Despite nanomolar potency and a promising selectivity profile, our prototype inhibitor has limitations including insufficient non-covalent binding affinity, moderate solubility and limited druglikeness. Moreover, and despite being very versatile covalent reactive groups, SNAr electrophiles are underinvestigated in the context of covalent inhibitors and lack guidelines for their optimization. Here, we aim to develop high-quality chemical probes enabling the investigation of S6K2's role in health and disease. Thorough optimization of the covalent reactive group and investigation of structure–activity relationships will provide valuable insights which will help to broaden the scope and applicability of SNAr electrophiles in covalent inhibitor design. We will further optimize the piperazine-linked "upper side chain" and the "hinge-binding motif" of our prototype compound to increase binding affinity and solubility.

In combination, our optimizations are expected to provide inhibitors fulfilling the community's stringent criteria for high-quality chemical probes. Accompanied by suitable negative control compounds, our probe set may lay the basis for pharmacological validation of S6K2 as a drug target and open new avenues in cancer treatment and for combatting therapy resistance.