SignificanceMutants of RAS are major oncogenes and occur in many human cancers, but efforts to develop drugs that directly inhibit the corresponding constitutively active RAS proteins have failed so far. We therefore focused on SOS1, the guanine nucleotide exchange factor (GEF) and activator of RAS. A combination of high-throughput and fragment screening resulted in the identification of nanomolar SOS1 inhibitors, which effectively down-regulate active RAS in tumor cells. In cells with wild-type KRAS, we observed complete inhibition of the RAS-RAF-MEK-ERK pathway. In a mutant KRAS cell line, SOS1 inhibition resulted in a reduction of phospho-ERK activity by 50%. Together, the data indicate that inhibition of GEFs may represent a viable approach for targeting RAS-driven tumors.

Polo-like kinase 1 (Plk1) is a key regulator of mitotic progression and cell division in eukaryotes. It is highly expressed in tumor cells and considered a potential target for cancer therapy. Here, we report the discovery and application of a novel potent small-molecule inhibitor of mammalian Plk1, ZK-Thiazolidinone (TAL). We have extensively characterized TAL in vitro and addressed TAL specificity within cells by studying Plk1 functions in sister chromatid separation, centrosome maturation, and spindle assembly. Moreover, we have used TAL for a detailed analysis of Plk1 in relation to PICH and PRC1, two prominent interaction partners implicated in spindle assembly checkpoint function and cytokinesis, respectively. Specifically, we show that Plk1, when inactivated by TAL, spreads over the arms of chromosomes, resembling the localization of its binding partner PICH, and that both proteins are mutually dependent on each other for correct localization. Finally, we show that Plk1 activity is essential for cleavage furrow formation and ingression, leading to successful cytokinesis. INTRODUCTIONThe error-free segregation of chromosomes during cell division is necessary for the maintenance of correct ploidy and genomic integrity, and errors in cell division are presumed to lead to aneuploidy and cancer (Rajagopalan and Lengauer, 2004). To ensure that daughter cells receive the correct complement of chromosomes, two key events need to be coordinated. First, chromosomes must be equally segregated, a process that depends on the mitotic spindle. Second, cytokinesis, the process dividing the cell into two, must occur between the two sets of segregated chromosomes. Both of these processes require the activity of a key cell cycle regulator, the Polo-like kinase 1 (Plk1). Plks form a conserved subfamily of serine/threonine protein kinases. The first member to be identified was Polo in Drosophila melanogaster (Llamazares et al., 1991) and, subsequently, four Plk family members have been identified in mammals Barr et al., 2004).Plk1 contains an N-terminal kinase domain and a phosphopeptide-binding C-terminal regulatory polo-box domain (PBD; Leung et al., 2002;Elia et al., 2003b). In vertebrates Plk1 has been implicated in the activation of Cdk1-cyclin B upon entry into mitosis, centrosome maturation via the recruitment of the ␥-tubulin ring complex (␥-TuRC), spindle formation, sister chromatid separation by cohesin removal from the chromosome arms, promotion of anaphase onset through direct phosphorylation of the APC/C complex as well as the inhibition of the APC/C inhibitor Emi1, and finally, mitotic exit and cytokinesis (reviewed in Barr et al., 2004). Fitting with these diverse functions, Plk1 localizes to the centrosomes, spindle poles, and kinetochores in prophase and metaphase, the central spindle in anaphase, and the midbody during cytokinesis. These localizations require the function of the PBD (Jang et al., 2002;Seong et al., 2002) and priming-kinases to generate phosphorylated docking sites that are subsequently recog...

The DNA damage response (DDR) secures the integrity of the genome of eukaryotic cells. DDR deficiencies can promote tumorigenesis but concurrently may increase dependence on alternative repair pathways. The ataxia telangiectasia and Rad3-related (ATR) kinase plays a central role in the DDR by activating essential signaling pathways of DNA damage repair. Here, we studied the effect of the novel selective ATR kinase inhibitor BAY 1895344 on tumor cell growth and viability. Potent antiproliferative activity was demonstrated in a broad spectrum of human tumor cell lines. BAY 1895344 exhibited strong monotherapy efficacy in cancer xenograft models that carry DNA damage repair deficiencies. The combination of BAY 1895344 with DNA damage-inducing chemotherapy or external beam radiotherapy (EBRT) showed synergistic antitumor activity. Combination treatment with BAY 1895344 and DDR inhibitors achieved strong synergistic antiproliferative activity in vitro, and combined inhibition of ATR and PARP signaling using olaparib demonstrated synergistic antitumor activity in vivo. Furthermore, the combination of BAY 1895344 with the novel, nonsteroidal androgen receptor antagonist darolutamide resulted in significantly improved antitumor efficacy compared with respective singleagent treatments in hormone-dependent prostate cancer, and addition of EBRT resulted in even further enhanced antitumor efficacy. Thus, the ATR inhibitor BAY 1895344 may provide new therapeutic options for the treatment of cancers with certain DDR deficiencies in monotherapy and in combination with DNA damage-inducing or DNA repair-compromising cancer therapies by improving their efficacy.

Targeted protein degradation (TPD), the ability to control a proteins fate by triggering its degradation in a highly selective and effective manner, has created tremendous excitement in chemical biology and drug discovery within the past decades. The TPD field is spearheaded by small molecule induced protein degradation with molecular glues and proteolysis targeting chimeras (PROTACs) paving the way to expand the druggable space and to create a new paradigm in drug discovery. However, besides the therapeutic angle of TPD a plethora of novel techniques to modulate and control protein levels have been developed. This enables chemical biologists to better understand protein function and to discover and verify new therapeutic targets. This Review gives a comprehensive overview of chemical biology techniques inducing TPD. It explains the strengths and weaknesses of these methods in the context of drug discovery and discusses their future potential from a medicinal chemist's perspective.

Deprotonation of enantiomerically pure hydrazones and subsequent trapping with suitable electrophiles generates new stereogenic centers with excellent stereoselectivity. To liberate the original carbonyl functionality in the final products, it is necessary to cleave the hydrazone moiety. In recent years, many reagents have been developed to regenerate carbonyl compounds from the corresponding dialkylhydrazones which are compatible with a wide range of functionalities. This has allowed the use of hydrazones in the total synthesis of complex natural products. This Account is meant to be an overview of methods which are classified as oxidative, hydrolytic, and reductive cleavage procedures.