Selective CDK6 degradation mediated by cereblon, VHL, and novel IAP-recruiting PROTACs.

Inhibitors of CDK4 and CDK6 have emerged as important FDA-approved treatment options for breast cancer patients. The properties and pharmacology of CDK4/6 inhibitor medicines have been extensively profiled, and investigations into the degradation of these targets via a PROTAC strategy have also been reported. PROTACs are a novel class of small-molecules that offer the potential for differentiated pharmacology compared to traditional inhibitors by redirecting the cellular ubiquitin-proteasome system to degrade target proteins of interest. We report here the preparation of palbociclib-based PROTACs that incorporate binders for three different E3 ligases, including a novel IAP-binder, which effectively degrade CDK4 and CDK6 in cells. In addition, we show that the palbociclib-based PROTACs in this study that recruit different E3 ligases all exhibit preferential CDK6 vs. CDK4 degradation selectivity despite employing a selection of linkers between the target binder and the E3 ligase binder.

The utility of pharmacokinetic-pharmacodynamic modeling in the discovery and optimization of selective S1P(1) agonists.

Sphingosine-1-phosphate (S1P(1)) receptor agonists such as Fingolimod (FTY-720) are a novel class of immunomodulators that have clinical utility in the treatment of remitting relapsing multiples sclerosis. This class of compound act by inducing peripheral lymphopenia. Using an integrated pharmacokinetic/pharmacodynamic (PK-PD) approach based on an in vivo rat model, novel S1P(1) agonists were identified with a predicted more rapid rate of reversibility of lymphocyte reduction in human compared to Fingolimod. The in vivo potency of 15 compounds based on PK-PD modelling of the rat lymphocyte reduction model was correlated with in vitro measures of potency at the S1P(1) receptor using ß arrestin recruitment and G-protein signalling. A structurally novel S1P(1) agonist was identified and predictions of human pharmacokinetics and clinical dose are presented.

A Potent and Selective Kallikrein-5 Inhibitor Delivers High Pharmacological Activity in Skin from Patients with Netherton Syndrome.

Regulation of proteolytic activity in the skin plays a pivotal role in epidermal homeostasis. This is best exemplified in Netherton syndrome, a severe genetic skin condition caused by loss-of-function mutations in the gene serine protease inhibitor Kazal-type 5 encoding lympho-epithelial Kazal-type-related inhibitor, a serine protease inhibitor that regulates kallikrein (KLK)-related peptidase 5, 7, and 14 activities. KLK5 plays a central role in stratum corneum shedding and inflammatory cell signaling, activates KLK7 and KLK14, and is therefore an optimal therapeutic target. We aimed to identify a potent and selective small-molecule inhibitor of KLK5 amenable to epidermal delivery. GSK951 was identified using a structure-based design strategy and showed a half maximal inhibitory concentration of 250 pM for KLK5 and greater than 100-fold selectivity over KLK7 and KLK14. Cocrystal structure analysis identified the critical catalytic site interactions to a surrogate for KLK5. Topical application of GSK951-containing cream inhibited KLK5 activity in TgKLK5 mouse skin, reduced transepidermal water loss, and decreased proinflammatory cytokine expression. GSK951 achieved high concentrations in healthy human epidermis following topical application in a cream formulation. Finally, KLK5 protease activity was increased in stratum corneum of patients with Netherton syndrome and significantly inhibited by GSK951. These findings unveil a KLK5-specific small-molecule inhibitor with a high therapeutic potential for patients with Netherton syndrome.

Structural Insights into PROTAC-Mediated Degradation of Bcl-xL.

The Bcl-2 family of proteins, such as Bcl-xL and Bcl-2, play key roles in cancer cell survival. Structural studies of Bcl-xL formed the foundation for the development of the first Bcl-2 family inhibitors and FDA approved drugs. Recently, Proteolysis Targeting Chimeras (PROTACs) that degrade Bcl-xL have been proposed as a therapeutic modality with the potential to enhance potency and reduce toxicity versus antagonists. However, no ternary complex structures of Bcl-xL with a PROTAC and an E3 ligase have been successfully determined to guide this approach. Herein, we report the design, characterization, and X-ray structure of a VHL E3 ligase-recruiting Bcl-xL PROTAC degrader. The 1.9 Å heterotetrameric structure, composed of (ElonginB:ElonginC:VHL):PROTAC:Bcl-xL, reveals an extensive network of neo-interactions, between the E3 ligase and the target protein, and between noncognate parts of the PROTAC and partner proteins. This work illustrates the challenges associated with the rational design of bifunctional molecules where interactions involve composite interfaces.

Extended pharmacodynamic responses observed upon PROTAC-mediated degradation of RIPK2.

Proteolysis-Targeting Chimeras (PROTACs) are heterobifunctional small-molecules that can promote the rapid and selective proteasome-mediated degradation of intracellular proteins through the recruitment of E3 ligase complexes to non-native protein substrates. The catalytic mechanism of action of PROTACs represents an exciting new modality in drug discovery that offers several potential advantages over traditional small-molecule inhibitors, including the potential to deliver pharmacodynamic (PD) efficacy which extends beyond the detectable pharmacokinetic (PK) presence of the PROTAC, driven by the synthesis rate of the protein. Herein we report the identification and development of PROTACs that selectively degrade Receptor-Interacting Serine/Threonine Protein Kinase 2 (RIPK2) and demonstrate in vivo degradation of endogenous RIPK2 in rats at low doses and extended PD that persists in the absence of detectable compound. This disconnect between PK and PD, when coupled with low nanomolar potency, offers the potential for low human doses and infrequent dosing regimens with PROTAC medicines.

Internalization of the chemokine receptor CCR4 can be evoked by orthosteric and allosteric receptor antagonists.

The chemokine receptor CCR4 has at least two natural agonist ligands, MDC (CCL22) and TARC (CCL17) which bind to the same orthosteric site with a similar affinity. Both ligands are known to evoke chemotaxis of CCR4-bearing T cells and also elicit CCR4 receptor internalization. A series of small molecule allosteric antagonists have been described which displace the agonist ligand, and inhibit chemotaxis. The aim of this study was to determine which cellular coupling pathways are involved in internalization, and if antagonists binding to the CCR4 receptor could themselves evoke receptor internalization. CCL22 binding coupled CCR4 efficiently to ß-arrestin and stimulated GTPγS binding however CCL17 did not couple to ß-arrestin and only partially stimulated GTPγS binding. CCL22 potently induced internalization of almost all cell surface CCR4, while CCL17 showed only weak effects. We describe four small molecule antagonists that were demonstrated to bind to two distinct allosteric sites on the CCR4 receptor, and while both classes inhibited agonist ligand binding and chemotaxis, one of the allosteric sites also evoked receptor internalization. Furthermore, we also characterize an N-terminally truncated version of CCL22 which acts as a competitive antagonist at the orthosteric site, and surprisingly also evokes receptor internalization without demonstrating any agonist activity. Collectively this study demonstrates that orthosteric and allosteric antagonists of the CCR4 receptor are capable of evoking receptor internalization, providing a novel strategy for drug discovery against this class of target.