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Table S1. Statistics for the crystallographic analysis; Table S2. Biochemical and cellular potencies of the second generation CDK-directed drug AG-024322; Table S3. In vitro analysis of binding potency of abemaciclib, dinaciclib, and palbociclib to non-kinase proteins; Table S4. Broad kinase selectivity of selective CDK4/6 drugs; Table S5. Biochemical dose-response follow-up was conducted for a subset of kinases inhibited by the three CDK4/6 drugs using a Km concentration of ATP (Carna Biosciences) (CDK proteins excluded); Table S6. Kinase selectivity toward endogenous human kinases using irreversible ATP analog target engagement assay; Table S7. Biochemical potencies of selective CDK4/6 drugs palbociclib (PD-0332991); Table S8. Human pharmacokinetic properties of CDK-targeted drugs; Table S9. Isothermal titration calorimetry results for CDK-directed drugs binding to CDK6 in the absence of cyclin D.

Detailed description of the methods used in the manuscript

Summary The CDK4/6 inhibitor, palbociclib (PAL), significantly improves progression-free survival in HR+/HER2− breast cancer when combined with anti-hormonals. We sought to discover PAL resistance mechanisms in preclinical models and through analysis of clinical transcriptome specimens, which coalesced on induction of MYC oncogene and Cyclin E/CDK2 activity. We propose that targeting the G1 kinases CDK2, CDK4, and CDK6 with a small-molecule overcomes resistance to CDK4/6 inhibition. We describe the pharmacodynamics and efficacy of PF-06873600 (PF3600), a pyridopyrimidine with potent inhibition of CDK2/4/6 activity and efficacy in multiple in vivo tumor models. Together with the clinical analysis, MYC activity predicts (PF3600) efficacy across multiple cell lineages. Finally, we find that CDK2/4/6 inhibition does not compromise tumor-specific immune checkpoint blockade responses in syngeneic models. We anticipate that (PF3600), currently in phase 1 clinical trials, offers a therapeutic option to cancer patients in whom CDK4/6 inhibition is insufficient to alter disease progression.

Therapeutically targeting aberrant intracellular kinase signaling is attractive from a biological perspective but drug development is often hindered by toxicities and inadequate efficacy. Predicting drug behaviors using cellular and animal models is confounded by redundant kinase activities, a lack of unique substrates, and cell-specific signaling networks. Cyclin-dependent kinase (CDK) drugs exemplify this phenomenon because they are reported to target common processes yet have distinct clinical activities. Tumor cell studies of ATP-competitive CDK drugs (dinaciclib, AG-024322, abemaciclib, palbociclib, ribociclib) indicate similar pharmacology while analyses in untransformed cells illuminates significant differences. To resolve this apparent disconnect, drug behaviors are described at the molecular level. Nonkinase binding studies and kinome interaction analysis (recombinant and endogenous kinases) reveal that proteins outside of the CDK family appear to have little role in dinaciclib/palbociclib/ribociclib pharmacology, may contribute for abemaciclib, and confounds AG-024322 analysis. CDK2 and CDK6 cocrystal structures with the drugs identify the molecular interactions responsible for potency and kinase selectivity. Efficient drug binding to the unique hinge architecture of CDKs enables selectivity toward most of the human kinome. Selectivity between CDK family members is achieved through interactions with nonconserved elements of the ATP-binding pocket. Integrating clinical drug exposures into the analysis predicts that both palbociclib and ribociclib are CDK4/6 inhibitors, abemaciclib inhibits CDK4/6/9, and dinaciclib is a broad-spectrum CDK inhibitor (CDK2/3/4/6/9). Understanding the molecular components of potency and selectivity also facilitates rational design of future generations of kinase-directed drugs. Mol Cancer Ther; 15(10); 2273–81. ©2016 AACR.

Human rhinoviruses (HRV), the predominant members of the Picornaviridae family of positive-strand RNA viruses, are the major causative agents of the common cold. Given the lack of effective treatments for rhinoviral infections, virally encoded proteins have become attractive therapeutic targets. The HRV genome encodes an RNA-dependent RNA polymerase (RdRp) denoted 3Dpol, which is responsible for replicating the viral genome and for synthesizing a protein primer used in the replication. Here the crystal structures for three viral serotypes (1B, 14, and 16) of HRV 3Dpol have been determined. The three structures are very similar to one another, and to the closely related poliovirus (PV) 3Dpol enzyme. Because the reported PV crystal structure shows significant disorder, HRV 3Dpol provides the first complete view of a picornaviral RdRp. The folding topology of HRV 3Dpol also resembles that of RdRps from hepatitis C virus (HCV) and rabbit hemorrhagic disease virus (RHDV) despite very low sequence homology.

Phosphoinositide-dependent kinase-1 (PDK1) is a critical enzyme in the PI3K/AKT pathway and to the activation of AGC family protein kinases, including S6K, SGK, and PKC. Dysregulation of this pathway plays a key role in cancer cell growth, survival and tumor angiogenesis. As such, inhibitors of PDK1 offer the promise of a new therapeutic modality for cancer treatment. Fragment based drug screening has recently become a viable entry point for hit identification. In this work, NMR spectroscopy fragment screening of PDK1 afforded novel chemotypes as orthogonal starting points from HTS screening hits. Compounds identified as hits by NMR spectroscopy were tested in a biochemical assay, and fragments with activity in both assays were clustered. The Pfizer compound file was mined via substructure and 2D similarity search, and the chemotypes were prioritized by ligand efficiency (LE), SAR mining, chemical attractiveness, and chemical enablement of promising vectors. From this effort, an isoquinolone fragment hit, 5 (IC(50) 870 μM, LE = 0.39), was identified as a novel, ligand efficient inhibitor of PDK1 and a suitable scaffold for further optimization. Initially in the absence of crystallographic data, a fragment growing approach efficiently explored four vectors of the isoquinolone scaffold via parallel synthesis to afford a compound with crystallographic data, 16 (IC(50) 41.4 μM, LE = 0.33). Subsequent lead optimization efforts provided 24 (IC(50) 1.8 μM, LE = 0.42), with greater than fivefold selectivity against other key pathway kinases.

Human rhinoviruses (HRV), the predominant members of the Picornaviridae family of positive-strand RNA viruses, are the major causative agents of the common cold. Given the lack of effective treatments for rhinoviral infections, virally encoded proteins have become attractive therapeutic targets. The HRV genome encodes an RNA-dependent RNA polymerase (RdRp) denoted 3Dpol, which is responsible for replicating the viral genome and for synthesizing a protein primer used in the replication. Here the crystal structures for three viral serotypes (1B, 14, and 16) of HRV 3Dpol have been determined. The three structures are very similar to one another, and to the closely related poliovirus (PV) 3Dpol enzyme. Because the reported PV crystal structure shows significant disorder, HRV 3Dpol provides the first complete view of a picornaviral RdRp. The folding topology of HRV 3Dpol also resembles that of RdRps from hepatitis C virus (HCV) and rabbit hemorrhagic disease virus (RHDV) despite very low sequence homology.