Asako Nagata

Covalent EGFR inhibitor analysis reveals importance of reversible interactions to potency and mechanisms of drug resistance

Significance Covalent kinase inhibition strategies are reemerging, but critical gaps in the understanding of molecular determinants of potency still persist. A kinetic approach is developed to describe the components of overall inhibitor potency (reversible binding and chemical reactivity). Detailed kinetic descriptions of EGFR covalent drugs are provided. Reversible interactions of covalent inhibitors are found to be essential to biochemical and cellular potency. A dynamic linkage between available affinity and necessary reactivity is proposed. Cysteine oxidation is an emerging type of posttranslational modification. Specific oxidation of the EGF receptor cysteine nucleophile causes highly variable effects on inhibitor potency. Two mechanisms of drug resistance are identified (reversible cysteine–inhibitor warhead interactions and specific cysteine oxidation) as well as a rational framework for understanding and designing covalent inhibitors. Covalent inhibition is a reemerging paradigm in kinase drug design, but the roles of inhibitor binding affinity and chemical reactivity in overall potency are not well-understood. To characterize the underlying molecular processes at a microscopic level and determine the appropriate kinetic constants, specialized experimental design and advanced numerical integration of differential equations are developed. Previously uncharacterized investigational covalent drugs reported here are shown to be extremely effective epidermal growth factor receptor (EGFR) inhibitors (kinact/Ki in the range 105–107 M−1s−1), despite their low specific reactivity (kinact ≤ 2.1 × 10−3 s−1), which is compensated for by high binding affinities (Ki < 1 nM). For inhibitors relying on reactivity to achieve potency, noncovalent enzyme–inhibitor complex partitioning between inhibitor dissociation and bond formation is central. Interestingly, reversible binding affinity of EGFR covalent inhibitors is highly correlated with antitumor cell potency. Furthermore, cellular potency for a subset of covalent inhibitors can be accounted for solely through reversible interactions. One reversible interaction is between EGFR-Cys797 nucleophile and the inhibitor’s reactive group, which may also contribute to drug resistance. Because covalent inhibitors target a cysteine residue, the effects of its oxidation on enzyme catalysis and inhibitor pharmacology are characterized. Oxidation of the EGFR cysteine nucleophile does not alter catalysis but has widely varied effects on inhibitor potency depending on the EGFR context (e.g., oncogenic mutations), type of oxidation (sulfinylation or glutathiolation), and inhibitor architecture. These methods, parameters, and insights provide a rational framework for assessing and designing effective covalent inhibitors.

Spectrum and Degree of CDK Drug Interactions Predicts Clinical Performance

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.

Protein redox chemistry: post-translational cysteine modifications that regulate signal transduction and drug pharmacology.

The perception of reactive oxygen species has evolved over the past decade from agents of cellular damage to secondary messengers which modify signaling proteins in physiology and the disease state (e.g., cancer). New protein targets of specific oxidation are rapidly being identified. One emerging class of redox modification occurs to the thiol side chain of cysteine residues which can produce multiple chemically distinct alterations to the protein (e.g., sulfenic/sulfinic/sulfonic acid, disulfides). These post-translational modifications (PTM) are shown to affect the protein structure and function. Because redox-sensitive proteins can traffic between subcellular compartments that have different redox environments, cysteine oxidation enables a spatio-temporal control to signaling. Understanding ramifications of these oxidative modifications to the functions of signaling proteins is crucial for understanding cellular regulation as well as for informed-drug discovery process. The effects of EGFR oxidation of Cys797 on inhibitor pharmacology are presented to illustrate the principle. Taken together, cysteine redox PTM can impact both cell biology and drug pharmacology.

Discovery of N-((3R,4R)-4-Fluoro-1-(6-((3-methoxy-1-methyl-1H-pyrazol-4-yl)amino)-9-methyl-9H-purin-2-yl)pyrrolidine-3-yl)acrylamide (PF-06747775) through Structure-Based Drug Design: A High Affinity Irreversible Inhibitor Targeting Oncogenic EGFR Mutants with Selectivity over Wild-Type EGFR.

Mutant epidermal growth factor receptor (EGFR) is a major driver of non-small-cell lung cancer (NSCLC). Marketed first generation inhibitors, such as erlotinib, effect a transient beneficial response in EGFR mutant NSCLC patients before resistance mechanisms render these inhibitors ineffective. Secondary oncogenic EGFR mutations account for approximately 50% of relapses, the most common being the gatekeeper T790M substitution that renders existing therapies ineffective. The discovery of PF-06459988 (1), an irreversible pyrrolopyrimidine inhibitor of EGFR T790M mutants, was recently disclosed.1 Herein, we describe our continued efforts to achieve potency across EGFR oncogenic mutations and improved kinome selectivity, resulting in the discovery of clinical candidate PF-06747775 (21), which provides potent EGFR activity against the four common mutants (exon 19 deletion (Del), L858R, and double mutants T790M/L858R and T790M/Del), selectivity over wild-type EGFR, and desirable ADME properties. Compound 21 is currently being evaluated in phase-I clinical trials of mutant EGFR driven NSCLC.

Discovery of 3-aryloxy-lactam analogs as potent androgen receptor full antagonists for treating castration resistant prostate cancer.

High throughput cell-based screening led to the identification of 3-aryloxy lactams as potent androgen receptor (AR) antagonists. Refinement of these leads to improve the ADME profile and remove residual agonism led to the discovery of 12, a potent full antagonist with greater oral bioavailability. Improvements in the ADME profile were realized by designing more ligand-efficient molecules with reduced molecular weights and lower lipophilicities.

Discovery of Allosteric, Potent, Subtype Selective, and Peripherally Restricted TrkA Kinase Inhibitors

Tropomyosin receptor kinases (TrkA, TrkB, TrkC) are activated by hormones of the neurotrophin family: nerve growth factor (NGF), brain derived neurotrophic factor (BDNF), neurotrophin 3 (NT3), and neurotrophin 4 (NT4). Moreover, the NGF antibody tanezumab has provided clinical proof of concept for inhibition of the TrkA kinase pathway in pain leading to significant interest in the development of small molecule inhibitors of TrkA. However, achieving TrkA subtype selectivity over TrkB and TrkC via a Type I and Type II inhibitor binding mode has proven challenging and Type III or Type IV allosteric inhibitors may present a more promising selectivity design approach. Furthermore, TrkA inhibitors with minimal brain availability are required to deliver an appropriate safety profile. Herein, we describe the discovery of a highly potent, subtype selective, peripherally restricted, efficacious, and well-tolerated series of allosteric TrkA inhibitors that culminated in the delivery of candidate quality compound 23.