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Drug discovery opportunities where loss-of-function alleles of a target gene link to a disease-relevant phenotype often require an agonism approach to up-regulate or re-establish the activity of the target gene. Antibody therapy is increasingly recognized as a favored drug modality due to multiple desirable pharmacological properties. However, agonistic antibodies that enhance the activities of the target enzymes are rarely developed because the discovery of agonistic antibodies remains elusive. Here we report an innovative scheme of discovery and characterization of human antibodies capable of binding to and agonizing a circulating enzyme lecithin cholesterol acyltransferase (LCAT). Utilizing a modified human LCAT protein with enhanced enzymatic activity as an immunogen, we generated fully human monoclonal antibodies using the XenoMouse(TM) platform. One of the resultant agonistic antibodies, 27C3, binds to and substantially enhances the activity of LCAT from humans and cynomolgus macaques. X-ray crystallographic analysis of the 2.45 Å LCAT-27C3 complex shows that 27C3 binding does not induce notable structural changes in LCAT. A single administration of 27C3 to cynomolgus monkeys led to a rapid increase of plasma LCAT enzymatic activity and a 35% increase of the high density lipoprotein cholesterol that was observed up to 32 days after 27C3 administration. Thus, this novel scheme of immunization in conjunction with high throughput screening may represent an effective strategy for discovering agonistic antibodies against other enzyme targets. 27C3 and other agonistic human anti-human LCAT monoclonal antibodies described herein hold potential for therapeutic development for the treatment of dyslipidemia and cardiovascular disease.

LCAT is intimately involved in HDL maturation and is a key component of the reverse cholesterol transport (RCT) pathway which removes excess cholesterol molecules from the peripheral tissues to the liver for excretion. Patients with loss-of-function LCAT mutations exhibit low levels of HDL cholesterol and corneal opacity. Here we report the 2.65 A crystal structure of the human LCAT protein. Crystallization required enzymatic removal of N-linked glycans and complex formation with a Fab fragment from a tool antibody. The crystal structure reveals that LCAT has an α/β hydrolase core with two additional subdomains that play important roles in LCAT function. Subdomain 1 contains the region of LCAT shown to be required for interfacial activation, while subdomain 2 contains the lid and amino acids that shape the substrate binding pocket. Mapping the naturally occurring mutations onto the structure provides insight into how they may affect LCAT enzymatic activity.

Fms-like tyrosine kinase 3 (FLT3) is implicated in the pathogenesis of acute myeloid leukemia (AML). FLT3-activating internal tandem duplication (ITD) mutations are found in approximately 30% of patients with AML and are associated with poor outcome in this patient population. Quizartinib (AC220) has previously been shown to be a potent and selective FLT3 inhibitor. In the current study, we expand on previous observations by showing that quizartinib potently inhibits the phosphorylation of FLT3 and downstream signaling molecules independent of FLT3 genotype, yet induces loss of viability only in cells expressing constitutively activated FLT3. We further show that transient exposure to quizartinib, whether in vitro or in vivo, leads to prolonged inhibition of FLT3 signaling, induction of apoptosis, and drastic reductions in tumor volume and pharmacodynamic endpoints. In vitro experiments suggest that these prolonged effects are mediated by slow binding kinetics that provide for durable inhibition of the kinase following drug removal/clearance. Together these data suggest quizartinib, with its unique combination of selectivity and potent/sustained inhibition of FLT3, may provide a safe and effective treatment against FLT3-driven leukemia. Mol Cancer Ther; 12(4); 438–47. ©2013 AACR.

Serine/threonine protein kinases Aurora A, B, and C play essential roles in cell mitosis and cytokinesis. Currently a number of Aurora kinase inhibitors with different isoform selectivities are being evaluated in the clinic. Herein we report the discovery and characterization of 21c (AC014) and 21i (AC081), two structurally novel, potent, kinome-selective pan-Aurora inhibitors. In the human colon cancer cell line HCT-116, both compounds potently inhibit histone H3 phosphorylation and cell proliferation while inducing 8N polyploidy. Both compounds administered intravenously on intermittent schedules displayed potent and durable antitumor activity in a nude rat HCT-116 tumor xenograft model and exhibited good in vivo tolerability. Taken together, these data support further development of both 21c and 21i as potential therapeutic agents for the treatment of solid tumors and hematological malignancies.

Activating mutations in the receptor tyrosine kinase FLT3 are present in up to approximately 30% of acute myeloid leukemia (AML) patients, implicating FLT3 as a driver of the disease and therefore as a target for therapy. We report the characterization of AC220, a second-generation FLT3 inhibitor, and a comparison of AC220 with the first-generation FLT3 inhibitors CEP-701, MLN-518, PKC-412, sorafenib, and sunitinib. AC220 exhibits low nanomolar potency in biochemical and cellular assays and exceptional kinase selectivity, and in animal models is efficacious at doses as low as 1 mg/kg given orally once daily. The data reveal that the combination of excellent potency, selectivity, and pharmacokinetic properties is unique to AC220, which therefore is the first drug candidate with a profile that matches the characteristics desirable for a clinical FLT3 inhibitor.

Treatment of AML patients with small molecule inhibitors of FLT3 kinase has been explored as a viable therapy. However, these agents are found to be less than optimal for the treatment of AML because of lack of sufficient potency or suboptimal oral pharmacokinetics (PK) or lack of adequate tolerability at efficacious doses. We have developed a series of extremely potent and highly selective FLT3 inhibitors with good oral PK properties. The first series of compounds represented by 1 (AB530) was found to be a potent and selective FLT3 kinase inhibitor with good PK properties. The aqueous solubility and oral PK properties at higher doses in rodents were found to be less than optimal for clinical development. A novel series of compounds were designed lacking the carboxamide group of 1 with an added water solubilizing group. Compound 7 (AC220) was identified from this series to be the most potent and selective FLT3 inhibitor with good pharmaceutical properties, excellent PK profile, and superior efficacy and t...

The Ras/RAF/MEK/ERK mitogen-activated protein kinase (MAPK) signaling pathway plays a central role in the regulation of cell growth, differentiation, and survival. Expression of mutant BRAF(V600E) results in constitutive activation of the MAPK pathway, which can lead to uncontrolled cellular growth. Herein, we describe an SAR optimization campaign around a series of quinazoline derived BRAF(V600E) inhibitors. In particular, the bioisosteric replacement of a metabolically sensitive tert-butyl group with fluorinated alkyl moieties is described. This effort led directly to the identification of a clinical candidate, compound 40 (CEP-32496). Compound 40 exhibits high potency against several BRAF(V600E)-dependent cell lines and selective cytotoxicity for tumor cell lines expressing mutant BRAF(V600E) versus those containing wild-type BRAF. Compound 40 also exhibits an excellent PK profile across multiple preclinical species. In addition, significant oral efficacy was observed in a 14-day BRAF(V600E)-dependent human Colo-205 tumor xenograft mouse model, upon dosing at 30 and 100 mg/kg BID.

Aryl phenyl ureas with a 4-quinazolinoxy substituent at the meta-position of the phenyl ring are potent inhibitors of mutant and wild type BRAF kinase. Compound 7 (1-(5-tert-butylisoxazol-3-yl)-3-(3-(6,7-dimethoxyquinazolin-4-yloxy)phenyl)urea hydrochloride) exhibits good pharmacokinetic properties in rat and mouse and is efficacious in a mouse tumor xenograft model following oral dosing.

The gamma-tubulin ring complex (gammaTuRC), consisting of multiple protein subunits, can nucleate microtubule assembly. Although many subunits of the gammaTuRC have been identified, a complete set remains to be defined in any organism. In addition, how the subunits interact with each other to assemble into gammaTuRC remains largely unknown. Here, we report the characterization of a novel gammaTuRC subunit, Drosophila gamma ring protein with WD repeats (Dgp71WD). With the exception of gamma-tubulin, Dgp71WD is the only gammaTuRC component identified to date that does not contain the grip motifs, which are signature sequences conserved in gammaTuRC components. By performing immunoprecipitations after pair-wise coexpression in Sf9 cells, we show that Dgp71WD directly interacts with the grip motif-containing gammaTuRC subunits, Dgrips84, 91, 128, and 163, suggesting that Dgp71WD may play a scaffolding role in gammaTuRC organization. We also show that Dgrips128 and 163, like Dgrips84 and 91, can interact directly with gamma-tubulin. Coexpression of any of these grip motif-containing proteins with gamma-tubulin promotes gamma-tubulin binding to guanine nucleotide. In contrast, in the same assay Dgp71WD interacts with gamma-tubulin but does not facilitate nucleotide binding.

Publisher Summary This chapter discusses the purification and reconstitution of Drosophila γ-tubulin complexes. To clone the individual Xenopus and Drosophila γTuRC subunits there are two approaches: (1) raising mouse polyclonal antibodies against purified γTuRCs and (2) obtaining protein sequences for individual γTuRC subunits. To date, four of the Drosophila γTuRC subunits are cloned, sequenced, and characterized. The isolation of γTuRC from Drosophila embryos involves an affinity purification step, using antipeptide antibodies specific for the C-terminal 17 amino acids of Drosophila γ-tubulin. An advantage of using Drosophila extracts is that once fly population cages are established, embryos are readily available in large quantities and flies do not require hormone injections. Another advantage is that Drosophila embryo extracts are less variable in quality than Xenopus egg extracts. Although the γ-tubulin-containing complexes—namely, γTuRC and γTuSC—can be purified from Drosophila embryos with relative ease, a system is developed that would allow to express the Dgrip proteins individually and in various combinations to study the organization and function of the γ-tubulin complexes biochemically. The chapter describes methods used to culture and infect the Sf9 cells and focuses on the general methods used to construct and assay the baculovirus.

Activating mutations in the receptor tyrosine kinase FLT3 are present in up to approximately 30 % of acute myeloid leukemia (AML) patients, implicating FLT3 as a potential target for kinase inhibitor therapy. While several compounds have been evaluated in the clinic as FLT3 inhibitors, including CEP-701 (lestaurtinib), PKC-412 (midostaurin), MLN-518 (tandutinib) and most recently sorafenib, none of these was originally developed as a FLT3 inhibitor. We report here the characterization of AC220, a novel compound that has been expressly optimized as a selective FLT3 inhibitor. We demonstrate that this second generation FLT3 inhibitor has a unique combination of high potency, exceptional selectivity, bioavailability, and pharmacokinetic properties compatible with once a day oral dosing. In an in vitro binding assay AC220 interacted with FLT3 with high affinity (Kd = 1.6 nM). In the FLT3 dependent human leukemia cell line MV4-11, which harbors a homozygous activating FLT3 internal tandem duplication (ITD) mutation, AC220 inhibited FLT3 autophosphorylation and cellular proliferation with subnanomolar potency. Inhibition of proliferation of a FLT3 independent cell line was several hundred-fold less potent, demonstrating cellular selectivity for FLT3. A biochemical screen against a panel of 402 kinase assays representing almost 80 % of human protein kinases revealed a highly focused and selective interaction pattern. The only targets with affinity for AC220 within 10-fold that for FLT3 were closely related class III receptor tyrosine kinases (KIT, PDGFR, RET, CSF1R), and the only targets with affinity within 100-fold that for FLT3 were four additional receptor tyrosine kinases (FLT1, FLT4, DDR1, VEGFR2). When orally administered to mice at a dose of 10 mg/kg, AC220 achieved a peak plasma concentration (Cmax) of 3.8 μM (2,100 ng/mL) within two hours of dosing. When corrected for plasma protein binding, the concentration of AC220 in plasma remained above the cellular IC50 for FLT3 inhibition 24 hours after dosing. Total exposure (AUC0-24 h) as well as Cmax increased proportionally with the administered dose from 0.1 to approximately 30 mg/kg. At higher doses, both Cmax and AUC0-24 h continued to increase, approaching a plateau above 100 mg/kg. In a FLT3-ITD-dependent MV4-11 tumor xenograft model, AC220 showed substantial, dose dependent efficacy when dosed at 1, 3 and 10 mg/kg orally once a day for 28 days. Tumors regressed at 3 and 10 mg/kg, and remained static at 1 mg/kg. In a follow-on study at the 10 mg/kg oral dose, tumor size was monitored for an additional 60 days after dosing was discontinued. By the end of the study eight complete responses and two partial responses were observed in the ten animals treated with AC220. AC220 also had activity in a leukemia tumor model at doses as low as 1 mg/kg given orally once a day. A direct comparison of AC220 with the first generation FLT3 inhibitors CEP-701, MLN-518, PKC-412, sorafenib and sunitinib revealed that the combination of potency, selectivity and pharmacokinetic properties is unique to AC220. AC220 is a second generation FLT3 inhibitor that has been explicitly optimized for the combination of properties believed to be required for the successful treatment of FLT3-dependent AML, and specifically to test the hypothesis that selective FLT3 inhibition will result in clinical benefit. AC220 is currently being evaluated in a phase I clinical trial in relapsed or refractory AML patients.