Pre-clinical evaluation of the MDM2-p53 antagonist RG7388 alone and in combination with chemotherapy in neuroblastoma

Neuroblastoma is a predominantly p53 wild-type (wt) tumour and MDM2-p53 antagonists offer a novel therapeutic strategy for neuroblastoma patients. RG7388 (Roche) is currently undergoing early phase clinical evaluation in adults. This study assessed the efficacy of RG7388 as a single-agent and in combination with chemotherapies currently used to treat neuroblastoma in a panel of neuroblastoma cell lines. RG7388 GI₅₀ concentrations were determined in 21 p53-wt and mutant neuroblastoma cell lines of varying MYCN, MDM2 and p14^ARF status, together with MYCN-regulatable Tet21N cells. The primary determinant of response was the presence of wt p53, and overall there was a >200-fold difference in RG7388 GI₅₀ concentrations for p53-wt versus mutant cell lines. Tet21N MYCN+ cells were significantly more sensitive to RG7388 compared with MYCN− cells. Using median-effect analysis in 5 p53-wt neuroblastoma cell lines, selected combinations of RG7388 with cisplatin, doxorubicin, topotecan, temozolomide and busulfan were synergistic. Furthermore, combination treatments led to increased apoptosis, as evident by higher caspase-3/7 activity compared to either agent alone. These data show that RG7388 is highly potent against p53-wt neuroblastoma cells, and strongly supports its further evaluation as a novel therapy for patients with high-risk neuroblastoma and wt p53 to potentially improve survival and/or reduce toxicity.

Abstract 2835: MDM2 antagonist-based therapeutic response is discriminated by a 4-gene signature in acute myeloid leukemia patients

The activity of p53, a key tumor suppressor is tightly controlled by MDM2-mediated ubiquination and degradation. Nutlins, a class of small-molecule MDM2 antagonists, have been characterized as drivers of p53 re-activation. Acute myeloid leukemia (AML) is uniquely sensitive to p53 re-activation as ∼90% of cases have wild-type TP53 and frequent MDM2 overexpression to overcome mechanisms of oncogene addiction. Personalized theranostic strategies may distinguish patients likely to clinically benefit from MDM2-antagonist therapy.

Association between MDM2 antagonist (RG7112) growth inhibition (IC50s) in 287 human cancer cell lines (Cell Lines for Oncology/Chugai Accumulative Tumor Encyclopedia), and pretreatment RNAseq profiling established a classifier comprising MDM2, XPC, BBC3, and CDKN2A. This signature significantly associated with cell-line efficacy to MDM2 antagonist (odds ratio = 2.53; P<0.001) and discriminated MDM2 antagonist-sensitive (IC50 10 uM) better than MDM2 mRNA or TP53 mutation status alone (AUC = 0.92). Multivariate logistic regression indicated signature scores were significant (odds ratio = 1.84; P<0.001) when adjusted for TP53 mutation status.

RG7112 treatment was assessed in a phase 1 dose escalation trial in relapsed/refractory AML patients (NO21279). Signature scores of AML patient blood specimens at baseline significantly associated with clinical response (PD<HI<MLFS<CR; Spearman correlation coefficient 0.58; P<0.001); and with pharmacodynamic biomarker response, defined as change in MDM2 mRNA expression in blood (Spearman correlation coefficient 0.41; P = 0.02). Patient response was discriminated by the signature with 100% sensitivity/71% specificity (Wilcoxon p = .0007, AUC = 0.72). Monitoring of signature with clinical response in a second AML Phase 1/1b clinical trial NP28679 with a next generation nutlin-class MDM2 Antagonist RG7388 validated this panel as significantly discriminating MDM2 antagonist therapeutic responders and non-responders in AML patients (Wilcoxon Rank Sum Test P = 0.008; AUC = .75).

In summary, we demonstrate that a biological classifier discriminates response broadly to MDM2-antagonist therapy. The level of evidence attained by cell line efficacy modeling and response assessments in trial NO21279 (with MDM2 antagonist RG7112) and now in trial NP28679 (with MDM2 antagonist RG7388) adds substantial weight to the validity of this panel.

Citation Format: Hua Zhong, Gong Chen, Lori Jukofsky, David Geho, Sung Won Han, Fabian Birzele, Sabine Bader, Lucia Himmelein, James Cai, Zayed Albertyn, Mark Rothe, Laurent Essioux, Helmut Burtscher, Steven A. Middleton, Lin-Chi Chen, Markus Dangl, William E. Pierceall, Gwen Nichols. MDM2 antagonist-based therapeutic response is discriminated by a 4-gene signature in acute myeloid leukemia patients. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2835. doi:10.1158/1538-7445.AM2015-2835

A selective small molecule NOP (ORL-1 receptor) partial agonist for the treatment of anxiety

Small molecule (1) has been identified as a selective partial agonist of Opioid Receptor Like-1 (ORL-1) with potential utility for the treatment of anxiety and other disorders. Nociceptin (orphanin FQ) is an endogenous peptide ligand that binds to ORL-1, however it does not bind the classical δ, μ and κ opioid receptors with high affinity. The synthesis of 1 involved using a molecular diversity approach, to rapidly advance a library of compounds for biological testing. A lead selective potent partial agonist (35-fold ORL-1/Mu) progressed to ORL-1 (NOP or OP4) proof of concept testing in advanced studies. The synthetic approach and biological data for the related chemical series will be presented.

Structurally diverse MDM2-p53 antagonists act as modulators of MDR-1 function in neuroblastoma

BACKGROUND

A frequent mechanism of acquired multidrug resistance in human cancers is overexpression of ATP-binding cassette transporters such as the Multi-Drug Resistance Protein 1 (MDR-1). Nutlin-3, an MDM2-p53 antagonist, has previously been reported to be a competitive MDR-1 inhibitor.

METHODS

This study assessed whether the structurally diverse MDM2-p53 antagonists, MI-63, NDD0005, and RG7388 are also able to modulate MDR-1 function, particularly in p53 mutant neuroblastoma cells, using XTT-based cell viability assays, western blotting, and liquid chromatography-mass spectrometry analysis.

RESULTS

Verapamil and the MDM2-p53 antagonists potentiated vincristine-mediated growth inhibition in a concentration-dependent manner when used in combination with high MDR-1-expressing p53 mutant neuroblastoma cell lines at concentrations that did not affect the viability of cells when given alone. Liquid chromatography-mass spectrometry analyses showed that verapamil, Nutlin-3, MI-63 and NDD0005, but not RG7388, led to increased intracellular levels of vincristine in high MDR-1-expressing cell lines.

CONCLUSIONS

These results show that in addition to Nutlin-3, other structurally unrelated MDM2-p53 antagonists can also act as MDR-1 inhibitors and reverse MDR-1-mediated multidrug resistance in neuroblastoma cell lines in a p53-independent manner. These findings are important for future clinical trial design with MDM2-p53 antagonists when used in combination with agents that are MDR-1 substrates.

Phase I study of RO4929097, a gamma secretase inhibitor of Notch signaling, in patients with refractory metastatic or locally advanced solid tumors

PURPOSE

To determine the maximum-tolerated dose (MTD) and assess safety, pharmacokinetics, pharmacodynamics, and evidence of antitumor activity of RO4929097, a gamma secretase inhibitor of Notch signaling in patients with advanced solid malignancies.

PATIENTS AND METHODS

Patients received escalating doses of RO4929097 orally on two schedules: (A) 3 consecutive days per week for 2 weeks every 3 weeks; (B) 7 consecutive days every 3 weeks. To assess reversible CYP3A4 autoinduction, the expanded part of the study tested three dosing schedules: (B) as above; modified A, 3 consecutive d/wk for 3 weeks; and (C) continuous daily dosing. Positron emission tomography scans with [(18)F]fluorodeoxyglucose (FDG-PET) were used to assess tumor metabolic effects.

RESULTS

Patients on schedule A (n = 58), B (n = 47), and C (n = 5; expanded cohort) received 302 cycles of RO4929097. Common grade 1 to 2 toxicities were fatigue, thrombocytopenia, fever, rash, chills, and anorexia. Transient grade 3 hypophosphatemia (dose-limiting toxicity, one patient) and grade 3 pruritus (two patients) were observed at 27 mg and 60 mg, respectively; transient grade 3 asthenia was observed on schedule A at 80 mg (one patient). Tumor responses included one partial response in a patient with colorectal adenocarcinoma with neuroendocrine features, one mixed response (stable disease) in a patient with sarcoma, and one nearly complete FDG-PET response in a patient with melanoma. Effect on CYP3A4 induction was observed.

CONCLUSION

RO4929097 was well tolerated at 270 mg on schedule A and at 135 mg on schedule B; the safety of schedule C has not been fully evaluated. Further studies are warranted on the basis of a favorable safety profile and preliminary evidence of clinical antitumor activity.

Discovery of novel 4-amino-6-arylaminopyrimidine-5-carbaldehyde oximes as dual inhibitors of EGFR and ErbB-2 protein tyrosine kinases

We herein disclose a novel series of 4-aminopyrimidine-5-carbaldehyde oximes that are potent and selective inhibitors of both EGFR and ErbB-2 tyrosine kinases, with IC(50) values in the nanomolar range. Structure-activity relationship (SAR) studies elucidated a critical role for the 4-amino and C-6 arylamino moieties. The X-ray co-crystal structure of EGFR with 37 was determined and validated our design rationale.

Aminocyclohexylsulfonamides: Discovery of metabolically stable α1a/1d-selective adrenergic receptor antagonists for the treatment of benign prostatic hyperplasia/lower urinary tract symptoms (BPH/LUTS)

Benign prostatic hyperplasia/lower urinary tract symptoms (BPH/LUTS) can be effectively treated by alpha(1) adrenergic receptor antagonists, but these drugs also produce side effects that are related to their subtype non-selective nature. To overcome this limitation, it was hypothesized that an alpha(1a/1d) subtype-selective antagonist would be efficacious while keeping side effects to a minimum. To discover alpha(1a/1d)-selective antagonists and improve metabolic stability of our previously reported compounds, we have designed and synthesized a series of (phenylpiperazinyl)- or (phenylpiperidinyl)-cyclohexylsulfonamides. By incorporating the information obtained from metabolism studies, we were able to discover several compounds that are both alpha(1a/1d) adrenoceptor subtype selective and show increased stability toward human liver microsomal metabolism. The selectivity profile of these compounds provides great improvement over the commercial drug tamsulosin, hence may pave the way to the development of new and efficacious therapeutic agents with reduced side effects.

Cellular and in vivo activity of JNJ-28871063, a nonquinazoline pan-ErbB kinase inhibitor that crosses the blood-brain barrier and displays efficacy against intracranial tumors

JNJ-28871063 is a potent and highly selective pan-ErbB kinase inhibitor from a novel aminopyrimidine oxime structural class that blocks the proliferation of epidermal growth factor receptor (EGFR; ErbB1)- and ErbB2-overexpressing cells but does not affect the growth of non-ErbB-overexpressing cells. Treatment of human cancer cells with JNJ-28871063 inhibited phosphorylation of functionally important tyrosine residues in both EGFR and ErbB2 and blocked downstream signal transduction pathways responsible for proliferation and survival. A single dose of compound reduced phosphorylation of ErbB2 receptors in tumor-bearing mice, demonstrating target suppression in vivo. Tissue distribution studies show that JNJ-28871063 crosses the blood-brain barrier and penetrates into tumors, where it is able to accumulate to higher levels than those found in the plasma. JNJ-28871063 showed oral antitumor activity in human tumor xenograft models that overexpress EGFR and ErbB2. In an intracranial ErbB2-overexpressing tumor model, JNJ-28871063 extended survival relative to untreated animals. The brain is a primary site of metastasis for EGFR-overexpressing lung cancers and ErbB2-overexpressing breast cancers. Therefore, the ability to penetrate into the brain could be an advantage over existing therapies such as trastuzumab (Herceptin) and cetuximab (Erbitux), which are antibodies and do not cross the blood-brain barrier. These results show that JNJ-28871063 is orally bioavailable, has activity against EGFR and ErbB2-dependent tumor xenografts, and can penetrate into the brain and inhibit ErbB2-overexpressing tumor growth.

(Phenylpiperidinyl)cyclohexylsulfonamides: Development of α1a/1d-selective adrenergic receptor antagonists for the treatment of benign prostatic hyperplasia/lower urinary tract symptoms (BPH/LUTS)

Although alpha(1) adrenergic receptor blockers can be very effective for the treatment of benign prostatic hyperplasia/lower urinary tract symptoms (BPH/LUTS), their usage is limited by CV-related side-effects that are caused by the subtype non-selective nature of the current drugs. To overcome this problem, it was hypothesized that a alpha(1a/1d) subtype selective antagonist would bring more benefit for the therapy of BPH/LUTS. In developing such selective alpha(1a/1d) ligands, a series of (phenylpiperidinyl)cyclohexylsulfonamides has been synthesized and evaluated for binding to three cloned human alpha(1)-adrenergic receptor subtypes. Many compounds showed equal affinity for both alpha(1a) and alpha(1d) subtypes with good selectivity versus the alpha(1b) subtype.

(Arylpiperazinyl)cyclohexylsufonamides: Discovery of α1a/1d-selective adrenergic receptor antagonists for the treatment of Benign Prostatic Hyperplasia/Lower Urinary Tract Symptoms (BPH/LUTS)

Benign Prostatic Hyperplasia/Lower Urinary Tract Symptoms (BPH/LUTS) can be effectively treated by alpha(1)-adrenergic receptor antagonists. Unfortunately, all currently marketed alpha(1) blockers produced CV related side effects that are caused by the subtype non-selective nature of the drugs. To overcome this problem, it was postulated that a alpha(1a/1d) subtype selective antagonist would bring more benefit for the treatment of BPH/LUTS. In developing selective alpha(1a/1d) ligands, (arylpiperazinyl)cyclohexylsulfonamides were synthesized and their binding profiles against three cloned human alpha(1)-adrenergic receptor subtypes were evaluated. Many compounds show equal affinity for both alpha(1a) and alpha(1d) subtypes with good selectivity against the alpha(1b) subtype. They also overcome the problem of dopamine receptor affinity that previous analogues had exhibited.

4-Aryl-5-cyano-2-aminopyrimidines as VEGF-R2 inhibitors: Synthesis and biological evaluation

A novel series of 4-aryl-5-cyano-2-aminopyrimidines were synthesized and found to have potent VEGF-R2 kinase inhibitory activity. Structure-activity relationships were investigated and compound 14a was shown to be efficacious in a mouse model of corneal neovascularization.

Synthesis and evaluation of pyrazolo[3,4-b]pyridine CDK1 inhibitors as anti-tumor agents

A series of 3,5-disubstituted pyrazolo[3,4-b]pyridine cyclin-dependent kinase (CDK) inhibitors was synthesized. These compounds showed potent and selective CDK inhibitory activities and inhibited in vitro cellular proliferation in cultured human tumor cells. Selected compounds were evaluated in an in vivo tumor xenograft model. The synthesis and biological evaluation of these pyrazolo[3,4-b]pyridines and related compounds are reported.

Synthesis and biological study of 2-amino-4-aryl-5-chloropyrimidine analogues as inhibitors of VEGFR-2 and cyclin dependent kinase 1 (CDK1)

The series of 2-amino-4-aryl-5-chloropyrimidines was discovered to be potent for both VEGFR-2 and CDK1. Described here are the chemistry for analogue synthesis, SAR study, and its kinase selectivity prolifing. The full rat PK data and in vivo efficacy study are also included.

Metastin (KiSS-1) mimetics identified from peptide structure-activity relationship-derived pharmacophores and directed small molecule database screening

Metastin, also known as KiSS-1, the cognate ligand for the metastin receptor GPR54, is a peptide known to dramatically reduce metastasis in experimental models. Despite this, there is no reported structure for metastin nor any small molecule modulators of metastin function that could be used either clinically or experimentally. Here we report the NMR solution structure of a 13-residue metastin peptide in a membrane-like environment (SDS micelles) and find it to have a relatively stable helix conformation from residues 7 to 13. In assays for metastin receptor binding and calcium flux with receptor-transfected HEK-293 cells, we demonstrate through alanine scanning and amino acid substitutions that the peptide C-terminus shows helix periodicity in an NMR structural model and that Phe9, Arg12, and Phe13 are crucial to the activity of the peptide. These three residues lie on one face of the helix and define a pharmacophore site for metastin. We used these pharmacophore features in small molecule database searches to identify hits with submicromolar affinity for the metastin receptor. We also show here that molecules mimicking key elements of this pharmacophore site bind to the metastin receptor and act as full agonists, albeit with reduced potency compared to that of metastin itself. Together this structure-activity approach may yield pharmacologically useful compounds relevant in defining and modulating metastin receptor function.

1-Arylpiperazinyl-4-cyclohexylamine derived isoindole-1,3-diones as potent and selective α-1a/1d adrenergic receptor ligands

Subtype-selective alpha-1a and/or alpha-1d adrenergic receptor antagonists may be useful for the treatment of benign prostatic hyperplasia (BPH) and lower urinary tract symptoms (LUTS) with fewer adverse effects than non-selective drugs. A series of 1-arylpiperazinyl-4-cyclohexylamine derived isoindole-1,3-diones has been synthesized, displaying in vitro alpha(1a) and alpha(1d) binding affinity K(i) values in the range of 0.09-38nM with K(i)(alpha1b)/K(i)(alpha1a) and K(i)(alpha1b)/K(i)(alpha1d) selectivity ratios up to 3607-fold.

Synthesis of 3-(1H-benzimidazol-2-yl)-5-isoquinolin-4-ylpyrazolo[1,2-b]pyridine, a potent cyclin dependent kinase 1 (CDK1) inhibitor

The novel compound 3-(1H-benzimidazol-2-yl)-5-isoquinolin-4-ylpyrazolo[1,2-b]pyridine was discovered to be a potent CDK1 inhibitor. Described here is the chemistry for its synthesis, including Pd(II) catalyzed Stille coupling reaction and sulfur(0) induced benzimidazole formation. Its effects on VEGFR-2 kinase activity and tumour cell proliferation are also described.

Role of the ABCG2 drug transporter in the resistance and oral bioavailability of a potent cyclin-dependent kinase/Aurora kinase inhibitor

Cell cycle kinase inhibitors have advanced into clinical trials in oncology. One such molecule, JNJ-7706621, is a broad-spectrum inhibitor of the cyclin-dependent kinases and Aurora kinases that mediate G(2)-M arrest and inhibits tumor growth in xenograft models. To determine the putative mechanisms of resistance to JNJ-7706621 that might be encountered in the clinic, the human epithelial cervical carcinoma cell line (HeLa) was exposed to incrementally increasing concentrations of JNJ-7706621. The resulting resistant cell population, designated HeLa-6621, was 16-fold resistant to JNJ-7706621, cross-resistant to mitoxantrone (15-fold) and topotecan (6-fold), and exhibited reduced intracellular drug accumulation of JNJ-7706621. ABCG2 was highly overexpressed at both the mRNA ( approximately 163-fold) and protein levels. The functional role of ABCG2 in mediating resistance to JNJ-7706621 was consistent with the following findings: (a) an ABCG2 inhibitor, fumitremorgin C, restored the sensitivity of HeLa-6621 cells to JNJ-7706621 and to mitoxantrone; (b) human embryonic kidney-293 cells transfected with ABCG2 were resistant to both JNJ-7706621 and mitoxantrone; and (c) resistant cells that were removed from the drug for 12 weeks and reverted to susceptibility to JNJ-7706621 showed near-normal ABCG2 RNA levels. ABCG2 is likely to limit the bioavailability of JNJ-7706621 because oral administration of JNJ-7706621 to Bcrp (the murine homologue of ABCG2) knockout mice resulted in an increase in the plasma concentration of JNJ-7706621 compared with wild-type mice. These findings indicate that ABCG2 mediates the resistance to JNJ-7706621 and alters the absorption of the compound following administration.

Synthesis of 2-amino-4-(7-azaindol-3-yl)pyrimidines as cyclin dependent kinase 1 (CDK1) inhibitors

A novel series of 2-amino-4-(7-azaindol-3-yl)pyrimidines was discovered as cyclin dependent kinase 1 (CDK1) inhibitors. The core structure was synthesized via Pd(II) catalyzed coupling reaction. A number of analogues showed good potency for CDK1 and exhibited cellular antiproliferation activity. The structure-activity relationship is described.

3-(4-Piperidinyl)indoles and 3-(4-piperidinyl)pyrrolo-[2,3-b]pyridines as ligands for the ORL-1 receptor

A novel series of indoles and 1H-pyrrolo[2,3-b]pyridines having a piperidine ring at the 3-position were synthesized and found to bind with high affinity to the ORL-1 receptor. Structure-activity relationships at the piperidine nitrogen were investigated in each series. Substitution on the phenyl ring and nitrogen atom of the indole and 1H-pyrrolo[2,3-b]pyridine cores generated several selective high-affinity ligands that were agonists of the ORL-1 receptor.

(6,7-Dimethoxy-2,4-dihydroindeno[1,2-c]pyrazol-3-yl)phenylamines: platelet-derived growth factor receptor tyrosine kinase inhibitors with broad antiproliferative activity against tumor cells

A series of (6,7-dimethoxy-2,4-dihydroindeno[1,2-c]pyrazol-3-yl)phenylamines has been optimized to preserve both potent kinase inhibition activity against the angiogenesis target, the receptor tyrosine kinase of Platelet-Derived Growth Factor-BB (PDGF-BB), and to improve the broad tumor cell antiproliferative activity of these compounds. This series culminates in the discovery of 17 (JNJ-10198409), a compound with anti-PDGFR-beta kinase activity (IC(50)=0.0042 microM) and potent antiproliferative activity in six of eight human tumor cell lines (IC(50) < 0.033 microM).

8-(Heteroaryl)phenalkyl-1-Phenyl-1,3,8-triazaspiro[4.5]decan-4-ones as Opioid Receptor Modulators

A series of N-biarylalkyl-1-phenyl-1,3,8-triazaspiro[4.5]decan-4-ones were prepared and evaluated for biological activity at opioid (mu, delta, kappa) and opioid receptor like-1 (ORL-1) G-protein coupled receptors. Substitution on the biaryl moiety produced enhanced affinity for the mu-opioid receptor.

Preparation of 3-spirocyclic indolin-2-ones as ligands for the ORL-1 receptor

A novel series of indolin-2-ones having a spirocyclic piperidine ring at the 3-position was synthesized and found to bind with high affinity to the ORL-1 receptor. Structure-activity relationships at the piperidine nitrogen were investigated. Substitution on the phenyl ring and nitrogen atom of the indolin-2-one core generated several selective high-affinity ligands that were antagonists of the ORL-1 receptor.

TheIn vitroandIn vivoEffects of JNJ-7706621: A Dual Inhibitor of Cyclin-Dependent Kinases and Aurora Kinases

Modulation of aberrant cell cycle regulation is a potential therapeutic strategy applicable to a wide range of tumor types. JNJ-7706621 is a novel cell cycle inhibitor that showed potent inhibition of several cyclin-dependent kinases (CDK) and Aurora kinases and selectively blocked proliferation of tumor cells of various origins but was about 10-fold less effective at inhibiting normal human cell growth in vitro. In human cancer cells, treatment with JNJ-7706621 inhibited cell growth independent of p53, retinoblastoma, or P-glycoprotein status; activated apoptosis; and reduced colony formation. At low concentrations, JNJ-7706621 slowed the growth of cells and at higher concentrations induced cytotoxicity. Inhibition of CDK1 kinase activity, altered CDK1 phosphorylation status, and interference with downstream substrates such as retinoblastoma were also shown in human tumor cells following drug treatment. Flow cytometric analysis of DNA content showed that JNJ-7706621 delayed progression through G1 and arrested the cell cycle at the G2-M phase. Additional cellular effects due to inhibition of Aurora kinases included endoreduplication and inhibition of histone H3 phosphorylation. In a human tumor xenograft model, several intermittent dosing schedules were identified that produced significant antitumor activity. There was a direct correlation between total cumulative dose given and antitumor effect regardless of the dosing schedule. These results show the therapeutic potential of this novel cell cycle inhibitor and support clinical evaluation of JNJ-7706621.

1-Acyl-1H-[1,2,4]triazole-3,5-diamine analogues as novel and potent anticancer cyclin-dependent kinase inhibitors: synthesis and evaluation of biological activities

A series of 1-acyl-1H-[1,2,4]triazole-3,5-diamine analogues were synthesized as cyclin-dependent kinase (CDK) inhibitors. These compounds showed potent and selective CDK1 and CDK2 inhibitory activities and inhibited in vitro cellular proliferation in various human tumor cells. Representative compound 3b demonstrated in vivo efficacy in a human melanoma A375 xenograft model in nude mice.

Synthesis and identification of [1,3,5]triazine-pyridine biheteroaryl as a novel series of potent cyclin-dependent kinase inhibitors

On the basis of previous studies, we identified pyrazine-pyridine A as a potent vascular endothelial growth factor inhibitor and pyrimidine-pyridine B as a moderately potent cyclin dependent kinase (CDK) inhibitor. A proposed combination of CGP-60474 and compound B led to the discovery of [1,3,5]triazine-pyridine as a new series of potent CDK inhibitors. Palladium-catalyzed C-C bond formation reactions, particularly the Negishi coupling reaction, were used to assemble various triazine-heteroaryl analogues effectively. Among them, compound 20 displayed high inhibitory potency at CDK1 (IC(50) = 0.021 microM), CDK2, and CDK5 and submicromolar potency at CDK4, CDK6, and CDK7. Compound 20 also displayed high potency at GSK-3beta. It demonstrated potent antiproliferative activity on various tumor cell lines, including HeLa, HCT-116, U937, and A375. When 20 was administered intraperitoneally at 150 and 125 mg/kg to nude mice bearing human A375 xenografts, the compound produced a significant survival increase. Molecular docking studies were conducted in an attempt to enhance the understanding of the observed structure-activity relationship.

Synthesis and Structure−Activity Relationships of Pyrazine-Pyridine Biheteroaryls as Novel, Potent, and Selective Vascular Endothelial Growth Factor Receptor-2 Inhibitors

There is much evidence that direct inhibition of the kinase activity of vascular endothelial growth factor receptor-2 (VEGFR-2) will result in the reduction of angiogenesis and the suppression of tumor growth. Palladium-catalyzed C-C bond, C-N bond formation reactions were used to assemble various pyrazine-pyridine biheteroaryls as potent VEGFR-2 inhibitors. Among them, 4-{5-[6-(3-chloro-phenylamino)-pyrazin-2-yl]-pyridin-3-ylamino}-butan-1-ol (39) and N-{5-[6-(3-chloro-phenylamino)-pyrazin-2-yl]-pyridin-3-yl}-N',N'-dimethyl-ethane-1,2-diamine (41) exhibited the highest kinase selectivity against fibroblast growth factor receptor kinase, platelet-derived growth factor receptor kinase, and glycogen synthase kinase-3. All of these compounds showed good cellular potency to inhibit VEGF-stimulated proliferation of human umbilical vein endothelial cells (HUVEC) but with modest effects on the unstimulated growth of HUVEC. The low inhibition of these compounds to the growth of tumor cell lines, such as HeLa, HCT-116, and A375 further confirms that these VEGFR-2 inhibitors are not cytotoxic agents. The in vivo antitumor activity of 39 and 41 were demonstrated in the A375 human melanoma xenograft nude mice model. Molecular modeling (QSAR analysis) was conducted in an attempt to rationalize the observed structure-activity relationship.

Synthesis and discovery of pyrazine-pyridine biheteroaryl as a novel series of potent vascular endothelial growth factor receptor-2 inhibitors

Pathological angiogenesis is associated with disease states such as cancer, diabetic retinopathy, rheumatoid arthritis, endometriosis, and psoriasis. There is much evidence that direct inhibition of the kinase activity of vascular endothelial growth factor receptor-2 (VEGFR-2) will result in the reduction of angiogenesis and the suppression of tumor growth. Attempts to optimize a cyclin-dependent kinase-1 (CDK1) inhibitor by using palladium-catalyzed C-C bond, C-N bond formation reactions to assemble diverse biheteroaryl molecules led to the unexpected discovery of a pyrazine-pyridine biheteroaryl as a novel series of potent VEGFR-2 inhibitors. Compound 15, which had IC(50) = 0.084 microM at VEGFR-2, showed very modest selectivity against fibroblast growth factor receptor-2 (IC(50) = 0.21 microM), platelet-derived growth factor receptor (IC(50) = 0.36 microM), and glycogen synthase kinase-3 (IC(50) = 0.478 microM), while it exhibited more than 10-fold selectivity against epidermal growth factor receptor (IC(50) = 1.36 microM) and insulin-R kinase (IC(50) = 1.69 microM). On the other hand, compound 15 exhibited more than 100-fold selectivity against calmodulin kinase 2; casein kinase-1 and -2; CDK1 and -4; mitogen-activated protein kinase; and protein kinase A, Cbeta2, and Cgamma (IC(50) >10 microM). Compound 15 also displayed high inhibitory potency on VEGF-stimulated human umbilical vein endothelial cell (HUVEC) proliferation (IC(50) = 0.005 microM) and good selectivity against cell lines such as HUVEC, human aortic smooth muscle cells, and MRC5 lung fibroblasts. Molecular docking studies were conducted in an attempt to rationalize the unexpected high VEGFR-2 selectivity of 15.

3-Acyl-2,6-diaminopyridines as cyclin-dependent kinase inhibitors: synthesis and biological evaluation

A novel series of 2,6-diamino-3-acylpyridines were designed and synthesized as cyclin-dependent kinase (CDK) inhibitors. The representative compounds 2r and 11 showed potent CDK1 and CDK2 inhibitory activities and inhibited cellular proliferation in HeLa, HCT116, and A375 tumor cells.

The nociceptin pharmacophore site for opioid receptor binding derived from the NMR structure and bioactivity relationships

Nociceptin, a 17 amino acid opioid-like peptide that has an inhibitory effect on synaptic transmission in the nervous system, is involved in learning, memory, attention, and emotion and is also implicated in the perception of pain and visual, auditory, and olfactory functions. In this study, we investigated the NMR solution structure of nociceptin in membrane-like environments (trifluoroethanol and SDS micelles) and found it to have a relatively stable helix conformation from residues 4-17 with functionally important N-terminal residues being folded aperidoically on top of the helix. In functional assays for receptor binding and calcium flux, alanine-scanning variants of nociceptin indicated that functionally important residues generally followed helix periodicity, consistent with the NMR structural model. Structure-activity relationships allowed identification of pharmacophore sites that were used in small molecule data base searches, affording hits with demonstrated nociceptin receptor binding affinities.

A Vascular Endothelial Growth Factor Receptor-2 Kinase Inhibitor Potentiates the Activity of the Conventional Chemotherapeutic Agents Paclitaxel and Doxorubicin in Tumor Xenograft Models

Inhibition of angiogenesis may have wide use in the treatment of cancer; however, this approach alone will not cause tumor regression but may only slow the growth of solid tumors. The clinical potential of antiangiogenic agents may be increased by combining them with conventional chemotherapeutics. 4-[4-(1-Amino-1-methylethyl)phenyl]-2-[4-(2-morpholin-4-yl-ethyl)phenylamino]pyrimidine-5-carbonitrile (JNJ-17029259) represents a novel structural class of 5-cyanopyrimidines that are orally available, selective, nanomolar inhibitors of the vascular endothelial growth factor receptor-2 (VEGF-R2) and other tyrosine kinases involved in angiogenesis, such as platelet-derived growth factor receptor, fibroblast growth factor receptor, VEGF-R1, and VEGF-R3, but have little activity on other kinase families. At nanomolar levels, JNJ-17029259 blocks VEGF-stimulated mitogen-activated protein kinase signaling, proliferation/migration, and VEGF-R2 phosphorylation in human endothelial cells; inhibits the formation of vascular sprouting in the rat aortic ring model of angiogenesis; and interferes with the development of new veins and arteries in the chorioallantoic membrane assay. At higher concentrations of 1 to 3 microM, this compound shows antiproliferative activity on cells that may contribute to its antitumor effects. JNJ-17029259 delays the growth of a wide range of human tumor xenografts in nude mice when administered orally as single-agent therapy. Histological examination revealed that the tumors have evidence of reduced vascularity after treatment. In addition, JNJ-17029259 enhances the effects of the conventional chemotherapeutic drugs doxorubicin and paclitaxel in xenograft models when administered orally in combination therapy. An orally available angiogenesis inhibitor that can be used in conjunction with standard chemotherapeutic agents to augment their activity may have therapeutic benefit in stopping the progression of cancer and preventing metastasis.

Synthesis and discovery of macrocyclic polyoxygenated bis-7-azaindolylmaleimides as a novel series of potent and highly selective glycogen synthase kinase-3beta inhibitors

Attempts to design the macrocyclic maleimides as selective protein kinase C gamma inhibitors led to the unexpected discovery of a novel series of potent and highly selective glycogen synthase kinase-3beta (GSK-3beta) inhibitors. Palladium-catalyzed cross-coupling reactions were used to synthesize the key intermediates 17 and 22 that resulted in the synthesis of novel macrocycles. All three macrocyclic series (bisindolyl-, mixed 7-azaindoleindolyl-, and bis-7-azaindolylmaleimides) were found to have submicromolar inhibitory potency at GSK-3beta with various degrees of selectivity toward other protein kinases. To gain the inhibitory potency at GSK-3beta, the ring sizes of these macrocycles may play a major role. To achieve the selectivity at GSK-3beta, the additional nitrogen atoms in the indole rings may contribute to a significant degree. Overall, the bis-7-azaindolylmaleimides 28 and 29 exhibited little or no inhibitions to a panel of 50 protein kinases. Compound 29 almost behaved as a GSK-3beta specific inhibitor. Both 28 and 29 displayed good potency in GS cell-based assay. Molecular docking studies were conducted in an attempt to rationalize the GSK-3beta selectivity of azaindolylmaleimides.

New epoetin molecules and novel therapeutic approaches

Erythropoietin (EPO) is a 34 kDa protein that is the primary regulator of red blood cell production. EPO facilitates its effect by binding to the cell surface EPO receptor which initiates the JAK-STAT signal transduction cascade. The search for small mimetic molecules of EPO has led to the discovery of a family of peptides that demonstrate EPO mimetic activity. A member of this peptide family, EMP1 (EPO mimetic peptide 1), was used to solve the crystal structure of the soluble EPO receptor in complex with this peptide. The structure revealed a 2:2 stoichiometry of receptor to peptide, with each peptide contacting both receptor molecules in a symmetrical fashion. The potency of the EMPs could be improved through the covalent dimerization of two peptide molecules. Further investigations of EMP EPO receptor complex structures revealed the formation of a non-productive receptor dimer using an inactive peptide. An alternative approach towards the identification of an EPO-like mimetic is to target an intracellular signalling molecule such as haematopoietic cell phosphatase (HCP), also known as SHP1. Inhibiting HCP causes responsive cells to be hypersensitive to EPO. The cloned HCP protein has been utilized in screening assays to identify small molecule inhibitors of HCP.

Shared and unique determinants of the erythropoietin (EPO) receptor are important for binding EPO and EPO mimetic peptide

We have shown previously that Phe93 in the extracellular domain of the erythropoietin (EPO) receptor (EPOR) is crucial for binding EPO. Substitution of Phe93 with alanine resulted in a dramatic decrease in EPO binding to the Escherichia coli-expressed extracellular domain of the EPOR (EPO-binding protein or EBP) and no detectable binding to full-length mutant receptor expressed in COS cells. Remarkably, Phe93 forms extensive contacts with a peptide ligand in the crystal structure of the EBP bound to an EPO-mimetic peptide (EMP1), suggesting that Phe93 is also important for EMP1 binding. We used alanine substitution of EBP residues that contact EMP1 in the crystal structure to investigate the function of these residues in both EMP1 and EPO binding. The three largest hydrophobic contacts at Phe93, Met150, and Phe205 and a hydrogen bonding interaction at Thr151 were examined. Our results indicate that Phe93 and Phe205 are important for both EPO and EMP1 binding, Met150 is not important for EPO binding but is critical for EMP1 binding, and Thr151 is not important for binding either ligand. Thus, Phe93 and Phe205 are important binding determinants for both EPO and EMP1, even though these ligands share no sequence or structural homology, suggesting that these residues may represent a minimum epitope on the EPOR for productive ligand binding.

Crystallographic evidence for preformed dimers of erythropoietin receptor before ligand activation

Erythropoietin receptor (EPOR) is thought to be activated by ligand-induced homodimerization. However, structures of agonist and antagonist peptide complexes of EPOR, as well as an EPO-EPOR complex, have shown that the actual dimer configuration is critical for the biological response and signal efficiency. The crystal structure of the extracellular domain of EPOR in its unliganded form at 2.4 angstrom resolution has revealed a dimer in which the individual membrane-spanning and intracellular domains would be too far apart to permit phosphorylation by JAK2. This unliganded EPOR dimer is formed from self-association of the same key binding site residues that interact with EPO-mimetic peptide and EPO ligands. This model for a preformed dimer on the cell surface provides insights into the organization, activation, and plasticity of recognition of hematopoietic cell surface receptors.

Identification of a 13 amino acid peptide mimetic of erythropoietin and description of amino acids critical for the mimetic activity of EMP1

To obtain information about the functional importance of amino acids required for effective erythropoietin (EPO) mimetic action, the conserved residues of a peptide mimetic of EPO, recently discovered by phage display, were subjected to an alanine replacement strategy. Further, to identify a minimal mimetic peptide sequence, a series of truncation peptides has been generated. One EPO mimetic peptide sequence, EMP1, was targeted and more than 25 derivatives of this sequence were evaluated for their ability to compete with [125I]EPO for receptor binding and for their ability to support the proliferation of two EPO-responsive cell lines. Two hydrophobic amino acids, Tyr4 and Trp13, appear essential for mimetic action, and aromatic residues appear to be important at these sites. These findings are consistent with the previously reported X-ray crystal structure of EMP1 complexed with the extracellular domain of the EPO receptor (EPO binding protein; EBP). In our efforts to define the structural elements required for EPO mimetic action, a 13 amino acid peptide was identified which possesses mimetic properties and contains a minimal agonist epitope. The ability of this peptide to effectively serve as a mimetic capable of the induction of EPO-responsive cell proliferation appears to reside within a single residue, equivalent to position Tyr4 of EMP1, when present in a sequence that includes the cyclic core peptide structure. Although these peptides are less potent than EPO, they should serve as an excellent starting point for the design of compounds with EPO mimetic activity.

Mutagenesis studies of the human erythropoietin receptor. Establishment of structure-function relationships

Mutagenesis of the erythropoietin receptor (EPOR) permits analysis of the contribution that individual amino acid residues make to erythropoietin (EPO) binding. We employed both random and site-specific mutagenesis to determine the function of amino acid residues in the extracellular domain (referred to as EPO binding protein, EBP) of the EPOR. Residues were chosen for site-specific alanine substitution based on the results of the random mutagenesis or on their homology to residues that are conserved or have been reported to be involved in ligand binding in other receptors of the cytokine receptor family. Site-specific mutants were expressed in Escherichia coli as soluble EBP and analyzed for EPO binding in several different assay formats. In addition, selected mutant proteins were expressed as full-length EPOR on the surface of COS cells and analyzed for 125I-EPO binding in receptor binding assays. Using these methods, we have identified residues that appear to be involved in EPO binding as well as other residues, most of which are conserved in receptors of the cytokine receptor family, that appear to be necessary for the proper folding and/or stability of the EPOR. We present correlations between these mutagenesis data and the recently solved crystal structure of the EBP with a peptide ligand.

Functional mimicry of a protein hormone by a peptide agonist: the EPO receptor complex at 2.8 A

The functional mimicry of a protein by an unrelated small molecule has been a formidable challenge. Now, however, the biological activity of a 166-residue hematopoietic growth hormone, erythropoietin (EPO), with its class 1 cytokine receptor has been mimicked by a 20-residue cyclic peptide unrelated in sequence to the natural ligand. The crystal structure at 2.8 A resolution of a complex of this agonist peptide with the extracellular domain of EPO receptor reveals that a peptide dimer induces an almost perfect twofold dimerization of the receptor. The dimer assembly differs from that of the human growth hormone (hGH) receptor complex and suggests that more than one mode of dimerization may be able to induce signal transduction and cell proliferation. The EPO receptor binding site, defined by peptide interaction, corresponds to the smaller functional epitope identified for hGH receptor. Similarly, the EPO mimetic peptide ligand can be considered as a minimal hormone, and suggests the design of nonpeptidic small molecule mimetics for EPO and other cytokines may indeed be achievable.

Identification of a critical ligand binding determinant of the human erythropoietin receptor. Evidence for common ligand binding motifs in the cytokine receptor family

The erythropoietin receptor (EPOR) is a member of a family of cytokine and growth factor receptors that share conserved features in their extracellular and cytoplasmic domains. We have used site-specific mutagenesis within the extracellular domain of the EPOR to search for amino acid residues involved in erythropoietin (EPO) binding. Mutant proteins were expressed in bacteria as soluble EPO binding proteins (EBP) and characterized for EPO binding activity in a number of different assays. Substitution of phenylalanine at position 93 (Phe93) with alanine (F93A mutation) resulted in a drastic reduction in EPO binding in the EBP. More conservative tyrosine or tryptophan substitutions at Phe93 resulted in much less dramatic effects on EPO binding. Biophysical studies indicated that the F93A mutation does not result in gross structural alterations in the EBP. Furthermore, the F93A mutation in full-length EPOR expressed in COS cells abolished detectable EPO binding. This was not a result of processing or transport defects, since mutant receptor was present on the surface of the cells. Mutations in the region immediately around Phe93 and in residues homologous to other reported ligand binding determinants of the cytokine receptor family had small to moderate effects on EPO binding. These data indicate that Phe93 is a critical EPO binding determinant of the EPOR. Furthermore, since Phe93 aligns with critical ligand binding determinants in other receptors of the cytokine receptor family, these data suggest that receptors of this family may use common structural motifs to bind their cognate ligands.

Refolding, purification, and characterization of human erythropoietin binding protein produced in Escherichia coli

The extracellular domain of the human erythropoietin receptor (EPO binding protein (EBP)) has been expressed and overproduced in Escherichia coli. Regardless of the presence ofpelB or ompT signal sequences the recombinant protein produced in this fashion appears, as with many other recombinant eukaryotic proteins produced in E. coli as an insoluble product in laboratory scale fermentations. The induction product of the pelB protein expression system appears as two protein forms with slightly different molecular weights. Based on N-terminal sequence analysis of recovered protein, these forms represent two variants, one with the signal sequence properly processed to yield the expected "native" amino terminus and another which retains the signal sequence. Both forms appear as insoluble fermentation products. Control of oxygen levels and pH during high density fermentation allows the production of only the protein variant with the native amino terminus. Methods reported here permit the efficient recovery of purified EBP which quantitatively binds EPO in solution as determined by high performance size exclusion chromatography. A long-lived refolding intermediate was observed which penultimately collapses into an active conformation. The active purified protein competes with membrane associated EPO receptor for binding [125I]EPO and neutralizes EPO-dependent stimulation in a cell based proliferation assay. Further, the radioligand equilibrium binding constant for this interaction has been determined by immobilizing EBP on agarose gel via a free cysteine. The production of EBP by these methods should facilitate the structural determination of the protein by NMR or crystallography and may serve as a guide for the refolding of other hematopoietic receptors.

Erythropoietin receptor: application in drug development

Erythropoietin (EPO) is the primary hormone responsible for the growth and maturation of red blood cells in mammals. In contrast to many other growth factors, the specificity of EPO for mature erythroid cells has lead to its development as a safe and efficacious therapeutic, EPREX. The medical benefits of EPREX have been well established in the treatment of anaemic chronic renal failure patients, anaemic HIV patients treated with AZT, cancer chemotherapy patients, and patients wishing to donate their own blood prior to elective surgery (autologous predonation). Due to the chronic nature of EPO therapy, it would be desirable to have an orally administered 'second generation' molecule. An understanding of the structural basis of the interaction of EPO with its receptor will aid in the design of an oral anaemia drug. In this study, a series of mutations have been generated in a truncated form of the receptor comprising the extracellular region, termed EPO binding protein (EBP). One mutant, in which alanine replaces phenylalanine at position 93 (F93A) has a 500-fold reduction in binding compared to wild-type EBP. A neutralizing anti-EBP antibody binds poorly to the F93A mutant, while a non-neutralizing anti-EBP antibody binds wild-type and F93A equally well. Information from this mutational analysis can be applied to a receptor 3-D model and ultimately used in drug development.

Structural consequences of the replacement of Glu239 by Gln in the catalytic chain of Escherichia coli aspartate transcarbamylase

Low-angle X-ray scattering in solution has been used to probe the quaternary structure of a mutant version of Escherichia coli aspartate transcarbamylase in which Glu239 of the catalytic chain was replaced by glutamine by site-directed mutagenesis. X-ray crystallographic studies of the wild-type enzyme have shown that one set of intersubunit interactions involving Glu239 are lost, and are replaced by another set of intrachain interactions when the enzyme undergoes the allosteric transition from the T to the R state. Functional analysis of the mutant enzyme with glutamine in place of Glu239 indicates that homotropic co-operativity is lost without altering the maximal specific activity. The radius of gyration of the unligated mutant enzyme is larger than the unligated wild-type, indicating an alteration in quaternary structure of the mutant. However, the radius of gyration of the mutant enzyme in the presence of N-(phosphonoacetyl)-L-aspartate (PALA) is identical with the value for the wild-type enzyme in the presence of PALA. X-ray scattering at larger angles indicates that the mutant enzyme is in a new structural state different from the wild-type T and R structures. The scattering pattern in the presence of saturating concentrations of PALA is identical with that of the wild-type R structure. Saturating concentrations of carbamyl phosphate alone are sufficient to convert most of the mutant enzyme to the R structure, in the absence of aspartate. CTP shifts the scattering pattern of the mutant enzyme in the presence of saturating carbamyl phosphate towards the scattering curve of the unligated enzyme, but CTP has no effect on the scattering curve in the absence of carbamyl phosphate or in the presence of subsaturating PALA. However, in the presence of subsaturating PALA, ATP causes a strong shift towards the R structure. Neither ATP nor CTP has any effect on the activity of the mutant enzyme. These data suggest that the replacement of Glu239 by glutamine results in a new quaternary structure. These data also explain, on a structural basis, why co-operativity is lost in this mutant enzyme.

Regulatory behavior of Escherichia coli aspartate transcarbamylase altered by site-specific mutation of Tyr240----Phe in the catalytic chain

Isotopic exchange kinetics at chemical equilibrium have been used to identify changes in the regulatory properties of aspartate transcarbamylase (ATCase) caused by site-specific mutation of Tyr240----Phe (Y240F) in the catalytic chain. With both wild-type and the mutant enzymes, ATP activates both [14C]Asp in equilibrium N-carbamyl-L-aspartate (C-Asp) and the [32P]carbamyl phosphate (C-P) in equilibrium Pi exchanges. In contrast, with wild-type enzyme, CTP inhibits both exchanges, but with Y240F mutant enzyme CTP inhibits Asp in equilibrium C-Asp exchange and activates C-P in equilibrium Pi exchange. The bisubstrate analog N-(phosphonacetyl-L-aspartate), PALA, activates Asp in equilibrium C-Asp at a lower concentration with the Y240F enzyme, but the extent of activation is decreased, relative to wild-type enzyme. PALA activation of C-P in equilibrium Pi observed with wild-type enzyme disappears completely with the Y240F mutant enzyme. Analysis of perturbations of exchange rates by ATP and CTP were carried out by systematic methods plus computer-based simulations with the ISOBI program. These analyses indicate that (a) ATP increases the rates of association and dissociation for both C-P and Asp, but (b) CTP differentially increases the rate of C-P association to a greater degree than dissociation, but also decreases the rates for Asp association and dissociation in equal proportion. In addition, Arrhenius plots for Y240F ATCase suggest that ATP and CTP act by different mechanisms: ATP increases Vmax (decreases delta G not equal to) uniformly at all temperatures, whereas CTP does not alter either Vmax (delta G not equal to) or the Arrhenius slope (delta H not equal to).

Site-specific mutation of Tyr240----Phe in the catalytic chain of Escherichia coli aspartate transcarbamylase. Consequences for kinetic mechanism

In the catalytic chain of Escherichia coli aspartate transcarbamylase, Tyr240 helps stabilize the T-state conformation by an intrachain hydrogen bond to Asp271. Changes in kinetic characteristics of ATCase that result from disruption of this bond by site-specific mutation of Tyr240----Phe have been investigated by isotopic exchanges at chemical equilibrium. The Tyr240----Phe (Y240F) mutation caused the rate of the [32P] carbamyl phosphate (C-P) in equilibrium Pi exchange to decrease by 2-8-fold, without altering the [14C]Asp in equilibrium N-carbamyl-L-aspartate (C-Asp) rate. The mutation also caused the S0.5 and Hill nH values to decrease in virtually every substrate saturation experiment. Upon increasing the concentrations of the C-P,Pi or C-P,C-Asp reactant-product pairs, inhibition effects observed with the C-P in equilibrium Pi exchange for wild-type enzyme were not apparent with the Y240F mutant enzyme. In contrast, upon increasing the concentrations of the Asp,C-Asp and Asp,Pi pairs, inhibition effects on C-P in equilibrium Pi observed with wild-type enzyme became stronger with the Y240F mutant enzyme. These data indicate that the Tyr240----Phe mutation alters the kinetic mechanism in two different ways: on the reactant side, C-P binding prior to Asp shifts from preferred to compulsory order, and, on the product side, C-Asp and Pi release changes from preferred to nearly random order. These conclusions were also confirmed on a quantitative basis by computer simulations and fitting of the data, which also produced an optimal set of rate constants for the Y240F enzyme. The Arrhenius plot for wild-type holoenzyme was biphasic, but those for catalytic subunits and Y240F enzyme were linear (monophasic). Taken together, the data indicate that the Tyr240----Phe mutation destabilizes the T-state and shifts the equilibrium for the T-R allosteric transition toward the R-state by increasing the rate of T----R conversion.