Kinetic and mechanistic characterisation of Choline Kinase-α

Choline Kinase is a key component of the Kennedy pathway that converts choline into a number of structural and signalling lipids that are essential for cell growth and survival. One member of the family, Choline Kinase-α (ChoKα) is frequently up-regulated in human cancers, and expression of ChoKα is sufficient to transform cells. Consequently ChoKα has been studied as a potential target for therapeutic agents in cancer research. Despite great interest in the enzyme, mechanistic studies have not been reported. In this study, a combination of initial velocity and product inhibition studies, together with the kinetic and structural characterisation of a novel ChoKα inhibitor is used to support a mechanism of action for human ChoKα. Substrate and inhibition kinetics are consistent with an iso double displacement mechanism, in which the γ-phosphate from ATP is transferred to choline in two distinct steps via a phospho-enzyme intermediate. Co-crystal structures, and existing site-specific mutation studies, support an important role for Asp306, in stabilising the phospho-enzyme intermediate. The kinetics also indicate a distinct kinetic (isomerisation) step associated with product release, which may be attributed to a conformational change in the protein to disrupt an interaction between Asp306 and the phosphocholine product, facilitating product release. This study describes a mechanism for ChoKα that is unusual amongst kinases, and highlights the availability of different enzyme states that can be exploited for drug discovery.

The TARC/sICAM5 Ratio in Patient Plasma is a Candidate Biomarker for Drug Resistant Epilepsy

Epilepsy is a common affliction that involves inflammatory processes. There are currently no definitive chemical diagnostic biomarkers in the blood, so diagnosis is based on a sometimes expensive synthesis of clinical observation, radiology, neuro-psychological testing, and interictal and ictal EEG studies. Soluble ICAM5 (sICAM5), also known as telencephalin, is an anti-inflammatory protein of strictly central nervous system tissue origin that is also found in blood. Here we have tested the hypothesis that plasma concentrations of select inflammatory cytokines, including sICAM5, might serve as biomarkers for epilepsy diagnosis. To test this hypothesis, we developed a highly sensitive and accurate electrochemiluminescent ELISA assay to measure sICAM5 levels, and measured levels of sICAM5 and 18 other inflammatory mediators in epilepsy patient plasma and controls. Patient samples were drawn from in-patients undergoing video-EEG monitoring, without regard to timing of seizures. Differences were defined by t-test, and Receiver Operating Condition (ROC) curves determined the ability of these tests to distinguish between the two populations. In epilepsy patient plasmas, we found that concentrations of anti-inflammatory sICAM5 are reduced (p = 0.002) and pro-inflammatory IL-1β, IL-2, and IL-8 are elevated. TARC (thymus and activation regulated chemokine, CCL17) concentrations trend high. In contrast, levels of BDNF and a variety of other pro-inflammatory mediators are not altered. Based on p-value and ROC analysis, we find that the ratio of TARC/sICAM5 discriminates accurately between patients and controls, with an ROC Area Under the Curve (AUC) of 1.0 (p = 0.034). In conclusion, we find that the ratio of TARC to sICAM5 accurately distinguishes between the two populations and provides a statistically and mechanistically compelling candidate blood biomarker for drug resistant epilepsy.

Targeting ATR in vivo using the novel inhibitor VE-822 results in selective sensitization of pancreatic tumors to radiation

Combined radiochemotherapy is the currently used therapy for locally advanced pancreatic ductal adenocarcinoma (PDAC), but normal tissue toxicity limits its application. Here we test the hypothesis that inhibition of ATR (ATM-Rad3-related) could increase the sensitivity of the cancer cells to radiation or chemotherapy without affecting normal cells. We tested VE-822, an ATR inhibitor, for in vitro and in vivo radiosensitization. Chk1 phosphorylation was used to indicate ATR activity, γH2AX and 53BP1 foci as evidence of DNA damage and Rad51 foci for homologous recombination activity. Sensitivity to radiation (XRT) and gemcitabine was measured with clonogenic assays in vitro and tumor growth delay in vivo. Murine intestinal damage was evaluated after abdominal XRT. VE-822 inhibited ATR in vitro and in vivo. VE-822 decreased maintenance of cell-cycle checkpoints, increased persistent DNA damage and decreased homologous recombination in irradiated cancer cells. VE-822 decreased survival of pancreatic cancer cells but not normal cells in response to XRT or gemcitabine. VE-822 markedly prolonged growth delay of pancreatic cancer xenografts after XRT and gemcitabine-based chemoradiation without augmenting normal cell or tissue toxicity. These findings support ATR inhibition as a promising new approach to improve the therapeutic ration of radiochemotherapy for patients with PDAC.

The novel ATR inhibitor VE-821 increases sensitivity of pancreatic cancer cells to radiation and chemotherapy

DNA damaging agents such as radiotherapy and gemcitabine are frequently used for the treatment of pancreatic cancer. However, these treatments typically provide only modest benefit. Improving the low survival rate for pancreatic cancer patients therefore remains a major challenge in oncology. Inhibition of the key DNA damage response kinase ATR has been suggested as an attractive approach for sensitization of tumor cells to DNA damaging agents, but specific ATR inhibitors have remained elusive. Here we investigated the sensitization potential of the first highly selective and potent ATR inhibitor, VE-821, in vitro. VE-821 inhibited radiation- and gemcitabine-induced phosphorylation of Chk1, confirming inhibition of ATR signaling. Consistently, VE-821 significantly enhanced the sensitivity of PSN-1, MiaPaCa-2 and primary PancM pancreatic cancer cells to radiation and gemcitabine under both normoxic and hypoxic conditions. ATR inhibition by VE-821 led to inhibition of radiation-induced G 2/M arrest in cancer cells. Reduced cancer cell radiosurvival following treatment with VE-821 was also accompanied by increased DNA damage and inhibition of homologous recombination repair, as evidenced by persistence of γH2AX and 53BP1 foci and inhibition of Rad51 foci, respectively. These findings support ATR inhibition as a novel approach to improve the efficacy and therapeutic index of standard cancer treatments across a large proportion of pancreatic cancer patients.

Exposure to CYP3A4-inducing and CYP3A4-non-inducing antiepileptic agents and the risk of fractures

PURPOSE

To evaluate whether exposure to Cytochrome P450, family 3, subfamily A, polypeptide 4 (CYP3A4)-inducing antiepileptics increases fracture risk compared to CYP3A4-non-inducing antiepileptics.

METHODS

We performed a retrospective cohort study of initiators of antiepileptic agents using a UK medical record database (The Health Improvement Network) from 1995 to 2007. We considered an antiepileptic user an initiator if he or she had not received a prescription for an antiepileptic agent within the first year after entry in the database. Proportional hazards regression was used to calculate hazard ratios for fracture during long-term (≥ 6 months) exposure to CYP3A4 inducing versus CYP3A4 non-inducing antiepileptics.

RESULTS

We identified 4077 initiators of CYP3A4-inducing antiepileptics and 6433 initiators of CYP3A4-non-inducing antiepileptics with at least 6 months of antiepileptic exposure. During 6006 person-years exposed to CYP3A4-inducing antiepileptics, 118 fractures were identified for an incidence rate of 1.96 (95% confidence interval (CI): 1.63-2.35) fractures per 100 person-years. During 7184 person-years exposed to CYP3A4-non-inducing antiepileptics, 127 fractures were identified, for an incidence rate of 1.77 (95% CI: 1.47-2.10) fractures per 100 person-years. The adjusted hazard ratio for CYP3A4-inducing antiepileptic versus CYP3A4-non-inducing antiepileptic was 1.21 (95% CI: 0.93-1.56). No duration-response relationship was evident.

CONCLUSIONS

Our results do not support the hypothesis that CYP3A4 induction by antiepileptic agents increases the fracture risk. Further research will be needed to evaluate whether mechanisms other than CYP3A4 induction might explain some of the elevated risk of fractures associated with long-term use of antiepileptic agents.

Abstract 5491: Evaluation of the first potent and highly selective inhibitor of ATR kinase: An approach to selectively sensitize cancer cells to genotoxic drugs

DNA damaging agents have been the cornerstone of solid cancer therapy for decades yet they provide only modest benefit for patients with many tumor types. This reflects, in part, the efficient repair of DNA damage via a complex signaling and repair network known as the DNA damage response (DDR). Key regulators of the DDR are the phosphoinositol 3-kinase-like serine/threonine protein kinase (PIKK) family members ATR, ATM and DNA-PK. The DDR acts to detect DNA lesions, enforce checkpoints to halt cell cycle progression, and stimulate repair. Recent data have shown that elements of the DDR are commonly defective in cancer cells. It is widely believed that these cells become dependent on the remaining DDR pathways for survival from DNA damage. Inhibitors have been reported for a number of DDR enzymes, including ATM, DNA-PK, CHK1 and PARP, however there are no reports of drug-like ATR inhibitors.

Here we disclose the in vitro characterization of a potent and highly selective ATR inhibitor (VE-821). This compound selectively blocks ATR signaling in cells (IC50 = 0.7 µM), but has little impact on ATM or DNA-PK signaling (IC50 >10 µM). Treatment with 10 µM VE-821 for 144 h causes little cell death in normal cell lines (5-11 %) but markedly higher death in cancer cell lines (28-46 %). VE-821 also dramatically sensitizes many cancer cells to multiple classes of genotoxic agents including antimetabolites, topoisomerase inhibitors and crosslinking agents; with over 10-fold increases genotoxic potency observed in some cases. In a panel of 36 lung cancer cell lines, VE-821 sensitized the cytotoxic effect of cisplatin to a far greater magnitude and over a broader subset of these lines than potent inhibitors of ATM, Chk1, or PARP. In over half of these cell lines, the IC50 of cisplatin was reduced by greater than 5 fold upon the addition of VE-821.

We show that a basis for the cancer-selective effects of VE-821 is a synthetic lethal interaction between loss of ATM signaling (a frequent event in cancer resulting from loss of function of proteins such as ATM or p53) and ATR inhibition when cells encounter DNA damage. In keeping with this, ATR inhibition does not sensitize normal cells (with functional ATM) to the cytotoxic effects of genotoxic therapy. In this case a compensatory DDR is activated that is associated with marked activation of ATM, which in turn leads to reversible checkpoint arrest and a strong survival response.

These studies show for the first time that a selective ATR inhibitor can preferentially sensitize cancer cells to genotoxic drugs by exploiting a synthetic lethal interaction between ATM and ATR signaling. This underpins the broad potential of ATR inhibition as a highly promising new strategy to improve the efficacy of genotoxic therapy.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5491. doi:10.1158/1538-7445.AM2011-5491

Selective killing of ATM- or p53-deficient cancer cells through inhibition of ATR

Here we report a comprehensive biological characterization of a potent and selective small-molecule inhibitor of the DNA damage response (DDR) kinase ATR. We show a profound synthetic lethal interaction between ATR and the ATM-p53 tumor suppressor pathway in cells treated with DNA-damaging agents and establish ATR inhibition as a way to transform the outcome for patients with cancer treated with ionizing radiation or genotoxic drugs.

Discovery of potent and selective inhibitors of ataxia telangiectasia mutated and Rad3 related (ATR) protein kinase as potential anticancer agents

DNA-damaging agents are among the most frequently used anticancer drugs. However, they provide only modest benefit in most cancers. This may be attributed to a genome maintenance network, the DNA damage response (DDR), that recognizes and repairs damaged DNA. ATR is a major regulator of the DDR and an attractive anticancer target. Herein, we describe the discovery of a series of aminopyrazines with potent and selective ATR inhibition. Compound 45 inhibits ATR with a K(i) of 6 nM, shows >600-fold selectivity over related kinases ATM or DNA-PK, and blocks ATR signaling in cells with an IC(50) of 0.42 μM. Using this compound, we show that ATR inhibition markedly enhances death induced by DNA-damaging agents in certain cancers but not normal cells. This differential response between cancer and normal cells highlights the great potential for ATR inhibition as a novel mechanism to dramatically increase the efficacy of many established drugs and ionizing radiation.

Antiepileptic drugs and other medications: what interactions may arise?

Many patients with epilepsy are on lifelong therapy with antiepileptic drugs (AEDs), and AEDs are used for other conditions such as mood stabilization and headache prophylaxis. These drugs have high potential for clinically significant interaction with nonepilepsy drugs. Interactions occur largely through altered pharmacokinetics. One drug may increase the hepatic clearance of another, leading to attenuated efficacy of the affected drug. Alternatively, inhibition of liver metabolism by one drug can cause acute toxicity by reducing clearance of another drug. To identify potential drug interactions before they lead to toxicity or therapy failure, the treating clinician should combine knowledge of the patient's overall history with a general knowledge of comorbid conditions in which significant interactions involving AEDs are most likely to occur. Treatments susceptible to interactions include anticoagulants, antiarrhythmics, antibiotics, antiretroviral drugs, immunosuppressives, antineoplastics, and contraceptives. Therefore, it is important to obtain periodically a thorough history of medical problems, use of medications or herbal remedies, and adverse effects of medications. Physicians managing epilepsy patients should also strive to avoid potential drug interactions by favoring low-interaction AEDs in patients taking many other types of drugs. There is quite a large degree of patient heterogeneity in the extent of any given interaction between an AED and another drug. Indeed, some groups of patients may have different susceptibilities to such interactions because of genetic and environmental influences on drug metabolism. Effective treatment with AEDs should include attention to drug interactions.

Seletracetam, a small molecule SV2A modulator for the treatment of epilepsy

UCB SA was developing the high-affinity synaptic vesicle glycoprotein 2A ligand, seletracetam, an analog of levetiracetam, for the potential oral treatment of epilepsy. Phase II epilepsy trials were underway, but in July 2007, the company stated that development of seletracetam had been put on hold and it is unknown whether planned phase IIb/III trials will begin.

Structural basis for potent inhibition of the Aurora kinases and a T315I multi-drug resistant mutant form of Abl kinase by VX-680

The small molecule inhibitor of the Aurora-family of protein kinases VX-680 or MK-0457, demonstrates potent anti-cancer activity in multiple in vivo models and has recently entered phase II clinical trials. Although VX-680 shows a high degree of enzyme selectivity against multiple kinases, it unexpectedly inhibits both Flt-3 and Abl kinases at low nanomolar concentrations. Furthermore VX-680 potently inhibits Abl and the Imatinib resistant mutant (T315I) that is commonly expressed in refractory CML and ALL. We describe here the crystal structure of VX-680 bound to Aurora-A and show that this inhibitor exploits a centrally located hydrophobic pocket in the active site that is only present in an inactive or "closed" kinase conformation. A tight association of VX-680 with this hydrophobic pocket explains its high affinity for the Aurora kinases and also provides an explanation for its selectivity profile, including its ability to inhibit Abl and the Imatinib-resistant mutant (T315I).

Antiepileptic drugs in development

BACKGROUND

Despite the success of several new antiepileptic drugs, about one third of patients with epilepsy are not seizure free on medication. Improvement in this situation might lie in drugs that are currently in development.

RECENT DEVELOPMENTS

Some new antiepileptic drugs are modifications of those already available, referred to in this Rapid Review as evolutionary drugs. These modifications of existing drugs are developed to improve effectiveness, often by increasing tolerability. Other drugs work by new mechanisms and are usually discovered through screening of animal models. WHERE NEXT? The large number of drugs currently in clinical trials provides a measure of hope for patients whose epilepsy is not controlled with currently available medication. In the future, this range of antiepileptic drugs will probably increase because of the use of new animal models, discovery of new basic mechanisms of epileptogenesis, acceleration of proof of principle studies in people, and development of new methods of drug delivery.

Racial differences in the prevalence of cardiac sources of embolism in subjects with unexplained stroke or transient ischemic attack evaluated by transesophageal echocardiography

Little is known about the distribution of cardiac sources of embolism among African-Americans with cryptogenic cerebrovascular events. We compared the prevalence of potential cardiac sources of embolism between black and white patients referred to our laboratory for transesophageal echocardiographic (TEE) evaluation of unexplained stroke or transient ischemic attack. Records were reviewed to exclude subjects with high-risk cardiac or vascular disorders likely to explain the index event. Of 297 patients satisfying the inclusion criteria, 196 were white and 87 black. Potential cardioembolic sources were significantly less common in blacks than in whites (adjusted odds ratio [OR], 0.44; 95% confidence interval [CI] 0.26 to 0.75), and related largely to the difference in prevalence of interatrial communication (OR 0.40; 95% CI 0.21 to 0.74). In contrast, African-Americans had a higher prevalence of left ventricular (LV) hypertrophy (OR 3.50; 95% CI 1.97 to 6.22), and particularly, moderate or severe hypertrophy (OR 4.03; 95% CI 1.88 to 9.65) compared with whites. In conclusion, in African-Americans with unexplained cerebrovascular events, the yield of TEE for potential cardioembolic sources, and especially interatrial communication, is lower than in their white counterparts. African-Americans exhibit a substantially higher prevalence of LV hypertrophy, which may be a marker for a higher burden of subclinical cerebrovascular disease involved in the pathogenesis of cryptogenic cerebral ischemia in this population.

Vertebrobasilar thrombolysis with intravenous tirofiban: case report

The use of thrombolytic agents in the setting of established cerebral infarction is limited by concerns for hemorrhagic transformation. Novel thrombolytic approaches, which have received minimal consideration, may be associated with lower risks of hemorrhage. We illustrate vertebrobasilar thrombolysis with intravenous tirofiban, a selective platelet glycoprotein IIb/IIIa receptor antagonist, and discuss the potential thrombolytic properties of this class of antithrombotics.

The 3-methylaspartase reaction probed using 2H- and 15N-isotope effects for three substrates: a flip from a concerted to a carbocationic amino-enzyme elimination mechanism upon changing the C-3 stereochemistry in the substrate from R to S

The mechanisms of the elimination of ammonia from (2S,3S)-3-methylaspartic acid, (2S)-aspartic acid and (2S,3R)-3-methylaspartic acid, catalysed by the enzyme L-threo-3-methylaspartase ammonia-lyase (EC 4.3.1.2) have been probed using 15N-isotope effects. The 15N-isotope effects for V/K for both (2S,3S)-3-methylaspartic acid and aspartic acid are 1.0246 +/- 0.0013 and 1.0390 +/- 0.0031, respectively. The natural substrate, (2S,3S)-3-methylaspartic acid, is eliminated in a concerted fashion such that the C(beta)-H and C(alpha)-N bonds are cleaved in the same transition state. (2S)-Aspartic acid appears to follow the same mechanistic pathway, but deprotonation of the conjugate acid of the base for C-3 is kinetically important and influences the extent of 15N-fractionation. (2S,3R)-3-Methylaspartic acid is deaminated via a stepwise carbocationic mechanism. Here we elaborate on the proposed model for the mechanism of methylaspartase and propose that a change in stereochemistry of the substrate induces a change in the mechanism of ammonia elimination.

Mechanism of 3-methylaspartase probed using deuterium and solvent isotope effects and active-site directed reagents: identification of an essential cysteine residue

The mechanism of the L-threo-3-methylaspartate ammonia-lyase (EC 4.3.1.2) reaction has been probed using deuterium and solvent isotope effects with three different substrates, (2S,3S)-3-methylaspartic acid, (2S)-aspartic acid and (2S,3R)-3-methylaspartic acid. Each substrate appears to form a covalent adduct with the enzyme through the amination of a dehydroalanine (DehydAla-173) residue. The true substrates are N-protonated and at low pH, the alkylammonium groups are deprotonated internally in a closed solvent-excluded pocket after K+ ion, an essential cofactor, has become bound to the enzyme. At high pH, the amino groups of the substrates are able to react with the dehydroalanine residue prior to K+ ion binding. This property of the system gives rise to complex kinetics at pH 9.0 or greater and causes the formation of dead-end complexes which lack Mg2+ ion, another essential cofactor. The enzyme-substrate adduct is subsequently deaminated in two elimination processes. Hydrazines act as alternative substrates in the reverse reaction direction in the presence of fumaric acid derivatives, but cause irreversible inhibition in their absence. Borohydride and cyanide are not inhibitors. N-Ethylmaleimide also irreversibly inactivates the enzyme and labels residue Cys-361. The inactivation process is enhanced in the presence of cofactor Mg2+ ions and Cys-361 appears to serve as a base for the removal of the C-3 proton from the natural substrate, (2S,3S)-3-methylaspartic acid. The dehydroalanine residue appears to be protected in the resting form of the enzyme by generation of an internal thioether cross-link. The binding of the substrate and K+ ion appear to cause a conformational change which requires hydroxide ion. This is linked to reversal of the thioether protection step and generation of the base for substrate deprotonation at C-3. The deamination reaction displays high reverse reaction commitments and independent evidence from primary deuterium isotope effect data indicates that a thiolate acts as the base for deprotonation at C-3.

Substrate selectivity and biochemical properties of 4-hydroxy-2-keto-pentanoic acid aldolase from Escherichia coli

4-Hydroxy-2-keto-pentanoic acid aldolase from Escherichia coli was identified as a class I aldolase. The enzyme was found to be highly selective for the acetaldehyde acceptor but would accept alpha-ketobutyric acid or phenylpyruvic acid in place of the pyruvic acid carbonyl donor.

Purification, characterisation and reaction mechanism of monofunctional 2-hydroxypentadienoic acid hydratase from Escherichia coli

2-Hydroxypentadienoic acid hydratase is found on many bacterial catabolic pathways responsible for the degradation of aromatic compounds. Monofunctional 2-hydroxypentadienoic acid hydratase from Escherichia coli has been purified 3800-fold to homogeneity, using enzymatically generated 2-hydroxypentadienoic acid as substrate. The purified 28-kDa protein requires a divalent metal ion for activity, optimum activity being obtained with Mn2+. Steady-state kinetic parameters were measured (Km = 41 +/- 4 microM, k(cat) = 450 s(-1)), the enzyme exhibiting substrate inhibition at high substrate concentrations. The pH/rate profile and inhibition by group-specific reagents were examined, and evidence was obtained for essential cysteine and tryptophan residues. An amino acid sequence alignment of the inferred amino acid sequence with nine other sequences was carried out and revealed several conserved sequence motifs. The substrate for the enzymatic reaction was found to be the dienol tautomer of 2-hydroxypentadienoic acid. Analysis of the reaction products by HPLC confirmed the identity of the 4-hydroxy-2-ketopentanoic acid product. Analogues of possible reaction intermediates were tested as inhibitors, and sodium oxalate was found to act as a potent enzyme inhibitor (Ki = 4.9 +/- 0.7 microM). The potent inhibition by oxalate is consistent with a mechanism in which tautomerisation to 2-ketopent-3-enoic acid takes place at the active site, followed by conjugate addition of water.