Robert J. Buckland

Relationship between degree of P2Y12 receptor blockade and inhibition of P2Y12-mediated platelet function

The thienopyridine P2Y12 receptor antagonists clopidogrel and prasugrel prevent arterial thrombosis and are routinely used following percutaneous coronary intervention. However, the optimal level of P2Y12 blockade to effectively inhibit platelet function is unknown. These studies utilised the active metabolite of prasugrel (R-138727) to achieve a range of P2Y12 blockade in vitro and assessed several aspects of platelet function. Blood from healthy volunteers was incubated with R-138727 (0-10 microM). P2Y12 receptor number was assessed using a 33P-2MeSADP binding assay. Platelet aggregation (PA) was measured by optical aggregometry with ADP 2-20 microM. VASP phosphorylation, annexin V binding, microparticle formation and P-selectin expression were assessed by flow cytometry. Increasing numbers of unblocked receptors were required for a sustained aggregation response with decreasing concentrations of ADP. A P2Y12 receptor blockade of 60-80% resulted in strong inhibition of final PA response, P-selectin expression, microparticle formation and vasodilator-stimulated phosphoprotein (VASP). PA induced by ADP 2 microM and P-selectin expression were particularly sensitive to low levels of receptor blockade whereas the VASP phosphorylation assay was relatively insensitive, requiring 60% receptor blockade to achieve substantial inhibition. Different assays varied in their ability to discriminate particular ranges of P2Y12 blockade and 80% or greater P2Y12 receptor blockade is required for consistently strong inhibition of several aspects of platelet function. These data guide the interpretation of results from different assays used to monitor the effects of P2Y12 receptor antagonists.

The active metabolite of prasugrel effectively blocks the platelet P2Y12 receptor and inhibits procoagulant and pro-inflammatory platelet responses.

The aim of these studies was to investigate the extent of platelet P2Y12 receptor inhibition by the thienopyridine active metabolite of prasugrel, R-138727. Blood was taken from healthy volunteers and pre-incubated with R-138727 or cangrelor (AR-C66931MX). Platelet aggregation was assessed in platelet rich plasma (PRP) and whole blood (WB). Vasodilator stimulated phosphoprotein (VASP) phosphorylation, platelet procoagulant activity (annexin V binding and microparticle formation) and calcium mobilisation were measured by flow cytometry. Platelet-leukocyte co-aggregate formation and sCD40L release, both pro-inflammatory responses of platelets, were measured by flow cytometry and ELISA, respectively. P2Y12 receptor antagonism was determined using a radioligand binding assay (33P 2-MeSADP) in resting and stimulated platelets and the effects of clopidogrel administration were also assessed. R-138727 yielded concentration-dependent inhibition of platelet aggregation, VASP phosphorylation inhibition, procoagulant activity and pro-inflammatory responses. In the presence of R-138727 or cangrelor there was increased calcium reuptake following agonist stimulation. R-138727 30 µmol/L and cangrelor 1 µmol/L completely inhibited 33P 2-MeSADP binding, compared to partial inhibition following clopidogrel administration. Platelet activation and granule secretion did not expose an additional pool of P2Y12 receptors. Prasugrels active metabolite effectively blocks the P2Y12 receptor with the highest concentrations tested yielding complete inhibition of P2Y12-mediated amplification of several important platelet responses.

Glycoprotein IIb/IIIa and P2Y12 receptor antagonists yield additive inhibition of platelet aggregation, granule secretion, soluble CD40L release and procoagulant responses

Glycoprotein IIb/IIIa (GPIIb/IIIa) antagonists, including abciximab and tirofiban, are administered concurrently with clopidogrel, a P2Y12 antagonist, and aspirin in some patients undergoing percutaneous coronary intervention. We studied the effects of, and interactions between, abciximab, tirofiban, aspirin and the P2Y12 antagonist cangrelor on platelet aggregation, α and dense granule secretion and procoagulant responses in vitro. Blood was obtained from healthy volunteers. Platelet aggregation, dense granulesecretion, α granule secretion (PAI-1 and soluble CD40 ligand levels) and procoagulant responses (annexin-V and microparticle formation) were assessed using collagen and thrombin receptor activating peptide (TRAP) as agonists. All the antagonists used singularly inhibited collagen-induced responses. Combinations of abciximab or tirofiban with aspirin and/or cangrelor gave additive inhibition with the greatest effect seen when abciximab or tirofiban was combined with both aspirin and cangrelor. Cangrelor inhibited TRAP-induced responses and, again, there was additive inhibition of these parameters when abciximab or tirofiban were combined with cangrelor. The GPIIb/IIIa receptor plays an important role in amplification of platelet activation such that there are important interactions between GPIIb/IIIa antagonists and inhibitors of both P2Y12 receptor activation and, to a lesser extent, thromboxane A2 generation. These interactions are likely to have important influences on the safety and efficacy of combination anti-platelet therapies.

Heterozygous colon cancer-associated mutations of SAMHD1 have functional significance.

Significance The three major DNA replication fidelity determinants are nucleotide selectivity, proofreading, and mismatch repair. Defects in the two latter determinants are now firmly associated with cancer. Nucleotide selectivity is affected by changes in the absolute or relative concentrations of dNTPs. Here, we show that hemizygous SAMHD1+/− mouse embryos have increased dNTP pools compared with wild-type controls and that heterozygous mutations that inactivate SAMHD1 are frequently found in colon cancers. We infer that such cancer cells have increased dNTP pools and, therefore, higher mutation rates. These observations suggest that changes in dNTP concentrations, which affect nucleotide selectivity, the first major determinant of DNA replication fidelity, are associated with cancer. Even small variations in dNTP concentrations decrease DNA replication fidelity, and this observation prompted us to analyze genomic cancer data for mutations in enzymes involved in dNTP metabolism. We found that sterile alpha motif and histidine-aspartate domain-containing protein 1 (SAMHD1), a deoxyribonucleoside triphosphate triphosphohydrolase that decreases dNTP pools, is frequently mutated in colon cancers, that these mutations negatively affect SAMHD1 activity, and that several SAMHD1 mutations are found in tumors with defective mismatch repair. We show that minor changes in dNTP pools in combination with inactivated mismatch repair dramatically increase mutation rates. Determination of dNTP pools in mouse embryos revealed that inactivation of one SAMHD1 allele is sufficient to elevate dNTP pools. These observations suggest that heterozygous cancer-associated SAMHD1 mutations increase mutation rates in cancer cells.

Increased and Imbalanced dNTP Pools Symmetrically Promote Both Leading and Lagging Strand Replication Infidelity

The fidelity of DNA replication requires an appropriate balance of dNTPs, yet the nascent leading and lagging strands of the nuclear genome are primarily synthesized by replicases that differ in subunit composition, protein partnerships and biochemical properties, including fidelity. These facts pose the question of whether imbalanced dNTP pools differentially influence leading and lagging strand replication fidelity. Here we test this possibility by examining strand-specific replication infidelity driven by a mutation in yeast ribonucleotide reductase, rnr1-Y285A, that leads to elevated dTTP and dCTP concentrations. The results for the CAN1 mutational reporter gene present in opposite orientations in the genome reveal that the rates, and surprisingly even the sequence contexts, of replication errors are remarkably similar for leading and lagging strand synthesis. Moreover, while many mismatches driven by the dNTP pool imbalance are efficiently corrected by mismatch repair, others are repaired less efficiently, especially those in sequence contexts suggesting reduced proofreading due to increased mismatch extension driven by the high dTTP and dCTP concentrations. Thus the two DNA strands of the nuclear genome are at similar risk of mutations resulting from this dNTP pool imbalance, and this risk is not completely suppressed even when both major replication error correction mechanisms are genetically intact.

Potentiation of clopidogrel active metabolite formation by rifampicin leads to greater P2Y12 receptor blockade and inhibition of platelet aggregation after clopidogrel.

Summary.  Background: The thienopyridine P2Y12 receptor antagonist clopidogrel reduces the risk of arterial thrombosis and individual pharmacodynamic responses to clopidogrel are believed to reflect the levels of active metabolite (AM) generated. Rifampicin increases the inhibitory effect of clopidogrel on platelet aggregation (PA). We studied the response to clopidogrel before and during administration of rifampicin in order to study the relationship between individual AM levels and P2Y12 blockade. Methods: Healthy volunteers received a 600‐mg loading dose of clopidogrel followed by 75 mg daily for 7 days and, after a washout period and treatment with rifampicin [300 mg twice a day (b.i.d.)], received the same regimen of clopidogrel. Clopidogrel AM levels were determined over 4 h after the clopidogrel loading dose and unblocked P2Y12 receptor number was assessed using a 33P‐2MeSADP binding assay. PA was measured by optical aggregometry with ADP and TRAP. Results: Rifampicin enhanced clopidogrel AM production [area‐under‐the‐curve (AUC): clopidogrel 89 ± 22 ng h mL−1, clopidogrel + rifampicin 335 ± 86 ng h mL−1, P < 0.0001], and P2Y12 blockade (unblocked receptors: clopidogrel 48 ± 24, clopidogrel + rifampicin 4 ± 2, P < 0.0001) and reduced PA (5 μmol L−1 ADP: clopidogrel 20 ± 4, clopidogrel + rifampicin 5 ± 2, P < 0.01). Increasing numbers of unblocked receptors were required for an aggregation response with a decreasing concentration of ADP. PA induced by ADP 2 μmol L−1 was particularly sensitive to low levels of receptor blockade. Conclusion: Potentiation of clopidogrel AM production by rifampicin leads to greater P2Y12 blockade and consequently greater inhibition of PA. PA responses to low concentrations of ADP are more sensitive to P2Y12 blockade.

Genome-wide analysis of the specificity and mechanisms of replication infidelity driven by imbalanced dNTP pools.

The absolute and relative concentrations of the four dNTPs are key determinants of DNA replication fidelity, yet the consequences of altered dNTP pools on replication fidelity have not previously been investigated on a genome-wide scale. Here, we use deep sequencing to determine the types, rates and locations of uncorrected replication errors that accumulate in the nuclear genome of a mismatch repair-deficient diploid yeast strain with elevated dCTP and dTTP concentrations. These imbalanced dNTP pools promote replication errors in specific DNA sequence motifs suggesting increased misinsertion and increased mismatch extension at the expense of proofreading. Interestingly, substitution rates are similar for leading and lagging strand replication, but are higher in regions replicated late in S phase. Remarkably, the rate of single base deletions is preferentially increased in coding sequences and in short rather than long mononucleotides runs. Based on DNA sequence motifs, we propose two distinct mechanisms for generating single base deletions in vivo. Collectively, the results indicate that elevated dCTP and dTTP pools increase mismatch formation and decrease error correction across the nuclear genome, and most strongly increases mutation rates in coding and late replicating sequences.

Cangrelor inhibits the binding of the active metabolites of clopidogrel and prasugrel to P2Y12 receptors in vitro.

Abstract Cangrelor is a rapid-acting, direct-binding, and reversible P2Y12 antagonist which has been studied for use during percutaneous coronary intervention (PCI) in patients with or without pretreatment with an oral P2Y12 antagonist. As cangrelor is administered intravenously, it is necessary to switch to an oral P2Y12 antagonist following PCI, such as the thienopyridines clopidogrel, and prasugrel or the non-pyridine ticagrelor. Previous studies have suggested a negative pharmacodynamic interaction between cangrelor and thienopyridines. This in vitro study evaluated the receptor-level interaction between cangrelor and the active metabolite (AM) of clopidogrel or prasugrel by assessing functional P2Y12 receptor number using a 33P-2MeSADP binding assay. All P2Y12 antagonists studied resulted in strong P2Y12 receptor blockade (cangrelor: 93.6%; clopidogrel AM: 93.0%; prasugrel AM: 97.9%). Adding a thienopyridine AM in the presence of cangrelor strongly reduces P2Y12 receptor blockade by the AM (clopidogrel AM: 7%, prasugrel AM: 3.2%). The thienopyridine AMs had limited ability to compete with cangrelor for binding to P2Y12 (% P2Y12 receptor blockade after co-incubation with cangrelor 1000 nmol/L: 11.7% for clopidogrel AM 3 µmol/L; 34.1% for prasugrel AM 3 µmol/L). In conclusion, in vitro cangrelor strongly inhibits the binding of clopidogrel and prasugrel AMs to the P2Y12 receptor, consistent with the previous observation of a negative pharmacodynamic interaction. Care may need to be taken to not overlap exposure to thienopyridine AMs and cangrelor in order to reduce the risk of thrombotic complications following PCI.

Determination of deoxyribonucleoside triphosphate concentrations in yeast cells by strong anion-exchange high-performance liquid chromatography coupled with ultraviolet detection.

DNA polymerase assays are commonly used for the detection of deoxyribonucleoside triphosphates (dNTPs) in biological samples. For better specificity and accuracy, high-performance liquid chromatography (HPLC) methods have been developed for the analysis of the four dNTPs in complex samples. Here we describe a simple method using isocratic strong anion-exchange (SAX) chromatographic separation coupled with ultraviolet detection (UV) for the analysis of the four dNTPs in budding yeast Saccharomyces cerevisiae. This method can be applied to other species of yeast or bacteria.

Molecular Basis of the Essential S Phase Function of the Rad53 Checkpoint Kinase

ABSTRACT The essential yeast kinases Mec1 and Rad53, or human ATR and Chk1, are crucial for checkpoint responses to exogenous genotoxic agents, but why they are also required for DNA replication in unperturbed cells remains poorly understood. Here we report that even in the absence of DNA-damaging agents, the rad53-4AQ mutant, lacking the N-terminal Mec1 phosphorylation site cluster, is synthetic lethal with a deletion of the RAD9 DNA damage checkpoint adaptor. This phenotype is caused by an inability of rad53-4AQ to activate the downstream kinase Dun1, which then leads to reduced basal deoxynucleoside triphosphate (dNTP) levels, spontaneous replication fork stalling, and constitutive activation of and dependence on S phase DNA damage checkpoints. Surprisingly, the kinase-deficient rad53-K227A mutant does not share these phenotypes but is rendered inviable by additional phosphosite mutations that prevent its binding to Dun1. The results demonstrate that ultralow Rad53 catalytic activity is sufficient for normal replication of undamaged chromosomes as long as it is targeted toward activation of the effector kinase Dun1. Our findings indicate that the essential S phase function of Rad53 is comprised by the combination of its role in regulating basal dNTP levels and its compensatory kinase function if dNTP levels are perturbed.