New parenteral anticoagulants in development

Coming soon to a pharmacy near you? FXI and FXII inhibitors to prevent or treat thromboembolism

Anticoagulants have been in use for nearly a century for the treatment and prevention of venous and arterial thromboembolic disorders. The most dreaded complication of anticoagulant treatment is the occurrence of bleeding, which may be serious and even life-threatening. All available anticoagulants, which target either multiple coagulation factors or individual components of the tissue factor (TF) factor VIIa or the common pathways, have the potential to affect hemostasis and thus to increase bleeding risk in treated patients. While direct oral anticoagulants introduced an improvement in care for eligible patients in terms of safety, efficacy, and convenience of treatment, there remain unmet clinical needs for patients requiring anticoagulant drugs. Anticoagulant therapy is sometimes avoided for fear of hemorrhagic complications, and other patients are undertreated due to comorbidities and the perception of increased bleeding risk. Evidence suggests that the contact pathway of coagulation has a limited role in initiating physiologic in vivo coagulation and that it contributes to thrombosis more than it does to hemostasis. Because inhibition of the contact pathway is less likely to promote bleeding, it is an attractive target for the development of anticoagulants with improved safety. Preclinical and early clinical data indicate that novel agents that selectively target factor XI or factor XII can reduce venous and arterial thrombosis without an increase in bleeding complications.

Total Synthesis and Biological Evaluation of Clavatadines C-E

We described herein the application of a convergent and protecting-group avoidant approach that led to the first total synthesis of the marine natural products clavatadine D (4) and E (5), and the second total synthesis of clavatadine C (3). In each case, a key amide-coupling afforded an immediate precursor of each natural product in a rapid manner from structurally similar western and eastern portions that derived from an ester of l-tyrosine and butane-1,4-diamine, respectively. A deprotection step free of detectable byproducts cleanly provided the remaining known members of the clavatadine family of natural products. Each total synthesis required five steps (longest linear sequence) with overall yields of 30-37%, 26-39%, and 28-50% for clavatadine C (3), D (4), and E (5), respectively. A screen of their potential anticancer activity against the NCI-60 cell line panel revealed cytotoxicity levels up to 38% across a broad spectrum of tumor types. Although clavatadine C (3) was relatively benign, clavatadine D (4) exhibited 20-38% growth inhibition against a wide array of cancer cell types including leukemia, non-small-cell lung, colon, ovarian, and breast. Clavatadine E (5) was active against two types of human brain tumors.

Porous dynamic covalent polymers as promising reversal agents for heparin anticoagulants

Heparins are natural and partially degraded polyelectrolytes that consist of sulfated polysaccharide backbones. However, as clinically used anticoagulants, heparins are associated with clinical bleeding risks and thus require rapid neutralization. Protamine sulfate is the only clinically approved antidote for unfractionated heparin (UFH), which not only may cause severe adverse reactions in patients, but also is only partially effective against low molecular weight heparins (LMWHs). We here present the facile synthesis of four porous multicationic dynamic covalent polymers (DCPs) from the condensation of tritopic aldehyde and acylhydrazine precursors. We show that, as new water-soluble polymeric antidotes, the new DCPs can effectively include both UFH and LMWHs and thus reverse their anticoagulating activity, which is confirmed by the activated partial thromboplastin time and thromboelastographic assays as well as mouse tail transection assay (bleeding model). The neutralization activities of two of the DCPs were found to be overall superior to that of protamine and have wider concentration windows and good biocompatibility. This pore-inclusion neutralization strategy paves the way for the development of water-soluble polymers as universal heparin binding agents.

Overview of the Therapeutic Potential of Aptamers Targeting Coagulation Factors

Aptamers are single-stranded DNA or RNA sequences that bind target molecules with high specificity and affinity. Aptamers exhibit several notable advantages over protein-based therapeutics. Aptamers are non-immunogenic, easier to synthesize and modify, and can bind targets with greater affinity. Due to these benefits, aptamers are considered a promising therapeutic candidate to treat various conditions, including hematological disorders and cancer. An active area of research involves developing aptamers to target blood coagulation factors. These aptamers have the potential to treat cardiovascular diseases, blood disorders, and cancers. Although no aptamers targeting blood coagulation factors have been approved for clinical use, several aptamers have been evaluated in clinical trials and many more have demonstrated encouraging preclinical results. This review summarized our knowledge of the aptamers targeting proteins involved in coagulation, anticoagulation, fibrinolysis, their extensive applications as therapeutics and diagnostics tools, and the challenges they face for advancing to clinical use.

Dimeric and Multimeric DNA Aptamers for Highly Effective Protein Recognition

Multivalent interactions frequently occur in biological systems and typically provide higher binding affinity and selectivity in target recognition than when only monovalent interactions are operative. Thus, taking inspiration by nature, bivalent or multivalent nucleic acid aptamers recognizing a specific biological target have been extensively studied in the last decades. Indeed, oligonucleotide-based aptamers are suitable building blocks for the development of highly efficient multivalent systems since they can be easily modified and assembled exploiting proper connecting linkers of different nature. Thus, substantial research efforts have been put in the construction of dimeric/multimeric versions of effective aptamers with various degrees of success in target binding affinity or therapeutic activity enhancement. The present review summarizes recent advances in the design and development of dimeric and multimeric DNA-based aptamers, including those forming G-quadruplex (G4) structures, recognizing different key proteins in relevant pathological processes. Most of the designed constructs have shown improved performance in terms of binding affinity or therapeutic activity as anti-inflammatory, antiviral, anticoagulant, and anticancer agents and their number is certainly bound to grow in the next future.

Novel aptamer to von Willebrand factor A1 domain (TAGX-0004) shows total inhibition of thrombus formation superior to ARC1779 and comparable to caplacizumab

von Willebrand factor (VWF) is a blood glycoprotein that plays an important role in platelet thrombus formation through interaction between its A1 domain and platelet glycoprotein Ib. ARC1779, an aptamer to the VWF A1 domain, was evaluated in a clinical trial for acquired thrombotic thrombocytopenic purpura (aTTP). Subsequently, caplacizumab, an anti-VWF A1 domain nanobody, was approved for aTTP in Europe and the United States. We recently developed a novel DNA aptamer, TAGX-0004, to the VWF A1 domain; it contains an artificial base and demonstrates high affinity for VWF. To compare the effects of these three agents on VWF A1, their ability to inhibit ristocetin- or botrocetin-induced platelet aggregation under static conditions was analyzed, and the inhibition of thrombus formation under high shear stress was investigated in a microchip flow chamber system. In both assays, TAGX-0004 showed stronger inhibition than ARC1779, and had comparable inhibitory effects to caplacizumab. The binding sites of TAGX-0004 and ARC1779 were analyzed with surface plasmon resonance performed using alanine scanning mutagenesis of the VWF A1 domain. An electrophoretic mobility shift assay showed that R1395 and R1399 in the A1 domain bound to both aptamers. R1287, K1362, and R1392 contributed to ARC1779 binding, and F1366 was essential for TAGX-0004 binding. Surface plasmon resonance analysis of the binding sites of caplacizumab identified five amino acids in the VWF A1 domain (K1362, R1392, R1395, R1399, and K1406). These results suggested that TAGX-0004 possessed better pharmacological properties than caplacizumab in vitro and might be similarly promising for aTTP treatment.

G-quadruplex-based aptamers targeting human thrombin: Discovery, chemical modifications and antithrombotic effects

First studies on thrombin-inhibiting DNA aptamers were reported in 1992, and since then a large number of anticoagulant aptamers has been discovered. TBA - also named HD1, a 15-mer G-quadruplex (G4)-forming oligonucleotide - is the best characterized thrombin binding aptamer, able to specifically recognize the protein exosite I, thus inhibiting the conversion of soluble fibrinogen into insoluble fibrin strands. Unmodified nucleic acid-based aptamers, in general, and TBA in particular, exhibit limited pharmacokinetic properties and are rapidly degraded in vivo by nucleases. In order to improve the biological performance of aptamers, a widely investigated strategy is the introduction of chemical modifications in their backbone at the level of the nucleobases, sugar moieties or phosphodiester linkages. Besides TBA, also other thrombin binding aptamers, able to adopt a well-defined G4 structure, e.g. mixed duplex/quadruplex sequences, as well as homo- and hetero-bivalent constructs, have been identified and optimized. Considering the growing need of new efficient anticoagulant agents associated with the strong therapeutic potential of these thrombin inhibitors, the research on thrombin binding aptamers is still a very hot and intriguing field. Herein, we comprehensively described the state-of-the-art knowledge on the DNA-based aptamers targeting thrombin, especially focusing on the optimized analogues obtained by chemically modifying the oligonucleotide backbone, and their biological performances in therapeutic applications.

Design, Synthesis and Characterization of Cyclic NU172 Analogues: A Biophysical and Biological Insight

NU172—a 26-mer oligonucleotide able to bind exosite I of human thrombin and inhibit its activity—was the first aptamer to reach Phase II clinical studies as an anticoagulant in heart disease treatments. With the aim of favoring its functional duplex-quadruplex conformation and thus improving its enzymatic stability, as well as its thrombin inhibitory activity, herein a focused set of cyclic NU172 analogues—obtained by connecting its 5′- and 3′-extremities with flexible linkers—was synthesized. Two different chemical approaches were exploited in the cyclization procedure, one based on the oxime ligation method and the other on Cu(I)-assisted azide-alkyne cycloaddition (CuAAC), affording NU172 analogues including circularizing linkers with different length and chemical nature. The resulting cyclic NU172 derivatives were characterized using several biophysical techniques (ultraviolet (UV) and circular dichroism (CD) spectroscopies, gel electrophoresis) and then investigated for their serum resistance and anticoagulant activity in vitro. All the cyclic NU172 analogues showed higher thermal stability and nuclease resistance compared to unmodified NU172. These favorable properties were, however, associated with reduced—even though still significant—anticoagulant activity, suggesting that the conformational constraints introduced upon cyclization were somehow detrimental for protein recognition. These results provide useful information for the design of improved analogues of NU172 and related duplex-quadruplex structures.

Targeted inhibition of thrombin attenuates murine neonatal necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is an inflammatory bowel necrosis of premature infants and an orphan disease with no specific treatment. Most patients with confirmed NEC develop moderate-severe thrombocytopenia requiring one or more platelet transfusions. Here we used our neonatal murine model of NEC-related thrombocytopenia to investigate mechanisms of platelet depletion associated with this disease [K. Namachivayam, K. MohanKumar, L. Garg, B. A. Torres, A. Maheshwari, Pediatr. Res. 81, 817-824 (2017)]. In this model, enteral administration of immunogen trinitrobenzene sulfonate (TNBS) in 10-d-old mouse pups produces an acute necrotizing ileocolitis resembling human NEC within 24 h, and these mice developed thrombocytopenia at 12 to 15 h. We hypothesized that platelet activation and depletion occur during intestinal injury following exposure to bacterial products translocated across the damaged mucosa. Surprisingly, platelet activation began in our model 3 h after TNBS administration, antedating mucosal injury or endotoxinemia. Platelet activation was triggered by thrombin, which, in turn, was activated by tissue factor released from intestinal macrophages. Compared to adults, neonatal platelets showed enhanced sensitivity to thrombin due to higher expression of several downstream signaling mediators and the deficiency of endogenous thrombin antagonists. The expression of tissue factor in intestinal macrophages was also unique to the neonate. Targeted inhibition of thrombin by a nanomedicine-based approach was protective without increasing interstitial hemorrhages in the inflamed bowel or other organs. In support of these data, we detected increased circulating tissue factor and thrombin-antithrombin complexes in patients with NEC. Our findings show that platelet activation is an important pathophysiological event and a potential therapeutic target in NEC.

Total Synthesis of Clavatadine B

The first total synthesis of clavatadine B (2), a natural product found to be a selective human blood coagulation factor XIa inhibitor, is described. A convergent approach that exemplifies the advantages of direct, early stage guanidinylation provided an immediate clavatadine B precursor, which was assembled in an efficient manner using known synthetic precursors of the structurally related natural product clavatadine A (1). Global deprotection cleanly provided clavatadine B in only four steps from a known derivative of homogentisic acid lactone (longest linear sequence, 75% overall yield).

Surfing the Blood Coagulation Cascade: Insight into the Vital Factor Xa

Factor Xa (FXa) plays a key role in haemostasis, it is a central part of the blood coagulation cascade which catalyzes the production of thrombin and leads to clot formation and wound closure. Therefore, FXa is an attractive target for the development of new anticoagulant agents. In this review, we will first describe the molecular features of this fundamental protein in order to understand its mechanism of action, an essential background for the design of novel inhibitors by means of synthetic organic chemistry or using peptides obtained from recombinant methodologies. Then, we will review the current state of the synthesis of novel direct FXa inhibitors along with their mechanisms of action. Finally, approved reversal agents that aid in maintaining blood haemostasis by using these commercial drugs will also be discussed.

A Mini-Review: Clinical Development and Potential of Aptamers for Thrombotic Events Treatment and Monitoring

The unique opportunity for aptamer uses in thrombotic events has sparked a considerable amount of research in the area. The short half-lives of unmodified aptamers in vivo remain one of the major challenges in therapeutic aptamers. Much of the incremental successful therapeutic aptamer stories were due to modifications in the aptamer bases. This mini-review briefly summarizes the successes and challenges in the clinical development of aptamers for thrombotic events, and highlights some of the most recent developments in using aptamers for anticoagulation monitoring.

Targeting hormone refractory prostate cancer by in vivo selected DNA libraries in an orthotopic xenograft mouse model

The targeting of specific tissue is a major challenge for the effective use of therapeutics and agents mediating this targeting are strongly demanded. We report here on an in vivo selection technology that enables the de novo identification of pegylated DNA aptamers pursuing tissue sites harbouring a hormone refractory prostate tumour. To this end, two libraries, one of which bearing an 11 kDa polyethylene glycol (PEG) modification, were used in an orthotopic xenograft prostate tumour mouse model for the selection process. Next-generation sequencing revealed an in vivo enriched pegylated but not a naïve DNA aptamer recognising prostate cancer tissue implanted either subcutaneous or orthotopically in mice. This aptamer represents a valuable and cost-effective tool for the development of targeted therapies for prostate cancer. The described selection strategy and its analysis is not limited to prostate cancer but will be adaptable to various tissues, tumours, and metastases. This opens the path towards DNA aptamers being experimentally and clinically engaged as molecules for developing targeted therapy strategies.

Advances in managing and preventing thromboembolic disease in cancer patients

PURPOSE OF REVIEW

To update on new data for low-molecular weight heparins (LMWHs) and the direct oral anticoagulants (DOACs) for the treatment and prevention of cancer-associated thrombosis (CAT), to discuss progress with the risk-adaptive management scores (RAMS) and update on increased dose primary thromboprophylaxis (IDPTP).

RECENT FINDINGS

In a pooled meta-analysis of 1132 cancer patients who received DOACs vs. vitamin K analogues (VKAs), recurrence of venous thromboembolism (VTE) was reduced from 6.0% on VKA schedules to 3.9% on DOACs. In a randomized trial of warfarin vs. once daily sc. tinzaparin (175 IU/kg), cumulative 6-month VTE incidence reduced from 10.5 to 7.2% [hazard ratio, 0.65 (95% confidence interval, 0.41-1.03); P = 0.07]. Despite early suggestions that DOACs may have a role in CAT, 3-6 months of LMWH remain the standard for initial treatment of CAT. A prospective comparison of RAMS found the Vienna CATS or the PROTECHT scores superior to the Khorana score but concluded that RAMS did not perform well enough to be used in the clinic. An efficacy scale of LMWHs in pancreatic cancer facilitates IDPTP. Practical implementation of IDPTP was needed to control the 40% VTE incidence of the HALO-109-202 study in metastatic pancreatic cancer.

SUMMARY

DOACs have some encouraging data, but LMWHs remain the standard for CAT treatment. RAMS generated to predict VTE occurrence or recurrence are still of unproven significance and IDPTP for advanced pancreatic cancer has tools and guidance for implementation.

Discovery of FIXa inhibitors by combination of pharmacophore modeling, molecular docking, and 3D-QSAR modeling

Human Coagulation Factor IXa (FIXa), specifically inhibited at the initiation stage of the blood coagulation cascade, is an excellent target for developing selective and safe anticoagulants. To explore this inhibitory mechanism, 86 FIXa inhibitors were selected to generate pharmacophore models and subsequently SAR models. Both best pharmacophore model and ROC curve were built through the Receptor-Ligand Pharmacophore Generation module. CoMFA model based on molecular docking and PLS factor analysis methods were developed. Model propagations values are q² = 0.709, r² = 0.949, and r²_pred = 0.905. The satisfactory q² value of 0.609, r² value of 0.962, and r²_pred value of 0.819 for CoMSIA indicated that the CoMFA and CoMSIA models are both available to predict the inhibitory activity on FIXa. On the basis of pharmacophore modeling, molecular docking, and 3D-QSAR modeling screening, six molecules are screened as potential FIXa inhibitors.

Heparin: Past, Present, and Future

Heparin, the most widely used anticoagulant drug in the world today, remains an animal-derived product with the attendant risks of adulteration and contamination. A contamination crisis in 2007-2008 increased the impetus to provide non-animal-derived sources of heparin, produced under cGMP conditions. In addition, recent studies suggest that heparin may have significant antineoplastic activity, separate and distinct from its anticoagulant activity, while other studies indicate a role for heparin in treating inflammation, infertility, and infectious disease. A variety of strategies have been proposed to produce a bioengineered heparin. In this review, we discuss several of these strategies including microbial production, mammalian cell production, and chemoenzymatic modification. We also propose strategies for creating "designer" heparins and heparan-sulfates with various biochemical and physiological properties.

New Direct Oral Anticoagulants (DOAC) and Their Use Today

The ideal anticoagulant is oral, has a wide therapeutic range, predictable pharmacokinetics and pharmacodynamics, a rapid onset of action, an available antidote, minimal side effects and minimal interactions with other drugs or food. With the development of the novel direct oral anticoagulants (DOAC), we now have an alternative to the traditional vitamin K antagonists (VKA) for the prevention and treatment of thrombosis. DOACs have limited monitoring requirements and very predictable pharmacokinetic profiles. They were shown to be non-inferior or superior to VKA in the prophylaxis or treatment of thromboembolic events. Particularly in terms of safety they were associated with less major bleeding, including intracranial bleeding, thus providing a superior benefit for the prevention of stroke in patients with atrial fibrillation. Despite these advantages, there are remaining limitations with DOACs: their dependence on renal and hepatic function for clearance and the lack of an approved reversal agent, whereas such antidotes are successively being made available. DOACs do not need regular monitoring to assess the treatment effect but, on the other hand, they interact with other drugs and interfere with functional coagulation assays. From a practical point of view, the properties of oral administration, simple dosing without monitoring, a short half-life allowing for the possibility of uncomplicated switching or bridging, and proven safety overwhelm the disadvantages, making them an attractive option for short- or long-term anticoagulation.

Factor VII promotes hepatocellular carcinoma progression through ERK-TSC signaling

We previously demonstrated PAR2 starts upstreamed with tissue factor (TF) and factor VII (FVII), inhibited autophagy via mTOR signaling in HCC. However, the mechanism underlying for merging functions of PAR2 with the coagulation system in HCC progression remained unclear. The present study aimed to investigate the role of TF, FVII and PAR2 in tumor progression of HCC. The expressions of TF, FVII and PAR2 from HCC specimens were evaluated by immunohistochemical stains and western blotting. We found that the expression of FVII, but not TF and PAR2, directly related to the vascular invasion and the clinical staging. Importantly, a lower level of FVII expression was significantly associated with the longer disease-free survival. The addition of FVII but not TF induced the expression of PAR2 and phosphorylation of ERK1/2, whereas knockdown of FVII decreased PAR2 expression and ERK1/2 phosphorylation in HCC cell lines. Furthermore, levels of phosphor-TSC2 (Ser664) were increased after treatment with FVII and PAR2 agonist whereas these were significantly abolished in the presence of a potent and specific MEK/ERK inhibitor U0126. Moreover, mTOR knockdown highly reduced Hep3B migration, which could be reverted by FVII but not TF and PAR2. These results indicated that FVII/PAR2 signaling through MEK/ERK and TSC2 axis for mTOR activation has potent effects on the migration of HCC cells. In addition, FVII/PAR2 signaling elicits an mTOR-independent signaling, which promotes hepatoma cell migration in consistent with the clinical observations. Our study indicates that levels of FVII, but not TF, are associated with tumor migration and invasiveness in HCC, and provides clues that evaluation of FVII expression in HCC may be useful as a prognostic indicator in patients with HCC and may form an alternative target for further therapy.

The modulation of coagulation by aptamers: an up-to-date review

Coagulation and anticoagulation system is kept in balance by the orchestrated action of a variety of biological factors, and the disruption of this balance leads to the risk of hemorrhage or thrombosis. Oligonucleotide aptamers are single-stranded DNA (ssDNA) or RNA ligands that are synthesized in vitro and bind to target molecules through dimensional structure with high specificity and affinity, and thus represent attractive candidates for the development of agents to maintain the balance of coagulation and anticoagulation. In this review, we summarize recent progress in aptamer-based application in the modulation of coagulation. The aptamers with specific chemical and biological characteristics have great potential to be explored as agents for the treatment of blood coagulation abnormalities.

Design and prediction of new anticoagulants as a selective Factor IXa inhibitor via three-dimensional quantitative structure-property relationships of amidinobenzothiophene derivatives

Factor IXa (FIXa), a blood coagulation factor, is specifically inhibited at the initiation stage of the coagulation cascade, promising an excellent approach for developing selective and safe anticoagulants. Eighty-four amidinobenzothiophene antithrombotic derivatives targeting FIXa were selected to establish three-dimensional quantitative structure–activity relationship (3D-QSAR) and three-dimensional quantitative structure–selectivity relationship (3D-QSSR) models using comparative molecular field analysis and comparative similarity indices analysis methods. Internal and external cross-validation techniques were investigated as well as region focusing and bootstrapping. The satisfactory q² values of 0.753 and 0.770, and r² values of 0.940 and 0.965 for 3D-QSAR and 3D-QSSR, respectively, indicated that the models are available to predict both the inhibitory activity and selectivity on FIXa against Factor Xa, the activated status of Factor X. This work revealed that the steric, hydrophobic, and H-bond factors should appropriately be taken into account in future rational design, especially the modifications at the 2′-position of the benzene and the 6-position of the benzothiophene in the R group, providing helpful clues to design more active and selective FIXa inhibitors for the treatment of thrombosis. On the basis of the three-dimensional quantitative structure–property relationships, 16 new potent molecules have been designed and are predicted to be more active and selective than Compound 33, which has the best activity as reported in the literature.

New orally active anticoagulant agents for the prevention and treatment of venous thromboembolism in cancer patients

Patients with cancer have a 6-7-fold higher risk of venous thromboembolism (VTE) as compared with non-cancer patients. Effective and safe anticoagulation for the prevention and treatment of VTE is the cornerstone of the management of patients with cancer, aiming to decrease morbidity and mortality and to improve quality of life. Unfractionated heparin, low molecular weight heparins, fondaparinux and vitamin K antagonists (VKAs) are used in the prevention and treatment of VTE in cancer patients. Heparins and fondaparinux are administered subcutaneously. VKAs are orally active, but they have a narrow therapeutic window, numerous food and drug interactions, and treatment requires regular laboratory monitoring and dose adjustment. These limitations among others have important negative impact on the quality of life of patients and decrease adherence to the treatment. New orally active anticoagulant (NOAC) agents are specific inhibitors of activated factor Xa (FXa) (rivaroxaban and apixaban) or thrombin (dabigatran). It is expected that NOACs will improve antithrombotic treatment. Cancer patients are a particular group that could benefit from treatment with NOACs. However, NOACs present some significant interactions with drugs frequently used in cancer patients, which might influence their pharmacokinetics, compromising their efficacy and safety. In the present review, we analyzed the available data from the subgroups of patients with active cancer who were included in Phase III clinical trials that assessed the efficacy and safety of NOACs in the prevention and treatment of VTE. The data from the Phase III trials in prophylaxis of VTE by rivaroxaban or apixaban highlight that these two agents, although belonging to the same pharmacological group (direct inhibitors of factor Xa), have substantially different profiles of efficacy and safety, especially in hospitalized acutely ill medical patients with active cancer. A limited number of patients with VTE and active cancer were included in the Phase III trials (EINSTEIN, AMPLIFY, and RE-COVER) which evaluated the efficacy and safety of NOACs in the acute phase and secondary prevention of VTE. Although, from a conceptual point of view, NOACs could be an attractive alternative for the treatment of VTE in cancer patients, the available data do not support this option. In addition, due to the elimination of the NOACs by the liver and renal pathway as well as because of their pharmacological interactions with drugs which are frequently used in cancer patients, an eventual use of these drugs in cancer patients should be extremely cautious and be restricted only to patients presenting with contraindications for low molecular weight heparins, fondaparinux, or VKAs. The analysis of the available data presented in this review reinforces the request for the design of new Phase III clinical trials for the assessment of the efficacy and safety of NOACs in specific populations of patients with cancer.

Development of aptamer oligonucleotides as anticoagulants and antithrombotics for cardiovascular diseases: current status

Aptamers are short DNA/RNA oligonucleotides selected by a process known as Systematic Evolution of Ligands by Exponential Enrichment (SELEX) based on affinity for target molecules. Since aptamers have several advantages over monoclonal antibodies, such as high specificity and affinity, flexible modification and stability, and lack of toxicity and immunogenicity, they are promising novel diagnostic and therapeutic agents. In this review, we will describe the development of aptamers against thrombin, von Willebrand factor (vWF), factor IX, and factor XII as potential anticoagulants or antithrombotics for cardiovascular diseases, especially those that have entered clinical trials.

Evolution of heparin anticoagulants to ultra-low-molecular-weight heparins: a review of pharmacologic and clinical differences and applications in patients with cancer

The burden of venous thromboembolism (VTE) is high in patients with cancer, particularly those with metastatic disease and those receiving chemotherapy. The use of heparin and heparin derivatives should be considered for primary prevention of VTE in hospitalized patients with cancer and in patients undergoing cancer surgery. Preliminary evidence also suggests that heparins may have direct anticancer benefits owing to effects on tumor growth, angiogenesis, and metastasis. Despite the potential benefits of heparin-derived anticoagulants, many at-risk patients do not receive adequate thromboprophylaxis. The evolution of unfractionated heparin to low-molecular-weight and ultra-low-molecular-weight heparins has provided practitioners with alternatives for VTE prevention in cancer, although these alternatives present challenges related to clinically relevant pharmacologic differences between agents. In this review, we present results from our review of the medical literature focusing on the use of the heparin-derived anticoagulants in prospective interventional studies of primary thromboprophylaxis in patients with cancer in surgical, hospitalized, and ambulatory settings.

Potential role of new anticoagulants for prevention and treatment of venous thromboembolism in cancer patients

Venous thromboembolism (VTE), encompassing deep vein thrombosis and pulmonary embolism, represents a major cause of morbidity and mortality in patients with cancer. Low molecular weight heparins are the preferred option for anticoagulation in cancer patients according to current clinical practice guidelines. Fondaparinux may also have a place in prevention of VTE in hospitalized cancer patients with additional risk factors and for initial treatment of VTE. Although low molecular weight heparins and fondaparinux are effective and safe, they require daily subcutaneous administration, which may be problematic for many patients, particularly if long-term treatment is needed. Studying anticoagulant therapy in oncology patients is challenging because this patient group has an increased risk of VTE and bleeding during anticoagulant therapy compared with the population without cancer. Risk factors for increased VTE and bleeding risk in these patients include concomitant treatments (surgery, chemotherapy, placement of central venous catheters, radiotherapy, hormonal therapy, angiogenesis inhibitors, antiplatelet drugs), supportive therapies (ie, steroids, blood transfusion, white blood cell growth factors, and erythropoiesis-stimulating agents), and tumor-related factors (local vessel damage and invasion, abnormalities in platelet function, and number). New anticoagulants in development for prophylaxis and treatment of VTE include parenteral compounds for once-daily administration (ie, semuloparin) or once-weekly dosing (ie, idraparinux and idrabiotaparinux), as well as orally active compounds (ie, dabigatran, rivaroxaban, apixaban, edoxaban, betrixaban). In the present review, we discuss the pharmacology of the new anticoagulants, the results of clinical trials testing these new compounds in VTE, with special emphasis on studies that included cancer patients, and their potential advantages and drawbacks compared with existing therapies.

Large inter-individual variation of the pharmacodynamic effect of anticoagulant drugs on thrombin generation

Anticoagulation by a standard dosage of an inhibitor of thrombin generation presupposes predictable pharmacokinetics and pharmacodynamics of the anticoagulant. We determined the inter-individual variation of the effect on thrombin generation of a fixed concentration of direct and antithrombin-mediated inhibitors of thrombin and factor Xa. Thrombin generation was determined by calibrated automated thrombinography in platelet-poor plasma from 44 apparently healthy subjects which was spiked with fixed concentrations of otamixaban, melagatran, unfractionated heparin, dermatan sulfate and pentasaccharide. The variability of the inhibitory effect of the different anticoagulants within the population was determined using the coefficient of variation, i.e. the standard deviation expressed as a percentage of the mean. The inter-individual coefficients of variation of the endogenous thrombin potential and peak height before inhibition were 18% and 16%, respectively and became 20%-24% and 24%-43% after inhibition. The average inhibition of endogenous thrombin potential and peak height (ETP, peak) brought about by the anticoagulants was respectively: otamixaban (27%, 83%), melagatran (56%, 63%), unfractionated heparin (43%, 58%), dermatan sulfate (68%, 57%) and pentasaccharide (25%, 67%). This study demonstrates that the addition of a fixed concentration of any type of anticoagulant tested causes an inhibition that is highly variable from one individual to another. In this respect there is no difference between direct inhibitors of thrombin and factor Xa and heparin(-like) inhibitors acting on the same factors.

Chemoenzymatic synthesis of the next generation of ultralow MW heparin therapeutics

Heparin, a sulfated glycosaminoglycan, is a widely used injectable anticoagulant. This polysaccharide is a natural product extracted from porcine intestinal tissue. A specific pentasaccharide sequence is responsible for heparin's high affinity towards anti-thrombin III, which undergoes a conformational change and, as a result, inhibits the blood coagulation Factor Xa, a critical serine protease at the convergence on the intrinsic and extrinsic activation pathway of the coagulation cascade. Due to its structural complexity and heterogeneity, the synthesis of the anti-thrombin III-binding sequence of heparin has been limited to a few approaches. The heparin contamination crisis in 2007 has motivated the development of alternative methods for the efficient preparation of safe heparin products. In this article, we discuss the current methods and recent advances in heparin and low MW heparin syntheses and the recent successful chemoenzymatic preparation of ultralow MW heparins.

Aptamer-based modulation of blood coagulation

Nucleic acid based aptamers are single-stranded oligonucleotide ligands isolated from random libraries by an in-vitro selection procedure. Through the formation of unique three-dimensional structures, aptamers are able to selectively interact with a variety of target molecules and are therefore also promising candidates for the development of anticoagulant drugs. While thrombin represents the most prominent enzymatic target in this field, also aptamers directed against other coagulation proteins and proteases have been identified with some currently being tested in clinical trials. In this review, we summarize recent developments in the design and evaluation of aptamers for anticoagulant therapy and research.

Inhibitors of propagation of coagulation: factors V and X

Cardiovascular diseases are still the most important cause of morbidity and mortality in western countries and antithrombotic treatment is nowadays widely used. Drugs able to reduce coagulation activation are the treatment of choice for a number of arterial and/or venous thromboembolic conditions. Some of the drugs currently used for this purpose, such as heparins (UFH or LMWH) and VKA, have limitations consisting of a narrow therapeutic window and an unpredictable response with the need of laboratory monitoring in order to assess their efficacy and safety. These drawbacks have stimulated an active research aimed to develop new drugs able to act on single factors involved in the coagulation network, with predictable response. Intense experimental and clinical work on new drugs has focused on synthetic agents, which could preferably be administered orally and at fixed doses. The most advanced clinical development with new anticoagulants has been achieved for those inhibiting FXa and some of them, like fondaparinux, are already currently used in clinical practice. Other agents, such as rivaroxaban, apixaban, otamixaban and edoxaban are under development and have already been studied or are currently under investigation in large scale phase III clinical trials for prevention and treatment of venous thromboembolism, atrial fibrillation and acute coronary syndromes. Some of them have proved to be more effective than conventional therapy. Data on some agents inhibiting FVa are still preliminary and some of these drugs have so far been considered only in patients with disseminated intravascular coagulation secondary to sepsis.