Effects of novel synthetic serine protease inhibitors on postoperative blood loss, coagulation parameters, and vascular relaxation after cardiac surgery

Fibrinolysis Inhibitors: Potential Drugs for the Treatment and Prevention of Bleeding

Hyperfibrinolytic situations can lead to life-threatening bleeding, especially during cardiac surgery. The approved antifibrinolytic agents such as tranexamic acid, ε-aminocaproic acid, 4-aminomethylbenzoic acid, and aprotinin were developed in the 1960s without the structural insight of their respective targets. Crystal structures of the main antifibrinolytic targets, the lysine binding sites on plasminogen's kringle domains, and plasmin's serine protease domain greatly contributed to the structure-based drug design of novel inhibitor classes. Two series of ligands targeting the lysine binding sites have been recently described, which are more potent than the most-widely used antifibrinolytic agent, tranexamic acid. Furthermore, four types of promising active site inhibitors of plasmin have been developed: tranexamic acid conjugates targeting the S1 pocket and primed sites, substrate-analogue linear homopiperidylalanine-containing 4-amidinobenzylamide derivatives, macrocyclic inhibitors addressing nonprimed binding regions, and bicyclic 14-mer SFTI-1 analogues blocking both, primed and nonprimed binding sites of plasmin. Furthermore, several allosteric plasmin inhibitors based on heparin mimetics have been developed.

Design of a DNA-Programmed Plasminogen Activator

Although the functional specificity and catalytic versatility of enzymes have been exploited in numerous settings, controlling the spatial and temporal activity of enzymes remains challenging. Here we describe an approach for programming the function of streptokinase (SK), a protein that is clinically used as a blood "clot buster" therapeutic. We show that the fibrinolytic activity resulting from the binding of SK to the plasma proenzyme plasminogen (Pg) can be effectively regulated (turned "OFF" and "ON") by installing an intrasteric regulatory feature using a DNA-linked protease inhibitor modification. We describe the design rationale, synthetic approach, and functional characterization of two generations of intrasterically regulated SK-Pg constructs and demonstrate dose-dependent and sequence-specific temporal control in fibrinolytic activity in response to short predesignated DNA inputs. The studies described establish the feasibility of a new enzyme-programming approach and serves as a step toward advancing a new generation of programmable enzyme therapeutics.

Recent advances in the discovery and development of factor XI/XIa inhibitors

Factor XIa (FXIa) is a serine protease homodimer that belongs to the intrinsic coagulation pathway. FXIa primarily catalyzes factor IX activation to factor IXa, which subsequently activates factor X to factor Xa in the common coagulation pathway. Growing evidence suggests that FXIa plays an important role in thrombosis with a relatively limited contribution to hemostasis. Therefore, inhibitors targeting factor XI (FXI)/FXIa system have emerged as a paradigm-shifting strategy so as to develop a new generation of anticoagulants to effectively prevent and/or treat thromboembolic diseases without the life-threatening risk of internal bleeding. Several inhibitors of FXI/FXIa proteins have been discovered or designed over the last decade including polypeptides, active site peptidomimetic inhibitors, allosteric inhibitors, antibodies, and aptamers. Antisense oligonucleotides (ASOs), which ultimately reduce the hepatic biosynthesis of FXI, have also been introduced. A phase II study, which included patients undergoing elective primary unilateral total knee arthroplasty, revealed that a specific FXI ASO effectively protects patients against venous thrombosis with a relatively limited risk of bleeding. Initial findings have also demonstrated the potential of FXI/FXIa inhibitors in sepsis, listeriosis, and arterial hypertension. This review highlights various chemical, biochemical, and pharmacological aspects of FXI/FXIa inhibitors with the goal of advancing their development toward clinical use.

Plasma Kallikrein Inhibitors in Cardiovascular Disease: An Innovative Therapeutic Approach

Plasma prekallikrein is the liver-derived precursor of the trypsin-like serine protease plasma kallikrein, and circulates in plasma bound to high molecular weight kininogen. Plasma prekallikrein is activated to plasma kallikrein by activated factor XII or prolylcarboxypeptidase. Plasma kallikrein regulates the activity of multiple proteolytic cascades in the cardiovascular system such as the intrinsic pathway of coagulation, the kallikrein-kinin system, the fibrinolytic system, the renin-angiotensin system, and the complement pathways. As such, plasma kallikrein plays a central role in the pathogenesis of thrombosis, inflammation, and blood pressure regulation. Under physiological conditions, plasma kallikrein serves as a cardioprotective enzyme. However, its increased plasma concentration or hyperactivity perpetuates cardiovascular disease (CVD). In this article, we review the biochemistry and cell biology of plasma kallikrein and summarize data from preclinical and clinical studies that have established important functions of this serine protease in CVD states. Finally, we propose plasma kallikrein inhibitors as a novel class of drugs with potential therapeutic applications in the treatment of CVDs.

Nafamostat mesilate promotes endothelium-dependent vasorelaxation via the Akt-eNOS dependent pathway

Nafamostat mesilate (NM), a synthetic serine protease inhibitor, has anticoagulant and anti-inflammatory properties. The intracellular mediator and external anti-inflammatory external signal in the vascular wall have been reported to protect endothelial cells, in part due to nitric oxide (NO) production. This study was designed to examine whether NM exhibit endothelium dependent vascular relaxation through Akt/endothelial nitric oxide synthase (eNOS) activation and generation of NO. NM enhanced Akt/eNOS phosphorylation and NO production in a dose- and time-dependent manner in human umbilical vein endothelial cells (HUVECs) and aorta tissues obtained from rats treated with various concentrations of NM. NM concomitantly decreased arginase activity, which could increase the available arginine substrate for NO production. Moreover, we investigated whether NM increased NO bioavailability and decreased aortic relaxation response to an eNOS inhibitor in the aorta. These results suggest that NM increases NO generation via the Akt/eNOS signaling pathway, leading to endothelium-dependent vascular relaxation. Therefore, the vasorelaxing action of NM may contribute to the regulation of cardiovascular function.

Recent advances on plasmin inhibitors for the treatment of fibrinolysis-related disorders

Growing evidence suggests that plasmin is involved in a number of physiological processes in addition to its key role in fibrin cleavage. Plasmin inhibition is critical in preventing adverse consequences arising from plasmin overactivity, e.g., blood loss that may follow cardiac surgery. Aprotinin was widely used as an antifibrinolytic drug before its discontinuation in 2008. Tranexamic acid and ε-aminocaproic acid, two small molecule plasmin inhibitors, are currently used in the clinic. Several molecules have been designed utilizing covalent, but reversible, chemistry relying on reactive cyclohexanones, nitrile warheads, and reactive aldehyde peptidomimetics. Other major classes of plasmin inhibitors include the cyclic peptidomimetics and polypeptides of the Kunitz and Kazal-type. Allosteric inhibitors of plasmin have also been designed including small molecule lysine analogs that bind to plasmin's kringle domain(s) and sulfated glycosaminoglycan mimetics that bind to plasmin's catalytic domain. Plasmin inhibitors have also been explored for resolving other disease states including cell metastasis, cell proliferation, angiogenesis, and embryo implantation. This review highlights functional and structural aspects of plasmin inhibitors with the goal of advancing their design.

The effects of MDCO-2010, a serine protease inhibitor, on activated clotting time in blood obtained from volunteers and cardiac surgical patients

BACKGROUND

The activated clotting time (ACT) is widely used for monitoring heparin anticoagulation during cardiac surgery. Celite-based ACT values are prolonged when aprotinin is administered. MDCO-2010, a novel serine protease inhibitor, is currently being evaluated as a possible alternative to aprotinin. Therefore, we evaluated the in vitro effects of this novel agent on ACT values using 3 different point-of-care instruments with kaolin or celite as an activator.

METHODS

The study was performed in 2 parts. In the first part, blood samples were obtained from 15 healthy volunteers. Samples were pipetted into small Eppendorf tubes and 2 concentrations of the MDCO-2010 (100 and 500 nM, final concentration) alone or with heparin (1.2 or 2.4 U/mL) were added. ACTs were measured using Helena (celite), Hemochron (kaolin), and Medtronic (kaolin) devices. In the second part of the study, blood samples were obtained intraoperatively, at 5 time points, from 15 patients undergoing cardiopulmonary bypass. MDCO-2010 at a final concentration of 100 or 500 nM was added and ACT testing was performed as before. Additional coagulation tests included prothrombin time, activated partial thromboplastin time, fibrinogen, antithrombin, prothrombin, and anti-Xa levels.

RESULTS

Addition of MDCO-2010 concentration-dependently prolonged ACTs in volunteers' and patients' blood samples regardless of the ACT activator or device used. In volunteer samples (no heparin) and in patient samples (baseline and intensive care unit) percent changes in ACTs due to MDCO-2010 were on average 3.1 ± 1.8 times higher (95% confidence interval 2.6-3.6; P < 0.001) for the celite-based Helena device compared with either Hemochron or Medtronic devices.

CONCLUSION

MDCO-2010 causes less ACT prolongation with kaolin than with celite activation.

Natural and engineered plasmin inhibitors: applications and design strategies

The serine protease plasmin is ubiquitously expressed throughout the human body in the form of the zymogen plasminogen. Conversion to active plasmin occurs through enzymatic cleavage by plasminogen activators. The plasminogen activator/plasmin system has a well-established function in the removal of intravascular fibrin deposition through fibrinolysis and the inhibition of plasmin activity; this has found widespread clinical use in reducing perioperative bleeding. Increasing evidence also suggests diverse, although currently less defined, roles for plasmin in a number of physiological and pathological processes relating to extracellular matrix degradation, cell migration and tissue remodelling. In particular, dysregulation of plasmin has been linked to cancer invasion/metastasis and various chronic inflammatory conditions; this has prompted efforts to develop inhibitors of this protease. Although a number of plasmin inhibitors exist, they commonly suffer from poor potency and/or specificity of inhibition that either results in reduced efficacy or prevents clinical use. Consequently, there is a need for further development of high-affinity plasmin inhibitors that maintain selectivity over other serine proteases. This review summarises clearly defined and potential applications for plasmin inhibition. The properties of naturally occurring and engineered plasmin inhibitors are discussed in the context of current knowledge regarding plasmin structure, specificity and function. This includes design strategies to obtain the potency and specificity of inhibition in addition to controlled temporal and spatial distribution tailored for the intended use.

Identification and characterisation of Kunitz-type plasma kallikrein inhibitors unique to Oxyuranus sp. snake venoms

As part of a wider study on Australian snake venom components, we have identified and characterised Kunitz-type protease inhibitors from the venoms of Oxyuranus scutellatus and Oxyuranus microlepidotus (Australian taipans) with plasma kallikrein inhibitory activity. Each inhibitor had a mass of 7 kDa and was purified from the venom as part of a protein complex. Mass spectrometry and N-terminal sequencing was employed to obtain amino acid sequence information for each inhibitor and a recombinant form of the O. scutellatus inhibitor, termed TSPI, was subsequently expressed and purified. TSPI was investigated for inhibition against a panel of 12 enzymes involved in haemostasis and estimates of the K(i) value determined for each enzyme. TSPI was found to be a broad spectrum inhibitor with most potent inhibitory activity observed against plasma kallikrein that corresponded to a K(i) of 0.057 ± 0.019 nM. TSPI also inhibited fibrinolysis in whole blood and prolonged the intrinsic clotting time. These inhibitors are also unique in that they appear to be found only in Oxyuranus sp. venoms.