Leading Article: Cardiovascular & Renal: Recombinant hirudin: A perspective

Predicting the metabolic characteristics of neorudin, a novel anticoagulant fusion protein, in patients with deep vein thrombosis

INTRODUCTION

Recombinant neorudin (EPR-hirudin, EH) is an inactive prodrug that is converted to its active metabolite, hirudin variant 2-Lys47 (HV2), at the thrombus site. We aimed to investigate the mechanism underlying site-selective bioconversion of EH to HV2 at the thrombus target site and metabolic transformation of EH in patients with deep vein thrombosis (DVT).

MATERIALS AND METHODS

Metabolites in healthy volunteer plasma and urine after intravenous administration of EH were determined to elucidate how EH was metabolised after releasing HV2 at the target site in patients with DVT. After intravenous administration of EH in rats with venous thrombosis, the concentrations of EH in the blood and thrombus and the antithrombotic activity of EH were measured to predict whether EH could release HV2 at the thrombus site to exert anticoagulant effect in patients with DVT.

RESULTS

In healthy volunteers, EH and HV2 were predominantly excreted in the urine. Nine EH metabolites and ten HV2 metabolites truncated at the C-terminal were identified as N-terminal fragments, and these had the same cleavage sites. In rats with venous thrombosis, the area under the curve ratio of HV2 between the thrombus and blood was 29.5. The weight of wet thrombus was decreased with the production of HV2 by the cleavage of EH. The prothrombin time (PT) and prothrombin time (TT) changed proportionally to the concentration of EH and HV2 in the blood.

CONCLUSION

EH selectively accumulates and releases HV2 in the thrombus to exert antithrombotic effects, thus lowering the bleeding risk. Moreover, after conversion, EH may follow the same metabolic profile as that of HV2 in patients with DVT.

Evaluating prodrug characteristics of a novel anticoagulant fusion protein neorudin, a prodrug targeting release of hirudin variant 2-Lys47 at the thrombosis site, by means of in vitro pharmacokinetics

Recombinant neorudin (EPR-hirudin, EH), a low-bleeding anticoagulant fusion protein, is an inactive prodrug designed to be converted to the active metabolite, hirudin variant 2-Lys47 (HV2), locally at the thrombus site by FXa and/or FXIa, following activation of the coagulation system. Our aim was to evaluate the prodrug characteristics of EH by comparing the biotransformation of EH and HV2 in biological matrices, including rat blood, liver, and kidney homogenates, demonstrating the cleavage of EH to HV2 by FXa and FXIa, and comparing the conversion of EH to HV2 between fresh whole blood and whole-blood clot homogenate, using ultra-performance liquid chromatography-mass spectrometry (UPLC-MS/MS). Both EH and HV2 were stable in blood and unstable in the liver and kidney homogenates. Eight EH metabolites and eight HV2 metabolites identified as N-terminal fragments were found in the liver and kidney. C-terminal proteolysis is therefore the major metabolic pathway, with serine/cysteine carboxypeptidases and metallocarboxypeptidases being responsible for the degradation of EH and HV2 in the liver and kidney, respectively. EH was cleaved to release HV2 by FXIa. Higher levels of HV2 were produced from EH in the whole-blood clot homogenate, in which the coagulation system was activated compared with those in fresh whole blood. In conclusion, the metabolism of EH and HV2 shares the same cleavage pattern, and EH is transformed into HV2 when the coagulation system is activated, where FXIa is a specific enzyme. Our in vitro study revealed the anticipated prodrug characteristics of EH newly designed as an inactive prodrug of hirudin.

Synthetic and recombinant antithrombin drugs

As the final enzyme in the activation of the coagulation system, the serine protease, thrombin, is believed to be an important target for the development of new anticoagulant/antithrombotic drugs. Direct thrombin inhibitors are either derived from natural sources, such as hirudin or are chemically synthesised, such as argatroban. The coupling of hirudin or parts of it with other entities leads to novel agents with different pharmacokinetic and pharmacodynamic characteristics, such as polyethylene glycol (PEG)-hirudin or the hirulogs. Due to the reversible or irreversible inactivation of the enzyme, thrombin inhibitors exert strong anticoagulant effects that can be measured in global clotting assays. Furthermore, these compounds inhibit thrombin-induced platelet reactions and influence other cellular, receptor-mediated actions of thrombin, e.g., on vascular cells. Directly acting thrombin inhibitors prevent blood clotting and are also capable of inhibiting clot-associated thrombin; however, they do not effectively block the further generation of the enzyme. Comprehensive experimental studies suggest that thrombin inhibitors may be effective drugs in a wide range of intravascular thrombus formation, also including the inhibition of vascular restenosis. Recent clinical trials revealed the effectiveness of direct thrombin inhibitors in various thrombotic and cardiovascular indications, but also a tendency to an increased risk of bleeding complications. At present, thrombin inhibitors are the most promising class of drugs for the initial therapy of patients with heparin-induced thrombocytopaenia (HIT) or the heparin-induced thrombocytopaenia and thrombosis syndrome (HITTS). They are also useful for the management of venous thrombosis and for acute ischaemic syndromes as well as for invasive procedures. However, with regard to the long-term outcome, a superiority of thrombin inhibitors over heparin has not yet been demonstrated. Several important issues, such as monitoring, pharmacological antagonism and drug interactions will also play an important role in the development of these new drugs. Further clinical trials are required to confirm the effectiveness of direct thrombin inhibitors in the prophylaxis and treatment of various thromboembolic and cardiovascular disorders.

The therapeutic potential of novel anticoagulants

Conventional anticoagulant therapy has been based on indirect inhibition of coagulation factors with heparin and warfarin. These agents display liabilities prompting the development of new anticoagulants over the last two decades. The first to be developed was a series of low molecular weight heparins(LMWHs). Their favourable pharmacokinetic profiles and risk/benefit ratios led to widespread use in Europe and, more recently, approval for their use in the USA. Paralleling the development of LMWHs has been the pursuit of a different strategy focused on direct rather than indirect inhibition of enzymes in the coagulation cascade. In contrast to heparin, LMWHs, or other glycosaminoglycans, direct inhibitors exert their effects independent of either antithrombin III (ATIII) or heparin cofactor II (HCII) and more effectively inhibit clot-bound thrombin or FXa. Highly potent, selective (versus other serine proteases)direct thrombin and FXa inhibitors have been identified and isolated from natural sources, such as leeches, ticks and hookworms. The recombinant forms and analogues of the senatural proteins have been produced using molecular biology techniques, i.e., rHirudin, Hirulogs, recombinant tick anticoagulant peptide (rTAP), recombinant antistasin (rATS) and recombinant nematode anticoagulant peptide-5 (rNAP-5). The design of novel structures or the modification of existing chemicals has led to the synthesis of many non-peptide, low molecular weight inhibitors of thrombin and FXa. Some of them are orally active and may be suitable for long-term clinical use. In addition, considerable progress has been made in developing specific TF/VIIa complex inhibitors. The anticoagulation properties of the new agents are being characterised in experimental studies. Some of them have been advanced to large scale clinical trials and their effectiveness, and sometimes relative ineffectiveness,in arterial and venous thromboembolic disorders has been demonstrated. They are being tested for their potential as new antithrombotic agents that act via direct enzyme inhibition. Thus,the clinician should in future be able to target different thrombotic conditions with proven, specific anticoagulant interventions.

Characterization of the postglomerular renal metabolism of lepirudin in healthy volunteers

INTRODUCTION

The anticoagulant r-hirudin lepirudin is eliminated exclusively via the kidneys. We examined the C-terminal amino acid degradation of lepirudin by the proximal kidney tubulus cells in humans as well as the antithrombotic efficacy of the metabolites and quantified the metabolite portions.

MATERIALS AND METHODS

In vitro metabolites of lepirudin were produced by adding 250 microg lepirudin to urine of three healthy volunteers and a concentration of 100 ml fresh urine to 1.5 ml and subsequent separation by high performance liquid chromatography. Anticoagulant activities of the mass spectrometrically identified metabolites were measured by ecarin clotting time and protein determination with bicinchoninic acid. In 10 healthy volunteers 1 mg lepirudin was administered intravenously, urine was collected during the following 2 h. The urine amount containing 50 microg lepirudin measured by ecarin clotting time was enzyme-inactivated and measured analogously to the in vitro samples.

RESULTS

The in vitro generated metabolites were shortened amino acid by amino acid at the C-terminal end, up to five amino acids. Their anticoagulant activity was reduced to 92.6% (M64), 80.1% (M63) and 74.4% (M60,61,62) in comparison to lepirudin. Lepirudin (57.9 +/- 8.6%) was eliminated unchanged via the kidneys. Identical to the in vitro situation metabolite fragments were built in the distribution M64 = 8.1 +/- 5.7%, M63 = 21.1 +/- 6.5%, and M60,61,62 = 12.9 +/- 4.5%.

CONCLUSIONS

Lepirudin is metabolized spontaneously in more than 10-fold concentrated urine. Metabolization of lepirudin takes place in the proximal tubulus cells as well. In vitro, the degradation takes place amino acid by amino acid, but in vivo even dipeptides and perhaps tripeptides are degraded.

Pharmacokinetics of recombinant hirudin in healthy horses

REASONS FOR PERFORMING STUDY

Recombinant (r)-hirudin is a specific inhibitor of thrombin that is independent of the activity of antithrombin.

OBJECTIVES

To evaluate pharmacokinetic properties and coagulatory changes of r-hirudin in healthy horses.

METHODS

Two clinically healthy horses received a single i.v. bolus of 0.4 mg/kg bwt r-hirudin and 6 clinically healthy horses received the same dose subcutaneously (subcut.) q. 12 h for 3 days. Coagulation times and r-hirudin plasma concentration were determined over 720 mins and 3 days after i.v. and subcut. administration, respectively.

RESULTS

In all horses, treatment with r-hirudin was not associated with systemic or local side effects. After i.v. injection, the 2 horses showed an elimination half-life of 58 and 80 mins, respectively. After subcut. administration, maximum plasma concentration of r-hirudin occurred at 128 +/- 55 mins and declined with a terminal half-life of 561 +/- 364 mins. Maximum response of activated partial thromboplastin time (aPTT) occurred 1.5 h after administration of r-hirudin. A prolongation of 1.9 +/- 0.2 times the pretreatment value was noted.

CONCLUSIONS

Pharmacokinetics of r-hirudin in healthy horses were similar to those in man and other animal species.

POTENTIAL RELEVANCE

The results of this study indicate that r-hirudin can be used in horses, but further studies should be performed in order to prove its effectiveness in diseased horses.

Continuous analysis in extracorporeally circulating blood--a rat model applying flow-through ion-selective electrodes for the measurement of Ca2+, K+, Na+ and pH

A rat model is introduced which enables investigations in anticoagulated blood with continuous measurements by a flow-through electrode system. In the present study, a potentiometric ion-selective electrode (ISE)-system was used for measuring Ca2+, K+, Na+ and pH in rats. The setup was adjusted to an extracorporeal blood-volume of 0.750 ml. This permits indirect measurements of the analytes via a dialysis membrane, with electrical separation of the ISE's and the animal. The flow-rates of blood and dialysis-solution were adjusted in such a way that water diffusing from the aqueous dialysis solution into the blood, across the dialysis membrane, does not alter the haematocrit. Polyethyleneglycol-hirudin was used for anticoagulation, since it was superior to heparin. The assembly enables continuous measurements in the living anaesthetized rat over a time period of at least 3 hours.