Biochemical determinants of clearance of tissue-type plasminogen activator from the circulation

Identification, characterization, and engineering of glycosylation in thrombolyticsa

Cardiovascular diseases, such as myocardial infarction, ischemic stroke, and pulmonary embolism, are the most common causes of disability and death worldwide. Blood clot hydrolysis by thrombolytic enzymes and thrombectomy are key clinical interventions. The most widely used thrombolytic enzyme is alteplase, which has been used in clinical practice since 1986. Another clinically used thrombolytic protein is tenecteplase, which has modified epitopes and engineered glycosylation sites, suggesting that carbohydrate modification in thrombolytic enzymes is a viable strategy for their improvement. This comprehensive review summarizes current knowledge on computational and experimental identification of glycosylation sites and glycan identity, together with methods used for their reengineering. Practical examples from previous studies focus on modification of glycosylations in thrombolytics, e.g., alteplase, tenecteplase, reteplase, urokinase, saruplase, and desmoteplase. Collected clinical data on these glycoproteins demonstrate the great potential of this engineering strategy. Outstanding combinatorics originating from multiple glycosylation sites and the vast variety of covalently attached glycan species can be addressed by directed evolution or rational design. Directed evolution pipelines would benefit from more efficient cell-free expression and high-throughput screening assays, while rational design must employ structure prediction by machine learning and in silico characterization by supercomputing. Perspectives on challenges and opportunities for improvement of thrombolytic enzymes by engineering and evolution of protein glycosylation are provided.

Development and Testing of Thrombolytics in Stroke

Despite recent advances in recanalization therapy, mechanical thrombectomy will never be a treatment for every ischemic stroke because access to mechanical thrombectomy is still limited in many countries. Moreover, many ischemic strokes are caused by occlusion of cerebral arteries that cannot be reached by intra-arterial catheters. Reperfusion using thrombolytic agents will therefore remain an important therapy for hyperacute ischemic stroke. However, thrombolytic drugs have shown limited efficacy and notable hemorrhagic complication rates, leaving room for improvement. A comprehensive understanding of basic and clinical research pipelines as well as the current status of thrombolytic therapy will help facilitate the development of new thrombolytics. Compared with alteplase, an ideal thrombolytic agent is expected to provide faster reperfusion in more patients; prevent re-occlusions; have higher fibrin specificity for selective activation of clot-bound plasminogen to decrease bleeding complications; be retained in the blood for a longer time to minimize dosage and allow administration as a single bolus; be more resistant to inhibitors; and be less antigenic for repetitive usage. Here, we review the currently available thrombolytics, strategies for the development of new clot-dissolving substances, and the assessment of thrombolytic efficacies in vitro and in vivo.

Glycosylation of therapeutic proteins: an effective strategy to optimize efficacy

During their development and administration, protein-based drugs routinely display suboptimal therapeutic efficacies due to their poor physicochemical and pharmacological properties. These innate liabilities have driven the development of molecular strategies to improve the therapeutic behavior of protein drugs. Among the currently developed approaches, glycoengineering is one of the most promising, because it has been shown to simultaneously afford improvements in most of the parameters necessary for optimization of in vivo efficacy while allowing for targeting to the desired site of action. These include increased in vitro and in vivo molecular stability (due to reduced oxidation, cross-linking, pH-, chemical-, heating-, and freezing-induced unfolding/denaturation, precipitation, kinetic inactivation, and aggregation), as well as modulated pharmacodynamic responses (due to altered potencies from diminished in vitro enzymatic activities and altered receptor binding affinities) and improved pharmacokinetic profiles (due to altered absorption and distribution behaviors, longer circulation lifetimes, and decreased clearance rates). This article provides an account of the effects that glycosylation has on the therapeutic efficacy of protein drugs and describes the current understanding of the mechanisms by which glycosylation leads to such effects.

Induction of protein aggregation in an early secretory compartment by elevation of expression level

A variety of valuable therapeutic proteins are expressed in mammalian cells. Currently, rate-limiting for secretion of recombinant glycoproteins are activities in the secretory pathway of eukaryotic cells, i.e., folding and glycosylation of the naked polypeptide chain. In this paper we provide evidence that elevation of expression level alone is sufficient to cause intracellular aggregation of a structurally relatively simple glycoprotein, antithrombin III (ATIII). Elevation of expression level by selection for increased drug resistance in Chinese hamster ovary cells stably expressing ATIII resulted in formation of disulfide-bonded aggregates of ATIII. Aggregated ATIII displayed incomplete sialylation and Endo H-sensitivity and located to the endoplasmic reticulum and the cis-Golgi compartment in subcellular fractionations. To explore possible causes for aggregation of ATIII at elevated expression levels we investigated the influence of the two major energy sources of cultured mammalian cells, D-glucose and L-glutamine, on the ATIII-yield. We found that utilization of D-glucose was not limiting for synthesis of ATIII at elevated expression levels. However, the amount of ATIII-synthesized per L-glutamine consumed did not seem to increase steadily with expression level for ATIII, indicating that secretion of ATIII may be limited by the capacity of the cell to utilize L-glutamine.

Plasminogen activator inhibitor type-1 in cardiovascular disease. Status report 2001

Plasminogen activator inhibitor type-1 (PAI-1) is known to contribute to thrombus formation and to the development and the clinical course of acute and chronic cardiovascular disease, as well as of other arterial and venous thromboembolic diseases. Recently, an important role of elevated pretreatment levels of PAI-1 for failure of thrombolytic therapy of acute myocardial infarction has been discussed. PAI-1 plasma levels depend on the one hand on gene regulation but are related on the other hand to known risk factors of atherosclerosis like insulin resistance, diabetes or hypertriglyceridemia, respectively. Furthermore, an activated renin-angiotensin-aldosterone system (RAAS) significantly contributes to the upregulation of PAI-1 concentration via a receptor-mediated mechanism. In accordance to the known mechanisms of regulation of PAI-1 plasma levels, the use of specific agents like antidiabetic drugs, fibrates, statins, ACE inhibitors and angiotensin II type-1 receptor-blockers may contribute to the downregulation of circulating PAI-1 and, therefore, increase the fibrinolytic capacity and consecutively counteract the thrombotic tendency. To further improve the efficacy of thrombolytic therapy, a PAI-1 resistant variant of t-PA, TNK-t-PA, has been developed and is now available for acute myocardial infarction.

Plasminogen activator inhibitor type-1 (part two): role for failure of thrombolytic therapy. PAI-1 resistance as a potential benefit for new fibrinolytic agents

Rapid and sustained reperfusion of an occluded coronary artery is the goal of thrombolytic therapy in acute myocardial infarction. However, the clot-dissolving efficacy of fibrinolytic agents such as tissue-type plasminogen activator (t-PA) is limited, in vivo, in part by the action of plasminogen activator inhibitor type-1 (PAI-1). A new generation of fibrinolytic agents has been genetically engineered to have greater resistance to PAI-1 inhibition. This article reviews the pathophysiologic role of PAI-1 in failure of thrombolytic therapy and describes the advantages that PAI-1-resistance may confer upon fibrinolytic agents such as TNK-t-PA, the new fibrinolytic agent with the most powerful PAI-1 resistance.

Secretory production of recombinant urokinase-type plasminogen activator-annexin V chimeras in Pichia pastoris

To produce a thrombi-targeting plasminogen activator, we expressed a fused gene that contains a modified pre-sequence of Mucor pussilus rennin (MPR) followed by a chimeric gene of single-chain urokinase-type plasminogen activator (scu-PA)::annexin V (AV). The fused gene was ligated into an integrative vector, under the control of the alcohol oxidase 1 (AOX1) promoter (p), and transformed into Pichia pastoris. Transformants were monitored for the secretion of fibrinolytic activity. The highest expressing clone, HB225, secreted as much as 600 international units (IU) of fibrinolytic activity per ml of culture medium under optimal conditions. It contained three tandem copies of the full-size vector disruptively integrated into the AOX1 sequence. Western blot analysis revealed that the secreted chimera was highly susceptible to proteolysis. Addition of excess amino acids (aa) to the culture medium minimized the degree of proteolysis. Two major species of chimera, 85 and 65 kDa, were then isolated from the culture medium. The former was the intact form consisting of a single-chain and showing full enzyme activity after activation by plasmin. The latter was an enzymatically processed form consisting of two chains held by a disulfide bond, having full enzyme activity without activation. Both chimeras exhibited calcium-dependent phospholipid (PL)-binding affinities similar to the parent AV.

Interaction of mutants of tissue-type plasminogen activator with liver cells: effect of domain deletions

The fibrin-specific thrombolyticum tissue-type plasminogen activator (t-PA) has proven to be a potent drug in several clinical trials, but its clinical application is complicated by the rapid clearance of t-PA from the circulation. The rapid plasma clearance of t-PA results from the uptake of t-PA in the liver. t-PA consists of several domains which may be involved in the interaction with the liver. Three domain-deletion mutants, which were produced by the use of a cassette gene system, were studied in vivo and in vitro for their capacity to bind to the various types of rat liver cells. The three mutants lacked, in comparison to control t-PA, the epidermal growth factor (G) domain, the finger (F) domain or the G domain plus the first kringle (K1). The plasma clearance of the three mutants was slower than that of control t-PA. The slower plasma clearance resulted from a decreased liver uptake: 50 and 80% for t-PA mutants and control t-PA respectively. It was found that the K1 domain was of major importance for the uptake of t-PA by liver endothelial cells in vivo and in vitro. The high-affinity binding of t-PA (and t-PA mutants) to parenchymal liver cells depended largely on the presence of the G domain. Other domain(s), like the F, K2 or protease domain, may be responsible for low-affinity, t-PA-specific binding to rat parenchymal liver cells.

A faster-acting and more potent form of tissue plasminogen activator

Current treatment with tissue plasminogen activator (tPA) requires an intravenous infusion (1.5-3 h) because the clearance of tPA from the circulation is rapid (t 1/2 approximately 6 min). We have developed a tPA variant, T103N,N117Q, KHRR(296-299)AAAA (TNK-tPA) that has substantially slower in vivo clearance (1.9 vs. 16.1 ml per min per kg for tPA in rabbits) and near-normal fibrin binding and plasma clot lysis activity (87% and 82% compared with wild-type tPA). TNK-tPA exhibits 80-fold higher resistance to plasminogen activator inhibitor 1 than tPA and 14-fold enhanced relative fibrin specificity. In vitro, TNK-tPA is 10-fold more effective at conserving fibrinogen in plasma compared to tPA. Arterial venous shunt models of fibrinolysis in rabbits indicate that TNK-tPA (by bolus) induces 50% lysis in one-third the time required by tPA (by infusion). TNK-tPA is 8- and 13-fold more potent in rabbits than tPA toward whole blood clots and platelet-enriched clots, respectively. TNK-tPA conserves fibrinogen and, because of its slower clearance and normal clot lysis activity, is effective as a thrombolytic agent when given as a bolus at a relatively low dose.

Glycosylation of two recombinant human uterine tissue plasminogen activator variants carrying an additional N-glycosylation site in the epidermal-growth-factor-like domain

Recombinant human uterine tissue plasminogen activator (tPA) glycosylation mutants carrying an additional N-glycosylation site in the epidermal-growth-factor-like domain due to the replacement of either Tyr67 by Asn (YN-tPA) or Gly60 by Ser (GS-tPA) were expressed in mouse epithelial cells (C127) in the presence of [6-3H]glucosamine. Glycopeptides comprising individual glycosylation sites were isolated and oligosaccharides attached were liberated by treatment with endo-beta-N-acetylglucosaminidase H or peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase F. Oligosaccharide alditols obtained after reduction were either directly characterized by high-pH anion-exchange chromatography (high-mannose and hybrid-type glycans) or preparatively subfractionated after enzymic desialylation and separation from sulphated asialooligosaccharides (complex-type sugar chains). Individual (sub)fractions of glucans were studied by methylation analysis, liquid secondary-ion mass spectrometry and, in part, by exoglycosidase digestion, whereas corresponding deglycosylated peptides were identified by amino acid analysis and N-terminal amino acid sequencing. The results revealed that Asn117 of YN-tPA carried exclusively high-mannose-type glycans with five to nine mannose residues similar to wild-type tPA expressed in this cell line [Pfeiffer, G., Schmidt, M., Strube, K.-H. & Geyer, R. (1989) Eur. J. Biochem. 186, 273-286]. In contrast, Asn117 of GS-tPA carried only small amounts (about 25%) of high-mannose and hybrid-type species and predominantly complex-type sugar chains (about 75%) which were partially incomplete and mostly devoid of fucose. Newly introduced N-glycosylation sites at Asn67 (YN-tPA) or Asn58 (GS-tPA) as well as those at Asn184 and Asn448 were solely substituted by complex-type glycans. Each carbohydrate attachment site displayed a peculiar oligosaccharide pattern with regard to branching and substitution by Gal alpha 3-residues, sulphate groups, intersecting GlcNAc and lactosamine repeats. Our study clearly demonstrates that creation of a new glycosylation site at Asn58 influenced the oligosaccharide processing and, hence, the glycosylation pattern at Asn117, whereas introduction of a new site at Asn67 did not. The relative amounts of complex-type glycans at Asn117 of GS-tPA correlated with the degree of carbohydrate substitution of Asn58. Therefore, it can be concluded that the presence of a sugar chain at the position and not the Gly to Ser mutation itself is responsible for the observed alteration of GS-tPA glycosylation.

Isolation and characterization of the mannose receptor from human liver potentially involved in the plasma clearance of tissue-type plasminogen activator

Various studies have shown that mannose receptors rapidly eliminate glycoproteins and microorganisms bearing high mannose-type carbohydrate chains from the blood circulation. The purpose of this study was to characterize the mannose receptor in the liver, which in vivo is involved in the rapid clearance of tissue-type plasminogen activator from the circulation. Human liver membranes were solubilized in Triton X-100, and the solution was applied to a tissue-type plasminogen activator Sepharose column. Bound proteins were eluted with ethylenediaminetetraacetate (10 mmol/L). A second, similar purification step rendered a single liver protein of 175,000 daltons. A combination of ligand blotting and a chromogenic assay for tissue-type plasminogen activator demonstrated that the identified liver protein is a mannose receptor because it bound tissue-type plasminogen activator, this tissue-type plasminogen activator binding being fully inhibited by 0.2 mol/L D-mannose. Western-blot analysis revealed that the isolated liver protein is immunologically identical to the human mannose receptor from placenta. Treatment of the liver protein and the placenta mannose receptor with trypsin yielded the same pattern of proteolytic degradation products as identified on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. We conclude that the physiologically relevant mannose receptor for tissue-type plasminogen activator clearance isolated from human liver is immunologically and structurally similar to or identical with the human mannose receptor isolated from placenta.

Complex carbohydrates in drug development

Recent advances in carbohydrate chemistry and biochemistry afford the opportunity to develop bioactive complex carbohydrates, per se, as drugs or as lead compounds in drug development. Complex carbohydrates are unique among biopolymers in their inherent potential to generate diverse molecular structures. While proteins vary only in the linear sequence of their monomer constituents, individual monosaccharides can combine at any of several sites on each carbohydrate ring, in linear or branched arrays, and with varied stereochemistry at each linkage bond. This chapter addresses some salient features of mammalian glycoconjugate structure and biosynthesis, and presents examples of the biological activities of complex carbohydrates. The chapter presents selected examples that will provide an accurate introduction to their pharmacological potential. In addition to their independent functions, oligosaccharides can modify the activities of proteins to which they are covalently attached. Many glycoprotein enzymes and hormones require glycosylation for expression and function. The chapter discusses the ancillary role of carbohydrates that is of great importance to the use of engineered glycoproteins as pharmaceuticals.

Mechanism of deiodination of 125I-human growth hormone in vivo. Relevance to the study of protein disposition

Examination of the disposition of proteins employing 125I-labeled tracers can be complicated by the in vivo deiodination of the tracer. The purpose of this study was to characterize the mechanism by which 125I-labeled proteins are deiodinated in vivo using 125I-human growth hormone (hGH) as a model compound. Intravenous (i.v.) administration of 125I-hGH resulted in a biphasic plasma kinetic pattern, with the majority of radioactivity removed from the plasma during the first 15 min. The level of circulating radioactivity at 2 hr was similar to that 15 min after administration. Radioactivity was eliminated from the animals almost exclusively in the urine. The chemical form of radioactivity in the plasma and urine was analyzed by HPLC, and precipitation of radioactivity with silver nitrate or trichloroacetic acid. Fifteen minutes after administration of 125I-hGH, 30% of the circulating radioactivity was present in the form of iodide (125I-). By 2 hr, the majority of radioactivity in the plasma was in the form of 125I-. The radioactivity in the urine was present exclusively in the form of 125I-. In vivo deiodination of 125I-hGH was reflected by the accumulation of radioactivity in the thyroid glands. There was no evidence for the presence of 125I-peptide intermediates in the plasma or urine of treated animals. In vitro, 125I-hGH was degraded to 125I-peptide intermediates by thyroid gland but not liver or kidney homogenates. In the absence of cofactors, 125I- was not observed as an in vitro metabolic product. However, in the presence of dithiothreitol and NADPH as cofactors, the predominant metabolic product formed by thyroid gland homogenates was 125I-. The deiodination of 125I-hGH by thyroid gland homogenates was inhibited by the serine protease inhibitor phenylmethylsulfonyl fluoride (PMSF), indicating that proteolysis of 125I-hGH was required for deiodination to occur. This was supported by the observation that 125I-labeled proteolytic fragments of 125I-hGH, but not 125I-hGH, were deiodinated by liver or kidney homogenates in the presence of these cofactors. Deiodination by thyroid gland homogenates was inhibited by the sulfhydryl-group blocking reagent, iodoacetate, in a concentration-dependent manner. The characteristics of the in vitro deiodination reaction suggest that a form of thyronine 5'-monodeiodinase may be involved in the in vivo deiodination of 125I-hGH and possibly other 125I-proteins. These data suggest that the disposition of proteins may be determined more accurately with 3H-, 14C- or 35S-labeled molecules which better represent the characteristics of the native protein.

Receptor-mediated endocytosis of tissue-type plasminogen activator (t-PA) by liver cells

Tissue-type plasminogen activator (t-PA) has a short half-life in the circulation because the enzyme is rapidly cleared by the liver. This short review summarizes recent literature concerning mechanisms of uptake and degradation of t-PA in the liver. In vivo studies in rats show that degradation takes place via a lysosomal pathway. Saturation of the uptake system at high t-PA concentrations suggests a receptor-mediated mechanism. Competition experiments with various glycoproteins indicate that the asialoglycoprotein receptor is not involved, but they point to a role for the mannose receptor, which recognizes t-PA via its high mannose-type oligosaccharide on the first kringle domain. Both in vivo and in vitro studies with isolated liver cells demonstrate that parenchymal cells, as well as liver endothelial cells, are involved in the clearance of t-PA. Parenchymal cells, as the hepatoma cell line Hep G2, endocytose t-PA via a still unknown, possibly t-PA specific receptor, while liver endothelial cells catabolize t-PA via the mannose receptor.

Thrombolytic therapy in acute myocardial infarction: Part II--rt-PA

Thrombolysis with pharmacologic agents is a valuable modality for treatment of acute myocardial infarction. The results of several clinical studies indicate that early recanalization can be elicited with intravenous agents and that it is associated with substantial reductions of infarct size, improvement of ventricular function, and reduction in mortality. The recently introduced fibrin-selective agent, rt-PA, appears to represent a significant pharmacologic advance. Its use intravenously elicits high recanalization rates without marked derangements of the coagulation system, reflecting its relative fibrin selectivity. The efficacy of thrombolysis with any drug given by any route, unfortunately, is not 100 per cent. Bleeding remains an important risk. The optimal approach to management of residual stenosis after thrombolysis has not yet been delineated. Currently available information appears to justify the following conclusions: 1. Transmural myocardial infarction usually is caused by an acute obstructing coronary thrombus superimposed on a chronic atherosclerotic lesion. Myocardial necrosis following interruption of blood flow generally is complete within several hours. 2. The thrombus can be lysed and blood flow can be restored with intravenous agents that activate plasminogen. Intravenous rt-PA, a relatively fibrin-specific agent, elicits recanalization in 70 to 75 per cent of infarct-related arteries. 3. Recombinant t-PA evokes only modest depletion of fibrinogen (16 to 36 per cent reduction from baseline). 4. Early reperfusion preserves myocardium and ventricular function and reduces mortality. 5. The extent of benefit after pharmacologic reperfusion is correlated strongly with the brevity of myocardial ischemia prior to initiation of therapy. The greatest benefit is realized in patients treated within the first few hours of onset of acute myocardial infarction. 6. The incidence and optimal means of prevention of reocclusion and reinfarction following successful pharmacologic reperfusion are not yet entirely clear. Mechanical recanalization with PTCA in conjunction with thrombolysis is promising, but its routine immediate use on an emergency basis does not appear to be beneficial. Vigorous educational efforts are needed to heighten the awareness of prospective patients and all members of the health care team to the value of prompt diagnosis of incipient or evolving infarction so that prompt implementation of thrombolysis in appropriate candidates can be facilitated.

Bolus dose response characteristics of single chain urokinase plasminogen activator and tissue plasminogen activator in a dog model of arterial thrombosis

Tissue plasminogen activator (t-PA) and single chain urokinase-plasminogen activator (scu-PA) are relatively "fibrin-specific" thrombolytic drugs with short plasma half lives of 6-8 minutes. Most treatment regimens with these agents utilize a bolus injection followed by continuous drug infusion, usually combined with anticoagulant therapy. The purpose of this study was to establish the dose-response characteristics for scu-PA and t-PA, when given as a single intravenous bolus injection, in a dog model of arterial thrombosis. Eight groups of 6 dogs each were given one of the following doses of scu-PA (mg/kg): 0.20, 0.50, 1.00, 2.00; or t-PA: 0.05, 0.10, 0.20; or an equivalent amount of saline (control group). All doses were given as a single bolus injection 60 minutes after formation of a totally occlusive femoral artery thrombus. Thrombolysis was measured by monitoring the continuous decrement of 125I activity from a radiolabelled thrombus. Ninety minutes after drug injection, all scu-PA treated dogs showed greater thrombolysis (30%, 45%, 56%, and 67%, respectively) than the control group (15%, p less than 0.01). The 0.10 and 0.20 mg/kg t-PA treated dogs showed greater thrombolysis (35% and 49%, respectively) than the control group (15%, p less than 0.01). Both scu-PA and t-PA caused a partial and dose-dependent decrease in alpha 2-antiplasmin activity but scu-PA caused a greater depletion (72% vs. 18%, respectively, p less than 0.05) at 60 minutes after the highest dose of drug administration. Both drugs showed a longer than expected thrombolytic effect based upon the known half lives. Neither drug caused significant changes in the prothrombin time, activated partial thromboplastin time, thrombin time, hematocrit, platelet count, or fibrin degradation product concentration. Single bolus injections of scu-PA and t-PA produce safe and effective thrombolysis in this dog model of arterial thrombosis.