Structure-function analysis with tissue-type plasminogen activator. Effect of deletion of NH2-terminal domains on its biochemical and biological properties

Design of Novel Thrombolytic Agents via Domain Modifications*

Generation of plasmin in the vicinity of a blood clot has proven to be an effective approach for treating thrombotic disorders, particularly myocardial infarction. Conceptually, the ideal thrombolytic agent would initiate the formation of plasmin, primarily in association with fibrin incorporated into the occlusive thrombus. Thus, thrombolytic agents that exhibit relative fibrin specificity and, thus, presumably clot selectivity (e.g., tissue plasminogen activator) were expected to have a marked clinical benefit compared to agents that do not display affinity for fibrin (e.g., streptokinase). However, results obtained recently from clinical trials indicate that these 2 agents essentially were equally effective in treating myocardial infarction. With these findings in mind, efforts are being made to develop novel thrombolytic agents that might achieve more rapid and specific thrombolysis than that achieved by presently available agents and, thus, could be administered earlier because of an improved margin of safety. The available data suggest that tissue-type PA (tPA) mutants possessing resistance to endogenous inhibitors, altered fibrin affinity, and/or slower rates of clearance may prove beneficial in this regard. In addition, adjunctive therapies (i.e., anti-platelet and anti-thrombin compounds) have been found to decrease the time necessary to achieve reperfusion and have reduced rates of reocclusion. These efforts are expected to yield therapeutic agents in the 1990s and beyond that, when administered in combination, would exhibit increased efficacy in the treatment of myocardial infarction and other thrombotic disorders.

High-level expression of a novel recombinant human plasminogen activator (rhPA) in the milk of transgenic rabbits and its thrombolytic bioactivity in vitro

The human tissue-type plasminogen activator (tPA) is a key kinase of fibrinolysis that plays an important role in dissolving fibrin clots to promote thrombolysis. The recombinant human plasminogen activator (rhPA) has more thrombolytic advantages than the wild type tPA. To increase the half-life and thrombolytic activity of tPA, a mutant containing only the essential K2 fibrin-binding and P activating plasminogen domains of the wild type tPA was cloned. This fragment was then inserted into goat β-casein regulatory sequences. Then, a mammary gland-specific expression vector, PCL25/rhPA, was constructed, and the transgenic rabbits were generated. In this study, 18 live transgenic founders (12♀, 6♂) were generated using pronuclear microinjection. Six transgenic rabbits were obtained, and the expression levels of rhPA in the milk had a range of 15.2-630 µg/ml. A fibrin agarose plate assay of rhPA showed that it had strong thrombolytic bioactivity in vitro, and the highest specific activity was >360 (360 times more than that of alteplase). The results indicated that the rhPA containing only the K2 and P domains is efficiently expressed with higher thrombolytic bioactivity in the milk of transgenic rabbits. Our study also demonstrated a new method for the large-scale production of clinically relevant recombinant pharmaceutical proteins in the mammary glands of transgenic rabbits.

ADAM9 promotes lung cancer metastases to brain by a plasminogen activator-based pathway

The transmembrane cell adhesion protein ADAM9 has been implicated in cancer cell migration and lung cancer metastasis to the brain, but the underpinning mechanisms are unclear and clinical support has been lacking. Here, we demonstrate that ADAM9 enhances the ability of tissue plasminogen activator (tPA) to cleave and stimulate the function of the promigratory protein CDCP1 to promote lung metastasis. Blocking this mechanism of cancer cell migration prolonged survival in tumor-bearing mice and cooperated with dexamethasone and dasatinib (a dual Src/Abl kinase inhibitor) treatment to enhance cytotoxic treatment. In clinical specimens, high levels of ADAM9 and CDCP1 correlated with poor prognosis and high risk of mortality in patients with lung cancer. Moreover, ADAM9 levels in brain metastases derived from lung tumors were relatively higher than the levels observed in primary lung tumors. Our results show how ADAM9 regulates lung cancer metastasis to the brain by facilitating the tPA-mediated cleavage of CDCP1, with potential implications to target this network as a strategy to prevent or treat brain metastatic disease.

Soluble expression of active recombinant human tissue plasminogen activator derivative (K2S) in Escherichia coli

CONTEXT

The kringle 2 plus serine protease domains (K2S) of human tissue plasminogen activator (tPA) is an efficacious thrombolytic drug, which has been used to treat heart attacks and strokes by breaking up the clots that cause them. It has nine disulfide bridges, which are needed for proper folding and be the bottleneck in improving the production in the Escherichia coli system. So far, few reports have described the production of soluble active K2S from E. coli.

OBJECTIVE

To achieve high-level expression of active K2S in the E. coli system.

MATERIALS AND METHODS

The DNA fragment coding for K2S was fused with the E. coli disulfide isomerase DsbC. The constructed fusion protein was expressed in E. coli, and then purified with the Ni(2+)-chelating affinity chromatography. K2S was released by cleavage with Factor Xa protease, and the thrombolytic activity was determined using the fibrin plate assay.

RESULTS

The fusion protein DsbC-K2S was found in the culture supernatant of recombinant E. coli as a soluble form of ~40%. The result of fibrinolysis fibrin plate assay showed that the purified recombinant K2S exhibited significant fibrinolysis activity in vitro.

DISCUSSION AND CONCLUSION

These works provided a novel approach for the production of active K2S in E. coli without the requirements of in vitro refolding process, and might establish a significant foundation for the following production of K2S.

High preoperative plasma concentration of tissue plasminogen activator (tPA) is an independent marker for shorter overall survival in patients with ovarian cancer

OBJECTIVE

The objective was to evaluate the preoperative blood concentration of tissue plasminogen activator (tPA) as a discriminator between malignant and benign ovarian tumors, and as a potential marker of postoperative prognosis in patients with ovarian cancer.

METHODS AND MATERIAL

The concentration of tPA was assayed with ELISA (Imulyse Biopool) in preoperative plasma samples obtained from 111 patients with adnexal lesions. Tumors were classified as benign (n = 25), borderline malignant (n = 11), well-differentiated (G1, n = 22), moderately differentiated (G2, n = 11), and poorly differentiated malignant (G3, n = 42). The median follow-up time of patients with malignant tumors was 5.6 years (range 2.1-13.2 years) and 37 patients died during the follow-up period.

RESULTS

Patients with moderately and poorly differentiated tumors had higher levels of plasma tPA compared to those with well-differentiated tumors (P = 0.004 and P = 0.005). No significant differences in the plasma tPA levels were observed between patients with benign, borderline, and well-differentiated tumors. The tPA levels were not different between stages nor within stage Ia-c. In a multivariate Cox proportional hazards model including stage, grade, age, and plasma tPA dichotomized at the median (> or =9 vs <9 ng/mL), high levels of tPA were significantly associated with shorter survival: HR = 4.4 (95% CI 2.0-9.8, P = 0.0003). In the univariate analyze high levels of tPA showed HR = 4.5 (95% CI 2.1-9.6, P = 0.0003).

CONCLUSIONS

High concentration of plasma tPA was an independent marker for poor prognosis in patients with ovarian cancer in our study. Plasma tPA did, however, not discriminate between benign and malignant adnexal lesions.

A catalytic switch and the conversion of streptokinase to a fibrin-targeted plasminogen activator

Plasminogen (Pg) activators such as streptokinase (SK) save lives by generating plasmin to dissolve blood clots. Some believe that the unique ability of SK to activate Pg in the absence of fibrin limits its therapeutic utility. We have found that SK contains an unusual NH(2)-terminal "catalytic switch" that allows Pg activation through both fibrin-independent and fibrin-dependent mechanisms. Unlike SK, a mutant (rSKDelta59) fusion protein lacking the 59 NH(2)-terminal residues was no longer capable of fibrin-independent Pg activation (k(cat)/K(m) decreased by >600-fold). This activity was restored by coincubation with equimolar amounts of the NH(2)-terminal peptide rSK1-59. Deletion of the NH(2) terminus made rSKDelta59 a Pg activator that requires fibrin, but not fibrinogen, for efficient catalytic function. The fibrin-dependence of the rSKDelta59 activator complex apparently resulted from selective catalytic processing of fibrin-bound Pg substrates in preference to other Pg forms. Consistent with these observations, the presence (rSK) or absence (rSKDelta59) of the SK NH(2)-terminal peptide markedly altered fibrinolysis of human clots suspended in plasma. Like native SK, rSK produced incomplete clot lysis and complete destruction of plasma fibrinogen; in contrast, rSKDelta59 produced total clot lysis and minimal fibrinogen degradation. These studies indicate that structural elements in the NH(2) terminus are responsible for SK's unique mechanism of fibrin-independent Pg activation. Because deletion of the NH(2) terminus alters SK's mechanism of action and targets Pg activation to fibrin, there is the potential to improve SK's therapeutic efficacy.

Regulation of tissue plasminogen activator activity by cells. Domains responsible for binding and mechanism of stimulation

A number of cell types have previously been shown to bind tissue plasminogen activator (tPA), which in some cases can remain active on the cell surface resulting in enhanced plasminogen activation kinetics. We have investigated several cultured cell lines, U937, THP1, K562, Molt4, and Nalm6 and shown that they bind both tPA and plasminogen and are able to act as promoters of plasminogen activation in kinetic assays. To understand what structural features of tPA are involved in cell surface interactions, we performed kinetic assays with a range of tPA domain deletion mutants consisting of full-length glycosylated and nonglycosylated tPA (F-G-K1-K2-P), DeltaFtPA (G-K1-K2-P), K2-P tPA (BM 06.022 or Reteplase), and protease domain (P). Deletion variants were made in Escherichia coli and were nonglycosylated. Plasminogen activation rates were compared with and without cells, over a range of cell densities at physiological tPA concentrations, and produced maximum levels of stimulation up to 80-fold with full-length, glycosylated tPA. Stimulation for nonglycosylated full-length tPA dropped to 45-60% of this value. Loss of N-terminal domains as in DeltaFtPA and K2P resulted in a further loss of stimulation to 15-30% of the full-length glycosylated value. The protease domain alone was stimulated at very low levels of up to 2-fold. Thus, a number of different sites are involved in cell interactions especially within finger and kringle domains, which is similar to the regulation of tPA activity by fibrin. A model was developed to explain the mechanism of stimulation and compared with actual data collected with varying cell, plasminogen, or tPA concentrations and different tPA variants. Experimental data and model predictions were generally in good agreement and suggest that stimulation is well explained by the concentration of reactants by cells.

Protein loop grafting to construct a variant of tissue-type plasminogen activator that binds platelet integrin alpha IIb beta 3

Protein-protein interactions can be guided by contacts between surface loops within proteins. We therefore investigated the hypothesis that novel protein-protein interactions could be created using a strategy of "loop grafting" in which the amino acid sequence of a biologically active, flexible loop on one protein is used to replace a surface loop present on an unrelated protein. To test this hypothesis we replaced a surface loop within an epidermal growth factor module with the complementarity-determining region of a monoclonal antibody. Specifically, the HCDR3 from Fab-9, an antibody selected to bind the beta 3-integrins with nanomolar affinity (Smith, J. W., Hu, D., Satterthwait, A., Pinz-Sweeney, S., and Barbas, C. F., III (1994) J. Biol. Chem. 269, 32788-32795), was grafted into the epidermal growth factor-like module of human tissue-type plasminogen activator (t-PA). The resulting variant of t-PA bound to the platelet integrin alpha IIb beta 3 with nanomolar affinity, retained full enzymatic activity, and was stimulated normally by the physiological co-factor fibrin. Binding of the novel variant of t-PA to integrin alpha IIb beta 3 was dependent on the presence of divalent cations and was inhibited by an RGD-containing peptide, demonstrating that, like the donor antibody, the novel t-PA binds specifically to the ligand-binding site of the integrin. These findings suggest that surface loops within protein modules can, at least in some cases, be interchangeable and that phage display can be combined with loop grafting to direct proteins, at high affinity, to selected targets. In principle, these targets could include not only other proteins but also peptides, nucleic acids, carbohydrates, lipids, or even uncharacterized markers of specific cell types, tissues, or viruses.

The role of the finger and growth factor domains in the clearance of tissue-type plasminogen activator by hepatocytes

The relative contribution of the finger/growth factor domains of tissue-type plasminogen activator (t-PA) and of the other t-PA domains to the clearance of t-PA by hepatocytes was investigated. A recombinant finger/growth factor construct inhibited t-PA and t-PA/plasminogen activator inhibitor type-1 degradation with an IC50 of 1800 nM, whereas a t-PA mutant lacking the finger and growth factor domains inhibited degradation with an estimated IC50 of 1200 nM. In comparison the IC50 of t-PA was found to be approximately 10 nM. Clearance of t-PA by human or rat hepatoma cells was not inhibited by high concentrations of fucose (50 mM), which suggests that the fucose on Thr-61 is not involved in clearance by these cells. These results suggest that the binding of t-PA involves several low affinity binding sites located on distinct domains of the t-PA molecule.

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.

The potential improvement of thrombolytic therapy by targeting with bispecific monoclonal antibodies: why they are used and how they are made

The generation of the proteolytic enzyme plasmin from its inactive precursor plasminogen, mediated by so called plasminogen activators, is the essential step in thrombolytic therapy. Plasmin is responsible for the degradation of the insoluble fibrin, the major component of a thrombus, to soluble fibrin degradation products. So far, the use of the more recently developed thrombolytic agents single-chain urokinase-type plasminogen activator (scu-PA) and tissue-type plasminogen activator (t-PA) were disappointing, mainly due to some of their negative properties in vivo, i.e., rapid inhibition and/or hepatic clearance. Besides some background information on the haemostatic balance; t-PA and scu-PA structure; and mechanisms of action, we here review some reported attempts to improve on these agents for thrombolytic therapy following various strategies. One of the more potential strategies, antibody-targeted thrombolytic therapy using bispecific monoclonal antibodies, is discussed somewhat more extensively, as are the several procedures that can be followed for bispecific antibody preparation.

Thrombolytic and pharmacokinetic properties of chimeric tissue-type and urokinase-type plasminogen activators

BACKGROUND

Chimeric molecules comprising the A-chain of tissue-type plasminogen activator (t-PA) and the catalytic domain of urokinase-type plasminogen activator (u-PA) have intact enzymatic characteristics of u-PA, partial fibrin-binding properties of t-PA, and thrombolytic properties in animal models comparable with but not superior to those of single-chain u-PA (scu-PA). Deletion of the finger and growth factor domains (t-PA-delta FE/scu-PA-e) in such chimeras further reduces their affinity for fibrin.

METHODS AND RESULTS

A detailed investigation of the thrombolytic potency and the pharmacokinetics of t-PA and u-PA chimeras was performed in quantitative animal models for thrombolysis. In hamsters with pulmonary embolism, in rabbits with jugular vein thrombosis, and in baboons with femoral vein thrombosis, the thrombolytic potency (percent lysis per milligram of compound administered per kilogram of body weight) of t-PA-delta FE/scu-PA-e was significantly higher than that of recombinant scu-PA (rscu-PA, Saruplase) as shown by a maximal rate of 720 +/- 170% versus 45 +/- 5% lysis per milligram of compound per kilogram of body weight (mean +/- SEM, p less than 0.01) in hamsters, 210 +/- 18% versus 49 +/- 3% lysis per milligram of compound per kilogram of body weight (mean +/- SEM, p less than 0.01) in rabbits, and 310 +/- 73% versus 90 +/- 0.3% lysis per milligram of compound per kilogram of body weight (p less than 0.01) in baboons. However, the specific thrombolytic activity (percent lysis per microgram per milliliter steady-state plasma antigen level) of t-PA-delta FE/scu-PA-e was not significantly different from that of rscu-PA in hamsters (210 +/- 57% versus 160 +/- 27% lysis per microgram per milliliter antigen level) and was lower than that of rscu-PA in rabbits (37 +/- 4% versus 130 +/- 5% lysis per microgram per milliliter antigen level; p less than 0.01). In dogs with a combined femoral vein blood clot and a platelet-rich femoral arterial eversion graft thrombosis, 0.25 mg/kg body wt bolus injections of t-PA-delta FE/scu-PA-e produced significantly more venous clot lysis (90 +/- 5%, n = 10) than 0.25 mg/kg rscu-PA (26 +/- 3%, n = 10) (p less than 0.001) and, at the arterial side, more frequent (10 of 10 dogs versus three of 10 dogs) and more persistent (six of 10 dogs versus none of 10 dogs) recanalization (p = 0.002). After bolus injection in hamsters, rabbits, or baboons, t-PA-delta FE/scu-PA-e had a fourfold to sixfold longer initial half-life than rscu-PA and a slower plasma clearance of sixfold in hamsters, 10-fold in rabbits, and more than 10-fold in baboons.

CONCLUSIONS

These results indicate that t-PA-delta FE/scu-PA-e has a markedly enhanced thrombolytic potency toward venous and arterial thrombi caused by a delayed in vivo clearance with relatively maintained specific thrombolytic activity. These properties suggest that the chimera may be clinically useful for thrombolytic therapy by bolus administration in patients with thromboembolic disease.

Thrombolytic therapy with tissue-type plasminogen activator: New modes and novel variant plasminogen activators

Tissue-type plasminogen activator produced by recombinant DNA technology, has been established as an important thrombolytic agent in the treatment of acute myocardial infarction. New approaches to increase the effectiveness of this agent, including rapid high dose administration are being investigated. Several novel protein engineered variant forms of plasminogen activators have been produced that have increased thrombolytic potency in animal models and offer the potential of a more effective lower dose agent than can be administered clinically as a single bolus intravenous injection.

Development of new thrombolytic agents using recombinant DNA technology

The increasing incidence of thromboembolic diseases has sustained the search for new agents able to stimulate the natural fibrinolytic system. The first generation of antithrombotic agents include bacterial streptokinase and human urine urokinase. Because these molecules lack specificity for the fibrin clot, important efforts have been made to produce, using recombinant DNA technology, agents presenting higher fibrin clot selectivity such as t-PA (tissue-type plasminogen activator) and scu-PA (single chain urokinase-type plasminogen activator). In parallel, several laboratories are presently attempting to create mutants and hybrids plasminogen activators displaying improved thrombolytic properties with respect to the natural molecules. In this paper, we describe briefly the mechanisms of fibrinolysis and the role of the different natural thrombolytic agents. In addition, we review the possibilities of genetic engineering for the production of natural and novel plasminogen activators.

Thrombolytic activity of a novel plasminogen activator, LY210825, compared with recombinant tissue-type plasminogen activator in a canine model of coronary artery thrombosis

LY210825, a recombinant tissue-type plasminogen activator (rt-PA), which contains the kringle-2 and serine protease functional domains of native tissue-type plasminogen activator, was previously produced by site-directed mutagenesis in a Syrian hamster cell line. We studied the thrombolytic potential of this molecule in a canine thrombosis model. Male hounds (16-22 kg) were anesthetized; a 2.0-cm segment of the left circumflex coronary artery (LCX) was isolated proximal to the first main branch, and the dogs were instrumented with an electromagnetic flow probe to measure coronary blood flow. An occlusive thrombus was formed after injury of the intimal surface of the LCX with an electrical current applied by a needle-tipped anode placed distal to the electromagnetic flow probe. After 1 hour of occlusion, either LY210825 or rt-PA was administered intravenously according to the following protocols: 1) a 1-hour infusion of either 0.25 mg/kg LY210825 or 0.4 mg/kg rt-PA, 2) single injections of 0.15-0.6 mg/kg LY210825, and 3) a single injection of 0.45 mg/kg LY210825 and a 3-hour infusion of 1.0 or 1.7 mg/kg rt-PA. Plasma half-lives of LY210825 and rt-PA were 58 +/- 7 and 3.3 +/- 0.3 minutes, respectively. LY210825 produced more rapid reperfusion of the LCX than did rt-PA. In the third study, 90% of the rt-PA-treated vessels reoccluded within 1 hour after cessation of drug, whereas only 25% of the LY210825-treated vessels reoccluded during a 4-hour washout period. There were significant, but relatively small, reductions produced by both plasminogen activators on plasma fibrinogen and plasminogen (25-35% decreases). Because of its longer plasma half-life, LY210825 could be administered intravenously as a single injection. In a canine model of coronary artery thrombosis, LY210825 was a more effective thrombolytic agent than was rt-PA.

Alterations in the Domain Structure of Tissue-Type Plasminogen Activator Change the Nature of Asparagine-Linked Glycosylation

The formation of N-linked oligosaccharides of eukaryotic glycoproteins starts with the attachment of a common precursor at the recognition site Asn-X-Ser/Thr. Subsequent processing, by yet unknown controlling factors, leads to the formation of three different glycans: the high mannose type, the complex type and the hybrid type. In order to gain insight into the processing mechanisms, we studied the glycan pattern of a panel of related molecules constructed by insertion, duplication or deletion of the domains encoded by the cDNA of a fibrinolytic glycoprotein, tissue-type plasminogen activator (t-PA). These variant molecules are identical in regard to the glycosylation sites originally situated in particular domains, but differ with respect to the sequential alignment of the domains. The variant and native t-PA genes were transfected into mouse C127 cells and their carbohydrate structures analyzed by the susceptibility to specific endoglycosidases and by reaction with sugar-specific lectins. We found that with one exception, all mutant activators lack the high mannose glycan found at asn 117 of native t-PA. The exception was a molecule that retains the original domain arrangement up to and through the glycosylation site at asn 117. These results demonstrate for the first time that structural alterations in the primary sequence distal to the actual glycosylation site can result in altered processing of N-linked oligosacharides.

Biological properties of hybrid plasminogen activators

A number of hybrid plasminogen activator genes were constructed from the t-PA and u-PA cDNAs and expressed using a bovine papilloma virus vector and mouse C-127 cells. Hybrid A was constructed by replacing the finger (F) and EGF domains of t-PA with the EGF and Ku domains of u-PA, while hybrids B and C had an extra Ku inserted before or after the double kringle (K1-K2) region of t-PA respectively. While all the hybrids showed comparable enzymatic activities towards a small substrate (S-2288), they had different activities in binding to fibrin clots as well in the fibrin-dependent plasminogen activation, the order of activities being: t-PA greater than or equal to hybrid B greater than hybrid C greater than hybrid A. Carbohydrate analysis showed that while hybrid C, like rt-PA, had at least one high-mannose type sugar chain (probably at residue 117 in K1), the other hybrids had only complex-type carbohydrates suggesting that domain interaction in t-PA might influence glycan processing. Pharmacokinetic studies in dog showed that hybrid B had a significantly longer plasma half-life than rt-PA. Thrombolytic efficacies of hybrid B and rt-PA were compared in dog model using an artificially induced coronary thrombus. Complete thrombolysis was achieved with 18 mg and 50 mg dosages for hybrid B and rt-PA respectively. These data show the superior pharmacokinetic and thrombolytic properties of hybrid B compared to rt-PA.

Synthesis and refolding of human tissue-type plasminogen activator in Bacillus subtilis

A 1.6 kb cDNA fragment encoding the mature part of the human tissue-type plasminogen activator (t-PA) was subcloned into a Bacillus subtilis dual plasmid expression system [Le Grice et al., Gene 55 (1987) 95-103]. Expression of the tPA gene in this vector was regulated by the inducible Escherichia coli lac elements, as well as a strong phage-T5-derived promoter and ribosome-binding site preceding the polylinker. The 5' end of the tPA gene corresponding to the N terminus of mature t-PA was fused in phase to the third codon present in the polylinker region of the expression vector, p602/22, to form p602-t-PA. B. subtilis containing p602-t-PA, when induced with isopropyl-beta-D-thiogalactopyranoside, produced large amounts of immunoreactive t-PA (approx. 20 micrograms/ml). As expected, t-PA was not secreted into the culture media, but was localized in intracellular inclusion bodies and was found to be enzymatically inactive. However, enzymatic activity could be regained following complete reduction followed by slow oxidation of the solubilized inclusion bodies. The recombinant t-PA (rt-PA) showed, after purification, a smaller molecular size than melanoma t-PA, probably due to lack of glycosylation in the Bacillus system. Like melanoma t-PA, rt-PA exhibited tremendous stimulation of plasminogen activation in the presence of fibrin. Our results illustrate that B. subtilis, when supplied with the proper transcriptional/translational regulatory elements, can be an effective system for expression of heterologous gene products.

After coronary thrombolysis and reperfusion, what next?

Thrombolytic therapy for the removal of intravascular thrombi was introduced when streptokinase was first given to humans 40 years ago, the same year the American College of Cardiology was founded. Streptokinase was first administered to patients with acute myocardial infarction in 1959. Today, thrombolytic therapy has been established to offer significant benefits to patients with acute myocardial infarction provided they are brought to medical attention early enough after the onset of symptoms. The two major agents, streptokinase and recombinant tissue-type plasminogen activator (rt-PA), have been shown to result in reperfusion of infarct-related arteries, to salvage ischemic myocardium, to improve myocardial performance and to reduce mortality. In spite of these impressive gains, this novel therapy has shortcomings. The interval from the start of thrombolytic treatment to coronary reperfusion varies significantly from patient to patient and may, at times, be too long to produce a real benefit in terms of salvage of ischemic myocardium. The rate of reocclusion lies somewhere between 10% and 20% and appears not to be influenced by concomitant heparin anticoagulation. The rate of bleeding complications even with the "fibrin-specific" rt-PA is higher than anticipated and may range from 10% to 30%. As a consequence, intensive efforts are being directed at the development of improved thrombolytic agents and for adjunctive therapy evaluating better anticoagulants than heparin and better antiplatelet agents than aspirin. This review is a status report summarizing where we are in thrombolytic therapy in acute myocardial infarction, where we need to improve treatment results and what is being done mainly at the preclinical level to bring about such improvements.

New strategies in the development of thrombolytic agents

Recombinant DNA technology has allowed large-scale production of the physiological, fibrin-specific, plasminogen activators tissue-type plasminogen activator (t-PA) and single-chain urokinase-type plasminogen activator (scu-PA). The results of clinical trials with these agents, mainly for the treatment of acute myocardial infarction, have revealed a limited fibrin specificity at the large therapeutic doses required for efficient thrombolysis. Mutants and variants of t-PA and scu-PA have given important information on structure-function relationships in these proteins and have resulted in rt-PA variants with significantly prolonged half-lives in vivo. Construction of chimaeric plasminogen activators containing various portions of t-PA and scu-PA has produced functionally active enzymes, however with a lower fibrin-affinity than wild-type t-PA. The promise of antibody targeting and the use of synergistic combinations of thrombolytic agents remains to be further investigated. We anticipate that eventually these research lines will yield artificial plasminogen activators with improved efficacy, risk/benefit and cost/benefit ratios.

Construction and expression of a hybrid plasminogen activator gene with sequences from non-protease region of tissue-type plasminogen activator (t-PA) and protease region of urokinase (u-PA)

There are two physiological plasminogen activators (PAs), tissue-type PA (t-PA) and urokinase (u-PA) which possess distinct immunological and biochemical characteristics. Using genetic engineering techniques a hybrid t:u-PA cDNA, comprised of amino acid (aa) sequences corresponding to the non-protease region (aa 1-261) of t-PA and the protease region (aa 132-411) of u-PA, was constructed. The t:u-PA gene after insertion into the SV40 expression vector was expressed in monkey Cos-1 cells. The 66-67 kDa t:u-PA was produced in an enzymatically active form. The fibrinolytic activity of the t:u-PA could be quenched by anti-urokinase as well as by anti-t-PA sera. Like urokinase, the t:u-PA showed a high intrinsic plasminogen activation. This activity, as in the case of t-PA, was stimulated by fibrin. The u-PA, on the other hand, stimulated plasminogen activation marginally in the presence of fibrin. Both the t:u-PA and t-PA showed binding affinity for fibrin clot. This study strongly suggests the autonomous nature of the structural domains in PA and also demonstrates the feasibility of shuffling these domains without loss of their functional activities.

Disposition of a novel recombinant tissue plasminogen activator, delta 2-89 TPA, in mice

The pharmacokinetic characteristics of delta 2-89 tPA, characterized by the deletion of the first 89 amino acids at the NH2-terminus of tPA, were evaluated and compared to those of recombinant tPA (rtPA). When they were administered intravenously to mice, a biexponential disposition curve was observed for both tPAs. The plasma half-lives of lambda 1 and lambda 2 phases of delta 2-89 tPA were 15 minutes and 180 minutes which are significantly higher than those of rtPA. A zymogram of mouse plasma taken at various time intervals showed that delta 2-89 tPA retained fibrinolytic activity up to 30 minutes, whereas rtPA could be detected only up to 5 minutes after injection. Autoradiography revealed that most of 125I-delta 2-89 tPA was associated with plasma protein complex.