Mutant t-PA with kringle replacement

Abstract: Tissue plasminogen activator analogs exhibiting greater specificity for fibrin than native t-PA are disclosed. The analogs include the K1 domain of native t-PA replaced with another kringle domain mediating the binding of the analog to fibrin. The kringle contains six cysteine residues. The t-PA analogs may further include a variety of substitutions and modifications. Pharmaceutical compositions containing one or more of the t-PA analogs along with a physiologically acceptable carrier or diluent are also disclosed.

Description: Technical Field The present invention relates to fibrinolytic agents, methods for their production, and pharmaceutical compositions containing them. More specifically, it relates to tissue plasminogen activator analogs having a modified kringle domain. Background Art Blood coagulation is a process consisting of a complex interaction of various blood components which eventually gives rise to a fibrin network, or clot. Degradation of the fibrin network can be accomplished by activation of the zymogen plasminogen into plasmin. Plasmin is a serine protease which acts directly to degrade the fibrin network and thereby regulate the coagulation process. Conversion of plasminogen into plasmin is normally catalyzed in vivo by tissue-type plasminogen activator (t-PA), a fibrin-specific serine protease which is believed to be the physiological vascular activator of plasminogen. Urokinase-type plasminogen activator (u-PA) is another member of the class of plasminogen activators characterized as serine proteases. t-PA and u-PA are functionally and immunologically distinguishable. t-PA normally circulates as a single polypeptide chain of M r = 72,000 daltons, which is converted to a two-chain form by cleavage of a peptide bond between amino acids 275 (Arg) and 276 (Ile). The heavy chain of t-PA (two variants of M r 40,000 and 37,000) is derived from the amino-terminus, while the light chain (M r 33,000) is derived from the carboxy-terminal end of the t-PA molecule. This cleavage is catalyzed by trypsin or plasmin, and is accompanied by an increase in activity as measured using synthetic substrates, and by an increase in fibrinolytic activity. Single-chain t-PA becomes active upon binding to fibrin, probably due to a conformational change in the activator induced by binding to fibrin. Cleavage to the two-chain form may be associated with rapid clearance of t-PA from the bloodstream, but conflicting reports on this have been published (see Wallen et al., Eur. J. Biochem. 132 : 681-686, 1983), and the clearance mechanism is poorly understood. A two-dimensional model of the potential precursor t-PA protein has been established (Ny et al., Proc. Natl. Acad. Sci. USA 81 : 5355-5359, 1984). From this model, it was determined that the heavy chain contains two triple disulfide structures known as "kringles." Similar kringle structures also occur in prothrombin, plasminogen and urokinase, and are believed to be important for binding to fibrin (Ny et al., ibid .). The second kringle (K2) of t-PA is believed to have a higher affinity for fibrin than the first kringle (K1) (Ichinose, Takio and Fujikawa, J. Clin. Invest. 78 :163-169, 1986; Verheijen, et al., EMBO. J. 5 : 3525-3530, 1986). In addition, the heavy chain of t-PA contains a "growth factor" domain, a triple disulfide-bonded structure which has homology to epidermal growth factor and to similar domains in protein C, factor VII, factor IX and factor X. It has been found that the growth factor domain participates in the rapid in vivo clearance of t-PA, and that deletion of the growth factor domain neither prevents the binding of the resultant molecule to fibrin nor blocks its ability to activate plasminogen. The heavy chain of t-PA also contains a "finger" domain that is homologous to the finger domains of fibronectin. Fibronectin exhibits a variety of biological activities, including fibrin binding; its fibrin-binding activity has been correlated to four or five of its nine finger domains. The light chain of t-PA contains the active site for serine protease activity, which is highly homologous to the active sites of other serine proteases. The precursor form of t-PA additionally comprises a pre-region followed downstream by a pro-region, which are collectively referred to as the "pre-pro" region. The pre-region contains a signal peptide which is important for secretion of t-PA by vascular endothelial cells (Ny et al., ibid .). The pre sequence is believed responsible for secretion of t-PA into the lumen of the endoplasmic reticulum, a necessary step in extracellular secretion. The pro sequence is believed to be cleaved from the t-PA molecule following transport from the endoplasmic reticulum to the Golgi apparatus. The use of t-PA for fibrinolysis in animal and human subjects has highlighted several shortcomings of the native molecule. The half-life of t-PA in vivo has been shown to be as short as three minutes in humans (Nilsson et al., Scand. J. Haematol. 33 : 49-53, 1984). Injected t-PA is rapidly cleared by the liver, and, within 30 minutes, most of the injected material is metabolized to low molecular weight forms. This short half-life may limit the effectiveness of t-PA as a thrombolytic agent by necessitating high dosages. Typically, native t-PA is administered at a dose of 30-150 mg per patient, and the low solubility of the protein necessitates prolonged infusion. Fuchs et al. ( Blood 65 : 539-544, 1985) concluded that infused t-PA is cleared by the liver in a process independent of the proteolytic site, and that infused t-PA will not accumulate in the body, that is, the clearance mechanism cannot be saturated. Furthermore, doses of t-PA sufficient to lyse coronary thrombi are far larger than normal physiological levels, and may cause activation of plasminogen throughout the body, leading to systemic degradation of fibrinogen (Sherry, ibid .), which results in dangerous bleeding episodes. This systemic activity is apparently due to the low specificity of the two-chain form of the molecule. Various workers have modified t-PA in attempts to enhance its clinical suitability. Rosa and Rosa (International Patent Application WO 86/01538) modified the Lys at position 277 of t-PA to stabilize the single-chain form of t-PA. Ile (277) t-PA produced in E . coli was found to be less active as a single-chain molecule, as compared to native t-PA. Wallen et al. ( ibid .) postulated that this lysine residue may be responsible for proteolytic activity of single-chain t-PA. Heyneker and Vehar (published British Patent Application 2,173,804) disclose amino acid substitutions around the cleavage site of t-PA. A variant t-PA comprising Glu at position 275 was shown to have greater specific activity, as compared to native t-PA. This variant t-PA also formed fewer complexes with t-PA inhibitor. The single-chain form was also shown to have greater affinity for fibrin than the two-chain form. Robinson (WO 84/01786) used enzymatic means to remove carbohydrate side chains from t-PA to increase plasma half-life. Van Zonneveld et al. ( Proc. Natl. Acad. Sci. USA 83 : 4670-4674, 1986) disclose modified forms of t-PA wherein portions of the heavy chain have been deleted. Robinson, et al. (EP 207,589 A1) disclose mutant forms of t-PA in which the growth factor domain has been deleted or otherwise modified. However, these variant forms of t-PA do not fully overcome the problems associated with the native protein. There remains a need in the art for a plasminogen activating protein with a long plasma half-life and an enhanced affinity for fibrin. The present invention fulfills this need by providing novel derivatives of tissue plasminogen activator in which the kringle 1 domain has been replaced by a kringle domain derived from another protein. The t-PA analogs described herein provide significant advantages over native t-PA when used as therapeutic agents. These advantages include an enhanced specificity for fibrin, i.e., an enhanced affinity for fibrin which results in increased specificity for clot lysis. Increased specificity can reduce the systemic bleeding effects seen with native t-PA. Replacement of the kringle 1 domain may also be combined with other mutations in t-PA to provide additional novel analogs having enhanced clinical suitability. Through the use of recombinant DNA technology, a consistent and homogeneous source of these proteins is provided. The proteins can be utilized to lyse existing clots in heart attack and stroke victims and in others where the need to lyse or suppress the formation of fibrin matrices is therapeutically desirable. Disclosure of the Invention Briefly stated, the present invention discloses tissue plasminogen activator analogs wherein the K1 domain of native t-PA is replaced with another kringle domain, the kringle domain mediating the binding of the analog to fibrin. The kringle contains six cysteine residues, and the analog exhibits greater specificity for fibrin than native t-PA. The kringle domain the plasminogen K1 domain. Within certain aspects of the present invention, the kringle domain may be composed of 78 - 82 amino acids, and may comprise the amino acid sequence: The t-PA analogs described herein may further contain a substitution of at least one amino acid within thirteen amino acid residues of the cleavage site, this substitution resulting in an increased resistance to cleavage by plasmin. In addition, the t-PA analogs described herein may contain a finger domain having an amino acid sequence selected from the group consisting of the sequence set forth in Figures 19 (A) - (I). In addition, the analogs described herein may further contain the K2 domain of native t-PA modified to contain serine and threonine at amino acid positions 183 and 186, respectively. Still further, the analogs described herein may lack a growth factor domain, or may contain a growth factor domain of a protein selected from the group consisting of native t-PA, protein C, factor VII, factor IX, and factor X. Within a particularly preferred embodiment, the growth factor domain is that of native t-PA, the growth factor domain being modified so that at least one cysteine residue is replaced with another amino acid. The amino acid is preferably serine or alanine. Within certain preferred embodiments described herein, the cysteine residues are located at positions 1 and 22 relative to the N-terminus of the kringle domain, and at positions 1, 6, 18 or 19, and any of 29, 30 or 31 relative to the C-terminus of the kringle domain. DNA sequences encoding the t-PA analogs described above, as well expression vectors containing such DNA sequences are also disclosed. Preferred expression vectors in this regard are Zem99-8000 or Zem99-8100. Host cells transfected or transformed with such an expression vector are also disclosed. The host cell may be E . coli or a mammalian host cell, such as BHK host cells. Still another aspect of the present invention disclosed a pharmaceutical composition comprising a t-PA analog as described herein, and a physiologically acceptable carrier or diluent. These and other aspects of the present invention will become evident upon reference to the following detailed description and attached drawings. Brief Description of the Drawings Figure 1 illustrates the pre-pro t-PA coding sequence constructed from cDNA and synthesized oligonucleotides, together with the amino acid sequence of the encoded protein. Numbers above the lines refer to nucleotide position and numbers below the lines refer to amino acid position. Figure 2 illustrates the construction of the vector Zem99. Figure 3 illustrates the amino acid sequence and DNA sequence of the K1 domain of plasminogen. Figure 4 shows partial restriction maps of clones #1-3 and #8-5, which encode portions of the plasminogen K1 domain. Figure 5 illustrates the construction of plasmid pPKA. Figure 6 illustrates the construction of a vector containing the plasminogen K1 coding sequence. Figure 7 illustrates the construction of plasmid Zem99-2020. Figure 8 illustrates the construction of the plasmids Zem99-8000 and Zem99-8100. Figures 9 and 10 show the cDNA sequences and amino acid sequences of representative t-PA analogs. Figure 11 illustrates the construction of the pMH series of plasmids, comprising mutant DNA sequences encoding t-PA analogs with altered cleavage sites. Figure 12 illustrates the construction of the plasmid Zem182b. Figure 13 illustrates the construction of plasmid Zem219b. Figure 14 illustrates the construction of plasmids Zem99-9100 and Zem99-9200. Figure 15 shows the mutated t-PA sequence in plasmid Zem99-9100, together with the amino acid sequence of the encoded t-PA analog. Numbers refer to amino acid position. Figure 16 illustrates the nucleotide sequence of the mutant DNA sequence in Zem99-9200, together with the amino acid sequence of the encoded t-PA analog. Numbers refer to amino acid position. Figure 17 shows a plot of plasma level vs. time for native t-PA and a representative t-PA analog which were administered to rats. (----) indicates native t-PA, (----) analog 8000. Figure 18 shows the results of a clot lysis assay on native t-PA and a representative t-PA analog of the present invention. (----) indicates native t-PA, (----) analog #8000. Figure 19(A)-(I) illustrates the amino acid sequences of the finger domain of native t-PA and of consensus finger domains. Figure 20 illustrates the homology among kringle domains of urokinase (U), native t-PA (tPA), plasminogen (PL), factor XII (FXII) and prothrombin (PT). (-) indicates a gap inserted to maximize sequence alignment. Best Mode for Carrying Out the Invention Prior to setting forth the invention, it may be helpful to an understanding thereof to set forth definitions of certain terms used herein. Complementary DNA or cDNA: A DNA molecule or sequence which has been enzymatically synthesized from the sequences present in an mRNA template, or a clone of such a molecule. DNA construct: A DNA molecule, or a clone of such a molecule, either single- or double-stranded, which has been modified through human intervention to contain segments of DNA combined and juxtaposed in a manner which would not otherwise exist in nature. Plasmid or vector : A DNA construct containing genetic information which provides for its replication when inserted into a host cell. Replication may be autonomous or achieved by integration into the host genome. A plasmid generally contains at least one gene sequence to be expressed in the host cell, as well as sequences which encode functions that facilitate such gene expression, including promoters, transcription initiation sites and transcription terminators. It may be a linear molecule or a closed, circular molecule. Pre-pro region : An amino acid sequence which generally occurs at the amino-termini of the precursors of certain proteins, and which is generally cleaved from the protein, at least in part, during secretion. The pre-pro region comprises, in part, sequences directing the protein into the secretory pathway of the cell, and generally contains a region which is rich in hydrophobic amino acids. Domain : A three-dimensional, self-assembling array of amino acids of a protein molecule, which contains structural elements necessary for a specific biological activity of that protein. Biological activity : The function or set of functions performed by a molecule in a biological context (i.e., in an organism, a cell, or an in vitro facsimile thereof). Biological activities of proteins may be divided into catalytic and effector activities. Catalytic activities of fibrinolytic agents often involve the activation of other proteins through specific cleavage of precursors. In contrast, effector activities include specific binding of the biologically active molecule to other molecules, such as fibrin, or to cells. Effector activity frequently augments, or is essential to, catalytic activity under physiological conditions. Catalytic and effector activities may, in some cases, reside in the same domain of the protein. For plasminogen activators, biological activity is characterized by the conversion of the pro-enzyme or zymogen plasminogen into plasmid, which in turn degrades fibrin matrices. Because fibrin acts as a cofactor in the activation of plasminogen by t-PA, single chain t-PA has relatively little activity in the absence of fibrin. Native t-PA: A protein having the structure and biological activity of tissue plasminogen activator as isolated from human melanoma cells (see EP 0041766 A2). Native t-PA has the amino acid sequence of the melanoma cell t-PA or may contain slight variations in sequence. Such variations, arising from, for example, genetic polymorphisms, will not substantially alter the structure or activity of the protein. Native t-PA may be isolated from cells which naturally produce it, or may be prepared from recombinant cells which have been transfected or transformed with a DNA sequence encoding native t-PA. The amino acid sequence of a representative native t-PA is shown in Figure 1. t-PA analog : A protein having the characteristic biological activity of plasminogen activators as defined above, further characterized by the presence of a specific artificially induced mutation in the amino acid sequence. The DNA sequence encoding a t-PA analog is referred to as a "mutant DNA sequence," and will generally be in the form of a cDNA. The term "specific artificially induced mutation" includes deletions, insertions and substitutions in the amino acid sequence, which may be introduced through manipulation of a cloned DNA sequence. In general, the biological activity of the t-PA analogs will be measurably altered from that of native t-PA. As discussed above, native t-PA contains two triple disulfide-bonded regions known as kringle domains, hereinafter referred to as "K1" and "K2." These domains are homologous to similar domains found in prothrombin, plasminogen, urokinase and other proteins and are believed to participate in the binding of t-PA to fibrin. The K2 domain has been shown to mediate the stimulatory effect of fibrin on the activation of plasminogen by native t-PA. The K1 domain of native t-PA may be characterized by the location and relationships of the cysteine residues. For purposes of the present invention, the K1 of native t-PA is defined as that portion of the protein from Cys (92) to Cys (173). This domain includes four other cysteine residues, the six cysteines being arranged in three intramolecular disulfide bonds. Bonds join cysteine residues 92 and 173, 113 and 155, and 144 and 168. The disulfide-bonded structure described above is characteristic of kringle domains found in other proteins, including plasminogen, factor XII, and prothrombin. Positions of cysteine residues are highly conserved. There is also a strong preference for the sequence Tyr-Arg-Gly, Tyr-Asp-Gly, or Tyr-Gln-Gly at the position corresponding to residues 100-102 of native t-PA, Gly at the position corresponding to residue 110, Trp at the position corresponding to residue 116, Leu at the position corresponding to residue 138, and Trp at the position corresponding to residue 154. There is also a region of homology spanning residues 142-148 of native t-PA. Figure 20 illustrates the homology among kringle domains of urokinase, native t-PA, plasminogen, factor XII and prothrombin. In particular, there is a preference for Tyr-Arg-Gly, Tyr-Asp-Gly or Tyr-Gln-Gly at position 9 to 11 from the N-terminus; a Gly residue at position 19 from the N-terminus; a Trp at position 25 from the N-terminus; a Leu at one of positions 44 through 47 from the N-terminus; a Trp immediately N-terminal to the Cys at position 18 or 19 from the C-terminus; Arg-Asn-Pro-Asp immediately C-terminal to the Cys located at position 29-31 from the C-terminus; and any of Asn-Tyr, Asn-Phe or Ala-Phe located immediately N-terminal to the Cys at position 29-31 from the C-terminus. The inventors have found that by replacing the K1 domain of native t-PA with a kringle domain derived from another protein, the fibrin affinity and specificity for clot lysis of the resulting protein are significantly enhanced. Suitable substitute kringle domains include the K1, K2, K3, K4 and K5 domains of plasminogen, the K2 domain of native t-PA, the K1 and K2 domains of prothrombin, and the kringle domain of factor XII. It is preferred to utilize the K1, K4 or K5 domain of plasminogen, with the K1 domain being particularly preferred. A partial cDNA sequence encoding plasminogen is disclosed by Malinowski et al., Biochemistry 23 : 4243, 1984. This sequence may be used as a probe to obtain a full-length clone. The amino acid sequences of kringle domains of plasminogen are disclosed by Sottrup-Jensen et al., Prog. Chem. Fibrinolysis Thrombolysis 3 : 191-209, 1978; Lerch et al., Eur. J. Biochem . 197 : 7-13, 1980; and DeMarco et al., J. Biol. Chem. 257 : 12716-12721, 1982. The amino acid sequence of the factor XII kringle domain is disclosed by McMullen and Fujikawa, J. Biol. Chem. 260 : 5328-5341, 1985. DNA sequences encoding the various kringle domains may be obtained by enzymatic digestion of cDNA or, preferably, may be constructed from synthesized oligonucleotides based on the known amino acid or DNA sequences. Methods for synthesizing DNA are well known in the art. Alternatively, suitable oligonucleotides may be constructed from synthesized oligonucleotides based on the known amino acid or DNA sequences. Methods for synthesizing DNA are well known in the art. Alternatively, suitable oligonucleotides may be purchased from commercial suppliers. According to the present invention, it is preferred to produce these novel proteins through the use of recombinant DNA technology, using cDNA clones or genomic clones as starting materials. Suitable DNA sequences can also be synthesized according to standard procedures. It is preferred to use cDNA clones because, by employing the full-length cDNA encoding native t-PA as starting material for producing modified t-PA, introns are removed so that all exons of the native t-PA are present and correctly oriented with respect to one another. The cDNA can also be used as a template for deletion, alteration or insertion of sequences via oligonucleotide-directed mutagenesis. In addition to native t-PA, variants of t-PA, including those previously described, may be modified according to the present invention to contain a modified kringle domain. In this way, the benefits of the modified kringle domain can be combined with the advantages of the t-PA variants to give particularly useful products. For example, as described in detail in the examples which follow, the K1 domain of a t-PA analog which is resistant to cleavage by plasmin may be modified, resulting in a highly clot-specific plasminogen activator. t-PA analogs resistant to cleavage by plasmin were generated by altering the amino acid sequence around the Arg (275) - Ile (276) cleavage site of native t-PA. Such alterations are in the form of amino acid substitutions and additions, generally within thirteen amino acid residues of the cleavage site. Some of these alterations result in t-PA analogs which can be cleaved by thrombin. Also as described herein, the K1 modification may be combined with modifications in the finger domain, also resulting in a highly specific activator; or the growth factor domain of a t-PA analog having a modified kringle domain may be modified or deleted to increase plasma half-life. A preferred modification of the growth factor domain is the replacement of one or more of the cysteine residues with another amino acid. In addition, growth factor domains from proteins having long plasma half-lives may be substituted for the native t-PA growth factor domain. Substitute growth factor domains may be derived from, for example, protein C, factor VII, factor IX and factor X. DNA sequences encoding these proteins have been described (see, for example, Hagen et al., EP 200,421; Foster et al., Proc. Natl. Acad. Sci. USA 82 : 4673-4677, 1985; Kurachi and Davie, Proc. Natl. Acad. Sci. USA 79 : 6461-6464, 1982; and Leytus et al., Biochemistry 25 : 5098-5102, 1986). Other preferred modifications include the modification of carbohydrate attachment sites to alter glycosylation patterns of the t-PA analogs. Native t-PA contains a carbohydrate addition site in the K2 domain which is glycosylated to varying degrees (Pohl et al., Biochemistry 23 : 3701-3707, 1984), leading to heterogeneity in preparations of the protein. This heterogeneity can be eliminated by altering the amino acid sequence, preferably by replacing Gly and Ser at positions 183 and 186, respectively, with Ser and Thr. These changes allow the production of a more uniform and soluble product. It has also been found that by blocking glycosylation on the K1 domain, plasma half-life of t-PA is enhanced. This is preferably achieved by replacing Ser (119) with Met. It is therefore desirable in t-PA analogs of the present invention to alter the K1 domain to block glycosylation if the K1 domain of the analog otherwise contains a carbohydrate addition site. Recombinant DNA technology allows the convenient enhancement of the fibrin-binding domain of native t-PA. t-PA analogs having kringle substitutions as described above may be further modified by the insertion of additional kringle structures, the addition of finger domains, or the substitution of the finger domain. This methodology provides a means for selecting the optimum combination of functional domains found in native t-PA or in related proteins, and thus provides fibrinolytic agents with enhanced biological activity with respect to fibrin binding and specificity of serine protease activity. Amine acid substitutions, additions or deletions are introduced by site-specific mutagenesis using the cloned t-PA DNA sequence or a portion thereof as a template. Techniques for oligonucleotide-directed in vitro mutagenesis are generally known in the art. A preferred such method is that of Zoller and Smith, DNA 3 : 479-488, 1984. The mutated sequence is then joined to the remainder of the t-PA coding sequence, and the reconstructed (mutant) coding sequence is then inserted into an expression vector. The mutant sequences may be expressed in various host cells, including mammalian cells, yeast and other fungi, and bacteria. Production of recombinant t-PA in bacteria, yeast, and mammalian cells is disclosed by, for example, Goeddel et al. (EP 93619 A1), Meyhack and Hinnen (EP 143,081 A2), and Gill (EP 174,835 A1). Methods for transfecting mammalian cells and for transforming bacteria and fungi with foreign DNA are well known in the art. Suitable expression vectors will comprise a promoter which is capable of directing the transcription of a foreign gene in a host cell and a functional transcription termination site. In some instances, it is preferred that expression vectors further comprise an origin of replication, as well as sequences which regulate and/or enhance expression levels, depending on the host cell selected. Suitable expression vectors may be derived from plasmids, RNA and DNA viruses or cellular DNA sequences, or may contain elements of each. Preferred prokaryotic hosts for use in carrying out the present invention are strains of the bacteria Escherichia coli , although Bacillus and other genera are also useful. Techniques for transforming these hosts, are for expressing foreign DNA sequences cloned in them, are well known in the art (see, for example, Maniatis et al., Molecular Cloning: A Laboratory Manual , Cold Spring Harbor Laboratory, 1982). Vectors used for expressing foreign DNA in bacterial hosts will generally contain a selectable marker, such as a gene for antibiotic resistance, and a promoter which functions in the host cell. Appropriate promoters include the trp (Nichols and Yanofsky, Meth. in Enzymology 101 : 155, 1983), lac (Casadaban et al., J. Bact. 143 : 971-980, 1980), TAC (Russell et al., Gene 20 : 231-243, 1982), and phage λ promoter systems. Plasmids useful for transforming bacteria include pBR322 (Bolivar et al., Gene 2 : 95-113, 1977), the pUC plasmids (Messing, Meth. in Enzymology 101 : 20-77, 1983; and Vieira and Messing, Gene 19 : 259-268, 1982), pCQV2 (Queen, J. Mol. Appl. Genet. 2 : 1-10, 1983), and derivatives thereof. Eukaryotic microorganisms, such as the yeast Saccharomyces cerevisiae , or filamentous fungi including Aspergillus , may also be used as host cells. Particularly preferred species of Aspergillus include A . nidulans , A . niger , A . oryzae , and A . terreus . Techniques for transforming yeast are described, for example, by Beggs ( Nature 275 : 104-108, 1978). Aspergillus species may be transformed according to known procedures, for example, that of Yelton et al. ( Proc. Natl. Acad. Sci. USA 81 : 1740-1747, 1984). Expression vectors for use in yeast include YRp7 (Struhl et al., Proc. Natl. Acad. Sci. USA 76 : 1035-1039, 1979), YEp13 (Broach et al., Gene 8 : 121-133, 1979), pJDB248 and pJDB219 (Beggs, ibid .), and derivatives thereof. Such vectors will generally comprise a selectable marker, such as the nutritional marker TRP1 , which allows selection in a host strain carrying a trp1 mutation. Preferred promoters for use in yeast expression vectors include promoters from yeast glycolytic genes (Hitzeman et al., J. Biol. Chem. 255 : 12073-12080, 1980; Alber and Kawasaki, J. Mol. Appl. Genet. 1 : 419-434, 1982; Kawasaki, U.S. Patent No. 4,599,311) or alcohol dehydrogenase genes (Young et al., in Genetic Engineering of Microorganisms for Chemicals , Hollaender et al., eds., p. 335, Plenum, New York, 1982; and Ammerer, Meth. in Enzymology 101 : 192-201, 1983). To facilitate purification of a modified t-PA protein produced in a yeast transformant and to obtain proper disulphide bond information, a signal sequence from a yeast gene encoding a secreted protein may be substituted for the t-PA pre-pro sequence. A particularly preferred signal sequence is the pre-pro region of the MF α1 gene (Kurjan and Herskowitz, Cell 30 : 933-943, 1982; and Singh (EP 123,544)). Higher eukaryotic cells may also serve as host cells in carrying out the present invention. Cultured mammalian cells, such as the BHK, CHO, NS-1, SP2/0, and J558L cell lines, are preferred. These and other cell lines are widely available, for example, from the American Type Culture Collection. A particularly preferred adherent cell line is the BHK cell line tk⁻ts13 (Waechter and Baserga, Proc. Natl. Acad. Sci. USA 79 : 1106-1110, 1982), hereinafter referred to as "tk⁻BHK cells." Expression vectors for use in mammalian cells comprise a promoter capable of directing the transcription of a foreign gene introduced into a mammalian cell. Particularly preferred promoters include the SV40 promoter (Subramani et al., Mol. Cell Biol. 1 : 854-64, 1981), the MT-1 promoter (Palmiter et al., Science 222 : 809-814, 1983), and the mouse kappa gene promoter (Bergman et al., Proc. Natl. Acad. Sci. USA 81 : 7041-7045, 1984). Also contained in the expression vectors is a transcription terminator, located downstream of the insertion site for the DNA sequence to be expressed. A preferred terminator is the human growth hormone (hGH) gene terminator (DeNoto et al., Nuc. Acids Res. 9 : 3719-3730, 1981). In addition, vectors will preferably contain enhancer sequences appropriate to the particular host cell line. For expression of mutant t-PAs in cultured mammalian cells, expression vectors containing cloned t-PA sequences are introduced into the cells by appropriate transfection techniques, such as calcium phosphate-mediated transfection (Graham and Van der Eb, Virology 52 : 456-467, 1973; as modified by Wigler et al., Proc. Natl. Acad. Sci. USA 77 : 3567-3570, 1980; or as described by Loyter et al., Proc. Natl. Acad. Sci. USA 79 :422, 1982) or electroporation (Neumann et al., EMBO J. 1 : 841-845, 1982). A small fraction of the cells integrate the DNA into the genome of the host cell or maintain the DNA in non-chromosomal nuclear structures. These transfectants can be identified by cotransfection with a gene that confers a selectable phenotype (a selectable marker). Preferred selectable markers include the DHFR gene, which imparts cellular resistance to methotrexate (MTX), an inhibitor of nucleotide synthesis; or the bacterial neomycin resistance gene, which confers resistance to the drug G-418, an inhibitor of protein synthesis. After the host cells have taken up the DNA, drug selection is applied to select for a population of cells that are expressing the selectable marker at levels high enough to confer resistance. Selectable markers may be carried on the same vector as the sequence encoding the t-PA analog, or may be carried on a separate vector, depending on the transfection protocol employed. The t-PA analogs of the present invention may be used within pharmaceutical compositions for the treatment of thrombosis. The pharmaceutical compositions will comprise the t-PA analogs in combination with a carrier or diluent, such as sterile water or sterile saline, and may also comprise appropriate excipients and/or solvents. The resulting aqueous solutions may be packaged for use or filtered under aseptic conditions and lyophilized, the lyophilized preparation being combined with a sterile aqueous solution prior to administration. Typically, an aqueous solution containing 3 g of mannitol and 10⁶ units of the t-PA analog is prepared under sterile conditions. One ml aliquots of this solution are pipetted into small vials, which are then lyophilized and sealed. For injection, the lyophilized material is combined with 2 ml of sterile water, the water being provided in a sealed ampoule. Administration is preferably by injection. The proteins of the present invention will typically be administered at doses of from about 6 mg to about 30 mg per patient, depending on the weight of the patient and the nature of the thrombus to be dissolved. However, the present invention is not restricted to the above range and the dose may be varied depending on the condition. Determination of proper dose will be apparent to the skilled practitioner. EXAMPLES Example 1 - Construction of a Full-Length t-PA Clone The sequence of a native human t-PA cDNA clone has been reported (Pennica et al., Nature 301 : 214-221, 1983). The sequence encodes a pre-pro peptide of 32-35 amino acids followed by a 527-530 amino acid mature protein. A cDNA clone comprising the coding sequence for mature t-PA was constructed using as starting material mRNA from the Bowes melanoma cell line (Rijken and Collen, J. Biol. Chem. 256 : 7035-7041, 1981). This cDNA was then used to construct the plasmid pDR1296. Escherichia coli strain JM83 transformed with pDR1296 has been deposited with the American Type Culture Collection under Accession No. 53347. Because the pre-pro sequence was not present in the cDNA clone pDR1296, it was constructed from synthesized oligonucleotides and subsequently joined to the cDNA. In the synthesized t-PA pre-pro sequence, cleavage sites for Bam HI and Nco I were introduced immediately 5′ to the first codon (ATG) of the pre-pro sequence, and a Bgl II (Sau 3A, Xho II) site was maintained at the 3′ end of the pre-pro sequence. The naturally occurring pre-pro sequence lacks a convenient restriction site near the middle; however, the sequence GGAGCA (coding for amino acids -20 and -19, Gly-Ala) can be altered to GGCGCC to provide a Nar I site without changing the amino acid sequence. To construct the pre-pro sequence, the following oligonucleotides were synthesized using an Applied Biosystems Model 380-A DNA synthesizer: Following purification, oligomers ZC131 and ZC132 were annealed to produce an overlap of 12 base pairs (Section 1). Oligomers ZC133 and ZC134 were similarly annealed (Section 2). The oligomers were mixed in Pol I buffer (Bethesda Research Labs), heated to 65°C for five minutes, and slowly cooled to room temperature for four hours to anneal. Ten units of DNA polymerase I were added and the reaction proceeded for two hours at room temperature. The mixtures were electrophoresed on an 8% polyacrylamide-urea sequencing gel at 1,000 volts for 2½ hours in order to size fractionate the reaction products. The correct size fragments (those in which the polymerase reaction went to completion) were cut from the gel and extracted. After annealing, Section 1 was cut with Bam HI and Nar I and cloned into Bam HI + Nar I-cut pUC8 (Vieira and Messing, Gene 19 : 259-268, 1982; and Messing, Meth. in Enzymology 101 : 20-77, 1983). Section 2 was reannealed and cut with Nar I and Bgl II and cloned into Bam HI + Nar I-cut pUC8. Colonies were screened with the appropriate labeled oligonucleotides. Plasmids identified as positive by colony hybridization were sequenced to verify that the correct sequence had been cloned. Section 1 was then purified from a Bam HI + Nar I double digest of the appropriate pUC clone. Section 2 was purified from a Nar I + Xho II digest. The two fragments were joined at the Nar I site and cloned into Bam HI-cut pUC8. The t-PA sequence of pDR1296 was then joined to the synthesized pre-pro sequence in the following manner (Figure 2). Plasmid pIC19R (Marsh et al., Gene 32 : 481-486, 1984) was digested with Sma I and Hind III. The ori region of SV40 from map position 270 (Pvu II) to position 5171 (Hind III) was then ligated to the linearized pIC19R to produce plasmid Zem67. This plasmid was then cleaved with Bgl II and the terminator region from the human growth hormone gene (De Noto et al., Nuc. Acids Res. 9 : 3719-3730, 1981) was inserted as a Bgl II-Bam HI fragment to produce plasmid Zem86. The synthesized t-PA pre-pro sequence was removed from the pUC8 vector by digestion with Bam HI and Xho II. This fragment was inserted into Bgl II-digested Zem86 to produce plasmid Zem88. Plasmid pDR1296 was digested with Bgl II and Bam HI and the t-PA cDNA fragment was isolated and inserted into Bgl II-cut Zem88. The resultant plasmid was designated "Zem94.". The vector Zem99, comprising the MT-1 promoter, complete t-PA coding sequence, and the hGH terminator, was then assembled in the following manner (Figure 2). A Kpn I-Bam HI fragment comprising the MT-1 promoter was isolated from MThGH111 (Palmiter et al., Science 222 : 809-814, 1983) and inserted into pUC18 to construct Zem93. Plasmid MThGH112 (Palmiter et al., ibid .) was digested with Bgl II and religated to eliminate the hGH coding sequence. The MT-1 promoter and hGH terminator were then isolated as an Eco RI fragment and inserted into pUC13 to construct Zem4. Zem93 was then linearized by digestion with Bam HI and Sal I. Zem4 was digested with Bgl II and Sal I and the hGH terminator was purified. The t-PA pre-pro sequence was removed from the pUC8 vector as a Bam HI-Xho II fragment. The three DNA fragments were then joined and a plasmid having the structure of Zem97 (Figure 2) was selected. Zem97 was cut with Bgl II and the Xho II t-PA fragment from Zem94 was inserted. The resultant vector is Zem99. Example 2: Construction of a DNA Sequence Encoding the K1 Domain of Plasminogen Plasmid pK1 comprises a coding sequence for the K1 domain of plasminogen, the sequence of which is shown in Figure 3. It was constructed from a series of eleven oligonucleotides designated "PK1-1, PK1-2, PK1-3----PK1-12," the sequences of which are shown in Table 1. The coding sequence for nucleotides 1 through 182 of the plasminogen K1 domain was constructed from oligonucleotides PK1-1 through PK1-7 in the following manner. 100 pmole each of the oligonucleotides PK1-1, PK1-2, PK1-3 and PK1-4 were phosphorylated at their 5′ termini. The phosphorylated oligonucleotides were mixed with 100 pmole each of PK1-5, PK1-6, and PK1-7. The mixture was precipitated with ethanol and the precipitate was resuspended in H₂O and heated for three minutes at 90°C. The solution was then left to stand at room temperature for ten minutes, then placed on ice. To the chilled mixture was added 10 µl of 660 mM Tris HCl, pH 7.6, containing 6.6 mM MgCl₂, 10 µl of 0.1 M dithiothreitol, 10 µl of 5 mM ATP, and 1000 units of T₄ DNA ligase. The mixture was incubated 15 hours at 14°C. Ethanol was added and the precipitate was resuspended in 20 µl of TE buffer (10 mM Tris HCl, 1 mm EDTA, pH 8.0), followed by the addition of an equal volume of alkali loading buffer (20 mM NaCl, 2 mM EDTA, 80% formamide, 0.1% xylene cyanol and 0.1% bromphenol blue). The mixture was heated for three minutes at 90°C and electrophoresed on a 6% polyacrylamide gel containing 8.4 M urea for one hour at 300 volts. The gel was stained with ethidium bromide, and a 250 bp band was recovered by electrophoretic transfer to DEAE-cellulose paper (Dretzen et al., Anal. Biochem. 112 : 295-298, 1981). The recovered DNA was solubilized in 100 µl of TE buffer and the fragment was designated "PK1-n." PK1-n was C-tailed at the 3′ terminus by combining 10 µl of the PK1-n solution with 2 µl of 100 mM sodium cacodylate-25 mM HEPES, pH 7.6, 6.2 µl of 1 mM dCTP, 10 units terminal deoxynucleotidyl transferase and 5 µl of H₂O. The reaction mix was incubated at 37°C for ten minutes, then extracted with phenol:chloroform (1:1). One µl of 3′-oligo (dG) tailed pUC9 (obtained from Pharmacia) was cleaved with Sma I. The linearized, tailed plasmid was added to the C-tailed PK1-n. The mixture was then ethanol-precipitated, and the DNA was resuspended in 0.5 µl of 2M KCl and 9.5 µl of TE buffer, and incubated at 65°C for 10 minutes, then cooled to room temperature. To the cooled mixture were added 5 µl of 0.2 M Tris HCl, pH 7.5, containing 0.1 M MgCl₂ and 0.1 M dithiothreitol, 20 µl of 2.5 mM dNTPs, 10 µl of 5 mM ATP, 53 µl H₂O, 5 units DNA polymerase 1 (Klenow fragment), and 300 units T₄ DNA ligase (final volume of 100 µl). The mixture was incubated at 14°C for 12 hours, then used to transfect E . coli JM83. The transfected JM83 cells were probed with PK1-6 using the method of Wallace et al. ( Nuc. Acids Res. 9 : 879-894, 1981). Twenty positive clones were sequenced and two were selected, #1-3, including base pairs 1-170, and #8-5, including base pairs 68-186 (see Figure 4). Referring to Figure 5, clone #1-3 was digested with Eco RI and Fok I, and a 130 bp fragment containing a Kpn I site was recovered. Similarly, clone #8-5 was digested with Fok I and Hind III, and a 90 bp fragment was recovered. The two fragments were joined to Eco RI, Hind III-digested pUC12, and the resultant plasmid was designated "pPKA." This plasmid thus contains a DNA sequence corresponding to nucleotides 1-182 of the plasminogen K1 sequence. The remainder of the K1 sequence was constructed using oligonucleotides PK1-9, PK1-10, PK1-11 and PK1-12. One pmole each of the oligonucleotides was phosphorylated at the 5′ end and the combined oligos were mixed with 40 ng of Bam HI, Sph I-digested M13tg130 RF (obtained from Amersham). To this mixture were added 4 µl of 660 mM Tris HCl, pH 7.6, containing 66 mM MgCl₂, and 22 µl of H₂O. The solution was heated for three minutes at 90°C and allowed to cool to room temperature over a period of one hour. Four µl of 0.1 M dithiothreitol, 4 µl of 5 mM ATP, and 300 units of T₄ DNA ligase were added, and the mixture was incubated for 12 hours at 14°C. The resulting phage clone, designated "M13PKB RF" (Figure 6), contained nucleotides 183 through 250 of the K1 sequence. The assembly of the complete K1 coding sequence is illustrated in Figure 6. Plasmid pPKA was digested with Mlu I and Sau 3AI, and a 176 bp fragment was recovered. M13PKB RF was digested with Sau 3AI and Eco RI, and an 88 bp fragment was recovered. These fragments were joined to Mlu I, Eco RI-digested M13um20 RF (obtained from IBI), and the resultant plasmid was designated "M13um20-PK1." The PK1 coding sequence was then inserted into the t-PA cDNA as a replacement for the t-PA Kringle 1 sequence (Figures 7 and 8). The t-PA sequence was first mutagenized to insert Mlu I and Eco RV sites. Plasmid pDR1496 was digested with Sph I and Xba I, and the 2.1 kb fragment comprising the alpha factor and t-PA sequences was isolated. ( S . cerevisiae strain E8-11c transformed with pDR1496 has been deposited with American Type Culture Collection under Accession Number 20728.) This fragment was joined to Sph I, Xba I-digested M13tg130 (RF), and the resultant phage was designated" M13tg130-W." Single-stranded phage DNA was then annealed to an oligonucleotide ( 5′ GCA CGT GGC ACG CGT ATC TAT TTC 3′ ), and mutagenesis was carried out according to standard procedures. The mutagenized phage was designated "M13tg130-PKA1." Single-stranded DNA of M13tg130-PKA1 was isolated and mutagenized with an oligonucleotide having the sequence 5′ CTC AGA GCA TTC CAG GAT ATC GCA GAA CTC 3′ . Single-stranded DNA was prepared from the mutagenized phage and sequenced. A clone containing an Mlu I site at the 5′ end and an Eco RV site at the 3′ end of the Kringle 1 coding sequence was selected and designated "M13tg130-PKA2" (Figure 7). Replicative form DNA was prepared from M13tg130-PKA2 and was digested with Bgl II and Apa I. The fragment containing the Mlu I and Eco RV sites was recovered and joined to Bgl II, Apa I-digested Zem99, as shown in Figure 7. The resultant plasmid was designated "Zem99-2020." The PK1 sequence was then inserted into the t-PA cDNA. M13um20-PK1 RF was digested with Mlu I and Eco RV, and the 336 bp fragment was recovered. This fragment was joined to Mlu I, Eco RV-digested Zem99-2020 to construct Zem99-8000 (Figure 8). The mutant t-PA coding sequence of Zem99-8000 and the encoded amino acid sequence are shown in Figure 9. E . coli transformed with Zem99-8000 has been deposited with the Fermentation Research Institute, Agency of Industrial Science and Technology, Ministry of International Trade and Industry, Japan (FRI), under Accession No. FERM BP-2160. Plasmids Zem99-8000 and pSV2-dhfr (Subramani et al., ibid .) were used to cotransfect tk⁻BHK cells by the method of Loyter ( Proc. Natl. Acad. Sci. USA 79 : 422, 1982). Transfectants were cloned by the limiting dilution method. The mutant t-PA protein, designated "#8000," was affinity-purified on a column containing an antibody to native t-PA. Example 3: Preparation of #8100 A second plasminogen K1 sequence encoding Asn at position 96 was constructed (Figure 8). Zem99-8000 was digested with Bam HI, and the fragment containing the Bgl II site was recovered. This fragment was joined to Bam HI-cut M13mp18 to construct M13-8000R. An oligonucleotide primer (sequence 5′ TTT TTA CCA TTA CCG GTC TT 3′ ) was annealed to single-stranded M13-8000R, and mutagenesis was carried out according to routine procedures. Clones were screened and sequenced, and double-stranded DNA, designated "M13-8000RF," was prepared from a positive clone. This phage was digested with Bgl II and Apa I, and the t-PA fragment was isolated and joined to Bgl II, Apa I-cut Zem99. The resultant plasmid was designated "Zem99-8100." The t-PA coding sequence present in Zem99-8100 and the encoded amino acid sequence are shown in Figure 10. E . coli RR1 transformed with Zem99-8100 has been deposited with the Fermentation Research Institute under Accession No. FERM p-9315. To express the 8100 mutant protein, Zem99-8100 and pSV2-dhfr were used to cotransfect tk⁻BHK cells by the method of Loyter ( ibid .). Transformants were cloned by the limiting dilution method. The protein was affinity-purified as described above. Example 4 - Substitutions of the K1 Domain in t-PA Analogs Resistant to Cleavage by Plasmin A: Mutagenesis For site-specific mutagenesis of the cleavage site, a 472 bp Eco RI fragment comprising the t-PA sequence from bp 802 to bp 1274 was isolated from Zem99 and cloned into the Eco RI site of M13mp18 (replicative form). The recombinant phage were transfected into E . coli (JM101), and anti-sense strand DNA was isolated. Site-specific mutagenesis was then carried out on the single-stranded anti-sense template DNA using one of the mutagenic primers shown in Table 2 and ZC87 ( 5′ TCC CAG TCA CGA CGT 3′ ) as second primer. Oligonucleotides ZC487, 488, 489 and 620 change the Phe at position 274 to Glu, Gly, Arg or Pro, respectively. Oligonucleotides ZC797, 874, 1013 and 1027 change the Arg at position 275 to Gly, Leu, Pro or Asp, respectively. Oligonucleotide 621 introduces a Leu in place of the Lys at position 277. Oligonucleotide 928 changes the Ile at position 276 to Pro. Oligonucleotide 875 changes Arg (275) to Leu and oligonucleotide 927 changes Phe (274) to Pro in the mutant which previously had Lys (277) converted to Leu. Thus, oligonucleotides 875 and 927 can be used to generate double mutations. Twenty pmoles of phosphorylated mutagenic primer and 20 pmoles of the second primer were combined with one pmole of single-stranded template in 10 µl of 20 mM Tris, pH 7.5, 10 mM MgCl₂, 50 mM NaCl, 1 mM DTT and incubated at 65°C for 10 minutes, then 5 minutes at room temperature, and placed on ice. Ten µl of 20 mM Tris, pH 7.5, 10 mM MgCl₂, 2 mM ATP, 10 mM DTT containing 1 mM dNTPs, 2.5 units Klenow polymerase, and 3.5 units DNA ligase were added to the annealed DNA, and the mixture was incubated 3 hours at 15°C. The DNA was then transfected into competent E . coli JM101, and the cells were plated on YT agar and incubated at 37°C. The DNA was then transferred to nitrocellulose and prehybridized at the Tm-4°C of the mutagenic primer for 1 hour in 6x SSC, 10x Denhardt's and hybridized to ³²P-labeled mutagenic primer at Tm-4°C in the same solution. After three washes at TM-4°C, filters were exposed to X-ray film overnight. Additional wash steps were performed at 5°C higher increments as necessary to identify mutant plaques. The mutated inserts were sequenced by the dideoxy method. B: Vector Construction Expression vectors for the altered sequences were then constructed (Figure 11). Plasmid Zem86 (described in Example 1) was digested with Hind III and the ends filled in using DNA polymerase I (Klenow fragment). The linearized DNA was then digested with Eco RI; and a ∼ 350 bp fragment, comprising the SV40 ori sequence, was gel-purified and ligated to Sma I + Eco RI-digested pUC13. The resultant vector was designated "pDR3001." Plasmid pDR3001 was digested with Sal I and Eco RI; and the ∼ 350 bp fragment, comprising SV40 ori and polylinker sequences, was gel-purified. Zem86 was partially digested with Eco RI and completely digested with Xho I to remove the SV40 ori sequence. The SV40 fragment from pDR3001 was then joined to the linearized Zem86. The resultant plasmid was designated "pDR3002" (Figure 11). The sequence just upstream of the ATG start codon of the t-PA sequence in Zem94 was altered by site-specific mutagenesis, resulting in the positioning of Hind III and Bam HI sites adjacent to the ATG. The resultant nucleotide sequence contains an adenine in the -3 position. Single-stranded M13 template DNA was prepared by inserting a ∼ 800 bp Hind III-Eco RI fragment from Zem94 comprising polylinker, pre-pro, and a portion of the mature t-PA sequences into M13mp19. Site-specific mutagenesis was carried out essentially as described by Zoller et al. ( Manual for Advanced Techniques in Molecular Cloning Course , Cold Spring Harbor Laboratory, 1983), using the oligonucleotide ZC444 ( 5′ CAT CCA TGG TGG ATC CAA GCT TGG C 3′ ) as mutagenic primer. Oligonucleotide ZC87 ( 5′ TCC CAG TCA CGA CGT 3′ ) was used as second primer. The mutated inserts were sequenced by the dideoxy method, and a clone in which polylinker sequences had been deleted and the Bam HI site at the 5′ end of the pre-pro sequence had been restored was selected. This phage clone was digested with Bam HI and Eco RI, and the 5′ t-PA sequence was isolated. Zem99 was digested with Eco RI, and the fragment comprising the 3′ portion of the t-PA sequence and the hGH terminator was isolated. The two fragments were then joined with Bam HI + Eco RI-digested pIC19H (Marsh et al., Gene 32 : 481-486, 1984) in a three-part ligation. A plasmid containing the t-PA fragments in the proper orientation was selected and designated "Zem182." Plasmid Zem182 was partially digested with Eco RI, and the ends were filled using DNA polymerase I (Klenow fragment). The linearized plasmid was gel-purified and recircularized using T₄ DNA ligase. A plasmid in which the Eco RI site at the 3′ end of the hGH terminator was destroyed was selected and designated "Zem182b" (Figure 12). Replicative form (RF) DNA was prepared from the mutagenized phage described in Example 4A, and the modified t-PA sequences were purified as Eco RI fragments. Plasmid Zem182b was digested with Eco RI, the vector sequences containing the 5′ and 3′ portions of the t-PA coding sequence were treated with calf alkaline intestinal phosphatase, and the modified t-PA sequences were inserted. The resultant plasmids were digested with Bam HI and Xba I, and the t-PA fragments were inserted into Bam HI + Xba I-cut pDR3002. The resultant vectors were designated "pMH7" through "pMH20" (Table 3 and Figure 11). The activation site mutations of pMH10, pMH13 and pMH17 are combined with the 8100 mutation disclosed in Example 3 to produce t-PA analogs having substantially increased specificity of clot binding and lysis. Vectors containing coding sequences for these analogs are constructed by combining, in a three-part ligation, the vector fragment from Bam HI, Xba I-digested Zem219b, the Bam HI-Apa I kringle fragment from Zem99-8100 and the Apa I-Xba I activation site fragment from the appropriate pMH vector. The resultant plasmids are used to transfect t k -BHK cells by electroporation. After selection and scale-up, the mutant proteins are purified and characterized. Example 5 - t-PA Analogs Having Substitutions in the K1 and Finger Domains Replacement of the t-PA finger domain with a consensus finger region results in the elimination of potential proteolytic cleavage sites at Arg-27 and Lys-49. Eight finger replacement sequences were constructed, based on an analysis of the finger domains of fibronectin and t-PA. The amino acid sequences of these "consensus" finger domains are shown in Figure 19 and Table 4. The consensus finger sequences were constructed from oligonucleotides as described below, then inserted into the t-PA coding sequence. To facilitate this insertion, a Kpn I site was introduced downstream (3′) of the region encoding the wild-type finger domain. Digestion of the resulting sequence with Bgl II and Kpn I resulted in the deletion of the finger domain. A. Kpn I Site Insertion Between the Finger and Growth Factor Domains In order to place a Kpn I site after the finger domain in t-PA, a mutagenesis was performed with oligonucleotide ZC986 ( 5′ TTT GAC AGG TAC CGA GTG GCA 3′ ). DNA of a phage M13 clone containing the 5′ Bam HI-Eco RI fragment of the native t-PA cDNA was prepared. 100 µl of the DNA solution was used to infect E . coli RZ1032 in 100 µl of YT medium supplemented with 0.1 µg/ml uridine. This culture was incubated at 37°C, with vigorous shaking, overnight. Growing the M13 in RZ1032 produces phage containing uridine which are viable in RZ1032 but not in JM101. The cells were spun out, and the phage supernatant was used to reinfect E . coli RZ1032. This second passage was performed to dilute out any JM101-derived phage which contained no uracil. Again, the cells were spun out and the phage were plated on JM101 and RZ1032. Normal viability was observed on RZ1032 plates (indicating phage at 10⁹ pfu/ml), but no plaques were observed on JM101 cells. A complementary strand primed with the mutagenic oligonucleotide was then produced in vitro . The new strand, containing the mutation, contained thymidine and was therefore viable in JM101; the wild-type template was not. Template DNA was prepared by PEG precipitation of the phage supernatant followed by phenol-chloroform extraction and ethanol precipitation. One µg of this template DNA was hybridized with 10 µg of oligonucleotide ZC986 by briefly boiling, incubating at 65°C for 5 minutes, and then slowly bringing the temperature down to 4°C before adding 10 µl 0.2 M HEPES, pH 7.8, 2 µl 100 mM DTT, 1 µl 1 M MgCl₂, 20 µl 2.5 mM each dNTP, 10 µl 10 mM ATP, 1 µl 2.5 U/µl Klenow, and 2 µl 1 U/µl T₄ DNA ligase, final volume adjusted to 100 µl with H₂O. After extension at 37°C for 2 hours, the DNA was transfected into competent JM101 cells. A control extension (minus oligonucleotide) was performed to compare the amount of background produced by extension by priming on contaminating RNA or DNA species. The transfection produced zero plaques with unmutagenized template, 150 on control extension (minus oligonucleotide) and 300 with mutagenized template. The plates were screened by hybridizing a plaque lift with ³²P-labeled mutagenic oligonucleotide and washing in 3 M TMACl (Wood et al., Proc. Natl. Acad. Sci. USA 82 : 1585-1588, 1985) at Tm-5°C for 30 minutes and also by sequencing randomly picked plaques. One positive clone was obtained. B. Production of Finger Replacement Domains The consensus finger region replacements shown in Table 4 and Figure 19 were constructed. The eight consensus sequences were generated from the indicated oligonucleotides. The oligonucleotides (Table 5) were produced using an Applied Biosystems Model 380A DNA synthesizer. First, the twelve oligonucleotides were kinased and simultaneously labeled to a low specific activity with γ-³²P ATP by incubating each with polynucleotide kinase at 37°C for ½ hour. Then the indicated eight combinations (ABC, DEF, ABF, AEC, AEF, DBF, DEC and DBC) were produced by mixing the appropriate oligonucleotides, adding DNA ligase, and incubating at 37°C for 1 hour. The products of this reaction were sorted out on a 6% polyacrylamide-8 M urea sequencing gel. The bands corresponding to the DNA coding for full-length finger domains were cut out, and the DNA was eluted in 2.5 M ammonium acetate. The DNA was ethanol-precipitated and resuspended in water to a concentration of 1 pmole/µl. RF DNA was prepared from the positive clone described in Example 5A, and the Bam HI to Eco RI t-PA fragment was purified. Plasmid Zem219a (described in Example 5D, below) was digested with Xba I and then partially digested with Eco RI. The 1010 bp fragment, containing the 3′ t-PA coding region, was purified. Plasmid Zem219b (described in Example 5D, below) was digested with Bam HI and Xba I and ligated to the 5′ t-PA fragment (Bam HI-Eco RI) and the 1010 bp Eco RI-Xba I fragment. The resulting vector, designated "Zem238," contains a Kpn I site after the finger domain. Zem238 was digested with Bgl II and Kpn I, gel-purified to remove the wild-type finger domain, and ligated with each of the eight consensus sequences to generate expression vectors 238-Fcon 1 to 238-Fcon 8. C. t-PA Analogs Having a Substituted K1 Domain in Combination with a Consensus Finger Domain Single-stranded M13-8100 DNA is used as a template for mutagenesis with oligonucleotide ZC986 (Example 5A) to insert a Kpn I site at the 3′ end of the finger region as described above. A correct clone (M13-8100-K) is identified by sequencing, and the RF DNA is digested with Kpn I and Xba I to isolate the mutant K1 fragment. The mutant K1 fragment is then inserted into Kpn I, Xba I-digested plasmids 238-Fcon 1 to 238-Fcon 8. Resultant plasmids 8100-F1 through 8100-F8 are transfected into tk⁻BHK cells by electroporation. The mutant proteins are purified and characterized. D. Construction of Zem219b Plasmid pSV2-DHFR (Subramani et al., ibid .) was digested with Cfo I, and the fragment containing the DHFR cDNA and the 3′ attached SV40 sequences was isolated, repaired, and ligated to Bam HI linkers. After digestion with Bam HI, an approximately 800 bp fragment containing the entire cDNA and the SV40 terminator region was purified and ligated to Bam HI-digested pUC8. Zem67 (Example 1) was digested with Bgl II and ligated with the Bam HI DHFR-SV40 fragment to generate plasmid Zem176. Plasmid Zem93 was digested with Sst 1 and re-ligated to generate plasmid Zem106, in which approximately 600 bp of sequence 5′ to the MT-1 promoter were eliminated. Plasmid Zem106 was digested with Eco RI and ligated to the Eco RI fragment containing the DHFR gene from plasmid Zem176. The resulting plasmid was designated "Zts14." Plasmid Zts14 was digested with Bam HI and ligated to the Bam HI fragment from plasmid Zem99 containing the entire t-PA coding region and hGH terminator sequence. The resulting plasmid was designated "Zts15." Zts15 was partially digested with Bam HI, repaired, re-ligated and transformed to generate plasmid Zem219, in which the 3′ Bam HI site was destroyed. Plasmid Zem219 was partially digested with Xba I, repaired, re-ligated and transformed to generate plasmid Zem219a, in which the 3′ Xba I site was destroyed. Plasmid Zem219a was digested with Bam HI and Xba I, the vector sequences purified away from the t-PA cDNA sequences, and ligated with an oligomeric Bam HI-Xba I adaptor to generate the expression vector Zem219b (Figure 13), into which mutant Bam HI-Xba I t-PA sequences were inserted. Example 6 - t-PA Analog Having a Substituted K1 Domain and Lacking a Growth Factor Domain A mutant sequence lacking the growth factor domain coding sequence is constructed by deletion mutagenesis using oligonucleotide ZC820 ( 5′ GTA GCA CGT GGC CCT GGT TTT GAC AGG CAC TGA GTG 3′ ) and the single-stranded phage template M13-8100. A correct clone is identified by sequencing and designated "M13-8100-820." RF DNA of M13-8100-820 is digested with Bam HI and Xba I, and the doubly-mutagenized DNA fragment is isolated. Zem219b is digested with Bam HI and Xba I, and the large (vector) fragment is isolated and joined to the mutant fragment with T₄ DNA ligase. After transformation, a correct clone is identified (8100-820) and transfected by electroporation into tk⁻BHK cells. The mutant protein is purified and characterized. Example 7 - t-PA Analogs Having a Substituted K1 Domain and a Modified Growth Factor Domain A. Replacement of Cys (83) The t-PA coding sequence in Zem99 was mutagenized to encode a serine at position 83 (amino acid numbers refer to the sequence shown in Figure 1). Zem99 was digested with Bam HI, and a 2.4 kb fragment comprising the t-PA coding sequence and the hGH terminator was isolated. This fragment was joined to Bam HI-digested M13mp18 (obtained from Pharmacia Japan Co.), and the resultant recombinant phage was used to transfect E . coli JM103. A phage clone having the desired insertion was designated "M13mp18/Bam-Zem99." For site-specific mutagenesis, an oligonucleotide (sequence 5′ CT GGT ATC GAT TTC ACA GCT CTT CCC AGC A 3′ was synthesized and used as a mutagenic primer. The oligonucleotide was annealed to single-stranded M13mp18/Bam-Zem99. Mutagenesis was carried out according to standard procedures, and single-stranded DNA was isolated for sequencing. The replicative form of the mutagenized phage, designated "M13-9100RF," was digested with Bgl II and Hind III. A 2.3 kb fragment containing the t-PA sequence was recovered and joined to Zem99, which had been digested with Bgl II and Hind III (Figure 14), and the DNA was used to transform E . coli TB1. A plasmid having the desired sequence alteration was recovered and designated "Zem99-9100" (Figure 14). The mutated t-PA sequence of Zem99-9100 and the encoded amino acid sequence are shown in Figure 15. Plasmids Zem99-9100 and pSV2-dhfr were used to transfect tk⁻BHK cells by the method of Loyter et al. ( ibid .). Transformants were subjected to cloning by the limiting dilution method. The mutant protein, designated "9100," was purified from the cell culture media by affinity purification. An E. coli TB1 transformant containing plasmid Zem99-9100 has been deposited with the Fermentation Research Institute, Agency of Industrial Science and Technology, Ministry of International Trade and Industry, Japan (FRI), under Accession No. FERM BP-2820. B. Replacement of Cys (84) The t-PA DNA sequence was mutagenized to encode serine at amino acid 84 by means of site-specific mutagenesis using the oligonucleotide 5′ CCT GGT ATC GAT TTC ACT GCA CTT CCC 3′ . The oligonucleotide was annealed to M13mp18/Bam-Zem99, and mutagenesis was carried out using standard procedures. Single-stranded mutagenized phage were sequenced, and a clone having the desired sequence alteration was selected. Replicative form DNA was prepared (designated "M13-9200RF") and digested with Bgl II and Hind III. The 2.3 kb t-PA fragment was isolated and joined to the Bgl II + Hind III-cut Zem99. The resultant vector was designated "Zem99-9200" (Figure 14). The altered t-PA coding sequence of Zem99-9200 and the encoded amino acid sequence are shown in Figure 16. Zem99-9200 and pSV2-dhfr were used to co-transfect tk⁻BHK cells by the method of Loyter ( ibid ). Transformants were subjected to cloning by the limiting dilution method. The mutant protein (9200) was purified by affinity purification. An E . coli RR1 transformant containing plasmid Zem99-9200 has been deposited with FRI under Accession No. FERM BP-2161. C. Combination of Cys Replacement and K1 Substitution Single-stranded DNA was isolated from M13-9200 and was mutagenized using the oligonucleotide 5′ GCA CGT GGC ACG CGT ATC TAT TTC 3′ to introduce an Mlu I site. The muagenized phage was designated "M13-92.05 PKA1." RF DNA was prepared from the mutant phage and was digested with Bgl II and Mlu I, and the 264 bp fragment was recovered. Zem99-8000 was digested with Mlu I and Apa I, and the 1126 bp fragment was recovered. These two fragments were joined to Bgl II, Apa I-digested Zem99, and the resultant plasmid was designated "Zem99-9280." This plasmid thus encodes a mutant t-PA with Ser at amino acid 84 and the K1 domain of plasminogen with Asp at position 96. A second vector, which contains a mutant t-PA sequence encoding a t-PA analog with Ser at amino acid 84 and the K1 domain of plasminogen with Asn at position 96 was constructed. RF DNA from M13-92.05 PKA1 was digested with Bgl II and Mlu I, and the 264 bp fragment was recovered. Zem 99-8100 was digested with Mlu I and Apa I, and the 1126 bp fragment was recovered. These two fragments were joined to Bgl II, Apa I-digested Zem99, and the resultant plasmid was designated "Zem99-9281." Example 8 - Modification of Carbohydrate Attachment Sites in t-PA Analogs An oligonucleotide primer ( 5′ ACG GTA GGC TGT CCC ATT GCT AAA GTA GCA 3′ ) was prepared in order to replace Gly (183) and Ser (186) with Ser and Thr, respectively. Site-directed mutagenesis was performed according to standard procedures on the template M13mp18/Bam-Zem99 (Example 7). Single-stranded mutated phage DNA was prepared and sequenced. A clone having the desired sequence alteration was designated "M13-6000." RF DNA of M13-600 was isolated, digested with Bgl II and Apa I, and a fragment of approximately 1.4 kb was isolated. This fragment was joined to Bgl II, Apa I-digested Zem99 to produce the vector Zem99-6000. E . coli RR1 transformed with Zem99-6000 has been deposited with the Fermentation Research Institute under Accession No. FERM P-9126. Single-stranded M13-6000 DNA was mutagenized to introduce an Eco RV site into the mutant t-PA sequence. Mutagenesis was carried out by the one-primer method using the oligonucleotide 5′ CTC AGA CGA TTC CAG GAT ATC GCA GAA CTC 3′ . The mutagenized phage was designated "M13-6000PKA2." RF DNA of M13-6000PKA2 as digested with Eco RV and Apa I, and the 890 bp fragment was recovered. Zem99-8000 was digested with Bgl II and Eco RV, and the 500 bp fragment was recovered. These two fragments were joined to Bgl II, Apa I-digested Zem99, and the resultant plasmid was designated "Zem99-8060." Example 9: Characterization of Protein Mutant protein #8000 was tested for plasma half-life, and the results were compared to a native t-PA (produced in recombinant BHK cells) control sample. Proteins to be tested were solubilized in a saline solution. The solutions were injected into the femoral veins of male Sprague-Dawley rats (230 g to 270 g each) at a dose of 0.4 mg/kg body weight. Blood samples (0.5 ml) were removed from the jugular veins, adjusted to 3.8% citric acid, centrifuged, and the plasma was removed. Plasma levels of t-PA protein were determined by a sandwich-type enzyme immunoassay. Figure 17 shows a graph of blood level curves for native t-PA and protein #8000. Changes in blood levels were analyzed by a two-compartment model (Shargel, L. and Yu, A.B.C., eds., Applied Biopharmaceutics and Pharmacokinetics , Appleton-Century-Crofts, N.Y., 1980, pp. 38-48). Half-lives were determined for the alpha and beta phases of clearance, and the back extrapolated intercept of the beta phase with the ordinate (B) and the area under the curve (AUC) were also determined. The values obtained for the various proteins are presented in Table 6. t-PA analog #8000 was tested for clot lysis activity using native recombinant t-PA as a control. A silk thread 3 cm in length was introduced into an Atom venous catheter (4Fr 3.5 cm) and the catheter was connected to an injection syringe. Human citrated blood was prepared by mixing blood and a solution of 3.8% sodium citrate in a 9:1 ratio. The citrated blood (0.5 ml) was combined with ¹²⁵l-fibrinogen (25 µ Ci in 50 µl of physiological saline solution), 50 µl of 0.25 M CaCl₂, and thrombin (5 U/10 µl of solution). Sixteen µl of the resulting solution was injected into the catheter, and the catheter was allowed to stand at room temperature for 60 minutes. The silk thread was then removed from the catheter and washed with a physiological saline solution. The radioactivity bound to the thread (the initial fibrin thrombus value) was determined. The thread was then introduced into a carotid arteriovein (A-V) shunt on a male Sprague-Dawley rat weighing between 200 and 300 grams. One ml samples of the protein in a saline solution containing 50 units heparin per ml were injected into the femoral vein of the animal. After two hours, the silk thread was removed from the shunt and the radioactivity (residual fibrin thrombus value) was determined. The residual thrombus ratio was determined according to the equation: Figure 18 is a graph of the results obtained using various doses of native t-PA and analog #8000. The data indicate that the mutant protein is superior to native t-PA in the ability to lyse clots in vivo . From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims. The features disclosed in the foregoing description, in the claims and/or in the accompanying drawings may, both separately and in any combination thereof, be material for realising the invention in diverse forms thereof.

Claims: A t-PA analog in which the K1 domain of native t-PA is replaced with the plasminogen K1 domain. A t-PA analog according to Claim 1, wherein the plasminogen K1 domain comprises the amino acid sequence: A DNA sequence encoding a t-PA analog according to Claim 1 or 2. An expression vector containing a DNA sequence according to Claim 3. An expression vector according to Claim 4, wherein the vector is Zem99-8000 or Zem99-8100 constructed as shown in Figures 7 and 8. A host cell transformed with the expression vector according to Claim 4 or 5. A host cell according to Claim 6, wherein the host cell is E. coli or a mammalian host cell. A process for the manufacture of a t-PA analog, which comprises the steps of (1) culturing a host cell transformed with an expression vector containing DNA encoding a t-PA analog according to Claim 1 or 2, and (2) collecting the resultant t-PA analog. A pharmaceutical composition comprising a t-PA analog according to Claim 1 or 2, and a pharmacologically acceptable carrier or diluent.

Inventors: HASHIMOTO AKIRA, IKEDA YASUNORI, MULVIHILL EILEEN R, NEXO BJORN A, SUZUKI SUGURU, YOSHITAKE SHINJI, YUZURIHA TERUAKI, HASHIMOTO, AKIRA, IKEDA, YASUNORI, MULVIHILL, EILEEN R.,, NEXO, BJORN A.,, SUZUKI, SUGURU, YOSHITAKE, SHINJI, YUZURIHA, TERUAKI
Applicants: EISAI CO LTD, NOVONORDISK AS, ZYMOGENETICS INC, EISAI CO., LTD., NOVO NORDISK A/S, ZYMOGENETICS, INC.
Application year: 1988
Main classification: C12N9
IPC codes: C07K14, C12N15, C12N1, A61P7, C12P21, C07K19, C12N9, A61K38, C12R1, C12N5