Novel inhibitors of vascular endothelial growth factor activity, their uses and processes for their production   

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of prior application Ser. No. 11/537,623 filed Sep. 30, 2006, which is a continuation of Application No. 11,043,693 filed Jan. 26, 2005, which is a continuation of application Ser. No. 10/105,901 filed Mar. 20, 2002, now U.S. Pat. No. 6,897,294, which is a continuation of application Ser. No. 09/348,886 filed Jul. 1, 1999, now U.S. Pat. No. 6,383,486, which is a divisional of application Ser. No. 08/643,839 filed May 7, 1996, now U.S. Pat. No. 6,100,071.

FIELD OF THE INVENTION

The present invention is directed to novel chimeric VEGF receptor proteins comprising amino acid sequences derived from the vascular endothelial growth factor (VEGF) receptors flt-1, KDR and the murine homologue of the human KDR receptor, FLK-1, wherein said chimeric VEGF receptor proteins bind to VEGF and antagonize the endothelial cell proliferative and angiogenic activity thereof. The present invention is also directed to nucleic acids and expression vectors encoding these chimeric VEGF receptor proteins, host cells harboring such expression vectors, pharmaceutically acceptable compositions comprising such proteins, methods of preparing such proteins and to methods utilizing such proteins for the treatment of conditions associated with undesired vascularization.

BACKGROUND OF THE INVENTION

The two major cellular components of the mammalian vascular system are the endothelial and smooth muscle cells. Endothelial cells form the lining of the inner surface of all blood vessels in the mammal and constitute a non-thrombogenic interface between blood and tissue. Therefore, the proliferation of endothelial cells is an important component for the development of new capillaries and blood vessels which, in turn, is a necessary process for the growth and/or regeneration of mammalian tissues.

One protein that has been shown to play an extremely important role in promoting endothelial cell proliferation and angiogenesis is vascular endothelial growth factor (VEGF). VEGF is a heparin-binding endothelial cell growth factor which was originally identified and purified from media conditioned by bovine pituitary follicular or folliculostellate (FS) cells. Ferrara and Henzel, Biochem. Biophys. Res. Comm. 161:851-858 (1989). VEGF is a dimer with an apparent molecular mass of about 46 kDa with each subunit having an apparent molecular mass of about 23 kDa. Human VEGF is expressed in a variety of tissues as multiple homodimeric forms (121, 165, 189 and 206 amino acids per monomer), wherein each form arises as a result of alternative splicing of a single RNA transcript. VEGF121 is a soluble mitogen that does not bind heparin whereas the longer forms of VEGF bind heparin with progressively higher affinity.

Biochemical analyses have shown that VEGF exhibits a strong mitogenic specificity for vascular endothelial cells. For example, media conditioned by cells transfected by human VEGF cDNA promoted the proliferation of capillary endothelial cells, whereas medium conditioned by control cells did not. Leung, et al., Science 246:1306 (1989). Thus, VEGF is known to promote vascular endothelial cell proliferation and angiogenesis, a process which involves the formation of new blood vessels from preexisting endothelium. As such, VEGF may be useful for the therapeutic treatment of numerous conditions in which a growth-promoting activity on the vascular endothelial cells is important, for example, in ulcers, vascular injuries and myocardial infarction.

In contrast, however, while vascular endothelial proliferation is desirable under certain circumstances, vascular endothelial proliferation and angiogenesis are also important components of a variety of diseases and disorders including tumor growth and metastasis, rheumatoid arthritis, psoriasis, atherosclerosis, diabetic retinopathy, retrolental fibroplasia, neovascular glaucoma, age-related macular degeneration, hemangiomas, immune rejection of transplanted corneal tissue and other tissues, and chronic inflammation. Obviously, in individuals suffering from any of these disorders, one would want to inhibit, or at least substantially reduce, the endothelial proliferating activity of the VEGF protein.

In the specific case of tumor cell growth, angiogenesis appears to be crucial for the transition from hyperplasia to neoplasia and for providing nourishment to the growing solid tumor. Folkman, et al., Nature 339:58 (1989). Angiogenesis also allows tumors to be in contact with the vascular bed of the host, which may provide a route for metastasis of tumor cells. Evidence for the role of angiogenesis in tumor metastasis is provided, for example, by studies showing a correlation between the number and density of microvessels in histologic sections of invasive human breast carcinoma and actual presence of distant metastasis. Weidner et al., New Engl. J. Med. 324:1 (1991). Thus, one possible mechanism for the effective treatment of neoplastic tumors is to inhibit or substantially reduce the endothelial proliferative and angiogenic activity of the VEGF protein.

The endothelial proliferative activity of VEGF is known to be mediated by two high affinity tyrosine kinase receptors, flt-1 and KDR, which exist only on the surface of vascular endothelial cells. DeVries, et-al., Science 225:989-991 (1992) and Terman, et al., Oncogene 6:1677-1683 (1991). Both the flt-1 and KDR tyrosine kinase receptors have seven immunoglobulin-like (Ig-like) domains which form the extracellular ligand-binding regions of the receptors, a transmembrane domain which serves to anchor the receptor on the surface of cells in which it is expressed and an intracellular catalytic tyrosine kinase domain which is interrupted by a “kinase insert”. While the KDR receptor binds only the VEGF protein with high affinity, the flt-1 receptor also binds placenta growth factor (PLGF), a molecule having significant structural homology with VEGF. An additional member of the receptor tyrosine kinases having seven Ig-like domains in the extracellular ligand-binding region is FLT4, which is not a receptor for either VEGF or PLGF, but instead binds to a different ligand; VH1.4.5. The VH1.4.5 ligand has been reported in the literature as VEGF-related protein (VRP) or VEGF-C.

Recent gene knockout studies have demonstrated that both the flt-1 and KDR receptors are essential for the normal development of the mammalian vascular system, although their respective roles in endothelial cell proliferation and differentiation appear to be distinct. Thus, the endothelial proliferative and angiogenic activity of the VEGF protein is mediated by binding to the extracellular ligand-binding region of the flt-1 and KDR receptors on the surface of vascular endothelial cells.

In view of the role of VEGF in vascular endothelial proliferation and angiogenesis, and the role that these processes play in many different diseases and disorders, it is desirable to have a means for reducing or inhibiting one or more of the biological activities of VEGF. As such, the present invention is predicated upon research intended to identify the Ig-like domain or domains of the flt-1 and KDR receptor extracellular ligand-binding region which mediate binding to the VEGF protein and inserting or fusing that domain or domains into amino acid sequences derived from another protein to produce a “chimeric VEGF receptor protein”. The chimeric VEGF receptor proteins of the present invention will bind to and inactivate endogenous VEGF, thereby providing a means for reducing or inhibiting endogenous VEGF activity and, in turn, reducing or inhibiting endothelial cell proliferation and angiogenesis. Thus, it is an object of the present invention to provide novel chimeric VEGF receptor proteins comprising amino acid sequences derived from the extracellular ligand-binding region of the flt-1 and KDR receptors, wherein said chimeric VEGF receptor proteins are capable of binding to and inhibiting the activity of VEGF.

Further objects of the present invention are to provide nucleic acids encoding chimeric VEGF receptor proteins of the present invention, replicable expression vectors capable of expressing such chimeric proteins, host cells transfected with those expression vectors, pharmaceutical compositions comprising the chimeric VEGF receptor proteins of the present invention, methods for preparing such chimeric proteins and method of using those chimeric proteins for the therapeutic treatment of an individual in need thereof.

SUMMARY

The objects of this invention, as generally defined supra, are achieved by the provision of chimeric VEGF receptor proteins which are capable of binding to VEGF and exerting an inhibitory effect thereon, wherein said chimeric VEGF receptor protein comprises Ig-like domains 1, 2 and 3 of the flt-1 and/or the KDR receptor (or the murine homologue of the KDR receptor, FLK-1) or functional equivalents thereof.

In a preferred embodiment, the chimeric VEGF receptor proteins of the present invention contain flt-1 or KDR receptor amino acid sequences corresponding only to Ig-like domains 1, 2 and 3 of the extracellular ligand-binding region thereof and each Ig-like domain is derived from the same VEGF receptor.

In other embodiments, however, the chimeric VEGF receptor proteins of the present invention comprise Ig-like domains 1, 2 and 3 of the extracellular ligand-binding region of the flt-1 or KDR receptor in addition to one or more of the remaining four immunoglobulin-tike domains thereof. Preferably, the Ig-like domains employed are derived from the same receptor, however, a combination of Ig-like domains derived from both the flt-1 and KDR receptors will find use.

In another embodiment of the present invention, the chimeric VEGF receptor proteins of the present invention comprise the extracellular ligand-binding region of the FLT4 receptor wherein at least Ig-like domain 2 of the FLT4 receptor is replaced with the Ig-like domain 2 of either the flt-1 or KDR receptor. Preferably, only Ig-like domain 2 of the FLT4 receptor is replaced by the corresponding Ig-like domain from either the flt-1 or KDR receptor, however, other domains may also be similarly replaced.

A further aspect of the present invention is directed to nucleic acid sequences encoding the chimeric VEGF receptor proteins described herein and functional equivalents thereof. It is well known to the ordinarily skilled artisan that such nucleic acids can vary due to the degeneracy of the genetic code and such nucleic acid variants are also encompassed by the present invention.

In still other embodiments, the present invention relates to replicable expression vectors encoding the various chimeric VEGF receptor proteins described supra, host cells transfected with those expression vectors and compositions comprising the chimeric VEGF receptor proteins described supra compounded with a pharmaceutically acceptable excipient.

In yet other embodiments, the present invention relates to methods for producing the chimeric VEGF receptor proteins described supra by introducing an expression vector encoding the desired chimeric protein into an appropriate expression systems and effecting the expression of said protein.

Yet another aspect of the invention provides for the use of the chimeric VEGF receptor proteins of the present invention for the treatment of conditions associated with inappropriate vascularization wherein an inhibition of vascularization and angiogenesis is desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an alignment of the amino acid sequences for the extracellular ligand-binding regions of the flt-1, KDR and FLT4 receptors. Amino acids are presented by their standard single letter designations. Dashes are inserted to create the best fit for alignment of the seven Ig-like domains. The seven Ig-like domains of each extracellular ligand-binding region of each receptor presented are shown as boxed areas.

FIG. 2 presents the oligonucleotides used in the oligonucleotide-directed mutagenesis-generated deletions of each of the seven Ig-like domains existing within the extracellular ligand-binding region of the flt-1 receptor. The restriction sites created in the DNA sequence are underlined and are listed above the underlined sequence. Amino acids are presented by their standard one-letter designations. Numbers shown in parentheses designate the amino acid number. In some cases, amino acids were changed when the restriction sites were designed into the oligonucleotides. These amino acid changes are underlined and the original amino acid at that position is presented below in parentheses.

FIG. 3 shows the ability of intact flt-1/IgG chimeric VEGF receptor protein and various flt-1/IgG domain deletion chimeric proteins to specifically bind to the VEGF ligand. Binding efficiency is presented as the total cpm bound per 5 ng of immunoreactive (ir) Fe. “wt” refers to the intact flt-1/IgG chimeric VEGF receptor protein. “d1” through “d7” refer to the flt-1/IgG domain deletion chimeras where the number corresponds to the Ig-like domain that is deleted.

FIG. 4 shows the ability of intact flt-1/IgG chimeric VEGF receptor protein and various flt-1/IgG domain deletion chimeric proteins to specifically bind to the VEGF ligand. Binding efficiency is presented as the total cpm bound per 4.5 ng of immunoreactive (ir) Fc. “flt-wt” refers to the intact flt-1/IgG chimeric protein. “flt(1,2)” is the chimeric protein having only flt-1 Ig-like domains 1 and 2 fused to the Fc of IgG. “flt(2)” is the chimeric protein having only flt-1 Ig-like domain 2 fused to the Fc of IgG. “flt(2,3)” is the chimeric protein having only flt-1 Ig-like domains 2 and 3 fused to the Fc of IgG. “flt(1,2,3)” is the chimeric protein having only flt-1 Ig-like domains 1, 2 and 3 fused to the Fc of IgG. Finally, “KDR(2)” is a chimeric VEGF receptor protein wherein only the Ig-like domain 2 of the extracellular ligand-binding region of the KDR receptor is fused to the Fc of IgG.

FIG. 5 shows the percent binding of the VEGF ligand to the intact flt-1/IgG chimeric VEGF receptor protein and to the flt(1,2,3) deletion chimera in the presence of increasing amounts of unlabeled VEGF competitor. “” designates binding by the flt(1,2,3) deletion chimera. “♦” designates binding by the intact flt-1/IgG chimeric protein.

FIG. 6 shows the ability of intact flt-1/IgG chimeric VEGF receptor protein and various Ig-like domain 2 “swap” mutants to specifically bind to the VEGF ligand. Binding efficiency is presented as the total cpm bound per 1 ng of immunoreactive (ir) Fc. “flt-1” refers to the native flt-1/IgG chimeric protein. “flt.d2” refers to the flt-1/IgG chimeric protein having a deletion of Ig-like domain 2. “flt.K2” refers to the “swap” chimera protein where the Ig-like domain 2 of the flt-1/IgG protein is replaced with the Ig-like domain 2 of the KDR receptor. Finally, “fltF4.2” refers to the “swap” chimera protein where the Ig-like domain 2 of the flt-1 protein is replaced with the Ig-like domain 2 of the FLT4 receptor.

FIG. 7 shows the percent inhibition of VEGF binding by either unlabeled VEGF competitor or unlabeled PLGF competitor with various flt-1/IgG “swap” chimeric proteins. “flt” and “KDR” designate the native flt-1 and native KDR receptor, respectively. “flt.K1” refers to the “swap” chimera wherein the Ig-like domain 1 of the flt-1 receptor is replaced by the Ig-like domain 1 of the KDR receptor. “flt.K2” refers to the “swap” chimera wherein the Ig-like domain 2 of the flt-1 receptor is replaced by the Ig-like domain 2 of the KDR receptor. “flt.K3” refers to the “swap” chimera wherein the Ig-like domain 3 of the flt-1 receptor is replaced by the Ig-like domain 3 of the KDR receptor. “flt.K5” refers to the “swap” chimera wherein the Ig-like domain 5 of the flt-1 receptor is replaced by the Ig-like domain 5 of the KDR receptor. Finally, “flt.K7” refers to the “swap” chimera wherein the Ig-like domain 7 of the flt-1 receptor is replaced by the Ig-like domain 7 of the KDR receptor.

FIG. 8 shows the entire amino acid sequence of the intact FLT4 receptor. Amino acid residues are presented in their standard one-letter designations.

FIG. 9 shows the entire amino acid sequence of the receptor encoded by the flt-1(1,2,3)/FLT4 expression construct. Underlined amino acid residues are those derived from Ig-like domains 1-3 of the flt-1 receptor and which replace the Ig-like domains 1-3 of the FLT4 receptor. The bolded amino acid residue differs from the wild type FLT4 amino acid residue normally at that position. Amino acid residues are presented in their standard one-letter designations.

FIG. 10 shows the entire amino acid sequence of the receptor encoded by the flt-1(2)/FLT4 expression construct. Underlined amino acid residues are those derived from Ig-like domain 2 of the flt-1 receptor and which replace the Ig-like domain 2 of the FLT4 receptor. The bolded amino acid residues differ from the wild type FLT4 amino acid residues normally at that position. Amino acid residues are presented in their standard one-letter designations.

DETAILED DESCRIPTION

As used herein, the term “chimeric VEGF receptor protein” means a receptor molecule having amino acid sequences derived from at least two different proteins, at least one of which is the flt-1 or KDR receptor, said receptor molecule being capable of binding to and inhibiting the activity of VEGF. Preferably, the chimeric VEGF receptor proteins of the present invention consist of amino acid sequences derived from only two different VEGF receptor molecules, however, amino acid sequences comprising Ig-like domains from the extracellular ligand-binding region of the flt-1 and/or KDR receptor can be linked to amino acid sequences from other unrelated proteins, for example, immunoglobulin sequences. Other amino acid sequences to which Ig-like domains are combined will be readily apparent to those of ordinary skill in the art.

The term “KDR receptor” as used herein is meant to encompass not only the KDR receptor but also the murine homologue of the human KDR receptor, designated FLK-1.

“Immunoglobulin-like domain” or “Ig-like domain” refers to each of the seven independent and distinct domains that are found in the extracellular ligand-binding region of the flt-1, KDR and FLT4 receptors. Ig-like domains are generally referred to by number, the number designating the specific domain as it is shown in FIG. 1. As used herein, the term “Ig-like domain” is intended to encompass not only the complete wild-type domain, but also insertional, deletional and substitutional variants thereof which substantially retain the functional characteristics of the intact domain. It will be readily apparent to those of ordinary skill in the art that numerous variants of the Ig-like domains of the flt-1 and KDR receptors can be obtained which will retain substantially the same functional characteristics as the wild type domain.

“Soluble” as used herein with reference to the chimeric VEGF receptor proteins of the present invention is intended to mean chimeric VEGF receptor proteins which are not fixed to the surface of cells via a transmembrane domain. As such, soluble forms of the chimeric VEGF binding proteins of the present invention, while capable of binding to and inactivating VEGF, do not comprise a transmembrane domain and thus generally do not become associated with the cell membrane of cells in which the molecule is expressed. A soluble form of the receptor exerts an inhibitory effect on the biological activity of the VEGF protein by binding to VEGF, thereby preventing it from binding to its natural receptors present on the surface of target cells.

“Membrane-bound” as used herein with reference to the chimeric VEGF receptor proteins of the present invention is intended to mean chimeric VEGF receptor proteins which are fixed, via a transmembrane domain, to the surface of cells in which they are expressed.

“Functional equivalents” when used in reference to the Ig-like domains of the extracellular ligand-binding regions of the flt-1, KDR or FLT4 receptors means the Ig-like domain or domains possess at least one particular alteration, such as a deletion, addition and/or substitution therein yet retains substantially the same functional characteristics as does the wild type Ig-like domain or domains with reference more specifically to Ig-like domains 1, 2 and 3 of the flt-1 and/or KDR receptor, “functional equivalents” intends scope of so much of such domains as to result in at least substantial binding to VEGF, i.e., a partial sequence of each of said domains that will produce a binding effect.

“Inhibitory effect” when used in reference to the activity of a chimeric VEGF receptor protein of the present invention means that the chimeric VEGF receptor protein binds to and substantially inhibits the activity of VEGF. Generally, the result of this inhibitory effect is a decrease in the vascularization and/or angiogenesis which occurs as a result of the VEGF protein.

“Undesired vascularization” refers to the endothelial proliferation and/or angiogenesis which is associated with an undesirable disease or disorder and which, if reduced or eliminated, would result in a reduction or elimination of the undesirable characteristics of the disease or disorder. For example, the vascularization and/or angiogenesis associated with tumor formation and metastasis and various retinopathies is undesirable.

“Transfection” refers to the taking up of an expression vector by a host cell whether or not any coding sequences are in fact expressed. Numerous methods of transfection are known to the ordinarily skilled artisan, for example, CaPO4 and electroporation. Successful transfection is generally recognized when any indication of the operation of this vector occurs within the host cell.

“Transformation” means introducing DNA into an organism so that the DNA is replicable, either as an extrachromosomal element or by chromosomal integration. Depending on the host cell used, transformation is done using standard techniques appropriate to such cells. The calcium treatment employing calcium chloride, as described by Cohen, S. N., Proc. Natl. Acad. Sci. (USA), 69, 2110 (1972) and Mandel et al. J. Mol. Biol. 53, 154 (1970), is generally used for prokaryotes or other cells that contain substantial cell-wall barriers. For mammalian cells without such cell walls, the calcium phosphate precipitation method of Graham, F. and van der Eb, A., Virology, 52, 456-457 (1978) is preferred. General aspects of mammalian cell host system transformations have been described by Axel in U.S. Pat. No. 4,399,216 issued Aug. 16, 1983. Transformations into yeast are typically carried out according to the method of Van Solingen, P., at al. J. Bact., 130, 946 (1977) and Hsiao, C. L., at al. Proc. Natl. Acad. Sci (USA) 76, 3829 (1979). However, other methods for introducing DNA into cells such as by nuclear injection, electroporation or by protoplast fusion may also be used.

“Site-directed mutagenesis” is a technique standard in the art, and is conducted using a synthetic oligonucleotide primer complementary to a single-stranded phage DNA to be mutagenized except for limited mismatching, representing the desired mutation. Briefly, the synthetic oligonucleotide is used as a primer to direct synthesis of a strand complementary to the phage, and the resulting double-stranded DNA is transformed into a phage-supporting host bacterium. Cultures of the transformed bacteria are plated in top agar, permitting plaque formation from single cells that harbor the phage. Theoretically, 50% of the new plaques will contain the phage having, as a single strand, the mutated form; 50% will have the original sequence. The plaques are hybridized with kinased synthetic primer at a temperature that permits hybridization of an exact match, but at which the mismatches with the original strand are sufficient to prevent hybridization. Plaques that hybridize with the probe are then selected and cultured, and the DNA is recovered.

“Operably linked” refers to juxtaposition such that the normal function of the components can be performed. Thus, a coding sequence “operably linked” to control sequences refers to a configuration wherein the coding sequence can be expressed under the control of these sequences and wherein the DNA sequences being linked are contiguous and, in the case of a secretory leader, contiguous and in reading phase. For example, DNA for a presequence or secretory leader is operably linked to DNA for a polypeptide if it is expressed as a preprotein that participates in the secretion of the polypeptide; a promoter or enhancer is operably linked to a coding sequence if it affects the transcription of the sequence; or a ribosome binding site is operably linked to a coding sequence if it is positioned so as to facilitate translation. Linking is accomplished by ligation at convenient restriction sites. If such sites do not exist, then synthetic oligonucleotide adaptors or linkers are used in accord with conventional practice.

“Control sequences” refers to DNA sequences necessary for the expression of an operably linked coding sequence in a particular host organism. The control sequences that are suitable for prokaryotes, for example, include a promoter, optionally an operator sequence, a ribosome binding site, and possibly, other as yet poorly understood sequences. Eukaryotic cells are known to utilize promoters, polyadenylation signals, and enhancers.

“Expression system” refers to DNA sequences containing a desired coding sequence and control sequences in operable linkage, so that hosts transformed with these sequences are capable of producing the encoded proteins. To effect transformation, the expression system may be included on a vector; however, the relevant DNA may then also be integrated into the host chromosome.

As used herein, “cell,” “cell line,” and “cell culture” are used interchangeably and all such designations include progeny. Thus, “transformants” or “transformed cells” includes the primary subject cell and cultures derived therefrom without regard for the number of transfers. It is also understood that all progeny may not be precisely identical in DNA content, due to deliberate or inadvertent mutations. Mutant progeny that have the same functionality as screened for in the originally transformed cell are included. Where distinct designations are intended, it will be clear from the context.

“Plasmids” are designated by a lower case p preceded and/or followed by capital letters and/or numbers. The starting plasmids herein are commercially available, are publicly available on an unrestricted basis, or can be constructed from such available plasmids in accord with published procedures. In addition, other equivalent plasmids are known in the art and will be apparent to the ordinary artisan.

“Digestion” of DNA refers to catalytic cleavage of the DNA with an enzyme that acts only at certain locations in the DNA. Such enzymes are called restriction enzymes, and the sites for which each is specific is called a restriction site. The various restriction enzymes used herein are commercially available and their reaction conditions, cofactors, and other requirements as established by the enzyme suppliers are used. Restriction enzymes commonly are designated by abbreviations composed of a capital letter followed by other letters representing the microorganism from which each restriction enzyme originally was obtained and then a number designating the particular enzyme. In general, about 1 mg of plasmid or DNA fragment is used with about 1-2 units of enzyme in about 20 ml of buffer solution. Appropriate buffers and substrate amounts for particular restriction enzymes are specified by the manufacturer. Incubation of about 1 hour at 37° C. is ordinarily used, but may vary in accordance with the supplier\'s instructions. After incubation, protein is removed by extraction with phenol and chloroform, and the digested nucleic acid is recovered from the aqueous fraction by precipitation with ethanol. Digestion with a restriction enzyme infrequently is followed with bacterial alkaline phosphatase hydrolysis of the terminal 5′ phosphates to prevent the two restriction cleaved ends of a DNA fragment from “circularizing” or forming a closed loop that would impede insertion of another DNA fragment at the restriction site. Unless otherwise stated, digestion of plasmids is not followed by 5′ terminal dephosphorylation. Procedures and reagents for dephosphorylation are conventional (T. Maniatis et al. 1982, Molecular Cloning: A Laboratory Manual (New York: Cold Spring Harbor Laboratory, 1982) pp. 133-134).

“Recovery” or “isolation” of a given fragment of DNA from a restriction digest means separation of the digest on polyacrylamide or agarose gel by electrophoresis, identification of the fragment of interest by comparison of its mobility versus that of marker DNA fragments of known molecular weight, removal of the gel section containing the desired fragment, and separation of the gel from DNA. This procedure is known generally. For example, see R. Lawn et al., Nucleic Acids Res. 9, 6103-6114 (1981) and D. Goeddel et al., Nucleic Acids Res. 8, 4057 (1980).

“Ligation” refers to the process of forming phosphodiester bonds between two double stranded nucleic acid fragments (T. Maniatis et al. 1982, supra, p. 146). Unless otherwise provided, ligation may be accomplished using known buffers and conditions with 10 units of T4 DNA ligase (“ligase”) per 0.5 mg of approximately equimolar amounts of the DNA fragments to be ligated.

“Preparation” of DNA from transformants means isolating plasmid DNA from microbial culture. Unless otherwise provided, the alkaline/SDS method of Maniatis et al. 1982, supra, p. 90, may be used.

“Oligonucleotides” are short-length, single- or double-stranded polydeoxynucleotides that are chemically synthesized by known methods (such as phosphotriester, phosphite, or phosphoramidite chemistry, using solid phase techniques such as described in EP Pat. Pub. No. 266,032 published May 4, 1988, or via deoxynucleoside H-phosphonate intermediates as described by Froehler et al., Nucl. Acids Res. 14, 5399-5407 [1986]). They are then purified on polyacrylamide gels.

1. Amino Acid Sequence Variants

It will be appreciated that various amino acid substitutions can be made in the Ig-like domain or domains of the chimeric VEGF receptor proteins of the present invention without departing from the spirit of the present invention with respect to the chimeric proteins\' ability to bind to and inhibit the activity of VEGF. Thus, point mutational and other broader variations may be made in the Ig-like domain or domains of the chimeric VEGF receptor proteins of the present invention so as to impart interesting properties that do not substantially effect the chimeric protein\'s ability to bind to and inhibit the activity of VEGF. These variants may be made by means generally known well in the art.

a. Covalent Modifications

Covalent modifications may be made to various amino acid residues of the Ig-like domain or domains present in the chimeric VEGF receptor protein, thereby imparting new properties to that Ig-like domain or domains without eliminating the capability to bind to and inactivate VEGF.

For example, cysteinyl residues most commonly are reacted with a-haloacetates (and corresponding amines), such as chloroacetic acid or chloroacetamide, to give carboxymethyl or carboxyamidomethyl derivatives. Cysteinyl residues also are derivatized by reaction with bromotrifluoroacetone, a-bromo-b-(5-imidozoyl)propionic acid, chloroacetyl phosphate, N-alkylmaleimides, 3-nitro-2-pyridyl disulfide, methyl 2-pyridyl disulfide, p-chloromercuribenzoate, 2-chloromercuri-4-nitrophenol, or chloro-7-nitrobenzo-2-oxa-1,3-diazole.

Histidyl residues are derivatized by reaction with diethylpyrocarbonate at pH 5.5-7.0 because this agent is relatively specific for the histidyl side chain. Para-bromophenacyl bromide also is useful; the reaction is preferably performed in 0.1 M sodium cacodylate at pH 6.0.

Lysinyl and amino terminal residues are reacted with succinic or other carboxylic acid anhydrides. Derivatization with these agents has the effect of reversing the charge of the lysinyl residues. Other suitable reagents for derivatizing a-amino-containing residues include imidoesters such as methyl picolinimidate; pyridoxal phosphate; pyridoxal; chloroborohydride; trinitrobenzenesulfonic acid; O-methylisourea; 2,4-pentanedione; and transaminase-catalyzed reaction with glyoxylate.

Arginyl residues are modified by reaction with one or several conventional reagents, among them phenylglyoxal, 2,3-butanedione, 1,2-cyclohexanedione, and ninhydrin. Derivatization of arginine residues requires that the reaction be performed in alkaline conditions because of the high pKa of the guanidine functional group. Furthermore, these reagents may react with the groups of lysine as well as the arginine epsilon-amino group.

The specific modification of tyrosyl residues per se has been studied extensively, with particular interest in introducing spectral labels into tyrosyl residues by reaction with aromatic diazonium compounds or tetranitromethane. Most commonly, N-acetylimidizol and tetranitromethane are used to form O-acetyl tyrosyl species and 3-nitro derivatives, respectively. Tyrosyl residues are iodinated using 125I or 131I to prepare labeled proteins for use in radioimmunoassay, the chloramine T method described above being suitable.

Carboxyl side groups (aspartyl or glutamyl) are selectively modified by reaction with carbodiimides (R′—N—C—N—R′) such as 1-cyclohexyl-3-(2-morpholinyl-(4-ethyl)carbodiimide or 1-ethyl-3-(4-azonia-4,4-dimethylpentyl)carbodiimide. Furthermore, aspartyl and glutamyl residues are converted to asparaginyl and glutaminyl residues by reaction with ammonium ions.

Derivatization with bifunctional agents is useful for crosslinking the chimeric VEGF receptor protein to a water-insoluble support matrix or surface for use in the method for purifying the VEGF protein from complex mixtures. Commonly used crosslinking agents include, e.g., 1,1-bis(diazoacetyl)-2-phenylethane, glutaraldehyde, N-hydroxysuccinimide esters, for example, esters with 4-azidosalicylic acid, homobifunctional imidoesters, including disuccinimidyl esters such as 3,3′-dithiobis-(succinimidylpropionate), and bifunctional maleimides such as bis-N-maleimido-1,8-octane. Derivatizing agents such as methyl-3-[(p-azidophenyl)dithio]propioimidate yield photoactivatable intermediates that are capable of forming crosslinks in the presence of light. Alternatively, reactive water-insoluble matrices such as cyanogen bromide-activated carbohydrates and the reactive substrates described in U.S. Pat. Nos. 3,969,287; 3,691,016; 4,195,128; 4,247,642; 4,229,537; and 4,330,440 are employed for protein immobilization.

Glutaminyl and asparaginyl residues are frequently deamidated to the corresponding glutamyl and aspartyl residues. Alternatively, these residues are deamidated under mildly acidic conditions. Either form of these residues falls within the scope of this invention.

Other modifications include hydroxylation of proline and lysine, phosphorylation of hydroxyl groups of seryl or threonyl residues, methylation of the a-amino groups of lysine, arginine, and histidine side chains (T. E. Creighton, Proteins: Structure and Molecular Properties, W.H. Freeman & Co., San Francisco, pp. 79-86 [1983]), acetylation of the N-terminal amine, and, in some instances, amidation of the C-terminal carboxyl group.

b. DNA Mutations

Amino acid sequence variants of the Ig-like domain or domains present in the chimeric VEGF receptor proteins of the present invention can also be prepared by creating mutations in the DNA encoding the chimeric protein. Such variants include, for example, deletions from, or insertions or substitutions of, amino acid residues within the amino acid sequence of the Ig-like domain or domains. Any combination of deletion, insertion, and substitution may also be made to arrive at the final construct, provided that the final construct possesses the desired activity. Obviously, the mutations that will be made in the DNA encoding the variant must not place the sequence out of reading frame and preferably will not create complementary regions that could produce secondary mRNA structure (see EP 75,444A).

At the genetic level, variants of the Ig-like domain or domains present in the chimeric VEGF receptor proteins of the present invention ordinarily are prepared by site-directed mutagenesis of nucleotides in the DNA encoding the Ig-like domain or domains, thereby producing DNA encoding the variant, and thereafter expressing the DNA in recombinant cell culture. The variants typically exhibit the same qualitative ability to bind to the VEGF ligand as does the unaltered chimeric protein.

While the site for introducing an amino acid sequence variation in the Ig-like domain or domains of the chimeric VEGF receptor protein is predetermined, the mutation per se need not be predetermined. For example, to optimize the performance of a mutation at a given site, random mutagenesis may be conducted at the target codon or region and the expressed chimeric protein variants screened for the optimal combination of desired attributes such as ability to specifically bind to the VEGF ligand, in vivo half-life, and the like. Techniques for making substitution mutations at predetermined sites in DNA having a known sequence are well known, for example, site-specific mutagenesis.

Preparation of variants in the Ig-like domain or domains of a chimeric VEGF receptor protein in accordance herewith is preferably achieved by site-specific mutagenesis of DNA that encodes an earlier prepared chimeric protein. Site-specific mutagenesis allows the production of Ig-like domain variants through the use of specific oligonucleotide sequences that encode the DNA sequence of the desired mutation, as well as a sufficient number of adjacent nucleotides, to provide a primer sequence of sufficient size and sequence complexity to form a stable duplex on both sides of the deletion junction being traversed. Typically, a primer of about 20 to 25 nucleotides in length is preferred, with about 5 to 10 residues on both sides of the junction of the sequence being altered. In general, the technique of site-specific mutagenesis is well known in the art, as exemplified by publications such as Adelman et al., DNA 2, 183 (1983), the disclosure of which is incorporated herein by reference.

As will be appreciated, the site-specific mutagenesis technique typically employs a phage vector that exists in both a single-stranded and double-stranded form. Typical vectors useful in site-directed mutagenesis include vectors such as the M13 phage, for example, as disclosed by Messing et al., Third Cleveland Symposium on Macromolecules and Recombinant DNA, Editor A. Walton, Elsevier, Amsterdam (1981), the disclosure of which is incorporated herein by reference. These phage are readily commercially available and their use is generally well known to those skilled in the art. Alternatively, plasmid vectors that contain a single-stranded phage origin of replication (Veira at al., Meth. Enzymol., 153, 3 [1987]) may be employed to obtain single-stranded DNA.

In general, site-directed mutagenesis in accordance herewith is performed by first obtaining a single-stranded vector that includes within its sequence a DNA sequence that encodes the relevant chimeric VEGF receptor protein. An oligonucleotide primer bearing the desired mutated sequence is prepared, generally synthetically, for example, by the method of Crea et al., Proc. Natl. Acad. Sci. (USA), 75, 5765 (1978). This primer is then annealed with the single-stranded chimeric protein-sequence-containing vector, and subjected to DNA-polymerizing enzymes such as E. coli polymerase I Klenow fragment, to complete the synthesis of the mutation-bearing strand. Thus, a heteroduplex is formed wherein one strand encodes the original non-mutated sequence and the second strand bears the desired mutation. This heteroduplex vector is then used to transform appropriate cells such as JM101 cells and clones are selected that include recombinant vectors bearing the mutated sequence arrangement.

After such a clone is selected, the mutated DNA encoding the variant chimeric VEGF receptor protein may be removed and placed in an appropriate vector for protein production, generally an expression vector of the type that may be employed for transformation of an appropriate host.

c. Types of Mutations

Amino acid sequence deletions generally range from about 1 to 15 residues, more preferably 1 to 7 residues, and typically are contiguous.

Amino acid sequence insertions include amino- and/or carboxyl-terminal fusions of from one residue to polypeptides of essentially unrestricted length, as well as intrasequence insertions of single or multiple amino acid residues. Intrasequence insertions (i.e., insertions within the Ig-like domain sequences) may range generally from about 1 to 10 residues, more preferably 1 to 5. An example of a terminal insertion includes a fusion of a signal sequence, whether heterologous or homologous to the host cell, to the N-terminus of the chimeric VEGF receptor protein to facilitate the secretion of the chimeric protein from recombinant hosts.

The third group of mutations which can be introduced into the Ig-like domain or domains present in the chimeric VEGF receptor protein are those in which at least one amino acid residue in the Ig-like domain or domains, and preferably only one, has been removed and a different residue inserted in its place. Such substitutions preferably are made in accordance with the following Table 1 when it is desired to modulate finely the characteristics of the Ig-like domain or domains.

TABLE 1 Original Residue Exemplary Substitutions Ala (A) gly; ser Arg (R) lys Asn (N) gln; his Asp (D) glu Cys (C) ser Gln (Q) asn Glu (E) asp Gly (G) ala; pro His (H) asn; gln Ile (I) leu; val Leu (L) ile; val Lys (K) arg; gln; glu Met (M) leu; tyr; ile Phe (F) met; leu; tyr Ser (S) thr Thr (T) ser Trp (W) tyr Tyr (Y) trp; phe Val (V) ile; leu Substantial changes in function or immunological identity are made by selecting substitutions that are less conservative than those in Table I, i.e., selecting residues that differ more significantly in their effect on maintaining (a) the structure of the polypeptide backbone in the area of the substitution, for example, as a sheet or helical conformation, (b) the charge or hydrophobicity of the molecule at the target site, or (c) the bulk of the side chain. The substitutions that in general are expected to produce the greatest changes in the properties of the Ig-like domains will be those in which (a) glycine and/or proline (P) is substituted by another amino acid or is deleted or inserted; (b) a hydrophilic residue, e.g., seryl or threonyl, is substituted for (or by) a hydrophobic residue, e.g., leucyl, isoleucyl, phenylalanyl, valyl, or alanyl; (c) a cysteine residue is substituted for (or by) any other residue; (d) a residue having an electropositive side chain, e.g., lysyl, arginyl, or histidyl, is substituted for (or by) a residue having an electronegative charge, e.g., glutamyl or aspartyl; (e) a residue having an electronegative side chain is substituted for (or by) a residue having an electropositive charge; or (f) a residue having a bulky side chain, e.g., phenylalanine, is substituted for (or by) one not having such a side chain, e.g., glycine.

Most deletions and insertions, and substitutions in particular, are not expected to produce radical changes in the characteristics of the Ig-like domain or domains of the chimeric VEGF receptor protein. However, when it is difficult to predict the exact effect of the substitution, deletion, or insertion in advance of doing so, one skilled in the art will appreciate that the effect will be evaluated by routine screening assays. For example, an Ig-like domain variant typically is made by site-specific mutagenesis of the nucleic acid encoding the intact chimeric VEGF receptor protein, expression of the variant nucleic acid in recombinant cell culture, purification of the variant chimeric VEGF receptor protein from the cell culture and detecting the ability of the variant chimeric VEGF receptor protein to specifically bind to a VEGF ligand. Binding assays which can be routinely employed to determine if a particular alteration or alterations in an Ig-like domain or domains affects the capability of the chimeric VEGF receptor protein to bind to and inhibit the activity of VEGF are described both in the Examples below and in the article by Park et al., J. Biol. Chem. 269:25646-25654 (1994) which is expressly incorporated by reference herein.

Thus, the activity of a variant chimeric VEGF receptor protein may be screened in a suitable screening assay for the desired characteristic. For example, a change in the ability to specifically bind to a VEGF ligand can be measured by a competitive-type VEGF binding assay. Modifications of such protein properties as redox or thermal stability, hydrophobicity, susceptibility to proteolytic degradation, or the tendency to aggregate with carriers or into multimers are assayed by methods well known to the ordinarily skilled artisan.

2. Recombinant Expression

The chimeric VEGF receptor proteins of the present invention are prepared by any technique, including by well known recombinant methods. Likewise, an isolated DNA is understood herein to mean chemically synthesized DNA, cDNA, chromosomal, or extrachromosomal DNA with or without the 3′- and/or 5′-flanking regions. Preferably, the desired chimeric VEGF receptor protein herein is made by synthesis in recombinant cell culture.

For such synthesis, it is first necessary to secure nucleic acid that encodes a chimeric VEGF receptor protein of the present invention. DNA encoding a flt-1 or KDR receptor may be obtained from vascular endothelial cells by (a) preparing a cDNA library from these cells, (b) conducting hybridization analysis with labeled DNA encoding the flt-1 or KDR receptor or fragments thereof (up to or more than 100 base pairs in length) to detect clones in the library containing homologous sequences, and (c) analyzing the clones by restriction enzyme analysis and nucleic acid sequencing to identify full-length clones. If full-length clones are not present in a cDNA library, then appropriate fragments may be recovered from the various clones using the nucleic acid and amino acid sequence information known for the flt-1 and KDR receptors and ligated at restriction sites common to the clones to assemble a full-length clone encoding the flt-1 or KDR domain. Alternatively, genomic libraries may provide the desired DNA.

Once this DNA has been identified and isolated from the library, this DNA may be ligated into an appropriate expression vector operably connected to appropriate control sequences. Moreover, once cloned into an appropriate vector, the DNA can be altered in numerous ways as described above to produce functionally equivalent variants thereof. Additionally, DNA encoding various domains, such as the intracellular, transmembrane and/or various Ig-like domains can be deleted and/or replaced by DNA encoding corresponding domains from other receptors. DNA encoding unrelated amino acid sequences, such as the Fc portion of an immunoglobulin molecule, may also be fused to the DNA encoding some or all of the VEGF receptor, thereby producing a chimeric VEGF receptor molecule.

In one example of a recombinant expression system, an Ig-like domain containing chimeric VEGF receptor protein is expressed in mammalian cells by transformation with an expression vector comprising DNA encoding the chimeric VEGF receptor protein. It is preferable to transform host cells capable of accomplishing such processing so as to obtain the chimeric protein in the culture medium or periplasm of the host cell, i.e., obtain a secreted molecule.

a. Useful Host Cells and Vectors

The vectors and methods disclosed herein are suitable for use in host cells over a wide range of prokaryotic and eukaryotic organisms.

In general, of course, prokaryotes are preferred for the initial cloning of DNA sequences and construction of the vectors useful in the invention.

For example, E. coli K12 strain MM 294 (ATCC No. 31,446) is particularly useful. Other microbial strains that may be used include E. coli strains such as E. coli B and E. coli X1776 (ATCC No. 31,537). These examples are, of course, intended to be illustrative rather than limiting.

Prokaryotes may also be used for expression. The aforementioned strains, as well as E. coli strains W3110 (F-, lambda-, prototrophic, ATCC No. 27,325), K5772 (ATCC No. 53,635), and SR101, bacilli such as Bacillus subtilis, and other enterobacteriaceae such as Salmonella typhimurium or Serratia marcesans, and various pseudomonas species, may be used.

In general, plasmid vectors containing replicon and control sequences that are derived from species compatible with the host cell are used in connection with these hosts. The vector ordinarily carries a replication site, as well as marking sequences that are capable of providing phenotypic selection in transformed cells. For example, E. coli is typically transformed using pBR322, a plasmid derived from an E. coli species (see, e.g., Bolivar et al., Gene 2, 95 [1977]). pBR322 contains genes for ampicillin and tetracycline resistance and thus provides easy means for identifying transformed cells. The pBR322 plasmid, or other microbial plasmid or phage, must also contain, or be modified to contain, promoters that can be used by the microbial organism for expression of its own proteins.

Those promoters most commonly used in recombinant DNA construction include the β-lactamase (penicillinase) and lactose promoter systems (Chang et al., Nature, 375, 615 [1978]; Itakura at al., Science, 198, 1056 [1977]; Goeddel et al., Nature, 281, 544 [1979]) and a tryptophan (trp) promoter system (Goeddel et al., Nucleic Acids Res., 8, 4057 [1980]; EPO Appl. Publ. No. 0036,776). While these are the most commonly used, other microbial promoters have been discovered and utilized, and details concerning their nucleotide sequences have been published, enabling a skilled worker to ligate them functionally with plasmid vectors (see, e.g., Siebenlist et al., Cell, 20, 269 [1980]).

In addition to prokaryotes, eukaryotic microbes, such as yeast cultures, may also be used. Saccharomyces cerevisiae, or common baker\'s yeast, is the most commonly used among eukaryotic microorganisms, although a number of other strains are commonly available. For expression in Saccharomyces, the plasmid YRp7, for example (Stinchcomb et al., Nature 282, 39 [1979]; Kingsman at al., Gene 7, 141 [1979]; Tschemper at al., Gene. 10, 157 [1980]), is commonly used. This plasmid already contains the trp1 gene that provides a selection marker for a mutant strain of yeast lacking the ability to grow in tryptophan, for example, ATCC No. 44,076 or PEP4-1 (Jones, Genetics, 85, 12 [1977]). The presence of the trp1 lesion as a characteristic of the yeast host cell genome then provides an effective environment for detecting transformation by growth in the absence of tryptophan.

Suitable promoting sequences in yeast vectors include the promoters for 3-phosphoglycerate kinase (Hitzeman et al., J. Biol. Chem. 255, 2073 [1980]) or other glycolytic enzymes (Hess et al., J. Adv. Enzyme Reg. 7, 149 [1968]; Holland at al., Biochemistry 17, 4900 [1978]), such as enolase, glyceraldehyde-3-phosphate dehydrogenase, hexokinase, pyruvate decarboxylase, phosphofructokinase, glucose-6-phosphate isomerase, 3-phosphoglycerate mutase, pyruvate kinase, triosephosphate isomerase, phosphoglucose isomerase, and glucokinase. In constructing suitable expression plasmids, the termination sequences associated with these genes are also ligated into the expression vector 3′ of the sequence desired to be expressed to provide polyadenylation of the mRNA and termination. Other promoters, which have the additional advantage of transcription controlled by growth conditions, are the promoter region for alcohol dehydrogenase 2, isocytochrome C, acid phosphatase, degradative enzymes associated with nitrogen metabolism, and the aforementioned glyceraldehyde-3-phosphate dehydrogenase, and enzymes responsible for maltose and galactose utilization. Any plasmid vector containing yeast-compatible promoter, origin of replication and termination sequences is suitable.

In addition to microorganisms, cultures of cells derived from multicellular organisms may also be used as hosts. In principle, any such cell culture is workable, whether from vertebrate or invertebrate culture. However, interest has been greatest in vertebrate cells, and propagation of vertebrate cells in culture (tissue culture) has become a routine procedure in recent years [Tissue Culture, Academic Press, Kruse and Patterson, editors (1973)]. Examples of such useful host cell lines are VERO and HeLa cells, Chinese hamster ovary (CHO) cell lines, and W138, BHK, COS-7, 293, and MDCK cell lines. Expression vectors for such cells ordinarily include (if necessary) an origin of replication, a promoter located in front of the gene to be expressed, along with any necessary ribosome binding sites, RNA splice sites, polyadenylation sites, and transcriptional terminator sequences.

For use in mammalian cells, the control functions on the expression vectors are often provided by viral material. For example, commonly used promoters are derived from polyoma, Adenovirus 2, and most frequently Simian Virus 40 (SV40). The early and late promoters of SV40 virus are particularly useful because both are obtained easily from the virus as a fragment that also contains the SV40 viral origin of replication [Fiers et al., Nature, 273, 113 (1978)]. Smaller or larger SV40 fragments may also be used, provided there is included the approximately 250-bp sequence extending from the Hind111 site toward the BgII site located in the viral origin of replication. Further, it is also possible, and often desirable, to utilize promoter or control sequences normally associated with the desired gene sequence, provided such control sequences are compatible with the host cell systems.

An origin of replication may be provided either by construction of the vector to include an exogenous origin, such as may be derived from SV40 or other viral (e.g., Polyoma, Adeno, VSV, BPV) source, or may be provided by the host cell chromosomal replication mechanism. If the vector is integrated into the host cell chromosome, the latter is often sufficient.

Satisfactory amounts of protein are produced by cell cultures; however, refinements, using a secondary coding sequence, serve to enhance production levels even further. One secondary coding sequence comprises dihydrofolate reductase (DHFR) that is affected by an externally controlled parameter, such as methotrexate (MTX), thus permitting control of expression by control of the methotrexate concentration.

In selecting a preferred host cell for transfection by the vectors of the invention that comprise DNA sequences encoding both chimeric protein and DHFR protein, it is appropriate to select the host according to the type of DHFR protein employed. If wild-type DHFR protein is employed, it is preferable to select a host cell that is deficient in DHFR, thus permitting the use of the DHFR coding sequence as a marker for successful transfection in selective medium that lacks hypoxanthine, glycine, and thymidine. An appropriate host cell in this case is the Chinese hamster ovary (CHO) cell line deficient in DHFR activity, prepared and propagated as described by Urlaub and Chasin, Proc. Natl. Acad. Sci. (USA) 77, 4216 (1980).

On the other hand, if DHFR protein with low binding affinity for MTX is used as the controlling sequence, it is not necessary to use DHFR-deficient cells. Because the mutant DHFR is resistant to methotrexate, MTX-containing media can be used as a means of selection provided that the host cells are themselves methotrexate sensitive. Most eukaryotic cells that are capable of absorbing MTX appear to be methotrexate sensitive. One such useful cell line is a CHO line, CHO-K1 (ATCC No. CCL 61).

b. Typical Methodology Employable

Construction of suitable vectors containing the desired coding and control sequences employs standard ligation techniques. Isolated plasmids or DNA fragments are cleaved, tailored, and religated in the form desired to prepare the plasmids required.