Patent Publication Number: US-2012039986-A1

Title: Compositions and methods for sirna inhibition of angiopoietin 1 and 2 and their receptor tie2

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims priority to U.S. patent application Ser. No. 10/827,759, filed Apr. 19, 2004, which claims the benefit of U.S. Provisional Application No. 60/463,981, filed on Apr. 18, 2003; the entire contents of which is hereby incorporated by reference in its their entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the regulation of angiopoietin 1, angiopoietin 2 and Tie2 gene expression by small interfering RNA, in particular for treating diseases or conditions involving angiogenesis. 
     BACKGROUND OF THE INVENTION 
     Angiogenesis or “neovascularization” is the formation of new blood vessels from the endothelial cells (EC) of preexisting blood vessels. This process involves EC migration, proliferation, and differentiation, which begins with localized breakdown of the basement membrane in the parent vessel. The EC then migrate away from the parent vessel into the interstitial extracellular matrix (ECM) to form a capillary sprout, which elongates due to continued migration and proliferation of the cells. 
     Angiogenesis is typically held under strict control, and under normal conditions occurs only under certain defined physiological processes. For example, angiogenesis occurs during embryogenesis, post-natal growth, wound repair, and menstruation. Uncontrolled angiogenesis, however, can result in pathogenic conditions where the developing blood vessels destroy the surrounding tissue or sustain malignancies. Such pathogenic conditions include diabetic retinopathy, psoriasis, exudative or “wet” age-related macular degeneration (“AMD”), inflammatory disorders, and most cancers. AMD in particular is a clinically important angiogenic disease. This condition is characterized by choroidal neovascularization in one or both eyes in aging individuals, and is the major cause of blindness in industrialized countries. 
     Two key regulators of angiogenesis are angiopoietin-1 (“Ang1”) and angiopoietin-2 (“Ang2”). These regulators can act in concert with vascular endothelial growth factor (“VEGF”) to regulate angiogenesis, although inhibition of Ang1 or Ang2 alone appears to block neovascularization. Ang1, Ang2 and VEGF exert their effect on EC through the two VEGF receptors and another tyrosine kinase receptor called “tyrosine kinase with immunoglobulin and epidermal growth factor homology domains 2” or “Tie2.” Hackett et al. (2002),  J. Cell. Phys.  192: 182-187. Whereas VEGF binding to its receptors is crucial for initiating the angiogenic process, Ang1 and Ang2 bind to Tie2 and modulate maturation of the new blood vessels. Ang1 and Ang2 are also involved in maintaining endothelial cell integrity. Lobov et al. (2002),  Proc. Nat. Acad. Sci USA  99: 11205-11210. As discussed below, agents which bind to and block the Tie2 receptor can also inhibit angiogenesis. 
     Ang1 and Ang2 are differentially expressed, and early studies indicated that Ang1 promoted neovascularization and Ang2 was an angiogenesis antagonist. However, evidence now shows that Ang2 can increase blood vessel diameter and promote remodeling of the basal lamina. Ang2 also appears to induce EC proliferation, migration and sprouting of blood vessels in the presence of VEGF. Lobov et al., 2002, supra. 
     Ang1 reportedly promotes angiogenesis during embryonic development, in particular through the modulation of endothelial-stromal cell communication and by regulating the maturation and stability of blood vessels. Lin P et al.,  Proc. Nat. Acad. Sci. USA  95: 8829-8834 (1998). However, the widespread expression of Ang1 and Tie2 in vascular endothelium, and phosphorylation of Tie2 in quiescent adult vasculature also suggest that Ang1 is involved in postnatal angiogenesis. Takagi et al. (2003),  Inv. Ophthalm. Vis. Sci.  44: 393-402. 
     In contrast to the more extensive expression patterns of Ang1 and Tie2, Ang2 appears to be expressed only at sites of vascular remodeling. Takagi et al. (2003), supra. For example, Ang2 expression is markedly increased in ovary, uterus and placenta during menstruation. Ang2 expression levels also follow a cyclical pattern of expression in the corpus luteum, which parallels the cycle of quiescence, angiogenesis and vascular regression of this structure (i.e., Ang2 levels are low during quiescence and high during angiogenesis and regression). Hackett et al., 2002, supra. Ang2 is also induced by hypoxic cytokines, including VEGF, and is expressed in tissues undergoing pathologic angiogenesis associated with tumors, AMD and in an animal model of retinal ischemia. Takagi et al., 2003, supra. Moreover, Ang2 is upregulated in the epiretinal membranes of patients with ischemic retinal disorders, but not in membranes from patients with non-ischemic retinal disorders. The expression of Ang1, however, remains similar in epiretinal membranes from patients with ischemic or non-ischemic disorders. Takagi et al., 2003, supra. 
     Ang2 and Tie2 are co-localized in the EC of highly vascularized regions, and Tie2 is overexpressed in areas of vascular remodeling. Asahara T. et al.,  Circ. Res.  83: 223-240 report that Ang1 and Ang2 have similar synergistic effects with VEGF to promote angiogenesis in a mouse corneal neovascularization assay. Thus, Ang1, Ang2 and Tie2 play an important role in both normal and pathogenic neovascularization in developing and adult organisms. 
     Ang1, Ang2 or Tie2 are therefore attractive therapeutic targets for treatment of pathogenic angiogenesis. For example, Lin Pet al. (1998), supra, inhibited tumor growth and metastasis in a mouse model by expressing a soluble recombinant Tie2 receptor. The recombinant Tie2 protein blocked ligand binding to endogenous Tie2 receptors, but likely produced only a stoichiometric reduction in Ang2/Tie2 binding. Takagi et al., 2003, supra inhibited of Tie2 signaling with a soluble fusion protein containing the ectoplasmic domain of Tie2, which suppressed hypoxia-induced retinal angiogenesis both in vitro and in vivo. Asahara et al. (1998), supra showed that administration of a soluble Tie2 receptor abolished the effects of Ang1 or Ang2 on VEGF-induced neovascularization in the mouse cornea. However, therapeutic strategies based on agents such as soluble Tie2 receptors are not preferred, however, because such agents would likely be overwhelmed by the high production of Ang2 or Tie2 in the EC of highly vascularized areas. 
     RNA interference (hereinafter “RNAi”) is a method of post-transcriptional gene regulation that is conserved throughout many eukaryotic organisms. RNAi is induced by short (i.e., &lt;30 nucleotide) double stranded RNA (“dsRNA”) molecules which are present in the cell (Fire A et al. (1998),  Nature  391: 806-811). These short dsRNA molecules, called “short interfering RNA” or “siRNA,” cause the destruction of messenger RNAs (“mRNAs”) which share sequence homology with the siRNA to within one nucleotide resolution (Elbashir S M et al. (2001),  Genes Dev,  15: 188-200). It is believed that the siRNA and the targeted mRNA bind to an “RNA-induced silencing complex” or “RISC”, which cleaves the targeted mRNA. The siRNA is apparently recycled much like a multiple-turnover enzyme, with 1 siRNA molecule capable of inducing cleavage of approximately 1000 mRNA molecules. siRNA-mediated RNAi degradation of an mRNA is therefore more effective than currently available technologies for inhibiting expression of a target gene. 
     Elbashir S M et al. (2001), supra, has shown that synthetic siRNA of 21 and 22 nucleotides in length, and which have short 3′ overhangs, are able to induce RNAi of target mRNA in a  Drosophila  cell lysate. Cultured mammalian cells also exhibit RNAi degradation with synthetic siRNA (Elbashir S M et al. (2001)  Nature,  411: 494-498), and RNAi degradation induced by synthetic siRNA has recently been shown in living mice (McCaffrey A P et al. (2002),  Nature,  418: 38-39; Xia H et al. (2002),  Nat. Biotech.  20: 1006-1010). The therapeutic potential of siRNA-induced RNAi degradation has been demonstrated in several recent in vitro studies, including the siRNA-directed inhibition of HIV-1 infection (Novina C D et al. (2002),  Nat. Med.  8: 681-686) and reduction of neurotoxic polyglutamine disease protein expression (Xia H et al. (2002), supra). 
     What is needed, therefore, are agents and methods which selectively inhibit expression of Ang1, Ang2 or Tie2 in catalytic or sub-stoichiometric amounts, in order to effectively decrease or block angiogenesis. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to siRNA which specifically target and cause RNAi-induced degradation of mRNA from Ang1, Ang2 or Tie2 genes. These siRNA degrade Ang1, Ang2 or Tie2 mRNA in substoichiometric amounts. The siRNA compounds and compositions of the invention are, thus used to inhibit angiogenesis. In particular, the siRNA of the invention are useful for treating cancerous tumors and disorders related to ocular neovascularization, such as age-related macular degeneration and diabetic retinopathy. 
     Thus, the invention provides an isolated siRNA which targets human Ang1, Ang2 or Tie2 mRNA, or an alternative splice form, mutant or cognate thereof. The siRNA comprises a sense RNA strand and an antisense RNA strand which form an RNA duplex. The sense RNA strand comprises a nucleotide sequence substantially identical to a target sequence of about 19 to about 25 contiguous nucleotides in the target mRNA. 
     The invention also provides recombinant plasmids and viral vectors which express the siRNA of the invention, as well as pharmaceutical compositions comprising the siRNA of the invention and a pharmaceutically acceptable carrier. 
     The invention further provides a method of inhibiting expression of human Ang1, Ang2 or Tie2 mRNA, or an alternative splice form, mutant or cognate thereof, comprising administering to a subject an effective amount of the siRNA of the invention such that the target mRNA is degraded. 
     The invention further provides a method of inhibiting angiogenesis in a subject, comprising administering to a subject an effective amount of an siRNA targeted to human Ang1, Ang2 or Tie2 mRNA, or an alternative splice form, mutant or cognate thereof. 
     The invention further provides a method of treating an angiogenic disease, comprising administering to a subject in need of such treatment an effective amount of an siRNA targeted to human Ang1, Ang2 or Tie2 mRNA, or an alternative splice form, mutant or cognate thereof, such that angiogenesis associated with the angiogenic disease is inhibited. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a histogram showing the silencing effect of siRNA candidates, as measured by the levels of human angiopoietin 2 (“hANG2”) protein in growth medium removed from tissue culture wells containing HEK-293 cells transfected with: twelve different siRNA targeted to hANG2 mRNA (hANG2#1-hANG2#12); with control nonspecific siRNA targeted to enhanced green fluorescent protein (“EGFP”); or with transfection reagent containing no siRNA (“no”). hANG2 protein level is given in picograms of protein per milliliter of growth medium (pg/ml), as measured by hANG2 ELISA at 48 hours post-transfection. 
         FIG. 2  is a histogram showing lack of cytotoxicity in HEK-293 cells transfected with twelve different siRNA targeted to hANG2 mRNA (hANG2#1-hANG2#12). Control cells were transfected with nonspecific siRNA targeted to enhanced green fluorescent protein mRNA (“EGFP”), or with transfection reagent containing no siRNA (“no”). Cytotoxicity is measured as percent growth of cells treated with siRNA vs. cells treated with transfection reagent alone. 
         FIG. 3  is a histogram showing the silencing effect of increasing doses of hANG2#2 and hANG2#3 on the level of hANG2 protein secreted by HEK-293 cells. The HEK-293 cells were transfected with 1 nanomolar (“nM”), 5 nM, or 25 nM hANG2#2 or hANG2#3 siRNA. Control cells were transfected with 25 nM nonspecific siRNA targeted to enhanced green fluorescent protein mRNA (“EGFP”), or with transfection reagent containing no siRNA (“no”). hANG2 protein level is given in picograms of protein per milliliter of growth medium (pg/ml), as measured by hANG2 ELISA at 48 hours post-transfection. 
         FIG. 4  is a histogram showing lack of cytotoxicity in HEK-293 cells transfected with increasing doses of hANG2#2 and hANG2#3 siRNA. Control cells were transfected with nonspecific siRNA targeted to enhanced green fluorescent protein mRNA (“EGFP”), or with transfection reagent containing no siRNA (“no”). Cytotoxicity is measured as percent growth of cells treated with siRNA vs. cells treated with transfection reagent alone. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Unless otherwise indicated, all nucleic acid sequences herein are given in the 5′ to 3′ direction. Also, all deoxyribonucleotides in a nucleic acid sequence are represented by capital letters (e.g., deoxythymidine is “T”), and ribonucleotides in a nucleic acid sequence are represented by lower case letters (e.g., uridine is “u”). 
     Compositions and methods comprising siRNA targeted to Ang1, Ang2 and Tie2 mRNA are advantageously used to inhibit angiogenesis, in particular for the treatment of angiogenic disease. The siRNA of the invention are believed to cause the RNA1-mediated degradation of these mRNAs, so that the protein products of the Ang1, Ang2 or Tie2 genes are not produced or are produced in reduced amounts. Because Ang1, Ang2 and Tie2 are involved in angiogenesis, the siRNA-mediated degradation of Ang1, Ang2 or Tie2 mRNA inhibits the angiogenic process. 
     As used herein, siRNA which is “targeted to the Ang1, Ang2 or Tie2 mRNA” means siRNA in which a first strand of the duplex is substantially identical to the nucleotide sequence of a portion of the Ang1, Ang2 or Tie2 mRNA sequence. It is understood that the second strand of the siRNA duplex is complementary to both the first strand of the siRNA duplex and to the same portion of the Ang1, Ang2 or Tie2 mRNA. 
     The invention therefore provides isolated siRNA comprising short double-stranded RNA from about 17 nucleotides to about 29 nucleotides in length, preferably from about 19 to about 25 nucleotides in length, that are targeted to the target mRNA. The siRNA&#39;s comprise a sense RNA strand and a complementary antisense RNA strand annealed together by standard Watson-Crick base-pairing interactions (hereinafter “base-paired”). As is described in more detail below, the sense strand comprises a nucleic acid sequence which is substantially identical to a target sequence contained within the target mRNA. 
     As used herein, a nucleic acid sequence “substantially identical” to a target sequence contained within the target mRNA is a nucleic acid sequence which is identical to the target sequence, or which differs from the target sequence by one or more nucleotides. Sense strands of the invention which comprise nucleic acid sequences substantially identical to a target sequence are characterized in that siRNA comprising such sense strands induce RNAi-mediated degradation of mRNA containing the target sequence. For example, an siRNA of the invention can comprise a sense strand comprise nucleic acid sequences which differ from a target sequence by one, two or three or more nucleotides, as long as RNAi-mediated degradation of the target mRNA is induced by the siRNA. 
     The sense and antisense strands of the present siRNA can comprise two complementary, single-stranded RNA molecules or can comprise a single molecule in which two complementary portions are base-paired and are covalently linked by a single-stranded “hairpin” area. Without wishing to be bound by any theory, it is believed that the hairpin area of the latter type of siRNA molecule is cleaved intracellularly by the “Dicer” protein (or its equivalent) to form a siRNA of two individual base-paired RNA molecules (see Tuschl, T. (2002), supra). As described below, the siRNA can also contain alterations, substitutions or modifications of one or more ribonucleotide bases. For example, the present siRNA can be altered, substituted or modified to contain one or more deoxyribonucleotide bases. 
     As used herein, “isolated” means synthetic, or altered or removed from the natural state through human intervention. For example, an siRNA naturally present in a living animal is not “isolated,” but a synthetic siRNA, or an siRNA partially or completely separated from the coexisting materials of its natural state is “isolated.” An isolated siRNA can exist in substantially purified form, or can exist in a non-native environment such as, for example, a cell into which the siRNA has been delivered. By way of example, siRNA which are produced inside a cell by natural processes, but which are produced from an “isolated” precursor molecule, are themselves “isolated” molecules. Thus, an isolated dsRNA can be introduced into a target cell, where it is processed by the Dicer protein (or its equivalent) into isolated siRNA. 
     As used herein, “target mRNA” means human Ang1, Ang2 or Tie2 mRNA, mutant or alternative splice forms of human Ang1, Ang2 or Tie2 mRNA, or mRNA from cognate Ang1, Ang2 or Tie2 genes. The human Ang1, Ang2 and Tie2 mRNA sequences are described in GenBank Record Accession Nos. AY124380, NM — 00147 and L06139, respectively, as the cDNA equivalents. The human Ang1, Ang2 and Tie2 mRNA sequences are given herein as SEQ ID NO: 1, SEQ ID NO: 2 and SEQ ID NO: 3, respectively, as the cDNA equivalents. One skilled in the art would understand that the cDNA sequence is equivalent to the mRNA sequence, and can be used for the same purpose herein; i.e., the generation of siRNA for inhibiting expression of Ang1, Ang2, or Tie2. 
     As used herein, a gene or mRNA which is “cognate” to human Ang1, Ang2 or Tie2 is a gene or mRNA from another mammalian species which is homologous to human Ang1, Ang2 or Tie2. For example, the partial sequence of Ang1 mRNA for the domesticated dog ( Canis familiaris ) is described as the cDNA equivalent in GenBank Record Accession No. AF345932, which is given herein as SEQ ID NO: 4. The  Mus musculus  (mouse) Ang2 mRNA is described as the cDNA equivalent in GenBank Record Accession No. NM — 007426, which is given herein as SEQ ID NO. 5. The  Mus musculus  (mouse) and  Rattus norvegicus  (rat) Tie2 mRNA sequences are described as the cDNA equivalents in GenBank Record Accession Nos. NM — 013690 and NW — 043856, respectively. The mouse and rat Tie2 mRNA sequences are given herein as SEQ ID NO. 6 and SEQ ID NO. 7, respectively. 
     Alternative splice forms of human Ang1, Ang2 and Tie2 are also known. See, e.g., GenBank Record Accession No. AY121504, which describes a splice variant of human Ang1 as the cDNA equivalent (SEQ ID NO: 8). Kim et al.,  J. Biol. Chem.  275 (24), 18550-18556 (2000) and GenBank Record Accession No. AF187858 describe an Ang2 splice variant encoding an Ang2 protein lacking amino acids 96-148, given as the cDNA equivalent (SEQ 1D NO: 9). See also GenBank Record Accession No. AB086825, which describes a splice variant of Tie2 encoding a Tie2 protein lacking the epidermal growth factor-like domain, given as the cDNA equivalent (SEQ ID NO: 10). 
     The mRNA transcribed from the human Ang1, Ang2 or Tie2 genes can also be analyzed for alternative splice forms using techniques well-known in the art. Such techniques include reverse transcription-polymerase chain reaction (RT-PCR), northern blotting and in-situ hybridization. Techniques for analyzing mRNA sequences are described, for example, in Busting S A (2000),  J. Mol. Endocrinol.  25: 169-193, the entire disclosure of which is herein incorporated by reference. Representative techniques for identifying alternatively spliced mRNAs are also described below. 
     For example, databases that contain nucleotide sequences related to a given disease gene can be used to identify alternatively spliced mRNA. Such databases include GenBank, Embase, and the Cancer Genome Anatomy Project (CGAP) database. The CGAP database, for example, contains expressed sequence tags (ESTs) from various types of human cancers. An mRNA or gene sequence from the Ang1, Ang2 or Tie2 genes can be used to query such a database to determine whether ESTs representing alternatively spliced mRNAs have been found. 
     A technique called “RNAse protection” can also be used to identify alternatively spliced Ang1, Ang2 or Tie2 mRNAs. RNAse protection involves translation of a gene sequence into synthetic RNA, which is hybridized to RNA derived from other cells; for example, cells which are induced to express Ang1, Ang2 or Tie2. The hybridized RNA is then incubated with enzymes that recognize RNA:RNA hybrid mismatches. Smaller than expected fragments indicate the presence of alternatively spliced mRNAs. The putative alternatively spliced mRNAs can be cloned and sequenced by methods well known to those skilled in the art. 
     RT-PCR can also be used to identify alternatively spliced Ang1, Ang2 or Tie2 mRNAs. In RT-PCR, mRNA from vascular endothelial cells or cells from other tissue known to express Ang1, Ang2 or Tie2 is converted into cDNA by the enzyme reverse transcriptase, using methods within the skill in the art. The entire coding sequence of the cDNA is then amplified via PCR using a forward primer located in the 3′ untranslated region, and a reverse primer located in the 5′ untranslated region. The amplified products can be analyzed for alternative splice forms, for example by comparing the size of the amplified products with the size of the expected product from normally spliced mRNA, e.g., by agarose gel electrophoresis. Any change in the size of the amplified product can indicate alternative splicing. 
     The mRNA produced from mutant Ang1, Ang2 or Tie2 genes can also be readily identified with the techniques described above for identifying Ang1, Ang2 or Tie2 alternative splice forms. As used herein, “mutant” Ang1, Ang2 or Tie2 genes or mRNA include human Ang1, Ang2 or Tie2 genes or mRNA which differ in sequence from the Ang1, Ang2 and Tie2 sequences set forth herein. Thus, allelic forms of the Ang1, Ang2 or Tie2 genes, and the mRNA produced from them, are considered “mutants” for purposes of this invention. See also WO 02/20734, which describes several mutants of Tie2, one of which is described in GenBank Record Accession No. AX398356, which is given herein as the cDNA equivalent in SEQ ID NO: 11. 
     It is understood that human Ang1, Ang2 or Tie2 mRNA may contain target sequences in common with its respective alternative splice forms, cognates or mutants. A single siRNA comprising such a common targeting sequence can therefore induce RNAi-mediated degradation of those different mRNAs which contain the common targeting sequence. 
     The siRNA of the invention can comprise partially purified RNA, substantially pure RNA, synthetic RNA, or recombinantly produced RNA, as well as altered RNA that differs from naturally-occurring RNA by the addition, deletion, substitution and/or alteration of one or more nucleotides. Such alterations can include addition of non-nucleotide material, such as to the end(s) of the siRNA or to one or more internal nucleotides of the siRNA; modifications that make the siRNA resistant to nuclease digestion (e.g., the use of 2′-substituted ribonucleotides or modifications to the sugar-phosphate backbone); or the substitution of one or more nucleotides in the siRNA with deoxyribonucleotides. siRNA which are exposed to serum, lachrymal fluid or other nuclease-rich environments, or which are delivered topically (e.g., by eyedropper), are preferably altered to increase their resistance to nuclease degradation. For example, siRNA which are administered intravascularly or topically to the eye can comprise one or more phosphorothioate linkages. 
     One or both strands of the siRNA of the invention can also comprise a 3′ overhang. As used herein, a “3′ overhang” refers to at least one unpaired nucleotide extending from the 3′-end of an RNA strand. 
     Thus in one embodiment, the siRNA of the invention comprises at least one 3′ overhang of from 1 to about 6 nucleotides (which includes ribonucleotides or deoxynucleotides) in length, preferably from 1 to about 5 nucleotides in length, more preferably from 1 to about 4 nucleotides in length, and particularly preferably from about 2 to about 4 nucleotides in length. 
     In the embodiment in which both strands of the siRNA molecule comprise a 3′ overhang, the length of the overhangs can be the same or different for each strand. In a most preferred embodiment, the 3′ overhang is present on both strands of the siRNA, and is 2 nucleotides in length. For example, each strand of the siRNA of the invention can comprise 3′ overhangs of dithymidylic acid (“TT”) or diuridylic acid (“uu”). 
     In order to enhance the stability of the present siRNA, the 3′ overhangs can be also stabilized against degradation. In one embodiment, the overhangs are stabilized by including purine nucleotides, such as adenosine or guanosine nucleotides. Alternatively, substitution of pyrimidine nucleotides by modified analogues, e.g., substitution of uridine nucleotides in the 3′ overhangs with 2′-deoxythymidine, is tolerated and does not affect the efficiency of RNAi degradation. In particular, the absence of a 2′-hydroxyl in the 2′-deoxythymidine significantly enhances the nuclease resistance of the 3′ overhang in tissue culture medium. 
     In certain embodiments, the siRNA of the invention comprises the sequence AA(N19)TT or NA(N21), where N is any nucleotide. These siRNA comprise approximately 30-70% GC, and preferably comprise approximately 50% G/C. The sequence of the sense siRNA strand corresponds to (N19)TT or N21 (i.e., positions 3 to 23), respectively. In the latter case, the 3′ end of the sense siRNA is converted to TT. The rationale for this sequence conversion is to generate a symmetric duplex with respect to the sequence composition of the sense and antisense strand 3′ overhangs. The antisense RNA strand is then synthesized as the complement to positions 1 to 21 of the sense strand. 
     Because position 1 of the 23-nucleotide sense strand in these embodiments is not recognized in a sequence-specific manner by the antisense strand, the 3′-most nucleotide residue of the antisense strand can be chosen deliberately. However, the penultimate nucleotide of the antisense strand (complementary to position 2 of the 23-nucleotide sense strand in either embodiment) is generally complementary to the targeted sequence. 
     In another embodiment, the siRNA of the invention comprises the sequence NAR(N17)YNN, where R is a purine (e.g., A or G) and Y is a pyrimidine (e.g., C or u/T). The respective 21-nucleotide sense and antisense RNA strands of this embodiment therefore generally begin with a purine nucleotide. Such siRNA can be expressed from pol III expression vectors without a change in targeting site, as expression of RNAs from pol III promoters is only believed to be efficient when the first transcribed nucleotide is a purine. 
     The siRNA of the invention can be targeted to any stretch of approximately 19-25 contiguous nucleotides in any of the target mRNA sequences (the “target sequence”). Techniques for selecting target sequences for siRNA&#39;s are given, for example, in Tuschl T et al., “The siRNA User Guide,” revised Oct. 11, 2002, the entire disclosure of which is herein incorporated by reference. “The siRNA User Guide” is available on the world wide web at a website maintained by Dr. Thomas Tuschl, Department of Cellular Biochemistry, AG 105, Max-Planck-Institute for Biophysical Chemistry, 37077 Göttingen, Germany, and can be found by accessing the website of the Max Planck Institute and searching with the keyword “siRNA.” Thus, the sense strand of the present siRNA comprises a nucleotide sequence substantially identical to any contiguous stretch of about 19 to about 25 nucleotides in the target mRNA. 
     Generally, a target sequence on the target mRNA can be selected from a given cDNA sequence corresponding to the target mRNA, preferably beginning 50 to 100 nt downstream (i.e., in the 3′ direction) from the start codon. The target sequence can, however, be located in the 5′ or 3′ untranslated regions, or in the region nearby the start codon. For example, a suitable target sequence in the human Ang2 cDNA sequence is: 
     
       
         
           
               
               
            
               
                   
                 (SEQ ID NO: 12) 
               
               
                   
                 AATGCTGTGCAGAGGGACGCG 
               
            
           
         
       
     
     Thus, an siRNA of the invention targeting SEQ ID NO: 12, and which has 3′ uu overhangs on each strand (overhangs shown in bold), is: 
     
       
         
           
               
               
            
               
                   
                 (SEQ ID NO: 13) 
               
               
                   
                 5′-tgctgtgcagagggacgcg uu -3′ 
               
               
                   
                   
               
               
                   
                 (SEQ ID NO: 14) 
               
               
                   
                 3′- uu ucgacacgucucccugcgc-5′ 
               
            
           
         
       
     
     An siRNA of the invention targeting SEQ ID NO: 12, but having 3′ TT overhangs on each strand (overhangs shown in bold) is: 
     
       
         
           
               
               
            
               
                   
                 (SEQ ID NO: 15) 
               
               
                   
                 5′-tgctgtgcagagggacgcg TT -3′ 
               
               
                   
                   
               
               
                   
                 (SEQ ID NO: 16) 
               
               
                   
                 3′- TT ucgacacgucucccugcgc-5′ 
               
            
           
         
       
     
     Another target sequence from the human Ang2 cDNA sequence is: 
     
       
         
           
               
               
            
               
                   
                 (SEQ ID NO: 17) 
               
               
                   
                 AAGTATTAAATCAGACCACGA 
               
            
           
         
       
     
     Thus, an siRNA of the invention targeting SEQ ID NO: 17, and which has 3′ uu overhangs on each strand (overhangs shown in bold), is: 
     
       
         
           
               
               
            
               
                   
                 (SEQ ID NO: 18) 
               
               
                   
                 5′-gtattaaatcagaccacga uu -3′ 
               
               
                   
                   
               
               
                   
                 (SEQ ID NO: 19) 
               
               
                   
                 3′- uu cauaauuuagucuggugcu-5′ 
               
            
           
         
       
     
     An siRNA of the invention targeting SEQ ID NO: 17, but having 3′ TT overhangs on each strand (overhangs shown in bold) is: 
     
       
         
           
               
               
            
               
                   
                 (SEQ ID NO: 20) 
               
               
                   
                 5′-gtattaaatcagaccacga TT -3′ 
               
               
                   
                   
               
               
                   
                 (SEQ ID NO: 21) 
               
               
                   
                 3′- TT cauaauuuagucuggugcu-5′ 
               
            
           
         
       
     
     A suitable target sequence in the human Ang1 cDNA sequence is: 
     
       
         
           
               
               
            
               
                   
                 (SEQ ID NO: 22) 
               
               
                   
                 AATGCAGTTCAGAACCACACG 
               
            
           
         
       
     
     Thus, an siRNA of the invention targeting SEQ ID NO: 22, and which has 3′ uu overhangs on each strand (overhangs shown in bold), is: 
     
       
         
           
               
               
            
               
                   
                 (SEQ ID NO: 23) 
               
               
                   
                 5′-tgcagttcagaaccacacg uu -3′ 
               
               
                   
                   
               
               
                   
                 (SEQ ID NO: 24) 
               
               
                   
                 3′- uu  acgucaagucuuggugugc-5′ 
               
            
           
         
       
     
     An siRNA of the invention targeting SEQ ID NO: 22, but having 3′ TT overhangs on each strand (overhangs shown in bold) is: 
     
       
         
           
               
               
            
               
                   
                 (SEQ ID NO: 25) 
               
               
                   
                 5′-tgcagttcagaaccacacg TT -3′ 
               
               
                   
                   
               
               
                   
                 (SEQ ID NO: 26) 
               
               
                   
                 3′- TT acgucaagucuuggugugc-5′ 
               
            
           
         
       
     
     Another target sequence from the human Ang1 cDNA is: 
     
       
         
           
               
               
            
               
                   
                 (SEQ ID NO: 27) 
               
               
                   
                 AACTTCTCGACTTGAGATACA 
               
            
           
         
       
     
     An siRNA of the invention targeting SEQ ID NO: 27, but having 3′ uu overhangs on each strand (overhangs shown in bold) is: 
     
       
         
           
               
               
            
               
                   
                 (SEQ ID NO: 28) 
               
               
                   
                 5′-cttctcgacttgagataca uu -3′ 
               
               
                   
                   
               
               
                   
                 (SEQ ID NO: 29) 
               
               
                   
                 3′- uu gaagagcugaacucuaugu-5′ 
               
            
           
         
       
     
     An siRNA of the invention targeting SEQ ID NO: 27, but having 3′ TT overhangs on each strand (overhangs shown in bold) is: 
     
       
         
           
               
               
            
               
                   
                 (SEQ ID NO: 30) 
               
               
                   
                 5′-cttctcgacttgagataca TT -3′ 
               
               
                   
                   
               
               
                   
                 (SEQ ID NO: 31) 
               
               
                   
                 3′- TT gaagagcugaacucuaugu-5′ 
               
            
           
         
       
     
     Other Ang1, Ang2 and Tie2 target sequences, from which siRNA of the invention can be derived, include those given herein: Suitable human Ang1 target sequences include those of SEQ ID NOS; 32-227; suitable human Ang2 target sequences include those of SEQ ID NOS: 228-427; and suitable human Tie2 target sequences include those of SEQ ID NOS: 428-739. It is understood that the target sequences given herein are with reference to the human Ang1, Ang2 or Tie2 cDNA, and thus these sequences contain deoxythymidines represented by “T.” One skilled in the art would understand that, in the actual target sequence of the mRNA, the deoxythymidines would be replaced by uridines (“u”). Likewise, a target sequence contained within an siRNA of the invention would also contain uridines in place of deoxythymidines. 
     The siRNA of the invention can be obtained using a number of techniques known to those of skill in the art. For example, the siRNA, can be chemically synthesized or recombinantly produced using methods known in the art, such as the.  Drosophila  in vitro system described in U.S. published application 2002/0086356 of Tuschl et al., the entire disclosure of which is herein incorporated by reference. 
     Preferably, the siRNA of the invention are chemically synthesized using appropriately protected ribonucleoside phosphoramidites and a conventional DNA/RNA synthesizer. The siRNA can be synthesized as two separate, complementary RNA molecules, or as a single RNA molecule with two complementary regions. Commercial suppliers of synthetic RNA molecules or synthesis reagents include Proligo (Hamburg, Germany), Dharmacon Research (Lafayette, Colo., USA), Pierce Chemical (part of Perbio Science, Rockford, Ill., USA), Glen Research (Sterling, Va., USA), ChemGenes (Ashland, Mass., USA) and Cruachem (Glasgow, UK). 
     Alternatively, siRNA can also be expressed from recombinant circular or linear DNA plasmids using any suitable promoter. Suitable promoters for expressing siRNA of the invention from a plasmid include, for example, the U6 or H1 RNA pol III promoter sequences and the cytomegalovirus promoter. Selection of other suitable promoters is within the skill in the art. The recombinant plasmids of the invention can also comprise inducible or regulatable promoters for expression of the siRNA in a particular tissue or in a particular intracellular environment. 
     The siRNA expressed from recombinant plasmids can either be isolated from cultured cell expression systems by standard techniques, or can be expressed intracellularly. The use of recombinant plasmids to deliver siRNA of the invention to cells in vivo is discussed in more detail below. 
     siRNA of the invention can be expressed from a recombinant plasmid either as two separate, complementary RNA molecules, or as a single RNA molecule with two complementary regions. 
     Selection of plasmids suitable for expressing siRNA of the invention, methods for inserting nucleic acid sequences for expressing the siRNA into the plasmid, and methods of delivering the recombinant plasmid to the cells of interest are within the skill in the art. See, for example Tuschl, T. (2002),  Nat. Biotechnol,  20: 446-448; Brummelkamp T R et al. (2002),  Science  296: 550-553; Miyagishi M et al. (2002),  Nat. Biotechnol.  20: 497-500; Paddison P J et al. (2002),  Genes Dev.  16: 948-958; Lee N S et al. (2002),  Nat. Biotechnol.  20: 500-505; and Paul C P et al. (2002),  Nat. Biotechnol.  20: 505-508, the entire disclosures of which are herein incorporated by reference. 
     In one embodiment, a plasmid expressing an siRNA of the invention comprises a sense RNA strand coding sequence in operable connection with a polyT termination sequence under the control of a human U6 RNA promoter, and an antisense RNA strand coding sequence in operable connection with a polyT termination sequence under the control of a human U6 RNA promoter. Such a plasmid can be used in producing an recombinant adeno-associated viral vector for expressing an siRNA of the invention. 
     As used herein, “in operable connection with a polyT termination sequence” means that the nucleic acid sequences encoding the sense or antisense strands are immediately adjacent to the polyT termination signal in the 5′ direction. During transcription of the sense or antisense sequences from the plasmid, the polyT termination signals act to terminate transcription. 
     As used herein, “under the control” of a promoter means that the nucleic acid sequences encoding the sense or antisense strands are located 3′ of the promoter, so that the promoter can initiate transcription of the sense or antisense coding sequences. 
     The siRNA of the invention can also be expressed from recombinant viral vectors intracellularly in vivo. The recombinant viral vectors of the invention comprise sequences encoding the siRNA of the invention and any suitable promoter for expressing the siRNA sequences. Suitable promoters include, for example, the U6 or HI RNA pol III promoter sequences and the cytomegalovirus promoter. Selection of other suitable promoters is within the skill in the art. The recombinant viral vectors of the invention can also comprise inducible or regulatable promoters for expression of the siRNA in a particular tissue or in a particular intracellular environment. The use of recombinant viral vectors to deliver siRNA of the invention to cells in vivo is discussed in more detail below. 
     siRNA of the invention can be expressed from a recombinant viral vector either as two separate, complementary RNA molecules, or as a single RNA molecule with two complementary regions. 
     Any viral vector capable of accepting the coding sequences for the siRNA molecule(s) to be expressed can be used, for example vectors derived from adenovirus (AV); adeno-associated virus (AAV); retroviruses (e.g, lentiviruses (LV), Rhabdoviruses, murine leukemia virus); herpes virus, and the like. The tropism of viral vectors can be modified by pseudotyping the vectors with envelope proteins or other surface antigens from other viruses, or by substituting different viral capsid proteins, as appropriate. 
     For example, lentiviral vectors of the invention can be pseudotyped with surface proteins from vesicular stomatitis virus (VSV), rabies, Ebola, Mokola, and the like. AAV vectors of the invention can be made to target different cells by engineering the vectors to express different capsid protein serotypes. For example, an AAV vector expressing a serotype 2 capsid on a serotype 2 genome is called AAV 2/2. This serotype 2 capsid gene in the AAV 2/2 vector can be replaced by a serotype 5 capsid gene to produce an AAV 2/5 vector. Techniques for constructing AAV vectors which express different capsid protein serotypes are within the skill in the art; see, e.g., Rabinowitz J E et al. (2002),  J Virol  76:791-801, the entire disclosure of which is herein incorporated by reference. 
     Selection of recombinant viral vectors suitable for use in the invention, methods for inserting nucleic acid sequences for expressing the siRNA into the vector, and methods of delivering the viral vector to the cells of interest are within the skill in the art. See, for example, Dornburg R (1995),  Gene Therap.  2: 301-310; Eglitis M A (1988),  Biotechniques  6: 608-614; Miller A D (1990),  Hum Gene Therap.  1: 5-14; Anderson W F (1998),  Nature  392: 25-30; and Rubinson D A et al.,  Nat. Genet.  33: 401-406, the entire disclosures of which are herein incorporated by reference. 
     Preferred viral vectors are those derived from AV and AAV. In a particularly preferred embodiment, the siRNA of the invention is expressed as two separate, complementary single-stranded RNA molecules from a recombinant AAV vector comprising, for example, either the U6 or H1 RNA promoters, or the cytomegalovirus (CMV) promoter. 
     A suitable AV vector for expressing the siRNA of the invention, a method for constructing the recombinant AV vector, and a method for delivering the vector into target cells, are described in Xia H et al. (2002),  Nat. Biotech.  20: 1006-1010. 
     Suitable AAV vectors for expressing the siRNA of the invention, methods for constructing the recombinant AV vector, and methods for delivering the vectors into target cells are described in Samulski R et al. (1987),  J. Virol.  61: 3096-3101; Fisher K J et al. (1996),  J. Virol.,  70: 520-532; Samulski R et al. (1989).  J. Virol.  63: 3822-3826; U.S. Pat. No. 5,252,479; U.S. Pat. No. 5,139,941; International Patent Application No. WO 94/13788; and International Patent Application No. WO 93/24641, the entire disclosures of which are herein incorporated by reference. 
     The ability of an siRNA containing a given target sequence to cause RNAi-mediated degradation of the target mRNA can be evaluated using standard techniques for measuring the levels of RNA or protein in cells. For example, siRNA of the invention can be delivered to cultured cells, and the levels of target mRNA can be measured by Northern blot or dot blotting techniques, or by quantitative RT-PCR. Alternatively, the levels of Ang1, Ang2 or Tie2 protein in the cultured cells can be measured by ELISA or Western blot. Suitable protocols for the delivery of siRNA to cultured cells, and assays for detecting protein and mRNA levels in cultured cells, are given in the Examples below. 
     For example, cells which naturally express Ang1, Ang2 or Tie2, or which are induced to express Ang1, Ang2 or Tie2, are grown to confluence in suitable cell culture vessels; e.g., 12- or 25-well culture plates or 96-well microtiter plates. siRNA of the invention can be administered to one group of Ang1, Ang2 or Tie2 expressing cells. A non-specific siRNA (or no siRNA) can be administered to a second group of Ang1, Ang2 or Tie2 expressing cells as a control. The cells are washed and directly fixed to the microtiter plate wells with 1 to 2% paraformaldehyde. Nonspecific binding sites on the microtiter plate are blocked with 2% bovine serum albumin, and the cells incubated with an Ang1, Ang2 or Tie2 specific monoclonal antibody. Bound Ang1, Ang2 or Tie2 antibody can be detected, for example, by incubation with a 1:1000 dilution of biotinylated goat anti-mouse IgG (Bethesda Research Laboratories, Gaithersberg, Md.) for 1 hour at 37° C. and with a 1:1000 dilution of streptavidin conjugated to beta-galactosidase (Bethesda Research Laboratories) for 1 hour at 37° C. The amount of beta-galactosidase bound to the Ang1, Ang2 or Tie2 specific monoclonal antibody is determined, for example, by developing the microtiter plate in a solution of 3.3 mM chlorophenol red-beta-D-galactopyranoside, 50 mM sodium phosphate, 1.5 mM MgCl 2 ; pH 7.2 for 2 to 15 minutes at 37° C., and measuring the concentration of bound antibody at 575 nm in an ELISA microtiter plate reader. 
     The ability of the present siRNA to down-regulate Ang1, Ang2 or Tie2 expression can also be evaluated in vitro by measuring tube formation by bovine retinal endothelial cells (BRECs), using techniques within the skill in the art. An inhibition of tube formation indicates a down-regulation of Ang1, Ang2 or Tie2 by the present siRNA. 
     A suitable BREC tube formation assay comprises culturing BRECs on fibronectin-coated dishes containing Dulbecco&#39;s modified Eagle&#39;s medium (DMEM) with 5.5 mM glucose, 10% platelet-derived horse serum (PDHS; Wheaton, Pipersville, Pa.), 50 mg/mL heparin, and 50 U/mL endothelial cell growth factor (Roche Molecular Biochemicals). BRECs suitable for use in the tube-formation assay exhibit endothelial homogeneity by immunoreactivity for factor VIII antigen, and remain morphologically unchanged under these conditions as confirmed by light microscopy. 
     The tube formation assay can be performed as described in King G L et al.,  J. Clin. Invest.  75:1028-1036 (1985) and Otani A et al.,  Circ. Res.  82: 619-628 (1998), the entire disclosures of which are herein incorporated by reference. Briefly, an 8:1:1 (400 microliter) mixture of Vitrogen 100 (Celtrix, Palo Alto, Calif.), 0.2 N NaOH and 200 mM HEPES in 10× RPMI medium (Gibco BRL, Gaithersburg, Md.), containing 5 microgram/mL fibronectin and 5 microgram/mL laminin, is added to 24-well plates. After polymerization of the gels, 1.0×10 5  of the cultured BRECs are seeded in the wells and incubated for 24 hours at 37° C. with DMEM containing 20% PDHS. The cell number is chosen to optimize the shape and tube length, as is known in the art (see King G L et al., 1985, supra and Otani A et al., 1998, supra). The medium is then removed, and additional collagen gel is introduced onto the cell layer. Before making the collagen gel, reference points can be randomly marked in the center area of the bottom of each well, in order to measure the density per surface area of any tubelike structures formed by the BRECs. Either VEGF or hypoxia-conditioned medium is then added to the wells to induce tube formation. One or more siRNA of the invention are then introduced into the BRECs of certain wells by any suitable procedure (see below). Other wells are treated with either no siRNA or a non-specific siRNA as controls. Inhibition of tube formation in the wells treated with siRNA as compared to the control wells indicates that expression of the target RNA has been has been inhibited. 
     RNAi-mediated degradation of Ang1, Ang2 or Tie2 mRNA by an siRNA of the invention can also be evaluated with animal models of neovascularization, such as the retinopathy of prematurity (“ROP”) or choroidal neovascularization (“CNV”) rat or mouse models. For example, areas of neovascularization in a CNV rat or mouse can be measured before and after administration of the present siRNA, as in Example 6 below. A reduction in the areas of neovascularization upon administration of the siRNA indicates the down-regulation of target mRNA and an inhibition of angiogenesis. Down-regulation of target mRNA and an inhibition of angiogenesis is also demonstrated below in the streptozotocin-induced diabetic retinopathy rat model (Example 3), a rat model of VEGF-induced retinal vascular permeability and leukostasis (Example 4), and a rat model of ocular neovascularization induced by corneal/limbal injury (Example 5). 
     The mouse model of ischemia-induced retinal neovascularization as described in Takagi et al., 2003, supra can also be used to detect RNAi-mediated degradation of Ang1, Ang2 or Tie2 with the present siRNA. Briefly, litters of 7-day-old (“postnatal day 7” or “P7”) C57BL/6J mice are exposed to 75%±2% oxygen for 5 days, and are then returned to room air at P12 to produce retinal neovascularization. Mice of the same age, maintained in room air, serve as a control. Maximal retinal neovascularization is typically observed at P17, S days after return to room air. One or more siRNA of the invention are injected subretinally into one eye of each treatment animal on P12 and P14. Either no siRNA, or a non-specific siRNA is injected into the contralateral eye as a control. At P17, the mice are killed by cardiac perfusion of 1 mL 4% paraformaldehyde in PBS, and the eyes are enucleated and fixed in 4% paraformaldehyde overnight at 4° C. before paraffin embedding. Serial sections of the paraffin-embedded eyes can be obtained for observation of the extent of neovascularization in the retina. Reduced neovascularization in the retinas of eyes treated with one or more siRNA of the invention, as compared to controls, indicate inhibition in expression of the target mRNA. 
     As discussed above, the siRNA of the invention target can cause the RNAi-mediated degradation of Ang1, Ang2 or Tie2 mRNA, or alternative splice forms, mutants or cognates thereof. Degradation of the target mRNA by the present siRNA reduces the production of a functional gene product from the Ang1, Ang2 and/or Tie2 genes. Thus, the invention provides a method of inhibiting expression of Ang1, Ang2 or Tie2 in a subject, comprising administering an effective amount of an siRNA of the invention to the subject, such that the target mRNA is degraded. As the products of the Ang1, Ang2 or Tie2 genes are involved in angiogenesis, the invention also provides a method of inhibiting angiogenesis in a subject by the RNAi-mediated degradation of the target mRNA by the present siRNA. 
     In the practice of the present methods, two or more siRNA comprising different target sequences in the Ang1, Ang2 or Tie2 mRNA can be administered to the subject. Likewise, two or more siRNA, each comprising target sequences from a different target mRNA (i.e., Ang1, Ang2 and Tie2 mRNA) can also be administered to a subject. 
     As discussed above, Ang1 or Ang2 in conjunction with Tie2 appear to promote angiogenesis in the presence of VEGF, and Ang2 in conjunction with Tie2 appears to promote angiogenesis under hypoxic conditions. However, it is not clear whether VEGF and/or hypoxic conditions are required for Ang1 Ang2- and Tie2-mediated angiogenesis. Also, downregulation of either Ang1, Ang2 or Tie2 expression alone can be sufficient to inhibit angiogenesis. It is therefore not necessary to verify the presence of VEGF or hypoxia in the practice of the present methods. 
     As used herein, a “subject” includes a human being or non-human animal. Preferably, the subject is a human being. 
     As used herein, an “effective amount” of the siRNA is an amount sufficient to cause RNAi-mediated degradation of the target mRNA, or an amount sufficient to inhibit the progression of angiogenesis in a subject. 
     RNAi-mediated degradation of the target mRNA can be detected by measuring levels of the target mRNA or protein in the cells of a subject, using standard techniques for isolating and quantifying mRNA or protein as described above. 
     Inhibition of angiogenesis can be evaluated by directly measuring the progress of pathogenic or nonpathogenic angiogenesis in a subject; for example, by observing the size of a neovascularized area before and after treatment with the siRNA of the invention. An inhibition of angiogenesis is indicated if the size of the neovascularized area stays the same or is reduced. Techniques for observing and measuring the size of neovascularized areas in a subject are within the skill in the art; for example, areas of choroid neovascularization can be observed by ophthalmoscopy. 
     Inhibition of angiogenesis can also be inferred through observing a change or reversal in a pathogenic condition associated with the angiogenesis. For example, in AMD, a slowing, halting or reversal of vision loss indicates an inhibition of angiogenesis in the choroid. For tumors, a slowing, halting or reversal of tumor growth, or a slowing or halting of tumor metastasis, indicates an inhibition of angiogenesis at or near the tumor site. Inhibition of non-pathogenic angiogenesis can also be inferred from, for example, fat loss or a reduction in cholesterol levels upon administration of the siRNA of the invention. 
     It is understood that the siRNA of the invention can mediate RNA interference (and thus inhibit angiogenesis) in substoichiometric amounts. Without wishing to be bound by any theory, it is believed that the siRNA of the invention induces the RISC to degrade the target mRNA in a catalytic manner. Thus, compared to standard therapies for cell adhesion or cell adhesion mediated pathologies, significantly less siRNA needs to be administered to the subject to have a therapeutic effect. 
     One skilled in the art can readily determine an effective amount of the siRNA of the invention to be administered to a given subject, by taking into account factors such as the size and weight of the subject; the extent of the neovascularization or disease penetration; the age, health and sex of the subject; the route of administration; and whether the administration is regional or systemic. Generally, an effective amount of the siRNA of the invention comprises an intercellular concentration at or near the neovascularization site of from about 1 nanomolar (nM) to about 100 nM, preferably from about 2 nM to about 50 nM, more preferably from about 2.5 nM to about 10 nM. Particularly preferred effective amounts of the siRNA of the invention can comprise an intercellular concentration at or near the neovascularization site of about 1 nM, about 5 nM, or about 25 nM. It is contemplated that greater or lesser effective amounts of siRNA can be administered. 
     The present methods can be used to inhibit angiogenesis which is non-pathogenic; i.e., angiogenesis which results from normal processes in the subject. Examples of non-pathogenic angiogenesis include endometrial neovascularization, and processes involved in the production of fatty tissues or cholesterol. Thus, the invention provides a method for inhibiting non-pathogenic angiogenesis; e.g., for controlling weight or promoting fat loss, for reducing cholesterol levels, an inhibitor of the menstrual cycle, or as an abortifacient. 
     The present methods can also inhibit angiogenesis which is associated with an angiogenic disease; i.e., a disease in which pathogenicity is associated with inappropriate or uncontrolled angiogenesis. For example, most cancerous solid tumors generate an adequate blood supply for themselves by inducing angiogenesis in and around the tumor site. This tumor-induced angiogenesis is often required for tumor growth, and also allows metastatic cells to enter the bloodstream. 
     Other angiogenic diseases include AMD, psoriasis, rheumatoid arthritis and other inflammatory diseases. These diseases are characterized by the destruction of normal tissue by newly formed blood vessels in the area of neovascularization. For example, in the wet form of AMD, the choroid is invaded and destroyed by capillaries. The angiogenesis-driven destruction of the choroid in AMD eventually leads to partial or full blindness. 
     In another embodiment, the invention provides a method of treating a subject for complications arising from type I diabetes, by the RNAi-mediated degradation of the target mRNA by the present siRNA. Preferably, the complications arising from type I diabetes to be treated by the present method are diabetic retinopathy, diabetic neuropathy, diabetic nephropathy, and macrovascular disease (including coronary artery disease, cerebrovascular disease, and peripheral vascular disease). 
     Preferably, an siRNA of the invention is used to inhibit the growth or metastasis of solid tumors associated with cancers; for example breast cancer, lung cancer, head and neck cancer, brain cancer, abdominal cancer, colon cancer, colorectal cancer, esophagus cancer, gastrointestinal cancer, glioma, liver cancer, tongue cancer, neuroblastoma, osteosarcoma, ovarian cancer, pancreatic cancer, prostate cancer, retinoblastoma, Wilm&#39;s tumor, multiple myeloma; skin cancer (e.g., melanoma), lymphomas and blood cancer. 
     More preferably, an siRNA of the invention is used to treat complications arising from type I diabetes, such as diabetic retinopathy, diabetic neuropathy, diabetic nephropathy and macrovascular disease. 
     Particularly preferably, an siRNA of the invention is used to inhibit ocular neovascularization, for example to inhibit choroidal neovascularization in AMD. 
     For treating angiogenic diseases, the siRNA of the invention can administered to a subject in combination with a pharmaceutical agent which is different from the present siRNA. Alternatively, the siRNA of the invention can be administered to a subject in combination with another therapeutic method designed to treat the angiogenic disease. For example, the siRNA of the invention can be administered in combination with therapeutic methods currently employed for treating cancer or preventing tumor metastasis (e.g., radiation therapy, chemotherapy, and surgery). For treating tumors, the siRNA of the invention is preferably administered to a subject in combination with radiation therapy, or in combination with chemotherapeutic agents such as cisplatin, carboplatin, cyclophosphamide, 5-fluorouracil, adriamycin, daunorubicin or tamoxifen. 
     In the present methods, the present siRNA can be administered to the subject either as naked siRNA, in conjunction with a delivery reagent, or as a recombinant plasmid or viral vector which expresses the siRNA. 
     Suitable delivery reagents for administration in conjunction with the present siRNA include the Mirus Transit TKO lipophilic reagent; lipofectin; lipofectamine; cellfectin; or polycations (e.g., polylysine), or liposomes. A preferred delivery reagent is a liposome. 
     Liposomes can aid in the delivery of the siRNA to a particular tissue, such as retinal or tumor tissue, and can also increase the blood half-life of the siRNA. Liposomes suitable for use in the invention are formed from standard vesicle-forming lipids, which generally include neutral or negatively charged phospholipids and a sterol, such as cholesterol. The selection of lipids is generally guided by consideration of factors such as the desired liposome size and half-life of the liposomes in the blood stream. A variety of methods are known for preparing liposomes, for example as described in Szoka et al. (1980),  Ann. Rev. Biophys. Bioeng.  9: 467; and U.S. Pat. Nos. 4,235,871, 4,501,728, 4,837,028, and 5,019,369, the entire disclosures of which are herein incorporated by reference. 
     Preferably, liposomes encapsulating the present siRNA comprise a ligand molecule that can target the liposome to cells such as endothelial cells which express Ang1, Ang2 or Tie2 at or near the site of angiogenesis. Ligands which bind to receptors prevalent in vascular EC, such as monoclonal antibodies that bind to EC surface antigens, are preferred. 
     Particularly preferably, the liposomes encapsulating the present siRNA&#39;s are modified so as to avoid clearance by the mononuclear macrophage and reticuloendothelial systems, for example by having opsonization-inhibition moieties bound to the surface of the structure. In one embodiment, a liposome of the invention can comprise both opsonization-inhibition moieties and a ligand. 
     Opsonization-inhibiting moieties for use in preparing the liposomes of the invention are typically large hydrophilic polymers that are bound to the liposome membrane. As used herein, an opsonization inhibiting moiety is “bound” to a liposome membrane when it is chemically or physically attached to the membrane, e.g., by the intercalation of a lipid-soluble anchor into the membrane itself, or by binding directly to active groups of membrane lipids. These opsonization-inhibiting hydrophilic polymers form a protective surface layer which significantly decreases the uptake of the liposomes by the macrophage-monocyte system (“MMS”) and reticuloendothelial system (“RES”); e.g., as described in U.S. Pat. No. 4,920,016, the entire disclosure of which is herein incorporated by reference. Liposomes modified with opsonization-inhibition moieties thus remain in the circulation much longer than unmodified liposomes. For this reason, such liposomes are sometimes called “stealth” liposomes. 
     Stealth liposomes are known to accumulate in tissues fed by porous or “leaky” microvasculature. Thus, tissue characterized by such microvasculature defects, for example solid tumors, will efficiently accumulate these liposomes; see Gabizon, et al. (1988),  P.N.A.S., USA,  18: 6949-53. In addition, the reduced uptake by the RES lowers the toxicity of stealth liposomes by preventing significant accumulation in the liver and spleen. Thus, liposomes of the invention that are modified with opsonization-inhibition moieties are particularly suited to deliver the present siRNA to tumor cells. 
     Opsonization inhibiting moieties suitable for modifying liposomes are preferably water-soluble polymers with a number average molecular weight from about 500 to about 40,000 daltons, and more preferably from about 2,000 to about 20,000 daltons. Such polymers include polyethylene glycol (PEG) or polypropylene glycol (PPG) derivatives; e.g., methoxy PEG or PPG, and PEG or PPG stearate; synthetic polymers such as polyacrylamide or poly N-vinyl pyrrolidone; linear, branched, or dendrimeric polyamidoamines; polyacrylic acids; polyalcohols, e.g., polyvinylalcohol and polyxylitol to which carboxylic or amino groups are chemically linked, as well as gangliosides, such as ganglioside GM 1 . Copolymers of PEG, methoxy PEG, or methoxy PPG, or derivatives thereof, are also suitable. In addition, the opsonization inhibiting polymer can be a block copolymer of PEG and either a polyamino acid, polysaccharide, polyamidoamine, polyethyleneamine, or polynucleotide. The opsonization inhibiting polymers can also be natural polysaccharides containing amino acids or carboxylic acids, e.g., galacturonic acid, glucuronic acid, mannuronic acid, hyaluronic acid, pectic acid, neuraminic acid, alginic acid, carrageenan; aminated polysaccharides or oligosaccharides (linear or branched); or carboxylated polysaccharides or oligosaccharides, e.g., reacted with derivatives of carbonic acids with resultant linking of carboxylic groups. 
     Preferably, the opsonization-inhibiting moiety is a PEG, PPG, or derivatives thereof. Liposomes modified with PEG or PEG-derivatives are sometimes called “PEGylated liposomes.” 
     The opsonization inhibiting moiety can be bound to the liposome membrane by any one of numerous well-known techniques. For example, an N-hydroxysuccinimide ester of PEG can be bound to a phosphatidyl-ethanolamine lipid-soluble anchor, and then bound to a membrane. Similarly, a dextran polymer can be derivatized with a stearylamine lipid-soluble anchor via reductive amination using Na(CN)BH 3  and a solvent mixture such as tetrahydrofuran and water in a 30:12 ratio at 60° C. 
     Recombinant plasmids which express siRNA of the invention are discussed above. Such recombinant plasmids can also be administered directly or in conjunction with a suitable delivery reagent, including the Minis Transit LT1 lipophilic reagent; lipofectin; lipofectamine; cellfectin; polycations (e.g., polylysine) or liposomes. Recombinant viral vectors which express siRNA of the invention are also discussed above, and methods for delivering such vectors to cells of a subject which are expressing Ang1, Ang2 or Tie2 are within the skill in the art. 
     The siRNA of the invention can be administered to the subject by any means suitable for delivering the siRNA to the cells expressing Ang1, Ang2 or Tie2. For example, the siRNA can be administered by gene gun, electroporation, or by other suitable parenteral or enteral administration routes. 
     Suitable enteral administration routes include oral, rectal, or intranasal delivery. 
     Suitable parenteral administration routes include intravascular administration (e.g. intravenous bolus injection, intravenous infusion, intra-arterial bolus injection, intra-arterial infusion and catheter instillation into the vasculature); peri- and intra-tissue administration (e.g., peri-tumoral and intra-tumoral injection, intra-retinal injection or subretinal injection); subcutaneous injection or deposition including subcutaneous infusion (such as by osmotic pumps); direct (e.g., topical) application to the area at or near the site of neovascularization, for example by a catheter or other placement device (e.g., a corneal pellet or a suppository, eye-dropper, or an implant comprising a porous, non-porous, or gelatinous material); and inhalation. Suitable placement devices include the ocular implants described in U.S. Pat. Nos. 5,902,598 and 6,375,972, and the biodegradable ocular implants described in U.S. Pat. No 6,331,313, the entire disclosures of which are herein incorporated by reference. Such ocular implants are available from Control Delivery Systems, Inc. (Watertown, Mass.) and Oculex Pharmaceuticals, Inc. (Sunnyvale, Calif.). 
     In a preferred embodiment, injections or infusions of the siRNA are given at or near the site of neovascularization. For example, the siRNA of the invention can be delivered to retinal pigment epithelial cells in the eye. Preferably, the siRNA is administered topically to the eye, e.g. in liquid or gel form to the lower eye lid or conjunctival cul-de-sac, or by electroporation or iontophoresis, as is within the skill in the art (see, e.g., Acheampong A A et al, 2002,  Drug Metabol. and Disposition  30: 421-429, the entire disclosure of which is herein incorporated by reference). 
     Typically, the siRNA of the invention is administered topically to the eye in volumes of from about 5 microliters to about 75 microliters, for example from about 7 microliters to about 50 microliters, preferably from about 10 microliters to about 30 microliters. The siRNA of the invention is highly soluble in aqueous solutions, and it is understood that topical instillation in the eye of siRNA in volumes greater than 75 microliters can result in loss of siRNA from the eye through spillage and drainage. Thus, it is preferable to administer a high concentration of siRNA (e.g., about 10 to about 200 mg/ml, or about 100 to about 1000 nM) by topical instillation to the eye in volumes of from about 5 microliters to about 75 microliters. 
     A particularly preferred parenteral administration route is intraocular administration. It is understood that intraocular administration of the present siRNA can be accomplished by injection or direct (e.g., topical) administration to the eye, as long as the administration route allows the siRNA to enter the eye. In addition to the topical routes of administration to the eye described above, suitable intraocular routes of administration include intravitreal, intraretinal, subretinal, subtenon, peri- and retro-orbital, trans-corneal and trans-scleral administration. Such intraocular administration routes are within the skill in the art; see, e.g., and Acheampong AA et al, 2002, supra; and Bennett et al. (1996),  Hum. Gene Ther.  7: 1763-1769 and Ambati J et al., 2002,  Progress in Retinal and Eye Res.  21: 145-151, the entire disclosures of which are herein incorporated by reference. 
     The siRNA of the invention can be administered in a single dose or in multiple doses. Where the administration of the siRNA of the invention is by infusion, the infusion can be a single sustained dose or can be delivered by multiple infusions. 
     One skilled in the art can also readily determine an appropriate dosage regimen for administering the siRNA of the invention to a given subject. For example, the siRNA can be administered to the subject once, such as by a single injection or deposition at or near the neovascularization site. Alternatively, the siRNA can be administered to a subject multiple times daily or weekly. For example, the siRNA can be administered to a subject once weekly for a period of from about three to about twenty-eight weeks, more preferably from about seven to about ten weeks. In a preferred dosage regimen, the siRNA is injected at or near the site of neovascularization (e.g., intravitreally) once a week for seven weeks. It is understood that periodic administrations of the siRNA of the invention for an indefinite length of time may be necessary for subjects suffering from a chronic neovascularization disease, such as wet AMD or diabetic retinopathy. 
     Where a dosage regimen comprises multiple administrations or the administration of two or more siRNA, each of which comprise a different target sequence, it is understood that the effective amount of siRNA administered to the subject can comprise the total amount of siRNA administered over the entire dosage regimen. 
     The siRNA of the invention are preferably formulated as pharmaceutical compositions prior to administering to a subject, according to techniques known in the art. Pharmaceutical compositions of the present invention are characterized as being at least sterile and pyrogen-free. As used herein, “pharmaceutical formulations” include formulations for human and veterinary use. Methods for preparing pharmaceutical compositions of the invention are within the skill in the art, for example as described in  Remington&#39;s Pharmaceutical Science,  17th ed., Mack Publishing Company, Easton, Pa. (1985), the entire disclosure of which is herein incorporated by reference. 
     The present pharmaceutical formulations comprise an siRNA of the invention (e.g., 0.1 to 90% by weight), or a physiologically acceptable salt thereof, mixed with a physiologically acceptable carrier medium. Preferred physiologically acceptable carrier media are water, buffered water, normal saline, 0.4% saline, 0.3% glycine, hyaluronic acid and the like. 
     Pharmaceutical compositions of the invention can also comprise conventional pharmaceutical excipients and/or additives. Suitable pharmaceutical excipients include stabilizers, antioxidants, osmolality adjusting agents, buffers, and pH adjusting agents. Suitable additives include physiologically biocompatible buffers (e.g., tromethamine hydrochloride), additions of chelants (such as, for example, DTPA or DTPA-bisamide) or calcium chelate complexes (as for example calcium DTPA, CaNaDTPA-bisamide), or, optionally, additions of calcium or sodium salts (for example, calcium chloride, calcium ascorbate, calcium gluconate or calcium lactate). Pharmaceutical compositions of the invention can be packaged for use in liquid form, or can be lyophilized. 
     For topical administration to the eye, conventional intraocular delivery reagents can be used. For example, pharmaceutical compositions of the invention for topical intraocular delivery can comprise saline solutions as described above, corneal penetration enhancers, insoluble particles, petrolatum or other gel-based ointments, polymers which undergo a viscosity increase upon instillation in the eye, or mucoadhesive polymers. Preferably, the intraocular delivery reagent increases corneal penetration, or prolongs preocular retention of the siRNA through viscosity effects or by establishing physicochemical interactions with the mucin layer covering the corneal epithelium. 
     Suitable insoluble particles for topical intraocular delivery include the calcium phosphate particles described in U.S. Pat. No. 6,355,271 of Bell et al., the entire disclosure of which is herein incorporated by reference. Suitable polymers which undergo a viscosity increase upon instillation in the eye include polyethylenepolyoxypropylene block copolymers such as poloxamer 407 (e.g., at a concentration of 25%), cellulose acetophthalate (e.g., at a concentration of 30%), or a low-acetyl gellan gum such as Gelrite® (available from CP Kelco, Wilmington, Del.). Suitable mucoadhesive polymers include hydrocolloids with multiple hydrophilic functional groups such as carboxyl, hydroxyl, amide and/or sulfate groups; for example, hydroxypropylcellulose, polyacrylic acid, high-molecular weight polyethylene glycols (e.g., &gt;200,000 number average molecular weight), dextrans, hyaluronic acid, polygalacturonic acid, and xylocan. Suitable corneal penetration enhancers include cyclodextrins, benzalkonium chloride, polyoxyethylene glycol lauryl ether (e.g., Brij® 35), polyoxyethylene glycol stearyl ether (e.g., Brij® 78), polyoxyethylene glycol oleyl ether (e.g., Brij® 98), ethylene diamine tetraacetic acid (EDTA), digitonin, sodium taurocholate, saponins and polyoxyethylated castor oil such as Cremaphor EL. 
     For solid compositions, conventional nontoxic solid carriers can be used; for example, pharmaceutical grades of mannitol, lactose, starch, magnesium stearate, sodium saccharin, talcum, cellulose, glucose, sucrose, magnesium carbonate, and the like. 
     For example, a solid pharmaceutical composition for oral administration can comprise any of the carriers and excipients listed above and 10-95%, preferably 25%-75%, of one or more siRNA of the invention. A pharmaceutical composition for aerosol (inhalational) administration can comprise 0.01-20% by weight, preferably 1%-10% by weight, of one or more siRNA of the invention encapsulated in a liposome as described above, and propellant. A carrier can also be included as desired; e.g., lecithin for intranasal delivery. 
     The invention will now be illustrated by the following non-limiting examples. 
     EXAMPLE 1 
     Inhibition of Ang2 Expression in Cultured Human Cells with siRNA Targeted to Ang2 mRNA 
     Human embryonic kidney (HEK-293 cells) were cultured in 24 well plates at 37° C. with 5% CO 2  overnight in standard growth medium. Transfections were performed the next day when the cells were about 70% confluent. The HEK-293 cells were separately transfected with twelve different siRNA (25 nM each) targeted to human Ang2 (“hANG2”) mRNA, mixed with a CaPi transfection reagent. These twelve siRNAs target the sequences listed in Table 1, and all siRNAs contained 3′ TT overhangs on each strand. Control cells were transfected with CaPi transfection reagent lacking siRNA, or a nonspecific siRNA targeted to enhanced green fluorescent protein (EGFP siRNA) mixed with CaPi transfection reagent. Forty eight hours post-transfection, the growth medium was removed from all wells, and a human ANG2 ELISA (R &amp; D systems, Minneapolis, Minn.) was performed as described in the Quantikine human ANG2 ELISA protocol, the entire disclosure of which is herein incorporated by reference. ELISA results were read on an AD340 plate reader (Beckman Coulter), and are reported in  FIG. 1 . 
     
       
         
           
               
             
               
                 TABLE 1  
               
             
            
               
                   
               
               
                 Target Sequences for hANG2 siRNAs Tested 
               
               
                 in HEK-293 Cells 
               
            
           
           
               
               
               
               
            
               
                   
                 Target Sequence 
                 SEQ ID NO: 
                 siRNA 
               
               
                   
                   
               
               
                   
                 AAGAGCATGGACAGCATAGGA 
                 232 
                 hANG2#1 
               
               
                   
                   
               
               
                   
                 AACCAGACGGCTGTGATGATA 
                 254 
                 hANG2#2 
               
               
                   
                   
               
               
                   
                 AAACGCGGAAGTTAACTGATG 
                 262 
                 hANG2#3 
               
               
                   
                   
               
               
                   
                 AACGCGGAAGTTAACTGATGT 
                 263 
                 hANG2#4 
               
               
                   
                   
               
               
                   
                 AAGAAGGTGCTAGCTATGGAA 
                 291 
                 hANG2#5 
               
               
                   
                   
               
               
                   
                 AATAGTGACTGCCACGGTGAA 
                 316 
                 hANG2#6 
               
               
                   
                   
               
               
                   
                 AATAACTTACTGACTATGATG 
                 323 
                 hANG2#7 
               
               
                   
                   
               
               
                   
                 AATCAGGACACACCACAAATG 
                 336 
                 hANG2#8 
               
               
                   
                   
               
               
                   
                 AAATGGCATCTACACGTTAAC 
                 337 
                 hANG2#9 
               
               
                   
                   
               
               
                   
                 AATGGCATCTACACGTTAACA 
                 338 
                  hANG2#10 
               
               
                   
                   
               
               
                   
                 AATTATTCAGCGACGTGAGGA 
                 344 
                  hANG2#11 
               
               
                   
                   
               
               
                   
                 AAGAACTCAATTATAGGATTC 
                 366 
                  hANG2#12 
               
               
                   
                   
               
            
           
         
       
     
     As can be seen from  FIG. 1 , the level of hANG2 protein secreted into the growth medium was reduced in HEK-293 cells transfected with hANG2#2, #3, #4, #9 and #10 siRNA. Transfection of HEK-293 cells with non-specific siRNA had no apparent effect on hANG2 protein levels. 
     After the growth medium was removed from each well, a cytotoxicity assay was performed on the cells as follows. Complete growth medium containing 10% AlamarBlue (Biosource, Camarillo, Calif.) was added to each well, and cells were incubated at 37° C. with 5% CO 2  for 3 hours. Cell proliferation was measured by detecting the color change of medium containing AlamarBlue which resulted from cell metabolic activity. The cytotoxicity assay results were read on an AD340 plate reader (Beckman Coulter), and are reported in  FIG. 2 . 
     As can be seen in  FIG. 2 , the transfection of HEK-293 cells with the hANG2#8 and #12 siRNA produced a slight reduction in cell growth as compared to control cells. The remaining hANG2 siRNAs showed no apparent cytotoxicity as compared with control cells. 
     After cytotoxicity assay was performed, the AlamarBlue-containing medium in each well was completely removed and RNA extractions were performed using the RNAqueous RNA isolation kit (Ambion, Austin, Tex.). The levels of hANG2 mRNA in the HEK-293 cells were measured by a quantitative reverse-transcriptase/polymerase chain reaction (RT-PCR) assay. Expression of human glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA was used as a internal control. The levels of hANG2 mRNA in HEK-293 cells were reduced by transfection with the hANG2 siRNA compared to control cells, in a pattern which correlated with the reduction in hANG2 protein shown in  FIG. 1 . 
     EXAMPLE 2  
     Dose-Response of hAng2#2 and #3 in Cultured Human Cells 
     HEK-293 cells were grown to about 70% confluency as in Example 1 above. The cells were then transfected with I nanomolar (“nM”), 5 nM or 25 nM doses of hANG2#2 or #3 siRNA in CaPi transfection reagent. Control cells were transfected with 25 nM nonspecific EGFP siRNA in CaPi transfection reagent, or with transfection reagent alone. hANG2 protein levels were measured in the growth medium at 48 hours post-transfection by ANG2 ELISA as described in Example 1 above, and the results are presented in  FIG. 3 . 
     As can be seen from  FIG. 3 , the levels of hANG2 protein level were reduced in HEK-293 cells transfected with the hANG2#2 and #3 siRNA, in a dose-dependent manner. All doses of hANG2#2 siRNA and the 5 and 25 nM doses of hANG2#3 siRNA reduced the level of hANG2 protein secreted into the growth medium, as compared to control cells. The 1 nM dose of hANG2#3 siRNA did not reduce the hANG2 protein level as compared to control cells mock-transfected with transfection reagent alone. However, the level of hANG2 protein secreted by cells transfected with 1 nM hANG2#3 siRNA was slightly reduced as compared to control cells transfected with the nonspecific siRNA. Transfections with the non-specific siRNA had no apparent effect on hANG2 protein levels. 
     A cytotoxicity assay was performed on the control HEK-293 cells and the HEK-293 cells transfected with the different doses of hANG2#2 and #3 siRNA as described above in Example 1. As can be seen in  FIG. 4 , the transfection of HEK-293 cells with 5 nM hANG2#2 siRNA produced a slight reduction in cell growth as compared to control cells mock-transfected with transfection reagent alone. There was no apparent toxicity of the 5 nM hANG2#2 siRNA dose as compared to control cells transfected with the nonspecific siRNA. The remaining doses of hANG2#2 or #3 siRNA showed no apparent cytotoxicity as compared with control cells transfected with nonspecific siRNA or with transfection reagent alone. 
     EXAMPLE 3  
     Treatment of Streptozotocin-Induced Diabetic Retinopathy with siRNA Targeted to Ang1, Ang2 or Tie2 
     Vascular leakage and non-perfusion in the retinas of individuals with diabetic retinopathy is spatially and temporally associated with leukocyte stasis. See, e.g., Miyamoto K et al. (1999),  Proc. Nat. Acad. Sci. USA  96(19):10836-41, the entire disclosure of which is herein incorporated by reference. It is expected that intravitreal injection of siRNA targeted to Ang1, Ang2 or Tie2 will decrease leukocyte stasis, and therefore reduce retinal vascular permeability, in diabetic rats. 
     Long-Evans rats (approximately 200 g) will be injected with streptozotocin in citrate buffer intravenously after an overnight fast to induce diabetes, as described in Miyamoto K et al. (1999), supra. Long-Evans rats (approximately 200 g) will be injected with citrate buffer alone after an overnight fast as a control. The serum blood-sugar will be measured and blood pressure will be recorded daily. Elevated levels of serum blood sugar as compared to control animals are considered diabetic. 
     Intravitreal injections of siRNA targeted to Ang1, Ang2 or Tie2 (“experimental siRNA”) will be performed OD in each rat. Non-specific siRNA will be injected as a control OS. The overall group scheme will be as shown in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Overall Group Scheme 
               
            
           
           
               
               
               
            
               
                   
                 OD 
                 OS 
               
               
                   
                 (experimental siRNA) 
                 (non-specific siRNA) 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                 Diabetic Rat (STZ) 
                 Experimental group 
                 Control 
               
               
                 Non-diabetic Rat 
                 Control 
                 Control 
               
               
                   
               
            
           
         
       
     
     At day 7 post treatment, the rats will be subjected to Acridine Orange Leukocyte Fluorography (AOLF), as described in Miyamoto K et al (1999), supra. Briefly, the rats will be anaesthetized, and their pupils dilated with tropicamide. The rats will then be injected intravenously with acridine orange suspended in sterile saline. The fundus of each eye will be observed and imaged with a scanning laser ophthalmoscope (argon blue laser as a light source) for leukocyte stasis. The rats will then be perfused with fluorescein dextran and the eyes will be further imaged. The density of leukocyte stasis will be calculated as a percentage of bright pixels in a 10 disk diameter radius. The density of leukocyte stasis will be used as an endpoint. 
     Also on day 7, the rats will undergo an isotope dilution technique to quantify vascular leakage, as described in Miyamoto K et al (1999), supra. Briefly, the rats will be injected intravenously with I 125  in BSA at one time point, and with I 131  at a second time point. The rats will be sacrificed minutes after the second injection, the retinas will be isolated, and arterial samples will be taken. The retinas and the arterial samples will be analyzed using γ-spectroscopy after correcting for activity in the retinas using a quantitative index of iodine clearance. The measurements will then be normalized for exact dose given, body weight and tissue weight. The corrected quantity of γ activity will be used as a marker of vascular leakage in the retina (second endpoint). It is expected that the γ activity will be decreased in the retinas of the experimental animals, indicating decreased vascular leakage. 
     EXAMPLE 4  
     Treatment of VEGF-Induced Vascular Permeability and Leukostasis with siRNA Targeted to Ang1, Ang2 or Tie2 
     The presence of VEGF in the eye causes retinal leukostasis that corresponds with increased vascular permeability and capillary non-perfusion in the retina. See, e.g., Miyamoto K et al. (2000), Am. J. Pathol. 156(5):1733-9, the entire disclosure of which is herein incorporated by reference. It is expected that intravitreal injection of siRNA targeted to Ang1, Ang2 or Tie2 will decrease the permeability and leukostasis created by intravitreal injection of VEGF in rats. 
     Long-Evans rats (approximately 200 g) will be anaesthetized and injected intravitreally with VEGF in buffer OU. siRNA targeted to Ang1, Ang2 or Tie2 (“experimental siRNA”) will be simultaneously delivered OD to each rat by intravitreal injection. Non-specific siRNA will be injected intravitreally as a control OS. Additional controls will include rats injected with buffer alone (no VEGF). The overall group scheme will be as shown in Table 3. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Overall Group Scheme 
               
            
           
           
               
               
               
            
               
                   
                 OD 
                 OS 
               
               
                   
                 (experimental siRNA) 
                 (Non-specific siRNA) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 VEGF 
                 Experimental group 
                 Control 
               
               
                   
                 Buffer 
                 Control 
                 Control 
               
               
                   
                   
               
            
           
         
       
     
     At 24 hours post injection the rats are subjected to AOLF and an isotope dilution technique as described in Example 3. 
     EXAMPLE 5  
     Treatment of Neovascularization in Eyes Subjected to Corneal/Limbal Injury with siRNA Targeted to Ang1, Ang2 or Tie2 
     Injury to the ocular surface can cause the destruction of corneal limbal stem cells. Destruction of these cells induces a VEGF-dependent corneal neovascularization, which can lead to blindness. The VEGF which drives the neovascularization is supplied by neutrophils and monocytes that infiltrate the cornea after injury to the ocular surface. See, e.g., Moromizato Y et al. (2000), Am. J. Pathol. 157(4):1277-81, the entire disclosure of which is herein incorporated by reference in its entirety. It is expected that siRNA targeted to Ang1, Ang2 or Tie2 applied to the cornea after limbal injury will decrease the resultant area of neovascularization of the cornea in mice. The area of neovascularization can be measured directly. Alternatively, a reduction in corneal neovascularization can be inferred from a decrease in the number of VEGF-producing polymorphonuclear cells in the cornea. 
     Corneal neovascularization will be induced in C57Bl/6 by damaging the limbus, as described in Moromizato Y et al., supra. Briefly, the mice will be anaesthetized and sodium hydroxide will be applied to the cornea. The corneal and limbal epithelia will be debrided using a corneal knife OU. siRNA targeted to Ang1, Ang2 or Tie2 will be applied to the corneal surface OD immediately, after removal, and 3 times a day for the duration of the study (7 days). Non-specific siRNA will be administered OS with the same dosing regimen as a control. 
     On days 2, 4 and 7 after debridement of the corneal and limbal epithelia, mice will be evaluated for the degree of corneal neovascularization as described in Moromizato Y et al., supra. Briefly, endothelial-specific, fluorescein-conjugated lectin will be injected intravenously. Thirty minutes after injection, mice will be sacrificed, and the eyes will be harvested and fixed in formalin for 24 hours. Flat mounts of the corneas will be made, and pictures of the corneal flat mounts will be taken under fluorescent microscopy and imported into Openlab software for analysis. Using the Openlab software, threshold level of fluorescence will be set, above which only vessels are seen. The area of fluorescent vessels and the area of the cornea (demarcated by the limbal arcade) will be calculated. The area of vessels will be divided by the total corneal area, and this value will equal the percent neovascular area. The percent neovascular area of the treatment and control groups will be compared. 
     On days 2, 4 and 7 after debridement of the corneal and limbal epithelia, additional mice will be sacrificed for quantification of corneal polymorphonuclear cells (PMNs) as described in Moromizato Y et al., supra. Briefly, mice will be sacrificed, and the eyes will be harvested and fixed in formalin for 24 hours. After formalin fixation, the enucleated eyes will be embedded in paraffin and sectioned. One paraffin section from each eye which correlates to the corneal anatomical center will be chosen and used for microscopy. The PMNs (identified as multilobulated cells) will be counted on this one section, and the number of PMNs in the sections from the treatment and control groups will be compared. 
     EXAMPLE 6  
     Treatment of Laser-Induced Choroidal Neovascularization with siRNA Targeted to Ang1, Ang2 or Tie2 
     Laser photocoagulation that ruptures Bruch&#39;s membrane will induce choroidal neovascularization (CNV) similar to that seen in wet macular degeneration. It is expected that intravitreal injection of siRNA targeted to Ang1, Ang2 or Tie2 will decrease the area of laser-induced CNV in mice. 
     CNV will be induced in mice by the procedure described in Sakurai E et al. (2003),  Invest. Ophthalmol.  &amp;  Visual Sci.  44(61:2743-9, the entire disclosure of which is herein incorporated by reference. Briefly, C57Bl/6 mice will be anaesthetized, and their pupils will be dilated with tropicamide. The retinas of the mice will be laser photocoagulated with one laser spot at the 9, 12, and 3 o&#39;clock positions of each retinal OU. Immediately following laser photocoagulation, inject siRNA targeted to Ang1, Ang2 or Tie2 will be injected intravitreally OD. Non-specific siRNA will be injected intravitreally OS as a control. 
     Fourteen days after laser photocoagulation, the mice will be sacrificed and retinal flat mounts will be prepared for CNV area quantification as described in Sakurai E et al. (2003), supra. Briefly, the mice will be anaesthetized, the chest will be opened, and the descending aorta will be cross-clamped. The right atrium will then be clipped and fluorescein-labeled dextran will be injected slowly into the left ventricle. 
     After injection of the fluorescein-labeled dextran, the eyes will be enucleated and fixed in paraformaldehyde for 24 hours. The anterior chamber and retina will then be removed, and a flat mount of each choroid will be prepared for analysis. Choroidal flat mounts will be analyzed by taking a picture of each under fluorescent microscopy, and importing the picture into Openlab software. Using the Openlab software, the area of neovascularization will be outlined and quantified, being sure known laser location is compared to the fluorescent tuft. The neovascular area of the treatment animals will be compared to that of the control animals.