PREPARATION OF STABLE FORMULATIONS OF LIPID-NUCLEIC ACID COMPLEXES FOR EFFICIENT IN VIVO DELIVERY

Abstract: The present invention provides for lipid:nucleic acid complexes that have increased shelf life and high transfection activity in vivo following intravenous injection, and methods of preparing such complexes. The methods generally involve contacting a nucleic acid with an organic polycation to produce a condensed nucleic acid, and then combining the condensed nucleic acid with a lipid comprising an amphiphilic cationic lipid to produce the lipid:nucleic acid complex. This complex can be further stabilized by the addition of a hydrophilic polymer attached to hydrophobic side chains. The complex can also be made specific for specific cells, by incorporating a targeting moiety such as a Fab' fragment attached to a hydrophilic polymer.

Description: TITLE OF THE INVENTION PREPARATION OF STABLE FORMULATIONS OF LIPID- NUCLEIC ACID COMPLEXES FOR EFFICIENT IN VIVO DELIVERY CROSS REFERENCE TO RELATED APPLICATIONS This application claims the benefit of the filing date of USSN 60/030,578, filed November 12, 1996. FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT Not applicable. FIELD OF THE INVENTION The present invention relates to the field of cationic lipid:DNA complexes ("CLDC"). In particular, the present invention relates to lipid:nucleic acid complexes that contain (1) hydrophilic polymer; (2) nucleic acid that has been condensed with organic polycations; and (3) hydrophilic polymer and nucleic acid that has been condensed with organic polycations. The lipid: nucleic acid complexes of this invention show high transfection activity in vivo following intravenous injection and an unexpected increase in shelf life, as determined by in vivo transfection activity. BACKGROUND OF THE INVENTION Liposomes that consist of amphiphilic cationic molecules are useful non-viral vectors for gene delivery in vitro and in vivo (reviewed in Crystal, Science 270: 404-410 (1995); Blaese et al., Cancer Gene Ther. 2: 291- 297 (1995); Behr et al, Bioconjugate Chem. 5: 382-389 (1994); Remy et al., Bioconjugate Chem. 5: 647-654 (1994); and Gao et al., Gene Therapy 2: 710- 722 (1995)). In theory, the positively charged liposomes complex to negatively charged nucleic acids via electrostatic interactions to form lipid:nucleic acid complexes. The lipid:nucleic acid complexes have several advantages as gene transfer vectors. Unlike viral vectors, the lipid:nucleic acid complexes can be used to transfer expression cassettes of essentially unlimited size. Since the complexes lack proteins, they may evoke fewer immunogenic and inflammatory responses. Moreover, they cannot replicate or recombine to form an infectious agent and have low integration frequency. There are a number of publications that demonstrate convincingly that amphiphilic cationic lipids can mediate gene delivery in vivo and in vitro, by showing detectable expression of a reporter gene in culture cells in vitro (Feigner et al, Proc. Natl Acad. Sci. USA 84: 7413-17 (1987); Loeffler et al, Methods in Enzymology 217: 599-618 (1993); Feigner et al, J. Biol. Chem. 269: 2550-2561 (1994)). Because lipid:nucleic acid complexes are on occasion not as efficient as viral vectors for achieving successful gene transfer, much effort has been devoted in finding cationic lipids with increased transfection efficiency (Behr, Bioconjugate Chem. 5: 382-389 (1994); Remy et al, Bioconjugate Chem. 5: 647-654 (1994); Gao et al, Gene Therapy 2: 710-722 (1995)). Lipid:nucleic acid complexes are regarded with enthusiasm as a potentially useful tool for gene therapy. Several groups have reported the use of amphiphilic cationic lipid:nucleic acid complexes for in vivo transfection both in animals, and in humans (reviewed in Gao et al, Gene Therapy 2: 710-722 (1995); Zhu et al, Science 261: 209-211 (1993); and Thierry et al, Proc. Natl. Acad. Sci. USA 92: 9742-9746 (1995)). However, the technical problems for preparation of complexes that have stable shelf-lives have not been addressed. For example, unlike viral vector preparations, lipid:nucleic acid complexes are unstable in terms of particle size (Behr, Bioconjugate Chem. 5: 382-389 (1994); Remy et al., Bioconjugate Chem. 5: 647-654 (1994); Gao et al, Gene Therapy 2: 710-722 (1995)). It is therefore difficult to obtain homogeneous lipid:nucleic acid complexes with a size distribution suitable for systemic injection. Most preparations of lipid:nucleic acid complexes are metastable. Consequently, these complexes typically must be used within a short period of time ranging from 30 minutes to a few hours. In recent clinical trials using cationic lipids as a carrier for DNA delivery, the two components were mixed at the bed-side and used immediately (Gao et al, Gene Therapy 2: 710-722 (1995)). The structural instability along with the loss of transfection activity of lipid:nucleic acid complex with time have been challenges for the future development of lipid-mediated gene therapy. SUMMARY OF THE INVENTION The present invention provides a novel method of preparing cationic lipid:nucleic acid complexes that have increased shelf life. In one embodiment, these complexes are prepared by contacting a nucleic acid with an organic polycation, to produce a condensed or partially condensed nucleic acid. The condensed nucleic acid is then combined with an amphiphilic cationic lipid plus a neutral helper lipid such as cholesterol in a molar ratio from about 2:1 to about 1:2, producing the lipid:nucleic acid complex. Optionally, a hydrophilic polymer is subsequently added to the lipid:nucleic acid complex. Alternatively, the hydrophilic polymer is added to a lipid:nucleic acid complex comprising nucleic acid that has not been not condensed. These lipid:nucleic acid complexes have an increased shelf life, e.g., when stored at 22° C or below, as compared to an identical lipid:nucleic acid complex in which the nucleic acid component has not been contacted with the organic polycation and/or in which the lipid:nucleic acid complex has not been contacted with a hydrophilic polymer. In a particularly preferred embodiment, the polycation is a polyamine, more preferably a polyamine such as spermidine or spermine. In another preferred embodiment, the lipid:nucleic acid complexes are prepared by combining a nucleic acid with an amphiphilic cationic lipid and then combining the complex thus formed with a hydrophilic polymer. This lipid:nucleic acid complex has an increased shelf life, e.g., when stored at 22° C or below as compared to an identical complex that has not been combined with the hydrophilic polymer. In one embodiment, the hydrophilic polymer is selected from the group consisting of polyethylene glycol (PEG), polyethylene glycol derivatized with phosphatidyl ethanolamine (PEG-PE), polyethylene glycol derivatized with tween, polyethylene glycol derivatized with distearoylphosphatidyl- ethanolamine (PEG-DSPE), ganglioside G M1 and synthetic polymers. In one embodiment, the lipid:nucleic acid complex is lyophilized. In any of the methods and compositions of this invention, the nucleic acid can be virtually any nucleic acid, e.g., a deoxyribonucleic acid (DNA) or a ribonucleic acid (RNA), and peptide nucleic acid (PNA) etc., and is most preferably a DNA. In a particularly preferred embodiment, the DNA is an expression cassette capable of expressing a polypeptide in a cell transfected with the lipid:nucleic acid complex. In one embodiment the lipid:nucleic acid complexes are formed by first forming a liposome, and then combining the formed liposome with condensed or partially condensed nucleic acid to form a lipid:nucleic acid complex. Optionally, the lipid:nucleic acid complex is subsequently contacted with a hydrophilic polymer. The liposomes can alternatively be combined with an uncondensed nucleic acid to form a lipid: nucleic acid complex to which a hydrophilic polymer (e.g., PEG-PE) is later added. A lipid:nucleic acid complex prepared by the combination of nucleic acid and a liposome contacted with a hydrophilic polymer can be subsequently combined with additional hydrophilic polymer. In a preferred embodiment, the lipid and nucleic acid are combined in a ratio ranging from about 1 to about 20, more preferably from about 4 to about 16, and most preferably from about 8 to about 12 nmole lipid: μg nucleic acid. The lipid and hydrophilic polymer are combined in a molar ratio ranging from about 0.1 to about 10%, more preferably from about 0.3 to about 5% and most preferably from about 0.5% to about 2.0% (molar ratio of hydrophilic polymer to cationic lipid of the complex). It will be appreciated that a targeting moiety (e.g., an antibody or an antibody fragment) can be attached to the lipid and/or liposome before or after formation of the lipid:nucleic acid complex. In a preferred embodiment, the targeting moiety is coupled to the hydrophilic polymer {e.g., PEG), where the targeting moiety/hydrophilic polymer is subsequently added to the lipid ucleic acid complex. This provides a convenient means for modifying the targeting specificity of an otherwise generic lipid:nucleic acid complex. In a particularly preferred embodiment, the method of increasing the shelf life of the lipid:nucleic acid complex includes the steps of combining an expression cassette with spermidine or spermine with an amphiphilic cationic lipid plus a helper lipid such as cholesterol, and a Fab' fragment of an antibody attached to a spacer, e.g., polyethylene glycol, so that the complex has increased shelf life when stored at about 4°C. In one particularly preferred embodiment, the method of increasing the shelf life of the lipid:nucleic acid complex includes the steps of combining an expression cassette with spermidine or spermine with an amphiphilic cationic lipid, and a Fab' fragment of an antibody attached to a polyethylene glycol derivative. In another particularly preferred embodiment, includes the steps of combining an expression cassette with an amphiphilic cationic lipid, and a Fab' fragment of an antibody attached to a polyethylene glycol derivative so that the complex has increased shelf life when stored at about 4° C. This invention also provides for a method of transfecting a nucleic acid into a mammalian cell, the method comprising contacting the cell with any one of the lipid:nucleic acid complexes prepared as described above. In one embodiment, the method uses systemic administration of a lipid:nucleic acid complex into a mammal. In a preferred embodiment, the method of transfecting uses intravenous administration of the lipid:nucleic acid complex into a mammal. In a particularly preferred embodiment, the method comprises contacting a specific cell that expresses a ligand that recognizes the Fab' fragment. In yet another embodiment, this invention also provides for pharmaceutical composition comprising the lipidxondensed nucleic acid complex described above. The pharmaceutical compositions comprise a therapeutically effective dose of the lipid:nucleic acid complex and a pharmaceutically acceptable carrier or excipient. In yet another embodiment, the invention also provides a kit for preparing a lipid:nucleic acid complex, the kit comprising a container with a liposome; a container with a nucleic acid; and a container with a hydrophilic polymer, wherein the liposome and the nucleic acid are mixed to form the lipid: nucleic acid complex and wherein the lipid: nucleic acid complex is contacted with the hydrophilic polymer. In a preferred embodiment, the hydrophilic polymer is derivatized with a targeting moiety, preferably an Fab' fragment. In another preferred embodiment, the nucleic acid is condensed. This invention also provides for a lipidxondensed nucleic acid complex prepared using the method of increasing shelf life using nucleic acid condensed with an organic polycation, as summarized above. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1A and IB illustrate the role of neutral lipid in gene delivery. Three liposome formulations were tested for gene delivery to both culture cells (SKBR-3, human breast cancer cell) and mice (CD1, female, 20-25 g). Samples were: (1) DDAB/Chol (1:1); (2) DDAB/Chol/DOPE (1:0.5:0.5); (3) DDAB/DOPE (1:1); and (4) DDAB alone. Figure 1A illustrates cell transfection. SKBR-3 cells were plated at 50,000 cells per well in twelve- well plates and incubated overnight. Each well received 1 μg of P-CMWrVSLuc+ plasmid which was complexed with liposomes at 5 nmole of DDAB. Cells were harvested after 24 hr incubation with complexes at 37° C. Values presented are mean from 2 wells. Values ranged within 10-30% of mean. Figure IB illustrates in vivo transfection in mice. Mice received via tail vein injection 40 μg of P-CMVIVS-Luc+ plasmid, which was complexed with liposomes at 8 nmole DDAB per μg DNA ratio. Values presented are mean from 2 mice. Values ranged within 20-25% of mean. Figure 2 illustrates reporter gene expression in mouse tissue extracts. Mice received (via tail vein injection) 60 μg of P-CMVTVS-Luc-f- plasmid, which was complexed with DDAB/Chol (1:1) liposomes at 8-nmole DDAB per μg DNA ratio (without spermidine). Values presented are mean from 3 mice. Figure 3 illustrates the duration of reporter gene expression in mouse luna. Each animal received 40 μg of P-CMVIVS-Luc-r- plasmid, which was complexed with DDAB/Chol (1:1) liposomes at 8 nmole DDAB per μg DNA ratio. Figure 4 illustrates gene delivery in mouse lung by various stabilized complexes. Each mouse received 60 μg of P-CMVIVS-Luc+, which was complexed with DDAB/Chol (1:1) liposomes at 8 nmole DDAB/μg DNA ratio. Values presented are mean from 3 mice. Stippled bars: freshly made complexes; filled bars; one month-old samples. Samples are as follows: (1) No stabilizing agent was added; (2) PEG-PE was added at 1% of total lipid to the formed complexes; and (3) Spermidine (0.5 nmole per μg DNA) was added to the plasmid prior to the complex formation. Figure 5A and 5B illustrate in vitro transfection of cell lines with immunolipid:DNA complexes. The samples are as follows: (1) DDAB/DOPE (1:1), producing cationic liposomes complexed with DNA only; (2) DDAB/DOPE (1:1) with 1% PEG-PE derivatized with maleimide at the ultimate position of PEG, producing liposomes with the steric stabilization component added after complexation with the DNA; and (3) DDAB/DOPE (1:1) with 1% PEG-PE derivatized with the Fab' fragment of a humanized anti-Her-2 antibody attached to the ultimate position of PEG via the free thiol group to the maleimide residue. DEFINITIONS The following abbreviations are used herein: Choi, cholesterol; PA, phosphatidic acid; PC, phosphatidylcholine; PI, phosphatidylinositol; SM, sphinogmyelin; M-DPE, maleimide derivatized dipalmityolethanolamine; PBS, phosphate buffered saline; LUV, large unilamellar vesicles; MLV, multilamellar vesicles; PE, phosphatidylethanolamine; PEG, polyethylene glycol; PEG-PE, polyethylene glycol derivatized phosphatidylethanolamine, DC-chol, 3β [N-(N\ N'-dimethylaminoethane) carbanoyl]-cholesterol; DDAB, Dimethyidioctadecylammoniumbromide;DMEPC,Dimyristoylglycero-3-ethyl phosphocholine; DODAP, Dioleoyl-3-dimethylammonium propane; DOEPC, Dioleoylglycero-3-ethyl phosphocholine; DOGS, N,N-Dioctadecylamidoglycyl spermine; DOPE, Dioleoylphosphatidylethanolamine; DOTAP, Dioleoyl-3-trimethylammonium propane; DOTMA, N-[2,3-(dioleyloxy) propyl] N,N,N-trimethyl ammonium bromide; DSPE, Distearoylphosphatidylethanolamine; PEG-PE, N-[ω- methoxypoly(oxyethylene)-αoxycarbonyl]-DSPE; POEPC, Palmitoyloleoylglycero-3-ethyl phosphocholine. The term "amphiphilic cationic lipid" is intended to include any amphiphilic lipid, including synthetic lipids and lipid analogs, having hydrophobic and polar head group moieties, a net positive charge, and which by itself can form spontaneously into bilayer vesicles or micelles in water, as exemplified by phospholipids. The term also includes any amphiphilic lipid that is stably incorporated into lipid bilayers in combination with phospholipids with its hydrophobic moiety in contact with the interior, hydrophobic region of the bilayer membrane, and its polar head group moiety oriented toward the exterior, polar surface of the membrane. The term "specific binding" refers to that binding which occurs between such paired species as enzyme/substrate, receptor/agonist, antibody/antigen, and lectin/carbohydrate which may be mediated by covalent or non-covalent interactions or a combination of covalent and non-covalent interactions. When the interaction of the two species produces a non- covalently bound complex, the binding which occurs is typically electrostatic, hydrogen-bonding, or the result of lipophilic interactions. Accordingly, "specific binding" occurs between a paired species where there is interaction between the two which produces a bound complex having the characteristics of an antibody/antigen or enzyme/substrate interaction. In particular, the specific binding is characterized by the binding of one member of a pair to a particular species and to no other species within the family of compounds to which the corresponding member of the binding member belongs. Thus, for example, an antibody preferably binds to a single epitope and to no other epitope within the family of proteins. The terms "ligand" or "targeting moiety", as used herein, refer generally to all molecules capable of specifically binding to a particular target molecule and forming a bound complex as described above. Thus the ligand and its corresponding target molecule form a specific binding pair. Examples include, but are not limited to antibodies, lymphokines, cytokines, receptor proteins such as CD4 and CD8, solubilized receptor proteins such as soluble CD4, hormones, growth factors, and the like which specifically bind desired target cells, and nucleic acids which bind corresponding nucleic acids through base pair complementarity. Particularly preferred targeting moieties include antibodies and antibody fragments (e.g., the Fab' fragment). The term "lipid: nucleic acid complex" refers to the product made by mixing amphiphilic cationic lipids or liposomes with a nucleic acid. The term "CLDC," which stands for "cationic lipid:DNA complex" as used herein is not limited to DNA and is a convenient abbreviation for lipid: nucleic acid complex. The lipid:nucleic acid complex can also include a helper lipid. The helper lipid is often a neutral lipid such as DOPE or cholesterol with cholesterol being most preferred. The lipid:nucleic acid complex may also contain other compounds such as a polycation that are in contact with the nucleic acid of the complex, producing condensed nucleic acid, and hydrophilic polymers such as PEG and derivatized PEG. The terms "immunoliposome" and "immunolipid:nucleic acid complex" refer to a liposome or lipid:nucleic acid complex bearing an antibody or antibody fragment that acts as a targeting moiety enabling the lipid:nucleic acid complex to specifically bind to a particular "target" molecule that may exist in solution or may be bound to the surface of a cell. Where the target molecule is one that is typically found in relative excess (e.g., ≥ 10-fold) and in association with a particular cell type or alternatively in a multiplicity of cell types all expressing a particular physiological condition the target molecule is said to be a "characteristic marker" of that cell type or that physiological condition. Thus, for example, a cancer may be characterized by the overexpression of a particular marker such as the HER2 (c-erb -2/neu) proto-oncogene in the case of breast cancer. A "hydrophilic polymer" as used herein refers to long chain highly hydrated flexible neutral polymers attached to lipid molecules. Examples include, but are not limited to polyethylene glycol (PEG), polyethylene glycol derivatized with phosphatidyl ethanolamine (PEG-PE), polyethylene glycol derivatized with tween, polyethylene glycol derivatized with distearoylphosphatidylethanolamine (PEG-DSPE), ganglioside G M1 and synthetic polymers. Such polymers typically have a molecular weight in the range of 1000-10,000. Preferably, the molecular weight for PEG is approximately 2000. "Transfection" refers to contacting a living cell with a nucleic acid, for example, as part of a lipid:nucleic acid complex. "Transfection activity" refers to the efficiency of introducing a nucleic acid into a living cell. Transfection efficiency may be measured by determining the amount of expression of a reporter gene that has been transfected into the cell as part of a lipid:nucleic acid complex, for example, by fluorescent or functional assays. The terms "condensed nucleic acid" and "partially condensed nucleic acid" are used to refer to a nucleic acid that has been contacted with an organic cation for example, polyamines, including spermine and spermidine, polyammonium molecules such as Polybrene (hexadimethrine bromide), basic polyamino acids, and basic proteins. Condensed nucleic acids typically occupy a significantly smaller volume than non-condensed nucleic acids. It is recognized, however, that the degree of condensation may vary with local environment (e.g., lipid as opposed to aqueous environment). The term "shelf life" when used to refer to the lipid:nucleic acids disclosed herein refers to the period of time the lipid: nucleic acid complex can be stored (under defined conditions e.g., at 4°C) before losing its biological activity. The biological activity assayed for determination of shelf life in the present invention is the ability of the lipid:nucleic acid complex to transfect mammalian cells in vivo after intravenous administration. The "shelf life" of a lipid:nucleic acid complex is conveniently determined by assaying by gene expression from reporter nucleic acids in the lipid:nucleic acid complex as described herein. An "expression cassette" refers to a promoter operably linked to a DNA molecule, containing all the elements required for expression of that DNA molecule in a living cell. The expression cassette may contain additional elements such as enhancers, replication origins and the like, forming an expression vector. "Organic polycation" or "polycation" refer to a an organic polymeric structure where more than one unit of the polymer bears a negative charge and the net charge of the polymer is positive. Examples of such an organic cation are polyamines, including spermine and spermidine, polyammonium molecules such as Polybrene (hexadimethrine bromide), basic polyamino acids, or basic proteins. A "pharmaceutically acceptable carrier" is a material that is not biologically or otherwise undesirable, i.e., the material can be administer to an individual along with the lipid:nucleic acid complex without causing unacceptable biological effects or interacting in a deleterious manner with any of the other components of the pharmaceutical composition in which it is contained. The term "nucleic acid" refers to a polymer or oligomer composed of nucleotide units (ribonucleotides, deoxyribonucleotides or related structural variants or synthetic analogs thereof) linked via phosphodiester bonds (or related structural variants or synthetic analogs thereof). Thus, the term refers to a nucleotide polymer in which the nucleotides and the linkages between them are naturally occurring (DNA or RNA), as well as various analogs, for example and without limitation, peptide-nucleic acids (PNAs), phosphoramidates, phosphorothioates, methyl phosphonates, 2-O-methyl ribonucleic acids, and the like. The term "mole percent" when referring to the percentage of hydrophilic polymer in a liposome is expressed relative to the cationic lipid in the liposome unless otherwise stated. Thus, for example, in a liposome comprising a ratio of DDAB to cholesterol (Choi) of 100:100, a 4 mole percent of hydrophilic polymer (e.g., PEG) would represent a ratio of DDAB:Chol:PEG of about 100:100:4. The term "identical" refers to a composition that is formed using the same compounds as another composition, where the compositions do not differ in a statistically significant manner. The term "systemic administration" refers to a method of administering a compound or composition to a mammal so that the compound or composition is delivered to many sites in the body via the circulatory system. DETAILED DESCRIPTION This invention provides methods of increasing the shelf life of cationic lipid:nucleic acid complexes, and in vivo and/or in vitro transfection efficiency of these complexes. Such complexes have attracted considerable interest as a means of delivering nucleic acids expressing various therapeutic polypeptides as a means of delivering therapeutic (e.g., antisense) nucleic acids themselves. Unfortunately, it has been difficult to maintain and store homogeneous lipid:nucleic acid complexes suitable for in vivo administration. The complexes tend to aggregate rapidly or decompose within a relatively short time. This instability has required use of these complexes within a short period of time after preparation, often as little as 30 minutes up to a few hours. Thus, for example, in recent clinical trials using cationic lipids as a carrier for DNA delivery, the DNA and lipid components were mixed at the bedside and used immediately (Gao et al, Gene Therapy 2: 710-722 (1995)). This lipid: nucleic acid complex instability provides a significant hindrance to the widespread acceptance of cationic lipid:nucleic acid complexes as therapeutics. The necessity of preparation of the complex shortly before use requires that a pharmaceutical facility be in relatively close proximity to the area of use. Alternatively, combination of the lipid and nucleic acid at the bedside imposes a substantial labor burden, introduces quality control problems in insuring adequate complexation, creates a source of potential error. The present invention solves these problems by providing methods of significantly increasing the shelf (storage) life of lipid:nucleic acid complexes. The methods generally involve: (1) condensing the nucleic acid before incorporation into the lipid:nucleic acid complex; (2) combining a lipid:nucleic acid complex with a hydrophilic polymer (e.g., PEG); and (3) both condensing the nucleic acid prior to complex formation and combining the complex with a hydrophilic polymer. While condensation of nucleic acids may lead to stability of the nucleic acid in isolation (e.g., in an aqueous buffer), it was a surprising discovery of this invention that the use of a condensing agent (e.g., an organic polycation) provides a lipid:nucleic acid complex that remains capable of transfecting a cell in vivo even after a period of prolonged storage (e.g., cold storage at a temperature of about 22° C or below, more preferably ranging from about 0°C to about 22° C, and most preferably at about 4°C). It was also a surprising discovery that lipid:nucleic acid complexes combined with a hydrophilic polymer attached to an amphipathic lipid (e.g., PEG-PE) also show an increased shelf life. Without being bound by a particular theory, it is believed that when the cationic lipid:DNA complex ("CLDC") is contacted with the hydrophilic polymer, the hydrophilic polymer locates and is incorporated into hydrophobic pockets in the complex via its hydrophobic side chains, while leaving the hydrophilic part at the exterior surface, thereby stabilizing the entire complex. In view of these discoveries, this invention provides methods of increasing the shelf life of cationic lipid:nucleic acid complexes. The methods generally involve either condensing the nucleic acid using a polycation and/or contacting, e.g., coating, the lipid:nucleic acid complex with a hydrophilic polymer. This invention also include the lipid:nucleic acid complexes thus prepared. I. Cationic lipid:nucleic acid complexes As explained above, this invention provides methods of increasing the storage life (shelf life) of lipid:nucleic acid complexes. In a preferred embodiment the complexes are formed by combination of a nucleic acid with a liposome. It is recognized, however, that the lipids need not be provided as a liposome. It is also recognized that after complexation, the lipid:nucleic acid complex may no longer exist as a true vesicle and therefore is not generally regarded as a liposome. The preparation of lipid:nucleic acid complexes is well known to one of skill in the art (see, e.g., reviewed in Crystal, Science 270: 404-410 (1995); Blaese et al, Cancer Gene Ther. 2: 291- 297 (1995); Behr et al, Bioconjugate Chem. 5: 382-389 (1994); Remy et al, Bioconjugate Chem. 5: 647-654 (1994); and Gao et al, Gene Therapy 2: 710- 722 (1995)). The various components and construction of the stabilized lipid:nucleic acid complexes of the invention are described in detail below. A. Amphiphilic cationic lipids As indicated above, the methods of this invention involve complexing a cationic lipid with a nucleic acid. The term "cationic lipid" refers to any of a number of lipid species which carry a net positive charge at physiological pH. Such lipids include, but are not limited to, DODAC, DOTMA, DDAB, DOTAP, DC-Choi and DMRIE. Additionally, a number of commercial preparations of cationic lipids are available which can be used in the present invention. These include, for example, LIPOFECTIN® (commercially available cationic liposomes comprising DOTMA and DOPE, from GIBCO/BRL, Grand Island, New York, USA); LIPOFECTAMINE® (commercially available cationic liposomes comprising DOSPA and DOPE, from GIBCO/BRL); and TRANSFECTAM® (commercially available cationic lipids comprising DOGS in ethanol from Promega Corp., Madison, Wisconsin, USA). The cationic lipid can be used alone, or in combination with a "helper" lipid. Preferred helper lipids are non-ionic or uncharged at physiological pH. Particularly preferred non-ionic lipids include, but are not limited to cholesterol and DOPE, with cholesterol being most preferred. The molar ratio of cationic lipid to helper can range from 2:1 to about 1:2, more preferably from about 1.5:1 to about 1:1.5 and most preferably is about 1:1. Additional cationic and nonionic lipids suitable for use in the lipid:nucleic acid complexes of the present invention are well known to persons of skill in the art and are cited in a variety of well known sources, e.g., McCutcheon's Detergents and Emulsiβers and McCutcheon's Functional Materials, Allured Publishing Co., Ridgewood, N.J. Preferred lipids include DDAB: cholesterol or DOTAP: cholesterol at a molar ratio of 1:1. B. Nucleic acid The lipid:nucleic acid complexes contain a nucleic acid, typically an expression cassette that is constructed using recombinant techniques. A recombinant nucleic acid is prepared by first isolating the nucleic acid of interest. The isolated nucleic acid is then ligated into a cassette or vector suitable for expression of the gene. Methods for preparing a recombinant nucleic acid are known by those skilled in the art (see Sambrook et al, Molecular Cloning: A Laboratory Manual (2d ed. 1989)). The gene of interest, for example, a gene encoding a therapeutic polypeptide, or a reporter gene, can be inserted into an "expression vector," "cloning vector," or "vector," terms which usually refer to plasmids or other nucleic acid molecules that are able to replicate in a chosen host cell and express the gene of interest. Expression vectors can replicate autonomously, or they can replicate by being inserted into the genome of the host cell. Often, it is desirable for a vector to be usable in more than one host cell, e.g., in E. coli for cloning and construction, and in a mammalian cell for expression. Additional elements of the vector can include, for example, selectable markers and enhancers. Selectable markers, e.g., tetracycline resistance or hygromycin resistance, permit detection and/or selection of those cells transformed with the desired DNA sequences (see, e.g., U.S. Patent 4,704,362). The particular vector used to transport the genetic information into the cell is also not particularly critical. Any of the conventional vectors used for expression of recombinant proteins in prokaryotic or eukaryotic cells can be used. The expression vectors typically have a transcription unit or expression cassette that contains all the elements required for the expression of the nucleic acid in the host cells. A typical expression cassette contains a promoter operably linked to the DNA sequence encoding a protein. The promoter is preferably positioned about the same distance from the heterologous transcription start site as it is from the transcription start site in its natural setting. As is known in the art, however, some variation in this distance can be accommodated without loss of promoter function. In the expression cassette, the nucleic acid sequence of interest can be linked to a sequence encoding a cleavable signal peptide sequence to promote secretion of the encoded protein by the transformed cell. The expression cassette should also contain a transcription termination region downstream of the structural gene to provide for efficient termination. The termination region can be obtained from the same gene as the promoter sequence or can be obtained from a different gene. For more efficient translation in mammalian cells of the mRNA encoded by the structural gene, polyadenylation sequences are also commonly added to the expression cassette. Termination and polyadenylation signals that are suitable for the present invention include those derived from SV40, or a partial genomic copy of a gene already resident on the expression vector. In addition to the expression cassette, many expression vectors optimally include enhancer elements that can stimulate transcription up to 1,000 fold from linked homologous or heterologous promoters. Many enhancer elements derived from viruses have a broad host range and are active in a variety of tissues. For example, the SV40 early gene enhancer is suitable for many cell types. Other enhancer/promoter combinations that are suitable for the present invention include those derived from polyoma virus, human or murine cytomegalo virus, the long terminal repeat from various retroviruses such as murine leukemia virus, murine or Rous sarcoma virus, and HIV (see Enhancers and Eukaryotic Expression (1983)). In addition to the recombinant nucleic acids discussed above, synthetic nucleic acids or oligonucleotides can also be used in the invention. As a general point regarding the nucleic acids used in the invention, those of skill in the art recognize that the nucleic acids used in the invention include both DNA and RNA molecules, as well as synthetic, non-naturally occurring analogs of the same, and heteropolymers, of deoxyribonucleotides, ribonucleotides, and/or analogues of either. The particular composition of a nucleic acid or nucleic acid analogue will depend upon the purpose for which the material will be used and the environment in which the material will be placed. Modified or synthetic, non-naturally occurring nucleotides have been designed to serve a variety of purposes and to remain stable in a variety of environments, such as those in which nucleases are present, as is well known in the art. Modified or synthetic non-naturally occurring nucleotides, as compared to naturally occurring ribo- or deoxyribonucleotides may differ with respect to the carbohydrate (sugar), phosphate bond, or base portions of the nucleotide, or may even contain a non-nucleotide base (or no base at all) in some cases (see, e.g., Arnold et al., PCT patent publication no. WO 89/02439). For example, the modified or non-naturally occurring nucleic acids of the invention can include biotinylated nucleic acids, O-methylated nucleic acids, methylphosphonate backbone nucleic acids, phosphorothioate backbone nucleic acids, or polyamide nucleic acids. Oligonucleotides, such as antisense RNA described below, preferably are synthesized on an Applied BioSystems or other commercially available oligonucleotide synthesizer according to specifications provided by the manufacturer. Oligonucleotides may be prepared using any suitable method, such as the phosphotriester and phosphodiester methods, or automated embodiments thereof. In one such automated embodiment, diethylphosphoramidites are used as starting materials and may be synthesized as described by Beaucage et al, Tetrahedron Letters 22: 1859 (1981), and U.S. Patent No. 4,458,066. C. Condensed nucleic acid Small polycationic molecules are known to condense nucleic acids via electrostatic charge-charge interactions (Plum et al, Biopolymers 30: 631-643 (1990)). The pretreatment of nucleic acid with polyamines can therefore reduce the number of charge sites for complexing with cationic liposomes. However, condensing nucleic acid prior to lipid complex formation produced the surprising result of increased shelf life for lipid:nucleic acid complexes, as measured by transfection efficiency. The lipid:nucleic acid complexes formed with such pretreatment were stable at a lower ratio of lipid to DNA without aggregation. Organic polycations such as polyamines, polyammonium molecules, and basic polyamino acids, and their derivatives are used to condense the nucleic acid prior to lipid complex formation. A preferred embodiment uses polyamines such as spermidine and spermine to condense the nucleic acid (see, e.g., Example 1). D. Hydrophilic polymer It has been established recently that PEG-PE incorporation in liposomes produces steric stabilization resulting in longer circulation times in blood (Allen et al, Biochim. Biophys. Ada 1066: 29-36 (1991); Papahadjopoulos et al, Proc. Natl Acad. Sci. USA 88: 11460-11464 (1991)). In the present invention, inserting PEG-PE (e.g., 1% of total lipid) into the freshly formed lipid:nucleic acid complexes prevents the complexes from aggregating during storage. It was a surprising discovery, however, that the incorporation of PEG-PE did not inhibit transfection activity in vivo and also that the in vitro transfection activity, which was inhibited, was regained by the incorporation of Fab' fragment conjugated at the end of the PEG-PE. The presence of hydrophilic polymers in the lipid:nucleic acid complex provides increased shelf life, as measured by transfection efficiency after storage. Thus, it is desirable to add a hydrophilic polymer such as polyethylene glycol (PEG)-modified lipids or ganglioside G M1 to the liposomes. PEG may also be derivatized with other amphipathic molecules such as fatty acids, sphingolipids, glycolipids, and cholesterol. Addition of such components prevents liposome aggregation during coupling of the targeting moiety to the liposome. These components also provides a means for increasing circulation lifetime of the lipid:nucleic acid complexes. A number of different methods may be used for the preparation of PEG for incorporation into liposomes. In one preferred embodiment, PEG is incorporated as PEG derivatized phosphatidylethanolamine (PEG-PE) or PEG derivatized distearoyl phosphatidylethanolamine (PEG-DSPE) . Methods of preparing PEG-PE are well known and typically involve using an activated methoxy PEG (with only one reactive end) and PE. Thus PEG-succinimidyl succinate may be reacted in a basic organic solvent (Klibanov et al, FEBS Lett. 268: 235-237 (1990)). A particularly preferred method of PEG-PE preparation is based on reaction of the PEG with carbonyldiimidazole followed by addition of PE (see Woodle et al, Proc. Intern. Symp. Control. Rel. Bioact. Mater. 17: 77-78 (1990); Papahadjopoulos et al, Proc. Natl. Acad. Sci. USA 88: 11460-11464 (1991); Allen et al, Biochim. Biophys. Ada. 1066: 29-36 (1991); Woodle et al, Biochim. Biophys. Ada 1105: 193-200 (1992); and Woodle et al, Period. Biol. 93: 349-352 (1991)). Similarly, cyanuric chloride activated PEG in a basic organic solvent is described by Blume et al, Biochim. Biophys. Ada. 1029: 91-97 (1990) and U.S. Patent No. 5,213,804. A completely different approach is based on coupling the PEG with preformed liposomes utilizing tresyl chloride activated PEG which is then added to liposomes containing PE at high pH (Senior et al, Biochim. Biophys. Ada. 1062: 77-82 (1991)). Derivatized PEG is also commercially available. Thus, for example, PEG-PE is available from Avanti Polar lipids (Alabaster, Alabama) . One of skill in the art will recognize that many other linkages are available, e.g., PEG linked detergents such as tweens and insertion of PEG derivatized lipid into formed lipid: nucleic acid complexes. E. Fab' antibody fragment In a preferred embodiment, the lipid:nucleic acid complexes of the present invention are conjugated to the Fab' fragment of an antibody, which acts as a targeting moiety enabling the lipid:nucleic acid complex to specifically bind a target cell bearing the target molecule (e.g., characteristic marker) to which the Fab' antibody fragment is directed. Smaller peptides from the hypervariable region or from another peptide interacting with a specific cell surface ligand may also be conjugated to the complexes. In general terms, the Fab' fragment of an antibody represents a monomer comprising the variable regions and the C H 1 region of one arm of an antibody. One such preferred embodiment is described in Example 2. An "antibody" refers to a protein consisting of one or more polypeptides substantially encoded by immuno globulin genes or fragments of immuno globulin genes. The recognized immuno globulin genes include the kappa, lambda, alpha, gamma, delta, epsilon and mu constant region genes, as well as the myriad immunoglobulin variable region genes. Light chains are classified as either kappa or lambda. Heavy chains are classified as gamma, mu, alpha, delta, or epsilon, which in turn define the immunoglobulin classes, IgG, IgM, IgA, IgD and IgE, respectively. The basic immunoglobulin (antibody) structural unit is known to comprise a tetramer. Each tetramer is composed of two identical pairs of polypeptide chains, each pair having one "light" (about 25 kD) and one "heavy" chain (about 50-70 kD). The N-terminus of each chain defines a variable region of about 100 to 110 or more amino acids primarily responsible for antigen recognition. The terms variable light chain (V L ) and variable heavy chain (V H ) refer to these light and heavy chains respectively. Antibodies may exist as intact immunoglobulins or as a number of well characterized fragments produced by digestion with various peptidases. In particular, pepsin digests an antibody below the disulfide linkages in the hinge region to produce F(ab)' 2 a dimer of Fab' which itself is a light chain joined to V H -C H 1 by a disulfide bond. The F(ab)' 2 may be reduced under mild conditions to break the disulfide linkage in the hinge region thereby converting the F(ab)' 2 dimer into an Fab' monomer. The Fab' monomer is essentially an Fab with part of the hinge region (see Fundamental Immunology, W.E. Paul, ed., Raven Press, N.Y. (1993) for more antibody fragment terminology). While the Fab' fragment is defined in terms of the digestion of an intact antibody, one of skill will appreciate that such Fab' fragments may be synthesized de novo either chemically or by utilizing recombinant DNA methodology. The Fab' fragments used in the present invention may be derived from antibodies of animal (especially mouse or rat) or human origin or may be chimeric (Morrison et al, Proc Natl. Acad. Sci. USA 81: 6851-6855 (1984)) or humanized (Jones et al, Nature 321: 522-525 (1986), and published UK patent application No. 8707252). The Fab' fragment is selected to specifically bind to a molecule or marker characteristic of the surface of the cells to which it is desired to deliver the contents of the cationic lipid:nucleic acid complex. A molecule is characteristic of cell, tissue, or physiological state when that molecule is typically found in association with that cell type or alternatively in a multiplicity of cell types all expressing a particular physiological condition (e.g., transformation). A specific characteristic marker is preferably found on the surfaces of cells of a particular tissue or cell type or on the surfaces of tissues or cells expressing a particular physiological condition and on no other tissue or cell type in the organism. One of skill will recognize however, that such a level of specificity of the marker is often not required. For example a characteristic cell surface marker will show sufficient tissue specificity if the only non-target tissues are not accessible to the lipid:nucleic acid complex. Alternatively, effective specificity may be achieved by overexpression of the marker in the target tissue as compared to other tissues. This will result in preferential uptake by the target tissue leading to effective tissue specificity. Thus for example, many cancers are characterized by the overexpression of cell surface markers such as the HER2 (c-erδB-2, neu) proto-oncogene encoded receptor in the case of breast cancer. One of skill will recognize that there are numerous cell surface markers that provide good characteristic markers depending on the particular tissue it is desired to target. These cell surface markers include, but are not limited to carbohydrates, proteins, glycoproteins, MHC complexes, interleukins, and receptor proteins such as HER, CD4 and CD8 receptor proteins as well as other growth factor and hormone receptor proteins. Growth factor receptors are particularly preferred characteristic cell surface markers. Growth factor receptors are cell surface receptors that specifically bind growth factors and thereby mediate a cellular response characteristic of the particular growth factor. The term "growth factor", as used herein, refers to a protein or polypeptide ligand that activates or stimulates cell division or differentiation or stimulates biological response like motility or secretion of proteins. Growth factors are well known to those of skill in the art and include, but are not limited to, platelet-derived growth factor (PDGF), epidermal growth factor (EGF), insulin-like growth factor (IGF), transforming growth factor β (TGF-β), fibroblast growth factors (FGF), interleukin 2 (IL2), nerve growth factor (NGF), interleukin 3 (IL3), interleukin 4 (IL4), interleukin 1 (IL1), interleukin 6 (IL6), interleukin 7 (IL7), granulocyte/macrophage colony-stimulating factor (GM-CSF), granulocyte colony-stimulating factor (G-CSF), macrophage colony-stimulating factor (M-CSF), erythropoietin, interleukin 13 receptor (IL13R), and the like. One of skill in the art recognizes that the term growth factor as used herein generally includes cytokines and colony stimulating factors. Particularly preferred markers are found in the HER family of growth factor receptors. More specifically HER1, HER2, HER3 and HER4 are more preferred with HER2 most preferred. The HER receptors comprise protein tyrosine kinases that themselves provide highly specific antibody targets. Thus, in one embodiment, the P185 tyrosine kinase of HER2 provides a most preferred target for the Fab' fragment of the utilized in the immunolipid:nucleic acid complexes of the present invention. It will be appreciated that the characteristic marker need not be a naturally occurring marker, but rather may be introduced to the particular target cell. This may be accomplished by directly tagging a cell or tissue with a particular marker (e.g., by directly injecting the particular target tissue with a marker, or alternatively, by administering to the entire organism a marker that is selectively incorporated by the target tissue. In one embodiment, the marker may be a gene product that is encoded by a nucleic acid in an expression cassette. The marker gene may be under the control of a promoter that is active only in the particular target cells. Thus introduction of a vector containing the expression cassette will result in expression of the marker in only the particular target cells. One of skill in the art will recognize that there are numerous approaches utilizing recombinant DNA methodology to introduce characteristic markers into target cells. In one preferred embodiment, the targeting moiety will specifically bind products or components of a growth factor receptor, in particular products of the HER2 (c-erδB-2, neu) proto-oncogene. It is particularly preferred that the targeting moiety bind the growth factor receptor-tyrosine kinase encoded by HER2, protein pl85 HER2 , which is commonly overexpressed in breast cancers (Slamon et al, Science 235: 177- 182 (1987). Other suitable targets for the targeting moiety include, but are not limited to EGFR (HER1), HER3, and HER4, combinations of these receptors, and other markers associated with cancers. Other antibodies of interest include, but are not limited to BR96 (Friedman et al, Cancer Res. 53: 334-339 (1993), e23 to erbB2 (Batra et al, Proc. Natl. Acad. Sci. USA 89: 5867-5871 (1992)), PR1 in prostate cancer (Brinkmann et al, Proc. Natl Acad. Sci. USA. 90: 547-551 (1993)), and Kl in ovarian cancer (Chang et al. Int. J. Cancer 50: 373-381 (1992). Immunolipid: nucleic acid complexes of the present invention may be prepared by incorporating the Fab' fragment into the liposomes or lipids by a variety of techniques well known to those of skill in the art. The Fab' is added to the lipid: nucleic acid complex either before or after complex formation. For example, a biotin conjugated Fab' may be bound to a liposome containing a streptavidin. Alternatively, the biotinylated Fab' may be conjugated to a biotin derivatized liposome by an avidin or streptavidin linker. Thus, for example, a biotinylated monoclonal antibody was biotinylated and attached to liposomes containing biotinylated phosphatidylethanolamine by means of an avidin linker (see, e.g., Ahmad et al, Cancer Res. 52: 4817-4820 (1992)). Typically about 30 to 125 and more typically about 50 to 100 Fab' fragments per lipid:nucleic acid complex are used. In a preferred embodiment, the targeting moiety may be directly conjugated to the liposome. Such means of direct conjugation are well known to those of skill in the art (see, e.g., Gregoriadis, Liposome Technology (1984) and Lasic, Liposomes: from Physics to Applications (1993)). Particularly preferred is conjugation through a thioether linkage. This may be accomplished by reacting the antibody with a maleimide derivatized lipid such as maleimide derivatized phosphatidylethanolamine (M-PE) or dipalmitoylethanolamine (M-DEP). This approach is described in detail by Martin et al. J. Biol Chem. 257: 286-288 (1982). II. Preparation of liposomes A variety of methods are available for preparing liposomes as described in, e.g., Szoka et al, Ann. Rev. Biophys. Bioeng. 9:467 (1980); U.S. Pat. Nos. 4,186,183, 4,217,344, 4,235,871, 4,261,975, 4,485,054, 4,501,728, 4,774,085, 4,837,028, 4,946,787; PCT Publication No. WO 91/17424; Szoka & Papahadjopoulos, Proc. Natl Acad. Sci. USA 75: 4194-4198 (1978); Deamer & Bangham, Biochim. Biophys. Ada 443: 629-634 (1976); Fraley et al, Proc. Natl. Acad. Sci. USA 76: 3348-3352 (1979); Hope et al, Biochim. Biophys. Ada 812: 55-65 (1985); Mayer et al, Biochim. Biophys. Ada 858: 161-168 (1986); Williams et al, Proc. Natl. Acad. Sci. USA 85: 242-246 (1988), Liposomes, ch. 1 (Ostro, ed., 1983); and Hope et al, Chem. Phys. Lip. 40: 89 (1986). Suitable methods include, e.g., sonication, extrusion, high pressure/homogenization, microfluidization, detergent dialysis, calcium- induced fusion of small liposome vesicles, and ether-infusion methods, all well known in the art. One method produces multilamellar vesicles of heterogeneous sizes. In this method, the vesicle-forming lipids are dissolved in a suitable organic solvent or solvent system and dried under vacuum or an inert gas to form a thin lipid film. If desired, the film may be redissolved in a suitable solvent, such as tertiary butanol, and then lyophilized to form a more homogeneous lipid mixture which is in a more easily hydrated powder- like form. This film is covered with an aqueous buffered solution and allowed to hydrate, typically over a 15-60 minute period with agitation. The size distribution of the resulting multilamellar vesicles can be shifted toward smaller sizes by hydrating the lipids under more vigorous agitation conditions or by adding solubilizing detergents such as deoxycholate. In a preferred embodiment, mostly unilammellar liposomes are produced by the reverse phase evaporation method of Szoka & Papahadjopoulos, Proc. Natl Acad. Sci. USA, 75: 4194-4198 (1978). Unilamellar vesicles are generally prepared by sonication or extrusion. Sonication is generally performed with a bath-type sonifier, such as a Branson tip sonifier at a controlled temperature as determined by the melting point of the lipid. Extrusion may be carried out by biomembrane extruders, such as the Lipex Biomembrane Extruder. Defined pore size in the extrusion filters may generate unilamellar liposomal vesicles of specific sizes. The liposomes may also be formed by extrusion through an asymmetric ceramic filter, such as a Ceraflow Microfilter, commercially available from the Norton Company, Worcester MA. Following liposome preparation, the liposomes that have not been sized during formation may be sized by extrusion to achieve a desired size range and relatively narrow distribution of liposome sizes. A size range of about 0.2-0.4 microns allows the liposome suspension to be sterilized by filtration through a conventional filter, typically a 0.22 micron filter. The filter sterilization method can be carried out on a high through-put basis if the liposomes have been sized down to about 0.2-0.4 microns. Several techniques are available for sizing liposomes to a desired size. One sizing method is described in U.S. Pat. Nos. 4,529,561 or 4,737,323. Sonicating a liposome suspension either by bath or probe sonication produces a progressive size reduction down to small unilamellar vesicles less than about 0.05 microns in size. Homogenization is another method which relies on shearing energy to fragment large liposomes into smaller ones. In a typical homogenization procedure, multilamellar vesicles are recirculated through a standard emulsion homogenizer until selected liposome sizes, typically between about 0.1 and 0.5 microns, are observed. The size of the liposomal vesicles maybe determined by quasi-electric light scattering ( ELS) as described in Bloomfield, Ann. Rev. Biophys. Bioeng., 10: 421-450 (1981). Average liposome diameter maybe reduced by sonication of formed liposomes. Intermittent sonication cycles may be alternated with QELS assessment to guide efficient liposome synthesis. Extrusion of liposome through a small-pore polycarbonate membrane or an asymmetric ceramic membrane is also an effective method for reducing liposome sizes to a relatively well-defined size distribution. Typically, the suspension is cycled through the membrane one or more times until the desired liposome size distribution is achieved. The liposomes may be extruded through successively smaller-pore membranes, to achieve a gradual reduction in liposome size. For use in the present invention, liposomes having a size of about 0.05 microns to about 0.5 microns. More preferred are liposomes having a size of about 0.05 to 0.2 microns. III. Formation of lipidmucleic acid complexes It was a discovery of this invention that stabilized lipid:nucleic acid complexes (e.g., having condensed nucleic acid and/hydrophilic polymer) tended not to form visible large aggregates and had increased transfection efficiency and shelf life. Nucleic acid/liposome ratios for preparing lipid: nucleic acid complexes that do not form visible large aggregates can be determined by one of skill in the art. Typically, the ratio is determined by mixing fixed amounts of a nucleic acid, e.g., a plasmid, to various amounts of liposomes (see Example 1). In general, lipid:nucleic acid complexes are formed by pipetting the nucleic acid (e.g., plasmid DNA) into a liposome suspension of equal volume and mixing rapidly. Routinely, liposomes containing 5-15 nmole of a lipid such as DDAB or DOPE (as described above) form a complex with 1 μg plasmid, without forming visible large aggregates. Inspection for visible large aggregates is typically performed without the aid of a microscope. The endpoint of the titration of the amounts of lipid and nucleic acid is also achieved by assaying for increased transfection efficiency, either in vitro or in vivo, as compared to a non-stabilized control (as described below). To keep the lipid:nucleic acid complexes from forming large aggregates and losing transfecting activity with time, two approaches are taken: (1) incorporating a small amount of a hydrophilic polymer such as PEG-PE (approx. 1 % mole ratio) into lipid:nucleic acid complexes within a few minutes after their preparation; and/or (2) condensing the nucleic acid with a polycation such as a polyamine (e.g., approximately 0.05 to 5.0 nmole of spermidine per μg DNA) prior to mixing with the liposomes. The optimal amount of the polyamines and hydrophilic polymer can be determined by one of skill in the art by titrating the polyamine or hydrophilic polymer with the nucleic acid so that the formed complexes do not form large, e.g., visible, aggregates. The size of these lipid:nucleic acid complexes can be estimated by dynamic light scattering to be in the range of 410 ± 150 nm. The endpoint of the titration is also achieved by assaying for increased transfection efficiency either in vitro or in vivo, as compared to a non-stabilized control (as described below). IV. Transfection and gene therapy with lipid:nucleic acid complexes The present invention provides lipid: nucleic acid complexes that have increased shelf life, for transfection of mammalian cells in vitro, in vivo, and ex vivo, and methods of production and transfection of such complexes. In particular, this invention relies in part on the unexpected discovery that a lipid:nucleic acid complex comprising nucleic acid that has been condensed by contact with an organic polycation demonstrates an increased shelf life. In addition, this invention relies on the unexpected discovery that a lipid:nucleic acid complex, which is mixed with a hydrophilic polymer after lipid:nucleic acid complex formation, exhibits high transfection activity and increased shelf life, as measured by transfection activity after storage. Such lipid:nucleic acid complexes having increased shelf life are useful, e.g., for in vitro and ex vivo transfection of cells, and for delivery of nucleic acids into cells for mammalian gene therapy in vivo and following intravenous administration. Using lipid: nucleic acid complexes to deliver nucleic acids into different mammalian cell types results in a safe method of transfer, and high efficiency of gene transfer. Transfection of cells in vivo with lipid:nucleic acid complexes is known to those skilled in the art and can be performed using standard techniques, as discussed in Example 1 (see, e.g., Sambrook et al., Molecular Cloning: A Laboratory Manual (2d ed. 1989); Ausubel et al., Current Protocols in Molecular Biology (1995)). Any heterologous nucleic acid that is suitable for introduction into a host cell can be used in the present invention by one skilled in the art. Genes useful for gene therapy can be introduced into mammals using the methods and vectors of this invention. Genes encoding blood proteins, enzymes, hormones, ribozymes, antisense RNA, viral inhibitors, and ion channel proteins are examples of heterologous nucleic acids useful in gene therapy. A functional heterologous gene can be used to replace a mutated gene in a mammal using gene therapy. For example, the gene encoding β- globin can be used to treat β-thalassemia; and the gene encoding CFTR can be used to treat cystic fibrosis. Genes encoding selectable markers, such as those that confer antibiotic resistance, can be used to detect and isolate cells transfected with the lipid: nucleic acid complex. Reporter genes such as luciferase, β-galactosidase, chloramphenicol acetyl transferase (CAT), human growth hormone (hGH), and the green fluorescent protein (GFP) are preferred examples of genes that can be used in assays to determine transfection efficiency. In one embodiment of the invention, luciferase can be used as a reporter gene to determine transfection efficiency. Transfection efficiency of a reporter gene can be determined with an assay that is appropriate for the reporter gene in use. Such assays are known to those skilled in the art. For example, the HGH reporter assay is immunologically based and employs commercially available radioimmunoassay kits. In a preferred embodiment of the invention, the luciferase assay is used to detect transfection and expression of the luciferase reporter gene. The luciferase assay is preferred because it is highly sensitive and does not use radioactivity. A luminometer can be used to measure the luciferase enzyme activity, as described in Example 1. Gene therapy provides methods for combating chronic infectious diseases such as HIV infection, as well as non-infectious diseases such as cancer and birth defects (see generally Anderson, Science 256: 808-813 (1992); Yu et al., Gene Ther. 1: 13-26 (1994)). Gene therapy can be used to transduce cells with either an ex vivo or an in vivo procedure. Ex vivo methods for gene therapy involve transducing the cell outside of the mammal with a lipid:nucleic acid complex of this invention, and introducing the cell back into the organism. The cells can be hematopoietic stem cells isolated from bone marrow or other cells that can be transfected by lipid:nucleic acid complexes. In humans, hematopoietic stem cells can be obtained from a variety of sources including cord blood, bone marrow, and mobilized peripheral blood. Purification of CD34 + cells can be accomplished by antibody affinity procedures (see Ho et al, Stem Cells 13 (suppl. 3): 100-105 (1995); see also Brenner, J. Hematotherapy 2: 7-17 (1993)). Cells can also be isolated and cultured from patients. Alternatively, the cells used for ex vivo procedures can be those stored in a cell bank (e.g., a blood bank). The advantage to using stem cells is that they can be differentiated into other cell types in vitro, or can be introduced into a mammal (such as the donor of the cells) where they will engraft in the bone marrow. Methods for differentiating bone marrow cells in vitro into clinically important immune cell types using cytokines such a GM-CSF, IFN-γ and TNF-α are known (see, e.g., Inaba et al, J. Exp. Med. 176: 1693-1702 (1992)). Delivery of a nucleic acid can also be achieved using in vivo gene therapy. The lipid:nucleic acid complexes of the invention can be administered directly to a patient, preferably a human. In vivo and ex vivo administration is by any of the routes normally used for introducing a molecule or cell into ultimate contact with blood or tissue cells. Lipid: nucleic acid complexes of the invention are administered in any suitable manner, preferably with pharmaceutically acceptable carriers. Suitable methods of administering such non-viral particles in the context of the present invention to a patient are known to those skilled in the art. Preferably, the pharmaceutical compositions are administered using aerosol administration (e.g., using a nebulizer or other aerosolization device), and parenterally, i.e., intraarticularly, intravenously, intraperitoneally, subcutaneously, or intramuscularly. More preferably, the pharmaceutical compositions are administered via aerosol administration or intravenously or intraperitoneally by a bolus injection. Particular formulations which are suitable for this use are found in Remington's Pharmaceutical Sciences (17th ed. 1985). Typically, the formulations will comprise a solution of the lipid: nucleic acid complexes suspended in an acceptable carrier, preferably an aqueous carrier. V. Pharmaceutical compositions Pharmaceutical compositions comprising the lipid:nucleic acid complexes of the invention are prepared according to standard techniques and further comprise a pharmaceutically acceptable carrier. Generally, normal saline will be employed as the pharmaceutically acceptable carrier. Other suitable carriers include, e.g., water, buffered water, isotonic solution (e.g., dextrose), 0.4% saline, 0.3% glycine, and the like, including glycoproteins for enhanced stability, such as albumin, lipoprotein, globulin, etc. These compositions may be sterilized by conventional, well known sterilization techniques. The resulting aqueous solutions may be packaged for use or filtered under aseptic conditions and lyophilized, the lyophilized preparation being combined with a sterile aqueous solution prior to administration. The compositions may contain pharmaceutically acceptable auxiliary substances as required to approximate physiological conditions, such as pH adjusting and buffering agents, tonicity adjusting agents and the like, for example, sodium acetate, sodium lactate, sodium chloride, potassium chloride, calcium chloride, etc. Additionally, the lipid:nucleic acid complex suspension may include lipid- protective agents which protect lipids against free-radical and lipid- peroxidative damages on storage. Lipophilic free-radical quenchers, such as alphatocopherol and water-soluble iron-specific chelators, such as ferrioxamine, are suitable. The concentration of lipid:nucleic acid complexes in the pharmaceutical formulations can vary widely, i.e., from less than about 0.05%, usually at or at least about 2-5% to as much as 10 to 30% by weight and will be selected primarily by fluid volumes, viscosities, etc., in accordance with the particular mode of administration selected. For example, the concentration may be increased to lower the fluid load associated with treatment. This may be particularly desirable in patients having atherosclerosis-associated congestive heart failure or severe hypertension. Alternatively, immunolipid:nucleic acid complexes composed of irritating lipids may be diluted to low concentrations to lessen inflammation at the site of administration. The amount of lipid:nucleic acid complex administered will depend upon the particular Fab' used, the disease state being treated, and the judgement of the clinician. Generally the amount of lipid:nucleic acid complexes administered will be sufficient to deliver a therapeutically effective dose of the nucleic acid. The quantity of lipid:nucleic acid complex necessary to deliver a therapeutically effective dose can be determined by one skilled in the art. Typical lipid:nucleic acid complex dosages will generally be between about 0.01 and about 50 mg nucleic acid per kilogram of body weight, preferably between about 0.1 and about 10 mg nucleic acid/kg body weight, and most preferably between about 2.0 and about 5.0 mg nucleic acid/kg of body weight. For administration to mice, the dose is typically 50-100 μg per 20 g mouse. VI. Assaying blood half-life One aid for lipid:nucleic acid complex localization in a target tissue is an extended lipid:nucleic acid complex lifetime in the bloodstream following administration. One measure of lipid:nucleic acid complex lifetime in the bloodstream is the blood/RES ratio determined at a selected time after complex administration. Typically lipid:nucleic acid complexes containing a label (e.g., fluorescent marker, electron dense reagent, or radioactive marker), either internal in the complex or bound to a lipid comprising the complex are injected into the test organism. A fixed period of time later, the organism is sacrificed and the amount of label detected in the blood (e.g., by measuring luminescence, or scintillation counting) is compared to that localized in particular tissues (e.g., liver or spleen). The time course of retention of lipid:nucleic acid complexes in the blood may also simply be determined by sampling blood at fixed intervals after administration of label-containing lipid:nucleic acid complexes and determining the amount of label remaining in the circulation. The result may be expressed as the fraction of the original dose. VII. Assaying tissue transfection by the lipidrnucleic acid complexes Transfection of target cells by the lipidrnucleic acid complexes of this invention may similarly be determined by administering lipid:nucleic acid complexes containing a nucleic acid that is itself detectable or that encodes a detectable product. Biological samples (e.g., tissue biopsies or fluid samples) are then collected and assayed for transfection by detecting the presence of the transfected nucleic acid itself or by detecting the presence of the expressed product of the nucleic acid. The nucleic acid itself can be selected to have a sequence that is readily detectable, e.g., by nucleic acid amplification. In this instance, the nucleic acid would be selected that has primer sites selected so as to permit unique amplification of the subject nucleic acid and no other in the sample of the biological tissue that is to be assayed for transfection. Means for detecting specific DNA sequences are well known to those of skill in the art. For instance, oligonucleotide probes chosen to be complementary to a select subsequence with the region can be used. Alternatively, sequences or subsequences may be amplified by a variety of DNA amplification techniques including, but not limited to polymerase chain reaction (PCR) (Innis et al, PCR Protocols: A guide to Methods and Application (1990)), ligase chain reaction (LCR) (see Wu & Wallace, Genomics 4: 560 (1989); Landegren et al, Science 241: 1077 (1988); Barringer et al, Gene 89: 117 (1990), transcription amplification (Kwoh et al, Proc. Natl. Acad. Sci. USA 86: 1173 (1989)), and self-sustained sequence replication (Guatelli et al, Proc. Nat. Acad. Sci. USA 87: 1874 (1990)). In a particularly preferred embodiment, transfection is evaluated by detecting the presence or absence or quantifying a gene product in one or more tissues. Any gene that expresses an easily assayable product will provide a suitable indicator for the present assay. Suitable reporter genes are well known to those of skill in the art. They include, but are not limited to, bacterial chloramphenicol acetyl transferase (CAT), beta-galactosidase, or luciferase (see, e.g., Alam et al, Analytical Biochemistry 188: 245-254 (1990)). One particularly preferred reporter gene is the Fflux gene as illustrated in the Examples. VIII. Assaying shelf life As indicated above, the term "shelf life" is used herein to refer to the period of time the lipid:nucleic acid complex can be stored (under defined conditions e.g., in a buffer at 4° C) before losing its biological activity. The biological activity assayed for determination of shelf life in the present invention is the ability of the lipid: nucleic acid complex to transfect mammalian cells in vivo after intravenous administration. In a preferred embodiment the shelf life is determined by storing the lipid:nucleic acid complexes for varying periods of time, injecting one or more test animals with the complex and assaying selected tissues in the animal for transfection (e.g., expression of a reporter gene) as described above and as illustrated in the examples. It will be appreciated that shelf life can be expressed in absolute terms, i.e., the length of time the composition can be stored before loosing activity. Alternatively, shelf life can be expressed in relative terms by reference to a different composition. Thus, for example, when the subject complex shows transfection activity after a fixed period of storage and this activity is greater than the activity of a different complex similarly stored for the same amount of time, the subject complex is said to have an increased shelf life as compared to the different complex. IX. Targeting lipidrnucleic acid complexes to specific tissues Specific targeting moieties can be used with the lipidrnucleic acid complexes of the invention to target specific cells or tissues. In one embodiment, the targeting moiety, such as an antibody or antibody fragment, is attached to a hydrophilic polymer and is combined with the lipidrnucleic acid complex after complex formation. Thus, the use of a targeting moiety in combination with a generic effector lipidrnucleic acid complex provides the ability to conveniently customize the complex for delivery to specific cells and tissues. Examples of effectors in lipidrnucleic acid complexes include nucleic acids encoding cytotoxins (e.g., diphtheria toxin (DT), Pseudomonas exotoxin A (PE), pertussis toxin (PT), and the pertussis adenylate cyclase (CYA)), antisense nucleic acid, ribozymes, labeled nucleic acids, and nucleic acids encoding tumor suppressor genes such as p53, pllORb, and p72. These effectors can be specifically targeted to cells such as cancer cells, immune cells (e.g., B and T cells), and other desired cellular targets with a targeting moiety. For example, as described above, many cancers are characterized by overexpression of cell surface markers such as HER2, which is expressed in breast cancer cells, or IL17R, which is expressed in gliomas. Targeting moieties such as anti-HER2 and anti-IL17R antibodies or antibody fragments are used to deliver the lipidrnucleic acid complex to the cell of choice. The effector molecule is thus delivered to the specific cell type, providing a useful and specific therapeutic treatment. X. Lipid:nucleic acid complex kits The present invention also provides for kits for preparing the above-described lipidrnucleic acid complexes. Such kits can be prepared from readily available materials and reagents, as described above. For example, such kits can comprise any one or more of the following materials: liposomes, nucleic acid (condensed or uncondensed), hydrophilic polymers, hydrophilic polymers derivatized with targeting moieties such as Fab' fragments, and instructions. A wide variety of kits and components can be prepared according to the present invention, depending upon the intended user of the kit and the particular needs of the user. For example, the kit may contain any one of a number of targeting moieties for targeting the complex to a specific cell type, as described above. The kit may optionally include instructional materials containing directions (i.e., protocols) providing for the use of the cationic lipid:nucleic acid complex for transfecting cells in vivo, ex vivo, or in vitro. Typically, the instruction materials describe the procedure for preparing the lipid:nucleic acid complex from liposomes and nucleic acid, as described above. The instruction materials also describe how to mix the hydrophilic polymer with the lipidrnucleic acid complex. Additionally, the instruction materials can describe procedures for transfecting cells with the lipidrnucleic acid complex. While the instructional materials typically comprise written or printed materials, they are not limited to such. Any medium capable of storing such instructions and communicating them to an end user is contemplated by this invention. Such media include, but are not limited to electronic storage media (e.g., magnetic discs, tapes, cartridges, chips), optical media (e.g., CD ROM), and the like. Such media may include addresses to internet sites that provide such instructional materials. EXAMPLES The invention is illustrated by the following examples. These examples are offered to illustrate, but not to limit the present invention. Example 1: Preparation of stable lipid:plasmid DNA complexes for in vivo gene delivery A. Materials and methods 1. Lipids & other reagents DOPE was purchased from Avanti (Alabaster, AL). Highly purified Cholesterol was obtained from Calbiochem (San Diego, CA). DDAB and dextran (M.W. 40,000) were purchased from Sigma (St. Louis, MO). DDAB was recrystalized once from acetone-methanol solution. D-luciferin was obtained from Boehringer Mannheim. PEG-PE was a gift from Sequus Pharmaceuticals (Menlo Park, CA). DC-Choi, MMCE and DOGS were obtained from UCSF Gene Transfer Vehicle Core of Gene Therapy Center. ESPM, DOTAP, POEPC, DOEPC, DMEPC and DODAP were gifts from Avanti (Alabaster, AL). Chloroform solution of each lipid was stored under argon in sealed ampules at -40° C. Other reagents of possible highest grade were purchased and used without further purification. 2. Preparation of liposomes Small cationic liposomes were prepared in 5% (w/v) dextrose solution in the following fashion. DDAB or other cationic lipids in chloroform was mixed with DOPE or/and cholesterol in a desired molar ratio, and the solvent was removed slowly under reduced pressure at 50° C on a rotary evaporator. The dry lipid film was hydrated with 5% dextrose solution prewarmed to 50° C and the container was sealed under argon. The hydrated lipid suspension was sonicated in a bath sonicator (Lab Supplies, Hicksville, N.Y.) for 5-10 min at 50° C. The final concentration of liposomes was 5 mM cationic lipid and the size of liposomes was measured by dynamic light scattering to be 195 ± 65 nm. Sonicated liposomes were stored under argon at 4°C until use. 3. Luciferase reporter system Plasmid, pCMV/TVS-luc + , was constructed as follows. A fragment containing the CMV promoter and synthetic IgE intron was excised from pBGt2.CAT using Spe I and Hind III and cloned into pBSIIKS + . The cDNA encoding the modified firefly luciferase (luc +) including SV40 late poly (A) signal was cut from the pGL3-Basic Vector (Promega) with Hind III and Sal I and was put into the pBS-CMV-IVS clone downstream of the splice. Plasmids were purified using alkaline lysis procedures adopted and devised by Qiagen Corp. (Chats worth, CA). Plasmid purity was measured by the ratio of absorbance at 260 nm vs 280 nm, and stored in buffer containing 10 mM Tris-Cl and lmM EDTA at pH 8.0 at concentrations of 1-2 mg/ml. 4. Preparation of transfection complexes Prior to the transfection experiments, the optimal DNA/liposome ratio for forming complexes which were not large aggregates was determined by mixing fixed amount plasmid to various amount of liposomes. In general, the transfection complexes were formed by pipetting plasmid into liposome suspension of equal volume and mixing rapidly. Routinely, liposomes containing 8-12 nmole of DDAB could complex with 1 μg plasmid without forming visible large aggregates. Such complexes have excess positive charge, but still tend to aggregate with time during storage at 4°C and lose transfection activity in 4 days. For in vitro experiments, which called for much dilute complexes, cationic lipid:plasmid DNA complexes ("CLDC") at 5 nmole DDAB per μg DNA were used. To keep the lipid:plasmid DNA complexes from forming large aggregates and losing transfecting activity with time, two approaches were taken: (1) incorporating a small amount of PEG-PE (approx. 1 % mole ratio) into lipid:plasmid DNA complexes within a few minutes after their preparation; and/or (2) condensing plasmid with polyamines (e.g., 0.05 to 5.0 nmole of spermidine per μg DNA) prior to mixing with liposomes. The optimal amount of the polyamines was determined by titrating polyamines to DNA before forming large aggregates. The size of these complexes was estimated by dynamic light scattering to be in the range of 410 ± 150 nm. 5. Assay of reporter gene expression Purified luciferase was purchased from Boehringer Mannheim as a standard for calibrating the luminometer and constructing a control standard for the relative specific activity of luciferase. Reporter gene expression in a tissue extract was presented in nanogram quantities by converting relative light unit measured from a luminometer into weight unit according to a standard curve. Luciferase expressed in cells or tissues was extracted with chemical cell lysis. Effective lysis buffer consisted of 0.1 M potassium phosphate buffer at pH 7.8, 1 % Triton X-100, 1 mM DTT and 2 mM EDTA. Female CD1 mice (4-6 weeks old, weighing approx. 25 g) were obtained from Charles River Laboratory. Mice received lipid:plasmid DNA complexes by tail vein injection and were sacrificed 24 h later. The anesthetized animals were perfused with cold phosphate-buffered saline (PBS) via heart puncture. Each tissue was dissected and washed in PBS, and then homogenized in 6 ml round-bottomed culture tube containing 500 μl of lysis buffer. The samples were kept at room temperature for 20 min with occasional mixing. The homogenized samples were centrifuged for 10 min at 3000 rpm in an Eppendorf centrifuge. Luciferase activity of each tissue was measured by mixing 100 μl of the reconstituted luciferase substrate (Promega, Madison, WI) with 20 μl of the supernatant of tissue homogenate in the injection system of a luminometer. Peak light emission was measured for 10 sec. at 20° C. Relative light units of each sample were converted to the amount of luciferase in the tissue extract by comparing with a standard curve which was established for each set of experiments. The protein content of the extract was determined using protein assay kits (BioRad, Richmond, CA). Background was the count of lysis buffer only. SK-BR-3 cells (Park et al, Proc. Natl. Acad. Sci. USA 92: 1327-1331 (1995)) were cultured in McCoy's 5 A medium supplemented with 10% heat-inactivated bovine calf serum and in 5% C0 2 . SK-BR-3 cells in monolayer culture were plated at 50,000 cells per well in 12-well plates and incubated overnight. Each well received 0.5 ~ 1 μg of pCMV/IVS-luc + within 20 min of complex formation. Cells were harvested after 24 hr of incubation with complexes at 37° C. Luciferase activity in cells was determined as described above. B. Results 1. Optimizing the "helper" lipid The use of cationic liposomes for in vitro gene transfer has become widespread since Feigner et al. published their study (Feigner et al, Proc. Natl. Acad. Sci. USA 84: 7413-17 (1987)). It was established later (Feigner et al, J. Biol Chem. 269: 2550-2561 (1994)) that DOPE is by far the most efficient "helper" lipid for in vitro gene transfection and this result has confirmed by several laboratories (Farhood et al, in Gene therapy for Neoplastic Diseases, pp 23-55 (Huber & Lazo eds., 1994); Zhou et al, Biochim. Biophys. Ada 1189: 195-203 (1994)). It has been suggested, on the basis of in vitro studies, that DOPE may facilitate the cytoplasmic delivery via membrane fusion once positively charged lipid:plasmid DNA complexes are bound to the cell membrane (Zhou et al, Biochim. Biophys. Ada 1189: 195-203 (1994)). Even though Friend et al did not obtain any morphological evidence that the DOTMA/DOPE lipid:plasmid DNA complexes fuse directly with the plasma membrane, they do not exclude the possibility of fusion events (Friend et al, Biochim. Biophys. Ada 1278: 41-50 (1996)). They suggested that the complexes are endocytosed and the cationic lipids disrupt the endosomal/lysosomal membranes and then facilitate an escape of the DNA complexes into the cytoplasm and eventually into the nucleus. Contrary to most expectations, the "helper" role of DOPE established from in vitro studies is not evident for in vivo gene delivery following i.v. injection of the complexes. When DOPE was included in DDAB cationic liposomes, the in vivo gene transfection was inhibited. This DOPE-dependent inhibition is shown in Figure 1. Cholesterol, not DOPE, was found to be effective as "helper" lipid for in vivo gene delivery. There was a ten-fold reduction in luciferase expression in mouse lungs when half of the cholesterol was replaced with DOPE. The in vivo results of DDAB and other cationic liposomes are not consistent with the general assumption that DOPE is a suitable "helper" lipid. On the contrary, DOPE in cationic lipid:plasmid DNA complexes attenuates the in vivo transfection to such a great degree that DOPE is considered as an inhibitory agent in formulations for in vivo gene delivery. Cholesterol has been chosen for in vivo studies in recent published reports (Liu et al, J. Biol. Chem. 270: 24864-70 (1995); Solodin et al, Biochemistry 34: 13537-44 (1995)) in which the authors do not elaborate on how and why they selected different "helper" lipids for their experimental designs, i.e. DOPE for in vitro and cholesterol for in vivo studies. Stabilization of anionic and neutral liposomes in blood by cholesterol has been known for a long time (Mayhew et al, Cancer Treat. Rep. 63: 1923-1928 )1979)). It is therefore obvious that for systemic gene delivery, one has to consider the stability of lipid:plasmid DNA complexes in blood, various components of which are known to react with macromolecular complexes. In fact, the preliminary study of various formulations of lipid:plasmid DNA complexes using freeze-fracture electron microscopy has shown that the cholesterol-containing complexes were structurally more stable than the DOPE-containing complexes in the presence of serum. Using DDAB/Chol lipid :plasmid DNA complexes (8 nmole DDAB/μg DNA) for in vivo transfection experiments, detectable luciferase expression in the lung of 25 g mouse required a DNA dose ranging from 30 μg to 60 μg. Routinely 40 ~ 60 μg plasmid DNA per mouse gave consistent gene expression. The amount of DDAB usually associated with 80 μg DNA (or more) per mouse was found to be too toxic to the animal. The expression of luciferase in various tissues is shown in Figure 2. As observed before (Zhu et al, Science 261: 209-211 (1993); Liu et al, J. Biol, Chem. 270: 24864-70 (1995); Solodin et al, Biochemistry 34: 13537-44 (1995)), maximal expression was found in lung tissue. For 60 μg plasmid injected, 1-2 ng luciferase per mg tissue protein was routinely obtained. Figure 3 shows the duration of reporter gene expression in lung tissue. Expression of luciferase decreased rapidly and reached undetectable levels in 2 weeks. Zhu et al reported that following i.v. injection of DOTMA/DOPE (1:1) - plasmid complexes into adult mice, the expression of the reporter gene (CAT) is widespread among various tissues and the maximum expression is from complexes with a ratio of 1 1 μg plasmid to 8 nmole total lipids (Zhu et al, Science 261: 209-211 (1993)). However, at this ratio (corresponding to 1 μg plasmid to 4 nmole cationic lipid), DDAB/Chol lipid:plasmid DNA complexes tended to aggregate and did not produce measurable gene expression in this investigation. Since different reporter genes have been used among different laboratories, it has been difficult to attribute the variations in the efficiency of in vivo gene delivery to changes in the formulation of liposomes. For a direct comparison of the results in the literature, the relative light units of luciferase activity measured from a luminometer was converted to a standard of purified luciferase. By doing so, the peak transfection activity of DDAB/Chol formulations was 3 orders of magnitude higher than values reported recently in comparable experiments (Thierry et al. , Proc. Natl. Acad. Sci. USA 92: 9742-9746 (1995)). Given that same reporter gene along with same promoter in the experimental design, the difference in expression may reflect the selection of liposome formulation. In fact, DDAB/Chol was one of the most efficient gene delivery vehicle among many formulations from 18 different cationic lipids which was screened recently. Preliminary results of expression in mouse lung following i.v. injection indicated that DOTMA/Chol, DOTAP/Chol, MMCE/Chol and ESPM/Chol gave 10-100% transfection activity of DDAB/Chol, DOGS/Chol, POEPC/Chol, LYSPE/DOPE and DC-Chol/DOPE gave 1-10% of DDAB/Chol. DOEPC/Chol, DMEPC/Chol, DODAP/Chol and DDAB/DOPE did not give any measurable activity. In parallel with the transfection studies, the morphology of these complexes in serum and in cell medium was examined by freeze-fracture electron microscopy. When examined in 50% mouse serum (10 minute incubation time), non-stabilized, one day old CLDC are as small as they are in buffer at low ionic strength (100-250 nm) but show very few protrusions. Six day old, non stabilized CLDC incubated in 50% mouse serum appeared as densely packed aggregates of spherical particles, with a high number of attached particles. Such formulations have lost all of their in vivo transfection activity within 4 days. Residual fibrillar protrusions are not observed. PEG-PE stabilized CLDC incubated in 50% mouse serum were small (100-200 nm) even at six days. Similarly, CLDC prepared with condensed DNA were also quite small even after six days of storage. Specifically, the CLDC were shaped like "map pins" that were structurally stable in the presence of serum. After incubation in cell medium (RPMI-1640 with 10% FCS), non-stabilized six day old CLDC were morphologically similar to those incubated in mouse serum, as described above. These complexes, however, were more loosely packed and showed no fibrillar protrusions. Similar morphology was observed with PEG-PE stabilized CLDC and condensed DNA CLDC incubated in cell medium. 2. Increasing shelf life for transfection activity The relationship between structural stability and transfection activity of lipid :plasmid DNA complexes has not been detailed in the published reports so far. Screening procedures have been established to avoid large aggregates of lipid:plasmid DNA complexes by changing the ratio of DNA to lipid from net negatively charged to positively charged. Lipid:plasmid DNA complexes of each particular cationic lipid at various ratios of DNA/lipid were prepared and the resulting stable and metastable formulations were used for in vivo transfection. Complexes which contained 8 to 12 nmole of cationic lipid per μg DNA were found to have the highest in vivo transfection activity. However, the transfection activity of these complexes decreased with time. Without modifying the procedures of forming the lipid:plasmid DNA complexes, there was a visible aggregation within a few days, and the transfection activity decreased by more than a thousand fold to almost background levels after one month's storage at 4°C (Figure 4). Therefore, formulation of stabilized lipid:plasmid DNA complexes was undertaken, which could maintain high in vivo transfection activity during storage. i. Increasing transfection stability: PEG-PE Inserting PEG-PE (1% of total lipid) into the freshly formed lipid:plasmid DNA complexes not only could prevent the complexes from aggregating during storage, but that the PEG-PE containing complexes exhibited reasonably high transfection activity in vivo, only slightly lower activity as compared to the complexes without PEG-PE (Figure 4). Incorporation of PEG-PE into the complexes is evident in view of the dose-related inhibition of the transfection activity with increasing percentage of PEG-PE (results not shown). Unexpectedly, storage of the complexes containing PEG-PE at 4° C has slowly restored the original activity, as shown in Figure 4. The mechanistic aspects of the inhibition effect on transfection by PEG-PE, as well as the recovery of the activity following storage at low temperature, are not known at present time. ii. Increasing transfection stability: polyamines In addition to the role of PEG-PE in increasing the shelf life of lipid:nucleic acid complexes, condensing nucleic acid with polyamines also gave a similar unexpected increase in shelf life of the complexes. The lipid :plasmid DNA complexes formed with condensed DNA were stable at a lower ratio of lipid to DNA without aggregation. Figure 4 shows the level of in vivo transfection activity of such preparation, and its fate during storage. Again, An unexpected increase of the transfection activity was found in aged polyamine-treated lipid:plasmid DNA complexes, when compared to that of the samples which were not pretreated with polyamines and used immediately after complexes were formed. A different approach to obtain stable cationic lipid/DNA complexes by complexing plasmid with lipid in lipid-detergent micelles was published recently (Hofland et al. , Proc. Natl. Acad. Sci. USA 93: 7305-7309 (1996)). However, only 30% of the transfection efficiency was maintained by such complexes in 15% serum for in vitro, and no in vivo results were reported. Hi. Increasing transfection stability: Ivophilization Finally, conditions have been established for the stabilization of lipid:plasmid DNA complexes by lyophilization. Liposomes composed of DDAB/Chol suspended by sonication in 5% (w/v) of dextran in water, when mixed with DNA in 1:10 ratio (μg DNA per nmole DDAB) as described in methods, could be lyophilized without loss of activity. The final concentration of dextran in which lipid:plasmid DNA complexes were formed was 8% (w/v). The lyophilized preparations were reconstituted by adding distilled water and their transfection activity in the lungs of mice after i.v. injection was measured by luciferase reporter gene expression. Freezing and thawing the reconstituted preparation did not affect the activity (usually 1-2 ng luciferase protein per mg tissue protein). Several of the cationic lipid:plasmid DNA complexes described herein are stable and can give consistent in vivo transfection activity (ranging from 0.5 to 2 ng luciferase per mg tissue protein) even after long storage at 4°C or lyophilization. Formulations containing cholesterol as the "helper" lipid generate much higher in vivo transfection efficiency. Stabilizing the complex structure by PEG-PE maintains the complex activity in storage and may prolong the circulation time in blood for targeting to specific tissues. Condensing the DNA with polyamines before lipid complexation enhances in vitro storage and levels of activity in vivo. The methodical approach for producing stable formulations of lipid:plasmid DNA complexes exhibiting high transfection activity in vivo confers advantages for establishing pharmaceutically acceptable preparations, and therefore facilitates liposome - based gene therapy. Example 2: In vitro transfection of lipidrplasmid DNA complexes with targeting ligands A. Preparation of Fab' fragments Cloned rhuMAbHER2 sequences for heavy and light chain were co-expressed in E. coli as previously described (Carter et al, Biotechnology 10: 163-167 (1992)). The antibody fragment, rhuMAbHER2-Fab', was recovered from E. coli fermentation pastes by affinity chromatography with Streptococcal protein G (Carter et al, Biotechnology 10:163-167 (1992)), typically yielding Fab' with 60-90% containing reduced free thiol (Fab'-SH). B. Preparation of liposomes Condensed DNA was complexed with three different lipid compositions, using the methods described above in Example 1, with the following modifications. The first complex was made with DDAB/DOPE (1/1), which produced cationic liposomes complexed with DNA only, as described above. The second complex was made with DDAB/DOPE (1/1) with 1% PEG- PE derivatized with maleimide at the ultimate position of PEG, producing CLDC with the steric stabilization component added after complexation with the DNA. The third complex was made with DDAB/DOPE (1/1) with 1% PEG-PE derivatized with the Fab' fragment of a humanized anti-Her-2 antibody attached to the ultimate position of PEG via the free thiol group to the maleimide residue. This produced CLDC with the targeting ligand attached to the steric stabilization component added after the complexation with the DNA. C. Transfection and results Cells were transfected as described above in Example 1, but without storage of the lipid:plasmid DNA complex. Two cell lines were used in this Example. The first cell line was MCF-7, which do not overexpress the HER-2 receptor. These cells were cultured in DME H-21 with 10% bovine calf serum and in 5% C0 2 . The second cell line was SK-BR3 cells, which overexpress the HER-2 receptor, cultured in McCoy's 5A medium with bovine calf serum in 5% C0 2 . In both cases, the cells ( ~ 5 x 10 4 cells per well) were transfected and incubated with 12 μg plasmid DNA complexed with lipid as described above (PCMV/TVS-luc+, luciferase reporter gene described above) for 4 hours at 37° C. The supernatant was then aspirated, fresh medium was added and the cells were incubated for 24 hours at 37° C. Cells were then harvested by washing with PBS (Ca/Mg free) and then suspended in lysis buffer for the luciferase assay, as described above. Figure 5A shows that transfection of non-target cells, not overexpressing the HER-2 receptor, was inhibited by the addition of PEG-PE, even in the presence of the targeting ligand conjugated at the tip of PEG via the terminal maleimide residue. Figure 5B shows that transfection of target cells overexpressing the HER-2 receptor was also inhibited by the addition of PEG-PE, but the transfection activity was restored and augmented when the PEG-PE was conjugated to a targeting ligand, which recognizes the HER-2 receptor. Comparison of Figures 5A and 5B indicates that the targeted immuno-CLDC were active in transfecting target cells much more efficiently than non-target cells. This result occurs because the addition of the ligand- carrying stabilizing agent (PEG-PE) conjugated to anti-HER-2-Fab'), which inhibits the transfection of non-target cells (Figure 5A) but augments transfection of the target cells (Figure 5B). The above examples are provided to illustrate the invention but not to limit its scope. Other variants of the invention will be readily apparent to one of ordinary skill in the art and are encompassed by the appended claims. All publications, patents, and patent applications cited herein are hereby incorporated by reference.

Claims: WHAT IS CLAIMED IS: 1. A method of increasing the shelf life of a lipid:nucleic acid complex, said method comprising the steps of: contacting a nucleic acid with an organic polycation to produce a condensed nucleic acid; and combining said condensed nucleic acid with a lipid comprising an amphiphilic cationic lipid to produce said lipid: nucleic acid complex, wherein said lipid: nucleic acid complex comprising said condensed nucleic acid has increased shelf life as compared to an identical lipid:nucleic acid complex lacking said organic polycation. 2. The method of claim 1, wherein said organic polycation is selected from the group consisting of polyamine, polyammonium, and basic polyamino acid. 3. The method of claim 2, wherein said polyamine is selected from the group consisting of spermine and spermidine. 4. The method of claim 1, wherein said nucleic acid is selected from the group consisting of DNA and RNA. 5. The method of claim 1, wherein said nucleic acid is DNA. 6. The method of claim 1, wherein said step of combining said condensed nucleic acid with said lipid comprises first forming a liposome comprising said amphiphilic cationic lipid. 7. The method of claim 1, wherein said step of combining said condensed nucleic acid with said lipid comprises combining said lipid and said nucleic acid in a ratio ranging from about 1 to about 20 nmole lipid: μg nucleic acid. 8. The method of claim 1, wherein said step of contacting said nucleic acid with said organic polycation comprises contacting said organic polycation and said nucleic acid in a ratio ranging from about 0.05 to about 5.0 nmole organic polycation: μg nucleic acid. 9. The method of claim 1, wherein said method comprises the steps of: contacting an expression cassette with a polyamine selected from the group consisting of spermidine and spermine to produce a condensed expression cassette; and combining said condensed expression cassette with a lipid comprising DDAB and cholesterol from about 2:1 to about 1:2 molar ratio to produce a lipid expression cassette complex, wherein said lipid:expression cassette complex comprising said condensed expression cassette has an increased shelf life at about 4°C as compared to an identical lipid:expression cassette complex lacking said polyamine. 10. The method of claim 9, wherein said lipid complex is mixed with an amphipathic lipid attached to polyethylene glycol conjugated to a Fab' fragment of an antibody. 11. The method of claim 1, wherein said lipid:nucleic acid complex is lyophilized. 12. A method of increasing the shelf life of a lipid:nucleic acid complex, said method comprising the steps of: combining a nucleic acid with a lipid comprising an amphiphilic cationic lipid to produce said lipid:nucleic acid complex; and mixing said lipid:nucleic acid complex with a hydrophilic polymer, wherein said lipid:nucleic acid complex has increased shelf life as compared to an identical lipid:nucleic acid complex lacking said hydrophilic polymer. 13. The method of claim 12, wherein said hydrophilic polymer is selected from the group consisting of polyethylene glycol (PEG), polyethylene glycol derivatized with phosphatidyl ethanolamine (PEG-PE), polyethylene glycol derivatized with tween, polyethylene glycol derivatized with distearoylphosphatidylethanolamine (PEG-DSPE), and ganglioside G M1 . 14. The method of claim 12, wherein said nucleic acid is selected from the group consisting of DNA and RNA. 15. The method of claim 12, wherein said nucleic acid is DNA. 16. The method of claim 12, wherein said step of combining said nucleic acid with said lipid comprises first forming a liposome comprising said amphiphilic cationic lipid. 17. The method of claim 12, wherein said step of combining said nucleic acid with said lipid comprises combining said lipid and said nucleic acid in a ratio ranging from about 1 to about 20 nmole lipid: μg nucleic acid. 18. The method of claim 12, wherein said step of mixing said lipid:nucleic acid complex with said hydrophilic polymer comprises mixing said hydrophilic polymer and said lipid:nucleic acid complex in a molar ratio from about 0.1 to about 10% hydrophilic polymer to lipid within the lipid: nucleic acid complex. 19. The method of claim 12, wherein said method comprises the steps of: combining an expression cassette with said lipid comprising said amphiphilic cationic lipid to produce a lipid:expression cassette complex; and mixing said lipid:expression cassette complex with polyethylene glycol derivatized with phosphatidyl ethanolamine (PEG-PE), wherein said lipid expression cassette complex has increased shelf life at about 4°C as compared to an identical lipid expression cassette complex lacking said polyethylene glycol derivatized with phosphatidyl ethanolamine (PEG-PE). 20. The method of claim 19, wherein said lipid comprises polyethylene glycol attached to a Fab' fragment of an antibody. 21. The method of claim 12, wherein said lipid:nucleic acid complex is lyophilized. 22. A method of transfecting a nucleic acid into a cell, said method comprising the step of contacting said cell with a lipid: nucleic acid complex, said lipid:nucleic acid complex comprising: said nucleic acid contacted with an organic polycation to produce a condensed nucleic acid; and a lipid comprising an amphiphilic cationic lipid, wherein contacting said cell with said lipid:nucleic acid complex transfects said nucleic acid into said cell. 23. The method of claim 22, wherein said lipid:nucleic acid complex is stored at a temperature of about 22° C or below. 24. The method of claim 22, wherein said organic polycation is selected from the group consisting of polyamine, polyammonium, and basic polyamino acid. 25. The method of claim 24, wherein said polyamine is selected from the group consisting of spermine and spermidine. 26. The method of claim 22, wherein said nucleic acid is DNA. 27. The method of claim 22, wherein said step of contacting said cell with said lipid:nucleic acid complex comprises systemic administration of said lipid:nucleic acid complex into a mammal. 28. The method of claim 22, wherein said step of contacting said cell with said lipid:nucleic acid complex comprises intravenous administration of said lipid:nucleic acid complex into a mammal. 29. The method of claim 22, wherein said method comprises the step of contacting said cell with a lipid:expression cassette complex, said lipid:expression cassette complex comprising: an expression cassette contacted with a polyamine selected from the group consisting of spermidine and spermine to produce a condensed expression cassette; and said lipid comprising said amphiphilic cationic lipid, wherein said lipid:expression cassette complex is stored at a temperature of about 22° C or below. 30. The method of claim 29, wherein said lipid comprises polyethylene glycol attached to a Fab' fragment of an antibody. 31. A method of transfecting a nucleic acid into a cell, said method comprising the step of contacting said cell with a lipid:nucleic acid complex, said lipid:nucleic acid complex comprising: said nucleic acid; a lipid comprising an amphiphilic cationic lipid; and a hydrophilic polymer, wherein contacting said cell with said lipid:nucleic acid complex transfects said nucleic acid into said cell. 32. The method of claim 31, wherein said lipid: nucleic acid complex is stored at a temperature of about 22° C or below. 33. The method of claim 31, wherein said hydrophilic polymer is selected from the group consisting of polyethylene glycol (PEG), polyethylene glycol derivatized with phosphatidyl ethanolamine (PEG-PE), polyethylene glycol derivatized with tween, polyethylene glycol derivatized with distearoylphosphatidylethanolamine (PEG-DSPE), and ganglioside G M1 . 34. The method of claim 31, wherein said nucleic acid is DNA. 35. The method of claim 31, wherein said step of contacting said cell with said lipid:nucleic acid complex comprises systemic administration of said lipid:nucleic acid complex into a mammal. 36. The method of claim 31, wherein said step of contacting said cell with said lipid:nucleic acid complex comprises intravenous administration of said lipid:nucleic acid complex into a mammal. 37. The method of claim 31, wherein said method comprises the step of contacting said cell with a lipid expression cassette complex, said lipidexpression cassette complex comprising: an expression cassette; said lipid comprising said amphiphilic cationic lipid; and polyethylene glycol derivatized with phosphatidyl ethanolamine (PEG- PE), wherein said lipidexpression cassette complex is stored at a temperature of about 22° C or below. 38. The method of claim 37, wherein said lipid comprises polyethylene glycol attached to a Fab' fragment of an antibody. 39. A lipid:nucleic acid complex comprising: a nucleic acid contacted with an organic polycation to produce a condensed nucleic acid; and a lipid comprising an amphiphilic cationic lipid, wherein said lipid:nucleic acid complex has an increased shelf life as compared to an identical lipid:nucleic acid complex lacking said organic polycation. 40. The complex of claim 39, wherein said organic polycation is selected from the group consisting of polyamine, polyammonium, and basic polyamino acid. 41. The complex of claim 40, wherein said polyamine is selected from the group consisting of spermine and spermidine. 42. The complex of claim 39, wherein said nucleic acid is selected from the group consisting of DNA and RNA. 43. The complex of claim 39, wherein said nucleic acid is DNA. 44. The complex of claim 39, wherein the amount of said lipid and said nucleic acid is in a ratio ranging from about 1 to about 20 nmole lipid: μg nucleic acid. 45. The complex of claim 39, wherein said nucleic acid is contacted with said organic polycation in a ratio ranging from about 0.05 to about 5.0 nmole organic polycation: μg nucleic acid. 46. A complex of claim 39, wherein said complex comprises: an expression cassette contacted with a polyamine selected from the group consisting of spermidine and spermine to produce a condensed expression cassette; and said lipid comprising said amphiphilic cationic lipid, wherein said lipidexpression cassette complex has an increased shelf life at about 4°C as compared to an identical lipidexpression cassette complex lacking said spermidine. 47. The complex of claim 46, wherein said lipid comprises polyethylene glycol attached to a Fab' fragment of an antibody. 48. A pharmaceutical composition comprising: a nucleic acid condensed with a polycation and complexed with a a lipid comprising an amphiphilic cationic lipid, in a pharmaceutically acceptable carrier, wherein said pharmaceutical composition has an increased shelf life as compared to an identical pharmaceutical composition lacking said organic polycation. 49. The composition of claim 48, wherein said organic polycation is selected from the group consisting of polyamine, polyammonium, and basic polyamino acid. 50. The composition of claim 49, wherein said polyamine is selected from the group consisting of spermine and spermidine. 51. The composition of claim 48, wherein said nucleic acid is DNA. 52. The composition of claim 48, wherein the amount of said lipid and said nucleic acid is in a ratio ranging from about 1 to about 20 nmole lipid: μg nucleic acid. 53. The composition of claim 48, wherein said composition comprises: an expression cassette contacted with a polyamine selected from the group consisting of spermidine and spermine to produce a condensed expression cassette; and said lipid comprising said amphiphilic cationic lipid, wherein said composition has an increased shelf life at about 4° C as compared to an identical composition lacking said polyamine. 54. The composition of claim 53, wherein said lipid comprises polyethylene glycol attached to a Fab' fragment of an antibody. 55. A kit for preparing a lipid:nucleic acid complex, said kit comprising: (i) a container containing a liposome; (ii) a container containing a nucleic acid; and (iii) a container containing a hydrophilic polymer, wherein the liposome and the nucleic acid are mixed to form the lipid:nucleic acid complex and wherein the lipid:nucleic acid complex is contacted with the hydrophilic polymer. 56. A kit of claim 55, wherein said hydrophilic polymer is derivatized with a targeting moiety. 57. A kit of claim 56, wherein said targeting moiety is an Fab' fragment. 58. A kit of claim 55, wherein said nucleic acid is condensed nucleic acid.

Inventors: HONG DEELUNG, PAPAHADJOPOULOS DEMETRIOS, ZHENG WEIWEN, HONG, DEELUNG, PAPAHADJOPOULOS, DEMETRIOS, ZHENG, WEIWEN
Applicants: UNIV CALIFORNIA, THE REGENTS OF THE UNIVERSITY OF CALIFORNIA
Application year: 1997
Main classification: A61K9
IPC codes: A61K9, A61K31, A61K47, A61K48, C07H21