Abstract:
The invention relates to the use of a composition comprising a corticosteroid encapsulated in a vesicle for the manufacture of a medicament for treating cancer, such as the use of a composition comprising a corticosteroid and liposomes, the liposomes comprising a non-charged vesicle-forming lipid and, optionally, an amphipathic vesicle-forming lipid and/or a negatively charged vesicle-forming lipid. The invention further relates to a new pharmaceutical composition suitable for treating cancer, especially, solid primary and secondary tumors.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of PCT International Application No. PCT/NL2003/000596, filed Aug. 25, 2003 designating the United States of America, published in English as PCT International Publication No. WO 2004/019916 A1 on Mar. 11, 2004, which application claims priority to European Patent Application No. 02078525.9, filed Aug. 27, 2002, the contents of both of which are incorporated by this reference. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates to the use of a composition comprising a corticosteroid in the treatment of cancer or the inhibition of cancer growth. More specifically, the invention relates to a method for targeting a corticosteroid to tumor tissue. Even more specifically, the present invention aims to provide the use of such compositions in the treatment of non-lymphatic cancers and, preferably, solid (non-hematological) malignancies and metastases (solid primary and secondary tumors), and is hence in the field of oncology.  
       BACKGROUND  
       [0003]     Corticosteroids, such as glucocorticoids, have been proposed as active ingredients in the treatment of cancer. For example, Coleman has described in  Biotherapy  4(1) (1992), 37-44, that in tumor therapy, glucocorticoids are often used for their anti-inflammatory and anti-emetic potential and for the treatment of hematological malignancies due to their efficient cytolytic activity on cells of lymphoid origin.  
         [0004]     In the treatment of lymphatic cancer types, e.g., chronic and acute lymphatic leukemia, Hodgkin and non-Hodgkin lymphomas, steroids have shown antitumor activity. In view of the fact that cells of lymphatic origin can be suppressed by corticosteroids (immunosuppressive activity), this is not surprising.  
         [0005]     An example of such a treatment is disclosed in U.S. Pat. No. 6,090,800, wherein vesicles, liposomes and micelles are described that contain lipid-soluble steroid prodrugs. It is noted in this document that “steroids such as cortisone and dexamethasone are potent immune suppressants and are used to treat conditions such as auto-immune diseases, organ transplant rejection, arthritis, skin, mucosal membrane and ophthalmic inflammation, as well as neoplastic conditions such as lymphoma.” In addition, this U.S. patent teaches targeting to IL-2 receptors on T-cells.  
         [0006]     Further reports in the 1980&#39;s and 1990&#39;s demonstrated that glucocorticoids could also decelerate solid tumor growth. Several studies showed that this effect was not directly aimed at the tumor cells, but rather mediated by interference with the tumor vascularization (see in this light, e.g., Folkman et al. in  Science  221 (4612) (1983), 719-723; Lee et al. in  Cancer Res.  47(19) (1987), 5021-5024; McNatt et al. in  J. Ocul. Pharmacol. Ther.  15(5) (1999) 413-423; and Crowley et al. in  Oncology  45(4) (1988), 331-335). The exact mechanism of interference is, however, unclear. It has been suggested that it is mediated by inhibition of endothelial cell proliferation and migration (Cariou et al. in  Cell. Biol. Int. Rep.  12(12) (1988), 1037-1047), basement membrane turnover (Folkman et al. (ibid); Ingber et al. in  Endocrinology  119(4) (1986), 1768-1775), and/or inhibition of pro-angiogenic factors (like plasminogen activator and vascular endothelial growth factor) (Blei et al. in  J Cell Physiol.  155(3) (1993), 568-578; Nauck et al. in  Eur. J. Pharmacol.  341(2-3) (1998), 309-315). Furthermore, the hypothesis that inflammatory processes in and around the tumor are important in the angiogenic cascade suggests that glucocorticoids&#39; immunosuppressive action may also be involved (Bingle et al. in  J Pathol.  196(3) (2002), 254-265; O&#39;Byrne &amp; Dalgleish in  Br. J. Cancer  85(4) (2001), 473-483).  
         [0007]     One of the drawbacks of the strategy of using corticosteroids in solid tumor therapy is the need for high dosing (typically 100-200 mg/kg per day) for prolonged periods of time to obtain significant tumor growth inhibition. The doses inevitably lead to severe side effects and have been shown to result in morbidity and mortality as a result of severe immune suppression in experimental animals.  
         [0008]     It has been proposed that selective delivery of glucocorticoids to tumor tissue could be an attractive strategy to increase local drug concentrations. This would reduce the overall dose and, hence, decrease the likelihood of side effects. For instance, U.S. Pat. No. 5,762,918 aimed at selective delivery of steroids to tumor endothelial cells via conjugation to polyanionics and especially to heparin (fragments). A maximal tumor inhibition of approximately 65% at a dose of cortisol of 35 mg/kg per day for eight days is indicated.  
         [0009]     More particularly, the corticosteroid-heparin conjugate described in U.S. Pat. No. 5,762,918 is claimed to be capable of delivering the corticosteroid more selectively to vascular endothelial cells. In vivo studies in mice demonstrated that a daily dose of 1 mg heparin-cortisol over a nine day period resulted in a tumor reduction of 65%, compared to 40% reduction in mice treated with free cortisol and free heparin. The dosage per kg body weight was not specified but it has been found in both a mice model and a murine model that a cortisol-heparin conjugate is capable of obtaining a 65% reduction in tumor growth rate when administered at a daily dose of 35 mg/kg over an eight day period.  
         [0010]     A drawback of this approach is the chemical derivatization required to bind the corticosteroid to the polyanionic and particularly to heparin. The resultant binding between corticosteroid and heparin should be sufficiently stable to reach the tumor, but sufficiently reversible to allow dissociation of the corticosteroid. Furthermore, in order to overcome problems relating to the anticoagulant effect of heparin, specified heparins or its derivatives should be selected.  
         [0011]     Although the patentee mentioned in U.S. Pat. No. 5,762,918 claims that the corticosteroid has an improved activity because it is specifically delivered by the heparin to endothelial cells in and around the tumor, this mechanism is not demonstrated or supported by data. On the basis of the data presented, the only conclusion that can be drawn is that the heparin-corticosteroid conjugate has a higher activity. Moreover, it is noted that the pharmacokinetic behavior of the conjugate has not been studied. The heparin used has a low molecular weight, leading to efficient clearance by the kidneys. This means that the conjugate probably will also be excreted by the kidneys, resulting in the fact that specific targeting of the conjugate to the tumor is not highly effective.  
         [0012]     The conjugate is, as stated in U.S. Pat. No. 5,762,918, dependent on a stable coupling between corticosteroid and heparin and the cleavage of this coupling at the tumor site. The nature of the bond is critical and difficult to control. A premature cleavage or too little degradation of the coupling leads to less effective activity.  
         [0013]     U.S. Pat. No. 5,762,918 additionally states, while referring to the problems associated with too high of dose of corticosteroids in the treatment of, among other conditions, cancer, that 
        “. . . another means of altering the pharmacologic properties of a drug, including altered biodistribution and reduced toxicity. Liposomal encapsulation involves the incorporation of molecules of the drug into a “capsule” of lipid materials, usually composed of uni- and multilamellar vesicles of various phospholipids. Unfortunately, the ability to form stable liposomes of a particular agent is somewhat limited, in that liposome stability is a function of a variety of parameters such as drug lipophilicity and other structural consideration. Thus, liposomal encapsulation has not proved to be applicable to a broad spectrum of agents. Furthermore, the ability of liposomal encapsulation to alter biodistribution is unpredictable at best. Thus, such encapsulation has not provided a particularly satisfactory broadly applicable means of targeting a selected drug in a uniform, tissue-specific manner.”       
 
         [0015]     There is an ongoing need for alternative therapies and therapeutical compositions.  
       SUMMARY OF THE INVENTION  
       [0016]     The present invention provides a method to target corticosteroids to tumor tissue for or in the treatment, retardation or inhibition of cancer. Particularly, to treat, retard or inhibit the growth of solid and non-lymphatic cancer types.  
         [0017]     More specifically, the present invention is directed to a method to use microvesicles to encapsulate corticosteroids and use these systems for targeted tumor delivery.  
         [0018]     In accordance with the present invention, it has now been found that a composition comprising a corticosteroid, encapsulated in particular types of microvesicles, can be used to manufacture a medicament useful in the treatment of cancer.  
         [0019]     Accordingly, the present invention relates to the use of a composition comprising a corticosteroid encapsulated in a long-circulating microvesicle for the manufacture of a medicament useful in the treatment of cancer, and especially in oncology. The composition was found to inhibit tumor growth, especially solid tumor growth, and can hence be used for this effect in the treatment of cancer, especially in oncology.  
         [0020]     The long-circulating microvesicles have very favorable pharmacokinetics, a favorable tissue distribution behavior and an efficient half-life. Additionally, a stable association between corticosteroid and the carrier system, the microvesicles, is observed, while the loading with corticosteroid is efficient. Further, a good biological availability at the site where activity is required is observed. Without wishing to be bound by any theory, it is hypothesized that the microvesicles have an interaction with macrophages in the tumor. This would mean that the cell type could be down-regulated in tumor therapy or that it may liberate the encapsulated corticosteroids to allow interaction with other cell types in and around the tumor.  
         [0021]     In a preferred embodiment, the long-circulating microvesicle is a liposome, a nano-capsule or a polymeric micelle.  
         [0022]     Other suitable long-circulating microvesicles can be based on lipoproteins, especially high-density lipoproteins and low-density lipoproteins, and on lipoprotein mimetics or neo-lipoproteins.  
         [0023]     It has been found that long-circulating microvesicles, and especially long-circulating liposomes, nano-capsules and polymeric micelles, are capable of efficiently delivering corticosteroids drugs to a specific site, i.e., to a tumor. In particular, the present invention provides a medicament for, or in the treatment of, cancer suitable to administer corticosteroids, and especially glucocorticoids, in relatively low dosages. In accordance with the invention, effective inhibitions in tumor growth have been observed in particular embodiments with relatively low dosages of only 20 mg/kg body weight per week.  
         [0024]     In addition, it has been found that by the use of corticosteroids present in the microvesicles of the invention, massive apoptosis of tumor cells in the center of the tumor is observed, which may be related to an impaired blood supply to the tumor. Further, there is an indication that the tumor is encapsulated by connective tissue. Particularly in a mouse model, a loss of binding of the tumor to the underlying tissue is observed. Microscopic evaluation appeared to show formation of connective tissue encapsulating the tumor.  
         [0025]     Without wishing to be bound by any theory, it is believed that microvesicles used in the present invention accumulate at sites of malignant tissues such as tumors as a result of the enhanced permeability of tumor vasculature as compared to healthy endothelium, allowing an improved localization and improved retention of the corticosteroid at these sites.  
         [0026]     The long-circulating microvesicles used in accordance with the present invention typically have a mean particle diameter of less than 450 nm, and preferably less than 300 nm, as determined by dynamic light scattering using a Malvern 4700™ system equipped with a He/Ne laser, and preferably in a range of about 40 nm to 200 nm. Moreover, as can be seen in the working examples (vide infra), the microvesicles of the invention have a rather small polydispersity, which means that the particle size distribution is narrow. Preferably, the polydispersity, which is calculated by the software belonging to the dynamic light scattering equipment, is less than 0.25, and more preferably less than 0.2.  
         [0027]     Preferably, the long-circulating microvesicles used in the compositions of the invention are long-circulating liposomes. With such types of long-circulating microvesicles, it has been found (vide infra) that tumor growth can be reduced by more than 65% and even up to 90%, compared to controls, at a dose of only 20 mg/kg body weight per week, within two to three weeks.  
         [0028]     Such long-circulating liposomes are already known in the art, even in combination with corticosteroids, especially water-soluble corticosteroids. More particularly, these known liposome systems are described to be useful in site-specific treatment of inflammatory disorders in WO-A-02/45688. For the preparation of suitable compositions to be used in the present invention, the preparation methods described in WO-A-02/45688 are incorporated herein by reference.  
         [0029]     In WO-A-02/45688, the liposome systems described in EP-A-0 662 820 are adapted to become “long-circulating.” 
         [0030]     EP-A-1 044 679 (also incorporated herein by this reference) relates to liposomes having a drug included therein, which are said to have an ensured stability in blood. In addition, these liposomes have an active targeting property to proteoglycan-rich areas. These areas are created because with some diseases, an over-production of proteoglycans occurs, the proteoglycans keeping cell surfaces anionic. To target liposomes to these anionic surfaces, the liposomes need to be cationic in nature. Thereto, the liposomes require the presence of a basic compound taking positive charge within a physiological pH range.  
         [0031]     The liposomes of the present invention do not require the active targeting property described in EP-A-1 044 679. That is, no specific homing groups are required to selectively bring the microvesicles to the tumor sites. However, to increase the selectivity to an even higher extent, it is possible to attach or incorporate tumor-specific antibodies or receptor ligands or food compounds at the outside surface of the microvesicles so as to increase the interaction possibilities with the tumor cells or the cells in or around the tumor.  
         [0032]     The “targeting” of the neutral or optionally negatively charged liposomes of the present invention is ruled by the above-identified increased permeability in the tumor vasculature. That is, the present invention is, for a major part, based on passive accumulation, rather than active targeting.  
         [0033]     The liposomes useful in the present invention should not have a positive charge and hence, should not comprise components that give the liposomes a positive charge at physiological pH; that is, at physiological pH, being a pH of between 6 and 8, the overall charge of the liposome to be used in the present invention should be neutral or negatively charged.  
         [0034]     Preferred liposomes are based on non-charged vesicle-forming lipids. Neutral or non-charged vesicle-forming lipids lead to a suitable long circulation time. Typically, 5 to 10 mole % of negatively charged lipids may be present. Preferred lipids to be used to prepare the microvesicles used in the invention comprise saturated phospholipids and sphingolipids in combination with cholesterol and/or ergosterol and derivatives thereof.  
         [0035]     To secure a suitable stability in the blood circulation system, 10 to 50 mole % sterols should be present in the microvesicle material. Suitable liposome constituents are described in the above-identified WO-A-02/45688 and EP-A-0 662 820.  
         [0036]     More preferably, the liposomes contain at least one type of polymer lipid conjugates, such as lipids derivatized with polyalkylene glycol, preferably with polyethylene glycol (PEG). Suitable polymer-lipid conjugates have a molecular weight of between 200 and 30,000 Dalton.  
         [0037]     Other suitable candidates to be used in these polymer-lipid conjugates or water-soluble polymers are poly ((derivatized) carbohydrate)s, water-soluble vinylpolymers (e.g., poly(vinylpyrrolidone), polyacrylamide and poly(acryloylmorpholine)), and poly(methyl/ethyl oxazone). These polymers are coupled to the lipid through conventional anchoring molecules.  
         [0038]     Suitably, the concentration of polymer-lipid conjugates is 0 to 20 mole %, and preferably 1 to 10 mole %, based upon the total molar ratio of the vesicle-forming lipids.  
         [0039]     The presence of these polymer-lipid conjugates has a favorable effect on the circulation time. However, by carefully selecting specific lipid compositions and physical specifications, suitable long circulation times can be obtained without using a polymer-lipid conjugate, for example, 50 to 100 nm liposomes of distearylphosphatidylcholine and cholesterol and/or sphingolipids like sphingomyelin.  
         [0040]     The liposomes may additionally contain one or more types of charged vesicle-forming lipids, e.g. phosphatidylglycerol, phosphatidylethanolamine, (di)stearylamine, phosphatidylserine, dioleoyl trimethylammonium propane, phosphatidic acids and cholesterol hemisuccinate.  
         [0041]     Typically, the concentration of charged vesicle-forming lipids is 0 to 15 mole %, preferably 0 to 10 mole %, based upon the molar ratio of the vesicle-forming lipids.  
         [0042]     In this description where reference is made to charged/uncharged/amphiphatic, and so on, this reference relates to physiological conditions.  
         [0043]     Hence, in a preferred embodiment, the present invention relates to the use of a composition, wherein the microvesicle is a liposome comprising a non-charged vesicle-forming lipid, 0 to 20 mole % of a polymer-lipid conjugate, and preferably a polyethyleneglycol, 0 to 50 mole % of a sterol, and 0 to 10 mole % of a charged vesicle-forming lipid. The liposomes have a preferred mean particle diameter in the size range between about 40 to 200 nm.  
         [0044]     Polymeric micelles to be used in the present invention can be made in accordance with the method described in EP-A-1 072 617 adapted in accordance with the above-described method for the preparation of liposomes.  
         [0045]     The long-circulating microvesicles have a circulation half-life of at least three hours and especially at least six hours. The circulation half-life is, as the person skilled in the art appreciates, defined as the time at which the second linear phase of the logarithmic microvesicle, for instance liposomal, clearance profile reaches 50% of its initial concentration, which is the extrapolated plasma concentration at t=0.  
         [0046]     In a preferred embodiment, the medicament to be used for or in the treatment of cancer is a medicament for parenteral or local application. Oral or pulmonal application are, however, also possible.  
         [0047]     Contrary to, for instance, the system taught in U.S. Pat. No. 6,090,800 that requires lipid-soluble steroid prodrugs, the corticosteroid is most preferably a water-soluble corticosteroid. The term “water-soluble” is defined herein as having a solubility at a temperature of 25° C. of at least 10 g/l water or water buffered at a neutral pH.  
         [0048]     Water-soluble corticosteroids, which can be advantageously used in accordance with the present invention, are alkali metal and ammonium salts prepared from corticosteroids having a free hydroxyl group and organic acids, such as (C 2 -C 12 ) aliphatic, saturated and unsaturated dicarbonic acids, and inorganic acids, such as phosphoric acid and sulphuric acid. Also, acid-addition salts of corticosteroids can advantageously be encapsulated in the vesicles, preferably liposomes, more preferably, long-circulating PEG-liposomes. If more than one group in the corticosteroid molecule is available for salt formation, mono-, as well as di-, salts may be useful. As alkaline metal salts, the potassium and sodium salts are preferred. Other positively or negatively charged derivatives of corticosteroids can also be used. Specific examples of water-soluble corticosteroids are betamethasone sodium phosphate, desonide sodium phosphate, dexamethasone sodium phosphate, hydrocortisone sodium phosphate, hydrocortisone sodium succinate, methylprednisolone disodium phosphate, methylprednisolone sodium succinate, prednisolone sodium phosphate, prednisolone sodium succinate, prednisolamate hydrochloride, prednisone disodium phosphate, prednisone sodium succinate, triamcinolone acetonide disodium phosphate and triamcinolone acetonide disodium phosphate. Besides water-soluble corticosteroids, lipophilic corticosteroid derivatives prepared from corticosteroids having one or more free hydroxyl group and lipophilic aliphatic or aromatic carbon acids can also be advantageously used. Corticosteroids esterified with one or two alkyl carbon acids, such as palmityl and stearyl acid, containing more than 10 C-atoms are preferred.  
         [0049]     Suitable corticosteroids include, for example, alclomethasone dipropionate, amcinonide, beclomethasone monopropionate, betamethasone 17-valerate, ciclomethasone, clobetasol propionate, clobetasone butyrate, deprodone propionate, desonide, desoxymethasone, dexamethasone acetate, diflucortolone valerate, diflurasone diacetate, diflucortolone, difluprednate, flumetasone pivalate, flunisolide, fluocinolone acetonide acetate, fluocinonide, fluocortolone pivalate, fluormetholone acetate, fluprednidene acetate, halcinonide, halometasone, hydrocortisone acetate, medrysone, methylprednisolone acetate, mometasone furoate, parametasone acetate, prednicarbate, prednisolone acetate, prednylidene, rimexolone, tixocortol pivalate and triamcinolone hexacetonide.  
         [0050]     Of these corticosteroids, prednisolone disodium phosphate, prednisolone sodium succinate, methylprednisolone disodium phosphate, methylprednisolone sodium succinate, dexamethasone disodium phosphate and betamethasone disodium phosphate are preferred.  
         [0051]     Steroids, devoid of glucocorticoid or mineralocorticoid action, termed “angiostatic steroids,” have been shown to inhibit tumor growth with less glucocorticoid/mineralocorticoid related side effects. Very good results have been achieved with such angiostatic corticosteroids, preferably steroids having a C20 ketone but lacking a C3 ketone, such as tetrahydrocortisone, tetrahydrocortisol and tetrahydro S or a functional analogue thereof.  
         [0052]     Topical corticosteroids which undergo fast, efficient clearance as soon as these drugs become available in the general circulation, are of special interest. Examples thereof are budesonide, flunisolide and fluticasone proprionate, rimexolone, butixocort and beclomethasone and its derivatives. By preparing a water-soluble form or a lipophilic derivative of the above-mentioned topical steroids and encapsulating these into microvesicles, preferably PEG liposomes, in accordance with the present invention, it is now possible to systemically administer such corticosteroids in order to come to tumor site-specific drug delivery. By doing so, adverse effects associated with systemic treatment and overcoming problems that are inherent to the corticosteroid, such as a fast clearance, are avoided. In this respect, budesonide disodiumphosphate has appeared to be a salt of great interest.  
         [0053]     As said, the microvesicles used in accordance with the present invention may be prepared according to methods used in the preparation of conventional liposomes and PEG liposomes, as disclosed in, e.g., EP-A-0 662 820 or WO 02/45688. Passive loading of the active ingredients into the liposomes by dissolving the corticosteroids in the aqueous phase is sufficient in order to reach an encapsulation as high as possible, but other methods can also be used. The lipid components used in forming the liposomes may be selected from a variety of vesicle-forming lipids, such as phospholipids, sphingolipids and sterols. Substitution (complete or partial) of these basic components by, e.g., sphingomyelines and ergosterol appeared to be possible. For effective encapsulation of the preferably water-soluble corticosteroids in the microvesicles, thereby avoiding leakage of the drug from the microvesicles, phospholipid components having saturated, rigidifying acyl chains have appeared to be especially useful. The beneficial effects observed after one single injection of the water-soluble corticosteroid-containing PEG liposomes according to the invention are very favorable.  
         [0054]     In addition, a composition used in accordance with the present invention may comprise one or more additional components.  
         [0055]     In this respect, the present invention also relates to the use of a composition according to the invention, which composition additionally comprises a heparin (derivative). Preferably, the heparin derivative does not affect the blood clotting.  
         [0056]     In another preferred embodiment, the composition to be used in the present invention additionally comprises a component facilitating the delivery of the corticosteroid to the tumor, preferably a heparin or a heparin fragment. These components facilitating the delivery of the corticosteroids are described in more detail in U.S. Pat. No. 5,762,918, which document is, for the description of these components, incorporated herein by reference.  
         [0057]     Compositions to be used in accordance with the present invention may also suitably contain or comprise at least one compound selected from the group consisting of cytostatic agents and cytotoxic agents, preferably at least one compound selected from the group consisting of doxorubicin and taxol.  
         [0058]     Moreover, suitable use can be made of compositions comprising at least one component selected from the group consisting of immunomodulators and immunosuppressants. Examples of such components are methotrexate, cyclophosphamide, cyclosporin, muramyl peptides, cytokines and penicillamine.  
         [0059]     In an additional aspect, the present invention also relates to novel pharmaceutical compositions. For instance, the invention relates to a pharmaceutical composition comprising a long-circulating microvesicle, a corticosteroid contained therein and at least one compound selected from the group consisting of heparin and heparin derivatives.  
         [0060]     Further, the present invention encompasses pharmaceutical compositions comprising a long-circulating microvesicle and a corticosteroid contained therein, wherein the corticosteroid is tetrahydrocorticosterone.  
         [0061]     In yet another aspect, the present invention is directed to pharmaceutical compositions comprising a long-circulating microvesicle, a corticosteroid contained therein and at least one cytostatic and/or cytotoxic agent (other than a corticosteroid) preferably selected from the group consisting of anthracyclins (derivatives), topoisomerase I inhibitors and vinca-alkaloids.  
         [0062]     Without wishing to be bound by any theory, it is hypothesized that apoptosis in the tumor core effectuated by the microvesical corticosteroids gives rise to a decreased efflux of the cytostatic agents out of the tumor. In addition, perhaps the above-mentioned encapsulation by connective tissue stimulated by the microvesical corticosteroids may also play a role.  
         [0063]     Preferably, the pharmaceutical compositions of the invention comprise a long-circulating microvesicle in the form of a liposome comprising a non-charged vesicle-forming lipid, 0 to 20 mole % of an amphipathic vesicle-forming lipid derivatized with polyethyleneglycol and 0 to 10 mole % of a negatively charged vesicle-forming lipid, which liposomes have a selected mean particle diameter in the size range between about 40 nm to 200 nm. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0064]     The present invention will now further be illustrated by the following non-limiting examples and the drawings, wherein:  
         [0065]      FIG. 1 : Tissue distribution of  111 In-labeled liposomes at 24 hours after intravenous administration in B16 tumor-bearing C57B1/6 mice or C26 tumor-bearing Balb/c mice. Tumors weighed approximately 1 g. Mean±S.D., n=5 animals/experimental group;  
         [0066]      FIG. 2 : Effect of dose of free or liposomal PLP on tumor growth in B16- or C26-bearing mice. Mice received a single injection with the indicated dose and formulation of PLP on the day tumors became palpable. Tumor volume one week later is reported. Mean±S.D. N=5 mice/experimental group;  
         [0067]      FIG. 3 : Effect of tumor size on anti-tumor effects of free or liposomal PLP in B16- or C26-bearing mice. Mice received either single or multiple injections with 20 mg/kg PLP of the indicated formulations. Arrows indicate treatment. Mean±S.D. is reported. N=5 mice/experimental group; and  
         [0068]      FIG. 4 : Microscopic images of tumor tissue one week after treatment with 20 mg/kg liposomal PLP. Mice received a single injection of liposomal PLP at day seven after tumor cell inoculation. 
        A: Arrows indicate occurrence of massive apoptosis in the tumor core. Asterisks indicate blood vessels;     B: Magnification of A. Arrow indicates blood vessel obstruction.         
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
     EXAMPLES  
       [0000]     Materials and Methods  
         [0071]     Liposome Preparation  
         [0072]     Long-circulating liposomes were prepared by dissolving dipalmitoyl-phosphatidylcholine (DPPC) (Lipoid GmbH, Ludwigshafen, Germany), cholesterol (Chol) (Sigma, St. Louis, USA), and poly (ethylene) glycol 2000-distearoylphosphatidylethanolamine (PEG-DSPE) (Lipoid GmbH) in a molar ratio of 1.85:1.0:0.15, respectively, in chloroform:methanol (2:1 vol:vol) in a round-bottom flask. Typically, batch sizes of 1000 to 2000 μmol total lipid were used. For the short-circulating liposome formulation, PEG-DSPE was replaced by a corresponding amount of egg phosphatidylglycerol (EPG) (Lipoid GmbH). A lipid film was made under reduced pressure on a rotary evaporator and dried under a stream of nitrogen. Liposomes were formed by addition of an aqueous solution of 100 mg/ml prednisolone disodium phosphate (PLP) (BUFA B.V. Uitgeest, The Netherlands). The water-soluble phosphate derivative of prednisolone was used to ensure stable encapsulation in the liposomes. In the case of labeling of the liposomes with  111 In-oxine (Mallinckrodt Medical, Petten, The Netherlands), liposomes were formed by addition of 5 mM DTPA/10 mM Hepes/135 mM NaCl-buffer pH 7 to the lipid film, according to a procedure described by Boerman et al. in  J. Nucl. Med.  36(9) (1995), 1639-44. Liposome size was reduced by multiple extrusion steps through polycarbonate membranes (Nuclepore, Pleasanton, US) with a final pore size of 50 nm. The pore was 400 mm in the preparation of the short-circulating liposomes. Unencapsulated material was removed by dialysis with repeated change of buffer against 10 mM Hepes/135 mM NaCl-buffer pH 7 at 4° C.  
         [0073]     The mean particle size of the long-circulating liposomes was determined by dynamic light scattering to be 0.1 μm with a polydispersity value of ˜0.1, whereas the short-circulating liposomes had a particle size of 0.5 μm and a polydispersity of ˜0.2. The polydispersity value varies between 0 and 1. A value of 1 indicates large variations in particle size, whereas a value of 0 indicates a complete monodisperse system. Thus, the present preparations showed limited variation in particle size. The amount of lipid in the liposome dispersion was determined by colorimetric phosphate determination according to Rouser ( Lipids  5 (1970), 494-496).  
         [0074]     The concentration of PLP in the liposomes was determined by HPLC. PLP was extracted with ethylacetate and concentrated under a stream of nitrogen. The samples were diluted in an appropriate volume of ethanol: water (1:1 mol:mol) and 100 μl sample or standard was injected on an RP-HPLC system equipped with a 100 RP-18 column (5 μm, 125×4 mm). Acetonitrile:water (75%:25% vol:vol) pH 2 was calculated using Millenium software (Waters Associates Inc.). Liposomes contained 25 to 35 μg PLP/μmol lipid.  
         [0075]     Cells  
         [0076]     B16 murine melanoma and C26 murine colon carcinoma cells were cultured at 37° C. in a 5% CO 2 -containing humidified atmosphere in DMEM medium (Gibco, Breda, The Netherlands) containing 10% (v/v) fetal calf serum supplemented with 2 mM L-glutamine, 100 IU/ml penicillin, 100 μg/ml streptomycin and 0.25 μg/ml amphotericin B (Gibco).  
         [0077]     Animals  
         [0078]     Male Balb/c and C57B1/6 mice (6 to 8 weeks of age) were obtained from Charles River, kept in standard housing with standard rodent chow and water available ad libitum, and a 12 hour light/dark cycle. Experiments were performed according to national regulations and approved by the local animal experiments ethical committee.  
         [0079]     Tissue distribution of  111 In-labeled PEG liposomes in tumor-bearing mice. 1×10 5  B 16 or C26 cells were inoculated subcutaneously in the flank of C57B1/6 of Balb/c mice, respectively. At a tumor volume of approximately 1 cm 3 , mice were injected i.v. with 25 μmol lipid/kg (corresponding to 30×10 6  cpm/mouse) of  111 In-labeled liposomes. At 6 hours and 24 hours after injection, animals were sacrificed, a blood sample was taken, and tumor, lung, liver, spleen and kidneys were dissected, weighed and radioactivity was counted. See, in this light,  FIG. 1  for the results.  
         [0080]     Tumor Growth Inhibition  
         [0081]     Effect of Dose  
         [0082]     Mice received a single intravenous injection of an indicated dose of free PLP or liposomal PLP at the time when the tumor became palpable. At seven days after treatment, tumor size was measured and tumor volume calculated according to the equation V=0.52×a 2 ×b, wherein a is the smallest and b is the largest superficial diameter.  
         [0083]     Effect of Tumor Size  
         [0084]     Free PLP or liposomal PLP were i.v. administered at a dose of 20 mg/kg at day 1, 7 and 14 or by single injection at day 7 or day 14 after tumor cell inoculation. As a reference, B16F10 tumors became palpable around seven days and C26 tumors around eleven days after tumor cell inoculation. Tumor size was measured regularly and tumor volume was calculated as described above.  
         [0085]     Statistical Analysis  
         [0086]     Data were analyzed by one-way ANOVA with Dunnett&#39;s post-test using GraphPad InStat version 3.05 for Windows, GraphPad Software (San Diego, USA). Data were logarithmically transformed to correct for significant differences between SD of groups when appropriate according to Bartlett&#39;s test. Spearman rank correlation coefficient was calculated to identify dose response.  
         [0000]     Results  
         [0087]     Tissue Distribution of Long-Circulating Liposomes  
         [0088]     The tissue distribution of long-circulating liposomes after i.v. injection in C26 or B16 tumor-bearing mice is shown in  FIG. 1 . 15% of the injected dose of radioactively labeled liposomes was still present in the circulation in both mouse models at 24 hours after injection.  
         [0089]     Approximately 7 to 10% of the injected dose could be recovered from tumor tissue in both the C26 and B16F10 model. Approximately the same amount was present in the livers of both experimental groups. Relatively low amounts of liposomes were recovered from spleen, kidney and lung in the two mouse strains.  
         [0090]     Effect of Dose of Free Liposomal PLP on Tumor Growth  
         [0091]     To compare the effect of different doses of free PLP or liposomal PLP on tumor growth, B16 or C26 tumor-bearing mice received a single injection of either formulation at the moment that the tumor became palpable. At one week after injection, tumor volume was smaller with increasing dose of liposomal PLP in both mouse models (B16: Spearman correlation coefficient r=−0.92 (p&lt;0.001); C26 Spearman correlation coefficient r=−0.82 (p&lt;0.01)). 20 mg/kg PLP was the maximum dose that could be administered for the liposomal formulation in view of injection volume.  
         [0092]     Treatment of B16 or C26 tumor-bearing mice with 20 mg/kg or 50 mg/kg free PLP did not result in significantly different tumor volumes compared to buffer-treated control animals. See, in this light,  FIG. 2  showing the results.  
         [0093]     Effect of Tumor Size  
         [0094]     To determine the effect of time of injection of free or liposomal PLP on tumor growth inhibition, the formulations were injected at a dose of 20 mg/kg at day 1, 7 and 14 or single injection at day 7 or day 14. See, in this light,  FIG. 3  showing the results.  
         [0095]     B 16-Model  
         [0096]     Neither liposomal PLP nor free PLP inhibited tumor growth in B 16 tumor-bearing mice between day 1 and day 7. Tumors were just palpable at this time-point in all groups.  
         [0097]     Between day 7 and day 14, after a second injection at day 7, liposomal PLP resulted in 92% tumor growth inhibition as compared to controls (p&lt;0.05), whereas free PLP did not reduce tumor volume. On day 14, mice received a third injection. At day 17, some of the mice in the free PLP and control group had to be culled because of large tumor sizes (&gt;2 cm 3 ), whereas average tumor volume in the liposomal PLP group was approximately 79% smaller (p&lt;0.01).  
         [0098]     After single injection of liposomal of free PLP at day 7, a significantly smaller tumor volume was only seen after treatment with liposomal PLP, with average inhibition of tumor growth of 89% at day 14 and 67% at day 17, as compared to controls (p&lt;0.05, both time-points). Single injection at day 14 showed a similar image, at day 17 liposomal PLP-treated tumors were 58% smaller than controls (p&lt;0.05).  
         [0099]     C26-Model  
         [0100]     Neither liposomal PLP nor free PLP inhibited tumor growth in C26 tumor-bearing mice between day 1 and day 7. Mice received a second injection on day 7. Tumors became palpable around day 10. Both at day 14 and day 21, average tumor volume in liposomal PLP-treated animals was 89% smaller than that of controls, but it was only significantly smaller at day 21 (p&lt;0.01). Free PLP did not inhibit tumor growth. After single injection of liposomal of free PLP at day 7, tumor volume was not significantly smaller with either treatment compared to controls, although average tumor volume was 66% smaller at day 14 and 67% smaller at day 21 for liposomal PLP-treated mice as compared to controls. Single injection at day 14 resulted in a 78% smaller tumor volume at day 21 for liposomal PLP-treated animals (p&lt;0.05).  
         [0101]     Effect of Liposomal Circulation Time  
         [0102]     To determine whether the liposome formulation is important in therapeutic efficacy, both a short-circulating and a long-circulating liposome formulation of PLP were tested. Both were injected at day 14 after tumor cell inoculation in C26 tumor-bearing mice. It appeared that the tumor inhibition of short-circulating liposomes was not as pronounced and lasted shorter than that of long-circulating liposomes and was not significantly different from saline-treated animals.  
         [0103]     Analysis of Amount of PLP or PL in Tissues  
         [0104]     PLP and prednisolone (PL) concentrations at 24 hours after injection of liposomal PLP in liver, spleen and tumor tissue were determined by HPLC analysis.  FIG. 4  shows that the highest amount of PLP (±5 μg) was present in the tumor, which was a similar amount as present in the form of PL. Levels of PLP or PL in the spleen were relatively low. In liver tissue, hardly any PLP could be detected, but a high amount of PL was present. As PLP added to control liver tissue to prepare the standard line could also not be detected, the PL content of this tissue is likely overrated as a result of high enzymatic activity in liver homogenates. Neither PLP nor PL was detected in any of these tissues at 24 hours after injection of free PLP.  
         [0105]     Microscopic evaluation of tumor tissue treated with liposomal PLP at one week after a single dose of 20 mg/kg showed massive apoptosis of tumor cells in the tumor core. In addition, the presence of blood clots in larger blood vessels obstructing tumor blood flow is shown. The results are shown in  FIG. 4 .