Patent Publication Number: US-2021177802-A1

Title: Pharmaceutical composition for treating tumor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Japanese patent application No. 2019-138041 filed on Jul. 26, 2019, the disclosure of which is herein incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a pharmaceutical composition for treating tumor. 
     BACKGROUND 
     Eribulin represented by formula (I) is used as a therapeutic agent for breast cancer and soft tissue tumor. 
     
       
         
         
             
             
         
       
     
     Patent Literature 1 discloses eribulin or a pharmaceutically acceptable salt thereof and a method of producing the same. Patent Literatures 2 and 3 disclose methods for producing eribulin and eribulin mesylate, which is a mesylate (methanesulfonate) thereof. Patent Literature 4 discloses a method of inhibiting growth of cancer in a patient by administering eribulin or a pharmaceutically acceptable salt thereof to the patient. Patent Literature 5 discloses a method of treating cancer in a patient by administering eribulin or a pharmaceutically acceptable salt thereof to the patient in combination with a certain second anticancer agent. Patent Literature 6 discloses a method of treating cancer in a patient by administering eribulin or a pharmaceutically acceptable salt thereof to the patient in combination with a second therapeutic approach. Patent Literatures 7 and 8 disclose liposomal compositions comprising eribulin mesylate. Patent Literature 9 discloses a method for treating breast cancer, comprising administering a combination of eribulin or a pharmaceutically acceptable salt thereof and a programmed cell death 1 protein (PD-1) antagonist. 
     PD-1 is recognized as an important factor in maintenance of immunoregulation and peripheral tolerance. PD-1 is moderately expressed in naive T cells, B cells, and NK T cells, and upregulated by T/B cell receptor signal transduction in lymphocytes, monocytes, and myeloid cells (Non Patent Literature 1). Meanwhile, PD-L1 is expressed in various cancer cells or T/B cells, macrophages, mDCs, plasmacytoid DCs:(pDCs), bone marrow mast cells, and the like. 
     The two known PD-1 ligands PD-L1 (B7-H1) and PD-L2 (B7-DC) are expressed in human cancer occurring in various tissues. For example, in a large amount of sample sets of ovarian cancer, renal cancer, colorectal cancer, pancreatic cancer, liver cancer, and melanoma, the PD-L1 expression has been shown to correlate with poor prognosis and decreased overall survival, regardless of subsequent treatment (Non Patent Literatures 2 to 13). Similarly, it was found that the PD-1 expression in tumor-infiltrating lymphocytes is characteristic of functionally impaired T cells in breast cancer and melanoma (Non Patent Literatures 14 to 15) and correlates with poor prognosis in kidney cancer (Non Patent Literature 16). Therefore, it has been proposed to block the immunosuppression mechanism that cancer cells bring, such as the interaction of tumor cells expressing PD-L1 with T cells expressing PD-1, and thereby bring the immune response to tumor. 
     Several monoclonal antibodies that inhibit the interaction between PD-1 and either or both of PD-1 ligands PD-L1 and PD-L2 are under clinical development for treating cancer. It has been proposed that the efficacy of such antibodies may be increased when administered in combination with another approved or experimental cancer therapy, for example, radiation, surgery, a chemotherapeutic agent, a targeted therapy, an agent that inhibits another signaling pathway that is dysregulated in tumor, and another immunostimulant.
     Patent Literature 1: WO 99/65894   Patent Literature 2: WO 2005/118565   Patent Literature 3: WO 2011/094339   Patent Literature 4: U.S. Pat. No. 6,469,182   Patent Literature 5: U.S. Application Publication No. 2006/104984   Patent Literature 6: U.S. Pat. No. 6,653,341   Patent Literature 7: WO 2010/113984   Patent Literature 8: WO 2017/188350   Patent Literature 9: WO 2016/141209   Non Patent Literature 1: Sharpe, A. H, Wherry, E. J., Ahmed R., and Freeman G. J., The function of programmed cell death 1 and its ligands in regulating autoimmunity and infection. Nature Immunology (2007); 8: 239-245.   Non Patent Literature 2: Dong H et al., Tumor-associated B7-H1 promotes T-cell apoptosis: a potential mechanism of immune evasion. Nat Med. 2002 August; 8(8): 793-800.   Non Patent Literature 3: Yang et al., PD-1 interaction contributes to the functional suppression of T-cell responses to human uveal melanoma cells in vitro. Invest Ophthalmol Vis Sci. 2008 June; 49(6 (2008): 49: 2518-2525.   Non Patent Literature 4: Ghebeh et al., The B7-H1 (PD-L1) T lymphocyte-inhibitory molecule is expressed in breast cancer patients with infiltrating ductal carcinoma: correlation with important high-risk prognostic factors. Neoplasia (2006) 8: 190-198.   Non Patent Literature 5: Hamanishi J et al., Programmed cell death 1 ligand 1 and tumor-infiltrating CD8+T lymphocytes are prognostic factors of human ovarian cancer. Proceeding of the National Academy of Sciences (2007): 104: 3360-3365.   Non Patent Literature 6: Thompson R H et al., Significance of B7-H1 overexpression in kidney cancer. Clinical genitourin Cancer (2006): 5: 206-211.   Non Patent Literature 7: Nomi, T. Sho, M., Akahori, T., et al., Clinical significance and therapeutic potential of the programmed death-1 ligand/programmed death-1 pathway in human pancreatic cancer. Clinical Cancer Research (2007); 13: 2151-2157.   Non Patent Literature 8: Ohigashi Y et al., Clinical significance of programmed death-1 ligand-1 and programmed death-1 ligand 2 expression in human esophageal cancer. Clin. Cancer Research (2005): 11: 2947-2953.   Non Patent Literature 9: Inman et al., PD-L1 (B7-H1) expression by urothelial carcinoma of the bladder and BCG-induced granulomata: associations with localized stage progression. Cancer (2007): 109: 1499-1505.   Non Patent Literature 10: Shimauchi T et al., Augmented expression of programmed death-1 in both neoplasmatic and nonneoplastic CD4+ T-cells in adult T-cell Leukemia/Lymphoma. Int. J. Cancer (2007): 121: 2585-2590.   Non Patent Literature 11: Gao et al., Overexpression of PD-L1 significantly associates with tumor aggressiveness and postoperative recurrence in human hepatocellular carcinoma. Clinical Cancer Research (2009) 15: 971-979.   Non Patent Literature 12: Nakanishi J., Overexpression of B7-H1 (PD-L1) significantly associates with tumor grade and postoperative prognosis in human urothelial cancers. Cancer Immunol Immunother. (2007) 56: 1173-1182.   Non Patent Literature 13: Hino et al., Tumor cell expression of programmed cell death-1 is a prognostic factor for malignant melanoma. Cancer (2010): 116: 1757-1766.   Non Patent Literature 14: Ghebeh H., Foxp3+ tregs and B7-H1+/PD-1+T lymphocytes co-infiltrate the tumor tissues of high-risk breast cancer patients: implication for immunotherapy. BMC Cancer. 2008 Feb. 23; 8:57.   Non Patent Literature 15: Ahmadzadeh M. et al., Tumor antigen-specific CD8 T cells infiltrating the tumor express high levels of PD-1 and are functionally impaired. Blood (2009) 114: 1537-1544.   Non Patent Literature 16: Thompson R H et al., PD-1 is expressed by tumor infiltrating cells and is associated with poor outcome for patients with renal carcinoma. Clinical Cancer Research (2007) 15: 1757-1761.   

     SUMMARY 
     The present invention is directed to provide a new pharmaceutical composition for treating tumor. 
     The present inventors have studied diligently and, as a result, found that the combined administration of a liposomal composition comprising eribulin or a pharmaceutically acceptable salt thereof and a PD-1 antagonist exhibits unexpected antitumor effect, thereby completing the present invention. 
     Accordingly, the present disclosure is as follows. 
     [1] A pharmaceutical composition for treating tumor, comprising a liposomal composition comprising eribulin or a pharmaceutically acceptable salt thereof, wherein the pharmaceutical composition is administered in combination with a PD-1 antagonist.
 
[2] A pharmaceutical composition for treating tumor, comprising a PD-1 antagonist, wherein the pharmaceutical composition is administered in combination with a liposomal composition comprising eribulin or a pharmaceutically acceptable salt thereof.
 
[3] The pharmaceutical composition according to [1] or [2] above, wherein the liposomal composition comprising eribulin or a pharmaceutically acceptable salt thereof and the PD-1 antagonist are administered simultaneously, separately, continuously, or at a time interval.
 
[4] The pharmaceutical composition according to any of [1] to [3] above, wherein eribulin or the pharmaceutically acceptable salt thereof is eribulin mesylate.
 
[5] The pharmaceutical composition according to any of [1] to [4] above, wherein the PD-1 antagonist is an anti-PD-1 antibody.
 
[6] The pharmaceutical composition according to [5] above, wherein the anti-PD-1 antibody is selected from the group consisting of nivolumab, pembrolizumab, cemiplimab, sintilimab, and toripalimab.
 
[7-1] The pharmaceutical composition according to any of [1] to [6] above, wherein the tumor is selected from the group consisting of breast cancer, gastric cancer, esophageal cancer and small cell lung cancer.
 
[7-2] The pharmaceutical composition according to any of [1] to [6] above, wherein the tumor is breast cancer.
 
[8] A therapeutic agent for tumor, comprising a liposomal composition comprising eribulin or a pharmaceutically acceptable salt thereof wherein the pharmaceutical composition is administered in combination with a PD-1 antagonist.
 
[9] A therapeutic agent for tumor, comprising a PD-1 antagonist, wherein the therapeutic agent is administered in combination with a liposomal composition comprising eribulin or a pharmaceutically acceptable salt thereof.
 
[10] The therapeutic agent according to [8] or [9] above, wherein the liposomal composition comprising eribulin or a pharmaceutically acceptable salt thereof and the PD-1 antagonist are administered simultaneously, separately, continuously, or at a time interval.
 
[11] The therapeutic agent according to any of [8] to [10] above, wherein eribulin or the pharmaceutically acceptable salt thereof is eribulin mesylate.
 
[12] The therapeutic agent according to any of [8] to [11] above, wherein the PD-1 antagonist is an anti-PD-1 antibody.
 
[13] The therapeutic agent according to [12] above, wherein the anti-PD-1 antibody is selected from the group consisting of nivolumab, pembrolizumab, cemiplimab, sintilimab, and toripalimab.
 
[14-1] The therapeutic agent according to any of [8] to [13] above, wherein the tumor is selected from the group consisting of breast cancer, gastric cancer, esophageal cancer and small cell lung cancer.
 
[14-2] The therapeutic agent according to any of [8] to [13] above, wherein the tumor is breast cancer.
 
[15] A method for treating tumor, comprising administering a liposomal composition comprising eribulin or a pharmaceutically acceptable salt thereof and a PD-1 antagonist to a patient in need thereof.
 
[16] The method according to [15] above, wherein the liposomal composition comprising eribulin or a pharmaceutically acceptable salt thereof and the PD-1 antagonist are administered simultaneously, separately, continuously, or at a time interval.
 
[17] The method according to [15] or [16] above, wherein eribulin or the pharmaceutically acceptable salt thereof is eribulin mesylate.
 
[18] The method according to any of [15] to [17] above, wherein the PD-1 antagonist is an anti-PD-1 antibody.
 
[19] The method according to [18] above, wherein the anti-PD-1 antibody is selected from the group consisting of nivolumab, pembrolizumab, cemiplimab, sintilimab, and toripalimab.
 
[20-1] The method according to any of [15] to [19] above, wherein the tumor is selected from the group consisting of breast cancer, gastric cancer, esophageal cancer and small cell lung cancer.
 
[20-2] The method according to any of [15] to [19] above, wherein the tumor is breast cancer.
 
[21] Use of eribulin or a pharmaceutically acceptable salt thereof in the manufacture of a pharmaceutical composition for treating tumor, wherein the pharmaceutical composition is administered in combination with a PD-1 antagonist.
 
[22] Use of a PD-1 antagonist in the manufacture of a pharmaceutical composition for treating tumor, wherein the pharmaceutical composition is administered in combination with a liposomal composition comprising eribulin or a pharmaceutically acceptable salt thereof.
 
[23] The use according to [21] or [22] above, wherein the liposomal composition comprising eribulin or a pharmaceutically acceptable salt thereof and the PD-1 antagonist are administered simultaneously, separately, continuously, or at a time interval.
 
[24] The use according to any of [21] to [23] above, wherein eribulin or the pharmaceutically acceptable salt thereof is eribulin mesylate.
 
[25] The use according to any of [21] to [24] above, wherein the PD-1 antagonist is an anti-PD-1 antibody.
 
[26] The use according to [25] above, wherein the anti-PD-1 antibody is selected from the group consisting of nivolumab, pembrolizumab, cemiplimab, sintilimab, and toripalimab.
 
[27-1] The use according to any of [21] to [26] above, wherein the tumor is selected from the group consisting of breast cancer, gastric cancer, esophageal cancer and small cell lung cancer.
 
[27-2] The use according to any of [21] to [26] above, wherein the tumor is breast cancer.
 
[28] A liposomal composition comprising eribulin or a pharmaceutically acceptable salt thereof for use in tumor treatment, wherein the liposomal composition is administered in combination with a PD-1 antagonist.
 
[29] A PD-1 antagonist for use in tumor treatment, wherein the PD-1 antagonist is administered in combination with a liposomal composition comprising eribulin or a pharmaceutically acceptable salt thereof.
 
[30] The liposomal composition or PD-1 antagonist for use according to [28] or [29] above, wherein the liposomal composition comprising eribulin or a pharmaceutically acceptable salt thereof and the PD-1 antagonist are administered simultaneously, separately, continuously, or at a time interval.
 
[31] The liposomal composition or PD-1 antagonist for use according to any of [28] to [30] above, wherein eribulin or the pharmaceutically acceptable salt thereof is eribulin mesylate.
 
[32] The liposomal composition or PD-1 antagonist for use according to any of [28] to [31] above, wherein the PD-1 antagonist is an anti-PD-1 antibody.
 
[33] The liposomal composition or PD-1 antagonist for use according to [32] above, wherein the anti-PD-1 antibody is selected from the group consisting of nivolumab, pembrolizumab, cemiplimab, sintilimab, and toripalimab.
 
[34-1] The liposomal composition or PD-1 antagonist for use according to [28] to [33] above, wherein the tumor is selected from the group consisting of breast cancer, gastric cancer, esophageal cancer and small cell lung cancer.
 
[34-2] The liposomal composition or PD-1 antagonist for use according to [28] to [33] above, wherein the tumor is breast cancer.
 
[35] A kit for treating tumor, comprising a formulation comprising a liposomal composition comprising eribulin or a pharmaceutically acceptable salt thereof and a formulation comprising a PD-1 antagonist.
 
[36] The kit according to [35] above, wherein the liposomal composition comprising eribulin or a pharmaceutically acceptable salt thereof and the PD-1 antagonist are administered simultaneously, separately, continuously, or at a time interval.
 
[37] The kit according to [35] or [36] above, wherein eribulin or the pharmaceutically acceptable salt thereof is eribulin mesylate.
 
[38] The kit according to any of [35] to [37] above, wherein the PD-1 antagonist is an anti-PD-1 antibody.
 
[39] The kit according to [38] above, wherein the anti-PD-1 antibody is selected from the group consisting of nivolumab, pembrolizumab, cemiplimab, sintilimab, and toripalimab.
 
[40-1] The kit according to any of [35] to [39] above, wherein the tumor is selected from the group consisting of breast cancer, gastric cancel; esophageal cancer and small cell lung cancer.
 
[40-2] The kit according to any of [35] to [39] above, wherein the tumor is breast cancer.
 
[41] The pharmaceutical composition according to any of [1] to [7-2] above or therapeutic agent according to any of [8] to [14-2] above, further comprising a pharmaceutically acceptable carrier.
 
     The combined administration of a liposomal composition comprising eribulin or a pharmaceutically acceptable salt thereof and a PD-1 antagonist exhibits unexpected antitumor effect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph illustrating effect of treatment with a combination of a liposomal formulation comprising eribulin mesylate at a dose of 0.1 mg/kg and an anti-PD-1 antibody on tumor growth. 
         FIG. 2  is a graph illustrating effect of treatment with a combination of a liposomal formulation comprising eribulin mesylate at a dose of 0.1 mg/kg and an anti-PD-1 antibody on T×5. 
         FIG. 3  is a graph illustrating effect of treatment with a combination of a liposomal formulation comprising eribulin mesylate at a dose of 0.3 mg/kg and an anti-PD-1 antibody on tumor growth. 
         FIG. 4  is a graph illustrating effect of treatment with a combination of a liposomal formulation comprising eribulin mesylate at a dose of 0.3 mg/kg and an anti-PD-1 antibody on T×5. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will be described below. The following embodiments are illustrations for the purpose of describing the present disclosure and not intended to limit the present disclosure only to these embodiments. The present disclosure can be carried out in various forms, unless they deviate from its spirit. 
     The liposomal compositions in the present disclosure comprises eribulin or a pharmaceutically acceptable salt thereof (hereinafter referred to as “eribulin or the like”). 
     In the present disclosure, the “pharmaceutically acceptable salt” may be either an inorganic acid salt or an organic acid salt and is not particularly limited, as long as it forms a salt with eribulin, and examples thereof include hydrochloride, sulfate, citrate, hydrobromide, hydroiodide, nitrate, bisulfate, phosphate, superphosphate, isonicotinate, acetate, lactate, salicylate, tartrate, pantothenate, ascorbate, succinate, maleate, fumarate, gluconate, saccharinate, formate, benzoate, glutamate, mesylate (methanesulfonate), ethanesulfonate, benzenesulfonate, p-toluenesulfonate, and pamoate. In one embodiment, the pharmaceutically acceptable salts are hydrochloride, sulfate, acetate, phosphate, citrate, and mesylate. In a particular embodiment, the pharmaceutically acceptable salt is mesylate. 
     The pharmaceutically acceptable salt of eribulin may be a salt of eribulin and aluminum, calcium, lithium, magnesium, sodium, zinc, or diethanolamine. 
     In the present disclosure, examples of eribulin or the like include eribulin mesylate. 
     Eribulin or the like is a compound or a salt thereof described in Patent Literature 1 or U.S. Pat. No. 6,214,865 and has pharmacological activities including antitumor and antimitotic activities. Patent Literature 1 discloses that eribulin or the like has, as an antitumor agent, anti-tumor activity against melanoma, fibrosarcoma, monocytic leukemia, colon cancer, ovarian cancer, breast cancel; osteosarcoma, prostate cancer, lung cancer, and ras-transformed fibroblasts. Eribulin or the like is obtained by a method of production described in Patent Literatures 1 to 3. 
     In the present disclosure, the “liposome” means a closed microvesicle having an inner phase surrounded by a lipid bilayer. The liposomes include small unilamellar liposomes (SUVs: small unilamellar vesicles), large unilamellar liposomes (LUVs: large unilamellar vesicles), further large unilamellar liposomes (GUVs: giant unilamellar vesicles), multi-lamellar liposomes having a plurality of concentric membranes (MLVs: multi lamellar vesicles), liposomes having a plurality of non-concentric, irregular membranes (MVVs: multivesicular vesicles), and the like. 
     In the present disclosure, the “liposomal inner phase” means an aqueous region surrounded by a liposomal lipid bilayer and is used synonymously with an “inner aqueous phase” and a “liposomal inner aqueous phase”. The “liposomal outer phase” means a region that is not surrounded by a liposomal lipid bilayer (that is, the region except the inner phase and the lipid bilayer) when the liposome is dispersed in a liquid. 
     In the present disclosure, the “liposomal composition” means a composition comprising a liposome and further comprising eribulin or the like in the liposomal inner phase. In the present disclosure, the liposomal composition includes solid and liquid compositions. 
     In the present disclosure, the “liposomal dispersion liquid” means a composition comprising a liposome in which eribulin or the like is not yet encapsulated into the liposomal inner phase. 
     In the present disclosure, the “liposomal preparatory liquid” means a composition comprising a liposome in which an adjustment of the liposome outer phase in order to encapsulate eribulin or the like into the liposome inner phase is not yet performed. 
     [Lipid] 
     In one embodiment, the liposome preferably comprises a phospholipid and/or a phospholipid derivative as a membrane component. 
     Examples of the phospholipid and/or phospholipid derivative include phosphatidylethanolamine, phosphatidylcholine, phosphatidylserine, phosphatidylinositol, phosphatidylglycerol, cardiolipin, sphingomyelin, ceramide phosphoryl ethanolamine, ceramidephosphorylglycerol, ceramidephosphorylglycerolphosphate, 1,2-dimyristoyl-1,2-deoxyphosphatidyl choline, plasmalogen, and phosphatidate. 
     The phospholipid and/or phospholipid derivative may be one or a combination of two or more of these. 
     Fatty acid residues in the phospholipid and/or phospholipid derivative are not particularly limited, and examples thereof include saturated or unsaturated fatty acid residues having 12 to 20 carbon atoms, and specific examples thereof include acyl groups derived from fatty acids such as lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, and linoleic acid. As the phospholipid and/or phospholipid derivative, a phospholipid derived from a natural product such as egg yolk lecithin and soy lecithin, and partially hydrogenated egg yolk lecithin, (fully) hydrogenated egg yolk lecithin, partially hydrogenated soy lecithin, and (fully) hydrogenated soybean lecithin, in which unsaturated fatty acid residues are partially or fully hydrogenated, or the like may be used. 
     The amount (molar fraction) of the phospholipid and/or phospholipid derivative, used in the preparation of the liposome, that are/is blended is not particularly limited, and is, in one embodiment, 10 to 80% and, in a particular embodiment, 30 to 60% based on the total ribosomal membrane components. 
     In the present disclosure, the liposome may comprise, as a membrane component, a sterol such as cholesterol and cholestanol and a fatty acid having a saturated or unsaturated acyl group having 8 to 22 carbon atoms as a membrane stabilizing agent, and an antioxidant such as α-tocopherol, besides the phospholipid and/or phospholipid derivative. 
     The amount (molar fraction) of the sterol, used in the preparation of the liposome, that is blended is not particularly limited, and is, in one embodiment, 1 to 60%, 10 to 50%, or 30 to 50% based on the total liposomal membrane components. 
     The amount (molar fraction) of the fatty acid blended is not particularly limited, and is, in one embodiment, 0 to 30% and 0 to 20% or 0 to 10% based on the total liposomal membrane components. 
     The amount (molar fraction) of the antioxidant blended is not particularly limited, as long as an amount that provides the antioxidant effect is added, and it is, in one embodiment, 0 to 15%, 0 to 10%, or 0 to 5% based on the total liposomal membrane components. 
     In the present disclosure, the liposome may comprise a functional lipid or a modified lipid as a membrane component. 
     Examples of the functional lipid include a blood-retaining lipid derivative, a temperature change-sensitive lipid derivative, and a pH-sensitive lipid derivative. 
     Examples of the modified lipid include a PEGylated lipid, a glycolipid, an antibody-modified lipid, and a peptide-modified lipid. 
     Examples of the blood-retaining lipid derivative include polyethylene glycol derivatives (such as methoxy polyethylene glycol condensates) such as condensation products of phosphoethanolamine and methoxy polyethylene glycol: N-{carbonyl-methoxy polyethylene glycol-2000}-1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine, N-{carbonyl-methoxy polyethylene glycol-5000}-1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine, N-{carbonyl-methoxy polyethylene glycol-750}-1,2-distearoyl-sn-glycero-3-phosphoethanolamine, N-{carbonyl-methoxy polyethylene glycol-2000}-1,2-distearoyl-sn-glycero-3-phosphoethanolamine, (MPEG2000-distearoylphosphatidylethanolamine), and N-{carbonyl-methoxy polyethylene glycol-5000}-1,2-distearoyl-sn-glycero-3-phosphoethanolamine. 
     The blending amount (molar fraction) of the blood-retaining lipid derivative, used in the preparation of the liposome, is not particularly limited, and is, in one embodiment, 0 to 50%, 0 to 30%, or 0 to 20% based on the total liposomal membrane components. 
     Examples of the temperature change-sensitive lipid derivative include dipalmitoylphosphatidylcholine. By including a temperature change-sensitive lipid derivative in a liposome, it becomes possible to disrupt the liposome at a particular temperature, to change the surface properties of the liposome, and the like. Furthermore, by combining it with heating of a target site such as tumor, it becomes possible to disrupt the liposome at the target site and have an active compound released at the target site, and the like. 
     Examples of the pH-sensitive lipid derivative include dioleoylphosphatidylethanolamine. By including a pH-sensitive lipid derivative in a liposome, it becomes possible to promote membrane fusion of the liposome and an endosome when the liposome is taken in a cell by endocytosis and improve the delivery of the active compound to the cytoplasm, and the like. 
     Examples of the glycolipid, antibody-modified lipid, and peptide-modified lipid include lipids linked to a sugar, an antibody, or a peptide having an affinity for a target cell or a target tissue. Using a modified lipid allows to deliver the liposome actively to the target cell or the target tissue. 
     The composition of membrane components for liposome having a practically acceptable level of membrane permeability can be set by a person skilled in the art as appropriate depending on the active compound, the target tissue, and the like (see Hiroshi Kikuchi et al. “Liposome I—How to prepare and assay—(in Japanese)” Cell technology (1983) 2 (9): pp. 1136-1149 and the references cited in the reference, and the like). The liposomal composition may be used not only in targeting at a target tissue such as solid cancel; but also in the delivery of an active compound to blood cancer or the like. 
     The liposomal membrane components include, in one embodiment, phospholipid, cholesterol, and a methoxy polyethylene glycol condensation product. 
     [Liposomal Composition] 
     In the liposomal composition in the present disclosure, eribulin or the like is encapsulated in a liposome having a lipid membrane. In the liposomal composition, eribulin or the like may be distributed in the lipid bilayer. 
     The liposomal composition according to the present disclosure can be obtained by a method described in Patent Literature 7. 
     If the liposomal composition is solid, the solid liposomal composition may be dissolved or suspended in a certain solvent described below to prepare a liquid liposomal composition. In case that the liposomal composition is a frozen solid, the frozen solid liposomal composition may be thawed by leaving the composition at room temperature or the like to prepare a liquid liposomal composition. 
     The liposomal composition according to the present disclosure is not limited, as long as it comprises (1) eribulin or the like. The liposomal composition according to the present disclosure may further comprise (2) at least one ammonium salt and (3) at least one acid, salt, base, and/or amino acid. 
     Examples of the at least one ammonium salt (2) include ammonium chloride, ammonium borate, ammonium sulfate, ammonium formate, ammonium acetate, ammonium citrate, ammonium tartrate, ammonium succinate, and ammonium phosphate and, in one embodiment, the at least one ammonium salt (2) is ammonium sulfate, ammonium citrate, and ammonium tartrate. 
     As for the acid, salt, base, and/or amino acid (3), examples of the acid include ascorbic acid, benzoic acid, succinic acid, citric acid, glutamic acid, phosphoric acid, acetic acid, propionic acid, tartaric acid, carbonic acid, lactic acid, boric acid, maleic acid, fumaric acid, malic acid, adipic acid, hydrochloric acid, and sulfuric acid; examples of the salt include sodium salts of the aforementioned acids, potassium salts of the aforementioned acids, and ammonium salts of the aforementioned acids; examples of the base include trishydroxymethylaminomethane, ammonia, sodium hydroxide, and potassium hydroxide; and examples of the amino acid include arginine, histidine, and glycine. 
     In one embodiment of the liposomal composition according to the present disclosure, the acid, salt, base, and/or amino acid (3) in the liposomal inner phase is hydrochloric acid, acetic acid, lactic acid, tartaric acid, succinic acid, citric acid, and phosphoric acid, sodium salts of the aforementioned acids, and sodium hydroxide and ammonia, and, in a particular embodiment, the acid, salt, base, and/or amino acid (3) is acetic acid, lactic acid, tartaric acid, citric acid, and phosphoric acid, sodium salts of the aforementioned acids, and sodium hydroxide and ammonia. 
     An example of the components of the liposomal composition is set forth in Table 1. In another specific example, 96 mg/mL sucrose may be used, instead of 9 mg/mL sodium chloride, as an osmotic agent (liposomal outer phase). 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Component 
                 Concentration 
                 Purpose of inclusion 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Eribulin mesylate 
                 0.2 
                 mg/mL 
                 Drug 
               
               
                 HSPC [1]   
                 7.1 
                 mg/mL 
                 Lipid membrane component 
               
               
                 Cholesterol 
                 2.3 
                 mg/mL 
                 Lipid membrane component 
               
               
                 MPEG2000-DSPE [2]   
                 2.7 
                 mg/mL 
                 Lipid membrane component 
               
               
                 Ammonium sulfate 
                 100 
                 mM 
                 Liposomal inner phase component 
               
               
                 Citric acid monohydrate 
                 30 
                 mM 
                 Liposomal inner phase component 
               
               
                 Sodium chloride 
                 9 
                 mg/mL 
                 Liposomal outer phase component 
               
               
                 L-histidine 
                 1.6 
                 mg/mL 
                 Liposomal outer phase component 
               
            
           
           
               
               
               
            
               
                 Sodium hydroxide/hydrochloric acid 
                 q.s. 
                 pH adjuster 
               
               
                   
               
               
                   [1] Hydrogenated soy phosphatidylcholine 
               
               
                   [2] N-{carbonyl-methoxy polyethylene glycol-2000}-1,2-distearoyl-sn-glycero-3-phosphoethanolamine (MPEG2000-distearoylphosphatidylethanolamine) 
               
            
           
         
       
     
     The liposomal composition according to the present disclosure may be administered by injection (intravenous injection, intraarterial injection, local injection), orally, nasally, transdermally, transpulmonarily, ophthalmically, and the like, and examples thereof include injection such as intravenous injection, subcutaneous injection, intradermal injection, intraarterial injection, as well as local injection to a target cell and organ. Examples of the dosage form of the liposomal composition for oral administration include tablets, powders, granules, syrups, capsules, and oral solutions. Examples of the dosage form of the liposomal composition for parenteral administration include injections, drip infusions, ophthalmic liquids, ointments, suppositories, suspensions, cataplasms, lotions, aerosols, and plasters, and, in one embodiment, the liposomal composition for parenteral administration is an injection or a drip infusion. The liposomal composition according to the present disclosure may be formulated by a method, for example, described in Japanese Pharmacopoeia (JP) 17th edition, United States Pharmacopoeia (USP), or European pharmacopoeia (EP). 
     In a case that the liposomal composition is a liquid, the liposomal composition may be used as it is. To use the liposomal composition as a medicine, for example, a solvent may be injected by a physician or a patient into a vial in which a solid formulation is encapsulated to do such preparation upon use. A solid formulation obtained by freezing a liquid liposomal composition may be stored in a frozen state and thawed by leaving at room temperature or thawed rapidly with heating back into a liquid upon use to be used as a liquid. 
     The dose upon administration of the liposomal composition alone vary markedly depending on the kind of the target disease, the age, sex, body weight of the patient, the severity of symptoms, and the like. The liposomal composition is administered, for example, at 0.1 to 10 mg/m 2  (body surface) in terms of eribulin mesylate per day for an adult. In one embodiment, the liposomal composition is administered at a dose of 0.5 to 3 mg/m 2  (body surface) in terms of eribulin mesylate once every 1 week, 2 weeks, or 3 weeks. In a particular embodiment, the liposomal composition is more preferably administered at a dose of 0.5 to 2 mg/m 2  (body surface) in terms of eribulin mesylate once every 1 week, 2 weeks, or 3 weeks. 
     In another aspect, the liposomal composition is preferably administered at a dose of approximately 1.5 mg/m 2  (body surface) in terms of eribulin mesylate once every 1 week, 2 weeks, or 3 weeks. 
     More specifically, the liposomal composition is administered intravenously at 0.5 to 1.4 mg/m 2  on day 1 of a 21-day cycle or administered intravenously at 0.5 to 1.5 mg/m 2  on day 1 and day 15 of a 28-day cycle in terms of eribulin mesylate. 
     Eribulin or the like contained in the liposomal composition may be administered once a day or in several divided daily doses. 
     The liposomal composition may be a liposomal composition comprising, for example, 0.01 to 300 mg/mL of eribulin or the like in the liposomal inner phase. 
     The liposomal composition is formulated, for example, as an injection comprising 0.20 mg/mL eribulin mesylate (0.18 mg/mL eribulin) incorporated in a liposome having a lipid membrane consisting of HSPC, cholesterol, and MPEG2000-DSPE. Such an injection may comprise sucrose or sodium chloride as an isotonizing agent, ammonium sulfate, citric acid, and L-histidine, and sodium hydroxide and hydrochloric acid to adjust pH. The injection is directly administered to a patient or diluted with physiological saline to a concentration in the range of 0.0035 mg/mL or higher and lower than 0.2 mg/mL before the administration to a patient. 
     The PD-1 antagonist in the present disclosure may comprise any compound or biological molecule that blocks the binding of PD-L1 expressed in cancer cells to PD-1 expressed in immune cells (T cells, B cells, or natural killer T (NKT) cells), or that blocks the binding of PD-L2 expressed in cancer cells to PD-1 expressed in immune cells. The PD-1 antagonist blocks the binding of human PD-L1 to human PD-1 and, in one embodiment, blocks the binding of both human PD-L1 and PD-L2 to human PD-1. The amino acid sequence of human PD-1 can be found in NCBI Locus No.: NP_005009. The amino acid sequences of human PD-L1 and PD-L2 can be found in NCBI Locus No: NP_054862 and NP_079515, respectively. 
     The PD-1 antagonist useful in the present disclosure may comprise a monoclonal antibody (mAb) or an antigen-binding fragment thereof that specifically binds to PD-1 or PD-L1 or that specifically binds to human PD-1 or human PD-L1. The mAb may be a human antibody, a humanized antibody, or a chimeric antibody and may comprise a human constant region. The human constant region is selected from the group consisting of IgG1, IgG2, IgG3, and IgG4 constant regions and, in one embodiment, the human constant region is an IgG1 or IgG4 constant region. The antigen-binding fragment may be selected from the group consisting of Fab, Fab′-SH, F(ab) 2 , scFv, and Fv fragment. 
     An example of useful PD-1 antagonists is an anti-PD-1 antibody, which is, in one embodiment, an anti-human PD-1 antibody and, in a particular embodiment, an anti-human PD-1 monoclonal antibody (anti-human PD-1 mAb). Examples of the human PD-1-binding mAb binding are described in U.S. Pat. Nos. 7,488,802, 7,521,051, 8,008,449, 8,354,509, 8,168,757, WO 2004/004771, WO 2004/072286, WO 2004/056875, and US Patent Application Publication No. 2011/0271358. Anti-human PD-1 monoclonal antibodies useful as the PD-1 antagonist according to the present disclosure include nivolumab, pembrolizumab, cemiplimab, sintilimab, and toripalimab. 
     The PD-1 antagonist according to the present disclosure may be administered by injection (intravenous injection, intraarterial injection, local injection), orally, nasally, transdermally, transpulmonarily, ophthalmically, and the like and examples thereof include injection such as intravenous injection, subcutaneous injection, intradermal injection, intraarterial injection, as well as local injection to target cells and organ. Examples of the dosage form of the PD-1 antagonist for oral administration include tablets, powders, granules, syrups, capsules, and oral solutions. Examples of the dosage form of the PD-1 antagonist for parenteral administration include injections, drip infusions, ophthalmic liquids, ointments, suppositories, suspensions, cataplasms, lotions, aerosols, and plasters and in one embodiment, the dosage form of the PD-1 antagonist for parenteral administration is an injection or a drip infusion. The PD-1 antagonist according to the present disclosure may be formulated by a method, for example, described in Japanese Pharmacopoeia (JP) 17th edition, United States Pharmacopoeia (USP), or European pharmacopoeia (EP). 
     If the PD-1 antagonist according to the present disclosure is an anti-PD-1 antibody, the anti-PD-1 antibody may be provided as a liquid preparation or prepared by rehydrating freeze-drying powder with sterile water for injection before use. 
     Upon administration of an anti-human PD-1 mAb alone as the PD-1 antagonist to a patient, the dose thereof varies markedly depending on the kind of the target disease, the age, sex, body weight of the patient, the severity of symptoms, and the like. The anti-human PD-1 mAb is administered, for example, at a dose of 1, 2, 3, 5, or 10 mg/kg at intervals of approximately 14 days (±2 days), approximately 21 days (±2 days), or approximately 30 days (±2 days). 
     When pembrolizumab is administered as the PD-1 antagonist, pembrolizumab is, for example, administered intravenously at a dose selected from the group consisting of 1 mg/kg Q2W, 2 mg/kg Q2W, 3 mg/kg Q2W, 5 mg/kg Q2W, 10 mg of Q2W, 1 mg/kg Q3W, 2 mg/kg Q3W, 3 mg/kg Q3W, 5 mg/kg Q3W, and 10 mg Q3W. Pembrolizumab is administered as a liquid medicine, for example, comprising 25 mg/ml pembrolizumab, 7% (w/v) sucrose, and 0.02% (w/v) polysorbate 80 in a 10 mM histidine buffer, pH 5.5, and a selected dose of the medicine is administered by IV injection over a period of approximately 30 minutes. 
     When nivolumab is administered as the PD-1 antagonist, nivolumab is, for example, administered intravenously at a dose selected from the group consisting of 1 mg/kg Q2W, 2 mg/kg Q2W, and 3 mg/kg Q2W. 
     The doses of the liposomal composition and the PD-1 antagonist in the combined administration of the present disclosure may usually be set at doses lower than the doses when they are administered alone. Specific doses, administration routes, administration frequencies, and administration cycles are determined as appropriate in consideration of the kind of the target disease, the age, sex, body weight of the patient, the severity of symptoms, and the like. 
     The mode of administration of the liposomal composition and the PD-1 antagonist in the present disclosure is not particularly limited, as long as the liposomal composition and the PD-1 antagonist are administered in combination when they are administered. For example, the liposomal composition and the PD-1 antagonist are administered to a patient simultaneously, separately, continuously, or at a time interval. Here, “simultaneously” means that each component is administered in the same period of time or strictly simultaneously or via the same administration route. “Separately” means that each component is administered at different dose intervals or frequencies or via different administration routes. “Continuously” means that each component is administered via the same administration route or different administration routes in any order within a certain period of time. “At a time interval” means that each component is administered via the same administration route or different administration routes, with each component administered at a time interval. When the PD-1 antagonist is administered in a period of 1 cycle of the administration of the liposomal composition or in a period in which the cycle is repeated, it is considered that both are administered in combination. 
     Tumors to be treated in the present disclosure are, for example, breast cancer, gastric cancer, esophageal cancer, small cell lung cancer, colorectal cancer, and kidney cancer. 
     EXAMPLES 
     [Example 1] Antitumor Effect of Combined Administration of Low Dose of Eribulin Mesylate (0.1 mg/kg) or Low Dose of Eribulin Mesylate Liposomal Formulation (0.1 mg/kg) and Anti-Mouse PD-1 Antibody in Syngeneic Transplantation Model of Murine Breast Cancer 4T1 Cell Line (Pgp-KO 4T1) with P Glycoprotein Knock-Out 
     A P glycoprotein-knockout cell line produced from murine breast cancer 4T1 cells (purchased from ATCC) was cultured using RPMI1640 medium (SIGMA) containing 10% of FBS (fetal bovine serum), 1 mM sodium pyruvate, and antibiotics, under conditions at 37° C. in a 5% carbon dioxide gas incubator. The cells were collected using trypsin-EDTA when the cells reached to approximately 80% confluency. The medium described above (RPMI1640) was added to the collected cells, a suspension was prepared at 1.0×10 7  cells/mL, and 0.1 mL of the suspension was subcutaneously transplanted at the right body side into 6 mice (BALB/cAJcl, CLEA Japan, Inc.) per each group of the control group, eribulin mesylate alone administration group, eribulin mesylate liposomal formulation alone administration group, the anti-mouse PD-1 antibody (Bio X cell) alone administration group, eribulin mesylate and anti-mouse PD-1 antibody combined administration group, and eribulin mesylate liposomal formulation and anti-mouse PD-1 antibody combined administration group. From day 5 post-transplantation, eribulin mesylate (0.1 mg/kg, once a week, twice in total, tail vein injection), eribulin mesylate liposomal formulation (0.1 mg/kg, once a week, twice in total, tail vein injection), and the anti-mouse PD-1 antibody (200 μg/mouse, once a week, twice in total, tail vein injection) were each administered alone or in combination to the alone administration groups or the anti-mouse PD-1 antibody combined administration groups. No drug was administered to the control group. The maximum tolerated dose of eribulin mesylate liposomal formulation in mice is 2.5 mg/kg and the dose in this experiment was set very low at 0.1 mg/kg, which is 1/25 of the maximum tolerated dose. 
     The liposomal composition comprising eribulin mesylate was prepared with the components set forth in Table 1 in accordance with the following method. 
     &lt;Preparation of Aqueous Solution for Liposomal Inner Phase&gt; 
     Ammonium sulfate and citric acid monohydrate were dissolved and diluted with pure water to prepare an aqueous solution of 200 mM ammonium sulfate/60 mM citric acid. The aqueous solution of 200 mM ammonium sulfate/60 mM citric acid was adjusted to pH 5.5 with an aqueous ammonium solution and then diluted with pure water to obtain an aqueous solution of 100 mM ammonium sulfate/30 mM citric acid. 
     &lt;Preparation of Liposomal Preparatory Liquid&gt; 
     Hydrogenated soy phosphatidylcholine, cholesterol, and MPEG2000-distearoylphosphatidylethanolamine were weighted in accordance with a weight ratio of 71:23:27, respectively. These were each dissolved in chloroform and these solutions were mixed. Chloroform was then evaporated under reduced pressure in a rotary evaporator to prepare a lipid film. To the obtained lipid film, the prepared aqueous solution for liposomal inner phase heated to approximately 80° C. was added and the resulting mixture was stirred to prepare a liposomal preparatory liquid. Sizing was performed using an extruder (a product made by Lipex Biomembranes Inc.) heated to approximately 80° C. to obtain a sized liposomal preparatory liquid. 
     &lt;Preparation of Liposomal Dispersion Liquid&gt; 
     By eluting the obtained liposomal preparatory liquid through a Sephadex G-50 column with an aqueous solution of 0.9% sodium chloride/10 mM histidine (pH=7.6), the liposomal outer phase was exchanged into an aqueous solution of 0.9% sodium chloride/10 mM histidine. After exchanging the liposomal outer phase, the liquid was centrifuged at 400,000×g for 30 minutes. After the centrifugation, re-dispersion was performed and the liquid volume was adjusted with an aqueous solution of 0.9% sodium chloride/10 mM histidine to obtain a liposomal dispersion liquid. 
     &lt;Preparation of Eribulin Mesylate Solution&gt; 
     0.9% of eribulin mesylate was dissolved in an aqueous solution of sodium chloride/10 mM histidine to obtain an eribulin mesylate solution. 
     &lt;Preparation of Liposomal Composition&gt; 
     The liposomal dispersion liquid and eribulin mesylate solution were mixed in a glass container and incubated in a water bath at 60° C. for 3 minutes to obtain a liposomal composition with a liposomal inner phase in which eribulin mesylate was introduced. An aqueous solution of 0.9% sodium chloride/10 mM histidine was added to the liposomal composition and filter sterilization was performed with a 0.22 μm polyvinylidene fluoride (PVDF) filter to obtain an eribulin mesylate liposomal composition. 
     On day 3, day 7, day 10, day 13, day 16, day 20, day 23, day 27, day 30, and day 34 after administration, with the starting date of administration being day 0, the longest diameter and the short axis of the tumor grown in each mouse were measured with a digimatic caliper (a product made by Mitutoyo Corporation). 
     The tumor volume was calculated in accordance with the following formula. 
       Tumor volume (mm 3 )=longest diameter (mm)×short axis 2  (mm 2 )/2
 
     The results of measurement of the tumor volume in each group are illustrated as mean and standard deviation (SD) in  FIG. 1 . As statistical analysis, repeated measures analysis of variance (ANOVA) followed by Dunnett&#39;s multiple comparison was conducted in comparison with the control group for tumor volumes on all measurement days in all groups (*: p&lt;0.05, ***: p&lt;0.001). The statistical comparison between the two groups of eribulin mesylate and anti-mouse PD-1 antibody combined administration group and eribulin mesylate liposomal formulation and anti-mouse PD-1 antibody combined administration group was conducted by repeated measures ANOVA (#: p&lt;0.05). 
     As a result, the combined administration of a low dose of eribulin mesylate liposomal formulation (0.1 mg/kg) and an anti-mouse PD-1 antibody exhibited a remarkable antitumor effect in comparison with the control group in the Pgp-KO 4T1 syngeneic tumor transplantation model. * and *** in  FIG. 1  indicate that the combined administration of eribulin mesylate liposomal formulation and the anti-mouse PD-1 antibody statistically significantly inhibited tumor growth in comparison with the control group. # indicates that the combined administration of eribulin mesylate liposomal formulation and the anti-mouse PD-1 antibody statistically significantly inhibited tumor growth in comparison with the combined administration of eribulin mesylate and the anti-mouse PD-1 antibody. In contrast, no antitumor effect was observed at low doses with eribulin mesylate (0.1 mg/kg) alone administration, eribulin mesylate liposomal formulation (0.1 mg/kg) alone administration, the anti-mouse PD-1 antibody alone administration, and even the combined administration of eribulin mesylate (0.1 mg/kg) and the anti-mouse PD-1 antibody. 
     The result of comparison of the groups for the time until the tumor volume exceeds 5 times that on the starting date of administration (T×5) in the experiment is shown in  FIG. 2  and the median of T×5 in each group and the percentage (%) thereof to the control group are set forth in Table 2. For statistical analysis, a log-rank test was conducted in comparison with the control group to calculate the Bonferroni-corrected p-value (*: p&lt;0.05). 
     As a result, the combined administration of eribulin mesylate liposomal formulation (0.1 mg/kg) and the anti-mouse PD-1 antibody exhibited the effect of extending (243%) the time of suppressing tumor growth (T×5) in Pgp-KO 4T1 syngeneic tumor transplantation model. In contrast, no extension effect was observed at low doses with eribulin mesylate (0.1 mg/kg) alone administration, eribulin mesylate liposomal formulation (0.1 mg/kg) alone administration, the anti-mouse PD-1 antibody alone administration, and further the combined administration of eribulin mesylate and the anti-mouse PD-1 antibody. * in Table 2 indicates that the combined administration of eribulin mesylate liposomal formulation (0.1 mg/kg) and the anti-mouse PD-1 antibody statistically significantly extended the time of tumor growth suppression in comparison with the control group. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Effect of the combined administration of eribulin mesylate liposomal 
               
               
                 formulation (0.1 mg/kg) and anti-mouse PD-1 antibody on T × 5 
               
            
           
           
               
               
               
            
               
                   
                   
                 Percentage to 
               
               
                 Group 
                 T × 5 (days) 
                 control 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Control 
                 10.5 
                 100% 
               
               
                 Eribulin mesylate Alone 
                 12.0 
                 114% 
               
               
                 Eribulin mesylate liposomal formulation Alone 
                 11.5 
                 110% 
               
               
                 Anti-mouse PD-1 antibody Alone 
                 14.5 
                 138% 
               
               
                 Eribulin mesylate + Anti-mouse 
                 14.5 
                 138% 
               
               
                 PD-1 antibody Combined administration 
               
               
                 Eribulin mesylate liposomal formulation + 
                 25.5* 
                 243% 
               
               
                 Anti-mouse PD-1 antibody Combined administration 
               
               
                   
               
            
           
         
       
     
     [Example 2] Antitumor Effect of Combined Administration of Low Dose of Eribulin Mesylate (0.3 mg/kg) or Low Dose of Eribulin Mesylate Liposomal Formulation (0.3 mg/kg) and Anti-Mouse PD-1 Antibody in Pgp-KO 4T1 Cell Line Syngeneic Transplantation Model 
     Pgp-KO 4T1 cells were cultured with the RPMI1640 medium containing 10% FBS, 1 mM sodium pyruvate, and antibiotics under conditions at 37° C. in a 5% carbon dioxide gas incubator. The cells were collected using trypsin-EDTA when the cells reached to approximately 80% confluency. The medium described above was added to the collected cells to prepare a suspension at 1.0×10 7  cells/mL. 0.1 mL of the cell suspension was subcutaneously transplanted at the right body side into 6 mice (BALB/cAJcl, CLEA Japan, Inc.) per each group of the control group, eribulin mesylate liposomal formulation alone administration group, the anti-mouse PD-1 antibody (Bio X cell) alone administration group, and the combined administration of eribulin mesylate liposomal formulation and the anti-mouse PD-1 antibody. From day 4 post-transplantation, eribulin mesylate liposomal formulation (0.3 mg/kg, once a week, twice in total, tail vein injection) and the anti-mouse PD-1 antibody (200 μg/mouse, once a week, twice in total, tail vein injection) were each administered alone or in combination to the alone administration groups or the combined administration group. No drug was administered to the control group. The liposomal composition comprising eribulin mesylate was prepared in accordance with the method as that in Example 1. 
     On day 3, day 7, day 9, day 13, day 17, day 20, day 24, day 27, day 31, day 34, day 38, day 41, day 44, day 48, and day 51 after administration, with the starting date of administration being day 0, the longest diameter and the short axis of the tumor grown in each mouse were measured with a digimatic caliper (a product made by Mitutoyo Corporation). 
     The tumor volume was calculated in accordance with the following formula. 
       Tumor volume (mm 3 )=longest diameter (mm)×short axis 2  (mm 2 )/2
 
     The mean and standard deviation (SD) of the results of measurement of the tumor volume in each group are illustrated in  FIG. 3  and the frequencies of mice with tumor disappearance are set forth in Table 3. For statistical analysis, the statistical comparison between the two groups of eribulin mesylate liposomal formulation alone administration group or the anti-mouse PD-1 antibody alone administration group and eribulin mesylate liposomal formulation and anti-mouse PD-1 antibody combined administration group was conducted by repeated measures ANOVA (t, #: p&lt;0.05). 
     As a result, in the Pgp-KO 4T1 syngeneic tumor transplantation model, the combined administration of a low dose of eribulin mesylate liposomal formulation (0.3 mg/kg) and the anti-mouse PD-1 antibody exhibited excellent antitumor effect in comparison with eribulin mesylate liposomal formulation alone administration group or the anti-mouse PD-1 antibody alone administration group. In  FIG. 3 , (t) indicates that the combined administration of eribulin mesylate liposomal formulation and the anti-mouse PD-1 antibody statistically significantly inhibited tumor growth in comparison with eribulin mesylate liposomal formulation alone administration and (#) indicates that the combined administration of eribulin mesylate liposomal formulation and the anti-mouse PD-1 antibody statistically significantly inhibited tumor growth in comparison with the anti-mouse PD-1 antibody alone administration. The tumor disappearance was observed in the combined administration group of eribulin mesylate liposomal formulation (0.3 mg/kg) and the anti-mouse PD-1 antibody at a frequency higher than other groups. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Frequency of appearance of mice with tumor disappearance in each group 
               
            
           
           
               
               
            
               
                   
                 Frequency (%) of mice with tumor 
               
               
                 Group 
                 disappearance 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Control 
                 0/6 
                 (0%) 
               
               
                 Eribulin mesylate liposomal formulation Alone 
                 1/6 
                 (17%) 
               
               
                 Anti-mouse PD-1 antibody Alone 
                 0/6 
                 (0%) 
               
               
                 Eribulin mesylate liposomal formulation + 
                 4/6 
                 (67%) 
               
               
                 Anti-mouse PD-1 antibody Combined administration 
               
               
                   
               
            
           
         
       
     
     The result of comparison of the groups for the time until the tumor volume exceeds 5 times that on the starting date of administration (T×5) in the experiment is shown in  FIG. 4  and the median of T×5 in each group and the percentage (%) thereof to the control group are set forth in Table 4. For statistical analysis, a log-rank test between the 2 groups of the combined administration group of eribulin mesylate liposomal formulation and the anti-mouse PD-1 antibody to the anti-mouse PD-1 antibody, alone administration group was conducted (#: p&lt;0.05). 
     As a result, in the Pgp-KO 4T1 syngeneic tumor transplantation model, the combined administration of eribulin mesylate liposomal formulation (0.3 mg/kg) and the anti-mouse PD-1 antibody exhibited the effect of extending suppression time of tumor growth (T×5) in comparison with the control group, eribulin mesylate liposomal formulation alone administration group, the anti-mouse PD-1 antibody alone administration group. # in  FIG. 4  indicates that the combined administration of eribulin mesylate liposomal formulation and the anti-mouse PD-1 antibody statistically significantly extended suppression time of tumor growth in comparison with the anti-mouse PD-1 antibody alone administration. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Effect of combined administration of eribulin mesylate liposomal 
               
               
                 formulation (0.3 mg/kg) and anti-mouse PD-1 antibody to T × 5 
               
            
           
           
               
               
               
            
               
                 Group 
                 T × 5 (days) 
                 Ratio to control 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Control 
                 11.5 
                 100% 
               
               
                 Eribulin mesylate liposomal formulation Alone 
                 33.0 
                 287% 
               
               
                 Anti-mouse PD-1 antibody Alone 
                 18.0 
                 157% 
               
               
                 Eribulin mesylate liposomal formulation + 
                 &gt;51.0# 
                 &gt;443%  
               
               
                 Anti-mouse PD-1 antibody Combined administration