the present invention will now be described in reference to embodiments set forth herein and in the figures . these embodiments are merely for the purposes of illustration and are not to be interpreted as limiting the invention as defined by the claims . in one aspect , the invention provides a liposomal preparation , comprising a suspension of liposomes having a gel - phase lipid bilayer and an active agent entrapped inside the liposomes ; said lipid bilayer comprising : ( i ) one or more phospholipids selected from the group consisting of phosphatidyl cholines , phosphatidyl glycerols , phosphatidyl inositols , and phosphatidyl ethanolamines ; ( ii ) one or more phospholipids derivatized with a hydrophilic polymer ; and ( iii ) one or more lysolipids selected from the group consisting of monoacylphosphatidyl cholines , monoacylphosphaticlylglycerols , monoacylphosphatidylinositols , and monoacylphosphatidyl - ethanolamines ; wherein the active agent is selected from the group consisting of doxorubicin , bleomycin , dacarbazine , daunorubicin , dactinomycin , fludarabine , gemcitabine , idarubicin , methotrexate , mitomycin , mitoxantrone , vinblastine , vinorelbine , and vincristine , and wherein the lipid bilayer constituents are provided in a molar ratio of about 80 - 90 : 2 - 8 : 2 - 18 ; and wherein the size of the liposomes in the suspension is between about 50 and about 150 nm . in one embodiment , the active agent is doxorubicin , and the relative concentration of impurity a after 6 months of storage at less than or equal to 8 ° c . is less than 0 . 5 %, wherein impurity a is a peak with a relative retention time approximately 1 . 4 in a high performance liquid chromatography ( hplc ) with a c18 reverse phase column with an acetic acid / methanol solvent gradient elution conditions . in one embodiment , the relative concentration of impurity a after 6 months of storage at less than or equal to 8 ° c . is less than about 0 . 5 %, or less than 0 . 4 %, or less than 0 . 3 %, or less than 0 . 2 %. in another embodiment , the relative concentration of impurity a after about 1 year of storage at less than or equal to 8 ° c . is less than about 0 . 5 %, or less than 0 . 4 %, or less than 0 . 3 %, or less than 0 . 2 %. in another embodiment , the relative concentration of impurity a after about 2 years of storage at less than or equal to 8 ° c . is less than about 1 %, 0 . 75 %, 0 . 5 %, or less than 0 . 4 %, or less than 0 . 3 %, or less than 0 . 2 %. in one embodiment , the relative concentration of 8 - desacetyl - 8 - carboxy daunorubicin after 6 months of storage at less than or equal to 8 ° c . is less than about 0 . 5 %, less than 0 . 4 %, less than 0 . 3 %, or less than 0 . 2 %. in another embodiment , the relative concentration of 8 - desacetyl - 8 - carboxy daunorubicin after about 1 year of storage at less than or equal to 8 ° c . is less than about 0 . 5 %, less than 0 . 4 %, less than 0 . 3 %, or less than 0 . 2 %. in another embodiment , the relative concentration of 8 - desacetyl - 8 - carboxy daunorubicin after about 2 years of storage at less than or equal to 8 ° c . is less than about 2 . 0 %, less than 1 . 6 %, less than 1 . 5 %, less than 1 . 0 %, less than 0 . 5 %, less than 0 . 4 %, less than 0 . 3 %, or less than 0 . 2 %. in a further embodiment , the concentration of doxorubicin after 150 days of storage at a temperature of about less than or equal to 8 ° c . is greater than 95 %, greater than 96 %, greater than 97 %, greater than 98 %, greater than 99 %, or greater than 99 . 5 %, of the initial doxorubicin concentration , as determined by hplc with a c18 reverse phase column with an acetic acid / methanol solvent gradient elution conditions . in another embodiment , the concentration of doxorubicin after about six months of storage at a temperature of about less than or equal to 8 ° c . is greater than 95 %, greater than 96 %, greater than 97 %, greater than 98 %, greater than 99 %, or greater than 99 . 5 %, of the initial doxorubicin concentration , as determined by hplc with a c18 reverse phase column with an acetic acid / methanol solvent gradient elution conditions . in another embodiment , the concentration of doxorubicin after about one year of storage at a temperature of about less than or equal to 8 ° c . is greater than 95 %, greater than 96 %, greater than 97 %, greater than 98 %, greater than 99 %, or greater than 99 . 5 %, of the initial doxorubicin concentration , as determined by hplc with a c18 reverse phase column with an acetic acid / methanol solvent gradient elution conditions . in another embodiment the concentration of doxorubicin after about two years of storage at a temperature of about less than or equal to 8 ° c . is greater than 95 %, greater than 96 %, greater than 97 %, greater than 98 %, greater than 99 %, or greater than 99 . 5 %, of the initial doxorubicin concentration , as determined by hplc with a c18 reverse phase column with an acetic acid / methanol solvent gradient elution conditions . in another embodiment , the invention is a pharmaceutical composition , wherein the formation of total degradation products after 150 days of storage at a temperature of about less than or equal to 8 ° c . is less than 1 %, or less than 0 . 5 %. in a further embodiment , the invention is a pharmaceutical composition , wherein the formation of total degradation products after about six months of storage at a temperature of about less than or equal to 8 ° c . is less than 1 %, or less than 0 . 5 %. in a further embodiment , the invention is a pharmaceutical composition , wherein the formation of total degradation products after about one year of storage at a temperature of about less than or equal to 8 ° c . is less than 1 %, or less than 0 . 5 %. in a further embodiment the invention is a pharmaceutical composition , wherein the formation of total degradation products after about two years of storage at a temperature of about less than or equal to 8 ° c . is less than 2 . 5 %, less than 1 % or less than 0 . 5 %. in yet another embodiment , the liposomes are suspended in a buffer comprising a saccharide . the saccharide may be a monosaccharide , such as lactose , or a disaccharide such as sucrose . in another embodiment , the buffer further comprises histidine . in another aspect , the invention provides a method for loading an active agent into temperature sensitive liposomes , comprising : ( a ) preparing a suspension of liposomes having a gel - phase lipid bilayer and a greater concentration of ammonium ions inside the liposomes than outside the liposomes , said lipid bilayer comprising : ( i ) one or more phospholipids selected from the group consisting of phosphatidyl cholines , phosphatidyl glycerols , phosphatidyl inositols , and phosphatidyl ethanolamines ; ( ii ) one or more derivatized with a hydrophilic polymer ; and ( iii ) one or more lysolipids selected from the group consisting of monoacylphosphatidyl cholines , monoacylphosphaticlylglycerols , monoacylphosphatidylinositols , and monoacylphosphatidyl - ethanolamines ; wherein the lipid bilayer constituents are provided in a molar ratio ( i ):( ii ):( iii ) of about 80 - 90 : 2 - 8 : 2 - 18 ; and where said preparing includes reducing the size of the liposomes in the suspension to an average particle size of between about 50 and about 150 nm ; ( b ) adding a solution of the active agent to the suspension of liposomes , wherein the active agent is taken up into the liposomes , wherein the active agent is selected from the group consisting of doxorubicin , bleomycin , dacarbazine , daunorubicin , dactinomycin , fludarabine , gemcitabine , idarubicin , methotrexate , mitomycin , mitoxantrone , vinblastine , vinorelbine , and vincristine . in one embodiment , the active agent is doxorubicin . in one embodiment , at least 90 %, at least 91 % at least 92 %, at least 93 %, at least 94 %, at least 95 %, at least 96 %, at least 97 %, or at least 98 % of the doxorubicin present in the solution is taken up into the liposomes . in another embodiment , the concentration of doxorubicin taken up into the liposomes is about 1 mm to about 200 mm , preferably about 10 to about 65 mm , and most preferably about 45 mm to about 55 mm . in a further embodiment the concentration of doxorubicin taken up into the liposomes is about 50 mm . in another embodiment the concentration of doxorubicin taken up into the liposomes is about 75 mm . liposomes of the present invention are composed of phospholipids selected from the group consisting of phosphatidyl cholines , phosphatidyl glycerols , phosphatidyl inositols , and phosphatidyl ethanolamines . the phospholipids preferably possess a solid or gel form to liquid transition temperature in the lower end of the hyperthermic range ( e . g ., the range of from approximately 38 ° c . to approximately 45 ° c .). more preferred are phospholipids whose acyl groups are saturated . in one embodiment , the one or more phospholipids have two same or different c 14 - c 20 acyl groups , such as , for example dipalmitoylphosphatidylcholine ( dppc ), distearoylphosphatidyl glycerol ( dspg ), or a combination thereof . the liposomes of the present invention are composed of one or more lysolipids . in one embodiment , the lysolipid is monopalmitoylphosphatidylcloline ( mppc ), monolaurylphosphatidylcholine ( mlpc ), monornyristoylphosphatidylcholine ( mmpc ), monostearoylphosphatidylcholine ( mspc ), or a mixture thereof . in one embodiment of the invention , the total concentration of lipids in the final liposomal formulation is about 10 - 50 mg / ml , about 20 - 50 mg / ml , about 30 - 40 mg / ml , about 20 mg / ml , about 30 mg / ml , or 40 mg / ml . in another embodiment , the concentration of doxorubicin in the liposomal formulation is about 0 . 2 - 40 mg / ml , about 0 . 5 - 30 mg / ml , about 1 - 20 mg / ml , about 2 - 10 mg / ml , about 1 mg / ml , about 2 mg / ml , about 3 mg / ml , about 4 mg / ml or about 5 mg / ml . in one embodiment of the invention the doxorubicin to lipid ratio is 0 . 02 - 10 , about 0 . 05 , about 1 , about 2 , about 3 , about 4 , about 5 , about 6 , about 7 , about 8 , about 9 or about 10 . liposomes of the present invention include polymer - derivatized lipids to decrease liposome uptake by the res and thus increase the circulation time of the liposomes . suitable polymers include hydrophilic polymers such as polyethylene glycol , polyvinylpymlidine , olylactic acid , polyglycolic acid , copolymers of polylactic acid and polyglycolic acid , polyvinyl alcohols , polyvinylpyrrolidone , dextrans , oligosaccharides , along with mixtures of the above . in one embodiment , the one or more phospholipids derivatized with a hydrophilic polymer is a polyethylene glycol derivatized ( pegylatal ) lipid . preferably , the pegylated lipid is 1 , 2 - distearoyl - sn - glycero - 3 - phosphoethanolamine - n -[ poly ( ethyleneglycol ) 2000 ]. in one embodiment , the invention provides a method for loading a liposome with an active agent which is bleomycin , dacarbazine , daunorubicin , dactinomycin , fludarabine , gemcitabine , idarubicin , methotrexate , mitomycin , mitoxantrone , vinblastine , vinorelbine , or vincristine . in one embodiment , the said preparing comprises preparing the liposomes in the presence of an ammonium salt , provided as an ammonium sulfate solution . in one embodiment , the concentration of ammonium sulfate in the solution is about 100 mm to about 300 mm , preferably about 200 mm . in another embodiment , the ammonium salt is provided as a salt of adipic acid . l - ascorbic acid , l - aspartic acid , citric acid , fumaric acid , glutamic acid , glutaric acid , hippuric acid , hydrochloric acid , d , l - lactic acid , maleic acid , l - malic acid , phosphoric acid , succinic acid , or l - tartaric acid . in a further embodiment , the ammonium salt in the solution is about 100 mm to about 300 mm , preferably about 200 mm . the ammonium ions outside the liposomes are replaced with a monosaccharide or disaccharide solution . in a further embodiment , the concentration of the monosaccharide or disaccharide solution is about 5 - 15 %, preferably about 10 %. this replacement or exchange can be earned out by techniques such as dialysis or diafiltration . in a further embodiment , the ammonium ions outside the liposomes are replaced with a monosaccharide solution , such as for example , a lactose solution . in another embodiment , the ammonium ions outside the liposomes are replaced with a disaccharide solution , such as for example , a sucrose solution . in one embodiment , a histidine buffer is added to the liposomal preparation after step ( b ). in a further embodiment , the concentration of the histidine buffer is about 5 mm to about 15 mm , preferably about 10 mm . a method of preparing a liposomal formulation according to the present invention comprises mixing the bilayer components in the appropriate proportions in a suitable organic solvent . useful solvents include chloroform , acetone , methanol or methylene chloride . the solvent is then evaporated to form a dried lipid film . the film is rehydrated ( at temperatures above the phase transition temperature of the lipid mixture ) using an aqueous solution containing an equilibrating amount of the lysolipid and a desired active agent , e . g ., doxorubicin . the liposomes formed after rehydration are extruded to form liposomes of a desired size . for example , when liposomes composed of 80 : 20 dppc : mspc are produced , rehydration is carried out at a temperature above the phase transition temperature of this particular lipid mixture ( above 39 ° c .). the aqueous solution used to rehydrate the lipid film comprises an equilibrating amount of lysolipid monomers ( e . g ., a concentration equal to the critical micelle concentration of mspc , about 1 micromolar ). the manufacturing process for large scale batches of the ammonium loaded formulation is described below . the process can be employed to produce various size batches of formulation , for example , a 2 - 2000 l scale batch . a proposed manufacturing process is illustrated schematically in fig2 . 1 . prepare an ammonium sulfate buffer by dissolving appropriate quantities of ammonium sulfate in water for injection ( wfi ) followed by a bioburden reduction filtration . the molarity of the buffer may be , for example , 200 mm . 2 . hydrate the lipids utilizing the ammonium sulfate buffer from step 1 for an appropriate amount of time at an elevated temperature ( 45 - 70 ° c .). for example , the lipids are hydrated for 1 hour at 60 ° c . 3 . extrude the hydrated lipid mixture through filter membranes having a certain pore size at an elevated temperature , in order to obtain liposomes of desired size . for example , the hydrated lipid mixture is extruded through 80 nm polycarbonate filter membranes at 65 ° c . to form ˜ 100 nm liposomes . 4 . exchange the non - liposome entrapped ammonium sulfate against a saccharide solution , for example a 10 % sucrose solution , followed by sterile filtration through a preheated filter , such as a sartobran p filter . 5 . prepare a histidine hcl buffer , for example , a 100 mm histidine buffer at ph 6 , by dissolving appropriate quantities of histidine hcl in wfi , followed by sterile filtration . 6 . prepare a doxorubicin hcl solution , for example at a concentration , of 5 . 0 mg / ml , by dissolving appropriate quantity of doxorubicin hcl in wfi , followed by sterile filtration . 7 . mix 1 . 0 parts sterile liposome with 0 . 8 parts sterile doxorubicin hcl solution , and incubate at 35 ° c . for 4 hours . in one embodiment , the invention is a liposomal preparation made by a method for loading doxorubicin into temperature sensitive liposomes , comprising : ( a ) preparing a suspension of liposomes having a gel - phase lipid bilayer and a gi eater concentration of ammonium ions inside the liposomes than outside the liposomes , said lipid bilayer comprising : ( i ) one or more phospholipids selected from the group consisting of phosphatidyl cholines , phosphatidyl glycerols , phosphatidyl inositols , and phosphatidyl ethanolamines ; ( ii ) one or more phospholipids derivatized with a hydrophilic polymer ; and ( iii ) one or more lysolipids selected from the group consisting of monoacylphosphatidyl cholines , monoacylphosphatidylglycerols , monoacylphosphatidylinositols , and monoacylphosphatidyl - ethanolamines ; wherein the lipid bilayer constituents are provided in a molar ratio ( i ):( ii ):( iii ) of about 80 - 90 : 2 - 8 : 2 - 18 ; and where said preparing includes reducing the size of the liposomes in the suspension to an average particle size of between about 50 and about 150 nm ; ( b ) adding a doxorubicin solution to the suspension of liposomes , wherein the doxorubicin is taken up into the liposomes . liposomes of between 0 . 05 to 0 . 3 microns in diameter , have been reported as suitable for tumor administration ( u . s . pat . no . 5 , 527 , 528 to allen et al .). sizing of liposomes according to the present invention may be carried out according to methods known in the art , and taking into account the active agent contained therein and the effects desired ( see , e . g ., u . s . pat . no . 5 , 225 , 212 to martin et al ; u . s . pat . no . 5 , 527 , 528 to allen et al ., the disclosures of which are incorporated herein by reference in their entirety ). in a preferred embodiment of the present invention , liposomes are from about 0 . 05 microns or about 0 . 1 microns in diameter , to about 0 . 3 microns or about 0 . 4 microns in diameter . liposome preparations may contain liposomes of different sizes . advantageously , these liposomes comprise , lipid mixtures set forth herein and are therefore temperature - sensitive , with an ability to release contained drug , as described . in one aspect of the present invention , the liposomes are prepared to have substantially homogeneous sizes in a selected size range . one effective sizing method involves extruding an aqueous suspension of the liposomes through a series of polycarbonate membranes having a selected uniform pore size ; the pore size of the membrane will correspond roughly with the average sizes of liposomes produced by extrusion through that membrane . see e . g ., u . s . pat . no . 4 , 737 , 323 . in another preferred embodiment of the present invention , liposomes are from about 50 nm , 100 nm , 120 nm , 130 nm , 140 nm or 150 nm , up to about 175 nm , 180 nm , 200 nm , 250 nm , 300 nm , 350 nm , 400 nm or 500 nm in diameter . in one embodiment , the liposomal preparation of the present invention is stored at a temperature of less than or equal to 8 ° c ., from about 2 ° c . to about 8 ° c ., from about − 80 ° c . to about − 15 ° c ., from about − 30 ° c . to about − 15 ° c ., or from about − 15 ° c . to about 2 ° c . in another aspect , the liposomal preparation comprises doxorubicin and an imaging or diagnostic agent . the ability to encapsulate an imaging agent in a liposome or an imaging agent in combination with a therapeutic is desirable for a number of reasons . first , the therapeutic efficacy of the active agent will be increased with the ability to visualize release of the imaging agent and thus infer the release of drug . this would provide the tools to determine the drug &# 39 ; s tissue penetration and concentration . further , combining a drug with an imaging agent in a liposome will permit monitoring and quantitation of drug release over time , tissue distribution , and drug clearance . secondly , a liposome carrying and releasing imaging agent will allow for the opportunity to pre - screen patients . for example , a select patient population may be identified as likely to benefit from the therapeutic liposome based on the “ leakiness ” of tumor vasculature . this leakiness , as visualized using an imaging agent , is an indicator of ability of the active agent to extravasate across the microvasculature and any fibrotic tissue to access and treat the tumor . examples of imaging or diagnostic agents that may be employed include , but are not limited to , agents for x - ray imaging , magnetic resonance imaging ( mri ), ultrasound imaging or nuclear medicine imaging . in x - ray imaging , including applications such as computed tomography ( ct ) and digital subtraction angiography ( dsa ), contrast is based on differences in electron density . in one aspect of the invention , the liposomal preparation comprises doxorubicin and an x - ray contrast agent . x - ray contrast agents are generally based on heavy elements , and include barium salts such as barium sulphate , which may be used to enhance visualization of the gastrointestinal system and iodinated contrast agents , which may be used in visualization of the gastrointestinal system and in parenteral studies . iodinated x - ray contrast agents include , but are not limited to , iohexyl , iopentol , iopamidol , iodixanol , iopromide , iotrolan , metrizamide , metrizoic acid , diatriazoic acid , iothalamic acid , ioxaglic acid and salts of these acids . in another aspect of the invention , the liposomal preparation comprises doxorubicin and an mri contrast agent . mri contrast agents include paramagnetic chelates , for example based on manganese ( 2 +), gadolinium ( 3 +) or iron ( 3 +). hydrophilic chelates such as gddtpa , gddota , gdhpdo3a and gddtpa - bma are distributed extracellularly and eliminated renally . such compounds are useful in , for example , visualizing lesions in the central nervous system . other more organ - or tissue - specific agents include mndpdp , gdbopa , gdeob - dtpa , paramagnetic porphyrins , macromolecular compounds , particles and liposomes . in yet another aspect of the invention , the liposomal preparation comprises doxorubicin and an ultrasonic imaging , agent . ultrasonic imaging is based on penetration of ultrasound waves , e . g . in the frequency range 1 - 10 mhz , into a human or animal subject via a transducer , the ultrasound waves interacting with interfaces of body tissues and fluids . contrast in an ultrasound image derives from differential reflection / absorption of the sound waves at such interfaces ; results may be enhanced by the use of doppler techniques , including the use of color doppler to evaluate blood flow . examples of ultrasound contrast agents include echovist ®, based on gas - containing galactose microcrystals ; levovist ®, comprising gas - containing galactose microcrystals coated with fatty acid ; and infoson ®, which comprises gas bubbles encapsulated by partially denatured human serum albumin . other imaging or diagnostic agents that may be used in the present invention include , but are not limited to , fluorescent agents such as 6 - carboxyfluorescem , radioactive agents ( such as radioisotopes or compounds containing radioisotopes , including iodo - octanes , halocarbons , and renografin ), and the like . in another aspect of the invention , the liposomal preparation further comprises an additional active agent , for e . g ., another chemotherapeutic drug . preparation of doxorubicin loaded temperature - sensitive liposomes by nh 4 + - loading liposomes containing 1 , 2 - dipalmitoyl - sn - glycero - 3 - phosphatidyl choline ( dppc ), which comprises 86 % ( mole %) of the liposome membrane ; 1 , 2 - distearoyl - sn - glycero - 3 - phosphoethanolamine - n - polyethylene glycol 2000 ( dspe - mpeg ), at approximately 4 % ( mole %); and 1 - stearoyl - 2 - hydroxy - sn - glycero phosphatidyl choline ( mspc ) at approximately 10 % ( mole %) are prepared by the following technique : the appropriate lipid composition is first hydrated in 200 mm ammonium sulfate buffer , forming multi - lamellar liposomes . small uni - lamellar liposomes are then formed by extrusion through 80 nm filters to form approximately 100 nm spheres in 200 mm ammonium sulfate buffer . the liposomes prepared in the previous step were then subjected to a dialysis or diafiltration step exchanging the ammonium sulfate that is external to the liposome with a 10 % sucrose solution , forming an ammonium concentration gradient across the liposome membrane ( i . e . 200 mm inside , less than 1 mm outside ). it is known ( haran g , cohen r , bar l k and barenholz y , transmembrane ammonium sulfate gradients in liposomes produce efficient and stable entrapment of amphipathic weak bases , biochimica et biophysica acta , 1151 ( 1993 ) 201 - 215 201 ) that the ammonium concentration can effectively , and near quantitatively , promote the loading of an added doxorubicin solution to the internal volume of the liposome at elevated temperatures . doxorubicin was entrapped within the inner aqueous volume of the liposomes by incubation at 35 - 39 ° c . at the completion of loading , the liposomal solution was buffered with a histidine buffer to stabilize the product ph during storage . liposomes with doxorubicin loaded using a ph gradient are prepared according to the method described in wo 2007 / 024826 , liposomes containing 1 , 2 - dipahnitoyl - sn - glycero - 3 - phosphatidyl choline ( dppc ), which comprises 86 % ( mole %) of the liposome membrane ; 1 , 2 - distearoyl - sn - glycero - 3 - phosphoethanolamine - n - polyethylene glycol 2000 ( dspe - mpeg ), at approximately 4 % ( mole %); and 1 - stearoyl - 2 - hydroxy - sn - glycero phosphatidyl choline ( mspc ) at approximately 10 % ( mole %) are prepared by the following technique : the appropriate lipid composition is first hydrated in 300 mm citrate buffer ( ph = 4 ), forming multi - lamellar liposomes . small uni - lamellar liposomes are then formed by extrusion through 80 nm filters to form approximately 100 nm spheres in 300 mm citrate buffer . a 500 mm sodium carbonate solution is then added to the liposomes prepared in the previous step , increasing the external solution to a ph of ˜ 7 . 5 . it is known ( see for example , mayer l b , bally m b , cullis p r ., uptake of adriamyacin into large unilamellar liposomes in response to a ph gradient , biochimica et biophysiea acta 857 ( 1986 ) 123 - 126 ) that the ph gradient formed across the membrane can effectively , and near quantitatively , promote the loading of an added doxorubicin solution to the internal volume of the liposome at elevated temperatures . doxorubicin was entrapped within the inner aqueous volumes of liposomes by incubation at 35 - 39 ° c . table 1 displays a comparison between formulations according to example 1 , and a conventional ph loaded liposome , according to example 2 . as seen from table 1 , both formulations contain 2 . 0 mg / ml of doxorubicin . the formulation according to the present invention compares well to a more conventional liposomal doxorubicin formulation . all raw materials used were of pharmaceutical grade . the final product is characterized for total doxorubicin content , doxorubicin degradation products , ph , osmolality , particle size distribution , mspc content , dppc content , dspe - mpeg content , % encapsulated doxorubicin , drug release at 37 ° c ., and drug release at 41 ° c . to effectively complete assessment of the product . the target total doxorubicin content is between about 1 . 8 to about 2 . 2 mg / ml . the drug encapsulation was typically greater than 90 %, and showed limited release , e . g . & lt ; 10 %, at normal body temperature ( i . e . 37 ° c . ), and exhibited enhanced release , typically & gt ; 80 %, at 41 . 0 ° c . the volume averaged particle size of the liposomes as measured by dynamic light scattering is between about 50 to about 150 nm . the physicochemical properties of the liposomes formed in the above example 1 are comparable to a liposomal preparation formed using a conventional buffer . as shown in fig3 , the particle size distribution of ammonium sulfate hydrated liposome is essentially identical to a citrate buffer hydrated liposome . as shown in table 1 above , the lipid composition of the liposomal preparation of the present invention is identical to the lipid composition of the liposomal , preparation known in the art . the functionality of the lipid membrane composition is also confirmed by testing the differential drug release at both 37 ° c . and 41 . 0 ° c . the present invention provides a liposomal product designed to utilize a remote loading , procedure ( see for example , haran g , cohen r , bar l k and barenholz transmembrane ammonium sulfate gradients in liposomes produce efficient and stable entrapment of amphipathic weak bases , biochimica et biophysica acta , 1151 ( 1993 ) 201 - 215 201 ), to encapsulate greater than 90 % of the doxorubicin in the internal aqueous core . the % of doxorubicin encapsulated is calculated by measuring unencapsulated doxorubicin ( free dox ), separated by ultrafiltration , and the total doxorubicin in the product . current studies have shown that greater than 95 % encapsulation can b achieved for the ammonium loaded formulation . additionally , the thermal release properties of each batch , % release at 37 ° c . % release at 41 ° c ., have been very reproducible from batch to batch , and are comparable , as shown in fig8 . in addition to the list of finished product characterization tests mentioned above , several other properties of the new formulation have been evaluated . first , due to the importance of the liposome membrane in the key design parameters for the drug product , differential scanning calorimetry was performed on the ph - loaded ( shown in fig4 ) and nh 4 + - loaded ( shown in fig5 ) formulations . each thermogram shows one major exotherm , at about 41 ° c ., and suggests that the membrane for new formulation is quite similar to that for the ph - loaded liposomes , as to be expected , as the buffer solution should have negligible effects on the overall structure of the membrane order . the overall size and morphology of the two formulations were also compared using the high resolution technique of tunneling electron microscopy ( tem ). again , the comparison between ph - loaded product produced in a gmp manufacturing facility at the current manufacturing scale ( fig6 ), which is currently being used in phase iii clinical studies , to product made using the nh 4 + - loaded formulation at the laboratory scale at celsion ( fig7 ) was performed . the liposomes for the two formulations show similar vesicle diameters , predominately unilamellar membranes , and exhibit a classical single crystal inside each liposome , which is attributed to the doxorubicin drug complex formation inside the liposome during the loading step . overall , the tems show that the liposomes generated using either ph or nh 4 + - loading system are quite similar . the temperature release profiles measuring the amount of doxorubicin released as a function of temperature from 35 to 45 ° c . was determined by incubating each sample at the specified temperature for 10 minutes . the results of the tests are shown in fig8 . as in the previous tests , the comparison was made between ph - loaded product produced in a gmp manufacturing facility at the current manufacturing scale , which is currently being used in phase iii clinical studies , to product made using the nh 4 + - loaded formulation at the laboratory scale at celsion ( fig8 ). the release curves are very similar for the two formulations , both showing minimal release at temperatures below 39 ° c ., and near 90 % release at 41 . 0 ° c . and above . clearly , both formulations support the design target of limiting doxorubicin release at normal body temperature , i . e . 37 ° c ., with the majority of the drug being released with mild hyperthermia , or temperatures in the 41 - 45 ° c . range . the temperature release data is also the best measure of the microscopic uniformity of the lipid membrane composition . in order for a formulation to release greater than 90 % of the drug at 41 . 0 ° c ., the majority of the liposomes ( i . e . the 100 nm vesicles ) must have the appropriate lipid composition to demonstrate the thermal triggered release for the bulk product . it is known that incorrect levels of dspe - mpeg or mspc will adversely affect the extent and rate of release for doxorubicin from these liposomes . furthermore , the fact that the transition temperatures are nearly identical , in conjunction with the comparative dsc scans ( fig4 and 5 ), leads to the conclusion that the change in the buffer sys em has negligible impact on liposome membrane and , therefore should have negligible impact on its drug release properties . comparison of levels of 8 - desacetyl - 8 - carboxy daunorubicin and impurity a for the ph - loaded and nh 4 + - loaded formulations laboratory experiments were performed to examine the levels of 8 - desacetyl - 8 - carboxy daunorubicin and impurity a produced in the ph - loaded and nh 4 + - loaded formulations ( fig9 ). excipients sourced from two providers , excipients a and b , were examined for the ph - loaded formulation . three independent preparations of the nh 4 + - loadecl formulations were also examined . in all cases , and both for 8 - clesacetyl - 8 - carboxy daunorubicin and impurity a , the levels formed were significantly higher for the ph - loaded formulations than the nh 4 + - loaded formulations . reduced levels of 8 - desacetyl - 8 - carboxy daunorubicin were observed for the ph - loaded and nh 4 + - loaded formulations with the new source of excipients , with no change on the levels of impurity a . furthermore , the combined levels of 8 - desacetyl - 8 - carboxy daunorubicin and impurity a for the nh 4 + - loaded formulations were less than 0 . 2 %, even with four hour incubation times at 35 ° c . the levels of degradate formation are shown as the initial time point in the stability data shown in fig1 , and correlate well with the doxorubicin values shown in fig1 . comparative stability data were generated for the ph - loaded and nh 4 + - loaded formulations . while the ph - loaded formulation requires storage at − 15 ° c . to − 30 ° c ., the stability comparison as generated both at − 20 ° c . and under accelerated stability condition , i . e ., at + 5 ° c . storage . the results of the doxorubicin assay after 739 days showed a loss of ˜ 4 % doxorubicin for the ammonium - loaded formulation . in contrast , the loss of doxorubicin after the same time period was ˜ 60 % for the ph loaded formulation . the loss of doxorubicin assay data is summarized in fig1 and table 2 . the total degradate growth supports the same trend , i . e . significant increase in degradates are observed for the ph - loaded formulation , with very low levels of degradate , growth for the nh 4 + - loaded formulation ( fig1 and table 2 in addition to the stability at 2 - 8 ° c ., fig1 and fig1 show the loss of doxorubicin assay data at − 20 ° c . the data demonstrate that the nh 4 + - loaded formulation exhibits very low levels of degradate growth and increased doxorubicin stability compared to the ph - loaded formulation . it has also been observed that the identity of the degradation products formed from the ph - loaded and nh 4 + - loaded formulations are the same confirmed by lc / ms , although formation occurs to a lesser extent for the nh 4 + - loaded formulation . furthermore , the nh 4 + - loaded formulation exhibits improved doxorubicin hcl stability , in addition to lower levels of degradation product growth , through at least two years of storage . the solution ph , liposome particle size , % encapsulation , and % release of doxorubicin at 41 . 0 ° c . for the nh 4 + - loaded formulation remain through at least two years storage at temperatures of less than or equal to 8 ° c . the cumulative stability data outlined above , support the assertion that the nh 4 + - loaded formulation can be provided commercially as a refrigerated product , stored at temperatures of less than or equal to 8 ° c . it is expected that the new , minimized total degradate formation will yield an acceptable product for commercial use with a shelf life of up to 2 years . the decreased degradation levels will also translate into improved maintenance of product potency . overall , the combined effects of these improvements to the drug product are considered to enhance dosing reproducibility , achieve better shipping and storage compliance , and thus lead to a higher quality commercial product . it is to be appreciated that the detailed description section , and not the summary and abstract sections , is intended to be used to interpret the claims . the summary and abstract sections may set forth one or more , but not all exemplary embodiments of the present invention as contemplated by the inventor ( s ), and thus , are not intended to limit the present invention and the appended claims in any way . the foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can , by applying knowledge within the skill of the art , readily modify and / or adapt for various applications such specific embodiments , without undue experimentation , without departing from the general concept of the present invention . therefore , such adaptations and modifications are intended to be within the meaning , and range of equivalents of the disclosed embodiments , based on the teaching and guidance presented herein . it is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation , such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance . the breadth and scope of the present invention should not be limited by any of the above - described exemplary embodiments , but should be defined only in accordance with the following claims and their equivalents .
the present invention relates to a formulation of thermosensitive liposomes , and more specifically to a formulation of liposomes comprising phospholipids and a surface active agent , wherein the liposomes support long term storage at temperatures less than or equal to about 8 ° c ., control degradate formation to maximize product potency and release their contents at mild hyperthermic temperatures . methods of making formulations are also described .