as discussed in the furneaux et al . references cited above , aluminum can be anodized in acid to produce a uniform array of cells or openings having cylindrical pores which preferably branch from larger pore - size openings in one face of the film to smaller pore - size openings in the other face of the film . such filters are available in a variety of pore sizes , and ones having a pore size of the smaller pores of less than about 1000 nm are preferred for use in the present invention , although smaller or larger pore - sizes can be used depending on the final application of the liposomes . ( hereinafter , unless otherwise noted , “ pore size ” for such filters shall refer to the minimum pore size of the smaller pores .) at present , of the anopore ™ anodized aluminum porous filters commercially available from anotec separations , those of pore size under 200 nm are of the branched - pore structure , and those of pore size of 200 nm or larger are of uniform pore size from one surface to the other . when the desired average particle size of the liposomes is less than about 200 nm , then the preferred pore size of the filter is less than about 100 nm . these filters are also of the preferred branched - pore type structure . these aluminum oxide filters are hydrophilic ; they do not swell in aqueous solvents ; they have good organic solvent resistance ; and they have pores of uniform size which promote high flow - through characteristics . because of the properties of such films , it was found that liposomes could be extruded through them at relatively high flow rates under relatively low pressure ( see example 3 , below ). thus , aluminum oxide filters are shown to be superior for extruding liposomes over the previously known polymeric filters . in the present invention , the branched - pore filters are preferably used so that the liposomes enter the face with the smaller size pores and exit though the face with the larger size pores . however , as shown in the examples below , good extrusion can also be obtained with a filter mounted in the inverted position , so that the liposomes enter the large pore - size face . in accordance with the process of present invention , a population of liposomes substantially free of liposomes above a predetermined maximum size is produced from a suspension of liposomes , a portion of which are of sizes larger than the predetermined maximum size . the process includes passing the suspension of liposomes under pressure one or more times through an aluminum oxide porous film , such as one of the type described above . to determine whether a population of liposomes is “ substantially free of liposomes above a predetermined maximum size ”, the liposomes can be tested using a standard sizer . one such standard sizer is a malvern sizer , available from malvern instruments , malvern , england , which is described and used in some of the examples below . another sizer which can used to determine particle size distributions is a nicomp ™ laser particle sizer , available from hiac / royco instruments , menlo park , calif ., which is also described and used in some of the examples below . in the particle size distributions reported in the examples below , a test result indicating that 0 . 0 percent of the liposomes present in the population are above a given size indicates that the population is “ substantially free ” of such large - size liposomes . although not required , the particle size , and particularly the particle - size distribution , of liposomes may be made smaller and more uniform by extruding though a larger filter as a first step . for example , extruding the liposomes through a suitable filter of 2 - 10 micrometer size , such as one made from polytetrafluoroethylene ( ptfe ), as is well known in the art , will reduce the particle size and particle - size distribution prior to extruding through the aluminum oxide filter , and may thereby reduce the time for extrusion . the pressure during the extrusion will be varied depending upon the liposomes employed , their mean particle diameter and particle - size distribution , and the rate of flow desired . extrusion pressures may vary from about 200 to about 1000 psi ( 1 . 4 - 6 . 9 mpa ), but pressures of less than about 600 psi ( 4 . 2 mpa ) are preferred . in general , fewer extrusion passes are required when using the branched - pore type aluminum oxide filters of the present invention , as opposed to the previously known polycarbonate filters , for similar results in terms of particle size , particle - size distribution and flow rates . however , repeated extrusion passes through the aluminum oxide filters may be used to obtain a narrower particle - size distribution , and particularly to reduce all liposomes to below a predetermined maximum size . for example , 2 - 10 extrusions of the liposomes through the filters are preferred to decrease the particle - size distribution , thereby producing relatively uniform liposomes of high capacity in a rapid , efficient and inexpensive manner . multiple extrusions may also convert multilamellar vesicles to more desired oligolamellar or unilamellar forms . subsequent to the extrusion process of the invention , any free unencapsulated therapeutic agent or other solution can be readily removed , as by dialysis or diafiltration , leaving stable drug encapsulating liposomes of relatively uniform size . the resultant liposomes may be readily measured into uniform dosages for administration parenterally or orally . the invention will be further illustrated by the following examples , but the invention is not meant to be limited to the details described therein . three batches of liposomes ( hereinafter designated a , b and c ) were prepared as follows : 71 . 3 mg / ml egg phosphatidylcholine ( obtained from princeton lipids , princeton , n . j .) and 28 . 7 mg / ml cholesterol ( j . t . baker , phillipsburg , n . j .) were dissolved in 0 . 15 to 0 . 5 ml of methylene chloride and added to a 300 - mm citrate buffer solution ( ph 4 . 0 ) to make up a 1 - ml volume . the methylene chloride was removed by heating the mixture to about 40 ° c . to aid in the removal of the solvent , nitrogen was sparged through batches a and c , while batch b was heated under partial vacuum . the resultant liposomes were vesicles of various sizes and various size distributions . the initial size distributions of these liposomes prior to size reduction were measured on a malvern sizer 3600 e type with a 63 - mm lens , available from malvern instruments , malvern , england . the results are presented in table i , in which the mean diameters and size distribution ranges are expressed in micrometers ( um ). before extrusion , batch b was presized through a 5 - um pore - size mitex ™ ptfe filter ( millipore corp ., bedford , mass . ), and the mean diameter and distribution range for the batch b liposomes after presizing is included in the table . batches a and c were not put through presizing . the results show a considerable batch - to - batch variation in the size distribution of the unsized liposomes . in accordance with the present invention , the liposomes of example 1 were extruded five times under pressure through an anopore ™ 90 - mm diameter , 100 - nm pore size ( small pores ) branched - pore aluminum oxide filter of the type described above . the placement of the filter for batches a and b was with the input through the small - pore surface . good results were also obtained for batch c , which was extruded with the filter inverted so that the input was through the large - pore surface . the mean diameter of the liposomes and the particle - size distributions of the liposomes were measured after the indicated passes through the filter . the size distributions were measured on a nicomp ™ model 370 laser particle sizer , available from hiac / royco instruments , menlo park , calif . the results measured after each extrusion pass are summarized in table ii : for comparison , a sample of the liposomes prepared in batch c of example i was extruded through a total of eight passes , first five times through a 90 - mm diameter , 200 - nm pore size nuclepore ™ polycarbonate filters , two times through a 100 - nm nuclepore ™ filter , and once though a 220 - nm sterilization filter . ( nuclepore ™ filters are commercially available from nuclepore , inc ., pleasanton , calif .) a second sample was extruded in four passes through a 90 - mm diameter , 100 - nm pore size anopore ™ filter , in accordance with the present invention . as with the first sample , this sample was then passed through a 220 - nm sterilization filter . size data was measured after the pass numbers indicated in the first column . all of the extrusions were conducted at the same pressure of 400 psi ( 2 . 8 mpa ). the results are presented in table iii : the branched - pore type aluminum oxide filters of the present invention required fewer passes with a higher flow rate than the polycarbonate filters to obtain a similar particle - size distribution . an additional test was conducted to compare the size - reduction capabilities of a branched - pore type anodized aluminum oxide film with an equivalent pore - sized polycarbonate filter . the tests were performed using egg phosphatidylcholine and cholesterol liposomes , made in accordance with example 1 , with the liposomes in aqueous suspension at 100 mg / ml . for this example , the cholesterol was obtained from croda chemicals , new york , n . y . the initial size distribution , as measured on the malvern sizer , showed a median diameter of 10 . 9 um , and a range of diameters of 2 . 4 to 118 um . to facilitate submicron size reduction , the batch was processed twice through a 5 - um mitex ™ ptfe filter at a pressure of 100 psi ( 0 . 7 mpa ). after this step , the median diameter of the liposomes was measured as 3 . 5 um , and the range of diameters was 1 . 9 to 11 um . the batch was divided into two portions . portion number one was extruded through a 0 . 1 micrometer anopore ™ filter , and portion two was extruded through a 0 . 1 micrometer nuclepore ™ polycarbonate filter . the starting extrusion pressure for both portions was 300 psi ( 2 . 1 mpa ). the extrusion flow rate , particle size , and particle size distribution were measured for each pass through the filters . a total of five passes were performed on each portion . the results are presented in table iv : these data demonstrate that both filters are capable of producing similar size distributions with the same number of passes . however , the aluminum oxide filter used for the first portion required less pressure and operated at a much higher flow rate than the polycarbonate filter used for the second portion . all of the passes through the aluminum oxide filter were conducted at 300 psi ( 2 . 1 mpa ), with flow rates of 0 . 6 - 0 . 8 liters / min . when the extrusion was repeated using the polycarbonate filter , the initial pressure had to be increased from 300 psi ( 2 . 1 mpa ) to 500 psi ( 3 . 4 mpa ) just to complete the first pass through the filter at a very low flow rate of 0 . 02 liter / min . for passes 2 through 5 , a higher pressure of 700 psi ( 4 . 8 mpa ) was needed to maintain a flow rate of 0 . 2 - 0 . 3 liter / min . a further test was conducted to study the differences in the extrusion properties of liposomes with respect to the orientation of the 100 nm anopore ™ branched - pore filter used to size reduce the liposomes . as discussed above , the anopore 0 . 1 um branched - pore filter has a small - pore side , having 100 nm pores , and a large - pore side , having 200 nm pores . in this test , a comparison was made to determine the effects of passing aliquots of the same liposome material through the 100 nm anopore filters with the small - pore side upstream or with the large - pore side upstream . the test was performed using egg phosphatidylcholine and cholesterol liposomes , made in accordance with example 1 , with the liposomes in aqueous suspension at 100 mg / ml . the material was prepared in a single - five liter lot , mixed well , and divided into four 750 ml samples ( a - d ). samples a and b were extruded using the 100 - nm upstream orientation , and samples c and d used the 200 - nm upstream orientation . extrusion of the liposomes was carried out by passing them twice though the branched - pore filters , under 400 psig ( 2 . 8 mpa ) pressure . the particle size distributions of the filtered materials from each of the test samples were measured using a nicomp ™ sizer , as described in example 2 , and found to be generally equivalent . however , the time required to size reduce the liposomes was significantly less for samples a and b , with the 100 - nm filter side upstream , as opposed to samples c and d , with the 200 - nm filter side upstream . table v presents the mean particle size diameter in nanometers ( nm ) and the filtration time in minutes ( min ): the extruded material was then sterile filtered through a 220 - nm sterilization filter , of the same type used in the comparative example . the sterilization filter used in this test , and in the above comparative example , was a commercially available millipak ™ 200 filter supplied by millipore corp ., bedford , mass , and described as having a durapore ™ polyvinylidene difluoride ( pvdf ) tortuous path membrane . sterile filtration was considered complete when all of the material had passed through the sterilization filter , or when the steady stream of material had broken into a slow drip . the mean particle diameter ( nm ), the time required to pass through the filter ( min ), and the percent volume of material which passed through the sterilization filter were measured for each sample , and the results are presented in table vi : these data demonstrate that the sterile filtration was very efficient for samples a and b , in which almost all of the material successfully passed through the sterilization filter . in contrast , only about half of the volume of samples c and d was able to pass through the sterilization filter before the flow stopped . fig1 is a simplified top view , not to scale , of a filter assembly ( 10 ) made in accordance with a particular embodiment of the present invention , designed for use with a 90 - mm diameter anopore ™ filer . it should also be recognized that this filter assembly could also be used to house other types of filters as well , such as , for example , the nuclepore ™ filters used in the comparative tests in example iii above . fig2 is a simplified cross - sectional side view , not to scale , of figure assembly ( 10 ) cut along line 2 — 2 of fig1 . filter assembly ( 10 ) comprises a filter housing top half ( 11 ) and a filter housing bottom half ( 12 ), held together by a plurality of fastening screws ( 13 ). referring to fig2 filter unit ( 14 ) represents a 90 - mm anopore filter mounted on a drain disk ( nuclepore catalog # 231700 ) cut to 90 mm , which is in turn mounted on a 90 - mm teflon r coated mesh filter support ( millipore catalog # yy30 090 54 ). filter unit ( 14 ) is in turn mounted on a stainless steel filter support plate ( 15 ), which is provided with fluid passage means , such as transverse channels ( 16 ). support plate ( 15 ) sits into a seat portion ( 17 ) of filter housing bottom half ( 12 ), with the seat portion ( 17 ) provided with radial grooves ( not shown ) to channel the liquid which passes through the filter into liquid outlet ( 18 ). filter housing top half ( 11 ) includes a ring portion ( 19 ) which holds filter unit ( 14 ) in place when top half ( 11 ) is tightened down onto bottom half ( 12 ) by screws ( 13 ). in operation , the liquid to be filtered enters filter assembly ( 10 ) through liquid inlet ( 20 ), flows through filter unit ( 14 ) and filter support plate ( 15 ), and then is channeled out through filter outlet ( 18 ). preferably , housing top half ( 11 ) is provided with a relief outlet ( 21 ), by which a relief valve ( not shown ) can be connected to the housing . although filter assembly ( 10 ) has been described in terms of a “ top half ” and a “ bottom half ”, these references are for purposes of describing the structure , and do not reflect the orientation of the housing in operation . because the liquid being filtered is sent to the assembly at such relatively high pressures , it is believed that the assembly can be used in any orientation . fig3 is a schematic representation of an extrusion system made in accordance with the present invention . the liposome composition to be filtered is contained in a high pressure supply vessel ( 31 ), and the filtrate is collected in a similar receiving vessel ( 32 ). both of these vessels are shown as being equipped with stirrers to maintain the liposome mixtures , and heat jacketed for temperature control . the liquid exits supply vessel ( 31 ) through a bottom outlet , and is carried through a one or more filter assemblies ( 33 ), of the type described above , containing 90 - mm anopore ™ filters . the filter assemblies ( 33 ) are connected in parallel , with the number of such assemblies used determined by the desired total flow rate from supply vessel ( 31 ) to receiving vessel ( 32 ). from receiving vessel ( 32 ), the filtrate is forced through a sterilization filter ( 35 ), such as a millipak ™ 200 filter as described in example 4 above , and is then collected in a stirred holding vessel ( 36 ). in addition , a recycle line ( 34 ) may be provided to allow a portion of the output of receiving vessel ( 32 ) to be recycled to supply vessel ( 31 ). the terms and expressions which have been employed are used as terms of description and not of limitation , and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof , but it is recognized that various modifications are possible within the scope of the invention claimed .
a method of sizing liposomes by passing a suspension of liposomes through an aluminum oxide porous film by entering the film via the smaller pores and extruded via the larger pores , under pressure , is disclosed . in a preferred embodiment , the porous film is a branched - pore type anodic aluminum oxide porous film . the process produces a population of liposomes substantially free of liposomes above a predetermined maximum size . also disclosed is an apparatus for carrying out the invention .