the present invention is a novel approach to drug delivery using receptor - mediated carriers . in the preferred embodiment , magnetic particles are included so that , under the influence of a suitable magnetic field , the magnetic particles will be stationary , thereby enhancing binding of the carrier to the capillary endothelial cell receptors . the retained magnetic microspheres may then be taken up by endocytosis and by passage through adjacent endothelial cell gaps in fenestrated or discontinuous capillary beds . in continuous capillary beds , i . e ., the blood - brain barrier , the receptor - mediated binding of the particle to the capillary wall may create a local concentration gradient as drug is released from the particle . the concentration gradient may lead to increased brain concentrations for drugs permeable to the blood - brain barrier . the basis for this capillary endothelial receptor - mediated delivery system is the formation of a complex between the anionic heparin - related glycosaminoglycans on the luminal surface of the capillary endothelial cells and cationic magnetic microspheres . a strong interaction between cationic microspheres and anionic glycosaminoglycan receptors should retain the microspheres in the capillary region . other polymers which exhibit similar interactions with glycosaminoglycan receptors could be utilized in place of the polysaccharides , such as some of the polypeptide lectins . cationic magnetic microspheres were prepared from approximately 83 % deacetylated chitin , herein referred to as chitosan . chitin ( poly ( 1 -& gt ; 4 ) n - acetyl - d - glucosamine ), and its deacetylated derivative , chitosan , are naturally - occurring polysaccharides . chitosan , low viscosity grade ( m . w . 652 , 000 ) was purchased from proton laboratories , redmond , wash . the following were obtained from sigma chemical co ., st . louis , mo . ; sorbitan sesquioleate ( arlacel 83 , s3386 ), bovine serum albumin ( a - 7030 ), methylene blue ( mb1 ) and heparin sodium ( from porcine intestinal mucosa , h7005 ). ferrofluid ( emg iiii ) was purchased from ferrofluidics corp ., nashua , n . h . double - distilled deionized water was obtained from a millipore ( bedford , mass .) system . extra heavy mineral oil was purchased from ruger chemical co ., irvington , n . j . all other chemicals were analytical grade . the ultrasonic water bath was a bransonic 220 ; the ultrasonic probe was a branson sonifier . a brinkman rotovapor - r vacuum drier was used . all absorbance data were obtained from a varian 2200 spectrophotometer . iron analyses were completed with a perkin - elmer 5000 atomic absorption spectrophotometer . a philips 505 scanning electron microscope was used for particle size analysis . one hundred and fifty milligrams of chitosan were dissolved in 6 ml of 10 % ( v / v ) acetic acid in deionized water . to this solution , 4 ml of acetone were added and the contents vortexed until a clear solution was obtained . in a 75 ml double - walled beaker , one milliliter of chitosan solution was sonicated with 100 μl of a 10 % ( w / v ) ferrofluid suspension in a water bath for 10 min . to this mixture , 50 ml of mineral oil was added , and an emulsion was formed by sonication for 3 min at 200 w with an ultrasonic probe . the temperature of the emulsion was controlled with a circulating water system maintained at 4 ° c . 100 μl of arlacel 83 was then added to the emulsion as a stabilizer . the emulsion was then placed in a rotary vacuum drier under nitrogen atmosphere at 70 ° c . for 2 hrs to evaporate the acetone and water in the internal phase . following evaporation , the microsphere suspension was diluted with 60 ml of hexanes containing 1 % v / v arlacel 83 and centrifuged at 4500 rpm for 15 min . the supernatant was decanted and the microsphere pellet was washed twice in 80 ml of hexanes containing the arlacel 83 . the solid microsphere pellet was then suspended in 15 ml of anhydrous ether and transferred with two 300 g bar magnets in place to a 20 ml centrifuge tube to retain any free magnetite . after centrifugation at 2000 rpm for 5 min , the ether was removed and the microspheres were stored at room temperature . the method of magnetic chitosan microsphere preparation is similar to techniques used for magnetic albumin microspheres . the primary difference is that solid chitosan microspheres are obtained by solvent evaporation rather than by protein denaturation of the emulsion . the nitrogen atmosphere increased both the rate of solvent evaporation and the stability of the microspheres . arlacel 83 was added to prevent breaking of the emulsion at temperatures above 50 ° c . the surfactant also increased the recovery of the microspheres by preventing their adhesion to glass surfaces . approximately 10 mg of microspheres were recovered per batch . particle size analyses were done with a scanning electron microscope . approximately 600 μg of the microspheres were digested in 1 ml of concentrated hydrochloric acid for 12 hrs and the magnetite content of the microspheres was obtained from atomic absorption spectroscopy . the samples were diluted 20 times with deionized water and measured for iron at a wavelength of 248 . 3 nm . numerous batches of microspheres were prepared by the above method . it was found that the mean particle diameter was 0 . 70μ =/- 0 . 20μ ( n = 350 ). the magnetite ( fe 3 0 4 ) content of the microspheres was 16 % w / w ± 2 % ( n = 4 ). the chitosan : heparin complex was studied on the basis of the competitive binding displacement method , using methylene blue as the competing marker . the following solutions were prepared : 570 μg / ml of methylene blue in deionized water ( mb1 ), 5 . 7 μg / ml of methylene blue in 1 % v / v acetic acid in deionized water ( mb2 ), 2 . 25 mg / ml of heparin in deionized water ( h ), 2 . 25 mg / ml of chitosan in 1 % v / v acetic acid in deionized water ( c ), and 2 . 25 mg / ml of bovine serum albumin in deionized water ( bsa ). the ph of 1 % v / v acetic acid in deionized water was 3 . 5 . the following solutions were mixed in stoppered vials : 1 ml of h and 9 ml of mb2 ; 0 . 1 ml of mb1 , 7 . 9 ml of 1 % v / v acetic acid , 1 ml h and 1 ml c ; 0 . 1 ml mb1 , 7 . 9 ml 1 % v / v acetic acid , 1 ml h and 1 ml bsa . the visible absorbance spectrum was obtained for each mixture between 520 nm to 700 nm with 1 % v / v acetic acid in the reference cell . spectra of mb1 and ten - fold dilutions of h , c , and bsa in 1 % v / v acetic acid were also obtained . the interaction between heparin and the magnetic chitosan microspheres was also studied by the competitive binding displacement method . a mixture of 0 . 1 ml h , 0 . 1 ml mb1 , 9 . 8 ml of deionized water ( ph = 7 ) and 4 mg of magnetic chitosan microspheres was prepared and the absorbance scan was recorded . an exactly analogous mixture was prepared with alkali treated microspheres obtained after mixing the magnetic chitosan microspheres with 2 % v / v ammonium hydroxide in acetone for 10 min . the basic treatment was used to neutralize the ammonium ions on the chitosan molecules . characterization of the complex ratio was completed as follows . to 0 . 1 ml of mb1 , volume increments of a 0 . 225 mg / ml heparin solution was added . after each volume addition of heparin , 1 % v / v acetic acid in deionized water was added to obtain a final volume of 15 ml . the absorbance of the resulting solution was recorded at 660 nm . in a second study , a 0 . 225 mg / ml chitosan solution was added in volume increments to the methylene blue : heparin complex ( 0 . 1 ml of mb1 and 1 . 5 ml of 0 . 225 mg / ml heparin ). the resulting mixture was diluted to 15 ml with 1 % v / v acetic acid , and the absorbance measured at 660 nm . due to the heterogeneity of the heparin polymer , the chitosan : heparin complex ratio calculation was based on a heparin tetrasaccharide with a molecular weight of 1026 daltons . the corresponding molecular weight of the chitosan tetramer was 632 daltons . the absorbance spectra from the complex studies indicates the formation of a complex . addition of chitosan to the heparin : methylene blue complex produced the methylene blue spectrum , indicating the formation of a chitosan : heparin complex . bovine serum albumin did not affect the absorbance spectrum of the methylene blue : heparin complex . similar to the results for the addition of chitosan to the heparin : methylene blue complex , the magnetic chitosan microspheres displaced methylene blue from heparin binding sites causing the methylene blue spectrum to be regenerated . the methylene blue : heparin spectrum remained the same after the addition of the alkali - treated microspheres , indicating that a complex was not formed between alkali - treated microspheres and heparin . as shown in fig1 the addition of heparin causes the absorbance to decrease , a hypochromic effect , indicating that methylene blue is being bound to heparin . the slower decline in absorbance most likely represents rearrangement of methylene blue molecules on heparin binding sites . the curve reaches a plateau when all methylene blue is bound to heparin . addition of chitosan to the methylene blue : heparin complex causes an increase in absorbance consistent with methylene blue &# 39 ; s displacement from heparin binding sites . the sharp increase in absorbance represents the complete displacement of methylene blue from heparin binding sites by chitosan . the small increase in absorbance prior to the plateau region of the curve is most likely due to polymer - dye interactions . the chitosan : heparin complex ratio was determined to be 1 : 1 based on charge . the magnetic chitosan microspheres were less than one micron in diameter , allowing distribution to capillary endothelial cells following intravascular administration . the magnetite content of 16 % w / w should be sufficient to retain the particles at capillary blood flow rates under the influence of a suitable magnetic field . the amount of magnetite which can be utilized can be varied , however , optimizing as desired using techniques available to those skilled in the art . heparin was used as a structural analogue for the glycosaminoglycans . since the cationic moiety of methylene blue attacks anionic heparin binding sites , the competitive binding displacement method is valid to study the chitosan : heparin complex . a polylysine : heparin complex has been extensively studied and a chitosan : heparin complex has been observed , both complexes indicative of macromolecular ion complexes . the reaction between chitosan and heparin was studied at an acid ph in which chitosan is soluble . the similar spectra obtained from the methylene blue : heparin complex and following the addition of albumin indicate that albumin does not specifically bind to anionic sites on heparin . thus , albumin microspheres should not interact with glycosaminoglycan receptors at ionic binding sites . the complex formed between the magnetic chitosan microspheres and heparin was formed in deionized water at ph 7 . at this ph , so 3 - and coo - groups are present on heparin and will complex with ammonium ions on the microspheres . a physiological ph of 7 . 4 could not be used since methylene blue is not ionized at ph values above 7 , thus preventing the formation of a methylene blue : heparin complex . chitosan is insoluble at ph 7 but ammonium ions are apparently present due to the association of the acetic acid with the microspheres . acetic acid is used in the formulation of the microspheres to dissolve the polymer prior to emulsification , and may also serve to control the ionization of the amino group on the chitosan molecule . the association of the acetic acid with the microspheres will provide a local acid environment in vivo to facilitate microsphere binding to the glycosaminoglycan receptors . the cationic : anionic nature of the microsphere : heparin complex is supported by the fact that neutralized microspheres did not displace methylene blue from heparin . the neutralization procedure will produce ammonium acetate and water and make available free amino groups on chitosan . it is thought that the ionic and hydrogen bonding forces between the chitosan microspheres and the glycosaminoglycans on the capillary endothelial cells will be sufficient to retain the microspheres in the capillaries . heparan sulfate proteoglycans are considered to form microdomains on capillary endothelial cell surfaces , and are involved in endocytotic and transcytotic events . heparan sulfate proteoglycans are involved in the receptor - mediated uptake of low - density lipoproteins . thus , binding of chitosan microspheres to glycosaminoglycans should result in endocytosis in peripheral capillary beds . further , the carrier : receptor complex may trigger the receptor - mediated endocytosis of the glycosaminoglycans . this later process is a normal turnover mechanism for damaged or aged glycosaminoglycans on the cell surface . the complex of polylysine : heparin is taken up at a greater rate than the unassociated species into hamster ovarian cells . cerebral bovine microvessel endothelial cells were isolated from the gray matter of the cerebral cortex and cultured according to the methods of audus and borchardt . endothelial cells were grown on 35 mm plastic culture dishes and homogenous monolayers were obtained 10 - 14 days after seeding . the protein content of cultured endothelial cells was determined using the biorad - bradford protein assay . cells were digested with 1 . 8 ml 2n sodium hydroxide for two hours prior to protein assay . bovine serum albumin was used as the standard . the cultured endothelial monolayers were washed with serum free culture medium followed by ph 7 . 4 phosphate buffered aline ( pbs ), three times each . each monolayer was incubated with 2 ml 0 . 1 % collagenase in pbs for 10 minutes at 37 ° c . cells were gently scraped by a rubber policeman and transferred to a 15 ml centrifuge tube . cells were separated by centrifugation at 1000 rpm and washed twice with serum free culture medium followed by two washes with pbs . the final pellet was resuspended in pbs to yield a protein concentration of 38 . 2 μg / ml . all fluorometric titrations were measured with spectrofluorometer at an excitation wavelength of 495 nm and an emission wavelength of 530 nm . the fluorescence polarization of increasing concentrations of fluorescein labeled concanavalin a were measured in the presence of a 3 . 3 μg / ml chitosan solution , 1 % acetic acid , and in pbs . fluorescence polarization measurements were also obtained by titration of three mls of 0 . 066 μg / ml fluorescein labeled concanavalin a solution with 1 % acetic acid , 1 mg / ml chitosan solution in 1 % acetic acid and the endothelial cell suspension . to study the chitosan - endothelial cell interaction , 0 . 2 ml of cell suspension and 2 . 8 ml of 0 . 06 μg / ml fluorescein labeled concanavalin a in pbs were stirred with a magnetic stirrer for five minutes then titrated with 10 μl increments of a 1 mg / ml chitosan solution . steady - state fluorescence polarization were directly recorded after each addition of titrant . after addition of the fourth increment of the chitosan solution , 0 . 2 ml of cell suspension was added and the titration was continued . chitosan magnetic microspheres were prepared as above and suspended in 10 mm pbs just before use . the prepared endothelial cell monolayers were washed with three portions of 2 ml of pbs after aspiration of the culture medium . 0 . 8 ml of freshly prepared microsphere suspension equivalent to a chitosan concentration of 75 μg / ml was then added to each washed monolayer and incubated undisturbed at 37 c . in an atmosphere of 5 % co 2 and 95 % humidity . after 20 , 40 , 60 , 90 , 180 and 300 minutes of incubation , the percentage of unbound chitosan microspheres was determined as follows : the microsphere suspension was poured into a 10 ml centrifuge tube and the monolayer was washed three times by adding 1 ml of pbs , swirling for one minute and then pouring the wash medium into the 10 ml centrifuge tube . any remaining washing buffer in the dish was then collected by aspiration and combined with the other washings in the 10 ml centrifuge tube . the tube was then centrifuged at 2000 rpm for 30 min to collect the unbound microspheres . the microsphere pellet was dissolved in 1 % acetic acid and assayed for chitosan using the colormetric method of ride and drysdale physiol . plant . path . 2 , 7 - 15 ( 1972 ). the fraction of microspheres bound to the cells was calculated as the difference between the initial amount of chitosan ( added as microspheres to the cells ) and the amount of chitosan recovered from the washes ( due to unbound microspheres ) divided by the initial amount of chitosan . the above procedure was also used to examine the effect of heparin and ruthenium red on the binding characteristics of chitosan magnetic microspheres to endothelial cell monolayers . microspheres were suspended in pbs containing 250 μg / ml heparin or ruthenium red . unbound microspheres were collected , the chitosan concentration determined , and the fraction of microspheres bound in the presence of heparin and ruthenium red was calculated as described above . the chitosan concentration ( as microspheres ) was 36 . 3 μg / ml in the pbs control and in the presence of heparin and 45 . 7 μg / ml in the presence of ruthenium red . fluorescein labeled concanavalin a was used as a surface marker to cerebral endothelial cells because concanavalin a binds to specific carbohydrate receptors on cell membranes ( glucose and mannose containing domains ). it has also been shown that concanavalin a binds to rat heart capillary endothelial cell surfaces . this binding is reversible and could be inhibited by methyl - d - mannoside . concanavalin a remains at cell surfaces for at least four hours before it is transferred into the cell . changes in fluorescence polarization of increasing concentrations fluorescein labeled concanavalin a ( flc ), in the presence of molecular chitosan , acetic acid or pbs was measured . the decrease in flc polarization in the chitosan and acetic acid media compared to the pbs medium indicated a lack of interaction between chitosan and flc . thus , any increase in fluorescence polarization upon addition of chitosan to the endothelial cell suspension marked with flc is solely due to binding of chitosan to cell surfaces . when flc solution was titrated with endothelial cell suspension , fluorescence polarization was increased as a result of binding of flc to the endothelial cell surface . fig2 shows the fluorescence polarization of cell suspension marked with flc in the presence of chitosan . upon increase of the chitosan concentration an increase in fluorescence polarization occurred as a result of the binding of chitosan to the cell surfaces . after reaching a peak , the fluorescence polarization decreased . the decrease in polarization at high chitosan concentrations may be due to the release of flc from cell surfaces by chitosan molecules . this possibility is supported by the fact that addition of more cell suspension followed by continuing titration with chitosan solution leaded to a curve with the same characteristics as the previous one . the binding of chitosan magnetic microspheres to endothelial cell monolayers , expressed in terms of the percentage of bound microspheres , is illustrated in fig3 . the percentage of microspheres bound to the monolayers increased with an increase in the incubation time , reaching a maximum of 81 . 9 % after five hours . this plateau value apparently represents the microspheres saturating the glycosaminoglycan receptors on the cell surface . the time dependency of the percentage of microspheres bound is probably due to the time required for the microspheres to sediment to the cell monolayer . both heparin and ruthenium red inhibited the binding of the chitosan microspheres to the cultured monolayers . heparin sharply decreased the percentage of bound microspheres to 33 %, as compared to 80 . 7 % in pbs . heparin inhibited the binding of the microspheres to the cell surface glycosaminoglycan receptors by occupying chitosan binding sties , supporting the idea that the microspheres primarily bind to the glycosaminoglycans through ionic interactions . as shown in example 1 , heparin and chitosan form an anionic : cationic complex through so 3 and coo and nh 3 + groups . ruthenium red is a cationic dye which binds to anionic domains of cell membranes ( e . g . sialoglycoconjugates and glycosaminoglycans ). the partial inhibition of microsphere binding due to ruthenium red is therefore indicative of interactions between the chitosan microspheres and endothelial cells through the glycosaminoglycan receptors . in conclusion , polysaccharides such as chitosan , either in solution or as magnetic microspheres , bind to cerebral microvessel endothelial cell surfaces . the binding is mediated by anionic glycosaminoglycan receptors . other polysaccharides and polypeptides such as lectins can be used to form similar drug delivery systems , using the above - described methods . these systems should have a number of applications in controlled drug delivery , especially to hard to reach areas such as the brain . modifications and variations of the present invention , a drug delivery system utilizing microcapsules which bind to glycosaminoglycan receptors on the surface of cells , will be obvious to those skilled in the art from the foregoing detailed description of the invention . such modifications and variations are intended to come within the scope of the appended claims .
a delivery system for biologically active materials consisting of polysaccharide or polypeptide microspheres which bind to glycosaminoglycan receptors on cell surfaces . the delivery system can be used to localize drugs by both biochemical and physical means . in one embodiment , microspheres prepared from the polysaccharide chitosan are bound to anionic polysaccharides similar to the glycosaminoglycan receptors found on the surface of capillary endothelial cells . the microspheres were formulated to have a controlled cationic character , and had a mean diameter of 0 . 70 u and a magnetite content of 16 % w / w . binding to a model polysaccharide , heparin , was demonstrated in vitro . binding to cultured cerebral capillary endothelial cells was also demonstrated .