the present invention describes pharmaceutical compositions composed of block polymers composed of a poly ( ethylene oxide ) forming the hydrophilic segment and a poly ( butyl ( alkyl ) acrylate - co -( alkyl ) acrylic acid ) forming the hydrophobic segment ; and at least one biologically active agent . the molecular weight of the hydrophilic segment can be in the range of 200 to 80 , 000 da , more preferably in the range of 500 to 10 , 000 da , still more preferably in the range of 2 , 000 to 5 , 000 da . the hydrophobic segments of the polymers in the present invention are composed of butyl ( alkyl ) acrylate and ( alkyl ) acrylic acid , where the alkyl chain is composed of from 0 to about 10 carbon atoms , inclusively , more preferably with 0 or 1 carbon atom . the butyl segment of the butyl ( alkyl ) acrylate can be a linear or branched chain including without limitation n - butyl and tert - butyl groups . the mole ratio of the butyl ( alkyl ) acrylate : ( alkyl ) acrylic acid in the hydrophobic segment is in the range of about 5 : 95 to 95 : 5 , more preferably in the range of about 30 : 70 to 70 : 30 . the length of the hydrophobic segment can be in the range of about 200 to 50 , 000 , more preferably in the range of about 500 to 80 , 000 da . amphiphilic block copolymers have a tendency to self - assemble in water forming micelles . upon micellization , the hydrophobic segment forms a core and the hydrophilic segment forms the corona of the micelles . the core of these micelles can be used as a reservoir of hydrophobic compounds protecting them form the external environment . if the hydrophobic segment of the polymer contains reversibly ionizable moieties , then the hydrophobicity of the segment could be manipulated by controlling the ionization of the moiety . polymers of the present invention differ from other block copolymers in this aspect . in the polymers of the present invention , the hydrophobic segment is composed of the mixture of two monomers , one of them is butyl ( alkyl ) acrylate , which confers hydrophobicity to the segment . butyl ( alkyl ) acrylate monomer is more preferably butyl acrylate or butyl methacrylate . the other monomer , ( alkyl ) acrylic acid has a pendant carboxyl group that can be reversibly ionized by changing the environmental ph . thus , when the environmental ph is below the pka of the carboxyl group , it will remain mostly in the unionized form and will confer hydrophobicity to the segment . this results in spontaneous aggregation of polymeric chains forming stable supramolecular assemblies or micelles in aqueous environment . however , when environmental ph is increased above pka of the carboxyl group , its ionization will impart hydrophilicity to the hydrophobic segment . this may result in the dissociation of the micelle . ( alkyl ) acrylic acid monomer is more preferably acrylic acid or methacrylic acid . these supramolecular assemblies are in the size range of from about 5 to 1000 nanometers . hydrophobic drugs are incorporated in the core of such supramolecular assemblies by methods that are known to one of ordinary skill in the art ( see for example lavasanifar et al j controlled release 2001 , 77 , 155 – 60 ; kohori et j controlled release 2002 , 78 , 155 – 63 ). manipulation of the composition of the hydrophobic segment results in variation in the hydrophobicity of the polymer allowing control of the incorporation efficiencies of the hydrophobic drugs . loadings in the range of 0 . 1 to 50 % w / w more preferably in the range of 1 to 20 % w / w of hydrophobic drugs are obtained using different drug loading procedures . block copolymers of the present invention are used to prepare pharmaceutical compositions containing hydrophobic molecules . non - limiting examples of the hydrophobic molecules includes hypolipidemic agents such as fenofibrate , anticancer agents such as doxorubicin , paclitaxel , docetaxel , camptothecin , megestrol acetate , teniposide , etoposide , antihypertensive agents such as candesartan cilexetil , non - steroidal anti - inflammatory agents such as indomethacin , celecoxib , antiviral agents such as retinovir , amprenavir , indinavir , efavirenz , immunosuppressive agents such as cyclosporin a , sirolimus , tacrolimus , and similar agents belonging to other therapeutic classes . in an alternative embodiment of the present invention , the poly ( butyl ( alkyl ) acrylate - co -( alkyl ) acrylic acid ) segment of the polymer will bear a negative charge at a ph above the pka of carboxyl groups and form complexes with cationic molecules including without limitation polycations , peptides and proteins bearing cationic residues by electrostatic interactions . this will result in the partial or complete charge neutralization of polymer and / or cationic molecule thereby forming supramolecular assemblies or micelles . the cation or polycationic molecule will be entrapped in the core of such supramolecular assemblies . the term “ cationic residues ” refers to the functional groups imparting positive charge to the molecule such as cationic amino acids e . g . lysine , arginine , histidine or other functional groups such as primary , secondary , tertiary or quaternary amine groups present in the molecule . the complexes of poly ( ethylene oxide )- block - poly ( n - butyl acrylate - co - methacrylic acid ) with poly - l - lysine are prepared in a buffer solution at ph 7 . 4 . supramolecular assemblies having unimodal size distribution within the size range of about 20 to 50 nm are obtained depending upon the molecular weight of the poly - l - lysine and composition of the polymer . on the other hand , complexes of poly ( ethylene oxide )- block - poly ( ethyl acrylate - co - methacrylic acid ) with poly - l - lysine in ph 7 . 4 buffer results in formation of aggregates having multimodal size distribution with sizes above about 200 nm . in yet another embodiment of the present invention , block copolymers are used to form stable coordination complexes with biologically active agents illustrated as metallic compounds such as cisplatin , carboplatin above the pka of the carboxyl groups . the presence of butyl ( alkyl ) acrylate in the hydrophobic segment plays several crucial roles in forming stable supramolecular assemblies . it confers hydrophobicity to the polymer chain , which is one of the important driving forces in the self - assembly of polymeric chains . it also increases the incorporation of hydrophobic drugs in the supramolecular assemblies . it is well known that carboxylic acid groups form intra - and / or intermolecular hydrogen - bonding complexes with oxygen present in the polyethylene oxide chain ( see for example donini et al , int j pharm , 2002 , 245 , 83 – 91 ; lele et al , j . controlled release , 2000 , 69 , 237 – 248 ). this results in the formation of large aggregates or sometimes in precipitation of the complex . this problem could be possible in poly ( ethylene oxide )- block - poly ( aspartic acid ) polymers ( nishiyama et al pharm res 2001 , 18 , 1035 – 1041 ; yokoyama et al j controlled release 1996 , 39 , 351 – 356 ). it was evident in polymers having the composition poly ( ethylene oxide )- block - poly ( methacrylic acid ) as reported previously ( ranger et al j polymer science : part a : polymer chemistry , 2001 , 39 , 3861 – 3874 ). one method for overcoming this problem is by incorporating hydrophobic monomers such as ethyl acrylate in the hydrophobic segment , as disclosed in our earlier u . s . patent application ( ser . no . 09 / 877 , 999 jun . 8 , 2001 ). in accordance with the instantly disclosed invention , it was observed that polymers with improved characteristics could be obtained by incorporating butyl ( alkyl ) acrylate in the hydrophobic segment . one of the major advantages of polymers in accordance with the present invention is the presence of the butyl chain of the butyl ( alkyl ) acrylate that largely minimizes formation of such hydrogen bonding complexes and can prevent formation of aggregates . this aids in the formation of stable supramolecular assemblies having uniform size range . for example , poly ( ethylene oxide )- block - poly ( n - butyl acrylate - co - methacrylic acid ) with 50 : 50 mole ratio of n - butyl acrylate : methacrylic acid having molecular weight of about 5300 da forms micelles of 30 nm at ph 5 . 0 while poly ( ethylene oxide )- block - poly ( ethyl acrylate - co - methacrylic acid ) with 50 : 50 mole ratio of ethyl acrylate : methacrylic acid having molecular weight of about 5100 da forms micelles of 120 nm at ph 5 . 0 , which are possibly aggregates of several micelles . an oral route is the most preferred route of administration for a pharmaceutically active agent . for oral delivery , the compositions can be used in the form of tablets , capsules , powders , lozenges , solutions , suspensions , syrups , elixirs , and the like . the pharmaceutical compositions of the present invention are administered orally . the pharmaceutical compositions of the present invention can also be administered by a number of other routes , including without limitation , rectally , vaginally , topically , by pulmonary route , parenterally , including but not limited to intravenous , intra - arterial , intramuscular , intraperitoneal or subcutaneous route . the polymers in the present invention can be modified to attach targeting ligands such as lectin , antibodies or fragments of antibodies , peptides , vitamins or sugar molecules . in all further text , figures appearing as subscript in the polymer composition indicate the mole ratio of that monomer present in the hydrophobic segment of the polymer . in vitro release of 3 h - progesterone from peo - b - poly ( nba 50 - co - maa 50 ) supramolecular assemblies at different ph : progesterone was used as a model hydrophobic drug to evaluate the effect of ph on drug release from supramolecular assemblies . 3 h - progesterone was loaded in the supramolecular assemblies of peo - b - poly ( nba 50 - co - maa 50 ) of molecular weight 5300 da by film casting method . briefly , 10 mg polymer , 1 mg progesterone and 1 μci 3 h - progesterone were dissolved in a mixture of dichloromethane , ethanol and water in a scintillation vial . the solvents were evaporated under reduced pressure to cast a film of polymer and drug on the glass surface . the film was hydrated with water to obtain the supramolecular assemblies , this solution was filtered through 2 μm filter to remove precipitated drug . for in vitro release study , the solution of progesterone loaded supramolecular assemblies was filled in a dialysis bag ( 6000 – 8000 da molecular weight cut off ) and the bag was put in a beaker containing 200 ml of simulated gastric fluid , ph 1 . 2 maintained at 37 ° c . the release medium was magnetically stirred . after 2 hours , the ph of medium was adjusted to 7 . 2 by addition of sodium hydroxide and potassium dihydrogen phosphate . during the entire release experiment , 1 ml samples of release medium were withdrawn periodically to measure the radioactivity of 3 h - progesterone . as a control , the release of 3 h - progesterone from supramolecular assemblies was also measured at ph 1 . 2 , ph 7 . 2 and at ph 1 . 2 in absence of polymer . the results of the release experiment are shown in fig1 . as shown in fig1 , the progesterone is released rapidly in the absence of polymer at ph 1 . 2 , suggesting that the dialysis bag does not form a barrier for the drug release . further , the progesterone release from peo - b - poly ( nba 50 - co - maa 50 ) supramolecular assemblies is very rapid at ph 7 . 2 , while slow at ph 1 . 2 . on the other hand , when the ph of the release medium is changed from 1 . 2 to 7 . 2 after 2 hours , the release rate increases significantly . this is evidentiary of ph dependent dissociation of supramolecular assemblies . at ph 1 . 2 , the polymer exists in the form of supramolecular assemblies due to unionized carboxyl groups and the drug is released slowly from the core of supramolecular assemblies . however , when the ph is increased to 7 . 2 , the carboxyl groups become ionized resulting in the dissociation of supramolecular assemblies and the drug is released rapidly . to support this data , ph dependent aggregation behavior of peo - b - poly ( nba 50 - co - maa 50 ) was studied using dynamic light scattering . polymer solutions ( 0 . 5 mg / ml ) were prepared in citrate phosphate universal buffer and the ph was adjusted between about 2 . 2 – 7 . 0 . the intensity of scattered light from these solutions at different ph was measured at 25 ° c . and 90 ° angle and was plotted as a function of ph . the results are shown in fig2 . from fig2 , it is evident that the scattered light intensity is negligible at ph above ˜ 5 . 5 while when the ph is decreased below 5 . 5 , the intensity increases significantly suggesting association of polymeric chains . this indicates that below ph 5 . 5 the polymer is present in the form of supramolecular assemblies . the size of these supramolecular assemblies is in the range of 30 – 100 nm depending upon the environmental ph . bioavailability studies of fenofibrate entrapped in supramolecular assemblies upon oral administration to rats fenofibrate ( fnb ) was used as a model poorly water - soluble hydrophobic drug to evaluate the effect of drug incorporation in supramolecular assemblies on the bioavailability upon oral administration to rats . in a series of experiments , fnb incorporation was studied in different peo - b - poly ( ea - co - maa ) and peo - b - poly ( nba 50 - co - maa 50 ) polymers by emulsion and film casting methods . the fnb loading was higher in peo - b - poly ( nba 50 - co - maa 50 ) polymers . therefore these polymers were used to evaluate relative bioavailability of fnb loaded supramolecular assemblies in sprague - dawley rats . the study was conducted on 3 fenofibrate formulations , namely fnb supramolecular assemblies , fnb standard formulation and resuspended fnb . fnb loaded supramolecular assemblies were prepared from peo - b - poly ( nba 50 - co - maa 50 ) of about molecular weight 5300 da by film casting method . size of the supramolecular assemblies was in the range of about 100 – 300 nm . fnb standard formulation was prepared by suspending the powder from lipidil macro ® ( fournier ) capsule in 0 . 5 % w / v carboxymethyl cellulose sodium ( cmc na ) solution to obtain uniform suspension . fnb powder ( sigma ) was also suspended in 0 . 5 % w / v cmc na solution to prepare resuspended fnb formulation which acts as a negative control . rats were divided into 3 groups of 6 animals each . the rats were fasted overnight and fed with standard diet throughout the study . each formulation was administered orally at a dose of 7 . 5 mg / kg to 6 rats from a group . blood was removed periodically from each rat , plasma was separated and stored at − 80 ° c . till further use . fnb content from the plasma was determined and plotted against time , the results of which are shown in fig3 . the results show that fnb incorporated in supramolecular assemblies results in highest peak plasma level , ie . 10 . 9 μg / ml compared to 8 . 4 μg / ml for standard formulation . also , t max was achieved rapidly by fnb loaded supramolecular assemblies compared to standard formulation . overall , the relative bioavailability of fnb was enhanced by 19 % upon entrapment in supramolecular assemblies compared to standard fnb formulation , and the bioavailability enhancement was 133 % compared to resuspended fnb powder . this enhancement in relative bioavailability is possibly due to release of drug from supramolecular assemblies in the nanoscopic size range , which increases the rate of dissolution of drug . formation of polyion complex micelles of peo - b - p ( nba 50 - co - maa 50 ) with poly - l - lysin poly - l - lysine ( pll ) of molecular weight 16 , 100 was used as a model cationic compound for formation of polyion micelles with peo - b - p ( ea 50 - co - maa 50 ) and peo - b - p ( nba 50 - co - maa 50 ) copolymers with molecular weights of 5100 and 5700 da , respectively . polymer : pll (−/+) charge ratios ( mole : mole ) of 1 : 1 and 2 : 1 were used for complex formation . stock solutions of polymer and pll ( molecular weight 16 , 100 ) having concentration of 2 . 5 mg / ml were prepared in phosphate buffer ( ph 7 . 4 ) and mixed at room temperature to obtain 1 mg / ml final polymer concentration . the solution was filtered through 0 . 2 μm filter and size measurements were performed at 25 ° c . using dynamic light scattering ( dls ). the results are shown in table 1 . the results of table 1 show that complexation of pll with peo - b - p ( ea 50 - co - maa 50 ) at different charge ratios results in formation of relatively large aggregates which could be attributed to the hydrogen bonding between poly ( ethylene oxide ) chain and carboxyl groups . in contrast , the complexation of pll with peo - b - p ( nba 50 - co - maa 50 ) at similar ratios results in formation of micelles having unimodal size distribution and low polydispersity indices . similar complexes are obtained with pll of different molecular weights . verapamil hydrochloride was used as a model cationic drug . solutions of peo - b - p ( nba 50 - co - maa 50 ) and verapamil hydrochloride in universal buffer were mixed to obtain final polymer concentration of 0 . 5 mg / ml and verapamil hydrochloride concentration of 0 . 8 mg / ml . the solution ph was adjusted to 6 . 1 and size was measured using dls . polyion complex micelles of 38 ± 10 . 3 nm were obtained . all patents and publications mentioned in this specification are indicative of the levels of those skilled in the art to which the invention pertains . all patents and publications are herein incorporated by reference to the same extent as if each individual publication was specifically and individually indicated to be incorporated by reference . it is to be understood that while a certain form of the invention is illustrated , it is not to be limited to the specific form or arrangement of parts herein described and shown . it will be apparent to those skilled in the art that various changes may be made without departing from the scope of the invention and the invention is not to be considered limited to what is shown and described in the specification and drawings . one skilled in the art will readily appreciate that the present invention is well adapted to carry out the objects and obtain the ends and advantages mentioned , as well as those inherent therein . the compounds , compositions , biologically related compounds , methods , procedures and techniques described herein are presently representative of the preferred embodiments , are intended to be exemplary and are not intended as limitations on the scope . changes therein and other uses will occur to those skilled in the art which are encompassed within the spirit of the invention and are defined by the scope of the appended claims . although the invention has been described in connection with specific preferred embodiments , it should be understood that the invention as claimed should not be unduly limited to such specific embodiments . indeed , various modifications of the described modes for carrying out the invention which are obvious to those skilled in the art are intended to be within the scope of the following claims .