Patent Publication Number: US-2011059921-A1

Title: Oligomer-Nitrogenous Base Conjugates

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority under 35 U.S.C. §119(e) to Provisional Application Ser. No. 61/072,171, filed 27 Mar. 2008, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention comprises (among other things) chemically modified nitrogenous bases that possess certain advantages over nitrogenous bases lacking the chemical modification. The chemically modified nitrogenous bases described herein relate to and/or have application(s) in (among others) the fields of drug discovery, pharmacotherapy, physiology, organic chemistry and polymer chemistry. 
     BACKGROUND OF THE INVENTION 
     Viral infections, including influenza and AIDS are of great importance, both from the health care and economic perspective. Treatment of viral diseases includes both vaccines and anti-viral medicines. 
     Processes of replication of viral genetic material (DNA or RNA, single-stranded or double-stranded), are sufficiently different from that of the eukaryotes that much research has been conducted at developing novel antiviral compounds that act by modulating replication. Nitrogenous bases which are the building blocks of all genetic material represent such a target class. Many antiviral drugs belong to this category, including acyclovir and ganciclovir, among others. These drugs are thought to act through: i) competitive inhibition of viral DNA polymerase and/or reverse transcriptase; ii) incorporation into the DNA/RNA chain as a base; and iii) inactivation of the viral DNA polymerase and/or reverse transcriptase. 
     As these drugs target, directly or indirectly, replication of genetic material, they also modulate growing cell metabolism. Thus, some of these antiviral drugs are also prescribed as anti-neoplastic or in the treatment of various cancers. 
     Pharmacologically, nitrogenous base drugs represent an important class of agents employed in the management of viral diseases and cancers. However, viruses are known to develop resistance to these drugs over time. Many of these drugs demonstrate side effects, including anemia, reduced white cell count, and nausea, among others. They also exhibit low oral bioavailability and hepatic toxicity. 
     Thus, a modification of any one of these or other characteristics would enhance their desirability as therapeutic agents. Therefore, pharmacotherapy with nitrogenous bases would be improved if these and/or other side effects associated with their use could be decreased or if their pharmacology could be improved. Thus, there is a large unmet need for developing novel nitrogenous base compounds. 
     The present invention seeks to address these and other needs in the art. 
     SUMMARY OF THE INVENTION 
     In one or more embodiments of the invention, a compound is provided, the compound comprising a nitrogenous base residue covalently attached via a stable or degradable linkage to a water-soluble, non-peptidic oligomer. 
     Exemplary compounds of the invention include those having the following structure: 
     B-R 1 -X-POLY, wherein: B is a nitrogenous base;
 
R 1  is a substitution on the nitrogenous base; the substitution is selected from the group consisting of
 
     
       
         
         
             
             
         
       
     
     wherein A is S or O; R 3  is hydrogen, straight or branched chain or cyclic alkyl, hydroxyalkyl, benzyloxyalkyl or phenyl; R 4  is hydrogen, hydroxy, or lower alkyl; R 5  is hydrogen, hydroxy, amino; alkyl, hydroxyalkyl, benzoyloxy, benzoyloxyalkyl, benzyloxy, sulphamoyloxy, phosphate, carboxypropionyloxy, or acetoxy; R 6  is hydrogen, alkyl, or hydroxyalkyl; 
     
       
         
         
             
             
         
       
     
     wherein R 7  is —CH 2 —CH(R 9 )—CH 2 —, wherein R 9  is hydrogen, methyl, or hydroxymethyl; R 8  is selected from the group consisting of CH 2 OC(O)R 10  and CH(R 10 )OC(O)R 10 , wherein R 10  is selected from the group consisting of C 1 -C 20  alkyl aryl, and aryl-alkyl which may be substituted or unsubstituted by hydroxy or halogen; 
     
       
         
         
             
             
         
       
     
     wherein R 11  and R 12  are each independently hydrogen, or a carboxylic acyl or R 11  and R 12  are joined to form a cyclic acetal or a cyclic carbonate group;
 
-(-dihydroxypropan-2-yloxymethyl);
 
-(4-hydroxy-3-(hydroxymethyl)butyl;
 
-methoxyethyl-L-valinate;
 
     
       
         
         
             
             
         
       
     
     wherein Z is independently —OC(R 13 ) 2 OC(O)Y(R 14 ) α , an ester, an amidate or —H; 
     Y is N or O; 
     R 13  independently is —H, C 1 -C 12  alkyl, C 5 -C 12  aryl, C 2 -C 12  alkenyl, C 2 -C 12  alkynyl, C 7 -C 12  alkenylaryl, C 7 -C 12  alkynylaryl, or C 6 -C 12  alkaryl, any one of which is unsubstituted or is substituted with 1 or 2 halo, cyano, azido, nitro or —OR 16  in which R 16  is C 1 -C 12  alkyl, C 5 -C 12  aryl, C 2 -C 12  alkenyl, C 2 -C 12  alkynyl;
 
R 14  is independently —H, C 1 -C 12  alkyl, C 5 -C 12  aryl, C 2 -C 12  alkenyl, C 2 -C 12  alkynyl, C 7 -C 12  alkenylaryl, C 7 -C 12  alkynylaryl, or C 6 -C 12  alkaryl, any one of which is unsubstituted or is substituted with 1 or 2 halo, cyano, azido, nitro, —N(R 15 ) 2  or —OR 16 , where R 15  independently is —H or C 1 -C 8  alkyl; and
 
α is 1 or 2; and
 
-methoxy-3-hydroxy-1-propanyl-L-valinate;
 
X is a spacer moiety; and
 
POLY is a water-soluble, non-peptidic oligomer.
 
     In another embodiment, exemplary compounds of the invention include those having the following structure: 
     B-R 1 -X-POLY, wherein: B is a nitrogenous base selected from the group consisting of uracil-1-yl, cytosin-1-yl, 5-methylcytosin-1-yl, thymin-1-yl, 5-fluoruracil-1-yl, guanin-9-yl, guanin-7-yl, adenine-9-yl, adenin-3-yl, hypoxanthin-9-yl, 2-methyladenin-9-yl, 2-methylthioadenin-9-yl, 2-aminoadenin-9-yl, 2-aminopurin-9-yl, N 6 -dimethyladenin-9-yl, 8-bromoadenin-9-yl, 8-hydroxyadenin-9-yl, 6-hydroxylaminopurin-9-yl, 6-hydrazinopurin-9-yl, 6-thiopurin-9-yl, purin-9-yl, and xanthin-9-yl;
 
R 1  is as defined above;
 
X is a spacer moiety; and
 
POLY is a water-soluble, non-peptidic oligomer.
 
     In another embodiment, exemplary compounds of the invention include those having the following structure: 
     R 1 -B-X-POLY, wherein: R 1  is as defined above;
 
B is a nitrogenous base comprising an amino group;
 
X is a spacer moiety attached to the amino group of the nitrogenous base; and
 
POLY is a water-soluble, non-peptidic oligomer.
 
     The “nitrogenous base residue” is a compound having a structure of a nitrogenous base compound that is altered by the presence of one or more bonds, which bonds serve to attach (either directly or indirectly) one or more water-soluble, non-peptidic oligomers. 
     In this regard, any nitrogenous base compound having antiviral or anti-cancer activity can be used as a nitrogenous base moiety. Exemplary nitrogenous base moieties have a structure encompassed by Formula: 
     B-R 1 , wherein: B is a nitrogenous base; and
 
R 1  is a substitution on the nitrogenous base; the substitution is selected from the group consisting of
 
     
       
         
         
             
             
         
       
     
     wherein A is S or O; R 3  is hydrogen, straight or branched chain or cyclic alkyl, hydroxyalkyl, benzyloxyalkyl or phenyl; R 4  is hydrogen, hydroxy, or lower alkyl; R 5  is hydrogen, hydroxy, amino, alkyl, hydroxyalkyl, benzoyloxy, benzoyloxyalkyl, benzyloxy, sulphamoyloxy, phosphate, carboxypropionyloxy, or acetoxy; R 6  is hydrogen, alkyl, or hydroxyalkyl; 
     
       
         
         
             
             
         
       
     
     wherein R 7  is —CH 2 —CH(R 9 )—CH 2 —, wherein R 9  is hydrogen, methyl, or hydroxymethyl; R 8  is selected from the group consisting of CH 2 OC(O)R 10  and CH(R 10 )OC(O)R 10 , wherein R 10  is selected from the group consisting of C 1 -C 20  alkyl aryl, and aryl-alkyl which may be substituted or unsubstituted by hydroxy or halogen; 
     
       
         
         
             
             
         
       
     
     wherein R 11  and R 12  are each independently hydrogen, or a carboxylic acyl or R 11  and R 12  are joined to form a cyclic acetal or a cyclic carbonate group;
 
-(-dihydroxypropan-2-yloxymethyl);
 
-(4-hydroxy-3-(hydroxymethyl
 
-methoxyethyl-L-valinate;
 
     
       
         
         
             
             
         
       
     
     wherein Z is independently —OC(R 13 ) 2 OC(O)Y(R 14 ) α , an ester, an amidate or —H; 
     Y is N or O; 
     R 13  independently is —H, C 1 -C 12  alkyl, C 5 -C 12  aryl, C 2 -C 12  alkenyl, C 2 -C 12  alkynyl, C 7 -C 12  alkenylaryl, C 7 -C 12  alkynylaryl, or C 6 -C 12  alkaryl, any one of which is unsubstituted or is substituted with 1 or 2 halo, cyano, azido, nitro or —OR 16  in which R 16  is C 1 -C 12  alkyl, C 5 -C 12  aryl, C 2 -C 12  alkenyl, C 2 -C 12  alkynyl;
 
R 14  is independently —H, C 1 -C 12  alkyl, C 5 -C 12  aryl, C 2 -C 12  alkenyl, C 2 -C 12  alkynyl, C 7 -C 12  alkenylaryl, C 7 -C 12  alkynylaryl, or C 6 -C 12  alkaryl, any one of which is unsubstituted or is substituted with 1 or 2 halo, cyano, azido, nitro, —N(R 15 ) 2  or —OR 16 , where R 15  independently is —H or C 1 -C 8  alkyl; and
 
α is 1 or 2; and
 
-methoxy-3-hydroxy-1-propanyl-L-valinate.
 
     In one or more embodiments of the invention, a composition is provided, the composition comprising a compound comprising a nitrogenous base residue covalently attached via a stable or degradable linkage to a water-soluble and non-peptidic oligomer, and optionally, a pharmaceutically acceptable excipient. 
     In one or more embodiments of the invention, a dosage form is provided, the dosage form comprising a compound comprising a nitrogenous base residue covalently attached via a stable or degradable linkage to a water-soluble, non-peptidic oligomer, wherein the compound is present in a dosage form. 
     In one or more embodiments of the invention, a method is provided, the method comprising covalently attaching a water-soluble, non-peptidic oligomer to a nitrogenous base moiety. 
     In one or more embodiments of the invention, a method is provided, the method comprising administering a compound comprising a nitrogenous base residue covalently attached via a stable or degradable linkage to a water-soluble, non-peptidic oligomer. 
     Additional embodiments of the present conjugates, compositions, methods, and the like will be apparent from the following description, examples, and claims. As can be appreciated from the foregoing and following description, each and every feature described herein, and each and every combination of two or more of such features, is included within the scope of the present disclosure provided that the features included in such a combination are not mutually inconsistent. In addition, any feature or combination of features may be specifically excluded from any embodiment of the present invention. Additional aspects and advantages of the present invention are set forth in the following description and claims, particularly when considered in conjunction with the accompanying examples and drawings. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As used in this specification, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. 
     In describing and claiming the present invention, the following terminology will be used in accordance with the definitions described below. 
     “Water soluble, non-peptidic oligomer” indicates an oligomer that is at least 35% (by weight) soluble, preferably greater than 70% (by weight), and more preferably greater than 95% (by weight) soluble, in water at room temperature. Typically, an unfiltered aqueous preparation of a “water-soluble” oligomer transmits at least 75%, more preferably at least 95%, of the amount of light transmitted by the same solution after filtering. It is most preferred, however, that the water-soluble oligomer is at least 95% (by weight) soluble in water or completely soluble in water. With respect to being “non-peptidic,” an oligomer is non-peptidic when it has less than 35% (by weight) of amino acid residues. 
     The terms “monomer,” “monomeric subunit” and “monomeric unit” are used interchangeably herein and refer to one of the basic structural units of a polymer or oligomer. In the case of a homo-oligomer, a single repeating structural unit forms the oligomer. In the case of a co-oligomer, two or more structural units are repeated—either in a pattern or randomly—to form the oligomer. Preferred oligomers used in connection with present the invention are homo-oligomers. The water-soluble, non-peptidic oligomer typically comprises one or more monomers serially attached to form a chain of monomers. The oligomer can be formed from a single monomer type (i.e., is homo-oligomeric) or two or three monomer types (i.e., is co-oligomeric). 
     An “oligomer” is a molecule possessing from about 1 to about 30 monomers. Specific oligomers for use in the invention include those having a variety of geometries such as linear, branched, or forked, to be described in greater detail below. 
     “PEG” or “polyethylene glycol,” as used herein, is meant to encompass any water-soluble poly(ethylene oxide). Unless otherwise indicated, a “PEG oligomer” or an oligoethylene glycol is one in which substantially all (preferably all) monomeric subunits are ethylene oxide subunits, though, the oligomer may contain distinct end capping moieties or functional groups, e.g., for conjugation. PEG oligomers for use in the present invention will comprise one of the two following structures: “—(CH 2 CH 2 O) n —” or “—(CH 2 CH 2 O) n-1 CH 2 CH 2 —,” depending upon whether or not the terminal oxygen(s) has been displaced, e.g., during a synthetic transformation. As stated above, for the PEG oligomers, the variable (n) ranges from about 1 to 30, and the terminal groups and architecture of the overall PEG can vary. When PEG further comprises a functional group, A, for linking to, e.g., a small molecule drug, the functional group when covalently attached to a PEG oligomer does not result in formation of (i) an oxygen-oxygen bond (—O—O—, a peroxide linkage), or (ii) a nitrogen-oxygen bond (N—O, O—N). 
     The terms “end-capped” or “terminally capped” are interchangeably used herein to refer to a terminal or endpoint of a polymer having an end-capping moiety. Typically, although not necessarily, the end-capping moiety comprises a hydroxy or C 1-20  alkoxy group. Thus, examples of end-capping moieties include alkoxy (e.g., methoxy, ethoxy and benzyloxy), as well as aryl, heteroaryl, cyclo, heterocyclo, and the like. In addition, saturated, unsaturated, substituted and unsubstituted forms of each of the foregoing are envisioned. Moreover, the end-capping group can also be a silane. The end-capping group can also advantageously comprise a detectable label. When the polymer has an end-capping group comprising a detectable label, the amount or location of the polymer and/or the moiety (e.g., active agent) of interest to which the polymer is coupled, can be determined by using a suitable detector. Such labels include, without limitation, fluorescers, chemiluminescers, moieties used in enzyme labeling, colorimetric moieties (e.g., dyes), metal ions, radioactive moieties, and the like. Suitable detectors include photometers, films, spectrometers, and the like. In addition, the end-capping group may contain a targeting moiety. 
     The term “targeting moiety” is used herein to refer to a molecular structure that helps the conjugates of the invention to localize to a targeting area, e.g., help enter a cell, or bind a receptor. Preferably, the targeting moiety comprises of vitamin, antibody, antigen, receptor, DNA, RNA, sialyl Lewis X antigen, hyaluronic acid, sugars, cell specific lectins, steroid or steroid derivative, RGD peptide, ligand for a cell surface receptor, serum component, or combinatorial molecule directed against various intra- or extracellular receptors. The targeting moiety may also comprise a lipid or a phospholipid. Exemplary phospholipids include, without limitation, phosphatidylcholines, phospatidylserine, phospatidylinositol, phospatidylglycerol, and phospatidylethanolamine. These lipids may be in the form of micelles or liposomes and the like. The targeting moiety may further comprise a detectable label or alternately a detectable label may serve as a targeting moiety. When the conjugate has a targeting group comprising a detectable label, the amount and/or distribution/location of the polymer and/or the moiety (e.g., active agent) to which the polymer is coupled can be determined by using a suitable detector. Such labels include, without limitation, fluorescers, chemiluminescers, moieties used in enzyme labeling, colorimetric (e.g., dyes), metal ions, radioactive moieties, gold particles, and quantum dots. 
     “Branched,” in reference to the geometry or overall structure of an oligomer, refers to an oligomer having two or more polymer “arms” extending from a branch point. 
     “Forked,” in reference to the geometry or overall structure of an oligomer, refers to an oligomer having two or more functional groups (typically through one or more atoms) extending from a branch point. 
     A “branch point” refers to a bifurcation point comprising one or more atoms at which an oligomer branches or forks from a linear structure into one or more additional arms. 
     The term “reactive” or “activated” refers to a functional group that reacts readily or at a practical rate under conventional conditions of organic synthesis. This is in contrast to those groups that either do not react or require strong catalysts or impractical reaction conditions in order to react (i.e., a “nonreactive” or “inert” group). 
     “Not readily reactive,” with reference to a functional group present on a molecule in a reaction mixture, indicates that the group remains largely intact under conditions that are effective to produce a desired reaction in the reaction mixture. 
     A “protecting group” is a moiety that prevents or blocks reaction of a particular chemically reactive functional group in a molecule under certain reaction conditions. The protecting group may vary depending upon the type of chemically reactive group being protected as well as the reaction conditions to be employed and the presence of additional reactive or protecting groups in the molecule. Functional groups which may be protected include, by way of example, carboxylic acid groups, amino groups, hydroxyl groups, thiol groups, carbonyl groups and the like. Representative protecting groups for carboxylic acids include esters (such as a p-methoxybenzyl ester), amides and hydrazides; for amino groups, carbamates (such as tert-butoxycarbonyl) and amides; for hydroxyl groups, ethers and esters; for thiol groups, thioethers and thioesters; for carbonyl groups, acetals and ketals; and the like. Such protecting groups are well-known to those skilled in the art and are described, for example, in T. W. Greene and G. M. Wuts,  Protecting Groups in Organic Synthesis , Third Edition, Wiley, New York, 1999, and references cited therein. 
     A functional group in “protected form” refers to a functional group bearing a protecting group. As used herein, the term “functional group” or any synonym thereof encompasses protected forms thereof. 
     A “physiologically cleavable” or “hydrolyzable” or “degradable” bond is a relatively labile bond that reacts with water (i.e., is hydrolyzed) under physiological conditions. The tendency of a bond to hydrolyze in water may depend not only on the general type of linkage connecting two central atoms but also on the substituents attached to these central atoms. Appropriate hydrolytically unstable or weak linkages include but are not limited to carboxylate ester, phosphate ester, anhydrides, acetals, ketals, acyloxyalkyl ether, imines, orthoesters, peptides, oligonucleotides, thioesters, and carbonates. 
     An “enzymatically degradable linkage” means a linkage that is subject to degradation by one or more enzymes. 
     A “stable” linkage or bond refers to a chemical bond that is substantially stable in water, that is to say, does not undergo hydrolysis under physiological conditions to any appreciable extent over an extended period of time. Examples of hydrolytically stable linkages include but are not limited to the following: carbon-carbon bonds (e.g., in aliphatic chains), ethers, amides, urethanes, amines, and the like. Generally, a stable linkage is one that exhibits a rate of hydrolysis of less than about 1-2% per day under physiological conditions. Hydrolysis rates of representative chemical bonds can be found in most standard chemistry textbooks. 
     “Substantially” or “essentially” means nearly totally or completely, for instance, 95% or greater, more preferably 97% or greater, still more preferably 98% or greater, even more preferably 99% or greater, yet still more preferably 99.9% or greater, with 99.99% or greater being most preferred of some given quantity. 
     “Monodisperse” refers to an oligomer composition wherein substantially all of the oligomers in the composition have a well-defined, single molecular weight and defined number of monomers, as determined by chromatography or mass spectrometry. Monodisperse oligomer compositions are in one sense pure, that is, substantially having a single and definable number (as a whole number) of monomers rather than a large distribution. A monodisperse oligomer composition possesses a MW/Mn value of 1.0005 or less, and more preferably, a MW/Mn value of 1.0000. By extension, a composition comprised of monodisperse conjugates means that substantially all oligomers of all conjugates in the composition have a single and definable number (as a whole number) of monomers rather than a large distribution and would possess a MW/Mn value of 1.0005, and more preferably, a MW/Mn value of 1.0000 if the oligomer were not attached to the therapeutic moiety. A composition comprised of monodisperse conjugates may, however, include one or more nonconjugate substances such as solvents, reagents, excipients, and so forth. 
     “Bimodal,” in reference to an oligomer composition, refers to an oligomer composition wherein substantially all oligomers in the composition have one of two definable and different numbers (as whole numbers) of monomers rather than a large distribution, and whose distribution of molecular weights, when plotted as a number fraction versus molecular weight, appears as two separate identifiable peaks. Preferably, for a bimodal oligomer composition as described herein, each peak is generally symmetric about its mean, although the size of the two peaks may differ. Ideally, the polydispersity index of each peak in the bimodal distribution, Mw/Mn, is 1.01 or less, more preferably 1.001 or less, and even more preferably 1.0005 or less, and most preferably a MW/Mn value of 1.0000. By extension, a composition comprised of bimodal conjugates means that substantially all oligomers of all conjugates in the composition have one of two definable and different numbers (as whole numbers) of monomers rather than a large distribution and would possess a MW/Mn value of 1.01 or less, more preferably 1.001 or less and even more preferably 1.0005 or less, and most preferably a MW/Mn value of 1.0000 if the oligomer were not attached to the therapeutic moiety. A composition comprised of bimodal conjugates may, however, include one or more nonconjugate substances such as solvents, reagents, excipients, and so forth. 
     A “nitrogenous base moiety” refers to an organic, inorganic, or organometallic compound typically having a molecular weight of less than about 1000 Daltons (and typically less than 500 Daltons) and having some degree of activity as nitrogenous base activity. Nitrogenous base compounds of the invention encompass oligopeptides and other biomolecules having a molecular weight of less than about 1000 Daltons. 
     A “biological membrane” is any membrane made of cells or tissues that serves as a barrier to at least some foreign entities or otherwise undesirable materials. As used herein a “biological membrane” includes those membranes that are associated with physiological protective barriers including, for example: the blood-brain barrier (BBB); the blood-cerebrospinal fluid barrier; the blood-placental barrier; the blood-milk barrier; the blood-testes barrier; and mucosal barriers including the vaginal mucosa, urethral mucosa, anal mucosa, buccal mucosa, sublingual mucosa, and rectal mucosa. Unless the context clearly dictates otherwise, the term “biological membrane” does not include those membranes associated with the middle gastro-intestinal tract (e.g., stomach and small intestines). 
     A “biological membrane crossing rate,” provides a measure of a compound&#39;s ability to cross a biological membrane, such as the blood-brain barrier (“BBB”). A variety of methods may be used to assess transport of a molecule across any given biological membrane. Methods to assess the biological membrane crossing rate associated with any given biological barrier (e.g., the blood-cerebrospinal fluid barrier, the blood-placental barrier, the blood-milk barrier, the intestinal barrier, and such), are known, described herein and/or in the relevant literature, and/or may be determined by one of ordinary skill in the art. 
     A “reduced rate of metabolism” refers to a measurable reduction in the rate of metabolism of a water-soluble oligomer-small molecule drug conjugate as compared to the rate of metabolism of the small molecule drug not attached to the water-soluble oligomer (i.e., the small molecule drug itself) or a reference standard material. In the special case of “reduced first pass rate of metabolism,” the same “reduced rate of metabolism” is required except that the small molecule drug (or reference standard material) and the corresponding conjugate are administered orally. Orally administered drugs are absorbed from the gastro-intestinal tract into the portal circulation and may pass through the liver prior to reaching the systemic circulation. Because the liver is the primary site of drug metabolism or biotransformation, a substantial amount of drug may be metabolized before it ever reaches the systemic circulation. The degree of first pass metabolism, and thus, any reduction thereof, may be measured by a number of different approaches. For instance, animal blood samples may be collected at timed intervals and the plasma or serum analyzed by liquid chromatography/mass spectrometry for metabolite levels. Other techniques for measuring a “reduced rate of metabolism” associated with the first pass metabolism and other metabolic processes are known, described herein and/or in the relevant literature, and/or may be determined by one of ordinary skill in the art. Preferably, a conjugate of the invention may provide a reduced rate of metabolism reduction satisfying at least one of the following values: at least about 30%; at least about 40%; at least about 50%; at least about 60%; at least about 70%; at least about 80%; and at least about 90%. A compound (such as a small molecule drug or conjugate thereof) that is “orally bioavailable” is one that preferably possesses a bioavailability when administered orally of greater than 25%, and preferably greater than 70%, where a compound&#39;s bioavailability is the fraction of administered drug that reaches the systemic circulation in unmetabolized form. 
     “Alkyl” refers to a hydrocarbon chain, ranging from about 1 to 20 atoms in length. Such hydrocarbon chains are preferably but not necessarily saturated and may be branched or straight chain. Exemplary alkyl groups include methyl, ethyl, propyl, butyl, pentyl, 2-methylbutyl, 2-ethylpropyl, 3-methylpentyl, and the like. As used herein, “alkyl” includes cycloalkyl when three or more carbon atoms are referenced. An “alkenyl” group is an alkyl of 2 to 20 carbon atoms with at least one carbon-carbon double bond. 
     The terms “substituted alkyl” or “substituted C q-r  alkyl” where q and r are integers identifying the range of carbon atoms contained in the alkyl group, denotes the above alkyl groups that are substituted by one, two or three halo (e.g., F, Cl, Br, I), trifluoromethyl, hydroxy, C 1-7  alkyl (e.g., methyl, ethyl, n-propyl, isopropyl, butyl, t-butyl, and so forth), C 1-7  alkoxy, C 1-7  acyloxy, C 3-7  heterocyclic, amino, phenoxy, nitro, carboxy, acyl, cyano. The substituted alkyl groups may be substituted once, twice or three times with the same or with different substituents. 
     “Lower alkyl” refers to an alkyl group containing from 1 to 6 carbon atoms, and may be straight chain or branched, as exemplified by methyl, ethyl, n-butyl, i-butyl, t-butyl. “Lower alkenyl” refers to a lower alkyl group of 2 to 6 carbon atoms having at least one carbon-carbon double bond. 
     “Non-interfering substituents” are those groups that, when present in a molecule, are typically non-reactive with other functional groups contained within the molecule. 
     “Alkoxy” refers to an —O—R group, wherein R is alkyl or substituted alkyl, preferably C 1 -C 20  alkyl (e.g., methoxy, ethoxy, propyloxy, etc.), preferably C 1 -C 7 . 
     “Pharmaceutically acceptable excipient” or “pharmaceutically acceptable carrier” refers to component that may be included in the compositions of the invention causes no significant adverse toxicological effects to a patient. 
     The term “aryl” means an aromatic group having up to 14 carbon atoms. Aryl groups include phenyl, naphthyl, biphenyl, phenanthrenyl, naphthalenyl, and the like. “Substituted phenyl” and “substituted aryl” denote a phenyl group and aryl group, respectively, substituted with one, two, three, four or five (e.g. 1-2, 1-3 or 1-4 substituents) chosen from halo (F, Cl, Br, I), hydroxy, cyano, nitro, alkyl (e.g., C 1-6  alkyl), alkoxy (e.g., C 1-6  alkoxy), benzyloxy, carboxy, aryl, and so forth. 
     Chemical moieties are defined and referred to throughout primarily as univalent chemical moieties (e.g., alkyl, aryl, etc.). Nevertheless, such terms are also used to convey corresponding multivalent moieties under the appropriate structural circumstances clear to those skilled in the art. For example, while an “alkyl” moiety generally refers to a monovalent radical (e.g., CH 3 —CH 2 —), in certain circumstances a bivalent linking moiety can be “alkyl,” in which case those skilled in the art will understand the alkyl to be a divalent radical (e.g., —CH 2 —CH 2 —), which is equivalent to the term “alkylene.” (Similarly, in circumstances in which a divalent moiety is required and is stated as being “aryl,” those skilled in the art will understand that the term “aryl” refers to the corresponding multivalent moiety, arylene). All atoms are understood to have their normal number of valences for bond formation (i.e., 1 for H, 4 for carbon, 3 for N, 2 for O, and 2, 4, or 6 for S, depending on the oxidation state of the S). 
     “Pharmacologically effective amount,” “physiologically effective amount,” and “therapeutically effective amount” are used interchangeably herein to mean the amount of a water-soluble oligomer-small molecule drug conjugate present in a composition that is needed to provide a desired level of active agent and/or conjugate in the bloodstream or in the target tissue. The precise amount may depend upon numerous factors, e.g., the particular active agent, the components and physical characteristics of the composition, intended patient population, patient considerations, and may readily be determined by one skilled in the art, based upon the information provided herein and available in the relevant literature. 
     A “difunctional” oligomer is an oligomer having two functional groups contained therein, typically at its termini. When the functional groups are the same, the oligomer is said to be homodifunctional. When the functional groups are different, the oligomer is said to be heterodifunctional. 
     A basic reactant or an acidic reactant described herein include neutral, charged, and any corresponding salt forms thereof. 
     The term “patient,” refers to a living organism suffering from or prone to a condition that can be prevented or treated by administration of a conjugate as described herein, and includes both humans and animals. 
     “Optional” or “optionally” means that the subsequently described circumstance may but need not necessarily occur, so that the description includes instances where the circumstance occurs and instances where it does not. 
     “Nil” refers to the absence of a substituent group. Thus, when a substituent is nil, the substituent may be represented in the structure as a chemical bond or hydrogen in the resulting structure. 
     As indicated above, the present invention is directed to (among other things) a compound comprising a nitrogenous base residue covalently attached via a stable or degradable linkage to a water-soluble, non-peptidic oligomer. 
     The “nitrogenous base residue” is a compound having a structure of a nitrogenous base compound that is altered by the presence of one or more bonds, which bonds serve to attach (either directly or indirectly) one or more water-soluble, non-peptidic oligomers. Exemplary nitrogenous bases have a structure encompassed by at least one of the structures defined herein: 
     B-R 1 , wherein: B is a nitrogenous base; and
 
R 1  is a substitution on the nitrogenous base; the substitution is selected from the group consisting of
 
     
       
         
         
             
             
         
       
     
     wherein A is S or O; R 3  is hydrogen, straight or branched chain or cyclic alkyl, hydroxyalkyl, benzyloxyalkyl or phenyl; R 4  is hydrogen, hydroxy, or lower alkyl; R 5  is hydrogen, hydroxy, amino, alkyl, hydroxyalkyl, benzoyloxy, benzoyloxyalkyl, benzyloxy, sulphamoyloxy, phosphate, carboxypropionyloxy, or acetoxy; R 6  is hydrogen, alkyl, or hydroxyalkyl; 
     
       
         
         
             
             
         
       
     
     wherein R 7  is —CH 2 —CH(R 9 )—CH 2 —, wherein R 9  is hydrogen, methyl, or hydroxymethyl; R 8  is selected from the group consisting of CH 2 OC(O)R 10  and CH(R 10 )OC(O)R 10 , wherein R 10  is selected from the group consisting of C 1 -C 20  alkyl aryl, and aryl-alkyl which may be substituted or unsubstituted by hydroxy or halogen; 
     
       
         
         
             
             
         
       
     
     wherein R 11  and R 12  are each independently hydrogen, or a carboxylic acyl or R 11  and R 12  are joined to form a cyclic acetal or a cyclic carbonate group;
 
-(-dihydroxypropan-2-yloxymethyl);
 
-(4-hydroxy-3-(hydroxymethyl)butyl;
 
-methoxyethyl-L-valinate;
 
     
       
         
         
             
             
         
       
     
     wherein Z is independently —OC(R 13 ) 2 OC(O)Y(R 14 ) α , an ester, an amidate or —H; 
     Y is N or O; 
     R 13  independently is —H, C 1 -C 12  alkyl, C 5 -C 12  aryl, C 2 -C 12  alkenyl, C 2 -C 12  alkynyl, C 7 -C 12  alkenylaryl, C 7 -C 12  alkynylaryl, or C 6 -C 12  alkaryl, any one of which is unsubstituted or is substituted with 1 or 2 halo, cyano, azido, nitro or —OR 16  in which R 16  is C 1 -C 12  alkyl, C 5 -C 12  aryl, C 2 -C 12  alkenyl, C 2 -C 12  alkynyl; 
     R 14  is independently —H, C 1 -C 12  alkyl, C 5 -C 12  aryl, C 2 -C 12  alkenyl, C 2 -C 12  alkynyl, C 7 -C 12  alkenylaryl, C 7 -C 12  alkynylaryl, or C 6 -C 12  alkaryl, any one of which is unsubstituted or is substituted with 1 or 2 halo, cyano, azido, nitro, —N(R 15 ) 2  or —OR 16 , where R 15  independently is —H or C 1 -C 8  alkyl; and 
     α is 1 or 2; and
 
-methoxy-3-hydroxy-1-propanyl-L-valinate.
 
     In one or more embodiment of the invention, a compound is provided, the compound comprising a residue of a nitrogenous base covalently attached via a linker to a water-soluble and non-peptidic oligomer, wherein the nitrogenous base has a structure encompassed by the following formula: 
     
       
         
         
             
             
         
       
     
     wherein A is S or O; R 3  is hydrogen, straight or branched chain or cyclic alkyl, hydroxyalkyl, benzyloxyalkyl or phenyl; R 4  is hydrogen, hydroxy, or lower alkyl; R 5  is hydrogen, hydroxy, amino, alkyl, hydroxyalkyl, benzoyloxy, benzoyloxyalkyl, benzyloxy, sulphamoyloxy, phosphate, carboxypropionyloxy, or acetoxy; R 6  is hydrogen, alkyl, or hydroxyalkyl. 
     In one or more embodiment of the invention, a compound is provided, the compound comprising a residue of a nitrogenous base covalently attached via a linker to a water-soluble and non-peptidic oligomer, wherein the nitrogenous base has a structure encompassed by the following formula: 
     
       
         
         
             
             
         
       
     
     wherein B is selected from the group consisting of adenyl (A), guanyl (G), and 2,6-diaminopurinyl;
 
R 7  is —CH 2 —CH(R 9 )—CH 2 —, wherein R 9  is hydrogen, methyl, or hydroxymethyl; R 8  is selected from the group consisting of CH 2 OC(O)R 10  and CH(R 10 )OC(O)R 10 , wherein R 10  is selected from the group consisting of C 1 -C 20  alkyl aryl, and aryl-alkyl which may be substituted or unsubstituted by hydroxy or halogen.
 
     In one or more embodiment of the invention, a compound is provided, the compound comprising a residue of a nitrogenous base covalently attached via a linker to a water-soluble and non-peptidic oligomer, wherein the nitrogenous base has a structure encompassed by the following formula: 
     
       
         
         
             
             
         
       
     
     wherein B is a nitrogenous base selected from the group consisting of uracil-1-yl, cytosin-1-yl, 5-methylcytosin-1-yl, thymin-1-yl, 5-fluoruracil-1-yl, guanin-9-yl, guanin-7-yl, adenine-9-yl, adenin-3-yl, hypoxanthin-9-yl, 2-methyladenin-9-yl, 2-methylthioadenin-9-yl, 2-aminoadenin-9-yl, 2-aminopurin-9-yl, N 6 -dimethyladenin-9-yl, 8-bromoadenin-9-yl, 8-hydroxyadenin-9-yl, 6-hydroxylaminopurin-9-yl, 6-hydrazinopurin-9-yl, 6-thiopurin-9-yl, purin-9-yl, and xanthin-9-yl. 
     In one or more embodiment of the invention, a compound is provided, the compound comprising a residue of a nitrogenous base covalently attached via a linker to a water-soluble and non-peptidic oligomer, wherein the nitrogenous base has a structure encompassed by the following formula: 
     
       
         
         
             
             
         
       
     
     wherein R 11  and R 12  are each independently hydrogen, or a carboxylic acyl or R 11  and R 12  are joined to form a cyclic acetal or a cyclic carbonate group. 
     In one or more embodiment of the invention, a compound is provided, the compound comprising a residue of a nitrogenous base covalently attached via a linker to a water-soluble and non-peptidic oligomer, wherein the nitrogenous base has a structure encompassed by the following formula: 9-(1,3-dihydroxy-2-propoxymethyl)guanine. 
     In one or more embodiment of the invention, a compound is provided, the compound comprising a residue of a nitrogenous base covalently attached via a linker to a water-soluble and non-peptidic oligomer, wherein the nitrogenous base has a structure encompassed by the following formula: 9-(4-hydroxy-2-hydroxymethylbut-1-yl)guanine. 
     In one or more embodiment of the invention, a compound is provided, the compound comprising a residue of a nitrogenous base covalently attached via a linker to a water-soluble and non-peptidic oligomer, wherein the nitrogenous base has a structure encompassed by the following formula: 2-[(2-amino-1,6-dihydro-6-oxo-9H-purin-9-yl)methoxy]ethyl L-valinate. 
     In one or more embodiment of the invention, a compound is provided, the compound comprising a residue of a nitrogenous base covalently attached via a linker to a water-soluble and non-peptidic oligomer, wherein the nitrogenous base has a structure encompassed by the following formula: 2-(2-amino-1,6-dihydro-6-oxo-9-purin-9-yl)methoxy-3-hydroxy-1-propanyl L-valinate hydrochloride. 
     In one or more embodiment of the invention, a compound is provided, the compound comprising a residue of a nitrogenous base covalently attached via a linker to a water-soluble and non-peptidic oligomer, wherein the nitrogenous base has a structure encompassed by the following formula: 
     
       
         
         
             
             
         
       
     
     wherein Z is independently —OC(R 13 ) 2 OC(O)Y(R 14 ) α ,an ester, an amidate or —H;
 
A is the residue of a phosphonomethoxy nucleotide analog;
 
     Y is N or O; 
     R 13  independently is —H, C 1 -C 12  alkyl, C 5 -C 12  aryl, C 2 -C 12  alkenyl, C 2 -C 12  alkynyl, C 7 -C 12  alkenylaryl, C 7 -C 12  alkynylaryl, or C 6 -C 12  alkaryl, any one of which is unsubstituted or is substituted with 1 or 2 halo, cyano, azido, nitro or —OR 16  in which R 16  is C 1 -C 12  alkyl, C 5 -C 12  aryl, C 2 -C 12  alkenyl, C 2 -C 12  alkynyl;
 
R 14  is independently —H, C 1 -C 12  alkyl, C 5 -C 12  aryl, C 2 -C 12  alkenyl, C 2 -C 12  alkynyl, C 7 -C 12  alkenylaryl, C 7 -C 12  alkynylaryl, or C 6 -C 12  alkaryl, any one of which is unsubstituted or is substituted with 1 or 2 halo, cyano, azido, nitro, —N(R 15 ) 2  or —OR 16 , where R 15  independently is —H or C 1 -C 8  alkyl; and
 
α is 1 or 2.
 
     Exemplary nitrogenous base residue-containing compounds also include, but are not limited to, acyclovir, adefovir, cidofovir, famciclovir, ganciclovir, penciclovir, tenofovir, valacyclovir, and valganciclovir. 
     In some instances, nitrogenous bases can be obtained from commercial sources. In addition, nitrogenous bases can be obtained through chemical synthesis. Examples of nitrogenous bases as well as synthetic approaches for preparing nitrogenous bases are described in the literature and in, for example, U.S. Pat. Nos. 4,199,574, 5,663,159, 5,142,051, 5,246,937, 4,355,032, 5,075,445, 5,922,695, 4,957,924, and 6,083,953. 
     Each of these (and other) nitrogenous bases can be covalently attached (either directly or through one or more atoms) to a water-soluble and non-peptidic oligomer. 
     Exemplary compounds of the invention include those having the following structure: 
     B-R 1 -X-POLY, herein, B is a nitrogenous base;
 
R 1  is a substitution on the nitrogenous base, the substitution is selected from the group
 
consisting of
 
     
       
         
         
             
             
         
       
     
     wherein A is S or O; R 3  is hydrogen, straight or branched chain or cyclic alkyl, hydroxyalkyl, benzyloxyalkyl or phenyl; R 4  is hydrogen, hydroxy, or lower alkyl; R 5  is hydrogen, hydroxy, amino, alkyl, hydroxyalkyl, benzoyloxy, benzoyloxyalkyl, benzyloxy, sulphamoyloxy, phosphate, carboxypropionyloxy, or acetoxy; R 6  is hydrogen, alkyl, or hydroxyalkyl; 
     
       
         
         
             
             
         
       
     
     wherein R 7  is —CH 2 —CH(R 9 )—CH 2 —, wherein R 9  is hydrogen, methyl, or hydroxymethyl; R 8  is selected from the group consisting of CH 2 OC(O)R 10  and CH(R 10 )OC(O)R 10 , wherein R 10  is selected from the group consisting of C 1 -C 20  alkyl aryl, and aryl-alkyl which may be substituted or unsubstituted by hydroxy or halogen; 
     
       
         
         
             
             
         
       
     
     wherein R 11  and R 12  are each independently hydrogen, or a carboxylic acyl or R 11  and R 12  are joined to form a cyclic acetal or a cyclic carbonate group;
 
-(-dihydroxypropan-2-yloxymethyl);
 
-(4-hydroxy-3-(hydroxymethyl)butyl;
 
-methoxyethyl-L-valinate;
 
     
       
         
         
             
             
         
       
     
     wherein Z is independently —OC(R 13 ) 2 OC(O)X(R 14 ) α , an ester, an amidate or —H; 
     X is N or O; 
     R 13  independently is —H, C 1 -C 12  alkyl, C 5 -C 12  aryl, C 2 -C 12  alkenyl, C 2 -C 12  alkynyl, C 7 -C 12  alkenylaryl, C 7 -C 12  alkynylaryl, or C 6 -C 12  alkaryl, any one of which is unsubstituted or is substituted with 1 or 2 halo, cyano, azido, nitro or —OR 16  in which R 16  is C 1 -C 12  alkyl, C 5 -C 12  aryl, C 2 -C 12  alkenyl, C 2 -C 12  alkynyl;
 
R 14  is independently —H, C 1 -C 12  alkyl, C 5 -C 12  aryl, C 2 -C 12  alkenyl, C 2 -C 12  alkynyl, C 7 -C 12  alkenylaryl, C 7 -C 12  alkynylaryl, or C 6 -C 12  alkaryl, any one of which is unsubstituted or is substituted with 1 or 2 halo, cyano, azido, nitro, —N(R 15 ) 2  or —OR 16 , where R 15  independently is —H or C 1 -C 8  alkyl; and
 
α is 1 or 2;
 
-methoxy-3-hydroxy-1-propanyl-L-valinate;
 
X is a spacer moiety; and
 
POLY is a water-soluble, non-peptidic oligomer.
 
     In another embodiment, exemplary compounds of the invention include those having the following structure: 
     B-R 1 -X-POLY, wherein, B is a nitrogenous base selected from the group consisting of uracil-1-yl, cytosin-1-yl, 5-methylcytosin-1-yl, thymin-1-yl, 5-fluoruracil-1-yl, guanin-9-yl, guanin-7-yl, adenine-9-yl, adenin-3-yl, hypoxanthin-9-yl, 2-methyladenin-9-yl, 2-methylthioadenin-9-yl, 2-aminoadenin-9-yl, 2-aminopurin-9-yl, N 6 -dimethyladenin-9-yl, 8-bromoadenin-9-yl, 8-hydroxyadenin-9-yl, 6-hydroxylaminopurin-9-yl, 6-hydrazinopurin-9-yl, 6-thiopurin-9-yl, purin-9-yl, and xanthin-9-yl;
 
R 1  is as defined above;
 
X is a spacer moiety; and
 
POLY is a water-soluble, non-peptidic oligomer.
 
     In another embodiment, exemplary compounds of the invention include those having the following structure: 
     R 1 -B-X-POLY, wherein: R 1  is as defined above;
 
B is a nitrogenous base comprising an amino group;
 
X is a spacer moiety attached to the amino group of the nitrogenous base; and
 
POLY is a water-soluble, non-peptidic oligomer.
 
     Use of oligomers (e.g., from a monodisperse or bimodal composition of oligomers, in contrast to relatively impure compositions) to form oligomer-containing compounds may advantageously alter certain properties associated with the corresponding small molecule drug. For instance, a compound of the invention, when administered by any of a number of suitable administration routes, such as parenteral, oral, transdermal, buccal, pulmonary, or nasal, exhibits reduced penetration across the blood-brain barrier. It is preferred that the compounds of the invention exhibit slowed, minimal or effectively no crossing of the blood-brain barrier, while still crossing the gastro-intestinal (GI) walls and into the systemic circulation if oral delivery is intended. Moreover, the compounds of the invention maintain a degree of bioactivity as well as bioavailability in comparison to the bioactivity and bioavailability of the compound free of all oligomers. 
     With respect to the blood-brain barrier (“BBB”), this barrier restricts the transport of drugs from the blood to the brain. This barrier consists of a continuous layer of unique endothelial cells joined by tight junctions. The cerebral capillaries, which comprise more than 95% of the total surface area of the BBB, represent the principal route for the entry of most solutes and drugs into the central nervous system. 
     For compounds whose degree of blood-brain barrier crossing ability is not readily known, such ability may be determined using a suitable animal model such as an in situ rat brain perfusion (“RBP”) model as described herein. Briefly, the RBP technique involves cannulation of the carotid artery followed by perfusion with a compound solution under controlled conditions, followed by a wash out phase to remove compound remaining in the vascular space. (Such analyses may be conducted, for example, by contract research organizations such as Absorption Systems, Exton, Pa.). In one example of the RBP model, a cannula is placed in the left carotid artery and the side branches are tied off. A physiologic buffer containing the analyte (typically but not necessarily at a 5 micromolar concentration level) is perfused at a flow rate of about 10 mL/minute in a single pass perfusion experiment. After 30 seconds, the perfusion is stopped and the brain vascular contents are washed out with compound-free buffer for an additional 30 seconds. The brain tissue is then removed and analyzed for compound concentrations via liquid chromatograph with tandem mass spectrometry detection (LC/MS/MS). Alternatively, blood-brain barrier permeability can be estimated based upon a calculation of the compound&#39;s molecular polar surface area (“PSA”), which is defined as the sum of surface contributions of polar atoms (usually oxygens, nitrogens and attached hydrogens) in a molecule. The PSA has been shown to correlate with compound transport properties such as blood-brain barrier transport. Methods for determining a compound&#39;s PSA can be found, e.g., in, Ertl, P., et al.,  J. Med. Chem.  2000, 43, 3714-3717; and Kelder, J., et al.,  Pharm. Res.  1999, 16, 1514-1519. 
     With respect to the blood-brain barrier, the water-soluble, non-peptidic oligomer-small molecule drug conjugate exhibits a blood-brain barrier crossing rate that is reduced as compared to the crossing rate of the small molecule drug not attached to the water-soluble, non-peptidic oligomer. Exemplary reductions in blood-brain barrier crossing rates for the compounds described herein include reductions of: at least about 5%; at least about 10%; at least about 25%; at least about 30%; at least about 40%; at least about 50%; at least about 60%; at least about 70%; at least about 80%; or at least about 90%, when compared to the blood-brain barrier crossing rate of the small molecule drug not attached to the water-soluble oligomer. A preferred reduction in the blood-brain barrier crossing rate for a conjugate of the invention is at least about 20%. 
     Assays for determining whether a given compound (regardless of whether the compound includes a water-soluble, non-peptidic oligomer or not) can act as a nitrogenous base are known and/or may be prepared by one of ordinary skill in the art and are further described infra. 
     Each of these (and other) diarylpiperazine moieties can be covalently attached (either directly or through one or more atoms) to a water-soluble and non-peptidic oligomer. 
     Exemplary molecular weights of small molecule drugs include molecular weights of: less than about 950; less than about 900; less than about 850; less than about 800; less than about 750; less than about 700; less than about 650; less than about 600; less than about 550; less than about 500; less than about 450; less than about 400; less than about 350; and less than about 300 Daltons. 
     The small molecule drug used in the invention, if chiral, may be obtained from a racemic mixture, or an optically active form, for example, a single optically active enantiomer, or any combination or ratio of enantiomers (i.e., scalemic mixture). In addition, the small molecule drug may possess one or more geometric isomers. With respect to geometric isomers, a composition can comprise a single geometric isomer or a mixture of two or more geometric isomers. A small molecule drug for use in the present invention can be in its customary active form, or may possess some degree of modification. For example, a small molecule drug may have a targeting agent, tag, or transporter attached thereto, prior to or after covalent attachment of an oligomer. Alternatively, the small molecule drug may possess a lipophilic moiety attached thereto, such as a phospholipid (e.g., distearoylphosphatidylethanolamine or “DSPE,” dipalmitoylphosphatidylethanolamine or “DPPE,” and so forth) or a small fatty acid. In some instances, however, it is preferred that the small molecule drug moiety does not include attachment to a lipophilic moiety. 
     The nitrogenous base moiety for coupling to a water-soluble, non-peptidic oligomer possesses a free hydroxyl, carboxyl, thio, amino group, or the like (i.e., “handle”) suitable for covalent attachment to the oligomer. In addition, the nitrogenous base moiety may be modified by introduction of a reactive group, preferably by conversion of one of its existing functional groups to a functional group suitable for formation of a stable covalent linkage between the oligomer and the drug. 
     Accordingly, each oligomer is composed of up to three different monomer types selected from the group consisting of: alkylene oxide, such as ethylene oxide or propylene oxide; olefinic alcohol, such as vinyl alcohol, 1-propenol or 2-propenol; vinyl pyrrolidone; hydroxyalkyl methacrylamide or hydroxyalkyl methacrylate, where alkyl is preferably methyl; α-hydroxy acid, such as lactic acid or glycolic acid; phosphazene, oxazoline, amino acids, carbohydrates such as monosaccharides, alditol such as mannitol; and N-acryloylmorpholine. Preferred monomer types include alkylene oxide, olefinic alcohol, hydroxyalkyl methacrylamide or methacrylate, N-acryloylmorpholine, and α-hydroxy acid. Preferably, each oligomer is, independently, a co-oligomer of two monomer types selected from this group, or, more preferably, is a homo-oligomer of one monomer type selected from this group. 
     The two monomer types in a co-oligomer may be of the same monomer type, for example, two alkylene oxides, such as ethylene oxide and propylene oxide. Preferably, the oligomer is a homo-oligomer of ethylene oxide. Usually, although not necessarily, the terminus (or termini) of the oligomer that is not covalently attached to a small molecule is capped to render it unreactive. Alternatively, the terminus may include a reactive group. When the terminus is a reactive group, the reactive group is either selected such that it is unreactive under the conditions of formation of the final oligomer or during covalent attachment of the oligomer to a small molecule drug, or it is protected as necessary. One common end-functional group is hydroxyl or —OH, particularly for oligoethylene oxides. 
     The water-soluble, non-peptidic oligomer (e.g., “POLY” in various structures provided herein) can have any of a number of different geometries. For example, the water-soluble, non-peptidic oligomer may be linear, branched, or forked. Most typically, the water-soluble, non-peptidic oligomer is linear or is branched, for example, having one branch point. Although much of the discussion herein is focused upon poly(ethylene oxide) as an illustrative oligomer, the discussion and structures presented herein can be readily extended to encompass any water-soluble, non-peptidic oligomers described above. 
     The molecular weight of the water-soluble, non-peptidic oligomer, excluding the linker portion, is generally relatively low. Exemplary values of the molecular weight of the water-soluble polymer include: below about 1500; below about 1450; below about 1400; below about 1350; below about 1300; below about 1250; below about 1200; below about 1150; below about 1100; below about 1050; below about 1000; below about 950; below about 900; below about 850; below about 800; below about 750; below about 700; below about 650; below about 600; below about 550; below about 500; below about 450; below about 400; below about 350; below about 300; below about 250; below about 200; and below about 100 Daltons. 
     Exemplary ranges of molecular weights of the water-soluble, non-peptidic oligomer (excluding the linker) include: from about 100 to about 1400 Daltons; from about 100 to about 1200 Daltons; from about 100 to about 800 Daltons; from about 100 to about 500 Daltons; from about 100 to about 400 Daltons; from about 200 to about 500 Daltons; from about 200 to about 400 Daltons; from about 75 to 1000 Daltons; and from about 75 to about 750 Daltons. 
     Preferably, the number of monomers in the water-soluble, non-peptidic oligomer falls within one or more of the following ranges: between about 1 and about 30 (inclusive); between about 1 and about 25; between about 1 and about 20; between about 1 and about 15; between about 1 and about 12; between about 1 and about 10. In certain instances, the number of monomers in series in the oligomer (and the corresponding conjugate) is one of 1, 2, 3, 4, 5, 6, 7, or 8. In additional embodiments, the oligomer (and the corresponding conjugate) contains 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 monomers. In yet further embodiments, the oligomer (and the corresponding conjugate) possesses 21, 22, 23, 24, 25, 26, 27, 28, 29 or 30 monomers in series. Thus, for example, when the water-soluble and non-peptidic polymer includes CH 3 —(OCH 2 CH 2 ) n —, “n” is an integer that can be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 or 30, and can fall within one or more of the following ranges: between about 1 and about 25; between about 1 and about 20; between about 1 and about 15; between about 1 and about 12; between about 1 and about 10. 
     When the water-soluble, non-peptidic oligomer has 1, 2, 3, 4, 5, 6, 7, 8, 9, or monomers, these values correspond to a methoxy end-capped oligo(ethylene oxide) having a molecular weights of about 75, 119, 163, 207, 251, 295, 339, 383, 427, and 471 Daltons, respectively. When the oligomer has 11, 12, 13, 14, or 15 monomers, these values correspond to methoxy end-capped oligo(ethylene oxide) having molecular weights corresponding to about 515, 559, 603, 647, and 691 Daltons, respectively. 
     When the water-soluble, non-peptidic oligomer is attached to the nitrogenous base (in contrast to the step-wise addition of one or more monomers to effectively “grow” the oligomer onto the nitrogenous base), it is preferred that the composition containing an activated form of the water-soluble, non-peptidic oligomer be monodisperse. In those instances, however, where a bimodal composition is employed, the composition will possess a bimodal distribution centering around any two of the above numbers of monomers. For instance, a bimodal oligomer may have any one of the following exemplary combinations of monomer subunits: 1-2, 1-3, 1-4, 1-5, 1-6, 1-7, 1-8, 1-9, 1-10, and so forth; 2-3, 2-4, 2-5, 2-6, 2-7, 2-8, 2-9, 2-10, and so forth; 3-4, 3-5, 3-6, 3-7, 3-8, 3-9, 3-10, and so forth; 4-5, 4-6, 4-7, 4-8, 4-9, 4-10, and so forth; 5-6, 5-7, 5-8, 5-9, 5-10, and so forth; 6-7, 6-8, 6-9, 6-10, and so forth; 7-8, 7-9, 7-10, and so forth; and 8-9, 8-10, and so forth. 
     In some instances, the composition containing an activated form of the water-soluble, non-peptidic oligomer will be trimodal or even tetramodal, possessing a range of monomers units as previously described. Oligomer compositions possessing a well-defined mixture of oligomers (i.e., being bimodal, trimodal, tetramodal, and so forth) can be prepared by mixing purified monodisperse oligomers to obtain a desired profile of oligomers (a mixture of two oligomers differing only in the number of monomers is bimodal; a mixture of three oligomers differing only in the number of monomers is trimodal; a mixture of four oligomers differing only in the number of monomers is tetramodal), or alternatively, can be obtained from column chromatography of a polydisperse oligomer by recovering the “center cut”, to obtain a mixture of oligomers in a desired and defined molecular weight range. 
     It is preferred that the water-soluble, non-peptidic oligomer is obtained from a composition that is preferably unimolecular or monodisperse. That is, the oligomers in the composition possess the same discrete molecular weight value rather than a distribution of molecular weights. Some monodisperse oligomers can be purchased from commercial sources such as those available from Sigma-Aldrich, or alternatively, can be prepared directly from commercially available starting materials such as Sigma-Aldrich. Water-soluble, non-peptidic oligomers can be prepared as described in Chen Y., Baker, G. L., J. Org. Chem., 6870-6873 (1999), WO 02/098949, and U.S. Patent Application Publication 2005/0136031. 
     When present, the spacer moiety (through which the water-soluble, non-peptidic polymer is attached to the nitrogenous base moiety) may be a single bond, a single atom, such as an oxygen atom or a sulfur atom, two atoms, or a number of atoms. A spacer moiety is typically but is not necessarily linear in nature. The spacer moiety, “X,” is hydrolytically stable, and is preferably also enzymatically stable. Preferably, the spacer moiety “X” is one having a chain length of less than about 12 atoms, and preferably less than about 10 atoms, and even more preferably less than about 8 atoms and even more preferably less than about 5 atoms, whereby length is meant the number of atoms in a single chain, not counting substituents. For instance, a urea linkage such as this, R oligomer —NH—(C═O)—NH—R′ drug , is considered to have a chain length of 3 atoms (—NH—C(O)—NH—). In selected embodiments, the linkage does not comprise further spacer groups. 
     In some instances, the spacer moiety “X” comprises an ether, amide, urethane, amine, thioether, urea, or a carbon-carbon bond. Functional groups such as those discussed below, and illustrated in the examples, are typically used for forming the linkages. The spacer moiety may less preferably also comprise (or be adjacent to or flanked by) other atoms, as described further below. 
     More specifically, in selected embodiments, a spacer moiety of the invention, X, may be any of the following: “—” (i.e., a covalent bond, that may be stable or degradable, between the nitrogenous base residue and the water-soluble, non-peptidic oligomer), —O—, —NH—, —S—, —C(O)—, —C(O)O—, —OC(O)—, —CH 2 —C(O)O—, —CH 2 —OC(O)—, —C(O)O—CH 2 —, —OC(O)—CH 2 —, C(O)—NH, NH—C(O)—NH, O—C(O)—NH, —C(S)—, —CH 2 —, —CH 2 —CH 2 —, —CH 2 —CH 2 —CH 2 —, —CH 2 —CH 2 —CH 2 —CH 2 —, —O—CH 2 —, —CH 2 —O—, —O—CH 2 —CH 2 —, —CH 2 —O—CH 2 —, —CH 2 —CH 2 —O—, —O—CH 2 —CH 2 —CH 2 —, —CH 2 —O—CH 2 —CH 2 —, —CH 2 —CH 2 —O—CH 2 —, —CH 2 —CH 2 —CH 2 —O—, —O—CH 2 —CH 2 —CH 2 —CH 2 —, —CH 2 —O—CH 2 —CH 2 -CH 2 —, —CH 2 —CH 2 -O—CH 2 —CH 2 —, —CH 2 —CH 2 —CH 2 —O—CH 2 —, —CH 2 —CH 2 —CH 2 —CH 2 —O—, —C(O)—NH—CH 2 —, —C(O)—NH—CH 2 —CH 2 —, —CH 2 —C(O)—NH—CH 2 —, —CH 2 —CH 2 —C(O)—NH—, —C(O)—NH—CH 2 —CH 2 —CH 2 —, —CH 2 —C(O)—NH—CH 2 —CH 2 —, —CH 2 —CH 2 —C(O)—NH—CH 2 —, —CH 2 —CH 2 —CH 2 —C(O)—NH—, —C(O)—NH—CH 2 —CH 2 —CH 2 —CH 2 —, —CH 2 —C(O)—NH—CH 2 —CH 2 —CH 2 —, —CH 2 —CH 2 —C(O)—NH—CH 2 —CH 2 —, —CH 2 —CH 2 —CH 2 —C(O)—NH—CH 2 —, —CH 2 —CH 2 —CH 2 —C(O)—NH—CH 2 —CH 2 —, —CH 2 —CH 2 —CH 2 —CH 2 —C(O)—NH—NH—C(O)—CH 2 —, —CH 2 —NH—C(O)—CH 2 —, —CH 2 —CH 2 —NH—C(O)—CH 2 —, —NH—C(O)—CH 2 —CH 2 —, —CH 2 —NH—C(O)—CH 2 —CH 2 , —CH 2 —CH 2 —NH—C(O)—CH 2 —CH 2 , —C(O)—NH—CH 2 —, —C(O)—NH—CH 2 —CH 2 —, —O—C(O)—NH—CH 2 —, —O—C(O)—NH—CH 2 —CH 2 —, —NH—CH 2 —, —NH—CH 2 —CH 2 —, —CH 2 —NH—CH 2 —, —CH 2 —CH 2 —NH—CH 2 —, —C(O)—CH 2 —, —C(O)—CH 2 —CH 2 —, —CH 2 —C(O)—CH 2 —, —CH 2 —CH 2 —C(O)—CH 2 —, —CH 2 —CH 2 —C(O)—CH 2 —CH 2 —, —CH 2 —CH 2 —C(O)—, —CH 2 —CH 2 —CH 2 —C(O)—NH—CH 2 —CH 2 —NH—, —CH 2 —CH 2 —CH 2 —C(O)—NH—CH 2 —CH 2 —NH—C(O)—, —CH 2 —CH 2 —CH 2 —C(O)—NH—CH 2 —CH 2 —NH—C(O)—CH 2 —, bivalent cycloalkyl group, —N(R 6 )—, R 6  is H or an organic radical selected from the group consisting of alkyl, substituted alkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, aryl and substituted aryl. Additional spacer moieties include, acylamino, acyl, aryloxy, alkylene bridge containing between 1 and 5 inclusive carbon atoms, alkylamino, dialkylamino having about 2 to 4 inclusive carbon atoms, piperidino, pyrrolidino, N-(lower alkyl)-2-piperidyl, morpholino, 1-piperizinyl, 4-(lower alkyl)-1-piperizinyl, 4-(hydroxyl-lower alkyl)-1-piperizinyl, 4-(methoxy-lower alkyl)-1-piperizinyl, guanidine, ester, carbonate, and phosphate. In some instances, a portion or a functional group of the drug compound may be modified or removed altogether to facilitate attachment of the oligomer. In some instances, it is preferred that X is not an amide, i.e., —CONR— or —RNCO—). 
     For purposes of the present invention, however, a group of atoms is not considered a linkage when it is immediately adjacent to an oligomer segment, and the group of atoms is the same as a monomer of the oligomer such that the group would represent a mere extension of the oligomer chain. 
     The linkage “X” between the water-soluble, non-peptidic oligomer and the small molecule is typically formed by reaction of a functional group on a terminus of the oligomer (or nascent oligomer when it is desired to “grow” the oligomer onto the nitrogenous base) with a corresponding functional group within the nitrogenous base. Illustrative reactions are described briefly below. For example, an amino group on an oligomer may be reacted with a carboxylic acid or an activated carboxylic acid derivative on the small molecule, or vice versa, to produce an amide linkage. Alternatively, reaction of an amine on an oligomer with an activated carbonate (e.g. succinimidyl or benzotriazolyl carbonate) on the drug, or vice versa, forms a carbamate linkage. Reaction of an amine on an oligomer with an isocyanate (R—N═C═O) on a drug, or vice versa, forms a urea linkage (R—NH—(C═O)—NH—R′). Further, reaction of an alcohol (alkoxide) group on an oligomer with an alkyl halide, or halide group within a drug, or vice versa, forms an ether linkage. In yet another coupling approach, a small molecule having an aldehyde function is coupled to an oligomer amino group by reductive amination, resulting in formation of a secondary amine linkage between the oligomer and the small molecule. 
     A particularly preferred water-soluble, non-peptidic oligomer is an oligomer bearing an aldehyde functional group. In this regard, the oligomer will have the following structure: CH 3 O—(CH 2 —CH 2 —O) n —(CH 2 ) p —C(O)H, wherein (n) is one of 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10 and (p) is one of 1, 2, 3, 4, 5, 6 and 7. Preferred (n) values include 3, 5 and 7 and preferred (p) values 2, 3 and 4. 
     The termini of the water-soluble, non-peptidic oligomer not bearing a functional group may be capped to render it unreactive. When the oligomer includes a further functional group at a terminus other than that intended for formation of a conjugate, that group is either selected such that it is unreactive under the conditions of formation of the linkage “X,” or it is protected during the formation of the linkage “X.” 
     As stated above, the water-soluble, non-peptidic oligomer includes at least one functional group prior to conjugation. The functional group typically comprises an electrophilic or nucleophilic group for covalent attachment to a small molecule, depending upon the reactive group contained within or introduced into the small molecule. Examples of nucleophilic groups that may be present in either the oligomer or the small molecule include hydroxyl, amine, hydrazine (—NHNH 2 ), hydrazide (—C(O)NHNH 2 ), and thiol. Preferred nucleophiles include amine, hydrazine, hydrazide, and thiol, particularly amine. Most small molecule drugs for covalent attachment to an oligomer will possess a free hydroxyl, amino, thio, aldehyde, ketone, or carboxyl group. 
     Examples of electrophilic functional groups that may be present in either the oligomer or the small molecule include carboxylic acid, carboxylic ester, particularly imide esters, orthoester, carbonate, isocyanate, isothiocyanate, aldehyde, ketone, thione, alkenyl, acrylate, methacrylate, acrylamide, sulfone, maleimide, disulfide, iodo, epoxy, sulfonate, thiosulfonate, silane, alkoxysilane, and halosilane. More specific examples of these groups include succinimidyl ester or carbonate, imidazoyl ester or carbonate, benzotriazole ester or carbonate, vinyl sulfone, chloroethylsulfone, vinylpyridine, pyridyl disulfide, iodoacetamide, glyoxal, dione, mesylate, tosylate, and tresylate (2,2,2-trifluoroethanesulfonate). 
     Also included are sulfur analogs of several of these groups, such as thione, thione hydrate, thioketal, 2-thiazolidine thione, etc., as well as hydrates or protected derivatives of any of the above moieties (e.g. aldehyde hydrate, hemiacetal, acetal, ketone hydrate, hemiketal, ketal, thioketal, thioacetal). 
     An “activated derivative” of a carboxylic acid refers to a carboxylic acid derivative that reacts readily with nucleophiles, generally much more readily than the underivatized carboxylic acid. Activated carboxylic acids include, for example, acid halides (such as acid chlorides), anhydrides, carbonates, and esters. Such esters include imide esters, of the general form —(CO)O—N[(CO)—] 2 ; for example, N-hydroxysuccinimidyl (NHS) esters or N-hydroxyphthalimidyl esters. Also preferred are imidazolyl esters and benzotriazole esters. Particularly preferred are activated propionic acid or butanoic acid esters, as described in co-owned U.S. Pat. No. 5,672,662. These include groups of the form —(CH 2 ) 2-3 C(═O)O-Q, where Q is preferably selected from N-succinimide, N-sulfosuccinimide, N-phthalimide, N-glutarimide, N-tetrahydrophthalimide, N-norbornene-2,3-dicarboximide, benzotriazole, 7-azabenzotriazole, and imidazole. 
     Other preferred electrophilic groups include succinimidyl carbonate, maleimide, benzotriazole carbonate, glycidyl ether, imidazoyl carbonate, p-nitrophenyl carbonate, acrylate, tresylate, aldehyde, and orthopyridyl disulfide. 
     These electrophilic groups are subject to reaction with nucleophiles, e.g., hydroxy, thio, or amino groups, to produce various bond types. Preferred for the present invention are reactions which favor formation of a hydrolytically stable linkage. For example, carboxylic acids and activated derivatives thereof, which include orthoesters, succinimidyl esters, imidazolyl esters, and benzotriazole esters, react with the above types of nucleophiles to form esters, thioesters, and amides, respectively, of which amides are the most hydrolytically stable. Carbonates, including succinimidyl, imidazolyl, and benzotriazole carbonates, react with amino groups to form carbamates. Isocyanates (R—N═C═O) react with hydroxyl or amino groups to form, respectively, carbamate (RNH—C(O)—OR′) or urea (RNH—C(O)—NHR′) linkages. Aldehydes, ketones, glyoxals, diones and their hydrates or alcohol adducts (i.e., aldehyde hydrate, hemiacetal, acetal, ketone hydrate, hemiketal, and ketal) are preferably reacted with amines, followed by reduction of the resulting imine, if desired, to provide an amine linkage (reductive amination). 
     Several of the electrophilic functional groups include electrophilic double bonds to which nucleophilic groups, such as thiols, can be added, to form, for example, thioether bonds. These groups include maleimides, vinyl sulfones, vinyl pyridine, acrylates, methacrylates, and acrylamides. Other groups comprise leaving groups that can be displaced by a nucleophile; these include chloroethyl sulfone, pyridyl disulfides (which include a cleavable S—S bond), iodoacetamide, mesylate, tosylate, thiosulfonate, and tresylate. Epoxides react by ring opening by a nucleophile, to form, for example, an ether or amine bond. Reactions involving complementary reactive groups such as those noted above on the oligomer and the small molecule are utilized to prepare the conjugates of the invention. 
     In some instances the nitrogenous base may not have a functional group suited for conjugation. In this instance, it is possible to modify (or “functionalize”) the “original” nitrogenous base so that it does have a functional group suited for conjugation. For example, if the nitrogenous base has an amide group, but an amine group is desired, it is possible to modify the amide group to an amine group by way of a Hofmann rearrangement, Curtius rearrangement (once the amide is converted to an azide) or Lossen rearrangement (once amide is concerted to hydroxamide followed by treatment with tolyene-2-sulfonyl chloride/base). 
     It is possible to prepare a conjugate of small molecule nitrogenous base bearing a carboxyl group wherein the carboxyl group-bearing small molecule nitrogenous base is coupled to an amino-terminated oligomeric ethylene glycol, to provide a conjugate having an amide group covalently linking the small molecule nitrogenous base to the oligomer. This can be performed, for example, by combining the carboxyl group-bearing small molecule nitrogenous base with the amino-terminated oligomeric ethylene glycol in the presence of a coupling reagent, (such as dicyclohexylcarbodiimide or “DCC”) in an anhydrous organic solvent. 
     Further, it is possible to prepare a conjugate of a small molecule nitrogenous base bearing a hydroxyl group wherein the hydroxyl group-bearing small molecule nitrogenous base is coupled to an oligomeric ethylene glycol halide to result in an ether (—O—) linked small molecule conjugate. This can be performed, for example, by using sodium hydride to deprotonate the hydroxyl group followed by reaction with a halide-terminated oligomeric ethylene glycol. 
     Further, it is possible to prepare a conjugate of a small molecule nitrogenous base moiety bearing a hydroxyl group wherein the hydroxyl group-bearing small molecule nitrogenous base moiety is coupled to an oligomeric ethylene glycol bearing an haloformate group [e.g., CH 3 (OCH 2 CH 2 ) n OC(O)-halo, where halo is chloro, bromo, iodo] to result in a carbonate [—O—C(O)—O—] linked small molecule conjugate. This can be performed, for example, by combining a nitrogenous base moiety and an oligomeric ethylene glycol bearing a haloformate group in the presence of a nucleophilic catalyst (such as 4-dimethylaminopyridine or “DMAP”) to thereby result in the corresponding carbonate-linked conjugate. 
     In another example, it is possible to prepare a conjugate of a small molecule nitrogenous base bearing a ketone group by first reducing the ketone group to form the corresponding hydroxyl group. Thereafter, the small molecule nitrogenous base now bearing a hydroxyl group can be coupled as described herein. 
     In still another instance, it is possible to prepare a conjugate of a small molecule nitrogenous base bearing an amine group. In one approach, the amine group-bearing small molecule nitrogenous base and an aldehyde-bearing oligomer are dissolved in a suitable buffer after which a suitable reducing agent (e.g., NaCNBH 3 ) is added. Following reduction, the result is an amine linkage formed between the amine group of the amine group-containing small molecule nitrogenous base and the carbonyl carbon of the aldehyde-bearing oligomer. 
     In another approach for preparing a conjugate of a small molecule nitrogenous base bearing an amine group, a carboxylic acid-bearing oligomer and the amine group-bearing small molecule nitrogenous base are combined, typically in the presence of a coupling reagent (e.g., DCC). The result is an amide linkage formed between the amine group of the amine group-containing small molecule nitrogenous base and the carbonyl of the carboxylic acid-bearing oligomer. 
     While it is believed that the full scope of the conjugates disclosed herein behave as described, an optimally sized oligomer can be identified as follows. 
     First, an oligomer obtained from a monodisperse or bimodal water soluble oligomer is conjugated to the small molecule drug. Preferably, the drug is orally bioavailable, and on its own, exhibits a non-negligible blood-brain barrier crossing rate. Next, the ability of the conjugate to cross the blood-brain barrier is determined using an appropriate model and compared to that of the unmodified parent drug. If the results are favorable, that is to say, if, for example, the rate of crossing is significantly reduced, then the bioactivity of conjugate is further evaluated. Preferably, the compounds according to the invention maintain a significant degree of bioactivity relative to the parent drug, i.e., greater than about 30% of the bioactivity of the parent drug, or even more preferably, greater than about 50% of the bioactivity of the parent drug. 
     The above steps are repeated one or more times using oligomers of the same monomer type but having a different number of subunits and the results compared. 
     For each conjugate whose ability to cross the blood-brain barrier is reduced in comparison to the non-conjugated small molecule drug, its oral bioavailability is then assessed. Based upon these results, that is to say, based upon the comparison of conjugates of oligomers of varying size to a given small molecule at a given position or location within the small molecule, it is possible to determine the size of the oligomer most effective in providing a conjugate having an optimal balance between reduction in biological membrane crossing, oral bioavailability, and bioactivity. The small size of the oligomers makes such screenings feasible and allows one to effectively tailor the properties of the resulting conjugate. By making small, incremental changes in oligomer size and utilizing an experimental design approach, one can effectively identify a conjugate having a favorable balance of reduction in biological membrane crossing rate, bioactivity, and oral bioavailability. In some instances, attachment of an oligomer as described herein is effective to actually increase oral bioavailability of the drug. 
     For example, one of ordinary skill in the art, using routine experimentation, can determine a best suited molecular size and linkage for improving oral bioavailability by first preparing a series of oligomers with different weights and functional groups and then obtaining the necessary clearance profiles by administering the conjugates to a patient and taking periodic blood and/or urine sampling. Once a series of clearance profiles have been obtained for each tested conjugate, a suitable conjugate can be identified. 
     Animal models (rodents and dogs) can also be used to study oral drug transport. In addition, non-in vivo methods include rodent everted gut excised tissue and Caco-2 cell monolayer tissue-culture models. These models are useful in predicting oral drug bioavailability. 
     To determine whether the nitrogenous base or the conjugate of a nitrogenous base and a water-soluble non-peptidic polymer has activity as a nitrogenous base therapeutic, it is possible to test such a compound. 
     HSV Assay: A virus-induced cytopathic effects (CPE)-inhibition assay procedure using Promega&#39;s Cell Titer Aqueous One Solution (MTS, metabolic dye) is employed to evaluate compounds for antiviral activity against herpes simplex virus type 1 (strain HF) or herpes simplex virus type 2 (strain MS) in Vero cells. Vero cells are pre-grown in 96-well tissue culture plates using Dulbecco&#39;s Modified Eagle&#39;s Medium (DMEM) supplemented with 10% heat-inactivated fetal bovine serum (FBS), L Glutamine, penicillin, and streptomycin. Antiviral assays are performed at a FBS concentration of 2% and are designed to test six half-log dilutions of each compound in triplicate against the challenge virus in microtiter plate wells containing host cell monolayers. To each of the replicate cell cultures is added 50 μl of the test drug solution and 50 μl of virus suspension. Cell controls containing medium alone, virus-infected controls containing medium and virus, drug cytotoxicity controls containing medium and each drug concentration, reagent controls containing culture medium only (no cells), and drug colorimetric controls containing drug and medium (no cells) are run simultaneously with the test samples. The plates are incubated at 37° C. in a humidified atmosphere containing 5% CO2 until maximum CPE is observed in the untreated virus control cultures (Day 5). 
     MTS Staining for PBMC Viability to Measure Cytotoxicity: CPE inhibition is determined by a dye (MTS) uptake procedure. This method measures cell viability and is based on the reduction of the tetrazolium-based MTS by mitochondrial enzymes of viable host cells to MTS formazan. MTS (10 μl) is added to each of the plate wells. The plates are incubated at 37° C. for 4 hours. The purple color of the MTS formazan is then measured spectrophotometrically at 490/650 nm with a Molecular Devices Vmax or SpectraMaxPlus plate reader. The optical density (OD) value of each culture is a function of the amount of formazan produced, which is proportional to the number of viable cells. 
     Data Analysis: Using an in-house computer program, 1050 (50%, inhibition of virus replication), TC50 (50% cytotoxicity) and a therapeutic index (TI, TC50/IC50) are provided. Raw data for both antiviral activity and toxicity with a graphical representation of the data are provided in a printout summarizing the individual compound activity. Acyclovir was evaluated in parallel as a relevant positive control compound in the anti-HSV assay. 
     Similarly, conjugates of the present invention may be tested for HIV-1 antiviral cytoprotection assay using CEM-SS cells and the laboratory-adapted HIV-1 IIIB  strain. The activity may also be tested using other viruses and suitable cell lines. 
     Conjugates of the invention may also be tested for anti-neoplastic or anti-cancer activity using the known assays. For example, the conjugates may be tested for inhibition of PMA-induced cell growth, PMA-induced colony formation in soft agar, inhibition of metastatic tumor growth in the mouse models of neoplasm and others. 
     The present invention also includes pharmaceutical preparations comprising a conjugate as provided herein in combination with a pharmaceutical excipient. Generally, the conjugate itself will be in a solid form (e.g., a precipitate), which can be combined with a suitable pharmaceutical excipient that can be in either solid or liquid form. 
     Exemplary excipients include, without limitation, those selected from the group consisting of carbohydrates, inorganic salts, antimicrobial agents, antioxidants, surfactants, buffers, acids, bases, and combinations thereof. 
     A carbohydrate such as a sugar, a derivatized sugar such as an alditol, aldonic acid, an esterified sugar, and/or a sugar polymer may be present as an excipient. Specific carbohydrate excipients include, for example: monosaccharides, such as fructose, maltose, galactose, glucose, D-mannose, sorbose, and the like; disaccharides, such as lactose, sucrose, trehalose, cellobiose, and the like; polysaccharides, such as raffinose, melezitose, maltodextrins, dextrans, starches, and the like; and alditols, such as mannitol, maltitol, lactitol, xylitol, sorbitol, myoinositol, and the like. 
     The excipient can also include an inorganic salt or buffer such as citric acid, sodium chloride, potassium chloride, sodium sulfate, potassium nitrate, sodium phosphate monobasic, sodium phosphate dibasic, and combinations thereof. 
     The preparation may also include an antimicrobial agent for preventing or deterring microbial growth. Nonlimiting examples of antimicrobial agents suitable for the present invention include benzalkonium chloride, benzethonium chloride, benzyl alcohol, cetylpyridinium chloride, chlorobutanol, phenol, phenylethyl alcohol, phenylmercuric nitrate, thimersol, and combinations thereof. 
     An antioxidant can be present in the preparation as well. Antioxidants are used to prevent oxidation, thereby preventing the deterioration of the conjugate or other components of the preparation. Suitable antioxidants for use in the present invention include, for example, ascorbyl palmitate, butylated hydroxyanisole, butylated hydroxytoluene, hypophosphorous acid, monothioglycerol, propyl gallate, sodium bisulfite, sodium formaldehyde sulfoxylate, sodium metabisulfite, and combinations thereof. 
     A surfactant may be present as an excipient. Exemplary surfactants include: polysorbates, such as “Tween 20” and “Tween 80,” and pluronics such as F68 and F88 (both of which are available from BASF, Mount Olive, N.J.); sorbitan esters; lipids, such as phospholipids such as lecithin and other phosphatidylcholines, phosphatidylethanolamines, fatty acids and fatty esters; steroids, such as cholesterol; and chelating agents, such as EDTA, zinc and other such suitable cations. 
     Pharmaceutically acceptable acids or bases may be present as an excipient in the preparation. Nonlimiting examples of acids that can be used include those acids selected from the group consisting of hydrochloric acid, acetic acid, phosphoric acid, citric acid, malic acid, lactic acid, formic acid, trichloroacetic acid, nitric acid, perchloric acid, phosphoric acid, sulfuric acid, fumaric acid, and combinations thereof. Examples of suitable bases include, without limitation, bases selected from the group consisting of sodium hydroxide, sodium acetate, ammonium hydroxide, potassium hydroxide, ammonium acetate, potassium acetate, sodium phosphate, potassium phosphate, sodium citrate, sodium formate, sodium sulfate, potassium sulfate, potassium fumerate, and combinations thereof. 
     The amount of the conjugate in the composition will vary depending on a number of factors, but will optimally be a therapeutically effective dose when the composition is stored in a unit dose container. A therapeutically effective dose can be determined experimentally by repeated administration of increasing amounts of the conjugate in order to determine which amount produces a clinically desired endpoint. 
     The amount of any individual excipient in the composition will vary depending on the activity of the excipient and particular needs of the composition. Typically, the optimal amount of any individual excipient is determined through routine experimentation, i.e., by preparing compositions containing varying amounts of the excipient (ranging from low to high), examining the stability and other parameters, and then determining the range at which optimal performance is attained with no significant adverse effects. 
     Generally, however, excipients will be present in the composition in an amount of about 1% to about 99% by weight, preferably from about 5%-98% by weight, more preferably from about 15-95% by weight of the excipient, with concentrations less than 30% by weight most preferred. 
     These foregoing pharmaceutical excipients along with other excipients and general teachings regarding pharmaceutical compositions are described in “Remington: The Science &amp; Practice of Pharmacy”, 19 th  ed., Williams &amp; Williams, (1995), the “Physician&#39;s Desk Reference”, 52 nd  ed., Medical Economics, Montvale, N.J. (1998), and Kibbe, A. H., Handbook of Pharmaceutical Excipients, 3 rd  Edition, American Pharmaceutical Association, Washington, D.C., 2000. 
     The pharmaceutical compositions can take any number of forms and the invention is not limited in this regard. Exemplary preparations are most preferably in a form suitable for oral administration such as a tablet, caplet, capsule, gel cap, troche, dispersion, suspension, solution, elixir, syrup, lozenge, transdermal patch, spray, suppository, and powder. 
     Oral dosage forms are preferred for those conjugates that are orally active, and include tablets, caplets, capsules, gel caps, suspensions, solutions, elixirs, and syrups, and can also comprise a plurality of granules, beads, powders or pellets that are optionally encapsulated. Such dosage forms are prepared using conventional methods known to those in the field of pharmaceutical formulation and described in the pertinent texts. 
     Tablets and caplets, for example, can be manufactured using standard tablet processing procedures and equipment. Direct compression and granulation techniques are preferred when preparing tablets or caplets containing the conjugates described herein. In addition to the conjugate, the tablets and caplets will generally contain inactive, pharmaceutically acceptable carrier materials such as binders, lubricants, disintegrants, fillers, stabilizers, surfactants, coloring agents, flow agents, and the like. Binders are used to impart cohesive qualities to a tablet, and thus ensure that the tablet remains intact. Suitable binder materials include, but are not limited to, starch (including corn starch and pregelatinized starch), gelatin, sugars (including sucrose, glucose, dextrose and lactose), polyethylene glycol, waxes, and natural and synthetic gums, e.g., acacia sodium alginate, polyvinylpyrrolidone, cellulosic polymers (including hydroxypropyl cellulose, hydroxypropyl methylcellulose, methyl cellulose, microcrystalline cellulose, ethyl cellulose, hydroxyethylcellulose, and the like), and Veegum. Lubricants are used to facilitate tablet manufacture, promoting powder flow and preventing particle capping (i.e., particle breakage) when pressure is relieved. Useful lubricants are magnesium stearate, calcium stearate, and stearic acid. Disintegrants are used to facilitate disintegration of the tablet, and are generally starches, clays, celluloses, algins, gums, or crosslinked polymers. Fillers include, for example, materials such as silicon dioxide, titanium dioxide, alumina, talc, kaolin, powdered cellulose, and microcrystalline cellulose, as well as soluble materials such as mannitol, urea, sucrose, lactose, dextrose, sodium chloride, and sorbitol. Stabilizers, as well known in the art, are used to inhibit or retard drug decomposition reactions that include, by way of example, oxidative reactions. 
     Capsules are also preferred oral dosage forms, in which case the conjugate-containing composition can be encapsulated in the form of a liquid or gel (e.g., in the case of a gel cap) or solid (including particulates such as granules, beads, powders or pellets). Suitable capsules include hard and soft capsules, and are generally made of gelatin, starch, or a cellulosic material. Two-piece hard gelatin capsules are preferably sealed, such as with gelatin bands or the like. 
     Included are parenteral formulations in the substantially dry form (typically as a lyophilizate or precipitate, which can be in the form of a powder or cake), as well as formulations prepared for injection, which are typically liquid and requires the step of reconstituting the dry form of parenteral formulation. Examples of suitable diluents for reconstituting solid compositions prior to injection include bacteriostatic water for injection, dextrose 5% in water, phosphate-buffered saline, Ringer&#39;s solution, saline, sterile water, deionized water, and combinations thereof. 
     In some cases, compositions intended for parenteral administration can take the form of nonaqueous solutions, suspensions, or emulsions, each typically being sterile. Examples of nonaqueous solvents or vehicles are propylene glycol, polyethylene glycol, vegetable oils, such as olive oil and corn oil, gelatin, and injectable organic esters such as ethyl oleate. 
     The parenteral formulations described herein can also contain adjuvants such as preserving, wetting, emulsifying, and dispersing agents. The formulations are rendered sterile by incorporation of a sterilizing agent, filtration through a bacteria-retaining filter, irradiation, or heat. 
     The conjugate can also be administered through the skin using conventional transdermal patch or other transdermal delivery system, wherein the conjugate is contained within a laminated structure that serves as a drug delivery device to be affixed to the skin. In such a structure, the conjugate is contained in a layer, or “reservoir,” underlying an upper backing layer. The laminated structure can contain a single reservoir, or it can contain multiple reservoirs. 
     The conjugate can also be formulated into a suppository for rectal administration. With respect to suppositories, the conjugate is mixed with a suppository base material which is (e.g., an excipient that remains solid at room temperature but softens, melts or dissolves at body temperature) such as coca butter (theobroma oil), polyethylene glycols, glycerinated gelatin, fatty acids, and combinations thereof. Suppositories can be prepared by, for example, performing the following steps (not necessarily in the order presented): melting the suppository base material to form a melt; incorporating the conjugate (either before or after melting of the suppository base material); pouring the melt into a mold; cooling the melt (e.g., placing the melt-containing mold in a room temperature environment) to thereby form suppositories; and removing the suppositories from the mold. 
     In some embodiments of the invention, the compositions comprising the conjugates may further be incorporated into a suitable delivery vehicle. Such delivery vehicles may provide controlled and/or continuous release of the conjugates and may also serve as a targeting moiety. Non-limiting examples of delivery vehicles include, adjuvants, synthetic adjuvants, microcapsules, microparticles, liposomes, and yeast cell wall particles. Yeast cells walls may be variously processed to selectively remove protein component, glucan, or mannan layers, and are referred to as whole glucan particles (WGP), yeast beta-glucan mannan particles (YGMP), yeast glucan particles (YGP), Rhodotorula yeast cell particles (YCP). Yeast cells such as  S. cerevisiae  and  Rhodotorula  sp. are preferred; however, any yeast cell may be used. These yeast cells exhibit different properties in terms of hydrodynamic volume and also differ in the target organ where they may release their contents. The methods of manufacture and characterization of these particles are described in U.S. Pat. Nos. 5,741,495; 4,810,646; 4,992,540; 5,028,703; 5,607,677, and US Patent Applications Nos. 2005/0281781, and 2008/0044438. 
     The invention also provides a method for administering a conjugate as provided herein to a patient suffering from a condition that is responsive to treatment with the conjugate. The method comprises administering, generally orally, a therapeutically effective amount of the conjugate (preferably provided as part of a pharmaceutical preparation). Other modes of administration are also contemplated, such as pulmonary, nasal, buccal, rectal, sublingual, transdermal, and parenteral. As used herein, the term “parenteral” includes subcutaneous, intravenous, intra-arterial, intraperitoneal, intracardiac, intrathecal, and intramuscular injection, as well as infusion injections. 
     In instances where parenteral administration is utilized, it may be necessary to employ somewhat bigger oligomers than those described previously, with molecular weights ranging from about 500 to 30K Daltons (e.g., having molecular weights of about 500, 1000, 2000, 2500, 3000, 5000, 7500, 10000, 15000, 20000, 25000, 30000 or even more). 
     The method of administering may be used to treat any condition that can be remedied or prevented by administration of the particular conjugate. Those of ordinary skill in the art appreciate which conditions a specific conjugate can effectively treat. The actual dose to be administered will vary depend upon the age, weight, and general condition of the subject as well as the severity of the condition being treated, the judgment of the health care professional, and conjugate being administered. Therapeutically effective amounts are known to those skilled in the art and/or are described in the pertinent reference texts and literature. Generally, a therapeutically effective amount will range from about 0.001 mg to 1000 mg, preferably in doses from 0.01 mg/day to 750 mg/day, and more preferably in doses from 0.10 mg/day to 500 mg/day. 
     The unit dosage of any given conjugate (again, preferably provided as part of a pharmaceutical preparation) can be administered in a variety of dosing schedules depending on the judgment of the clinician, needs of the patient, and so forth. The specific dosing schedule will be known by those of ordinary skill in the art or can be determined experimentally using routine methods. Exemplary dosing schedules include, without limitation, administration five times a day, four times a day, three times a day, twice daily, once daily, three times weekly, twice weekly, once weekly, twice monthly, once monthly, and any combination thereof. Once the clinical endpoint has been achieved, dosing of the composition is halted. 
     All articles, books, patents, patent publications and other publications referenced herein are incorporated by reference in their entireties. In the event of an inconsistency between the teachings of this specification and the art incorporated by reference, the meaning of the teachings in this specification shall prevail. 
     EXPERIMENTAL 
     It is to be understood that while the invention has been described in conjunction with certain preferred and specific embodiments, the foregoing description as well as the examples that follow are intended to illustrate and not limit the scope of the invention. Other aspects, advantages and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains. 
     All non-PEG chemical reagents referred to in the appended examples are commercially available unless otherwise indicated. The preparation of PEG-mers is described in, for example, U.S. Patent Application Publication No. 2005/0136031. 
     All  1 H NMR (nuclear magnetic resonance) data was generated by an NMR spectrometer. A list of certain compounds as well as the source of the compounds is provided below. 
     Example 1 
     Synthesis of PEG-Acyclovir Monophosphate Conjugates 
     
       
         
         
             
             
         
       
     
     Acyclovir is mixed with benzoyl chloride and the solution is stirred for three hours at room temperature. The reaction mixture is concentrated to an oily residue, which is dissolved in a solution of triethylamine and methanol. This solution is stirred for 1.5 hours at room temperature. The product is purified by flash chromatography. 
     
       
         
         
             
             
         
       
     
     Benzoyl—acyclovir and DIPEA are dissolved in anhydrous DCM. At room temperature, 2-cyanoethyl diisopropylchlorophosphoramidite is added to the solution dropwise. The solution is stirred at room temperature for 30 minutes and then washed with saturated NaHCO 3 . The mixed solution is extracted with DCM. The organic phases are combined and dried with anhydrous Na 2 SO 4 . After filtering off solid, the solvent is removed under reduced pressure and the resulting residue is used directly for next step without further purification. 
     
       
         
         
             
             
         
       
     
     2-Cyanoethyldiisopropylchloro-phosphoramidite and (ethylene glycol) n  monomethyl ether (“n” may be from 1 to 30), are dissolved in anhydrous acetonitrile. At room temperature, tetrazole solution is added to the reaction solution. The reaction solution is stirred for 4 hours at room temperature. Iodine solution is added. After 10 minutes, Na 2 S 2 O 3  solution is added. After 5 minutes, the mixed solution is extracted with DCM. The organic phases are combined and dried with anhydrous Na 2 SO 4 . After filtering off solid, the solvent is removed under reduced pressure and the resulting residue was subjected to flash chromatography (e.g., acetone/EtOAc=10%˜50%) to obtain PEG-benzoyl-acyclovir monophosphate conjugate. 
     
       
         
         
             
             
         
       
     
     At room temperature, ammonia in methanol is added to the mPEG n -benzoyl-acyclovir monophosphate conjugate solution. The reaction solution is stirred at room temperature for 18 hours. The solvent is evaporated under reduced pressure. The resulting residue is dissolved in methanol. HCl in ethyl ether (1 N) is added to the solution. The solvent is removed under reduced pressure and the crude residue is subjected to flash chromatography. 
     Example 2 
     Synthesis of PEG-O-Acyclovir Conjugates Via a Stable Ether Linkage 
     
       
         
         
             
             
         
       
     
     Benzoylation of acyclovir is described above. Benzoyl-acyclovir is dissolved in a mixture of acetone and toluene. Potassium carbonate is added, followed by addition of mPEG 3 -Br at room temperature. The resulting mixture is stirred at room temperature for ˜27.5 hours. More potassium carbonate is added. The mixture is heated with CEM microwave such that 60° C. for 20 minutes is achieved, and then such that 100° C. for 30 minutes is achieved. DMF is added. The mixture is heated with microwave at 60° C. for 20 minutes, at 100° C. for 30 minutes. The reaction is concentrated to remove the organic solvents, the residue is mixed with water, extracted with dichloromethane. The combined organic solution is washed with brine, dried over Na 2 SO 4 , and concentrated. The crude product is checked with HPLC and LC-MS. The residue is mixed again with water, adjusted the pH to 2.3 with 1N HCl, and washed with dichloromethane. The aqueous solution is adjusted to pH 10.4, extracted with dichloromethane. The combined organic solution is washed with brine, dried over Na 2 SO 4 , and concentrated. The residue is purified by flash column chromatography resulting in the desired product. The acyclovir conjugate is deprotected using HCl in methanol. 
     Example 3 
     Synthesis of PEG-O-Acyclovir Conjugates Via a Hydrolyzable linkage 
     
       
         
         
             
             
         
       
     
     Benzoylation of acyclovir is described above. Using mPEG n -O-Linker-X— as a reagent in the scheme described above, synthesis of PEG-O-acyclovir conjugates with a hydrolysable linkage is achieved. 
     Example 4 
     Synthesis of PEG-N-Acyclovir Conjugates 
     
       
         
         
             
             
         
       
     
     Acyclovir is dissolved in THF, and NaH is added to the solution. The mixture is stirred for 5 min. before mPEG n -Br [n=1-7] is added. The resulting mixture is stirred at room temperature for 14 h. The solids are removed and dichloromethane is added. The organic phase is washed with H 2 O, dried over Na 2 SO 4  and solvent is removed under reduced pressure. The crude product is purified by flash column chromatography.