NOVEL ANTIVIRAL COMPOUNDS

Abstract: Novel compounds are disclosed, useful as antiviral agents.

Description: PATENT NOVEL ANTIVIRAL COMPOUNDS Cross Reference to Related Applications This application claims the benefit of United States Provisional Application Serial Numbers 60/088,448, filed June 8, 1998, and 60/093,072, filed July 16, 1998, both of which are incorporated by reference in their entirety. Field of the Invention This invention relates to novel therapeutic compounds that act as antiviral agents by virtue of their ability to inhibit viral DNA polymerase, thus hindering the replicative cycle of DNA viruses. More particularly, the invention relates to novel antiviral compounds that inhibit viral DNA polymerase by acting as bivalent agents. The bivalent agents of the invention comprise two different ligands covalently connected by a linker, wherein the first ligand in its monovalent (i.e. unlinked) state is a competitive inhibitor of the deoxynucleotide triphosphate (dNTP) substrate, and the second ligand is a non- competitive inhibitor of the dNTP substrate. The manner in which the ligands are linked is such that the bivalent agents so constructed demonstrate an increased biological and/or therapeutic effect as compared to either of the unlinked ligands, separately or as a simple mixture. The compounds of the invention are particularly useful in mammals for treating disease states that are mediated by DNA viruses. Accordingly, the invention also relates to pharmaceutical compositions comprising a pharmaceutically acceptable excipient and an effective amount of a compound of the invention, and to methods of using such compounds and pharmaceutical compositions containing them for the treatment of such conditions. Still further, the invention relates to methods of preparing such compounds. Background DNA viruses, a group of viruses including heφes simplex virus (HSV), varicella zoster virus (VZV), cytomegaloviruses (CMV), herpetic encephalitis virus, poxviruses, hepadanaviruses, adenoviruses and papilloma viruses, are the etiological agents in diseases such heφes, chicken pox, shingles, monocutaneous heφes, heφes encephalitis, heφes zoster, smallpox, conjunctivitis, sore throat, hepatitis B, warts, and mononucleosis. These common disorders cause significant morbidity and mortality in human populations. DNA viruses are structurally related and consist of double-stranded DNA enclosed in a proteinaceous capsid. These viruses infect host cells by attaching to the membrane, shedding their capsids, and injecting their DNA into the cells. The virus then uses host cellular mechanisms to integrate the viral DNA into cellular DNA. The integrated viral DNA acts as a template for the synthesis of proteins involved with viral replication, including enzymes known as DNA polymerases. Viral DNA polymerase is an enzyme that catalyzes the replication of the viral DNA. Inhibition of the activity of viral DNA polymerase results in premature termination of the viral DNA replication strand and consequently, the production of defective, non-infectious, viral particles. Thus, viral DNA polymerase has been used as a target for the treatment of heφes viruses infections. Acyclovir, valacylcovir, cidofovir, penciclovir, ganciclovir, penicilovir, and famciclovir, are commercially available inhibitors of DNA polymerases. These nucleoside analogs share a common mechanism of action. Briefly, infection of the cells by heφes viruses stimulates the production of virally encoded phosphorylases that phosphorylate these drugs. The monophosphorylated drugs are further phosphorylated by cellular phosphorylases to the triphosphate stage. The triphosphorylated drugs, which are dNTP analogs, are incoφorated into the replicating viral DNA. The viral DNA polymerase is unable to continue the elongation of this adulterated DNA and viral replication stops. Other viral DNA polymerase inhibitors such idoxuridine, trifluorothymidine and adenine-arabinoside are also nucleoside analogues. They are mechanistically distinct from the drugs listed above in that they are phosphorylated by cellular kinases. They also inhibit DNA polymerases resulting in chain termination. The current viral DNA polymerase inhibitors have numerous defects. For example the side effects of acyclovir, valacylcovir, cidofovir, penciclovir, ganciclovir, penicilovir, and famciclovir include neutropenia, thrombocytopenia, hepatopathology, headaches, rash, GI disturbances, phlebitis, nausea, diarrhea, bone marrow suppression, renal insufficiency, nephrotoxicity, hypocalcemia, vomiting and seizure. Idoxuridine and trifluorothymidine are incoφorated into both viral and cellular DNA and, therefore interfere with both viral and cellular replication. Side effects of Idoxuridine include local irritation and discomfort. Trifluorothymidine also has side effects including edema, burning keratopathy and occasionally allergic reactions. Foscarnet (phosphonoformate) is a pyrophosphate analogue that inhibits DNA polymerases by complexing with the enzyme at its pyrophosphate-binding site. This prevents further cleavage of the pyrophosphate from substrate nucleotide triphosphates, which is required for extension of the DNA chain. Treatment with foscarnet, while effective, can lead to renal insufficiency, hypocalcemia, nausea, vomiting and seizures. In addition, foscarnet has low selectivity for viral DNA polymerases. Related Disclosures Synthesis and antiviral activity of 1 -O-octadecyl 2-O-alkyl-sn-glycero 3-foscarnet conjugates in human cytomegalovirus-infected cells. Synthesis and antiviral evaluation of S ATE-foscarnet prodrugs and new foscarnet- AZT conjugates, Meier, C, et al. 1998. Antiviral Chem. Chemother. 9(1):41). Synthesis and antiviral activity of new carbonylphosphonate 2',3',-dideoxy 3'- thiacytidine conjugates.1994. Charvet, AS., et al. Antiviral Res. 25(2): 161). The disclosures of these and other documents referred to throughout this application are incoφorated herein by reference. Summary of the Invention This invention addresses the above needs by providing novel bivalent agents. Accordingly, in one aspect, the present invention relates to novel bivalent agents: wherein a bivalent agent is a compound represented by Formula I: L'-X-L 2 Formula I in which: L 1 is a ligand that is a competitive inhibitor of deoxynucleotide triphosphate incoφoration by DNA polymerase; L 2 is a ligand that is a non-competitive inhibitor of deoxynucleotide triphosphate incoφoration by DNA polymerase; and X is a linker, or a salt thereof; In a second aspect, the invention relates to a method of treatment of mammals having a disease state that is treatable by an antiviral agent, comprising administering a therapeutically effective amount of a compound of Formula I thereto; In a third aspect, the invention relates to a pharmaceutical composition comprising a therapeutically effective amount of a compound of Formula I, or a pharmaceutically acceptable salt thereof, admixed with at least one pharmaceutically acceptable excipient. In a fourth aspect, the invention relates to processes for preparing the compounds of Formula I. In a fifth aspect, the invention relates to a method for identifying a bivalent antiviral agent, comprising preparing an array of dimeric agents, screening the dimeric agent array for enzyme inhibitory and antiviral activity, and selecting a bivalent agent based upon such activity. DETAILED DESCRIPTION OF THE INVENTION Viral DNA polymerases are enzymes that are essential for DNA replication and transcription in viruses. Accordingly, viral DNA polymerases are targets for chemotherapeutics designed to have antiviral activity. Antiviral drugs that exert their antiviral activity by the inhibition of viral DNA polymerases are well known, and include nucleoside analogs. For example, acyclovir inhibits the synthesis of viral DNA by competing with 2 '-deoxyguanosine triphosphate as a substrate for viral DNA polymerase, acting as a competitive inhibitor. Acyclovir and its analogs also exert a second mode of inhibition, namely chain termination. Such compounds lack a 3'-hydroxy group (or its equivalent), and are consequently unable to support chain elongation after their incoφoration. Another class of antiviral drugs includes organic analogs of inorganic pyrophosphate, in particular phosphono formic acid salts (foscarnet) and its phosphonoacetic acid analog. Foscarnet forms complexes with viral DNA polymerase at its pyrophosphate-binding site, thus preventing cleavage of pyrophosphate from additional nucleoside triphosphates and thus acting as a non- competitive inhibitor The compounds of the invention act in a bivalent fashion, and although not wishing to be bound or restricted by any particular theory or proposed mechanism of action, it is believed that the suφrising activity of these compounds at least in part arises from their ability to bind to their target in a bivalent fashion. The bivalent binding interactions of the present invention are characterized by the concurrent interaction of two different ligands of a bivalent agent with two ligand binding sites, i.e., the nucleoside binding site and the pyrophosphate binding site. Bivalent interactions differ from collections of individual monovalent interactions by the phenomenon of energetic coupling, wherein the binding of one ligand of a bivalent agent to its receptor will typically affect the thermodynamics of binding of a second ligand of the same bivalent agent, giving rise to an enhanced biological effect. Definitions As used herein: The term "alkyl" refers to a monoradical branched or unbranched saturated hydrocarbon chain, preferably having from 1 to 20 carbon atoms, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, secondary butyl, tert-butyl, n-hexyl, n-octyl, n-decyl, n- dodecyl, 2-ethyldodecyl, tetradecyl, and the like, unless otherwise indicated. The term "substituted alkyl" refers to an alkyl group as defined above having from 1 to 5 substituents selected from the group consisting of alkoxy, substituted alkoxy, cycloalkyl, substituted cycloalkyl, cycloalkenyl, substituted cycloalkenyl, acyl, acylamino, acyloxy, amino, aminoacyl, aminoacyloxy, oxyaminoacyl, azido, cyano, halogen, hydroxyl, keto, thioketo, carboxyl, carboxylalkyl, thioaryloxy, thioheteroaryloxy, thioheterocyclooxy, thiol, thioalkoxy, substituted thioalkoxy, aryl, aryloxy, heteroaryl, heteroaryloxy, heterocyclic, heterocyclooxy, hydroxyamino, alkoxyamino, nitro, -SO-alkyl, -SO-substituted alkyl, -SO-aryl, -SO-heteroaryl, -SO 2 - alkyl, -SO -aryl, -SO -heteroaryl, and mono- and di-alkylamino, mono- and di- cycloalkylamino, mono- and di-substituted cycloalkylamino, mono- and di-alkenylamino, mono- and di-substituted alkenylamino, mono- and di-cycloalkenylamino, mono- and di- substituted cycloalkenylamino, mono- and di-alkynylamino, mono- and di-substituted alkynylamino, mono- and di-arylamino, mono- and di-heteroarylamino, mono- and di- heterocyclic amino, and unsymmetric di-substituted amines having different substituents selected from alkyl, cycloalkyl, substituted cycloalkyl, alkenyl, substituted alkenyl, cycloalkenyl, substituted cycloalkenyl, alkynyl, substituted alkynyl, aryl, heteroaryl and heterocyclic. The term "alkylene" refers to a diradical of a branched or unbranched saturated hydrocarbon chain, preferably having from 1 to 40 carbon atoms, preferably 1-10 carbon atoms, more preferably 1-6 carbon atoms. This term is exemplified by groups such as methylene (-CH 2 -), ethylene (-CH 2 CH 2 -), the propylene isomers (e.g., -CH 2 CH 2 CH 2 - and -CH(CH 3 )CH 2 -) and the like. The term "substituted alkylene" refers to: (a) an alkylene group as defined above having from 1 to 5 substituents selected from the group consisting of alkoxy, substituted alkoxy, cycloalkyl, substituted cycloalkyl, cycloalkenyl, substituted cycloalkenyl, acyl, acylamino (including, for example, N- glucosaminecarbonyl, benzoylamino, biphenylcarbonylamino, and the like), acyloxy, amino, aminoacyl, aminoacyloxy, oxyacylamino, azido, cyano, halogen, hydroxyl, keto, thioketo, carboxyl, carboxylalkyl, thiol, thioalkoxy, substituted thioalkoxy, aryl, aryloxy, thioaryloxy, heteroaryl, heteroaryloxy, thioheteroaryloxy, heterocyclic, heterocyclooxy, thioheterocyclooxy, nitro, and -NR a R b , wherein R a and R b may be the same or different and are chosen from hydrogen, optionally substituted alkyl, cycloalkyl, alkenyl, cycloalkenyl, alkynyl, aryl, heteroaryl and heterocyclic. Additionally, such substituted alkylene groups include those where 2 substituents on the alkylene group are fused to form one or more cycloalkyl, substituted cycloalkyl, cycloalkenyl, substituted cycloalkenyl, aryl, heterocyclic or heteroaryl groups fused to the alkylene group. (b) an alkylene group as defined above that is interrupted by 1 -20 atoms or substituents independently chosen from oxygen, sulfur and NR a -, wherein R a is chosen from hydrogen, optionally substituted alkyl, cycloalkyl, alkenyl, cycloalkenyl, alkynyl, aryl, heteroaryl and heterocyclic; or (c) an alkylene group as defined above that has both from 1 to 5 substituents as defined above and is also interrupted by 1-20 atoms as defined above. Examples of substituted alkylenes are chloromethylene (-CH(Cl)-), aminoethylene (-CH(NH 2 )CH 2 -), l-(dodecanoylamino)propylene (-CH[NHC(O)-(CH 2 )π-CH 3 ] CH 2 -), l-(4-phenylbenzoylamino)pentylene (-CH[NHC(O)-Z] (CH 2 ) 4 ) ,2-carboxypropylene isomers (-CH 2 CH(CO 2 H)CH 2 -), ethoxyethyl (-CH 2 CH 2 O-CH 2 CH 2 -), -), ethylmethylaminoethyl (-CH 2 CH 2 N(CH 3 ) CH 2 CH 2 -), 1 -ethoxy-2-(2-ethoxy- ethoxy)ethane (-CH 2 CH 2 O-CH 2 CH 2 -O-CH 2 CH 2 O-CH 2 CH 2 -), and the like. The term "alkaryl" or "aralkyl" refers to the groups -alkylene-aryl and -substituted alkylene-aryl in which alkylene and aryl are as defined herein. Such alkaryl groups are exemplified by benzyl, phenethyl and the like. The term "alkoxy" refers to the groups alkyl-O-, alkenyl-O-, cycloalkyl-O-, cycloalkenyl-O-, and alkynyl-O-, where alkyl, alkenyl, cycloalkyl, cycloalkenyl, and alkynyl are as defined herein. Preferred alkoxy groups are alkyl-O- and include, by way of example, methoxy, ethoxy, n-propoxy, iso-propoxy, n-butoxy, tert-butoxy, sec-butoxy, n-pentoxy, n-hexoxy, 1 ,2-dimethylbutoxy, and the like The term "substituted alkoxy" refers to the groups substituted alkyl-O-, substituted alkenyl-O-, substituted cycloalkyl-O-, substituted cycloalkenyl-O-, and substituted alkynyl-O- where substituted alkyl, substituted alkenyl, substituted cycloalkyl, substituted cycloalkenyl and substituted alkynyl are as defined herein. The term "alkylalkoxy" refers to the groups -alkylene-O-alkyl, alkylene-O-substituted alkyl, substituted alkylene-O-alkyl and substituted alkylene-O- substituted alkyl wherein alkyl, substituted alkyl, alkylene and substituted alkylene are as defined herein. Examples of such groups are methylenemethoxy (-CH 2 OCH 3 ), ethylenemethoxy (-CH 2 CH OCH 3 ), n-propylene-iso-propoxy (-CH 2 CH 2 CH 2 OCH(CH 3 ) 2 ), methylene-t-butoxy (-CH 2 -O-C(CH 3 ) 3 ) and the like. The term "alkylthioalkoxy" refers to the group -alkylene-S-alkyl, alkylene-S-substituted alkyl, substituted alkylene-S-alkyl and substituted alkylene-S- substituted alkyl wherein alkyl. substituted alkyl, alkylene and substituted alkylene are as defined herein. Preferred alkylthioalkoxy groups are alkylene-S-alkyl and include, by way of example, methylenethiomethoxy (-CH SCH ), ethylenethiomethoxy (- CH 2 CH 2 SCH 3 ), n-propylene-iso-thiopropoxy (-CH 2 CH CH 2 SCH(CH 3 ) 2 ), methylene-t- thiobutoxy (-CH 2 SC(CH 3 ) 3 ) and the like. "Alkenyl" refers to a monoradical of a branched or unbranched unsaturated hydrocarbon preferably having two to twenty carbon atoms, and preferably having 1 -6 double bonds. This term is further exemplified by such radicals as vinyl, prop-2-enyl, pent-3-enyl, hex-5-enyl, 5-ethyldodec-3,6-dienyl, and the like. The term "substituted alkenyl" refers to an alkenyl group as defined above having from 1 to 5 substituents selected from the group consisting of alkoxy, substituted alkoxy. acyl, acylamino, acyloxy, amino, aminoacyl, aminoacyloxy, oxyaminoacyl, azido, cyano, halogen, hydroxyl, keto, thioketo, carboxyl, carboxylalkyl, thiol, thioalkoxy, substituted thioalkoxy, aryl, heteroaryl, heterocyclic, aryloxy, thioaryloxy, heteroaryloxy, thioheteroaryloxy, heterocyclooxy, thioheterocyclooxy, nitro, -SO-alkyl, -SO-substituted alkyl, -SO-aryl, -SO-heteroaryl, -SO 2 -alkyl, -SO 2 -substituted alkyl, -SO 2 -aryl, -SO 2 - heteroaryl, and mono- and di-alkylamino. mono- and di-(substituted alkyl)amino, mono- and di-cycloalkylamino, mono- and di-substituted cycloalkylamino, mono- and di- alkenylamino, mono- and di-cycloalkenylamino, mono- and di-substituted cycloalkenylamino, mono- and di-alkynylamino, mono- and di-substituted alkynylamino, mono- and di-arylamino, mono- and di-heteroarylamino, mono- and di- heterocyclic amino, and unsymmetric di-substituted amines having different substituents selected from alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl, alkenyl, cycloalkenyl, substituted cycloalkenyl, alkynyl, substituted alkynyl, aryl, heteroaryl and heterocyclic. "Alkenylene" refers to a diradical of an unsaturated hydrocarbon, preferably having two to twenty carbon atoms and preferably having 1-6 double bonds. This term is further exemplified by such radicals as 1,2-ethenyl, l,3-prop-2-enyl, l,5-pent-3-enyl, 1,4- hex-5-enyl, 5-ethyl-l,12-dodec-3,6-dienyl, and the like. The term "substituted alkenylene" refers to an alkenylene group as defined above having from 1 to 5 substituents, selected from the group consisting of alkoxy, substituted alkoxy, acyl, acylamino, acyloxy, amino, aminoacyl, aminoacyloxy, oxyacylamino, azido, cyano, halogen, hydroxyl, keto, thioketo, carboxyl, carboxylalkyl, thiol, thioalkoxy, substituted thioalkoxy, aryl, aryloxy, thioaryloxy, heteroaryl, heteroaryloxy, thioheteroaryloxy, heterocyclic, heterocyclooxy, thioheterocyclooxy, nitro, and mono- and di-alkylamino, mono- and di-(substituted alkyl)amino, mono- and di- cycloalkylamino, mono- and di-substituted cycloalkylamino, mono- and di-alkenylamino, mono- and di-substituted alkenylamino, mono- and di-cycloalkenylamino, mono- and di- substituted cycloalkenylamino, mono- and di-alkynylamino. mono- and di-substituted alkynylamino, mono- and di-arylamino, mono- and di-heteroarylamino, mono- and di- heterocyclic amino, and unsymmetric di-substituted amines having different substituents selected from alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl, alkenyl, substituted alkenyl, cycloalkenyl, substituted cycloalkenyl. alkynyl, substituted alkynyl, aryl, heteroaryl and heterocyclic. Additionally, such substituted alkenylene groups include those where 2 substituents on the alkenylene group are fused to form one or more cycloalkyl, substituted cycloalkyl, cycloalkenyl, substituted cycloalkenyl, aryl, heterocyclic or heteroaryl groups fused to the alkenylene group. "Alkynyl" refers to a monoradical of an unsaturated hydrocarbon, preferably having two to twenty carbon atoms, and preferably having 1-6 triple bonds. This term is further exemplified by such radicals as acetylenyl, prop-2-ynyl, pent-3-ynyl, hex-5-ynyl, 5-ethyldodec-3,6-diynyl, and the like. The term "substituted alkynyl" refers to an alkynyl group as defined above having from 1 to 5 substituents, selected from the group consisting of alkoxy, substituted alkoxy, acyl, acylamino, acyloxy. amino; aminoacyl, aminoacyloxy, oxyacylamino, azido, cyano, halogen, hydroxyl, keto, thioketo, carboxyl, carboxylalkyl, thiol, thioalkoxy, substituted thioalkoxy, aryl, aryloxy, thioaryloxy, heteroaryl, heteroaryloxy, thioheteroaryloxy, heterocyclic, heterocyclooxy, thioheterocycloxy, nitro, -SO-alkyl, -SO-substituted alkyl, -SO-aryl, -SO-heteroaryl, -SO 2 -alkyl, -SO 2 -substituted alkyl, -SO 2 -aryl, -SO 2 -heteroaryl, SO2-heterocyclic, mono- and di-alkylamino, mono- and di-(substituted alkyl)amino, mono- and di-cycloalkylamino, mono- and di-substituted cycloalkylamino, mono- and di-alkenylamino, mono- and di-substituted alkenylamino, mono- and di- cycloalkenylamino, mono- and di-substituted cycloalkenylamino, mono- and di- alkynylamino, mono- and di-substituted alkynylamino, mono- and di-arylamino, mono- and di-heteroarylamino, mono- and di-heterocyclic amino, and unsymmetric di- substituted amines having different substituents selected from alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl, alkenyl, substituted alkenyl, cycloalkenyl, substituted cycloalkenyl, alkynyl, substituted alkynyl, aryl, heteroaryl and heterocyclic. "Alkynylene" refers to a diradical of an unsaturated hydrocarbon radical, preferably having two to twenty carbon atoms, and preferably having 1-6 triple bonds. This term is further exemplified by such radicals as l,3-prop-2-ynyl, l,5-pent-3-ynyl, 1,4- hex-5-ynyl, 5-ethyl-l,12-dodec-3,6-diynyl, and the like. The term "acyl" refers to the groups alkyl-C(O)-, substituted alkyl-C(O)-, cycloalkyl-C(O)-, substituted cycloalkyl-C(O)-, cycloalkenyl-C(O)-, substituted cycloalkenyl-C(O)-, aryl-C(O)-, heteroaryl-C(O)- and heterocyclic-C(O)- where alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl, cycloalkenyl, substituted cycloalkenyl, aryl, heteroaryl and heterocyclic are as defined herein. The term "acylamino" refers to the group -C(O)NRR where each R is independently hydrogen, alkyl, substituted alkyl, aryl, heteroaryl, heterocyclic or where both R groups are joined to form a heterocyclic group (e.g., moφholino) wherein alkyl, substituted alkyl, aryl, heteroaryl and heterocyclic are as defined herein. The term "aminoacyl" refers to the group -NRC(O)R where each R is independently hydrogen, alkyl, substituted alkyl, aryl, heteroaryl, or heterocyclic wherein alkyl, substituted alkyl, aryl, heteroaryl and heterocyclic are as defined herein. The term "aminoacyloxy" refers to the group -NRC(O)OR where each R is independently hydrogen, alkyl, substituted alkyl, aryl, heteroaryl, or heterocyclic wherein alkyl, substituted alkyl, aryl, heteroaryl and heterocyclic are as defined herein. The term "acyloxy" refers to the groups alkyl-C(O)O-, substituted alkyl-C(O)O-, cycloalkyl-C(O)O-, substituted cycloalkyl-C(O)O-, aryl-C(O)O-, heteroaryl-C(O)O-, and heterocyclic-C(O)O- wherein alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl, aryl, heteroaryl, and heterocyclic are as defined herein. The term "aryl" refers to an unsaturated aromatic carbocyclic group of from 6 to 14 carbon atoms having a single ring (e.g., phenyl) or multiple condensed (fused) rings (e.g., naphthyl or anthryl). Unless otherwise constrained by the definition for the aryl substituent, such aryl groups can optionally be substituted with from 1 to 5 substituents selected from the group consisting of acyloxy, hydroxy, thiol, acyl, alkyl, alkoxy, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, substituted alkyl, substituted alkoxy, substituted alkenyl, substituted alkynyl, substituted cycloalkyl, substituted cycloalkenyl, amino, aminoacyl, acylamino, alkaryl, aryl, aryloxy, azido, carboxyl, carboxylalkyl, cyano, halo, nitro, heteroaryl, heteroaryloxy, heterocyclic, heterocyclooxy, aminoacyloxy, oxyacylamino, thioalkoxy, substituted thioalkoxy, thioaryloxy, thioheteroaryloxy, -SO-alkyl, -SO- substituted alkyl, -SO-aryl, -SO-heteroaryl, -SO 2 -alkyl, -SO -substituted alkyl, -SO 2 -aryl, -SO 2 -heteroaryl, trihalomethyl, mono-and di-alkylamino, mono- and di-(substituted alkyl)amino, mono- and di-cycloalkylamino, mono- and di-substituted cycloalkylamino, mono- and di-alkenylamino, mono- and di-substituted alkenylamino, mono- and di- cycloalkenylamino, mono- and di-substituted cycloalkenylamino, mono- and di- alkynylamino, mono- and di-substituted alkynylamino, mono- and di-arylamino, mono- and di-heteroarylamino, mono- and di-heterocyclic amino, and unsymmetric di- substituted amines having different substituents selected from alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl, alkenyl, substituted alkenyl, cycloalkenyl, substituted cycloalkenyl, alkynyl, substituted alkynyl, aryl, heteroaryl, heterocyclic and the like. Preferred aryl substituents include alkyl, alkoxy, halo, cyano, nitro, trihalomethyl, and thioalkoxy. The term "aryloxy" refers to the group aryl-O- wherein the aryl group is as defined above including optionally substituted aryl groups as also defined above. The term "arylene" refers to a diradical derived from aryl or substituted aryl as defined above, and is exemplified by 1,2-phenylene, 1,3-phenylene, 1 ,4-phenylene, 1,2- naphthylene and the like. The term "carboxyalkyl" refers to the group "-C(O)Oalkyl" where alkyl is as defined above. The term "cycloalkyl" refers to cyclic alkyl groups of from 3 to 20 carbon atoms having a single cyclic ring or multiple condensed rings. Such cycloalkyl groups include, by way of example, single ring structures such as cyclopropyl, cyclobutyl, cyclopentyl, cyclooctyl, and the like, or multiple ring structures such as adamantanyl, and the like. The term "substituted cycloalkyl" refers to cycloalkyl groups having from 1 to 5 substituents selected from the group consisting of alkoxy, substituted alkoxy, cycloalkyl, cycloalkenyl, substituted cycloalkenyl, acyl, acylamino, acyloxy, amino, aminoacyl, aminoacyloxy, oxyaminoacyl, azido, cyano, halogen, hydroxyl, keto, thioketo, carboxyl, carboxylalkyl, thioaryloxy, thioheteroaryloxy, thioheterocyclooxy, thiol, thioalkoxy, substituted thioalkoxy, aryl, aryloxy, heteroaryl, heteroaryloxy, heterocyclic, heterocyclooxy, hydroxyamino, alkoxyamino, nitro, -SO-alkyl. -SO- substituted alkyl, -SO-aryl, -SO-heteroaryl, -SO 2 -alkyl, -SO 2 -substituted alkyl. -SO 2 -aryl, -SO 2 -heteroaryl, and mono- and di-alkylamino, mono- and di-(substituted alkyl)amino, mono- and di-cycloalkylamino, mono- and di-alkenylamino, mono- and di-substituted alkenylamino, mono- and di-cycloalkenylamino, mono- and di-substituted cycloalkenylamino, mono- and di-alkynylamino, mono- and di-substituted alkynylamino, mono- and di-arylamino, mono- and di-heteroarylamino, mono- and di- heterocyclic amino, and unsymmetric di-substituted amines having different substituents selected from alkyl, substituted alkyl, cycloalkyl, alkenyl, substituted alkenyl, cycloalkenyl, substituted cycloalkenyl, alkynyl. substituted alkynyl, aryl, heteroaryl and heterocyclic. The term "cycloalkenyl" refers to cyclic alkenyl groups of from 4 to 8 carbon atoms having a single cyclic ring and at least one point of internal unsaturation. Examples of suitable cycloalkenyl groups include, for instance, cyclobut-2-enyl, cyclopent-3-enyl, cyclooct-3-enyl and the like. The term "substituted cycloalkenyl" refers to cycloalkenyl groups having from 1 to 5 substituents selected from the group consisting of alkoxy, substituted alkoxy, cycloalkyl, substituted cycloalkyl, cycloalkenyl, substituted cycloalkenyl, acyl, acylamino, acyloxy, amino, aminoacyl, aminoacyloxy, oxyaminoacyl, azido, cyano, halogen, hydroxyl, keto, thioketo, carboxyl, carboxylalkyl, thioaryloxy, thioheteroaryloxy, thioheterocyclooxy, thiol, thioalkoxy, substituted thioalkoxy, aryl, aryloxy, heteroaryl, heteroaryloxy, heterocyclic, heterocyclooxy, hydroxyamino, alkoxyamino, nitro, -SO-alkyl, -SO-substituted alkyl, -SO-aryl, -SO-heteroaryl, -SO 2 - alkyl, -SO 2 -substituted alkyl, -SO 2 -aryl, -SO -heteroaryl, and mono- and di-alkylamino, mono- and di-(substituted alkyl)amino, mono- and di-cycloalkylamino, mono- and di- substituted cycloalkylamino, mono- and di-alkenylamino, mono- and di-substituted alkenylamino, mono- and di-cycloalkenylamino, mono- and di-substituted cycloalkenylamino, mono- and di-alkynylamino, mono- and di-substituted alkynylamino, mono- and di-arylamino, mono- and di-heteroarylamino, mono- and di- heterocyclic amino, and unsymmetric di-substituted amines having different substituents selected from alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl, alkenyl, substituted alkenyl, cycloalkenyl, substituted cycloalkenyl, alkynyl, substituted alkynyl, aryl, heteroaryl and heterocyclic. The term "halo" or "halogen" refers to fluoro, chloro, bromo and iodo. "Haloalkyl" refers to alkyl as defined above substituted by 1 -4 halo groups as defined above, which may be the same or different, such as 3-fluorododecyl, 12,12,12- trifluorododecyl, 2-bromooctyl, -3-bromo-6-chloroheptyl, and the like. The term "heteroaryl" refers to an aromatic carbocyclic group of from 1 to 15 carbon atoms and 1 to 4 heteroatoms selected from oxygen, nitrogen and sulfur within at least one ring (if there is more than one ring). Unless otherwise constrained by the definition for the heteroaryl substituent, such heteroaryl groups can be optionally substituted with 1 to 5 substituents selected from the group consisting of acyloxy, hydroxy, thiol, acyl, alkyl, alkoxy, alkenyl, alkynyl, cycloalkyl, cycloalkenyl, substituted alkyl, substituted alkoxy, substituted alkenyl, substituted alkynyl, substituted cycloalkyl, substituted cycloalkenyl, amino, aminoacyl, acylamino, alkaryl, aryl, aryloxy, azido, carboxyl, carboxylalkyl, cyano, halo, nitro, heteroaryl, heteroaryloxy, heterocyclic, heterocyclooxy, aminoacyloxy, oxyacylamino, thioalkoxy, substituted thioalkoxy, thioaryloxy, thioheteroaryloxy, -SO-alkyl, -SO- substituted alkyl, -SO-aryl, -SO-heteroaryl, -SO 2 -alkyl, -SO 2 -substituted alkyl, -SO 2 -aryl, -SO 2 -heteroaryl, trihalomethyl, mono-and di-alkylamino, mono- and di-(substituted alkyl)amino, mono- and di-cycloalkylamino, mono- and di-substituted cycloalkylamino, mono- and di-alkenylamino, mono- and di-substituted alkenylamino, mono- and di- cycloalkenylamino, mono- and di-substituted cycloalkenylamino, mono- and di- alkynylamino, mono- and di-substituted alkynylamino, mono- and di-arylamino, mono- and di-heteroarylamino, mono- and di-heterocyclic amino, and unsymmetric di- substituted amines having different substituents selected from alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl, alkenyl, substituted alkenyl, cycloalkenyl, substituted cycloalkenyl, alkynyl, substituted alkynyl, aryl, heteroaryl, heterocyclic and the like. Preferred aryl substituents include alkyl, alkoxy, halo, cyano, nitro, trihalomethyl, and thioalkoxy. Such heteroaryl groups can have a single ring (e.g., pyridyl or furyl) or multiple condensed rings (e.g., indolizinyl or benzothienyl). Preferred heteroaryls include pyridyl, pyrrolyl and furyl. The term "heteroaryloxy" refers to the group heteroaryl-O-. The term "heteroarylene" refers to the diradical group derived from heteroaryl or substituted heteroaryl as defined above, and is exemplified by the groups 2,6-pyridylene, 2,4-pyridiylene, 1 ,2-quinolinylene, 1,8-quinolinylene, 1 ,4-benzofuranylene, 2,5- pyridinylene, 1,3-moφholinylene, 2,5-indolenyl, and the like. The term "heterocycle" or "heterocyclic" or "heterocyclo" refers to a monoradical saturated or unsaturated group having a single ring or multiple condensed rings, from 1 to 40 carbon atoms and from 1 to 10 hetero atoms, preferably 1 to 4 heteroatoms, selected from nitrogen, sulfur, phosphorus, and/or oxygen within the ring. Unless otherwise constrained by the definition for the heterocyclic substituent, such heterocyclic groups can be optionally substituted with 1 to 5, and preferably 1 to 3 substituents, selected from the group consisting of alkoxy, substituted alkoxy, cycloalkyl, substituted cycloalkyl, cycloalkenyl, substituted cycloalkenyl, acyl, acylamino, acyloxy, amino, aminoacyl, aminoacyloxy, oxyaminoacyl, cyano, halogen, hydroxyl, keto, thioketo, carboxyl, carboxylalkyl, thioaryloxy, thioheteroaryloxy, thioheterocyclooxy, thiol, thioalkoxy, substituted thioalkoxy, aryl, aryloxy, heteroaryl, heteroaryloxy, heterocyclic, heterocyclooxy, hydroxyamino, alkoxyamino, nitro, -SO-alkyl, -SO- substituted alkyl, -SO-aryl, -SO-heteroaryl, -SO 2 -alkyl, -SO 2 -substituted alkyl, -SO 2 -aryl, -SO 2 -heteroaryl, and mono- and di-alkylamino, mono- and di-(substituted alkyl)amino, mono- and di-cycloalkylamino, mono- and di-substituted cycloalkylamino, mono- and di- alkenylamino, mono- and di-substituted alkenylamino, mono- and di-cycloalkenylamino, mono- and di-substituted cycloalkenylamino, mono- and di-alkynylamino, mono- and di- substituted alkynylamino, mono- and di-arylamino, mono- and di-heteroarylamino, mono- and di-heterocyclic amino, and unsymmetric di-substituted amines having different substituents selected from alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl, alkenyl, substituted alkenyl, cycloalkenyl, substituted cycloalkenyl, alkynyl, substituted alkynyl, aryl, heteroaryl and heterocyclic. Such heterocyclic groups can have a single ring or multiple condensed rings. Examples of nitrogen heterocycles and heteroaryls include, but are not limited to, pyrrole, imidazole, pyrazole, pyridine, pyrazine, pyrimidine, pyridazine, indolizine, isoindole, indole, indazole, purine, quinolizine, isoquinoline, quinoline, phthalazine, naphthylpyridine, quinoxaline, quinazoline, cinnoline, pteridine. carbazole, carboline, phenanthridine, acridine, phenanthroline, isothiazole, phenazine, isoxazole, phenoxazine, phenothiazine, imidazolidine, imidazoline, piperidine, piperazine, indoline, moφholino, piperidinyl, tetrahydrofuranyl, and the like as well as N-alkoxy-nitrogen containing heterocycles. A preferred class of heterocyclics include "crown compounds" which refers to a specific class of heterocyclic compounds having one or more repeating units of the formula [-(CH 2 -) m Y-] where m is equal to or greater than 2, and Y at each separate occurrence can be O, N, S or P. Examples of crown compounds include, by way of example only, [-(CH 2 ) 3 -NH-] 3 , [-((CH 2 ) 2 -O) 4 -((CH 2 ) 2 -NH) 2 ] and the like. Typically such crown compounds can have from 4 to 10 heteroatoms and 8 to 40 carbon atoms. The term "heterocyclooxy" refers to the group heterocyclic-O-. The term "thioheterocyclooxy" refers to the group heterocyclic-S-. The term "heterocyclene" refers to the diradical group formed from a heterocycle as defined herein, and is exemplified by the groups 2,6-moφholino, 2,5-moφholino and the like. The term "oxyacylamino" refers to the group -OC(O)NRR where each R is independently hydrogen, alkyl, substituted alkyl, aryl, heteroaryl, or heterocyclic wherein alkyl, substituted alkyl, aryl, heteroaryl and heterocyclic are as defined herein. The term "thiol" refers to the group -SH. The term "thioalkoxy" refers to the group -S-alkyl. The term "substituted thioalkoxy" refers to the group -S-substituted alkyl. The term "thioaryloxy" refers to the group aryl-S- wherein the aryl group is as defined above including optionally substituted aryl groups also defined above. The term "thioheteroaryloxy" refers to the group heteroaryl-S- wherein the heteroaryl group is as defined above including optionally substituted aryl groups as also defined above. As to any of the above groups that contain one or more substituents, it is understood, of course, that such groups do not contain any substitution or substitution patterns which are sterically impractical and/or synthetically non-feasible. In addition, the compounds of this invention include all stereochemical isomers arising from the substitution of these compounds. "Alkyl optionally interrupted by 1-5 atoms chosen from O, S, or N" refers to alkyl as defined above in which the carbon chain is interrupted by O, S, or N. Within the scope are ethers, sulfides, and amines, for example 1 -methoxy decyl, 1 -pentyloxynonane, l-(2- isopropoxyethoxy)-4-methylnonane, l-(2-ethoxyethoxy)dodecyl, 2-(t-butoxy)heptyl, 1- pentylsulfanylnonane, nonylpentylamine, and the like. "Heteroarylalkyl" refers to heteroaryl as defined above linked to alkyl as defined above, for example pyrid-2-ylmethyl, 8-quinolinylpropyl, and the like. "Optional" or "optionally" means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances in which it does not. The compounds of the invention comprise two different ligands linked covalently. As used herein, "first ligand" denotes a compound that in its monovalent (i.e. unlinked) state is a competitive inhibitor of deoxynucleotide triphosphate incoφoration by DNA polymerase. Preferred first ligands include nucleoside analogs, for example acyclovir, valacylcovir, cidofovir, penciclovir, ganciclovir, and famciclovir. Such ligands are disclosed, for example, in U.S. Patent Nos. 4,199,574, 4,355,032, 5,817,807, 5,852,027, 5,840,743, and 5,859,021, the complete disclosures of which are hereby incoφorated by reference. Other ligands that fall within the scope of this definition are idoxuridine and trifluorothymidine. "Second ligand" denotes a compound that in its monovalent state is a non- competitive inhibitor of deoxynucleotide triphosphate incoφoration by DNA polymerase. Preferred second ligands include organic analogs of inorganic pyrophosphate, in particular phosphonoformic acid salts (foscarnet) and its phosphonoacetic acid analog, and their phosphinyl derivatives. Those skilled in the art will appreciate that portions of the ligand structures that are not essential for molecular recognition and binding activity (i.e. that are not part of the ligand domain) may be varied substantially, replaced or substituted with unrelated structures (for example, with ancillary groups as defined below), and, in some cases, omitted entirely without affecting the binding interaction. Accordingly, it should be understood that the term ligands is not intended to be limited to compounds known to be useful as antiviral compounds (for example, known drugs). Those skilled in the art will understand that the term ligand can equally apply to a molecule that is not normally recognized as having useful properties related to competitive or non-competitive inhibition of DNA polymerase, in that ligands that exhibit minimally useful properties as monomers can be highly active as bivalent compounds, due to the biological benefit (increased biological effect) conferred by bivalency. The primary requirement for a ligand as defined herein is that it has a ligand domain as defined below. It should be understood that the term "ligand" or "ligands", or "linker" or "linkers", is intended to include racemic ligands or linkers as well as the individual stereoisomers of the ligands or linkers, including pure enantiomers and non-racemic mixtures thereof. The scope of the invention as described and claimed encompasses the racemic forms of the ligands and linkers as well as the individual enantiomers and non- racemic mixtures thereof. "Bivalent agent" or "bivalent compound" as used herein refers to a compound that is capable of bivalency as defined herein, and which has 2 different ligands as defined herein connected by a covalent linker. A bivalent compound provides an improved biological and/or therapeutic effect as measured against that achieved by the same number of unlinked ligands available for binding to the ligand binding site of the oligonucleotide template-primer. Examples of increased biological and/or therapeutic effect with respect to the target include, for example, increased specificity, increased affinity, increased selectivity, increased potency, increased efficacy, increased therapeutic index, a change in the duration of action, decreased toxicity, decreased side effects, improved bioavailability, improved pharmacokinetics, improved activity spectrum, and the like. The bivalent compounds of the invention exhibit one or more of the foregoing effects. "Potency" as used herein refers to the minimum concentration at which a ligand is able to achieve a desirable biological or therapeutic effect. The potency of a ligand is typically proportional to its affinity for its ligand binding site. In some cases, the potency may be non-linearly correlated with its affinity. In comparing the potency of two drugs, e.g., a bivalent compound and the aggregate of its ligands in unlinked form, the dose- response curve of each is determined under identical test conditions (e.g., in an in vitro or in vivo assay or in an appropriate animal model). The finding that the bivalent compound produces an equivalent biological or therapeutic effect at a lower concentration than the aggregate unlinked ligand (e.g., on a per weight, per mole, or per ligand basis) is indicative of enhanced potency. "Univalency" as used herein refers to a single binding interaction between the ligand domain of one ligand as defined herein with the ligand recognition site of DNA polymerase. It should be noted that a compound having two ligands exhibits univalency when only one ligand of that compound is interacting with a ligand binding site. Examples of univalent interactions are depicted below. where the arrow represents a ligand domain and the indent represents the ligand binding site of a receptor (the DNA polymerase). "Bivalency" as used herein refers to the concurrent binding of 2 different linked ligands and two corresponding ligand binding sites of DNA polymerase. It should be understood that all compounds that contain two ligands attached to a linker do not necessarily exhibit the phenomena of bivalency, i.e. that improved biological and/or therapeutic effect of the bivalent compound that is obtained as measured against that produced by two unlinked ligands available for binding to a ligand binding site. For bivalency to occur, the ligand domains of the ligands that are connected by a linker have to be presented to the ligand binding site by the linker in a specific manner in order to bring about the desired ligand-orienting result, and thus produce a bivalent effect. Thus, the term "dimeric ligand compound" refers to two different ligands attached to a linker that may or may not exhibit the phenomena of bivalency. "Dimeric ligand compound library" refers to the collection of dimeric ligand compounds that are provided by the synthetic methods disclosed herein. "Selectivity" or "specificity" is a measure of the binding preferences of a ligand for different receptors and/or different ligands for the same receptor. The selectivity of a ligand with respect to its target receptor relative to another receptor is given by the ratio of the respective values of Ka (i.e., the dissociation constants for each ligand-receptor complex), or in cases where a biological effect is observed below the K , selectivity is given by the ratio of the respective EC 50 s (i.e. the concentrations that produce 50% of the maximum response for the ligand interacting with the two distinct receptors). The term "ligand recognition site" or "ligand binding site" as used herein denotes the site on the viral DNA polymerase-DNA template-DNA primer complex that recognizes a ligand domain and provides a binding partner for a ligand. The ligand binding site may be defined by monomeric or multimeric structures. As used herein, the terms "inert organic solvent" or "inert solvent" mean a solvent inert under the conditions of the reaction being described in conjunction therewith [including, for example, benzene, toluene, acetonitrile, tetrahydrofuran ("THF"), dimethylformamide ("DMF"), chloroform ("CHC1 3 "), methylene chloride (or dichloromethane or "CH 2 C1 2 "), diethyl ether, ethyl acetate, acetone, methylethyl ketone, methanol, ethanol, propanol. isopropanol, tert-butanol, dioxane, pyridine, and the like]. Unless specified to the contrary, the solvents used in the reactions of the present invention are inert solvents. "Pharmaceutically acceptable salt" means those salts which retain the biological effectiveness and properties of the bivalent compounds of the invention, and which are not biologically or otherwise undesirable. The bivalent compounds of the invention are capable of forming both acid and base salts by virtue of the presence of amino and carboxyl groups respectively. 1. Pharmaceutically acceptable base addition salts may be prepared from inorganic and organic bases. Salts derived from inorganic bases include, but are not limited to, the sodium, potassium, lithium, ammonium, calcium, and magnesium salts. Salts derived from organic bases include, but are not limited to, salts of primary, secondary and tertiary amines, substituted amines including naturally-occurring substituted amines, and cyclic amines, including isopropylamine, trimethyl amine, diethylamine, triethylamine, tripropylamine, ethanolamine, 2-dimethylaminoethanol, tromethamine, lysine, arginine, histidine, caffeine, procaine, hydrabamine, choline, betaine, ethylenediamine, glucosamine, N-alkylglucamines, theobromine, purines, piperazine, piperidine, and N- ethylpiperidine. It should also be understood that other carboxylic acid derivatives would be useful in the practice of this invention, for example carboxylic acid amides, including carboxamides, lower alkyl carboxamides, di(lower alkyl) carboxamides, and the like. 2. Pharmaceutically acceptable acid addition salts may be prepared from inorganic and organic acids. Salts derived from inorganic acids include hydrochloric acid, hydrobromic acid, sulfuric acid, nitric acid, phosphoric acid and the like. Salts derived from organic acids include acetic acid, propionic acid, glycolic acid, pyruvic acid, oxalic acid, malic acid, malonic acid, succinic acid, maleic acid, fumaric acid, tartaric acid, citric acid, benzoic acid, cinnamic acid, mandelic acid, methanesulfonic acid, ethanesulfonic acid, p-toluenesulfonic acid, salicylic acid and the like. The term "treatment" as used herein covers any treatment of a condition or disease in an animal, particularly a mammal, more particularly a human, and includes: (i) preventing the disease or condition from occurring in a subject which may be predisposed to the disease but has not yet been diagnosed as having it; (ii) inhibiting the disease or condition, i.e. arresting its development; (iii) relieving the disease or condition, i.e. causing regression of the condition; or. (iv) relieving the conditions caused by the disease, i.e. symptoms of the disease. The term "a disease or condition which is alleviated by treatment with an antiviral agent" as used herein covers all conditions and disease states which are generally acknowledged in the art to be usefully treated with the first ligand and/or the second ligand as defined, and those conditions and disease states which have been found to be usefully treated by the specific bivalent compounds of our invention. The term covers prophylactic treatment as well as relief or regression of the disease. Such disease states include, but are not limited to, diseases such as heφes, chicken pox, shingles, monocutaneous heφes, heφes encephalitis, heφes zoster, smallpox, conjunctivitis, sore throat, hepatitis B, warts, and mononucleosis, and the like. The term "therapeutically effective amount" refers to that amount of a bivalent compound that is sufficient to effect treatment, as defined above, when administered to a mammal or avian in need of such treatment. The therapeutically effective amount will vary depending on the subject and disease state being treated, the severity of the affliction and the manner of administration, and the like, and may be determined routinely by one of ordinary skill in the art. The term "protecting group" or "blocking group" refers to any group which when bound to one or more hydroxyl, thiol, amino or carboxyl groups of the compounds prevents reactions from occurring at these groups and which protecting group can be removed by conventional chemical or enzymatic steps to reestablish the hydroxyl, thio, amino or carboxyl group. The particular removable blocking group employed is not critical and preferred removable hydroxyl blocking groups include conventional substituents such as allyl, benzyl, acetyl, chloroacetyl, thiobenzyl, benzylidine, phenacyl, t-butyl-diphenylsilyl and any other group that can be introduced chemically onto a hydroxyl functionality and later selectively removed either by chemical or enzymatic methods in mild conditions compatible with the nature of the product. Protecting groups are disclosed in more detail in T.W. Greene and P.G.M. Wuts, "Protective Groups in Organic Synthesis" 2 nd Ed., 1991, John Wiley and Sons, N.Y. Preferred removable amino blocking groups include conventional substituents such as t-butyoxycarbonyl (t-BOC), benzyloxycarbonyl (CBZ), fiuorenylmefhoxycarbonyl (FMOC), allyloxycarbonyl (ALOC) and the like, which can be removed by conventional conditions compatible with the nature of the product. Preferred carboxyl protecting groups include esters such as methyl, ethyl, propyl, t-butyl etc. which can be removed by mild conditions compatible with the nature of the product. "Linker" or "linkers" as used herein, identified where appropriate by the symbol X, refers to a group that covalently links 2 different ligands (as defined herein) in a manner that provides a compound capable of bivalency. The linker is a ligand domain orienting entity that permits attachment of the ligands thereto. The extent to which bivalent binding is realized depends upon the efficiency with which the linker that joins the ligands permits the ligand domains to be presented to the ligand recognition sites. Beyond presenting ligand domains for bivalent interactions with receptors, the linker spatially constrains these interactions to occur within dimensions defined by the linker. Thus, the structural features of the linker (valency, geometry, orienting capabilities, size, flexibility, chemical composition) are features of bivalent compounds that play an important role in determining their activities. The term linker, however, does not include solid inert supports such as beads, resins, glass particles, rods, fibers, and the like, but it should be understood that the bivalent compounds of the invention can be attached to a solid support if desired to provide, for example, a material useful for separation and purification processes (e.g. affinity chromatography). The ligands are covalently attached to the linker using conventional chemical techniques, for example reaction between a carboxylic acid and an amine to form an amide, an amine and a sulfonyl halide to form a sulfonamide, an alcohol or phenol with an alkyl or aryl halide to form an ether, and the like. The linker is attached to the ligand at a position such that the ligand domain is permitted to orient itself appropriately in order to bind to the ligand binding site. The term linker embraces everything that is not considered to be part of the ligand. The relative orientation in which the ligand domains are displayed derives from the particular point or points of attachment of the ligands to the linker, and on the framework geometry. The determination of where acceptable substitutions can be made on a ligand is typically based on prior knowledge of structure-activity relationships of the ligand and/or congeners and/or structural information about ligand-receptor complexes (e.g., from X-ray crystallography, NMR). Such positions and the synthetic methods for covalent attachment are well known in the art. Suitable linkers are discussed below. "Biological effect" as used herein includes, but is not limited to, increased affinity, increased selectivity, increased potency, increased efficacy, increased duration of action, decreased toxicity, and the like. Combinatorial Libraries The methods described above lend themselves to combinatorial approaches for selecting compounds that have bivalent properties related to the inhibition of DNA polymerase from a library of dimeric compounds. Specifically, factors such as the proper juxtaposition of the individual ligands of a bivalent compound with respect to the relevant array of binding sites on a target or targets is important in optimizing the interaction of the bivalent compound with its target(s) and to maximize the biological advantage through bivalency. One approach is to identify a library of candidate bivalent compounds with properties spanning the parameters of bivalency that are relevant for a particular target. These parameters include: (1) the identity of ligand(s), (2) the orientation of ligands, (3) linker length, (4) linker geometry, (5) linker physical properties, and (6) linker chemical functional groups. Libraries of dimeric compounds potentially possessing bivalent properties (i.e., candidate bivalent compounds) and comprising a multiplicity of such variables are prepared and these libraries are then evaluated via conventional assays corresponding to the ligand selected and the parameters desired. Considerations relevant to each of these variables are set forth below: Selection of ligand(s) A set of first ligands that are capable of inhibiting the DNA polymerase in a competitive manner are selected for incoφoration into the libraries of candidate bivalent compounds. A set of second ligands that are capable of inhibiting the DNA polymerase in a non-competitive manner are likewise selected. The chosen ligands may be known drugs, modified forms of known drugs, substructures of known drugs, or other compounds. The ligands are preferably chosen based on known favorable properties that may be projected to be carried over to or amplified in bivalent forms. Favorable properties include demonstrated safety and efficacy in human patients, appropriate PK/ADME profiles, synthetic accessibility, and desirable physical properties such as solubility, logP, etc. However, it is crucial to note that ligands that display an unfavorable property from among the previous list may obtain a more favorable property through the process of bivalent compound formation; i.e., ligands should not necessarily be excluded on such a basis. For example, a ligand that is not sufficiently potent at a particular target so as to be efficacious in a human patient may become highly potent and efficacious when presented in bivalent form. A ligand that is potent and efficacious but not of utility because of a non-mechanism-related toxic side effect may have increased therapeutic index (increased potency relative to toxicity) as a bivalent compound. Compounds that exhibit short in vivo half-lives may have extended half-lives as bivalent compounds. Physical properties of ligands that limit their usefulness (e.g. poor bioavailability due to low solubility, hydrophobicity, hydrophilicity) may be rationally modulated in bivalent forms, providing compounds with physical properties consistent with the desired utility. Orientation: selection of ligand attachment points and linking chemistry Several points are chosen on each ligand at which to attach the ligand to the linker. The selected points on the ligand/linker for attachment are functionalized to contain complementary reactive functional groups. This permits probing the effects of presenting the ligands to their receptor(s) in multiple relative orientations, an important bivalent design parameter. The only requirement for choosing attachment points is that attaching to at least one of these points does not abrogate activity of the ligand. Such points for attachment can be identified by structural information when available. Alternatively, evaluation of ligand/target binding by nuclear magnetic resonance will permit the identification of sites non-essential for ligand/target binding. See, for example, Fesik, et al., U.S. Patent No. 5,891,643. When such structural information is not available, utilization of structure-activity relationships (SAR) for ligands will suggest positions where substantial structural variations are and are not allowed. In the absence of both structural and SAR information, a library is merely selected with multiple points of attachment to allow presentation of the ligand in multiple distinct orientations. Subsequent evaluation of this library will indicate what positions are suitable for attachment. It is important to emphasize that positions of attachment that do abrogate the activity of the monomeric ligand may also be advantageously included in candidate bivalent compounds in the library provided that such compounds bear at least one ligand attached in a manner which does not abrogate intrinsic activity. Another way to consider this is that the SAR of individual ligands within the context of a bivalent structure is often different from the SAR of those same ligands in monomeric form. Once the ligand attachment points have been chosen, one identifies the types of chemical linkages that are possible at those points. The most preferred types of chemical linkages are those that are compatible with the overall structure of the ligand (or protected forms of the ligand) readily and generally formed, stable and intrinsically innocuous under typical chemical and physiological conditions, and compatible with a large number of available linkers. Amide bonds, ethers, amines, carbamates, ureas, and sulfonamides are but a few examples of preferred linkages. Linkers: spanning relevant bivalent parameters through selection of valency, linker length, linker geometry, rigidity, physical properties, and chemical functional groups In the library of linkers employed to generate the library of candidate bivalent compounds, the selection of linkers employed in this library of linkers takes into consideration the following factors: Linker length. Linkers are chosen in a range of lengths to allow the spanning of a range of inter-ligand distances that encompass the distance preferable for a given bivalent interaction. In some instances the preferred distance can be estimated rather precisely from high-resolution structural information of targets, typically enzymes and soluble receptor targets. In other instances where high-resolution structural information is not available, one can make use of simple models to estimate the maximum distance between binding sites. In situations where two binding sites are present on the same target (or target subunit for multisubunit targets), preferred linker distances are 2-20 Angstroms, with more preferred linker distances of 2-5 Angstroms. In situations where two binding sites reside on separate target sites, preferred linker distances are 20-100 Angstroms, with more preferred distances of 30-70 Angstroms. Linker geometry and rigidity. The combination of ligand attachment site, linker length, linker geometry, and linker rigidity determine the possible ways in which the ligands of candidate bivalent compounds may be displayed in three dimensions and thereby presented to their binding sites. Linker geometry and rigidity are nominally determined by chemical composition and bonding pattern, which may be controlled and are systematically varied as another spanning function in a bivalent array. For example, linker geometry is varied by attaching two ligands to the ortho, meta, and para positions of a benzene ring, or in cis- or trans-arrangements at the 1,1- vs. 1,2- vs. 1,3- vs. 1,4- positions around a cyclohexane core or in cis- or trans-arrangements at a point of ethylene unsaturation. Linker rigidity is varied by controlling the number and relative energies of different conformational states possible for the linker. For example, a bivalent compound bearing two ligands joined by 1,8-octyl linker has many more degrees of freedom, and is therefore less rigid than a compound in which the two ligands are attached to the 4,4' positions of a biphenyl linker. Linker physical properties. The physical properties of linkers are nominally determined by the chemical constitution and bonding patterns of the linker, and linker physical properties impact the overall physical properties of the candidate bivalent compounds in which they are included. A range of linker compositions is typically selected to provide a range of physical properties (hydrophobicity, hydrophilicity, amphiphilicity, polarization, acidity, and basicity) in the candidate bivalent compounds. The particular choice of linker physical properties is made within the context of the physical properties of the ligands they join and preferably the goal is to generate molecules with favorable PK/ADME properties. For example, linkers can be selected to avoid those that are too hydrophilic or too hydrophobic to be readily absorbed and/or distributed in vivo. Linker chemical functional groups. Linker chemical functional groups are selected to be compatible with the chemistry chosen to connect linkers to the ligands and to impart the range of physical properties sufficient to span initial examination of this parameter. Combinatorial synthesis Having chosen a set of n ligands (n being determined by the sum of the number of different attachment points for each ligand chosen) and m linkers by the process outlined above, a library of (n!)m candidate bivalent compounds is prepared which spans the relevant design parameters for a particular target. For example, an array generated from a first ligand and a second ligand, the first ligand having two attachment points (Al, A2) and the second ligand having three attachment points (BI, B2, B3) joined in all possible combinations provide for at least 15 possible combinations of bivalent compounds: Al-Al A1-A2 Al-Bl A1-B2 A1-B3 A2-A2 A2-B1 A2-B2 A2-B3 Bl-Bl B1-B2 B1-B3 B2-B2 B2-B3 B3-B3 When each of these combinations is joined by 10 different linkers, a library of 150 candidate bivalent compounds results. Given the combinatorial nature of the library, common chemistries are preferably used to join the reactive functionalities on the ligands with complementary reactive functionalities on the linkers. The library therefore lends itself to efficient parallel synthetic methods. The combinatorial library can employ solid phase chemistries well known in the art wherein the ligand and/or linker is attached to a solid support. Alternatively and preferably, the combinatorial library is prepared in the solution phase. After synthesis, candidate bivalent compounds are optionally purified before assaying for activity by, for example, chromatographic methods (e.g., HPLC). Analysis of array by biochemical, analytical, pharmacological, and computational methods Various methods are used to characterize the properties and activities of the candidate bivalent compounds in the library to determine which compounds possess bivalent properties. Physical constants such as solubility under various solvent conditions and logD/clogD values can be determined. A combination of NMR spectroscopy and computational methods is used to determine low-energy conformations of the candidate bivalent compounds in fluid media. The ability of the members of the library to bind to the desired target and other targets is determined by various standard methods, which include radioligand displacement assays for receptor and ion channel targets, and kinetic inhibition analysis for many enzyme targets. In vitro efficacy can also be determined. Pharmacological data, including oral absoφtion, everted gut penetration, other pharmacokinetic parameters and efficacy data can be determined in appropriate models. In this way, key structure-activity relationships are obtained for bivalent design parameters that are then used to direct future work. The members of the library which exhibit bivalent properties, as defined herein, can be readily determined by conventional methods. First those members that exhibit bivalent properties are identified by conventional methods as described above including conventional assays (both in vitro and in vivo). Second, ascertaining the structure of those compounds that exhibit bivalent properties can be accomplished via art recognized procedures. For example, each member of the library can be encrypted or tagged with appropriate information allowing determination of the structure of relevant members at a later time. See, for example, Dower, et al., International Patent Application Publication No. WO 93/06121 ; Brenner, et al., Proc. Natl. Acad. Sci., USA, 89:5181 (1992); Gallop, et al., U.S. Patent No. 5,846,839; each of which are incoφorated herein by reference in its entirety. Alternatively, the structure of relevant bivalent compounds can also be determined from soluble and untagged libraries of candidate bivalent compounds by methods known in the art such as those described by Hindsgaul, et al, Canadian Patent Application No. 2,240,325 which was published on July 11, 1998. Such methods couple frontal affinity chromatography with mass spectroscopy to determine both the structure and relative binding affinities of candidate bivalent compounds to receptors. Follow-up synthesis and analysis of additional array(s) Based on the information obtained through analysis of the initial library, an optional component of the process is to ascertain one or more promising bivalent "lead" compounds as defined by particular relative ligand orientations, linker lengths, linker geometries, etc. Additional libraries can then be generated around these leads to provide for further information regarding structure to activity relationships. These arrays typically bear more focused variations in linker structure in an effort to further optimize target affinity and/or activity at the target (antagonism, partial agonism, etc.), and/or alter physical properties. By iterative redesign/analysis using the novel principles of bivalent design along with classical medicinal chemistry, biochemistry, and pharmacology approaches, one is able to prepare and identify optimal bivalent compounds that exhibit biological advantage towards their targets and as therapeutic agents. To further elaborate upon this procedure, suitable bivalent linkers include, by way of example only, those derived from dicarboxylic acids, disulfonylhalides, dialdehydes, diketones, dihalides, diisocyanates, diamines, diols. mixtures of carboxylic acids, sulfonylhalides, aldehydes, ketones, halides, isocyanates, amines and diols. In each case, the carboxylic acid, sulfonylhalide, aldehyde, ketone, halide, isocyanate, amine and diol functional group is reacted with a complementary functionality on the ligand to form a covalent linkage. Such complementary functionality is well known in the art as illustrated in the following table: COMPLEMENTARY BINDING CHEMISTRIES First Reactive Group Second Reactive Group Linkage hydroxyl isocyanate urethane amine epoxide β-hydroxyamine sulfonyl halide amine sulfonamide carboxyl acid amine amide hydroxyl alkyl/aryl halide ether aldehyde amine/NaCNBH 4 amine ketone amine/NaCNBH 4 amine amine isocyanate urea Exemplary linkers include those described in the Appendix. Pharmaceutical Formulations When employed as pharmaceuticals, the compounds of the invention are usually administered in the form of pharmaceutical compositions. These compounds can be administered by a variety of routes including oral, rectal, transdermal, subcutaneous, intravenous, intramuscular, and intranasal. These compounds are effective as both injectable and oral compositions. Such compositions are prepared in a manner well known in the pharmaceutical art and comprise at least one active compound. This invention also includes pharmaceutical compositions that contain, as the active ingredient, one or more of the compounds of formula I above associated with one or more pharmaceutically acceptable carriers. In making the compositions of this invention, the active ingredient is usually mixed with an excipient, diluted by an excipient or enclosed within such a carrier which can be in the form of a capsule, sachet, paper or other container. When the excipient serves as a diluent, it can be a solid, semi- solid, or liquid material, which acts as a vehicle, carrier or medium for the active ingredient. Thus, the compositions can be in the form of tablets, pills, powders, lozenges, sachets, cachets, elixirs, suspensions, emulsions, solutions, syrups, aerosols (as a solid or in a liquid medium), ointments containing, for example, up to 10% by weight of the active compound, soft and hard gelatin capsules, suppositories, sterile injectable solutions, and sterile packaged powders. In preparing a formulation, it may be necessary to mill the active compound to provide the appropriate particle size prior to combining with the other ingredients. If the active compound is substantially insoluble, it ordinarily is milled to a particle size of less than 200 mesh. If the active compound is substantially water soluble, the particle size is normally adjusted by milling to provide a substantially uniform distribution in the formulation, e.g. about 40 mesh. Some examples of suitable excipients include lactose, dextrose, sucrose, sorbitol, mannitol, starches, gum acacia, calcium phosphate, alginates, tragacanth, gelatin, calcium silicate, microcrystalline cellulose, polyvinylpyrrolidone, cellulose, sterile water, syrup, and methyl cellulose. The formulations can additionally include: lubricating agents such as talc, magnesium stearate, and mineral oil; wetting agents; emulsifying and suspending agents; preserving agents such as methyl- and propylhydroxy-benzoates; sweetening agents; and flavoring agents. The compositions of the invention can be formulated so as to provide quick, sustained or delayed release of the active ingredient after administration to the patient by employing procedures known in the art. The compositions are preferably formulated in a unit dosage form, each dosage containing from about 0.1 mg to about 1 g, more usually about 1 to about 100 mg, of the active ingredient. The term "unit dosage forms" refers to physically discrete units suitable as unitary dosages for human subjects and other mammals, each unit containing a predetermined quantity of active material calculated to produce the desired therapeutic effect, in association with a suitable pharmaceutical excipient. Preferably, the compound of formula I above is employed at no more than about 20 weight percent of the pharmaceutical composition, more preferably no more than about 15 weight percent, with the balance being pharmaceutically inert carrier(s). The active compound is effective over a wide dosage range and is generally administered in a pharmaceutically effective amount. It, will be understood, however, that the amount of the compound actually administered will be determined by a physician, in the light of the relevant circumstances, including the condition to be treated, the chosen route of administration, the actual compound administered, the age, weight, and response of the individual patient, the severity of the patient's symptoms, and the like. For preparing solid compositions such as tablets, the principal active ingredient is mixed with a pharmaceutical excipient to form a solid preformulation composition containing a homogeneous mixture of a compound of the present invention. When referring to these preformulation compositions as homogeneous, it is meant that the active ingredient is dispersed evenly throughout the composition so that the composition may be readily subdivided into equally effective unit dosage forms such as tablets, pills and capsules. This solid preformulation is then subdivided into unit dosage forms of the type described above containing from, for example, 0.1 to about 500 mg of the active ingredient of the present invention. The tablets or pills of the present invention may be coated or otherwise compounded to provide a dosage form affording the advantage of prolonged action. For example, the tablet or pill can comprise an inner dosage and an outer dosage component, the latter being in the form of an envelope over the former. The two components can be separated by an enteric layer which serves to resist disintegration in the stomach and permit the inner component to pass intact into the duodenum or to be delayed in release. A variety of materials can be used for such enteric layers or coatings, such materials including a number of polymeric acids and mixtures of polymeric acids with such materials as shellac, cetyl alcohol, and cellulose acetate. The liquid forms in which the novel compositions of the present invention may be incoφorated for administration orally or by injection include aqueous solutions, suitably flavored syrups, aqueous or oil suspensions, and flavored emulsions with edible oils such as corn oil, cottonseed oil, sesame oil, coconut oil, or peanut oil, as well as elixirs and similar pharmaceutical vehicles. Compositions for inhalation or insufflation include solutions and suspensions in pharmaceutically acceptable, aqueous or organic solvents, or mixtures thereof, and powders. The liquid or solid compositions may contain suitable pharmaceutically acceptable excipients as described supra. Preferably the compositions are administered by the oral or nasal respiratory route for local or systemic effect. Compositions in preferably pharmaceutically acceptable solvents may be nebulized by use of inert gases. Nebulized solutions may be inhaled directly from the nebulizing device or the nebulizing device may be attached to a face mask tent, or intermittent positive pressure breathing machine. Solution, suspension, or powder compositions may be administered, preferably orally or nasally, from devices that deliver the formulation in an appropriate manner. Actual methods of preparing such dosage forms are known, or will be apparent, to those skilled in this art; for example, see Remington's Pharmaceutical Sciences, Mack Publishing Company, Easton, Pennsylvania, 19th Edition, 1995, the complete disclosure of which is hereby incoφorated by reference. The composition or formulation to be administered will, in any event, contain a quantity of the active compound(s) in an amount effective to alleviate the symptoms of the subject being treated. UTILITY The bivalent agents of the invention are useful in medical treatments related to the inhibition of DNA polymerase, and accordingly exhibit biological effects well known to those skilled in the art, in particular antiviral activity, which can be demonstrated in the tests described herein. Assays for antiviral activity are well known to those skilled in the art. Examples of antiviral diseases that can be treated by the inhibition of oligonucleotide template primer include heφes, chicken pox, shingles, monocutaneous heφes, heφes encephalitis, heφes zoster, smallpox, conjunctivitis, sore throat, hepatitis B, warts, mononucleosis, and the like. TESTING The bivalent agents of the invention are useful in medical treatments and exhibit biological effects that can be demonstrated in vitro in assays well known to those skilled in the art. For example, the inhibition of virus replication and cytotoxicity in vitro is determined in a plaque reduction assay using MRC-5, HeLa S-3 cells or Vero cells (Vaghefi, MM., et al. 1986. J. Med. Chem. 29:1389 and Lowe, DM., et al., 1995. Antimicrob. Agents and Chemother. 39(8): 1802 and Elion, GB., et al., PNAS,74(12):5716). The effect of the compounds on the activity of the viral DNA polymerase is evaluated in vitro using the method of Larson, A et al 1986. Mol Pharmacol. 29(6):614, McKenna, CE., et al. 1987. Biochem. Pharmacol., 36(19):3103 and Xiong, X., 1997. Antimicrob. Agents Chemother, 41(3):594. The therapeutic effects of the bivalent ligands of the invention are also assessed in vivo in assays well known to those skilled in the art. For example, the effect of the compounds on animals infected with heφes virues is determined using the method of Bronson, JJ., et al., (1990. Nucleosides and Nucleotides, 9(6):745, Romanowski, EG., et al. 1999. Invest Ophthalmol. Vis Sci. 40(2):378,Jennings, R., et al. 1999 43(1 ):53 and Bolger, GT., et al. 1997. 35(3):157). Also, the pharmacokinetic effect of compounds in primates is also determined (Serabe, BM., et al. Cancer Chemother Pharmacol. 43(5):415 and Jankowski, A., et al. J. Pharm. Biomed. Anal. 1998 18(l-2):249 and Kimberlin, DF., et al. 1998. J. Obstet Gynecol. 1998 179(4):846). The effect of compounds on AIDS patients with CMV infections is assessed according to the method ofArevalo, JF., et al. 1995. 172(4):951. METHODS OF PREPARATION Linker The linker, when covalently connecting two ligands, provides a biocompatible, substantially non-immunogenic bivalent agent. The biological effects of the bivalent agent are highly sensitive to the valency, geometry, composition, size, flexibility or rigidity, the presence or absence of anionic or cationic charge, and similar considerations (including hydrophilicity and hydrophobicity as discussed below) with respect to the linker. Accordingly, the linker is preferably chosen to maximize the desired biological effect. The linker may be biologically "neutral", i.e. not itself contribute any biological activity to the compounds of the invention, or it may be chosen to enhance the biological effect of the molecule. In general, the linker may be chosen from any organic molecule that orients two ligands to the ligand binding sites, and permits bivalency. In this regard, the linker can be considered as a "framework" on which the ligands are arranged in order to bring about the desired ligand-orienting result, and thus produce a bivalent agent. It should be noted that the linker can simply be a covalent bond. For example, different orientations can be achieved by including in the framework groups containing monocyclic or polycyclic groups, including aryl and heteroaryl groups, or structures incoφorating one or more carbon-carbon multiple bonds (i.e., alkenes and alkynes). Other groups can also include oligomers and polymers that are branched- or straight-chain species. In preferred embodiments, rigidity is imparted by the presence of cyclic groups (e.g., aryl, heteroaryl, cycloalkyl, heterocycles, etc.). In other preferred embodiments, the ring is a six-or ten membered ring. In still further preferred embodiments, the ring is an aromatic group such as, for example, phenyl or naphthyl. Different frameworks can be designed to provide preferred orientations of the ligands. Such frameworks may be represented by using an array of dots (as shown below) wherein each dot may potentially be an atom, such as C, O, N, S, P, H, F, Cl, Br, and F, or the dot may alternatively indicate the absence of an atom at that position. To facilitate the understanding of the framework structure, the framework is illustrated as a two dimensional array in the following diagram, although clearly the framework is a three dimensional array in practice: Each dot is either an atom, chosen from carbon, hydrogen, oxygen, nitrogen, sulfur, phosphorus, or halogen, or the dot represents a point in space (i.e. an absence of an atom). Only certain atoms on the grid have the ability to act as an attachment point for the ligands, namely C, O, N, S, and P. Atoms can be connected to each other via bonds (single, double, or triple with acceptable resonance and tautomeric forms), with regard to the usual constraints of chemical bonding. Ligands may be attached to the framework via single, double, or triple bonds (with chemically acceptable tautomeric and resonance forms An example of a linker as represented by the grid is shown below for a biphenyl construct. Nodes (1,2), (2,0), (4,4), (5,2), (4,0), (6,2), (7,4), (9,4), (10,2), (9,0), (7,0) all represent carbon atoms. Node (10,0) is a chlorine atom. All other nodes (or dots) are points in space (i.e. represent an absence of atoms). Nodes (1,2) and (9,4) are attachment points. Hydrogen atoms are affixed to nodes (2,4), (4,4), (4,0), (2,0), (7,4), (10,2), and (7,0). Nodes (5,2) and (6,2) are connected by a single bond. The carbon atoms present are connected by either single or double bonds, taking into consideration the principle of resonance and/or tautomerism. The intersection of the framework (linker) and the ligand group, and indeed, the framework (linker) itself can have many different bonding patterns. Examples of acceptable patterns of three contiguous atom arrangements within the linker and at the linker-ligand interface are shown in the following diagram. CCC NCC CCN NCN OCC see PCC CCO NCO OCN SCN PCN CCS NCS OCO SCO PCO CCP CS NCP OCS S PCS OCP SCP PCP CNC NNC SNC CNN N NN ONC PNC CNO SNN NNO ONN PNN CNS S TO NNS ONO PNO CNP SNS NNH CTrTS- PNS ONP SNP PNP COC NOC CON SOC NON OOC P O C COO SON NUO UUN P O N COS SOO NOS UUU P O O COP SXTS- NOP OQ S FTPS ' ύϋ P SUP PTTP One skilled in the art would be able to identify bonding patterns that would produce bivalent compounds. Methods for producing these bonding arrangements are described in "Advanced Organic Chemistry, 4 th Edition" by March (Wiley-Interscience (New York), 1992). These arrangements are described in the grid of dots shown in the Scheme above. All of the possible arrangements for the five most preferred atoms are shown. Each atom has a variety of acceptable oxidation states. The bonding arrangements underlined are less acceptable and are not preferred. Examples of molecular structures in which the above bonding patterns could be employed as components of the linker are shown below. c ,c ^C C -C- ^c' 0 ^ -c^c- N c' c ^c- Figure 4 The identification of an appropriate framework geometry for ligand domain presentation is an important first step in the construction of a bivalent binding agent with enhanced activity. Systematic spatial searching strategies can be used to aid in the identification of preferred frameworks through an iterative process. Various strategies are known to those skilled in the art of molecular design and can be used for preparing the compounds of this invention. Having selected a preferred framework geometry, the physical properties of the linker can be optimized by varying the chemical composition. The composition of a linker can be varied in numerous ways to achieve the desired physical properties. It can therefore be seen that there is a plethora of possibilities for the composition of a linker. Examples of linkers include aliphatic moieties, aromatic moieties, steroidal moieties, peptides, and the like. Specific examples are peptides or polyamides, hydrocarbons, aromatic groups, ethers, lipids, cationic or anionic groups, or a combination thereof. Examples are given below, but it should be understood that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the invention. For example, properties of the linker can be modified by the addition or insertion of ancillary groups into the linker, for example, to change solubility of the bivalent agent (in water, fats, lipids, biological fluids, etc.), hydrophobicity, hydrophilicity, linker flexibility, antigenicity, molecular size, molecular weight, in vivo half-life, in vivo distribution, biocompatability, immunogenicity, stability, and the like. For example, the introduction of one or more poly or preferably oligo(ethylene glycol) (PEG) groups onto the linker enhances hydrophilicity and water solubility of the bivalent agent, increases both molecular weight and molecular size and, depending on the nature of the unPEGylated linker, may increase the in vivo retention time. Further, PEG may decrease antigenicity and potentially enhances the overall rigidity of the linker. Ancillary groups that enhance the water solubility/hydrophilicity of the linker are useful in practicing the present invention. Thus, it is within the scope of the present invention to use ancillary groups such as, for example, poly(ethylene glycol), alcohols, polyols (e.g., glycerin, glycerol propoxylate, saccharides, including mono-, oligo- and polysaccharides, etc.), carboxylates, polycarboxylates (e.g., polyglutamic acid, polyacrylic acid, etc.), amines, polyamines (e.g., polylysine, poly(ethyleneimine), etc) to enhance the water solubility and/or hydrophilicity of the compounds of Formula I. In preferred embodiments, the ancillary group used to improve water solubility/hydrophilicity will be a polyether. In particularly preferred embodiments, the ancillary group will be a poly(ethylene glycol). The incoφoration of lipophilic ancillary groups within the structure of the linker to enhance the lipophilicity and or hydrophobicity of the compounds of Formula I is within the scope of the present invention. Lipophilic groups of use in practicing the instant invention include, but are not limited to, aryl and heteroaryl groups. The aromatic groups may be either unsubstituted or substituted with other groups, but are at least substituted with a group that allows their covalent attachment to the linker. Other lipophilic groups of use in practicing the instant invention include fatty acid derivatives that do not form bilayers in aqueous medium until higher concentrations are reached. Also within the scope of the present invention is the use of ancillary groups that result in the compound of Formula I being incoφorated into a vesicle such as a liposome or a micelle. The term "lipid" refers to any fatty acid derivative that is capable of forming a bilayer or micelle such that a hydrophobic portion of the lipid material orients toward the bilayer while a hydrophilic portion orients toward the aqueous phase. Hydrophilic characteristics derive from the presence of phosphato, carboxylic, sulfato, amino, sulfhydryl, nitro, and other like groups. Hydrophobicity could be conferred by the inclusion of groups that include, but are not limited to, long chain saturated and unsaturated aliphatic hydrocarbon groups of up to 20 carbon atoms and such groups substituted by one or more aryl, heteroaryl, cycloalkyl and/or heterocyclic group(s). Preferred lipids are phosphoglycerides and sphingolipids. representative examples of which include phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine. phosphatidylinositol, phosphatidic acid, palmitoyloleoyl phosphatidylcholine, lysophosphatidylcholine, lysophosphatidyl-ethanolamine, dipalmitoylphosphatidylcholine, dioleoylphosphatidylcholine. distearoyl- phosphatidylcholine or dilinoleoylphosphatidylcholine could be used. Other compounds lacking phosphorus, such as sphingolipid and glycosphingolipid families are also within the group designated as lipid. Additionally, the amphipathic lipids described above may be mixed with other lipids including triglycerides and sterols. The flexibility of the linker can be reduced by the inclusion of ancillary groups that are bulky and/or rigid. The presence of bulky or rigid groups can hinder free rotation about bonds in the linker or bonds between the linker and the ancillary group(s) or bonds between the linker and the functional groups. Rigid groups can include, for example, those groups whose conformational lability is restrained by the presence of rings and/or multiple bonds, for example, aryl, heteroaryl, cycloalkyl, and/or heterocyclic. Other groups that can impart rigidity include polymeric groups such as oligo- or polyproline chains. Rigidity can also be imparted electrostatically. Thus, if the ancillary groups are either negatively or positively charged, the similarly charged ancillary groups will force the presenter linker into a configuration affording the maximum distance between each of the like charges. The energetic cost of bringing the like-charged groups closer to each other will tend to hold the linker in a configuration that maintains the separation between the like-charged ancillary groups. Further, ancillary groups bearing opposite charges will tend to be attracted to their oppositely charged counteφarts and potentially may enter into both inter- and intramolecular ionic bonds. This non-covalent bonding mechanism will tend to hold the linker into a conformation that allows bonding between the oppositely charged groups. The addition of ancillary groups which are charged, or alternatively, bear a latent charge which is unmasked, following addition to the linker, by deprotection, a change in pH, oxidation, reduction or other mechanisms known to those of skill in the art, is within the scope of the present invention. Rigidity may also be imparted by internal hydrogen bonding, or by hydrophobic collapse. Bulky groups can include, for example, large atoms and/or ions (e.g.. iodine, sulfur, metal ions, etc.) groups containing large atoms, polycyclic groups, including aromatic groups, non-aromatic groups and structures incoφorating one or more carbon- carbon multiple bonds (i.e., alkenes and alkynes). Bulky groups can also include oligomers and polymers that are branched- or straight-chain species. Species that are branched are expected to increase the rigidity of the structure more per unit molecular weight gain than are straight-chain species. Eliminating or reducing antigenicity of the compounds of Formula I by judicious choice of ancillary group(s) is within the scope of the present invention. In certain applications, the antigenicity of a compound of Formula I may be reduced or eliminated by the use of groups such as, for example, poly(ethylene glycol). The preferred linker length will vary depending upon the distance between adjacent ligand recognition sites, and the geometry, flexibility and composition of the linker. The length of the linker will preferably be in the range of about 2-100 Angstroms, more preferabl about 2-50 Angstroms, and even more preferably about 3-20 Angstroms. With the foregoing in mind, preferred linkers may be represented by the following formula: -X'-Z-(Y'-Z) m -Y"-Z-X'- in which: m is an integer of 0-20; X' at each separate occurrence is -O-, -S-, -S(O)-, -S(O) 2 -, -NR- (where R is as defined below), -C(O)-, or a covalent bond; Z at each separate occurrence is alkylene, cycloalkylene, alkenylene, alkynylene, arylene, heteroarylene, or a covalent bond; Y' and Y" at each separate occurrence are -S(0)„-CR'R"- ; -S(0) n -NR'-, S-S-, or a covalent bond; in which: n is 0, 1 or 2; and R, R' and R" at each separate occurrence are chosen from hydrogen, alkyl, cycloalkyl, alkenyl, cycloalkenyl, alkynyl, aryl, heteroaryl, and heterocyclo. Additionally, the linker moiety can be optionally substituted at any atom in the chain by alkyl, cycloalkyl, alkenyl, alkynyl, alkoxy, halo, nitro, aryl, heteroaryl, or heterocyclo. Preparation of Compounds of the Invention In general, a ligand or a ligand intermediate is reacted with a 'core' molecule having two or more functional groups with reactivity that is complementary to that of the functional groups on the ligand, thus linking ligands by a linker. Selecting different core molecules allows control of linker size, shape, orientation, rigidity, acidity/basicity, hydrophobicity /hydrophilicity, hydrogen bonding characteristics, and number of ligands connected. Examples of "cores" are shown below in Figure 2. The solid circle is used in the following figures and reaction schemes to represent any of the possible core molecules. That is, the solid (or shaded) circle is equivalent to a linker as defined above after reaction. Alkyl Cores r Γ~ - * « * ^ O V- Ether Cores Aromatic Cores • A 't H-bond Donor/ Acceptor Co Acidic/Basic Cores FIGURE 2 Preparation of Compounds of Formula I where the Linker is a Covalent Bond The preparation of a compound of Formula I where the linker is a covalent bond is exemplified in Reaction Scheme 1 , where the first ligand comprises a purine or pyrimidine base and deoxyribose, and the second ligand is derived from foscarnet. REACTION SCHEME 1 (12) Formula 1 (13) where B represents a purine or pyrimidine base, and PG and PG * represent protecting groups. The compound of formula (lla), dichlorophosphonocarboxylic acid, ethyl ester, is described in J. Med. Chem. 29, 1389-1393 (1986). For example, a compound of Formula I where the first ligand comprises deoxyguanosine, the second ligand is a phosphonoformate derivative linked to the 3'- position, is prepared as shown in Reaction Scheme IA. REACTION SCHEME IA Formula I (13a) (10a) where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl, the 2-amino group is protected by an acetyl group, and the 5'- hydroxy group is protected by triphenylmethyl. Treatment of the compound of formula (10a) (prepared as described in the Canadian Journal of Chemistry, 65, 1436 (1987), or J. Org. Chem. 61, 9207-9212 (1996)) with dichlorophosphonocarboxylic acid, ethyl ester, the compound of formula (l la), gives a compound of formula (12a), which upon hydrolysis followed by deprotection gives (13a), a compound of Formula I where the linker is a covalent bond. Preparation of Compounds of Formula I where the Linker is other than a Covalent Bond The preparation of a compound of Formula I where the linker is as defined above, the first ligand comprises a purine or pyrimidine base and deoxyribose, and the second ligand is derived from foscarnet, is exemplified in Reaction Scheme 2. REACTION SCHEME 2 Formula I (16) where B represents a purine or pyrimidine base, PG and PG' represent protecting groups, and the shaded circle represents a linker "core". For example, where the linker is ethylene (i.e., the shaded circle is methylene), reaction with 2-benzyloxy-l-bromoethane followed by the compound of formula (1 la) and subsequent hydrolysis and deprotection gives the following product: REACTION SCHEME 2A (16a) (10a) where the 6-carbonyl of deoxyguanine is protected as ah enol using diphenylaminocarbonyl, the 2-amino group is protected by an acetyl group, and the 5'- hydroxy group is protected by triphenylmethyl. Preparation of Compounds of Formula I where the Second Ligand is a Phosphinylformate Derivative of Foscarnet The preparation of a compound of Formula I where the linker is as defined above, the first ligand comprises a purine or pyrimidine base and deoxyribose, and the second ligand is a phosphinylformate, is exemplified in Reaction Scheme 3. REACTION SCHEME 3 OEt o 1 0 (21a) U (22b) PGO~ Etθ Cl CI^ >Et ^ P \ OEt gBr PG0~ - OEt (33) (34) 1 H 2 , Pd/C o MeOH Tsθ~ _ P \ 0Et 2 TsCI OEt pyπdine (36) (37) Formula I (38) where B represents a purine or pyrimidine base, PG and PG * represent protecting groups, and the shaded circle represents a linker "core". For example, where the linker is n-propylene (i.e., the shaded circle is ethylene): REACTION SCHEME 3A (10a) where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl, the 2-amino group is protected by an acetyl group, and the 5'- hydroxy group is protected by triphenylmethyl. Preparation of Compounds of Formula I where the Second Ligand is a Phosphonoacetate Derivative The preparation of a compound of Formula I where the linker is as defined above, the first ligand comprises a purine or pyrimidine base and deoxyribose, and the second ligand is derived from a phosphonoacetate derivative, is exemplified in Reaction Scheme 4. REACTION SCHEME 4 Fomula I (17) where B represents a purine or pyrimidine base, PG and PG' represent protecting groups, and the shaded circle represents a linker "core". A compound of formula (19), prepared similarly to the compound of formula (10) above, is reacted with ethoxycarbonylmethylphosphonyl dichloride, the compound of formula (l ib), which is then hydrolyzed and deprotected as shown above in Reaction Scheme 1 to give a compound of Formula I. For example, where the linker is n-propylene (i.e., the shaded circle is ethylene): REACTION SCHEME 4A where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl, the 2-amino group is protected by an acetyl group, and the 5'- hydroxy group is protected by triphenylmethyl. Preparation of Compounds of Formula I where the Second Ligand is a Phosphinylacetate Derivative The preparation of a compound of Formula I where the linker is as defined above, the first ligand comprises a purine or pyrimidine base and deoxyribose, and the second ligand is derived from a phosphinylacetic acid derivative, is exemplified in Reaction Scheme 5. REACTION SCHEME 5 (46) (48) Formula I where B represents a purine or pyrimidine base, PG and PG' represent protecting groups, and the shaded circle represents a linker "core". For example, where the linker is n-butylene (i.e., the shaded circle is n-propyl): REACTION SCHEME 5A where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl, the 2-amino group is protected by an acetyl group, and the 5'- hydroxy group is protected by triphenylmethyl. Preparation of Compounds of Formula I where the First Ligand is linked via the 5'- Hydroxy Group of Deoxyribose to the Second Ligand The preparation of a compound of Formula I where the linker is covalently attached to the 5'-hydroxy group of the first ligand (comprising a purine or pyrimidine base and deoxyribose), and the second ligand is derived from a foscarnet analog, is exemplified in Reaction Scheme 6. REACTION SCHEME 6 (30) Formula I where B represents a purine or pyrimidine base, PG and PG' represent protecting groups, and the shaded circle represents a linker "core". For example, where the linker is ethylene (i.e., the shaded circle is methylene): REACTION SCHEME 6A Formula I (30a) where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl, the 2-amino group is protected by an acetyl group, and the 3'- hydroxy group is protected by acetyl. Preparation of Compounds of Formula I where the Second Ligand is a Phosphinylformate Derivative, and where the First Ligand is linked via the 5'-Hydroxy Group of Deoxyribose to the Second Ligand The preparation of a compound of Formula I where the linker is as defined above, where the first ligand is linked via the 5'-hydroxy group of deoxyribose to the second ligand, where the first ligand comprises a purine or pyrimidine base and deoxyribose, and the second ligand is a phosphonoformate, is exemplified in Reaction Scheme 7. REACTION SCHEME 7 rmula I where B represents a purine or pyrimidine base, PG and PG' represent protecting groups, and the shaded circle represents a linker "core". The reaction is carried out in the same manner as for Reaction Scheme 3. For example, where the linker is n-propylene (i.e., the shaded circle is methylene): REACTION SCHEME 7A Formula I (30a) where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl, the 2-amino group is protected by an acetyl group, and the 3'- hydroxy group is protected by an acetyl group. The preparation of similar compounds in which the second ligand is a phosphonoacetate derivative is shown in Reaction Schemes 8 and 8A. REACTION SCHEME 8 I where B represents a purine or pyrimidine base, PG and PG' represent protecting groups, and the shaded circle represents a linker "core". The reaction is carried out in the same manner as for Reaction Scheme 4. For example, where the linker is ethylene (i.e., the shaded circle is methylene):: REACTION SCHEME 8A Formula 1 (30a) where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl, the 2-amino group is protected by an acetyl group, and the 3 ' hydroxy group is protected as an acetyl group. Preparation of Compounds of Formula I where the Second Ligand is a Phosphinylacetate Derivative, and where the First Ligand is linked via the 5 '-Hydroxy Group of Deoxyribose to the Second Ligand The preparation of a compound of Formula I where the linker is as defined above, where the first ligand is linked via the 5 '-hydroxy group of deoxyribose to the second ligand, where the first ligand comprises a purine or pyrimidine base and deoxyribose, and the second ligand is derived from a phosphinylacetic acid analog, is exemplified in Reaction Scheme 9. REACTION SCHEME 9 where B represents a purine or pyrimidine base, PG and PG' represent protecting groups, and the shaded circle represents a linker "core". The reaction is carried out in the same manner as for Reaction Scheme 5. For example, where the linker is n-propylene (i.e., the shaded circle is methylene): REACTION SCHEME 9A Formula I (30a) where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl, the 2-amino group is protected by an acetyl group, and the 3'- hydroxy group is protected as an acetyl group. Preparation of Compounds of Formula I where the Linker is a Covalent Bond The preparation of a compound of Formula I where the linker is a covalent bond is exemplified in Reaction Scheme 10, where the first ligand comprises a purine or pyrimidine base and 2-hydroxyethoxymefhyl, and the second ligand is derived from foscarnet. REACTION SCHEME 10 Formula I where B represents a purine or pyrimidine base, PG' represents a protecting group, and the shaded circle represents a linker "core". The reaction is carried out in the same manner as for Reaction Scheme 1. For example: REACTION SCHEME IOA where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl, and the 2-amino group is protected by an acetyl group. Preparation of Compounds of Formula I where the Linker is an Ether The preparation of a compound of Formula I where the linker is an ether is exemplified in Reaction Scheme 11 , where the first ligand comprises a purine or pyrimidine base and 2-hydroxyethoxymethyl, and the second ligand is derived from foscarnet. REACTION SCHEME 11 Formula I (40) where B represents a purine or pyrimidine base, PG' represents a protecting group, and the shaded circle represents a linker "core". The reaction is carried out in the same manner as for Reaction Scheme 2. For example: REACTION SCHEME 11 A (40a) Formula I where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl, and the 2-amino group is protected by an acetyl group. Preparation of Compounds of Formula I where the Second Ligand is a Phosphinylformate Derivative The preparation of a compound of Formula I where the first ligand comprises a purine or pyrimidine base and 2-hydroxyethoxymethyl, and the second ligand is derived from a phosphinylformate derivative is shown in Reaction Scheme 12. REACTION SCHEME 12 where B represents a purine or pyrimidine base, PG' represents a protecting group, and the shaded circle represents a linker "core". The reaction is carried out in the same manner as for Reaction Scheme 3. For example, REACTION SCHEME 12A Formula I (40a) where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl, and the 2-amino group is protected by an acetyl group. Preparation of Compounds of Formula I where the Second Ligand is a Phosphonoacetate Derivative The preparation of a compound of Formula I where the first ligand comprises a purine or pyrimidine base and 2-hydroxyethoxymethyl, and the second ligand is derived from a phosphonoacetate derivative is exemplified in Reaction Scheme 13 REACTION SCHEME 13 Formula I < 40 > where B represents a purine or pyrimidine base, PG' represents a protecting group, and the shaded circle represents a linker "core". The reaction is carried out in the same manner as for Reaction Scheme 4. For example: REACTION SCHEME 13A Formula I (40a) where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl, and the 2-amino group is protected by an acetyl group. Preparation of Compounds of Formula I where the Second Ligand is a Phosphinylacetate Derivative The preparation of a compound of Formula I is exemplified in Reaction Scheme 14, where the first ligand comprises a purine or pyrimidine base and 2- hydroxyethoxymethyl, and the second ligand is a phosphinylacetate derivative. REACTION SCHEME 14 (40) Formula where B represents a purine or pyrimidine base, PG' represents a protecting group, and the shaded circle represents a linker "core". The reaction is carried out in the same manner as for Reaction Scheme 5. For example: REACTION SCHEME 14A Formula I (40a) where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl, and the 2-amino group is protected by an acetyl group. Preparation of Compounds of Formula I where the First Ligand is a Ganciclovir Derivative and the Linker is a Covalent Bond The preparation of a compound of Formula I is exemplified in Reaction Scheme 15, where the first ligand comprises a purine or pyrimidine base and 2,3-dihydroxyprop- 1 -yl, the second ligand is a phosphonoformate derivative, and the linker is a covalent bond at the 3 -position. REACTION SCHEME 15 (50) Formula I where B represents a purine or pyrimidine base, and PG and PG' represent protecting groups. The reaction is carried out in the same manner as for Reaction Scheme 1. For example: REACTION SCHEME 15A where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl, the 2-amino group is protected by an acetyl group, and the 2- hydroxy group is protected by benzyl. Preparation of Compounds of Formula I where the First Ligand is a Ganciclovir Derivative The preparation of a compound of Formula I is exemplified in Reaction Scheme 16, where the first ligand comprises a purine or pyrimidine base and 2,3-dihydroxyprop- 1-yl, and the second ligand is a phosphonoformate derivative. REACTION SCHEME 16 (50) Formula where B represents a purine or pyrimidine base, PG and PG' represent protecting groups, and the shaded circle represents a linker "core". The reaction is carried out in the same manner as for Reaction Scheme 2. For example: REACTION SCHEME 16A where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl. the 2-amino group is protected by an acetyl group, and the 2- hydroxy group is protected by benzyl. Preparation of Compounds of Formula I where the First Ligand is a Ganciclovir Derivative The preparation of a compound of Formula I is exemplified in Reaction Scheme 17, where the first ligand comprises a purine or pyrimidine base and 2,3- dihydroxypropan-1-yl, and the second ligand is a phosphinylformate derivative. REACTION SCHEME 17 Formula where B represents a purine or pyrimidine base, PG and PG' represent protecting groups, and the shaded circle represents a linker "core". The reaction is carried out in the same manner as for Reaction Scheme 3. For example: REACTION SCHEME 17A where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl, the 2-amino group is protected by an acetyl group, and the 2- hydroxy group is protected by benzyl. Preparation of Compounds of Formula I where the First Ligand is a Ganciclovir Derivative The preparation of a compound of Formula I is exemplified in Reaction Scheme 18, where the first ligand comprises a purine or pyrimidine base and 2,3-dihydroxyprop- 1-yl, and the second ligand is a phosphonoacetate derivative. REACTION SCHEME 18 (50) Formula I where B represents a purine or pyrimidine base, PG and PG' represent protecting groups, and the shaded circle represents a linker "core". The reaction is carried out in the same manner as for Reaction Scheme 4. For example: REACTION SCHEME 18A where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl, the 2-amino group is protected by an acetyl group, and the 2- hydroxy group is protected by benzyl. Preparation of Compounds of Formula I where the First Ligand is a Ganciclovir Derivative The preparation of a compound of Formula I is exemplified in Reaction Scheme 19, where the first ligand comprises a purine or pyrimidine base and 2,3-dihydroxyprop- 1-yl, and the second ligand is a phosphinylacetate derivative. REACTION SCHEME 19 (50) Formula I where B represents a purine or pyrimidine base, PG and PG' represent protecting groups, and the shaded circle represents a linker "core". The reaction is carried out in the same manner as for Reaction Scheme 5. For example: REACTION SCHEME 19A Formula I where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl, the 2-amino group is protected by an acetyl group, and the 2- hydroxy group is protected by benzyl. Preparation of Compounds of Formula I where the First Ligand is a Ganciclovir Derivative and the Second Ligand is a Phosphoformate Derivative. The preparation of a compound of Formula I is exemplified in Reaction Scheme 20, where the first ligand comprises a purine or pyrimidine base and 2,3-dihydroxyprop- 1-yl, and the second ligand is a phosphoformate derivative. REACTION SCHEME 20 (60) Formula I where B represents a purine or pyrimidine base, PG and PG' represent protecting groups, and the shaded circle represents a linker "core". The reaction is carried out in the same manner as for Reaction Scheme 1. For example: REACTION SCHEME 20A Formula 1 (60a) where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl, the 2-amino group is protected by an acetyl group, and the 3- hydroxy group is protected by triphenylmethyl. Preparation of Compounds of Formula I where the First Ligand is a Ganciclovir Derivative The preparation of a compound of Formula I is exemplified in Reaction Scheme 21, where the first ligand comprises a purine or pyrimidine base and 2,3-dihydroxyprop- l-ol, and the second ligand is a phosphonoformate derivative. REACTION SCHEME 21 (60) Formula I where B represents a purine or pyrimidine base, PG and PG' represent protecting groups, and the shaded circle represents a linker "core". The reaction is earned out in the same manner as for Reaction Scheme 2. For example: REACTION SCHEME 21 A (60a) Formula I where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl, the 2-amino group is protected by an acetyl group, and the 3- hydroxy group is protected by triphenylmethyl. Preparation of Compounds of Formula I where the First Ligand is a Ganciclovir Derivative The preparation of a compound of Formula I is exemplified in Reaction Scheme 22, where the first ligand comprises a purine or pyrimidine base and 2,3-dihydroxyprop- 1-yl, and the second ligand is a phosphinylformate derivative. REACTION SCHEME 22 (60) Formula I where B represents a purine or pyrimidine base, PG and PG' represent protecting groups, and the shaded circle represents a linker "core". The reaction is carried out in the same manner as for Reaction Scheme 3. For example: REACTION SCHEME 22A Formula I (60a) where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl, the 2-amino group is protected by an acetyl group, and the 3- hydroxy group is protected by triphenylmethyl. Preparation of Compounds of Formula I where the First Ligand is a Ganciclovir Derivative The preparation of a compound of Formula I is exemplified in Reaction Scheme 23, where the first ligand comprises a purine or pyrimidine base and 2,3-dihydroxyprop- 1-yl, and the second ligand is a phosphonoacetate derivative. REACTION SCHEME 23 (60) Formula I where B represents a purine or pyrimidine base, PG and PG' represent protecting groups, and the shaded circle represents a linker "core". The reaction is carried out in the same manner as for Reaction Scheme 4. For example: REACTION SCHEME 23A 1 where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl, the 2-amino group is protected by an acetyl group, and the 3- hydroxy group is protected by triphenylmethyl. Preparation of Compounds of Formula I where the First Ligand is a Ganciclovir Derivative and the Second Ligand is a Phosphinoacetate Derivative. The preparation of a compound of Formula I is exemplified in Reaction Scheme 24, where the first ligand comprises a purine or pyrimidine base and 2,3-dihydroxyprop- 1-yl, and the second ligand is a phosphinoacetate derivative. REACTION SCHEME 24 (60) Formula I where B represents a purine or pyrimidine base, PG and PG' represent protecting groups, and the shaded circle represents a linker "core". The reaction is carried out in the same manner as for Reaction Scheme 5. For example: REACTION SCHEME 24A where the 6-carbonyl of deoxyguanine is protected as an enol using diphenylaminocarbonyl, the 2-amino group is protected by an acetyl group, and the 3- hydroxy group is protected by triphenylmethyl. Isolation and Purification of the Compounds Isolation and purification of the compounds and intermediates described herein can be effected, if desired, by any suitable separation or purification procedure such as, for example, filtration, extraction, crystallization, column chromatography, thin-layer chromatography, thick-layer chromatography, preparative low or high-pressure liquid chromatography or a combination of these procedures. Specific illustrations of suitable separation and isolation procedures can be had by reference to the Examples hereinbelow. However, other equivalent separation or isolation procedures could, of course, also be used. The following abbreviations have the following meanings. If an abbreviation is not defined, it has its generally accepted meaning. BOC = tert-butyloxycarbonyl Cbz = carbobenzyloxy DCC = N,N-dicyclohexylcarbodiimide DIPEA = diisopropylethylamine, Hunig's base DMA = N,N-dimethylacetamide DMF = N,N-dimethylformamide DMSO = dimethylsulfoxide DPPA = diphenylphosphoryl azide HATU = 6-(7-azabenzotriazol-l-yl)-N,N.N'N- tetramethyluronium hexafluorophosphate HBTU - 1 -hydroxybenzotriazole HOAT = l-hydroxy-7-azabenzotriazole PyBOP = pyridine benzotriazol-l-yloxy-tris(dimethyl- amino)phosphonium hexafluorophosphate TEA = triethylamine TFA = trifluoroacetic acid THF = tetrahydrofuran EXAMPLE 1 Preparation of Compounds of Formula I Example 1 (12a) Formula I PREPARATION OF COMPOUNDS OF FORMULA I Example 1 — Preparation of a compound of Formula I where the linker is a covalent bond, where one ligand is deoxyguanosine and the other ligand is phosphonoformate. A. To a solution of the dichloride of formula (1 la) (3 mmols) in trimethyl phosphate (10 mL) at 0 °C is added 2-acetylamino-6-diphenylaminocarbonyloxy-9-(3- hydroxy-2-triphenylmethoxy-tetrahydrofuran-5-yl)-purine (10a) (1 mmol). The mixture is stirred until dissolution is complete and then for an additional one hour. The mixture is then poured slowly into anhydrous diethyl ether (250 mL). The precipitate is separated by decantation and dried to give compound 12a. B. Intermediate 12a is dissolved in dichloromethane (10 mL), cooled to 0 °C, and gaseous HCI is bubbled through the solution for 5 minutes. The reaction mixture is diluted with 40 mL additional dichloromethane. A solution of saturated NaHCO 3 (50 mL) is added slowly and the mixture is stirred vigorously for 10 min. The organic layer is separated, dried over anhydrous NaSO 4. , filtered, and concentrated under reduced pressure. The residue is treated with IN NaOH (20 mL) and heated with stirring at 60 °C for 1 day. The reaction mixture is neutralized by the addition of concentrated HCI and the product is purified by anion exchange chromatography to afford 13a, a compound of Formula I where the linker is a covalent bond. [Compound (l la) is described in J. Med. Chem. 29, 1389-1393 (1986).] EXAMPLE 2 Preparation of Compounds of Formula I Example 2A d Od) (15a) (16a) Formula I Example 2A — Preparation of a compound of Formula I where the Linker is -(CH ) 2 -, where the First ligand is Deoxyguanine and the Second Ligand is Phosphonoformate. A. A solution of 20 mmols of 2-acetylamino-6-diphenylaminocarbonyloxy-9-(3- hydroxy-2-triphenylmethoxy-tetrahydrofuran-5-yl)-purine (10a) in DMF with 20 mmols of l-bromo-2-benzyloxyefhane (14a) and 20 mmols of diisopropylethylamine is heated at 80 C and the reaction followed by TLC. When judged complete, the mixture is extracted, washed, and dried by conventional technique. The residue is purified by chromatography to afford the desired alkylation product 1 Od. B. Compound lOd (10 mmols) is dissolved in methanol (100 mL) and hydrogenated overnight at 35 psi, in the presence of 10% Pd/C. The resulting mixture is worked up and purified by conventional technique to yield 2-acetylamino-6- diphenylaminocarbonyloxy-9-[3-(2-hydroxyethoxy)-2-triphenylmethoxy- tetrahydrofuran-5-yl)]-purine, compound 15a. Compound 15a is phosphorylated with compound l la and subsequently converted to compound 16a, a compound of formula I where the linker is -(CH2) -, one ligand is doexyguanosine and the other ligand is phosphonoformate by following steps A and B of example 1. Example 2B Formula (15b) Similarly, substituting EtO-C(O)-CH 2 -P(O)Cl 2 (l ib) for the compound of formula (l la) and l-bromo-4-benzyloxybutane (14b) for the compound of formula (14a), compound (17b) is prepared, a compound of formula I where the linker is -(CH 2 ) 4 -, one ligand is deoxyguamne and the other ligand is phosphonoacetate. (Shown in Example 2B) EXAMPLE 3: Preparation of Compounds of Formula I Example 3A (34a) (35a) (36a) (37a) Example 3 A — Preparation of a compound of Formula I where the linker is -(CH 2 ) 3 -, where one ligand is deoxyguanosine and the other ligand is phosphinylformate. A. A Grignard reagent is prepared from 3-bromo-l-benzyloxypropane (33 a) in diethyl ether by standard technique (40 mmols bromide, 40 mmols Mg, 2 mmols 1,2-dibromoethane). The reaction is refluxed for 20-30 minutes after addition of bromide is complete. Diethyl chlorophosphite (21) (2-3 equiv) is added and the mixture stirred at room temperature for 1 hour. This mixture is then treated with excess ethyl chloroformate (200 mmols). After and additional 4 hours, the reaction mixture is poured into a solution of saturated sodium bicarbonate solution (250 mL) and extracted with dichloromethane (250 mL). Compound 34a is recovered by chromatography. B. The benzyl protecting group of compound 34a is removed following the procedure for hydrogeno lysis of compound 1 Od outlined in Example 2 to afford compound 35 a. C. Compound 35a (10 mmols) is dissolved in anhydrous pyridine (10 mL) and treated with p-toluenesulfonyl chloride (11 mmols). The reaction is followed by tic. When judged to be complete, the reaction mixture is poured into ice water (100 mL) and extracted with ethyl acetate. The organic extract is washed with water, dried, filtered, and evaporated by conventional technique. Tosylate 36a is used without further purification. D. Tosylate 36a is coupled with 2-acetylamino-6-diphenylaminocarbonyloxy-9-(3- hydroxy-2-triphenylmethoxy-tetrahydrofuran-5-yl)-purine (10a) following the procedure for O-alkylation set forth in Example 2 to afford protected phosphinylformate analog 37a. E. Compound 37a (2 mmols) is dissolved in dichloromethane (10 mL), cooled to 0 C, and treated with and 2,4,6-collidine (2 mmols) and trimethylsilyl iodide (5 mmols). The reaction is monitored by tic. When it is judged to be complete, the reaction mixture is worked up by conventional technique. The residue is dissolved in dichloromethane (10 mL), cooled to 0 °C, and gaseous HCI is bubbled through the solution for 5 minutes. The reaction mixture is diluted with 40 mL additional dichloromethane. A solution of saturated NaHCO 3 (50 mL) is added slowly and the mixture is stirred vigorously for 10 min. The organic layer is separated, dried over anhydrous NaSO 4 ., filtered, and concentrated under reduced pressure. The residue is treated with IN NaOH (20 mL) and heated with stirring at 60 °C for 1 day. The reaction mixture is neutralized by the addition of concentrated HCI and the product is purified by anion exchange chromatography to afford 38a, a compound of Formula I where the linker is -(CH 2 ) 3 - one ligand is deoxyguanosine and the other ligand is phosphinylformate. Example 3B (44b) (45b) H 2 CI 2 (48b) Formula I Similarly, other compounds of Formula I are prepared by substituting alternative nucleophiles for the Grignard reagent derived from 3-bromo-l-benzyloxypropane and/or by substituting ethyl chloroacetate for ethyl chloroformate in the procedure described above. For example, use of 1 -bromo-4-benzyloxybutane in conjunction with ethyl chloroacetate compound 48b is prepared, a compound of formula I where the linker is - (CH 2 ) 4 -, one ligand is deoxyguanosine and the other ligand is phosphinylacetate. (Shown in Figure 3B) EXAMPLE 4 Preparation of Compounds of Formula I Following the procedures of Examples 1,2, and 3 and substituting compounds of formula (30) for the compound of formula (10a), compounds of Formula I in which the linker is attached to the 5 '-hydroxy group are made. EXAMPLE 5 Preparation of Compounds of Formula I Following the procedures of Examples 1 , 2, and 3 substituting compounds of formula (40) for the compound of formula (10a), compounds of Formula I in which the deoxyribose mimic is 2-hydroxyethoxymethyl are made, in which the linker is attached to the 2-hydroxy group. EXAMPLE 6 Preparation of Compounds of Formula I Following the procedures of Examples 1,2, and 3. substituting compounds of formula (50) for the compound of formula (10), compounds of Formula I in which the deoxyribose mimic is l,2-dihydroxy-prop-3-yl are made, in which the linker is attached to the 1 -hydroxy group. EXAMPLE 7 Preparation of Compounds of Formula I Following the procedures of Examples 1,2, and 3, substituting the compound of formula (60) for the compound of formula (10), compounds of Formula I in which the deoxyribose mimic is l,2-dihydroxy-prop-3-yl are made, in which the linker Is attached to the 2-hydroxy group. As is known to one skilled in the art, using different protecting groups (e.g. in Examples 4, 5, 6, 7) requires selection of appropriate deprotection conditions which may be the same or different as those in Examples 1, 2, 3. Preparation of Starting Materials (10), (30), (40), (50), and (60) These compounds are prepared by conventional technique, for example, by coupling a protected base to a sugar or sugar mimetic, followed by appropriate protection of hydroxy groups on the latter. For example, N,O-Bis(trimethylsilyl acetamide (0.5 mL) is added to a suspension of "protected deoxyguanosine" (1 mmol) in dichloromethane (10 mL). Stirring at 80 C for 15 minutes gives a clear solution that is evaporated in vacuo. The residue is dissolved in toluene (5 mL) and 0.25 mL of CF 3 SO 3 SiMe , and 1.2 mmol of "protected sugar" is added. The solution is stirred at 80 C for 1 hr then cooled. EtOAc (50 mL) is added followed by aqueous workup and conventional purification. In several cases, the base-sugar coupling produces a compound with both a primary alcohol and a secondary alcohol. The secondary alcohol can be accessed by protection of the primary alcohol. The primary alcohol can be coupled directly or can be accessed by protecting the primary alcohol, then orthagonally protecting the secondary alcohol, then deprotecting the primary alcohol. For example, the primary alcohol can be protected with a trityl group, the secondary alcohol protected with an acyl or benzyl group, and the trityl group selectively removed, for example: Precursors for coupling are described in the following references: (10a), (30a), (40a): Can. J. Chem. 65, 1436 (1987), J. Org. Chem. 61, 9207-9212 (1996). (50a), (60a): J. Med. Chem. 36, 2726-2738. EXAMPLE 6 This example illustrates the preparation of a representative pharmaceutical formulation for oral administration containing a bivalent compound of the invention. Ingredients Quantity per tablet, mgs. Active Compound 200 Lactose, spray-dried 148 Magnesium stearate 2 The above ingredients are mixed and introduced into a hard-shell gelatin capsule. Other compounds of the invention can be used as the active compound in the preparation of the orally administrable formulations of this example. EXAMPLE 7 This example illustrates the preparation of another representative pharmaceutical formulation for oral administration containing a bivalent compound of the invention. Ingredients Quantity per tablet, mgs. Active Compound 400 Cornstarch 50 Lactose 145 Magnesium stearate 5 The above ingredients are mixed intimately and pressed into single scored tablets. Other bivalent compounds of the invention can be used as the active compound in the preparation of the orally administrable formulations of this example. EXAMPLE 8 This example illustrates the preparation of a representative pharmaceutical formulation containing a bivalent compound of the invention An oral suspension is prepared having the following composition. Ingredients Active Compound 1.0 g Fumaric acid 0.5 g Sodium chloride 2.0 g Methyl paraben 0.1 g Granulated sugar 25.5 g Sorbitol (70% solution) 12.85 g Veegum K (Vanderbilt Co.) 1.0 g Flavoring 0.035 ml Colorings 0.5 mg Distilled water q.s. to 100 ml Other bivalent compounds of the invention can be used as the active compound in the preparation of the orally administrable formulations of this example. EXAMPLE 9 This example illustrates the preparation of a representative pharmaceutical formulation containing a bivalent compound of the invention. An injectable preparation buffered to a pH of 4 is prepared having the following composition: Ingredients Active Compound 0.2 g Sodium Acetate Buffer Solution (0.4 M) 2.0 ml HCL (IN) q.s. to pH 4 Water (distilled, sterile) q.s. to 20 ml Other bivalent compounds of the invention can be used as the active compound in the preparation of the injectable formulations of this example. ' EXAMPLE 10 This example illustrates the preparation of a representative pharmaceutical formulation for injection containing a bivalent compound of the invention. A reconstituted solution is prepared by adding 20 ml of sterile water to lg of the compound of Formula I. Before use, the solution is then diluted with 200 ml of an intravenous fluid that is compatible with the compound of Formula I. Such fluids are chosen from 5% dextrose solution, 0.9% sodium chloride, or a mixture of 5% dextrose and 0.9% sodium chloride. Other examples are lactated Ringer's injection, lactated Ringer's plus 5% dextrose injection, Normosol-M and 5% dextrose, Isolyte E, and acylated Ringer's injection Other bivalent compounds of the invention can be used as the active compound in the preparation of the injectable formulations of this example. EXAMPLE 1 1 This example illustrates the preparation of a representative pharmaceutical formulation for topical application containing a bivalent compound of the invention. Ingredients grams Active compound 0.2-10 Span 60 2 Tween 60 2 Mineral oil 5 Petrolatum 10 Methyl paraben 0.15 Propyl paraben 0.05 BHA (butylated hydroxy anisole) 0.01 Water q.s. to 100 All of the above ingredients, except water, are combined and heated to 60°C with stirring. A sufficient quantity of water at 60°C is then added with vigorous stirring to emulsify the ingredients, and water then added q.s. 100 g. Other biyalent compounds of the invention can be used as the active compound in the preparation of topical formulations of this example. EXAMPLE 12 This example illustrates the preparation of a representative pharmaceutical formulation containing a bivalent compound of the invention. A suppository totaling 2.5 grams is prepared having the following composition: Ingredients Active Compound 500 mg Witepsol H-15 balance ( triglycerides of saturated vegetable fatty acid; a product of Riches-Nelson, Inc., New York, NN.) Other bivalent compounds of the invention can be used as the active compound in the preparation of the suppository formulations of this example. While the present invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, process, process step or steps, to the objective, spirit and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto. Table of Linkers

Claims: IT IS CLAIMED: 1. A compound represented by Formula I: L'-X-L 2 Formula I in which: L 1 is a ligand that is a competitive inhibitor of DNA polymerase; L is a ligand that is a non-competitive inhibitor of DNA polymerase; and X is a linker. or a pharmaceutically acceptable salt thereof. 2. The compound of claim 1, wherein L 1 is a nucleoside analog. 3. The compound of claim 2, wherein the nucleoside analog is chosen from acyclovir, valacylcovir, cidofovir, penciclovir, ganciclovir, penicilovir, and famciclovir. 4. The compound of claim 1 , wherein L" is chosen from a phosphinylformate, a phosphonoformate, a phosphinylacetate, and a phosphonoacetate. 5. The compound of claim 4, wherein If is phosphonoformate. 6. The compound of claim 1, wherein the linker X is represented by the formula: -X'-Z , -(Y'-Z") m -Y"-Z'-X'- in which: m is an integer of 0-20; X' at each separate occurrence is -O-, -S-, -S(O)-, -S(O) -, -NR- (where R is as defined below), -C(O)-, or a covalent bond; Z' and Z" at each separate occurrence are alkylene, cycloalkylene, alkenylene, alkynylene, arylene, heteroarylene, heterocycloalkylene, or a covalent bond; Y' and Y" at each separate occurrence are: -S(0)„-CR'R"-. -S(0) n -NR'-. -S(0) n -CR'R"-. -S(0) π -NR'-. -O-Z'-O-, -N(R)-Z'-N(R), -S-S-, or a covalent bond; in which: n is 0, 1 or 2; and R, R' and R" at each separate occurrence are chosen from hydrogen, alkyl, cycloalkyl, alkenyl, cycloalkenyl, alkynyl, aryl, heteroaryl, and heterocyclo. 7. A method for treating a disease or condition which is alleviated by treatment with an antiviral agent, comprising administering to a subject in need of such treatment a therapeutically effective amount of a compound of claim 1. 8. The method of claim 7, wherein the disease or condition is heφes, chicken pox, shingles, monocutaneous herpes, heφes encephalitis, heφes zoster, smallpox, conjunctivitis, sore throat, hepatitis B, warts, or mononucleosis. 9. A pharmaceutical composition comprising a therapeutically effective amount of a compound of claim 1, or a pharmaceutically acceptable salt thereof; admixed with at least one pharmaceutically acceptable excipient. 10. A method for identifying dimeric ligand compounds that possess bivalent properties with respect to the inhibition of viral DNA polymerase enzymes, which method comprises: (a) identifying a first ligand or mixture of first ligands that are competitive inhibitors of DNA polymerase, and having at least one chemically reactive functional group; (b) identifying a second ligand or mixture of second ligands that are non-competitive inhibitors of DNA polymerase, and having at least one chemically reactive functional group; (c) identifying a library of linkers wherein each linker in said library comprises at least two functional groups having complementary chemical activity to at least one of the first ligand functional groups and at least one of the second ligand functional groups; (d) preparing a dimeric ligand compound by combining a first ligand or ligands identified in step (a) with a second ligand identified in step (b) by reaction with the library of linkers identified in step (c) under conditions sufficient to form covalent linkages between the complementary functional groups of the first ligand or ligands and the second ligand or ligands and the linker; and (e) assaying the dimeric ligand compound library produced in step (c) to identify dimeric ligand compounds that possess bivalent properties. 1 1. The method of claim 10, wherein each member of the library is isolated and identified by preparative liquid chromatography mass spectrometry (LCMS). 12. The method of claim 11, wherein the linker or linkers employed are selected from the group comprising flexible linkers, rigid linkers, hydrophobic linkers, hydrophilic linkers, linkers of different geometry, acidic linkers, basic linkers, linkers of different polarization-and/or polarizability and amphiphilic linkers. 13. The method of claim 12. wherein the linkers comprise linkers of different chain length and/or having different complementary reactive groups. 14. The method of claim 13, wherein the linkers are selected to have different linker lengths ranging from about 2 to 1 OOangstroms. 15. The method of claims 14, wherein said reactive functional groups of ligands is selected from the group consisting of carboxylic acids, carboxylic acid halides, carboxyl esters, amines, halides, pseudohalides, isocyanates, vinyl unsaturation, ketones, aldehydes, thiols, alcohols, anhydrides, boronates, and precursors thereof, wherein the reactive functionality on the ligand is selected to be complementary to at least one of the reactive groups on the linker so that a covalent linkage can be formed between the linker and the ligand. 16. A library of dimeric compounds which may possess bivalent properties, which library is prepared by the method comprising: (a) identifying a first ligand or mixture of first ligands that are competitive inhibitors of DNA polymerase, and having at least one chemically reactive functional group; (b) identifying a second ligand or mixture of second ligands that are non-competitive inhibitors of DNA polymerase, and having at least one chemically reactive functional group; (c) identifying a library of linkers wherein each linker in said library comprises at least two functional groups having complementary chemical activity to at least one of the first ligand functional groups and at least one of the second ligand functional groups; (d) preparing a dimeric ligand compound by combining a first ligand or ligands identified in step (a) with a second ligand identified in step (b) by reaction with the library of linkers identified in step (c) under conditions sufficient to form covalent linkages between the complementary functional groups of the first ligand or ligands and the second ligand or ligands and the linker; and (e) assaying the dimeric ligand compound library produced in step (c) to identify dimeric ligand compounds that possess bivalent properties. 17. The library of claim 16, wherein the linker or linkers employed are selected from the group comprising flexible linkers, rigid linkers, hydrophobic linkers, hydrophilic linkers, linkers of different geometry, acidic linkers, basic linkers, linkers of different polarization and/or polarizability and amphiphilic linkers. 18. The library of claim 17. wherein the linkers comprise linkers of different chain length and/or having different complementary reactive groups. 19. The library of claim 18, wherein the linkers are selected to have different linker lengths ranging from about 2 to 100 Angstroms. 20. The library of claim 19, wherein the ligand or mixture of ligands is selected to have reactive functionality at different sites on said ligands. 21. An iterative method for identifying dimeric ligand compounds possessing bivalent properties which method comprises: (a) preparing a first collection or iteration of dimeric compounds by the method of claim 10; (b) assaying said first collection or iteration of dimeric compounds to assess which if any of said dimeric compounds possess bivalent properties; (c) repeating the process of (a) and (b) above until at least one dimeric compound is found to possess bivalent properties; (d) evaluating what molecular constraints imparted or are consistent with imparting bivalent properties to the dimeric compound or compounds found in the first iteration recited in (a)- (c) above; (e) creating a second collection or iteration of dimeric compounds which elaborates upon the particular molecular constraints imparting bivalent properties to the dimeric compound or compounds found in said first iteration; (f) evaluating what molecular constraints imparted or are consistent with imparting enhanced bivalent properties to the dimeric compound or compounds found in the second collection or iteration recited in (e) above; (g) optionally repeating steps (e) and (f) to further elaborate upon said molecular constraints. 22. The method of claim 31 , wherein steps (e) and (f) are repeated from 2-50 times. 23. The method of claim 32, wherein steps (e) and (f) are repeated from 5-50 times.

Inventors: GRIFFIN JOHN H, TRAPP SEAN G, WRAY JONATHAN W, GRIFFIN, JOHN, H., TRAPP, SEAN, G., WRAY, JONATHAN, W.
Applicants: ADVANCED MEDICINE INC, GRIFFIN JOHN H, TRAPP SEAN G, WRAY JONATHAN W, ADVANCED MEDICINE, INC., GRIFFIN, JOHN, H., TRAPP, SEAN, G., WRAY, JONATHAN, W.
Application year: 1999
Main classification: A61K38
IPC codes: A01N57, A61K31, A61K38, A61K39, A61K47, A61K51, A61P31, A61P43, C07B61, C07C233, C07C237, C07C271, C07C275, C07C335, C07D207, C07D211, C07D215, C07D233, C07D235, C07D263, C07D265, C07D401, C07D405, C07D413, C07D453, C07D471, C07D473, C07D475, C07D477, C07D487, C07D493, C07D495, C07D498, C07D499, C07D501, C07D519, C07H15, C07H17, C07H19, C07K2, C07K4, C12N9, C12P19, C12Q1, C40B30, C40B40, G01N33, G01N37