DOI: 10.1002/cmdc.201100101

Design Strategies for Bivalent Ligands Targeting GPCRsJeremy Shonberg, Peter J. Scammells, and Ben Capuano*[a]

Introduction

G protein-coupled receptors (GPCRs) constitute a large familyof transmembrane receptors that have a great impact on anumber of disease states. As such, GPCRs have been exploitedas drug targets, and around 30 % of all drugs on the marketact on these receptors.[1] There are approximately 800 full-length members of the GPCR superfamily, which is generallysplit into five main families termed glutamate, rhodopsin, ad-hesion, secretin and frizzled/taste2.[2] The functional signifi-cance and the wide structural diversity of this superfamily ofreceptors has made them an attractive target in drug discov-ery,[3] and they continue to be explored.

The propensity for GPCRs to form dimeric and higher orderoligomers in natural tissues has been well established sincethe concept originated 30 years ago.[4–6] There is a broad con-sensus that class C receptors, in particular, exist as stabledimers,[7] both as homodimeric receptors, for example the co-valently bound metabotropic glutamate receptor (mGlu5)homodimer,[8] and as heterodimeric receptors,[9] as seen in theGABAB receptor 1 and GABAB receptor 2 heterodimer.[10] Anumber of other GPCRs have also been shown to dimerise,most notably the adrenergic receptors,[11, 12] opioid recep-tors,[13, 14] somatostatin receptors,[15, 16] purinergic receptors,[17–20]

and other druggable targets, such as the muscarinic recep-tors,[21, 22] and dopaminergic receptors.[23–27] New combinationsof receptors are continually being discovered to form homo-and heterodimers, as well as higher order oligomers in naturaltissues, for example the existence of heterotrimers combiningthe adenosine A2A (A2AR), dopamine D2 (D2R), and metabotropicmGlu5 receptors,[29] or heterotrimerization of A2AR/D2R/CB1 re-ceptors.[30]

There are significant pharmacological advantages in target-ing GPCR dimers with bivalent ligands designed to probe spe-cific dimeric binding sites.[31] Based on pioneering work by Por-toghese et al. targeting opioid receptor subtypes, such as ana-lysing the varying interreceptor distances between the sub-types (e.g. , m, k and d receptors),[32, 33] it has become apparentthat targeting dimeric GPCR units with bivalent ligands can

result in significant therapeutic advantages. Benefits can in-clude higher ligand affinity and selectivity, reduced reliance onmultiple drug regimens upon administration,[34] and an en-hanced or modified physiological response.[35]

Bivalent ligands are defined as compounds that contain twopharmacophoric units, an appropriately designed spacer toseparate and define the two pharmacophores, and a linkerunit to connect the pharmacophore to the spacer (Figure 1).They are designed to span across homo- or heterodimericGPCRs and therefore target two individual receptors simultane-ously. There are two general classifications for bivalent ligands:homobivalent, where the two pharmacophores are the same(1, Figure 1); and heterobivalent, where the two pharmaco-phores are different.[32, 36, 37] The increase in affinity that is attrib-uted to bivalent ligands (relative to the individual binding oftwo monovalent counterparts) arises from the thermodynamicadvantage that is inherent in a cooperative binding system. Inthis way, the overall entropy of the ligand–receptor complex issignificantly lowered by having the second pharmacophore lo-calised in the vicinity of its respective binding pocket uponbinding of the first pharmacophore.[38]

An interesting variation on the notion of cooperative bind-ing is the concept of bitopic ligands, in which one part of thepharmacophore targets the orthosteric, endogenous ligandbinding site, whilst the other pharmacophore section interactswith an allosteric binding site of the same receptor.[39–41] Someadvantages of simultaneously targeting the two topographical-ly distinct binding sites include the modulation of endogenousactivity, as well as the possibility of enhanced receptor-subtypeselectivity. Both of these properties are considered to reflect aconformational change in the GPCR caused by one pharmaco-

Specifically designed bivalent ligands targeting G protein-cou-pled receptor (GPCR) dimeric structures have become increas-ingly popular in recent literature. The advantages of the biva-lent approach are numerous, including enhanced potency andreceptor subtype specificity. However, the use of bivalent li-gands as potential pharmacotherapeutics is limited by prob-lematic molecular properties, such as high molecular weightand lipophilicity. This Minireview focuses on the design of biva-

lent ligands recently described in the literature; discussing thechoice of lead pharmacophore, the position and nature of theattachment point for linking the two pharmacophore units,and the length and composition of the spacer group. Further-more, this Minireview distils the molecular descriptors of thebivalent ligands that exhibit in vivo activity, as well as high-lights their ability to access the central nervous system.

[a] J. Shonberg, Prof. P. J. Scammells, Dr. B. CapuanoMedicinal Chemistry and Drug ActionMonash Institute of Pharmaceutical Sciences381 Royal Pde, Parkville, Victoria 3052 (Australia)Fax: (+61) 399-039-581E-mail : ben.capuano@monash.edu

ChemMedChem 2011, 6, 963 – 974 � 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 963

phore, which affects the interactive properties of the GPCRwith the other pharmacophore.[39] One excellent example of adesigned bitopic ligand was the fusion of the A1R orthostericendogenous agonist, adenosine, to a known A1R allostericmodulator, PD81,723.[42] The bitopic ligand 2 was optimisedwith short spacer groups (9 atoms) to indicate the likely loca-tion of the allosteric binding site to be in close proximity tothe orthosteric site (Figure 2). Another classic example of a bi-

topic ligand is McN-A-343 (3), a functionally selective muscarin-ic acetylcholine receptor (mAChR) partial agonist that can alsointeract allosterically at the M2 mAChR,[43] and it has beenshown to act as a bitopic orthosteric/allosteric ligand(Figure 2). Bitopic ligands are fast becoming an exciting newarea of research and are the focus of a number of recent re-views.[44, 45]

The large size and chemical nature of bivalent ligands in par-ticular, often results in an extremely challenging synthesis. Inattempting to extend the spacer across a dimeric cavity, therebegins to emerge an extensive range of problems from a syn-

thetic stand point that is oftennot discussed in the literature.There are some methods tocombat these problems, and thisMinireview aims to addresssome of these issues, as well asbeing a concise guide to a me-dicinal chemist beginning to ex-plore the design and synthesisof bivalent ligands. Furthermore,this Minireview aims to summa-rise the recent advances in thedesign of bivalent ligands andthe potential benefits that canbe achieved.

General Features of Bivalent Ligands

A recent review on bivalent ligands and receptor dimerisa-tion[46] highlighted that the design of a bivalent ligand requiresthe following selections: a good monomeric ligand that is se-lective and able to allow attachment of a linker; an appropriatespacer that can link the two monomers; and a linker thatwould not interfere with or contribute to receptor–ligand inter-actions. These aspects are to be described in more detail inthe following section, citing recent examples from the litera-ture to quantify these important aspects.

The monovalent counterparts

With the vast array of GPCR dimers as potential new drug tar-gets,[5] the choice of pharmacophore is extremely important.There are three main features that must be considered whenchoosing the individual ligands which are to be linked: molec-ular bulk, potency, and the ability to functionalise without dis-turbing the receptor–ligand interactions. Table 1 shows a sum-mary of some relatively recently reported monovalent ligandsto have been favourably modified in the synthesis of a bivalentligand, and gives an indication of the parameters that are gen-erally applicable for bivalent ligands. These can be summarisedto give the following two points.

Lead compounds for the synthesis of bivalent ligands aregenerally of low/medium molecular weight (300<MW<

400 Da, Table 1). The original lead compound must be suffi-ciently low in molecular weight to account for the significantadditional bulk added through the attachment of linker andspacer portions. For example, the 5-HT4 partial agonistML10302 has a molecular weight of 312.8 Da, allowing forchemical bulk to be added without detrimental effects.[47]

Lead compounds for the synthesis of bivalent ligands aregenerally of low nanomolar affinity (Table 1). It is commonlyobserved that significant reductions in affinity occur upon theattachment of a linker and spacer portion to the original leadcompound. This reduction in affinity has been shown to be upto two orders of magnitude; for example, in the synthesis ofbivalent ligands targeting the D2R/A2AR dimer.[48] In consideringthe mechanism of bivalent ligand binding to the dimeric com-

Figure 1. The general features of bivalent ligands, as superimposed on a CNS-active bivalent ligand targeting theCB1 receptor homodimer.[28] The pharmacophore is based on SR141716 (14), a highly potent and selective canna-binoid CB1 receptor (CB1R) antagonist/inverse agonist. The linker is an amide bond, and the spacer is an alkyla-mine of 15 atoms in length.

Figure 2. Chemical structures of designed bitopic ligand (2) based on theorthosteric agonist adenosine (7) and the allosteric ligand PD81,723,[42] andthe bitopic orthosteric/allosteric ligand McN-A-343 (3).[43]

964 www.chemmedchem.org � 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemMedChem 2011, 6, 963 – 974

MED J. Shonberg, P. J. Scammells, B. Capuano

Table 1. Key properties of lead compounds 4–14 for the synthesis of bivalent ligands.

Lead compound[a] Name Mechanism of action MW [Da] Ki [nm]

4 butorphan[49, 50] m/k receptors partial agonist 310.5m : 0.23k : 0.08

5 oxymorphone[51, 52] m receptor agonist 301.4 0.97

6 L-365,260[51, 53] cholecystokinin (CCK2) receptor antagonist 398.5 18

7 adenosine[42, 54] A1R agonist 373.4 7.3

8 XCC[48, 55] A2AR antagonist 386.4 1

9 (�)-PPHT-NH2[48, 56] D2R agonist 309.4 13.3

10 formoterol[54] b2-AR agonist 344.4 23

114-[2-(3-methoxyphenyl)ethyl]-1-(2-methoxyphenyl)piperazine[57]

5-HT7/5-HT1A receptor ligand 326.4 2.4

12 ML10302[35, 47, 58] 5-HT4 receptor partial agonist 312.8 5

ChemMedChem 2011, 6, 963 – 974 � 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemmedchem.org 965

Bivalent Ligands Targeting GPCRs

plex, it is essential that some affinity is retained from the initialbinding of the first pharmacophore. As such, more potency at-tributed to the original lead compound can result in enhancedbinding of the final bivalent ligand.

Attachment point

In the synthesis of bivalent ligands, a crucial feature is the posi-tion and nature of the attachment point for linking the twopharmacophore units (see arrows in Table 1). There are a fewkey questions that must be answered in order to begin thesynthesis of a bivalent ligand including, which is the best posi-tion for linker attachment? Is it better to modify or add tofunctionality present on the original pharmacophore? Whichfunctionality will serve as an appropriate linking portion?

Determining the best position for attachment of the linkergroup can be achieved using a good understanding of thestructure–activity relationships of the monovalent counter-parts. An example of this is the synthesis of bivalentb2 adrenergic (b2AR) and A1R ligands by Karellas et al.[54]

(Figure 3). It is known that the N6-position of adenosine (7) tol-erates large substituents,[60, 61] and therefore this site waschosen to be most suitable for the attachment of a linkergroup for the A1R. This was synthetically accessible considering

the commercial availability of 6-chloropurine riboside, the keyintermediate.

In another recent example by Zheng et al. ,[51] bivalent li-gands were synthesised to target a chemically induced hetero-dimer of m-CCK2 receptors, an interaction that does not exist innative tissue.[51] These bivalent ligands (16, Figure 4) were de-signed around the m-selective agonist oxymorphone (5),[62] andthe CCK2 antagonist L-365,260 (6).[53] The ketone portion of theoxymorphone derivative was chosen as the attachment point(Table 1), which was subsequently modified to an amine forease of connection of the linker and spacer portions via anamide bond. This was achieved with the added incorporationof an ether functionality for increased hydophilicity, using di-glycolic anhydride. The spacer group was then constructed bythe addition of varying length mono-Boc-diamines in the pres-ence of carbodiimide peptide coupling reagents, such as N,N’-

Table 1. (Continued)

Lead compound[a] Name Mechanism of action MW [Da] Ki [nm]

13 xanomeline[59] M1 receptor agonist 281.4 6.8

14 SR141716[28] CB1 receptor antagonist/inverse agonist 463.8 25

[a] Arrows indicate the point of attachment of linker and spacer groups. Components coloured grey indicate removal of that section in place of linker andspacer groups.

Figure 3. An example of a bivalent ligand (15) targeting the A1-b2 receptorheterodimer through a spacer. The N6-position of adenosine was chosen asthe best point of attachment.[54]

Figure 4. An example of a bivalent ligand (16) based on the oxymorphone(5) and L-365,260 (6) pharmacophores. The ketone of the oxymorphonecore and the aromatic methyl groups were the chosen points for attach-ment of a spacer.

966 www.chemmedchem.org � 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemMedChem 2011, 6, 963 – 974

MED J. Shonberg, P. J. Scammells, B. Capuano

dicyclohexylcarbodiimide (DCC),to give the final monovalent andbivalent ligands. Conversely, thearomatic methyl group of L-365,260 (6) was selected as theattachment point (Table 1),which was converted to a phe-nolic ether for attachment of thelinker and subsequent spacerfunctionality, in the samemanner as previously described.An example of the resulting bi-valent ligand (16) can be seen inFigure 4.

The examples given above in-dicate where linker attachmentwas achieved through eitherfunctionalisation of the existingpharmacophore (as in Figure 2),or the modification of existingfunctionality by the removal (oraddition) of a specific portion (asin Figure 3), to allow for attachment of the linker group. A gen-eral summary of linker attachment strategies can be found inthe 2008 review by Berque-Bestel et al. ,[63] which focuses onother general design strategies for the synthesis of bivalent li-gands. Ideally, it would be beneficial to have a standard set offunctional additions or modifications for the attachment of alinker that could be tolerated across a wide range of GPCRsand their respective hetero- and homodimers, and higherorder oligomers, but aside from those pointed out by Berque-Bestel et al.[63] this does not exist to the best of our knowledge.This means that the synthesis of bivalent ligands can be a slowand exhaustive process before the final bivalent ligands haveeven been synthesised, because a range of monovalent ligandsmust first be synthesised and extensively assessed.

Length of the spacer

Both the length and composition of the spacer unit employedin the synthesis of bivalent ligands can have a profound effecton the overall potency of the final compounds in terms ofbinding to dimeric receptors. The optimal distance range forspacer units connecting the two pharmacophores has beendetermined for a number of different targets, especially theopioid receptors.[36, 64, 65] In their synthesis of various bivalent li-gands targeting opioid receptors (m, d and k dimers), Portogh-ese et al.[32, 36, 65, 66] showed that an optimal distance to span theheteromeric cavities is approximately 21 atoms between phar-macophores. This can be seen in a recent example by Zhanget al. ,[65] where the k antagonist 5’-guanidinonaltrindole andthe m antagonist b-naltrexamine were used as lead compoundsfor the synthesis of selective k–m bivalent ligands (Figure 5).After synthesising spacers between 20 and 23 atoms in length,they found that the only bivalent ligand which showed an en-hanced binding affinity at the dimeric receptors was KMN-21(17), with a 21 atom spacer connecting the two pharmaco-

phores. This finding concurs with another study that revealeda selective k–d bivalent ligand, KDN-21 (18),[36] which also ex-perienced significantly enhanced potency and selectivity to-wards the target receptors in cultured cells with co-expressedreceptors when a spacer of 21 atoms was employed. Thesefindings suggest that for bivalent ligands targeting opioid re-ceptor dimers, 21 atoms is the ideal spacer length.

A different method was adopted in other studies, where thislength of 21 atom spacers was far surpassed, for example bySoriano et al.[48] In their synthesis of D2R agonist/A2AR antago-nist bivalent ligands (19), the minimum spacer length reportedwas 26 atoms ranging up to 118 atoms in length connectingthe two pharmacophores. It is perhaps no surprise that no cor-relation was found between spacer length and binding affinityof the bivalent ligands relative to the monovalent ligands.However, irrespective of this excessive length, a significant in-crease in binding affinity was observed for the bivalent ligandsrelative to their monovalent counterparts. The final structuresof these ligands display the nature of the Lys-Lys-[polyethyleneglycol (PEG)/polyamide]n-Lys-Glu spacer unit, which was em-ployed by the research group to span the vast distance be-tween the dimeric binding sites (Figure 6). Solid-phase peptidesynthesis was used for the synthesis of these elongated spacerunits, which were then connected to the pharmacophores sys-tematically using benzotriazol-1-yl-oxytripyrrolidinophos-phoni-um hexafluorophosphate (PyBOP)-mediated peptide coupling.The advantage of this method is the extreme length that canbe achieved through multiple coupling steps. However, thedisadvantage is the inability to build the spacer groups sys-tematically in incrementally increasing length to enable a me-thodical analysis of the exact distance between the bindingsites of the dimer. This series of bivalent ligands are the first re-ported molecules to target the D2R/A2AR heterodimer, whichmay require shorter spacer groups for a significant bivalentligand binding effect to be observed.

Figure 5. Structures of recently reported bivalent ligands targeting opioid receptor dimers with spacers of 21atoms in length. This length resulted in the best binding profiles of series targeting the k-m and k-d dimer.[36, 65]

ChemMedChem 2011, 6, 963 – 974 � 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemmedchem.org 967

Bivalent Ligands Targeting GPCRs

In another recent example, Tanaka et al.[67] used precise in-cremental additions to the spacer group of bivalent ligandstargeting the CXC chemokine receptor 4 (CXCR4). In doing so,the bivalent ligand could be used as a “molecular ruler” to fur-ther understand the details of CXCR4 oligomer formation.These incremental additions were on a large scale, with spacergroups spanning between two to eight nanometers in length.The research team found the optimal length between the li-gands to be six nanometers, which resulted in low nanomolarpotency (Figure 7).

The above three examples give a summary of some vastlydifferent spacer lengths reported in recent literature. There aregenerally no rules governing the precise distances that shouldbe achieved by bivalent ligands at various receptors. As such,these three examples give an outline of the general lengthsought after by researchers with a dimeric GPCR target.

Composition of the spacer

Spacer diversity is governed notonly by length, but also by thecomposition of spacer function-ality. A noteworthy list of spacergroups can be seen for the syn-thesis of 5-HT4 receptor bivalentligands based on the highlypotent and specific partial ago-nist ML10302 (12).[35, 47, 68] A di-verse range of spacer groupswere synthesised for connectingthis pharmacophore, includingpolyalkyl chains, polyamidechains, polar oxygenatedspacers, hydrophobic spacers,cyclic core alkyl spacers, flexible

aromatic containing spacers, and constrained aromatic con-taining spacers. All of these variations were synthesised in vari-ous lengths to probe the 5-HT4 receptor dimer for tolerancetowards various groups in the spacer region. It was suggestedthat a spacer length of 20–24 atoms was ideal when directedin a specific orientation to allow for binding of the two phar-macophores at the respective receptors.[35] The variety ofspacer groups are presented in Table 2, accompanied by addi-tional examples including some of the vast array of spacerssynthesised by Bonger et al.[69–71] who generated bivalent li-gands targeting the gonadotropin releasing hormone receptor(GnRHR) with rigid benzene-based scaffolds (21, 23, 26).[70]

These spacers could potentially be incorporated syntheticallyinto any bivalent ligand, provided the functionality is presentto join the portions. However, they all have their respective ad-vantages and disadvantages. The most significant of these ef-fects is on the lipophilicity (determined by the calculated logof the octanol/water partition coefficient, cLog P), and totalpolar surface area (tPSA) of the final compounds, as well as theflexibility in terms of the number of rotatable bonds. The po-larity of the spacer is extremely important for the solubility ofthe bivalent ligand, which can invariably become increasinglyhydrophobic with increasing length (as can be seen whenlengthening a polymethylene chain). This can result in prob-lematic aqueous solubility properties unless the appropriatemeasures are taken, and can have implications in the synthesis,pharmacological analysis, and potential optimisation as phar-macological tools or therapeutics.

Of equal importance is the interaction of the spacer with thedimeric structure of the target GPCR, which can significantlyvary based on the polarity of the spacer.[47] The rigidity of thespacer, represented in Table 2 as number of rotatable bonds, isalso of considerable interest as it can result in an ’all or noth-ing’ approach as described by Bobrovnik,[72] whereby the ap-propriately designed rigid spacer can selectively increase theconcentration of the latter binding pharmacophore to theactive site of the binding cavity. Both of these factors, cLog P/tPSA and rigidity of the spacer, are important considerationswhen designing a bivalent ligand, and the list in Table 2 can

Figure 6. Heterobivalent D2R/A2AR ligands (19) designed with spacer lengths ranging from 26–118 (n = 0–7) atomsin total length between the two pharmacophores.[48]

Figure 7. Graphic adapted from Tanaka et al.[67] showing the relationship be-tween potency of synthesised bivalent ligands targeting CXCR4 receptorsand the length of the linker group connecting the two pharmacophores.

968 www.chemmedchem.org � 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemMedChem 2011, 6, 963 – 974

MED J. Shonberg, P. J. Scammells, B. Capuano

Table 2. Examples of spacer groups that have been recently reported in the literature. The examples have been modified to have a total spacer length of15 atoms. The polarity, tPSA, and number of rotatable bonds act as a guide to the medicinal chemist searching for a suitable spacer.[a]

Spacer cLog P[b] tPSA [�2] Number of rotatable bonds Reference

20 �2.82 145.5 11 [28, 65]

21 0.19 58.2 6 [70, 78]

22 0.33 46.2 16 [28, 54, 71, 73, 74, 78, 79]

23 0.81 58.2 6 [70, 78]

24 1.89 98.3 8 [69]

25 2.28 27.7 16 [73]

26 2.67 58.2 6 [70, 78]

27 3.53 40.6 11 [80]

28 5.06 6.5 13 [80]

29 6.98 44.4 16 [47]

30 8.11 59.1 8 [35]

31 8.43 59.1 14 [35]

32 8.80 48.5 14 [47]

33 9.49 0 14 [47]

34 9.69 0 16 [50, 54, 57, 81–83]

35 9.85 9.2 11 [47]

[a] Properties were calculated using ChemBioDraw Ultra version 12.0 (CambridgeSoft 2010). Pharmacophores are represented as circles. [b] Spacer groupsare sorted in order of increasing cLog P values.

ChemMedChem 2011, 6, 963 – 974 � 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemmedchem.org 969

Bivalent Ligands Targeting GPCRs

serve as a list of standards for a medicinal chemist undertakinga project in the synthesis of bivalent ligands. From this list it isclear to see that some of the most hydrophilic spacer groupsinvolve polyamide (20) and PEG (22) units within the structure.This results in a significant reduction in the cLog P of thespacer, and can therefore be implemented when the pharma-cophoric units are overtly hydrophobic. However, polyamideand polyethylene glycol units have great variation in tPSA (seeTable 2), and this fact can be exploited. For example, bivalentligands targeting the opioid receptors (e.g. , 16–18) commonlycontain a combination of polyamide (most commonly polygly-cine) and alkyl chain spacers to have a combination effect onpolarity and tPSA.[14, 32, 36, 51, 65, 66] Furthermore, to accompany theadditional hydrophilicity of the spacer, flexibility can be in-creased using polyethylene glycol spacers or reduced usingpolyamide spacers to constrain the number of rotatablebonds.

It is interesting to note the use of polybutylene glycolspacer groups,[73] which have been designed to counter thenatural coil-like conformation obtained when using PEGspacers,[74–76] whilst retaining an element of hydrophilicity andflexibility. On the other hand, multiple oligoproline residues(24) have been used to form well-defined helical structures[77]

for the synthesis of specifically oriented bivalent ligands with acoiled spacer.[69] Another alternative to improving the hydro-philicity of the bivalent ligand (Table 2) is the incorporation ofpiperazine into the spacer group (28), which has slightly lessflexibility than an alkyl chain and PEG spacer, but slightly morethan polyamide chains, and for this reason it is an importantdesign consideration for bivalent ligands. A range of hydro-phobic spacer groups are also listed in Table 2, and this prop-erty can be introduced with varying levels of effect on rigidity.Indeed, a very rigid spacer can be synthesised with the incor-poration of a diyne central core (21), which hugely restricts thepossible conformations of the spacer.[35, 70, 78] Successful incorpo-ration of such rigidity can be immensely informative towardthe shape and nature of the spacer required for successful bi-valent ligand binding, however, only a few examples existwhere this is the case (see references in Table 2). Synthetically,all of the spacer groups listed in Table 2 are available to beused for any pharmacophore given the appropriate functionali-ty is in place, and this offers a broad range of diversity in thesynthesis of bivalent ligands. Invariably, the nature of thespacer group incorporated into the bivalent ligand plays animportant role in the overall function and capacity of the biva-lent ligand, the design of whichcan be tailored based on thesought after properties.

Bivalent Ligands In Vivo

The potential for bivalent ligandsto act as pharmaceutical thera-pies, as opposed to solely phar-macological tools, remains atopic of discussion.[63, 84] Someobvious hurdles exist in the de-

velopment of bivalent ligands as pharmaceutical therapies, in-cluding their design and synthesis (see above), their ability tosurvive in vivo, their propensity to cross the blood–brain barri-er (BBB), and other absorption, distribution, metabolism, andexcretion (ADME) issues. Furthermore, unfavourable physico-chemical characteristics, such as large size and molecularweight, add to this problem that probably limits the future useof bivalent ligands to purely pharmacological probes, as mostbivalent ligands are currently. However, whilst these barriersare seemingly impenetrable, they can potentially be overcomeby appropriately designed bivalent ligands, and the followingexcellent examples in the literature have shown this to be thecase.

Survival of bivalent ligands in vivo

The most extensive study of a bivalent ligand in vivo character-ised MDAN-21 (36), one of a series of bivalent ligands champi-oned by Portoghese et al.[64] designed as prospective analge-sics to investigate the physical relationship between m and d

receptor dimers. The compounds consist of the pharmaco-phores oxymorphone (5) and naltrindole as representatives ofa m agonist and d antagonist, respectively, connected via a des-ignated linker varying in chain length from 16 to 21 atoms,with the most active of these, MDAN-21, shown in Figure 8.The radiant heat tail flick assay, which is a centrally mediatedbehavioural model, was used to evaluate these compounds forantinociceptive properties. The data identified MDAN-21 (36)with a 21-atom linker as the best candidate with the potentialto be an effective drug, following intravenous (i.v.) administra-tion, for the treatment of pain. MDAN-21 was also analysedafter being administered intracerebroventricularly (i.c.v.), aroute often used for the analysis of compounds that do notcross the BBB, thereby circumventing the influence of the BBBand other peripheral metabolic processes on ligand access tothe brain. It was found that the i.v./i.c.v. potency ratio forMDAN-21 was identical to that of morphine, indicating thatboth morphine and MDAN-21 have similar access to neuronalopioid receptors, and that the bivalent ligand remains fully intact following administration. Furthermore, MDAN-21 wasfound to be 50-fold more potent than morphine, and devoidof tolerance and physical dependence following chronic ad-ministration.[64] This excellent example shows that an appropri-ately designed bivalent ligand can have a profound physiologi-cal effect, survive i.v. administration, and cross the BBB.

Figure 8. Structure of the centrally active MDAN-21 (36), which was found to be 50-fold more potent than mor-phine whilst sharing identical access to opioid receptors irrespective of the administration route (i.v. or i.c.v.).

970 www.chemmedchem.org � 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemMedChem 2011, 6, 963 – 974

MED J. Shonberg, P. J. Scammells, B. Capuano

Crossing the BBB

Many bivalent ligands are designed to target receptor dimerspresent in the central nervous system (CNS), in an attempt toact as pharmacological tools or potentially new therapies fordiseases such as Parkinson’s disease,[48] Alzheimer’s dis-ease,[47, 59, 81] schizophrenia,[81] migraine,[37, 85] and pain.[28, 51, 65]

Clearly, to have any pharmacological effect in vivo, these com-pounds must cross the BBB, an extensive network of capillarieslined with endothelial cells to prevent the CNS from potentiallyharmful xenobiotics.[86] Active transport pumps play a signifi-cant role in BBB penetration, with a number examples takingadvantage of this method, such as amino acid pumps to trans-port l-DOPA and gabapentin, and monocarboxylic acid trans-port systems to carry simvastatin acid, for example.[87] Passivediffusion is another common method for drugs to penetratethe BBB, and it has been stated that the probability of a libraryproviding favourable BBB permeability properties due to pas-sive diffusion can be enhanced by the application of theguidelines stated below:[88]

· tPSA (�2) <90· Hydrogen Bond Donors (HBD) <3· cLog P = 2–5· MW <450 Da

A number of reports have outlined that the most influentialmolecular properties responsible for BBB penetration are polar-ity/polarizability, and the number of hydrogen bond donors(HBD),[89, 90] whilst other parameters such as molecular weight(MW) and cLog P are less impactful.[88] Nonetheless, penetrationinto the CNS is not guaranteed even when the above criteriaare all satisfied, nor is access denied if they are unmet. Indeed,almost all bivalent ligands acting at CNS-based receptorsexceed these guidelines, revealing some excellent examples ofsuccessful BBB penetrable bivalent ligands in the literature.

Of note is an example reported by Perez et al. ,[85] where apotent 5-HT1B receptor bivalent ligand (38), based on theknown agonist sumatriptan (37), was designed, synthesisedand evaluated for the treatment of migraine (Figure 9). Follow-ing systemic and oral administration of the bivalent ligand inguinea pig, the bivalent ligand was found to induce strongerhypothermia and more potent 5-HT1B receptor agonism thanthe lead compound, sumatriptan, implying that CNS activitycould be achieved through BBB penetration by the bivalentligand (38).

The sumatriptan-based bivalent ligand (38) does not complywith many of the recommended limitations for BBB penetra-tion by passive diffusion,[88] with the relevant parametersstated below, as calculated using an online application (http://www.molinspiration.com/cgi-bin/properties).

· tPSA = 112.8 �2

· HBD = 2· cLog P = 5.2· MW = 721.0 Da

It is, however, possible that active transport played a role inBBB penetration, despite the large size and nature of the biva-lent ligand, but this has not been experimentally proven.Nonetheless, whether it is a result of passive diffusion or activetransport, this example shows a distinct case where BBB pene-tration can be achieved for large bivalent ligands.

Indeed, other examples in the literature also present central-ly active bivalent ligands, which are able to penetrate the BBBdespite published guidelines suggesting otherwise.[88] As previ-ously mentioned, MDAN-21 (36, Figure 8) has been shown toaccess neuronal opioid receptors following i.v. administration,which is suggestive of BBB penetration.[64] MDAN-21 surpassesthe general guidelines[88] for passive diffusion across the BBB,with properties stated below.

· tPSA = 256.5 �2

· HBD = 9· cLog P = 3.3· MW = 1058.2 Da

These values clearly show the extremely large molecularweight and polar surface area, as well as a large number of hy-drogen-bond donors relative to the generally accepted guide-lines for BBB penetration by passive diffusion. This suggeststhat either active transporters have assisted penetration, orthat large exceptions exist in the guidelines for passive diffu-sion of compounds across the BBB. Yet the sheer fact that biva-lent ligands with such extreme properties are able to penetratethe BBB is a remarkable achievement.

In a more recent example, Zhang et al. synthesised a rangeof bivalent ligands targeting the CB1 receptor based on the se-lective CB1 receptor antagonist/inverse agonist SR141716(14).[28] From SAR studies on the lead compound, it was foundthat replacements of the piperidinyl group with a variety ofeither alkyl or aryl groups, were generally well tolerated(Table 1).[91, 92] This enabled an appropriate linking point for syn-thesising bivalent ligands. Synthetically this could be achievedby (benzotriazol-1-yloxy)tris(dimethylamino)phosphonium hex-afluorophosphate (BOP)-mediated peptide coupling with a

Figure 9. The 5-HT1B receptor agonist sumatriptan (37) and the respectivepotent bivalent ligand (38) synthesised by Perez et al.[85] that showed BBBpenetration.

ChemMedChem 2011, 6, 963 – 974 � 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemmedchem.org 971

Bivalent Ligands Targeting GPCRs

range of primary amine containing linkers and spacers. Themost active bivalent ligand synthesised in the series (1,Figure 10) was shown to attenuate antinociceptive effects ofthe cannabinoid agonist CP55,940 in a tail flick assay after sys-temic administration, suggesting enhanced CNS penetration.Again, based on the general guidelines for central activity bypassive diffusion, this compound far surpasses the acceptable

limitations for BBB penetration. Most notable is the large mo-lecular weight, and the very hydrophobic nature of the finalcompound.

· tPSA = 105.9 �2

· HBD = 3· cLog P = 9.9· MW = 970.7 Da

This range of bivalent ligands, which are able to penetratethe BBB, all show properties that are inconsistent with generalfeatures associated with CNS penetrating compounds. In par-ticular, molecular weight and cLog P of many of the above-mentioned bivalent ligands, significantly exceed the generalguidelines that govern BBB penetration by passive diffusion.This should give confidence to the medicinal chemist targetingdimeric GPCRs in the CNS, given appropriately designed biva-lent ligands are able to pass into the CNS.

Outlook

Bivalent ligands targeting GPCRs have the potential to provideacademia and the pharmaceutical industry with a fresh ap-proach to pharmaceutical design. Dimeric GPCRs are continual-ly being discovered as potential drug targets that exist natural-ly, and it is only a matter of time before full advantage is takenof these targets. The possibility of gaining significantly in-creased selectivity and potency through the use of bivalent li-gands is an attractive pathway that is becoming more preva-lent in the literature.

The synthesis of bivalent ligands, however, poses a signifi-cant challenge due to the size and complexity of the final com-

pounds. Ultimately, the synthesis requires consideration of as-pects, including the choice of the initial monomeric lead com-pound, choice of attachment point for a linker, and choice ofthe length and composition of the linker/spacer. A successfulcombination of these aspects can allow for significant increas-es in potency of the final bivalent ligand, whilst still permittingCNS penetration, and therefore, these compounds have great

potential as pharmacologicaltools, or possibly therapeutics.Therefore, the synthesis of biva-lent ligands targeting GPCRdimers will likely continue to fea-ture as a prominent aspect ofmodern medicinal chemistry.

Acknowledgements

An Australian PostgraduateAward to JS is gratefully acknowl-edged for financial support. Theauthors also wish to thank Dr.Joseph Nicolazzo and Ms.Fiona M. McRobb for helpful dis-cussions and assistance in thepreparation of the manuscript.

Keywords: bivalent ligand · design strategies · dimerization ·drug design · G protein-coupled receptor

[1] J. P. Overington, B. Al-Lazikani, A. L. Hopkins, Nat. Rev. Drug Discovery2006, 5, 993 – 996.

[2] R. Fredriksson, M. C. Lagerstrçm, L.-G. Lundin, H. B. Schiçth, Mol. Phar-macol. 2003, 63, 1256 – 1272.

[3] M. C. Lagerstrçm, H. B. Schiçth, Nat. Rev. Drug Discovery 2008, 7, 339 –357.

[4] E. Hazum, K. Chang, P. Cuatrecasas, Science 1979, 206, 1077 – 1079.[5] V. Casad�, A. Cort�s, J. Mallol, K. P�rez-Capote, S. Ferr�, C. Lluis, R.

Franco, E. I. Canela, Pharmacol. Ther. 2009, 124, 248 – 257.[6] V. V. Gurevich, E. V. Gurevich, Trends Pharmacol. Sci. 2008, 29, 234 – 240.[7] J. P. Pin, T. Galvez, L. Pr�zeau, Pharmacol. Ther. 2003, 98, 325 – 354.[8] C. Romano, W.-L. Yang, K. L. O’Malley, J. Biol. Chem. 1996, 271, 28 612 –

28 616.[9] S. C. Prinster, C. Hague, R. A. Hall, Pharmacol. Rev. 2005, 57, 289 – 298.

[10] S. Balasubramanian, J. A. Teiss�re, D. V. Raju, R. A. Hall, J. Biol. Chem.2004, 279, 18840 – 18850.

[11] J. Xu, J. He, A. M. Castleberry, S. Balasubramanian, A. G. Lau, R. A. Hall, J.Biol. Chem. 2003, 278, 10770 – 10777.

[12] S. Angers, A. Salahpour, E. Joly, S. Hilairet, D. Chelsky, M. Dennis, M.Bouvier, Proc. Natl. Acad. Sci. USA 2000, 97, 3684 – 3689.

[13] D. Ramsay, E. Kellett, M. McVey, S. Rees, G. Milligan, Biochem. J. 2002,365, 429 – 440.

[14] P. S. Portoghese, J. Med. Chem. 2001, 44, 2259 – 2269.[15] M. Dur�n-Prado, M. M. Malag�n, F. Gracia-Navarro, J. P. CastaÇo, Mol.

Cell. Endocrinol. 2008, 286, 63 – 68.[16] M. Grant, U. Kumar, Regul. Pept. 2010, 159, 3 – 8.[17] M. Canals, J. BurgueÇo, D. Marcellino, N. Cabello, E. I. Canela, J. Mallol,

L. Agnati, S. Ferr�, M. Bouvier, K. Fuxe, F. Ciruela, C. Lluis, R. Franco, J.Neurochem. 2004, 88, 726 – 734.

[18] M. Canals, D. Marcellino, F. Fanelli, F. Ciruela, P. de Benedetti, S. R. Gold-berg, K. Neve, K. Fuxe, L. F. Agnati, A. S. Woods, S. Ferre, C. Lluis, M.Bouvier, R. Franco, J. Biol. Chem. 2003, 278, 46741 – 46749.

[19] J. Hillion, M. Canals, M. Torvinen, V. Casad�, R. Scott, A. Terasmaa, A.Hansson, S. Watson, M. E. Olah, J. Mallol, E. I. Canela, M. Zoli, L. F.

Figure 10. The CB1 receptor antagonist/inverse agonist SR141716 (14) and the respective, centrally active, bivalentligand (1) synthesised based on this pharmacophore.[28]

972 www.chemmedchem.org � 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemMedChem 2011, 6, 963 – 974

MED J. Shonberg, P. J. Scammells, B. Capuano

Agnati, C. F. Ib�Çez, C. Lluis, R. Franco, S. Ferr�, K. Fuxe, J. Biol. Chem.2002, 277, 18091 – 18097.

[20] S. Ferre, C. Quiroz, A. S. Woods, R. Cunha, P. Popoli, F. Ciruela, C. Lluis, R.Franco, K. Azdad, S. N. Schiffmann, Curr. Pharm. Des. 2008, 14, 1468 –1474.

[21] F.-Y. Zeng, J. Wess, Neuropsychopharmacology 2000, 23, S19 – S31.[22] J. A. Hern, A. H. Baig, G. I. Mashanov, B. Birdsall, J. E. T. Corrie, S. Lazare-

no, J. E. Molloy, N. J. M. Birdsall, Proc. Natl. Acad. Sci. USA 2010, 107,2693 – 2698.

[23] M. Scarselli, F. Novi, E. Schallmach, R. Lin, A. Baragli, A. Colzi, N. Griffon,G. U. Corsini, P. Sokoloff, R. Levenson, Z. Vogel, R. Maggio, J. Biol. Chem.2001, 276, 30308 – 30314.

[24] R. Maggio, M. Scarselli, F. Novi, M. J. Millan, G. U. Corsini, J. Neurochem.2003, 87, 631 – 641.

[25] S. P. Lee, C. H. So, A. J. Rashid, G. Varghese, R. Cheng, A. J. LanÅa, B. F.O’Dowd, S. R. George, J. Biol. Chem. 2004, 279, 35671 – 35678.

[26] S. Łukasiewicz, A. Polit, S. Kedracka-Krok, K. Wedzony, M. Mackowiak, M.Dziedzicka-Wasylewska, Biochim. Biophys. Acta 2010, 1803, 1347 – 1358.

[27] R. Maggio, G. Aloisi, E. Silvano, M. Rossi, M. J. Millan, Parkinsonism Relat.Disord. 2009, 15 (Suppl. 4), S2 – 7.

[28] Y. Zhang, A. Gilliam, R. Maitra, M. I. Damaj, J. M. Tajuba, H. H. Seltzman,B. F. Thomas, J. Med. Chem. 2010, 53, 7048 – 7060.

[29] N. Cabello, J. Gand�a, D. C. G. Bertarelli, M. Watanabe, C. Llu�s, R. Franco,S. Ferr�, R. Luj�n, F. Ciruela, J. Neurochem. 2009, 109, 1497 – 1507.

[30] P. Carriba, G. Navarro, F. Ciruela, S. Ferre, V. Casado, L. Agnati, A. Cortes,J. Mallol, K. Fuxe, E. I. Canela, C. Lluis, R. Franco, Nat. Methods 2008, 5,727 – 733.

[31] M. Erez, A. E. Takemori, P. S. Portoghese, J. Med. Chem. 1982, 25, 847 –849.

[32] P. S. Portoghese, D. L. Larson, L. M. Sayre, C. B. Yim, G. Ronsisvalle, S. W.Tam, A. E. Takemori, J. Med. Chem. 1986, 29, 1855 – 1861.

[33] P. S. Portoghese, G. Ronsisvalle, D. L. Larson, C. B. Yim, L. M. Sayre, A. E.Takemori, Life Sci. 1982, 31, 1283 – 1286.

[34] R. Morphy, C. Kay, Z. Rankovic, Drug Discovery Today 2004, 9, 641 – 651.[35] O. Russo, M. Berthouze, M. Giner, J. L. Soulier, L. Rivail, S. Sicsic, F. Le-

zoualc’h, R. Jockers, I. Berque-Bestel, J. Med. Chem. 2007, 50, 4482 –4492.

[36] R. G. Bhushan, S. K. Sharma, Z. H. Xie, D. J. Daniels, P. S. Portoghese, J.Med. Chem. 2004, 47, 2969 – 2972.

[37] M. Perez, C. Jorand-Lebrun, P. J. Pauwels, I. Pallard, S. Halazy, Bioorg.Med. Chem. Lett. 1998, 8, 1407 – 1412.

[38] P. S. Portoghese, Trends Pharmacol. Sci. 1989, 10, 230 – 235.[39] C. Valant, P. M. Sexton, A. Christopoulos, Mol. Interventions 2009, 9,

125 – 135.[40] L. T. May, K. Leach, P. M. Sexton, A. Christopoulos, Annu. Rev. Pharmacol.

Toxicol. 2007, 47, 1 – 51.[41] A. Christopoulos, T. Kenakin, Pharmacol. Rev. 2002, 54, 323 – 374.[42] R. Narlawar, J. R. Lane, M. Doddareddy, J. Lin, J. Brussee, A. P. Ijzerman, J.

Med. Chem. 2010, 53, 3028 – 3037.[43] C. Valant, K. J. Gregory, N. E. Hall, P. J. Scammells, M. J. Lew, P. M. Sexton,

A. Christopoulos, J. Biol. Chem. 2008, 283, 29312 – 29321.[44] P. Keov, P. M. Sexton, A. Christopoulos, Neuropharmacology 2011, 60,

24 – 35.[45] K. Mohr, C. Tr�nkle, E. Kostenis, E. Barocelli, M. De Amici, U. Holzgrabe,

Br. J. Pharmacol. 2010, 159, 997 – 1008.[46] A. Zhang, Z. L. Liu, Y. Kan, Curr. Top. Med. Chem. 2007, 7, 343 – 345.[47] J.-L. Soulier, O. Russo, M. Giner, L. Rivail, M. Berthouze, S. Ongeri, B. Mai-

gret, R. Fischmeister, F. Lezoualc’h, S. Sicsic, I. Berque-Bestel, J. Med.Chem. 2005, 48, 6220 – 6228.

[48] A. Soriano, R. Ventura, A. Molero, R. Hoen, V. Casad�, A. Cort�s, F. Fanel-li, F. Albericio, C. Llu�s, R. Franco, M. Royo, J. Med. Chem. 2009, 52,5590 – 5602.

[49] B. S. Fulton, B. L. Knapp, J. M. Bidlack, J. L. Neumeyer, Bioorg. Med.Chem. Lett. 2010, 20, 1507 – 1509.

[50] M. Decker, Y.-G. Si, B. I. Knapp, J. M. Bidlack, J. L. Neumeyer, J. Med.Chem. 2010, 53, 402 – 418.

[51] Y. Zheng, E. Akgun, K. G. Harikumar, J. Hopson, M. D. Powers, M. M.Lunzer, L. J. Miller, P. S. Portoghese, J. Med. Chem. 2009, 52, 247 – 258.

[52] R. Lattanzi, M. Spetea, F. Schllner, S. B. Rief, R. Krassnig, L. Negri, H.Schmidhammer, J. Med. Chem. 2005, 48, 3372 – 3378.

[53] M. G. Bock, R. M. DiPardo, B. E. Evans, K. E. Rittle, W. L. Whitter, V. M.Garsky, K. F. Gilbert, J. L. Leighton, K. L. Carson, J. Med. Chem. 1993, 36,4276 – 4292.

[54] P. Karellas, M. McNaughton, S. P. Baker, P. J. Scammells, J. Med. Chem.2008, 51, 6128 – 6137.

[55] K. N. Klotz, J. Hessling, J. Hegler, C. Owman, B. Kull, B. B. Fredholm, M. J.Lohse, Naunyn-Schmiedeberg’s Arch. Pharmacol. 1998, 357, 1 – 9.

[56] V. Bakthavachalam, N. Baindur, B. K. Madras, J. L. Neumeyer, J. Med.Chem. 1991, 34, 3235 – 3241.

[57] M. Leopoldo, E. Lacivita, N. A. Colabufo, M. Niso, F. Berardi, R. Perrone,Bioorg. Med. Chem. 2007, 15, 5316 – 5321.

[58] D. Yang, J.-L. Soulier, S. Sicsic, M. Mathe-Allainmat, B. Bremont, T. Croci,R. Cardamone, G. Aureggi, M. Langlois, J. Med. Chem. 1997, 40, 608 –621.

[59] Y. Cao, M. Zhang, C. Wu, S. Lee, M. E. Wroblewski, T. Whipple, P. I. Nagy,K. Takacs-Novak, A. Balazs, S. Toros, W. S. Messer, J. Med. Chem. 2003,46, 4273 – 4286.

[60] K. A. Jacobson, P. J. M. Van Galen, M. Williams, J. Med. Chem. 1992, 35,407 – 422.

[61] R. R. Ruffolo, W. Bondinell, J. P. Hieble, J. Med. Chem. 1995, 38, 3681 –3716.

[62] L. M. Sayre, P. S. Portoghese, J. Org. Chem. 1980, 45, 3366 – 3368.[63] I. Berque-Bestel, F. Lezoualc’h, R. Jockers, Curr. Drug Discovery Technol.

2008, 5, 312 – 318.[64] D. J. Daniels, N. R. Lenard, C. L. Etienne, P.-Y. Law, S. C. Roerig, P. S. Por-

toghese, Proc. Natl. Acad. Sci. USA 2005, 102, 19208 – 19213.[65] S. Zhang, A. Yekkirala, Y. Tang, P. S. Portoghese, Bioorg. Med. Chem. Lett.

2009, 19, 6978 – 6980.[66] D. J. Daniels, A. Kulkarni, Z. Xie, R. G. Bhushan, P. S. Portoghese, J. Med.

Chem. 2005, 48, 1713 – 1716.[67] T. Tanaka, W. Nomura, T. Narumi, A. Masuda, H. Tamamura, J. Am. Chem.

Soc. 2010, 132, 15899 – 15901.[68] F. Lezoualc’h, R. Jockers, I. Berque-Bestel, Curr. Pharm. Des. 2009, 15,

719 – 729.[69] K. M. Bonger, V. V. Kapoerchan, G. M. Grotenbreg, C. J. van Koppen,

C. M. Timmers, G. A. van der Marel, H. S. Overkleeft, Org. Biomol. Chem.2010, 8, 1881 – 1884.

[70] K. M. Bonger, R. J. B. H. N. van den Berg, A. D. Knijnenburg, L. H. Heit-man, A. P. Ijzerman, J. Oosterom, C. M. Timmers, H. S. Overkleeft, G. A.van der Marel, Bioorg. Med. Chem. 2008, 16, 3744 – 3758.

[71] K. M. Bonger, R. J. B. H. N. van den Berg, A. D. Knijnenburg, L. H. Heit-man, C. J. van Koppen, C. M. Timmers, H. S. Overkleeft, G. A. van derMarel, ChemMedChem 2009, 4, 1189 – 1195.

[72] S. A. Bobrovnik, J. Mol. Recognit. 2007, 20, 253 – 262.[73] A. L. LaFrate, K. E. Carlson, J. A. Katzenellenbogen, Bioorg. Med. Chem.

2009, 17, 3528 – 3535.[74] D. Han, F. Holger Fçrsterling, X. Li, J. R. Deschamps, D. Parrish, H. Cao, S.

Rallapalli, T. Clayton, Y. Teng, S. Majumder, S. Sankar, B. L. Roth, W. Sie-ghart, R. Furtmuller, J. K. Rowlett, M. R. Weed, J. M. Cook, Bioorg. Med.Chem. 2008, 16, 8853 – 8862.

[75] K. B. Wiberg, M. A. Murcko, K. E. Laidig, P. J. MacDougall, J. Phys. Chem.1990, 94, 6956 – 6959.

[76] S. Wolfe, Acc. Chem. Res. 1972, 5, 102 – 111.[77] P. M. Cowan, S. McGavin, Nature 1955, 176, 501 – 503.[78] K. M. Bonger, R. van den Berg, L. H. Heitman, A. P. Ijzerman, J. Oosterom,

C. M. Timmers, H. S. Overkleeft, G. A. van der Marel, Bioorg. Med. Chem.2007, 15, 4841 – 4856.

[79] K. M. Bonger, S. Hoogendoorn, C. J. van Koppen, C. M. Timmers, H. S.Overkleeft, G. A. van der Marel, ChemMedChem 2009, 4, 2098 – 2102.

[80] S. Halazy, M. Perez, C. Fourrier, I. Pallard, P. J. Pauwels, C. Palmier, G. W.John, J. P. Valentin, R. Bonnafous, J. Martinez, J. Med. Chem. 1996, 39,4920 – 4927.

[81] W. G. Rajeswaran, Y. Cao, X. P. Huang, M. E. Wroblewski, T. Colclough, S.Lee, F. H. Liu, P. I. Nagy, J. Ellis, B. A. Levine, K. H. Nocka, W. S. Messer, J.Med. Chem. 2001, 44, 4563 – 4576.

[82] P. C. Meltzer, O. Kryatova, D. P. Pham-Huu, P. Donovan, A. Janowsky,Bioorg. Med. Chem. 2008, 16, 1832 – 1841.

[83] J. Mathews, B. Fulton, S. Negus, J. Neumeyer, J. Bidlack, Neurochem. Res.2008, 33, 2142 – 2150.

[84] S. Halazy, Expert Opin. Ther. Pat. 1999, 9, 431 – 446.

ChemMedChem 2011, 6, 963 – 974 � 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemmedchem.org 973

Bivalent Ligands Targeting GPCRs

[85] M. Perez, P. J. Pauwels, C. Fourrier, P. Chopin, J.-P. Valentin, G. W. John,M. Marien, S. Halazy, Bioorg. Med. Chem. Lett. 1998, 8, 675 – 680.

[86] H. Potschk in Drug Delivery, vol. 197, (Ed. : M. Sch�fer-Korting), SpringerVerlag, Berlin Heidelberg, 2010, pp. 411 – 431.

[87] A. Tsuji, I. Tamai, Adv. Drug Del. Rev. 1999, 36, 277 – 290.[88] S. A. Hitchcock, Curr. Opin. Chem. Biol. 2008, 12, 318 – 323.[89] F. Atkinson, S. Cole, C. Green, H. van de Waterbeemd, Curr. Med. Chem.:

Cent. Nerv. Syst. Agents 2002, 2, 229 – 240.[90] M. H. Abraham, Eur. J. Med. Chem. 2004, 39, 235 – 240.

[91] M. E. Y. Francisco, H. H. Seltzman, A. F. Gilliam, R. A. Mitchell, S. L. Rider,R. G. Pertwee, L. A. Stevenson, B. F. Thomas, J. Med. Chem. 2002, 45,2708 – 2719.

[92] B. F. Thomas, M. E. Y. Francisco, H. H. Seltzman, J. B. Thomas, S. E. Fix, A.-K. Schulz, A. F. Gilliam, R. G. Pertwee, L. A. Stevenson, Bioorg. Med.Chem. 2005, 13, 5463 – 5474.

Received: March 18, 2011Published online on April 21, 2011

974 www.chemmedchem.org � 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemMedChem 2011, 6, 963 – 974

MED J. Shonberg, P. J. Scammells, B. Capuano