The' majority' of' investigator' groups' consider' multivalency' as' the' most' important' interaction'between'a'ligand'and'its'corresponding'receptor'(1);'(5);'(21).'It'is'relevant'to' distinguish'homoRmultivalency'and'heteroRmultivalency.'Multivalency'is'in'the'first'place' classified'on'the'basis'of'the'number'of'interactions'between'the'multivalent'ligand'and' its' specific' targeting' receptor,' and' then' on' the' basis' of' types' of' ligands' involved' in' the' same'binding'process.''
Homodimers,'or,'more'in'general,'homomultimers'are'constructs'presenting'two'or'more' identical'ligands'that'are'crossRlinked'with'high'avidity'values'(21);'(84).'The'best'example' of'a'homomultimer'concerns'RGD'peptide'multimers'radiolabeled'with'18F,'68Ga,'64Cu'and' evaluated' in' both' in! vitro' and' in! vivo' studies' (82);' (85R86).' There' is' a' controversy' with' regards' the' best' tumor' imaging' agent:' homomultimers' or' heteromultimers.' Because' cancer'is'characterized'by'simultaneous'expression'of'different'peptide'receptors,'many' authors' mention' heteromultimers' the' best' imaging' agents.' Others' consider' homomultimers'more'efficient'because'of'their'imaging'properties.'They'target'only'one' species'of'receptor'and'do'not'allow'the'targeting'of'other'sites'expressing'other'kinds'of' receptors.' Many' studies' focus' on' the' improvement' of' peptide' binding' affinity' via' heteromultimerization.' Currently' a' few' heterodimers' such' as' bombesinRRGD' or' DeltRIIR MSH(7)' are' under' investigation' (87).' ' A' bombesinRRGD' peptide' heterodimer' shows' promising'results'with'regards'patients'with'androgenRindependent'prostate'cancer'(82).' Imaging' results' gained' by' two' different' peptides' linked' in' the' same' multimer' may' improve'the'diagnostic'process.''Yan'et'al.'showed'that'a'BNRRGD'peptide'heterodimer,' linked' via' a' glutamate' linker' and' labeled' with' 18FRSFB' (N<succinimydilR4R(18F)' fluorobenzoate),'has'high'binding'affinities'for'both'GRPR'and'αvβ3'receptors,'but'displays' a' slower' washout' than' monomeric' peptide' counterparts' (82).' Recently,' Liu' et! al.,' synthesized' a' novel' BNRRGD' heterodimer' by' using' an' orthogonal' Alloc' protected' glutamate,'which'is'linked'to'aminocaproic'acid'(Aca)'of'AcaRBN(7R14).'After'removal'of'αR
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allyl'ester'and'coupling'to'RGD'peptide'by'using'lysine'aminoacid,'a'miniRPEG'spacer'was' attached'to'the'glutamate'linker;'introduction'of'this'spacer'lead'to'an'improved'yield'of' the' radio' labeling' (Fig.' 13).' Subsequent' tumor' and' tissue' uptake' studies' revealed' the' same' binding' affinities' of' BNRRGD' heterodimer,' but' a' faster' renal' clearance' due' to' the' attachment'of'miniRPEG'spacer'(85).'''
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' Figure( 13:' BNRRGD' peptide' heterodimer,' linked' via' a' miniRPEG' spacer' attached' to' the' glutamate'linker'and'labeled'with'18FRSFB'(N<succinimydilR4R(18F)'fluorobenzoate)'
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Yan'et!al.'developed'a'new'BNRRGD'heterodimer'by'introducing'the'AEADP'linker'[3,3`R(2R aminoethylazanediyl)' dipropanoic' acid]' (88).' This' linker' contains' one' amino' and' two' functional' carboxylic' acid' ' groups.' This' AEADPRBNRRGD' heterodimer' showed' good' receptor' binding' affinities' for' integrin' αvβ3' and' GRPR' receptors.' These' data' were' collected' by' in' vitro' cellRbinding' assays' on' U87MG' human' glioblastoma' cells' and' PCR3' human'prostate'carcinoma'cells.'In!vivo'blocking'studies'demonstrated'that'the'receptor' binding' of'18FRFBRAEADPRBNRRGD' for' integrin' αvβ3' and' GRPR' was' not' affected' by' the' AEADP'linker.'Substituting'the'glutamate'linker'with'AEADP'linker'did'not'show'any'other' effect'on'the'biological'activities'of'the'BNRRGD'heterodimer.'Both'GluRAcaRBN(7R14)'and' AEADPRBNRRGD' tracers' had' a' high' tumor' uptake,' but' also' a' nonRspecific' tissue' binding'
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uptake.' The' main' advantage' of' AEADPRBNRRGD' tracer' was' its' efficient' synthesis.' Other' studies'on'BNRRGD'heterodimers'were'performed'by'attachment'of'bifunctional'chelators' as'DOTA'and'NOTA,'and'radiolabeling'by'64Cu'and'68Ga'(82);'(85).'64CuRNOTARBNRRGD'was' documented' as' an' imaging' agent' with' good' binding' affinity,' high' binding' kinetics' and' showed'a'good'quality'of'imaging'and'low'backgrounds,'even'after'late'time'points'(Fig.' 14).''Radio'labeling'of'NOTARBNRRGD'by'68Ga'had'also'promising'results;'it'showed'a'high' tumor'uptake'and'a'lower'efflux'ratio'(82);'(85).''
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Figure(14:'DOTA'and'NOTA'BNRRGD'heterodimers.'These'ligands'have'been'evaluated'as' imaging'agents'with'good'binding'affinity,'high'binding'kinetics'and'showed'both'a'good' quality'of'imaging'and'low'backgrounds'even'after'late'time'points.'
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In'2008,'Handl'et!al.'described'the'binding'properties'of'DeltRIIRMSH(7)'heterodimers'(87).'' This'heterodimer'is'composed'of'ligands'connected'by'semiRrigid'or'flexible'linkers'and'is' complementary' to' the' human' melanocortin' receptor' Type' 4' (hMC4R)' and' the' cholecystokininR2' receptor' (CCKR2R).' The' receptor' binding' properties' of' ' DeltRIIRMSH(7)' was' evaluated' by' two' different' cell' lines:' human' embryonic' kidney' 293' (Hek293)' expressing'only'one'of'the'complementary'receptors'(Hek293/hMC4R'and'Hek293/CCKR 2R)' and' Hek293/hMC4R/CCKR2R.' Cells' expressing' both' complementary' receptors' had' a' twentyRfold'higher'affinity'compared'to'those'expressing'only'one'receptor.''
Neurotensin'(8–13)'peptide'homomultimer'is'well'documented'with'regards'its'synthesis' and'structure'(Fig.'15).''A'series'of'NT(8–13)'derivatives'with'a'multivalent'core'structure' have' been' synthesized' and' evaluated' via' linking' of' NT(8–13)' units' using' the' CR' or' NR termini' of' the' NT(8–13)' residues' (89).' The' attachment' of' the' NT(8–13)' moieties' to' the' core'unit'was'the'most'crucial'part'of'the'synthesis.'More'in'detail'the'dimeric'analogues' showed' an' approximately' two' times' higher' IC50' value' compared' to' the' tetrameric' compound.' Subsequent' radio' labeling' by' fluorineR18' showed' promising' results' of' the' sulfydrylRfunctionalized' peptides' using' (18F)FBAM.' However,' the' results' were' not' comparable'with'(18F)SFB'or'(18F)FBA'as'labeling'agents'because'both'reactions'strongly' depended'on'the'peptide'concentration'(89).'
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Figure( 15:' Neurotensin' (8–13)' peptide' homomultimers.' Monomer,' homodimer' and' homotetramer'have'been'synthesized'and'radiolabelled.'
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Recently' DOTARconjugated' monomeric,' dimeric,' and' tetrameric' (Tyr3)octreotideRbased' analogues' (Fig.' 16)' have' been' evaluated' (90)' A' relatively' long' retention' of' radiolabeled(Tyr3)octreotide' dimeric' conjugate' was' measured' in' sc' AR42J' inoculated' tumors' in' BALB/c' nude' mice.' Current' studies' address' the' receptor' binding' affinity' by' investigating' the' effects' of' different' kinds' of' spacer.' In' this' way' the' high' lipophilic' character'of'the'multimeric'conjugate'may'be'reduced.''
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Figure( 16:' DOTARconjugated' monomeric,' dimeric,' and' tetrameric' (Tyr3)octreotideRbased' analogues.'
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Specific'uptake'and'affinity,'washout'and'clearance,'imaging'and'therapeutic'value'are'all' important;' they' have' been' extensively' explained' in' the' previous' paragraphs.' When' the' goal' is' to' design' a' new' (radio)Rligand,' all' these' characteristics' need' to' be' evaluated' a!
priori'(Fig.'17).''
( Figure( 17:' Summary' of' important' characteristics' of' monomers,' multimers' and'
polymers/dendrimers.((Although'the'monomers'offer'best'imaging'properties,'dendrimers' and'highly'branched'multimers'have'the'best'therapeutic'applications.'They'are'able'to' deliver'a'payload'to'tumors.'''