Helsinn Healthcare Exhibit 2033
`Dr. Reddy's Laboratories, Ltd., et al. v. Helsinn Healthcare S.A.
`Trial PGR2016-00007
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`Page 1 of 10
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`Page 2 of 10
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`PTX-253.0002
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`TEVA—0088998
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`Page 2 of 10
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`357‘
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`S~Hydmxy1;1*ypta.n1ine (5-HT)3 Receptors: Molecular Biology, Pharniacology
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`and Therapeutic Ilnpvortamte
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`{'.C'.'tI"rem‘ PI:I:r::1m*e*ui’iax:i }’)'am‘ In, 1.996.
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`367-3711
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`R.M. Egleni‘ and DW. Bonhaus
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`z"ras:2itz;.te <2/']’i‘iarn:m<>(;Zog}>, 1\7em’(2bi0lo;,{,v {.e’m'i, Roche Biomience, 3401, Ifillview /lve._. 13am
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`Alto, CA 94304, USA.
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`Abstraét: The 5~HT_v,recep1.oris unique anmng known znm'zoarm‘im rece;>tIa>rs in tliat; ratlier than being :1
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`prot<3i:s—coupl.ed 1'6-C€:[?lO1'.
`it form a iigaiildgzztaci ion channel. in the
`this t'.e(:.e3_}ts)r is fmmcl in high deiisity
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`in‘ nuclei of the lmvei‘ brainstcm, area postmma anti nucleus of the trac:t:us .-;.ulitm“l’u:~;.
`I{.(iwe.r de.r13iti§§ of the
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`1‘c::r:;§iI0r are fmmd in the cerebral C'cu‘teK and limbic ?1l'\3é}S,illClIl=Jil.‘I.g the illp]_}()_CE1ii1pl3S. in the periphery, fS~I‘l'i‘3
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`z‘aceptor:a. are located on 1313- and postgangiiomic rmzrozzs. of both {~I€'3l’1Ee01‘}(' and f)zl{{:liiL\ xzerv<.>n..s: s;ysI;s1n:;. Tim.
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`3,‘8¢_34i{){()l‘
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`(1 p«:i1ta1ne1'ia: pratein ‘.~'llil“i 1n{ll_tif}l,&'3« agn>r1ist.aiaci allosimric ligand binding aim. Thus it has st:mi:l:m'al
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`and funcI.inm1,l 3in1_ila1-ities with nicotinic, C§ABA.ergic a11d,0‘tl’K’:rliga1*1c1 gated ion clizmnels.
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`efi:‘<—:L*.t:<
`in animal m<:>del:s of ztognim-rs and
`to pr«:»(l‘uce b<:zicl‘.ic.i;5il
`5-l~l'i‘3 re<3cpl:0z' amzigonists iiswe been f~Il}UWfl
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`psychiemic -ill-.‘I~()I‘C.5fi‘-1'5‘ Whether
`reccgitur antagonists may have si‘mil.ar pmfcvumi ef"i:‘ects in km: i.i'z:2.im«::n.t of
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`anxiety, cicpression or psyclmsis will 13:: dcte.11u1ine.cl by the (!lJCC()n].E1 af o11.g()iiig Clllllfii-ll trials,
`l’lOV\’C-V531‘, it is in
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`the treatmsmt of cancer <:hemoil‘xera;)y induced emrssis that S-HT3 receptzir a.zi§:ag01-nits lmve lléld tl3ei1“g:'eam{4:t
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`impact. Tlie cytotoxic A".-xgentvs used in cancer c.l1ei110the2‘a}3y pmvoke the r»3lease of 5-‘H'l‘ fmm szmzstzzcliromaf‘fi.z'i
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`cells in the gastrcilitezstinal
`trawl. This 5~HT acme on 5~HT3 :rece.pzors in the cemmi
`ixerxrzmm sys-21:31:} or on
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`p<31‘ipliz:ml vagal 23./l”fa:re.nt fibers to initiate vmnil: 1’E‘2llE2XGS. 5»H.T3 rzzceptor zmtagcmists block this actizm zmci
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`er:ab},:' greatly re«:iuce the munbsr of emetic episodes that 006111‘ during caiwer cl1e1n0I.he.x‘21py. Tim. l’ilf,i£‘1(l:)£l ciiiiicixl
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`my of 5—HT3 1'eceptc~2r aixtzzgmiists sauczh as ondam;e.i:z'cir2, grasaiseiroii and tropi3etr<m '£(3g6tl‘lL’-1‘ with their lack
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`of adverse Slilfi
`c1"l'e<;ts has rcvolutioiiized the treatimmt of caiicar
`cl12muih<:1*apy induced emcsis.
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`Introduction
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`Molecular Biology and Structulwz
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`irnportant.
`5—Hyzlmxytryptemiine (5-HT, serotoiiin) cx.<:rLs
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`inudulamry icffects an boil:
`cm'x:,1'al and pr:.1'ipl‘u:.i'al nervous
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`S}‘.S’i¢:1YIS, via activation of sevrzmi discrete receptor ihinilies. In
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`general. 5~llT receptors structurally conforrn to the arcliciypsxl
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`motif for G __m‘0I.ein~co.upleCi
`t.¢s:eptors and regulate cellular
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`r:::sg:ron.se.s via naodmation of aclenylyl ztyclaisiz or pii0.$plis)lip23se
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`C activity. The om: exception is tilt! 5~l-{T3 re::eI;:i:or subtype.
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`This .receptor, which licars significant SC:.ql.lC~|lvZ2(§ lioimlogy with
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`the 11i,<;olinic 1'CC6{,}(01‘ family, mediates a fast depolzimzatimi of
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`central and f?€}1‘iphE:{£ll
`l’l€ti!l‘.()ilS
`by increasing s<i<;iium. and
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`potassium perm::al:iliLy. in the last. decadc S-l~lT’3 rec '§7l;i.lI‘S have
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`been the focus of extensive rese.-:u‘cl1 e.fforls, not only becéiuse of
`i,:h«:ia" uimguc St!‘LlC),l;U{<3 amsmg monamine rcsccapters, but also
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`because of lilac clinical efficacy of selective ama.g0niszs in tin:
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`trcalrnmt of tslrmsis ll$.‘Sl3(2ilit(:‘iZ_l with ()Ell3CBl‘ (1li§)YliL‘1[l’1{3i"El{'lj/. 5—H'l‘3
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`;*m1ag0n.i:s1;s miijl also act as cngnitive 0l'll1E!l1C&)1‘S,
`reczeptzii‘
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`siltlmugh,
`i,’,Lll,5<i()lytlLl:S,
`anii.~}Jsyz;limic-s and as
`amxIgesi(:s,
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`still
`is
`cliniwl valis'laticm cal‘
`these pulraiive a}>;3iiczii:i(ms.
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`lacking. This lniicf review suliinwtizisr; smnc lcey Fcalures of the
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`rm3l'cculai' structure and pl1m'1‘na<::‘>].0gy oftllc S—l-’iT3 i‘cce.;3t0r.
`[1
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`also adcln->:sscs findings {ram rcc.e—’nt clinical studies examining
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`the lll‘c1t'2i))(:‘»l}tlC miiity oi‘ 5-HT3 re.c:z:plc:r aiiiagonists;
`in the
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`Lrc-.atn1em: oi’ acute. and -iiclayeicl
`l:1al'lC9?l' x:he1'm7tlier21py induced
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`mm-.sis, Savtwal reviews of 5-l'1’I‘3 recepi.m' pliam1a::0i0_gy and
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`mcclicinal ch:>,misti‘y lmve recently been pul>iisii:z.:l [1-3]. ‘Recent
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`1'ex«'iew's of the clinical
`trials te>;iin.g l1l'li3- <:l‘.‘flcacy of 5-HT}
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`1‘ecc:ptor zzntiagoriistel have also lxscan written [4~f5]. The reader is
`refrzrred to tilt?-SB revirws and the l>ibli0gmplaics cited therein for
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`ilélditionill.
`inl‘m'x11aLi<m on specific mpics.
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`E”=’E€§iE3‘{5:?3i may be
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`Page 3 of 10
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`1
`Cspyrigght iaw {létlza
`1381-6128./96 3ic\f;£)Oe%.li£}
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`5-l~l‘T;; recept0r.s* nizzciiate rapicl nmuumzl cle:])ol.ari2.atio1i by
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`i11L‘rc¢asing me-n1br;:m<:‘. })ei‘:ne.ability to ra<>d_inm anal p0ta.s3.i1n'n
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`in clcmal cell
`lines tli<3.y have also been shown to
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`Iiiediate calcium. ion flux [7~ i I]. The rapid 0:139: and 0i:‘l’".s:ct of thc
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`5-llT3z‘ecep1'.or ine.ciiate<l responses led to the initial suggestion
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`this re-..::<_=.pt<3r niagfbe an ion channel. However, only
`that
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`relativfly rec-am;I.y, by {h¢,‘. mar: of paicl -ciamp techniques. has‘
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`the 5vI~l'l"'3 reoeptoir brscn unequivocally shown to comprise an ion
`chsmiiszl per se, mllicai‘ than being a. C}-protein ctmplcci 1‘<:<;=capt<>i'
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`saccsnd
`that activated ism channels via m0hiliz»ai.im': cf 21
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`UlfiS8€i”1g{‘:l'
`{I2}. Thus,
`in isolated whole c-alis of guinea pig
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`submucosal plexus, 5-I~l'I‘3rcceptm'r-iiiedialed currents; were
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`ins::nsi1.ive to gmrmmfs toxin and i.ndcpr:.ndenI; of the presence of
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`:\:'1<.3rcm‘cz',
`GTP in ‘the. imrexcellulaz' :-mlution.
`in excised patches
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`of «mil zm~:mlm1_1'ics m)pisct3‘0n~scn5;‘il;ive Lrurrmits v./cm evokecl by
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`exi.mci2ll1Jlar
`5~-I-1"!‘ up to 5 lmurs l‘ol[ow_ing isolati<in of that
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`'l‘l1s::sr3
`daltzs
`_iI1-:ii<:ai:c,,<;l
`{lint
`(Ii"I‘"P or
`ctilixcr scilublc
`patcli.
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`c§,’mg>l.asi1iic ;u*0tcvix'zs were my; mc;i1iz'<*._(i for ‘aim cur3.‘x»3m.'-s i:</okzzd
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`by 5-HT and ctmvlncingly ;n'gued ihilif the l‘C¢(2t)}J(()l' was an ion
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`channel
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`Ull:ii1wi.ely, ti:-:3 »:lc:«nizzg and seqn<si3<:ii3g of 2: ftzmziiorml 5—I~i’i"3
`re(:c3>ts>r dt‘7lIl()l'lEiil‘ilt€'.d
`that
`the Sii'llCIlJ£'C of the l.'i‘)CC‘.pl(3)‘ had an
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`amino acid sequence and l_Clj)(lgl”E\[)lfl)’
`l1o1fm)!og<)i1s with other
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`ligzsml gzited ion Qlfifllilltlléh such as the 1i.i<:.;)tlni<: C.i).()¢l.ill£fi‘gl(Z or
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`GAi3/~\ 9:/zcepismfs {iii}. Ream: silxclitas using ele<*.t)'0n mi::ms<:o};sy
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`have slim-zcl light on the q_'u;itai'r:izu‘y SiJ‘l3(?{ilY\‘3 of this clmnncl f'm'tl1e2r
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`colifirmiug its
`inclusion in [hit family of ligem.(l gatccd ion
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`cl1am1c:l.s- [l4‘l5’}. }T’le.timzm maplica itnages oi? the charimzl as
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`imaged by romry s:i=.;1d§)vv'i,m;‘ of ;_>ur'sfictd 5—H"l"3 i'ec<>pl:0x‘e; Show
`the r::.c:<-lptm“ to be: as rosette sl1ag>.e<l particle, 3-9 nlvl’ in ciimnctmu
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`3
`:3; 15796 Bantlmm Scimice Pixblislicrs; B.V.
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`PTX-253.0003
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`Ta/A—0oaa999
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`Page 3 of 10
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`

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`368 Current Pharmaceutical Design, 1996, Vol. 2, No. 4
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`Eglen and Bmzlzatcs
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`with a length of approximately 11 nM (Fig. 1). This pentameric
`motif is, of course, similar to that determined for the nicotinic
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`cholinergie [I6] and GABAergic receptors [17].
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`Fig.
`receptors
`(1). Filtered electron microscopic images of four
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`demonstrating that the 5-HT3 receptor is a symmetrical pentamer with a
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`central core. This picture is taken, with permission, from Boess et al.,
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`1995.
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`The similarities in primary and quaternary structure of the 5-
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`HT3, GABA and nicotinic receptors suggest that these receptors
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`may share similar molecular pharmacology. Thus,
`it
`is
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`unsurprising that the 5-HT3 receptor possesses multiple agonist
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`binding sites subject to allosteric regulation and susceptible to
`noncompetitive blockade by ligands which may bind in the pore
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`of the channel. Based upon data from studies with nieotinic and
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`GABAergic receptors, it would also be expected that the 5-HT3
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`receptor would be a heteromeric structure with subtypes of the
`receptor being defined by the incorporation of different specific
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`subunits into the protein. However,
`in this last regard at least,
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`the 5—HT3 receptor may differ from other ligand gated channels,
`since the existence of heteropentameric receptors has not been
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`reported,
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`(Competitive, Non-
`Ligand Interactions
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`competitive, Cooperative and Allosteric)
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`The presence of multiple, positively cooperative agonist
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`first
`binding sites on? the
`receptor was
`indicated by
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`electrophysiological studies showing that
`the concentration-
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`response curves had Hill slopes markedly greater than unity.
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`Subsequent radioligand binding experiments, using radiolabeled
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`agonists, show similar indications of positive eooperativity
`[l8—24]. However, for reasons that are unclear, detection of this
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`phenomenon by radioligand binding depends upon the specific
`radioligand, membrane preparation and ionic content of the
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`buffer employed in the binding assay.
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`The interaction among agonist binding sites has also been
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`demonstrated using kinetic binding methods in that rates of
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`radioligand dissociation were found to be dependent upon the
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`competing ligand [25]. This finding is inconsistent with simple
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`the
`that
`it
`in
`competitive
`suggests
`that
`interactions
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`the_
`“displacement” of one agonist molecule modifies
`dissociation rate constants for the remaining bound agonist
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`molecules. The results from these kinetic experiments also
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`indicate that not all antagonists produce equivalent effects on
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`dissociation rates suggesting that
`the binding of different
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`antagonists can produce different conformations of the receptor.
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`A filial indication of multiple agonist binding sites comes from
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`the results of saturation binding experiments conducted with
`different radiolabelled agonists. These studies have shown that
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`the maximum number of detectable binding sites depends on the
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`raclioligand used to label the receptor [22,26]. One explanation
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`for these data is that the binding of certain ligands to one site on
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`the receptor precludes occupancy of adjacent sites either by a
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`steric hinderence or by allosterically modifying the affinity of
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`adjacent binding sites on the receptor. Since heterogeneity in
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`these agonist binding sites has not been detected, these multiple
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`binding sites may be pharmacologically equivalent. Taken
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`together, the findings from radieligand and electrophysiological
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`studies suggest a model of agonist interactions that are similar
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`to those which have been established for
`the nicotiuie
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`acetyleholine receptor. Namely,
`that occupancy of multiple
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`the
`binding sites is required for receptor activation and that
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`binding of one molecule of agonist facilitates the binding of
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`subsequent molecules [27].
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`Positive allosteric interactions among ligands binding to
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`sites distinct
`from the agonist recognition sites have been
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`suggested on the basis of findings made with ethanol and
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`ketamine [28,29]. Ethanol, a drug that modifies the function of a
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`number of ligand—gated ion channels, augments 5~HT3 receptor-
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`gated ion currents
`[30] and 5-HT3 receptor mediated
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`[l4C]guanidinium ion influx [31]. The concentrations of ethanol
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`required to achieve this action, while high, are not outside the
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`range of those achieved in humans; thus augmentation of 5—I-IT3
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`receptor mediated neurotransmission may occur during alcohol
`intoxication. Whether
`this interaction plays a role in the
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`psychobehavoral effects of alcohol is unclear. Howevei‘, 5~HT3
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`receptor antagonists have been shown to reduce the voluntary
`ethanol consumption in rats [32] and humans [33] and to disrupt
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`ethanol
`state-dependent
`learning [34]
`thus
`raising the
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`possibility that some of the reinforcing actions of ethanol are a
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`consequence of 5-HT3 receptor activation. The potentiating
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`actions of ketamine were detected at micromolar concentrations
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`and were reversible upon washout of the drug. Importantly the
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`effects of ketamine were not mimicked by a 5-HT uptake blocker.
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`The site of action of this compound is not known but
`the
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`mechanism may involve
`reduction in
`rate of
`a
`the
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`desensitization of the channel.
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`While these findings of positive modulatory actions
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`through
`strongly suggest direct actions on the receptor
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`allosteric ligand binding sites, these interactions have, for the
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`most part not yet been confirmed with ligand binding studies or
`isolated tissue patches. For
`this reason the possibility of
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`indirect interactions underlying these apparent allosteric effects
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`cannot be excluded.
`ln this regard,
`the finding that 5—HT3
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`receptor mediated currents can be enhanced by activators of
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`protein kinase C underscores
`the potential
`indirect
`for
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`interactions via activation or inhibition of cellular kinases [35].
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`riicotinic and
`The precedent established by NMDA,
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`GABAergic ligand gated ion channels suggests that 5-HT3
`should also be
`susceptible
`to blockade by
`receptors
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`Page4 of 10
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`TEVA—0089000
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`PTX-253.0004
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`Page 4 of 10
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`

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`Current Pharmaceutical Design, 1996. Vol. 2. No. 4
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`369
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`include several phenylbiguanides, notably metaehlorophenyl-
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`biguanide. A feature of these latter compounds is that, although
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`they possess relatively high potency,
`they are partial agonists
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`with respect to 5~HT. Therefore, the potency of these agonists
`depends on tissue factors such as the receptor reserve and or
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`efficiency of stimulus response coupling. Different effects on
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`desensitization rates may also contribute to the appearance of
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`partial efficacy. A lack of response to these agonists in a
`specific tissue .may not therefore necessarily indicate a lack of
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`involvement of the receptor.
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`Antagonists
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`~
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`Extensive research has centered around the identification of
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`silent, surmountable, antagonists for the 5-HT3 receptor (see
`below). The medicinal chemistry in this area has been
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`extensively reviewed previously and will be briefly summarized
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`here. Current 5~HT3 receptor antagonists comprise two major
`structural classes:
`the benzamides or benzoate esters, and the
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`6,5-heteroaromatics, of which MDL 72222 and tropise_tro_n are
`prototypic compounds.
`In the benzamide series,
`incorporation
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`of the 2-methoxy group and a 4-amino-5-chloro. substitution
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`results in compounds with high affinity. Several aromatic nuclei
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`have been incorporated in this molecule. including benzofurans,
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`benzothiophenes, quinolines and benzoxazines or pyridines.
`In
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`terms of the 6,5-lieteroaromaties, a large number of high affinity
`antagonists utilize this substitution. Granisetron, for example,
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`possess an indazole nucleus that acts as a bioisostere for the
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`indole ring.
`Indeed, a large number of nuclei may act as a
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`biosteric replacement of the indole. The position of the basic
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`nitrogen to the carbonyl
`linker is
`important
`in determining
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`affinity, particularly with the groups orientated close to
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`planarity. Several
`antagonists
`now available of
`are
`exceptionally high affinity i.e. pKi values greater than 9.0 [44-
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`46]
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`Receptor Subtypes
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`Pharmacological evidence from tissue~based functional
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`for several years, suggested the presence of
`studies have,
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`receptor subtypes. The principal finding being that tissues from
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`guinea—pig have lower affinity for nearly all 5—HT3 receptor
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`antagonists in comparison to equivalent tissues from rat, mouse
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`or man. Similar differences have been reported using high
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`. affinity radioligands to label 5-HT; receptors in guinea-pig and
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`other species. Differences in antagonist affinity between 5-HT;;
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`receptors in rat, rabbit or mouse, as well as between different
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`strains of mice have also been reported (see Table l) [47—49l.
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`The molecular basis for these affinity differences are unknown,
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`and cloning of a guinea-pig 5-HT3 receptor ot subunit has not yet
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`been reported. I-lowever,
`the sequence homology between the
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`cloned rat and rabbit 5—HT3 receptors is only 80 %. It is also
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`possible,
`though unproven,
`that these interspecies differences
`in affinity reflect different subunit assemblies.
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`that intraspecies
`Interestingly, several studies also report
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`in 5-HT3reeeptors may exist. However,
`differences
`the
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`pharmacological and eleetrophysiological data indicating
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`intraspecies differences are sparse and the differences in ligand
`affinity small. Nonetheless,
`there are unconfirmed reports of
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`differences in ligand affinity at 5-1-lT3rcceptors in mouse
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`cerebral cortex and ileum [50]. Electrophysiologically,
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`5—Hydr0x_ytryptamz'ne (5-HT) Rece tors
`
`
`3
`I’
`noncompetitive antagonists such as channel binding ligands.
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`And indeed, such interactions are now being found. Ifenprodil, a
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`non competitive blocker of NMDA receptor mediated currents,
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`also blocks 5-HT3 receptor mediated currents in NGl08-l5 cells
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`via a reversible, noncompetitive mechanism [36]. The most
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`likely site of interaction would seem to be the pore of the
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`channel itself, although voltage dependence of the blockade has
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`not been shown. A second compound which may block 5-HT3
`receptor mediated currents via an action in the channel is curare
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`[37—39]. This compound blocks other ligand—gated ion channels
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`and its block of 5—HT3 receptor gated channels shows both
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`competitive and noncompetitive components. Curare is
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`interesting in its own right
`in that,
`its potency for species
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`variants of the 5—HT3 receptor varies by nearly 2000 fold.
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`Other ligands, which interact with undefined binding sites on
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`the 5-HT3 receptor at relatively low affinities are some local
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`anesthetics [40,4l] and cannabinoids [42]. The finding that
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`local anesthetics interact with the channel, at concentrations
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`similar to those at which they interact with sodium channels,
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`implications
`for studies attempting to elucidate the
`has
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`mechanism by which this class of compounds produces
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`analgesia. The data showing that eannabinoids,
`including the
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`endogenous ligand anadamide, block 5-HT3 receptors, not only
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`suggests a possible mechanism by which these compounds
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`mediate tliéirantiemetic actions, but also raises the possibility
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`of endogenous antagonists for this receptor.
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`In summary the presence of multiple ligand binding sites on
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`the 5-HT3 receptor greatly enriches theipharmacology of the
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`receptor and provide mechanisms of regulation not found with
`the G-protein coupled 5-HT receptors. Additional studies are
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`clearly needed to address these intriguing interactions and to
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`further define the therapeutic potential of noncompetitive 5—HT3
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`receptor antagonists.
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`Pharmacology
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`the 5—HT3 receptor
`The discovery of
`is extensively
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`the
`documented elsewhere. However,
`it
`is worth noting that
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`pharmacological characterization of 5-HT3 receptors began forty
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`years ago with the recognition of a distinct 5-HT “M’ receptor
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`[43]. Interestingly, the ligand first used in the characterization
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`of the “M” receptor, namely cocaine, served as a lead for the one
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`of the first selective antagonists, MOL 72222. This research
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`extensively used tissue based functional assays such as those
`based on the guinea-pig isolated ileum or
`rabbit nodose
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`ganglion to identify selective 5-HT3 receptor antagonists. One
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`in vivo assay step that has been extensively used in studies of
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`the 5—HT3 receptor is the von Bezold Jarisch reflex. This is a
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`vago-vagal reflex, observed in in rats, dogs, ferrets or man,
`is
`mediated by afferent nerve terminals in the heart. Taken
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`together, it was only relatively late in the discovery process that
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`radioligands and expression cloning techniques were applied to
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`the characterization of 5-HT3 receptors.
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`Agonists
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`The 5—HT3 receptor is uniquely sensitive to the agonist 2-
`methyl 5—HT, and relatively insensitive to indoles such as 5-
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`methoxytryptamine or 5—carboxamidotryptamine. Indeed,
`the
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`response to 2-inethyl 5-HT is so selective that it is diagnostic of
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`activation of 5-HT3 receptors. Other agonists for the ijeceptor
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`Page 5 of 10
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`TEVA—008900i1
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`PTX-253.0005
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`Page 5 of 10
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`

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`370 Current Plzarrnacezltical Design. J 996, Vol. 2, No. 4
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`as
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`Eglen and Ban/zaus
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`Table 1.
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`Affinity Values (pKi) for Compounds Binding to 5—HT3 Receptors in Membrane Homogenates of Different Tissues
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`Compound
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`SD Rat Cortex
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`CD-I Mouse Cortex
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`CD-.1 Mouse Ileum
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`Rabbit Ileum
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`Guinea Pig Ileum
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`NGl08-15 Cells
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`8.6 (0.22)
`9.7 (0.26)
`RS-42358-197
`10.2 (0.12)
`8.4 (0.22)
`9.9 (0.05)
`9.8 (0.14)
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`9.2 (0.12)
`9.3 (0.16)
`(S)zacopride
`9.9 (0.04)
`8.4 (0.10)
`9.9 (0.06)
`9.7 (0.06)
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`--
`—-
`GR 65630
`9.1 (0.08)
`7.1 (0.14)
`9.7 (0.08)
`9.3 (0.12)
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`8.3 (0.15)
`9.2 (0.26)
`Tropisetron
`9.2 (0.05)
`7.8 (0.04)
`9.6 (0.11)
`9.0 (0.05)
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`7.2 (0.14)
`6.6 (0.06)
`mCPBG
`7.5 (002)
`5.6 (0.06)
`6.9 (0.11)
`8.9 (0.07)
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`—-
`——
`Q ICS 205—930
`9.0 (0.05)
`7.0 (0.15)
`9.7 (0.12)
`8.8 (0.06)
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`Ondansetron
`8.3 (0.05)
`6.9 (0.14)
`9.1 (0.06)
`7.3 (0.13)
`7.7 (0.18)
`8.5 (0.08)
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`(R)zacopride
`8.6 (0.06)
`7.0 (0.14)
`8.6 (0.18)
`7.9 (0.18)
`7.9 (0.31)
`8.3 (0.07)
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`6.0 (0.04)
`5.5 (0.19)
`5.9 (0.08)
`5.8 (0.25)
`5.4 (0.15)
`7.3 (0.03)
`PBG
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`curare
`7.2 (0.12)
`5.2 (0.19)
`6.] (0.13)
`6.4 (0.17)
`6.7 (0.15)
`6.6 (0.19)
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`5~HT
`6.8 (0.12)
`4.9 (0.45)
`6.7 (0.17)
`5.3 (0.08)
`5.7 (0.15)
`6.2 (0.15)
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`Radioligand binding studies were conducted with [3l~l]RS-42358-197 and tissue membrane homogenates. Expe1'i1nental details and additional data are contained in the
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`original papers [49,50]. Note that differences in rank order of affinities can be found for each species examined. The agonists phenylbiguanide (PBG) and
`metachorophenylbiguamde (mPBG) distinguished rat from mouse cortex while ondansetron could be used to distinguish rabbit from mouse ileum. Guinea-pig tissue had
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`relatively low affinity for most ligands tested but the differences between affinities in this species and others varied from 10 fold to over 1,000 fold depending upon the
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`ligand. Except for the agonists PEG and n1PBG'thc affinities of ligands in NGl08-15 cells were similar to those in rat corlcx. Note, by contrast the small differences in
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`affinities for ligands binding to mouse cortex and ileum.
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`discrepancies have also been observed in the potassium/sodium
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`ion permeability, unitary conductance, inward rectification and
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`rates of desensitization of 5—HT3 receptor as expressed in
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`different tissues from a single species. However, a number of
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`explanations, other than differences in the receptor protein or
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`subunit composition may account for these differences. Thus,
`the lipid environment in which the receptor sits may be different
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`or the degree to which the receptors are desensitized prior to the
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`experiment may differ. Molecular cloning approaches have not
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`yet provided evidence for intraspecies receptor heterogeneity.
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`Thus far, only a single 5—HT3 receptor subunit, termed 5-HT3R—A
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`and :1 splice variant have been cloned from any single s