CHOLINEACETYLTRANSFERASE INHIBITORS
March 1970
'221
Choline Acetyltransferase Inhibitors. Dimensional and Substituent Effects among Styrylpyridine -4nalogs C. J. CAVALLITO, H. S. YUN, School of Pharmacy, Cnioers1ty cf -Vorth Carolzna, Chapel Hill,S o r t h Carolina
T.
ICA4PL.4N, J.
CRISPIN SMITH,AND I?. F. FOLDES
Anesthesiology Research Laboratory, Jlontejiore Hospttal and Medical Center, Bronx, A'ew I'ork Received August 4, 1969 Among styrylpyridine analogs, choline acetylase (ChA) inhibitory potency is diminished by highly electronegative substituents ( C S , NO?) on the 3- or 4-position of the phenyl ring but is enhanced by halogens (Cl, Br) less electronegative than F. Substituents inducing deviation from coplanarity of the two ring systems are unfavorable for inhibitory act,ivity. 3-AIet.hyl substitution on the pyridine ring enhances potency. The nature of the pyrido--Y-attached quaternizing group is lioncritical and a hydrophilic substituent can provide potent, more water-soluble, derivatives. A naphthylvinylquinoline syst,em provides a high order of potency, but the same mass distributed as in pheiianthrylvinylpyl'idine is unfavorable. ChA inhibitory activity among t,hese compounds seems favored by thin flat molecules, one end of which teiids to have ?r-electron-excessive, the other end ?r-electron-deficieIIt, characteristics separated by a conjugating exocyclic bond. The photolability of some of these compounds in solut,ion requires appropriate precautionary measures in their evaluation.
I n a n earlier report,' derivatives and analogs of styrylpyridine were described that included some unusually potent inhibitors of choline acetylase (ChA) (choline acetyltransferase). The favorable influences of structural coplanarity, presence of a cationic charge, as well as some other features were related. The present report describes additional molecular modifications that provide the basis for more refined correlations of structural featureq with activities. These permit some speculations on the nature of possible interactions of these inhibitors with the enzyme a t the molecular level. A further aim was to prepare highly potent, selective, water-soluble inhibitors of ChA4. For the most part, these compounds are relatively poor inhibitors of acetylcholinesterase (AChE). The target compounds in this report are quaternary ammonium salts which may be considered as variations of prototype I.
I
It mas reported earlier' that the rioriquaternary amine analogs were influenced in the same directions as the quaternaries by structural variations, but were less potent and of limited solubility under the tebt condition?. Electronegativity of Substituents on a.-This influence is particularly evident from compounds in Table I. The distinctly unfavorable influences of S O , (VII, XI) and CX' (VI) substituents, arid progressive increases in potency in the order of E' < C1 < Br, implicate unfavorable effects of highly electronegative substituents which can withdraw electron density from the aromatic system, and potency enhancing characteristics of less electronegative atoms that can supply a-electron density to this system. The fluoro derivative (111) is comparable to the prototype in potency. The intermediate electronegativity of F in this series may be (1) C. J. Caiallito, H. C h e m , 12, 134 (1969).
such as to produce minimal resonance change in tlic aromatic system. I n the halogeri series, 3-substitution is slightly better than 4-; there is 110 :idvantage in disubstitution. Substitution in the ?-position is least desirable; however, here a st'eric influence may cause some deviation from coplanarity arid reduction in potency. Third dimensional bulk (thiclmess) contributed by these substituents probublj- is insufficient to be a negative factor (note I1 1's. I). The KO2 and C?; groups, of course, are coplanar. Heteroaromatic Systems at a.-In the earlier report' it n-as rioted that analogs in which either a or c \\-as fused bicyclic and the ot'her monocyclic yielded compounds of greater potency than that of bismonocyclic I. Compounds are now included in which a is an indolyl and c a pyridinium group (compare XXIV with XT'III) and where a is 3-pyridyl (XXX) or 2-thierij-1 ( X X I X ) and c is quinolinium (compare with S X Y I ) . In ezch of these the heteroaromatic subst>ituentat a is unfavorable compared with the carbocyclic prototype in influencing potency. The pyrido component of XXX has a pK, of the order of 3.8* and that moiety is essentially unionized under the test conditions. The nonbonding pair of n electrons on t'he hetero :itoms may be an unfavorable influence in a. Substitution on b.-It \vas show1 previously' that an interannular double or triple bond at b was a critical feature in providing active inhibitors. I t now is reported that replacement of H by CH, 011 the carbon of b linked to the pyridine ring yields a completely inactive compound (XXXI). This is further support for a requirement of coplanarity of the a-b-c Methyl subst'itution in this position sterically prevents coplanarity. Substitution a t c.-Most of the compounds in this and in the previous report's are h u b s t i t u t e d pyrido systems. A few examples of 2-isomers n-ere compared and found to be active inhibitors but less potent than the 4-derivatives. The 3-stilbazole methiodide (XXXII) was prepared for comparison (with I) and found to have an 16, of 4 X lop5against ChLL Although somewhat less potent, t>heactivity of the 3-isomer makes it
S Yun, J. C. Smith, and F. F. Foldes, J . M e d . (2) Estimated by titration with HC1.
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CHOLINEACETYLTRANSFERASE INHIBITORS
AIarch 1970
223
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a-donor (D) and a-acceptor (.4)moieties are present in the same molecule. The favorable influence on ac"sD C 3 3 tivity of the less electronegative halogens and un55 2 ; favorable effects of very strong electron-withdrawing 9 - m 3 % 5 7 5k C3 2g groups is compatible with, although no proof of, the 7 5 % ? T ;? 3 need for a D, structure at a. Slthough ring atoms o L ) 3 o og , .s .z?sk (X, S) with noribonding 11 electrons may enrich the a-elect,ron atmosphere, they may also maintain a more '5 ;, '5 $ 2 3 7 localized n-donor feature. In any event, these com.= a &; E $ pourids are poorer ChA inhibitors (SXX, XXIX). For 53 u -= g$s inhibitory activity an indole group as a (XXIV) is less favorable than an aromatic ring system (XVIII) but not .-$~6 2 2 .e % 5 ; as deleterious as thiophene ( X X I X 2's. XXVI). I n .3 2 2 5 I 2 3 addition to the unshared electrons on S ,and a possible >?Js j H-bonding K H feature, indole ha,? been attributed to s=-3 w - c 7 m .5z g s5 serve as a donor in charge transfer complexes in which ?t. N N T T N ; Zx' pg=S daccmI $ the C-3 carbon has been suggeatedj,6 as a region of N*.T?l 33 C l N ? a S-;" localization of charge. I n XXIY, the conjugated =c+ linkage through the C-3 of the indole group may not 3c =5 6 .s c: 5.g make the latter suggestion pertinent to this compound. h L 52 .= The greater potencies of the p!-ridinium derivatives H ~ a - k Z b- e; = % $ 5 than of the less ionized base forms is compatible with 4 ' a2 -i 2 A, characteristics of c associated v i t h the quaternary g 11 or the protonated onium moiety. The favorable inc h h t, fluerice of cationic character of the pyridinium group %% e & 95 . 70 - a = may riot be related to a need for bonding int,eractiori % E ,"%E = % - c Y, a a, a t an enzyme anionic site, but the pyridinium form can 6 62 %,E=$ provide the a-electron-acceptor characteristics. This gs 2 still would be compat,ible with the relative inactivities + 11' 5 ?4 6 .s c a reported' for the S-oxide form and of stilbenecarill '5.e boxylic acid. The apparent noncompetitive charact'er 8 of the inhibition does not require involvement of cat,ion ._k Q 6Y ;i 2 2 2 bonding compet'ition with choline. The wide latitude 0in dimensional and hydrophilic or hydrophobic char.-1 3 3c 3 s acteristics permissible for d further suggests that the .-2+ 2 5 a : . ; d pyridinium nitrogen locus would not be called upon to '=I1 2 react with a structurally discrimiiiating site of the 2 a2hz$s+ enzyme. The somewhat lower Ch;Z inhibitory potency ,CZbiPi Lc-c: A && ? 2 of 2- relative to 4-stilbazole isomers' need not be the result of increased steric hindrance to pyridinium bond2 22 - 4 G= ing t'o a recept'or anionic group but could derive from a * -
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( 5 ) A. Szent-Gyorgi, I. Isenberg and J. hIcLaughlin, Proc. S a t l . Acad. Scr. C. S . , 47, 1089 (1961). (6) Several review chapters on charge transfer complexes are provided in "-Molecular Associations in Biology." B . Pullman, E d . , Academic Press. S e w York, N . Y., 1968.
