November, 1964
A MOLECULAR THEORY OF DRUGACTIOX
for the simple reason that k3 would be insensitive to structural variations in the case of pure agonists but sensitive to moleoular structure in the case of partial agonists. There is no reason to believe that ks should not be sensitive to all types of structural variations, let it be in the agonist or partial-agonist series, since this is what corresponds to general experience in the field of enzyme Chemistry. The postulated intervention of a rate-limiting change in an initially inert drugreceptor complex requires that a unique energy barrier for the formation of the transition state for rearrangement would apply to all agonists regardless of structure. The lack of physico-chemical precedents for such a hypothesis suggests that the fitting of doseresponse curves to mathematical equations can hardly be used as evidence for the validity of a theory. With regard to Ariens’ initial interpretation of the phenomenon of intrinsic activity (or efficacy), it would seem that the hypothesis of the rate-limiting change of an inactive to an active drug-receptor complex is probably distantly related to the true mechanism of drug action. Interpretations of the physico-chemical significance of the parameters affinity and intrinsic activity are at best equivocal. Thus, ,4riens6 suggested that affinity could be determined initially by long-range electrostatic interactions between drug and receptor, interactions which would condition the subsequent operation of London dispersion forces culminating in complex formation. Such a stepwise mechanism of complex formation is without practical significance since the physico-chemical events occurring during the process of complex formation cannot in any way influence the ultimate response, the resulting drug-receptor complex supposedly becoming active after the binding process has occurred. With regard to the parameter intrinsic activity, the situation also appears ambiguous. The numerical values which reveal that some drugs are more or less intrinsically active were suggested recently by Ariens6 to reflect the fraction of collisions between drug and receptor that are effective in producing a stimulus. This model which is now a statistical one, was first developed by Janku and Mandl” and constitutes a mechanistic transition which is incompatible with the primary postulate of the formation of an initially inactive drug-receptor complex. It seems clear that if the parameter intrinsic activity is to be relevant to the rate-limiting change of an inactive complex to an active one, all collisions must initially produce an inactive complex. On the other hand, if the statistical model is accepted, intrinsic activity can no longer be made analogous to the rate-limiting step k , of enzyme reactions. Inconsistencies of this kind emphasize the need for suitable molecular theories of drug action. Partly because of the inadequacies of the AriensStephenson modification and partly on the basis of Croxatto’s postulate12 that a drug would be effective only at the moment of encounter with its receptor, Patong proposed his rate theory which states that drug activation of receptors would be proportional to the total number of encounters per unit of initial time. In other words, it would be the rate of complex forniation with receptors that would determine the response (11) I. Janku and P. Mandl, Cesk fyszol., 10, 338 (1961) (12) R . Croxatto and F. Huidobro, Arch. Intern. Pharmacodgn., 106, 207
(1956).
777
rather than it constituting an obIigatory step preceding the activating phase (as in the Ariens-Stephenson model). It is a requirement of this theory that high stimulant activity should reflect not only a high rate of combination with receptors but a still higher rate of dissociation of the drug. On that basis, the qualitative properties of drugs could be accounted for in terms of their characteristic rates of dissociation (k,) froni the receptors. Thus, agonists mould be characterized by a high k z , partial agonists by an interniediate, and antagonists by a low kz. It was noted that antagonists are usually large inolecules which because of their greater size would “stick” to the receptor substance. In addition, it was suggested that agonists would conibine with the receptor at a unique rate but would dissociate at variable rates, thus allowing for a wide range of encounters per unit time. This rate theory of drug action has niany attractive features but creates serious difficulties when analyzed at the molecular level. For instance, it is generally agreed that the action of acetylcholine (ACh) is catalytic a t the effector level. It is clear that ACh produces its effects by inducing a change in a theoretical receptor and that this change is conditioned by the presence of the molecule on that receptor. Also, the longer a catalyst is present in any reaction medium, the longer its effect is felt; it seems normal therefore to expect that the ACh-receptor complex should persist if a strong effect is to be observed. According to Paton, however, this mould not be so; the receptor acquires catalytic properties only during the act of combination but jniniediately loses this property if ACh should remain adsorbed. I t follows that ACh will have to dissociate at a high rate in order that the receptor can again acquire the potential property of a catalyst. Sow, if the rate of combination is not sensitive to structural variations in the substrate, the rate of dissociation has to be. Therefore, all strong stimulants (such as ACh) must be endowed with structural features that are characterized by a high degree of nuisance capacity towards complex formation since all must rapidly dissociate from the receptor. This conclusion contrasts with general experience in the field of enzyme chemistry, good substrates always possessing special structural features favoring an initially essential and specific affinity for binding sites rather than uniformly producing hindrance to adsorption. For instance, the ester methyl group of ACh which promotes affinity for acetylcholinesterase (AChE) would now be endowed with a considerable nuisance capacity with regard to adsorption on receptor sites, because when it is absent as in the analogous molecule of formylcholine, stimulating activity is sharply reduced. Hence, ACh would have a higher kz than formylcholine. But then, if instead of removing the methyl group of ACh which would result in a higher affinity for the receptor, we add two extra methylene groups (CH,) to obtain butyrylcholine, a lower potency results, and now this has to be attributed to increased force of binding (leading to low kz). Hence, both the removal and addition of CHI or CH2 groups would have a similar influence on kz. The nuisance capacity towards binding of the ACh methyl group would stand out as a unique phenomenon which is unlikely to be reproduced in the wide variety of known agonist molecules (such as carbachol
B. BELLEAV
778
which includes a hydrophilic NH2 group instead of a hydrophobic CH, in its molecule). Such dcviatioiis from coiiiniorily accepted physico-cheiiiical liiiordedgc~ serve to cast serious doubt on the practical significance. of the rate theory, and a return to some foriii of Clark’< original occupancy theory is i~idicated.
The Marcomolecular Perturbation Theory (MPT).--
A molecular theory of drug action will n o be~ dcmribed which is based oii biophysical priiiciple.; aiid which appears to account not only for drug receiptor kiiicitics hut whicli also s(~rvesto mplaiii stiuctui -acti\ itv relationships I\ liicli caii be illustrated for di ugq actiiig o i i the muscarinic choliiiergic rcwptor. I n additioii, tlic theory allows some iiitriguing coiiclusioiis coiiceriiiiig the probablr naturc of the. rcw>ptoraiitl of its idatiotiship with XChE. The thcory, whic3li iiiay 1)e corivetiicntly rcferrrd to as the iiiaciniiiolccnlar portiirbatioii thcory (.\IPT), is 1)ased oii the ~ i i a r l dm i d i i t i k l i i v coiifoririatioiial adaptability of cviayiiic’ ~itiiscariiiic rrcept or is a iiiacroiiioleculc with pro1 eiii-lilx properticand specifically designed for 1111cractioii 11i t li t hc natural substrate .‘ICh. Circuiiistantial (.videlice for this hypothesis is available a i d call t)p suiiuiiarizcd as follo.ivs. (a) The receptor subst aiicc can hind a n iclc variety of drug riioleculcs, t hiis suggest irig a d(grec1 of conforiiiational adaptability c1iarar.t(Iristic of pi’ot(4iih. (1)) Tleceptors display iiiarkd ahsoliitv atid relat i v c b st(~reospecificity,l3,I4 a propcrt y 4 i a i d by ~ ~ i z y i i i w . (c) Since both AChE and the cl-ioliiiorgic rcmlpt or arc’ specific for ACh, it seeiiis likely 1 hat t share the coiiiiiioii property of h i r i g protcinh. Thr investigations of TVascr’z o n t lie1 fivat i o i t of‘ labclecl curarine arid muscarorif’ i l l t he) ciid plat cs support t l i ~ hypoiliesis of a “strurtiii.aI Iiiil,” 1)ctu-cwi .lCIiI2 and choliiici*gicreccpi 01’s. Acceptance of the reasonable asauiiiption that tlio receptor is a protein of specific structure arid composition will tiow iiialw il possibk to a1 tempt an iiiteiprctation of drug action. The sj-striii ,‘ICh-reccptor call t lierefore be treat ed as an ciizynit.-substratc.-like systcni and it will rciiiairi to analvzc the basic conseqiimces of complex forinat ion in physico-cliciiiical teriiis. Ilur to the> elegaiit woik of Iiagram I represents priitein chains in the resting state xvhere drugs interact. 'Uie circles are hydrriphcibic. residues. T h c symhol X denotes water molecules of hydration; A and K are reactive speries representing specific. binding sites for ACh and other strong stimulants. Diagram I1 represents the structure of the complex (P*lI,) with the stimulant pentyltrimethylammonium (C6H,S + l l e 3 )(darkened circles). The driving force for binding originates in hydrophobic interactions. Some bonds between nonpolar chains in I are broken and new ones are created with the substrate. The shape of t'he protein in I1 represents a SCP whose formation is accmmpanied bj- the expulsion of water molecules (X = H?O). The hydrophobic character of the exposed protein surface would be iricreased in 11. Iliagram I11 illustrates the consequences of complex formation with the homcilogous nciii~-ltrimethylamrn(iriiumkin ICsH,& +Me3)(darkened c'ircles). The four extra methylene groups induce a KSCP because hydrophobir interiwtions in excess of that rreatecl by ACh or (CjHl,?;+Me) are operative. A different, kind of complex results ( P*AI;) in whit-h the outer surface would he less trydrciphobic-. Ijiagrams I V and T' represent the consequences of vomplex formation with a molecde of the SIs, type (heptl-ltrimcthyl:irnmonium, CiH15K+Me3); an equilibrium mixture of P*lI,i and P *AIai complexes would result. T h e rep1:wement of S-methyl by ethyl groups (not shown in t,he above diagrams) R-ould also disturb the hydrophobic periphery of t h e aiiioiiic~site arid a complex of the P viiriety would he favored.
*
poiiit of $1-eatest iiiterest lies in the fact that whereas t h(>ions of S a and K are tightly hydrated, quaternary (aatioris are actually hydrophobic.13 It may be, therefore, that the e,fe'ecl of the latter i s to disrupt the siructure c!f bound water. in the meinbranel lhus making it available ,for iVa+ and K + tl-ansporf. I t may be possible to label the watw in membranes aiid study its movements in relation t o stimulation by quaternary ions. Conceivably, in a P*M, complex, the protein mould assume that conformation which loosens a critical number of water niolecules. This n-ould riot apply t o P*lI, co 11ipl exes. The question of what factors will determine the affinity of AIs molecules for the receptor will be oxaniincd
ncxt. It will tw u d u I initially the ascertain to o\-ei,-all physico-chemical propwt ies of the ACh-specific binding region of the recept 01' Thr equally nonpolar character of this diffuse coiiipartnieni is readily evidenced by the gradual incrcast. in affinity for the receptoi' of alkyltriniethylaiiiiiioiiii~i~istimulants as the hydrocarbon chain i b iucrc~abcdfroni C1 to Cj.22 Similar i o the observations of Barlow, et U L . , ~ ~on the effect of added CH2 groups on the onium head of antagonists. the operation of AFt in the stimulant series serves to Pstahlish the hydrophobic nature of the ACh compart (22) J h i 7 an Kossurn and I,. J h e n s , A i c h . zntern. Pharmacidyn 110, 349 (1957); ,J. Rf van Rossuin, Doctoral Dissertation. Rornan Catholic 1 niTersity, Ullmegen, 19%
ii ~IOLECULAR THEORY OF DRUG ACTION
November, 1964
iiient. There is little doubt on that basis that Ing's rule of fivez3is relevant in a majority of cases to the phenomenon of hydrophobic interactions, the factor least exacting in its requirements. However, specific interactions between certain N8 molecules and the ACh compartment will be likely to occur as was previously demonstrated for the case of the muscarones and the dioxolane series of stin-~ulants.~~ The presence of shielded reactive functional groups jn the ACh conipartnient is required in order that the high potency of ACh can be accounted for. It is also conceivable that -1.1, molecules including 9-electrons, such as the furfuryltriinethylaininoniuni ion, niay engage into charge-transfer binding with the receptor surface and thus display increased affinity. Ideally, however, maximum efficiency in the induction of a SCP will be reserved to M smolecules producing true lock-and-key type of fits with the ACh Compartment. Sormally, such fits are a privilege of natural substrates (ACh in this case), but evidence has now been obtained that certain nonsubstrate molecules can interact in a substrate-like fashion with AChE.I3 Thus, the dioxolane inhibitor I of AChE was shown to alloiv the application of the distance-specific van der Waals attractions in its complex with AChE, a feature characteristic of ACh.13 This finding serves to explain the high potency of I at the receptor level (an observation which once more points to t,he similarity of AChE to the muscarinic receptor).
CH,
+
CH,N(CH,),
I
C. Mixed Complexes.-A problem of key importance consists in deciding whether the transition from eq. 1 to eq. 2 will be a continuous or discontinuous one. I n its present form, the AIPT requires that only one conformation for P*II, complexes is productive with respect to the criterion of pharmacological activity. I n contrast, an indefinite number of SSCP is encompassed by the symbol P*i\I,. It is conceivable that several related conformations niay characterize P*JI, complexes, but for the sake of preserving a minimum of rigor in this expos&,the tortuous paths of least resistance niay profitably be avoided and P*AI, complexes assigned unique structural features. The possibility therefore arises that certain molecules may induce the production of equilibrium iiiixtures of P* and l'+ complexes. Obviously, energy barriers must be overcome in order that a P*;\I, or P f N , complex can form and it is quite conceivable that for certain types of molecules, these energy barriers as well as the energies of the two types of complexes may be of coinparable magnitudes. The molecular features most likely to allow this phenomenon would be incorporated in an M, molecule carrying a substituent which does not protrude too deeply into the nonpolar periphery of the ACh conipartment (Fig. 1). On a probability basis, such a molecule would be as susceptible to induce a SSCP as it would be to favor a SCP, there being two comparable energy paths for the reaction. This class of molecules will be designated by the symbol (23) H. R. Ing, P Kordlk, and D P H T. Willianis, Bnt J . Pharmacol , 7, 103 (1952). (24) B. Belleau and .J Puranen, J .Wed Chem , 6, 726 (1963)
781
AI,, and is typified by the stimulant heptyltrimethylammonium (Fig. 1) which includes two CH2 groups susceptible to acconiniodation in the hydrophobic periphery. Equation 3 serves to rationalize this phenomenon of ambivalence. When K3 = 0, ey. 2 P*M.,
e -P + YIs> K4
K3
P*JL
(3)
applies, whereas when K , = 0, eq. 1 describes the process. I t will be realized at this point that the corner stone of the N P T rests on the question of transition in the mechanism of coiiiplex forination accompanying the formal cheniical conversion of an >Is niolecule to the Mi type. The validity of this hypothesis was tested using AChE as a inodel and the C1 to C12 series of alkyltriinethylamiiioniuni inhibitors as test substances. At the receptor level the transition from stimulant to antagonist in this series occurs with a chain length of 7-8 carbon atonis. I n agreement with expectations based on the APT, a sharp transition in thc mechanism of interaction of these ions with AChE was observed with a chain of 8 carbon atoms.25 These results constitute definitive evidence for the validity of the concepts forming the basis of the _\IPT. D. Polynary Complexes.-It will be expected that the structural integrity of P*lI, or l'*JIqL complexes will be maintained principally through the operation of hydrophobic interactions. Evidence for this notion is available in the work of Tanford26on the key role of hydrophobic forces in the stabilization of the tertiary structure of proteins. The multiplicity of nonpolar regions (as entities distinct from the active surface) in proteinsz6suggests that ati indefinite number of new binding sites may be created when additional molecules of nonpolar character are brought into contact with the protein. This will be especially true for charged substrate molecules which allow for initial contact with counter ions on the outer surface of the protein. Pertinent evidence supporting this view is available in the observations of Lovrie'g on the nonspecific binding of nonpolar chains by serum albumin, a protein devoid of known catalytic properties. It is of interest that the latter will create new biiiding sites only if it is initially perturbed by alkali. Extrapolation of these observations to the case of a P*& or P*& coniplex leads to the coiiclusion that suitable quaternary ions carrying hydrophobic chains will likely create additional binding sites in such complexes and therefore induce conformational deforniation (SSCP). The driving force for such reactions resides in the hydrophobic interactions exerted upon the substrate molecules. S o specific preformed binding sites are required for the reaction to occur. As pointed out above, if the forination of a P* complex implies the "loosening up" of water molecules, the nonpolar character of the entire protein surface would have to increase, thus facilitating the nonsprcific accoimiodation of additional hydrophobic ions. These considerations suggest that ey. 4 should be applicable to active receptor complexes. The saiiie ought l o apply to P*hi, or P*?.I,,
+ n M , or n X ,
tP+(J18)Jl or P*(?*I,,)" (4)
(25) B. Belleau and F. Lie, Pharmacol. Reu., in preparation. ( 2 6 ) C. Tanford, J. Am. Chem. Soc., 84, 4240 (1962).
782
B. BELLEAU '10 respons IOC
log [Msi]
a
b
%response IOC
n C
Fig. 2.-Dose-response partial agonists (eq. 3 ) ;
curves f o r : a , agonists ( e q . 1 ) : h, ternary complex formation (eq. 4).
(2)
I'*-\Il cotiiplexes, but this caw need not he considered further since such complexes are already iriactive as such. On a probability basis, a fairly large nuniber of h1, niolcculrs should exist out of which very few can I)(: expected to produce a lock-and-key type of fit with tlic specific binding surface of the protein. Siniilarly, an equally large nuniber of coiiipounds mill be of the -\I, type (since the requirements for the iriductioii of a SSCP are quite low) but oiily a restricted nunibcr will behave as -\Isl iiiolecules because of the more exacting requirements for the applicablity of eq. 3.
Derivation of Dose-Response Relationships Using the Macromolecular Perturbation Theory as a Basis.The application of an M, molecule (such as the natural substrate AICli)to the receptor protein will result in the forniatiori of a P*M, co~iiplex,the equilibrium coucentration of which will depcnd, as discussed above, on the structural features of the molecule. A niaxinial response will always be obtainable with this class of ~noleculeswhich therefore must belong to the category of pure agonists. The general form of the corresponding dosc-response curve sho~vnin Fig. 2a is tlie simple rcflectioii of the application of the mass action law. The parallel shift of the curves obtaitied with different *\laiiiolecules is an index of the relative efficiencies in the induction of a SCP in the protein. Li~iAI, type of irioleculc will 011 the other hand produce a S S C P i i i tlic protein (ey. 2 ) arid conipetitive antagonisin will he ohserved. However, an &I,, inoleculc will now induce the forination of an eyuilibriuiii mixture of P*& and l'*hI,, coinplexes (eq. 3) and only subinaxinial responses will obtain. This phenonicwon accounts for thc c~xistei~ce of partial agonists and t hc dose--rcsponse c n r v ( ~will have the general forni s1ion.n in Fig. 21). Viiially>the operation of cq. 4 a t tlic rccepior protein 1r:vcl trill be reflected in bell-shaped dosc-respoiise curves as illustrated in 1;ig. 2(.. This phenonieno~i
Vol. 7
A MOLECULAR THEORY OF DRGGACTION
November, 1964 TABLE I
Compd.” HCEC--N +hIer CH-CHN +&lea BrCHzCHaN +Ales Br(CH2)aN +Mea CHsOCCHzN +Mea
Nature of complexb P*M8 P*M. P*M,
P*M8 P*M.
Hydrophobic interactions favoring NSCP 0 0 0 0 0
Height of parasympathetic (muscarinic) stimulationC Maximal Maximal Maximal Maximal Maximal
II
CHz AcO(CHd2S +Mer dcNH(CHz)zN +Mea (ACh) Carbachol (Ac- -Me-Ch) (Muscarine) HFMea HFbIezEt dl-MeFbIes (L-crs-hleF.\Iea) EtFhlea MeFMezEt MeaFiUea Ch MeN +;\lea E t N +hies P r N +Mea B U N+hfe3 n-CsHi& +;\lea
P*M8 P*hI, P*M, P*hI, P*hI, P*M* P*M, P*M, P*M. P*RI, P*M, P*hl. P*M, P*M, P*hI, P*hI, P*hI, P*hI. P*M.
n-C6HnN +Mer
P*hI.
0 0
0 0 0
0 0 1 0 0 1 1 0 0 0
0 0 0 0 1
Maximal Maximal Rlaximal Maximal Maximal Maximal Maximal Rlaxirnal hlaximal hlaximal hlaximal hlaximal Maximal Maximal Maximal Maximal Maximal Maximal Maximal (BSC)‘ Maximal (BSC) 4
(av. = 0.15) HFhIeEta EtFhlezEt PrFhIea HFEtr hIeFRIeEtr Et?Fhler ChblezEt C h hIeE tz n-CvHuN +Ale3
2 2 2 3
2 2 1 2 2
s P*M,i
B U N+hle?Et n-CsH1iN +MezEt
P*,\I,, P*hI,i
BuFlles n-CeHuFhIea CsHsFhlea PrFhIerEt EtFMeEtt PrzFhlea Bu2FlIea (CsHdzFhler ChEtr n-CaHnX +Ilea
P *RIi
3
P*hIi P*Mi P*hli P *Mi P *Mi P *Mi P*Ni P *Mi P f M i (weak P*M.i) P *Mi (weak P*M,i) P*Mi P*Mi P *Mi P *Mi P *Mi
5 5 3
n-CoHisN +hler n-CloHnN +Mea n-CsHiaN +;\lelEt B U N+MeEt* n-CaHuN +;\leEtz B U N+Eta n-CIaHzsN +Me3
P *Mi
P*RI.,
2
1 (av. = 2.1)
Submaximal Submaximal Submaximal Submaximal Submaximal Submaximal Submaximal Submaximal Submaximal (BSC)’ Submaximal Submaxirnal
7 11 3 3
Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil (BSC)‘
4
Nil (BSC)I
5 2 2 2 3
Nil Nil Nil Nil Nil Nil
3 5
7
. = 4.3)
a Compounds enclosed in parentheses are taken as interacting ideally (or nearly so) with the protein;
F
=
g0>CH,p--.
Ch=choline.
P*M, = addition complex resulting in a SCP, P*M, = addition complex characterized by a NSCP, P*M,i and P*M.i are complexes resulting from an equilibrium mixture of SCP and NSCP. BSC = bell-shaped curve.
Some of the most efficient inducers of a P*Ms complex are enclosed in parentheses in Table I. It seems reasonable to ascribe their high potency to their ability to produce lock-and-key type of fits with the protein’s
783
specific binding surface or to engage in highly specific binding other than by van der Waals forces with that surface. Thus far, evidence that specific van der Waals binding has a marked favorable influence on potency has been adduced only in the case of the L-cisdioxolane quaternary salt (I) which is believedla to produce a true lock-and-key type of fit with the related protein AChE. The high potency of L-(+)muscarine2’ suggests that specific van der Waals attractions may be operative. It would be presumptuous at this stage to attempt a detailed interpretation of all the other factors controlling the relative potencies of stimulants. Much more will have to be learned about the factors influencing affinity and conditioning the induction of a SCP. The recognition of the key role of hydrophobic interactions (accounting for the AFt term) as a factor distinct from van der Waals attractions certainly constitutes an important step in this direction. I t will be noted that the special case of noncompetitive antagonism has not been considered in the treatment of the NPT. The reason for this lies in the fact that noncompetitive inhibition can usually be demonstrated only with substances bearing a distant structural analogy (such as papaverine, inorganic ions) to stimulants or competitive inhibitors. It is known that noncompetitive inhibitors of enzymes often do not interact with the same active sites that normally bind substrates or competitive inhibitors.28
The Nature of the Muscarinic Cholinergic Receptor. -It has long been noted by several investigators that many similarities exist between AChE and the cholinergic receptor. A number of new parallelisms between the two have been noted in this paper and el~ernhere.~~ The most striking of them consist in that (a) both active surfaces are similarly hydrophobic in character and (b) both display identical patterns of absolute and relative stereospecificities towards the dioxolane and muscarine series13 of stimulants. Other more revealing correlations will be discussed separately in the near future.25 The hypothesis of the identity of AChE and the cholinergic receptor was formulated first by Roepke in 1937.29 Since then, an impressive accumulation of conflicting data has cast considerable uncertainty on this hypothesis. Pertinent literature on this subject is a~ailable.~oOne of the key inconsistencies created by the AChE hypothesis for the receptor consists in that the latter is not inactivated by the organophosphorus drugs. However, some recent kinetic studies by Krupka and Laidler31 have revealed that acetylated AChE, the intermediate which is formed during hydrolysis of ACh, interacts equally well with inhibitors as free AChE does. It is currently believed that the acetyl group is attached to the same serine hydroxyl that serves as the eventual acceptor of phosphoryl groups. If it is now postulated that the induction of the physiologically significant SCP results from interactions with acetylated AChE, an explatia(27) P. Waser, Ezperientza, 17, 300 (1961). (28) K. J. Laidler, “The Chemical Kinetics of Enzyme Action,” Claren-
don Press, Oxford, 1958, p. 77. (29) M. H. Roepke, J . Pharmaool. E z p f l . Therap., 09, 264 (1937). see also W. C. Wescoe and W. F. Riker, Jr., Ann. N . 1‘. Acad. Sct., S4, 438 (1951). (30) “Handbuch der Experimentellen Pharrnaokologie,” Vol. 15, G. B. Koelle, Sub-Ed., Springer-Verlag, Berlin, 1963, Chapter8 9, 13. (31) R. hl Krupka and K. J. Laidler, J. A m . Chem. Soc., SS, 1445, 1448, 1454
(1961).
tion for the receptor resistancch to attack by organophosphorus drugs would lie iri the unavailability of a free serine hydroxyl for phosphorylat ioii. Soiiic i i i triguing features of this hypothesis may he suniniarizcd as follows. The receptor would havc its t,iocheinir:tl origin in the pool of AC'hE, a port ion of which ivould h(b trapped in the iiienibrarie network aud tiiaititainrd iti the acetylated form through a steady-staic quantal discharge of ACh from the syiapt ic cldt This n-oultl constitute a self-generating xystctii. a phcwoiiieuori riot uncoiiiiiion in bioclieiiiist rjv (as is i lie case f o r instance for hoiiie of t he catalytic iiiteriiicdia tricarboxylic acid cycle). dssuniiiig that s o n i ( ~drugb could cause release of additioiial cluarit i t ics of .\C'h?? (.%ZJ
G 13. Koplle, .I. I'ttaiin I'tiarnirz,o7
,
14,
(17
lW2l
A large number of 1,4,i-trisubstiluted piperidines have I ~ e e syiit ~ l lwsized hy lithium aluminurn hydride reduc,tion of the cwresponding glutarimides. Many of the piperidines :ire highly active as hypotensives when administered intraperitoneally t o intact conscious rabbits. The most active ronipounds were 1-(3,4-diebhoxyphenethg1)-4methyl-4-n-hes~lpiperidirie hydrochloride and l-(p-niethc lohesanepiperidine hydrochloride. Structure-activity relationships :ire discussed.
In recent years variations 011 the piperidine structui,c have been the subject of many investigations in addition to the earlier work which led to the introduction of 4-carboethoxy-1-methyl-A-phenylpiperidine (pethidim) as an analgesic. Other such cotiipowids haviiig potent arialgesic activity have. since heen reported, e.g., I 2 mid II.3 Hypotenhive activity has also been denioiistratrd iii this class, 1.2,2,6,6;-pentaiiirthylpiperidine (Peiiipidine) being the iiiost proiiiineiit example. Other piperidines havirig hypot~iisiveactiv-
qJ-
C350coc N-CH-CH.
NH-
CbH,
been reported to haw lieurosedative as well as hypotensive and aiitieiiietic action
