Richard E. Rice University
of
Kansas Lawrence
Drug Receptors
A
drug may be broadly defined as asubstance which in a relatively small quantity is able to elicit a response from living tissue. The biological mechanism by which a drug molecule acts is one of the most interesting and important problems in modern pharmacology. In spite of the large amount of research currently pursued in this area, there is still very little known about drug action at its most fundamental level. The molecular description of drug activity may be viewed as an extension of chemistry and is consequently often termed molecular or chemical pharmacology. Simple calculations made by Clark (I) indicated that in many instances only a relatively small portion of the affected tissue could be covered by the amount of drug known to be present. Clark noted that this idea of a drug acting on a small fraction of available surface was very similar to a theory of active sites on contact catalysts introduced by Taylor (9). The idea of a drug molecule interacting with an active site on the cellular surface, i.e., with a drug receptor, was first suggested by Langley (S),although it is usually associated with the early chemotherapeutic work of Ehrlich (4). One present concept of the receptor is that it is a means for attaching the drug molecule to a cell membrane. This type of mechanism would involve chemical and physical interactions such as covalent and hydrogen bonding, London forces, steric hindrance, electrostatic repulsion, i.e., both attractive and repulsive forces. Another viewpoint is that a receptor is simply a mechanism either in or on a cell which can cause a cellular response to a drug molecule. There is much evidence accumulated a t the present to indicate that a drug receptor is actually an enzyme. In particular, the cbolinesterases and monoamine oxidases have been extensively studied as target sites for drugs and much of this recent work has been reviewed by Zeller and Fouts
where k, and ka are the respective rate constants for adsorption and desorption of drug molecules and DR is the drug-receptor complex. The dissociation constant for the complex can be written
where [D,] and [R,] represent the concentrations of free drug and free receptors, respectively, and [DR] that of the drug-receptor complex. Using the total concentrations of drug and receptors it is possible to obtain an expression from eqn. (2) for the observed response to antagonist in terms of the maximum possible response Em:
The equation above is analogous to the expression
which relates the velocity of an enzyme reaction to the maximum possible velocity Vm, the total substrate concentration [St],and the Michaelis constant K,. Equations (3) and (4) give identical graphical representations as shown in Figures 1and 2. Equation (4) was derived from enzyme kinetics as originally introduced by
(5). Drug-Receptor Interaction
Early studies of both Clark (6) and Gaddum (7) have been used as the basis for most quantitative theories of drug action. These two investigators advanced and substantiated the hypothesis that drug-receptor interaction is described by the Langmuir adsorption isotherm (8). The main assumption involved was that a tissue response is directly dependent on the fraction of specific receptors occupied by the drug. The maximum response should thus occur when all available receptors are occupied. It was also assumed that the law of mass action describes receptor occupation, and that each drug molecule has an equal chance of access to any receptor. The interaction between a drug D and a receptor R which gives rise to a response E may be represented by
Figure 1 . Response of a drug-receptor interaction ~ l o t f ~agoinst d total drug concentration.
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Figuce 2. Velocity of an enzyme reaction concentration.
lotted
against total rubrtrote
Michaelis and Menten (9)and illustrates the kinetic similarity between enzyme action and drug-receptor interaction. It has also been shown that equations relating response to drug concentration in the presence of competitive and noncompetitive antagonists are analogous to the expressions describing competitive and noncompetitive enzyme inhibition, respectively (10). The mathematical relations derived from this theory are satisfactory for a qualitative description of drug dynamics but are not sufficient quantitatively. For this reason several persons have suggested extensions of the theory which attempt to account for the varying degrees of effectiveness of drug-receptor complexes. Ariens (11) introduced "intrinsic activity" as a measure of a drug's ability to cause an adequate stimulus when complexed with a receptor. Stephenson (1%)proposed the concept of "efficacy" which represents the capacity of a drug molecule to interact effectively with a receptor. The suggestion that a drug may produce its effect in two steps was made by del Castillo and Katz (IS). In the first step an inactive intermediate is formed by the combination of drug and receptor. This intermediate is then converted into an active form, and it is this rate of conversion that determines the final response. All three of these proposals are similar in that each attempts to explain drug response in terms of the number of receptors occupied. A diierent approach to quantitative drug action was presented by Paton (14), who suggested that response is a function of the rate of association between drug molecules and receutors. This has been termed rate 566
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theory in contrast to theolder occupation theory. Rate theory assumes that a drug molecule and receptor associate through an ionic exchange with the resulting stimulus proportional to the rate of association. Rate theory seems particularly appealing in that it can explain drug desorption, fade phenomena, and desensitization which occupation theory either could not explain or could explain only with additional assumptions. An interesting discussion of the inadequacies of both occupation and rate theories is given in a paper by Belleau (16) in which he also describes a macromolecular perturbation theory for the muscarinic cholinergic receptor. This theory is based on the conformational adaptibility of enzymes and the assumption that this receptor is in fact a protein-like macromolecule with acetylcholinesterase properties. Two types of drug-receptor interaction were considered, the first being a specific conformational perturbation of the protein involving van der Waals forces and the second a nonspecific conformational perturhation occurring through hydrophobic interactions only. The importance of this theory is increased when considered in conjunction with the theory described below. The dynamic receptor hypothesis of Bloom and Goldman (16) regards adrenergic receptors as enzyme-snbstrate complexes rather than simply enzymes. The drug-receptor interaction is thought to occur through phosphate ester hydrolysis resulting from agonist-enhanced substrate activity or perturbation of the enzyme as described by Belleau. Interaction results in receptor destruction, i.e., enzyme-substrate dissociation, but the overall process allows for immediate regeneration of active receptors provided that free substrate is available. The classical kinetic expressions for drug action were found to be inapplicable to the dynamic receptor hypothesis, and the correct kinetics had been originally derived for enzymes associated with two diierent suhstrates complexing in a given order. An idea closely related to the dynamic receptor hypothesis was described by Watkins (17) in connection with membrane permeability. The similarities between several pharmacologically active substances and the polar chains of certain phospholipids in the cell membrane were used as the basis of a molecular description. An appropriate agonist molecule associates with the protein of a phospholipid-protein complex in the cell membrane causing the original complex to dissociate as shown schematically in Figure 3. The protein is then conformationally altered which provides for the transport of the agonist into the cell where it dissociates from the protein. Thus the
protein is able t,o reassociate with the phospholipid, and the complex is again an active receptor.
(c) Information about receptors may he obtsjned from isolated degradation produots consisting of covalently hound drug molecules.
Drug Structure and Activity
If a drug-receptor interaction does result in some type of response then there must be a relation between the drug's molecular structure and the activity evoked. Unfortunately the structural formula of a drug molecule gives little indication of the compound's actual physicochemical properties. Considerably different structural formulae can represent compounds very similar physicochemically while quite similar formulas could represent diverse sets of properties. This lack of relationship h e tween structural formulas and physicochemical properties is a major obstacle in correlating structure and activity. Compounds with molecular structures that are fixed spatially may he compared more effectively than those able to undergo configurational changes. This latter type of molecule is able to exist in different conformations, some of which may fit a certain receptor and some of which may not. There has been much interest recently in the comparison of rigid and nonrigid structures which seemingly evoke identical responses. Waser (18) has tabulated results of the pharmacological action of a large number of various isomers, homologs, and derivatives of muscarine and muscarone on the cat and frog and on isolated tissues. These results were then discussed in terms of such parameters as stereoisomerism, presence or absence of certain functional groups, chain length, and aromaticity. In addition, Schueler (19) attempted a mathematical description of the activity of several acetylcholine derivatives on blood pressure by means of a statistical evaluation of possible influenceson spatial conformations. The case of biologically active stereoisomers poses an interesting and complex problem. The fact that two stereoisomers can differ markedly in pharmacological activity provides some of the best available evidence for the existence of receptors (to). However, two stereoisomers cannot differ greatly in their physicochemical properties because of their structural similarity, implying that the difference in their activities lies in the steric characteristics of the receptor. Receptors
The simple lock-and-key analogy is too static to serve as an acceptable model for drug-receptor interaction. The interaction is known to he a dynamic one since the receptor must undergo charge distribution and chemical changes in order that a stimulus and subsequent response may be realized. Thus a complete interpretation of the interaction requires knowledge of the charge distributions, spatial conformations, and steric factors relating to both drug and receptor molecules. Because of their inherent nature, receptors must generally be studied in situ which obviously presents many difficulties in their elucidation. Some of the general methods which can he applied to receptor studies ($1) are as follows: (a) The differences in chemical structures and activities of two drugs competing for the same receptor will give an indication of the complementary pasts of the receptor. (h) Some functional groups of a receptor may vary with the acidity or basicity of the surrounding medium so that pH-dependent drug action would yield information about these groups.
complex with a smaller number of different receptors.
There have been several recent attempts to isolate rec e ~ t o r s(9$-26) in order to studv their interactions with &gs in'vitro. ' I t should be noted that changes in the receptor during its interaction are transmitted to surrounding molecules which provide the stimulus for any subsequent biological response. This response is the result of a complicated interplay between the activated receptor and its immediate environment. The isolated receptor would he altered from its natural spatial arrangement and charge distribution, both of which are essential to interaction. In addition, the isolated receptor is no longer in a suitable biological environment which could give rise to any type of observable response even if interaction and its resulting stimulus were possible. Acknowledgment
I would like to thank Dr. Peter W. Ramwell of the Worcester Foundation for Experimental Biology, Shrewshury, Mass. for his many helpful discussions both before and during the preparation of this paper. Literature Cited (1) CLARK,A. J., "The Mode of Action of Drugs on Cells," Willirtms & Wilkins Co., Baltimore, 1933, chap. 11. (2) TAYLOR,H. S., PTOC.Roy. SOC. (London), AI08, 105 (1925). J. N., J. Physiol. (London), 33,374 (1905). (3) LANGLEY, (4) ALBERT,A,, "Sele~tiveToxicity," 3rd Ed., Methuen & Co., Ltd., London, 1965, p. 45. (5) ZELLER,E. A,, FOUTS,J. R., Ann. Rev. P h m c o l . , 3, 9 (1963). (6) CLARK,A. J., in "Handbuch der experimentellen Pharma, AND HEUBNER,H.), kologie" (Editom: H E F ~ E RA,, Springer, Berlin, 1937, Vol. IV. (7) GADDUM,J. H., Proe. Roy. Soe. (London), B121, 598 (1937). (1913). (10) HOLLAND, W. C., KLEIN,R. L., AND BRIGGB,A. A,,"Introduction to Molecular Pharmacology," The Macmillan Co., New York, 1964, p. 189. (11) ARIENB,E. J., Awh. Intern. P h r m a o d p . , 99, 32 (1954). R. P., Brit. J . Phrmacol., 11,379 (19.56). (12) STEPHENSON, (13) CASTILLO, J . D E 4 AND &TZ, B., PTOC.Roy. Soc. (London), B146,369 (1957). (14) PATON,W. D. M., PTOC.R w Sac. (London), B154, 21 (1961). (15) BELLEAU, B., J. Med. Chem., 7,776 (1964). I. M., Advan. Drug Res., 3, (16) BLOOM,B. M., AND GOLDMAN, 121 (1966). (17) WATKINS,J. C., J. Thewd. B i d , 9.37 (1965). (18) WASER,P. G., Phmmacol. Rev., 13,465 (1961). (19) SCRUELER, F. W., Arch. Intern. P h n n a c o d y . , 93, 417 ilQ.52) > - - - - ,. (20) BECKETP, A. H., Fortschr. 'Arzneimiltelforsch., 1, 455 t1UKO> ,A""",. (21) ARIENS,E. J., SIMONIS,A. M., *ND ROSSUM, J. M. VAN, in "Molecular Pharmacology" (Editw: ARIENS,E. J.), Academic Press, New York, 1964, Vol. I, p. 246. E., K., AND GARCIA, (22) CHAGAS, C., P E N N A - ~ A N C A NIsAIE, , E. J., Arch. B i o e h . Biophys., 75,251 (1958). S., in "Proceedings of the Firat International (23) EARENPREIS, Pharmaeologicd Meeting" (Editor: BRUNINGS,K. J.), The Macmillan Co., New York, 1963, Vol. 7, p. 119.
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(24) N A C H M A N ~ ~D., H Nin , "Proceedings of the First International Pharmacological Meeting," (Eddor: BRUNINQS, K. J.), The Macrnillan Co., New York, 1963, Vol. 7, p. 134.
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(25) TURPAJE:~, T. M., in "Proceedings of the First International Pharmacological Meeting," (Editor: BRUNINGS, K. J.), The Macmillan Co., New York, 1963, Vol. 7, p. 145.
