Occupancy Theory

The occupancy theory of Gaddum[49] and Clark[50] states

that the intensity of the pharmacological effect is directly

proportional to the number of receptors occupied by the

drug. The response ceases when the drug–receptor complex

dissociates. However, as discussed in Section 3.2.3, not

all agonists produce a maximal response. Therefore, this

theory does not rationalize partial agonists, and it does not

explain inverse agonists.

Ariëns[51] and Stephenson[52] modified the occupancy

theory to account for partial agonists, a term coined by Stephenson.

These authors utilized the original Langley concept

of a receptor, namely, that drug–receptor interactions involve two stages: first, there is a complexation
of the

drug with the receptor, which they both termed the affinity;

second, there is an initiation of a biological effect, which

Ariëns termed the intrinsic activity and Stephenson called

the efficacy. Affinity, then, is a measure of the capacity of

a drug to bind to the receptor, and depends on the molecular

complementarity of the drug and the receptor. Intrinsic

activity (α) now refers to the maximum response induced

by a compound relative to that of a given reference compound,

and efficacy is the property of a compound that

produces the maximum response or the ability of the drug–

receptor complex to initiate a response.[53] Because of the

slight change in definitions, we will use the term efficacy

response. In the original theory, this latter property was

considered to be constant. Examples of affinity and efficacy

are given in Figure 3.19. Figure 3.19A shows the theoretical

dose–response curves for five drugs with the same affinity

for the receptor (pKd=8), but having efficacies varying from

100% of the maximum to 20% of the maximum. The drug

with 100% efficacy is a full agonist; the others are partial

agonists. Figure 3.19B shows dose–response curves for four

drugs with the same efficacy (all full agonists), but having

different affinities varying from a pKd of 9 to 6.

Antagonists can bind tightly to a receptor (great affinity),

but be devoid of activity (no efficacy). Potent agonists

may have less affinity for their receptors than partial agonists or antagonists. Therefore, these two
properties,

affinity and efficacy, are uncoupled. Also, the terms agonist,

partial agonist, antagonist, and inverse agonist are biological

system dependent and not necessarily properties of

drugs. A compound that is an agonist for one receptor may

be an antagonist or inverse agonist for another receptor.

A particular receptor is considered to have an intrinsic maximum

response; this is the largest magnitude of response that

the receptor is capable of producing by any ligand. A compound

that elicits the maximum response is a full agonist;

a particular compound may be capable of exceeding the

maximum response of a tissue, but the observed response

A drug that is not capable of eliciting the maximum

response of the tissue, which depends on the structure of the

drug, is a partial agonist. A full agonist or partial agonist is

said to display positive efficacy, an antagonist displays zero

efficacy, and a full or partial inverse agonist displays negative

efficacy (depresses basal tissue response).

The modified occupancy theory accounts for the existence

of partial agonists and antagonists, but it does not account for

why two drugs that can occupy the same receptor can act

differently, i.e., one as an agonist, the other as an antagonist.

As an alternative to the occupancy theory, Paton[54] proposed

that the activation of receptors is proportional to

the total number of encounters of the drug with its receptor

per unit time. Therefore, the rate theory suggests that

the pharmacological activity is a function of the rate of

association and dissociation of the drug with the receptor

and not the number of occupied receptors. Each association

would produce a quantum of stimulus. In the case of

agonists, the rates of both association and dissociation

would be fast (the latter faster than the former). The rate of

association of an antagonist with a receptor would be fast,

but the dissociation would be slow. Partial agonists would

have intermediate drug–receptor complex dissociation

rates. At equilibrium, the occupancy and rate theories are

mathematically equivalent. As in the case of the occupancy

theory, the rate theory does not rationalize why the different

types of compounds exhibit the characteristics that they do.

3.2.4.3. Induced-Fit Theory

The induced-fit theory of Koshland[55] was originally proposed

for the action of substrates with enzymes, but it could

apply to drug–receptor interactions as well. According to this

theory, the receptor need not necessarily exist in the appropriate

conformation required to bind the drug. As the drug

approaches the receptor, a conformational change is induced,

conformational change in the receptor could be responsible

for the initiation of the biological response (movement

of residues to interact with the substrate). The receptor

(enzyme) was suggested to be elastic, and could return to its

original conformation after the drug (product) was released.

The conformational change need not occur only in the receptor

(enzyme); the drug (substrate) also could undergo deformation,

even if this resulted in strain in the drug (substrate).

According to this theory, an agonist would induce a conformational

change and elicit a response, an antagonist would

bind without a conformational change, and a partial agonist

would cause a partial conformational change. The inducedfit

theory can be adapted to the rate theory. An agonist would

induce a conformational change in the receptor, resulting in

a conformation to which the agonist binds less tightly and

from which it can dissociate more easily. If drug–receptor

complexation does not cause a conformational change in the

receptor, then the drug–receptor complex will be stable, and

an antagonist will result.

Other theories evolved from the induced-fit theory, such

as the macromolecular perturbation theory, the activation–

aggregation theory, and multistate models.

Having considered the conformational flexibility of receptors,

Belleau[56] suggested that in the interaction of a drug

with a receptor two general types of macromolecular perturbations could result: a specific
conformational perturbation

makes possible the binding of certain molecules

that produce a biological response (an agonist) and a nonspecific

conformational perturbation accommodates other

types of molecules that do not elicit a response (e.g., an

antagonist). If the drug contributes to both macromolecular

perturbations, a mixture of two complexes will result

(a partial agonist). This theory offers a physicochemical

basis for the rationalization of molecular phenomena that

involve receptors, but does not address the concept of

inverse agonism.

3.2.4.5. Activation–Aggregation Theory

An extension of the macromolecular perturbation theory

(which also is based on the induced-fit theory) is the

activation–aggregation theory of Monad, Wyman, and

Changeux[57] and Karlin.[58] According to this theory,

even in the absence of drugs, a receptor is in a state of

dynamic equilibrium between an activated form (Ro),

which is responsible for the biological response, and an

inactive form (To). Using this theory, agonists bind to the

Ro form and shift the equilibrium to the activated form,

antagonists bind to the inactive form (To), and partial agonists

binding site in the Ro conformation can be different

from the antagonist binding site in the To conformation. If

there are two different binding sites and conformations,

then this could account for the structural differences in

these classes of compounds and could rationalize why

an agonist elicits a biological response but an antagonist

does not. This theory can explain the ability of partial

agonists to possess both the agonistic and antagonistic

properties as depicted in Figure 3.16. In Figure 3.16B,

as the partial agonist interacts with the remaining unoccupied

receptors, there is an increase in the response up to

the maximal response for the partial agonist interaction.

In Figure 3.16C, the partial agonist competes with the

neurotransmitter for the receptor sites. As the partial agonist

displaces the neurotransmitter, it changes the amount

of Ro and To receptor forms (To increases and, therefore,

the response decreases) until all the receptors have the

partial agonist bound. This theory, however, also does not

address inverse agonists.[59]

Receptor Activation

The concept of a conformational change in a receptor

inducing a change in its activity has been viable for many

years.[60] The Monod–Wyman–Changeux idea described

above involves a two-state model of receptor activation, but

it does not go far enough. This model was revised based

mostly on observations with GPCRs (see Section 3.1).[61]

The revised two-state model of receptor activation proposes

that, in the absence of the natural ligand or agonist,

receptors exist in equilibrium (defined by equilibrium constant

L; Figure 3.21) between an active state (R*), which is

able to initiate a biological response, and a resting state (R),

which cannot. In the absence of a natural ligand or agonist, the equilibrium between R* and R defines
the basal activity

of the receptor. A drug can bind to one or both of these

conformational states, according to equilibrium constants

Kd and K*d for formation of the drug–receptor complex with

the resting (D·R) and active (D·R*) states, respectively. Full

agonists alter the equilibrium fully to the active state by

binding to the active state and causing maximum response;

partial agonists preferentially bind to the active state, but

not to the extent that a full agonist does, so maximum

response is not attained; full inverse agonists alter the equilibrium

fully to the resting state by binding to the resting

activity); partial inverse agonists preferentially bind to the

resting state, but not to the extent that a full inverse agonist

does; and antagonists have equal affinities for both states

(i.e., have no effect on the equilibrium or basal activity, and,

therefore, exhibit neither positive nor negative efficacy).[62]

A competitive antagonist is able to displace either an agonist

or an inverse agonist from the receptor.

Leff and coworkers further extended the two-state

receptor model to a three-state receptor model.[63] In this

model, there are two active conformations (this becomes a

multistate model by extension to more than two active conformations)

and an inactive conformation. This accommodates

experimental findings regarding variable agonist and

inverse agonist behavior (both affinities and efficacies) in

different systems containing the same receptor type (called

receptor promiscuity). According to this hypothesis, the

basis for differential agonist efficacies among different

agonists is their different affinities for the different active

states.