PHARMACODYNAMICS - MECHANISMS OF DRUG ACTION

GENERAL CONCEPTS

Pharmacodynamics: This is the study of drug effects (biochemical and physiological effects) and their mechanisms of actions. It attempts to elucidate the complete action – effect sequence and the dose – effect relationship. Modification of the effects of the one drug by another drug and by other factors is also a part of pharmacodynamics.

Action of drug: It is the process by which the chemical agents induce a change in pre-exisiting physiological functions or biochemical processes of the living organisms.

Effect of the drug: The series of events occurring after drug actions called as effects of drugs.

Bio-phase / Site of action: It is the location in the body or outside where the produces its effect.

PRINCIPLES OF DRUG ACTION

Drugs do not produce different actions in the body; rather, they protect or modify existing general or specific cellular functions, which usually return to their predosage states when the drug is removed. A thorough understanding of the biochemical and physiological effects induced by drugs is essential for their rational use.

Therapeutic agents may be applied either topically (externally) or used systemically (internally). They may then exert either a local or a general effect. The tissue responses produced may be immediate or delayed and can be due to the unchanged drug itself or to one or more of its metabolites. In addition, the desired therapeutic effect may result from a direct action on a target organ, or from a response elicited in another part of the body with indirect beneficial conse­quences for the affected organ.

A drug's effects may lead to an increase in cellular or tissue responsiveness (Stimulation) or may limit or depress cellular functions (Inhibition). A biphasic reaction is possible in which initial stimulation is followed by depression. This type of response is, quite commonly observed when CNS depressants are used. Some drugs produce a nonselective, often noxious effect and is particularly applied to less specialized cells viz., epithelium, connective tissue (irritation). Mild irritation may stimulate associated function e.g. bitters increase salivary secretion and gastric secretion; counter irritants increase blood flow to the site. But strong irritation results in inflammation, corrosion, necrosis and morphological damage. This may result in diminution or loss of function. The therapeutic effect sought from some agents is the actual destruction of cells (cytotoxicity) - either of the host (e.g., corrosives, blisters, antineoplastic agents) or of invading pathogens (e.g., antimicrobial agents, anthelmintics, external parasiti­cides). Certain pharmaceutical preparations may contain hormones, enzymes, vitamins, minerals, or electrolytes that are needed to correct deficiency states brought on by the lack of such endogenous substances. Such replacement ther­apy may also be appropriate for disease conditions in which there is an intrinsic lack of a neurotransmitter (e.g., the use of DOPA as a dopamine precursor to treat Parkinsonism in man).

The drugs employed in veterinary medicine fall into 2 broad general catego­ries viz., pharmacodynamic and chemotherapeutic.

Pharmacodynamic agents are those drugs that exert their actions directly on the host in one way or another to bring about the desired beneficial effects.

Chemotherapeutic agents produce toxic effects in pathogenic organisms or neoplastic cells without producing un­due harm to the host. However, many chemotherapeutic drugs also possess pharmacodynamic properties, particularly at higher dosage levels.

BASIC MECHANISMS OF DRUG ACTION

The sites of action vary considerably for the various classes of drugs used therapeutically. Many produce effects in the extracellular fluids as well as on cutaneous and mucosal surfaces without any direct involvement with the cellu­lar constituents of the body. Some drugs affect cellular functions through inter­actions with constituents of cell membranes, thereby altering the membrane's permeability characteristics and functions. Finally, drugs may also act intracellularly on very specific macromolecular structures bringing about their typical effects. Membrane and intracellular sites of drug action (receptors) are mainly composed of protein subunits arranged in a highly stereospecific fashion.

Mechanisms of Drug action

 

Non-cellular mechanisms of drug action                                  Cellular mechanisms of drug action

1. Physical effects                                1. Physico-chemical and bio-physical mechanisms

2. Chemical reactions                2. Modification of cell membrane structure and function

3. Physico-chemical mechanisms           3.Mechanisms associated with neurohumoral transmission

4. Modification of composition of          4. Mechanisms associated with enzyme inhibition

body fluids.                                          5. Regulatory molecule activation / inhibition through receptor mediated effects

Noncellular Mechanisms of Drug Action: Drug reactions that occur extracellularly and that involve noncellular constituents include the following:

Physical Effects: A physical property of the drug is responsible for its action, e.g.:

            Mass of the drug           - bulk laxatives (bran), protectives (dimethicone)

            Adsorptive property      - Charcoal, kaolin

            Osmotic activity            - mag. Sulphate, mannitol

            Radioactivity                 - Radioisotopes ¹³¹ I

Radioopacity                - Barium sulfate, urografin.

Chemical Reactions: A number of drugs produce their effects through a chemical union with an endogenous or foreign substance. Examples include the inactivation of heparin (an organic acid) by  protamine (an organic base); the chelation of lead by calcium disodium edentate; neutralization of hvdrochloric acid in the stomach by antacids such as aluminum hydroxide or sodium bicarbonate; treatment of alkali poisoning with weak acids; the conversion of hemoglobin to methaemoglobin by nitrites for the treatment of cyanide poisoning; the binding of bile salts in the intestinal tract by cholestyramine and colestipol; the precipitation of protein by astringents on the skin surface; and oxidation reactions initiated by certain antiseptics and disinfectants. 

Physicochemical Mechanisms: Certain drugs act by altering the physicochemical or biophysical properties of specific fluids or even components of cells. Examples of compounds that bring about physicochcmical changes in fluids include the surface-active agents or surfactants. Surfactants reduce the interfacial tension between 2 immiscible phases because their mol­ecules contain 2 localized regions, 1 being hydrophilic and the other hydrophobic in nature. Detergents or emulsifiers, antifoaming agents, and several antiseptics and disinfectants possess surfactant properties.

Modification of the Composition of Body Fluids: Several therapeutic ma­nipulations involve the administration of substances that, because of their in­trinsic physicochemical properties, exert osmotic effects across particular cellular membranes. Examples of osmotically active agents used in this manner include magnesium sulfate as an osmotic purgative, mannitol as an osmotic diuretic, hypertonic poultice applied on the skin, and the use of dextran 40 and dextran 70 as plasma-volume expanders. In addition, acid-base and electrolyte derangements, which occur in the extracellular fluid in many disease syn­dromes, may be corrected by the appropriate and judicious use of various electrolyte solutions. Also, acidifying or alkalinizing salts may be administered to alter the pH of the urine for specific therapeutic purposes.

Cellular Mechanisms of Drug Action: Most of the responses elicited by drugs take place at the cellular level and involve either functional constituents or more commonly, specific biochemical reactions.

Physicochemical and Biophysical Mechanisms: As noted above, certain drugs appear to act by altering the physicochemical or biophysical characteris­tics of specific components of cells. Examples include the effect of general inhalant anesthetics on the lipid matrix, and perhaps the hydrophobic proteins in neuronal membranes within the CNS. The purported ability of some of these same anesthetic agents to induce the formation of clathrates (molecules with erected crystal-like structures) within neurons would represent another exam­ple of a biophysical mechanism of drug action.

Modification of Cell Membrane Structure and Function: A variety of drugs may influence either the structure or specific functional components of cell membranes and thereby initiate their characteristic effects. These mechanisms of action may also involve enzyme systems or receptor-mediated reactions. A few examples will serve to illustrate how cell membranes can repre­sent a site of action. Local anesthetics bind to components of the sodium chan­nels in excitable membranes and prevent depolarization. Calcium channel blockers, e.g. verapamil, nifedipine, diltiazem, inhibit the entry of calcium into cells or its mobilization from intracellular stores. Insulin facilitates the trans­port of glucose into cells. Neurotransmitters characteristically increase or decrease sodium ion permeability of the excitable membranes that are in opposition to their sites of release and thereby either stimulate or inhibit neurotransmission, respectively. Polyene antifungal antibiotics disrupt the sterol component of fungal cell membranes and lead to loss of integrity and fatal cell leakage.

Mechanisms associated with Neurohumaral transmission: A number of drugs interfere in one way or another either with the synthesis, release, effects, or re-uptake of neurotransmitters. Once again, enzyme – and / or receptor-mediated effects may be responsible for the response that occurs. For example, reserpine blocks the transport system of adrenergic storage granules, while amphetamine displaces norepinephrine from axonal terminals. Botulinum toxin prevents the release of acetylcholine from cholinergic terminals, and bretylium inhibits the release of norepinephrine from adrenergic terminals. A large number of drugs can also selectively stimulate or block neurotransmitter receptor sites.

Enzyme Inhibition: Certain drugs exert their effects by inhibiting the ac­tivity of specific enzyme systems either in the host animal or in invading pathogens. This inhibition may be of a competitive (with normal substrate) or noncompetiitive nature. Noncompetitive enzyme inhibition may be reversible or irreversible. Allosteric inhibition is also possible when the drug influences enzy­matic function by interacting with a part of the enzyme remote from its usual active site. There are a large number of examples of drug action mediated through the inhibition of various enzyme systems. Only a selection of enzymes and drugs that inhibit them are noted here - just to illustrate the diversity of this type of drug action:

Acetylcholinesterase                                                      - Neostigmine(reversible)

                                                                                      Organophoaphateinhibitors (irreversible)

Membrane Na+, K+ - ATPase                             - Digitalis glycosides

Phosphodiesterase                                                       - Methylxanthines

Carbonic anhydrase                                                       - Acetazolamide

Cyclo-oxgenase                                                            - NSAIDs

Converting enzyme                                                        - Captopril

Xanthine oxidase                                                           - Allopurinol  

Vitamin K sensitive enzymes for clotting factor synthesis - Warfarin and other coumarins

Plsminogen-activating enzymes                                     - Aminocaproic acid

11 β- hydroxylase in corticosteroid biosynthesis - Metyrapone

Thymidine kinase (Viral)                                     - Acyclovir

Transpeptidase (bacterial)                                              - Beta lactam antibiotics

Dihydrofolate synthetase (bacterial)                   - Sulphonamides

DNA dependent RNA polymerase (bacterial)       - Rifampin

Thymidylate synthetase (fungal)                         - Flucytosine metabolite

Dihydrofolate reductase (protozoal)                               - Pyrimethamine

Regulatory Molecule Activation or Inhibition Through Receptor-Mediated Effects: Besides the ability of certain drugs to react with enzymes and to interfere with their function, it is also possible for drugs to interact with another set of very specific cellular proteins, commonly known as receptors.

Receptors are specific macromolecular components of the cell that regulate critical functions like enzyme activity, permeability, structural features, etc. These macromolecules or the sites on them bind and interact with the drug. Receptors are situated on the surface / inside the effector cell and specific agonists bind to them to initiate the characteristic response. Receptors may either be located on the cell membrane or may be present in the cytosol (steroid hormone receptors mainly). The normal role of receptors initially to interact with endogenous regulatory ligands (substances that bind to receptors), such as hormones, neurotransmitters, and autacoids, and thereby to propagate appropriate signals within the target cell i.e. Signal transduction. The effects may be direct or may depend on the synthesis and release of other intracellular regulatory molecules, often called second messengers. These second messengers then interact with closely associated cellular proteins (protein kinases) ultimately activating 1 or several enzyme systems, which then finally initiate the appropriate response. The arrangement constitutes a receptor-effector system. Receptors with their associated effector and coupling proteins may also act as integrators of extracellular information for the cell.

Properties of Receptors: It is extremely difficult to isolate and study receptors obtained from living cells, but much progress in characterizing these macro-molecules has been made in recent years. The use of receptor-specific, radio-labeled ligands has added a great deal to our understanding of receptors. Only a few of the general functional features of receptors are presented below.

Specificity: A receptor's specific molecular shape, form and ionic configura­tion will determine whether an agonist, or antagonist, with complementary mo­lecular size, shape and electrical charge, will bind with avidity to the site. Receptors are responsible for the selectivity of drug action.

Saturability: A finite number of receptors per cell should be present as revealed by a saturable binding curve. By adding increasing amounts of the drug, the number of drug molecules bound should form a plateau at the number of binding sites present.

Revesibility: The drug should bind to the receptor and then dissociate in its non-metabolised form. This property distinguishes receptor-drug interactions from enzyme-substrate interactions.

Number of Receptors: The number of functional receptors present in a tissue or cell is not necessarily fixed. Receptors may be induced - for example thyroid hormones increase the number of β-adrenergic receptors in cardiac muscle (Up-regulation). Certain agonists can promote a decrease in the number ("down-regulation") or coupling efficiency of receptors. The long-term use of select antago­nists may actually raise the number of receptors by preventing down-regulation. This leads to the "overshoot" phenomenon with exaggerated re­sponses following withdrawal of the drug. Steroid receptors present in the cytoplasm of target cells are both inducible and mobile.

Spare Receptors: In many cases, only a limited percentage of the receptors available need to be occupied by an agonist in order to produce a maximal response. The extras are known as spare receptors. It is possible to alter the sensitivity of tissues that possess spare receptors by changing the receptor's concentration. Agonists with low affinities are still able to produce maximal responses at low concentrations when spare receptors are present in the target tissue.

Classification of Receptors: A number of major receptor types have been iden­tified. Moreover, several subtypes also have been identified according to ligand affinity as well as by the actions produced by either agonists or antagonists. These differences between the responses elicited through various receptor subtypes are quantitatively selective but are very rarely absolute for  either agonists or antagonists. A selection of receptor types and subtypes that are important clinically is outlined below.

Receptor                      Type                                         Subtype

Cholinergic                    Muscarinic                                M₁, M₂, M₃, M₄ & M₅.

Nicotinic                                   N_n & N_m

Adrenergic                    Alpha (α)                                   α₁ & α₂

Beta (β)                                    β₁, β₂ & β₃

Dopaminergic               DA₁ & DA₂

Serotonergic                 5-HT₁, 5-HT₂

Histaminergic                H₁ H₂ & H₃

GABAergic                    GABA₁ & GABA₂

Adenosine                    A₁ & A₂

Opioid                          mu - μ

kappa - κ

delta - δ

sigma - σ

epsilon - ε

Steroid                         Progesterone

Estrogen

Testosterone

Glucocorticoid

Minentlocorticoid

Peptide hormones         Many

The basic mechanisms through which ligand-receptor interactions may initiate cellular responses are as follows:

1) Through Ligand gated ion channels: The receptor proteins themselves may be part of an ion channel across the plasma membrane. The ion channels open or close in response to a drug-receptor interaction and thereby change the cell's membrane potential and possibly its ionic equilibrium. Examples include the receptors for several neurotransmitters such as acetylcholine, gamma-aminobutyric acid, and glycine.

2) Through second messengers: Membrane proteins may act as receptors for agents that either stimulate or inhibit the closely associated membrane enzyme, adenylate cyclase. This leads to either an increase or a decrease in the intracellular concentration of the cyclic nucleotide, cyclic AMP (cAMP). The regulation of adenylate cyclase ac­tivity depends on distinct guanosine triphosphate (GTP)-binding regulatory proteins (G-proteins). There are 4 major types of G proteins:

i. Gs – which couples the stimulatory receptors to adenylate cyclase to increase its activity (e.g.) adrenoceptors(α1), histamine H1 receptors, and Dopamine D1 receptors. 

ii. Gi – which couples the inhibitory receptors to adenylate cyclase to decrease its activity (e.g.) adrenoceptors (α2), M2 muscarinic recptors anddopaminergic D2 receptors.

iii. Go – which is thought tocouple receptors to ion channels and

iv. Gq – which couples receptors to the activation of phospholipase for production of IP3 (Inositol tri phosphate) and DAG (Diaceyl glycerol).

A. Cyclic AMP acts in the cell to stimulate cAMP-dependent protein kinases, which catalyze in turn the activation of numerous intracellular enzymes, thus producing the specific effects associated with the endogenous ligand or; exogenous drug. A number of drug responses are mediated through adenylate cyclase mechanisms.

B. Stimulation of certain membrane receptors leads to the formation of inositol-triphosphate (Inos-P₃) and diacylglycerol (DAG) from membrane phospholipid. Inositol triphosphate in turn leads to the release of calcium from intracellular stores which, in conjunction with diacylglycerol, activates a dis­tinct protein kinase with resultant increased specific enzyme activity and re­sponses.

Several membrane-bound proteins are themselves protein kinases that can be activated by specific agents (Insulin, Epidermal growth factor, platlet derived growth factor & other tropic hormones) to produce their effects.

Agonist + Receptor à GDP displacement from G-protein & GTP replacement à GTP-G-protein complex à Regulates the activity of enzymes  / ion channels to produce a response.

C. Oxidative events in some cells, potentially produced by a variety of path­ways, lead to the activation of guanylate cyclase with consequent elevation of the intracellular concentration of cyclic GMP (cGMP). This cyclic nucleotide is an activator of yet another set of protein kinases. Several endogenous and exogenous substances can modulate cellular levels of cGMP.

D. Through regulation of Calcium entry: The concentration of calcium ion plays a major role in the regulation of many intracellular events. Calcium entry into cells through the plasma mem­brane is controlled by a distinct set of receptors. In some cases it is the calcium ion itself that regulates reactions directly, and in other instances calcium plays a role only when it is bound to an intracellular calcium-dependent regulatory protein known as calmodulin (CaM). Calmodulin also controls several cell func­tions directly and others through distinct protein kinase systems. A number of drug reactions have been shown to be mediated through calcium-dependent mechanisms.

Perturbation of the plasma membrane or the release of calcium ions may bring about the activation of membrane-associated phospholipases. This leads to the genesis of prostaglandins, leukotrienes, and related eicosanoids, which then play a major role in local cellular regulation. Several drugs may initiate the prostaglandin cascade in their target tissues - either as a primary effect or be­cause of cellular disruption.

            3. Through Intracellular receptors: These recptors are located in the cytoplasm. These are activated by a group of hormones – Steroid hormones, T3 & T4 and Vit-D and their synthetic congeners that are highlylipid soluble and thus are able to cross the cellular plasma membrane. The steroid ligands bind with a measurable affinity to the cytoplasmic receptor protein and form stroid – receptor complex. Following some modi­fication, the steroid-receptor complex is converted to a form that is translocated to the nucleus, where binding of the steroid-protein complex to chromatin occurs. As a result, specific mRNA and proteins (enzymes) are synthesized, which then lead to the characteristic steroid-hormone-induced effects for the particu­lar tissue. All steroid hormones used as drugs exert their primary effects through this mechanism. Here the response time may range from minutes to hours because new proteins must be synthesized.  Effects may persist for hours to days.

Drugs are capable of either stimulating or inhibiting receptor-mediated ef­fects provided their molecular structure is analogous, at least in part, to the physiological ligands that usually interact with specific receptors. This is the basis of modern receptor theory.

DRUG RECEPTORS AND PHARMACODYNAMICS

Most drugs are effective in extremely low concentrations and generally elicit very predictable responses that are dependent upon the concentration of the drug at the receptor sites. These reactions are usually mediated through the interaction of the drug with specific macromolecules (receptors) in the target cell activating or inhibiting 1 or more of the various mechanisms discussed above. Only the basic tenets of modem receptor theory, as they apply to clinical pharmacology, are briefly reviewed here to provide a foundation for the under­standing of drug action and reactions.

Nature of Drug Receptor Interactions: The selective action of a drug depends on its combination with a specific set of receptors. The receptor sites are almost invariably composed of proteins. They may be regulatory proteins, enzymes, transport proteins, and even structural proteins in rare cases. Drug-receptor interactions are usually reversible and are governed by the Law of Mass Action schematically represented as follows:

Receptor + Drug  ---------à Drug-Receptor complex -------à effect

The binding of drugs to receptors may involve all types of molecular interac­tions - Van der Waals forces, hydrophobic interactions, and hydrogen, dipole, ionic, and covalent bonds.

Covalent bond – High energy; cannot be broken; drug-receptor interaction is considered to be irreversible. Recovery from drug exposure usually depends on the body producing new receptors.

Electrostatic bond – Medium energy; occur between opposite charges; can act over a great distance; reversible; may be involved in attracting the drug to the receptor and lining it up properly prior to actual docking.

Hydrogen bond - Medium energy; occur when two molecules share a hydrogen atom. Usually between aminoacids with the H+ being shared between nitrogen (N) residue on one and an oxygen (O) residue on the other.

Van der Waals bond – When drug and receptor are in close proximity a dipole is induced in the non-polar regions that are closest. Each induced dipole has + and – charges that are attracted to the opposite charges in the other molecule’s dipole. Very weak, but may be many dipoles that interact, increasing drug-receptor specificity greatly.

Hydrophobic bond – Weak bonds between highly fat soluble (hydrophobic) regions of drug and receptor; these weak bonds are usually involved in very selective drug-receptor binding.

Bond Energy                                                    Bond Type

           

High                             N=C                              Covalent

                                                N~O-C                          Ionic

                                                N-H…O=C                     Hydrogen

                                                                                    Hydrophobic

            Low                              C-H                   H-N       Van der walls

Besides receptor proteins, drugs may also become bound in a similar fash­ion to other proteins. However, in these cases, no pharmacodynamic response is initiated ("silent receptors"). Examples of such drug acceptors include plasmaproteins, intracellular proteins, and membrane protein fractions. These macromolecules represent sites of drug loss or storage.

Drugs that are capable of reacting with specific receptors and which then produce a defined response are said to possess affinity as well as intrinsic ac­tivity and are termed "agonists". On the other hand, certain drugs are capable of combining with the same receptor complex, thus possessing affinity, but they lack intrinsic activity and no response occurs. These agents are termed pharma­cological "antagonists". Antagonists may act in several different ways. It is also possible for some drugs to interact with the same receptors as a full agonist but the response may be limited and less than maximal. These agents then possess affinity for the receptor but only intermediate activity and are termed "partial agonists". Finally, there are some special drugs that at certain concentrations act as agonists on 1 type of receptor population but as antagonists on other subsets of the receptor. These agents are known as "agonist-antagonists". Some of the drugs produce exactly opposite effect as seen with pure agonists, by acting on the same receptors and are called as “Inverse agonists”.

Properties of Agonists: Several typical properties of receptor agonists require further definition. The affinity of a ligand (drug or endogenous substance) for a receptor is a measure of its capacity to bind to the receptor. Affinity may vary greatly between agonists (as well as antagonists).

Intrinsic activity is a measure of the ability of the agonist-receptor complex to initiate the observed biological response. A full agonist has an intrinsic ac­tivity value (α) of 1.

Maximal efficacy reflects the upper limit of the dose-response relationship without toxic effects being evident. Agonists differ from each other in this re­gard.

Potency refers to the range of concentrations over which an agonist pro­duces increasing responses. Highly potent drugs produce their effects at lower concentrations and this may impart an advantage to their clinical use, provided the increase in potency is not accompanied by an increase in toxicity.

Selectivity and specificity: Few agonists are so specific that they interact only with a single subtype of receptor. However, several agonists (and antago­nists) do show evidence of selectivity for certain subpopulations of receptors.

Structure-activity relationships (SAR): The affinity of an agonist (or antago­nist) for a receptor as well as the agonist's intrinsic activity are intimately related to the chemical structure of the drug. The relationship is usually quite stringent and minor modifications in the drug molecule can result in significant differences in its pharmacological properties. Often, congeners of a parent drug molecule are developed for therapeutic use because of advantages in their therapeutic effects or reductions in the incidence of toxicity.

Drug-receptor theories: Whatever its actual nature, the conformational change that occurs when a receptor is occupied by an agonist is only 1 of several steps necessary for the expression of a full pharmacological response. The transduction process between occupancy of receptors and the drug re­sponse is called "coupling". Receptor-effector coupling is influenced by the ionic environment, several coupling factors, and the receptor itself. A number of theories have been advanced to explain how agonist-receptor interactions lead to effective receptor-effector coupling and the specific pharmacological response observed.

1. Occupancy theory: This is proposed by Clark (1937) after he studied the quantitative aspects of drug action. This theory is based on the Law of Mass action i.e. the drug action based on occupation of receptors by specific drugs and that the pace of a cellular function can be altered by interaction of these receptors with drugs. This theory postulated

i. The intensity of response is directly proportional to the fraction of acceptors occupied by a drug and maximal response occurs when all receptors are occupied.

ii. Drugs exert an "all or none" action on each receptor, i.e. either a receptor is fully activated or not at all there is no partial activation and

iii. A drug and its receptor have complementary structural features and stand in rigid "Lock and kev" relationship.

This theory gave the fundamental concept but the postulates were later found to be only partially correct and need to be modified.

2. Rate theory: This is introduced by W.D.M.Paton at 1961. This suggested that agonist action depended on the rate of agonist - receptor association and / or dissociation and this in turn decides the magnitude of drug effects. Paton suggested that the rate of receptor occupation rises sharply to start with, reaches a peak and  then there is a fall to steady state or equilibrium or the phenomenon of fade. This theory explains many aspects of the time-course of drug ejects.

3. Induced fit theory: This proposes that in the process of agonist - receptor interaction, a conformational change occurs that generates the active receptor site.

4. Perturbation theory: This differentiates between specific conformationaI chages indued by agonists Vs nonspecific perturbations produced by antagonists.

5. Activation - Aggregation theory: This proposes that receptors exist in dynamic equilibrium between different functional states and that agonists shift the equilibrium towards the activated form the receptor.

Properties of Antagonists: Many receptor antagonists are used therapeutically. However, a number of different forms of drug antagonism occur that need to be understood because of direct clinical implications.

Competitive antagonists combine reversibly with the same receptor site as agonists and progressively inhibit the response to agonists. Competitive antago­nists may even possess greater affinity for the receptor site than pure agonists. Typically, the blockade can be overcome by increasing the concentration of the agonist in the biophase.

Noncompetitive antagonists may act reversibly, or, more commonly, irre­versibly. Characteristically, high concentrations of agonist cannot completely overcome the antagonism and a maximal response cannot be produced. Several types of noncompetitive antagonism are recognized. The binding may oc­cur to the same receptor producing blockade. If covalent bonds are formed, receptor inhibition becomes permanent and the de novo synthesis of receptors is required for complete reversal of the antagonist's effects. This form of irre­versible noncompetitive antagonism is encountered with some organophosphates and cholinergic receptors. Binding of noncompetitive antagonists to a different part of a receptor macromolecule may lead to deformation of the active receptor site leading to a diminution in the affinity for the usual agonists. This effect is similar to allosteric inhibition of enzymes. Antagonists may also bind to an extrareceptor area in a membrane but, because of their molecular configuration, may obscure the receptor sites for 1 or more agonists. The phenothiazine neuroleptics appear to be multipotent receptor blockers of this type.

Physiological or functional antagonists are, in fact, agonists that elicit physiological responses that directly oppose those of the first drug adminis­tered, by stimulating a different class of receptors. Examples of physiological antagonism include the reversal of histamine's effects (histaminergic receptors) by epinephrine (adrenergic receptors) and the stimulation of intestinal smooth muscle by acetylcholine (cholinergic receptors) following its inhibition by nor-epinephrine (adrenergic receptors).

Properties of Partial Agonists and Agonist-Antagonists: Partial agonists pro­duce a lower than maximal response at full receptor occupancy notwithstand­ing high receptor affinities in many cases. They will also act as competitive antagonists in the presence of pure agonists. Nalorphine and pentazocine are partial agonists used in veterinary medicine. Agonist-antagonists that are used clinically include the opioid analgesics that possess selective affinity and intrin­sic activity for certain of the opioid receptor types but act as competitive an­tagonists at others, thus reducing some of the undesirable features of full opioid agonists. Butorphanol and nalbuphine are regarded as opioid agonist-antagonists.

Dose-Response Relationships

The response to a drug varies according to its dosage i.e. the magnitude of the drug effect is a function of the dose administered. The relation between the responses produced by different doses is expressed by graphical representation called dose response curves. Two forms of dose-response curves are recognized: 1.Graded response & 2.Quantal response

1. Graded dose response: It is the one in which the target tissue or even patient shows a progressively greater response with increasing doses of drug, ultimately reaching the maximum response. A plotted graph of a graded response is hyperbola on a linear scale and a sigmoid curve on semi-logarithmic scale. Graded dose response curve gives the relation between dose of the drug and intensity of the response in a single biological unit.

This curve depicts the

1) Threshold dose - The minimum dose at which observable changes arc noticed.

2) Ceiling dose - The minimum dose producing the maximal response and any further increase in the dose above the ceiling dose will not increase the level of response.

3) Potency – this refers to the dose (concentration) of a drug needed to produce an effect smaller the dose to produce the effect, the greater the potency. Potency is not an important property of a drug, provided the dosage form of the drug can be conveniently administered. If two drugs have similar pharmacological activities, the more potent drug is not necessarily the drug of choice. Consideration must also be given to other factors such as side effects, toxicities, cost and duration of action.

4) Slope - It is of both practical and theoretical importance. Slope represents the change in response to the change in dose of drugs. Drugs that have steep DRCs are potentially more difficult to use because small increase in the dose may produce toxity. Drugs that act on common receptor have DRCs with parallel slopes.

5) Variability - The Variability in the response can be related to the variation in dosage.

6) Maximal effect - is the maximum response possible for the effector.

Graded dose response curve is considered as a plot of efficacy Vs drug concentration. By plotting this we can calculate:

EC50: The drug concentration at which 50% efficacy is attained. The lower the EC50, the more potent the drug.

Emax: The maximum attained biological response out of the drug.

2. Quantal dose response: This is also known as All or none response. In this, the target tissue or patient, reacts either maximally or to a specific end point with in the therapeutic dose range. This represents the % response of animals in a group of population to the doses of the drug. The quantal D-R curve relates dose to an expression of the frequency with which any dose of the drug produces an all or none pharmacologic effect.

Each animal receiving a dosage is categorized as responding / non-responding.  The % responding to each dose are recorded (i.e. % alive, % dead, % responded or % not responded). These quantal responses (%) when plotted against log doses they do not show a linear regression. However when % is transformed into probits, the relationship becomes linear. This type of curve is used for estimating ED50/LD50 values of a drug.

Quantal DRC is a graph of discrete (yes/no) values, plotting the number of subjects attaining the condition (such as death or cure from disease)  vs drug conccntration.

ED50 - The drug dosage at which 50% of the population attains the desired characteristic.

LD50 - The drug dosage at which 50% of the population is killed by a drug.

Therapeutic index: It is the ratio used to evaluate the safety of the drug. TI = LD50 / ED50. Larger the TI, the safer the drug. i.e. the higher dose is required for lethality, compared to the dose required to be effective. However, if the LD50 and ED50 curves are not parallel, the TI may be misleading.

Margin of safety: This is the ratio of dosage required to kill 1% population, compared to the dosage i.e. effective in 99% of population. Margin of safety = LD1/ED99.

Standard safety margin: It is the per cent by which the ED99 must be increased before an LD1 is reached.

SSM = [(LD1- ED99) / ED99] x 100

Therapeutic ratios are also useful indices of drug safety but in this case the steepness of the dose response curves are taken into consideration. Therapeutic ratios are often calculated by dividing the LD25 by the ED75.