PHARMACODYNAMICS - MECHANISMS OF DRUG ACTION

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          

1. Physical effects                                              

2. Chemical reactions                                          

3. Physico-chemical mechanisms                         

4. Modification of composition of body fluids.                                        

 
Cellular mechanisms of drug action 
1. Physico-chemical and bio-physical mechanisms 
2. Modification of cell membrane structure and function   
3.Mechanisms associated with neurohumoral transmission 
4. Mechanisms associated with enzyme inhibition
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.