SOME TERMINOLOGIES
The term pharmacon means drugs; logos means studies.
Pharmacology is a branch of medicine which deals with drugs.
Drug (WHO)
Drug is a chemical substance or biological product that is used or intended to be used to modify or explore physiological systems or pathological states for the benefit of the recipient.
Drug is called medicine when used in proper dosage form for safe administration. All medicines are drugs but all drugs are not medicines.
Active Principle of Drug
Chemical constituent present in the drug which is responsible for pharmacological effect of the drug, e.g. alkaloids pilocarpine, atropine, physostigmine, quinine, etc.
Names of Drug
A drug may be named in various ways: chemical, generic or nonproprietory, proprietory, e.g. chemical name—acetaminophen or 4-acetamidophenol generic/nonproprietory name—Paracetamol, proprietory/trade name—Calpol.
Determination of Drug Routes
Routes of drug administration is determined primarily by—
- Properties of the drug, e.g. water or lipid solubility, degree of ionization, molecular weight, etc.
- Therapeutic objectives, e.g. rapidity of onset of action, duration, site of action, etc.
- Patient profile whether patient is conscious/unconscious, compliant/noncompliant, age of the patient, whether patient is vomiting/not vomiting (Flowchart 1.1 and Fig. 1.1).
BIOAVAILABILITY
It is the measure of the fraction of administered dose of a drug that reaches the systemic circulation in the unchanged form. Bioavailability (BA) is related to the rate and extent of absorption of a drug from its dosage form. Bioavailability of IV (intravenous) administered drug is 100% but it is not so, when administered orally. By plotting plasma concentrations of the drug vs time, one can measure area under the curve. This curve reflects the extent of absorption of the drug (Figs. 1.2A and B). So, bioavailability of a drug administered orally is the ratio of area under the curve calculated for oral administration compared with the area under the curve calculated for intravenous (IV) injection (Fig. 1.3).
Bioavailability of an orally administered drug can be assessed by the following (expressed as %):
Significance
- Variation in bioavailability affects drugs with narrow safety margin, toxicity may be precipitated.
- It influences the therapeutic efficacy of drugs, specially antibiotics.
- Enteric coated tablets are used to increase bioavailability of drugs destroyed by enzymes in GIT. Hence, route of administration and dosage form to be decided accordingly.
Factors that Influence Bioavailability
- First-pass hepatic metabolism—Drug absorbed from GIT enters portal circulation, before entering systemic circulation. If it is partly metabolized by the hepatic enzymes the amount of unchanged drug reaching systemic circulation is decreased, e.g. propranolol, lidocaine have high first pass hepatic metabolism.
- Solubility of drug—Hydrophilic drugs are poorly absorbed. Again extremely hydrophobic substitutes too are not absorbed because they are insoluble in aqueous body fluids.
- Chemical stability—Some drugs are destroyed in the GIT by degradative enzymes.
- pH of the drug—Some drugs are unstable at gastric pH.
- Properties of the drug and dosage form—Particle size, salt form, crystal polymorphism and presence of excipients (binders and dispersing agents) can influence rate of dissolution.
- Presence of food or other drugs—These influence the absorption of the drug.
- GI motility—It affects drug absorption hence bioavailability.
Two related drugs are bioequivalent if they show comparable bioavailability and similar times to achieve peak blood concentration. Two similar drugs which are bioequivalent may not be therapeutically equivalent.
Safety Profile of a Drug
A drug can be termed “risk free” if the precise action of the drug is known by the physician; drug was used correctly in appropriate dose and for appropriate indication; drug had biological selectivity or administered by selective targeted delivery.
Route | Bioavailability (%) | Characteristics |
---|---|---|
Intravenous (IV) | 100 | Most rapid onset |
Intramuscular (IM) | 75 ≤ 100 | Injection may be painful |
Subcutaneous (SC) | 75 ≤ 100 | Injection may be painful |
Oral | 5 <100 | Significant first-pass metabolism |
Rectal | 30 to <100 | Less first-pass metabolism than oral |
Inhalation | 5 to <100 | Rapid onset |
Transdermal | 80 to <100 | Slow onset |
The criteria of a risk-free drug is never achieved because—
- Drugs are not usually selective.
- If the action is selective, its action may be extended to other sites.
- Prolonged administration of a drug can lead to functional (receptor modification) and organic (iatrogenic disease) changes.
- Genetic variability may induce unpredictable responses.
- Need for dosage monitoring and adjustment.
- Physiological variables like age, sex, pregnancy, lactation affect disposition of a drug.
- Pathological variability, e.g. renal or hepatic disease also influences drug level.
- Ignorant and casual prescribing may be responsible for iatrogenic disease conditions.
Methods to Ensure Safety of Drug
- Target-oriented drug delivery.
- Therapeutic dose monitoring.
- Pharmacovigilance of adverse drug reactions (ADR).
- Proper information to the patient regarding the usage details.
- It is advisable not to prescribe a drug about which the prescriber is not fully conversant.
First-pass Metabolism
Most of the drugs administered orally, after absorption from the gastro-intestinal tract (GIT), enters the portal circulation first before reaching the systemic circulation. As the drug gains access to the liver via the portal circulation, it is exposed to the drug metabolizing enzymes (also in the intestinal wall) of the liver and a considerable fraction of the administered dose is metabolized. This metabolism is called first-pass metabolism.
Sites of First-pass Metabolism
(a) Intestine, (b) liver (mainly), (c) skin, (d) lungs.
The extent of first-pass metabolism differs for different drugs and is an important determinant of oral bioavailability. Consequences of first-pass metabolism—
- Oral dose required is higher than sublingual or parenteral route.
- There is marked individual variation in bioavailability, depending on the extent of metabolism.
- Oral bioavailability is increased in severe liver disease.
- Oral bioavailability is increased if another drug competing with it in first-pass metabolism is administered concurrently, e.g. chlorpromazine and propranolol.
Examples
Examples of drugs with high first-pass metabolism—isoprenaline, lignocaine, hydrocortisone, testosterone, propranolol, nitroglycerin and verapamil, etc.
Hence, these drugs with high first pass metabolism should either be given in high oral dose or preferably oral administration should be avoided. Routes which bypass first-pass metabolism are preferred.
Routes which Bypass First-pass Metabolism
- Parenteral → IV, IM, SC and routes like intravenous intramuscular, or subcutaneous.
- Sublingual.
- Transdermal.
- Topical.
DRUG ANTAGONISM
When one drug decreases or inhibits the action of the other, they are said to be antagonists. The types of antagonism may be classified according to mechanism as—(a) chemical antagonism, (b) pharmacokinetic antagonism, (c) antagonism by receptor block and (d) physiological antagonism—e.g. insulin and glucose on blood glucose level.
Chemical Antagonism
Condition where two substances combine in solution, as a result of which the effect of active drug is lost, e.g. thiopentone + succinylcholine → precipitate. Drug chemical interaction leads to drug inactivation formation.
Pharmacokinetic Antagonism
In this antagonism, antagonist effectively reduces the concentration of active drug at the site of action, either by affecting drug absorption, increasing the rate of metabolism or increasing the rate of renal excretion of the active drug.
Antagonism by Receptor Block
It is of two types; competitive and noncompetitive. In competitive receptor antagonism. Drug reception is blocked by antagonist either reversibly or irreversibly (Flowchart 1.2 and Fig. 1.4).
Competitive Antagonism
- Antagonist binds with the same receptor as the agonist.
- Antagonist resembles chemically with the agonist.
- Intensity of response depends on the concentration of both agonist and antagonist.
Reversible antagonism: The antagonist has affinity for the same receptor as the agonist but lacks efficacy.
- Parallel shift of the agonist log dose concentration-effect curve without any decrease in the maximal response.
- Rate of dissociation of the antagonist molecule is sufficiently high.
- New equilibrium is rapidly established on addition of the agonist.
Irreversible antagonism: The antagonist bears high affinity to the receptor and binds with strong covalent bonds so that it cannot be detached easily.
- Antagonism is not surmountable.
- Antagonist dissociates very slowly.
- No change in antagonist occupancy takes place when agonist is applied.
- Reactive grouping of drug binds covalently to receptor.
Noncompetitive Antagonism
- Chemical structure of the antagonist is not similar to that of the agonist and binds to an allosteric site.
- Binds to another site of receptor.
- Does not resemble the agonist.
- Flattening of agonist drug response curve (DRC).
- Maximum response is suppressed.
- Response depends on concentration of antagonist.
Physiological/Functional Antagonism
The two drugs act on different receptors or by different mechanisms but have opposite effects on the same physiological function, e.g. glucagon and insulin on blood sugar level, ACE inhibitor and thiazide diuretic on serum potassium level.
DRUG AGONISM (FLOWCHART 1.3)
- Full agonists: Agonists which can produce maximum effects and have high efficacy.
- Partial agonists: Agonists which produce submaximal effects and have intermediate efficacy.
- Inverse agonists: It shows selectivity for the receptor but produces effect opposite to that of an agonist. Hence shows affinity but negative efficacy.
Agonists on binding to receptors initiate changes in cell function, to bring about various effects. Potency of an agonist depends on two parameters—
- Affinity: Ability to bind to receptors.
- Efficacy: Ability to initiate changes that bring about effects.
According to the two state model, agonists show selectivity for the activated state of the receptor while antagonists show no selectivity.
Drugs act on various types of receptors:
- G-protein coupled receptor (GPCR)—e.g. adrenergic, muscarinic receptors (cholinergic).
- Ion-channel receptor—e.g. nicotinic (cholinergic), GABAA, 5HT3, receptors.
- Transmembrane enzyme linked receptor—e.g. insulin, epidermal growth factor, nerve growth factor receptor.
- Transmembrane JAK–STAT binding receptor e.g. growth hormone, prolactin receptor.
- Nuclear receptor—e.g. steroids hormones, vitamin D.
G-protein Coupled Receptor
The G-protein coupled receptors are also known as metabotropic receptors or seven transmembrane spanning (heptahelical) receptors. They are membrane receptors that are coupled to intracellular effector system via G-protein. They constitute the largest family and include receptors for many hormones and slow transmitters.
G-protein coupled receptor (GPCR) consists of a single polypeptide chain of up to 1100 residues; the characteristic structure comprises seven transmembrane α-helices, with extracellular N-terminal domain of varying length and an intracellular C-terminal domain.
G-protein coupled receptor (GPCR) are divided into 3 distinct families. They share the same heptahelical structure but differ in other respects, e.g. length of the N-terminus—location of the agonist binding domain.
- Family A: Related to rhodopsin, most monoamine and neuropeptide receptors. It is by far the largest.
- Family C: The smallest, its main members being the metabotropic glutamate receptors and the Ca+2 sensitizing receptors.
- Family B: Secretin/glucagon receptor family. Receptors for peptide hormones including secretin, glucagon, calcitonin.
Examples: Muscarinic cholinergic receptors, adrenoceptors, chemokine receptors, neuropeptide receptors. The first G-protein coupled receptor (GPCR) to be fully characterized was the β-adrenoceptor which was cloned in 1986. So far, G-protein coupled receptor (GPCR) cannot be obtained in crystalline form, so powerful techniques of X-ray crystallography cannot yet be used to define the molecular structure of the receptors in detail.
The long third cytoplasmic loop is the region of the molecule that couples to the G-protein. Usually a particular receptor subtype couples selectively with a particular G-protein, swapping parts of the cytoplasmic loop between different receptors alters their G-protein selectivity. For small molecules such as nor adrenaline the ligand binding domain appears to reside not on the extracellular N-terminal region but buried in the cleft between the α-helical segments within the membrane. Peptide ligands such as substance P, bind more superficially to the extracellular loops.
Though activation of G-protein coupled receptor (GPCR) is normally the consequence of agonist binding, it can occur by other mechanism.
- Rhodopsin activated by light induced cis-trans isomerization.
- Thrombin initiates a variety of cellular response by binding to a GPCR.
- β-adrenoceptor—mutations in the third intracellular loop or simply overexpression of the receptor, result in constitutive receptor activation.
Inactivation occurs by desensitization involving phosphorylation after which the receptor is internalized and degraded to be replaced by newly synthesized protein. One of the intracellular loops is larger than the others and interacts with the G-protein. The G-protein is a membrane protein compromising three subunits (α, β, γ), the α subunit possessing guanosine triphosphate (GTP) ase activity. The α subunits of G-proteins differ in structure.
Coupling of the α-subunit to an agonist occupied receptor causes the bound guanosine diphosphate (GDP) to exchange with intracellular guanosine triphosphate (GTP), The α-GTP complex then dissociates from the receptor and from the βγ subunit complex and interacts with a target protein (target 1). The βγ complex may also activate a target protein (target 2). Guanosine triphosphate (GTP) ase activity of the α–subunit is increased when the target protein is bound, leading to hydrolysis of the bound guanosine triphosphate (GTP) to guanosine diphosphate (GDP), where upon the α-subunit reunites with the βγ complex.
MUSCARINIC CHOLINERGIC RECEPTORS
Two classes of receptors for acetylcholine are recognized—muscarinic and nicotinic. The muscarinic receptors are G-protein coupled receptor (GPCR) and are selectively stimulated by muscarine and blocked by atropine (Fig. 1.5).
- Sites: These receptors are primarily present on autonomic effector cells in periphery—heart, blood vessels, eye, smooth muscles, glands of GI, respiratory and urinary tract and sweat glands.
- CNS: Preganglionic nerve fibers, ganglia—modulatory role.
- History: Muscarinic receptors were characterized initially by analysis of the responses of cells and tissues in the periphery and the CNS. Differential effects of two muscarinic antagonists, bethanechol and MeN-A 343 on the tone of esophageal sphincter led to the initial designation M1 and M2 (effector cell) by Goyal and Rattan 1978. Subsequently, radiological binding studies definitively revealed distinct populations of antagonist binding sites.
- Types: By pharmacological as well as molecular cloning techniques, muscarinic receptors have been divided into 5 subtypes—M1, M2, M3, M4 and M5. The first 3 subtypes have been functionally characterized but responses mediated by M4 and M5 subtypes are not well defined.
M1 Receptors
- Main locations—CNS: cortex, hippocampus and corpus striatum, autonomic ganglia, gastric and salivary gland and enteric nerves.
- Receptor type— G-protein coupled receptor (GPCR).
Mechanism of Action
The muscarinic receptors are G-protein coupled receptor (GPCR) having characteristic membrane domains. The M1 and M3 subtypes function through Gq protein and activate membrane bound phospholipase C (PLC); generating inositol triphosphate (IP3) and diacylglycerol (DAG) which in turn release Ca+2 intracellularly, causing depolarization, glandular secretion and raise smooth muscle tone.
Functional Response
- Increase in cognitive function (learning and memory)
- Increase in seizure activity
- Decrease in dopamine release and locomotion
- Increase in depolarization of autonomic ganglia
- Increase in secretions.
M2 Receptors
Main Locations
Widely expressed in CNS and heart, also in visceral smooth muscle and autonomic nerve terminals.
Receptor Type
G-protein Coupled Receptor (GPCR)
Mechanism of action: The M2 and M4 receptor opens k+ channels (through βγ subunits of regulatory protein Gi) and inhibits adenylyl cyclase (through α subunit of Gi) resulting in hyperpolarization, decrease in pacemaker 12activity, slowing of conduction and decreased force of contraction in heart. Increased production of cyclic guanosine monophosphate (cGMP) and release of eicosanoids can also occur in certain tissues by activation of muscarinic receptors.
Functional Response
- Heart: Sinoatrial node (SA) node → slowed spontaneous depolarization and hyperpolarization, decrease in heart rate (HR). Atrioventricular node (AV node) → decrease in conduction velocity, atrium → decrease in refractory period, decrease in contraction.
- Smooth muscles: Increase in contraction.
- Peripheral nerves: Neural inhibition and decrease in ganglionic transmission.
- Central nervous system (CNS): Neural inhibition, increase in tremor, hypothermia and analgesia.
M3 Receptors
Site of Location
Widely expressed in CNS, abundant in smooth muscles and glands, vascular endothelium, iris, and ciliary muscle.
Mechanism of Action and Receptor as M1
Functional response—gastric and salivary secretion, GI smooth muscle contraction, vasodilation, ocular accommodation, increase in body weight, fat deposits, increase in food intake, and inhibition of dopamine release.
M4 Receptors
Site of Location
Vagal nerve endings, CNS—cortex and hippocampus, striatum.
MOA and Receptor
As M2.
Action
Increase in locomotion, facilitation of dopamine release.
M5 Receptors
Site of Location
Central nervous system (CNS), substantia nigra [Mechanism of action (MOA) as M3], vascular endothelium of cerebral vessels.
Action
Dilatation of cerebral arteries and arterioles, facilitates dopamine release, augmentation of drug seeking behavior and reward.
ION CHANNEL RECEPTORS
These cell surface receptors enclose ion selective channels (for Na+, K+, Ca+2 and Cl–) within their molecules. Agonist binding opens the channel and causes depolarization/hyperpolarization/changes in cytosolic ionic composition depending on the ion that flows through.
Examples of ion channel receptors are nicotinic cholinergic receptors, GABAA receptor, glycine receptors, NMDA (n methyl D-aspartate), 5HT3 receptors.
The receptor is usually a pentameric protein. In addition to the intra- and extracellular segments, the receptor has four membrane spanning domains, in each of which, amino acids (AA) chain traverses the width of the membrane six times. The subunits are arranged round the channel like a rosette and the α-subunits usually bear the agonist binding sites.
These are fast channels and takes milliseconds to produce action. The subunits are α2, β, γ, δ each with molecular weight of 40–58 kDa. The four subunits show marked sequence homology and analysis of the hydrophobicity profile. The acetylcholine (ACh) binding site lie at the interface between one of the two α-subunits and its neighbor. Both must bind ACh molecules in order to be activated.
Nicotinic Receptor
Most excitatory neurotransmitters, such as acetylcholine (ACh) at the neuromuscular (NM) junction or glutamate in the central nervous system (CNS), cause an increase in Na+ and K+ permeability. This results in net inward current carried mainly by Na+, which depolarizes the cell and increases the probability that it will generate action potential (AP).
The action can be indirect involving a G-protein and other intermediaries or direct, where the drug itself binds to the channel protein and alters its function, e.g. local anesthetics (LA) act on voltage gated Na+ channels, drug molecule plugs the channel physically blocking ion permeation.
Example are:
- Dihydropyridines inhibit L-type calcium channel.
- Benzodiazepines bind to GABA receptor/chloride channel.
- Sulfonylureas act on adenosine triphosphate (ATP) sensitive potassium channels of β cells of pancreas.
Nonreceptor-mediated Drug Action
Drugs may target enzymes to produce their action. They may affect carriers or reuptake mechanisms for their actions. Cellular proteins like tubulin or immunophilines may also be targets for drug action.
Nicotinic Cholinergic Receptors (Fig. 1.6)
These receptors are selectively activated by nicotine and blocked by tubocurarine and hexamethonium. These are rosette-like pentameric structures which enclose a ligand gated cation channel. Activation of the channel causes opening of the channel with rapid flow of cations resulting in depolarization and generation of action potential. On the basis of location and selective agonists and antagonists two subtypes Nm and Nn are recognized.
Figures 1.6A and B: Nicotinic acetylcholine receptor. (A) Longitudinal view; (B) Cross-sectional view.
Sites of Location
- Muscle receptors which are confined to the skeletal neuromuscular junction.
- Ganglionic receptors—sympathetic and parasympathetic ganglia.
- CNS type receptors—brain.
Muscle Type (Nm) Receptor
(α1)2 β1Єδ—Adult, (α1)2 β1γδ—Fetal.
- Site: Skeletal neuromuscular junction.
- Mechanism: Opening of channel → increase in permeability of Na+ and K+. Excitatory end plate potential → depolarization → skeletal muscle contraction.
- Agonists: Acetylcholine (ACh), nicotine.
- Antagonist: Atracurium, vecuronium, d-tubocurarine and pancuronium.
Nn Type (α3)2 (β4)3
- Location: Autonomic ganglia and adrenal medulla.
- MOA: Same—depolarization – secretion of catecholamines.
- Agonist: ACh and nicotine.
- Antagonist: Trimethaphan and mecamylamine.
CNS Type (α4)2, (β4)3, (α7)5
α-toxin Insensitive, α-toxin Sensitive
- Location: Pre- and postsynaptic.
- Agonist: Anatoxin A.
- Antagonist: Dihydro-β-erythrodine, mecamylamine for α-toxin insensitive; α-bungarotoxin, α-lonotoxin for α-toxin sensitive.
BIOTRANSFORMATION
Process by which the body brings about chemical changes in drug molecule is called biotransformation. It is needed to render nonpolar, lipid-soluble compounds more polar or lipid-insolubles, so that they are not reabsorbed in the renal tubules and are excreted. Most hydrophilic drugs, e.g. streptomycin, neostigmine, decamethonium, etc. are not biotransformed and are excreted unchanged. So, biotransformation reactions alter the physiochemical properties of a drug so that a drug with high lipid or water partition coefficient will be converted into a polar and water-soluble one for easy disposal from the body.
Organs Involved
Liver is the main organ involved. Other sites are intestinal mucosa, nasal epithelium, lungs, skin, (as cortisol, testosterone, betamethasone), kidney, (as insulin, vitamin D), brain (e.g. levodopa), plasma (e.g. succinylcholine, procaine).
Enzymes Involved
Microsomal
- These enzymes are present in the smooth endoplasmic reticulum (ER) of liver, kidneys and gastrointestinal tract (GIT). Important microsomal enzymes present in the hepatic cells are mixed function oxidase P450. Other enzymes are hydroxylase, reductase, dehydrogenase, glucuronyl transferase, glutathione S-transferase.The micorosomal enzymes are responsible for most of the oxidation reactions, some reduction, hydrolysis, glucuronidation and glutathione conjugation reactions. These enzymes are primarily the substrates of high lipid water partition coefficient. The microsomal enzymes can be inducible by drugs, diet and other factors.
Nonmicrosomal
- They are present in the cytoplasm, mitochondria and extracellular spaces of different organs. High concentration are found in the liver, plasma, kidneys and other tissues.Nonmicrosomal enzymes are esterase, amidase, hydrolase, sulfotransferase and glutathione S-transferase.These enzymes catalyze all conjugation reactions (except glucuronide and glutathione conjugation), hydrolysis, some oxidation and reduction reactions.These enzymes cannot be induced but can be inhibited by drugs.
Types
The chemical reactions involved in biotransformations are classified as—
Phase I Reactions
In which enzymes carry out oxidation, reduction on hydrolysis reactions. Phase I enzymes lead to the introduction of what are called functional groups, resulting in a modification of the drug, such that it now carries an OH, -COOH, -5H, -O, or NH2 group. Reactions in phase I lead to inactivation of an active drug. In certain instances metabolism specially, hydrolysis of ester or amide linkage results in bioactivation of a drug which are called prodrugs. If the phase I metabolite is sufficiently polar, then it will be excreted in the urine. Some metabolites not excreted are further metabolized by phase II reactions. A single drug may undergo several biotransformation steps. Phase I oxygenases are—cytochrome P450 s, flavin containing monooxygenases (FMOs), epoxide hydrolases (MEH, SEH). The CYPs and FMOs are super families of enzymes. Each super family comprises multiple genes.
Cytochrome P450: Cytochrome P450 enzymes are heme proteins, comprising a large family (super family) of related but distinct enzymes (each referred to as CYP followed by a defining set of numbers and letters, number designates family and letter denoting the sub family). The name cytochrome P450 (CYP) is due the spectral properties of the hemoprotein. In its ferous form, it binds to carbon monoxide to produce a complex which absorbs maximum light at the range of 450 nm. These enzymes differ from one another in—
- Amino acid sequence.
- In regulation by inhibitors and inducing agents.
- In the specificity of the reactions they catalyze.
Different members of the family have distinct but often overlapping substrate specificities. This unusual feature of extensive overlapping substrate specificities by the CYPs is one of the underlying reasons for the predominance of drug-drug interactions.
Purification of P450 enzymes and DNA cloning has yielded 74 CYP gene families of which 3 main ones CYP1, CYP2, CYP3 are involved in drug metabolism in human liver. CYPIA2 is one of the main enzymes.
Heme contains one atom of iron in a hydrocarbon cage that functions to bind oxygen in the CYP active site as part of the catalytic cycle of these enzymes. CYPs use O2 plus H+ derived from the cofactor reduced nicotamide adenine dinucleotide phosphate (NADPH) to carry out the oxidation of substrates. The H+ is supplied through the enzyme NADPH cytochrome P450 oxidoreductase. Metabolism of a substrate by a CYP consumes one molecule of molecular oxygen and produces an oxidized substrate and a molecule of water as a by product. The O2 is usually converted to water by the enzyme superoxide dismutase.
The reactions carried out by mammalian CYPs are—
N – dealkylation, O – dealkylation aromatic hydroxylation, N – oxidation, S – oxidation deamination dehalogenation.
Inducers of P450 | Inhibitors of P450 |
---|---|
Rifampicin | Quinidine |
Ethanol | Ketaconazole |
Carbamazepine | Cimetidine |
Phenobarbitone | Metronidazole |
Glucocorticoids | Disulfiram |
INH | Omeprazole |
Chloral hydrate | Allopurinol |
Phenylbutazone | Erythromycin |
Griseofulvin | Clarithromycin |
DDT | Chloramphenicol |
As a family of enzymes CYPs are involved in the metabolism of dietary and xenobiotic agents, as well as synthesis of steroids and metabolism of bile acids. In contrast to the drug metabolizing CYPs, the CYPs that catalyze steroid and bile acid synthesis are substrate specific. The CYPs that carry out xenobiotic metabolism have a tremendous capacity to metabolize a large number of structurally diverse chemicals. The most active CYPs for drug metabolism are those in CYP2C, CYP2D, CYP3A subfamilies. CYP3A4 is the most abundantly expressed and involved in the metabolism of about 50% of clinically used drugs. Some examples—
- CYP1A1: Theophylline.
- CYP1A2: Caffeine, paracetamol, tacrine and theophylline.
- CYP2A6: Methoxyflurane.
- CYP2C9: Ibuprofen, phenytoxin, warfarin and tolbutamide.
- CYP2C19: Omeprazole.
- CYP2D6: Clozapine, codeine and metaprolol.
- CYP3A4/5: Cyclosporine, losartan, nifedipine and terfenadine.
Within human populations there are major sources of interindividual variation in CYP 450 enzymes. The causes include—genetic polymorphism, environmental factors, enzyme inhibitors and inducers, their presence in diet, e.g. grape fruit juice and St. John's wort (in alternative medicine) inhibit enzymes but Brussels sprout and cigarette smoke induce CYP 450 enzymes.
(Possible uses of enzyme induction—(1) congenital nonhemolytic jaundice—phenobarbitone, (2) Cushing's syndrome—phenytoin decreases manifestations, (3) liver disease and (4) chronic poisoning.)
Phase II (Conjugation) Reaction
If a drug molecule has a suitable hydroxyl, thiol or amino group, it is susceptible to conjugation reaction. The resulting conjugate is almost always pharmacologically inactive and less lipid soluble than its precursor and is excreted in urine or bile.
Conjugation with glucuronic acid, glycine conjugation, methylation, acetylation and sulfate occurs in phase II reactions.
SECOND MESSENGERS
As first messenger binds with its specific receptor, the drug receptor complex is formed which subsequently causes the synthesis and release of another intracellular regulatory molecule termed second messenger. These are—cyclic AMP, cyclic GMP, calcium, inositol 1, 4, 5 triphosphate, diacylglycerol, calmodulin.
Function
Second messenger molecules are produced in response to neurotransmitter binding to a receptor, translate the extracellular signal into a response that may be further propagated or amplified within the cell.
- Cyclic AMP—
- First to be recognized.
- Synthesized by plasma membrane attached adenylyl cyclase. Adenylyl cyclase converts adenosine triphosphate (ATP) into cyclic adenosine monophosphate (AMP).
- Mediates responses, such as ionotropy, chronotropy of heart muscles, relaxation of smooth muscles, breakdown of carbohydrates in liver, breakdown of triglycerides (TG) in fat cells, calcium homeostasis, other endocrine and neural processes, acts exclusively through cyclic AMP dependent protein kinase (A kinase), phosphorylates enzymes and proteins involved in cell function-transfer of phosphate from ATP occurs.
- Calcium—intracellular calcium plays an important role in the function of most of the cells.
- Intracellular calcium occurs in both bound and free form.
- Free form of calcium is responsible for action.
- Intracellular free calcium is about 10,000 times less compared to extracellular.
- As cells are stimulated by agonist, intracellular Ca+2 concentration increases rapidly.
- Intracellular free calcium brings about cellular action while bound form is present in the inner surface of cell membrane, ER, mitochondria, and secretory granules.
- Responsible for neurotransmitter release, muscle contraction and various function.
- Cyclic GMP—
- The enzyme is activated when muscarinic receptors are occupied by an agonist.
- Cyclic GMP → activates cyclic GMP-dependent protein kinase (G kinase).
- Subsequent effect is not yet known.
- Inositol 1, 4, 5 triphosphate—
- Hydrolytic product of phosphatidyl (PI) inositol, a minor phospholipid of the cell membrane.
- Activation of enzyme phospholipase which causes hydrolysis of phosphatidylinositol 4, 5 biphosphate (PIP2).
- Formation of water-soluble IP3 and diacylglycerol.
- This IP3 stimulates release of Ca from ER, this Ca is responsible for effect.
- IP3 is then converted to IP2, IP1 inositol and finally PI.
- Diacylglycerol—
- It formed from the metabolic product of PIP2.
- This diacylglycerol activates directly intracellular located protein kinase C (C kinase).
- Calmodulin—
- Single peptide chain containing 148 amino acid residues.
- Considered to be the receptor for intracellular free calcium.
- It has four binding sites.
- Three or four of these need to be occupied by Ca+2 before calmodulin will activate the myosin light chain kinase (MLCK)
- As phosphorylated myosin forms cross bridges with actin and sliding of actin over myosin filaments occur—producing contraction of muscle.
ADVERSE DRUG REACTIONS
An adverse drug reaction may be defined as a harmful or significant effect caused by a drug at doses intended for therapeutic effect that warrants reduction of dose or withdrawal of the drug and foretells hazards from future administration. Adverse effect or reaction refer to all unwanted effects attributable to the drug.
All drugs are xenobiotics and there is nothing like a safe drug. Whenever a drug is administered a risk is undertaken. This risk may be due to the properties of the drug, patient factors of the environment.
Classification
Adverse drug reactions may be classified as—
- Type A or predictable reactions—based on the pharmacological properties which are “augmented”. Effect often is reversible.
- Type B or unpredictable reactions—unrelated to pharmacological actions, e.g. idiosyncratic reactions. These effects are usually irreversible and bizarre. Such indirect toxicity may be direct or immunologic in nature, e.g. agranulocytosis with carbimazole. Other effects are liver or kidney damage, bone marrow suppression, carcinogenesis and disordered fetal development.
According to cause adverse drug reactions may be classified as—
- Side effects: Unwanted often unavoidable pharmacodynamic effects that often occur at therapeutic doses.
- Secondary effects: These are indirect consequences of primary actions of the drug.
- Toxic effects: These are the result of excessive pharmacological actions of the drug due to either overdosage or due to prolonged or chronic use. The CNS, CVS kidney, liver, lung, skin and blood toxin organs are most commonly affected.
- Drug-drug interactions: With the use of polypharmacy, response of one drug may be altered due to the administration of another resulting in untoward effects.
- Allergic drug reactions: These are common form of adverse drug reactions. A drug or its metabolite can act as a hapten and can be immunogenic.
Type A Reactions
Over expression of normal pharmacological action, e.g.
- α1 antagonists—causes hypotension.
- Anticoagulants—causes bleeding.
- Glycosides—causes cardiac arrhythmia.
- Anxiolytic—causes sedation.
- Insulin—causes hypoglycemia.
Type B Reactions
Rare and unpredictable, e.g.
- Paracetamol—causes hepatotoxicity.
- Thalidomide—causes teratogenicity.
- Chloramphenicol—causes aplastic anemia.
- Practalol—causes mucocutaneous syndrome.
- Carbimazole—causes agranulocytosis.
Side Effects
Unwanted effects due to pharmacodynamic profile of the drug in therapeutic dose regimens, e.g.
- Promethazine—causes sedation.
- Codeine—causes constipation.
- ACEI—causes cough.
Secondary Effects
Indirect consequences of primary action of the drug, e.g.
- Corticosteroids—causes osteoporosis.
- Tetracycline—causes superinfection.
Toxic Effects
Due to excessive pharmacological actions or due to direct tissue injury. Drug- induced cell damage occurs due to—
Noncovalent reactions: Lipid peroxidation, generation of toxic oxygen radicals, reactions causing depletion of glutathione, modification of sulfhydryl groups.
Covalent reactions: Targets are DNA and protein or peptide molecules, e.g.
- Hepatotoxicity— Paracetamol— INH— Iproniazid— Halothane— Methotrexate.It occurs when hepatocytes are exposed to toxic metabolites from oxidation by CYP 450.
- Nephrotoxicity: Kidney is exposed to high concentration of drugs and drug metabolites. Renal damage occurs due to—interstitial nephritis, papillary or tubular necrosis, decrease in compensatory vasodilator PGs.Drug-induced nephrotoxicity occurs due to NSAIDs specially phenacetinACE – I, methicillin, caffeine, captopril, cyclosporine.
- Mutagenesis: Mutation is a change in the genotype of a cell that is passed on when the cell divides. Chemical agents cause mutation by covalent modification of DNA. Mutation in protooncogenes, tumor suppressor genes result in carcinogenesis and usually involves more than one mutation. Drugs are relatively uncommon causes of birth defects and cancers.
- Carcinogenesis: Alteration of DNA is the first step in the complex multistage process of carcinogenesis. Carcinogens are chemical substances that cause cancer and can interact directly with DNA or act at a later stage to increase the likelihood that mutation will result in the production of a tumor. Most chemical carcinogens act by modifying bases in DNA particularly guanine, the O6 and N7 positions of which readily combine covalently with reactive metabolites of chemical carcinogens. Some therapeutic agents increase the risk of cancer—estrogen, pyrimethamine and methoxsalen.
- Teratogenesis: The term teratogenesis signifies production of gross structural malformations during fetal development. The timing of the teratogenic insult in relation to the stage of fetal development is critical in determining the type and extent of damage produced. It is during organogenesis days (17–60) that drugs can cause gross malformations. The structural organization of the embryo occurs in a well defined sequence, i.e. eye and brain, skeleton and limbs, heart and major blood vessels, palate and genitourinary system. The type of malformation produced thus depends on the time of exposure to the teratogen.
Drugs with teratogenic potential are thalidomide, hydantoin, alcohol, nicotine, antithyroid drugs and steroids (stilbestrol).
Allergic Reaction to Drugs
It is the most common form of adverse responses to drugs. Most drugs being low molecular weight substances are not immunogenic in themselves. A drug or its metabolites act as a hapten by interacting with protein to form a stable conjugate that is immunogenic.
Drug-induced allergic reactions may be Ab-mediated (types I, II, III) or cell-mediated (type IV). Important clinical manifestations include—
- Anaphylactic shock: It may be life threatening due to respiratory obstruction. Most deaths are caused by penicillin. Drugs causing anaphylaxis are penicillin, streptokinase, asparginase, hormones, like ACTH, heparin, dextran, radiological contrast agents and vaccines.Treatment is done with injection of adrenaline which is lifesaving. Penicilloyl polylysine is used as a skin test reagent for penicillin allergy. Other type I reactions are—bronchospasm and urticaria.
- Drug-induced hematological reactions are produced by type II, type III, type IV hypersensitivity.
- Type II reactions can affect any or all of the formed elements of the blood. Hemolytic anemia is related with sulfonamides and methyldopa.
- Agranulocytosis which can be irreversible is caused by sulfonamides, chloramphenicol and carbimazole.
- Thrombocytopenia is caused by quinine, heparin and thiazide diuretics.
Adverse Effects due to Chronic Use
- Eye—
- Cataract—may be caused by chloroquine, steroids, phenothiazine and anticancer drugs.
- Corneal opacity—may be caused by chloroquine, phenothiazine and amiodarone.
- Retinal injury—may be caused by chloroquine, ethambutol and thioridazine.
- Kidney: Analgesic nephropathy caused by NSAIDs.
- Allergic liver damage: Type II, III—hypersensitivity reaction as in halothane hepatitis due to reactive metabolite of halothane which couples to liver protein to form an immunogen. Enflurane (trifluoroacetyl chloride) may also cause Ab—mediated liver damage.
- Drug-induced SLE: Type III reaction which involves antibody to nuclear material and is a multisystem disorder affecting skin, lung, kidney, CNS. Subsides when offending drug is stopped of hydralazine and procainamide.
Adverse Drug Reaction
Adverse drug reaction (ADR) is a major consequence of pharmacotherapy and monitoring is an integral part of good clinical practice. If adverse drug reactions are not noticed, unnecessary morbidity and mortality occurs, hence comes the importance of pharmacovigilance.
Prevention
- Easy availability of ‘over the counter’ drugs to be stopped.
- Genetic screening for status of GL-6-PO4 dehydrogenase, pseudocholinesterase deficiency, acetylator status to be noted.
- Knowledge of the drug prescribed.
- Polypharmacy to be restricted to minimize drug-drug interactions.
- Recognition and reporting system of ADR to be practiced.
- Good clinical practices with detection of toxicities during preclinical as well as clinical trials.
- Finally risk–benefit ratio to be weighed before initiation of therapy.