Pharmacology Basic Principles

PHARMACOLOGY BASIC PRINCIPLES

Drug: a molecule that when introduced into body alters body’s functions by interaction at molecular level; most are molecular weight 100-1000, which allows efficient absorption and distribution; 25% of drugs are chiral (stereoisomeric)

Xenobiotics: chemicals not synthesized in living system

Drug-body interactions

Pharmacodynamic: effects of drug on body (drug receptor concept, dose-response relationships)

Pharmacokinetic: the way the body handles the drug (absorption, distribution, metabolism, elimination)

Methods of Drug Permeation

Body protected by membrane barriers which drugs must cross

1) Aqueous diffusion: limited capacity

Epithelial cells: only molecules MW 100-150 can pass through as cells joined by tight junctions

(eg. Li, methanol)

Capillaries: v large pores; MW 20,000-30,000 can pass (most brain capillaries aren’t this leaky except at pituitary and pineal gland, median eminence, choroid plexus)

2) Lipid diffusion: through membrane driven by conc grad; must be lipid soluble, though must be dissolved in water in order to reach membrane

3) Facilitated diffusion (via carriers): eg. Needed to cross BBB, for weak acids in PCT of kidney

4) Pinocytosis (receptor mediated endocytosis): for drugs MW>1000

Factors Affecting Drug Permeation

Rate of diffusion determined by Fick’s law of diffusion

Related to area of diffusion (eg. Lung>stomach)

thickness of barrier

conc grad – determined by gradient source (ie. Amount of drug administered)

gradient sink (ie. Rate of removal of drug near source)

- so high blood flow will keep high conc grad, so vasodilating drugs

absorbed faster

pKa (for lipid diffusion):

Weak acids/bases are more water-soluble when ionized (polar) and more lipid soluble when unionized à pH of environment determines ionization according to Henderson-Hasselbalch equation which measures amount of dissociation:

Log (protonated form/unprotonated form) = pKa – pH

IN WEAK ACIDS: the PROTONATED is LIPID-SOLUBLE and UNIONISED

If result of this calculation is >0 (before doing antilog) then more is PROTONATED (ie. Reabsorbed, not excreted)

If result is <0 (before antilog) then more is UNPROTONATED (ionised à water soluble à excreted)

IN WEAK BASES: the PROTONATED is WATER SOLUBLE and IONISED

If result of calculation is >0 then still more protonated, but this time it’s excreted

If result is <0 then still more is unprotonated, but this time it’s lipid soluble and reabsorbed

Acids: HA (protonated form) ↔ H+ + A- (unprotonated form)

Alkali: H+ + RNH2 (unprotonated form) ↔ RNH3 (protonated form)

pKa: the pH at which conc of ionized and unionized forms are equal

If pKA is alkaline à in alkaline urine it will be ionized à poorly soluble in lipids à not reabsorbed à excreted; in acidic urine it will be unionized à lipid-soluble à reabsorbed

Vice versa if pKa is acidic

Degree of ionization v important in body compartments where pH often changes (eg. Gastric, urine) (eg. Phenobarbitol cleared in alkaline urine as pKa is 7.4)

Note, some drugs are NOT significantly influenced by pH (eg. Penicillin)

Drug Absorption

Route of administration:

PO: absorption in gut is via lipid diffusion as cells joined by tight junctions; absorption favoured by large SA of GI tract, mixing of contents, pH differences at different levels; can be destroyed by acid (eg. Some penicillins) and enzymes (eg. Insulin is hydrolysed) and microbial activity

INH: can be used from agents that vaporize easily (eg. Amyl nitrite) or can be dispersed in aqueous droplets (eg. Ergot derivatives); large SA and high blood flow of lungs aid rapid absorption

TOP: transdermal route is using top drugs for systemic effects, slow absorption can cause prolonged drug levels; absorption through skin is through lipid barrier

BUCCAL: for drugs that are too rapidly deactivated by liver to be administered PO as portal circ will remove 90% therefore can use lower dose; high blood flow in buccal mucosa, into veins NOT portal circ

IV: bypass absorption barriers

IM: rapid absorption; depot injections are dissolved in oil to slow absorption

SC: slow absorption; if wish to give large amounts must add hyaluronidase to aid spread of injected solution

Intra-arterial or intra-thecal: for high local conc of drug; can use higher conc of drug than would be tolerated systemically

Drugs Distribution

Blood à compartments

· Held in blood if strongly binds plasma proteins (eg. Heparin)

· V. large molecules also stay in blood

· Water-soluble molecules freely distribute in TBW (eg. Ethanol, mannitol)

· Lipid-soluble molecules distribute to fat (eg. DDT)

· Some ions (eg. Heavy metals, lead, fluoride) go to bone

· Otherwise rate at which leaves blood determined by permeability principles

Apparent volume of distribution: total drug present in body / measured plasma conc of drug

Since some drugs are 100% bound to tissue structures à small amount in soln à v large apparent vol of distribution

Determining factors:

1) Protein binding: measured as % drug in blood that is not dialyzable; drugs is bound to inert binding sites on proteins à can’t diffuse/interact with receptors; in equilibrium with free drug, so alterations in free drug will change amount but not % bound; inert binding sites aren’t specific so diff drugs can compete (inert binding sites must be concentrated for drug to reach therapeutic levels à if another drug added and displaces drug à toxic levels); may also bind to proteins outwith vascular compartment

2) Blood flow: determines rapidity of delivery and conc gradient between blood and tissue; liver and kidneys have high blood flow; heart and skin have low blood flow; brain has mod flow, but since small receives high % cardiac output to reaches high conc; muscle has high flow but since large vice versa

3) Membrane permeation

4) Tissue solubility

Termination of Drug Effect

1) Excretion

a) Kidneys: by glomerular filtration: passive; non-saturatable; removes small molecules; cleared

from blood at rate equal to CrCl; can’t occur if bound to plasma protein

by tubular secretion: active; in PCT; saturatable (eg. Weak acids such as diuretics,

may have to compete with endogenous acid such as uric acid)

once in urine, lipid-soluble drug may be reabsorbed, water-soluble readily excreted;

metabolism of drugs can produce less lipid-soluble metabolites

b) Liver: important in drug metabolism à drug and metabolites secreted into bile (eg. Cardiac

glycosides, ABs) à duodenum à some drug may be reabsorbed from intestine into blood (enterohepatic circulation)

c) GI tract: drugs may cross lipid membrane from blood to lumen via passive diffusion (eg. Weakly basic morphine (pKa 7.9) diffuses into acidic stomach à 7.9 – 2.0 = 5.9 therefore more is protonated, therefore it’s 100% ionized and water-soluble à won’t be reabsorbed and can be lavaged (not excreted, as when passes to alkaline intestine will become un-ionised again)

d) Lungs: mostly for gaseous anaesthetics

e) Minor routes: sweat and salivary glands, milk

Early distribution phass = Extravascular equilibration: steep initial decr in conc of drug as drug passes from vascular into other compartments; this wouldn’t be demonstrated if drug remained in vascular compartment, only in 2-compartment kinetics

Slower elimination phase = Exponential decay curve: steep rise in conc when add drug à slow decr in levels as excreted, rate of decr decreases as the conc gradient driving drug out decreases as it level decreases in blood stream

2) Biotransformation: from active to inactive products

DRUG RECEPTORS AND PHARMACODYNAMICS

Receptors

The component of a cell/MO that interacts with a drug à biochemical events à drug effect

· Receptors determine relation between conc of drug and effect of drug

o A receptors affinity for a drug determines conc of drug required to form significant no of drug-receptor complexes

o Total no of receptors limits max effect a drug can produce

· Receptors determine selectivity of drug action

o Change in chemical structure of drug change affinities for diff classes of receptors

· Receptors mediate actions of pharmacological antagonists

o Pure antagonists bind receptors without altering them à prevent binding of agonists

Receptor may be:

a) Enzyme – can be inhibited/activated by binding of drug (eg. Dihydrofolate reductase)

b) Transport protein – eg. Digitalis on Na-K ATPase

c) Structural protein – eg. Colchicine attaches to tubulin

Receptor Desensitisation:

Usually reversible, which separates it from down-regulation

Eg. β-receptor – binding of receptor à begins to interact with G protein à phosphorylation of cytoplasmic OH terminal via serine β- adrenoceptor kinase à receptor gets high affinity for β-arrestin à bind, decreasing receptors ability to interact with Gs protein à reversed when agonist unbinds; Nicotinic Ach receptor

NB. There may be multiple receptors for any endogeous ligand, drugs often need to be more selective

Signalling Mechanisms and Drug Action

Mechanisms of transmembrane signaling:

1) Using a lipid soluble ligand: cross membrane and act on intracellular receptor (can be sited in cytoplasm or nucleus) à bind to DNA sequences (enhancers) à stimulate transcription of genes

There is a lag-time before effect as it takes time to make proteins; effects can persist long after drug eliminated from system 2Y to slow turnover of enzymes and proteins

Eg. Corticosteroids (binding to receptor triggers release of Hsp90 which was covering DNA-binding portion of receptor), mineralocorticoids, sex steroids, vit D, thyroid hormones

2) Using a transmembrane receptor protein:

Protein causes intracellular enzymatic actions

Consists of extracellular hormone-binding domain and cytoplasmic enzyme domain with protein tyrosine kinase activity

Hormone binds à conformational change in receptor à receptors bind together à protein kinases brought together à become enzymatically active à tyrosine residues become phosphorylated (cross-phosphorylation which may last longer than receptor activated) à cellular events (eg. Phosphorylation of substrate protein, transcription factor activation etc…)

Regulated by down-regulation (endocytosis and breakdown of receptors) – this process requires tyrosine kinase activity

Eg. Insulin, epidermal GF, PDGF

3) Using a ligand-gated channel:

Ligand binding opens/closes channel à change in ion transport à change in membrane potential

V fast response; for transfer across synpases

Eg. Ach (opening of nicotinc Ach channel, a pentamer, Ach binds α subunit à Na into cells à depolarisation , gamma-aminobutyric acid, excitatory aa’s (all synaptic transmitters)

4) Using a transmembrane receptor protein à G protein:

Ligand binds serpentine receptor (extracellular amino terminal, intracellular carboxyl terminal) à receptor activates (via interaction at 3rd cytoplasmic loop of receptor) G protein on cytoplasmic side of cell membrane àswapping of GDP for GTP (length of response may be related to length of GTP-bound Gs molecule as opposed to length of binding of ligand-receptor à amplification of original signal; this creates spare receptors) à via enzyme / ion channel G protein generates intracellular 2nd messenger à effect terminated by hydrolysis of GTP

Eg. Gs (α-receptors, glucagons, histamine, serotonin), Go (NT’s), Gq (Ach, serotonin), G1+2 (rods and cones), G1+3 (α1-receptors, Ach, opioids, serotonin)

Second messengers

Pathways may oppose eachother or work together

Reversible phosphorylation is common theme and can result in:

· Amplification: attachment of phosphoryl group to serine/threonine/tyrosine à amplification via creation of molecular memory, memory erased by dephosphorylation

· Flexible regulation: multiple protein kinases regulated by multiple 2nd messengers, many branches, many diff responses possible

a) cAMP made via adenyly cyclase:

à stimulate cAMP-dependent protein kinases via cAMP binding to R dimer of kinase à C chains released which diffuse through cytoplasm à transfer phosphate from ATP to (eg.) Lipase in adipocytes, myosin in smooth muscle, glycogen synthase in liver à agonist binding stops à cAMP effects terminated by phosphatases, cAMP degraded to 5’-AMP by cyclic nucleotide phosphodiesterases (NB. Caffeine and theophylline prevent this degradation)

Effects: mobilization of stored E, conservation of water by kidney, Ca homeostasis, increased contractility of heart, regulates production of adrenal/sex steroids, relaxation of smooth muscle

Activated by: ACTH, catecholamines via β-receptors, FSH, glucagons, LH, histamine, PTH, vasopressin via V2, serotonin via 5-HT1

b) Ca/phosphoinositides:

May be via G proteins/tyrosine kinase; stimulation of phospholipase C à hydrolyses PIP2 into diacylglycerol (à activates protein kinase C) and IP3 (triggers release of internal Ca stores) à Ca binds calmodulin à activates other enzymes. Reversed by dephosphorylation of IP3, phosphorylation of diacylglycerol, active pumping of Ca from cytoplasm

Activated by: lithium, Ach, angiotensin, catecholamines via α-receptors, PDGF, serotonin via 5-HT2, vasopressin via V1

c) cGMP:

Ligand binds à guanylyl cyclase produces cGMP à cGMP-dependent kinase activated; action terminated by dephosphorylation of cGMP and breakdown of substrate

Effects: relaxation of vascular smooth muscle (via dephosphorylation of myosin light chains)

Activated by: ANF, EDRF, NO (in response to Ach and histamine)

Receptor-Effector Coupling

Coupling: the transduction process between occupying receptors and drug response; effect of full agonists are more efficiently coupled to receptor occupancy than partial agonists; also determined by biochemical events that transduce receptor occupancy à cellular response

Spare receptors:

When max response achieved by agonist when not all available receptors occupied – increase sensitivity to drug

eg. Max inotropic response to catecholamine can be achieved even if 90% β-receptors blocked by irreversible antagonist

Allows agonists with low affinity for receptors and hence rapidly dissociate from receptors, to produce full responses at low concs

Occurs because receptor activation may greatly outlast agonist-receptor interaction or if intracellular components rather than receptor limits coupling

Explains why sensitivity of tissue to certain conc of agonist doesn’t only depend on affinity of receptor/drug but on concentration of receptors. Eg. There are 100 receptors:

· If 90% receptors are spare à 10 receptors need to be occupied for full response à EC50 occurs at 5 receptors occupied à EC50 < Kd

· If receptor conc doubled to 200 and 5 receptors occupied needed for EC50 à drug will still bind same percentage as above which will be higher no of receptors now and only 2.5% of receptors need be occupied à ↓EC50 as lower conc drug needed

A: max response achieved

B: higher conc needed to achieve max response, EC50 increasing

C: max response still achieved 2Y to spare receptors used, EC50 still higher

D + E: can no longer achieve max response as spare receptors filled with antagonist

HAgonists

a) Partial agonists:

Produce lower response at full receptor occupancy

Curve similar to that of full agonist in presence of irreversible antagonist

May occupy all binding sites but fail to produce maximal effect despite having high affinity for receptors

May competitively inhibit responses produce by full agonists

Changes receptor confirmation but not to extent for full activation of receptor

Efficacy of drug: relation between occupancy of receptors and pharmacologic response

b) Full agonists:

Produce higher response at full receptor occupancy

Competitive/Irreversible Antagonists

Antagonists bind receptors but don’t activate them à prevent agonists from binding.

a) Competitive Antagonist: