Pharmacokinetics and Pharmacodynamics

ACEM Primary

July 22, 2021

Table of Contents

Pharmacokinetics:

Difficult to give drugs directly to target tissue-> often drug is administered into one body compartment (gut) and must move to its site of action in another compartment (brain)
Drug absorbed from site administration and distributed to site of action-> effect-> eliminated by metabolic inactivation + excretion

Bioavailability = fraction of unchanged drug reaching systemic circulation

For IV dose, bioavailability is 100%
For PO dose, it must be <100% due to incomplete absorption across gut wall and first pass elimination by liver
Influenced by many factors
- Pharmaceutical prep – particle size, liquid, physicochemical interactions – concurrent drug or food ingestion, patient factors – malabsorption syndromes or gastric stasis, pharmacokinetic interactions and first pass metabolism (buccal and rectal mucosa spared)
- Extent of absorption: depends on drug characteristics and site of administration
  - Hydrophilic (atenolol) – drug cannot cross lipid cell membrane
  - Lipophilic (acyclovir) – not soluble enough to cross water layer adjacent to cell
  - Reverse transporter P-glycoprotein (normally pumps drug out of gut wall into lumen)-> inhibited by grapefruit juice-> increased absorption
- First pass elimination
  - Portal blood delivers drug to liver prior to entry into systemic circulation
  - Drug metabolism in gut wall (CYP3A4 system), in portal blood or in liver
  - Liver can excrete drug into bile
  - Bioavailability F = f (extent of absorption) x (1- ER extraction ratio)
  - Extraction ratio = fraction of drug removed from blood by liver, depends on hepatic blood flow + uptake into hepatocytes + enzyme metabolic capacity
  - Michaelis constant = activity of enzyme
  - Eg. Morphine is completely absorbed:
    - f=1 – thus loss in gut is negligible
    - hepatic extraction ratio = morphine clearance/ hepatic blood flow = 60 L/h/ 70kg / 90L/h/70kg = 0.67
    - F = 1 x (1- 0.67) = 33%

Absorption

Variety of routes: oral, sublingual, rectal, topical, subcutaneous, intramuscular, intravenous, intrathecal, epidural 
PO may be given for systemic or local effects 
IV provides direct and reliable method of systemic drug delivery = no dependence on gastrointestinal absorption, first pass metabolism, or skin or muscle perfusion
Rate of absorption
- Zero order: rate of drug absorption is constant until depleted from gut (eg. Controlled release or determined by rate of gastric emptying)
- First order: rate is proportional to gastrointestinal fluid concentration
Extraction ratio and First-Pass Effect
- Clearance is not affected by bioavailability
- However, clearance can affect extent of availability as determines extraction ratio
- PO dose increased-> may reach therapeutic blood concentrations-> higher concentrations of drug metabolites vs. if given IV form
  - Eg. Lidocaine and verapamil both used to treat cardiac arrythmias, bioavailability <40% = lidocaine never given PO as metabolites cause CNS toxicity
- High extraction by liver-> bypassing hepatic sites of elimination (hepatic cirrhosis with portosystemic shunting)-> increases in drug availability
- Poor extraction by liver-> little change in availability (warfarin, diazepam, phenytoin)
Alternative routes of administration and first-pass effect
- For convenience (PO), maximise concentration at site of action (topic), prolong duration of drug absorption (transdermal) or avoid first pass effect (rectal/ SL)
- Hepatic first pass effect avoided via sublingual tablets (straight to systemic circulation)/ transdermal/ suppositories (lower rectum-> vessels that drain IVC, bypass liver however upper rectum-> liver)
- Lungs are also site of first pass loss
3 distinct groups of drugs:
- Rapid hepatocyte uptake and high metabolic capacity (dependent on liver blood flow)
- Low metabolic activity and high protein binding (extraction most dependent on protein unbinding)
- Low metabolic capacity and low protein binding (unaffected by blood flow or protein binding)

Distribution

Depends on passage of drug across cell membrane and on regional blood flow

Physiochemical factors include: molecule size, lipid solubility, degree of ionisation and protein binding
3 general groups:
- Confined to plasma (Dextran 70 due to size, warfarin due to extensive protein binding)
- Limited distribution (non-depolarising muscle relaxants are polar, poorly lipid soluble and bulky; can only go through fenestrae in muscle)
- Extensive distribution – sequestered by tissues (amiodarone by fat, iodine by thyroid, tetracyclines by bone) = removed from circulation

BBB anatomical and functional barrier
- 2 main methods of transfer – active transport and facilitated diffusion
- Only lipid soluble, low molecular weight drugs can cross by simple diffusion
- Usually penicillin penetrates poorly, however in meningitis, inflammation increases permeability-> allows therapeutic access 
Foetal membranes (phospholipid) are less selective than blood brain barrier
- Lower pH of foetal blood = protein drug binding differs = high protein binding increases drug transfer across membrane as free drug levels are low 
- Newborn may have anaesthetic or analgesic drugs in circulation post labour – bupivacaine used commonly in epidurals = crosses placenta less readily than lignocaine due to its higher pKa

Volume of distribution = amount of drug in body to concentration of drug in blood/ plasma

V = (amount of drug in body)/ C

Volume apparently necessary to contain amount of drug homogenously at concentration found in blood/ plasma/ water
“High” volume of distribution = higher concentrations in extravascular tissue than vascular compartment ~ not homogenously distributed

Drug permeation; several mechanisms

Aqueous diffusion = occurs in larger aqueous compartments of body (interstitial space, cytosol) + across epithelial membrane tight junctions/ endothelial membrane aqueous pores. Driven by concentration gradient.
Lipid diffusion = determined by lipid:aqueous partition coefficient for how readily molecules moves between the two. Varies with pH of medium for weak acids/bases ~ Henderson-Hasselbalch equation.

Special carriers = exist for substances too important for cell function/ too large or insoluble in lipid to diffuse passively eg. Amino acids, glucose.
- ABC (ATP- binding cassete) family of transporters bind ATP
- MRP (multidrug resistance associated protein) transporters help excretion of drugs into urine and bile
Endo + exocytosis (vitamin b12)

Fick’s Law of Diffusion

Passive flux of molecules down concentration gradient
Flux (molecules per unit time) = (C1-C2) x (area x permeability coefficient)/ thickness
C1 = higher concentration, C2 = lower concentration

Ionisation of Weak Acids and Weak Bases; the Henderson- Hasselbalch Equation

Electrostatic charge of ionised molecule attracts water dipoles-> polar, relatively water soluble and lipid insoluble complex
Lipid diffusion requires high lipid solubility, ionization of drugs reduces ability to permeate membranes
Uncharged form is more lipid soluble

Weak acid = neutral molecule than can reversibly dissociate into anion + proton
- Eg. Aspirin: C₈H₇O₂COOH <–> C₈H₇O₂COO- + H+
- Protonated form = neutral, more lipid soluble form
Weak base = neutral molecule that can combine with proton to form a cation
- Eg. Pyrimethamine: C₁₂H₁₁CIN₃NH₃⁺ <–> C₁₂H₁₁CIN₃NH₂+ H+
- Unprotonated form = neutral
Law of mass action – reactions move to the LEFT in an acidic environment and RIGHT in alkaline environment
Henderson- Hasselbalch equation relates ratio of pronated to unprotonated weak acid to weak base to the molecules pKa and pH of medium
- Lower pH relative to pKa-> greater fraction of drug in protonated form
- Acid pH: more of weak acid will be in lipid soluble form
- Basic pH: more of weak base will be lipid soluble form

Application of principle – manipulation of drug excretion by kidney
- Drug filtered by glomerulus-> Renal tubule: lipid soluble drugs reabsorbed by passive diffusion
- If excretion is to be accelerated, must prevent reabsorption by changing urine pH to ensure drugs are in ionized state = lipid insoluble-> trapped in urine
- Weak acids usually excreted faster in alkaline urine, weak bases in acidic urine

Drugs which are weak acids – paracetamol, ampicillin, aspirin, ibuprofen
Drugs which are weak bases – salbutamol, allopurinol, diazepam
- Amine containing molecules – nitrogen has three atoms associated plus pair of unshared e-
- Primary amine ( 1 carbon + 2 H+)
- Secondary amine (2 carbons + 1H+)
- Tertiary amine (3 carbons)
- Quaternary amine – permanently charged with no unshared electrons to reversible bond proton = always in poorly lipid soluble charged form

Biotransformation

Renal excretion can terminate biologic activity of some drugs – however not all drugs
- Lipophilic molecules are reabsorbed in renal tubule (barbiturates)-> prolonged duration of action

Metabolic biotransformation can alter biologic activity and prevent this
- Occurs between absorption and renal elimination
- Liver principal organ-> first pass metabolism
- Produces more polar (water soluble) molecule that can be excreted in urine
- Prodrug = no inherent activity before metabolism – is converted into more active moiety
Two major categories:
- Phase I (non synthetic) = oxidation, reduction, hydrolysis
  - Can be excreted at this stage if polar enough
- Phase II (synthetic) = introduce functional group (OH, NH2, SH) to convert into more polar metabolite (usually more inactive form)

Some PO drugs are extensively metabolised in intestine-> lower gut harbours intestinal microorganisms capable of this (eg. gastric acid metabolises penicillin, digestive enzymes – insulin)
Most transformations catalysed by specific cellular enzymes in liver eg. ER, mitochondria, cytosol, lysosomes, nuclear envelope or plasma membrane
- ER – lamellar membranes reform into vesicles = microsomes
  - Rough for protein synthesis
  - Smooth for oxidative drug metabolism containing MFOs (mixed function oxidases)

PHASE I: Two microsomal enzymes important, found in liver, gut, brain, kidney
- Cytochrome P450 reductase (flavoprotein) using NADPH as reducing agent
- Cytochrome P450 (haemoprotein) serves as terminal oxidase
- Relative abundance of P450 oxidase vs reductase in liver = P450 haem reduction is rate limited step in hepatic drug oxidations
- Process: 1. Oxidized Fe3+ P450 + drug substrate = binary complex. 2. NADPH donates e- to flavoprotein P450 reductase = reduces oxidised P450- drug complex. 3. NADPH gives second e- via P450 reductase which reduces molecule oxygen = activated oxygen-P450-drug substrate complex 4. Complex transfers activated oxygen to drug substrate to form oxidized product.

Drug oxidation requires = P450 reductase and oxidase, NADPH and oxygen
P450 enzymes are classified into sub/families by degree of shared amino acid sequences
- Eg. CYP1A2: 1= family, A= subfamily, 2= isoform

Enzyme induction = enhanced rate of P450 expression (increase synthesis/ reduce degradation)
- Caused by repeated administration of chemically dissimilar P450 substrate drugs, environmental chemicals (hydrocarbons)
- Induction-> accelerated substrate metabolism-> usually decrease in pharmacologic action OR toxicity if active metabolites
- Increase first pass metabolism = decreased bioavailability

Enzyme inhibition = reduce metabolism by tight binding to P450 haem iron (eg. Imidazole containing drugs)
PHASE II: Coupling reactions with endogenous substances to yield drug conjugates = polar and readily excreted
- Glucuronidation (morphine, propofol), sulphation (quinol, metabolite of propofol), acetylation (sulphonamides, isoniazid) and methylation (catecholamines)
- Enzymes (transferases) present in cytosol or microsomes
- Monoamines (adrenaline, noradrenaline, dopamine) metabolised by mitochondrial enzyme monoamine oxidase, not P450s
- Liver failure affects phase I, phase II metabolism less affected
- Faster than phase I

Metabolism of drugs to toxic products: Paracetamol
- Acetaminophen undergoes glucuronidation and sulfation (95% excreted metabolites) + P450 dependent conjugation (5%)
- Supratherapeutic doses – glucuronidation/sulfation pathways are saturated and P450 pathway becomes more active-> hepatic GSH depleted-> formation of reactive metabolite NAPQI (type of ROS)-> hepatotoxicity and death
- Antidote is NAC within 8-16 hours post overdose

Drug dose and frequency to achieve therapeutic levels varies due to patient factors – different distribution/ rates of metabolism/ elimination

Elimination Kinetics

Distribution and metabolism = process of removal of drug from plasma + Excretion = removal of drug from body

Urine and bile are main sites of excretion
Depends on structure and weight of drug – high molecule weight compounds are not filtered by kidney and excreted in bile
Renal excretion:
- Glomerulus: filtration of small non protein bound, poorly lipid soluble but readily water soluble drugs-> PCT: active processes to secrete molecules against concentration gradients-> DCT: passive diffusion down concentration gradient, acidic drugs excreted in alkaline urine (increases ionised form) and basic drugs excreted in acidic urine (trapped as cations)
- Renal disease-> accumulation of drugs normally excreted by kidneys, require dose reduction
Biliary excretion:
- High molecular weight compounds-> bile into enterohepatic circulation-> hydrolysed in small bowel by glucuronidase (lipid soluble drug may be reabsorbed)
- Eg. OCP + antibiotics = reduced intestinal flora = reduced reabsorption in enterohepatic circulation = OCP failure

Volume of distribution increased (fluid retention) – increase loading dose
Apparent volume of distribution same – same loading dose, may require repeated dosing/ increased interval
Dose reduction = usual dose x (impaired clearance/ normal clearance)

Clearance

Volume of blood cleared of drug per unit time
Is also the rate of elimination in relation to the serum concentration

CL = rate of elimination/ C
- CL – clearance, C – plasma drug concentration

Separate clearances at various organs (kidneys/ liver)

Elimination

Amount of drug cleared from the body per unit time
Rate of elimination = clearance x C
Thus elimination and clearance rate of the same drug can be different
Elimination can be first order (flow dependent) or zero order (capacity limited)

Half life = time required to change amount of drug in body by one half during elimination

Drug Accumulation:

Repeat drug dosing = drug will accumulate until dosing stops
If dosing interval is shorter than four half lives, accumulation will be detectable

Accumulation factor = 1/ (1- fraction remaining)
For drug given one every half life, the accumulation factor is 1/0.5 = 2
Predicts the ratio of steady- state concentration to be seen at same time following first dose
Peak concentrations after intermittent dosing at steady state = peak concentration after first dose x accumulation factor

Liver Disease – alter many aspects of pharmacological profile:

Protein synthesis decreased = decreased plasma protein binding
Phase I and II affected = metabolism reduced
Ascites increased volume of distribution
Presence of portocaval shunts = reduced hepatic clearance = increased bioavailability

Difficult to measure function: synthetic (INR, PT, albumin) vs inflammatory change in hepatocyte (LFT)-> can have inflammatory change without reduced synthetic function
Illness = increased synthesis of acute phase proteins (albumin is not one of these, thus reduced)

Neonates – different pharmacokinetics:

Higher absolute proportion of water-> relatively increased ECF

 Reduced metabolising capacity of liver
Reduced plasma protein levels = less binding
Lower blood pH = affects proportions of ionised and unionised drugs
Enzymes not matured, activity of P450 system is reduced
Creatinine clearance is < 10% of adult per unit weight + nephrons not mature for months

Pharmacokinetics in the elderly

Altered volume of distribution ß reduced muscle mass and increase in fat
Reduced activity of hepatic enzymes = relative decrease in hepatic drug clearance
Reduced creatinine clearance
Multiple coexisting diseases altering pharmacokinetics and drug interactions
- Eg. Gentamicin

Pharmacodynamics

MECHANISMS OF ACTION

Drug must bind receptor to bring effect
Two properties of drug determine pharmacologic effect:
- Affinity – how well drug binds to R = Kd
- Intrinsic activity – magnitude of effect once drug bound = IA (0 – 1)

Types of interactions:
- Agonist = bind and activate receptor
  - Full agonist ~ activate R systems to maximum extent = shift all R into activated pool (Ra) (IA =1)
  - Partial = bind to R but do not stabilise Ra configuration as fully -> low intrinsic efficacy and reduced response (not due to decreased affinity) as competitively inhibit action of full agonists (0 < IA < 1)
    - Most resembles curve of full agonist + competitive antagonist
    - Eg. Buprenorphine (partial agonist of m opioid R) = less respiratory depression in overdose. However is anti-analgesia drug when administered with full agonist opioids – may precipitate withdrawal in opioid dependant patients.

Antagonist = bind but do not activate receptor (IA = 0)
- Eg. Atropine-> prevent access of ACh to R thus stabilise R inactive state
- Competitive = increasing concentrations progressively inhibit agonist response, EC50 shifts right
  - Schild equation C’/C = 1 + ([I]/Ki)
  - Degree of antagonism depends on [antagonist]
  - Response to antagonist depends on [agonist]
- Non-competitive (irreversible) = reduces maximal effect agonist can achieve, may not change EC50
  - Once R bound, agonist cannot inhibit antagonist regardless of concentration
  - Depends on spare R as to maximal response to agonist

Eg. Phenoxybenzamine (irreversible a adrenoceptor antagonist) = control HTN caused by catecholamine release from phaeochromocytoma. If drug lowers BP, blockade will remain despite higher concentrations of catecholamine from tumour-> ability to prevent response to varying concentration of agonist is advantageous.

Antagonists continued…
- Allosteric inhibition = binding to R site separate from agonist binding site = modifies R activity without blocking agonist binding
  - Negative allosteric modulators – inhibit R activity
  - Positive allosteric modulators – potentiate R activity (eg. BZP bind non-competitively to GABA-> enhance net GABA effect)
- Agonists that inhibit their binding molecules
  - Mimic agonist molecules which terminate action of endogenous agonist
  - Anticholinesterase inhibitors – slow destruction of ACh – cholinomimetic effects
- Inverse agonists ~higher affinity for R inactivated form, stabilise Ri configuration (-1 < IA < 0)
  - GABA_A R activated by GABA-> inhibition of postsynaptic cells. Exogenous agonists = BZP, reversed by neutral antagonist = flumazenil. Inverse agonists = anxiety, agitation.

Duration of drug action:
- Duration of drug occupying R – dissociation terminates effect
- Action may persist however, some coupling molecule is still active
- Drug R complex destroyed or new enzymes synthesised

Concentration- Effect Curve:
- Dose of drug µ response-> but as dose increases, response increment decreases
- Dose may be reached at which there is no response
- Hyperbolic relationship: E = (Emax x C)/ (C + EC50)
- Agonists bind by occupying R molecules ~ B; drugs bound to R
- B = (Bmax x C)/ (C + Kd)
- Low Kd = increased binding affinity

Receptor- Effector Coupling = drug occupancy of R + pharmacologic response
- Efficiency depends on R and degree of spare R, type of agonist, agonist affinity, downstream signalling
- Non linear relationship
- Spare Receptors = “free” receptors not bound to drug molecule, when concentration of agonist produces maximum biologic response
  - What accounts for this phenomenon?
    - R are spare in number relative to number of downstream molecules-> maximal response occurs without occupancy of ALL R
    - Temporal = maximal response can be elicited by relatively few R, because response initiated by signalling intermediate lasts longer than agonist-R interaction

Agonist concentration effect curve – Spare R
- A = agonist only
- B = agonist + low concentration antagonist-> concentration at which 50% maximal response if produced is higher. Spare R still around.
- C = increasing concentration antagonist-> maximal response can still be achieved, however nil spare R anymore
- D and E = increasing concentration antagonist-> diminished maximal response, reduced available R for agonist to bind
- Eg. β adrenoceptors
  - If 90% R occupied by irreversible antagonist-> maximal inotropic response can still be achieved in response to catecholamines = myocardial cells have lots of spare β adrenoceptors
  - Spareness is temporal = β adrenoceptors activation by agonist-> GTP, molecule outlives R-agonist interaction
Spare R number – possible to change tissue sensitivity
- Cell with 4 x R and 4 x E (nil spare R)-> agonist present at concentration = Kd will occupy 50% R = half E activated = half maximal response
- Cell with 40 x R and 4 x E (lots spare R)-> lower concentration of agonist needed to elicit half maximal response (2 of 4 effectors activated) – lower Kd ~ higher binding affinity

Degree of spareness = total number of R compared to number actually needed to elicit maximal response
Kd of agonist-receptor interactions determines fraction of total R occupied regardless of R concentration

RECEPTORS AND THEIR REGULATION

Five different types:
- Intracellular R for lipid soluble ligands = corticosteroids/mineralocorticoids/sex steroids, thyroid hormone and vitamin D-> bind to DNA sequences to stimulate transcription of genes (takes 0.5-several hours) and effects last for hours to days (slow turnover for enzymes/ proteins made)
- Ligand regulated transmembrane enzymes (R have hormone binding domain + enzyme domain eg. tyrosine kinase/serine kinase/ guanylyl cyclase) = insulin, EGF, PDGF, ANP and transforming growth factor -b
  - Insulin-> tyrosine kinase R-> increase uptake of glucose/ amino acids into cell
  - “Down regulation” = ligand binding induces accelerated endocytosis of R to cell surface, however if too many ligands-> R cannot be made in time-> surface R reduced-> ligands effect reduced
- Cytokine R = growth hormone, EPO, IFN. Mechanism similar to R tyrosine kinase except separate protein tyrosine kinase (janus kinase JAK family) binds to R
- Ligand gated channels = ACh, serotonin, GABA and glutamate-> increases transmembrane conductance of ion = altered electrical potential of membrane
  - Ach-> nicotinic Ach R (nAChR) which allows Na+ to flow down concentration gradient into cells-> depolarisation
- G Proteins + second messengers (below)

SECOND MESSENGERS/G PROTEINS

Ligands act by increasing intracellular concentrations of second messengers such as cyclic adenosine -3’5’-monophosphate (cAMP), calcium ions or phosphoinositides
Ligand binds to cell surface R-> activation of G protein-> alters activity of effector element (enzyme or ion channel)-> alters intracellular second messenger concentration
- Hormone – Gs protein-coupled R (b adrenoceptors/ glucagon/subtypes of DA/5HT2)– activates G protein s – alters adenylyl cyclase – converts ATP to cAMP
Ligand promiscuity = endogenous ligand can bind and stimulate various receptors that couple different subsets of G proteins = different responses in different cells
- NE/E-> Gs coupled b adrenoceptors and Gq coupled a1 adrenoceptors-> increase heart rate and cause vasoconstriction

Second messengers:

Cyclic adenosine monophosphate (cAMP)	Phosphoinositides and calcium	Cyclic guanosine monophosphate (cGMP)
cAMP stimulates adenylyl cyclase Eg. b adrenomimetic catecholamines increasing HR/ stimulate CHO breakdown in liver/ TG in fat cells	Diacylglycerol (DAG) = activates protein kinase C in membrane Inositol-1,4,5-triphosphate (IP3) = diffuses into cytoplasm to trigger release of calcium channels	cGMP stimulates cGMP dependant protein kinase Eg. NO binds cGMP causing smooth muscle relaxation and vasodilation (nitroglycerin in cardiac ischaemia)

RECEPTOR REGULATION

G protein mediated responses to drug/ hormones attenuate with time
Initially high level of response (cellular cAMP accumulation, Na+ influx)-> diminishes over seconds/ minutes even if agonist is present = desensitisation-> reversible if second exposure to agonist few minutes after first.
- Gs coupled b adrenoceptors regulated by phosphorylation – agonist induced change in conformation of R causes it to bind/activate kinases-> phosphorylates serine residues in R terminal tail-> binds beta arrestin-> reduces interaction with agonist response

DOSE RESPONSE

Potency = drug concentration/ dose needed to produce 50% maximal effect (EC50 or ED50) on DOSE AXIS
- Curves B/A are more potent then C/D because less drug is required to produce ED50
- A is less potent than B (partial agonist) – A EC50 > B EC50
- Potency depends on affinity (Kd) of R and efficiency in which D-R interaction is coupled with response
- Certain doses of A = larger effects than any dose of B because A has larger maximal efficacy
Efficacy = limit of dose-response relation on RESPONSE AXIS
- A/C/D have equal maximal efficacy > B
- Determined by mode of interactions with R or characteristics of R-E system
- Practical efficacy for achieving therapeutic endpoint (Eg. increased cardiac contractility) is limited by drugs propensity for toxic effect (arrhythmia)

DOSING ISSUES

Repeated doses of a drug-> increase or decrease pharmacological response for the same dose
Tachyphylaxis = rapid decrease in response to repeated doses over a short time period
- Eg. Decrease of stores of a neurotransmitter before resynthesis can take place
Desensitisation = chronic loss of response over longer period due to structural change in receptor or absolute loss of receptor numbers
Tolerance = larger doses are required to produce the same pharmacological effect-> altered sensitivity of R in CNS due to reduction in R density/ affinity

Last Updated on September 24, 2021 by Andrew Crofton

Andrew Crofton, Tanvi Karnik

Andrew Crofton