pkpd 1153

biological matrices used in PK studies

• invasive

  • plasma

  • serum’

  • blood

  • how its done: anticoagulant (eg heparin, EDTA) added to tube to prevent coagulation of blood

  • sample in centrifuged and unclotted rbc removed

  • supernatant (ie the liquid lying above the solid residue) is plasma

• non invasive

  • breath

  • milk

  • saliva

  • urine

  • feces

  • no anti-coagulant added —> blood clots

  • sample centrifuged —> clotted rbc removed

  • supernatant is serum (diff protein composition from plasma)

plasma vs blood conc

plasma drug conc is NOT unbound drug plasma conc IS NOT blood conc

unbound drug

unbound drugs —> free drugs

• exerts pharmacological or toxicological activity

• adme (absorbed, distributed, metabolised and excreted

however, in clinical studies total C is usually determined.

• total C is determined by adding solvents to extract all drug molecules and precipitate and remove protein

• we dont usually measure unbound drug conc

Pk exposure time

IV —> starts high and decreases

oral —> starts low and increases slowly then decreases

ADME

absorption, distribution, metabolism and excretion

• absorption —> for drugs administered extravascularly, systemic absorption needs to occur before it can enter systemic circulation

• distribution —> after systemic absorption, drugs get distributed via systemic circulation to various tissues and organs in the body

• metabolism—> drugs converted into a more water soluble form so it can readily be absorbed by the body

• excretion —> drugs (usually POLAR) or water soluble metabolites are excreted

metabolism and excretion = elimination

• irreversible loss of parent drug from systemic circulation

• elimination prevents toxicity

when a dose is administered, the overall exposure time profile observed is the outcome of dynamic and simultaneous process of ADME (processes dont wait for whole dose to be absorbed before it begins)

for extravascular dosing, even though the conc of drug is increasing due to net absorption, distribution and elimination processes are simultaneously ongoing to drug molecules that have been systemically absorbed.

IV bolus

whole dose contained in a small vol of diluent directly injected into vein at one go

1st order kinetics

after IV bolus injection, the body burden (amt of drug in body) decreases over time as elimination is ongoing —> elimination rate decreases correspondingly

1st order elimination allows the body to cope w body burden of the drug

• high body burden = high rate of elimination

  • not exposed to high drug conc for long time

• low body burden = low rate of elimination so drug is allowed to persist

1st order kinetics has a mono exponential decline

overall elinination rate k

• tldr, drug elimination does not occur at a fixed rate

• instead rate of elim changes continuously over time in relation to the amt of drug in the body

• k = rate of elim/ body burden

mode of transport across a membrane

drug must be

• drug must be able to cross the phospholipid bilayer

• drug must be in solution

• drug must be FREE

• drug transport is bidirectional

carrier mediated transport

1. passive diffusion

   1. no need energy

   2. high conc to lower conc until equilibrium is reached (unbound conc is equal on both sides)

   3. equilibrating transporters

2. active transport

   1. req energy, ATP dependant and requires concentrating transporters

   2. substrates move against conc gradient

   3. directional —> influx (uptake) or efflux

   4. diff transporters located in diff organs

   5. but it is saturable —> have a transport maximum

   6. transporters

      1. the most abundant EFFLUX transporter (pump out of cells) is P glycoprotein (PGP) coded by MDRI gene. it is abundant in the intestines and brain

      2. the most abundant INFLUX transporter is organic anion transporting peptide (OATP) [influx = uptake = intestines > blood > cells]

factors affecting movement across membrane

1. size

   1. MW more than 500 daltons is considered big

   2. bigger molecules have harder time crossing

2. lipophilicity

   1. lipophilic drugs have easier time crossing membrane, but more lipophilic means less soluble

   2. LogP 3-5 is lipophilic, below that is hydrophilic

3. charge

   1. charged molecules, more diff to cross and move slower

   2. most drugs are weak acids and weak bases —> exist in solutions and as an equilibrium of ionised and unionised forms

      1. depends on physiological pH and pKa (human blood pH = 7.4)

      2. carrier mediated transport is important to move charged molecules across membrane

4. solubility

   1. drugs need to be soluble before they can cross a membrane

for the above characteristics, the blood brain barrier and gastrointestinal system and highly dependant on size, charge and lipophilicity

membane characteristics

1. brain

   1. the tightest junctions

   2. continuous

2. renal glomerulus

   1. fenestrated (have small holes)

   2. permeability up will 500g/mol

3. liner sinusoids

   1. leaky

physiochemical vs physiological membrane characteristics

physiochermical

1. size

2. charge

3. lipophilicity

4. solubility

physiological

1. membrane porosity

2. membrane thickness

3. involvement of transporters

systemic absorption

for extravascular absorption like oral absorption, systemic absorption is said to be occured when the unchanged drug travels from site of administration to the site of measurement (blood/ plasma from vein)

oral systemic bioavailability , F

oral systemic bioavailability F, is the fraction of an orally administered drug that reaches the systemic circulation

it is derived from Ff x Fg x Fh

Ff: fraction of drugs that enters the intestinal tissues

Fg: fraction of drug that escapes destruction by GIT and metabolism by CYP450 enzymes and fraction of drug that reaches portal vein

Fh: fraction of drug that reaches liver but escapes extraction within liver

absorption sites and terms

portal vein is intestinal absorption

liver is 1st pass metabolism and biliary excretion

plasma protein binding

1. ALBUMIN

   1. molecular weight 67

   2. drug selectivity —> WIDE

   3. MOST COMMON PLASMA PROTEIN

2. α-1 acid glycoprotein

   1. drug selectivity —> LIPOPHILIC AMINES

   2. SECOND MOST COMMON PLASMA PRTN

3. lipoproteins

   1. drug selectivity —> LIPOPHILIC DRUGS

fraction unbound

Fu : fraction unbound in plasma. it ranges from 0-1.

• the higher the Fu, the lower the protein binding

• lower Fu —> high protein binding

• every drug has their own Fu value

Fut: fraction unbound in tissues

Fu and Fut are independant from each other

a drug might have good affinity for plasma, but if it has GREATER affinity for tissue proteins, then itll primarily be located at the tissues

Fu is a variable that affects primary pk parameters like CLEARANCE & VOLUME since drug can be ELIMINATED & DISTRIBUTED

PHYSIOLOGICAL BODY water composition

for a 70kg adult,

• 60% body weight = total body water

• means 42kg of body water is in a 70kg adult

• 4% of body weight is plasma water

• means 3kg of plasma water is in a 70kg adult

apparent vol of distribution V

sometimes the apparent vol of distribution of drugs are greater than 42L. the rate and extent of distribution is diff at diff sites

apparent V definition: fluid volume in which a drug seems to be distributed to account for its plasma concentration

• relates the amount of drug in the body to conc of drug in the blood/plasma

how well a drug distributes depends on

1. its affinity for tissue or plasma proteins

2. how well it associates with hydrophilic and lipophilic compartments depending on physiochemical properties

V and its affinity for tissue/ plasma

• HIGH V = HIGH AFFINITY FOR TISSUE (likely to distribute out of plasma into tissue)

  • so for example if u calc V is greater than physiological body water of 42L, then its a sign that most drug molecules are distributed into tissues

• qn: interpret why apparent V is larger than physiological body water volume

  • ans: “the V of drug is larger than 42L bc it is the apprent volume in which the drug seems to be distributed to account for its obsevred conc. We can infer that the drug has most likely distributed into the tissues”

metabolism

• metabolism via biotransformation converts a xenobiotic into

  • smth less reactive or inactive so they cause less harm to the body —> liver detox

  • a more water soluble entity so it can be readily excreted from the body —> urine and feces

• metabolites might be active (good) or reactive (bad! toxic!)

• pathway: xenobiotic parent drug (lipophilic and non polar) > inactive metabolite (hydrophilic, polar) > excreted

overview of drug metabolism:

• enzymatic process

• most drug metabolising enz are promiscuous

  • one enzyme targets many substrates

  • handles an array of foreign substances

• targets are mainly xenobiotics

  • many DME do not have endogenous substrates (substrate developed within an organism that serves as a target molecule for enzyme or metabolic process) —> basically they taregt foreign molecules as substrates and not substrates found inside the body

• occurs in liver (abundance of CYP450 enz)

• secondary organs (mucosa and lung and skin)

phase 1 and phase 2 metabolism

phase 1:

• if u see CYP450, means its phase 1

• oxidation, reduction, dealkylation, hydroxylation and dehalogenation

• the purpose of phase 1 is substrate functionalisation —> adding a functional grp or remove or modify the structure to expose functional group for easier conjugation in phase 2 metabolism

• OH groups are added to make the toxic substances or less soluble molecules more soluble to make them easier to flush out

phase 2:

• -transferases

• substrate conjugation: conjugates parent drug or metabolites from phase 1 metabolism with water soluble moieties, eg sulfates and glucuronic acid

depending on chemical structure of a drug, it can undergo phase 1 and/or phase 2 for drug metabolism

most drugs are metabolised by CYP450 enzymes. metabolism is the main clearance mechanism for 73% of drugs. for the CYP mediated clearance, 46% is by the CYP3A family

renal excretion

renal excretion

• most of the water soluble drugs and drug metabolites are made more soluble by oxidation or conjugation and are typically eliminated by kidneys

• the nephron is the functional unit of the kidney thats responsible for elimination of drug

• urine formation and drug renal excretion occurs from 3 processes

  • glomerular filtration

    • glomerulus is leaky and allows large molecules to pass through

  • tubular secretion

    • involves transporters

  • tubular reabsorption

    • the drug molecule being reabsorbed must interact with the membrane and be non-polar, hydrophobic and not charged

    • passive process

• drug has to be unbound to be filtered—> means albumin should NOT appear in the urine of a healthy person

• tubular secretion involves transporters

• tubular reabsorption is usually passive and only non polar entities are reabsorbed

bilary excretion

• for drugs secreted into bile, it will be stored in gall bladder

• gall bladder will release bile and drug into GI tract via the common bile duct and it enters the stomach—> feces

• bile salts help w fat digestion

• some drugs, metabolites and glucuronide conjugates are secreted into bile

  • polar

  • MW more than 350g/mol

• biliary secretion is an active transport process w specific transport mechanisms for anions and cations and neutral organic compounds

  • transport may be saturated or completely inhibited

• drug in bile enters small intestine via common bile duct and can end up undergoing

  • enterohepatic cycling —> drug reabsorbed (DISTRIBUTION, can still illict effect) (basically means bile acids travel from liver to small intestine via bile duct then back to liver again via portal vein)

  • excreted via feces —> elimination

• to tell if a drug is biliary secreted, see bioavailability and excretion.

  • if it has high bioavailability, then most drug shld be absorbed.

  • so if excretion is high (eg 30%) then the drug must have been secreted into bile —> intestines —> feces

  • if the liver is not metabolising the enzyme but fecal excretion is high, its likely that it secreted the unchanged drug into bile rather than metabolize it

clearance

follows first order kinetics

blood clearance Clb : VOLUME of blood that is being cleared of drug per unit time. it is a CONSTANT

the rate of clearance is proportionate to the amount of drug in the body

rate of entry is 1

eliminating organ is E

rate of output is 1-E

so when E is high (close to 1), outflow is very low. Clb is limited by blood flow

• flow limited / perfusion limited CL

when E is low, (less than 0.1), drug is not extracted efficiently by the organ

• conc of drug is arnd the same as conc of drug leaving

• Clb is only a small fraction of blood flow Q

• it is independent of blood flow (blood flow will not impact Clb)

• capacity -limited CL —> depends largely on eliminating organ and process

every drug has its own E value and can be categorised into high medium and low extraction ratio based the organ it is extracted in

E, extraction ratio

E is the fraction of drug removed from blood as it passes through eliminating organ

0<E<1

clearance

CL of a drug is the volume of blood being cleared of drug per unit time (same as Clb)

clearance of a drug is the proportionality factor that predicts rate of elimination in relation to drug concentration

total systemic CL, CLt, is the sum of all diff organ clearances = rate of hepatic elim + rate of renal elim + rate of elim by other organs (EXCEPT PULMONARY CLEARANCE bc it does not receive the same arterial drug conc as other organs)

each drug has its own CL value. it indicates how efficiently a drug is being cleared from the body.

high clearance = means more efficient as more blood is being cleared of drug per unit time

linear kinetics

• it is termed linear kinetics bc following first order kinetics, we all a dose proportionality in conc and AUC

  • if a drug follows first order kinetics, doubling the dose will double the drug conc and exposure in the body

primary Pk parameters

CL and V are independent primary pk parameters that influence elimination half life t1/2. elimination half life is dependant on the combination of CL and V values

elimination half life (t1/2) and elimination rate constant k are secondary pk parameters that depend on CL and V