mfda exam 3

what has to occur in order for a drug to exhibit a therapeutic effect?

1. drug molecules must reach the target in the body

2. drug molecules must stay there for a proper time and in sufficient concentration

ADME

the processes the drug molecules undergo in the body that represent pharmacokinetics

• A: Adsorption

• D: Distribution

• M: Metabolism

• E: Excretion

absorption

drug transport from the administration site to the bloodstream

• occurs in GI and pulmonary tracts,

  mucous membranes, and cornea

  • trans-cellular transport

• or, occurs on skin surface

  • paracellular transport!

absorption in the bloodstream

• convection and diffusion, binding to formed particles (erythrocytes, leukocytes, platelets), and (lipo)proteins

• mostly passive, trans-bilayer diffusion

distribution

drug transport from the bloodstream to tissues

• tight capillaries; drug transport into cells

  • trans-cellular transport

• leaky capillaries; drug transport into the extracellular space

  • paracellular transport

• mostly passive, trans-bilayer diffusion

metabolism

enzymatic and spontaneous (e.g., hydrolysis)

• the process by which the body chemically alters the structure of a drug, usually in the liver, to facilitate its elimination

• bilayer buildup in some cases

• typically, no transport is involved

excretion

elimination of a drug into urine, sweat, expired air, feces..

• filtration → paracellular transport

• secretion and reabsorption → trans-cellular transport

trans-cellular transport

-the movement of drugs through the actual cell membranes, crossing both the apical (outer) and basolateral (inner) sides of the cell.

-this process typically includes passing through various cellular structures, such as the lipid bilayer and cellular organelles.

-lipophilic (fat-soluble) drugs tend to utilize this transport more readily because they can dissolve in the lipid membrane.

-crucial for drugs that need to reach intracellular targets or enter the bloodstream from the site of administration

paracellular transport

-the movement of drugs between adjacent cells, passing through the intercellular space or tight junctions that connect neighboring cells.

-this pathway is particularly relevant for hydrophilic (water-soluble) drugs that may have difficulty crossing lipid-rich cell membranes

• tight junctions between cells

elimination

metabolism + excretion

disposition

distribution + elimination (+metabolism)

what must a drug be able to do?

• dissolve to an acceptable extent

• survive a range of pHs

• cross membranes in villi and capillary walls

• avoid excessive binding to plasma proteins and lipoproteins

• survive liver metabolism

• avoid active transport to bile

• cross membranes of capillaries and tissue cells

• partition into the target organ

• avoid partition into undesired places

• be selective for its target

• avoid binding to significant antitargets (causing toxicity)

what are the minimum requirements for drug bioavailability?

-solubility in water to have a sufficient free concentration

• in the stomach and GI juice for GI absorption

• in the bloodstream, tissues, and organs to accomplish distribution

-permeability for the membranes mostly by passive trans-bilayer diffusion, for which the drug needs

• appropriate lipophilicity and hydrophilicity balance

  • only drugs with optimal balance pass through several bilayers as required for absorption and distribution

• appropriate ionization – ionized molecules do not pass the membrane

biopharmaceutic classification system

-provides assurance of in vivo (in the body) therapeutic equivalence based on extensive in vitro (in the lab) evaluation

-importance of permeability and aqueous solubility of drug substance (including the rate of dissolution of the drug product) for oral bioavailability

-class I drug candidates can obtain FDA bio-waivers reducing human testing

-class II is subdivided for low dissolution rate (IIa) and low product (IIb) solubility

solubility

-the concentration (mol/L) of a drug (solid, liquid, or gas) in a saturated solution, at given temperature and pressure.

-historically, and for practical reasons, it is often given as % w/w or % w/v

saturated solution

when a solid/liquid/gaseous drug is in equilibrium with the solid/liquid/gas phase:

• the solvent is not capable of dissolving more drug under the given conditions

• the dissolved drug concentration has reached the maximum and does not increase anymore

very soluble

-parts of solvent required to dissolve 1 part of solute = less than 1

-solubility (% w/w) = greater than 50

freely soluble

-parts of solvent required to dissolve 1 part of solute = 1-10

-solubility (% w/w) = 10-50

soluble

-parts of solvent required to dissolve 1 part of solute = 10-30

-solubility (% w/w) = 3-10

sparingly soluble

-parts of solvent required to dissolve 1 part of solute = 30-100

-solubility (% w/w) = 1-3

slightly soluble

-parts of solvent required to dissolve 1 part of solute = 100-1,000

-solubility (% w/w) = 0.1-1

very slightly soluble

-parts of solvent required to dissolve 1 part of solute = 1,000-10,000

-solubility (% w/w) = 0.01-0.1

(practically) insoluble

-parts of solvent required to dissolve 1 part of solute = greater than 10,000

-solubility (% w/w) = less than 0.01

what factors determine solubility?

energy of solution

1. energy to bring the drug molecule out of the pure drug phase (loss)

   • for gaseous and liquid drugs – usually small

   • for solid drugs – can be significant, the higher the energies, the higher the melting points of crystals

2. energy to create a cavity in the solvent (loss)

   • significant only in water because of H-bonds

   • increases with the size of the drug molecule

3. energy of solute-solvent interactions (gain) → significant in water, low in lipoid phases - solvation

drug-solvent interactions (solute-solvent interactions)

weak, mostly attractive interactions, which may include

• electrostatic, H-bonding, dispersion, and

  hydrophobic interactions in water

• only dispersion interactions in lipoid phases

-solvation is strongest between similar drug and solvent molecules → the “like dissolves like” principle

polymorphs

-the same compound crystallizes in different crystal forms

-they differ in the energy of release from the solute phase

• they often have different solubilities

metastable polymorphs

-(less stable forms)

-usually have higher solubility than more stable forms because the release from the solid form requires less energy and cavity and solvation term are independent of the solid form

-they are polymorphs that persist at conditions where the most stable polymorph would not naturally form

amorphous powders

-lack the crystalline structure and thus, have higher solubility than crystalline forms

• due to differences in their molecular arrangement and structure

hydrated crystals

solvates of water

• tend to exhibit lower aqueous solubility than their anhydrate forms (water-less forms)

solid forms of drugs

often available in several forms

• crystalline forms

• polymorphs (more than one crystalline form)

• amorphous forms (lacks crystalline form)

-if feasible, amorphous form is chosen because of highest solubility

dissolution

is often the rate limiting step in getting the drug into systemic circulation after oral administration

• its rate is a critical parameter for solid dosage forms, especially for oral administration

USP dissolution apparatuses that are used to reduce the hydration layer

• rotating basket

• paddle

• reciprocating cylinder

• flow-through cell

the dissolution medium may imitate the content of GI tract where the formulation is expected to dissolve

• (0.1 M HCl, simulated gastric or intestinal juice), temperature 37C

what are strategies to improve drug solubility?

1. chemical structure modifications – only in early stages of drug development

2. modification of physical properties of solid dosage forms

• utilization of polymorphism or amorphous state

• micronization: break down the crystal lattice of the solid (it is more than just reducing the particle size)

• salt formation

3. formulation of solvent system (solution dosage forms)

• adjust pH (use buffer systems): keep drug in ionized state

• use appropriate cosolvents (i.e. alcohol, hydrogen bonding organic solvents)

• + surfactants (as excipients)

• + complexation: association between two or more molecules to form a coordination complex

polarized confluent cell monolayer

when the apical (outer) side and basolateral (inner) side have different morphology and membrane protein content

Caco-2 cells

a human colon epithelial cancer cell line, gold standard for intestinal absorption simulations:

• differentiated and polarized with intercellular tight junctions

• a well-differentiated brush border

• typical small-intestinal nutrient transporters

• resembling the enterocytes lining the small intestine

Madin-Darby canine kidney (MDCK) cells

susceptible to viral infection, including influenza serotypes

• development of new influenza vaccine candidates for humans

• identification of P-glycoprotein substrates and inhibitors

• cell lines forming confluent monolayer of polarized cells, i.e., having different

  apical (AP) and basolateral (BL) sides

absorptive direction of cell monolayer

apical (AP) → basolateral (BL)

• cellular uptake of tested compounds

• involvement of transporters

• the result is the intrinsic

  permeability coefficient

  • comparison with the apparent permeability coefficient (without

    inhibition) informs about the efflux or influx

where does drug metabolism occur?

in liver via drug metabolizing enzymes

• two phases: oxidation (I) and conjugation (II)

how do most drugs pass most membranes?

passive trans-bilayer transport

what lipid molecule is most prevalent in mammalian bilayers?

phosphatidylcholine

mammalian bilayers are mainly composed of…

1. phospholipids

2. sphingomyelins

3. glycolipids

4. cholesterol

main chain-melting temperature (Tm)

the temperature required to change the bilayer from a gel phase to a fluid phase

• between 20 and 60 degrees Celsuis

how to increase fluidity in bilayer?

presence of:

1. unsaturated fatty acids

2. molecules dissolved in the bilayer

how to decrease fluidity in bilayer?

presence of:

1. cholesterol

2. saturated fatty acid chains in phospholipids

3. rigid hydrophobic molecules that can intercalate between the fatty acid chains

gel phase of bilayer

below Tm (colder)

• phospholipid chains extended

• tight packing

  • slow passive transport

liquid/fluid phase of bilayer

above Tm (hotter)

• more conformational freedom

• more movement

• smaller thickness of the bilayer

• larger area per phospholipid

• more hydration in the headgroup region

faster passive transport

mixed phase (gel and liquid/fluid) of bilayer

at Tm

• fastest passive transport!

headgroup agitated water stratum (phosphatidylcholine bilayer)

-12-16 water molecules per headgroup just above the transition temperature (Tm)

-number of water molecules increases with increasing temperature

headgroup stratum/layer

-H-bond acceptors in high concentration

-all hydrating waters are engaged in H-bond

core soft polymer stratum/layer

the first 6-8 methylene segments of the chains

core alkanes status/layer

-the thickness rests on the length of fatty acid chains

-density of hexadecane

what molecules hop between thermal kinks in the bilayer?

small, nonionized molecules (like gases)

• water, urea

• oxygen, carbon monoxide, carbon dioxide, nitric oxide

• nonionized formic acid

what molecules diffuse through larger, water-filled pores?

monovalent ions

• protons, sodium, potassium, etc

what molecules diffuse through a heterogenous bilayer?

larger molecules (up to 1000 g/mol)

-most drugs

• solubility/diffusion mechanism

solubility/diffusion mechanism

1. fast drug interaction (“dissolution”) with the headgroups + interface + core depending on drug structure

2. slower diffusion of the drug to the opposite side of the bilayer

fast drug transport if…

compounds exhibit intermediate strength of interactions in the headgroup strata and in the hydrocarbon core

accumulation of drugs in the bilayer strata is…

a measure of interaction strength between drug and bilayer strata and is characterized by three drug properties:

1. lipophilicity

2. amphiphlicity

3. cephalophilicity

lipophilicity

tendency to accumulate in the hydrophobic core of the bilayer

• expressed as the 1-octanol/water partition coefficient P

amphiphilicty

tendency to adsorb to the headgroup/core interface

• the polar part interacts with the phospholipid head groups and the lipophilic part is protruding into the bilayer core composed of fatty acyl chains

cephalophilicity

tendency to interact with the phospholipid head groups

role of drug lipophilcity

drug needs to have intermediate strength of interactions with the core (= intermediate logP value) to pass the bilayer

• too hydrophilic (polar) drugs (low logP) will not enter the core

• too lipophilic (greasy) drugs (high logP) will get stuck in the core

molecules that are too hydrophilic (low logP value)…?

will not enter the core of the bilayer

molecules that are too lipophilic (high logP value)…?

will get stuck in the core of the bilayer

partition coefficient (Nernst’s distribution law)

P = Co/Cw

• cO is the concentration in the organic (nonpolar) phase

• cW is the concentration in the aqueous phase

  • the concentrations are measured at equilibrium and refer to the same molecular species

Nernst’s distribution law and solubility

drugs with high P have low solubility in water

• can be used to calculate solubility in one phase if the solubility in another phase is known

1-octanol/water partition coefficients history shiii

-discovered by Hansch in 1964

-Albert Leo maintains a database of the partition coefficients

-approach for calculation of the partition coefficient from the drug structure is ClogP

1-octanol/water reference system

• resembles membrane/water system with regard to drug partitioning

• describes well binding of drugs to proteins

• has practical advantages

  • dissolves most drugs well (water content 2 mol/L)

  • does not absorb UV VIS light

    • drug analysis by UV VIS spectrophotometry is straightforward

  • is easily purified by distillation

shake-flask method (1-octanol/water)

-the phases are pre-equilibrated, and then the drug is added to the phase where it has higher solubility

• if a drug solubility in a phase is low, the phase volume can be increased to make the transfer into this phase measurable as the loss from the other phase

• the test tube is shaken until equilibrium is achieved (no concentration changes)

• the equilibrium drug concentration in both phases is determined in ideal case

-disadvantages

• formation of emulsions possible

• no verification of the equilibrium

slow-stir method (1-octanol/water)

-the phase volumes are larger than in the shake-flask method

-the phases are gently stirred to remove concentration gradients without disturbing the interface

-drug concentration in each phase is determined

-the time course of the drug partitioning is measured

• verifying the equilibrium

• P for unstable drugs (hydrolysis, photolysis...) can be determined by fitting

the sieving effect

the peak becomes narrower after each addition of the bilayer

metabolism of drugs

-the liver is responsible for a major portion of drug metabolism, receives blood from:

• hepatic artery (25%) carries oxygen

• hepatic portal vein (75%) carries nutrients and drugs from the GI tract

-first-pass effect – drug loss in the first contact with liver

-difficult to relate to drug structure

• the drug candidates have susceptible functional groups modified

metabolism makes drug molecules…

1. more hydrophilic

2. less prone to protein binding

3. more susceptible to excretion

excretion of drugs

-kidney is the main excretory organ, but GI tract, lungs and skin are also involved

-renal excretion includes

• glomerular filtration

• active tubular secretion

  • (carrier-mediated → structure-specific, energy driven)

• tubular reabsorption (active or passive)

-filtration is size limited

-passive reabsorption behaves as passive drug transport from filtrate (pH 4.5 – 8.0) to plasma (pH 7.2) – optimum logP value

-active processes are difficult to relate to drug structure

upper limit of drug transport rates (fast drug transport)

~1 second per bilayer

• intermediate strength of interactions in the headgroup region, in the hydrocarbon core, and at the interface between them

lower limit of drug transport rates (slow drug transport)

1-2 days

drugs that are supposed to have oral bioavailability and fast general distribution in the body should have…

• intermediate amphiphilicity

• intermediate lipophilicity

• intermediate cephalophilicity

drugs designed for limited distribution close to the site of administration or release from a dosage form should have…

• low or high amphiphilicity

• low or high lipophilicity

• low or high cephalophilicity

if a drug with limited availability is supposed to be given orally…

it needs to be a substrate for active GI transport

number of cell layers served by a capillary in the lungs

1

number of cell layers served by a capillary in the kidneys

2

number of cell layers served by a capillary in the intestines, liver, heart, brain, and spleen

5

number of cell layers served by a capillary in skin, muscles, fat, and bones

15

-a good estimate of the number of membranes the drug molecules need to cross

the number of cell layers ×10

five layers of intestinal wall (apical/inner side)

• mucosa – contains villi with capillaries and lacteals inside

• submucosa – contains blood and lymph vessels

• circular muscle

• longitudinal muscle

• serosa

internal surface area of intestine is increased by…

• plicae (submucosal folds) - several millimeters in depth

• villi (~ 1 mm long) with the surface layer formed by enterocytes

• microvilli - brush border membrane

  (~ 1 micrometer tall); covers the apical side of enterocytes

stomach transit time (gastric emptying) depends on…

• temperature

• consistency of food

• digestibility of food

• volume

the stomach prevents the intestine from ‘nonphysiologic’ conditions

low volume of any consistency and composition of food results in…

prolonged stay in the stomach

liquid intake of standard volume (100 - 200 mL) with low nutrient content, hypotonic

-the stomach begins to empty immediately

-the rate at which the stomach empties is described as exponential. this means that the emptying follows a pattern where the remaining volume decreases by a constant proportion over a specific time period.

-the rate of emptying is directly proportional to the volume of the liquid ingested. in other words, larger volumes will empty at a faster rate.

-the time it takes for the stomach to empty (transit time) ranges from 5 to 20 minutes.

-taking a pill with a glass of water not only provides a dissolution medium for the pill but also facilitates quick gastric emptying

liquid intake of standard volume (100 - 200 mL) with high nutrient content, hypertonic, acidic

-result in slower and non-exponential emptying

-the nutrient content contributes to a more gradual and potentially irregular emptying pattern

-the stomach takes more time to process and release the contents into the small intestine.

soft meals

stay in stomach for 30-60 minutes and then the emptying is linear for 1 - 2 hours

chunky and fatty meals

have greatly prolonged transit time – up to 12 hours

solid dosage forms

needs to be disintegrated in stomach, and ideally the particles should be dissolved before entering small intestine

delay in emptying of the stomach will cause…

• delayed the drug absorption

• degradation of compounds unstable in acidic media (penicillins, cephalosporins)

• irritation of gastric mucosa (aspirin)

interdigestive or fasted state

-alternating cycles of

• ~1 hour quiescence

  • a state of rest or inactivity in the GI tract

• ~1 hour activity known as migrating motor complex (regular contractions with high amplitude housekeeper waves (4-5 per min), preceded and followed by irregular contractions)

digestive or fed state

• regular contractions:

• same frequency of housekeeper waves, however, the amplitude of these contractions is lower compared to the housekeeper waves, suggesting that the contractions are not as forceful

GI motility

-drugs need sufficient residence time in their absorption window for significant absorption.

• modified-release dosage forms have to release the drug before or in the absorption window

• the residence time can be increased or decreased in diseases with constipation and diarrhea, respectfully

residence time in the small intestine

~4-5 hours

residence time in the large intestine (colon)

~20 hours or more

number of membranes for a drug to pass

a series of membranes (~30 or more)

-microvilli: a part of the cell membrane of enterocytes

-endoplasmic reticulum inside the enterocytes (a series of membranes)

-the cell membrane at the distant (basolateral) end of enterocytes

- mucosal tissue (a series of membranes)

-wall of blood capillaries: for bilayer-crossing compounds (~ two membranes); other compounds cross through the fenestrae

variations along small intestine

• pH variation

• specific area affected by plicae, villi, and microvilli

• lengths and radii of individual segments

• the length of small intestine varies between 3 to 7 m, with average 5 m