1.2PHARMACOKINETICS, BLOODBRAIN BARRIER & MOLECULAR APPROACHES TO DRUG DELIVERY

Pharmacokinetics

study of how drugs are moved through the body (absorption, distribution, metabolism, excretion)

Absorption basic

drug enters systemic circulation

Distribution basic

moves between blood/ tissues

Metabolism basic

drug removed by enzymes

Excretion basic

drug removed from body as waste

How drugs move across membranes

most cross layers of cells

How do hydrophilic drugs move

paracellular transport

paracellular transport

move between space between cells

How do small, uncharged, lipid soluble molecules cross

passive lipid diffusion

Absorption

drug movement from the site of administration into the systemic circulation (whole-body blood supply)

What if a drug is administered directly into blood(intravenously)?

no absorption occurs

Types of drug administration routes

local, systemic, enteral,parenteral

Local drug administration

drug applied directly at intended site of action

Systemic drug administration

drug must enter the systemic circulation in order to get to site of action

can either be enteral or parenteral

parenteral

outside of the GI tract

such as subcutaneous and intravenous

enteral

along the GI tract

such as oral and rectal

Distribution

drug that has been absorbed into systemic circulation moves back and forth between blood and tissues

Importance of distribution

can bring drug to site of action, non-target sites, and clearing organs

Distribution rate

how rapidly (or slowly) the drug enters a tissue or space

Distribution extent

the amount of drug that “hangs out” in a tissue or space

Although rate and extent of distribution are different both can be influenced by:

Physicochemical properties of the drug that affect movement across membranes

Differing capacities of various tissues to interact with the drug

Rate of drug delivery to individual organs or spaces

Tissue perfusion

Blood vessel structure differs widely across various organs and tissues in terms of permeability to substrates

Types of blood vessels

Continuous capillaries

Fenestrated capillaries

Sinusoidal capillaries

Continuous capillaries

selectively let only a few small molecules through

Fenestrated capillaries

large opening between cells, for quick exchange of substrate

(intestine, endocrine gland)

Sinusoidal capillaries

large gaps allow extensive substrate exchange

liver

Tissue reservoir

a tissue that accumulates or retains drug preferentially

Accumulation in tissue reservoirs

may be due to high solubility, binding, ionization

Lipid solubility

Adipose (fat) is the most lipid-dense tissue

Brain is 2nd most lipid-rich; 50% of brain dry weight is lipid (e.g. myelin). In most other organs, lipid content is low

Binding to proteins

Drugs can bind onto proteins in blood or tissue.Drugs cannot cross membranes while bound, leading to accumulation on side of membrane with higher binding

BARRIERS TO DRUG DISTRIBUTION

Lymph nodes

Reproductive organs

Brain & spinal cord (blood-brain barrier)

Features of tissue barriers:

Vasculature

Transporters

Enzymes

Vasculature

tight junctions, basement membrane, no fenestra, wrapped with glia

Transporters

pump drug out

Enzymes

degrade drug

Drug-metabolizing enzymes catalyze a change in chemical structure

urning parent drug molecules into metabolites that are more hydrophilic, and thus more easily eliminated via excretion in urine or bile.

Many drugs are lipophilic

which enables movement across cell membranes and tissues. However, for this reason, lipophilic chemicals tend to be retained in the body.

What is the purpose of drug metabolism

to make drugs easier to excrete – the ability to rid the body of foreign substances (xenobiotics) is an important protective mechanism.

Changes to the chemical structure of a drug nearly always alter the biological activity of a drug as well (by changing the fit to its receptor)

Inactivation

Activation

Toxication

Inactivation

the most common effect of drug metabolism; most drugs are inactivated by metabolism

Activation

an already active drug can be metabolized into a product that is also active (“active metabolite”)

prodrug

molecule that is inactive until the body metabolizes it

Toxication

some drugs undergo toxication reactions (metabolism into a more toxic product)

Excretion

physically removing the drug or its metabolites to outside of the body

Kidney Excretion

major drug-excreting organ; drugs can be cleared from the body by excretion into urine or bile

Polar compounds and excretion

usually excreted more efficiently than substances with high lipid solubility

Lipophilic drugs are metabolized to more polar compounds

metabolites are more readily cleared by excretion

CNS is one of several sites in the body where exchange of chemicals with blood is limited or tightly regulated by

specialized endothelial or epithelial cells (termed blood-tissue barriers)

Why do we have a blood brain barrier?

Protected organs still need to take up some substances from blood (e.g., nutrients) and release waste into blood – so these are not impermeable barriers, but dynamic systems that tightly regulate chemical exchange between blood and tissue

pharmacokinetic perspective and BBB

regulates distribution; movement of chemicals (nutrients, waste, drugs, poisons) between tissue and blood

CNS VASCULATURE & BLOOD SUPPLY

Brain is heavily vascularized

Brain glucose utilization accounts for approximately 25% of whole-body glucose usage – varies by region

Brain capillaries

non-fenestrated (tightly regulates substrate flux between blood and brain tissue)

Brain Major blood supply comes from

internal carotid and vertebral arteries

To get from blood into brain, chemicals must move

(1) From blood into brain endothelial cells (BEC) then

(2) Out the other side of the BEC and into brain tissue

Routes into brain

Most substances enter brain tissue (parenchyma) by moving across blood vessels

paracellular diffusion in the brain

extremely rare (due to tight seals between cells); uptake of water-soluble molecules into brain is very limited

transcellular lipid diffusion

allows many small lipophilic and gaseous molecules to cross BEC into brain (most substances that enter brain enter by this route)

What if molecules dont use transcellular lipid diffusion to enter the brain?

use catalyzed transport

Transport proteins

which allow uptake of specific key nutrients and substrates

Transcytosis

receptor-mediated and adsorptive

which involve bulk packaging of many types of substrates into vesicles for delivery across BBB

CNS capillary structural features strictly limit entry of

fluids/water-soluble compounds into the brain

Physical/structural elements of the BBB

No fenestra (pores)

Few pinocytotic vesicles

A thick basement membrane

Pericytes (contractile cells that wrap around blood vessels)

Tight junctions

Microglia near vessels

Astrocytes

A net negative charge on brain endothelial cells

Glycocalyx

No fenestra (pores)

limits entry of small molecules and proteins

Few pinocytotic vesicles

minimizes entry of extracellular fluid, which contains sugars and proteins

Tight junctions

specialized structures that form a nearly-impenetrable seal between endothelial cells, preventing movement of ions and other water-soluble molecules between endothelial cells

Microglia near vessels

restrict pathogen entry into brain

Astrocytes

end-feet completely surround brain vessels; they can release chemicals that alter BBB permeability within seconds

Glycocalyx

a negatively Supporting cells within the brain parenchyma (tissue) also provide physical / structural barriers to transport between blood and braincharged layer of fibrous chains of macromolecules on the luminal surface of endothelial cells

Repels negatively charged molecules

A net negative charge on brain endothelial cells

limits entry of anions into brain

Pericytes (contractile cells that wrap around blood vessels)

these help to regulate vessel diameter & blood flow

Supporting cells within the brain parenchyma (tissue) also provide physical / structural barriers to transport between blood and brain

Microglia near vessels

Astrocytes

BIOCHEMICAL ELEMENTS OF THE BBB

Certain metabolizing enzymes are expressed (in limited amounts) in brain endothelial cells (BEC), astrocyte foot processes and microglia

Transporter proteins

Transporter proteins

the most important biochemical element of the BBB; these are expressed on Nature Rev. Drug BEC and other BBB structures

Certain metabolizing enzymes are expressed (in limited amounts) in brain endothelial cells (BEC), astrocyte foot processes and microglia

they metabolize substances before they can enter brain

Transport proteins

membrane-bound proteins that mediate the translocation of substrates across biological membranes

two main transporter superfamilies

ATP-binding cassette (ABC) superfamily and the solute carrier (SLC) superfamily.

ABC transporters

use energy from ATP and usually act as efflux transporters

SLC transporters

usually mediate uptake of small molecules into cells

Uptake transporters

bring molecules like glucose and certain amino acids into brain, and allow ion exchange

Efflux transporters

bind many chemicals that would otherwise be able to penetrate into brain, and pump them back into blood

Uptake transporter example

GLUT1 (glucose), and LAT1 (large neutral amino acid transporter)

Efflux transporters example

P-glycoprotein

Efflux transporters can also help clear waste products from brain

Efflux transporter example

organic anion transporters OATs

Transporters in the periphery

P-glycoprotein (an ATP-binding cassette or ABC transporter) and multi-drug resistance proteins (MRPs) also play major roles in limiting uptake of antineoplastic drugs into tumors and into portal vein from gut lumen

Organic anion transporters (OATs) are highly expressed in organs like kidney, where they clear drugs and waste from the body

P-gp

most important efflux transporter at the BBB; can nearly eliminate brain penetration of strong P-gp substrates

embedded in the membrane of brain endothelial cells, oriented directionally to pump out into blood

Uses energy (ATP) to move substrates (a large, diverse range of smaller lipophilic molecules) against their concentration gradient

Lipid-soluble drugs about to diffuse across the BEC membrane can be recognized by P-gp and actively transported out of the BEC, back into blood

P-GP SUBSTRATES AND INHIBITORS

include endogenous substances and drugs of diverse classes

transporters are functional proteins that can be induced or inhibited

Drugs can induce expression of P -gp, resulting in increased transporter activity

Drugs can inhibit P -gp’s function

Drugs can inhibit P -gp’s function example

cyclosporin

Drugs can induce expression of P -gp example

rifampin

CVO

circumventricular organs

Some structures near the ventricles(CVO)

have fenestrated capillaries, enabling certain chemo-sensory or secretory functions

CVOs with chemosensory functions, such as area postrema

enable the brain to respond to changes in blood chemistry

CVOs with secretory functions, such as the pituitary and pineal gland

enable a direct pathway for neuro-endocrine communication

Choroid plexus (CP)

layer of ependymal cells that produces CSF and clears waste from brain

Capillaries that supply CP

have few barrier features, like those in periphery

CP ependymal cells have most of the same features as BBB

tight junctions, transporters, enzymes (only a little leakier than the BBB)

Regional variability in BBB: CSF interface

Ependyma is the interface between brain tissue and CSF, made of a single layer of ependymal cells

Ependyma has incomplete tight junctions that allow some exchange between CSF and nearby brain/spinal cord tissue

Ependymal cells are ciliated glia that produce CSF and move it in one direction (away from brain surface) to clear waste

Regional variability in BBB: Nose-brain interface

Olfactory neurons extend through skull into nasal cavity

Interface allows selective rapid entry of a few specific substances into brain (faster than they can enter systemic circulation and cross BBB)

Possible routes from nose to brain

Absorption into systemic circulation

Retrograde neuronal transport

Poorly-understood “direct” route

Absorption into systemic circulation

(still has to cross BBB)