Pharmacology - Pharmacokinetics, Pharmacogenomics, and Membrane Transporters

What is drug absorption?

The movement of drugs into the bloodstream (central compartment) from the site of delivery

What are some factors that effect absorption?

Physiochemical properties of the drug:

Molecular size and shape

Degree of ionization

Lipid solubility

What must most drugs do in order to reach the central compartment and site of action?

Cross the lipid bilayer of cell membranes

What are some types of passive transport?

Paracellular transport

Diffusion

Facilitated diffusion

With concentration gradient

What is primary active transport?

Energy for transport across membranes supplied by ATP hydrolysis (ATPase)

What are some examples of primary active transport?

ABC transporters - move drugs against concentration gradient

Na+, K+ -ATPase - moves ions against concentration gradient

What affects absorption ability?

State of ionization - charged forms do not cross as easily, uncharged forms penetrate membranes

Distribution across membranes depends on pKa of drug and pH on either side of membrane

What is bioavailability?

The fractional extent to which a dose of drug reaches the systemic circulation

What factors influence bioavailability?

Route of administration, absorption (impact of pH and pKa), metabolism and/or excretion

What is area under the curve (AUC)?

Overall amount of drug in blood after a given dose (over time). Measures drug exposure

What are some characteristics of oral routes of administration?

Easiest route of administration, usually safe

Variable absorption

Subject to metabolism and patient compliance

What can be done to increase rate of absorption of oral medications?

Accelerate gastric emptying

What results from controlled-release medications?

Reduced frequency of administration

More uniform blood levels

Reduces peaks and eliminates troughs

What is a limitation of controlled-release preparations?

Inter-patient variability in systemic drug concentrations - dosage form may fail.

How are sublingual administrations absorbed?

Through the oral mucosa (protects drugs from first-pass metabolism)

What is the function of transdermal absorption?

Intact skin provides a lipid barrier for transdermal absorption

What are some characteristics of rectal routes of administration?

Is useful when oral ingestion is precluded, or patient is unconscious or vomiting

Absorption may be irregular and incomplete

Can cause irritation of rectal mucosa

First-pass metabolism avoided

What are some different types of parenteral administrations?

Intravenous, subcutaneous, intramuscular, intraarterial,

intrathecal

What are some characteristics of intravenous injections?

Causes immediate effects, can be useful in emergencies, not subject to first-pass metabolism, adjustment of dosage possible

Absorption is circumvented by direct injection of drug

Slow injections usually required. Is difficult to go back once injected

What are some characteristics of subcutaneous injections?

Is useful for poorly soluble suspensions and depot formulations

Aqueous dosage has prompt absorption, slower from extended-release formulations

Not suitable for large volumes, potential pain or necrosis at injection site

What are some characteristics of intramuscular injections?

Suitable for irritating substances, can be self-administered (ex. insulin), not subject to first pass metabolism in the liver

Absorbed faster from aqueous solutions, slower from depot formulations

Precluded during anticoagulant therapy, variability due to local heating, exercise, levels of subcutaneous fat

What are some characteristics of intraarterial injections?

Diagnostic agents, chemotherapy

Absorption is circumvented to deliver drug directly to tissue or organ, useful for treating liver tumors

Is reserved for experts

What are some characteristics of intrathecal injections?

Is an injection into space surrounding spinal cord

Useful for drugs that cannot cross the blood brain barrier of limiting effect to spinal cord or meninges

Absorption is circumvented

Expertise required

What are some characteristics of pulmonary absorption?

Inhalation - absorption through pulmonary epithelium and mucous membranes of respiratory tract, rapid access to circulation

Avoids first-pass hepatic metabolism, direct application of drug to target tissue, treatment of asthma, allergic rhinitis

Relatively few limitations, bacterial contamination of devices

What are some characteristics of topical application?

Mucous membranes - mainly for topical or local action, but occasionally for systemic effects

Eye - local effects administered as drops, suspensions, ointments, or inserts

Inserts - provide continuous delivery of drugs such as pilocarpine, a muscarinic agonist used in the treatment of glaucoma

What is bioequivalence?

Pharmaceutical equivalent drugs that also have similar rates and extent of bioavailability of the active pharmaceutical ingredient (API)

What is drug distribution?

The extent to which drugs reach potential target sites in the body. Depends on cardiac output, blood flow, capillary permeability, tissue volume

What are the two phases of drug distribution?

First phase - well-perfused organs: brain, lungs, liver, kidney

Second phase: muscle, viscera, fat

What determines drug distribution?

Partitioning of drug between bloodstream and tissues

How does protein binding limit drug availability and glomerular filtration?

Only unbound drug can cross membranes

Rapid equilibration occurs due to renal tubular secretion and biotransformation of free drug

How do drugs react with tissues?

Some drugs accumulate in tissues by active transport or binding to tissue proteins

Lipid soluble drugs may be stored in body fat

How do some drugs react with bone?

Some drugs such as tetracycline and aminoglycoside antibiotics accumulate in bone by adsorption onto the bone surface and later the crystal lattice

What is the blood-brain barrier composed of? What determines regulation of distribution of chemicals to the brain?

Composed of capillary endothelial cells (tight junctions between cells)

Distribution of chemicals to the brain is determined by:

lipid solubility

charge (ionization)

size

active transport for key amino acids

What are some membrane transporter?

Efflux transporters (moving away from brain) : P-glycoprotein (P-gp, ABCB1)

Influx transporters (moving into the brain):

organic anion-transporting polypeptide (OATP2, SLC21A3)

What is the function of the blood brain barrier?

Tight junctions of capillary endothelial cells in the brain prevent the passage of drugs and other substances into the brain

What are supporting cells in the blood-brain barrier?

Astrocytes, microglia

Supporting glial cells also help limit drug access to the brain

What is cerebrospinal fluid?

A barrier similar to the blood-brain barrier at the choroid plexus with epithelial cells separating blood from the CSF. Consists of tight junctions and efflux transporters.

Drugs also leave CNS via the arachnoid villi

How does the placenta react with drugs?

Drugs can cross the placenta causing potential problems for the developing fetus. Drug transfer across placenta determined by: lipid solubility, plasma binding, and ionization

What is biotransformation?

Drug metabolism - the chemical modification of drug molecules

What are some characteristics of metabolism?

Liver microsomal enzymes involved in drug metabolism

Metabolism increases water solubility of lipophilic drugs

Involved metabolites (can be active or inactive)

What is involved in phase 1 functionalization reactions?

Introduce or expose functional groups (ex. hydrolysis)

Result in both active and inactive compounds

Net result is water-soluble compounds that can be easily excreted into urine

ex. cytochrome P450

What is involved in phase 2 biosynthetic (conjugation) reactions?

Covalent linkage between functional groups on drug or metabolite and glucuronic acid, sulfate gluthathione, etc.

Polar conjugates are usually inactive and excreted rapidly

ex. UDP-glucuronosyl transferases, sulfotransferases

What are prodrugs?

Drugs that are given in an inactive state and metabolized by enzymes into pharmacologically active compounds

ex. ACE inhibitors for hypertension

enalapril is a prodrug that is converted to enalaprat, the active compound

What are the characteristics of first order metabolism kinetics?

Drug metabolism proportional to plasma concentration

Constant fraction of drug metabolized per unit time

What are the characteristics of zero order metabolism kinetics?

Limited metabolic capacity

Constant amount of drug metabolized per unit time

What processes are involved in renal excretion?

Glomerular filtration

Active tubular secretion

Active tubular reabsorption

Passive tubular reabsorption

What is biliary excretion?

Active transport of drugs and metabolites into bile

What is volume of distribution?

Apparent space of body available to contain a drug based on amount given and the concentration in the systemic circulation

What is clearence?

A measure of the body's efficiency in eliminating a drug from the systemic circulation

What is elimination half-life?

A measure of the rate of removal of a drug from the systemic circulation

What is steady state concentration?

The concentration achieved when intake of drug is in dynamic equilibrium with elimination of drug

(dosing rate equals elimination rate)

What determines hepatic clearance?

Hepatic blood flow

ex. diltiazem, imipramine, morphine, propranolol

What determines renal clearance?

Glomerular filtration, secretion, reabsorption, and glomerular blood flow

What is clearance?

A measure of the body's efficiency in eliminating a drug from the systemic circulation

What is pharmacogenetics?

Study of the basis for variation in drug responses

What is pharmacogenomics?

General study of all of the many different genes that determine drug behavior

What are the approaches to pharmacogenetics?

Phenotype to genotype - identify outliers in responses to drugs, search for pharmacological basis, identify families with inherited traits

Genotype to phenotype - takes advantage of explosion in genomic technologies, genomic polymorphisms serve as the starting point to examine impact of genetic variation on drug responses

What are polymorphisms?

Genetic variations occurring at a frequency of at least 1% in the human population

What are the definitions for autosomal, X-linked, exon, and intron?

Autosomal - pertaining to non-sex hormones

X-linked - mutations or genes on the X chromosome

Exon - segment of gene containing coding information

Intron - segment of gene that does not code for protein and interrupts gene sequnce

What are the inheritance patterns?

Dominant - inherited trait from one parent

Recessive - inherited trait from both parents

Codominant - two different alleles can be expressed with mixed phenotypic patterns

What are some polymorphism forms? What can they impact?

May be promoter, coding region, noncoding region

Impact may be on protein structure, function or pattern of expression

What are some characteristics of single nucleotide polymorphisms (SNPs)?

May change coding for proteins or impact expression

Insertions/deletions (indels)

Copy number variations - gene duplications or deletions

What are the different types of coding regions SNPs (cSNPs)?

Non-synonymous (missense)

Synonymous (sense)

What are non-synonymous cSNPs?

Change impacts protein sequence

Amino acid substitution or stop translation

What are synonymous cSNPs?

Change does not impact protein sequence

May impact expression

What are some characteristics of non-coding SNPs?

3' or 5' untranslated regions - promoter or enhancement regions, may regulate translation of proteins

Intronic regions - may impact splicing

Intergenic regions

What results from copy number variations?

Variable number of genes - result in genomic rearrangement (duplications, deletions, inversions)

What is an example of a copy number variation?

Gene duplications of CYP2D6

What does warfarin metabolized by?

CYP2C9

Which enzyme metabolizes about 20-25% of drugs?

CYP2D6

Which P-glycoprotein is involved with cSNPs?

ABCB1

What is the role of membrane transporters?

Regulate influx of ions and nutrients and efflux of cellular byproducts, environmental toxins and xenobiotics (foreign substances, including drugs)

Approximately how many transporter genes are in the human genome?

2000

What are the two major superfamilies of drug transporters?

ATP binding cassette (ABC) transporters

Solute carrier (SL) transporters

What are some characteristics of ABC transporters?

Active transport, require ATP for pumping substrates

Seven subclasses

Ex. P-glycoprotein (ABCB1), cystic fibrosis transmembrane regulator (CFTR, ABCC7)

What are some characteristics of solute carrier transporters?

Facilitated and active transporters

43 families with about 300 transporters

Ex. seratonin and dopamine transporters (SERT, DAT)

What are some examples of passive and active transport?

Passive - diffusion, facilitated diffusion (SLC transporters)

Active - primary active transport (ABC transporters), secondary active transport (SLC co-transporters)

What are some characteristics of passive diffusion?

Relative solubility in membrane vs. aqueous environment

Electrochemical potential gradient

What are some characteristics of facilitated diffusion?

Membrane SLC transporters facilitate diffusion

Does not require energy

What is the difference between primary and secondary active transport?

Primary - ABC transporters hydrolyze ATP through intrinsic ATP activity

Secondary - SLC transporters move one solute against ECG by transporting another solute with ECG

What is vectorial transport?

Asymmetrical transport across a layer of cells (magnitude and direction)

SLC transporters mediate drug uptake and efflux

ABC transporters mediate undirectional efflux

What is the role of type II nuclear receptors? What is their mechanism of action?

Form heterdimers with 9-cis-retinoic acid receptors (RXR)

1. Ligand binding and formation of heterodimers

2. Binding to specific elements in enhancer regions of target genes

3. Changes in transcription of transporters and other proteins

What are some structural characteristics of ABC transporters?

Span multiple membranes with domains for ATP, Mg2+, and substrate binding. Form dimers fro activity

Substrates interact with cytoplasmic domains with ATP bound to nucleotide binding domains (NBDs) - ATP hydrolysis produces steric changes that translocate substrates to exterior surface

What are some structural characteristics of SLC transporters?

Membrane-spanning - 12 putative transmembrane domains

Include channels, facilitators, and secondary active transporters

Mechanism is a gated pore with interchangeable binding sites

What are multidrug resistant-associated proteins?

Multi-gene family of transporters

What are the roles of ABCC1 (MRP1) and ABCC2 (MRP2)?

Substrates are generally organic anions

Conjugates of bile salts

Ex. vinca alkaloids, anthracyclines, folate

What are the roles of ABCC3, ABCC4, and ABCC5? (MRP 3,4,5)

MRP3 - substrates inclue monovalent bile salts (ex. cholate, glycocholate)

MRP4 and MRP5 - important for HIV drugs

What is the role of ABCG2 (BCRP)?

Breast cancer resistance protein

Expressed in liver, small intestine, colon, placenta

review membrane transporters!

What enzymes are involved with phase 1 pathways?

Cytochrome P450

Flavin-containing monoxygenases

Epoxide hydrolases

Add or remove functional groups to modestly increase hydrophilicity, provide sites for subsequent conjugation reactions, and possibly inactivate drugs

What are some enzymes involved with phase 2 pathways?

Transferases

Aid in conjugation reactions

What are the reactions involved in phase 1?

Oxidation and hydrolysis

What is the main reaction of phase 2?

Conjugation

Where does drug metabolism occur? What may alter drug metabolism?

Mostly the GI tract, some in nasal mucosa and lungs

Route of administration may alter drug metabolism

What enzyme does grapefruit inhibit? Which drug is an inducer for this enzyme?

CYP3A4 - atorvastatin

Which drug resulted in arrhythmias due to its interactions with CYP3A4 enzymes?

terfenadine (Seldane)

Which enzyme polymorphism has led to the withdrawal of the drugs debrisoquine and perhexiline?

CYP2D6

What are the two forms of hydrolytic enzymes? What is their purpose?

Soluble epoxide hydrolases (EH) in cytosol

Microsomal EH in ER

Deactivate reactive epoxides that can bind to protein, RNA or DNA and produce toxicity or transformation

Which enzyme is important for glucouronidation and thus clearance of bilirubin?

UGT1A1