Lecture 2 - Pharmacokinetics

Question 1

Summarise drug movement and what affects it?

Answer 1

Bulk flow (blood, lymph, CSF) or diffusion (mol by mol, over short distances).
Transfer by bulk flow not affected by chemical nature of drug - lipid solubility is the main factor in determining diffusion (molecular weight is less important)

Question 2

Plasma membranes?

Answer 2

Amphipathic - 4 routes through which transfer can happen.

1. Passive diffusion
2. Diffusion through aquaporins
3. Carrier mediated transport
4. Vesicle mediated transport

GI mucosa, renal tubule, BBB, placenta - express solute carrier transporters than facilitate movement down EC gradient, and ATP-binding cassettes (ABC) for active transport.

Question 3

What is absorption? Routes of administration? Factors affecting absorption from gut?

Answer 3

• Passage of drug from site of administration into general/systemic circulation.
• Oral, sublingual, rectal, inhalation, application to epithelium, injection
• GI motility, GI pH, particle size and formulation, physiochemical factors, food (decreases rate)

Question 4

Oral and IV administration?

Answer 4

• 80% drugs oral - convenient, safe, economical. Bad if first pass metabolism or irritates gut. Absorbed by stomach –> portal –> liver for metab. –> general circulation.
• IV = fastest and has 100% bioavailability and immediate pharmacological effect.

Question 5

Lipinski’s rule of 5?

Answer 5

Qualitative concept that describes how ‘drug-like’ a substance is or determine if a chemical compound with certain biological/pharmacological activity has properties that would make it a likely orally active drug.

Membrane permeability more likely if:

• more than 5 H-bond donors
• Mol weight >500
• Octanol - water partition coefficient logp >5
• More than 10 H-bond acceptors (N/O atoms)

Candidate drugs that conform to this have lower rates of attrition in clinical trials. Increased chance of reaching market.

Question 6

pH and ionisation?

Answer 6

Many drugs are weak acids. Only the uncharged species can diffuse across membranes, leads to pH partition. Weak acids tend to accumulate in compartments of relatively increased pH, weak bases do the opposite.

Question 7

What is distribution

Answer 7

Process by which drug is transferred reversibly from circulation to tissues. Once drug enters systemic circulation, must be distributed into intestinal and intracellular fluids.

Question 8

Drug plasma protein binding? (distribution)

Answer 8

Four main body fluid compartments: plasma, interstitial, lymph (extracellular) and intracellular fluid.

Many drugs bind to circulating plasma proteins. Unbound (free form) is pharmacolgically active, can diffusr through membranes and bind to drug targets.
Equilibrium pattern between compartments depends on: permeability across tissue barriers, binding within compartments, pH partition, fat:water partition.

Lipid insoluble drugs are confined mainly to plasma and interstitial fluids (no brain). Lipid soluble go everywhere and accumulate in fat.

Special drug delivery systems can improve drug delvery and localise drug to target tissue - biologically erodible nanoparticles, prodrugs, antibody-drug conjugates, packaging in liposomes, coated implantable devices.

Question 9

Summarise metabolism?

Answer 9

Absorbed –> hepatic portal –> liver –> systemic circulation.

First pass = metabolism that happens before drug gets into systemic circulation.
First pass drugs = aspirin, GTN, levodopa, lidocaine, morphine, propanolol, salbutamol, verapamil.

Question 10

Summarise bioavailability?

Answer 10

% of drug that gets into systemic circulation.

Oral

Question 11

What are phase 1 reactions?

Answer 11

CATABOLIC - oxidation or reduction of drug - products are usually more chemically reactive than parent drug. P450 monooxygenase system. Introduce a reactive group (-OH) which serves as a point of attack for conjugation (phase 2).

Question 12

What are cytochrome P450 enzymes?

Answer 12

Haem proteins - superfamily of related but distinct enzymes. Membrane associated endoplasmic reticiulum; mitochondrial inner membrane. (75% of drugs).

Add one atom of Oxygen from O2 to form hydroxylated product.
Exhibit common polymorphisms at the genomic level that are associated with changes in drug effects.

Question 13

What are the three P450 enzymes that have common polymorphisms?

Answer 13

CYP2D6
CYP2C9
CYP2C19

Question 14

Summarise CYP2D6?

Answer 14

CODEINE, DEBRISOQUINE, S-METOPROLOL
- Involved in metabolism of about 25% of all medicines. Difficult to predict how a particular person will respond due to genetic variation (over 70 alleles).

• 10% caucasians have gen variant that decreases enzyme activity - POOR METABOLISERS
• 7% have copies arranged in tandem, metabolise very quickly - ULTRAFAST METABOLISERS

Psychiatrc - antidepressants, antipsychotics, analgesics, antitussives.
Cardio - Antiarrhythmics, beta blockers.

PRODRUGS - responsible for converting codeine to morphine (fetal morphine toxicity for rapid mothers, or inactive in poor metabolisers), and metabolising tamoxifen (poor metabolisers don’t respond as well, more likely to have recurrent cancer).

Question 15

Summarise CYP2C9?

Answer 15

IBUPROFEN, TOLBUTAMIDE, WARFARIN

• 10% poor metabolisers
• 1/4 people on warfarin have adverse drug reactions, leading to transfusions and hospitalisation.

Question 16

Summarise CYP2C19?

Answer 16

OMEPRAZOLE, PHENYTOIN, DIAZEPAM

• 3% poor metabolisers (caucasian), 20% in asians.
• Antipsychotics, antiepileptics, antimalarial, anaesthetics.

Question 17

What will happen to prodrugs in poor/ultrafast metabolisers?

Answer 17

Poor - converted to active drug at a slower rate, lower efficacy and higher dose needed to achieve therapeutic effect.

Ultrafast - prone to toxicity, accumulate active drug at a greater rate.

Question 18

What are phase 2 reactions?

Answer 18

ANABOLIC - involve conjugation (attachment of a substituent group) –> inactive products.
If drug molecule from phase 1 has suitable ‘handle’, it is susceptible to conjugation.

Conjugation reactions = GLUCURONIDATION, sulphation, acetylation, amino acid conjugation, gluathione conjugation, fatty acid conjugation.

Question 19

Most important phase 2 reaction?

Answer 19

GLUCURONIDATION

Involves formation of high energy phosphate compound; uridine diphosphate glucuronic acid (UDPGA), from whch GA is transferred an electron rich atom on the substrate, forming an amide, ester or thiol bond, catalysed by UDP-GLUCONORYL TRANSFERASE - broad specificity, many drugs and molecules.

Question 20

Other phase 2 reactions

Answer 20

Acetylation (+ acetyl CoA) and methylation (+ S-adenosyl methionine) - donor compounds. Many occur in liver but can be in lung/kidney.

Question 21

What are conjugated metabolites like in comparison to active?

Answer 21

Polar and less soluble and less pharmacologically active. Excreted in urine.
Some are excreted in bile and reactivated in intestine and then reabsorbed (enterohepatic circulation).

Question 22

Active metabolites - prodrugs?

Answer 22

Enalapril –> enalaprilat (active met) - dicarboxylate-containing ACEi with SH-moeity replaced by -COO moeity. Only for IV due to poor oral BioA.

Developed because CAPTOPRIL was shite. Not a prodrug. Poor pharmacokinetic profile, short half life = 2/3 a day dosing –> poor compliance. Only 70% oral drug absorbed and BioA reduced by food in stomach.

Question 23

Toxic metabolites - paracetamol

Answer 23

Hepatotoxicity after paracetamol = common cause of death.
Metabolised by 3 pathways - with toxic doses, enzymes catalysing normal conjugation reactions are saturated ad mixed-function oxidases convert the drug to the active metabolite - N-acetyl-p-benzoquinoneimine (NAPQ) - which is toxic and can cause liver damage (CYP2E1 and CYP3A4) - can be quickly detoxified by conjugation with GSH.

Increased in patents in whom P450 enzymes have been induced.
Risk increased in patients with gluathione reserves as a result of genetic variation, HIV+, malnutrition or liver disease.

Question 24

Other types of toxic metabolites?

Answer 24

Bladder toxicity by cyclophosphamide (toxic met = acrolein)
Methanol and ethylene - toxic mets formed by alcohol dehydrogenase. Poisoning treated with ethanol which competes for active site.

Question 25

Inducers of CYP450 enzymes?

Answer 25

Phenytoin, phenobarbitone, carbamezapine, rifampicin, griseofulvin, st john’s wort, alcohol (chronic), smoking

Question 26

Inhibitors of CYP450 enzymes?

Answer 26

Erythromycin, ketoconazole, sulphonamides, cimetidine, omeprazole, chloramphenicol, alcohol (acute), oral contraceptives.

Question 27

CYP1A1 expression mediated through a specific cytosolic receptor - Aryl Hydrocarbon Receptor (AHR)…

Answer 27

AHR = part of cytosolic complex protein. In presence of exogenous ligand BENZO[a]PYRENE or industrial product TCDD, complex translocates to nucleus where it heterodimerises to another protein (ARYL HYDROCARBON NUCLEAR TRANSLATOR - ARNT).
Binds to consensus