Pharmacokinetics 2

Question 1

1. What is drug distribution?

Answer 1

This is the process by which drugs exit the blood stream and enter in to the extracellular fluid and/or tissues. The rate of drug delivery is dependent on the cardiac output, regional blood flow, capillary permeability, tissue volume, degree of binding of drugs to plasma and tissue proteins (most important!!) and hydrophobicity of the drug. Drug distribution to the muscle, viscera, skin and fat is slower compared to other well-perfused organs (liver, kidney, brain).

Question 2

2. What factors influence drug distribution?

Answer 2

1. cardiac output
2. regional blood flow
3. capillary permeability
4. tissue volume
5. degree of binding of drugs to plasma and tissue proteins
6. hydrophobicity of drug

Question 3

3. What is the most abundant plasma protein for acidic and basic drugs?

Answer 3

Acidic drugs → plasma albumin
Basic drugs → a1-acid glycoprotein
*only unbound drugs are pharmacologically active

Question 4

4. Why is Warfarin drug toxicity worrisome?

Answer 4

Warfarin has a very small therapeutic index and is usually highly bound to albumin within the blood. If another drug (ex. sulfonamide) is given, albumin may be displaced from the warfarin and bind to the sulfonamide increasing the amt of free warfarin in the blood. This increases the risk of bleeding within the patient.

Question 5

5. How may sulfonamide treatment lead to bilirubin encephalopathy?

Answer 5

When treating with sulfonamides, it may bind to albumin and displace unconjugated bilirubin. The release of unconjugated bilirubin may increase the risk of bilirubin encephalopathy in newborns (aka kernicterus).

Question 6

6. What is the effect of tissue binding of drugs?

Answer 6

Many drugs accumulate in tissues at high concentration due to active transport and binding. Tissue binding occurs on the cellular level with proteins, phospholipids and nuclear proteins, and is reversible. A large fraction of drug in the body end up being bound in this manner leading to drug accumulation and possible local toxicity. Ex. antibiotic gentamicin accumulates in kidney and vestibular system
Liposoluble drugs are stored in fat thereby serving as its reservoir. Ex. 70% of barbiturate thiopental is present in fat 3 hours post administration possibly creating no therapeutic/anesthetic effect

Question 7

7. What is the structure of the BBB?

Answer 7

Endothelial cells of brain capillaries with tight junctions and no intercellular pores or pinocytic vesicles. Astrocytic “end feet” surround the capillary preventing movement. Movement in to the capillaries depends on transcellular movement rather than paracellular. The more liposoluble, the more likely it is the cross the BBB.

Question 8

8. What is the role of P-glycoprotein and the L-transport system within the BBB?

Answer 8

P-glycoprotein is a transmembrane transporter that removes a lot of drugs from the cells shuttling them out of the brain. The L-transport system is an active/facilitative transporter for neutral amino acids.

Question 9

9. Can strongly ionized agents enter the CNS from circulation?

Answer 9

No, and an example are quarternary ammonium compounds

Question 10

10. How can the BBB be bypassed to direct drugs to the brain?

Answer 10

Intrathecal drug infusion – direct delivery to brain

Question 11

12. What are the two major routes of drug elimination?

Answer 11

1. Metabolism in liver

2. excretion as unchanged drugs in kidneys

Question 12

13. What is the general biotransformation reaction of drugs for elimination?

Answer 12

Most drugs are lipophilic which allows them to easily pass through cell membranes, but not be excreted because they end up being reabsorbed in the tubular membrane of the kidneys. Therefore, the lipophilic drugs need to be made more polar to prevent reabsorption and become an inactive metabolic to prevent continued functioning.

Question 13

14. How does drug transformation alter drugs?

Answer 13

1. active drug converted to inactive drug
2. unexcretable drug converted to excretable metabolite
3. active drug converted to active or toxic metabolite
4. inactive prodrug converted to active drug

Question 14

15. What are the reactions of drug metabolism?

Answer 14

Phase I → oxidations, reductions, decarboxylations, deaminations, and hydrolytic reactions to make the drug more polar by adding a functional group that may then be used in phase II reactions – reaction usually makes an inactive metabolite but in the case of a prodrug consumption, it may make it an active metabolite
Phase II → conjugation reactions to form covalent bonds b/t functional group and glucuronate, acetate, glutathione, AA, or sulfate

Question 15

16. How are prodrugs usually altered?

Answer 15

Hydrolysis of an ester or amide linkage – ex. cyclophosphamide which is an inactive drug turns in to an active anticancer metabolite via metabolism

Question 16

17. What is an exception to phase II metabolism where the metabolite is not inactive?

Answer 16

Morphine-6-glucuronide which is a more potent analgesic

Question 17

11. How do drugs cross the placenta?

Answer 17

Simple diffusion of liposoluble, nonionized drugs

Question 18

18. Give an example of a drug where phase II precedes phase I.

Answer 18

Isoniazid is acetylated by N-acetyltransferase then hydrolysed to isonicotinic acid

Question 19

19. What is first-pass metabolism?

Answer 19

Metabolism in the liver prior to reaching circulation therefore decreases bioavailability of orally administered drugs. 1st pass metabolism occurs in the liver and GI tract.

Question 20

20. Where does drug metabolism occur?

Answer 20

Liver (primary site), skin, lungs, GI tract, kidneys

Question 21

21. Where is the cell does drug metabolism occur?

Answer 21

Phase I → ER
Phase II → cytosol
Can also occur in the mitochondria, nuclear envelope and plasma membrane

Question 22

22. What is the main enzyme that catalyzes phase I reactions?

Answer 22

Oxidases such as heme protein mono-oxygenases of the cytochrome P450 class. Cytochrome P450is a superfamily of heme proteins that fx as microsomal mixed-function oxidases. Humans have 18 families of cytochrome P450 genes diving in to 43 subfamilies.

Question 23

23. What are the most important human liver isoforms of cytochrome P450?

Answer 23

CYP1A2, CYP2A6, CYP2C9, CYP2D6, CYP3A4

• CYP3A4 metabolizes approximately 30% of therapeutic drugs and consists of 30% of all isoforms.
• CYP1-3 are most active in metabolism of xenobiotics where as other families are involved with endogenous fx

Question 24

24. What is P450 enzyme induction?

Answer 24

Some P450 enzymes are constitutively on, but others need to be induced to increase their expression. Induction generally occurs via increased transcription (primarily), increased translation, or decreased degradation. Binding to the xenobiotic receptors in the hepatocytes causes its translocation to the nucleus and binding of promoters to various enzymes

Question 25

25. Give examples of inducers, receptors and the gene targets.

Answer 25

Inducer → receptor → gene target
PAHs, TCDD → AhR → CYP1A1, CYP1A2, CYP1B1
Sterioids, hyperforin, rifampin, phenobarbital, mifepristone → PXR → CYP3A4, CYP3A7
Phenobarbital, phenytoin → CAR → CYP2B6, CYP3A4

Question 26

26. What is the result of the cytochrome P450 induction?

Answer 26

Phenobarbital, rifampin and carbamazepine all increase number P450 isoforms leading to increase the drugs own metabolism (ex. carbamazepine via CYP3A4), increasing metabolism of coadministered drugs reducing plasma concentration below therapeutic levels, and inducing cytochromes P450 creating toxic level of metabolites with tissue damage.

Question 27

27. What are the major effects of enzyme inhibition?

Answer 27

1. Allow drug levels to reach toxic concentrations

2. Prolong presence of active drug in body

Question 28

28. Which drugs significantly inhibit P450?

Answer 28

Amiodarone, cimetidine, azole antifungal (ex. ketoconazole), macrolide antibiotics (ex. erythromycin), chroramphenicol, HIV protease inhibitor ritonavir, furanocoumarins in grapefruit juice

Question 29

29. When is the inhibition of P450 enzymes used for a therapeutic advantage?

Answer 29

Protease inhibitor lopinavir has severe 1st pass metabolism, therefore when administered with a CYP3A4 inhibitor ritonavir, lopinavir can reach therapeutic concentrations

Question 30

30. How does the P-glycoprotein (MDR1) effect the bioavailability of drugs?

Answer 30

Decreases bioavailability esp with digoxin and HIV-1 protease inhibitor because the drugs are actively transported back into the intestinal lumen. P-glycoproteins can be induced and inhibited by drugs. Ex. macrolide antibiotics inhibit the transporter therefore increasing serum levels of digoxin.

Question 31

31. How is P-glycoprotein transcriptionally regulated?

Answer 31

Via PXR, drugs that induce P45- enzymes via the PXR pathway concomitantly increase transcription of the P-glycoprotein drug transporter

Question 32

32. Discuss the toxic metabolism with large doses of acetaminophen.

Answer 32

Post ingestion of acetaminophen, 95% is glucoronylated or sulfated and excreted by the kidneys. About 5% is metabolized by CYP2E1 to yield N-acetyl-p-benzoquinoneimine (NAPQI) which is toxic to the liver. When normal levels of acetaminophen are ingested, NAPQI can be conjugated to glutathione which detoxifies it. When large doses of acetaminophen are consumed, the amt of glutathione is depleted increasing the amt of unconjugated NAPQI causing cell damage and hepatocyte death.

Question 33

33. What is the antidote for acetaminophen poisoning?

Answer 33

N-acetylcysteine which is a precursor for glutathione therefore increases its production and it also directly interacts/detoxifies NAPQI

Question 34

34. What individual factors affect drug metabolism?

Answer 34

1. genetic factors – pharmacokinetic variation (drug metabolism or transport), pharmacodynamics variation (drug target variation), variation with idiosyncratic drug effect
2. diet and environmental factors – grapefruit juice inhibits CYP3A4 and P-glycoprotein in small intestines, charcoal-broiled foods induce CYP1A, cigarette smoke induces AhR-mediated P450
3. age – reactions slowed in young children and elderly
4. disease affecting drug metabolism – fat accumulation, alcoholic hepatitis, acute or inactive alcoholic cirrhosis, hemochromatosis, chronic active hepatitis, biliary cirrhosis,
5. drug-drug interactions during metabolism

Question 35

35. What are the different forms of drug excretion?

Answer 35

1. renal (most common)
2. bile (small number of drugs)
3. fecal (with orally administered drugs that are incompletely absorbed from GI tract)
4. breast milk
5. lungs (esp with elimination of anesthetic gases)
6. sweat, saliva and tears
7. hair and skin

Question 36

36. What are the 3 basic processes involved in renal excretion of drugs?

Answer 36

1. glomerular filtration – free drug with MW less than 20,000 diffuse easily (not influenced by liposolubility and pH)
2. active tubular secretion – non-selective active transport from peritubular capillaries in proximal tubule for organic anions and organic cations
3. tubular reabsorption – high liposolubility drugs are excreted slowly b/c they are reabsorbed, highly polar and filtered drugs remain in tubules to be excreted (passive reasbsorption)

Question 37

37. Discuss biliary and fecal excretion.

Answer 37

Drug metabolites move from liver → bile via ATP binding cassettes (ABC) which are then excreted in the feces or reabsorbed in to blood via glucuronide conjugation in the intestines and excreted in urine. Ultimately biliary excretion is only the route of elimination if the drug cannot be reabsorbed.