term test 2 Flashcards
1
Q
Describe the steps before writing a prescription.
A
- Assess patient + decide if drug therapy is required
- Choose the best drug therapy after consideration.
- Choose the most appropriate route of administration.
- Determine appropriate dosage for the patient. (dose+frequency)
- Decide if follow-up or monitoring is required.
2
Q
Drug administration general steps.
A
- Figure out your drug target (receptor) and agent.
2. Figure out how to get your desired drug to your target.
3
Q
How is a dosage determined?
A
Follow recommended guidelines and consider patient factors.
propertied of the drug+target drug concentration from lit and use a formula
4
Q
How do you identify the target concentration?
A
Based on published literature or product monograph
+ Patient’s sign and symptoms or a surrogate marker
(recommended drug dosage is based on a target concentration)
5
Q
Does the plasma drug concentrations indicate the drug concentration at the site of action or the action of the drug?
A
No, not always.
6
Q
Define dose
A
Amount of drug
7
Q
Define dosing interval (τ)
A
Time between doses or frequency of dosing
8
Q
Define dosage
A
Amount of drug to be administered in a defined time interval
9
Q
Formula for calculating concentrations, define variables
A
Ct = C0e^(-ket)
or ln C = ln C0 – ket
t= time Ct = plasma concentration at given time C0 = initial plasma concentration ke = elimination rate constant
10
Q
Formula for calculating a single dose.
A
Dose = (target drug concentration)(Vd)
if rearranging for Vd use C0 instead of Cplasma
11
Q
Assumptions made about a single dose.
A
- all of the dose is distributed before any chance of elimination
- use initial concentration because drug will be eliminated over time
12
Q
Formula for single dose through oral administration.
A
Dose = (C0)(Vd)/F
F - bioavailability
13
Q
What is steady-state plasma concentration?
A
The concentration at which the rate of drug entry is equal to the rate of drug exit. Looks like a plateau on a graph
IN=OUT
14
Q
How long does steady-state plasma concentration take to achieve?
A
5 half-lives
15
Q
Formula for dosing for constant infusions.
A
Css = Q/Cl
steady-state plasma concentration = rate of infusion / rate of clearance
16
Q
What is a loading dose?
A
A loading dose is an initial does given to reach steady state faster than 5 half-lives.
17
Q
Formula for loading dose.
A
loading dose = (Css)(Vd)
18
Q
Formula for repeated IV dosing
A
Css = Dm / (Cl)(Tm)
Dm = maintenance dose Tm = time between doses
19
Q
Formula for repeated oral dosing.
A
Css = (Dm)(F) / (Cl)(Tm) OR = (Dm)(F)/(ke)(Vd)(Tm)
20
Q
Formula for ORAL loading dose
A
LD= (C)(Vd)/F
21
Q
What is considered when determine the maintenance dose and interval?
A
- available products
- safety
- convenience of drug administration
- concentrations fluctuate more if interval is less frequent
- dose is proportional to interval
22
Q
What is the common maintenance interval?
A
the half-life
23
Q
What is bioequivalence?
A
Comparable plasma concentration versus time profiles between drug products
24
Q
What is therapeutic equivalence?
A
Comparable clinical effectiveness (and safety) between related drug products
25
How do you achieve bioequivalence?
- bioavailability must be the same
- Cmax and AUC must have no more than a 20% difference between drugs
- confidence intervals must be within 80-125% of the original drug
26
List some factors affecting drug dosing.
age, body mass, sex, adherence, genetics, administration, tolerance, psychological factors, diet, cost, immune responses, etc.
27
What are the special populations we must consider in drug dosing?
Breastfeeding and pregnant women, infant/children, elderly, the very ill and those with genetic variants of drug targets, enzymes, transporters, etc.
28
What are current prescribing issues?
- inappropriate prescribing (e.g wrong drug)
- over-prescribing
- under-prescribing
29
How do we balance and meausure safety and efficacy of dosing?
Efficacy is based on population average
Toxicity - individual risk
Prioritize the type, quantity, quality of evidence , and time to both
30
Define drug biotransformation.
Conversion of a foreign chemical from one form into another within a living organism, usually be an enzyme-catalyzed rxn
31
How are the drug metabolism system and immune system analogous?
Both are meant to protect the organism from foreign substances, but they can be harmful if not well-regulated and well-balanced.
ex. transformation into a more active/dangerous form
32
What are the consequences of drug biotransformation?
- production of stable metabolites that have DECREASED, increased, or maintained activity
- production of chemically reactive metabolites
33
Example of biotransformation that decreases pharmacologic activity.
Acetaminophen (active parent drug)
- by UDP-glucuronosyl-transferase becomes Acetaminophen glucuronide (inactive)
- by phenolsulfotransferase becomes Acetaminophen sulfate (inactive)
34
Example of biotransformation of a pro-drug into an active metabolite (increases pharmacologic activity)
Cyclophosphamide (nitrogen mustards)
- with CYP3A4 and CYP2B6, stable to travel through body
- in tumor becomes phosphoramide mustard which is the main cytotoxic metabolite that acts to kill cancer cells
35
Example of biotransformation that maintains pharmacologic activity.
Propafenone (anti-arrhythmic drug)
- add O becomes 5-hydroxypropafenone (anti-arrhythmic drug)
- remove propyl in chain becomes N-Depropylpropafenone
36
What happens with chemically-reactive metabolites?
They covalently bind to cellular macromolecules and have toxic consequences including hepatic toxicity, carcinogenic, teratogenicity, etc.
37
Example of biotransformation into chemically-reactive metabolite
Aromatic anticonvulsant drugs (used to treat seizures) ex.phenytoin
- mediated by cytochrome P450
- arene oxide is formed, covalently binds cellular macromolecule
body now recognizes drug-macromolecule complex as foreign - immune response
which can lead to rejection of own skin or liver
38
Where does biotransformation take place?
almost every tissue but mostly the LIVER especially for oral administration
GI tract, lungs, kidneys, skin and eyes are also important (entry/exit portals for drugs)
39
What may cause tissue-specific toxicity?
Enzyme expression in afflicted tissues.
40
What is the first-pass effect?
Biotransformation by liver and GI tract before reaching systemic circulation
- alters bioavailability
41
How do we biotransform so many different foreign chemicals?
1. Enzyme multiplicity
| 2. Broad substrate selectivity - one enzyme can accept many substrates
42
What is Phase 1 of biotransformation?
exposes or introduces FUNCTIONAL GROUPS
- oxidation, reduction, hydrolysis
- drug becomes metabolite
43
What is Phase 2 of biotransformation?
synthesis ran, conjugation
| results in conjugated metabolite
44
What are the possible pathways of biotransformation?
Drug - phase 1 - phase 2
Drug - phase 2
Excreted unchanged or after phase 1 or if water-soluble
45
Example of phase 1 then phase 2 biotransformation
Phase 1: codeine (prodrug) with P450 becomes morphine
Phase 2: morphine with UGT becomes morphine-3-glucuronide
OR
1: heroin is hydrolyzed(deacetylated) becomes morphine
2: morphine UGT - morphine-3-glucuronide
46
Example of direct conjugation biotransformation
morphine with UGT in phase 2 becomes morphine-3-glucuronide
47
List the major enzymes involved in Phase 1.
- Monooxygenases ex. cytochromes P450
- Oxidases and Dehydrogenases
- Reductases
- Hydrolases
48
List the major phase 2 conjugating enzymes.
* **TRANSFERASES
- UDP-glucuronosyltransferases
- Methyltransferases
- Sulfotransferases
49
Define microsomes.
A subcellular fraction NOT found in intact living cells
50
Who discovered cytochrome P450 and who was the first to show that it is an enzyme that metabolizes steroids and drugs?
- Tsuneo Omura and Ryo Sato
| - Ronald Estabrook
51
What is similar between all reactions catalyzed by cytochrome P450?
An oxygen is being inserted.
52
What is the general formula that cytochrome P450 mediates?
RH (substrate) + O2 + NADPH + H+ -> ROH (product) +H2O + NADP+
53
Describe the process of the P450 reaction.
1. P450 with ferric (oxidized) iron binds substrate
2. NADPH- cytochrome P450 reductase transfers electron to reduce iron to ferrous state.
3. Now ferrous iron can bind molecular oxygen.
4. Rearrangements
5. Transfer of second electron from P450 reductase (or b5 reductase)
6. Oxygen now activated so oxygen-oxygen bond splits. One forms water.
7. Other oxygen is highly reactive and abstracts hydrogen.
8. Monooxygenase - carbon radical and hydroxyl radical recombine.
9. Hydroxylated product is formed and ferric iron is regenerated.
54
What is a rate-limiting step in P450 reactions?
The NADPH-cytochrome P450 reductase (POR) due to its abundance compared to P450
55
Where are P450 and POR located?
On the cytosolic side of the membrane of ER
56
How did we obtain so many P450s?
they evolved by duplication of ancestral P450 genes and accumulation of mutation that change structure over a long period of time.
57
What are the main roles of the families within the human P450 gene superfamily?
1-3 are the major P450s for xenobiotic metabolism
| 4-51 are involved in biosynthetic reactions plus metabolism of endogenous physiologic substrates
58
How are the human P450 genes classified into families and subfamilies?
Based on extent of similar gene sequences
ie. to be in the same family: >40% amino acid identity
same subfamily: >55%
59
CYP3A4
- most abundant P450 in human liver and small intestine
- CYP3A4/5 metabolize 50% of known drugs ie. erythromycin, codeine, lovastatin, diazepam, taxol
- important in drug-drug interactions and bioavailability
- many substrates
- inducible by many xenobiotics
- CYP3A7: major P450 in human fetal liver
60
CYP1A2
- expressed only in the liver
- close relative 1A1 is in tissues other than the liver
- induced by polycyclic aromatic hydrocarbons (PAHs found in cigarette smoke) therefore CYP1A1 involved in lung cancer
- biotransforms caffeine, acetaminophen, aromatic amine carcinogens, aflatoxin (liver carcinogen)
61
CYP2C9
- main 2C in liver
- polymorphic in humans
- catalyzes hydroxylation of tolbutamide (hypoglycemic agent), phenytoin (anticonvulsant), warfarin (anti-coagulant)
62
CYP2C19
- mainly in liver
- polymorphic
- catalyzes hydroxylation of S-mephenytoin (anticonvulsant)
63
CYP2D6
- highly polymorphic
- low abundance in liver
- BUT catalyzes biotransformation of drugs of many therapeutic categories:
antiarrhythmics, antidepressants, beta-blockers, analgesics
64
CYP2E1
- originally discovered as the "microsomal ethanol oxidizing system" (MEOS)
- induced by heavy alcohol intake
- expressed in liver, kidney, lung and some other extra hepatic tissues
- catalyzes biotransformation of ethanol and many other small organic molecules
ie. halogenated alkanes;converts benzene and chloroform into hepatotoxic metabolites
65
Describe the detoxification of the anticancer drug, doxorubicin, reduction formula.
- mediated by NAD(P)H: quinone oxidoreductase
- quinone
Quinone by POR gains an electron to become Semiquinone (unstable), can be reversed in the presence of oxygen but
produces hydroxyl radical leading to oxidative stress and cytotoxicity
detoxification pathway:
Quinone by quinone reductase gains 2 electrons to form hydroquinone
66
What are epoxides?
chemically-reactive electrophilic molecules that can bind covalently to proteins and nucleic acids leading to cytotoxicity/cancer
67
What does epoxide hydrolase do and where are its enzymes?
conjugates the expocide with water (acts like phase 2 enzyme)
in nearly all tissues: microsomal forms and cytosolic forms
68
Describe the epoxide hydrolase formula.
epoxide (arene oxide) +H2O -> dihydrodiol
| become s safe and not chemically reactive
69
What are the enzyme, substrates and cofactor of glucuronidation(phase2)?
Enzyme: UDP-glucuronosyltransferase (UGT)
- microsomal (ER lumen side)
Substrates: hydroxyl, carboxyl, amine, sulfhydryl functional groups ie. morphine, acetaminophen
Cofactor: uridine diphosphate-glucuronic acid (UDPGA)
- donates a functional group
70
What are the genetic deficiencies in glucuronidation?
- Gilberts syndrom
- Crigler-Najar syndrome: may lead to hyperbilirubinemia and impaired drug clearance
possible aplastic anemia from exposure to chloramphenicol
- newborns are deficient in glucuronidation so may get grey baby syndrome from chloramphenicol
71
What are the enzyme, substrates, and cofactors involved in glutathione conjugation (phase2)?
Enzyme: glutathione S-transferase (GST)
- cytosolic, microsomal, mitochondrial
Substrates: electrophilic centres such as epoxides, arena oxides, nitro groups, and hydroxylamines ie. ethacrynic acid, bromobenzene
Cofactor: reduced glutathione
72
What are the enzyme, substrates, and cofactors involved in sulfation (phase2)?
Enzyme: sulfotransferase (SULT)
- cytosolic
Substrates: phenols and aliphatic alcohols ie. acetaminophen, steroid hormones
Cofactor: 3'-phosphosadenosine-5'phospholsulfate (PAPS)
73
What is an example of integration of multiple enzyme pathways?
Acetaminophen hepatotoxicity: high dose acetaminophen causes death of hepatocytes
74
Describe the integrated acetaminophen pathways.
parent drug is usually undergo glucuronidation or sulfation to form nontoxic compounds.
CYP2E1 and CYP3A4 can mediate formation of NAPQI (a reactive toxic intermediate) which can potentially bind cellular macromolecules that may lead to cellular cell death
```
N-acetylcysteine (NAC) can be life saving as it restores depleted GSH pools.
adds glutathione (GSH-conjugation) to NAPQI.
```
75
What are the enzyme, substrates, and cofactors involved in acetylation (phase2)?
Enzyme: N-acetyltransferase (NAT)
- cytosolic
- fast and slow acetylators (ie. NAT2 and aromatic amine-inducer bladder cancer)
Substrates: aromatic amine and hydrazine groups ie. isoniazid, sulfamethoxazole
Cofactor: acetyl-CoA
76
What are the enzyme, substrates, and cofactors involved in methylation (phase 2)?
Enzyme: methyltransferase (MT)
- mostly cytosolic
Substrates: phenol, catechol, aliphatic and aromatic amine, N-hetericyclic, sulfhydryl ie. levodopa, 6- mercaptopurine
Cofactor: S-adenosylmethionine (SAM)
77
What are the potential effects of induction or inhibition of drug-metabolizing enzymes?
1. Alter the efficacy of a therapeutic agent
| 2. Alter the toxicity of drugs or environmental chemicals
78
Are P450s inducible?
Majority are non-inducible or weakly
| 2E1 and 1A2 are strongly inducible
79
What are some chemicals that activate the aryl hydrocarbon receptor and are inducers of drug-metabolizing enzymes?
TCDD (environmental toxicant), MC, benzo a pyrene (carcinogen), and carbazole (cabbage family)
80
How does benzo[a]pyrene induce its own biotransformation?
Prior exposure will increase the levels of certain drug-metabolizing enzymes, which increases the capacity of that organism to biotransform benzo[a]pyrene
81
What induces CYP1A1 activity?
Cigarette smoke
82
At what stages are P450 enzymes regulated?
Mostly in gene transcription
| but also during processing, mRNA stabilization, translation and enzyme stabilization/ degradation
83
Describe the aryl hydrocarbon receptor (AHR) response pathway.
AHR is in the cytosol in a complex with HSP90 and AIP
inducing agent (TCDD) diffuses through membrane and directly binds to AHR, making it more active
chaperone proteins are lost
AHR-TCDD enter nucleus and find AH receptor nuclear translocator forming a DNA binding factor
Leads to upregulation of CYP1A1,1A2,1B1,2S1 and phase 2 enzymes, GST and UGT
homeostatic mechanism -> enhances biotransformation to eliminate drug
84
Describe the relationship between St. John wort and Indlnavlr.
St. John's wort can speed up metabolism of Indlnavlr, which compromises its therapeutic effect
85
Describe the pathway of pregnane X receptor.
pregnane x receptor resides in cytosol
inducer ie. hyperforin diffuses into cytosol binds PXR, activating it and removing chaperone proteins
they enter the nucleus and bind to RXR (its nuclear dimerization partner) (retinoid X receptor)
then bind to specific DNA sequences
ex. may induce CYP3A4 which increases metabolism of many drugs
86
What is a therapeutic benefit of inducing drug-metabolizing enzymes?
treatment: treating neonates with hyperbilirubinemia with phenobarbital
action: induces UDP-glucuronosyltransferase
goal: increase bilirubin conjugation and clearance by elevating the rate of glucuronidation
87
What is a possible prophylactic benefit of inducing drug-metabolizing enzymes?
induce CYP1A2 in liver of premenopausal women with high risk for breast cancer by introducing indole-3-carbinol
goal: reduce risk of breast cancer by reducing circulating levels of 17beta-estradiol (biotransformed by CYP1A2
88
Describe the metabolism of estradiol to catechol estrogens.
AHR induces CYP1A1, 1A2, and 1B1
protective pathway in breast cancer:
17beta-estradiol form a less active estrogen (2-OH) through mediation of 1A1-1A2 dimer
but if mediated by CYP1B1 4-OH is formed which is a DNA damaging agent
89
What is a protection mechanism of the potent carcinogen, DMBA?
pretreatment with the pesticide, DDT, enhances first-pass clearance of DMBA
90
What is the relationship between the antihistamine, seldane ie.terfenadine, and the antifungal ketoconazole?
When taken at the same, it results in dangerous heart rhythm abnormalities
91
Describe the mechanism of terfenadine.
blocks cardiac hERG K+ channels and results in a long Q-T interval - can be fatal
prodrug that relies on metabolism of CYP3A4 but drugs such as erythromycin and ketoconazole are inhibitors of CYP3A4 so no anti-histaminic benefits
92
What is the metabolic bottleneck?
when too many compounds compete for the same enzyme
93
What is an example of a dietary agent?
Grapefruit - inhibits metabolism of many medications especially those whose main route of metabolism is via CYP3A4
94
How do we determine which human enzymes metabolize a particular drug?
1. in vitro tests with human tissues
2. in vitro tests with an individual cloned human enzyme expressed in a heterologous system (note:p450s also need reductase)`
95
What is the XenoTech approach to matching substrate to enzyme?
1. pure enzyme expressed from cDNA
2. correlation analysis in liver microsome bank
3. inhibition of the candidate enzyme with selective antibodies or selective chemical inhibitors
96
How can you ensure correct enzyme to substrate matching if in vitro and not in vivo?
- appropriate substrate concentration was tested in vitro
- all competing pathways are considered
- individual enzyme variation is considered
BUT in vitro only ever shows potential
97
Which enzymes would pharmaceutical companies prefer are NOT the primary route of metabolism for a new drug?
1. CYP3A4 - too much competition so increased potential drug-drug interactions
2. CYP2D6 - high degree of genetic polymorphism leads to some individuals being "rapid" metabolizers while others are slow
98
How is caffeine used as a probe drug?
Safe for intake and can tell you about the levels and activity of multiple drug-metabolizing enzymes.
- CYP1A2, N-acetyltransferase (NAT2) and XO
