Enzymes and Transporters Flashcards
What is the equation for Cls?
Cls=Clh+Clr
What are the hepatic clearance mechanisms?
phase 1 (P450)
phase 2 (conjugating enzymes)
phase 3 (biliary excretion)
What are the renal clearance mechanisms?
glomerular filtration
tubular secretion and reabsorption
What is metabolism?
refers to chemical modification of a drug
results in formation of a metabolite
-typically results in inactivation of drug
What is the primary site of metabolism?
the liver
metabolism in other organs/tissues may be important for site specific toxicity or drug activity
How does formation of a metabolite help in excretion?
the metabolite is more polar so facilitates excretion by the kidney
Differentiate between activation and bioactivation.
activation:
-process when a drug is metabolized to a metabolite that has greater pharmacological activity than the administered drug
bioactivation:
-usually reserved to the metabolic activation of a drug to a toxic metabolite
What is phase 1 metabolism?
phase 1 pathways primarily involve CYP P450 enzymes
-uncover or introduce a functional group on the drug molecule
Aside from CYP P450, what are some other phase 1 enzymes?
xanthine oxidase
alcohol and aldehyde dehydrogenase
epoxidases
esterases
What is the action of CYP P450 enzymes?
superfamily of heme containing enzymes responsible for the hydroxylation, oxidation, or reduction of structurally diverse drugs
often referred to as mixed-function oxidases
What is the reason why CYP enzymes metabolize lipophilic drugs?
they are typically associated with the smooth ER and active site of the enzyme lies within the ER membrane
Why can CYP P450 enzymes saturate? What is the significance of this?
they demonstrate high affinity but low capacity
means many potential drug interactions occur at P450 enzymes
What is the most important CYP enzyme in drug metabolism?
CYP 3A4
What is phase 2 metabolism?
mediates the conjugation of an endogenous molecule to a functional group on a drug molecule or metabolite
-many of these endogenous molecules are derived from compounds involved in CHO, fat, and protein metabolism
What are the important phase 2 enzymes?
UGT
ST
GST
NAT
methyltransferases
amino acid conjugates
What is the action of UGT enzyme?
mediate the conjugation of UDPA to drug substrates
-family of enzymes found in smooth ER
-two families: UGT1A and UGT2B
-broad and overlapping drug substrate specificities (many drugs are metabolised by UGT enzymes); includes endogenous cpds
Are UGT enzymes easily saturated?
UGT enzymes demonstrate high capacity and therefore are difficult to saturate
-this means few drug interactions occur at UGT enzymes
What is the action of sulfotransferase (ST) enzymes?
catalyze the conjugation of inorganic sulfate to drugs containing hydroxyl functional groups
-found in the cell cytosol
-the different ST enzymes exhibit overlapping substrate specificities that overlap as well with the UGT enzymes
Are ST enzymes easily saturated?
ST enzymes show lower capacity and may saturate at higher drug concentrations
What is the action of GST enzymes?
catalyze the conjugation of glutathione to electrophilic or reactive substrates
-protective function in the cell by mediating the conjugation of glutathione to reactive intermediates and alkylating agents that arise during normal cell metabolism
What is the action of NAT enzymes?
mediates the conjugation of the acetyl moiety of acetyl-CoA to NAT substrates
-two families: NAT1 and NAT2
-NAT2 demonstrates a genetic polymorphism, which contributes to wide interindividual variability associated with NAT substrates
What percentage of drugs on the market are metabolized by CYP3A4? What about CYP2Cs, CYP2D6, and CYP1A2?
CYP3A4: ~50% of drugs on the market
CYP2Cs: ~30% of drugs on the market
CYP2D6 and CYP1A2: significant fraction
What are the three important reasons to have knowledge of the drug metabolizing enzymes that mediate the elimination of a drug?
drug-drug interactions
interpatient variability in drug PK (genetic polymorphism)
effect of disease (major source of variability in response)
When do drug-drug interactions occur?
when either the PK or PD of one drug is altered by a second drug
What do drug-drug interactions impact?
absorption, distribution, and/or elimination
may lead to therapeutic ineffectiveness or toxicity
True or false: most drug interactions have limited clinical significance
true
What does the degree of a drug interaction depend on?
dose (concentration) or each of the interacting drugs and their time of administration relative to each other
Differentiate between competitive inhibition and induction.
competitive inhibition:
-acute decrease in metabolism/transport of drug by a simultaneously present drug
=reduction in the elimination of a drug and its accumulation in the body to potentially toxic concentrations
inhibition:
-net increase in enzyme/transporter activity upon exposure to drug due to an increased amount of functional protein
=increased metabolism/transport of a drug such that drug concentrations may drop below the TW and the drug is no longer effective
What is the etiology of interpatient variability in drug PK?
genetic
physiological/pathophysiological
environmental
What are genetic polymorphisms?
minor (single nucleotide polymorphism) or major (gene deletion or duplication) change in the nucleotide sequence of a gene encoding a drug metabolizing enzyme/transporter
What are the consequences of genetic polymorphisms on PK/PD?
reduction in Cls, which will lead to increased t1/2 and potential for drug accumulation
altered metabolite profiles and the production of active or toxic metabolites
altered drug-drug interactions
potential reduction in 1st pass metabolism which increases F
alteration in therapeutic effects of a drug
What are the four phases of drug metabolism?
phase 0:
-expression of transporters on basolateral membrane to move free drug into hepatocytes
phase 1:
-unbound drug diffuses in and meets CYP enzymes in ER membrane
-hydroxylation, oxidation, reduction
phase 2:
-conjugating enzymes
-polar molecule added for excretion
-polar molecule leaves via transporter
phase 3:
-parent cpd or phase 2 metabolite enters bile via efflux transporters
What are transporters?
proteins that move solutes (like drugs) across membranes (plasma membrane, organelle membrane)
widely expressed in all cell types
play a role in ADME for many drugs
Which phases of metabolism involve transporters?
phase 0 and 3
Describe solute carrier (SLC) transporters.
> 50 families
facilitated diffusion or secondary active
mediate bidirectional movement of drugs; uptake transporters
substrates usually charged or uncharged weakly acidic or basic drugs, inorganic ions, ammonia, other endogenous cps
ex: OCTs, OATs, OATPs, PEPTs
Describe ATP binding cassette (ABC) transporters.
> 25 families
primary active
efflux; moves drug against a conc gradient
substrates usually lipophilic drugs, endogenous cpds, other polar or charged cpds
ex: MDR1, MRPs, BCRP
What is the general importance of transporters?
influence drug absorption, distribution, and elimination and therefore overall exposure of the body to the drug
exert a protective function at blood-organ barriers
are determinants of drug levels in tissues/cells
are a site of potential drug-drug interactions or drug-endogenous cpd interactions
saturable which influence F, extent of distribution, and elimination characteristics of a drug
have a host of endogenous functions
How much bile does the liver secrete per day?
0.25-1L
-stored in gallbladder; secreted into duodenum
What kind of molecules are secreted into bile?
anions, cations, nonionized molecules
-some drugs; metabolites-glucuronide conjugates
What is enterohepatic recirculation?
conjugated compound is deconjugated to parent drug and reabsorbed back into systemic circulation
-may lead to a double peak phenomenon when meal consumed after drug administration
What is enterohepatic recycling a component of?
distribution
-functions to prolong the residence of drugs in the body
