Midterm 1: Drug Metabolism Introduction Flashcards
non-specific enzymes that carry out biotransformation reactions on lipophilic compounds
- What 3 processes are dominated by the action of these enzymes?
- Biotransformation
- Clearance
- Elimination
What rxns do drug metabolism enzymes carry out? Why do they do this?
- oxidation
- hydrolysis
- conjugation
- convert non-polar drugs into polar metabolites
- polar drug metabolites are more readily cleared from the systemic circulation by the kidneys than the parent drugs
- also see some metabolites in the bile.
Definition of **drug metabolism **
- process of enzymatic biotransformation of drugs
- single drug converted into multiple metabolites by multiple enzymes
- Usually these metabolites are inactive and rapidly eliminated
- Sometimes the metabolites are also active.
Why does drug metabolism play a critical role in achieving therapeutic drug concentrations?
- concentration of a drug at the site of action determines effect
- steady state levels of drugs depends on **input **
- dose size
- formulation
- frequency
- site of administration
- and output
- rate of metabolism and renal clearance
- What do families of drug metabolizing enzymes do?
- Variability in the concentrations of the enzymes in any given tissue?
- Where is the primary site of drug metabolism?
- There are families of drug metabolizing enzymes where each family carries out characteristic types of reactions.
- There is a wide inter-individual variability in the concentrations of the enzymes in any given tissue.
- The liver is richly endowed with drug metabolizing enzymes and is the primary site of drug metabolism.
metabolism of the beta-blocker propranolol
- 3 note worthy things
- 90% of a dose is eliminated as metabolites in the urine and <1% as unchanged drug. (think about were the other 9% went).
- Most of the metabolites are more polar (glucuronides (gluc) and sulfates (sulf) are charged).
- At least 6 different enzymes participate in metabolite formation and we observe both primary and secondary metabolites in the urine and feces. Thus the parent drug is converted to many metabolites. This is a good thing as it reduces the likelihood that a metabolite will be toxic.
Path of an orally administered drug through the body
- absorbed through the cells lining the small intestine (major) or stomach (minor) into the blood
- immediately enters the liver via the portal vein.
- Transits liver and enters the systemic blood circulation
- Undergoes many passes through the liver until it is finally captured by an enzyme and metabolized.
- The cells that line the intestinal lumen are called enterocytes.
- Enterocytes have families of drug transporters and drug metabolising enzymes that can act to promote or reduce the absorption of a given drug into the portal circulation.
Definition of pre-systemic metabolism
- drug metabolized by enzymes in the enterocytes or hepatocytes before it can enter the systemic circulation and travel to the site of action
- aka: first pass effect
- reduces the amount of absorbed drug that enters the systemic circulation and reduces **bioavailablity **
- the fraction of an oral dose that reaches the systemic circulation
- This set up prevents upwanted dietary compenents from reaching systemic circulation
Diagram of Drug Pathway
- Clearance: anything that take drug to metabolite
- Every drug has to pass through the liver
- In high clearance, portal vein concentration is higher than plasma concentration
2 reasons why 1st pass metabolism is something to be avoided
- leads to
- (1) highly interindividual variability in blood levels of drugs
- (2) an exaggerated sensitivity to drug-drug interactions.
When a drug molecule drug molecule enters the liver what are it’s possible fates?
- evading metabolism and transiting the liver to enter the systemic circulation via the central vein unchanged
- being transformed to a metabolite. Usually the metabolite will also exit the liver via the central vein and then enters the systemic circulation for eventual excretion by the kidneys
- metabolites of drugs and drugs themselves will exit the liver via the bile duct for excretion in the feces.
Definition of hepatic clearance
- drug is converted to a metabolite in the liver
- designed to provide lipophilic molecules with maximum access to the drug metabolizing enzymes
- rapid hepatic metabolism is a high extraction ratio
- ratio of the concentration of drug entering the liver divided by the concentration of drug that leaves the liver.
Cutaway diagram of liver
- Blood flow 1.2 L/min
- Blood volume 6-7 L
- RBC transits every 5 min or so
- Liver good at exposing blood molecules to enzymes and also putting molecules in the blood
Representation of Hepatocytes
- Capillary bed in liver is efficient in exposing molecules in blood to membrane and allowing them to enter the haptocytes
- Every cell has border on the bile. Metabolites secreted into the bile. Molecules in the bile go back to the GI tract. Pancreatic juices are also in the bile. Also bile salts produced by the liver to help us emulsify fats.
- concentration of drug in hepatocyte available for metabolism
- Where does drug enter and hepatocytes?
- drugs in the blood are bound to plasma proteins
- albumin
- fraction that is not bound is called the free drug concentration
- [drugs] in the hepatocytes available to drug metabolizing enzymes ~ [free drug]blood
- Drug enters and leaves the hepatocyte by passing through the sinusoidal membrane.
- Passage through membranes is normally much faster than drug metabolism rates
- (Some drugs are also taken up into the heptaocytes via transporters.)
Drugs and their metabolites leave the hepatocyte via:
- Passing through the sinusoidal membrane where they eventually enter the hepatic vein and systemic circulation (major route)
- Passing through the cannicular membrane into the bile. Passage into the bile usually uses transporters so it is much more selective than entry into the hepatocyte from the blood (minor).
Two Factors in Bioavailability
- Absortion
- Poorly absorbed go into feces
- First Pass Metabolism
Explain This Figure
- Area under curve smaller for oral dose of propanol (high 1st pass metabolism)
Big Picture of Liver in Circulatory System
- Note: bile flow from the liver into the GI tract (green)
- Placement of the liver so that it gets the first look at orally absorbed drugs before they enter the systemic circulation.
- drug metabolizing enzymes in the liver hepatocytes are located in one of two places
-
ER
- contains the membrane-bound P450 enzymes
- some forms of glucuronyl transferases.
- Microsomes are the workhorse of drug metabolism research.
-
Cytosol
- glutathione S-transferases (GST)
- epoxide hydrolases (EH)
- various dehydrogenases
- esterases
- 50% of the cell volume is cytosol.
Making microsomes
- homogenize liver which
- lyses the cells and shears the ER into small donut shaped vesicles
- centifuged at 10,000 x g and the pellet is discarded
- supernatant contains the enzymes.
- centrifuged again a 100,000 x g
- cytosolic enzymes are in the supernatant
- microsomal enzymes are in the pellet
- cytosolic enzymes (supernatant) and the microsomal enzymes (pellet) for metabolic studies.
- P450 enzymes and glucuronyl transferases are in the microsomal fraction.
- can study metabolism in each type of fraction
- 10,000 x g supernatent contains all of th enzymes of interest and all cofactors
- can identify the number and concentrations of metabolites upon incubation with drugs.
- can measure the rate of loss of drug and estimate how rapidly the drug will be metabolised
- in vivo by scaling rates of metabolism observed in vitro. The in vitro systems we use provide lots of useful information about interindividual variablity and drug-drug interactions.
- two basic types of drug metabolism reactions which can occur in cytosol or microsomes
-
Phase I reactions
- create a polar functional groups
- more readily excreted in urine and bile
- Cytochrome P450 enzymes
- Oxidation, reduction or hydrolysis
- create a polar functional groups
- **Phase II reactions **
- generate highly polar derivatives known as conjugates of drugs and metabolites of drugs
- deactivates highly reactive products of P450 reactions
- epoxide hydrolase (EH)
- glutathione-S-transferases (GSTs)
- Highly reactive products can bind to protein/DNA, causing hepatic necrosis and cancer
Phase II Transformations
- Drugs and/or their metabolites are combined in covalent reactions with endogenous hydrophilic compounds. The products are substances with sufficient hydrophilic character for excretion.
- aka: stick on something polar
- Conjugation reactions can occur with a variety of substances, usually intermediates in the organism’s metabolism (e.g. glucuronic acid, sulfate and glutathione are common added groups).
Glucuronidation by glucuronyl transferases (GTs):
- Phase II Reaction
- These enzymes are microsomal
- Conjugation of a polar group ( alcohol, phenol, carboxylic acid, amine) of a drug or a metabolite with glucuronic acid
- metabolic clearance of NSAIDs dominated by glucuronidation
- substrate is uridine diphosphate glucuronic acid (UDPGA)
- UDPGA is co-substrate for the enzyme-catalyzed rxn
- uridine diphosphate UDGP is a good leaving group.
- recycled after rxn
- Glucuronide conjugates are anions
- readily excreted into the bile and/or the urine
- heme metabolite bilirubin diglucuronide which is made from heme and excreted in the bile.
- inability to make these glucuronides due to a deficiency in the enzyme or hepatic failure leads to accumulation of unconjugated bilirubin (jaundice, yellow babies, kernicturis).
- readily excreted into the bile and/or the urine
Sulfate conjugation by sulfotransferases (SULTs)
- Phase II Reaction
- Phenolic drugs and metabolites are converted to highly polar sulfate conjugates by sulfotransferases
- co-factor in these reaction is an endogenous compound 3’- phosphoadenosine-5’-phosphosulfate (PAPS)
- product PAP is also “recycled” to PAPS.
- Sulfate conjugates (anions at body pH) are excreted mainly in the urine.
Glutathione conjugation by glutathione-S- transferases (GSTs) (Mercapturic acid formation)
- Phase II Reaction
- GSTs catalyze the attack of the sulfhydryl group of the co-substrate glutathione at electrophillic sites on drugs (sometimes), their metabolites (often) as well as environmental toxins and their metabolites.
- cytosolic enzymes
- Glutathione (GSH) is a novel highly polar (nucleophilic) tripeptide (γ-glutamylcysteinylglycine) substrate that is present at high levels in cells (5 mM). Glutathione can react with substrates directly or via GST catalysis.
- The main purpose of GSH and the family of GSTs appears to be “deactivate” reactive groups on molecules (such as epoxides, quinones and halides) and in so doing to create a water soluble metabolite. Conjugation with glutathione is a major defense pathway for reactive compounds created by oxidative metabolism.
- Glutathione conjugates are rarely excreted intact into the urine due to their high molecular weight. Instead they converted to N-acetyl-L-cysteine (mercapturic acid) conjugates. Thus the glycine and glutamate of glutathione are recycled but the cysteine is excreted as the mercapturate.