Week 2: Drug Elimination and Metabolism

Question 1

Drug Elimination Pathways

Answer 1

Metabolism
Urinary excretion
Biliary excretion

The liver is the major organ for drug metabolism and for biliary excretion
The kidney is a key site for drug excretion in urine.
Other organs can do it as well (e.g. intestine)
Drugs can also be excreted into saliva, milk and sweat.

Question 2

Xenobiotics

Answer 2

Substances foreign to the body

Metabolized by the same enzymatic pathways and transport systems that are utilized for dietary constituents.

Question 3

Lipophilic (hydrophobic) chemicals

Answer 3

Many xenobiotics and drugs are lipophilic chemicals that, in the absence of metabolism, would not be efficiently eliminated and would accumulate in the body, possibly causing toxicity.

Most drugs are subjected to metabolic pathways that convert these hydrophobic chemicals into more hydrophilic derivatives that are readily eliminated in urine or bile.

Question 4

Drug Metabolism Process

Answer 4

Phase 1: Bioconversion of drug metabolites through redox, hydrolytic reactions

• May create metabolite that is similarly active as the administered drug
• Some drugs become pharmacologically active after phase I metabolism (after being inactive) - prodrugs

Phase 2: Drug metabolism through conjugation reactions (adding a hydrophilic structure onto a drug molecule)

Both Phase I and II → formation of products that are pharmacologically inactive

Sometimes, Phase I → metabolite → Phase II

But sometimes, phase II can occur without phase I; sometimes, phase I can happen AFTER phase II

Question 5

Drug elimination by the Kidney

Answer 5

1. Free (unbound) drug in plasma is filtered at Bowman’s capsule (plasma proteins that bind drugs are too big to get past the glomerular sieve)
   - Blood that remains goes through efferent arterioles
   - Plasma protein binding affects a drug’s specific glomerular filtration rate
2. Drugs can be secreted from blood into urine
   - Secretion typically occurs in the proximal tubules
   - Drug secretion occurs through the actions of membrane transporters in the kidney
3. Drug reabsorption from the tubular lumen space back to blood occurs in the distal tubules of the kidney
   - Reabsorption normally occurs through passive diffusion process
   - Passive reabsorption is driven by increased solute concentration in the distal tubule as a result of water reabsorption
   - Lipophilic and un-ionized are usually reabsorbed and retained by the body (otherwise, it is excreted from the body)

Question 6

Drug elimination by Liver

Answer 6

Drugs that enter the liver or the metabolites that are produced can be excreted into the biliary tract for fecal elimination.

The liver receives two blood supplies: from intestines (portal vein) and from the heart (hepatic artery).

Hepatocytes have membranes that face the bloodstream and the bile canaliculi (headwater for bile flow).

• Drugs will move into hepatocytes by diffusion or transporters
• May be metabolized once inside
• Metabolites/drugs are recognized by localized transporters (on the canalicular side)
• Bile containing drugs will drain into the acinus → gallbladder → gut lumen → feces

Question 7

Sinusoids

Answer 7

Capillary bed of liver that is highly fenestrated and lined with rows of hepatocytes

Question 8

Entra-hepatic circulation

Answer 8

Continuous excretion/absorption cycle

Question 9

First-order kinetics of drug elimination

Answer 9

Rate of drug elimination by the body is not constant but is dependent on the concentration of the drug in the blood at a given time

The rate of drug elimination at a given time is directly proportional to the concentration of drug in the blood at that time.

(ex the rate of elimination of this drug at 1 hour is greater than at 4 hours (concentration of drug in blood is higher at 1 hour))

Question 10

Blood levels - drug elimination

Answer 10

After an intravenous dose of a drug, the elimination phase of the drug concentration vs time curve shows an exponential decline.

Exponential decline in drug levels conforms to what is considered “first-order” kinetics.

99.9% of meds have concentrations in the body that is MUCH LOWER than respective Km values for metabolic eliminations
Rate of metabolism is directly proportional to drug concentration

Question 11

Rate of elimination (Michaelis-Menten Equation)

Answer 11

Rate of elimination = Vmax x C/Km + C

C &laquo_space;Km: Vmax x C/km

C&raquo_space; Km: Vmax

• C = drug concentration
• Vmax (mass/time) = constant; maximum metabolic rate depending on how much enzyme is in the body, and efficiency of enzyme at turning over reaction
• Km (mass/time) = drug binding affinity to metabolizing enzyme (lower Km = higher affinity)
• Km of drug = conc. at half maximal rate of metabolism

Question 12

Zero order kinetics

Answer 12

When C»Km and the drug concentrations change within a high concentration range, the rate of metabolism remains relatively the same (rate of elimination = Vmax)

Very rare (Ethanol)

Question 13

Clearance (CL)

Answer 13

• Describes the kinetics of drug elimination by the body.
• Clearance (CL) is a measure of the efficiency of drug removal expressed as a VOLUME of blood from which drug is completely removed per unit time.
• NOT synonymous with RATE of drug removal
• CL has units of volume / unit time (e.g. L/hr or mL/min) → same as volumetric flow rate

CL = Rate of Drug Elimination / Concentration of Drug in Blood

• The higher the value for clearance, the greater the efficiency of drug removal.
• Usually, clearance is constant for a given drug in a given patient (regardless of time) given that they are in the same health state
• This may change with drug-drug interactions or disease (e.g. liver disease or kidney failure).

Question 14

Estimating Clearance

Answer 14

CL = Dose(iv) / AUC

• Clearance of any chemical in the body can be determined by giving an intravenous dose, calculating the AUC and using the above relationship.
• Pharmacokinetic experiment where the drug is administered intravenously to determine this
• Usually, clearance of a drug is constant in an individual over a wide range of clinically relevant doses.
• At high doses, the metabolic enzymes will be saturated and the value for clearance will no longer be constant
• Given constant drug clearance in an individual, the AUC is directly proportional to the DOSE (double dose = double AUC)

Question 15

Half Life of a Drug (Hybrid Parameter)

Answer 15

Time for the blood concentration of a drug to decline by one-half. For all practical purposes, the dose is removed after 3-5 half lives.

The half-life of a drug determines, in part, the duration of drug action (and concentration) and hence the frequency a drug needs to be taken to maintain drug effects.

It is independent of dose

T1/2 = 0.693 x Vd / CL

The higher the clearance, the shorter the half-life.
–> The more efficient the body is at eliminating the drug (metabolism or excretion), the more rapidly a drug is removed from the circulation.

The larger the volume of distribution, the longer the half-life.
–> One can conceptualize this as the following: when a drug is highly distributed in tissues, it is less available in blood and hence there is less drug being delivered to the eliminating organs such as the liver and kidneys.

Diseases and drug-drug interactions can affect either or both Vd and CL to cause changes in drug half-life.

Question 16

Single Fixed Dose

Answer 16

Rises, peaks, falls (almost all drugs are eliminated by the body within 48 hours)

Question 17

Repeated Fixed Dose

Answer 17

8 hours after first dose, there is still drug in body as you take the second dose

Eventually, with continued dosing, drug levels do not differ from one dose to another.
At this point, a patient is said to be in “steady-state”.

Question 18

Steady State (Drug dosage)

Answer 18

Occurs when the rate of drug input in the body equals the rate of drug output (elimination).
Plateau of concentration is achieved

Question 19

Constant Rate Drug Infusion

Answer 19

• When drug is dissolved in fluid, inserted in patients’ vein
• Beginning - no drug in the body
• With constant rate intravenous infusion, drug concentrations rise then plateau to a “steady-state”.
• The time it takes to reach steady-state depends solely on half-life.
• The time it takes for the drug to be eliminated after infusion stops is equal to the time to reach steady-state during infusion (about 3 to 5 half-lives).

Question 20

Effect of Changing the Drug Infusion Rate

Answer 20

Doubling the drug infusion rate doubles the steady-state drug level (Css).

Time to steady-state is unaffected by the rate of drug infusion.

The concentrations differ - high rate infusion is double the low rate infusion

Steady state drug concentration is proportional to the drug infusion rate

Question 21

Relationship between Dose Rate and Css

Answer 21

Drugs with first-order kinetics (99%+ of drugs at normal doses)

• -> The Css increases in direct proportion to the infusion rate
• -> However, when drug concentration becomes higher, zero order kinetics prevails

Drugs with zero-order kinetics (very few drugs; overdose)

• -> Steady state no longer proportional to infusion rate - unpredictable
• -> Very difficult to get just right

Question 22

Drug level fluctuations

Answer 22

• Css for intermittent dosing is like the “ average ” concentration during the dosing interval.
• For every dosing regimen with the same daily dose rate (mg/day), Css is the same but the “peak” and “trough” fluctuation is different.
• For some drugs, larger peak to trough differences are preferable (e.g. certain antibiotics) while for others, minimizing fluctuations between doses is ideal (cardiovascular drugs).

Question 23

Elimination mechanisms of drugs

Answer 23

70% metabolism
25% renal
5% biliary

Question 24

Phase I Metabolism

Answer 24

Introducing or exposing a functional group (e.g. OH) on the substrate drug

Question 25

Drug Metabolism

Answer 25

Removal of xenobiotics (foreign chemicals) from systemic circulation by making lipophilic molecules more hydrophilic to promote excretion

Chemical conversion of a compound into another compound called a metabolite.

For the most part, metabolism creates compounds that are pharmacologically inactive
–> Sometimes, the metabolite that is formed is the actual compound that elicits the pharmacological effect.

Question 26

Phase II Metabolism

Answer 26

Conjugation reactions, a large polar substituent (e.g. sugar - glucuronide, charged - sulfate, peptide - glutathione) is added to a functional group

*drugs can have Phase II metabolism without Phase I, but it is usually sequential (I before II)

• -> Conjugation reactions involve drug, cofactor, and enzyme
• -> Increases hydrophilicity of molecules (exception – acetylation)
• -> Products of Phase II metabolism are prone to excretion in bile or urine
• -> Generally considered a detoxification mechanism (because they are inactive)
• -> Process often terminates the biological activity of a drug
• -> Not often involved in pharmacokinetic drug-drug interactions
• -> Not usually susceptible to inhibition

The Phase II reactions with drugs most often involve one of 4 major enzymes (UGT, SULT, GST and NAT)

Question 27

Metabolites

Answer 27

More polar than the parent drug

Generally speaking, polar metabolites are more efficiently eliminated from the body than the original drug (especially true for renal elimination since hydrophilic molecules are less apt to be reabsorbed here)

In many cases, metabolic conversion of a drug can create metabolites that are toxic (e.g. acetaminophen overdose).

Question 28

Prodrug

Answer 28

Drug that is completely inactive at ingestion. It is converted to the active metabolite

Question 29

CYP enzymes

Answer 29

Found in many tissues but highly expressed in hepatocytes and enterocytes.
–> Superfamily of enzymes important for synthesizing cholesterol, hormones, bile acids - some can metabolize drugs as well

They are membrane-bound proteins residing in the endoplasmic reticulum (ER), with the active site facing the cytoplasm.

• -> CYP are associated with another protein called NADPH-P450 oxidoreductase in the ER.
• -> CYPs are heme containing enzymes that bind molecular oxygen on the incorporated Fe2+.
• -> The CYP enzymes are called “P450s” because the protein can also bind carbon monoxide (CO), and when it does, the absorption spectrum of the protein has a peak at 450 nm

Question 30

P450 (or CYP) Mediated Reactions

Answer 30

Aliphatic hydroxylation: adds OH to aliphatic carbon in drugs

Aromatic hydroxylation: adds OH to aromatic carbon to form a phenol

N-Dealkylation: oxidation of a carbon next to heteroatoms such as O or N form one aldehyde metabolite + amine

O-Dealkylation: oxidation of a carbon next to heteroatoms such as O or N form one aldehyde metabolite + hydroxyl

N-Oxidation: oxidation of a N group forms hydroxyl-amine metabolite

S-Oxidation: oxidation of an S group forms sulphoxide metabolites

Features:

• -> Each CYP enzyme can metabolize many drugs.
• -> Each CYP enzyme can metabolize a single drug to many metabolites (can oxidize many positions on the drug).
• -> A single drug may be metabolized by several CYP enzymes.
• -> Taking these drugs with other medications may result in pharmacokinetic drug-drug interaction that → increased drug conc., heightened response, toxicity
• -> Many CYP enzymes are genetically polymorphic - you could inherit a CYP enzyme with poor metabolic activity

Question 31

CYP3A4

Answer 31

Most important CYP enzyme for human metabolism

Question 32

Grapefruit study

Answer 32

Grapefruit juice destroys intestinal CYP3A4

Since felodipine is subject to significant intestinal first pass effect, lower CYP3A4 activity = lower bioavailability and increased plasma concentrations

Chemical in grapefruit (Bergamottin) that becomes chemically reactive after being metabolized by CYP3A4

• -> Covalently binds to CYP3A4 enzyme
• -> Cells recognize that the enzyme is modified and rapidly process CYP3A4 for cellular degradation
• -> Once the enzyme is eliminated from enterocytes by grapefruit juice, the cells are unable to make more CYP3A4
• -> To regain CYP3A4 activity, you must wait 4 days for the enterocytes to be sloughed off and be renewed with new cells

Question 33

Summary of Phase II Metabolism

Answer 33

Drug/metabolite + Cofactor (enzyme) –> Drug/Metabolite-Conjugate

1. Glucuronidation: catalyzed by UGT (found in ER) where they physically interact with CYP enzymes
   - -> Close proximity of CYP and UGT is why oxidation and glucuronidation reactions are highly coupled
   - -> UDP-GA is the cofactor required - provides the acid sugar moiety after conjugation
   - -> Abundant in intestine and liver
2. Sulfation: uses SULT that are cytosolic
   - -> PAPS cofactor relays sulfate groups onto drugs
   - -> Abundant in intestine and liver
3. Glutathione conjugation: uses GST which adds glutathione to drug or metabolite through a sulfur linkage
   - -> GSTs are found in every cell of the body but abundant liver (cytosol or mitochondrial membranes)
   - -> Occurs spontaneously as a purely chemical reaction
4. Acetylation: uses NAT enzymes in the liver and kidney
   - -> NATs use Acetyl Coenzyme A to transfer acetyl group to drugs
   - -> Liver, Kidneys

Question 34

Conjugation in Acetaminophen

Answer 34

At normal doses, metabolism occurs via sulfation and glucuronidation

• -> Products are readily excreted from the body in bile and urine
• -> Minor amount of drug oxidation occurring by CYP2E1 enzyme in the liver
• -> This pathway is detrimental because it forms a toxic metabolite → forms NAPQI
• -> Normally, NAPQI is quickly conjugated with glutathione for excretion into the bile

Acetaminophen overdose (>10g) will cause saturation of sulfation and glucuronidation pathways, leading to enhanced oxidative bioactivation by CYP enzymes to a toxic metabolite (N-acetyl-p-benzoquinoneimine, NAPQI)
–> Diversion of metabolism from phase II to phase I → increased production of NAPQI
–> NAPQI is efficiently eliminated by GSH conjugation, but when liver GSH is depleted (overdose; used up by conjugation reactions), more covalent binding to the hepatocyte → hepatotoxicity occurs → cell death
–> Doctors use graph to decide how to manage patients
–> N-acetylcysteine is the antidote which is thought to replete liver GSH stores → detoxify NAPQI metabolite
Liver damage occurs