Why is drug biotransformation necessary?
Pharmacologically active organic molecules are usually lipophilic and remain unionized or partially ionized at physiological pH. To be excreted by the renal system (and not reabsorbed) these molecules must have a charge, otherwise their duration of action
How can the body's own metabolic processes be used pharmacologically?
Some biotransformation products are more active than their parent drug. Enzymes involved in metabolic pathways are exploited by some drugs in that the parent drug is inactive and after biotransformation the drug is in its active state. Similar to zymogens
When does biotrabsformation of a drug occur?
Between absorption of the drug and elimination by the renal system.
What are phase 1 reactions?
Phase 1 reactions typically convert the parent drug into a more polar metabolite by adding or exposing functional groups. This when a drug usually becomes inactive, although some drugs become more active at this phase. Some drugs are sufficiently polar to
When do phase 1 reactions occur?
Between absorption and metabolism.
What are phase 2 reactions?
Reactions subsequent to phase 1 reactions in which an endogenous substrate, such as glucuronic acid, sulfuric acid, acetic acid or an amino acid combines with the newly incorporated functional group to form a highly polar conjugate. Some parent drugs are
When do phase 2 reactions occur?
Between metabolism and elimination.
Phase 1 reactions summary
-Add or expose functional groups
-Drug is inactivated, prodrug is activated
-Occurs in smooth ER
-Enzymes involved are typically mixed function oxidases, monooxygenases or microsomal oxidases
-Cytochrome P450 family
Phase 2 reactions summary
-Biosynthetic reactions
-Covalent linkage (conjugations) with various molecules
-Mostly takes place in the cytosol
-Can precede phase 1 reactions
Describe a drug whose biotransformation involves phase 2 reactions preceding phase 1 reactions
The hydrazide moiety of isoniazid directly forms an N-acetyl conjugate in a phase 2 reaction. This conjugate serves as a substrate in a phase 1 reaction in which the conjugate is hydrolyzed to form isonicotinic acid.
Where does drug biotransformation occur?
The liver is the principal organ of drug metabolism. The kidneys and GI tract also play a minor role in drug metabolism.
What is the first pass effect?
The concentration of the drug is greatly reduced before reaching systemic circulation.
Where can the first pass effect occur?
The liver and the gut. Many drugs, when orally administered, are absorbed intact by the small intestine and transported by the portal vein to the liver where they undergo extensive metabolism. Some drugs are more extensively metabolized in the intestine.
How does the first pass effect affect drug administration?
First pass effect can greatly limit the bioavailability of orally adminstered drugs, and another route of administration must be used.
What characteristics of the smooth endoplasmic reticulum make it ideal for drug biotransformation?
It is rich in mixed function oxidases.
How do mixed function oxidases work?
The activity of these enzymes requires a reducing agent (NADPH) and molecular oxygen. One molecule of oxygen is reduced per substrate molecule. One oxygen atom is incorporated into the product and the other is incorporated into water.
What enzymes comprise the mixed function oxidases?
NADPH-cytochrome P450 oxidoreductase and a hemoprotein, cytochrome P450.
What are the functional requirements of NADPH-cytochrome P450 oxidoreductase?
One mole of this enzyme contains flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). NADPH is required for electron donation.
What are the functional requirements of cytochrome P450?
The heme group must bind iron.
Describe the first step in the Cytochrome P450 cycle of drug oxidation
Oxidized P450 (Fe3+ is bound to heme group) combines with drug substrate to form a binary complex.
Describe the second step in the Cytochrome P450 cycle of drug oxidation
NADPH donates an electron to the flavoprotein reducatse (reducing agent component of mixed function oxidase), which in turn reduces the binary drug-P450 complex.
Describe the third step in the Cytochrome P450 cycle of drug oxidation
P450 reductase reduces molecular oxygen with an electron donated from NADPH to form an activated oxygen-P450-substrate complex. At this point both the drug substrate and the activated oxygen are bound to P450.
Describe the fourth step in the Cytochrome P450 cycle of drug oxidation
The activated oxygen-P450-substrate complex transfers its activated oxygen to the drug substrate to form the oxidized product.
Why is it important to understand the induction and inhibition of cytochrome P450?
Cytochrome P450 is implicated in drug-drug interactions. The activity of certain drugs at the cytochrome P450 can interfere with the effects of co-administered drugs.
Explain P450 enzyme induction
Repeated administration of certain drugs results in the induction of P450 expression. P450 expression induction results in accelerated substrate metabolism and a decrease in phramacologic action (tolerance) of the inducer and also of co-administered drugs
Explain P450 enzyme inhibition
Certain drug substrates inhibit cytochrome P450 by binding to the P450 heme iron and effectively reducing the metabolism of endogenous substances and co-administered drugs through competitive inhibition. Some drug substrates irreversibly inhibit P450s by
What are the most important forms of cytochrome P450?
CYP1A2
CYP2A6
CYP2C9
CYP2E1
CYP3A4
CYP1A2
Substrate example: 12% of drugs, acetaminohen and theophylline
Inducers: Smoking (aromatic hydrocarbons), cruciferous vegetables, omeprazole
Inhibitors: Fluvoamine
CYP2C9
Substrate example: 4% of drugs, phenytoin and warfarin
Inducers: General inducers
Inhibitors: N/A
CYP2D6
Substrate example: 28% of drugs, many cardiovascular and CNS drugs
Inducers: St. John's Wort and rifampin
Inhibitors: Quinidine and paroxetine
CYP2E1
Substrate example: Acetaminophen and gas anesthetics
Inducers: Ethanol and isoniazid
Inhibitors: disulfiram
CYP3A4
Substrate example: 50% of all drugs in the physician's desk reference
Inducers: General inducers
Inhibitors: General inhibitors and grapefruit juice
What are general inducers of cytochrome P450?
Barbiturates
Phenytoin
Rifampin
Griseofulvin
Carbamazepine
Board mnemonic to remember general inducers
Barb takes Phen-phen and Refuses Greasy Carbs
The four R's of rifampin
RNA polymerase inhibitor
Revs up microsomal P450s
Red/orange body fluids
Rapid resistance
What is drug tolerance?
Loss of drug effect after use. Necessitates the need to use more drug to achieve desired effect.
What is pharmacokinetic tolerance?
Induction of enzymes by drugs increases drug metabolism and can result in tolerance.
What is pharmacodynamic tolerance?
Altered receptor response.
Examples of drug-drug interaction: Inducers
-Barbiturate users require more warfarin.
-St. John's wort users had reduced oral contraceptive levels.
-Epileptics on phenytoin require more cortisol.
Examples of drug-drug interaction: Inhibitors
-Ketoconazole, erythromycin, or grapefruit juice produced cardiac arrhythmias in patients on antihistamine terfenadine or serotonin antagonist cisapride.
-Cimetidine potentiates the action of anticoagulants and sedatives.
-Cimetidine and ketoconazole inhi
Negative consequences that can result from using drugs that are inducers of P450s
Tolerance and cross tolerance.
Negative consequences that can result from using drugs that are inhibitors of P450s
Increased toxicity.
What kind of enzymes are required for phase 2 conjugate formation?
Energy requiring transferases.
What are the most common types of phase 2 reactions?
-Glucuronidation
-Acetylation
-Sulfation
-Glutathione (GSH) conjugation
What is a negative consequence of drug metabolism?
Metabolism inactivates most drugs, but can transform drugs into cytotoxic compunds.
Summarize why acetaminophen use is a contraindication for alcohol abusers
Chronic heavy alcohol abusers may be at increased risk of liver toxicity from excessive acetaminophen use. Reports almost invariably involve cases of severe chronic alcoholics and the dosages of acetaminophen most often exceed recommended doses and often
What is the normal metabolism of acetaminophen?
Acetaminophen normally undergoes glucuronidation and sulfation. This makes up 95% of excretion, the other 5% occurs through the P450 (CYP2E1) dependent GSH conjugation pathway.
What happens when acetaminophen intake exceeds therapeutic doses?
The glucuronidation and sulfation pathways become saturated and the excretion of acetaminophen becomes dependent on the P450 (CYP2E1) dependent GSH conjugation pathway.
When does hepatotoxicity occur during excessive intake of acetaminophen?
GSH is depleted faster than it can be regenerated. When GSH is not available for conjugation a reactive, toxic metabolite accumulates causing hepatotoxicity.
What are the antidotes to acetaminophen overdose?
Cysteamine and N-acetylamine (the safer of the two).
Why is GSH administration not effective in treating acetaminophen overdose?
It does not cross cell membranes readily.
What is the mechanism of action behind N-acetylamine?
Restores glutathione which can inactivate the reactive metabolite of acetaminophen by forming an excretable conjugate with it.
What is the mechanism of action behind cysteamine?
Cysteamine also restores glutathione, though in a longer energy dependent pathway. L-cysteine combines with glutamate to form L-gamma-glutamylcysteine at the expense of one ATP. L-gamma-glutamylcysteine is then transformed into glutathione by glutathione