Biotransformation of Drugs
September 10, 2007
Frank F. Vincenzi
Learning objectives: drug biotransformation
• Xenobiotic, endoplasmic reticulum & microsomes
• First pass effect, bioavailability
• Pro-drug, parent compound, metabolite (active or
inactive)
• Lipophilicity vs. hydrophilicity
• Cytochrome P-450
(CYP1A2, CYP2C9 CYP2C19, CYP2D6, CYP2E1,
CYP3A4* )
• Phase I vs. phase II drug metabolism
• (debrisoquine) polymorphism
• “Extensive” and “poor” metabolizers
Really fundamental concepts in drug
biotransformation
• Lipid soluble drugs are poorly excreted in the urine.
They tend to store in fat and/or circulate until they
are converted (phase I biotransformation) to more
water soluble metabolites or metabolites that
conjugate (phase II biotransformation) with water
soluble substances.
• Water soluble drugs are more readily excreted in the
urine. They may be metabolized, but generally not
by the CYP enzyme systems.
Biotransformation & urinary excretion
• Drugs with major first pass effect
(bioavailability) [% urinary excretion]
imipramine (39)
[<2]
lidocaine (35)
[2]
morphine (24)
[8]
propranolol (26)
[<0.5]
• Drugs with little or no first pass effect
(bioavailability) [% urinary excretion]
clonidine (95)
[62]
flucytosine (84)
[99]
metronidazole (99)
[10]
penicillin G (0)
[>90]* (when injected)
Evolution of drug metabolizing enzymes
• Terrestrial life necessitated a substantial increase
in the capacity and generality of how to deal with
environmental xenobiotics*, including compounds
present in food.
* compounds not normally present in a particular
living system, ‘foreign compounds’
Major biotransformation sites
• Liver
• Other….
– Lungs
– Plasma
Mixed function oxidases (monooxygenases)
Cytochrome P450s or CYPs)
• Add one atom of molecular oxygen to a drug
substrate, another to protons to form water.
• Are linked by NADPH-cytochrome P450
reductase* to the oxidation of NADPH to NADP+
• Have relatively low substrate selectivity; drugs
may be metabolized by more than one CYP; the
common characteristic is lipophilicity
• *also called NADPH cytochrome c reductase
A simple minded view of the liver
endoplasmic reticulum (microsomes)
Phase I biotransformation reactions*
• Oxidation (mainly by mixed function oxidases)
• Reduction
• Hydrolysis
*may result in inactivation, activation, or no
change in pharmacological potency/activity
Various enzymes involved in
metabolism of drugs
Metabolism of codeine (normal)
Phase II biotransformation reactions*
• Acetylation
• Glucuronidation
• Glutathione conjugation
• Sulfation
*almost always result in decreased potency and half life
(notable exception morphine-6-glucuronide ~ 100 x more
potent than morphine - may be created in the brain)
Patient case
• Donny Oregoni 12 y/o white male treated with
‘low dose’ imipramine for behavior problems and
enuresis. Rx for 1 month, renewable 3x.
• At three weeks Donny complains of tiredness,
weakness.
• At the end of a month of treatment, Donny feels
poorly; Stepmother gives Donny a warm bath; he
collapses and dies
• Imipramine plasma level in Donny was
>> 1 mg/L (therapeutic 0.1-0.3 mg/L)
Genetic polymorphism of
drug metabolizing enzymes
• Donny - had abnormal CYP2D6; about 3-10% of
whites, autosomal recessive
• Abnormal CYP 2D6 is also known as
‘debrisoquine polymorphism’ - such patients are
“slow metabolizers”, also called “poor
metabolizers” of debrisoquine
• Some ‘drug list’ items metabolized by 2D6:
carvedilol, cevimeline, chlorpheniramine,
efavirenz, imipramine, metoprolol, promethazine
and timolol)
Graphical representation of what happened
to Donny Oregoni
CYP2D6 deficiency also affects
metabolism of codeine
Therapeutic drug monitoring
• Potentially lifesaving
• Particularly important anytime dosage is changed
or drugs are added or withdrawn
• Expensive
• For some drugs, there is no simple relationship
between plasma level and therapeutic effect
GeneChip®* CYP450 Assay
(applied pharmacogenomics)
• A matrix on a chip (high density miniaturized
array of oligonucleotides) that can identify the
CYP phenotype of a patient. By the year 2000
enabled rapid detection of 18 known mutations of
CYP2D6 and CYP2C19. Could easily do
hundreds.
• In 2005 Roche AmpliChip® CYP 450 Test was
approved. Cost estimated at that time was about
$950 - cost will come down significantly
• Something like this is likely to become a part of
initial patient screening in the near future.
•
*http://www.affymetrix.com/
Patient Case
• Roberto Bolli, 22 y/o white male treated with
theophylline for control of asthma. Stable on
theophylline for 8 years.
• Transient skin infection - prescribed 10 doses of
ciprofloxacin to be taken over 5 days.
• At the end of day 5, Roberto was treated in the ER
for headache, nausea & vomiting and sent home.
Theophylline level not determined.
• Later that night, status epilepticus - theophylline
level >60 mg/L (10-20 mg/L ‘therapeutic’);
permanent brain damage.
Inhibition of biotransformation: A basis for
many different kinds of drug interaction
• Ciprofloxacin (and many other compounds)
inhibit CYP enzyme(s) involved in the
metabolism of theophylline (CYP1A2
&/CYP3A4)
• Roberto Bolli represents a tragic, but all too
common, case of drug interaction. Many Rx
drugs alter biotransformation. Even
grapefruit juice contains an inhibitor of
CYP3A4.
Increased bioavailability of some drugs in
patients who drink grapefruit juice
Induction of drug biotransformation: Another
basis of various drug interactions
• polycyclic aromatic hydrocarbons, etc. - CYP1A2
phenobarbital - CYP2C9
• ethanol - CYP2E1
• phenytoin - CYP3A4
• glucocorticoids, macrolide antibiotics
(erythromycin, etc.), anticonvulsants - CYP3A4
Some consequences of enzyme induction
• Smokers metabolize theophylline faster than nonsmokers - polycyclic aromatic hydrocarbons
induce CYP1A1
• Chronic consumption of alcohol induces CYP2E1
- promotes ‘pharmacokinetic tolerance’
• CYP3A is inducible by macrolide antibiotics,
some steroids and some anticonvulsants - may
increase metabolism of a wide variety of drugs
Examples of Substrates, Inhibitors and
Inducers of CYPs
Patient case, Heidi Raines, day 1
• Heidi Raines: 33-year-old white female, previously
healthy. Two days ago developed a bad headache with
nausea and vomiting. Yesterday developed a stiff
neck and was told over the telephone to take ExtraStrength Excedrin according to directions on the
bottle. Today made an ER visit (Dr. W) and was
treated with Demerol for headache pain and, finally,
was able to rest. She was felt to have a migraine and a
skeletal headache and was sent home with a
prescription for Demerol tablets (50 mg tabs, 1-2 q 34 h prn pain). Her husband was very concerned about
her and wanted to do all that he could to help.
Heidi Raines, day 2
• Mrs. Raines was seen (by Dr. X) in the Clinic with
headache, stiff neck, nausea, and vomiting.
Demerol was stopped, and she was treated with
Darvon without help. Continue Extra-Strength
Excedrin, 2 e 4 h prn pain.
Heidi Raines, day 5
• Day 5: Mrs. Raines was seen in the Clinic (by Dr
Y) and was given a prescription of Midrin®* (1
stat, then 1 q 3 h, prn). Her husband was
beginning to show increased anxiety concerning
her welfare. He said she had not really eaten for
days because of this 'sick headache'. Once again
he was reassured, and was told that he could help
by taking care of her at home. He promised to be
helpful.
*Isometheptene, 65 mg
Dichloralphenazone, 100 mg
Acetaminophen, 325 mg
Heidi Raines, day 6
• Day 6: Mrs Raines continued her medications and
seemed to improve, although she had been unable
to keep any food down for about one week. Her
headache worsened and on a Clinic visit (Dr. Z)
she was treated with Meprobamate and three Extra
Strength Tylenol and was sent home with
instructions to take three Extra Strength Tylenol
every 3-4 hours, prn. Same old story with the
husband.
Heidi Raines, day 7
• Day 7: Continuing medications as prescribed, Mrs.
Raines felt somewhat better on the day prior to
admission with little headache, nausea or
vomiting. However, she then developed crampy
abdominal pain, anorexia, and nausea and, as per a
telephone conversation, was treated with DiGel®,
but, because of persistent symptoms, was admitted
to the hospital.
Heidi Raines, day 8
• Physical exam on admission showed a thin, 33year-old, cigarette smoking, white female
complaining of abdominal pain, nausea, vomiting,
and headache. She talked in a weak voice. Her
neck was supple. The chest was clear. The heart
showed a sinus rhythm. There was some
abdominal tenderness with no rigidity and no
masses. The patient appeared pale and rather
drowsy, but had no focal neurologic findings.
Heidi Raines, hospital course
• Liver enzymes were massively elevated with SGOT
over 20,000 units. Fibrinogen level became
nondetectable and platelet count continued to fall.
Hematocrit and hemoglobin dropped. Bilirubin stayed
stable around 3.2 mg%. Creatinine rose from 2.5 to
3.5 mgm%. The BUN was 9 or 10 mg%. Total
protein was 4.5 with albumin of 3.1 grams%. Blood
ammonia was 239 mg%. Neurologically, the patient
deteriorated and became unresponsive. The cause of
death was felt to be a respiratory arrest with
subsequent cardiac arrest on the basis of severe
massive hepatic necrosis (emphasis added) with
metabolic changes and metabolic encephalopathy.
Heidi Raines: Autopsy
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Acute massive necrosis of the liver
Severe metabolic acidosis, lactic acidosis, treated.
Acute pulm. edema, with interalveolar hemorrhage
Diffuse intravascular coagulation.
Acute renal failure secondary to acute tubular necrosis.
Metabolic encephalopathy, secondary to hepatic coma
Metabol. acidosis severe lactic acidosis, H20 depletion
Anemia, ? hemolytic, ? due to blood loss into lungs.
Why did Heidi Raines die?
• Failure to properly diagnose and treat
The husband did it!!
• N-acetyl-phenylquinone imine (NAPQI) is a
MINOR, but potentially toxic, metabolite of
acetaminophen. NAPQI reacts with glutathione
unless there is no glutathione, then NAPQI reacts
with thiol groups on liver cell membranes - Voila!
Liver cell death.
• Normally, glutathione is rapidly synthesized in the
liver from various foodstuffs. However, in
animals (and people) deprived of food for a few
days, synthesis of glutathione will not occur (and
the toxicity of acetaminophen decreases by more
than ten-fold!).
Mechanism of Heidi Raines’ death
Learning objectives: drug biotransformation
•
•
•
•
•
Xenobiotic, endoplasmic reticulum & microsomes
First pass effect, bioavailability
Pro-drug, parent compound, metabolite (active or inactive)
Lipophilicity vs. hydrophilicity
Cytochrome P-450
(CYP1A2, CYP2C9 CYP2C19, CYP2D6, CYP2E1,
CYP3A4* )
• Phase I vs. phase II drug metabolism
• (debrisoquine) polymorphism
• “Extensive” and “poor” metabolizers