1. List of 10 extra slides shown by Professor Printz - January 2014
SLIDE # DESCRIPTION
Pharmacokinetic Lecture
Diffusion process / Brief Intro to Fick’s Law / Drug permeability concept
Partition Coefficient modification of original drug distribution slide
Henderson-Hasselbach calculation – examples
Easy thought process of ion trapping
Drug Metabolism Lecture
Drug/Xenobiotic metabolism overview
Examples of metabolism consequences
Phenacetin  acetaminophen  NAPQI  toxicity
Discussion of potential consequences of drug distribution
Cytochrome P450 isozymes – drugs affected
10. Outline of benzopyrene metabolism to carcinogenic products

2. Drug diffusion through phases:
Aqueous phase – passive by concentration gradient*
Lipid phase – passive by both concentration gradient* & lipophilicity of drug
Across barriers:
Carriers
Endocytosis / pinocytosis
Fick’s Law:
Flux = [C1 – C2] X { P } X (Area)
P is Permeability
P = Permeability coefficient
Thickness
Luellmann, 2011

3. Drug Distribution Membrane Permeation: [solute]oct
Passive Diffusion:
Requires some degree of lipid solubility, which is in part determined by the charge of the molecule
For weak acids or bases (which account for the vast majority of drugs), the charge of the molecule in dependent on the pH of the medium as determined by the Partition coefficient and the Henderson-Hasselbalch Equation:
Log ([H+Drug] / [Drug]) = pKa - pH
Active Transport: Drugs “highjack” cellular transporter (e.g. L-DOPA uptake via L-amino acid carrier)
Receptor-mediated Endocytosis: Clathrin-coated pits form endosomal vesicles; receptor gets “recycled” to the cell surface
[solute]oct
Log P(oct/water)=Log _________
[solute]water
Values negative = more water soluble
Values positive = more octanol soluble
Only the non-protonated form of the drug can move across the lipid membrane to establish an equilibrium. Due to the lower pH of the urine compared to the blood, more of the drug will be converted into the protonated form in the urine. Consequently, the protonated drug can not move back across the membrane and is trapped in the urine --> elimination (this principle is applied when urine acidification is used to detoxify OD victims).

4. WHICH DIRECTION WILL THE DRUG MOVE, BLOOD TO URINE OR URINE TO BLOOD?
Acid : HA A- + H+ Base : BH B + H+
Diffusible Form = HA Diffusible Form = B
Henderson-Hasselbach Equation:
Log [protonated form] = pKa - pH
[unprotonated form]
Consider 2 compartments – with different pH values
blood pH = urine pH = 5.4
WHICH DIRECTION WILL THE DRUG MOVE, BLOOD TO URINE OR URINE TO BLOOD?
Always set X to = nondiffusible form, set the diffusible form concentration = 1.0 uM
Consider weak base: Unprotonated form is readily diffusible. Assume a pKa = 7.0
BLOOD: URINE:
Log {X} = 7.0 – 7.4 = log {X} = 7.0 – 5.4 = 1.6
{1} {1}
X = X = 39.81
Total species = = 1.398uM = = 40.81uM
As the base moves from blood into urine, it gets trapped there
Consider weak acid. Protonated form is readily diffusible; Assume a pKa = 4.4
BLOOD: STOMACH:
Log {1} = 4.4 – 7.4 = log {1} = 4.4 – 1.4 = 3.0
{X} {X}
Log X = 3 X = Log X = -3.0 ; X = 0.001
Total species = = 1001uM = = uM
As the acid moves from stomach into blood, it gets trapped there

5. Easy thought process for direction of drug movement “Ion Trapping”
For weak acid drugs:
if compartment pH is LESS THAN pKa, less ionized, more of drug will pass bilayer membranes – increased flux across bilayer
if compartment pH is MORE THAN pKa, more ionized, less of drug will pass bilayer membranes – decreased flux
For weak base drugs:
if compartment pH is LESS THAN pKa, more ionized, less of drug will pass bilayer membranes – decreased flux
if compartment pH is MORE THAN pKa, less ionized, more of drug will pass bilayer membranes – increased flux

6. Drug Metabolism
NEW
Why is drug metabolism so important? Elimination of drugs and chemicals by the kidney is often compromised because the drug/chemical is too nonpolar, lipophilic and readily “reabsorbed” from tubular fluid. Metabolism can convert the drug to a more hydrophilic compound reducing reabsorption.
Most metabolic products are less pharmacologically active
Important exceptions:
Where the metabolite is more active - 3 examples
(Prodrugs, e.g. Erythromycin-succinate (less irritation of GI) --> Erythromycin, enaliprilat -> enalapril, codeine)
Where the metabolite is toxic (acetaminophen)
Where the metabolite is carcinogenic
Close relationship between the biotransformation of drugs and normal biochemical processes occurring in the body:
Metabolism of drugs involves many pathways associated with the synthesis of endogenous substrates such as steroid hormones, cholesterol and bile acids
Many of the enzymes involved in drug metabolism are principally designed for the metabolism of endogenous compounds
These enzymes metabolize drugs only because the drugs resemble the natural compound

7. Examples of more active metabolites
Not Responsible For
Examples of more active metabolites
Illustration Only
Erythromycin – gram+ antibiotic; pH sensitive (enteric coating), nonpolar, esterified (succinic acid, proprionic acid); converted by cell esterases
Enaliprilat - ACE-Inhibitor; prodrug; esterase converts to Enalapril (active)
Codeine – O-demethylation to morphine – more active analgesic than codeine; CYP2D6 metabolic enzyme; deficient in 10% caucasians, 2% in asians; reduced analgesia for same dosage

8. Different Metabolites: 1 More active, 1 More Toxic
New – General information
Phenacetin: 1887, analgesic, antipyretic, negative inotropic.
Present in APC headache mix: aspirin+phenacetin+caffeine
Use today: “cutting” cocaine, adulterant; chronic use leads to renal papillary necrosis due to metabolites
Ethyl ester  de-ethylation to acetaminophen (CYP2A13, CYP1A2)
Acetaminophen more potent than phenacetin
Phenacetin & acetaminophen conjugated with glucuronic acid or sulfate for elimination
Phenacetic metabolized by monooxygenase hydroxylation to toxic metabolites –
NAPQI = N-acetyl-(1,4) benzoquinone imine and epoxides
Acetaminophen also conjugated and hydroxylated (CYP2E1, CYP2A6, CYP1A2)
Hydroxylation leads to toxic metabolite imine and epoxide
Detoxification: conjugation with Hepatic cell glutathione (GSH)
If Overdose – deplete hepatic GSH; metabolites  mitochondrial dysfunction, oxidative damage to proteins, liver cell necrosis and hepatic failure
Therapy: N-acetyl cysteine or methionine

9. Drug Metabolism A B
A. Hydrophilic drugs absorbed in GI tract, pass through liver, get excreted by kidney.
B. Lipophilic drug without metabolism, absorbed into blood, may get excreted into tubular fluid in kidney but reabsorbed – no elimination.
C. Lipophilic drug slowly metabolized by liver enzymes; hydrophilic metabolite only one eliminated, unchanged drug recirculates.
D. Lipophilic drug rapidly metabolized by liver enzymes, nearly complete elimination by kidney. C D

10. Not Responsible For

11. Not Responsible For