• Absorption

  • Fick's Law

• Routes of Administration

• First-Pass Effect

• Pulmonary Effects

• Pharmacokinetics

  • Volume of distribution

  • Clearance

    • Renal clearance: clearance of unchanged drug and metabolites

      • Other Factors Affecting Renal Clearance

    • Factors Affecting Hepatic Clearance

    • Capacity-Limited Elimination

    • Half-life

    • Drug Accumulation

  • Bioavailablity

    • Extent of Absorption

    • First-Pass Elimination

    • Rate of Aborption

  • Some Pharmacokinetic Equations

  • Placental Transfer

  • Redistribution

  • Drug-Plasma Protein Binding

  • Renal Clearance

• Drug Metabolism

  • Introduction

  • Phase I and Phase II Reaction Overview:

  • Phase I characteristics

  • Phase II characteristics

  • Conjugates

  • Principal organs for biotransformation

    • Sequence I

    • Sequence II

  • Bioavailability

  • Microsomal Mixed Function Oxidase System and Phase I Reactions

    • The Reaction

    • flavoprotein--NADPH cytochrome P450 reductase

    • Cytochrome P450: -- terminal oxidase

    • P450 Enzyme Induction

    • P450 Enzyme Inhibition

    • Human Cytochrome P450

  • Phase II Reactions

    • Toxicities

• Individual Variation in Drug Responses

• Genetic Factors in Biotransformation

• Effects of Age on Drug Responses

• Drug-Drug Interactions

Pharmacokinetics and some IV Anesthetics Agents

• Barbiturates

  • Thiopental

• Benzodiazepines

• Ketamine and Etomidate

• Propofol

• Opioids

  • Membrane Bilayer Structure

• Introduction:

  • Lipophilic drug properties that promote passage through biological membranes and facilitate reaching site to drug action inhibit drug excretion.

    • Note: renal excretion of unchanged drug contributes only slightly to elimination, since the unchanged, lipophilic drug is easily reabsorbed through renal tubular membranes.

  • Biotransformation of drugs to more hydrophilic molecules is required for elimination from the body

    • Biotransformation reactions produces more polar, hydrophilic, biologically inactive molecules -- that are more readily excreted.

      • Sometimes metabolites retain biological activity and may be toxic.

    • Drug biotransformation mechanisms are described as either phase I or phase II reaction types.

• Phase I and Phase II Reactions -- Overview

  • Phase I characteristics:

    •  Parent drug is altered by introducing or exposing a functional group (-OH,-NH₂, -SH)

    •  Drugs transformed by phase I reactions usually lose pharmacological activity

    •  Inactive, prodrugs are converted by phase I reactions to biologically-active metabolites

    •  Phase I reaction products may:

      • be directly excreted in the urine

      • react with endogenous compounds to form water soluble conjugates.

• Phase II characteristics:

  • Parent drug participates in conjugation reactions that:

    • form covalent linkage between a parent compound functional group and:

      • glucuronic acid

      • sulfate

      • glutathione

      • amino acids

      • acetate

• Conjugates are:

  • highly polar

  • generally inactive

    • exception to the rule: morphine glucuronide metabolite-- more potent analgesic then parent compound

  • rapidly excreted in the urine

  • High molecular weight conjugates:

    1. excreted in the bile

    2. conjugate bond may be cleaved by intestinal flora

    3. parent drug released back to the systemic circulation

    4. this process, "enterohepatic recirculation":

       1. delayed parent drug elimination

       2. prolongation of drug effect

Principal Organs for Biotransformation:

• Principal Organ: Liver

  • Other metabolizing organs:

    • gastrointestinal tract

    • lungs

    • skin

    • kidney

• Sequence I

  1. Oral administration (isoproterenol (Isuprel), meperidine (Demerol), pentazocine (Talwain), morphine)

  2. Absorbed intact (small intestine)

  3. Transported first to the liver (portal system) and

  4. Extensive metabolism -- first-pass effect

• Sequence II

  1. oral administration: (clonazepam (Klonopin), chlorpromazine (Thorazine))

  2. absorbed intact (small intestine)

  3. extensive intestinal metabolism -- contributing to overall first-pass effect

• Issues in bioavailability: reduced bioavailability

  • First pass effect: bioavailability of orally administered drugs -- so limited -- alternative routes of administration must be used

  • Intestinal flora may metabolize drugs

  •  unstable in gastric acid-- penicillin

  •  metabolized by digestive enzymes -- insulin

  •  metabolized by intestinal wall enzymes-- sympathomimetic catecholamines

Correia, M.A., Drug Biotransformation. in Basic and Clinical Pharmacology, (Katzung, B. G., ed) Appleton-Lange, 1998, pp 50-61.

Benet, Leslie Z, Kroetz, Deanna L. and Sheiner, Lewis B The Dynamics of Drug Absorption, Distribution and Elimination. In, Goodman and Gillman's The Pharmacologial Basis of Therapeutics,(Hardman, J.G, Limbird, L.E, Molinoff, P.B., Ruddon, R.W, and Gilman, A.G.,eds) TheMcGraw-Hill Companies, Inc.,1996, pp. 3-27

Mixed function oxidase System (cytochrome 450 System)--Phase I Reactions

• Microsomes have been used to study mixed function oxidases

  • Drug metabolizing enzymes:

    • located in lipophilic, hepatic endoplasmic reticulum membranes

    • smooth endoplasmic reticulum: contains enzymes responsible for drug metabolism

• The reaction:

  • one molecule oxygen is consumed per substrate molecule

  • one oxygen atom -- appears in the product; the other in the form of water

  • Oxidation-Reduction Process:

    • Two important microsomal enzymes:

      1. flavoprotein--NADPH cytochrome P450 reductase

      2. Cytochrome P450: -- terminal oxidase

         1. multiple forms

         2. named cytochrome P450 because:

            • the reduced (ferrous) form, binds carbon monoxide: -- the resulting complex exhibits of absorption maximum at 450 nm.

         3. NOTE in the Figure Below the CONVERSION OF RH to ROH representing DRUG OXIDATION

Cytochrome p450 cycle (diagram by  Matthew Segall, 1997)

1. "The binding of a substrate to a P450 causes a lowering of the redox potential by approximately 100mV, which makes the transfer of an electron favourable from its redox partner, NADH or NADPH.

2. The first reduction -The next stage in the cycle is the reduction of the Fe³⁺ ion by an electron transfered from NAD(P)H via an electron transfer chain.

3. Oxygen binding An O₂ molecule binds rapidly to the ion Fe²⁺ forming Fe²⁺-O₂

4. Second reduction A second reduction is required by the stoichiometry of the reaction. This has been determined to be the rate-determining step of the reaction

5. O₂ cleavage: The O₂ reacts with two protons from the surrounding solvent, breaking the O-O bond, forming water and leaving an Fe-O³⁺ complex.

6. Product formation The Fe-ligated O atom is transferred to the substrate forming an hydroxylated form of the substrate.

7. Product release The product is released from the active site of the enzyme which returns to its initial state."--Matthew Segall, 1997

• "The active site of substrate-free cytochrome p450:Note the water molecule (which can be seen as a single oxygen atom) that forms the sixth axial ligand of the haem iron. Oxygen atoms are shown in red, nitrogen in light blue, sulphur in yellow and iron in dark blue. Carbon atoms are shown in grey as bonds only and hydrogens have been omitted from this figure for clarity."

• "The active site of camphor-bound cytochrome p450_cam , an example of a substrate-bound system. Note the absence of the water molecule which formed the sixth axial ligand of the haem iron in the substrate-free enzyme."

• "A representation of with bound camphor. The enlarged active site region shows the camphor substrate, haem moiety and cysteine residue which forms the distal haem ligand. In the representation of the full enzyme the protein backbone is shown in green, the haem moiety in blue and the substrate is coloured according to atomic species. Oxygen atoms are shown in red, carbon in grey, nitrogen in light blue, sulphur in yellow and iron in dark blue."-diagrams and text  by  Matthew Segall, 1997

• Cytochrome P450 Enzyme Induction:

  • Following repeated administration, some drugs induce cytochrome P450 (increase amount of P450 enzymes) usually by:

    •  increase enzyme synthesis rate

    • reduced enzyme degradation rate

• Cytochrome P450 enzyme inhibition:

  • Certain drugs, by binding to the cytochrome component, act to competitively inhibit metabolism. Examples:

    •  Cimetidine (Tagamet) (anti-ulcer --H₂ receptor blocker) and Ketoconazole (Nizoral) (antifungal) bind to the heme iron a cytochrome P450, reducing the metabolism of:

      • testosterone

      • other coadministered drugs

      • Mechanism of Action: competitive inhibition

  •  Catalytic inactivation of cytochrome P450.

    •  Macrolide antibiotics (troleandomycin, erythromycin estolate (Ilosone)), metabolized by a cytochrome P450:

      • metabolites complex with cytochrome heme-iron: producing a complex that is catalytically inactive.

    •  Chloramphenicol (Chloromycetin): metabolized by cytochrome P450 to an alkylating metabolite that inactivates cytochrome P450

    •  Other inactivators: Mechanism of Action: -- targeting the heme moiety:

      • steroids:

        • ethinyl estradiol (Estinyl)

        • norethindrone (Aygestin)

        • spironolactone (Aldactone)

      • others:

        • propylthiouracil

        • ethchlorvynol (Placidyl)

Phase II Metabolism

• Overview: Phase II reactions:Non-microsomal enzymes

  • Reaction types:

    1. conjugation

    2. hydrolysis

    3. oxidation

    4. reduction

  • Location (non-microsomal enzymes): primarily hepatic (liver); also plasma & gastrointestinal tract

  • Non-microsomal enzymes catalyze all conjugation reactions except glucuronidation

• Conjugation reactions: Usually "detoxification reaction

• Conjugates:

  •  more polar

  •  easily excreted

  •  typically inactive

•  Conjugation:

  • Involves "high-energy" intermediates and specific transfer enzymes (microsomal or cytosolic transferases)

  •   Conjugation with glucuronic acid requires cytochrome P450 enzymes.

    • glucuronic acid: available from glucose

    • glucuronic acid conjugated to lipid-soluble drug results in lipophilic glucuronic acid derivative:

      • pharmacologically inactive

      • more water-soluble; more easily excreted in urine & bile

  • Transferases:

    • catalyzes coupling of an endogenous substance with a drug

      • uridine-5'-diphosphate (UDP) derivative of glucuronic acid with a drug

    • catalyzes inactivated drug within endogenous substrate

      • for example: S-CoA derivative of benzoic acid within endogenous substrate.

•  Toxicity:

  • Certain conjugation reactions: form toxic reactive species (hepatotoxicity)

    • Example:

      1. acyl glucuronidation nonsteroidal antiinflammatory drugs

      2. N-acetylation of isoniazid

  •  Drugs metabolized to toxic products:

    • Acetaminophen hepatotoxicity -- normally safe in therapeutic doses

    • Therapeutic doses:

      • glucuronidation + sulfation to conjugates (95% of excreted metabolites); 5% due to alternative cytochrome P450 depending glutathione (GSH) conjugation pathway

    •  At high doses:

      • Glucuronidation and sulfation pathways become saturated

      •  Cytochrome P450 dependent pathway: now more important

        • with depletion of hepatic glutathione, hepatotoxic, reactive, electrophilic metabolites are formed

        • Antidotes: N-acetylcysteine, cysteamine

          • N-acetylcysteine: protects patients from fulminant hepatotoxicity and death following acetaminophen overdose.