Lecture 15-16 ADME Biochem

Biomolecules and Enzymes

ADME (Absorption, Distribution, Metabolism, Excretion)

• ADME Overview:
  • For a drug to be effective, it must reach its site of action by traveling from the administration site to the target organ(s) or tissue(s).
  • The ADME process significantly influences drug design.
• Absorption:
  • The process by which a drug moves from its administration site into the systemic circulation.
• Distribution:
  • The reversible transfer of a drug to and from the systemic circulation.
• Metabolism:
  • Any chemical alteration of a drug by a living system, enhancing water solubility for excretion.
• Excretion (Elimination):
  • The irreversible transfer of a drug from the systemic circulation, such as through renal excretion or sweat.
• ADME Processes:
  • May involve specific processes like metabolic processes, carrier-mediated uptake, and active transport.
  • May also involve nonspecific absorption by passive diffusion.
• Importance in Drug Design:
  • ADME considerations are integral to drug design to ensure the drug retains sufficient activity from administration to the target site.
• Requirements for Biologically Active Compounds:
  1. Sufficient quantity at the site of action.
  2. Suitable duration of action; otherwise, the drug is not clinically useful.

Drug Dynamics and Kinetics

• Factors Influencing Drug Effect:Dosage Regimen \rightarrow ADME \rightarrow Concentration \space in \space plasma \rightarrow Concentration \space at \space site \space of \space action
• Pharmacokinetics:
  • Relates the dosage regimen to the concentration in the plasma and subsequently at the site of action.
• Pharmacodynamics:
  • Relates the concentration at the site of action to the effect.

Absorption Through Membranes

• Routes of Administration:
  • Apart from intravenous or muscular injections, drugs generally must pass through membranes.
• Membranes for Drug Absorption:
  • Gastric mucosa (oral drugs).
  • Buccal cavity (e.g., sublingual).
  • Rectal membranes (suppositories).
  • Vaginal membranes (pessaries).
  • Skin (transdermal patches).
  • Pulmonary epithelium (inhaled drugs).

Drug Delivery and Bioavailability

• Factors Affecting Drug Delivery:
  • Destruction in the gut.
  • Failure to be absorbed.
  • Destruction by the gut wall.
  • Destruction by the liver.
• Bioavailability:
  • Refers to the fraction of the drug available in the bloodstream after administration.
• Requirements for Oral Drugs:
  • Dissolve.
  • Survive a range of pH levels (1.5 to 8.0).
  • Survive intestinal bacteria.
  • Cross membranes.
  • Survive liver metabolism.
  • Avoid active transport to bile.
  • Avoid excretion by kidneys.
  • Partition into the target organ.
  • Avoid partitioning into undesired places (e.g., brain, fetus).

Solubility

• Impact of Solubility:
  • Solubility in different media (aqueous vs. lipid, pH ranges) impacts absorption, distribution, metabolism, and excretion.
• Solubility Ranges (mg/mL):
  • Practically insoluble: < 0.1
  • Very slightly soluble: 0.1 – 1
  • Slightly soluble: 1 – 10
  • Sparingly soluble: 10 – 33
  • Soluble: 33 – 100
  • Freely soluble: 100 - 1000
  • Very soluble: > 1000
• Testing Media:
  • Water, polyethylene glycol, glycerol, sorbitol, ethanol, benzyl alcohol, Tweens (surfactants), peanut oil, buffers of various pH.
  • Tested at 25 °C and 37 °C.

Solutions for Poor Solubility

• Salt Creation:
  • If the compound is ionizable, creating a salt may increase solubility, particularly with the right counter ion.
  • Example of intestinal flux with different counter ions:
    • Sodium: 3.09 μg cm-1 hr-1
    • Ethylamine: 5.42 μg cm-1 hr-1
    • Diethylamine: 7.91 μg cm-1 hr-1
    • Triethylamine: 48.4 μg cm-1 hr-1

Plasma Concentration/Time Profiles

• Concentration Dynamics:
  1. The compound is absorbed and enters the bloodstream, causing the concentration to rise.
  2. The compound distributes into tissues, and absorption rates begin to slow.
  3. The drug is metabolized and excreted from systemic circulation.
• Key Concentrations:
  • MEC (Minimum Effective Concentration): Minimum concentration required for therapeutic effect.
  • MTC (Maximum Tolerated Concentration): Maximum concentration that can be tolerated without causing significant side effects or toxicity.
• Aims:
  1. Achieve activity at the desired target sufficient to reach the MEC.
  2. Ensure the drug concentration does not reach levels that might cause side effects/toxicity (exceeding the MTC).

Multiple Dosing and Steady State

• Considerations for Oral Drugs:
  • Most oral drugs are given once or twice daily, necessitating consideration of multiple dosing effects.
  • The compound may not be entirely eliminated from the body before the subsequent dose.
• Steady State (Css):
  • The concentration builds up over time until it reaches a steady state, where the amount eliminated equals the amount dosed.
  • Css - Steady-state concentration.

Factors Affecting Drug Absorption

• Key Factors:
  • Acid stability.
  • Solubility.
  • Permeability.
  • Lipophilicity.
  • Metabolism.
• Gastrointestinal Environment:
  • Stomach pH: ~2
  • Intestine pH: ~7
• Intestinal Wall Structure:
  • Epithelial cells have microvilli on their luminal surface, forming the brush border membrane.
  • Compounds passing across the gut wall are taken by a network of capillaries via the hepatic portal vein to the liver.
• Acid Stability:
  • Oral drugs must pass through the stomach (pH ~2) before entering systemic circulation.
  • The drug must be stable under these acidic conditions at body temperature.
  • The small intestine’s pH is ~7 (majority of absorption often takes place here due to large surface area).
• Solubility:
  • The drug requires sufficient aqueous solubility for dissolution, as only dissolved compounds can be absorbed.
  • Fat globules have low surface area, resulting in poor surface contact and reduced absorption.
• Permeability:
  • Poor permeability, gut wall metabolism, and/or efflux can lead to poor absorption across the intestinal wall.
• Lipophilicity:
  • Drugs absorbed passively through the gut wall need to be sufficiently lipophilic to cross cell membranes but polar enough to be water-soluble.
  • log \space P = 1 – 4 (octanol/water partition coefficient measures ionization and intrinsic lipophilicity).
• Metabolism – Gut Wall / First Pass Metabolism:
  • Blood from the stomach, small and large intestine passes to the liver via the hepatic portal vein.
  • Some of the compound may be metabolized (first-pass metabolism) before reaching systemic circulation.

pH Variation in the Body and Drug Absorption

• Neutral Form Absorption: The neutral form of compounds is passively absorbed due to its lipophilic nature.
• Ionization and Absorption:
  • The same compound will ionize differently in different parts of the body.
  • Basic compounds are less well absorbed in the stomach compared to acidic compounds since it is generally the unionized form that diffuses into the bloodstream.
• pH Levels in Different Fluids:
  • Aqueous humor: 7.2
  • Blood: 7.4
  • Colon: 5-8
  • Duodenum (fasting): 4.4-6.6
  • Duodenum (fed): 5.2-6.2
  • Saliva: 6.4
  • Small intestine: 6.5
  • Stomach (fasting): 1.4-2.1
  • Stomach (fed): 3-7
  • Sweat: 5.4
  • Urine: 5.5-7.0
• Effect of pH on Drug Absorption:
  • In an acidic medium, an acidic compound will be less ionized and more passively absorbed.
  • In an acidic medium, a basic compound will be more ionized and less passively absorbed.
  • Acidic compounds generally have better fraction absorbed and bioavailability than bases, with neutral compounds in between.

Absorption Mechanisms

• Transcellular Absorption:
  • Main route for most oral drugs.
  • Drug must be in solution at the cell surface.
  • pKa is important, as the drug must be unionized.
  • Lipophilicity is important.
  • H-bonds need to be dispersed.
  • Occurs along the concentration gradient.
• Paracellular Absorption:
  • Drug passes through gaps between cells.
  • Inefficient due to pores with less surface area than cellular surface.
  • Restricted to low MW hydrophilic molecules (MW < 200).
• Active Transport:
  • Drugs are carried through the membrane by a transporter, requiring energy (ATP).
  • Occurs against a concentration gradient.
  • Many transporters exist for nutrient molecules, e.g., glucose, amino acids.
  • SAR (Structural Activity Relationship) specific, so few drugs are absorbed by this route.
  • ATP \rightarrow ADP + Pi

Distribution

• Distribution Process:
  • Compounds distribute out of plasma into tissues.
• Factors Influencing Distribution:
  • pKa, lipophilicity, plasma protein binding (only unbound drug in the tissue is free to distribute).
  • Tissue pH is slightly lower than plasma (blood) pH.
  • Basic compounds tend to distribute out of plasma into tissue more than acidic compounds.
• Distribution Definition:
  • The reversible transfer of a drug to and from the systemic circulation.

Interactions with Blood Proteins

• Plasma Protein Binding (PPB):
  • Drugs can bind to macromolecules in the blood.
  • Compounds with high PPB are retained in the plasma and cannot distribute into the tissues.
  • Only unbound compound is available for distribution into tissues (bioavailable).
• Binding Levels:
  • 0-50% bound = negligible
  • 50-90% = moderate
  • 90-99% = high
  • >99% = very high
• Blood Components:
  • Blood also contains enzymes that may degrade drugs.

Plasma Protein Binding Specifics

• Serum Albumin:
  • Most abundant protein in blood.
  • Binds various hydrophobic molecules non-specifically, with a preference for acids (e.g., fatty acids) and steroids.
• Alpha-1 Acid Glycoprotein:
  • Basic drugs tend to bind to alpha-1 acid glycoprotein.

Drug-Drug Interactions

• Competition for Binding:
  • Occurs when multiple drugs compete for binding blood proteins, changing baseline binding levels.
  • Usually increases the bioavailability of one or both drugs.
• Clinical Implications:
  • Co-administration of probenecid can increase the bioavailability of penicillin.
  • A sudden increase in bioavailability can lead to toxicity.
• Albumin Binding:
  • Can increase the long-term solubility and release of hydrophobic drugs.

Drug Metabolism Concepts

• Metabolism Definition:
  • Chemical reactions in the body to maintain life.
  • Allows organisms to grow and reproduce, maintain structures, and respond to environments.
• Categories:
  • Catabolism: Breaks down organic matter.
  • Anabolism: Uses energy to build up or construct components of cells, such as proteins and nucleic acids.
• Drug Metabolism:
  • Catabolism: breakdown of drug molecules.
  • Anabolism: modification with the addition of water-solubilizing groups.
• Drug Origin:
  • Drugs are synthesized in the laboratory.
  • Not an endogenous event but a xenogenous one, originating outside the organism.
• Lipid Solubility:
  • Lipid-soluble drugs require more metabolism to become polar, ionizable, and easily excretable, involving both Phase I and Phase II mechanisms.

Drug Metabolism Details

• Cellular Site of Metabolism:
  • Endoplasmic reticulum and endosomes.
• Purpose:
  • To enhance water solubility (hydrophilicity) and hence excretability.
• Definition:
  • Any chemical alteration of a drug by the living system.
• Main Site:
  • Liver (first-pass effect) involves cytochrome p450.
• Other Sites:
  • Gastrointestinal wall, kidneys, blood.

Factors Affecting Metabolism

• Drug Structure:
  • Influences its physicochemical properties.
  • The more complex the structure, the more potential sites for metabolism.
• Availability:
  • Blocking/altering sites of metabolism can improve the availability of the drug.

Phase I and Phase II Metabolism

• Phase I - Oxidative Transformations:
  • New polar groups are introduced or exposed by oxidation, reduction, hydrolysis.
• Phase II - Conjugation:
  • The original drug or its metabolite is made more polar by conjugation reactions.
• Process:
  • Hydrophobic compounds are metabolized during two metabolic phases.

Phase I Metabolism Details

• Oxidation Catalysis:
  • Catalyzed by the enzyme cytochrome p450 (CYP).
• Enzyme Location:
  • Membrane-bound enzyme on the endoplasmic reticulum.
• Important Enzymes:
  • Two most important enzymes are CYP3A4 and CYP2D6.
• Interactions:
  • Interactions with warfarin, antidepressants, antiepileptic drugs, and statins often involve cytochrome p450 enzymes.

Redox Processes

• Oxidation Definition:
  • Loss of electrons or an increase in oxidation state by a molecule, atom, or ion.
• Reduction Definition:
  • Gain of electrons or a decrease in oxidation state by a molecule, atom, or ion.
• Biological Oxidation:
  • Most biological oxidation reactions involve loss of hydrogen ions – dehydrogenation reactions.

Cytochrome p450 Enzymes (CYP-450)

• Active Site:
  • Heme-containing proteins with a porphyrin ring coordinated to iron at the active site.

CYP Inhibition or Induction

• CYP Inhibition:
  • A drug binds to a particular CYP and inhibits it, preventing it from metabolizing other substrates.
  • Important when multiple drugs are dosed together.
• CYP Induction:
  • A drug leads to the expression of greater levels of a particular CYP, affecting the pharmacokinetics of co-administered compounds.

Phase I Metabolism Examples

• Oxidation:
  • Aliphatic or aromatic hydroxylation.
• Reduction:
  • Nitro reduction to hydroxylamine/amine.
• Hydrolysis:
  • Ester or amide to acid and alcohol or amine.

Phase II Metabolism

• Process:
  • The original drug or its metabolite is made more polar through conjugation reactions catalyzed by transferase enzymes.
  • The resulting conjugates are usually inactive.
• Addition:
  • Addition of a group to the molecule.
• Conjugation Examples:
  • Glucuronide formation (Glucuronyl transferase).
  • Sulphate (Sulfotransferase).
  • Glutathione derivatives (Glutathione).

Phase II Metabolism Details

• (i) Glucuronidation:
  • Carboxylic acid, alcohol, phenol, amine.
• (ii) Sulphation:
  • Alcohol, phenol, amine.
• (iii) Glutathione Conjugation (gly-cys-glu):
  • Halogenated compounds and epoxides.

Metabolism of Codeine – Phase I

• Metabolic Pathways:
  1. Norcodeine by CYP3A4.
  2. Codeine-6-glucuronide by glucuronidation (~80%).
  3. Morphine via CYP2D6 represents only 10% of codeine clearance.
• Morphine Conversion:
  • Codeine is routinely converted to morphine in the body to be an effective painkiller.
  • The metabolism of codeine to morphine takes place in the liver.

Metabolism of Codeine – Phase II

• Further Metabolism:
  • Morphine is further metabolized to morphine-6-glucuronide and morphine-3-glucuronide, followed by renal elimination.

Metabolism of Codeine – Inhibitors and Genetics

• Factors Affecting Metabolism:
  • Ultra-rapid CYP2D6 metabolism, inhibition of CYP3A4, and acute renal failure can lead to unexpected high morphine concentrations.
• Genetic Variability:
  • Most people have normal CYP2D6 activity and their response to codeine is as expected.
  • A substantial minority of people have CYP2D6 activity that is higher or lower than normal, potentially resulting in excessive or inadequate response to codeine.

Prodrugs

• Definition:
  • Inactive compounds converted to an active compound in the body.
  • Usually happens through metabolic processes but can occur by simple chemical reaction.
• Purpose:
  • Used to improve ADME properties.

Prodrugs – Membrane Permeability

• Improving Permeability:
  • Reduce polarity using esters, which are broken down by many endogenous esterases.

Prodrugs - Slow Release

• Prolonging Drug Activity:
  • Releasing the active compound slowly.

Prodrugs – Mask Toxicity

• Reducing Side Effects:
  • Can be safely taken orally, subsequently metabolized into a toxic compound.

Prodrugs – Alter Physiochemical Properties

• Modifying Properties:
  • Examples include chloramphenicol palmitate and chloramphenicol succinate to alter water solubility and taste.

Prodrugs - Targeting

• Specific Site Targeting:
  • Methenamine is used to treat urinary tract infections; it is activated only when it passes into a urinary tract which is acidic because of an infection.

Prodrugs – Tap into Metabolic Pathways

• Utilizing Metabolic Processes:
  • Example: 5-Fluorouracil (5-FU) is metabolized to FdUMP, which blocks the synthesis of dTMP, and F(d)UTP, which is incorporated into DNA/RNA, leading to cell death.

Excretion (Elimination)

• Definition:
  • The irreversible transfer of a drug from the systemic circulation.
• Major Routes of Elimination:
  • Renal excretion (for free drug, i.e., low logD).
  • Biliary excretion.
  • Also: lungs, sweat, etc.

Urine Formation

• Nephrons:
  • Functional unit of the kidney, with 1,000,000 nephrons in each kidney.
  • Each nephron consists of a glomerulus and a system of tubules.
• Tubular Reabsorption:
  • The removal of water (~99%) and solutes from the filtrate.
  • The water and solutes return to the blood via the peritubular capillaries.
• Tubular Secretion:
  • Transport of excess solutes and wastes from the peritubular fluid into the tubular fluid.

Renal Excretion of Drugs

• Process:
  1. All unbound drug in plasma is filtered in the glomerulus (significant for very polar compounds, log P < 0).
  2. Some compounds are actively secreted into urine along the proximal tubule.
  3. Un-ionized drug can undergo passive reabsorption from urine into blood along the length of the nephron (net excretion may be zero).
  4. Drug that is bound to plasma proteins is not filtered.

Solubility - Revisited

• Refer to solubility information previously presented.

Solubility – Polymorphs Problem

• Polymorphism:
  • One compound can crystallize in a variety of ways; each version has different solubility.
• Examples:
  • >60 forms of Lipitor known.
  • ~1/3 drug molecules have known polymorphs.
  • Carbamazepine (anticonvulsant) has at least four polymorphs; Form II is 10% more soluble than Form III.

Solubility - Solutions

• 1. Change the Compound Design:
  • Tune pKas by changing substituents.
  • Acids: electron withdrawing = more acidic / electron donating = less acidic.
  • Bases: electron withdrawing = less basic / electron donating = more basic.
• 2. Create a Salt:
  • If the compound is ionizable, a salt may be more soluble, particularly if you choose the right counter ion.
• 3. Change Formulation:
  • Particle engineering, solid dispersions, liposomes and micelles, complex formation, cosolvents.

Formulation

• Dosage Forms:
  • Liquid: Solutions, syrups, suspensions, emulsions.
  • Semisolid: Creams, ointments, gels.
  • Solid: Tablets, capsules, suppositories, transdermal patches.
• Importance:
  • Getting the formulation right is critical for clinical testing and marketing of a drug.

Formulation Details

• Components:
  • API (active pharmaceutical ingredient) - the drug itself.
  • Excipients - other ingredients which improve its delivery quality.
• Excipient Types:
  • Fillers: Bulking agents to make small quantities easier to handle (e.g., starch, calcium salts, sugars like lactose).
  • Lubricants/antiadherents: Reduce friction during manufacturing (e.g., magnesium stearate).
  • Binders: Polymers used to turn powders into granules and pills (e.g., gelatin, polyethylene glycol, polysaccharides).
  • Preservatives: Usually anti-oxidants (e.g., vitamin C, citric acid) or antifungal/bacterials (parabens).
  • Flavorings: Sweeteners (usually artificial) or others to mask unpleasant taste.
  • Coatings: Protect from unpleasant taste; usually hydroxypropyl methylcellulose. Capsules are coated in gelatin.
  • Vehicles: In liquids and gels, the rest of the liquid: water, mineral oil, DMSO.

Summary

• Drug Metabolism:
  • All drugs will be metabolized by the body in some way.
  • Phase I metabolism involves adding/removing functionality to make the molecule more processable and polar.
  • Phase II metabolism involves adding larger groups to further increase solubility.
  • Both processes can be used to control drug activity and induce the right function at the right time (prodrugs).
• Excretion:
  • Compounds of high solubility are readily excreted in urine.
• ADME Influence:
  • All ADME aspects are influenced by the drug’s solubility under various conditions.
• Optimization:
  • Getting this right can involve changing the molecule or fine-tuning the formulation.
• Drug Delivery:
  • Drugs need to get to their targets to have the desired effects.
  • Administered in a range of ways to provide either direct access to the tissue of interest or through the circulatory system.
• Absorption Requirements:
  • Drugs also need to be absorbed by the appropriate tissues.
  • Requires tuning their physicochemical properties to pass through the right membranes, considering physiological variations in conditions.
• Distribution Determinants:
  • The distribution of drugs throughout the body is determined by physicochemical properties of tissues (e.g., pH) and by blood plasma proteins.