Pharmacokinetics: Membrane Transport, Metabolism, and Distribution

Membrane diffusion and drug design fundamentals

• Drug movement across membranes is essential to target action, metabolism, and toxicity; design molecules with characteristics that favor reaching the site of action.
• LogP and LogD are key descriptors of a molecule’s behavior in membranes:
  • $\log P = \log<em>{10}\left(\frac{[\text{Drug}]</em>{\text{octanol}}}{[\text{Drug}]_{\text{water}}}\right)$, describing lipophilicity of the neutral form.
  • $\log D<em>{pH} = \log</em>{10}\left(\frac{[\text{Drug}]<em>{\text{lipophilic}}}{[\text{Drug}]</em>{\text{aqueous}}}\right)$ at a given pH; accounts for ionization. At physiological pH (roughly 7.5), LogD is used to reflect actual distribution in vivo.
• Cell membranes are complex barriers: not just a lipid bilayer but contain cholesterol (provides strength and integrity), proteins, and carbohydrates (glycoproteins). The liver synthesizes most cholesterol; membranes support structure, signaling, and hormone synthesis.
• Lipophilicity promotes diffusion into lipid environments, but excessive lipophilicity can reduce aqueous dispersal and overall absorption if a molecule clumps in gut contents. A practical guideline is a balance where logP is around the mid-range (neither too high nor too low).
• Octanol is a common membrane model because it is immiscible with water but can be wetted by a small amount of water due to the alcohol group (octanol is described as “wet”). This models how drugs partition into membranes from the aqueous environment.
• Transport across membranes occurs by multiple routes: passive diffusion through the lipid phase, aqueous diffusion through aqueous pathways, and transporter-mediated processes.

Lipophilicity, diffusion, and modeling membrane crossing

• Bigger logP values generally indicate greater lipophilicity and a greater tendency to partition into the lipophilic phase, up to a practical limit (~logP ≈ 5).
• Diffusion across membranes involves both lipid diffusion (lipophilic pathway) and aqueous diffusion (hydrophilic pathway).
• Aqueous diffusion is important for movement within the aqueous compartments of the body (gastrointestinal tract lumen, interstitial fluid, intracellular water). If a drug is too lipophilic, it can aggregate in gut water and hinder diffusion.

Fick’s law and diffusion across membranes

• Simple diffusion is often described by a form of Fick’s law:
  • $J = P\,(C<em>1 - C</em>2)$
    where J is the flux, P is the permeability (a composite parameter including diffusion and partitioning), and (C1 - C2) is the concentration difference across the membrane.
• Diffusion is driven by Brownian motion and concentration gradients; over time, diffusion tends toward equilibrium with equal concentrations on both sides.
• Ionization reduces membrane permeability: charged species cross lipid membranes poorly, which is why pH and pKa strongly influence absorption and distribution.

Henderson–Hasselbalch, ionization, and pH-dependent distribution

• The Henderson–Hasselbalch equation describes the ratio of ionized to non-ionized species:
  • $\mathrm{pH} = \mathrm{p}K<em>a + \log</em>{10}\left(\frac{[A^-]}{[HA]}\right)$ for acids (and a similar form for bases).
• Drugs are often weak acids or bases; their ionization state changes with pH as they traverse different body compartments (stomach, small intestine, urine).
• Examples and implications:
  • The stomach is highly acidic (low pH); the small intestine is around pH 7.5–8; urine pH can be acidic or basic depending on physiology and renal handling.
  • For an acidic drug like aspirin, ionization state changes along the GI tract, altering its permeability and absorption.
• LogP vs LogD distinction: logP is for the neutral form; logD at physiological pH (e.g., 7.5) accounts for ionization, giving a more realistic picture of behavior in vivo.
• Example note: Warfarin’s LogD can differ across pH; it illustrates how ionization affects distribution.
• If a drug cannot become charged in a given environment, logD at that pH resembles logP; for many drugs, logD7.5 is used in medicinal chemistry discussions.

Routes of administration and absorption considerations

• Oral: most common; not required to be sterile. The drug goes through the GI tract and the liver before reaching systemic circulation (first-pass effect).
• Intravenous (IV): direct access to systemic circulation; sterility is essential.
• Sublingual: avoided first-pass liver metabolism; GTN (glycerol trinitrate) is a classic example.
• Rectal: partial first-pass metabolism; sterility not strictly required; absorption can be variable (e.g., hemorrhoids affect consistency).
• Nasal: non-sterile formulations often acceptable; used for some agents (e.g., opioids in palliative care).
• Eye drops: sterile because local effect in eye; safety and contamination are critical.
• Dermal patches: no sterility required; used for hormone replacement therapy, nicotine patches, and some strong opioids.
• Key concept: whether absorption is fed or fasted affects gastric motility, splanchnic blood flow, and first-pass metabolism; fasted vs fed state can significantly alter oral absorption.
• Splanchnic blood flow feeds the intestines and is connected to the portal vein leading to the liver; this pathway is central to first-pass metabolism.

Carrier-mediated transport in absorption and distribution

• Solute carrier (SLC) transporters mediate passive transport of anions/cations along concentration gradients; they do not hydrolyze ATP themselves.
• SLC-22 family includes:
  • Organic anion transporters (OATs, SLC22A)
  • Organic cation transporters (OCTs, SLC22A)
• SLC transporters regulate uptake of neurotransmitters and many drugs; they can be drug targets themselves.
• ATP-binding cassette (ABC) transporters are mainly exporters (efflux pumps) that hydrolyze ATP to pump substrates out of cells.
• P-glycoprotein (P-gp, also known as MDR1 or ABCB1) is a well-studied ABC transporter that often limits drug brain penetration by effluxing drugs back toward the bloodstream.
• Example impact: paclitaxel exposure is reduced in brain due to P-gp activity; drug design must consider P-gp interactions to avoid limited CNS exposure or adverse drug interactions.
• Transporters influence oral absorption and tissue distribution; they interact with dietary state and formulation.

First-pass metabolism, bioavailability, and hepatic relevance

• First-pass metabolism: oral drugs encounter gastric acid, intestinal enzymes (flora and intestinal wall enzymes), then hepatic metabolism via the portal circulation before reaching systemic circulation.
• The liver is the major site of drug metabolism; hepatic extraction ratio (ER) describes the fraction of a drug removed during hepatic passage:
  • ER communicates how much of the entering drug is metabolized by the liver; commonly linked to hepatic clearance and liver blood flow.
  • A higher ER reduces oral bioavailability, unless alternative routes bypass first pass (e.g., sublingual GTN).
• Bioavailability:
  • $F = \frac{AUC<em>{po}}{AUC</em>{iv}}$, i.e., the systemic exposure after oral administration relative to IV dosing.
• Hepatic metabolism and enzymes (Phase I and II) alter the drug before it reaches the systemic circulation; variability in liver function, genetics, and drug interactions can drastically change F.
• Many drugs are available in oral forms despite high hepatic extraction due to balancing factors like dosing strategy, formulation, and alternative routes.

Volume of distribution and tissue distribution concepts

• Volume of distribution (V_d) is an apparent volume into which a drug distributes; not a real physical space but a theoretical one that helps explain plasma concentrations after dosing.
  • $V_d = \frac{\text{amount of drug in body}}{\text{plasma drug concentration}}$
• Examples:
  • Aspirin: $V_d \approx 0.15\ \text{L/kg}$ (relatively limited distribution due to acidity and plasma protein binding).
  • Chlorpromazine: $V_d \approx 21\ \text{L/kg}$ (extensive distribution into tissues; high lipophilicity).
• Higher logP often correlates with larger V_d because drug molecules partition into tissues more readily.
• Tissue affinity and reservoirs can dramatically alter apparent duration of action:
  • Thiopental and fentanyl distribute into fatty tissues, acting as reservoirs and releasing slowly over time.
  • Tissue affinity can also be for specific areas (e.g., eye melanin, neuromelanin, bone).
• Body water compartments relevant to distribution:
  • Total body water (TBW) ≈ 40 L.
  • Extracellular fluid (ECF) ≈ plasma (3 L) + interstitial (12 L) = ~15 L, which is about 37.5% of TBW.
  • Intracellular fluid (ICF) ≈ ~25 L.
• Implication: physicochemical properties of a drug determine penetration into these compartments and thus its clinical exposure and effects.

Drug metabolism: Phase I and Phase II

• The liver is the primary site of drug metabolism; metabolism converts lipophilic molecules into more water-soluble entities for elimination.
• Phase I (catabolic): introduces or unmasks functional groups; often oxidation (cytochrome P450 system), sometimes reduction or hydrolysis; typically makes molecules more polar but not always inactive.
• Phase II (conjugation): attaches polar groups (glucuronide, sulfate, glutathione, acetyl, glycine, etc.) to increase water solubility; usually inactive and readily excreted.
• A general tendency: many drugs are oxidized in Phase I, then conjugated in Phase II to facilitate excretion.

Cytochrome P450 enzymes: major players in Phase I metabolism

• The Cytochrome P450 system (often referred to as the mixed-function oxidases, MFOs) comprises multiple isoforms with overlapping substrate specificities.
• Core components include:
  • CYP enzyme (e.g., CYP3A4, CYP2D6, CYP2C9, CYP1A2, CYP2A6, CYP2C19, CYP2E1, CYP2C8)
  • Cytochrome P450 reductase
  • A phospholipid-rich membrane environment in the smooth endoplasmic reticulum
• The catalytic cycle involves electron transfer from NADPH via the reductase to the heme iron of the P450, enabling oxidation of RH to ROH with O2 and release of water.
• Naming and prevalence:
  • The numeric part (e.g., 3A4, 2D6) identifies family, subfamily, and isoform; P450s are named with a family (e.g., CYP3A) and an enzyme (e.g., CYP3A4).
  • CYP3A4 is responsible for the largest share of drug metabolism; CYP2D6 handles many clinically important drugs (e.g., codeine to morphine).
• Not all CYPs metabolize drugs equally; some contribute only modestly (e.g., 1A2, 2C19, 2E1).
• A single drug is often metabolized by multiple CYPs; metabolic redundancy is common (e.g., warfarin is metabolized by 1A2, 2C9, 2C19, 3A4).

Codeine, prodrugs, and pharmacogenomics

• Codeine is a prodrug activated to morphine primarily by CYP2D6.
• Genetic polymorphisms in CYP2D6 create distinct metabolizer phenotypes:
  • Poor metabolizers: limited conversion to morphine → reduced analgesia from codeine.
  • Extensive (normal) metabolizers: expected analgesia from morphine formation.
  • Ultra-rapid metabolizers: faster conversion to morphine → higher morphine exposure and risk of toxicity (nausea, respiratory depression) in some individuals.
• Population variability: ultra-rapid metabolizers exist in several populations; Kirchner et al. demonstrated significant AUC differences for morphine exposure among phenotypes.
• Prodrugs and activation: several important drugs require activation by metabolism (examples include tamoxifen, clopidogrel, ramipril).
• Implications: genetic testing and personalized dosing can be clinically important for drugs with CYP2D6 involvement.

Phase II conjugation and detoxification pathways

• Glucuronidation: UDP-glucuronosyltransferases (UGTs) attach glucuronic acid to substrates (glucuronidation) to form water-soluble glucuronides.
• Sulfation: sulfotransferases add sulfate groups to increase solubility.
• Glutathione conjugation: glutathione-S-transferases facilitate conjugation with glutathione (GSH); this pathway is particularly important for detoxification and is non-enzymatic in parts of the reaction, relying on the abundant hepatic GSH pool (~10 mM).
• Paracetamol (acetaminophen) metabolism as a classic example:
  • Most paracetamol is metabolized via glucuronidation and sulfation (≈90%).
  • A small fraction undergoes Phase I oxidation by CYP2E1 to form the reactive metabolite N-acetyl-p-benzoquinone imine (NAPQI).
  • NAPQI is detoxified by glutathione conjugation; in overdose, glutathione stores are depleted, leading to hepatotoxicity and potential liver failure.
• Typical pathway example: paracetamol → N-acetyl-p-benzoquinone imine (NAPQI) via CYP2E1 (Phase I); NAPQI conjugated with glutathione (Phase II) and excreted.
• Other Phase II conjugation examples include glucuronide and sulfate conjugates for many drugs; glucuronidation often increases clearance and reduces activity.
• Aspirin example: hydrolysis of the ester bond (Phase I-like hydrolysis) yields salicylic acid; subsequent conjugation alters activity and excretion.

Pharmacokinetic concepts tied to drug distribution and safety

• Volume of distribution (V_d) is strongly influenced by lipophilicity, tissue binding, and tissue affinity; it is not a physical space but a theoretical construct that helps predict plasma concentration changes after dosing.
• Tissue affinity and compartments influence drug action duration; fat can serve as a reservoir for lipophilic drugs (e.g., thiopental, fentanyl).
• The blood-brain barrier (BBB) restricts entry into the CNS:
  • Tight junctions between endothelial cells
  • Continuous basement membrane
  • Few paracellular aqueous pores
  • Efflux pumps (e.g., P-gp) limit CNS penetration
  • Near absence of pinocytosis

Blood-brain barrier and CNS penetration considerations

• For many CNS-acting drugs, crossing the BBB is essential; lipophilicity and transporter interactions determine CNS exposure.
• P-gp and other efflux transporters can limit brain penetration of substrates, impacting efficacy and risk of interactions.

Practical and diagnostic implications for drug development and prescribing

• Lipinski’s Rule of Five (empirical guidelines) summarize factors that influence oral bioavailability:
  • Lipophilicity (logP), molecular weight, hydrogen-bond donors/acceptors, and related properties influence oral absorption.
  • A balance of lipophilicity, molecular size, and transporter interactions is generally favorable for oral drugs; excessively lipophilic or large molecules may have poor bioavailability.
• Drug design must consider transporter interactions (SLCs and ABCs), BBB penetration, first-pass metabolism, and tissue distribution to optimize efficacy and minimize toxicity.
• The liver’s central role in metabolism means any hepatic impairment, drug interactions, or genetic variation affecting CYPs can drastically alter drug exposure and safety.
• Ethical and practical implications include pharmacogenomic testing to tailor therapy (e.g., CYP2D6 genotype guiding codeine use) and considering population differences in metabolizer status.

Numerical highlights and foundational facts to remember

• Major body water compartments:
  • Plasma volume: $\approx 3\ \text{L}$
  • Interstitial fluid: $\approx 12\ \text{L}$
  • Extracellular fluid (plasma + interstitial): $\approx 15\ \text{L}$ (about 37.5% of TBW)
  • Intracellular fluid: $\approx 25\ \text{L}$
  • Total body water (TBW): $\approx 40\ \text{L}$
• Examples of tissue distribution related to lipophilicity:
  • Aspirin, logP relatively low; V_d ≈ $0.15\ \text{L/kg}$
  • Chlorpromazine, high logP; V_d ≈ $21\ \text{L/kg}$ (extensive tissue distribution)
• Common transporter and enzyme players:
  • SLC transporters: Organic anion transporters (OATs) and organic cation transporters (OCTs); mediate uptake along concentration gradients
  • ABC transporters: P-glycoprotein (P-gp/ABCB1) exports substrates and can limit brain penetration
  • Major CYPs: $\text{CYP3A4},\ 2D6,\ 2C9,\ 1A2,\ 2A6,\ 2C19,\ 2E1,\ 2C8$
• Prodrug examples requiring metabolic activation:
  • Tamoxifen, clopidogrel, ramipril
• Classic pharmacogenomics example: Codeine → morphine by CYP2D6; phenotypes include poor, extensive, and ultra-rapid metabolizers; AUC differences reflect morphine exposure variability.
• Paracetamol metabolic pathways:
  • Major routes: glucuronidation and sulfation (approx. 90%)
  • Minor route: CYP2E1 to NAPQI (toxic metabolite); detoxified by glutathione conjugation; overdose can cause hepatotoxicity.
• First-pass metabolism and bioavailability are critical for oral dosing strategies and can be bypassed by sublingual or other routes when appropriate.

Key connections to foundational principles and real-world relevance

• The balance between lipophilicity and hydrophilicity governs absorption, distribution, and elimination; this is a practical application of physical chemistry in pharmacology.
• The presence of transporters (SLCs and ABCs) adds a layer of selectivity and potential for drug–drug interactions; transporter biology is central to pharmacokinetics and toxicity.
• Pharmacogenomics explains inter-individual variability in drug response; genotype-informed therapy improves safety and efficacy.
• Understanding metabolism (Phase I/II) explains how prodrugs become active, how some drugs acquire or lose activity, and how metabolites can contribute to toxicity or therapeutic effect.
• The BBB and CNS pharmacokinetics have direct consequences for treating CNS disorders and for minimizing CNS side effects of non-CNS drugs.
• Practical pharmacokinetic concepts (F, V_d, ER, AUC, bioavailability) connect laboratory measurements to clinical dosing strategies and patient outcomes.

Quick reference equations (LaTeX)

• Diffusion flux across a membrane:
  $J = P\,(C<em>1 - C</em>2)$
• Henderson–Hasselbalch (acid form):
  $\mathrm{pH} = \mathrm{p}K<em>a + \log</em>{10}\left(\frac{[A^-]}{[HA]}\right)$
• Lipophilicity and distribution descriptors (illustrative):
  $\log P = \log<em>{10}\left(\frac{[\text{Drug}]</em>{\text{octanol}}}{[\text{Drug}]<em>{\text{water}}}\right)$$\log D</em>{pH} = \log<em>{10}\left(\frac{[\text{Drug}]</em>{\text{lipophilic}}}{[\text{Drug}]_{\text{aqueous}}}\right)\quad (pH = 7.5)$
• Bioavailability:
  $F = \frac{\text{AUC}<em>{\text{po}}}{\text{AUC}</em>{\text{iv}}}$
• Volume of distribution:
  $V<em>d = \frac{\text{Amount of drug in body}}{C</em>p}$
• Hepatic extraction ratio (conceptual):
  $ER = \frac{\text{CL}<em>{\text{H}}}{Q</em>H}$
• Phase I/Phase II general framework (no single universal equation beyond descriptive categories).