PPT3 Principles of Drug Absorption - Comprehensive Study Notes

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  • Transition to new topic: the session moves on to a detailed topic—principles of drug absorption.

Topic: Principles of drug absorption

  • Context
    • We had covered the drug development process and preformulation studies (solubility and dissolution).
    • Dissolution is essential: absorption cannot occur if the drug is not dissolved.
  • Core question
    • How are drugs absorbed from the gastrointestinal (GI) tract and reach the target site?
  • Big picture view
    • Absorption is the process by which a drug moves from the site of administration into the bloodstream, enabling distribution to tissues and organs where it exerts effects.
  • Learning objectives for this section
    • Understand the barriers a drug must cross to be absorbed.
    • Learn two major mechanisms of diffusion across membranes: passive diffusion and specialized transport (facilitated diffusion and active transport).
    • Apply Henderson–Hasselbalch equations to predict drug ionization in GI tract environments.
    • Introduce the pH-partition (pH-dependent ionization) hypothesis and its implications for absorption.
    • Discuss how membrane surface area and thickness influence absorption (Fick’s considerations).

What a drug has to endure to be absorbed

  • Absorption is the initial step in a cascade: absorption → distribution → metabolism (liver detoxification) → excretion → target-site delivery.
  • Key barriers to absorption
    • Cross multiple cell layers (e.g., skin patches must cross several layers of skin).
    • In the GI tract, typically a single epithelial cell layer (intestinal lining) must be crossed.
    • In some dosage forms, entry into a single cell and then through the cell membrane is required.
  • Absorption pathways depend on the route of administration
    • Oral route: drug must cross the intestinal epithelium into the blood.
    • Transdermal (topical) routes: must cross skin layers to reach systemic circulation.
  • After absorption, distribution, metabolism, and excretion follow (the ADME sequence), with the liver playing a major detoxification role.

Basic diffusion and transport concepts

  • Two main routes for absorption across membranes
    • Diffusion (passive diffusion): movement down a concentration gradient; energy-free.
    • Specialized transport mechanisms: transport proteins/channels that mediate diffusion or active transport.
  • Diffusion fundamentals (recap)
    • Passive diffusion is driven by a concentration gradient from high concentration to low concentration.
    • In a simplified GI tract example: high drug concentration in the intestinal lumen (absorption side) vs. very low concentration in blood initially → diffusion occurs until equilibrium is reached.
    • In real biology, there is always a persistent gradient driving diffusion while absorption continues.
  • Key terms in diffusion
    • Concentration gradient: driving force for diffusion.
    • Permeability coefficient (P): governs how easily a drug crosses the membrane.
    • Membrane thickness (h): thicker membranes hinder diffusion (inversely related to diffusion rate).
    • Surface area (A): larger surface area increases diffusion rate (directly proportional).
    • The general idea: diffusion rate increases with higher driving concentration difference, higher permeability, and larger contact surface area; it decreases with greater membrane thickness.
  • A compact view of diffusion rate (as discussed in the lecture)
    • Rate of diffusion (motion of drug across the membrane) is proportional to the permeability coefficient and the concentration on the absorption side: ext{Rate} \propto P \, C_1
    • Permeability coefficient relation: P = rac{K}{h} where K is the partition coefficient and h is membrane thickness.
    • If surface area increases, the total amount diffusing per unit time increases as well: larger A → larger overall absorption.
    • The two-door analogy: more surface area (more doors) allows faster throughput; similarly, more interfaces increase diffusion.
  • The practical takeaway
    • A drug’s absorption is enhanced by high surface area (e.g., microvilli and villi in the small intestine) and by high solubility and partitioning into the membrane, but hindered by high membrane thickness.

Surface area and the GI tract microarchitecture

  • Duodenum and small intestine as major absorption sites
    • The intestinal lining has a zigzag, folded architecture that significantly increases surface area.
    • Villi and microvilli are brush-like protrusions that dramatically enlarge the absorptive surface area.
    • This increased surface area raises the probability that drug molecules contact and cross the membrane, boosting absorption.
  • Contrast with other sites
    • Stomach, colon, and rectum have absorption, but surface area is much smaller than in the duodenum and proximal small intestine.
  • Real-world analogy
    • Coastline analogy: a straight coastline has less distance to travel than a jagged coastline. Similarly, a folded intestinal surface (zigzag) increases surface area and diffusion opportunities.

Ionization, Henderson–Hasselbalch, and drug absorption

  • Why ionization matters
    • Cell membranes are nonpolar; nonionized (uncharged) molecules pass more easily through lipid membranes than ionized ones.
    • Many drugs are weak acids or weak bases; their ionization state depends on the local pH and the drug’s pKa.
  • Henderson–Hasselbalch equations (conceptual forms used in the lecture)
    • For a weak acid (HA ⇌ H+ + A−):
    • pH = pK_a + \log\left(\frac{[A^-]}{[HA]}\right)
    • For a weak base (B + H+ ⇌ BH+):
    • Using the conjugate acid BH+ with pKa(BH+): pH = pKa(BH^+) + \log\left(\frac{[B]}{[BH^+]}
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  • Practical simplified forms used in teaching (from the lecture)
    • The ionization ratio forms can be expressed as:
    • Weak acid: \frac{[A^-]}{[HA]} = 10^{\left(pH - pK_a\right)}
    • Weak base (using BH+): \frac{[BH^+]}{[B]} = 10^{\left(pH - pK_a\right)}
  • Useful derived fractions
    • For weak acid: fraction ionized (A−) = f{ionized} = \frac{10^{(pH-pKa)}}{1 + 10^{(pH-pK_a)}}
    • Fraction unionized (HA) = f{unionized} = \frac{1}{1 + 10^{(pH-pKa)}}
    • For base (using BH+): fraction ionized (BH+) = f{ionized} = \frac{10^{(pH-pKa(BH^+))}}{1 + 10^{(pH-pK_a(BH^+))}}
    • Fraction unionized (B) = f{unionized} = \frac{1}{1 + 10^{(pH-pKa(BH^+))}}
  • Worked example from the lecture (acid with pKa = 4 in environment pH = 1)
    • Ratio: \frac{[A^-]}{[HA]} = 10^{(pH - pK_a)} = 10^{(1 - 4)} = 10^{-3} = 0.001
    • Interpretation: mostly unionized HA (about 99.9%), very small fraction ionized A− (about 0.1%). So in very acidic environments, a weak acid tends to be largely unionized and more readily absorbed.
  • Additional example from the lecture (base with pKa = 7 in very acidic environment, pH ≈ 0)
    • Ratio: \frac{[BH^+]}{[B]} = 10^{(pH - pK_a)} = 10^{(0 - 7)} = 10^{-7}
    • Interpretation: mostly unionized B in this framing would be extremely low; however the lecture notes described the opposite intuition for a base in acid as highly ionized (BH+ abundant). The key is to consistently apply the Henderson–Hasselbalch form using the correct pKa for BH+ or the base form. The main takeaway is: ionization state is highly pH-dependent and directly affects absorption via membrane permeability.
  • Conceptual takeaway: pH partition hypothesis
    • Drug absorption from the GI tract is primarily via passive diffusion, but membrane permeation depends on the drug’s ionization state at intestinal pH.
    • Since membranes poorly permit ionized species, absorption depends on the fraction that remains unionized at intestinal pH.
    • Therefore, absorption is a function of both the intestinal pH and the drug’s pKa, linking preformulation data with in vivo absorption.
  • Practical implications for preformulation and formulation design
    • In many cases, converting a drug to a salt form can alter solubility and ionization, thereby improving dissolution and absorption.
    • Salt forms (ionic, polar) may rely more on specialized diffusion or transport mechanisms; nonionized forms may diffuse more readily, depending on the membrane.

Specialized transport mechanisms (beyond simple diffusion)

  • Why specialized transport exists
    • Many essential nutrients (amino acids, glucose) and some drugs are highly polar and would struggle to cross lipid membranes by simple diffusion.
    • Specialized transport proteins facilitate the movement of these molecules across membranes.
  • Facilitated diffusion (a type of passive transport)
    • Characteristics:
    • Requires a specific transport protein/channel in the membrane.
    • Does not require energy (ATP).
    • Movement is along the concentration gradient (high to low).
    • Highly selective for particular substrates (e.g., glucose channel for glucose).
    • How it works (conceptual): outside the cell, a drug or nutrient binds to a carrier protein; the protein undergoes conformational change, passes the molecule through the membrane, and releases it inside.
    • Important nuance: even within facilitated diffusion, there is a strict specificity to the transporter (structure, size, charge) – a glucose transporter won’t reliably transport a different molecule.
  • Active transport
    • Characteristics:
    • Requires energy (usually ATP) to move a molecule against its concentration gradient (low to high).
    • Involves a transport protein that undergoes conformational changes upon energy input (often phosphorylation via ATP).
    • Can move ions/molecules against their gradient and is governed by transporter specificity.
    • Mechanistic analogy used in the lecture: climbing a mountain requires energy; diffusion down the gradient is easy, but climbing up requires energy.
    • Examples discussed: sodium/glucose transport and transporter proteins that couple with ATP hydrolysis to move substances into the cell against gradients.
  • Summary of absorption pathways
    • Passive diffusion: high to low concentration; energy-free; rate depends on concentration gradient, membrane thickness, surface area, and lipophilicity/ionization state.
    • Facilitated diffusion: diffusion via specific transporter proteins; no energy; substrate-specific channels.
    • Active transport: transport against the gradient; energy-dependent; uses ATP and transport proteins; can move polar/charged molecules when diffusion is unfavorable.
  • A drug-absorption scenario (conceptual problem from the lecture)
    • MCP propane (a fictional drug): salt form (MCP propane sodium) is highly polar.
    • Primary mechanism for transient absorption of the salt form is facilitated diffusion (via a transporter/channel) along the concentration gradient (low to high). Energy is not required for facilitated diffusion.
    • For a non-salt (neutral) form, passive diffusion may suffice if the molecule is sufficiently lipophilic.
    • When a drug is very polar and ionized, transporters often play a key role in enabling absorption, whereas nonpolar forms can diffuse more readily across membranes.
  • Key exam-style takeaway
    • Determine which transport mechanism would dominate based on the drug’s form (neutral vs salt/polar) and the local environment (concentration gradients).
    • For polar/ionized drugs, facilitated diffusion or active transport is often required; for nonpolar/unionized drugs, passive diffusion is typically sufficient.

Worked practice problems and exam-style notes (concepts you’ll encounter)

  • About the types of questions you’ll see
    • Some questions test direct recall of the Henderson–Hasselbalch equations and the pH/pKa relationships.
    • Others test application: predicting ionization and absorption for given pH, pKa values, and environments.
    • Some questions are designed as scenario-based problems, where you must apply transport mechanism concepts to decide the most likely absorption pathway.
  • Example problem style (from the lecture)
    • Given a weak base with a certain pKa in an acidic stomach environment, determine whether it will be highly ionized or not and what that implies for absorption.
    • Use the salt form data (e.g., a sodium salt) to infer whether facilitated diffusion or active transport is likely the primary mechanism.
    • Given a drug with a known pKa and a pH value, calculate the ionized-to-unionized ratio:
    • For acids: \frac{[A^-]}{[HA]} = 10^{(pH - pK_a)}
    • For bases (using BH+): \frac{[BH^+]}{[B]} = 10^{(pH - pK_a(BH^+))}
    • Then infer the likely absorption behavior: unionized fraction favors diffusion; highly ionized favors transporter-mediated absorption or poor diffusion.
  • Tip for practice problems
    • Always compute the ratio and then convert to percent ionized vs unionized if needed:
    • If ratio r = 10^(pH - pKa) for acid, fionized = r/(1+r), funionized = 1/(1+r).
    • If r is very small (e.g., pH << pKa), most of the drug is unionized; if r is very large (pH >> pKa), most is ionized.
  • Note on study aids mentioned in the lecture
    • Practice problems will be posted, along with answers for self-checking.
    • The instructor emphasizes using practice questions to reinforce understanding of absorption concepts and to prepare for quizzes/exams.

Key takeaways to memorize for exams

  • Absorption is the entry point for drug action; without absorption, ADME cannot proceed.
  • Diffusion (passive) is driven by a concentration gradient and requires no energy; rate depends on: membrane thickness (h), surface area (A), and the permeability coefficient (P).
    • Permeability: P \propto \frac{K}{h} where K is the partition coefficient and h is membrane thickness.
    • Total rate of absorption is proportional to surface area: larger A increases diffusion opportunities.
  • Ionization state strongly influences absorption because membranes are lipid-rich and nonpolar; unionized species cross more readily than ionized species.
  • Henderson–Hasselbalch equations link pH, pKa, and the ratio of ionized/unionized forms; this governs how much drug is available for diffusion in a given environment:
    • For weak acid: \frac{[A^-]}{[HA]} = 10^{(pH - pK_a)}
    • For weak base (using BH+): \frac{[BH^+]}{[B]} = 10^{(pH - pK_a(BH^+))}
  • Fractional ionization concepts help predict absorption efficiency:
    • For an acid: f{ionized} = \frac{10^{(pH - pKa)}}{1 + 10^{(pH - pK_a)}}
    • For a base: analogous expressions apply with the base’s pKa (of BH+).
  • The pH partition hypothesis: drug absorption from the GI tract is a function of the drug’s ionization status at intestinal pH, which depends on pKa and the local environment.
  • The small intestine (duodenum and jejunum) is the primary site of absorption due to its large surface area created by villi and microvilli; the colon and stomach contribute less surface area for absorption.
  • Absorption can involve three main mechanisms:
    • Passive diffusion (high to low concentration, no energy).
    • Facilitated diffusion (requires a transporter/channel, no energy, substrate-specific).
    • Active transport (requires ATP, moves against the concentration gradient).
  • Practical implications for formulation design:
    • Salt formation can increase solubility and alter absorption profiles.
    • Transporter-targeted strategies can be important for highly polar drugs.

Quick recap: what to study and remember

  • Definitions and roles
    • Absorption, diffusion, diffusion constants, permeability, surface area, and membrane thickness.
  • Equations to know
    • Diffusion rate concepts and simplified forms used in class:
    • \text{Rate} \propto P \times (C1 - C2) with P = \frac{K}{h} and total rate also proportional to surface area A\to \text{Rate} = J\cdot A.
    • Henderson–Hasselbalch forms for acids and bases:
    • Acid: pH = pK_a + \log\left(\frac{[A^-]}{[HA]}\right)
    • Base: pH = pK_a(BH^+) + \log\left(\frac{[B]}{[BH^+]}
      ight)
  • Conceptual checks
    • In acidic environments, weak acids tend to stay mostly unionized (favorable for diffusion), whereas weak bases tend to be protonated (BH+) and may require transporters.
    • The presence of salt forms can shift the balance toward transporter-mediated absorption due to increased polarity.
  • Practice mentality
    • Be prepared to estimate ionization fractions from pH and pKa values, then infer likely absorption pathways.
    • Expect a mix of recall and application questions, including some calculation-based problems and transporter-related scenarios.