Biopharmaceutics & Pharmacakinetics Dosage form Design & Evaluation

Main ideas / lessons conveyed

• Link between biopharmaceutics & pharmacokinetics and dosage form design

  • Dosage form performance depends on how the drug behaves in the body (absorption, distribution, metabolism, excretion—ADME) and on physicochemical properties of the active drug, the drug product, and human anatomy/physiology.
  • The lecture focuses on how these factors guide dosage form principles, biopharmaceutical considerations, evaluation, and analysis.

• Biopharmaceutics: rational design to achieve desired drug levels

  • Drug product design should consider physiological interplay so the right amount reaches the body.
  • Particle size and surface area, solubility, permeability, stability, and release behavior affect bioavailability and onset/intensity/duration of action.

• Bioavailability as the core outcome

  • Bioavailability = fraction of administered dose reaching systemic circulation in active form.
  • Key determinants include:
    • Absorption-related factors
    • First-pass metabolism
    • Food–drug interactions
    • Solubility/dissolution
    • Permeability/transport mechanisms
    • Drug degradation (physical/chemical stability)
    • Release kinetics and modified release strategies
    • Physiological parameters and excipient effects

• BCS-based formulation strategy

  • Drugs are categorized by solubility and permeability (BCS classes 1–4) and formulation choices are adjusted accordingly:
    • Class 1: immediate release often works
    • Class 2: improve solubility
    • Class 3: improve permeability
    • Class 4: both are challenging → consider size/system changes or alternative routes

• Pharmacokinetic considerations guide “what to build”

  • PK parameters (e.g., Cmax, Tmax, AUC, half-life, clearance) inform:
    • Absorption rate/extent (onset/intensity/duration)
    • Therapeutic window safety (MEC–MSC)
    • Whether a drug should be delivered by certain routes (e.g., IV for very poorly absorbed drugs)
    • How dosage forms can increase safety and efficacy while maintaining stability (in vitro and in vivo)

• Evaluation and testing ensure performance and safety

  • In vitro evaluation covers physical/chemical properties, dissolution/disintegration, and permeability.
  • In vivo pharmacokinetic and bioavailability/bioequivalence studies quantify exposure.
  • Pharmacodynamic/toxicology studies evaluate therapeutic response, biomarkers/endpoints, distribution, and safety margins.

• Modern approaches

  • Use of in silico tools (AI/ML, molecular docking, PBPK modeling) to predict drug–excipient interactions and PK/ADME behavior, including support for regulatory submissions.

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Detailed methodology / instructional content (design & evaluation framework)

A) Dosage form design principles (guided by biopharmaceutics & PK)

• Start from learning objectives

  • Understand principles of dosage form design
  • Consider biopharmaceutical factors
  • Plan evaluation of the dosage form
  • Analyze performance characteristics

• Understand biopharmaceutics for optimization

  • Use physicochemical characteristics of the API (active drug) plus desired product and physiology to ensure correct drug levels in the body.
  • Consider key biopharmaceutical characteristics to optimize:
    • Particle size alteration → increase surface area → improve absorbability/penetrability → improve bioavailability and therapeutic efficacy

• Address absorption and bioavailability

  • Absorption
    • Ensure drug moves from administration site into systemic circulation.
    • Consider:
      • Solubility at the site
      • Permeation properties
  • Bioavailability meaning
    • Fraction of dose reaching systemic circulation in active form.
  • Factors affecting bioavailability and formulation responses
    • Drug physicochemical properties
      • Solubility, pKa, partition coefficient
    • Dosage form type
      • Solid: disintegration and dissolution rates dominate
      • Liquid: solubility and solubilization rates dominate
    • Route of administration
      • Oral vs parenteral vs transdermal affects absorption environment and rate
    • GI pH and ionization
      • Ionized form is less absorbed; formulate to release at the correct pH region
      • Strategy: enteric coating (intestinal-release) if drug is unstable in gastric pH
      • If unstable throughout GI tract: bypass oral route (e.g., injection)
    • First-pass metabolism
      • Occurs before systemic circulation (commonly liver, intestinal wall, lungs)
      • Examples mentioned: propranolol, nitroglycerin, morphine
      • Strategies:
        • Use alternative routes (sublingual, transdermal, IV)
        • Convert to prodrug (release active moiety at target; requires prodrug lysis/dissociation)
        • Use enzyme inhibitors
    • Food–drug interactions
      • Food can enhance or delay absorption
      • High-fat meals:
        • Increase absorption of lipophilic drugs (example: griseofulvin)
        • Provide lipophilic environment to dissolve poorly soluble drugs
      • Gastric emptying time:
        • Heavy food can slow emptying (more time for dissolution/absorption)
        • Lighter food can accelerate emptying (faster transit)
      • Gastric pH effects:
        • Acidic drugs absorb better in acidic environment; basic drugs in alkaline
        • Ionization at wrong pH reduces absorption
        • Food may mitigate ionization-related issues
    • Solubility and dissolution rate
      • Definitions:
        • Solubility (saturation solubility): max dissolved drug under conditions
        • Dissolution rate: speed at which drug enters solution → impacts absorption
      • Strategies to improve solubility/dissolution:
        • Salt formation (example: naproxen sodium dissolves faster than naproxen)
        • Particle size reduction / micronization → higher surface area
        • Surfactants (examples: polysorbates, SLS, cetrimide)
        • Choose solubilizer type by route:
          • Some for oral use, others safer for topical use only
    • Permeability and transport mechanisms
      • Passive transport
        • Downhill diffusion (no energy expenditure)
        • Depends on lipophilicity and molecular size
      • Carrier-mediated transport
        • Facilitated/active transport (against gradient; energy required)
      • Endocytosis and efflux transporters
        • Example efflux transporter: P-glycoprotein (P-gp)
        • Example drug: paclitaxel (efflux back into gut)
    • BCS-based transporter/solubility strategy
      • Drug classes (solubility + permeability):
        • Class 1: high solubility + high permeability (example: metoprolol)
        • Class 2: low solubility + high permeability (example: ketoconazole)
        • Class 3: high solubility + low permeability (example: ranitidine)
        • Class 4: low solubility + low permeability (example: amphotericin B)
      • Formulation actions:
        • Class 1: minimal concern → likely easier absorption
        • Class 2: increase solubility (micronization, polymeric dispersion, solid dispersions)
        • Class 3: increase permeability (use permeation enhancers)
        • Class 4: modify particle size and delivery system or use alternative route
    • Drug solubility vs degradation pathways (stability)
      • Physical instability examples:
        • Phase separation, crystallization, polymorphic change
      • Chemical instability examples:
        • Hydrolysis, oxidation, photodegradation
      • Examples given:
        • Hydrolysis: esters/amides (example: aspirin → salicylic acid)
        • Oxidation: phenolic/conjugated structures (epinephrine, vitamin C)
        • pH-dependent degradation: omeprazole degrades in acidic environments
      • Stabilization strategies:
        • Antioxidants (examples: BHT, ascorbic acid)
        • Buffers (e.g., phosphate buffer)
        • Coatings/encapsulation (e.g., enteric coating for acid-sensitive drugs)

• Design drug release kinetics & modified release formulations

  • Select release approach based on drug class and absorption issues:
    • BCS class 1: fast onset → immediate release (example: paracetamol tablets)
    • If prolonged action is desired or solubility/permeation issues exist:
      • Extended release: slow release → improved solubilization/absorption over time
      • Controlled-release and delayed-release: address poor solubility/permeation
  • System types:
    • Matrix systems
      • Drug dispersed in polymeric matrix (example: HPMC-based hydrophilic matrix tablets)
    • Reservoir systems
      • Drug enclosed in membrane; gradual release
      • Example: transdermal reservoir patch (example mentioned: “Band-Aid”)

• Incorporate physiological parameters affecting performance

  • GI transit time:
    • Drugs with short half-life can benefit from sustained release
    • Slower release reduces rapid elimination and maintains drug presence
  • pH-dependent solubility:
    • Acidic drugs: stomach; basic drugs: intestine
  • Enzymatic degradation:
    • Peptide drugs (example: insulin) degraded in GI tract → require parenteral administration
  • Food interactions:
    • Fat-rich meals can enhance absorption of lipophilic drugs but may delay gastric emptying

• Select excipients to support stability, bioavailability, and compliance

  • Examples of excipient categories and roles:
    • Binders (tablet cohesion): PVP, starch, agar, gums
    • Disintegrants: cross povidone, sodium starch glycolate
    • Lubricants: magnesium stearate (reduce friction; form hydrophobic layer)
    • Surfactants: polysorbates (improve wetting/dissolution)
    • pH modifiers/buffers: citric acid, sodium bicarbonate (especially in solutions; maintain stability)
    • Prodrugs (inactive derivatives converted to active form):
      • Improve solubility (prednisolone phosphate → prednisolone)
      • Enhance permeability (enalapril → enalaprilat)
      • Avoid first-pass metabolism (clopidogrel)

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B) Pharmacokinetic modeling and parameter usage for dosage form design

• Goal of PK in dosage form design

  • Determine absorption, distribution, metabolism, excretion to ensure:
    • high bioavailability
    • safety
    • patient compliance

• If a drug is poorly absorbed

  • Consider route changes:
    • Avoid extravascular routes if absorption is too low
    • Use intravascular route where absorption is not involved

• Absorption modeling concepts

  • Onset/intensity/duration depend on:
    • rate and extent of absorption
  • Plasma concentration changes over time:
    • plotted as curves
    • AUC (area under curve) indicates total exposure
    • Cmax = maximum plasma concentration
    • Tmax = time to reach Cmax
  • Therapeutic window
    • MSC (maximum safe concentration)
    • MEC (minimum effective concentration)
    • Drug concentration must stay between MSC and MEC to avoid toxicity and maintain efficacy

• Absorption kinetics models

  • Zero-order
    • constant release rate, independent of drug concentration (typical of certain controlled-release formulations)
  • First-order
    • rate depends on drug concentration
  • Higuchi model
    • dissolution relates to square root of time
  • Hixson–Crowell model
    • used for diffusion-based release understanding (cube-root relationships described)

• Distribution / VD

  • Apparent volume of distribution (VD) influenced by:
    • plasma protein binding (albumin, acid glycoproteins)

• Metabolism overview

  • Primary organ: liver
  • Phase 1 (catabolic): oxidative, reductive, hydrolytic
  • Phase 2: conjugation (glucuronidation, sulfation, acetylation, methylation)
  • Consider targeting metabolic phases to reduce metabolism (as desired)
  • Factors affecting metabolism:
    • genetic polymorphisms / enzyme variability
    • enzyme inhibition/induction (example: rifampicin inducing CYP enzymes)
    • hepatic disorders reducing metabolism

• Excretion overview

  • Identify route: renal, hepatic, pulmonary
  • Mechanism details:
    • filtration
    • reabsorption
    • enteropathic recycling
  • Clearance: volume of plasma cleared per time
  • Half-life:
    • time for plasma concentration to halve
  • Use knowledge of clearance/half-life/excretion pathways to adjust dosage forms:
    • increase half-life
    • decrease clearance
    • improve bioavailability

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C) Dosage form evaluation methodology (in vitro and in vivo)

1) In vitro evaluation

• Physical evaluation

  • Appearance and uniformity:
    • check color, shape, texture, odor
    • detect defects: cracks, capping, etc.
  • Weight variation:
    • dose-to-dose consistency
  • Hardness and friability:
    • mechanical strength and resistance to breaking
    • hardness relates to disintegration behavior (too hard → poor disintegration)
  • Thickness and diameter:
    • uniform packing and handling

• Chemical evaluation

  • Content uniformity:
    • correct drug amount per unit
    • test with analytical techniques (UV-visible spectroscopy, HPLC)
  • For liquids/semi-solids (ointments, injectables, eye drops):
    • pH compatibility with tissue pH to avoid irritation
  • Moisture content analysis:
    • critical for hygroscopic/effervescent systems
    • effervescent tablets lose function if they absorb moisture (swell, become soggy, gases release prematurely)

• In vitro drug release

  • Dissolution testing apparatus:
    • USP basket, paddle, reciprocating cylinder (chosen based on formulation type)
  • Purpose:
    • measure release rate over time
    • infer bioavailability potential

• Disintegration testing

  • For tablets/capsules:
    • measure time to disintegrate and release contents
  • For tablets:
    • disintegration into granules → informs solubilization/absorption behavior

• Permeability tests

  • Use Caco-2 monolayer model and other tissue/cell-line models
  • Aim:
    • estimate permeability through biologically relevant systems

2) In vivo evaluation

• Pharmacokinetic studies

  • Determine: Cmax, Tmax, AUC, half-life, clearance, residence time

• Bioavailability and bioequivalence

  • Absolute bioavailability
    • compare plasma concentrations after oral vs IV
  • Relative bioavailability
    • compare between two oral formulations or two semi-solid formulations
  • Bioequivalence testing (especially generics)
    • demonstrate similar dissolution profile → expected similar bioavailability to the reference innovator
    • sometimes “abridged studies” to show similar PK properties

3) Pharmacodynamic studies (efficacy/safety response)

• Evaluate therapeutic response and side effects through:
  • preclinical and clinical trials
• Biomarker examples:
  • blood glucose for diabetics
• Clinical endpoints:
  • symptom relief or disease progression
• Toxicity assessment:
  • monitor adverse reactions (examples: analgesic pain relief; anti-diabetic blood glucose reduction)

4) Distribution and toxicity testing

• Tissue distribution studies

  • radiolabeling + imaging:
    • PET/SPECT
  • microdialysis:
    • concentration in tissues (brain, muscles, skin, etc.)
  • whole-body autoradiography:
    • preclinical localization

• Toxicity studies

  • Determine safety profile across doses and durations
  • Metrics for regulatory approval:
    • maximum tolerable dose
    • NOAEL (no observed adverse effect level)
    • LD50 (lethal dose for 50% of test animals)
  • Types and key points:
    • Acute toxicity
      • short-term high-dose effects
      • OECD 423 (acute oral fixed-dose method)
      • OECD 425 (up-and-down procedure for dose finding)
      • endpoints: tremors, sedation, hyperactivity, weight changes, organ damage, biochemical changes
    • Sub-acute toxicity (repeated dose)
      • daily dosing 14–18 days
      • OECD 407
      • endpoints: hematology (WBC/RBC/platelets), liver/kidney function, histopathology
    • Chronic toxicity
      • long-term 90 days to 2 years
      • endpoints: carcinogenicity, nephrotoxicity, behavior, reproductive toxicity
      • OECD 451

5) In silico evaluation (modern methods)

• AI/ML-enabled approaches:

  • molecular docking and modeling to predict drug–excipient interactions
  • predict binding stability in vitro and in vivo
  • support design of nano-formulations, polymer carriers, lipid systems

• Physiologically based pharmacokinetic (PBPK) modeling

  • simulate absorption/metabolism/clearance in silico using physiological parameters
  • estimate organ/tissue concentrations
  • increasingly used for regulatory submissions to predict bioavailability/bioequivalence

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