Biopharmaceutics and Formulation Development - Bioavailability and Methods

Bioavailability: Definitions and Core Concepts

• Formal Definition:

  • Bioavailability refers to the extent a substance or drug becomes completely available to its intended biological destination(s).
  • More accurately, it is a measure of the rate and fraction of the initial dose of a drug that successfully reaches either the site of action or the bodily fluid domain from which the drug's intended targets have unimpeded access.

• General Concepts and Interpretations:

  • It measures the rate and extent at which therapeutically active agents reach systemic circulation.
  • It represents the fraction of the administered dose that reaches the systemic circulation in contrast to the amount stated on the label.
  • It assesses the rate and extent of absorption of unchanged drug from its dosage form.
  • It is a measure relative to a standard (rate and amount) of drug reaching systemic circulation unchanged following administration.

• Bioavailability Value Ranges:

  • Values range from $0$ to $1$.
  • Usually expressed as percentages ($\%$).
  • An absolute bioavailability of $1$ (or $100 \%$)) indicates complete absorption.
  • A relative absorption of $1$ ($100 \%$)) indicates that the bioavailability of the drug from both compared dosage forms is the same; however, this does not necessarily indicate that systemic drug absorption is complete.

Rationale and Timing for Bioavailability Studies

• Primary Reasons (Why):

  1. Safety and Efficacy: Regulatory agencies (FDA and EMA) require data proving the drug product is safe and effective.
  2. Generic Drugs: Bioavailability is measured via the ratio of $\text{AUC/dose}$ to compare generics to reference products.
  3. Absolute Availability: For drugs with an approved New Drug Application (NDA), studies are required for new formulations to show bioequivalence to the branded reference.
  4. Relative Availability: For drugs without a full NDA, studies establish bioequivalence to the reference drug in a standard formulation.

• Clinical Stages and Monitoring (When):

  1. Primary Development: During the early stages of developing a suitable dosage form for a new drug entity (‐drug‐).
  2. Factor Identification: Determination of the influence of excipients, patient-related factors, and possible interactions with other drugs on absorption efficacy.
  3. Reformulation: During the development of new formulations for existing drugs.
  4. Quality Control: During early marketing stages to determine the influence of processing factors, storage, and stability on drug absorption.

Types of Bioavailability: Absolute vs. Relative

• Absolute Availability ($F$):

  • The systemic availability of a drug administered orally is determined by comparing it with intravenous (IV) administration.
  • For the same dose (IV vs. Oral), the bioavailability is given by:         $F = \frac{[AUC]_{\text{extravascular}}}{[AUC]_{\text{intravenous}}} \times \frac{\text{Dose}_{\text{intravenous}}}{\text{Dose}_{\text{extravascular}}}$
  • IV Exception: While drugs administered via IV are assumed to have $100 \%$) bioavailability, this is only true if the substance reaches arterial blood without loss. The lungs can be a site of extensive first-pass effect (e.g., for prostaglandins and amines) as drugs must cross pulmonary circulation before reaching arterial blood.

• Relative Availability ($F_{rel}$):

  • The systemic availability of a drug administered orally is compared with that of an oral standard of the same drug.
  • For the same dose (Oral vs. Oral standard), the calculation is:         $F_{rel} = \frac{[AUC]_{\text{extravascular 1}}}{[AUC]_{\text{extravascular 2}}} \times \frac{\text{Dose}_{\text{extravascular 2}}}{\text{Dose}_{\text{extravascular 1}}}$

• Practice Problem Data Collection:

  • Scenario: Investigational drug studied in $12$ volunteers.
  • Groups: Single oral tablet ($200\,mg$), $5\,mL$ pure aqueous solution ($200\,mg$), or single IV bolus ($200\,mg$).
  • Calculation Goal: Relative bioavailability (Tablet vs. Solution) and Absolute bioavailability (Tablet).

Factors Influencing Bioavailability

• 1. Gastric Emptying:

  • Generally, increased gastric emptying enhances the bioavailability of orally administered drugs.
  • Variables affecting gastric emptying include:
    • Liquid intake volume.
    • Solid food intake volume and fat content.
    • Viscosity of stomach contents.
    • pH of the stomach.
    • Concomitant drug intake.
    • Patient age, weight, physical activity, and emotional state.
    • Disease states.

• 2. Presystemic and Systemic Metabolism:

  • First-pass metabolism: Occurs when an absorbed drug passes through the liver before reaching systemic circulation.
  • Intestinal metabolism: Metabolism within the intestine or during passage through the intestinal wall.
  • Gastric Hydrolysis: Breakdown of the drug in stomach fluids.
  • Transporters: P-glycoprotein can influence drug bioavailability.

• 3. GI Tract Interactions:

  • Complexation with other agents within the gastrointestinal tract.

• 4. Formulation Factors:

  • Use of inert ingredients.
  • Specific manufacturing processes.
  • Use of surfactants.

The 3R Principle in Bioavailability Assessment

• Definition: Established in $1959$ by Russell and Burch, the "3R" rule stands for Replace, Reduce, and Refine.
• Application:
  • Replace: Use in vitro or ex vivo models instead of animals.
  • Reduce: Decrease the number of animals required for experiments.
  • Refine: Improve methods to minimize animal distress.
• Regulatory Stance: Authorities (EMA, FDA) suggest non-animal models due to ethical concerns, animal welfare, and practical issues (animal tests are costly, time-consuming, and can provide misleading/inaccurate results).

In Vitro Drug Dissolution Testing Models

• Significance: Dissolution rate is the physicochemical property with the greatest influence on drug absorption from the GIT.

• Utility: While in vivo determination is ideal for new formulations, in vitro testing is used for batch-to-batch consistency because it is less costly, less tedious, and avoids exposing healthy subjects to drug hazards.

• Limitations: Simple disintegration tests are unreliable; the in vitro dissolution test is the best available tool to quantitatively assure biological availability.

• Design Factors for Dissolution Tests:

  1. Apparatus Factors: Design, size of container (several $mL$ to several liters), shape (round or flat bottom), nature of agitation (stirring, rotating, oscillating), and agitation speed.
  2. Fluid Factors: Composition (Water, $0.1\,N$ $HCl$, phosphate buffer, simulated gastric/intestinal fluid), viscosity, volume (larger than required for total dissolution), and temperature (usually $37\,^∘C$).
  3. Process Parameters: Method of introduction, sampling techniques, and fluid replacement.

• Ideal Features of an Apparatus:

  • Reproducible fabrication and dimensions.
  • Simple, easy to operate, and versatile.
  • Sensitive to formulation/process changes but repeatable.
  • Controlled, non-turbulent liquid agitation.
  • Maintenance of sink conditions.
  • Ease of introduction and immersion of dosage form.
  • Minimum mechanical abrasion to avoid disrupting the drug's microenvironment.
  • Elimination of solvent evaporation.
  • Capable of testing disintegrating, non-disintegrating, dense, or floating forms.

Sink Conditions and the Noyes-Whitney Equation

• Definition: Sink condition is the ability of the dissolution media to dissolve at least $3\,\times$ (three times) the amount of drug present in the dosage form.

• Noyes-Whitney Equation:     $R = \frac{D \times S}{V \times h} \times (C_s - C_t)$

  • $R$: Dissolution Rate
  • $k_2$: Intrinsic Dissolution Rate
  • $D$: Diffusion Coefficient
  • $S$: Surface Area
  • $V$: Volume
  • $h$: Thickness of Stagnant Layer
  • $C_s$: Saturation Constant of API
  • $C_t$: API Concentration at time $t$

• Significance of the 3x Multiplier:

  • Dissolution Rate ($R$) is proportional to $(C_s - C_t)$. As concentration approaches saturation, the rate slows.
  • In vivo, drug products rarely reach saturation because the dosage form moves through the body.
  • Met Sink Conditions: Rate slows by only $1/3$ over the course of the test (e.g., term goes from $3-0$ to $3-1$).
  • Unmet Sink Conditions: It becomes difficult to match in vivo performance and creates robustness issues (minor changes in media volume or temperature cause massive variability in results).

In Vivo Assessment Methods

• Single Dose Bioavailability Studies:

  • Common, easy, less drug exposure.
  • Limitation: Difficult to predict steady-state characteristics and intersubject variability.

• Multiple Dose Bioavailability Studies:

  • Requirement: Drug must be administered for $5-6$ elimination half-lives to ensure steady state is reached.
  • Advantages:
    • Reflects actual drug use.
    • Evaluates controlled-release performance.
    • Detects pharmacokinetic non-linearity.
    • Easy to predict peak and valley (steady-state) characteristics.
    • Fewer blood samples required.
    • Ethically viable in patients due to therapeutic benefit.
  • Limitations: Tedious, time-consuming, expensive, subject compliance issues, and increased risk of side reactions.

• Subject Selection: Healthy Volunteers vs. Patients:

  • Healthy volunteers ($20-40$ years, male, body weight within $\pm 10 \%$) of standard) are used for standardization.
  • Patient Advantages: Direct reflection of therapeutic efficacy and disease-state absorption patterns, avoids ethical issues of dosing healthy people.
  • Patient Drawbacks: Diseases and other medications may modify drug absorption patterns unexpectedly.

Measurement of Bioavailability (Pharmacokinetic vs. Pharmacodynamic)

• 1. Pharmacokinetic Methods (Indirect):

  • Plasma Level-Time Studies:
    • Assumes two dosage forms with superimposable plasma profiles have identical activity.
    • Serial blood samples are collected for $2-3$ biological half-lives.
    • Parameters:
      • $C_{max}$: Peak concentration; indicates if absorption provides therapeutic response. Function of rate and extent.
      • $T_{max}$: Peak time; indicates absorption rate (decreases as rate increases).
      • $AUC$: Measure of extent of absorption.
  • Urinary Excretion Studies:
    • Principle: Urinary excretion of unchanged drug is proportional to plasma concentration.
    • Rule: At least $20 \%$) (corrected from transcript text "200%") of dose should be excreted unchanged to use this method.
    • Parameters:
      • $\frac{dC_{xu}}{dt_{max}}$: Maximum excretion rate.
      • $T_{u,max}$: Time of maximum excretion rate.
      • $X_u^{\infty}$: Cumulative amount of drug excreted (related to $AUC$).

• 2. Pharmacodynamic Methods (Direct):

  • Acute Pharmacological Response: Measures effects like EEG, ECG, or pupil diameter for at least $3$ biological half-lives. Hard to correlate accurately with formulation availability.
  • Therapeutic Response: Observes clinical response in patients. Most definitive theoretically, but difficult to quantify and complicated by multi-drug regimens.

Advanced Assessment Techniques (In Silico, Ex Vivo, In Situ)

• In Silico Models:

  • Uses mathematical models and computer simulations to predict bioavailability based on chemical structure, physical properties, and body interactions.
  • Speeds up assessment and reduces costs of physical testing.

• Ex Vivo Models:

  • Provide theoretical estimates of absorption using three main methods: diffusion chambers, everted gut sac, and intestinal perfusion.
  • Ussing Chamber: Gold standard ex vivo method. Measures transepithelial transport and intestinal metabolism. Bi-directional transport and barrier integrity (transepithelial resistance) can be monitored.
  • Franz Diffusion Cell: Major method for assessing skin permeability. Used for topical and transdermal delivery research. Uses excised skin or synthetic membranes.

• Synthetic Membranes:

  • Polymethylsiloxane (PDMS): Hydrophobic membrane mimicking skin rate-limiting properties.
  • Porous Membranes: Act as a support for quality control, offering minimum diffusion resistance.

Summary of Assessment Techniques