Bioavailability, Bioequivalence & Dissolution Testing

Core Definitions and Concepts

• Therapeutic effectiveness depends on the dosage form delivering drug to the site of action at a rate/amount sufficient for the desired response.
• Bioavailability (BA): “Rate and extent of absorption of unchanged drug from its dosage form.” • Directly linked to plasma‐level profile. • Expressed as an absolute term.
• Bioavailable fraction (F): $F=\frac{\text{Bioavailable dose}}{\text{Administered dose}}$ (Eq. 11.1)
• Rate vs. extent
  • Rapid absorption → rapid onset (e.g., asthma, pain).
  • Slow absorption → prolonged effect / ↓ side-effects.
  • Extent critical in chronic therapy (hypertension, epilepsy).

Factors Governing Bioavailability

• Pharmaceutical factors: physicochemical properties + formulation variables.
• Patient-related factors.
• Route of administration (typical order): Parenteral > Oral > Rectal > Topical
  • IV gives ≈ 100 % BA (absorption bypassed).

Objectives of Bioavailability Studies

1. Support dosage-form development for new entities.
2. Assess influence of excipients, patient variables, drug–drug interactions.
3. Support reformulation of existing drugs.
4. Monitor quality during early marketing (processing, storage, stability).
5. Compare different dosage forms / manufacturers.

Absolute vs. Relative Bioavailability

• Absolute (F): Compare oral (or other extravascular) dose with IV standard.
  $F=\frac{AUC<em>{oral}\,D</em>{iv}}{AUC<em>{iv}\,D</em>{oral}}$ (Eq. 11.2)
• Relative (Fr): Compare two non-IV oral products.
  $F</em>r=\frac{AUC<em>{test}\,D</em>{std}}{AUC<em>{std}\,D</em>{test}}$ (Eq. 11.3)
• Drawbacks of using oral solution as “standard” instead of IV: limits PK modelling, difficult to separate metabolism vs. non-absorption, may obscure true elimination k.

Single- vs. Multiple-Dose BA Studies

Single-dose

• Simple, quick, ↓ exposure, but poor prediction of steady-state (SS) behaviour.
• Need long sampling (≥ 2–3 t½) for reliable terminal phase + AUC.

Multiple-dose

• Advantages: mirrors clinical use; blood levels at therapeutic range; fewer samples; detects non-linearity; evaluates controlled-release (CR) products; smaller inter-subject variability; ethical in patients; no long washout between formulations.
• Limitations: tedious, costly, compliance issues, ↑ adverse-event risk.
• Confirm SS (5–6 t½) before sampling.
• Extent at SS: $F=\frac{[AUC]<em>{ss,test}\,D</em>{std}}{[AUC]<em>{ss,std}\,D</em>{test}}$ (Eq. 11.4)
• Peak at SS: $F=\frac{C<em>{ss,max,test}\,\tau\,D</em>{std}}{C<em>{ss,max,std}\,\tau\,D</em>{test}}$ (Eq. 11.5)

Choice of Volunteers

• Preferably patients when ethically/clinically justified (multiple-dose studies, topical/non-systemic drugs).
• Practical considerations → young (20–40 y), healthy, male, ±10 % ideal body-weight, fasting control, fixed diet/activity.
• Female subjects only when drug targeted (e.g., OCs).
• Wash-out ≥ 10 t½ between periods; no other meds ≥ 1 week; medical screening essential.

Measurement Strategies

1. Pharmacokinetic Methods (indirect)

• Plasma level–time: gold standard.
• Urinary excretion: use only if ≥ 20 % dose excreted unchanged.

2. Pharmacodynamic Methods (direct)

• Acute pharmacological response (ECG, pupil, etc.).
• Therapeutic response in patients.

Plasma Level–Time Study Essentials

• Serial sampling for ≥ 2–3 t½.
• IV: first sample ≤ 5 min; q15 min thereafter.
• ≥ 3 points on ascending oral phase for reliable $k_a$; 3–6 on descending.
• Key parameters:
  1. $C_{max}$ – reflects rate & extent (↑ with dose and faster absorption).
  2. $t_{max}$ – inversely related to rate.
  3. AUC – measure of extent.

Urinary Excretion Method

• Requirements: ≥ 20 % dose unchanged in urine; complete bladder emptying each interval; frequent early samples; 7 t½ total collection.
• Analogous parameters:
  • $(dX<em>u/dt)</em>{max}$ ↔ $C_{max}$ (rate & extent).
  • $(t<em>u)</em>{max}$ ↔ $t_{max}$ (rate).
  • $X_u(\infty)$ ↔ AUC (extent).
• Extent equations:
  $F=\frac{X<em>{u,oral}}{X</em>{u,iv}}\frac{D<em>{iv}}{D</em>{oral}}$ (Eq. 11.6) – single dose
  $F=\frac{X<em>{u,ss,test}\,D</em>{std}}{X<em>{u,ss,std}\,D</em>{test}}$ (Eq. 11.8) – multiple dose

Pharmacodynamic Alternatives

• Useful when plasma assay impractical/unreliable.
• Challenges: variability, active metabolites, imprecise quantitation.

In Vitro Dissolution & BA

• In vitro test desirable for batch-to-batch QC; dissolution rate is major determinant for many drugs.
• Disintegration test unreliable → employ dissolution testing.

Ideal Apparatus Features

1. Precisely reproducible construction & positioning.
2. Simple, versatile, sensitive, repeatable.
3. Mild, uniform, non-turbulent agitation; variable speed.
4. Maintains near-perfect sink.
5. Easy sample introduction & minimal abrasion.
6. $37\,^{\circ}\mathrm{C}$, no evaporation.
7. Easy sampling without flow disruption.

USP / Compendial Apparatus & Uses

Dissolution Acceptance (Q values)

• Stage S1: 6 units ≥ $Q+5\%$ → pass.
• Stage S2: 12 units (6 + 6) → mean ≥ $Q$; no unit < $Q-15\%$.
• Stage S3: 24 units (6 + 6 + 12) → mean ≥ $Q$; ≤ 2 units < $Q-15\%$; none < $Q-25\%$.

Profile Comparison – f-factors

• Difference factor $f<em>1 =100\,\frac{\sum</em>{t=1}^{n}|R<em>t-T</em>t|}{\sum<em>{t=1}^{n}R</em>t}$ (Eq. 11.9)
• Similarity factor f2 =50\,\log\left{\bigg[1+\frac{1}{n}\sum{t=1}^{n}(Rt-Tt)^2\bigg]^{-0.5}\times100\right} (Eq. 11.10)
• Interpretation:
  • $f<em>1\le15$ and $f</em>2\ge50$ → profiles similar/equivalent.
  • Conditions: ≥ 3 time-points; 12 units each product; SD ≤ 10 % (except first point); no mean > 85 % (except final).

In Vitro–In Vivo Correlation (IVIVC)

• “Predictive mathematical model linking in-vitro property (e.g., dissolution) with in-vivo response (plasma profile or absorbed amount).”
• Applications: batch QC surrogate, guide formulation, set specs.
• Development approaches
  1. Direct relationship (often linear) between dissolution & BA parameters.
  2. Modify dissolution method to fit existing BA data.

Correlation Levels

• Level A: point-to-point superimposable curves; most useful – dissolution becomes surrogate QC; justifies post-approval changes.
• Level B: statistical moments (MDT vs. MRT); not point-to-point.
• Level C: single-point (e.g., $t_{50\%}$ vs. AUC). Limited.
• Multiple Level C: several in-vitro points vs. multiple PK parameters.

Biopharmaceutics Classification System (BCS)

• Categorises drugs by solubility & permeability; informs IVIVC & biowaiver.
  

Critical Dimensionless Numbers

BCS-Based Biowaiver Criteria

• Rapid & similar dissolution (≥ 85 % in 15 min).
• High solubility.
• High permeability (≥ 90 % absorbed).
• Wide therapeutic window.
• Same excipients as reference.

Bioequivalence (BE)

• Pharmaceutical equivalence: same API strength, quality, dissolution; excipients may differ.
• Bioequivalence: identical plasma rate & extent → no significant difference (statistically).
• Therapeutic equivalence: identical clinical effect.

Need for BE Studies

• Substitute product for approved reference; ensure performance; mitigate risk of therapeutic failure.

In Vivo vs. In Vitro

• In vivo required if: serious condition, NTI drug, complicated PK, poor solubility, documented BA issues, modified-release, non-oral.
• In vitro (biowaiver) acceptable when criteria above not met.

Experimental Designs

1. Completely Randomised – easiest; requires homogeneous subjects.
2. Randomised Block – subjects blocked by characteristics; ↑ precision.
3. Repeated-Measures / Cross-Over – each subject receives all treatments; control inter-subject variability; need adequate wash-out (≈ 10 t½); risk of order & carry-over.
4. Latin Square (balanced cross-over) – controls period, sequence, subject effects; typical in BE (e.g., 3 formulations A-B-C across 6 or 12 subjects).

Wash-Out Period

• ≥ 10 elimination half-lives to nullify carry-over.

Bioequivalence Study Protocol (Key Elements)

• Title, investigators, objectives.
• Design: products, regimen, sampling schedule, fasting/meal control, housing.
• Population: inclusion/exclusion, consent, ethics, IRB approval.
• Clinical procedures, adverse-event handling.
• Analytical methods: validated assay, stability, calibration, QA.
• Statistics: ANOVA; confidence intervals; acceptance.

Statistical Assessment

• ANOVA: detect differences; significance if $p\le0.05$.
• Two one-sided tests / 90 % CI: geometric-mean ratios (test/reference) for AUC & $C_{max}$ must lie within 80 – 125 % (bioequivalence interval).
  • Wider limits possible for metabolite or very variable drugs with regulatory approval.

Enhancing Bioavailability (Overview)

• Class II: micronisation, solid dispersions, cyclodextrins, lipid vehicles, nanotech.
• Class III: permeability enhancers, prodrugs, transporter targeting.
• Class IV: combine solubility + permeability strategies.
• Class V/unstable: prodrugs, enteric protection, enzyme inhibitors, lymphatic delivery, lipid systems.

These bullet-point notes integrate all major and minor details, equations, examples, study designs, regulatory criteria, apparatus descriptions, statistical thresholds, and BCS/IVIVC principles necessary for an exam on bioavailability, bioequivalence and dissolution testing.