BCS classification, dissolution apparatus and media (including SLS/surfactant justification), sampling schedules, acceptance criteria, and both the f2 similarity factor and f1 difference factor formulas explained with worked examples — with references to FDA, EMA, WHO, ICH and USP guidance.
Comparative Dissolution Profile (CDP) is an in vitro study used to compare the rate and extent of drug release from a test product and a reference product under the same dissolution conditions. It helps determine whether the two products have similar dissolution behavior and supports formulation development, quality control, and regulatory approval.
Comparing the dissolution behavior involves evaluating the rate and extent of drug release from the test product and the reference product under the same experimental conditions. This comparison helps determine whether both products exhibit similar drug release profiles over time.
Similarity in drug release means showing that the test product and the reference product have comparable dissolution profiles under the same test conditions. Similarity is commonly evaluated using the similarity factor (f2), where an f2 value between 50 and 100 indicates that the two dissolution profiles are similar.
Comparative dissolution studies help optimize and improve the formulation by evaluating how changes in formulation or manufacturing affect drug release.
Comparative dissolution studies ensure consistent drug release between production batches and verify that the product meets established quality standards.
Comparative dissolution studies provide evidence to regulatory authorities that the test product has a dissolution profile comparable to the reference product, supporting product approval or post-approval changes.
Comparative Dissolution Profile (CDP) is performed to compare the in vitro drug release behavior of a test product and a reference product under identical conditions. It is used to determine whether both products have similar dissolution characteristics, which is important for ensuring product quality, batch consistency, formulation equivalence, and regulatory compliance. CDP also supports formulation development, post-approval changes (SUPAC), and bioequivalence justification by providing evidence that any formulation or manufacturing changes do not significantly affect drug release performance.
Comparative dissolution testing is used to determine whether the in-vitro drug-release profiles of two pharmaceutical products are similar, commonly comparing a test product with a reference product.
The similarity factor, f2, is the most commonly used statistical approach for comparing dissolution profiles.
Where:
Interpretation of f2
The closer the f2 value is to 100, the greater the similarity between the two dissolution profiles.
The difference factor, f1, measures the percent difference between the test and reference profiles at each sampling point.
Where:
Interpretation of f1
| Parameter | Difference factor (f1) | Similarity factor (f2) |
|---|---|---|
| Measures | Difference between profiles | Similarity between profiles |
| Ideal value | 0 | 100 |
| Common criterion | 0–15 | 50–100 |
| Higher value means | Greater difference | Greater similarity |
| Main use | Quantifies difference | Determines profile similarity |
For a valid f2 comparison, the sampling conditions and dissolution method should be appropriately standardized. Typically, at least three time points are used, excluding the zero-time point, and the comparison should cover the relevant portion of the dissolution profile.
A common rule of thumb is:
f1 ≤ 15 and f2 ≥ 50 → dissolution profiles are considered similar.
However, f2 has assumptions and limitations, particularly regarding variability, the number and selection of sampling points, and highly variable dissolution data. Regulatory guidance should therefore be followed when making a formal biowaiver or product-equivalence decision.
f1 = Difference → lower is better → ≤15
f2 = Similarity → higher is better → ≥50
In comparative dissolution testing, f2 (similarity factor) and f1 (difference/dissimilarity factor) are used to quantitatively compare the dissolution profile of a test formulation with a reference formulation.
The main purpose is to determine whether two formulations release the drug in a similar way over time.
This is useful because two tablets can have similar assay/content but still release the drug differently. Comparing dissolution profiles helps assess whether a change in formulation or manufacturing process has altered drug release.
A simple way to remember:
f1 asks: “How different are they?”
f2 asks: “How similar are they?”
f2 is commonly used when you want to demonstrate that a test dissolution profile is similar to a reference profile.
Typical situations include:
Generally, f2 ≥ 50 is interpreted as indicating similarity, provided the applicable regulatory requirements and assumptions are satisfied.
f1 is often calculated along with f2 to quantify the difference between the profiles.
The commonly used interpretation is:
So, for example:
| Result | Interpretation |
|---|---|
| f1 = 5, f2 = 75 | Profiles are similar |
| f1 = 12, f2 = 58 | Generally similar |
| f1 = 20, f2 = 42 | Profiles are not considered similar |
f1 and f2 are not normally used to prove that a formulation is clinically bioequivalent by themselves. They are tools for comparing in-vitro dissolution profiles. Whether they are appropriate, and the exact acceptance criteria, depend on the regulatory guideline and the product.
Perform comparative dissolution profile testing when you need to establish that two products/batches have similar drug-release profiles, for example:
FDA specifically describes dissolution-profile comparison for assessing changes to a product, while also allowing its use for generic vs. reference comparisons.
https://www.fda.gov/media/70936/download
The similarity factor f2 is the primary model-independent approach commonly used.
FDA and EMA both use 50 as the conventional similarity threshold.
https://www.fda.gov/media/70936/download
https://www.ema.europa.eu/en/documents/scientific-guideline/guideline-investigation-bioequivalence-rev1_en.pdf
f1 measures the difference between the two profiles.
The commonly used criterion is:
FDA describes f1 as a measure of the relative error/difference between the two dissolution curves.
https://www.fda.gov/media/70936/download
According to the EMA guideline, the conventional f2 approach requires:
This is an important exam point:
If ≥85% of drug is dissolved within 15 minutes, f2 comparison is generally not necessary because the products are considered rapidly dissolving under the relevant guideline approach.
https://www.fda.gov/media/121311/download
https://www.ema.europa.eu/en/documents/scientific-guideline/guideline-investigation-bioequivalence-rev1_en.pdf
If dissolution variability is too high for reliable conventional f2 calculation, you should not simply calculate f2 and ignore the variability.
Current EMA guidance discusses bootstrap confidence-interval approaches when variability exceeds the conditions for conventional f2. Similarity can be concluded when the lower limit of the 90% CI for expected f2 is ≥50 under that approach.
Don’t automatically conclude that the products are clinically different.
A result of f2 <50 means similarity has not been demonstrated using the conventional f2 criterion. Depending on the regulatory situation, additional scientific/statistical justification or an alternative method may be appropriate. FDA explicitly notes that an f2 below 50 does not necessarily prove lack of similarity in certain SUPAC situations.
https://www.fda.gov/media/70956/download
One important update: the EMA bioequivalence framework has transitioned some immediate-release bioequivalence provisions to ICH M13A, effective in the EU from January 2025, so the exact guideline applicable to your product/study should be checked rather than relying on the older EMA text alone.
https://www.ema.europa.eu/en/investigation-bioequivalence-scientific-guideline
https://www.fda.gov/media/70936/download
https://www.ema.europa.eu/en/documents/scientific-guideline/guideline-investigation-bioequivalence-rev1_en.pdf
Select appropriate test and reference products for comparison to evaluate the similarity of their dissolution profiles.
A test product is the pharmaceutical formulation under investigation in a comparative dissolution study. It may be a newly developed, generic, or modified formulation whose dissolution profile is compared with that of the reference product to assess the similarity of drug release.
The reference product is the approved marketed formulation, usually the innovator (original brand) product, that is used as a standard for comparison in a comparative dissolution study. It represents the established product with known quality, safety, and efficacy, against which the test product’s dissolution profile is evaluated.
Comparative Dissolution Profile (CDP) testing, the role of the Biopharmaceutics Classification System (BCS) class is important because it influences when dissolution profile comparison and particularly the similarity factor (f2) is expected.
| BCS Class | Solubility | Permeability | CDP / f2 Considerations |
|---|---|---|---|
| Class I | High | High | If both products dissolve ≥85% in 15 minutes, an f2 calculation is generally not necessary. If not, compare profiles using f2. |
| Class II | Low | High | Comparative dissolution testing is important. f2 is commonly used to compare profiles in each dissolution medium. |
| Class III | High | Low | If both products dissolve ≥85% in 15 minutes, f2 is generally not required. Otherwise, profile comparison may be needed depending on regulatory expectations. |
| Class IV | Low | Low | Comparative dissolution testing is performed, but these drugs are generally not eligible for BCS-based biowaivers. f2 may still be used to compare profiles during formulation development or regulatory evaluation. |
“Dissolution is the rate-limiting step for absorption” means that the drug must dissolve before it can be absorbed, and the speed of dissolution determines how quickly and how much drug is absorbed into the bloodstream.
For an orally administered solid dosage form (e.g., a tablet or capsule), the sequence is:
If the drug dissolves slowly, absorption cannot occur rapidly, even if the intestinal membrane is highly permeable. Therefore, dissolution becomes the rate-limiting (slowest) step in the absorption process.
BCS Class II (Low Solubility, High Permeability):
BCS Class I (High Solubility, High Permeability):
BCS Class III (High Solubility, Low Permeability):
BCS Class IV (Low Solubility, Low Permeability):
Selection of an appropriate official dissolution testing equipment (such as Apparatus I – Basket or Apparatus II – Paddle) based on pharmacopeial guidelines and the nature of the dosage form to ensure proper and reproducible drug release testing.
| Apparatus | Name | Definition / Use |
|---|---|---|
| Apparatus I | Basket Method | A dissolution testing system where the dosage form is placed inside a rotating wire mesh basket submerged in dissolution medium; mainly used for capsules and floating tablets. |
| Apparatus II | Paddle Method | A system where a paddle stirs the dissolution medium while the dosage form remains at the bottom of the vessel; commonly used for tablets and capsules. |
| Apparatus III | Reciprocating Cylinder | A system where dosage units are placed in cylinders that move up and down in different media; used for modified or extended-release dosage forms. |
| Apparatus IV | Flow-Through Cell | A system where dissolution medium continuously flows through a cell containing the dosage form; used for poorly soluble or special formulations. |
In comparative dissolution profiling, it refers to choosing an appropriate dissolution fluid (such as 0.1 N HCl, pH 4.5 acetate buffer, or pH 6.8 phosphate buffer) that maintains sink conditions and simulates gastrointestinal environments to ensure accurate comparison of drug release between test and reference products under identical conditions.
| Dissolution Medium (USP CDP Conditions) | Preparation (Step-by-Step) | pH | Purpose in CDP |
|---|---|---|---|
| 0.1 N Hydrochloric Acid | 1. Measure about 8.5 mL of concentrated HCl. 2. Transfer into a volumetric flask containing ~800 mL purified water. 3. Mix carefully and allow it to cool if heated. 4. Make up the volume to 1000 mL with purified water. 5. Mix well. | ~1.2 | Simulates gastric conditions for drug release comparison. |
| Acetate Buffer | 1. Prepare acetic acid solution and sodium acetate solution separately. 2. Mix required volumes of both solutions. 3. Adjust pH to 4.5 using acetic acid or sodium hydroxide if needed. 4. Make up to final volume with purified water. 5. Mix thoroughly. | 4.5 | Simulates weak acidic intestinal environment for CDP testing. |
| Phosphate Buffer | 1. Weigh required quantity of KH2PO4 and dissolve in purified water. 2. Prepare a separate solution of K2HPO4 in purified water. 3. Mix both solutions in the required proportion as per USP method. 4. Check pH and adjust to pH 6.8 using dilute HCl or NaOH if needed. 5. Make up final volume with purified water and mix well. | 6.8 | Simulates intestinal fluid conditions for dissolution comparison. |
In comparative dissolution profile studies, surfactants, dispersing agents, or solubility enhancers may be incorporated into the dissolution medium when the active pharmaceutical ingredient (API) or dosage form is hydrophobic or poorly water-soluble, or when conventional aqueous media fail to achieve sink conditions or adequate dispersion. Their use improves the wettability, dispersion, and apparent solubility of the drug, enabling complete and reproducible dissolution. The selected agent and its concentration must be scientifically justified, using the lowest effective concentration to maintain the discriminatory power of the dissolution method. Both the test and reference products should be evaluated under identical dissolution conditions, including the same surfactant type and concentration, to ensure a valid comparison of dissolution profiles.
When sodium lauryl sulfate (SLS, also called sodium dodecyl sulfate, SDS) is required in a comparative dissolution profile, its use must be scientifically justified rather than added routinely. Regulatory agencies (FDA, EMA, USP, ICH) expect the dissolution medium to be biorelevant, discriminatory, and capable of maintaining sink conditions with the lowest effective concentration of surfactant.
SLS is added only when the drug has poor aqueous solubility and sink conditions cannot be achieved using physiological pH media alone.
Typical situations include:
FDA and USP recommend first evaluating aqueous media across physiological pH values before considering surfactants. SLS should be used only when necessary to maintain sink conditions.
The objectives are:
FDA
FDA recommends:
For example:
EMA
EMA states:
USP <1092>
USP recommend:
Common concentrations are:
| Solubility problem | Typical SLS concentration |
|---|---|
| Mild | 0.1% |
| Moderate | 0.25% |
| Poor | 0.5% |
| Very poor | 1.0% |
| Extremely poor | up to 2.0% (only with justification) |
Most pharmacopeial methods use 0.25–1.0% SLS.
A regulatory justification should include:
Step 1
Determine API solubility in
Step 2
Calculate sink conditions.
Sink condition means the dissolution medium should dissolve at least three times the amount of drug present in the dosage unit.
Step 3
If sink conditions are not achieved,
Evaluate
Step 4
Select the lowest concentration providing:
When comparing Test vs Reference products:
The same dissolution conditions must be used for both:
Too many SLS may:
Research has shown that lower SLS concentrations may be better reflected in vivo behavior than higher concentrations that simply create sink conditions.
When SLS is used:
| Guideline | Recommendation |
|---|---|
| FDA Dissolution Guidance | Use surfactant only when needed for sink conditions; justify type and concentration. |
| EMA Reflection Paper | Avoid surfactants if possible; if used, use the lowest justified concentration. |
| USP <1092> | Select surfactant based on API properties and avoid excessive concentrations that reduce method discrimination. |
| ICH M9 / EMA Bioequivalence Q&A | Dissolution profile comparisons should be used justified, validated, and discriminatory test conditions. |
Reference– USP General Chapter. Capsules—Dissolution Testing and Related Quality Attributes, Section 3.2.2: Use of Surfactants/Dispersing Agents/Solubility Enhancers.
Temperature: Maintain at 37 ± 0.5°C throughout the test.
Dissolution Medium: Use a suitable medium (e.g., 0.1 N HCl, pH 4.5 acetate buffer, pH 6.8 phosphate buffer) prepared as per USP and ensure sink conditions.
Volume: Typically, 500, 900, or 1000 mL, as specified in the monograph.
Apparatus: Use USP Apparatus I (Basket) or II (Paddle) unless otherwise specified in the monograph.
Rotation Speed: Maintain specified rpm (e.g., 50–100 rpm) as per USP method.
Sampling Time Points: Select multiple time points to adequately describe the release profile (early, intermediate, and late stages).
Uniformity: Ensure identical conditions for both test and reference products to allow valid profile comparison.
A minimum of 12 individual dosage units from both test and reference products must be evaluated. Each unit is tested individually (not pooled) to assess unit-to-unit variability and ensure reliable comparison of dissolution profiles.
This requirement is consistent with USP dissolution testing principles (e.g., USP <711>) and regulatory guidance for comparative dissolution studies.
| Dosage Form | USP / Guideline-Based Sampling Schedule Selection |
|---|---|
| Immediate Release Tablets/Capsules | Sampling should include early, intermediate, and late-time points (e.g., 5, 10, 10, 15, 30, 45, 60 min). At least 3–5 time points before 85% dissolution are recommended to describe the full release profile. |
| Delayed Release (Enteric-Coated) Dosage Forms | Sampling should include testing in acid stage (e.g., 0–2 h, no drug release expected) followed by buffer stage with multiple time points (e.g., 5, 10, 15, 30, 45, 60 min) to capture release after coat dissolution. |
| Extended/Controlled Release Dosage Forms | Sampling should be performed over an extended period (e.g., 1–24 h or longer) with multiple time points selected to define release kinetics (early, mid, and late phase). More frequent early sampling is recommended. |
| Orally Disintegrating Tablets (ODTs) | Very early sampling is required (e.g., 1, 2, 3, 5, 10, 15 min) to capture rapid disintegration and drug release characteristics. |
| Suspensions | Sampling should include multiple time points (e.g., 5, 10, 15, 30, 45, 60 min) ensuring adequate mixing before each withdrawal to capture dissolution of dispersed particles. |
| Step | USP / Regulatory Guideline Requirement |
|---|---|
| 1. Data Requirement | Dissolution profiles of both test and reference products must be generated using 12 individual dosage units each, under identical test conditions. |
| 2. Time Point Selection | At least 3 time points (excluding zero) must be used, with a sufficient number of points to describe the full profile. Not more than one time point should be considered after both products reach ≥85% dissolution. |
| 3. Variability Check (RSD) | The % Relative Standard Deviation (%RSD) should generally be ≤20% at early time points and ≤10% at later time points for both test and reference products. |
| 4. Similarity Factor (f2) | The similarity factor (f2) should be calculated. A value of 50–100 indicates similarity between test and reference dissolution profiles. |
| 5. Profile Shape Comparison | The overall shape of the dissolution curves for test and reference should be comparable, showing no significant difference in rate and extent of drug release. |
| 6. Sink Conditions Compliance | Dissolution study must be conducted under sink conditions, ensuring medium volume is sufficient to dissolve at least three times the drug concentration. |
| 7. Conclusion Criteria | If f2 ≥ 50, acceptable variability limits are met, and profiles are similar, the test product is considered to have an equivalent dissolution profile to the reference product. |
| Step | Details |
|---|---|
| 1. Formula | f2 = 50 × log {[1 + (1/n) Σ (Rt − Tt)²]⁻⁰·⁵ × 100} |
| 2. Meaning of terms | n = number of time points Rt = % drug dissolved of reference at time t Tt = % drug dissolved of test at time t |
| 3. Condition | Minimum 3 time points, and only one time point allowed after 85% dissolution |
| Time (min) | Reference (R) | Test (T) | R − T | (R − T)² |
|---|---|---|---|---|
| 10 | 30 | 28 | 2 | 4 |
| 15 | 42 | 40 | 2 | 4 |
| 20 | 55 | 53 | 2 | 4 |
| 30 | 75 | 73 | 2 | 4 |
| 45 | 90 | 88 | 2 | 4 |