Dissolution — The Performance Test
Assay tests ask what is in the tablet. Dissolution asks what gets out of it, and how fast — a surrogate for the rate and extent of absorption in a patient. It is the one routine test in this course that is about the patient’s experience rather than the molecule’s identity, and it is where the coating decisions from Week 2’s solid-dosage manufacturing section get proven or disproven.
The one idea
A dissolution method that passes every batch you have ever made is not necessarily good news — it may just mean it isn’t discriminating enough to tell a good batch from a bad one.
The apparatus (USP / Ph. Eur.)
| Apparatus | Name | Typical use |
|---|---|---|
| 1 | Basket | Capsules, floating dosage forms |
| 2 | Paddle | The default for tablets |
| 3 | Reciprocating cylinder | Extended-release, pH-change profiles |
| 4 | Flow-through cell | Low-solubility drugs, implants, modified-release; open or closed loop |
| 5–7 | Paddle-over-disk, cylinder, reciprocating holder | Transdermals and other special forms |
What has to be controlled
The medium (volume, pH, surfactant, degassing), temperature (37 °C), agitation, and — most easily overlooked — sink conditions: enough medium that the dissolved drug never approaches its saturation solubility, or the measured rate is limited by the medium, not by the product. Violate sink conditions and the result describes the bath, not the tablet.
Biorelevant and discriminating media
Simple buffers are used for routine QC; biorelevant media (FaSSIF/FeSSIF, simulating fasted/fed intestinal fluid) are used in development to predict in-vivo behaviour. A good QC method is discriminating — it responds to the formulation and process changes that would matter clinically, and ignores the ones that wouldn’t. Building that discrimination, and then proving it, is the hard part of method development, harder by far than running the test itself.
IVIVC and the biowaiver
An in-vitro / in-vivo correlation links the dissolution profile to a pharmacokinetic profile. A validated Level A IVIVC can support a biowaiver — a formulation or manufacturing-site change approved on dissolution data alone, instead of a new bioequivalence study in humans. This is the direct payoff of a discriminating method: it lets a change be defended with a bench test instead of a clinical one.
Acceptance criteria, staged
USP ⟨711⟩ builds a sampling design directly into the acceptance criteria — not a single pass/fail: test 6 units (S1), and only if that’s inconclusive, 6 more (S2), and only if still inconclusive, 12 more (S3), with the allowed variability widening at each stage. A batch can pass at S1 cleanly, pass at S3 only marginally, or fail outright — and each of those tells you something different about how close to the edge the batch really is.
Where the analyst sits
Is this dissolution method actually discriminating, or does it pass every batch including the ones that would underperform in a patient? That question has no compendial answer — it is answered by deliberately manufacturing batches with known defects (over-compressed, under-coated, wrong particle size) and confirming the method tells them apart. A method that has never been challenged that way hasn’t earned its trust yet, no matter how many batches it has passed.
For discussion
- A dissolution method passes every batch you have ever made. How would you go about finding out whether it actually discriminates?
- “Sink conditions” — why does violating them make a dissolution result meaningless, and how would you detect that you had?
- A batch fails at S1, is retested at S2, and passes comfortably. What does that sequence tell you about the batch that a single S1 result would not?
- A functional (extended-release) coating passes appearance and weight-gain checks but the batch fails dissolution — connect this back to Week 2’s coating section: what upstream step would you investigate first?
Source note. Dissolution: USP ⟨711⟩ / ⟨724⟩ / ⟨1092⟩, Ph. Eur. 2.9.3, the FDA dissolution and BCS-biowaiver guidances.