Albendazole is practically insoluble in water. That single fact decides the whole formulation strategy: dissolution is the slow step, not absorption, and particle size is therefore a functional property rather than a cosmetic one. Both the human and veterinary specifications require 90% of particles below 20 micrometre for that reason – it is the one parameter the two grades agree on.
Why particle size dominates this molecule
In a BCS Class II molecule, the drug dissolves slowly but crosses membranes readily. What limits exposure is the rate at which solid material goes into solution in the gut. Reducing particle size raises specific surface area, and by the Noyes-Whitney relationship that raises dissolution rate – which is why micronisation is written into the monograph rather than left to the formulator.
| Property | Consequence | Where it shows up |
|---|---|---|
| Very low aqueous solubility | Dissolution is rate-limiting | Exposure depends on particle size as much as on dose |
| High permeability | Once dissolved, absorption is not the barrier | Formulation effort goes into dissolution, not permeation enhancers |
| Polymorphism | Different lattice forms dissolve at different rates | Particle size alone does not guarantee the dissolution profile your development used |
| Oxidation sensitivity | Milling generates local heat and fresh surface | Degradation can rise during micronisation – re-test after milling |
Reading a particle size specification properly
| Figure | What it says | What it does not say |
|---|---|---|
| D90 max 20 micrometre | 90% of the volume is below 20 micrometre | Nothing about the coarsest 10% – a coarse tail can carry most of the undissolved mass |
| 90% below 20 micrometre | The same criterion expressed for the veterinary grade | Nothing about the fine end, which drives agglomeration and dust |
| Full distribution (D10 / D50 / D90) | The shape of the distribution, which is what dissolution responds to | Nothing about the measurement method – laser diffraction and sieving give different numbers on the same lot |
| Method statement | Whether the number is comparable with your own historical data | Nothing about dispersion during measurement, which is the usual source of disagreement between labs |
The practical instruction: ask for the full distribution and the method, and specify both in the purchase order. A certificate that reports only “D90 < 20” is compliant and still uninformative.
What goes wrong on the line
| Symptom | Likely cause | Action |
|---|---|---|
| Dissolution results drift between lots of API | Distribution shape changed even though D90 stayed inside the limit | Compare full distributions across lots, not only the single figure |
| Poor flow and dust at the tablet press | Over-micronised material with high fine fraction | Balance the fine end against dissolution – a wet granulation step may be needed |
| Assay fine, oxidative impurities rising after milling | Heat and fresh surface during micronisation | Re-test sulfoxide and sulfone after milling; consider cooled milling |
| Blend uniformity problems at low dose | Fine cohesive API agglomerating in the blender | Geometric dilution, or a granulation route rather than direct compression |
| Bioequivalence failure against the reference | Polymorph or distribution difference from the innovator material | Qualify against dissolution profile, not against a single D90 figure |
Micronisation and the oxidative impurities
Milling is a mechanical operation with a chemical consequence. Freshly created surfaces are reactive, local temperatures can rise, and the material spends time in air. For an oxidation-sensitive molecule with a sulfoxide and a sulfone already on the specification, that is a real risk rather than a theoretical one.
| Control | What it does | What to ask the supplier |
|---|---|---|
| Cooled or inert milling | Limits the thermal and oxidative load during size reduction | Whether milling is temperature-controlled |
| Post-milling testing of the oxidative pair | Confirms the size reduction did not consume the stability budget | Whether the certificate reflects pre- or post-milling material |
| Sealed transfer and packing | Prevents moisture and air ingress between mill and drum | Whether the liner is closed immediately after milling |
| Shelf-life data on micronised material | Micronised product can age differently from coarse material | Stability data generated on the micronised grade, not extrapolated from coarse |
Specifying it on the order
| Item | What to write |
|---|---|
| Distribution | D90 max 20 micrometre, with D10 and D50 reported |
| Method | The particle size method, stated, so the numbers are comparable with your own data |
| Post-milling impurities | The oxidative pair reported on the milled material |
| Polymorph | The form your development was based on, if your route is polymorph-sensitive |
| Sampling | How the sample is taken from the drum – particle size segregates in handling |
Both the human and the veterinary grade carry a 20 micrometre criterion, which is the clearest signal that this is about the molecule rather than the market. Whichever grade you buy, the dissolution behaviour is what the patient or the animal actually receives.