Dissolution and Stability
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Direct answer
A tablet that never dissolves might as well be a pebble — dissolution is the gateway between a swallowed dosage form and absorbed drug, and the Noyes-Whitney equation, dC/dt = DA(Cs − C)/h, is its mathematical statement: rate rises with surface area, diffusion coefficient and the gap between saturation and bulk concentration, and falls with the thickness of the diffusion layer. Stability is the second half of the same quality question: chemical and physical change over time, described by zero-order and first-order kinetics, predicted at room temperature through the Arrhenius relationship, and certified by storage studies — with India sitting in ICH climatic zone IVb, meaning long-term testing at 30 °C and 75 per cent relative humidity.
What you must remember
- Noyes-Whitney: dC/dt = DA(Cs − C)/h, where D is diffusion coefficient, A the surface area, Cs the saturation solubility, C the bulk concentration and h the diffusion layer thickness — every formulation trick maps to one symbol.
- Sink conditions keep C below about 10 per cent of Cs so dissolution stays efficient; the official medium is usually 900 mL at 37 ± 0.5 °C.
- IP/USP apparatus: Apparatus 1 (basket) run commonly at 100 rpm, Apparatus 2 (paddle) at 50 rpm; specifications are written as Q values, for example NLT 75 per cent dissolved in 45 minutes for many immediate-release products.
- Particle size increases A ( micronisation of griseofulvin is the standard example); polymorph choice changes Cs — chloramphenicol palmitate's polymorph B is more bioavailable than A.
- Degradation pathways: hydrolysis (esters, amides — aspirin), oxidation (catechol drugs such as adrenaline, ascorbic acid), photolysis (riboflavin, nifedipine), racemisation or epimerisation (tetracycline epimerises to toxic epianhydrotetracycline), and polymerisation (ampicillin in concentrated solution).
- Kinetics: zero order loses a constant amount per unit time; first order loses a constant fraction, with t1/2 = 0.693/k and t90 = 0.105/k.
- Shelf-life is the time to fall to 90 per cent of label claim — the t90 concept — and expiry dating is built on it.
- Arrhenius: k = A e^(−Ea/RT); accelerated studies at 40 ± 2 °C and 75 ± 5 per cent RH project long-term shelf-life, confirmed by real-time data.
- India is Zone IVb: long-term stability testing at 30 °C/75 per cent RH; packaging (amber glass, blisters, desiccants) is part of stability engineering.
A shelf-life calculated, not asserted
Take a first-order degradation with k = 0.001 per day at 25 °C. The fraction remaining after time t is e^(−kt), and the time to 90 per cent remaining is t90 = 0.105/k = 105 days. That is the arithmetic behind every expiry date: measure k, divide 0.105 by it, and the product's shelf-life falls out. Now raise the storage temperature and k grows according to Arrhenius — this is why a 40 °C/75 per cent RH cabinet can compress years into months: run the accelerated study, plot ln k against 1/T (kelvin), extrapolate the straight line to 25 °C, read k, and compute t90. The method's honesty caveat earns viva marks: accelerated data are provisional — suspensions, emulsions and moisture-sensitive products behave non-ideally, so real-time studies at zone conditions (30 °C/75 per cent RH for India) must confirm the projection.
Dissolution uses the same logic of controlled conditions in reverse: fix the medium, agitation and temperature so that only the product's variables — particle size, polymorph, excipients, coating — move the curve. A batch failing Q = 75 per cent in 45 minutes at paddle 50 rpm is rejected not because its content is wrong but because its release is wrong; and for a BCS class II drug (low solubility, high permeability) dissolution is the rate-limiting step to absorption, which is why micronised griseofulvin is several times better absorbed than the plain drug.
The confusions that decide the marks
First, disintegration versus dissolution: breaking into particles is not dissolving — a tablet may disintegrate in seconds and still release nothing if the drug cannot enter solution. Second, t90 versus expiry of safety: 90 per cent potency is the regulatory convention, not a cliff edge of toxicity; but products like tetracycline are the exception students must know, where the degradant (epianhydrotetracycline) is harmful and expiry is a safety date. Third, the Arrhenius shortcut: extrapolating from one elevated temperature assumes the same mechanism operates across the range, and moisture-driven reactions break the assumption — hence the humility of confirming with real-time zone IVb data.
The polymorphism trap completes the set: same molecule, different crystal lattice, different solubility — so stability and dissolution both shift between forms, and process changes (milling, drying, granulation) can convert forms during manufacture. That is why the answer to "what did you protect in this formulation?" may be a crystal habit, not just a molecule.
Frequently asked questions
State the Noyes-Whitney equation and how formulation improves dissolution.
Dissolution rate = DA(Cs − C)/h; increase surface area by micronisation, raise Cs by salt or polymorph selection, improve wetting with surfactants, or decrease h with agitation.
Compare IP dissolution Apparatus 1 and Apparatus 2.
Apparatus 1 is a rotating basket, typically 100 rpm, suiting floating or disintegrating dosage forms; Apparatus 2 is a paddle at typically 50 rpm, suiting most tablets and capsules; both use 900 mL medium at 37 ± 0.5 °C.
Define t90 and calculate it for k = 0.0005 per day.
t90 is the time for a first-order loss to reach 90 per cent remaining; t90 = 0.105/k = 210 days.
What is the significance of ICH climatic zone IVb for India?
Zone IVb (hot and humid) sets long-term stability testing at 30 °C and 75 per cent RH, more stressing than the 25 °C/60 per cent RH of zone II, so packaging and shelf-life must be designed for Indian conditions.
Why can accelerated stability data not simply replace real-time data?
Extrapolation assumes a single, temperature-independent mechanism; moisture-mediated and physical changes deviate from Arrhenius behaviour, so projections must be verified by real-time studies.