pH-responsive delivery

pH-responsive delivery is a system that keeps the drug locked away in one acidity and lets it out at another, like a material that stays solid in acid but dissolves in a more alkaline solution. The body's varying acidity is used as a built-in switch for when and where to release.

Acidity differs greatly along the body. The stomach is strongly acidic (about pH 1–3), the small intestine is mildly alkaline, the colon is somewhat acidic again, and tumour tissue and certain compartments inside cells are slightly more acidic than normal. pH-responsive materials are usually weakly acidic or weakly basic polymers whose ionisation, and therefore their swelling or solubility, changes sharply across a particular pH range. The familiar example is enteric coating, which stays intact in the acidic stomach and dissolves in the more alkaline small intestine.

pH is one of the most practical triggers because the differences along the gut are large and reliable, which is why enteric and colon-targeted products work reasonably well. The limits show up when the pH differences are small, as between tumour and healthy tissue, where a slight, variable acidity is a weak and unreliable switch. Gut pH also varies with food, disease, and from person to person, so even oral pH-responsive systems are not perfectly precise.