It is most important to determine suitable pH for a given drug in solution at early stages of drug discovery. If the drug is to be administered as a solution dosage form then the pH of the solution should be chosen carefully. Solution dosage forms include eye drops, nasal drops, ear drops, oral solutions, injectables...etc.
The following is a list of considerations to be taken while choosing a suitable pH for a drug solution:
1. Solubility of the drug. If the drug is a weak acid or a weak base then the pH of the solution affects its solubility. solubility of weak acids increase (to a limit) as pH increase and solubility of weak bases increase (to a limit) as pH decrease.
2. Stability of the drug. The pH of the solution can affect the degradation rate of the drug. Depending on the drug, a pH at which the drug is most stable can vary. For example, aspirin is most stable (has lowest degradation rate) at pH = 2.5.
3. Permeability of the drug through biological membranes. This depends on the extent of ionization of the drug. The pH of the solution can affect the extent of ionization of weakly acidic or weakly basic drugs. A Drug in the non-ionized form is more permeable than in its ionized form.
4. Tissue irritation. The pH of a pharmaceutical solution can not be too acidic or too basic. The further away we get from the physiological pH (7.4) the greater the irritation. For oral solutions the acceptable pH range can be larger than that for eye drops or injectable solutions given the delicacy of the tissues coming in touch with the solution.
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