Formulation Factors to Consider
The physiologic pH of blood and tears is approximately 7.4. Thus, from a comfort
and safety standpoint, this would be the optimal pH of ophthalmic and parenteral
solutions. This may not be possible, however, from a perspective of solubility,
chemical stability or therapeutic activity. Thus, some compromise must be made
and product stability must be considered paramount.
When a formulation is administered to the eye, it stimulates the flow of tears.
Tear fluid is capable of quickly diluting and buffering small volumes of added
substances, thus the eye can tolerate a fairly wide pH range. Ophthalmic solutions
may range from pH 4.5 - 11.5. But the useful range to prevent corneal damage
is 6.5 to 8.5.
Once we have determined the optimal pH of a product, we need a mechanism for
adjusting and maintaining the pH of the solution.