Vitamin B-2-sensitised photooxidation of the ophthalmic drugs Timolol and Pindolol: kinetics and mechanism

The kinetics and mechanistic aspects of the riboflavin-photosensitised oxidation of the topically administrable ophthalmic drugs Timolol (Tim) and Pindolol (Pin) were investigated in water-MeOH (9;1, v/v) solution employing light of wavelength > 400 nm. riboflavin, belonging to the vitamin B-2 complex, is a known human endogenous photosensitiser. The irradiation of riboflavin in the presence of ophthalmic drugs triggers a complex picture of competitive reactions which produces the photodegradtion of both the drugs and the pigment itself. The mechanism was elucidated employing stationary photolysis, polarographic detection of dissolved oxygen, stationary and time-resolved fluorescence spectroscopy, and laser flash photolysis. Ophthalmic drugs quench riboflavin-excited singlet and triplet states. From the quenching of excited triplet riboflavin, the semireduced form of the pigment is generated, through an electron transfer process from the drug, with the subsequent production of superoxide anion radical (O2(.-)) by reaction with dissolved molecular oxygen. through the interaction of dissolved oxygen with excited triplet riboflavin, the species singlet oxygen (O-2((1)Delta(g))) is also generated to a lesser extend. Both O-2(.-) and O-2((1)Delta(g)) induce photodegradation of ophthalmic drugs, Tim being similar to3-fold more easily photooxidisable than Pin, as estimated by oxygen consumption experiments.