The dye sensitized photooxidation in water (pH 6), of the pharmaceutical topical antiglaucoma drugs, Timolol and Pindolol, which act as β-adrenergic receptor antagonists, were studied by means of static and time-resolved spectroscopic methods and polarographic determinations. O2(1Δg)-mediated photooxidation of Timolol and Pindolol takes place with quantum efficiencies of 0.035 and 0.16, respectively, which raises concern about the possible day-light-mediated photodamaging of the drugs, in the presence of sensitizing agents. Pindolol behaves kinetically as a typical indole derivative, for which the intermediacy of a polar complex is proposed. Solvent effects on the kinetics of photooxidation suggests that the same mechanism could operate for the case of Timolol. Upon direct ultraviolet-light irradiation Timolol and Pindolol generate O2(1Δg), with quantum yields of 0.027 and 0.11 respectively. The former comprises three desirable properties for an external-use ocular drug: a reduced efficiency of O2(1Δg) photooxidation, a relatively high power as O2(1Δg) physical deactivator and a relatively low propensity to O2(1Δg) generation upon direct light irradiation.
