On the antioxidant properties of therapeutic drugs: quenching of singlet molecular oxygen by aminosalicylic acids.

The ability of the widely employed therapeutic drugs 4-aminosalicylic acid and 5-aminosalicylic acid to act as singlet molecular oxygen (O(2)((1)delta(g))) scavengers was investigated at pH 7 and pH 12. The isomer 3-aminosalicylic acid was also included in the study for comparative purposes. All three compounds quench photochemically generated O(2)((1)delta(g)) with rate constants in the range of 10(7)-10(8) x M(-1)s(-1), depending on the experimental conditions. No chemical reaction (oxidation of the aminosalicylic acids) was detected at the neutral pH, whereas at pH 12 both chemical and physical interactions with O(2)((1)delta(g)) operated. The physical process implies the de-activation of the oxidant species without destruction of the aminosalicylic acid. The quotients between the overall and reactive rate constants for O(2)((1)delta(g)) quenching at pH 12 (k(r)/k(t) ratios), which account for the actual effectiveness of photodegradation, were relatively low (0.22, 0.04, and 0.06 for 3-, 4- and 5-aminosalicylic acids, respectively). This indicates that the drugs, particularly the 4- and 5-amino derivatives, de-activate the excited oxygen species, at both pH values studied, mainly in a physical fashion, preventing its photodegradation and providing an antioxidative protection for possible photo-oxidizable biological targets in the surroundings.