Primary steps of the photochemical reactions of 2-cyano-10-(3-[dimethylamino, N-oxide]-2-methylpropyl)-5-oxide-phenothiazine, the photoproduct of cyamemazine, a phototoxic neuroleptic: comparison with the sulfoxide.

The UVA-absorbing photoproduct resulting from the oxidation of the sulfur atom and of the side chain nitrogen of the phototoxic drug cyamemazine (CMZ) (2-cyano-10-(3-[dimethylamino]-2 methylpropyl)-phenothiazine) is a potent photodynamic photosensitizer. The photophysical and photochemical properties of this photoproduct (P) (2-cyano-10-(3-[dimethylamino, N-oxide]-2-methylpropyl)-5-oxide-phenothiazine)) have been investigated in neutral buffered aqueous solutions and in ethanol and compared to those of the sulfoxide (S) (2-cyano-10-(3-[dimethylamino]-2 methylpropyl)-5-oxide-phenothiazine), a CMZ oxidation product of cells. The fluorescence quantum yield (PhiF) of P is 0.25 and 0.21 in pH 7 phosphate buffer and ethanol, respectively. By contrast, S (PhiF = 0.14 in buffer) is practically unfluorescent in alcohol. In buffer, the fluorescence lifetimes of P and S are 10.5 and 11.8 ns, respectively. The transient absorbance of the first excited triplet state (3P1) with a characteristic absorption band peaking at 660 nm (epsilon = 5,300 M(-1) cm(-1)) has been observed by 355 nm laser flash spectroscopy of deaerated phosphate buffer or ethanol solutions. In buffer, the 3P1 lifetime is 0.5 micros. The energy transfer which occurs from the 3P1 to naproxen suggests that the 3P1 energy is greater than 62 kcal mol(-1). Triplet quenching by dioxygen occurs at rate 2.3 x 10(9) M(-1) s(-1). With the triplet benzophenone as actinometer, the 3P1 formation quantum yield is found to be 0. 40 in buffer. The 3P1 state is quenched by ethanol and 2-propanol with bimolecular reaction rate constants of 1.6 and 2.4 x 10(6) M(-1) s(-1), respectively. In buffer, P and S triplet states react with tryptophan, indole and cysteine at rate constants of the order of 10(9) M(-1) s(-1) for Trp and indole and 10(8) M(-1) s(-1) for Cys.