Riboflavin-mediated Photosensitization of Vinca Alkaloids Distorts Drug Sensitivity Assays1

Poor reproducibility of cytotoxicity tests with Vinca alkaloids has fre quently been reported. A commonly presumed light sensitivity of the drugs could not be confirmed. However, we found that they are photo sensitized by riboflavin (vitamin li.i, an obligatory component of cell culture media. Light of wavelengths below 500 nm triggered rapid photoreactions of riboflavin with vinblastine, \ im risiine, and vindesine in aqueous solutions. The photoreactions altered the absorption spectra of these alkaloids and yielded degradation products that could be separated by TLC. In cell cultures, both immediate and persisting, riboflavin-mediated photoreactivity could be distinguished. They preclude reliable determina tions of sensitivity and resistance to Vinca alkaloids, a.s exemplified on chemosensitive and multidrug-resistant mouse ascites cells. In experi ments involving photosensitization, the 50% inhibitory concentration val ues of sensitive and resistant cells were overlapping and fluctuated in the ranges from 3 to 30 nM and 15 to 360 TIMvinblastine, respectively. Corresponding values from series of experiments protected from photo sensitization were 1.02 ±0.22 nM and 18.5 ±3.42 nM. Hence, riboflavinmediated photoreactions must be fully prevented in assays of cellular drug sensitivity. Procedures for eliminating immediate as well as persisting photoreactivity were established.