Adaphostin and other anticancer drugs quench the fluorescence of mitochondrial potential probes

Fluorescent dyes are widely used to monitor changes in mitochondrial transmembrane potential (ΔΨ_m). When MitoTracker Red CMXRos, tetramethylrhodamine methyl ester (TMRM), and 3,3′dihexyloxacarbocyanine iodide (DiOC₆(3)) were utilized to examine the effects of the experimental anticancer drug adaphostin on intact cells or isolated mitochondria, decreased fluorescence was observed. In contrast, measurement of tetraphenylphosphonium uptake by the mitochondria using an ion-selective microelectrode failed to show any effect of adaphostin on ΔΨ_m. Instead, further experiments demonstrated that adaphostin quenches the fluorescence of the mitochondrial dyes. Structure-activity analysis revealed that the adamantyl and p-aminobenzoic acid moieties of adaphostin are critical for this quenching. Anticancer drugs containing comparable structural motifs, including mitoxantrone, aminoflavone, and amsacrine, also quenched the mitochondrial probes. These results indicate the need for caution when mitochondrial dyes are utilized to examine the effects of xenobiotics on ΔΨ_m and suggest that some previously reported direct effects of anticancer drugs on mitochondria might need re-evaluation.