Fluorophore Labeling Affects the Cellular Accumulation and Gene Silencing Activity of Cholesterol-Modified Small Interfering RNAs In Vitro.

The objective of this study was to analyze the effects of fluorophores on the intracellular accumulation and biological activity of small interfering RNA (siRNA) and its cholesterol conjugates. In this study, we used stem-loop real-time PCR and calibration curves to quantitate cellular siRNA accumulation. Attachment of fluorophores significantly affected both the accumulation and biological activity of siRNA conjugates. The severity of this effect depended significantly on the structure of the conjugate; fluorophores (Cy5.5 or Alexa-488) attached to siRNA, facing the side of the duplex opposite to cholesterol, enhanced the unproductive intracellular accumulation of the conjugate when delivered in carrier-free mode. Enhanced cellular accumulation of siRNA conjugates did not result in enhanced biological activity of the conjugate. Moreover, the attachment of a hydrophobic fluorophore, such as Cy5.5, to conventional siRNA also enhanced its apparent intracellular accumulation, but not its biological activity. Thus, the use of fluorescent labels for the study of the intracellular accumulation of siRNA and its conjugates formed with different molecules is possible only for a limited range of structures, and requires verification using alternative methods.