Reducing the background fluorescence in mice receiving fluorophore/ inhibitor DNA duplexes

In principle, a DNA duplex consisting of an antisense fluorophore- conjugated major strand hybridized to a shorter complementary inhibitor-conjugated minor strand should provide fluorescence only in the tumor after intravenous administration if designed to remain intact except in the presence in tumor of its mRNA target. While we have obtained impressive tumor images in mice using this approach, there remains some background fluorescence. In this study, tissue homogenates of selected mouse organs were incubated with a test duplex and the kinetics of duplex dissociation in normal tissues were measured. In this manner we were able to identify the liver as the likely major source responsible for the duplex dissociation providing this fluorescence background. Thereafter liver homogenates were used to screen a series of duplex candidates with variable-length minor strands, and dissociation was measured by gel electrophoresis. The selected fluorophore/inhibitor duplex with improved stability displayed an insignificant (P ≥ 0.05) background fluorescence after administration to SKH-1 normal mice and apparently without affecting target mRNA binding in vitro in cell culture or in vivo in tumor bearing mice.