Metabolism of antisense oligonucleotides in rat liver homogenates

Phosphorothioate antisense oligodeoxynucleotides are novel therapeutic agen ts designed to selectively and specifically inhibit production of various d isease-related gene products. In vivo pharmacokinetic experiments indicate that these molecules are widely distributed in many species, with the major ity of oligomers accumulating within liver and kidney. To better understand the metabolism of these agents, we studied the stability of several phosph orothioate oligodeoxynucleotides, their congeners, and second generation ol igomer chemistries in rat liver homogenates. To examine metabolism, backgro und nuclease activity was characterized in whole liver homogenates by using ISIS 1049, a 21-mer phosphodiester oligodeoxynucleotide. Nuclease activity could readily be detected in liver homogenates. Under optimized conditions , the predominant enzymatic activity was 3'-exonucleolytic and could be inf luenced by pH and ionic conditions. However, in addition to 3'-exonucleases , 5' exo- and endonuclease activities were also observed. Our data indicate that metabolism of phosphorothioate oligodeoxynucleotides was more complex than that of phosphodiesters for many reasons, including phosphorothioate oligodeoxynucleotide inhibition of nucleases and the presence of R-p and S- p stereoisomers. The rate of phosphorothioate metabolism also appeared to b e influenced by sequence, with pyrimidine-rich compounds being metabolized to a greater extent than purine-rich oligomers. Other factors affecting sta bility included oligomer chemistry and length. Concomitant experiments perf ormed in rats dosed systemically with the same compounds mimic the activiti es seen in vitro and suggest that this liver homogenate system is a valuabl e model with which to study the mechanism of metabolism of antisense oligon ucleotides.