Purpose: To investigate whether transdermal iontophoresis may be potentially useful for delivery of oligonucleotide drugs, the electrotransport of representative bases (uracil and adenine), nucleosides (uridine and adenosine) and nucleotides (AMP, ATP, GTP and imido-GTP) across mammalian skin in vitro has been considered.
Methods: While the passive permeability of all compounds investigated (from 1 mM solutions at pH 7.4) was very low, the application of constant current iontophoresis (0.55 mA/cm2) significantly enhanced the transport of both charged and uncharged species.
Results: The efficiency of delivery depended only weakly upon lipophilicity, varied quite linearly with concentration (for AMP and ATP), was inversely sensitive to molecular weight, and was strongly influenced by charge. Neutral solutes were delivered better from the anode than the cathode, as expected; post-iontophoresis, passive permeabilities were greater than those of the untreated controls, suggesting that iontophoretically-induced changes in barrier function cannot be completely repaired in in vitro model systems. The triphosphate nucleotides, ATP and GTP, were essentially completely metabolized (presumably to their corresponding mono-phosphates) during their iontophoretic delivery, while imido-GTP was apparently resistant to enzymatic attack; however, comparison of the transport data from AMP and ATP suggested that ATP metabolism occurred primarily after the rate-limiting step of iontophoresis.
Conclusions: The results obtained are consistent with the general patterns of behavior previously observed in investigations of amino acid and peptide electrotransport. It remains to be seen whether extension of the research described here to larger oligonucleotide species is a feasible long-term objective.