Therapeutic Nucleic Acids: A Potential Source of Resistance to Cancer, Antiviral and Immunosuppressive Therapy.

According to their unique structural and chemical properties nucleic acids were recognized to provide inherent therapeutic potential beyond sole information storage. In the meantime an increasing number of nucleic acid based drugs achieved FDA approval and entered clinical trails (e.g. “antisense” or “immunosustimulatory CpG” oligodeoxynucletides (ODNs), aptamers, ribozymes, RNA interference or defibrotide). But so far, no or less information is available whether these compounds might

• compete with chemically and structurally related drugs, e.g. nucleoside analogues (NA) widely used in cancer or antiviral therapy, or

• interfere with the intracellular nucleic acid metabolism.

In the present report we provide evidence, that nucleic-acid based drugs antagonize fludarabine, acyclovir or mycophenolate mofetil (MMF). In presence of defibrotide (DF), a polydisperse mixture of single-stranded oligodeoxyribonucleotides (15 to 30 kD) e.g. used for treatment of hepatic veno-occlusive disease and other endothelial disorders, fludarabine treated lymphocytes or myeloid blasts where rescued from apoptosis. According to nucleic acid degradation the resulting metabolite deoxycytidine turned out to be the key substrate competing with fludarabine for phosphorylation by deoxycitidine kinase (dCK) and suggested interference with nucleic acid metabolism rather than direct competition with the drug for cellular uptake. Moreover, in standard drug resistance assays using acyclovir sensitive herpes simplex virus (HSV) strains (V0631508), 4 mM of DF restored viral replication in presence of 50 mM acyclovir. This was confirmed by quantitative PCR of viral DNA. Here, deoxythymidine turned out to be the main competitor for intracellular phosphorylation mediated by virus thymidine kinase. To further extent our findings, that an increase of the extracellular concentration of nucleic acids directly interfere with the intracellular nucleic acid metabolism, mixed lymphocyte reactions (MLRs) were performed, to test whether the immunosuppressive effects of mycophenolate mofetil (MMF) could be reversed. As control cyclosporine A (CsA) was used. Here, addition of singular ribonucleotides almost completely antagonized the T cell inhibitory effects of mycophenolic acid (MPA), respectively its prodrug MMF, but not of CsA. As MPA is known to be a potent, selective, uncompetitive and reversible inhibitor of inosine monophosphate dehydrogenase, a key enzyme for the de novo pathway of guanosine nucleotide synthesis, addition of guanosine to the MLRs was found to be effective and sufficient to reverse the immunosuppressive effects of MPA. We conclude that treatment with nucleic-acid-based drugs interfere with the intracellular nucleic acid metabolism, thus decreasing the efficacy of NAs used for cancer and antiviral therapy or the immunosuppressive therapy using MMF. Prospective clinical trials are required to confirm these in vitro findings.