RNA viruses exhibit extraordinarily high mutation rates during genome replication. Nonnatural ribonucleosides that can increase the mutation rate of RNA viruses by acting as ambiguous substrates during replication have been explored as antiviral agents acting through lethal mutagenesis. We have synthesized novel N-6-substituted purine analogues with ambiguous incorporation characteristics due to tautomerization of the nucleobase. The most potent of these analogues reduced the titer of poliovirus (PV) and coxsackievirus (CVB3) over 1,000-fold during a single passage in HeLa cell culture, with an increase in transition mutation frequency up to 65-fold. Kinetic analysis of incorporation by the PV polymerase indicated that these analogues were templated ambiguously with increased efficiency compared to the known mutagenic nucleoside ribavirin. Notably, these nucleosides were not efficient substrates for cellular ribonucleotide reductase in vitro, suggesting that conversion to the deoxyriboucleoside may be hindered, potentially limiting genetic damage to the host cell. Furthermore, a high-fidelity PV variant (G64S) displayed resistance to the antiviral effect and mutagenic potential of these analogues. These purine nucleoside analogues represent promising lead compounds in the development of clinically useful antiviral therapies based on the strategy of lethal mutagenesis.Natural nucleotides exist as tautomers in solution, and tautomerization of the nucleobases in DNA has been recognized as a likely mutagenic mechanism ever since the double-helical structure of the DNA molecule was first deduced by Watson and Crick (33,36). The keto (for G and U) and amino (for A and C) tautomers of the natural nucleotides are the predominant species with tautomeric constants (K T ) on the order of 10 5 . However, tautomeric conversion to the rare enol or imino forms of the nucleobases can lead to altered hydrogen bonding specificity and thus mutagenesis through noncanonical basepairing interactions. This has become known as the "rare tautomer" hypothesis of mutation (19,29).The tautomerization of bases to yield ambiguous base-pairing properties has also been exploited in the design of novel nucleoside drugs, including 5-hydroxy-2Ј-deoxycytidine (22) and 5-aza-5,6-dihydro-2Ј-deoxycytidine (KP-1212) (18, 25) (Fig. 1A). However, attempts to design clinically useful antiviral compounds around this premise have met with only limited success (13,16,18,25).Nucleobases exhibiting multiple conformations due to rotation or tautomerization have received attention as potential antiviral agents acting through lethal mutagenesis (11,22). Since the discovery that ribavirin (Fig. 1B) can act as a lethal mutagen (7), likely through rotation of the exocyclic carboxamide moiety, the concept of lethal mutagenesis as an antiviral strategy has received considerable attention (5,11,12).Recently, we have demonstrated that the ribonucleoside analogue rP (Fig. 1C) can act as a potent mutagen of poliovirus (PV) in vitro (13). Tautomerization of the nucleobas...