Seems like replacing cytosine could have some negative side effects if it occurs in the patient's cells as well as the virus's.
"Even though sequencing clearly shows the virus trying to resist the drug, no resistant strains developed."
I don't understand how that works. How does it "try" to resist? Isn't this all-or-nothing to the virus? Either it reproduces or it doesn't?
They mutate. 'Resist' implies intent which they don't have. But they mutate when they reproduce. Some mutations will given them drug resistance but they haven't lucked into them yet.
If this formulation encourages mutations, doesn't that feel like it will play right into your described method of lucking into resistance?