The present invention provides methods of preparing an oligonucleotide, nucleoside or nucleoside analog for selective introduction into a subject's cells, the method comprising (1) selecting a targeted aptamer, internalizing nucleic acid or tumor-homing nucleic acid via iterative rounds of selection, and (i) hybridizing it to an oligonucleotide, (ii) replacing one or more nucleotide with a nucleoside or nucleoside analog, or (iii) synthesizing the it with one or more nucleoside or nucleoside analogs; or (2) preparing a naive combinatorial aptamer, internalizing nucleic acid or tumor-homing nucleic acid prodrug library, and running iterative rounds of selection for the cells. The present invention also provides the agent, the pharmaceutical composition, and methods of treating or preventing cancer and/or viral infection, the method comprising administration of the oligonucleotide, nucleoside or nucleoside analog for selective introduction into a subject's cells.