All nucleoside analogs are pro-drugs.
They have to enter the cell and get phosphorilated and be converted to triphosphates in order to work. Hydroxyurea is a ribonucleotidese reductase inhibitor. At least that is what we know from in vitro studies. Hydroxyurea probably also plays an immunomodulator role that may be related to its antiviral activity when used as part of combination regimens. This was a 24-patient study.