The invention provides lipophilic phosphonoacid/nucleoside conjugates that exhibit exceptional antiviral activity, including activity against drug-resistant HIV strains. Compounds of the invention include phosphonoacid/nucleoside conjugates where the carboxyl group and phosphonyl groups of the phosphonacid are esterified whereby the compound contains at least one lipophilic group and at least one nucleoside group.
A N.sup.4 -alkoxycarbonylarabinofuranosylcytosine represented by the formula: ##STR1## wherein R is an aliphatic hydrocarbon group having 4 to 22 carbon atoms. These compounds are useful as anti-tumor agents.