We disclose a study on nucleosidetriphosphate (NTP) analogues in which the γ-phosphate is covalently modified by two different biodegradable masking units and d4T as nucleoside analogue that enable the delivery of d4TTP with high selectivity in phosphate buffer (pH 7.3) and by enzyme-triggered reactions in human CD4+ T-lymphocyte CEM cell extracts. This allows the bypass of all steps normally needed
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.