Continuing our investigations on inhibitors of ribonucleotide reductase (RNR), the crucial enzyme that catalyses the reduction of ribonucleotides to deoxyribonucleotides, we have now prepared and evaluated 5'-phosphonoacetic acid, amide and ester analogues of adenosine, uridine and cytidine with the aim to verify both substrate specificity and contribution to biological activity of diphosphate mimic moieties. A molecular modelling study has been conducted on the RNR R1 subunit, in order to verify the possible interaction of the proposed bioisosteric moieties. The study compounds were finally tested on the recombinant murine RNR showing a degree of inhibition that ranged from 350 microM for the UDP analogue 5'-deoxy-5'-N-(phosphon-acetyl)uridine sodium salt (amide) to 600 microM for the CDP analogue 5'-O-[(diethyl-phosphon)acetyl]cytidine (ester). None of the tested compounds displayed noteworthy cytostatic activity at 100-500 microM concentrations, whereas ADP analogue 5'-N-[(diethyl-phosphon) acetyl]adenosine (amide) and 5'-deoxy-5'-N-(phosphon-acetyl)adenosine sodium salt (amide) showed a moderate inhibitory activity (EC50: 48 microM) against HSV-2 and a modest inhibitory activity (EC50: 110 microM) against HIV-1, respectively.