Regulation of ribonucleotide reductase in mammalian cells by chemotherapeutic agents

The reductive conversion of ribonucleotides to deoxyribonucleotides is a prime target for the development of a cancer chemotherapeutic agent. A new series of inhibitors based on the polyhydroxy or polyamino aromatic ring has been developed. These compounds are effective ribonucleotide reductase inhibitors and possess antitumor activity if there are adjacent hydroxy or amino groups. The most effective enzyme inhibitor, 2,3,4-trihydroxybenzohydroxamic acid, is 145 times more effective than hydroxyurea. However, the best antileukemic compound is 3,4-dihydroxybenzohydroxamic acid, which increased the life span of L1210 leukemic mice more than 100%. Structure-activity studies have revealed that the hydroxamic moiety is not essential for activity. The polyhydroxybenzene derivatives reduced the pool sizes of all four deoxynucleotides. This contrasts with the action of hydroxyurea, which depletes only the deoxypurines. The mechanism of inhibition by this group of compounds appears to be related to their ability to trap free radicals, since there is good correspondence between reductase inhibition and free radical destruction. Some naturally occurring cytotoxic and neurological agents which have antineoplastic activity, dopa analogs and phenolic compounds isolated from mushrooms, bear a structural similarity to the new reductase inhibitors. We found that the dopa analogs were also inhibitory to ribonucleotide reductase. Another consequence of polyhydroxybenzoic acid derivative treatment is elevated reductase levels in the cell. Other cell cycle inhibitors that block from late G₁ through early G₂ also cause an enhanced level of ribonucleotide reductase. However, agents that block in early or mid-G₁ or mid or late G₂ and mitosis produce lower reductase levels. These data suggest that reductase synthesis is initiated at the G₁/S transition point and this enhanced level of activity continues until late S or G₂.