The methods of the present invention allow for the measurement of ribonucleotide reductase (RR) activity, an important enzyme in the de novo DNA synthesis pathway. Ribonucleotide reductase converts all four ribonucleotides to their deoxy form and is a rate-controlling step in this pathway. Biosynthetic pathways of deoxyribonucleotides (dN) have received considerable attention in the context of anti-proliferative chemotherapy. Inhibitors of various steps in dN biosynthesis, including inhibitors of RR are among the most useful chemotherapeutic agents in cancer, viral infections, and other therapeutic uses. DNA synthesis from the dN salvage pathway is also an important component to DNA replication. The relative contributions from RR vs. salvage pathways are critical to the actions and effectiveness of chemotherapeutic agents that act on nucleoside metabolic pathways. Until now, however, it has not been possible to study these metabolic processes in vivo. Disclosed within are methods of measuring RR activity in vivo and in vitro which find use, among other things, in drug discovery, development, and approval.
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