Harned and Gaynon Figure 1 Structures of clofarabine and other nucleoside analogs. Derived from Jeha et al (2004). release of the neurotoxic halogenated adenine (Parker et al 1991; Carson et al 1992; Plunkett and Gandhi 2001; Jeha et al 2004; Jeha and Kantarjian 2007; Kantarjian et al 2007). In addition, clofarabine has greater afﬁnity than cladribine or ﬂudarabine for deoxycytidine kinase, the enzyme required for intracellular phosporylation to its monophosphorylated form. Clofarabine is then serially phosphorylated by other kinases to its active metabolite, clofarabine triphosphate (Bonate et al 2004, 2005). Clofarabine triphosphate is retained in acute leukemia cells for longer periods of time than the metabolites of the other purine nucleoside analogs (Xie and Plunkett 1995, 1996; Bonate et al 2006; Gandhi et al 2006,). Mechanism of action Purine analog triphosphates inhibit ribonucleotide reductase and are substrates for DNA polymerases for incorpora- tion into DNA (Gandhi et al 2006). Fludarabine primarily inhibits DNA polymerases and cladribine mainly inhibits ribonucleotide reductase, while clofarabine inhibits both DNA polymerases and ribonucleotide reductase (Parker et al 1991, 1999; Xie and Plunkett 1995, 1996; Jeha et al 2004). Inhibition of DNA polymerases leads to impairment of DNA synthesis and/or repair through the incorporation of clofarabine monophosphate into the DNA chain (Xie and Plunkett 1996; Bonate et al 2006). Inhibition of ribonucleo- tide reductase depletes the deoxynucleotide pool primarily of dCTP and dATP (Xie and Plunkett 1996; Bonate et al 2006). The reduction in dCTP concentrations limits DNA synthesis, while the reduction in dATP creates a favorable environment for clofarabine triphosphate to compete with 330 dATP for incorporation into DNA (Xie and Plunkett 1996; Bonate et al 2006). A third mechanism of action is induc- tion of apoptosis initiated by DNA strand breaks, resulting in the release of cytochrome c and activation of apoptotic pathways (Genini et al 2000; Bonate et al 2006). Clofarabine has also been shown to act directly on the mitochondria in primary chronic lymphocytic leukemia (CLL) cells, altering the transmembrane potential and releasing pro-apoptotic mitochondrial factors, which may explain the mechanism of cell death in cells not actively synthesizing DNA for cell division or DNA repair. This direct mitochondrial activity was not seen with ﬂudarabine and may partly explain the enhanced cytotoxicity of clofarabine (Carson et al 1992; Genini et al 2000; Bonate et al 2006). Pharmacodynamics In the blast cells of adults with refractory leukemias, the accu- mulation of clofarabine triphosphate was associated with a decrease in DNA synthesis (Gandhi et al 2003). DNA synthesis was 75% to 95% inhibited at the end of an infusion with clo- farabine at doses ranging from 22.5 to 55 mg/m2. At 24 hours, partial recovery of DNA synthesis occurred in the blasts of patients treated with 22.5 and 30 mg/m2, while the inhibition of DNA synthesis was maintained at 24 hours after 40–55 mg/m2 doses (Gandhi et al 2003; Kantarjian et al 2007). Pharmocokinetics and metabolism (Table 2) The population pharmacokinetics of plasma clofarabine and intracellular clofarabine triphosphate was studied in Therapeutics and Clinical Risk Management 2008:4(2)