In vitro activity: Clofarabine is efficiently transported into cells via two facilitative or equilibrative nucleoside transporters, hENT1 and hENT2, and a concentrative nucleoside transporter, hCNT253. Clofarabine is phosphorylated in a stepwise manner by cytosolic kinases to the nucleotide analogues clofarabine 5′-mono-, di- and triphosphate following entry into cells, with Clofarabine triphosphate being the active form. Clofarabine 5′-mono-, di- and triphosphate are not substrates for nucleoside transporters and must be enzymatically converted by 5′-nucleotidase back to their dephosphorylated nucleoside form for transport out of the cell. Clofarabine triphosphate is a potent inhibitor of ribonucleotide reductase (IC50 = 65 nM), presumably by binding to the allosteric site on the regulatory subunit. Clofarabine has also been shown to act directly on mitochondria by altering the transmembrane potential with release of cytochrome c, apoptotic-inducing factor (AIF), apoptosis protease-activating factor 1 (APAF1) and caspase 9 into the cytosol. Clofarabine demonstrates strong in vitro growth inhibition and cytotoxic activity (IC50 values = 0.028–0.29 μM) in a wide variety of leukaemia and solid tumour cell lines. Clofarabine has been shown to increase the activity of dCK in HL60 cells, and increases the formation of the mono-, di-, and triphosphates of ara-C in K562 cells36. Clofarabine (10 μM) inhibits the repair initiated by 4-hydroperoxycyclophosphamide (4-HC), with inhibition peaking at the intracellular concentrations of 5 μM in chronic lymphocytic leukemia (CLL) lymphocytes. Clofarabine (10 μM) combined with 4-hydroperoxycyclophosphamide (4-HC) produces more than additive apoptotic cell death than the sum of each alone. Clofarabine (1 μM) combined with ara-C (10 μM) results in a biochemical modulation of ara-CTP and synergistic cell kill in K562 cells

Nat Rev Drug Discov. 2006 Oct;5(10):855-63; Clin Cancer Res. 2001 Nov;7(11):3580-9.