Clofarabine

Clofarabine is effectively transferred into cells by the concentrative nucleoside transporter hCNT253 as well as two facilitative or equilibrative nucleoside transporters, hENT1 and hENT2. After entering cells, cytosolic kinases phosphorylate clofarabine in a stepwise manner to its nucleotide analogues, clofarabine 5′-mono-, di-, and triphosphate; clofarabine triphosphate is the active form. Clofarabine 5′-mono-, di-, and triphosphate must be enzymatically converted back to their dephosphorylated nucleoside form by 5′-nucleotidase in order to be transported out of the cell. These compounds are not substrates for nucleoside transporters. With an IC50 of 65 nM, clofarabine triphosphate effectively inhibits ribonucleotide reductase, most likely through binding to the regulatory subunit's allosteric site. It has also been demonstrated that clofarabine directly affects mitochondria by changing the transmembrane potential and causing the cytosol to release caspase 9, cytochrome c, apoptotic-inducing factor (AIF), and apoptosis protease-activating factor 1 (APAF1). Strong in vitro growth inhibition and cytotoxic activity (IC50 values = 0.028–0.29 μM) are demonstrated by clofarabine in a range of solid tumor and leukemia cell lines. It has been demonstrated that clofarabine increases dCK activity in HL60 cells and increases ara-C mono-, di-, and triphosphate formation in K562 cells36.[1] In chronic lymphocytic leukemia (CLL) lymphocytes, clofarabine (10 μM) inhibits the repair that 4-hydroperoxycyclophosphamide (4-HC) started. In CLL lymphocytes, the inhibition peaks at intracellular concentrations of 5 μM. The combined effects of clofarabine (10 μM) and 4-hydroperoxycyclophosphamide (4-HC) result in more apoptotic cell death than the sum of their individual effects. The combination of clofarabine (1 μM) and ara-C (10 μM) causes synergistic cell death in K562 cells by biochemically modulating ara-CTP.[3]