Superoxide dismutase as a target for the selective killing of cancer cells.

Superoxide dismutases (SOD) are essential enzymes that eliminate superoxide radical (O2-) and thus protect cells from damage induced by free radicals. The active O2- production and low SOD activity in cancer cells may render the malignant cells highly dependent on SOD for survival and sensitive to inhibition of SOD. Here we report that certain oestrogen derivatives selectively kill human leukaemia cells but not normal lymphocytes. Using complementary DNA microarray and biochemical approaches, we identify SOD as a target of this drug action and show that chemical modifications at the 2-carbon (2-OH, 2-OCH3) of the derivatives are essential for SOD inhibition and for apoptosis induction. Inhibition of SOD causes accumulation of cellular O2- and leads to free-radical-mediated damage to mitochondrial membranes, the release of cytochrome c from mitochondria and apoptosis of the cancer cells. Our results indicate that targeting SOD may be a promising approach to the selective killing of cancer cells, and that mechanism-based combinations of SOD inhibitors with free-radical-producing agents may have clinical applications.

Fludarabine and arabinosylcytosine therapy of refractory and relapsed acute myelogenous leukemia.

There is a strong association between ability of leukemia blasts to accumulate ara-CTP, the active metabolite of ara-C, and response to ara-C in patients with relapsed or refractory AML. Ara-C dose rates in excess of 0.5 g/m2/h do not produce further ara-CTP formation. In contrast, when given 4 h prior to ara-C at this dose rate, fludarabine, at doses that are free of neurotoxicity in CLL, enhances ara-CTP accumulation. This led us to administer fludarabine and ara-C to 59 patients with AML in relapse or unresponsive to initial therapy. Fludarabine was given at 30 mg/m2 once daily for 5 doses and ara-C at 0.5 g/m2/h for 2-6 h daily for 6 doses. Doses of fludarabine preceded those of ara-C by 4 h. Results with fludarabine and ara-C (FA) were compared with those of patients treated at M.D. Anderson with high-dose ara-C (HDAC) or intermediate-dose ara-C (IDAC). The complete remission rate with FA was 21/59 (36%) and the actuarial median CR duration 39 weeks. FA produced significantly higher remission rates than HDAC or IDAC in patients with initial remissions > 1 yr (14/20 vs 9/23 vs 6/18, p < 0.05). Response rates were similar for all three treatments in patients with initial remissions < 1 yr or with primary refractory disease. The regimen was well tolerated; one patient developed peripheral neuropathy. This low level of toxicity encourages combination with other antileukemia agents.

Comparison of the cellular pharmacokinetics and toxicity of 2',2'-difluorodeoxycytidine and 1-beta-D-arabinofuranosylcytosine.

2',2'-Difluorodeoxycytidine (dFdC) is a new deoxycytidine analogue with good activity against human leukemic cell lines and murine solid tumors, while the activity of 1-beta-D-arabinofuranosylcytosine (ara-C) is established in experimental systems and for the treatment of human adult leukemia. This study compared the cellular metabolism and cytotoxic properties of dFdC and ara-C in Chinese hamster ovary cells. In wild-type cells, dFdC was significantly more cytotoxic than ara-C after both 4- and 18-h incubations. The 5'-triphosphate of dFdC (dFdCTP) was the major cellular metabolite (85-90%), reaching cellular concentrations up to 20-fold greater than those observed for ara-C 5'-triphosphate at equimolar concentrations of the parent drug. A deoxycytidine kinase-deficient mutant neither accumulated dFdCTP nor showed any cytotoxic response up to drug concentrations of 100 microM. The cytotoxicity of dFdC could be competitively reversed by deoxycytidine further suggesting that dFdC, like ara-C, required phosphorylation by deoxycytidine kinase for biological activity. Several explanations for the different cellular accumulation of the drug triphosphates were established: (a) nucleoside transport studies demonstrated that the membrane permeation of dFdC was 65% more rapid than that of ara-C; (b) deoxycytidine kinase had a higher affinity for dFdC (Km = 3.6 microM) than for ara-C (Km = 8.8 microM), while the Km for deoxycytidine was 1.4 microM; (c) the elimination of intracellular dFdCTP was biphasic with t1/2 alpha = 3.9 and t1/2 beta greater than 16 h while the degradation of ara-CTP was monophasic and significantly faster (t1/2 = 0.7 h). The comparatively long half-life of dFdCTP was related to the prolonged inhibition of DNA synthesis after removal of exogenous nucleoside. Together these factors contribute to the more potent cytotoxicity of dFdC compared with ara-C.