Comparative study of a novel nucleoside analogue (Troxatyl, troxacitabine,BCH-4556) and AraC against leukemic human tumor xenografts expressing highor low cytidine deaminase activity

Purpose: Troxacitabine (beta -L-dioxolane cytidine, BCH-4556; Troxatyl, Bio Chem Pharma Inc.) is a novel nucleoside analogue, which in experiments demo nstrated potent antitumor activity against both leukemias and solid tumors. Since troxacitabine is a cytidine nucleoside analogue like AraC (1-beta -D -arabinofuranosylcytosine), which is currently used in the treatment of acu te myelogenous leukemia, we compared the in vivo antileukemic activity of t roxacitabine with that of AraC in human leukemia xenograft models. Methods: The antiproliferative activity of troxacitabine and AraC was analyzed on h emapoietic cell lines by use of a thymidine incorporation assay. For in viv o studies, we compared troxacitabine with AraC by using equitotoxic schedul es of the two nucleosides optimized for therapeutic activity. The antileuke mic activity of both drugs was evaluated by measurement of their effect on the percent increased lifespan. Results: AraC had good in vitro antiprolife rative activity (IC50=14 nM) but was ineffective in vivo against the HL60 p romyelocyte leukemia cell line (treated vs control, T/C = 105%). Troxacitab ine, which in contrast to AraC is not a substrate for cytidine deaminase, s howed potent in vitro and in vivo activity in the same model (IC50=53 nM an d T/C=272% to 422%). The poor in vivo activity of AraC against HL60 leukemi a cells could be due to the high cytidine deaminase (CDA; EC 3.5.4.5) activ ity in this cell line. This hypothesis was tested with CCRF-CEM T-lymphobla stoid leukemia cells which have undetectable levels of CDA activity. Short- term exposure of these leukemia cell lines to both drugs indicated that Ara C was indeed significantly more effective in the CCRF-CEM cell line than in HL60. In contrast, the antiproliferative activity of troxacitabine was sim ilar for both cell lines. These observations were extended to in vivo studi es. Mice bearing CCRF-CEM tumor xenografts were treated with AraC and troxa citabine. In this model, T/C values were comparable for both drugs and rang ed from 138% to 157%. Conclusions: Our findings indicate that troxacitabine is likely to be effective not only against solid tumors with high CDA acti vity but also in leukemias which have developed resistance to AraC due to i ncreased CDA levels; this suggests that troxacitabine is a promising agent for the treatment of cancer. Indeed, significant antileukemic activity has been observed with troxacitabine in a phase I clinical trial in patients wi th primary refractory or relapsed acute myeloid leukemias (AML).