The radiosensitizing effect of fluorocyclopentenyl-cytosine (RX-3117) in ovarian and lung cancer cell lines

ABSTRACT

RX-3117 (fluorocyclopentenyl-cytosine) is a novel cytidine analog currently being evaluated in a Phase Ib clinical trial in cancer patients with solid tumors. The radiosensitizing effect of RX-3117 was studied in A2780 ovarian cancer cells and non-small cell lung cancer cell lines and related to cell survival and the effect on cell cycle and cell cycle proteins. RX-3117 has a schedule-dependent radiosensitizing effect, but only at pre-incubation (dose modifying factors: 1.4–1.8), observed at pulse and fractionated irradiation. Radiosensitizion was also seen in a three-dimensional spheroid model. At the low radiosensitizing concentration, RX-3117 in combination with radiation led to an accumulation of cells in S-phase, which was accompanied with an increase of cell cycle proteins such as p-Chk2 and p-cdc25C. In addition, RX-3117 caused DNA damage and increased apoptosis. In conclusion, our in vitro experiments showed a radiosensitizing effect of RX-3117.     

Sensitive liquid chromatography mass spectrometry (LC-MS) assay reveals novel insights on DNA methylation and incorporation of gemcitabine,its metabolite difluorodeoxyuridine,deoxyuridine, and RX-3117 into DNA

ABSTRACT

Antimetabolites are incorporated into DNA and RNA, affecting their function. Liquid-chromatography-mass-spectrometry (LC-MS-MS) permits the sensitive, selective analysis of normal nucleosides. The method was adapted to measure the incorporation of deoxyuridine, gemcitabine (difluorodeoxycytidine), its metabolite difluorodeoxyuridine (dFdU), and the novel compound fluorocyclopentenylcytosine (RX3117). DNA was degraded to its deoxynucleotides for quantification by LC-MS-MS, gradient chromatography on a Phenomenex prodigy-3-ODS with positive ionization. The range of deoxyuridine DNA-mis-incorporation varied nine-fold in 27 cell lines (leukemia, colon, ovarian, lung cancer). At low-folate conditions a 2.1-fold increase in deoxyuridine was observed. Global methylation (given as % 5-methyl-deoxycytidine) was comparable between the cell lines (4.6–6.5%). Exposure of A2780 cells to 1 μM gemcitabine (4 hours) resulted in 3.6 pmol gemcitabine/μg DNA, but in AG6000 cells (deoxycytidine-kinase-deficient) no incorporation was found. However, when A2780, AG6000, or CCRF-CEM cells were exposed to 100 μM dFdU we found it as gemcitabine, 20.5, 19.6, and 0.51 pmol gemcitabine/μg DNA, respectively. Preincubation of CCRF-CEM cells with cyclopentenyl-cytosine (a CTP-synthetase inhibitor) increased dFdU incorporation four-fold. Apparently dFdU is activated independently of deoxycytidine-kinase and possibly converted in-situ to dFdCMP. RX3117 was incorporated into both DNA and RNA (0.0037 and 0.00515 pmol/μg, respectively). In summary, a sensitive method to quantify the incorporation of gemcitabine, deoxyuridine, and RX-3117 was developed, which revealed that dFdU was incorporated into DNA as the parent compound gemcitabine.