Abstract C205: Persistent cell cycle perturbations and apoptosis mediate the antitumor activity of ST1968 in pediatric models.
Authors: Meco Daniela , Di Francesco Angela Maria , Cusano Gabriella , Bucci Federica , Torella Anna Rita , Pratesi Valentina , Pierri Filomena , Pisano Claudio , Riccardi Riccardo 
Published: June 2012

Introduction. ST1968 (Namitecan) is a new water-soluble camptothecin analog belonging to the 7-oxyiminomethyl series that has been shown to have activity against a wide spectrum of adult tumor xenografts. Based on its therapeutic efficacy, Namitecan was selected for clinical development.

Methods. We investigated the cellular pathways underlying the antitumor activity of ST1968 in the following pediatric tumor models: neuroblastoma (SK-N-DZ, SK-N-AS, SK-N-BE(2)c, HTLA and SHEP-1); primitive neuroectodermal tumors/PNET (PFSK) and medulloblastoma (DAOY). Cell growth inhibitory effect was evaluated with the 5-day MTT assay. The genotoxic potential of the camptothecin analog was evaluated in vitro and in vivo with the alkaline Comet assay. Cell cycle perturbation were studied by conventional propidium iodide staining whereas apoptosis was assayed by hypodiploid cell population and PARP cleavage. For in vivo studies, tumor xenografts were treated with the compounds at the following schedule: i.v. q4d×4, at the concentration of 30mg/kg for ST1968 and 50mg/kg for CPT-11. The in vivo tumor distribution of Namitecan was studied in the responsive neuroblastoma model SK-N-AS that was selected also for preliminary evaluations of drugs combination with platinum agents.

Results. In the initial growth inhibition screening ST1968 appeared between 2 and 22-fold less effective than SN38, the active metabolite of irinotecan. However, the cellular response to the novel analog increased with prolonged drug incubation (24 and 72h) in line with the reported evidence that ST1968 uptake is poor and would therefore be favoured by longer exposure times.

Overall, we observed a heterogeneous cellular response to ST1968. Cell cycle studies suggested that the enhanced cellular sensitivity to Namitecan in our models appeared mediated by more persistent cell cycle perturbations (late-S/G2 accumulation up to 72h) and increased sub-G1 peak indicative of apoptosis commitment. These effects were likely a consequence of the strong DNA damaging properties of the drug as shown by the rapid kinetic of DNA breaks induction. Interestingly this toxic cellular pathways appeared p53-independent as indicated by the lack of correlation between cellular sensitivity and p53 status of the cell lines from our panel. The antitumor potency of Namitecan was superior to CPT-11 in almost all our xenografts models where the drug was well tolerated. In SK-N-AS xenografts the pharmacokinetic study showed prolonged ST1968 retention in tumors consistently with a rapid and persistent DNA damage induction, G2-block (48–72h) and apoptosis. Combination studies with platinum agents showed a significant enhancement of antitumor activity of the drugs combination vs single agents. Particularly impressive was the observation of a very high rate of complete regression with the ST1868-carboplatin combination that persisted for 25–35 days after the cessation of treatment.

Conclusions. Our data confirm the superior activity of Namitecan as compared to irinotecan that is currently utilized in pediatric oncology, strongly supporting further investigations of the novel camptothecin derivative as a single agent or in combination.

Supported by FOP.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr C205.