Abstract 5356: Pediatric preclinical testing program (PPTP) evaluation of the DNA methylating agent temozolomide

Abstract

Abstract Background: Temozolomide is DNA methylating agent that has been approved in the United States for treatment of astrocytoma. This drug in many ways resembles more established compounds, such as dacarbazine and procarbazine, in that it gives rise to a methyl diazonium ion that attacks nucleophilic sites including the O6-guanine position in DNA. Temozolomide, however, differs from these drugs, which have to be activated by enzymatic oxidation, in that it degrades spontaneously via base-catalyzed hydrolysis to the final active methylating species. Methods: The PPTP includes a molecularly characterized in vitro panel of cell lines (n=27) and in vivo panel of xenografts (n=61) representing most of the common types of childhood solid tumors and childhood acute lymphoblastic leukemia (ALL). Temozolomide (provided by the NCI Drug Repository) was tested in vitro at concentrations from 100 nM to 1 mM. Temozolomide was administered PO using a daily × 5 schedule repeated at 21 days at a dose of 100 mg/kg, or 66 mg/kg. Three measures of antitumor activity were used: 1) an objective response measure modeled after the clinical setting; 2) a treated to control (T/C) tumor volume measure; and 3) a time to event (4-fold increase in tumor volume) measure based on the median event-free survival (EFS) of treated and control animals for each xenograft. Biomarkers of temozolomide sensitivity, MGMT, MLH1, MSH2 and p53 genotype were determined. Results: The median temozolomide IC50 value for the PPTP cell lines was 380 μM (range 2 to > 1000 μM), with the neuroblastoma cell line NB-1643 having the lowest IC50 value. There were no significant differences in IC50 values between the rhabdomyosarcoma, neuroblastoma, Ewing sarcoma, and ALL cell lines. In vivo temozolomide induced significant toxicity at 100 mg/kg resulting in exclusion of 10/42 lines from analysis. For the 32 lines evaluable there were 13 maintained complete responses (MCR) and 2 CR in the solid tumor panels and 3 MCR and 2 CR in the ALL panel. Retesting the excluded lines at 66 mg/kg resulted in excessive toxicity leading to exclusion of 2/9 lines. At this dose there were 2 MCR and progressive disease in the remaining 5 lines. Overall 17/30 tumor models demonstrated objective responses (≥PR). Of these 7 are MGMT-negative, 3 are MLH1-negative, and 11 have wild type p53. All 7/7 MGMT-negative tumors responded, irrespective of p53 genotype (5/7 wild type). Conclusions: In vitro temozolomide induced cytotoxicity with no apparent cell type specificity. In vivo temozolomide demonstrated high activity, but with a steep dose response curve, consistent with other DNA damaging agents. Responses poorly correlated with MGMT, MLH1, MSH2 levels, or p53 genotype. However, all MGMT-negative tumors were responsive, irrespective of MLH1 or p53 status. The results suggest that temozolomide may have potential for treatment of a range of pediatric malignancies. (Supported by NO1-CM91003-03) Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5356. doi:10.1158/1538-7445.AM2011-5356