Preclinical Efficacy and Toxicology Studies of APR-246, a Novel Anticancer Compound Currently In Clinical Trials for Refractory Hematological Malignancies and Prostate Cancer

Abstract 1806

The tumor suppressor protein p53 is a transcription factor involved in cell cycle arrest, senescence and apoptosis. The p53 gene is frequently mutated in cancer, and cancer cells carrying defects in p53 are generally more resistant to conventional chemotherapy. Thus, restoration of wild type function of p53 is a promising novel strategy for cancer therapy. APR-246 belongs to a new class of small molecules (quinuclidinones) that reactivates non-functional p53 by promoting its correct folding and triggering apoptosis (Lambert et al. Cancer Cell 15, 2009). The lead compound of APR-246, PRIMA-1 (p53 Reactivation and Induction of Massive Apoptosis) was identified by a cellular screen of a NCI (National Cancer Institute) library, and an optimization program led to the discovery of the analog APR-246 (PRIMA-1^MET). In various in vitro,ex vivo andin vivo cancer models, APR-246 has shown good antitumor activity. It reduces cell viability and/or induces apoptosis in a large number of human cancer cell lines with different p53 status, including leukemia, lymphoma and myeloma cell lines (Mohell et al. Blood 114, 2009). Ex vivo efficacy of APR-246 alone and in combination with conventional chemotherapeutic drugs has been shown in primary cells from patients with acute myeloid leukemia (AML) (Jonsson-Videsater et al. Blood 114, 2009). Ex vivo efficacy of APR-246 has also been shown in primary cells from patients with chronic lymphocytic leukemia (CLL). APR-246 was 4–8 fold more potent in killing malignant than normal lymphocytes, whereas common cytostatics often have negative ratio (Mohell et al. Blood 114, 2009). In vivo efficacy of APR-246/PRIMA-1 has been demonstrated in xenograft studies using many solid tumor cell lines (Mohell et al. Blood 114, 2009). Here we present results from studies with APR-246 using in vivo systemic and metastasic xenograft model with the human AML primary cell line AML-PS. This model was established by Giovazzi et al. (Int. J. Cancer 61, 1995) and is considered as a predictive in vivo model for human AML. In addition, some key results from preclinical safety and toxicology studies are reported. Briefly, SCID (severe combined immunodeficiency) mice were inoculated i.v. with 5×10⁶ human AML-PS primary cells. Three days after inoculation treatment with i.v. injections of APR-246 (200 and 300 mg/kg), twice daily for 10 days, was initiated. Mice were monitored daily for health status and mortality. Blood samples were collected for determination of the percentage of circulating human leukemia cells by FACS analysis. Human leukemic cells were detected using a fluorescent antibody against the major histocompatibility complex (HLA). In parallel, pharmacokinetic experiments to measure the concentration of APR-246 in the blood were performed. We found that APR-246 had a statistically significant antitumor effect by decreasing the percentage of circulating human AML-PS cells and increasing the survival time of the mice (P=0.0024, n=10). A good correlation between increase in survival time and decrease in circulating tumor cells in the blood was observed. Further in vivo efficacy studies using various treatment schedules and combinations with conventional cytostatics are ongoing. APR-246 was also investigated in pivotal toxicology studies using single and repeat-dose regimen. In dogs, APR-246 was well tolerated when administered as 2 h infusion with NOAEL (no observed adverse effect level) of 200 mg/kg/day (4000 mg/m2/day). In both dogs and mice, C_max levels less than 100 μg/ml did not induce any toxicity, regardless of the administration protocol. No systemic target organ toxicity was observed in mice or dogs, including blood and bone-marrow parameters. In conclusion, APR-246 has in various efficacy models demonstrated significant antitumor activity and a unique pharmacological profile. In preclinical safety/toxicity studies no toxicity at predicted therapeutic plasma concentrations was observed. Thus, APR-246 appears to be a promising novel anticancer compound to treat patients resistant to common chemotherapy. Currently, APR-246 is investigated in a dose escalating Phase I/IIa First-in-Man study for refractory hematological malignancies and prostate cancer. The Phase II Proof of Concept study is planned to start in 2011.