Purpose
RG7112 is a small-molecule MDM2 antagonist. MDM2 is a negative regulator of the tumor suppressor p53 and frequently overexpressed in leukemias. Thus, a Phase I study of RG7112 in patients with hematologic malignancies was conducted.
Experimental Design
Primary study objectives included determination of the dose and safety profile of RG7112. Secondary objectives included evaluation of pharmacokinetics, pharmacodynamics, such as TP53-mutation status and MDM2 expression, and preliminary clinical activity. Patients were divided into 2 cohorts: Stratum A (relapsed/refractory AML (except APL), ALL, and CML) and Stratum B (relapsed/refractory CLL/sCLL). Some Stratum A patients were treated at the MTD to assess clinical activity.
Results
RG7112 was administered to 116 patients (96 patients in Stratum A and 20 patients in Stratum B). All patients experienced at least 1 adverse event, and 3 DLTs were reported. PK analysis indicated that twice-daily dosing enhanced daily exposure. Anti-leukemia activity was observed in the 30 patients with AML assessed at the MTD included 5 patients who met IWG criteria for response. Exploratory analysis revealed TP53 mutations in 14% of Stratum A patients and in 40% of Stratum B patients. Two patients with TP53 mutations exhibited clinical activity. p53 target genes were induced only in TP53 wild-type leukemic cells. Baseline expression levels of MDM2 correlated positively with clinical response.
Conclusions
RG7112 demonstrated clinical activity against relapsed/refractory AML and CLL/sCLL. MDM2 inhibition resulted in p53 stabilization and transcriptional activation of p53-target genes. We provide proof-of-concept that MDM2 inhibition restores p53 function and generates clinical responses in hematologic malignancies.
SUMMARY
Evasion of apoptosis is a hallmark of cancer. Bcl-2 and p53 represent two important nodes in apoptosis signaling pathways. We find that concomitant p53 activation and Bcl-2 inhibition overcome apoptosis resistance and markedly prolong survival in three mouse models of resistant acute myeloid leukemia (AML). Mechanistically, p53 activation negatively regulates the Ras/Raf/MEK/ERK pathway and activates GSK3 to modulate Mcl-1 phosphorylation and promote its degradation, thus overcoming AML resistance to Bcl-2 inhibition. Moreover, Bcl-2 inhibition reciprocally overcomes apoptosis resistance to p53 activation by switching cellular response from G1 arrest to apoptosis. The efficacy, together with the mechanistic findings, reveals the potential of simultaneously targeting these two apoptosis regulators and provides a rational basis for clinical testing of this therapeutic approach.
Telomerase, a ribonucleic acid-protein complex, adds hexameric repeats of 5'-TTAGGG-3' to the ends of mammalian chromosomal DNA (telomeres) to compensate for the progressive loss that occurs with successive rounds of DNA replication. Although somatic cells do not express telomerase, germ cells and immortalized cells, including neoplastic cells, express this activity. To determine whether the phenotypic differentiation of immortalized cells is linked to the regulation of telomerase activity, terminal differentiation was induced in leukemic cell lines by diverse agents. A pronounced downregulation of telomerase activity was produced as a consequence of the differentiated status. The differentiation-inducing agents did not directly inhibit telomerase activity, suggesting that the inhibition of telomerase activity is in response to induction of differentiation. The loss of telomerase activity was not due to the production of an inhibitor, since extracts from differentiated cells did not cause inhibition of telomerase activity. By using additional cell lineages including epithelial and embryonal stem cells, downregulation of telomerase activity was found to be a general response to the induction of differentiation. These findings provide the first direct link between telomerase activity and terminal differentiation and may provide a model to study regulation of telomerase activity.The ends of eukaryotic chromosomes, called telomeres, consist of an array of tandem repeats of the hexanucleotide 5'-TTAGGG-3'. It is currently assumed that telomeres were evolved to protect the ends of chromosomes against exonucleases and ligases, to prevent the activation of DNA-damage checkpoints, and to counter the loss of terminal DNA segments that occurs when linear DNA is replicated (for a review, see refs.
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