Selective Induction of the p53-Pathway as Treatment for Multiple Myeloma.

The p53 pathway is central to cellular defences against neoplastic transformation, and mutations that impair p53 function are prominent oncogenic events. However, certain malignancies, such as multiple myeloma (MM), rarely present with p53 defects except in late stages. MM might therefore be vulnerable to p53 induction therapy, but it is unknown if the p53 pathway remains actually functional in this disease, and if it can be sufficiently well triggered to induce tumor cell death. We have used nutlin-3, a newly developed small-molecule antagonist against the p53 suppressor murine double minute 2 (MDM2), to analyze the effect on myeloma cell lines and a large panel of patient tumor samples. Nutlin-3 specifically and exclusively induced p53 downstream targets in myeloma cells with wild-type p53, and a large majority of primary medullary myeloma samples was susceptible to nutlin-3-mediated apoptosis. Comparison with the clinically relevant genotoxic drugs melphalan and etoposide showed that nutlin-3 was as effective or better at inducing apoptosis of primary tumor cells, and that combinations of nutlin-3 with genotoxic cytostatics at low doses produced better than additive anti-tumor effects. Importantly, broad anti-tumor activity of nutlin-3 persisted even when primary tumor cells were kept in coculture with bone marrow stromal cells (BMSCs). However, primary tumor isolates often contain a fraction of cells apparently unaffected by drug treatment, and some primary samples were conspicuously less sensitive to nutlin exposure when the cells were kept in coculture with BMSCs, as compared to culture in medium supplemented with interleukin-6. We have begun to analyze to what extent heterogeneity for p53 induction within the tumor cell population may underlie these differences, and if the capacity to activate intracellular growth and survival pathways has an influence on the result of p53 pathway activation. Nutlin-3, as a non-genotoxic agent to specifically induce or strengthen p53-mediated apoptotic responses may be a promising agent to complement standard genotoxic regimens for the treatment of MM.