Abstract
Multiple myeloma (MM) remains an incurable neoplasia and exhibits high propensity for de novo/acquired refractoriness even to novel agents, e.g. thalidomide (Thal) or proteasome inhibitors. This may be due to complex and evolving patterns of molecular lesions potentially conferring hyperactive antiapoptotic signaling with high degree of redundancy upon inhibition of isolated targets within those pathways. We thus hypothesized that, for genetically complex neoplasias such as MM, patient outcome might improved by addition, in the therapeutic armamentarium, of agents that simultaneously inhibit multiple proliferative/antiapoptotic targets. Towards this effort of multi-targeted therapies, we studied the tyrosine kinase inhibitor PKC412 (midausporin, Novartis, Basel, Switzerland). Low-nM levels of PKC412 selectively inhibit tyrosine kinase receptors, such as FLT3. But higher PKC412 concentrations (in μM range), which presumably inhibit (at least partly) other kinases, are achieved in clinical trials without catastrophic toxicities. This suggests that PKC412 can safely suppress in vivo the activity of, not just FLT3, but a broader spectrum of kinases, some of which (individually or cooperatively) might be critical for survival/proliferation of MM cells. Indeed, in vitro kinase activity assays showed that clinically achievable (high nM or low μM) PKC412 concentrations inhibit by >20% important kinases, including p70S6K, IKK-a and Akt,. Consistent with such multi-targeted effect, in vitro MTT colorimetric survival assays showed activity of PKC412 (at sub-μM levels) against panel of 40 MM cell lines and 10 primary tumor cells from MM patients (including cells resistant to Dex, alkylating agents, anthracyclines, Thal or its immunomodulatory derivatives, bortezomib, and/or Apo2L/TRAIL), and cell lines from hematologic neoplasias, e.g. B- and T-ALL, CML, various non-Hodgkin’s lymphoma subtypes, and solid tumors (e.g. breast, prostate, lung, thyroid, ovarian, renal Ca, retinoblastoma and sarcomas)). Mechanistic studies confirmed that PKC412 blocks key growth/survival pathways (e.g. PI-3K/Akt, IKK-α/NF-κB), coupled with by downstream effects on suppression of diverse inhibitors of apoptosis (e.g. FLIP, XIAP, cIAP-2, survivin). These molecular sequelae explain, at least partly, the ability of PKC412 to sensitize MM cells to other anti-MM agents (such as Dex, cytotoxic chemotherapy or proteasome inhibitor bortezomib) and overcome protective effects of cytokines (e.g. IL-6) or bone marrow stromal cells. Importantly, PKC412 significantly prolonged the overall survival (p<0.03, Kaplan-Meier analysis) of SCID/NOD mice in a clinically relevant model of diffuse MM bones lesions. These studies provide basis for clinical studies of PKC412 in MM and indicate that kinase inhibitors selectively blocking specific targets at low drug levels, may also have potent anti-tumor activities related to inhibition of multiple other, less specific, nonetheless important targets, thus allowing for anti-tumor activity in a much broader spectrum of tumor types than previously appreciated.
The FMS like Tyrosine Kinase 3 (FLT3) Is Overexpressed in a Subgroup of Multiple Myeloma Patients with Inferior Prognosis
