Drugs in early clinical development for the treatment of osteosarcoma

Biochemical and genetic characterization of D-type cyclins, their cyclin D-dependent kinases (CDK4 and CDK6), and the polypeptide CDK4/6 inhibitor p16(INK4)over two decades ago revealed how mammalian cells regulate entry into the DNA synthetic (S) phase of the cell-division cycle in a retinoblastoma protein-dependent manner. These investigations provided proof-of-principle that CDK4/6 inhibitors, particularly when combined with coinhibition of allied mitogen-dependent signal transduction pathways, might prove valuable in cancer therapy. FDA approval of the CDK4/6 inhibitor palbociclib used with the aromatase inhibitor letrozole for breast cancer treatment highlights long-sought success. The newest findings herald clinical trials targeting other cancers.

To determine whether the addition of ifosfamide and/or muramyl tripeptide (MTP) encapsulated in liposomes to cisplatin, doxorubicin, and high-dose methotrexate (HDMTX) could improve the probability for event-free survival (EFS) in newly diagnosed patients with osteosarcoma (OS). Six hundred seventy-seven patients with OS without clinically detectable metastatic disease were treated with one of four prospectively randomized treatments. All patients received identical cumulative doses of cisplatin, doxorubicin, and HDMTX and underwent definitive surgical resection of the primary tumor. Patients were randomly assigned to receive or not to receive ifosfamide and/or MTP in a 2 double dagger 2 factorial design. The primary end point for analysis was EFS. Patients treated with the standard arm of therapy had a 3-year EFS of 71%. We could not analyze the results by factorial design because we observed an interaction between the addition of ifosfamide and the addition of MTP. The addition of MTP to standard chemotherapy achieved a 3-year EFS rate of 68%. The addition of ifosfamide to standard chemotherapy achieved a 3-year EFS rate of 61%. The addition of both ifosfamide and MTP resulted in a 3-year EFS rate of 78%. The addition of ifosfamide in this dose schedule to standard chemotherapy did not enhance EFS. The addition of MTP to chemotherapy might improve EFS, but additional clinical and laboratory investigation will be necessary to explain the interaction between ifosfamide and MTP.

Rapamycin and several analogs, such as CCI-779 and RAD001, are currently undergoing clinical evaluation as anticancer agents. In this study, we show that inhibition of mammalian target of rapamycin (mTOR) signaling by rapamycin leads to an increase of Akt phosphorylation in Rh30 and RD human rhabdomyosarcoma cell lines and xenografts, and insulin-like growth factor (IGF)-II-treated C2C12 mouse myoblasts and IGF-II-overexpressing Chinese hamster ovary cells. RNA interference-mediated knockdown of S6K1 also results in an increase of Akt phosphorylation. These data suggest that mTOR/S6K1 inhibition either by rapamycin or small interfering RNA (siRNA) triggers a negative feedback loop, resulting in the activation of Akt signaling. We next sought to investigate the mechanism of this negative feedback regulation from mTOR to Akt. Suppression of insulin receptor substrate (IRS)-1 and tuberous sclerosis complex-1 by siRNAs failed to abrogate rapamycin-induced upregulation of Akt phosphorylation in both Rh30 and RD cells. However, pretreatment with h7C10 antibody directed against insulin-like growth factor-1 receptor (IGF-1R) led to a blockade of rapamycin-induced Akt activation. Combined mTOR and IGF-1R inhibition with rapamycin and h7C10 antibody, respectively, resulted in additive inhibition of cell growth and survival. These data suggest that rapamycin mediates Akt activation through an IGF-1R-dependent mechanism. Thus, combining an mTOR inhibitor and an IGF-1R antibody/inhibitor may be an appropriate strategy to enhance mTOR-targeted anticancer therapy.