MLN4924 showed potent in vitro
activity against the PPTP cell line panel, with IC50
values generally less than 200 nM. The range of IC50
values observed for the PPTP cell lines are comparable to those observed for adult cancer cell lines [4
]. There were clear differences in response to MLN4924 by histotype, with the Ewing sarcoma and ALL cell lines showing more complete cytotoxic responses compared with the rhabdomyosarcoma and neuroblastoma cell lines. There was not a clear relationship between in vitro
sensitivity and in vivo
activity for those xenografts that are represented by cell lines in the in vitro
panel. The in vitro
sensitivity of the Ewing cell lines was not replicated in vivo
for the Ewing xenografts, which showed a disappointingly low level of activity. The GBM2 cell line did show relative in vitro
sensitivity to MLN4924 and also showed good tumor growth inhibition in vivo
to MLN4924, as did the other glioblastoma xenografts, although tumor regressions were not observed in this panel. The ALL cell lines showed the clearest cytotoxic response to MLN4924 in vitro
, while in vivo
several of the ALL xenografts showed clear evidence of treatment effect, albeit no remissions, during the 3 weeks that MLN4924 was administered.
MLN4924 appears to be particularly effective against ABC-DLBCL cell lines and xenografts, which are known to have NF-κB pathway activation [5
]. The mechanism of action responsible for this impressive activity appears to be inhibition of NF-κB signaling by stabilization of IκBα [5
]. In the pediatric setting, histologies showing this same level of survival dependence on NF-κB signaling have not been described. NF-κB activation has been described for ALL [15
], although the PPTP ALL xenografts did not show the robust responses to MLN4924 that were previously demonstrated for ABC-DLBCL xenografts [5
]. The most consistent evidence of tumor growth inhibition was observed for the glioblastoma panel, for which 4 of 4 xenografts showed EFS T/C values greater than two. Previous reports have described NF-κB activation in glioblastomas, although discrepant results have been reported regarding whether inhibition results in inhibition of cell growth [17
]. Interpretation of the PPTP glioblastoma results must be made in light of the subcutaneous location of the tumors, and further evaluation of MLN4924 in orthotopic models may be warranted.
Target inhibition, determined by decreased levels of neddylated cullin proteins, was similar in each tumor model evaluated regardless of the level of tumor growth inhibition to MLN4924. Inhibition was maximal 2 hr after MLN4924 administration, and had recovered partially by 24 hr. It is likely that the accumulation of certain CRL substrates that are required for antitumor activity differs in models with greater versus lesser tumor growth inhibition; the accumulation of these substrates may depend on the activation of oncogenic pathways important to tumor survival. It will be important to identify the pathways that make tumors more sensitive to NAE inhibition.
MLN4924 has entered clinical evaluation in adults using a range of schedules administered at 21 day intervals (e.g., days 1 to 5; days 1, 2, 8, and 9; days 1, 3, and 5; days 1, 4, 8, and 11) [22
]. Dose-limiting toxicities have primarily been non-hematological, including transaminase elevation, hyperbilirubinemia, muscle cramps/myalgia, and elevated serum creatinine. Objective responses in MLN4924 phase 1 trials have been noted for patients with AML, MDS, Hodgkin lymphoma, and melanoma [22
]. In considering potential pediatric clinical applications of MLN4924, one focus could be on tumors that have a requirement for NF-κB activation. A number of mutations have been identified in adult cancers that result in NF-κB pathway activation, including somatic mutations in genes that positively (e.g., CARD11, CD79A, and CD79B) and negatively (A20/TNFAIP3) regulate the NF-κB pathway [26
]. Recurring somatic mutations in these genes are not known to occur in the pediatric solid tumor or ALL setting. Among the lymphomas known to show NF-κB activation in the adult setting, Hodgkin lymphoma and primary mediastinal B-cell lymphoma occur in the pediatric age range [31
]. Further preclinical evaluations of MLN4924 will depend upon emerging data from adult preclinical and clinical testing and on future advances in the understanding of the molecular basis of pediatric cancers.