MYCN and neuroblastoma: Consistent with their distinct release and activation rates, the prodrugs formulated in NP exhibited different NB cell growth inhibitory efficiencies: the proliferation of highly malignant, MYCN-amplified NB cells derived at diagnosis (IMR-32 [43]) was fully inhibited over 7 days following a 24-h exposure to NP[SN22-TOx] at a low dose equivalent to 5 ng SN22 per well, with potency similar to that of free SN22.