Neuroblastoma is the most common extracranial solid
tumor of childhood. The activity of J1 (
l-melphalanyl-p-l-fluorophenylalanine ethyl ester), an enzymatically activated
melphalan prodrug, was evaluated in
neuroblastoma models in vitro and in vivo. Seven
neuroblastoma cell lines with various levels of drug resistance were screened for cytotoxicity of J1 alone or in combination with standard cytotoxic drugs, using a fluorometric cytotoxicity assay. J1 displayed high cytotoxic activity in vitro against all
neuroblastoma cell lines, with IC(50) values in the submicromolar range, significantly more potent than
melphalan. The cytotoxicity of J1, but not
melphalan, could be significantly inhibited by the
aminopeptidase inhibitor
bestatin. J1 induced
caspase-3 cleavage and apoptotic morphology, had additive effects in combination with
doxorubicin,
cyclophosphamide,
carboplatin, and
vincristine, and synergistically killed otherwise
drug-resistant cells when combined with
etoposide. Athymic rats and mice carrying
neuroblastoma xenografts [SH-SY5Y, SK-N-BE(2)] were treated with equimolar doses of
melphalan, J1, or no
drug, and effects on
tumor growth and tissue morphology were analyzed.
Tumor growth in vivo was significantly inhibited by J1 compared with untreated controls. Compared with
melphalan, J1 more effectively inhibited the growth of mice with SH-SY5Y xenografts, was associated with higher
caspase-3 activation, fewer proliferating
tumor cells, and significantly decreased mean vascular density. In conclusion, the
melphalan prodrug J1 is highly active in models of
neuroblastoma in vitro and in vivo, encouraging further clinical development in this patient group.