Retinoids in Experimental Neuroblastoma Therapy

Retinoids are analogues of vitamin A, with documented activity against various malignant cell types. Neuroblastoma is a childhood tumour of the sympathetic nervous system that shows a complex clinical and biological heterogeneity, often with poor outcome despite intensive multimodal therapy. The aim of the thesis was to investigate effects of retinoid treatment in vitro on human neuroblastoma cells, and in vivo on human neuroblastoma xenografts in nude rats. The ultimate goal was to find a new retinoid treatment for children with neuroblastoma. Oral treatment with 9-cis RA in vivo resulted in a significant inhibition of neuroblastoma tumour growth, but with major toxic side effects. Further experiments showed that 9-cis RA might not be suitable for clinical use in children with neuroblastoma, because of its short half-life, low bioavailability and toxic profile in rats. Ro 13-6307 was established to be a morphologically differentiating retinoid, able to reduce proliferation and induce G1 growth arrest in both MYCN amplified and non-amplified neuroblastoma cell lines in vitro. Further experiments showed that oral Ro 13-6307 could inhibit neuroblastoma tumour growth in vivo with limited toxicity. In vitro and in vivo results indicated that Ro 13-6307 was at least as effective as the clinically established retinoid 13-cis RA. These results demonstrate that Ro 13-6307 is a potential retinoid for clinical oral therapy of children with neuroblastoma. Despite promising results demonstrating that fenretinide induces apoptosis in neuroblastoma cells in vitro, no significant reduction in neuroblastoma tumour growth was observed after oral treatment with fenretinide in vivo. Five different doses were evaluated, but no significant inhibiting effect on tumour growth or morphological changes were found in treated compared to untreated tumours. Other alternatives for fenretinide administration should be investigated in future experimental and clinical studies. Proton magnetic resonance spectroscopy was found to be a suitable method for detecting metabolic alterations in neuroblastoma cells in vitro undergoing fenretinide-induced apoptosis. It was possible to monitor the kinetics in the treatment response and to distinguish between fenretinide-sensitive and -resistant cells. These findings suggest that proton magnetic resonance spectroscopy is a potential clinical non-invasive tool to monitor early tumour response to retinoid treatments. In conclusion, retinoids were shown to inhibit growth of human neuroblastoma cells in vitro and in vivo, however the effect depends on the retinoid in use. Dosing, scheduling, and toxicity are important factors determining the therapeutic efficacy of retinoids in vivo. Ro 13-6307 may be a retinoid for future clinical therapy of children with neuroblastoma.