Outcomes of Children and Adolescents with Advanced Hereditary Medullary Thyroid Carcinoma Treated with Vandetanib

Purpose: Vandetanib is well-tolerated in patients with advanced medullary thyroid carcinoma (MTC). Long-term outcomes and mechanisms of MTC progression have not been reported previously.  Experimental Design:We monitored toxicities and disease status in patients taking vandetanib for hereditary, advanced MTC. Tumor samples were analyzed for molecular mechanisms of disease progression. Results: Seventeen patients (8 male, age 13 (9-17)* years) enrolled; 16 had a RET p.Met918Thr germline mutation. The duration of vandetanib therapy was 6.1 (0.1-9.7+)* years with treatment ongoing in nine patients. Best response was partial response (PR) in ten, stable disease (SD) in six, and progressive disease (PD) in one patient. Duration of response was 7.4 (0.6-8.7+)* and 4.9 (0.6-7.8+)* years in patients with PR and SD, respectively. Six patients died 2.0 (0.4-5.7)* years after progression. Median progression free survival (PFS) was 6.7 years (95% CI: 2.3 years-undefined) and 5-year overall survival (OS) was 88.2% (95% CI 60.6-96.9%). Of 16 patients with a RET p.Met918Thr mutation, progression free survival was 6.7 years (95% CI 3.1-undefined) and 5-year overall survival was 93.8% (95% CI 63.2-99.1%). No patients terminated treatment because of toxicity. DNA sequencing of tissue samples (n=11) identified an increase in copy number alterations across the genome as a potential mechanism of drug resistance. Conclusions:This study demonstrates that vandetanib is safe and results in sustained responses in children and adolescents with hereditary MTC. Our preliminary molecular data suggest that an increase in copy number abnormalities may be associated with tumor progression in hereditary MTC patients treated with vandetanib.

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