Background
Signaling through mTOR and somatostatin pathway is implicated in thyroid cancer development.
Method
We evaluated everolimus, an mTOR inhibitor and pasireotide, a multi receptor somatostatin analogue as potential therapy of thyroid cancer focusing on the
in vitro
and
in vivo
efficacy, as well as possible mechanism to explain any observed interaction.
Results
Both everolimus and pasireotide inhibit the growth of thyroid cancer cell lines
in vitro
with varied efficacy that correlates with tumor origin and somatostatin receptor (SSTR) expression profile of the cell lines.
In vitro
activity of everolimus show positive correlation with the expression of SSTR types 1, 4 and 5 (CC: 0.9; 0.85, 0.87) while pasireotide activity show negative correlation with SSTR2 (CC: -0.87). Although there is greater modulation of pS6 when pasireotide is combined with everolimus, there is no significant abrogation of the expected feedback upregulation of AKT induced by everolimus. Also, the combination is not significantly better than each agent alone in short and long term
in vitro
assays. Continuous administration of everolimus at a low dose as opposed to high intermittent dosing schedule has greater antitumor efficacy against thyroid cancer xenografts
in vivo
. Pasireotide LAR has modest in vivo efficacy and the combination of everolimus and pasireotide LAR achieve greater tumor growth inhibition than each agent alone in TPC-1 xenograft model of thyroid cancer (p = 0.048).
Conclusion
Our findings provide support for the clinical evaluation of everolimus and pasireotide in thyroid cancer and other neuroendocrine tumors.