Background:

Metastatic differentiated thyroid cancers (DTC) are notoriously resistant to traditional chemotherapy.  Kinase inhibitors have shown promise in patients with progressive DTC, but dose-limiting toxicity is common.  HSP90 regulates protein degradation of a number of growth-mediating kinases, and we hypothesized that an HSP90 inhibitor (AUY922) could inhibit RET-mediated cell growth in medullary thyroid cancer (MTC) cell lines, and/or radioactive iodine uptake in papillary thyroid cancer (PTC) cells.

Methods

AUY922 was obtained from Novartis.  Studies utilized MTC cell lines TT (C634) and MZ-CRC-1 (M918T) and the PTC cell line TPC-1 (RET/PTC1 rearrangement). Cell viability was assessed with MTS assays and apoptosis assessed with flow cytometry markers for DilC(5) and PI.  Analysis of signaling targets were achieved with western blots. Radioiodine assays were performed and relative activity calculated on gamma counter.

Results

AUY922 decreased cell viability in RET mutant medullary thyroid cancer cell lines and impaired signalling through the MAPK and mTOR pathways.  Prolonged treatment of AUY922 on MTC cell lines led to apoptosis (58.7% reduction in MZ-CRC-1 live cells and 78.7% reduction in TT live cells following 1µM AUY922; p<0.02). Similarly in the PTC cell line, growth and signalling targets were inhibited. TPC-1 cells have a 2.84 fold increase in radioiodine uptake following AUY922 administration (p=0.015).

Discussion

AUY922 demonstrates in vitro activity against MTC and PTC cell lines. We are now studying its potential in combination with kinase inhibition. We observed a potent dose dependent increase in apoptosis in MTC cell lines following drug administration. Western blots confirm inhibition of prosurvival proteins including AKT suggesting this as a mechanism of cell death.  In a functional study we observed an increase in radioiodine uptake in the PTC cell line following AUY922 treatment.  We believe this multitargeted approach is a better option for treatment of these resistant cancers.