Emerging molecular therapies of advanced thyroid cancer

Advanced thyroid cancer refers to thyroid tumors which are resistant to conventional therapies and do not respond to radioiodine and comprises metastatic or recurrent differentiated cancers, poorly differentiated and anaplastic tumors. Progress in the knowledge of genetic/epigenetic alterations in thyroid cancer cells is rapidly offering several opportunities to develop new drugs directed to specific targets. Drugs currently proposed for molecular therapy include: (a) monoclonal antibodies; (b) kinase inhibitors; (c) anti-angiogenetic drugs; (d) proteasome inhibitors; (e) retinoic acid and PPAR-gamma ligands; (f) radionuclide therapy; (g) epigenetic drugs (deacetylase inhibitors and demethylating agents). The results of several phase II trials using molecular drugs look promising. None of the treated patients, however, had a complete response, and only a minority of them had a partial response. The review will focus especially on epigenetic therapy, whose goal is to target the chromatin in rapidly dividing tumor cells and potentially restore normal cell functions. Deacetylases inhibitors modulate both epigenetic and multiple non-epigenetic mechanisms; they are, thus, viewed as a promising class of anticancer drugs. Experimental data show that deacetylase inhibitors are effective against advanced thyroid cancer. However, since multiple pathways need to be inhibited in order to substantially affect thyroid cancer growth, it is likely that a significant increase in the response rate to treatment of advanced thyroid cancer will be achieved through combinatorial drug therapies. Actually, many pre-clinical and clinical studies evaluate the combination of either two epigenetic drugs or a non-epigenetic chemotherapeutic and an epigenetic drug, in the effort to increase response rates.