Chasing Therapeutic Targets in Thymic Malignancies: Finding Needles in the Haystack to Frame a Comprehensive Canvas?

Thymic malignancies: emerging systemic therapies

PURPOSE OF REVIEW

The management of thymic malignancies is based on multidisciplinary collaboration. Systemic agents may be administered as an exclusive treatment if local treatment is not achievable. Novel and innovative agents are needed. Integrated genomic analyses reported the activation of targetable signaling pathways in thymomas and thymic carcinomas.

RECENT FINDINGS

Phase II trials reported the antitumor activity of phosphatidylinositol 3-kinase/mechanistic target of rapamycin kinase inhibitors, cyclin dependent kinase inhibitors, and antiangiogenic agents in advanced, refractory thymic malignancies. Meanwhile, a major challenge is the use of immune checkpoint inhibitors, given the frequent association of those tumors with autoimmune disorders.

SUMMARY

Although those innovative agents were assessed in phase II trials reporting on variable antitumor efficacy in terms of response and survival, in selected and limited cohorts of patients, a better understanding of systemic treatment sequences in a real-life setting is mandatory to analyze the actual efficacy of each line of treatment one after another, define the best clinical-pathological selection of patients for the administration of chemotherapy, targeted agents, and immunotherapy, and develop individualized decision-making to optimize the survival of patients with advanced thymic malignancies.

Activation of the mTOR/ Akt pathway in thymic epithelial cells derived from thymomas

The pathogenesis of thymic epithelial tumors remains poorly elucidated. The PIK3/Akt/mTOR pathway plays a key role in various cancers; interestingly, several phase I/II studies have reported a positive effect of mTOR inhibitors in disease control in thymoma patients. A major limit for deciphering cellular and molecular events leading to the transformation of thymic epithelial cells or for testing drug candidates is the lack of reliable in vitro cell system. We analyzed protein expression and activation of key players of the Akt/ mTOR pathway namely Akt, mTOR, and P70S6K in eleven A, B and AB thymomas as well as in normal thymuses. While only Akt and phospho-Akt were expressed in normal thymuses, both Akt and mTOR were activated in thymomas. Phospho-P70S6K was expressed in all thymic tumors whatever their subtypes, and absent in normal thymus. Interestingly, we report the activation of Akt, mTOR and P70S6 proteins in primary thymic epithelial cells maintained for short period of time after their derivation from seven AB and B thymomas. Finally, we showed that rapamycin (100 nM) significantly reduced proliferation of thymoma- derived epithelial cells without inducing cell death. Our results suggest that the activation of the Akt/ mTOR pathway might participate to the cell proliferation associated with tumor growth. Ultimately, our data enhance the potential role of thymic epithelial cells derived from tissue specimens for in vitro exploration of molecular abnormalities in rare thymic tumors.