Tropomyosin Receptor Kinase A Expression on Merkel Cell Carcinoma Cells

IMPORTANCE Merkel cell carcinoma (MCC) is a malignant neuroendocrine skin tumor frequently associated with the Merkel cell polyomavirus. Immune checkpoint therapy showed remarkable results, although not all patients are responsive to this therapy. Anti-tropomyosin receptor kinase A (TrkA)-targeted treatment has shown promising results in several tumor entities.& para;& para;OBJECTIVE To determine TrkA expression in MCC as a rationale for potential targeted therapy.& para;& para;DESIGN, SETTING, AND PARTICIPANTS This case series study investigated the MCC specimens of 55 patients treated at the Department of Dermatology, University Hospital of Schleswig-Holstein, Kiel, Germany, from January 1, 2005, through December 31, 2015, Thirty-nine of the 55 samples were suitable for further histopathologic examination. Expression of TrkA was explored by immunohistochemical analysis.& para;& para;EXPOSURE Diagnosis of MCC was confirmed by staining positive for cytokeratin 20 (CK20) and synaptophysin.& para;& para;MAIN OUTCOMES AND MEASURES Expression of TrkA on the tumor cells.& para;& para;RESULTS Specimens of 39 patients (21 women and 18 men; mean [SD] age, 75.0 [7.8] years) underwent immunohistochemical investigation. Thirty-eight of 38 specimens expressed CK20 and synaptophysin on the MCC tumor cells (100% expression). Merkel cell polyomavirus was detected in 32 of 38 specimens (84%). Tropomyosin receptor kinase A was found in all 36 evaluable specimens on the tumor cells; 34 (94%) showed a weak and 2 (6%) showed a strong cytoplasmic expression. In addition, strongly positive perinuclear dots were observed in 30 of 36 specimens (83%).& para;& para;CONCLUSIONS AND RELEVANCE Tropomyosin receptor kinase A was expressed on MCC tumor cells in 100% of evaluable specimens. This result may lead to the exploration of new targeted treatment options in MCC, especially for patients who do not respond to anti-programmed cell death protein 1 treatment.