Pembrolizumab plus platinum-based chemotherapy for unfavorable cancer of unknown primary site: Case report

INTRODUCTION: We report a case of sustained complete response in unfavorable cancer of unknown primary site (CUP) successfully treated with chemotherapy combining pembrolizumab, pemetrexed and platinum. CASE PRESENTATION: A 66-year-old man was presented with weight loss and cough for 3 months. Contrast-enhanced computed tomography (CT) confirmed a mass in the superior anterior mediastinum and multiple enlarged mediastinal and axillary lymph nodes. Positron emission tomography-CT (PET-CT) showed abnormal uptake in the corresponding lesions. Histopathological analysis of the left axillary nodule revealed poorly differentiated adenocarcinoma. Immunohistochemistry showed the tumor cells were positive for cytokeratin 7 and thyroid transcription factor-1 and negative for cytokeratin 20. Thus, the patient was diagnosed as poorly differentiated adenocarcinoma of unknown primary, and treated as non-small-cell lung cancer. Additional genetic testing revealed the patient was negative for EGFR, ALK fluorescence in situ hybridization, ROS1, BRAF, and PD-L1 22C3 IHC with Tumor Proportion Score (TPS) was less than 1%. The patient received six cycles of pembrolizumab, platinum, and pemetrexed intravenously. Cisplatin was switched to carboplatin because of cisplatin nephrotoxicity in one course. PET-CT after six cycles showed all lesions disappeared; complete response was considered to have been achieved. Maintenance therapy of pembrolizumab and pemetrexed has been continued for 6 months after the induction therapies to prevent progressive disease. Complete response has been maintained. DISCUSSION: Chemotherapy with pembrolizumab, platinum and pemetrexed could be valuable for treating unfavorable CUP. CONCLUSION: Chemotherapy with pembrolizumab, platinum, and pemetrexed helped achieved sustained complete response in a patient with unfavorable CUP.