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Acquired drug resistance: case report A 68-year-old man acquired resistance to erlotinib and osimertinib, while being treated for lung adenocarcinoma [routes and dosages not stated]. The man, who was smoker, was admitted to hospital due to relapse of stage IA lung adenocarcinoma in December 2014. He had undergone surgical resection of lung adenocarcinoma in December 2012. Subsequent computed tomography (CT) revealed mediastinal lymphadenopathy and multiple lung nodules. Thereafter, he underwent endobronchial ultrasound‑guided transbronchial needle aspiration of mediastinal lymph node which showed lung adenocarcinoma with an EGFR mutation (exon 19 deletion). In March 2015, he started receiving first line chemotherapy with osimertinib. The osimertinib treatment showed significant response to lung adenocarcinoma for 11 months. In February 2016, during routine follow-up, he was found to have ground‑glass attenuation (GGA) in the right lower lobe by CT scan. His treatment with osimertinib was discontinued. However, GGA was spontaneously regressed, mediastinal lymphadenopathy and multiple lung nodules aggravated. Hence, he started receiving second line chemotherapy with erlotinib in May 2016. After 8 months (January 2017), follow‑up CT scan showed re‑progression of the mediastinal lymph node and appearance of minor amounts of pericardial effusion. Subsequently, he underwent a lymph node biopsy which showed lung adenocarcinoma with EGFR T790M mutation in addition to the EGFR exon 19 deletion. A cobas EGFR Mutation test v2 showed only exon 19 deletion. Hence, a third line treatment with osimertinib was re-initiated. After 3 months, despite a tentative response, enlargement of mediastinal tumors with an elevated serum carcinoembryonic antigen (CEA) level of 24.3 ng/mL was observed. Then, he was treated with carboplatin in combination with paclitaxel, docetaxel and pemetrexed as fourth to sixth line therapy, respectively. These chemotherapies did not show desired effect and CEA level remained elevated. Thus, gimeracil/oteracil/tegafur [S‑1] monotherapy was initiated in August 2018 as seventh line treatment. After 1 month of initiation of therapy, CEA level decreased from 100.5 ng/mL to 30 ng/mL. However, dyspnea gradually appeared. Subsequent CT scan showed apparent enlargement of mediastinal tumors and increase of pericardial effusion which ultimately resulted in cardiac tamponade due to disease progression. Then, pericardial paracentesis showed poorly differentiated cells with a high nuclear‑to‑cytoplasmic ratio. Additionally, his blood test showed neuron specific enolase (NSE) of 39.4 ng/ml. Based on these clinical tests and immunohistochemistry, small‑cell lung carcinoma (SCLC) transformation was confirmed. At the same time, the molecular analysis of the obtained pericardial specimens showed positive exon 19 deletion despite negative T790M mutation by cobas EGFR Mutation test v2. A combination therapy of carboplatin and etoposide was initiated in September 2018. After