Exceptionally Durable Response to First-Line S-1 Monotherapy in Metastatic Pancreatic Adenocarcinoma: A Case Report

Pancreatic adenocarcinoma with distant metastases is associated with a poor prognosis, and exceptionally durable responses to systemic chemotherapy are uncommon. We report a rare case of pancreatic adenocarcinoma with radiologically suspected multiple liver metastases that achieved a prolonged response to first-line tegafur/gimeracil/oteracil (S-1) monotherapy. A 76-year-old man with a history of cerebral infarction and diabetes mellitus was referred to our department because of rapidly worsening diabetes. Imaging revealed an approximately 3-cm pancreatic head tumor and multiple hepatic lesions highly suspicious for metastases. Serum carbohydrate antigen 19-9 (CA19-9) was markedly elevated. Endoscopic ultrasound-guided fine-needle aspiration (EUS-FNA) of the pancreatic head tumor demonstrated adenocarcinoma on both cytological and histopathological examinations. Gemcitabine plus nanoparticle albumin-bound paclitaxel (nab-paclitaxel) was proposed; however, the patient preferred oral treatment. Considering his age, quality of life, and treatment preference, S-1 monotherapy was initiated at 100 mg/day on a 2-week-on/2-week-off schedule. Following initiation of S-1, the pancreatic and hepatic lesions progressively regressed, accompanied by rapid normalization of CA19-9. By approximately 14 months after treatment initiation, the previously identifiable pancreatic primary tumor and hepatic lesions were no longer clearly visualized on computed tomography (CT). S-1 monotherapy was continued with few interruptions, and no apparent radiological progression was documented for approximately 47 months. Approximately two and a half months after the last CT showing no apparent progression, the patient developed fever and dyspnea. CT demonstrated newly developed bilateral ground-glass and interstitial pulmonary opacities, along with new-onset ascites, without evidence of regrowth of the pancreatic or hepatic lesions. S-1-induced interstitial lung disease (ILD) was clinically suspected; however, occult malignant progression could not be definitively excluded. His respiratory condition deteriorated despite treatment, and he subsequently died. This case demonstrates that an exceptionally durable response may rarely occur with first-line S-1 monotherapy even in pancreatic adenocarcinoma with radiologically suspected metastatic disease.