Background: The impact of stroma-targeting therapy on tumor immune suppression is largely unexplored. An RNA oligonucleotide, STNM01, has been shown to repress carbohydrate sulfotransferase 15 (CHST15) responsible for tumor proteoglycan synthesis and matrix remodeling. This phase I/IIa study aimed to evaluate the safety and efficacy of STNM01 in patients with unresectable pancreatic ductal adenocarcinoma (PDAC)., Methods: This was an open-label, dose-escalation study of STNM01 as second-line therapy in gemcitabine plus nab -paclitaxel-refractory PDAC. A cycle comprised three 2-weekly endoscopic ultrasound-guided locoregional injections of STNM01 at doses of 250, 1,000, 2,500, or 10,000 nM in combination with S-1 (80-120 mg twice a day for 14 days every 3 weeks). The primary outcome was the incidence of dose-liming toxicity (DLT). The secondary outcomes included overall survival (OS), tumor response, changes in tumor microenvironment on immunohistopathology, and safety (jRCT2031190055)., Findings: A total of 22 patients were enrolled, and 3 cycles were repeated at maximum; no DLT was observed. The median OS was 7.8 months. The disease control rate was 77.3%; 1 patient showed complete disappearance of visible lesions in the pancreas and tumor-draining lymph nodes. Higher tumoral CHST15 expression was associated with poor CD3 + and CD8 + T cell infiltration at baseline. STNM01 led to a significant reduction in CHST15, and increased tumor-infiltrating CD3 + and CD8 + T cells in combination with S-1 at the end of cycle 1. Higher fold increase in CD3 + T cells correlated with longer OS. There were 8 grade 3 adverse events., Interpretation: Locoregional injection of STNM01 was well tolerated in patients with unresectable PDAC as combined second-line therapy. It prolonged survival by enhancing T cell infiltration in tumor microenvironment., Funding: The present study was supported by the Japan Agency for Medical Research and Development (AMED)., Competing Interests: Author TF received payments for lectures from 10.13039/501100013170Ono Pharmaceutical, Yakult Pharmaceutical Industry, Boston Scientific, Fujifilm Healthcare Corporation, Medico’s Hirata, and 10.13039/501100022274Daiichi Sankyo. Author TT received payments for lectures from 10.13039/100009734Olympus Japan, 10.13039/100016242Boston Scientific Japan, and Fujifilm medical, Kaneka medix Corporation, Gadelius Medical K.K., and supports for attending meetings from Olympus Japan, Boston Scientific Japan, and Fujifilm Medical. Author MK received payments for lectures from 10.13039/100016242Boston Scientific Japan, 10.13039/100009734Olympus Japan, AstraZeneca, Takeda Pharmaceutical, 10.13039/100014421EA Pharma, and 10.13039/501100022274Daiichi Sankyo, and supports for attending meetings from Olympus Japan. Author MM received payment for manuscript writing from 10.13039/100009734Olympus Japan. Author MN received consulting fees from 10.13039/100016242Boston Scientific, Olympus America, and Lumendi, and payment for the lecture from 10.13039/100008373Takeda Pharmaceutical Company Limited. Author SR received payments for lectures from 10.13039/100009734Olympus Japan, 10.13039/100016242Boston Scientific Japan. Author TI received research grants from 10.13039/501100012030Yakult, 10.13039/100007054MSD, 10.13039/100018046Nippon Kayaku, AstraZeneka, Medico’s Hirata Inc., 10.13039/100009734Olympus Japan, 10.13039/100016242Boston Scientific Japan K.K., and Gadelius Medical K.K. and consulting fees from 10.13039/100009734Olympus Japan, M.I. TECH Co.,Ltd., and J-MIT K.K., and payments for lectures from 10.13039/100009734Olympus Japan, 10.13039/100016242Boston Scientific Japan, Gadelius Medical K.K., 10.13039/100010740Asahi Kasei Pharma Corporation, Kaneka Medix Corporation, Medico’s Hirata Inc., 3-D Matrix Ltd., and Takeda Pharmaceutical Company Limited. Author HI received research grants from 10.13039/501100003769Eisai, 10.13039/501100007418Ajinomoto, Yakult Honsha, Taiho Pharmaceutical, and 10.13039/501100022274Daiichi Sankyo, and payments for lectures from Viatris, EA Pharma, 10.13039/100007054MSD, Abbie, Eizai, Otsuka Pharmaceutical, Ono Pharmaceutical, Yakult, Shionogi, 10.13039/501100022274Daiichi Sankyo, Taiho Pharmaceutical, Takeda Pharmaceutical, 10.13039/100010795Chugai Pharmaceutical, Teijin Healthcare, 10.13039/100004325AstraZeneca. Author NY received free provision of test agents from TME therapeutics, and a research grant for the present study from the 10.13039/100009619Japan Agency for Medical Research and Development (AMED), research grants from Sanwa Kagaku Kenkyujo and Kaigen Pharma, and consulting fees from Olympus Japan, Top corporation, Fujifilm and 10.13039/100016242Boston Scientific Japan, and payments for lectures from Olympus Japan, AstraZeneca, Daiichi Sankyo. Takeda Pharmaceuticals, Ohtsuka Pharmaceutical and 10.13039/100014421EA Pharma. Authors KT, HK, YM, TA, and HS have no declaration of interests., (© 2022 The Author(s).)