Abstract Background: Chemo-/radio-resistance is an important reason for prostate cancer (CaP) progression and metastasis. CD44 is a well-documented cancer stem cell (CSC) biomarker, and one of its variants, CD44 variant 6 (CD44v6) is closely associated with aggressive behaviour and correlates with poor prognosis in a variety of human cancers. Our previous study has demonstrated increased expressions of CD44v6 in metastatic CaP cell lines and human CaP tissues, which was associated with CaP progression and chemo-/radio-resistance in vitro. However, the role of CD44v6 in CaP progression and therapeutic resistance in vivo is still uncertain. Aim: The aim of this study was to investigate the role of CD44v6 in CaP development and chemo-/radioresistance as well as underlying pathways in vivo, and find whether it is a suitable therapeutic target for CaP therapy. Methods: CD44v6 gene was knocked down (KD) in PC-3M CaP cell line using short hairpin RNA (shRNA). Subcutaneous (s.c.) and orthotopic CaP animal models were established using PC-3M-CD44v6-KD and PC-3M-CD44v6-scramble (scr) control cells, to assess the effect of CD44v6 on CaP tumourigenesis, chemotherapy (docetaxel) response and radiation response. Signal transduction proteins in PI3K/Akt/mTOR pathway (Akt, p-Akt, mTOR and p-mTOR) as well as proliferation and apoptosis makers were assessed by immunohistochemistry in xenografts from both animal models. Results: Both s.c. and orthotopic xenografts from PC-3M-CD44v6-KD cells displayed supressed tumour growth and increased responsiveness to docetaxel (40mg/kg) and radiation (2Gy/day for 4 consecutive days) compared to control xenografts from PC-3M-CD44v6-scr cells in NOD/SCID male mice. Down-regulation of the PI3K/Akt/mTOR pathway proteins (p-Akt and p-mTOR), proliferation marker (Ki-67) and angiogenesis marker (CD31), as well as up-regulation of apoptotic responses (cleaved Caspase-3) to chemotherapy, DNA damage responses (γ-H2AX) to radiation were found in PC-3M-CD44v6-KD s.c. xenografts, respectively. Conclusions: CD44v6 is actively involved in CaP tumorigenesis and treatment responses to chemo-/radio-therapy via PI3K/Akt/mTOR signalling pathway in in vivo mouse models and holds promise as a therapeutic target for the treatment of CaP. Key words: CD44v6, prostate cancer, chemo-/radio-resistance, animal model, metastasis Citation Format: Jie Ni, Paul Cozzi, Julia Beretov, Joseph Bucci, Peter Graham, Yong Li. Study of CD44 variant 6 (CD44v6) in prostate cancer chemo-/radio resistance in vivo [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1999.