Preclinical Study using Malat1 Small Interfering RNA or Androgen Receptor Splicing Variant 7 Degradation Enhancer ASC-J9^® to Suppress Enzalutamide-resistant Prostate Cancer Progression

Background While androgen-deprivation-therapy with the recently developed antiandrogen enzalutamide (Enz) shows promising therapeutic benefits in men with metastatic castration-resistant prostate cancer (PCa), many patients develop resistance to Enz, which may involve the induction of the androgen receptor (AR) splicing variant 7 (AR-v7). Objective Our aim is to identify the mechanisms responsible for AR-v7 production and to develop novel preclinical approaches to suppress the Enz-resistant (EnzR) PCa. Design, setting, and participants We established EnzR-PCa cell lines and examined the long noncoding RNA Malat1 (Malat1) function in conferring Enz resistance. We also examined the in vivo effects of Malat1 short interfering RNA and the AR-v7 degradation enhancer, ASC-J9^®. Outcome measurements and statistical analysis Enz resistance and expression of Malat1 and AR-v7. All statistical comparisons were analyzed with a t-test or one way analysis of variance followed by t-test. Results and limitations We demonstrated that Malat1 is indispensable for Enz-induced AR-v7 production in VCaP and EnzR-C4-2 cells. We observed increased AR-v7 and Malat1 expression in our established EnzR-PCa cell lines and in some PCa patients who received Enz treatment. Targeting the Malat1/AR-v7 axis resulted in altering the PCa resistance to androgen deprivation therapy with Enz. The limitation of this study includes the small sample size from the same human patients before and after receiving Enz treatment. Conclusions Targeting the Malat1/AR-v7 axis via Malat1-short interfering RNA or AR-v7 degradation enhancer ASC-J9^® in EnzR-PCa cell lines and mouse models suppressed EnzR-PCa progression. Patient summary Androgen deprivation therapy-enzalutamide treatment may not be the best choice for prostate cancer patients who have higher expression of the Malat1/androgen receptor splicing variant 7 axis, and new therapies using Malat1-short interfering RNA or ASC-J9^® may be developed in the future to better suppress enzalutamide-resistant prostate cancer. Malat1 expression is linked to the enhanced androgen receptor splicing variant 7 expression, and their induction may play key roles in the development of enzalutamide resistance. Malat1 and the androgen receptor splicing variant 7 could serve as clinically relevant therapeutic targets for patients who received enzalutamide treatment and targeting AR-v7 with ASC-J9^® may further suppress enzalutamide resistant prostate cancer.