Impact of histological and molecular subtype on the potential therapeutic effect of buparlisib in canine mammary gland tumours

This study aims toevaluate the response ofprimary cells tobuparlisib, aPI3K inhibitor, atvarying concentrations and exposure durations across different histological and molecular subtypes ofCMGTs, and toassess PI3K/Akt/mTOR pathway activity bymeasuring Akt and mTOR expression. Three carcinomas (C), three sarcomas (S), and two carcinosarcomas (CS) tumours were collected from the dogs. The primary cells were produced from tissues and treated with buparlisib atdifferent doses. Subsequently, the WST-1 assay, AnnexinV, and AO/PI staining were performed sequentially toevaluate the effects ofbuparlisib. PI3K/Akt/mTOR signalling pathway inhibition was revealed atthe gene level inC, S, and CScells following 5µM buparlisib treatment byRT-PCR analysis. Our results demonstrated that C1 and C2 (basal-like) cells were more sensitive than C3, CS1, and CS2 cells (luminal A) upon buparlisib treatment. Liposarcoma S2cells responded more tobuparlisib than undifferentiated Scells (S1and S3). Buparlisib also induced apoptosis and inhibited Akt and mTOR mRNA levels inC2, C3, S2 and S3cells (P<0.05). Ahigher rate ofapoptotic cell death was observed inthe Chistological subtype and basal-like cells, with 62.9±0.8% apoptosis inC1and 79.1±0.3% inC2. The efficacy ofbuparlisib was more pronounced inC2basal-like CMGT cells and liposarcoma S2cells with the downregulation ofAkt and mTOR mRNA levels (P <0.001). Therefore, PI3K inhibitors could beused totreat CMGT, particularly the basal-like molecular subtype.