The PI3K/Akt/mTOR Signaling Pathway in Triple-Negative Breast Cancer: A Resistance Pathway and a Prime Target for Targeted Therapies

Triple-negative breast cancer is the most aggressive subtype of breast cancer and is associated with the worst prognosis. Conventional chemotherapy remains the gold standard treatment for this disease but is associated with a high relapse rate, highlighting the urgent need for effective targeted therapies. The PI3K/Akt/mTOR pathway, dysregulated in nearly 60% of these cancers, appears to be a prime target. It involves a signaling cascade beginning with PI3K activation followed by activating phosphorylation of Akt and then mTOR complex, which activates oncogenic processes by enhancing protein synthesis, inhibiting apoptosis, dysregulating autophagy and promoting DNA repair that supports tumor cell survival. Moreover, the PI3K/Akt/mTOR pathway plays a central role in the development of chemoresistance. Numerous alterations (activating the mutation of PIK3CA or the loss of tumor suppressor PTEN) may lead to its overactivation. Targeted inhibitors of PI3K, Akt and mTOR have been developed to counteract this dysregulation. However, numerous cancer resistance mechanisms have emerged, reducing their efficacy, for example, reactivation of Akt following mTOR blockade, reactivation of the pathway by insulin signaling or activation of compensatory pathways such as the MAPK pathway, thus limiting their integration into routine practice. To counteract these resistances, combination therapies currently being investigated in clinical trials aim to improve clinical outcomes of PI3K/Akt/mTOR pathway inhibition. The aim of this review was to summarize current therapies developed to target this pathway in TNBC, with a focus on the resistance mechanisms that limit their effectiveness.