When can mutation-negative breast cancers still respond?
The PI3K/AKT/mTOR pathway can be switched on even without a PIK3CA mutation. In triple-negative breast cancer (TNBC), the pathway is dysregulated in nearly 60% of cases, often due to loss of the tumor suppressor PTEN or other genetic changes [5]. This means that targeting the pathway with inhibitors can still be effective even if the PIK3CA gene is normal.
For example, a case study described two TNBC patients who had PIK3CA copy-number gains (extra copies of the gene, not mutations) and responded to the mTOR inhibitor everolimus or the PI3K inhibitor alpelisib, with visible tumor shrinkage on PET scans [4]. This suggests that looking beyond mutations to copy-number changes could identify more patients who might benefit from these drugs.
In HR+ breast cancer, does mutation status matter?
In hormone receptor-positive (HR+) breast cancer, the evidence is more nuanced. The approved drugs alpelisib and capivasertib are specifically indicated for tumors with PIK3CA mutations or other pathway alterations, and clinical trials have shown significant benefit in those groups [2][7]. For patients without these mutations, the benefit is less clear, and some inhibitors may not be approved for them.
However, the mTOR inhibitor everolimus is approved for HR+ advanced breast cancer regardless of PIK3CA status, based on trials showing benefit in a broader population [6]. This suggests that while mutation status can guide treatment choice, it is not an absolute barrier to using pathway inhibitors in HR+ disease.
Why do some mutation-negative cancers still resist treatment?
Resistance is a major problem, even when the pathway is overactive. The cancer can reactivate the pathway through feedback loops (e.g., AKT reactivation after mTOR blockade) or activate alternative pathways like MAPK, which limits the effectiveness of single-agent inhibitors [5][8]. This is why combination therapies are being explored.
For example, combining PI3K/AKT/mTOR inhibitors with endocrine therapy or CDK4/6 inhibitors is a promising strategy to overcome resistance and improve outcomes [3][7]. In TNBC, combining these inhibitors with chemotherapy or immunotherapy is also under investigation [1]. So, while multi-pathway blockade can work, it often needs to be part of a broader treatment plan to be effective.
About These Sources
This answer is built on 8 peer-reviewed studies — published from 2022 to 2025, 5 from 2024 or later, 4 in Q1 journals, collectively cited 345 times — selected as the most relevant from 8 studies that passed quality screening, drawn from 23 papers retrieved from a database of over 500 million.
Sources used in this answer
PI3K/AKT/mTOR signaling pathway: an important driver and therapeutic target in triple-negative breast cancer
Reviews the PI3K/AKT/mTOR pathway as a therapeutic target in TNBC, noting it is prevalently over-activated and discussing clinical studies of kinase inhibitors, but does not provide specific efficacy data for mutation-negative cases.
PI3K/AKT/mTOR inhibitors for hormone receptor-positive advanced breast cancer
Reviews PI3K/AKT/mTOR inhibitors in HR+ advanced breast cancer, highlighting that alpelisib and capivasertib are approved for this indication, and discusses diagnostic testing and clinical integration, implying benefit is linked to pathway aberrations.
Strategic advancements in targeting the PI3K/AKT/mTOR pathway for Breast cancer therapy
Reviews the pathway's dysregulation in breast cancer, including PIK3CA mutations and PTEN loss, and notes that combination therapies show promise in overcoming resistance, but does not specify outcomes for mutation-negative tumors.
<i>PIK3CA</i>copy-number gain and inhibitors of the PI3K/AKT/mTOR pathway in triple-negative breast cancer
Presents two clinical cases of TNBC patients with PIK3CA copy-number gains who responded to everolimus or alpelisib, suggesting that copy-number gains may be a predictive biomarker for response to PI3K/AKT/mTOR inhibitors.
The PI3K/Akt/mTOR Signaling Pathway in Triple-Negative Breast Cancer: A Resistance Pathway and a Prime Target for Targeted Therapies
States that the PI3K/Akt/mTOR pathway is dysregulated in nearly 60% of TNBCs, and discusses resistance mechanisms such as AKT reactivation and MAPK pathway activation, emphasizing the need for combination therapies.
Expert consensus on the clinical application of PI3K/AKT/mTOR inhibitors in the treatment of advanced breast cancer
An expert consensus that PI3K/AKT/mTOR inhibitors bring significant clinical benefit to advanced breast cancer patients, especially HR+/HER2- cases, and that everolimus is approved regardless of PIK3CA status, while alpelisib is expected to be approved.
Drugging the PI3K/AKT/mTOR Pathway in ER+ Breast Cancer
Reviews the role of the pathway in ER+ breast cancer, noting that alpelisib and capivasertib are approved in combination with fulvestrant, and discusses genomic contexts for superior activity, implying mutation status is important for some inhibitors.
Resistance to Targeted Inhibitors of the PI3K/AKT/mTOR Pathway in Advanced Oestrogen-Receptor-Positive Breast Cancer
Reviews resistance mechanisms to PI3K/AKT/mTOR inhibitors in ER+ advanced breast cancer, noting that most patients will develop resistance, and highlights the need for more studies on AKT1 inhibitor resistance.