Abstract: Capivasertib (AZD5363) is a novel, potent, ATP-competitive pan-AKT kinase inhibitor that has emerged as a significant therapeutic advancement for hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) advanced or metastatic breast cancer. Hyperactivation of the PI3K/AKT/mTOR signaling pathway, frequently driven by alterations in PIK3CA, AKT1, and PTEN, is a primary mechanism of resistance to standard endocrine therapy (ET) and CDK4/6 inhibitors. Capivasertib effectively blocks this oncogenic signaling by inhibiting all three AKT isoforms. Clinical trials, notably the phase II FAKTION and phase III CAPItello-291 studies, have demonstrated that the addition of capivasertib to fulvestrant significantly improves progression-free survival (PFS) and overall survival (OS), particularly in patients harboring PI3K/AKT/PTEN pathway alterations. These findings led to its FDA approval in 2023 for this specific patient population. While capivasertib exhibits a manageable safety profile, on-target toxicities such as diarrhea, rash, and hyperglycemia require careful clinical management. Ongoing research is exploring triplet combinations, expansion into other breast cancer subtypes like triple-negative breast cancer (TNBC), and the use of circulating tumor DNA (ctDNA) for dynamic monitoring to further optimize its clinical utility.
1. Introduction
Breast cancer remains the most commonly diagnosed cancer in women worldwide, with approximately 70% of cases classified as hormone receptor-positive (HR+) and human epidermal growth factor receptor 2-negative (HER2-) [33][54]. The standard first-line treatment for advanced or metastatic HR+/HER2- breast cancer involves endocrine therapy (ET) combined with a cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor [33][54]. However, resistance to these therapies inevitably develops, presenting a significant clinical challenge. A primary driver of this endocrine resistance is the hyperactivation of the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) signaling pathway [13][54]. Alterations in key genes within this pathway, such as PIK3CA, AKT1, and PTEN, occur in a substantial proportion of patients and are associated with disease progression and poor prognosis [13].
To overcome this resistance, capivasertib (AZD5363) was developed as a novel, potent AKT inhibitor [1]. By targeting the central node of this pathway, capivasertib has shown significant promise in restoring sensitivity to endocrine therapy. Based on compelling clinical trial data, capivasertib, in combination with the estrogen receptor antagonist fulvestrant, received US Food and Drug Administration (FDA) approval in 2023 for the treatment of adult patients with HR+/HER2- locally advanced or metastatic breast cancer harboring one or more PIK3CA, AKT1, or PTEN alterations following progression on at least one ET-based regimen [13][33].
2. Pharmacological Activity
Capivasertib has demonstrated robust pharmacological activity across both preclinical models and clinical trials. In vitro, the compound was tested across 182 cell lines and exhibited persistent anticancer effects, particularly in estrogen receptor (ER)-positive and HER2-positive breast cancer models [1][22]. In vivo studies further confirmed its efficacy in HER2-positive, PIK3CA-mutated breast cancer xenografts, where it also demonstrated synergy with anti-HER2 therapies (trastuzumab and lapatinib) and chemotherapy (docetaxel) [22].
The clinical efficacy of capivasertib in HR+/HER2- breast cancer was established through landmark trials. The phase II FAKTION trial evaluated capivasertib plus fulvestrant versus placebo plus fulvestrant in postmenopausal women progressing on an aromatase inhibitor. The combination significantly prolonged median progression-free survival (PFS) to 10.3 months compared to 4.8 months in the placebo arm [36]. Furthermore, the updated analysis revealed a statistically significant improvement in median overall survival (OS) (29.3 months vs. 23.4 months) [36].
These findings were confirmed by the phase III CAPItello-291 trial, which randomized 708 patients who had progressed on ET (with or without prior CDK4/6 inhibitors). The median PFS for the capivasertib/fulvestrant arm was 7.2 months compared to 3.6 months for the placebo/fulvestrant arm in the overall population [26][42]. Crucially, in the AKT pathway-altered subgroup (comprising 41% of the study population), the median PFS was 7.3 months versus 3.1 months, respectively, representing a 50% reduction in the risk of a PFS event [19][26][42]. The trial also noted that patients with prior CDK4/6 inhibitor exposure or liver metastases had numerically shorter PFS, highlighting the need for effective therapies in these higher-risk populations [19].
3. Molecular Mechanism of Action
The PI3K/AKT/mTOR pathway is a critical intracellular signaling cascade involved in regulating cellular growth, proliferation, metabolism, and survival [87]. AKT, a serine/threonine kinase belonging to the AGC (cAMP-dependent, cGMP-dependent, and protein kinase C) family, acts as the master switch of this pathway [1][13]. It exists in three isoforms (AKT1, AKT2, and AKT3) encoded by separate genes [1]. Hyperactivation of AKT, driven by activating mutations in PIK3CA and AKT1 or inactivating mutations/deletions in the PTEN tumor suppressor gene, leads to the phosphorylation of downstream substrates that promote cancer cell proliferation and survival [1][13].
Capivasertib exerts its mechanism of action by competitively binding to the ATP-binding site of the AKT kinase, thereby inhibiting all three AKT isoforms [1][13]. This blockade restricts oncogenic signaling from upstream PI3K, AKT, and PTEN alterations before they can propagate further downstream effects [13].
Importantly, preclinical evidence has elucidated a direct crosstalk between the AKT and ER pathways. AKT overactivity can cause ligand-independent phosphorylation of the estrogen receptor, altering ER transcription and contributing to endocrine resistance [22]. Inhibition of AKT by capivasertib reduces ER-related transcription by decreasing the recruitment of estrogen receptors and CREB-binding protein coactivators to estrogen response elements [81]. Consequently, the simultaneous inhibition of both pathways using capivasertib and fulvestrant exhibits synergistic inhibitory effects on ER-mediated tumor growth [81].
4. Structure-Activity Relationship (SAR)
The development of effective AKT inhibitors has historically been challenging due to the high structural similarity among the three AKT isoforms and the potential for dose-limiting toxicities [34]. Early allosteric inhibitors, such as MK-2206, demonstrated promising preclinical activity but failed to progress to regulatory approval due to limited clinical efficacy and significant toxicity in phase II trials [34].
In contrast, capivasertib is a highly potent, ATP-competitive pan-AKT inhibitor [1][62]. It possesses balanced nanomolar potency against all three isoforms, with half-maximal inhibitory concentrations (IC50) of 3 nmol/L for AKT1, 9 nmol/L for AKT2, and 3 nmol/L for AKT3 [62]. This structural design allows capivasertib to effectively inhibit downstream signaling even in the presence of robust pathway hyperactivation, distinguishing it from earlier allosteric compounds and enabling its successful clinical translation [34][62].
5. Current Limitations
Despite its clinical success, the use of capivasertib is accompanied by several limitations. Foremost is its toxicity profile. Because capivasertib inhibits physiologic PI3K/AKT pathway signaling, it induces on-target, off-tumor toxicities [13]. The most frequently reported adverse events (AEs) include diarrhea (up to 72%), cutaneous adverse reactions/rash (up to 38%), and hyperglycemia (up to 28%) [13][42]. In the CAPItello-291 trial, grade 3 or higher AEs such as rash (12.1%), diarrhea (9.3%), and hyperglycemia (2.0%) necessitated dose interruptions, reductions, and led to permanent treatment discontinuation in 13.0% of patients [13].
Another limitation is the strict biomarker dependency for optimal efficacy. The FDA approval is specifically limited to patients with PIK3CA, AKT1, or PTEN alterations, as the efficacy in the pathway-non-altered subgroup was found to be limited (hazard ratio 0.79) [19][23]. Considering that up to 40% of patients in this setting may lack these specific alterations, administering capivasertib to the broader population risks exposing patients to significant toxicity without confirmed therapeutic benefit [23]. Furthermore, acquired resistance to targeted inhibitors of the PI3K/AKT/mTOR pathway remains an ongoing challenge that eventually limits the duration of response [34].
6. Future Perspectives
Future research is focused on expanding the utility of capivasertib and overcoming current limitations. One major strategy is the investigation of triplet combination therapies to bypass resistance mechanisms and enhance efficacy. The ongoing phase Ib/III CAPItello-292 trial is evaluating the concurrent blockade of the PI3K/AKT/mTOR and CDK4/6 pathways by combining capivasertib, palbociclib, and fulvestrant in HR+/HER2- advanced breast cancer [23][63].
Additionally, capivasertib is being explored in other breast cancer subtypes. The phase III CAPItello-290 trial is currently investigating the combination of capivasertib and paclitaxel as a first-line therapy for metastatic triple-negative breast cancer (TNBC), building upon positive phase II data [51].
Advancements in biomarker testing, particularly the use of circulating tumor DNA (ctDNA) for dynamic monitoring, hold promise for better patient selection and early detection of resistance. Early changes in ctDNA levels have been shown to be predictive of PFS, which could guide timely therapeutic switches [63]. Finally, as the treatment landscape becomes more crowded with novel agents (e.g., alpelisib, everolimus, elacestrant, inavolisib), further studies are critical to determine the optimal sequencing of these therapies to maximize patient outcomes and minimize cumulative toxicity [51].
