Abstract: Axitinib (AG-013736) is a potent, selective, second-generation oral tyrosine kinase inhibitor (TKI) that primarily targets vascular endothelial growth factor receptors (VEGFR) 1, 2, and 3. While initially approved as a monotherapy for advanced renal cell carcinoma (RCC) following prior systemic therapy failure, its role has rapidly evolved into a cornerstone of combination immunotherapy. Recent landmark clinical trials have demonstrated that combining axitinib with immune checkpoint inhibitors (ICIs) targeting the PD-1/PD-L1 pathway—such as pembrolizumab, avelumab, and toripalimab—significantly improves objective response rates (ORR), progression-free survival (PFS), and overall survival (OS) in patients with metastatic RCC. Beyond its direct antiangiogenic effects, axitinib exhibits profound immunomodulatory properties, including the downregulation of myeloid-derived suppressor cells (MDSCs) and regulatory T cells (Tregs), and the promotion of vascular normalization, which collectively alleviate the immunosuppressive tumor microenvironment. This review comprehensively examines the pharmacological activity, molecular mechanisms, structure-activity relationship, current clinical limitations, and future perspectives of axitinib in the context of combination immunotherapy for advanced malignancies, including RCC and hepatocellular carcinoma (HCC).
1. Introduction
Genitourinary malignancies, particularly renal cell carcinoma (RCC), have witnessed a paradigm shift in their therapeutic landscape with the advent of targeted therapies and immunotherapies [35]. Axitinib (AG-013736) is an oral, highly selective second-generation tyrosine kinase inhibitor (TKI) of vascular endothelial growth factor receptors (VEGFRs) [1]. It was initially approved by the FDA and the European Medicines Agency (EMA) in 2012 as a second-line monotherapy for advanced RCC after the failure of prior systemic treatments like sunitinib or cytokines [1].
Recently, the treatment paradigm has shifted toward combination immunotherapy. The biological rationale stems from evidence that antiangiogenic agents can enhance antitumor immunity by increasing antigen-presenting cell function, enhancing immune cell tumor infiltration, and decreasing the effects of immunosuppressive cells [52]. Landmark phase III trials, such as KEYNOTE-426 (pembrolizumab plus axitinib) and JAVELIN Renal 101 (avelumab plus axitinib), have established these combinations as first-line standard-of-care treatments for metastatic RCC across all risk groups [2][8]. Furthermore, axitinib is being actively investigated in combination with ICIs for other hypervascular tumors, notably advanced hepatocellular carcinoma (HCC) [1].
2. Pharmacological Activity
Axitinib is a multitarget TKI that potently inhibits VEGFR-1, 2, and 3, as well as the platelet-derived growth factor receptor (PDGFR) and c-KIT [1][67]. Pharmacokinetically, oral administration of axitinib results in rapid absorption, reaching maximum plasma concentration within four hours. It exhibits a high protein binding rate exceeding 99%, with a strong preference for albumin. The drug is predominantly metabolized in the liver by CYP3A4/5, and to a lesser extent by CYP1A2, 2C19, and UGT1A1, producing pharmacologically inactive metabolites that are primarily excreted via the hepatobiliary route in feces [1].
In clinical settings, the standard starting dose is 5 mg twice daily. A notable on-target pharmacodynamic biomarker of axitinib's efficacy is an increase in diastolic blood pressure; a diastolic blood pressure > 90 mm Hg has been independently correlated with longer PFS, OS, and a higher probability of partial response [1]. In combination therapy, axitinib plus pembrolizumab demonstrated an objective response rate (ORR) of 59.3% and a median PFS of 15.1 months in treatment-naïve mRCC patients [8]. Similarly, the combination of axitinib and avelumab yielded an ORR of 55.2% and a median PFS of 13.8 months in PD-L1 positive mRCC patients [8]. Recent data from the RENOTORCH study in a Chinese population also showed that toripalimab plus axitinib significantly prolonged PFS (18.0 months) and achieved a high ORR (55.7%) [54].
3. Molecular Mechanism of Action
The molecular mechanism of axitinib in combination immunotherapy is dual-faceted, involving both direct antiangiogenic effects and profound immunomodulatory actions within the tumor microenvironment (TME).
Antiangiogenic and Direct Tumor Inhibition: Axitinib binds to the inactive conformation of the catalytic domain of VEGF receptor tyrosine kinases [1]. By blocking VEGFR signaling, it inhibits endothelial cell proliferation, prunes tumor vasculature, and deprives the tumor of oxygen and nutrients. In specific cancers like HCC, axitinib's biochemical effects are also linked to the modulation of several signaling cascades, including VEGFR2/PAK1, CYP1A2, CaMKII/ERK, Akt/mTor, and the miR-509-3p/PDGFRA axis [1].
Immunomodulation and Synergy with ICIs: The TME in RCC and HCC is highly immunosuppressive. Axitinib counteracts this by downregulating signal transducer and activator of transcription 3 (STAT3) expression, which hampers the accumulation and suppressive function of myeloid-derived suppressor cells (MDSCs) [4]. It also decreases the percentage of circulating regulatory T cells (Tregs) and promotes natural killer (NK) cell recognition and degranulation [2][4].
Crucially, axitinib promotes "vascular normalization." By reducing the tortuosity of tumor vessels and lowering interstitial fluid pressure, it enhances vessel maturation and pericyte coverage. This normalization alleviates tumor hypoxia and increases the infiltration of effector T lymphocytes into the tumor parenchyma, thereby priming the tumor to respond robustly to PD-1/PD-L1 blockade [4].
4. Structure-Activity Relationship (SAR)
Axitinib is an indazole derivative synthesized via chemical synthesis, possessing a molecular weight of 386.47 Da [1]. Its specific structural conformation allows it to act as a highly potent and selective second-generation inhibitor. Unlike some broader multikinase inhibitors, axitinib's structure is optimized to bind specifically to the inactive conformation of the catalytic domain of VEGF receptor tyrosine kinases (RTKs) [1]. This selective binding profile is responsible for its potent antiangiogenic properties and its specific on-target side effects, such as hypertension, which serves as a clinical surrogate for its target engagement and efficacy [1].
5. Current Limitations
Despite the remarkable success of axitinib in combination with ICIs, several clinical limitations persist:
Overlapping Toxicities: The combination of TKIs and ICIs adds significant complexity to adverse event (AE) management. Both axitinib and ICIs can cause overlapping toxicities such as diarrhea, fatigue, hepatotoxicity (transaminitis), and endocrine abnormalities (e.g., hypothyroidism and hyperthyroidism) [5]. Distinguishing whether an AE (like hepatitis or diarrhea) is immune-related (requiring corticosteroids) or axitinib-related (requiring dose interruption or reduction) is a major diagnostic dilemma [5]. Grade 3-4 AEs are common, occurring in over 70% of patients on these combination regimens [8].
Drug Resistance and Dosing Challenges: Both primary and acquired resistance to anti-PD-1/PD-L1 therapies remain significant hurdles [4]. Furthermore, standard high doses of antiangiogenic agents like axitinib can lead to rapid and excessive pruning of tumor vessels. This excessive pruning shortens the window of vascular normalization, exacerbates severe hypoxia, and ultimately worsens the immunosuppressive TME, which can compromise the efficacy of the partnered immunotherapy [4].
Lack of Predictive Biomarkers: There is a critical absence of reliable predictive biomarkers to select the optimal patient population for axitinib-ICI combinations. For instance, PD-L1 expression status does not consistently predict outcomes, as survival benefits with pembrolizumab plus axitinib were observed regardless of PD-L1 expression levels [8][40].
6. Future Perspectives
The future of axitinib in combination immunotherapy is focused on optimizing efficacy while mitigating toxicity and resistance:
Dose Optimization for Vascular Normalization: Emerging clinical and preclinical evidence suggests that low-dose axitinib might be more beneficial than standard-dose axitinib in assisting immunotherapy. Lower doses may better sustain vascular normalization, optimally alleviate hypoxia, and maximize the infiltration of immune cells without causing excessive vessel pruning or spiking Treg levels [4]. Tailoring the dose of axitinib to act primarily as an immunopotentiator rather than a strict tumoricidal agent is a promising research direction.
Expansion to Other Malignancies: While established in RCC, axitinib combined with ICIs is showing potential in other cancers. In advanced HCC, phase I/Ib trials combining axitinib with avelumab or radiotherapy have demonstrated manageable toxicity profiles and promising objective response rates, particularly in patients without baseline vascular invasion or with PD-L1 positive tumors [1]. Further large-scale randomized controlled trials are warranted to validate these findings.
Biomarker Discovery: Future research must prioritize the identification of precise biomarkers. Techniques such as dynamic contrast-enhanced ultrasound (DCE-US) to monitor early tumor fractional blood volume changes are being explored to predict overall survival and guide personalized treatment sequences [1].
7. References
