New insights into mechanisms and interventions of locoregional therapies for hepatocellular carcinoma

Abstract

Hepatocellular carcinoma (HCC) is responsible for a significant number of cancer-related deaths worldwide and its incidence is increasing. Locoregional treatments, which are precision procedures guided by imaging to specifically target liver tumors, play a critical role in the management of a substantial portion of HCC cases. These therapies have become an essential element of the HCC treatment landscape, with transarterial chemoembolization (TACE) being the treatment of choice for patients with intermediate to advanced stages of the disease. Other locoregional therapies, like radiofrequency ablation, are highly effective for small, early-stage HCC. Nevertheless, the advent of targeted immunotherapy has challenged these established treatments. Tyrosine kinase inhibitors (TKIs) and immune checkpoint inhibitors (ICIs) have shown remarkable efficacy in clinical settings. However, their specific uses and the development of resistance in subsequent treatments have led clinicians to reevaluate the future direction of HCC therapy. This review concentrates on the distinct features of both systemic and novel locoregional therapies. We investigate their effects on the tumor microenvironment at the molecular level and discuss how targeted immunotherapy can be effectively integrated with locoregional therapies. We also examine research findings from retrospective studies and randomized controlled trials on various combined treatment regimens, assessing their validity to determine the future evolution of locoregional therapies within the framework of personalized, comprehensive treatment.

Introduction

Despite significant advancements in survival rates over the past decade, cancer continues to be a major global public health challenge. While the prevalence of liver cancer is on the decline, the associated mortality rate remains elevated. In 2024, liver and intrahepatic bile duct cancers are projected to be the fifth most common cause of cancer death in men and the seventh in women in the United States (1). The approach to treating hepatocellular carcinoma (HCC) has shifted from a reliance solely on surgery to a comprehensive, multidisciplinary strategy, incorporating state-of-the-art surgical techniques, biological innovations, and the analytical power of big data. Yet, the prognosis for patients with HCC is still less than optimal. Surgical methods, including hepatectomy and liver transplantation, are the most definitive curative options for HCC to date (2). However, the surgical landscape is fraught with challenges, including the ineligibility of some patients for resection, postoperative recovery issues, and a high recurrence rate (3). The introduction of targeted therapy and immunotherapy has brought new optimism for those diagnosed with HCC. In 2022, the atezolizumab-bevacizumab combination was recommended as a first-line systemic therapy for HCC (4), and pembrolizumab, as well as the combination of nivolumab and ipilimumab, was approved as second-line treatments for patients whose disease progressed after a first-line TKI (5). A multitude of clinical trials are actively investigating immunotherapy and targeted therapy. However, no adjuvant therapy has yet to show a marked improvement in overall survival (OS) for early-stage HCC patients after surgical resection (6).

Locoregional therapies are typically defined as a series of techniques that utilize puncture needles, catheters, and other devices to introduce specific instruments directly to the lesion site within the human body. These procedures are performed through natural orifices or small incisions, guided and monitored by imaging equipment such as computed tomography (CT), ultrasound, and magnetic resonance imaging (MRI). Locoregional therapies have evolved from primarily transarterial chemoembolization (TACE) and ablation, for early and intermediate-stage HCC, to a range of treatments applicable to various stages of the disease (7). Particularly with the rapid advancements in locoregional therapy equipment and materials in recent years, these treatments have seen swift development. Traditional surgery and chemotherapy have faced challenges as preferred methods for HCC treatment, with locoregional therapies becoming the favored option for some HCC patients (8-10). In the age of molecular targeted therapy (MTT) and immunotherapy, an improved understanding of tumor progression mechanisms has offered new hope for patients with advanced HCC. The integration of locoregional therapies with targeted immunotherapy is gradually becoming more common in clinical settings, marking a transformative phase for traditional locoregional treatments. Numerous clinical trials have indicated a unique synergistic effect between locoregional therapies and targeted immunotherapy, yielding promising clinical outcomes (11). The findings from a large-scale retrospective cohort study (CHANCE001) (12), completed in 2023, demonstrated that combining TACE with programmed death-(ligand)1 [PD-(L)1] inhibitors and MTTs can significantly improve patients’ progression-free survival (PFS), OS, and objective response rate (ORR) compared to TACE alone. However, further research is essential to fully understand the specific mechanisms behind the synergy of locoregional therapies and targeted immunotherapy in HCC. This review aims to discuss the latest advancements in locoregional therapies for HCC, shedding light on the therapeutic mechanisms and intervention strategies from a molecular perspective.

Hepatic arterial infusion chemotherapy (HAIC)

HAIC, also known as hepatic arterial infusion chemotherapy, is a local regional chemotherapy method that involves the insertion of a catheter into the hepatic artery through the femoral artery, guided by digital subtraction angiography (DSA). This procedure enables the continuous infusion of chemotherapy drugs directly into the artery that supplies the tumor, over an extended period. Given that the blood supply to liver cancer tissue is almost exclusively (about 90%) derived from the hepatic artery, infusing chemotherapy drugs through this artery can create a high concentration within tumor cells, effectively killing them while sparing normal liver cells (13). Although HAIC demonstrates a significant antitumor effect, it is not typically recommended as a standard treatment for HCC due to the lack of large-scale randomized clinical trial evidence. Most published studies on HAIC originate from Asian countries, and the Barcelona Clinic Liver Cancer (BCLC) staging classification and treatment guidelines have yet included HAIC in the recommended HCC treatment options (9). However, it is challenging to achieve complete embolization of tumor tissues in cases of large HCC, and the use of a large number of embolization particles can pose greater risks to patients (14). Recent evidence suggests that the infusion of a chemotherapy regimen consisting of fluorouracil, leucovorin, and oxaliplatin (FOLFOX) via HAIC offers significant survival benefits for patients with advanced HCC. The results of a randomized phase III trial in China indicated that, compared with TACE, FOLFOX-HAIC significantly improved OS (23.1 vs. 16.1 months, P<0.001), response rate (46% vs. 18%, P<0.001), and PFS (9.6 vs. 5.4 months, P<0.001). In this trial, the incidence of serious adverse events was lower in patients receiving HAIC than those undergoing TACE (19% vs. 30%, P=0.03) (15). However, the treatment of advanced HCC with HAIC is not yet fully established and requires further investigation. To establish HAIC as a standard treatment modality for HCC, clinical research is necessary to confirm its efficacy relative to other therapies and to refine the treatment protocol.

TACE therapy

Conventional TACE (cTACE)

cTACE has become one of the standard treatments for patients with advanced HCC. Over 20 years of development, significant differences have emerged between the current conventional TACE and early TACE in terms of drugs, methods, materials, and other aspects, leading to improved OS rates for TACE patients (3,8,16,17). The first randomized controlled clinical trial of TACE, conducted by Llovet et al. in 2002, demonstrated a significant benefit of TACE concerning OS (18). Gelfoam and doxorubicin were used in this clinical trial. Among the 112 participants, the hazard ratio (HR) was 0.47 [95% confidence interval (95% CI), 0.25−0.91], and the 2-year survival rate with TACE was 63%. This trial established the foundation for cTACE in the interventional treatment of HCC. Another meta-analysis also reported improved 2-year survival compared with the control group. Sensitivity analysis revealed a significant effect of chemoembolization with cisplatin or doxorubicin, whereas embolization alone did not have a significant effect. Thus, chemoembolization is considered to improve the survival of patients with unresectable liver cancer and may become the standard treatment (19). Based on these data, TACE was recognized as the gold standard for treating intermediate-stage HCC in 2012, receiving the highest recommendation grade (1A) (20). In a 2022 phase III trial by Peng et al. (21), 338 patients from 12 Chinese medical centers were enrolled and randomly assigned to treatment groups, with 170 receiving lenvatinib in combination with TACE. The LEN-TACE group had a median OS (mOS) of 17.8 months, significantly longer than the 11.5 months in the control group, indicating improved outcomes for patients (HR: 0.45). This finding has renewed interest in cTACE, underscoring its efficacy and the potential to benefit a broader patient population.

Drug-eluting bead TACE (DEB-TACE)

In recent years, there has been significant advancement in TACE technology with the introduction of DEBs. These medically-used DEBs require chemotherapy drugs to be loaded prior to embolization. This process involves attaching drugs to microspheres, primarily through ion exchange, to minimize drug release into the bloodstream and extend the duration of drug-tumor interaction, thus enhancing treatment efficacy (22-25). The pharmacokinetic advantages and potential for fewer side effects make DEB-TACE superior to cTACE. Currently, the primary focus of in-depth research is the combination of different chemotherapy drugs with microspheres. Commercially available non-degradable microspheres used in clinical practice include DC Bead (BTG, UK), Calli Spheres (HENGRUI, China), and HepaSphere (Merit, USA). Due to the complex preparation process and sustained-release mechanism, there is ongoing preclinical research into chitosan-cellulose microspheres as a form of biodegradable microspheres (26).

Non-degradable DEB-TACE

DEB-TACE has been the subject of extensive clinical trials, both completed and ongoing. Varela et al. conducted a pivotal study in 2006, establishing the safety and efficacy of DEB-TACE when loaded with doxorubicin. These trials have yielded a wealth of data, further securing DEB-TACE’s role in medical research. The study included 27 patients with untreated HCC and Child-Pugh A cirrhosis. The response rate was 75%. After a median follow-up of 27.6 months, the 2-year and 5-year survival rates were 92.5% and 88.9%, respectively. Only two participants developed liver abscesses after treatment, with one fatality (27). This evidence suggests that DEB-TACE presents minor side effects in HCC patients. While no definitive inclusion guidelines have been established, the efficacy of DEB-TACE has been corroborated by multiple clinical trials. Current research focuses on evaluating DEB-TACE’s side effects and defining clearer indications, as well as exploring novel combination therapies. DEB-TACE has also been combined with transarterial radioembolization (TARE) and local immunotherapy, to potentially enhance each modality’s efficacy (16,28). Presently, the integration of multikinase inhibitors with DEB-TACE is being considered to potentially amplify effectiveness in patients with intermediate-stage HCC. A randomized phase II trial exploring the combination of sorafenib with doxorubicin-loaded DEB-TACE reported that, out of 307 patients randomized, the ORRs for sorafenib and placebo were 55.9% and 41.3%, respectively, while the DCRs were 89.2% and 76.1% (29). With ongoing advancements, DEB-TACE aims to minimize adverse effects and thus contributes to the future treatment of advanced HCC.

Biodegradable DEB-TACE

Advancements in materials science have introduced new prospects for DEBs. A variety of polymer compounds are gaining attention in biomedicine due to their superior biocompatibility and degradability (30). Laboratory and clinical trial results are increasingly validating biodegradable drug-eluting beads as potentially ideal DEBs (31). Evidence indicates that the currently available water-soluble, positively charged DEBs do not fulfill all clinical requirements. Hence, there is a need to enhance the drug-elution capabilities and diversify the types of DEBs. Numerous biodegradable DEBs are under exploration, including those made from chitosan (32), cellulose (33), poly(ethylene glycol) methacrylate (34-36), and poly(lactide-co-glycolide) (37-42). Each material offers distinct drug-loading capacities, solubilities, and safety profiles to accommodate various treatment strategies. However, their use is primarily based on animal studies, with clinical trials yet to affirm their efficacy. Among the most extensively researched and applied are the degradable starch microspheres (DSMs). DSMs consist of absorbable starch particles with an average diameter of about 45±7 μm, which can be combined with various chemotherapy drugs. These starch particles serve solely as carriers, without forming chemical bonds with the drugs. Once administered, DSMs are enzymatically degraded by amylase in the blood, with a half-life of approximately 35−50 min, and are fully absorbed within about 2 h. DSM’s stable chemical attributes and brief half-life contribute to its reliability in terms of safety. However, this also presents challenges (43). In the TACE procedure for HCC, it is critical to quickly deliver DSMs and drugs to the liver’s target area and to repeat treatments as necessary (44). EmboceptS (PharmaCept, Berlin, Germany) is a widely used commercial DSM for treating HCC, with numerous clinical trials supporting its efficacy and safety. However, whether DSM-TACE is superior to DEB-TACE in terms of indications and efficacy remains to be clarified (45,46). Notwithstanding, DSMs provide a novel perspective on the selection of drug carriers for TACE, and ongoing research is investigating various new DSM formulations. Given their excellent safety profile, biodegradable microspheres may become a preferred choice for TACE in the future.

TARE

TARE, also known as selective internal radiotherapy (SIRT), offers a promising and innovative option for managing HCC. This advanced technique involves the precise delivery of radioactive agents directly into the tumor’s blood supply, followed by the emission of concentrated radiation in proximity to the malignant cells (9,47). SIRT serves as an alternative to external beam radiation therapy, which, despite its effectiveness, can produce severe side effects due to the radiation sensitivity of normal liver tissue. Consequently, external beam radiation therapy plays a limited role in primary HCC treatment (48), and interest in internal radiation therapies has increased markedly in recent years (49,50). The mechanism underlying the use of radioactive isotopes to treat tumors involves the ionizing radiation produced during the decay of these isotopes, which damages the DNA within the tissue cells, ultimately leading to cell death (51). Clinically, radioactive embolic agents such as Yttrium-90 (Y-90) and Iodine-131 are commonly used. Y-90 is particularly well-suited for TARE, emitting β-rays that are high-energy, short-range, and have a half-life of 6 h to 7 d before decaying into harmless Zirconium-90. Y-90 microspheres can effectively destroy cancer cells with high-dose β-radiation while sparing healthy liver tissues (52,53). TARE is frequently used to treat advanced liver cancer, especially in patients with portal vein tumor thrombus (PVTT). Previously, it was believed that patients with PVTT were at risk of ischemic liver injury and thus unsuitable for cTACE treatment. However, TARE may be the preferred method for treating liver cancer with PVTT due to the small size of the embolic material and the low incidence of adverse reactions such as liver failure (54,55). A prospective cohort study from 2003 to 2017 included 1,000 patients who underwent Y-90 radioembolization therapy, and the results indicated that this therapy could extend the survival of patients with various stages of HCC (56). Consequently, TARE is recommended by the American Association for the Study of Liver Diseases as an alternative treatment option. The latest phase II randomized controlled trial results showed that the mOS after TARE was 30.2 months, compared to 15.6 months after DEB-TACE (HR, 0.48; P=0.006). Advances in radiotherapy technology and equipment have become critical for precise tumor targeting. Customizable techniques tailored to different tumor types and stages are improving patient management, reducing side effects, and optimizing treatment options.

Local ablation

Local ablation is crucial in the treatment of HCC and is widely regarded as a favorable alternative to achieve localized control following surgical intervention for HCC (57). Various ablation techniques are utilized, each with its own mechanism, including radiofrequency ablation (RFA), microwave ablation (MWA), cryoablation (CRA) (using an argon-helium knife or liquid nitrogen cryosurgery), high-intensity focused ultrasound (HIFU) ablation, laser ablation, and chemical ablation (such as anhydrous ethanol injection) (58-60). Over the past 30 years, local ablation technology has developed rapidly, and its application has expanded well beyond the treatment of early, unresectable, small HCC. As a result, the restrictive criteria such as tumor size, number, and location have ceased to be long-standing challenges and limitations for local ablation (61,62). In the era of MTT and immunotherapy, the potential synergistic effects of local ablation on changes within the tumor microenvironment (TME) may offer new insights for the systemic treatment of HCC.

RFA

RFA emerged in the 1990s as a treatment for HCC and has since seen significant advancements (7). Through years of exploration and practice, RFA has demonstrated substantial improvements in both safety and efficacy for HCC treatment. Initially used for unresectable HCC and recurrences after resection, RFA has now become one of the primary choices for managing small HCC (63). As the earliest thermal ablation technique used for HCC, RFA works by rapidly heating liver cancer tissue to 80−100 °C using a radiofrequency needle. This targeted heating induces coagulative necrosis in the liver cancer tissue, with the goal of achieving comprehensive treatment of the tumor lesion and hemostasis (64). With the advancement of percutaneous puncture technology, RFA has become increasingly less damaging to the patient’s organs, enhancing its safety (7). At present, there is still debate regarding the advantages and disadvantages of RFA compared with surgical treatment. A randomized controlled trial from Japan (65) compared RFA with surgery for small HCC measuring less than 3 cm. Among the 308 patients, the median recurrence-free survival (RFS) was 3.5 years (95% CI, 2.6−5.1) in the surgery group and 3.0 years (95% CI, 2.4−5.6) in the RFA group (HR, 0.92; 95% CI, 0.67−1.25; P=0.58). According to conventional guideline standards, the 5-year OS after RFA is 40%−68%, and the 10-year OS is about 30%, which is still lower than that of surgical treatment (66,67). A meta-analysis that included 16 studies with a total of 3,760 patients showed that the 3-year OS rate and 5-year OS rate of the RFA group were lower than those of the surgical treatment group, but the postoperative complication rate of the RFA group was lower (68). The size of the tumor is the most important factor affecting the efficacy of RFA. For solitary tumors less than 5 cm in diameter, RFA has achieved long-term efficacy comparable to surgical resection. For larger HCCs measuring more than 5 cm, early studies confirmed that the complete necrosis rate of RFA was only 52%−71%, significantly lower than the 95% rate for small HCC. Additionally, the local recurrence rate (4.2%−26.0%) and the incidence of serious complications (1.6%−5.8%) were significantly higher for large HCCs (69). Therefore, surgical intervention remains the preferred treatment for large HCCs. To address this challenge, interventional practitioners have progressively improved their arsenal of techniques, perioperative protocols, and surgical approaches, developing a range of effective strategies for the localized treatment of large HCCs. These strategies include the use of dual-electrode stepwise ablation, precise control of the ablation range guided by 3D imaging, and employing TACE as a preliminary measure before ablation (70).

MWA

MWA and RFA are based on similar principles and yield comparable effects. MWA involves inserting an electrode into the tumor using various interventional techniques, such as percutaneous, laparoscopic, or intraoperative methods, and is guided by imaging technologies like ultrasound, CT, or MRI. By applying an electromagnetic field at frequencies of either 915 MHz or 2.45 GHz, MWA causes polar molecules, such as water, to rotate rapidly. This rotation generates high temperatures exceeding 100 °C in a short period. As a result, cells near the electrode undergo protein denaturation and phospholipid bilayer rupture, leading to cell death (58,61,71,72). The increased temperature also obstructs blood flow, leading to blood stasis, thrombosis, and heat retention, which disrupts the blood supply to the tumor and ultimately causes ischemic necrosis of the tumor tissue (64). A retrospective study involving 1,289 patients from 12 hospitals compared the efficacy of MWA and laparoscopic liver resection (LLR) for solitary HCCs measuring 3−5 cm. The study found no difference in OS between MWA and LLR (73). Technically, MWA should be more effective than RFA, but according to the outcomes of the only two randomized controlled trials [NCT01340105 (74), ISRCTN73194360 (75)] comparing MWA to RFA, no significant differences in efficacy and safety were observed. Based on current evidence, MWA is prioritized over RFA for treating HCCs measuring 3−5 cm, although phase III research data are still lacking. For tumors larger than 5 cm in diameter, MWA is considered part of palliative or combined treatment only. However, with recent improvements in ablation equipment and imaging guidance technologies, there is increased interest in using MWA for tumors larger than 5 cm, with promising results. Medhat et al. (76) treated 26 patients with HCC lesions measuring 5−7 cm with MWA, achieving complete ablation in 73.1% (19/26) of cases and reporting a mean survival time of 21.5 months, with 1- and 1.5-year OS rates of 88.8% and 80.0%, respectively. Nevertheless, technical factors may introduce bias into clinical trial results. MWA’s image guidance primarily relies on ultrasound or CT, and the procedure’s success is heavily dependent on the operator’s experience. Common outcomes of empirically guided ablation using a single image modality include either incomplete tumor coverage or excessive ablation range, which can harm adjacent healthy tissues. The recent advent of 3D visualization technology offers a scientific, objective, quantifiable, and precise method for clinical applications, enhancing the role of MWA in HCC treatment (77,78). Moreover, due to its unique physical properties, novel materials such as thermosensitive hydrogels and liposome carriers (79) have been developed to effectively reduce adverse reactions and thermal damage caused by MWA. With ongoing advancements in medical imaging and microwave technology, MWA has the potential to become the preferred treatment for liver cancer in the future.

CRA

CRA mainly employs low-temperature gases or liquids as the cooling medium. This medium absorbs heat from the surroundings and rapidly reduces the local temperature to below −140 °C, effectively killing tumor cells. In recent years, CRA technology has emerged as a prevalent treatment for various cancers due to its minimally invasive nature, low levels of patient discomfort, and specific targeting. CRA is commonly used for cancers such as lung and breast cancer (80,81). For HCC, the technique used is targeted CRA therapy. However, the precise mechanism by which CRA eradicates tumors remains not fully understood. It is widely accepted that CRA’s extreme temperatures result in mechanical damage to cells. The dehydration of cells during freezing, coupled with a high concentration of solutes, leads to protein damage, destruction of cell membranes and cytoplasmic enzymes, and ultimately therapeutic effects. Additionally, the formation of ice crystals within the cells can damage organelles and the cell membrane (82). After freezing, the thawing of extracellular ice crystals can create a hypotonic environment in the cell, causing an influx of solvent into the previously dehydrated cells, which then re-swell and rupture, releasing their contents. Cells at the outermost periphery of the ablation zone that are exposed to sublethal temperatures undergo apoptosis, releasing apoptotic bodies that are cleared by macrophages or dendritic cells (DCs) (83). Should apoptotic cells not be promptly and effectively removed, they may undergo secondary necrosis due to loss of membrane integrity. This results in the release of damage-associated molecular patterns (DAMPs). The immune response elicited by CRA has been substantiated by some preclinical animal models and clinical cases (84,85). The tumor antigens released from necrotic tumor cells have a distinct immune effect, provoking the body to mount an immune response and formulating a subsequent antitumor effect from cryotherapy. It is reported that CRA can boost the antigen-presenting capacity of DCs and stimulate them to secrete cytokines such as interleukin (IL)-4 and IL-12, thus encouraging the proliferation and activation of T and B lymphocytes, and prompting the body’s immune system to exert antitumor effects (82,86). However, like thermal ablation, CRA does not induce such effects on the marginal cells, which fail to mature DCs and might even activate immune suppression signals, leading to T lymphocyte inactivation. Some researchers argue that the scope and duration of this immune response are confined, and that various immune suppression mechanisms enable tumor cells to evade the host’s immune attack. The antitumor immune response induced by CRA is insufficient to prevent tumor recurrence or reduce distant metastases (87,88). CRA, as a potential alternative to thermal ablation, is the subject of many ongoing large-scale clinical trials. A randomized controlled study involving 360 patients demonstrated that in HCC patients with tumors <4 cm, there were no significant differences in 1-year, 3-year, and 5-year OS rates and tumor-free survival rates between the CRA and RFA groups. However, for lesions >3 cm in diameter, the local tumor progression rate in the CRA group was significantly lower than that in the RFA group (89). In terms of safety, CRA may have potential advantages. Glazer et al. (90) reported that in the treatment of liver cancer patients with tumors ≥4 cm and <4 cm in diameter, the incidence of grade 3 or higher adverse events was 19.5% and 8.7%, respectively (P=0.04). While most patients with early-stage liver cancer are suitable for CRA, factors such as tumor size, location, and the skill level of the operator significantly influence the safety and effectiveness of the treatment.

Other ablative therapies

Conventional ablation therapies such as RFA, MWA, and CRA can cause indiscriminate and extensive damage to surrounding tissues due to the extreme temperatures generated during the ablation process. They are particularly prone to damaging critical structures such as the ductal system and nerves within the ablation zone. This limitation restricts the use of ablation therapy in HCC located near blood vessels and/or bile ducts. However, with technological advancements, new and improved ablation methods have been developed in recent years to address these issues (91). Irreversible electroporation (IRE) is a non-thermal ablation therapy that creates nanoscale irreversible pores in cell membranes using steep pulsed direct currents between electrodes. This technology is also referred to as a “nano-knife”. Unlike thermal ablation, IRE does not destroy tumor cells with high temperatures but instead uses high-voltage pulses to induce apoptosis in cells between electrodes while preserving connective tissues, elastic fibers, and collagen fibers. The cellular debris from apoptosis is then phagocytosed by phagocytes (92,93). IRE for the treatment of HCC is still in an exploratory phase, as it requires patients to have robust cardiopulmonary function, and the procedure is more complex. Currently, its use is limited to areas of liver cancer near blood vessels where traditional thermal ablation is not feasible, and it is applicable to cases with multiple tumor lesions (92,94-96). HIFU is not a new treatment method, but its application in HCC treatment has not become widespread (97). Studies have indicated that for HCC lesions smaller than 3 cm, HIFU is an effective alternative to RFA (98). A retrospective study (99) comparing the efficacy of HIFU and RFA showed that the 1-year, 2-year, and 3-year OS rates for the HIFU group were 96.3%, 81.5%, and 69.8%, respectively, while those for the RFA group were 92.1%, 76.1%, and 64.2% (P=0.19). However, HIFU’s effectiveness is limited for larger HCC lesions greater than 3 cm.

Effect and molecular mechanism of locoregional therapies on TME in HCC

TME is a complex network that surrounds the tumor, consisting of immune cells, extracellular matrix, blood vessels, and other cellular components. It represents a dynamic interaction between the tumor and its surroundings, exerting both favorable and hindering effects on tumor growth, progression, metastasis, and drug resistance. The TME’s formation and function are governed by complex reciprocal communication between malignant and non-malignant cells, involving chemokines, cytokines, growth factors, matrix remodeling enzymes, inflammatory mediators, and others. Extensive research has highlighted the crucial role of the TME in cancer progression, with various cancer types having similar microenvironments (100-103). The TME is a primary target for cancer immunotherapy, which has seen significant advancements in recent years, supported by extensive basic research and clinical trials. However, the impact of various locoregional therapies on the TME remains a complex issue, involving numerous signaling pathways and mechanisms. Unlike traditional treatment methods, locoregional therapy induces specific physical and chemical changes within the TME. Further research is needed to determine whether these changes are beneficial for targeted therapy and immunotherapy.

Mechanism of TACE on TME in HCC

TACE has been shown to influence tumor angiogenesis, interstitial fluid pressure (IFP), and immune function within the TME (Figure 1). The primary consequence of TACE is aggravated hypoxia in the TME postoperatively, leading to local metabolic disorders. This results in an increase in HIF-1α, which in turn upregulates cytokines such as vascular endothelial growth factor (VEGF), angiopoietin-2, and b-FGF, facilitating rapid growth of the remaining blood vessels around the tumor (104-106). Previous studies have demonstrated that serum VEGF levels before and after TACE positively correlate with tumor size, number, and/or vascular invasion. The activation of the HIF-1α pathway by TACE not only enhances angiogenesis but also promotes epithelial-mesenchymal transition (EMT), contributing to tumor metastasis (107). A retrospective analysis by Lin et al. (108) indicated that changes in serum HIF-1α have prognostic value for predicting treatment response, PFS, and OS in HCC patients after TACE. Hence, anti-angiogenic targeted drugs may be an effective countermeasure against the long-term severe hypoxia of the TME induced by TACE. Understanding the immune microenvironment of HCC after interventional therapy is crucial for uncovering resistance mechanisms. Present research mainly focuses on T lymphocytes and neutrophils. Results from Pinato et al. (109) showed that TACE significantly reduces peripheral CD8+ T cells (CD8+/PD-1+) involved in the inhibitory immune pathway, effectively improving immunotherapy resistance. Concurrently, TACE also modulates the immunosuppressive regulatory T cells (Tregs) in peripheral blood. Ren et al. (110) studied 33 HCC patients who underwent gelatin sponge microparticles (GSMs) TACE treatment and observed a decrease in the proportion of Tregs in peripheral blood from 11.74%±1.67% before surgery to 7.59%±1.27% after 3−5 weeks after TACE. However, the effects of TACE on different Treg subtypes require further investigation (111). The inflammatory environment after TACE is another significant factor that negatively influences the TME (112). The increased levels of tumor-associated neutrophils (TAN) (113) and tumor-associated macrophages (TAMs) (114) may suppress normal immune cells, fostering tumor growth and metastasis. Whether anti-inflammatory drugs should be combined is still under debate. Nevertheless, the multifaceted physiological impact of TACE on the TME is undeniable. This encourages the exploration of potential synergies with appropriate medications to expand its applications and improve patient outcomes.

Figure 1.

Effect of TACE on the TME: After TACE, HCC locally forms a hypoxic and inflammatory environment, causing HIF-1α to increase. Subsequently, elevated VEGF, b-FGF, angiopoientin-2 stimulate the vascular endothelial cells around the tumor tissue, leading to angiogenesis. Antigens and various pro-inflammatory factors change the immune microenvironment, PD-1+ CD8+ T cells, Tregs decrease, TANs, MDSCs, DCs and TAMs increase. TACE, transcatheter arterial chemoembolization; TME, tumor microenvironment; HCC, hepatocellular carcinoma; HIF-1α, hypoxia inducible factor-1α; VEGF, vascular endothelial growth factor; b-FGF, basic fibroblast growth factor; PD-1, programmed death-1; TAN, tumor associated neutrophils; TAM, tumor associated macrophage; Treg, regulatory T cell; MDSC, myeloid-derived suppressor cell; DC, dendritic cell.

Mechanism of TARE on TME in HCC

TARE impacts the TME in HCC similarly to TACE, particularly regarding tumor angiogenesis. However, numerous studies suggest that TARE affects HIF-1α and VEGF less than TACE does, as evidenced by the lower incidence of post-embolization syndrome associated with TARE (115). This might be attributed to the unique biological characteristics of the radiopharmaceutical used (116). Furthermore, TARE plays a significant role in modulating the immune system, with different radiation doses potentially having diametrically opposed effects on immune cells. Prior research indicates that an appropriate radiation dose can reshape the immune microenvironment, enhance the effect of immunotherapy, and exert an immunostimulatory role (117). Given the heterogeneity of HCC’s TME, there has been a shortage of systematic and comprehensive research on the immune alterations induced by radiation. Chew et al. (118) provided a comprehensive view of the local and systemic immune effects after Y-90 treatment in their multi-omics analysis, revealing the molecular mechanisms behind Y-90-induced sustained responses. The study included 41 HCC patients who underwent Y-90 treatment followed by surgical resection. Mass spectrometry flow analysis was utilized to examine the tumor-infiltrating lymphocytes (TILs) and peripheral blood mononuclear cells (PBMCs) of these patients. The results highlighted a significant increase in immune-activated cell subsets within the tumors after Y-90 treatment, as opposed to the control group, which showed a higher presence of regulatory T cells (Tregs) within the tumors. This research underscores Y-90’s potential to remodel the immune microenvironment of HCC, stimulate tumor cell apoptosis, and enhance the recruitment and activation of T cells.

Mechanism of ablation on TME in HCC

Studies have demonstrated that following RFA, three zones can be identified around the tumor’s center: the high-temperature central zone (>60 °C), the sublethal thermal therapy transition zone (43−50 °C), and the surrounding tissue area unaffected by ablation. In the high-temperature central zone, proteins rapidly denature, leading to coagulative necrosis and the release of antigenic degradation products. This creates a source of antigens for anti-tumor immunity, with the inactivated tumor cells acting as “vaccines” to induce the production of cytotoxic T cells. The thermal effect can also alter the spatial structure of antigenic determinants, activating the body’s specific immune response. In the sublethal thermal therapy transition zone, tissue suffers sublethal damage, and tumor cells within the migration zone undergo reversible damage, which could potentially lead to future tumor recurrence (119,120). This specialized TME significantly impacts immune cells. Mizukoshi et al. (121) demonstrated that local inflammation activates antigen-presenting cells in the tumor area. DCs present antigens to initial T cells, resulting in an increase in lymphocytes, primarily composed of CD8+ T cells, with tumor-associated antigen (TAA)-specific T cells constituting most of the increase. In the study by Zerbini et al. (122), co-culturing RFA-treated HCC tissue with APCs significantly enhanced APC maturity. This was evidenced by the RFA group’s co-stimulatory molecules, lymph node homing chemokine receptor, antigen-presenting effectiveness, and cytokine secretion being significantly higher than those in other groups (untreated HCC tissue and non-tumor liver tissue). Following RFA treatment, myeloid dendritic cells (MDCs) in HCC patients showed trends toward mature expression. Human leukocyte antigen-DR (HLA-DR) expression was significantly higher on the seventh day, with CD83, CD86, and CD40 expressions significantly higher on the fourteenth day. This increased the ability of MDCs to stimulate CD4+ T cells (123-125). Thus, RFA not only increases the number of DC cells but also promotes their maturation and enhances their presentation function. Kitahara et al. (126) attempted to extract DC cells from HCC patients, stimulate them with OK432 in vitro, and inject them back into the tumor after RFA. The results showed that the stimulated DC cells led to an increased TAA-specific T cell response. Patients in the RFA + OK432-stimulated DC cell group had a longer PFS than those in the control group (median: 24.8 months vs. 13.0 months; P=0.003). RFA has also been shown to enhance NK cell activity, effectively stimulating the anti-tumor effect of immune cells in vivo (127). Zerbini et al.’s study (128) indicated that the proportion of NK cells in the peripheral blood of HCC patients [20 cases, with 70% being hepatitis C virus (HCV)-HCC] significantly increased one month after RFA. Several studies have found that the numbers of CD3+, CD4+, CD8+ T cells, and the CD4+/CD8+ ratio generally trend upward after RFA (125,129). However, RFA does not immediately trigger a local and systemic immune response in the short term; rather, it sets off a progressive cascade reaction in the systemic immune environment, likely induced by local effects. Post-RFA treatment activates the secretion of growth factors and cytokines, ultimately causing inflammatory stress. Immunosuppressive cytokines [IL-10 and transforming growth factor (TGF)-β], typically secreted in the TME of HCC, appear to decline after RFA (130,131). Conversely, the level of interferon (IFN)-γ, an important pro-inflammatory molecule that stimulates anti-tumor immunity, is higher after RFA (132). However, the duration of this effect and whether it can support immunotherapy still require further evidence.

After RFA, a distinct TME may develop in the surrounding area. Treatment of HCC with tumors larger than 2 cm in diameter by RFA has been associated with higher rates of tumor residue and recurrence (124,133). The prevailing belief is that the main cause of tumor recurrence is thermal stimulation from increased tumor pressure, which promotes the spread of tumor cells to adjacent cancerous tissue. Additionally, RFA induces an inflammatory response and tissue regeneration in the adjacent tissue, which increases the release of pro-tumor cell factors and growth factors. This phenomenon is often referred to as residual tumor due to incomplete RFA (iRFA) (125,134,135). The proliferative and invasive abilities of residual tumor cells are enhanced after iRFA surgery, possibly as a self-protection mechanism of the tumor cells. Similarly to TACE, RFA postoperative conditions can also lead to the formation of a local inflammatory and hypoxic microenvironment. A portion of hypoxia-inducible factors (HIFs) may promote the EMT of local tumor tissue, and elevated HIF-1α can stimulate angiogenesis through the HIF-1α/VEGF signaling pathway, thereby facilitating tumor progression (136,137). With advancements in genomics, attention has been drawn to the immune suppressive cells and signaling pathways that may play a significant role in iRFA. Shi et al. (134) reported that after collecting post-RFA residual HCC, RNA sequencing analysis indicated an increase in the expression of genes related to the inflammatory response, immune response, immune system processing, leukocyte chemotaxis and migration, hypoxia, chemokine receptor 2 (CCR2) chemokine receptor binding, response to injury, and encoding of DAMP molecules. The expression of pro-inflammatory cytokines, chemokines, and immune suppression-related genes in the residual cancer also increased, suggesting that the inflammatory response may play a role in post-iRFA residual tumor progression. Additionally, Shi et al. (125) found that, using RNA-seq and proteomics, PRTN3 expression in post-treatment Kupffer cells facilitated sublethal heat stress-induced stimulation of HCC cell proliferation, migration, and invasion. Moreover, various immune cells such as Tregs, myeloid-derived suppressor cells (MDSCs), TAMs, and others have been implicated in this process (138). The progression of residual tumors after RFA is a critical factor affecting the long-term efficacy of HCC treatment (Figure 2). As such, it is essential to take appropriate measures to prevent this progression. The combination of RFA with other therapies is currently being extensively investigated as a treatment strategy.

Figure 2.

Effect of ablation on the TME: In the early stage after ablation, the local antigens of the tumor stimulate T cell proliferation, and some cytokines recruit NK cells, releasing IFN-γ and TNF-α. Radiofrequency ablation and microwave ablation also form a hypoxic environment after the operation, and the elevated HIF-1α and VEGF promote angiogenesis and EMT, and proinflammatory factors increase some immune suppressive cells such as TAMs, Tregs, MDSCs. While cryoablation will form ice crystals in the cells, promote cell apoptosis and release damage associated molecular patterns, DAMPs, recruit DCs and NK cells. And local freezing will cause vasoconstriction, resulting in local microthrombosis and local hypoxia. TME, tumor microenvironment; NK, natural killer; IFN-γ, Interferon-γ; TNF-α, tumor necrosis factor-α; HIF-1α, hypoxia inducible factor-1α; VEGF, vascular endothelial growth factor; TAM, tumor associated macrophages; Treg, regulatory T cell; MDSC, myeloid derived suppressor cell; DAMP, damage-associated molecular pattern; DC, dendritic cell; MHC, major histocompatibility complex; EMT, epithelial-mesenchymal transition.

Locoregional therapies combined with other therapies in HCC

TACE combined with immunotherapy

Given the unique impact of several locoregional therapies on the TME, the strategy of integrating them with immunotherapy is mechanistically feasible (139). Current research on immunotherapy for HCC is extensive, particularly in the area of immune checkpoint inhibitors like PD-(L)1 and cytotoxic T-lymphocyte antigen 4 (CTLA-4) inhibitors (140). The significance of immunotherapy in HCC management is now widely acknowledged. Nivolumab was the first immunotherapy drug approved by the FDA for HCC, and pembrolizumab has been authorized for use as a second-line treatment for HCC unresponsive to sorafenib (141,142). Other immunotherapeutic agents, such as camrelizumab, sintilimab, toripalimab, durvalumab, tremelimumab, ipilimumab, and atezolizumab, have successively been included in clinical guidelines. As research into immunotherapy deepens, challenges such as drug resistance and limited indications are becoming apparent. In current studies, the overall effective response rate of single-agent immunotherapy ranges from 17% to 20% (143). Consequently, it is vital to enhance local or systemic immune functions with combined treatments, including locoregional therapies and targeted drugs, to ensure safety and achieve a synergistic effect.

Few randomized controlled clinical trials on TACE combined with immunotherapy have concluded. However, the results indicate that TACE may augment the efficacy of immunotherapy drugs. In Qiao et al.’s single-arm phase II clinical trial, 36 HCC patients underwent TACE combined with ablation therapy followed by anti-PD1 treatment. The study revealed a 1-year RFS rate of 73.3% in the anti-PD1 treatment group, compared to 46.7% in the control group (144), suggesting that anti-PD1 treatment can effectively improve RFS after TACE. The HIMALAYA phase III trial (145) demonstrated that the dual immunotherapy regimen of PD-L1 (durvalumab) plus CTLA-4 (tremelimumab) provided a clinically meaningful OS advantage over sorafenib in the first-line treatment of patients with unresectable HCC. On December 9, 2022, the US National Comprehensive Cancer Network (NCCN) guidelines were updated, adding the dual immunotherapy regimen (durvalumab plus tremelimumab) as a category I recommendation for advanced HCC. Moreover, the combination therapy of dual immunotherapy with vascular interventions is under active investigation. The American Society of Clinical Oncology (ASCO) 2022 meeting reported the first combination of dual immunotherapy with TACE for recurrent HCC patients, involving 13 patients. Of these, 2 achieved partial remission at 8 weeks, 7 had stable disease, and 1 experienced disease progression, showing preliminary efficacy. Retrospective studies also support the efficacy of TACE combined with immunotherapy; for example, a study by Marinelli et al. (146) indicated that multimodal immunotherapy with TACE led to longer OS than ICI monotherapy, with a mOS of 35.1 (16.1− not evaluable) vs. 16.6 (15.7−32.6) months. The benefits of combining TACE with immunotherapy are increasingly recognized, but further validation through extensive clinical trials and basic research is necessary. Moreover, as HCC treatment paradigms shift, the focus is now on combination therapy. The specifics of combination therapy, including the dosing of immunotherapy drugs, and the timing for locoregional therapies or immunotherapy interventions, are still being explored. Determining how to maximize patient benefits from these treatments will require ongoing verification from relevant trials (8).

TACE combined with targeted therapy

Targeted therapy is a treatment method that targets cancer cells at the molecular level. In recent years, there have been significant breakthroughs in the diagnosis and treatment of HCC. The development of new biomarkers and detection technologies has effectively improved the efficiency of early diagnosis of HCC. Additionally, the application of targeted and immunotherapeutic drugs has provided more options for HCC treatment. Multi-omics technologies are helping researchers gain a deeper understanding of the mechanisms underlying the occurrence and development of HCC. Precision and personalized treatment methods have significantly extended the survival time of HCC patients (147). However, due to the heterogeneity of HCC and the complexity of the signaling pathways regulating liver cancer, it remains one of the malignancies with the poorest prognosis (148). Despite extensive scientific research, challenges persist, including low ORRs, short PFS, short OS, and insufficient benefits for patients (149). As mentioned earlier, vascular interventional therapy creates a unique hypoxic and pro-angiogenic TME in HCC. Targeted therapy can precisely target this environment for effective synergistic treatment. The therapeutic effect of TACE also reduces the dosage of targeted drugs, thereby decreasing drug side effects, prolonging OS, and improving patient prognosis (150) (Table 1). Additionally, targeted drugs can more effectively inhibit vascular growth at tumor sites, making tumor inactivation more thorough and further reducing the likelihood of tumor recurrence and metastasis (163).

Table 1. Randomized controlled study of TACE combined with targeted therapy.

Experimental arms	Stage	Endpoints and outcomes	Ref.
TACE, transcatheter arterial chemoembolization; DEB-TACE, drug-eluting Beads TACE; RT, external beam radiotherapy; BCLC, Barcelona Clinic Liver Cancer; uHCC, unresectable hepatocellular carcinoma; MVI, microvascular invasion; PVTT, portal vein tumor thrombus; TTP, time-to-progression; OS, overall survival; PFS, progression-free survival; ORR, objective response rate; HR, hazard ratio; 95% CI, 95% confidence interval.
DEB-TACE+sorafenib vs. DEB-TACE	BCLC-B/C, uHCC	TTP: 169 vs. 166 d (HR=0.797, P=0.072)	(151)
DEB-TACE+sorafenib vs. DEB-TACE	BCLC-B/C, uHCC	PFS: 238.0 d (95% CI, 221.0−281.0) vs. 235.0 d (95% CI, 209.0−322.0)	(152)
TACE+RT+sorafenib vs. sorafenib	HCC with MVI	PFS: 86.7% vs. 34.3% (P<0.001)	(153)
TARE+sorafenib vs. sorafenib	uHCC	OS: 12.1  vs. 11.4 months (HR=1.01; 95% CI, 0.81−1.25; P=0.9529)	(154)
TACE+sorafenib vs. sorafenib	uHCC	PFS: 25.2 vs. 13.5 months (P=0.006)	(155)
TACE+lenvatinib vs. TACE+sorafenib	HCC with PVTT	TTP: 4.7 vs. 3.1 months (HR=0.55; 95% CI, 0.32−0.95; P=0.029)	(156)
TACE+lenvatinib vs. lenvatinib	advanced HCC	OS: 17.8 vs. 11.5 months (HR=0.45; P<0.001)	(21)
TACE+bevacizumab vs. TACE	uHCC	PFS: At 16 weeks, 0.79 vs. 0.19 (P=0.021)	(157)
TACE+sorafenib vs. TACE	BCLC-B, HCC	TTP: 9.2 vs. 4.9 months (HR=2.5; 95% CI, 1.66−7.56; P<0.001)	(158)
TACE+brivanib vs. TACE	uHCC	OS: 26.4 vs. 26.1 months (HR=0.9; 95% CI, 0.66−1.23; P=0.528)	(159)
TACE+bevacizumab vs. TACE	BCLC-A/B, uHCC	OS: 5.3 vs. 13.7 months (HR=1.7; 95% CI, 0.8−3.6; P=0.195)	(160)
TACE+apatinib vs. TACE	uHCC	mPFS: 12.5 vs. 6 months (P<0.05)	(161)
TACE+orantinib vs. TACE	uHCC	OS: 31.1 vs. 32.3 months (HR=1.09; 95% CI, 0.878−1.352; P=0.435)	(162)
Yttrium-90 radioembolization + sorafenib vs. sorafenib	Not eligible for TACE	OS: 14.0 vs. 11.1 months (HR=0.86; P=0.2515)	(154)

Tyrosine kinase inhibitors (TKIs)

TKIs primarily target multiple receptor tyrosine kinases in tumor cells, including vascular endothelial growth factor receptor (VEGFR), epidermal growth factor receptor (EGFR), platelet-derived growth factor receptor, and fibroblast growth factor receptor. By doing so, they block the proliferation signals of tumor cells, inhibit tumor angiogenesis, and suppress tumor growth. Sorafenib, an oral multikinase inhibitor, controls tumor growth both directly, by targeting the RAF/MEK/ERK pathway, and indirectly, by inhibiting VEGFR and platelet-derived growth factor receptors, which suppresses the formation of new blood vessels in malignant tumors (164,165). It was the first targeted drug approved for use in HCC and remains a primary treatment for advanced HCC. However, the efficacy of sorafenib is often compromised by drug resistance due to the heterogeneity of HCC in patients. The application of sorafenib in treating unresectable HCC is theoretically sound (166). TACE can exacerbate hypoxia in the liver, leading to an upregulation of VEGFR and IGFR-2, which can promote disease progression after TACE. Several single-arm studies have confirmed that combining TACE with sorafenib is safe, effective, and feasible for treating unresectable HCC patients (167). However, early results from three prospective trials — the Japan-Korea Post-TACE (168), SPACE (151), and TACE2 (152) — did not demonstrate benefits of combining sorafenib with TACE in patients with intermediate-stage HCC. Yet, as clinical trials have expanded, the effectiveness of this combined approach has gained recognition (169). A randomized controlled trial by Kudo et al. (155) found that the median PFS (mPFS) for the group receiving both TACE and sorafenib was significantly longer than for the TACE-alone group (25.2 months, n=80 vs. 13.5 months, n=76; P=0.006). While a substantial body of research suggests that it is premature to routinely combine systemic sorafenib with TACE in clinical practice, and the strategy, efficacy evaluation criteria, and endpoint settings for such a combination require further investigation (169). Nevertheless, sorafenib, as one of the most frequently used targeted drugs for HCC, has gathered considerable clinical experience in post-TACE treatments. Lenvatinib, another TKI, also shows synergistic effects when combined with interventional therapies. Compared to other TKIs, lenvatinib is less detrimental to liver function and can prevent further deterioration after interventional therapy (170). A multicenter phase III clinical trial in 2021 (21) compared the efficacy of lenvatinib monotherapy with the combination of lenvatinib and TACE in treating advanced HCC patients. The trial enrolled a total of 338 patients. The study revealed that the combination of lenvatinib and TACE significantly improved the mOS of advanced HCC patients, with the combined treatment group experiencing an OS nearly 6 months longer than the monotherapy group. The mPFS in the combined treatment group was also about 4 months longer than in the monotherapy group. The ORR for the combined treatment group was 54.1%, nearly double that of the monotherapy group. Thus, TACE combined with lenvatinib presents a promising new treatment option for HCC patients. With encouraging results from phase III clinical trials of sorafenib or lenvatinib combined with TACE for advanced HCC, the integration of TKIs with TACE is attracting increasing attention, and many clinical trials are ongoing. As for donafenib, the latest first-line drug for unresectable HCC, large-scale clinical trials have yet to confirm its safety and efficacy when combined with TACE (171). Therefore, further clinical trials are necessary to verify whether donafenib can synergistically enhance the efficacy of TACE in HCC management. This innovative approach also provides new avenues for exploration in clinical research.

Anti-VEGF

Anti-VEGF drugs are an effective way to counteract angiogenesis and are highly selective. Their affinity for VEGFR-2 is significantly higher than that of conventional TKIs like sorafenib. Apatinib and bevacizumab are representative VEGF inhibitors (172). Studies have shown that apatinib’s IC50 for the VEGFR-2 receptor is only 1−2 nmol/L, while that of sorafenib is approximately 90 nmol/L, which is about 45 times higher, indicating apatinib’s substantially stronger inhibitory effect on VEGFR-2 compared to other targeted drugs (173). However, monotherapy with anti-VEGF drugs combined with TACE lacks reliable clinical evidence for its efficacy and safety, and some controversy exists. A phase II randomized controlled clinical trial investigated the efficacy of intravenous bevacizumab or placebo following TACE for HCC (160). This regimen did not lead to improved radiological tumor response or OS in HCC patients, and severe or even fatal sepsis and vascular adverse reactions occurred. Hence, bevacizumab monotherapy is not recommended as an adjuvant therapy for TACE. Given the potential safety issues, the rationale for combining anti-VEGF drugs with TACE must be carefully considered. Due to their exceptional selectivity, anti-VEGF medications may cause significant changes in tumor blood vessels and trigger various abnormal compensatory reactions or even resistance if not properly dosed. These adverse effects can worsen tumor progression and compromise the efficacy of TACE. Consequently, current research on anti-VEGF drugs is focusing on combining them with other targeted drugs and optimizing dose management to reduce these risks (172,174).

TACE combined with targeted therapy and immunotherapy

In recent years, the efficacy of TACE combined with targeted therapy and immunotherapy has shown to be superior to that of TACE alone or targeted therapy or immunotherapy alone in clinical trials (Table 2). HCC is a systemic disease, and local treatment alone can only alleviate the condition to a certain extent; therefore, the importance of systemic treatment is self-evident. From both the pathophysiological mechanisms and clinical data, we know that, for unresectable HCC, the efficacy of mono-targeted therapy or immunotherapy is limited, with response rates of less than 20%. Additionally, patients with late-stage HCC often have a poor physical condition, and extensive drug treatment can easily lead to serious side effects and drug resistance (6,186,187). Locoregional therapies create favorable conditions for exploring new systemic treatment methods. As mentioned earlier, from the perspective of the TME, TACE can cause tumor cell necrosis and release tumor antigens, which enhances the anti-tumor immune response and also forms a local pro-vascular environment, improving the efficacy of targeted drugs. After the administration of targeted drugs, the local tumor blood vessels tend to normalize, which promotes immune cell infiltration in the local area and transforms the immune-suppressive “cold tumor” into an immunogenic “hot tumor”. This enhances the response to immunotherapy. Therefore, combining TACE with targeted therapy and immunotherapy may have synergistic effects, and this combination might show a “1+1+1>3” effect, thus achieving better efficacy. A multicenter retrospective study from China (12), which included 376 patients with HCC who received TACE combined with anti-PD-(L)1 therapy and targeted therapy, and 450 patients who received TACE alone, showed that the mPFS of the combination group was 9.5 (95% CI, 8.4−11.0) months, while that of the TACE alone group was 8.0 (95% CI, 6.6−9.5) months. The results indicated that TACE combined with targeted therapy and immunotherapy can significantly improve the PFS, OS, and ORR of patients with intermediate and advanced HCC, providing important evidence and new strategies for the clinical management of intermediate and advanced HCC. However, controversy still exists over the combination therapy regimen, which remains a current focus of research for TACE combined with targeted therapy and immunotherapy. According to the latest CARES-310 study (188), apatinib combined with camrelizumab may be an ideal treatment regimen. In this study, the mOS of the combination group was 22.1 (95% CI, 19.1−27.2) months, while that of the sorafenib monotherapy group was 15.2 (95% CI, 13.0−18.5) months. This represents one of the best clinical trial outcomes for HCC patients in recent years. Concurrently, several clinical trials of apatinib and camrelizumab combined with TACE are underway. Despite the significant progress made in numerous studies, there are still many contentious issues within the realm of locoregional therapies that require further investigation. These issues include determining the optimal timing for integrating other treatment approaches after locoregional therapies, accurately predicting systemic efficacy, identifying suitable patient populations for different treatment combinations, and mitigating the adverse effects and drug resistance associated with systemic treatments, among others (189). Nevertheless, the integration of locoregional therapies with targeted therapy and immunotherapy remains a primary focus for future research.

Table 2. Retrospective study of TACE combined with immunotherapy.

Experimental arms	Stage	Endpoints and outcomes	Ref.
TACE, transcatheter arterial chemoembolization; SBRT, stereotactic body radiationtherapy; PD-(L)1, programmed death-(ligand)1; MTT, molecular targeted therapies; BCLC, Barcelona Clinic Liver Cancer; uHCC, unresectable hepatocellular carcinoma; OS, overall survival; PFS, progression-free survival; ORR, objective response rate.
TACE+camrelizumab vs. TACE	HCC	1-year PFS: 73.3% vs. 46.7% (P=0.027)	(144)
TACE+lenvatinib+sintilimab vs. TACE+lenvatinib	BCLC-C, uHCC	OS: 16.9 vs. 12.1 months (P=0.009); PFS: 7.3 vs. 4.0 months (P=0.002)	(175)
TACE+sorafenib+nivolumab/pembrolizumab vs. TACE+sorafenib	BCLC-B/C, uHCC	PFS: 16.26 vs. 7.30 months (P<0.001); OS: 23.3 vs. 13.8 months (P=0.012)	(176)
TACE+SBRT+toripalimab/sintilimab vs. TACE+toripalimab/sintilimab	BCLC-B	PFS: 19.6 vs. 10.1 months (P<0.001)	(177)
TACE+sorafenib+sintilimab/camrelizumab vs. TACE+sorafenib	BCLC-C	PFS: 7.63 vs. 2.9 months (P=0.0335), OS: 21.63 vs. 16.43 months (P=0.103)	(178)
TACE+lenvatinib+pembrolizumab/toripalimab vs. TACE+lenvatinib	uHCC	ORR: 67.9% vs. 29.6% (P<0.001); PFS: 11.9 vs. 6.9 months (P=0.003)	(179)
TACE+lenvatinib+sintilimab/toripalimab vs. TACE+lenvatinib	BCLC-B	PFS: 22.5 vs. 14.0 months (P<0.001)	(180)
TACE+nivolumab vs. nivolumab	HCC	PFS: 8.8 vs. 3.7 months (P<0.01)	(146)
TACE+sorafenib+camrelizumab vs. TACE+sorafenib	BCLC-C	PFS: 6.9 vs. 3.8 months (P=0.003)	(181)
TACE+lenvatinib+sintilimab/camrelizumab vs. TACE+lenvatinib	BCLC-B/C, uHCC	2 years OS rate: 74.1% vs. 43.4% (P=0.18)	(182)
TACE+apatinib+camrelizumab vs. TACE+apatinib	BCLC-B/C	OS: 25.5 vs. 18.5 months (P<0.01); PFS: 14.0 vs. 5.0 months (P<0.01)	(183)
TACE+lenvatinib+sintilimab/camrelizumab vs. TACE+lenvatinib	BCLC-B/C	PFS: 15.4 vs. 7.8 months (P=0.005); OS: 23.1 vs. 12.5 months (P=0.004)	(184)
TACE+apatinib+sintilimab vs. TACE+apatinib	HCC with MTT	PFS: 6.5 vs. 5.1 months (P<0.001); OS: 16.1 vs. 10.5 months (P<0.001)	(185)
TACE+PD-(L)1+MTT vs. TACE	BCLC-B/C	PFS: 9.5 vs. 8.0 months (P=0.002)	(12)

Ablation combined with targeted therapy and immunotherapy

Ablation therapy not only causes local tumor cell necrosis, producing immunogenic tumor-associated antigens, but also generates a large number of inflammatory cytokines and various immunogenic mediators. This promotes local infiltration and activation of immune cells, stimulating the immune system to produce a certain anti-tumor effect. The use of RFA combined with immune/targeted therapeutic drugs for adjuvant therapy can prevent tumor recurrence and further improve patient survival. In recent years, with the proliferation of targeted drugs on the market and the broadening of indications for ICIs, the combined application of RFA and systemic therapy has gradually evolved. A retrospective study by Feng et al. (190) included 128 patients with early-stage HCC (BCLC-0−B1), of whom 64 patients received both sorafenib and RFA treatment, and 64 patients received RFA alone. The results showed that the 3-year recurrence rate of the combined treatment group was 74.5%, while it was 92.7% for the RFA alone group, demonstrating that sorafenib can reduce the recurrence rate after RFA. However, Bockorny et al. (191) found that the use of sorafenib prior to RFA treatment did not improve the prognosis of HCC patients. This discrepancy may be related to the short duration of sorafenib treatment, insufficient dosage, and individual variation in sensitivity to sorafenib. According to the results of multiple studies, for HCC lesions larger than 3 cm in diameter, the synergistic effect of RFA and TKIs is evident, which can typically reduce the recurrence rate by 10%−20% and significantly improve OS (192-194). RFA can induce tumor-specific T-cell immune responses, but this alone is insufficient to halt the progression of cancer, making combined immunotherapy essential. Previous studies have found that if RFA is incomplete, the residual HCC can up-regulate the expression of PD-1 (195), thereby facilitating the immune escape of HCC. The combined use of RFA and immunotherapy also lacks robust data support; however, the most recent completed retrospective study in China (196) found that the combination of RFA and toripalimab (an anti-PD-1 therapy) extended PFS by approximately 7 months compared to the RFA alone group, with no difference in safety reported. Currently, more combinations of TKIs or ICIs with RFA are still in the clinical trial phase, and more results are expected within the next 1−2 years. Based on the currently available data, there is sufficient reason to believe in the synergistic effect of targeted immunotherapy and thermal ablation.

Combined treatment with multiple locoregional therapies

The combination of multiple locoregional therapies can complement the advantages of different treatment methods. Currently, TACE combined with HAIC or TACE combined with ablation therapy are commonly used in clinical practice. TACE-HAIC takes advantage of the complementary strengths of both, producing a synergistic effect. If a tumor artery has multiple sources of blood supply, TACE is usually used to embolize the non-main blood supply arteries, while HAIC is used to perfuse the main blood supply arteries. If the tumor has an exceptionally rich blood supply, TACE is typically used to embolize part of the blood supply arteries, followed by combined HAIC treatment (197). A propensity score-matching study in China compared the efficacy of TACE-HAIC combined with targeted immunotherapy to TACE alone in treating HCC patients with PVTT (198). The results showed that the overall remission rate of the combined treatment group was significantly higher than that of the TACE alone group (42.1% vs. 5.0%, P<0.001). Although the clinical application of HAIC is less frequent, it may be a potential ideal strategy when combined with other interventional therapies. Mechanistically, combining ablation therapy with TACE can also enhance efficacy. TACE occludes tumor blood vessels, reducing the loss of drugs or heat caused by blood flow, and the deposition of embolic agents provides a clear target for ablation, which is conducive to achieving complete ablation (16,192,199). As mentioned earlier, multiple TACE procedures can cause liver function deterioration and TACE refractoriness, and post-TACE ablation can improve this condition. There are currently two clinical TACE combined with thermal ablation treatment methods. Sequential ablation involves TACE treatment first, followed by ablation treatment within 1−4 weeks after the procedure; synchronous ablation involves ablation treatment at the same time as TACE treatment, which can significantly improve clinical efficacy and reduce liver function damage (59,192). A prospective randomized trial by Peng et al. (200) included 189 patients with primary HCC less than 7 cm in size. The results showed that the 1-year, 3-year, and 4-year OS rates of the TACE-RFA group were 92.6%, 66.6%, and 61.8%, respectively, while the RFA alone group had OS rates of 85.3%, 59%, and 45.0%. The OS and RFS rates of the TACE-RFA group were better than those of the RFA group. MWA combined with TACE also has a similar effect to RFA (201,202). However, at this stage, it is impossible to compare the advantages and disadvantages of the two methods directly. Research on TACE combination with CRA is lacking, which may become one of the hot research directions in the future. In summary, the combination of different locoregional therapies and targeted immunotherapy is the trend for future HCC treatment, gradually challenging the status of surgical operations. Of course, a significant amount of research is still required to develop rational and standardized treatment plans.

Locoregional therapy failure or refractoriness

Although locoregional therapy has become the recommended treatment for intermediate HCC, it is ineffective or even leads to tumor progression after TACE treatment in some patients (149). The Asia-Pacific Primary Liver Cancer Expert (APPLE) group has summarized three situations that are not suitable for TACE (TACE-unsuitability): 1) ineffectiveness of TACE treatment; 2) occurrence of TACE refractoriness; and 3) deterioration of liver function to Child-Pugh B or C level after TACE. “TACE refractoriness” was first defined by the Japan Society of Hepatology (JSH) in 2010 and has been in use after its revision in 2014 (203,204). The definition includes: 1) more than 50% activity in intrahepatic target lesions after two consecutive TACE treatments compared with before the first TACE, or continuous appearance of new intrahepatic lesions; 2) new vascular invasion or extrahepatic metastasis; and 3) continuous rise of postoperative tumor markers (even if there is a transient decline). However, with the widespread implementation of TACE, the understanding of TACE refractoriness by clinical doctors has evolved. In 2022, the Interventional Physicians Branch of the Chinese Medical Association (Chinese College of Interventionalists, CCI) redefined “TACE resistance”: after three or more standardized and meticulous TACE treatments, if the intrahepatic target lesions are still at the PD compared with before the first TACE treatment, as assessed by enhanced CT/MRI examination 1−3 months after treatment and based on the mRECIST standard, it is considered “TACE refractoriness” (205). This standardized definition assists clinicians in making better decisions and prevents unnecessary repeated treatments for patients. The molecular mechanisms behind TACE refractoriness are not fully understood. Current studies suggest that hypoxia induced by TACE remains a primary influencing factor. In a chronic hypoxic environment, TP53 mutation is thought to promote tumor angiogenesis and progression (206). Xue et al. (207) analyzed the gene mutation status of 38 advanced HCC patients who underwent TACE treatment. They found that the frequency of TP53 mutation was the highest (22/38, 57.9%). Additionally, functional enrichment analysis indicated that the mitogen-activated protein kinase signaling pathway and the apoptosis pathway were associated with TACE efficacy, suggesting that TP53 mutation may be an important link in causing TACE refractoriness. These molecular pathways could provide clues for identifying patients likely to benefit from TACE and may also represent potential new therapeutic targets for TACE refractory HCC. Effective countermeasures for TACE refractoriness are still lacking. Some scholars suggest that the embolic agents used might contribute to TACE refractoriness, and patients experiencing cTACE refractoriness might consider trying DEB-TACE (207). If apparent TACE refractoriness occurs, continuing TACE should be avoided to prevent worsening the patient’s condition. For patients unsuitable for TACE treatment, the APPLE consensus recommends first using MTT, followed by selective local treatment. Some retrospective studies suggest sorafenib as the preferred alternative for TACE-resistant patients. Ikeda et al. (208) conducted a retrospective study involving 56 patients with TACE refractoriness, of which 20 switched to sorafenib treatment and 36 continued TACE. The results showed that the median TTP was 22.3 months for the sorafenib group and 7.7 months for the TACE group (P=0.001), with mOS being 25.4 months vs. 11.5 months (P=0.003), respectively. In addition to targeted therapy, alternative treatment options such as radiotherapy, RFA, and hepatic artery infusion chemotherapy are available (209,210). However, sufficient evidence to confirm the effectiveness of these options for TACE refractoriness is still required.

Conclusions and prospects

Swift advancements in molecular biology have dramatically transformed our understanding of tumors. Given the complex nature of HCC and the limitations inherent in various treatment methods, there is a continuous need to discover and embrace new treatment philosophies for HCC. Managing HCC holistically requires a tailored approach that considers multiple aspects. Determining the optimal treatment strategy and achieving the desired outcomes necessitate an unending pursuit of novel insights and breakthroughs. The treatment landscape for hepatobiliary tumors has seen a considerable shift due to the integration of multiple disciplines. The introduction of locoregional treatments, such as TACE and RFA, has challenged the traditional surgical approach for early-stage HCC. Moreover, the advent of targeted therapy and immunotherapy has marked the beginning of a new era for hepatobiliary tumor treatments. The pace of advancements in systemic treatments has altered the conventional approach to advanced HCC, posing new challenges for locoregional procedures. Adapting to the changing landscape of systemic treatments has become essential for healthcare providers. As a result, the focus of HCC treatment has evolved from merely selecting an appropriate treatment option to combining different therapeutic approaches effectively. Although there is a continuous proposal of new combination strategies, there is a lack of studies on the interaction mechanisms and personalized dosing for different treatment modalities. The effectiveness of these combination treatment plans needs to be substantiated by more comprehensive basic and clinical research. However, one point remains clear: locoregional therapy maintains an indispensable and critical role in the treatment of HCC, both in the present and for the foreseeable future.