Challenges in the Treatment of Unresectable HCC:
The therapeutic landscape of advanced hepatocellular carcinoma (HCC) has progressed rapidly in the last decade, from multi-target tyrosine kinase inhibitors (TKIs) such as sorafenib to immunotherapies including adoptive cell therapy, and targeted therapy. However, despite the major advances in our understanding of the molecular biology of cancer cells, and the rapid development in precision oncology, drug resistance remains inevitable with single-agent therapies due to intratumor heterogenicity [
1,
2]. Over the past years, multiple combination therapies have been developed to overcome drug resistance, improve therapeutic efficacy, and minimize drug toxicity by dose reduction. In the 1960s Sidney Farber combined actinomycin D with radiotherapy (RT) in children with Wilm’s tumor, to reduce the dose of actinomycin D required for treatment and the risk of drug toxicity [
3]. Not too long after, Frei et al. combined multiple antileukemic agents to induce and maintain remission in children with acute leukemia [
4]. Since then, the number of potential combination therapies has increased exponentially.
The emergence of immunotherapy has redefined the field of oncology, including HCC. Multiple treatments have been used in combination with immunotherapy for the management of unresectable HCC, which includes the combination of more than one immune-checkpoint inhibitor (ICI) such as Durvalumab plus Tremelimumab, ICIs plus vascular endothelial growth factor (VEGF) inhibitors (Atezolizumab plus Bevacizumab), ICIs plus multi-kinase inhibitors, as well as combining immunotherapy with RT [
5,
6].
To better estimate the prognosis of HCC, The Barcelona Clinic Liver Cancer (BCLC) staging system was developed to stratify patients, predict survival, and guide the management of HCC. It is divided into 5 stages based on liver function, performance status, and tumor burden [
7]. Depending on the disease stage, multiple treatment modalities can be utilized, including surgery (liver transplantation or resection), Locoregional therapy (ablation, trans-arterial chemoembolization or radioembolization), and systemic therapy. Despite the simplicity and prognostic accuracy of the BCLC system, it has certain limitations due to treatment allocation in each stage, which may result in undertreatment [
8]. A multidisciplinary approach incorporating effective tumor biomarkers might offer better stratification of HCC patients and guide them to a more personalized treatment [
9].
The European Association for the Study of the Liver (EASL) provided a new concept termed “treatment stage migration” [
Figure 1]; this concept was applied to patients in each BCLC group who didn’t fulfill all the criteria for treatment allocated to them and it was recommended that patients might receive the subsequent treatment for next advanced stage (left to right). Later this concept was expanded to include bi-directional migration, where patients could receive treatment assigned to an earlier stage in certain situations (right to left) [
10].
Advanced HCC is characterized by significant heterogenicity in tumor burden (TB) and underlying liver dysfunction, which makes prognostic stratification and choice of treatment challenging. Several studies have evaluated TB in HCC, such as the up-to-seven, seven-eleven criteria, and metro-ticket prognostic tool [
11,
12,
13,
14,
15,
16]. These criteria incorporate the tumor size and number in the calculation of the TB and have proven to predict survival in HCC patients undergoing trans-arterial chemoembolization (TACE) and surgical resection.
Some studies found an association between the etiology of HCC and response to therapy; for example, HCV-positive HBV-negative HCC patients showed an improved overall survival (OS) when treated with Sorafenib compared to patients with HBV-positive HCV-negative HCC status [
17]. In contrast, patients with HBV-related HCC showed improved OS vs. HCV-related HCC when treated with Lenvatinib [
18]. Atezolizumab plus Bevacizumab demonstrated OS benefit vs. Sorafenib across patient subgroups according to the IMbrave150 phase III trial, except in patients with non-viral HCC (HR=1.05; 95% CI 0.68-1.63) [
19]. In addition, a meta-analysis conducted to evaluate the survival benefit of immunotherapy in patients with HCC, based on three randomized clinical trials, IMbrave150, KEYNOTE-240, and CheckMate-459, and found that programmed cell death protein-1 (PD-1) and programmed cell death ligand-1 (PDL-1) inhibitors did not improve OS in patients with non-viral HCC compared to viral HCC, particularly in patients with metabolic dysfunction-associated steatotic liver disease (MASLD) vs other etiologies (median OS= 5.4 vs 11 months; P=0.023) [
20].
One of major concerns in combining ICIs with other therapies is the emergence of treatment-related adverse events (TRAEs). A meta-analysis was conducted based on the analysis of 161 clinical trials, to investigate the safety of combining PD-1 and PD-L1 inhibitors with chemotherapy, targeted therapy, and RT. The incidence of grade 3 or higher adverse events was 68·3%, 47·3%, 35·9%, 12·4%, respectively [
21]. 142 trials out of 161 were evaluated for TRAEs and seven trials were included in the RT group; the most common combination therapy with ICIs in this group was SBRT (n=4/7), and cancer types included solid tumors and melanoma. In the RT group, the most common all-grade adverse event was dysphagia, and the most common grade 3 or higher TRAE was lymphopenia; the incidence of grade 3 or higher TRAEs was 14.9% in the stereotactic body radiation therapy (SBRT) subgroup vs 10.8% with other radiotherapies. Nonetheless, it is worth exploring the option of combining therapies to maximize the treatment response of advanced unresectable HCC with a poor prognosis.
HCC Immune Microenvironment:
The liver is the largest solid organ in the body and comprises 4 major cell types: hepatocytes, hepatic stellate cells, Kupffer cells, and sinusoidal endothelial cells [
22]. They are responsible for detoxification, coagulation, and immune response. However, chronic inflammation (e.g. chronic viral hepatitis, non-alcoholic steatohepatitis) disrupts the liver microenvironment and stimulates kupffer cells (resident liver macrophages) to secrete various cytokines and chemokines [
23]. Immune suppressive cytokines such as interleukin-10 (IL-10) and transforming growth factor-beta (TGF-β) promote angiogenesis and immune evasion in HCC [
24,
25].
The linkage between cancer development and inflammation was first suggested by Virchow in 1863 and subsequently validated by others [
26]. Several cytokines play an important role in shaping the tumor microenvironment (TME), Some cytokines induce the recruitment of tumor-associated macrophages (TAMs), and their polarization into the tumor-promoting M2-macrophage phenotype [
27], which in turn recruits CD4(+)CD25(+)FoxP3(+) T-regulatory cells into the TME. Increased regulatory T-cells were found to correlate with impaired CD8(+) T-cell and HCC progression [
28]. Intra-tumoral accumulation of IL-7-producing cells was also associated with angiogenesis and tumor progression [
29]. In addition, activation of the TGF-β-pathway promotes fibrogenesis and immune-cell exhaustion in the TME [
30]. A combination of a TGF-β-inhibitor and ICI induced a complete response in mouse models [
31].
SBRT in HCC-Does It Work?
For many years HCC was considered a radioresistant tumor, due to the inability to deliver more than 30 Gy of conventional external beam radiation therapy (EBRT) to the liver without risk of radiation-induced liver damage. With the advances in the field of radiotherapy, we can now deliver optimized doses in fractionated regimens by IMRT, 3D conformal radiation therapy, proton beam therapy (PBT), or SBRT, while using computed tomography to spare normal liver tissue.
PBT induces DNA damage and cell apoptosis by generating reactive oxygen species [
44]. In addition, it offers a dosimetric advantage including Bragg peak when compared to photon radiotherapy in HCC, thus allowing dose conformity and protecting organs at risk, including healthy liver parenchyma [
45]. A phase II prospective trial was conducted on 76 patients with liver cirrhosis and HCC after receiving PBT and showed PFS of 36 months (95% CI 30-42 months), and a 60% 3-year PFS rate in patients within the Milan criteria [
46]. This was followed by a phase III randomized clinical trial, in which 144 patients with recurrent HCC received either PBT or radiofrequency ablation (RFA) with crossover allowed, resulting in a 2-year local PFS rate of 94.8%
vs. 83.9% respectively (90% CI 1.8–20;
p <0.001) in the per-protocol population, and 2-year local PFS rate of 92.8%
vs. 83.2% (90% CI 0.7–18.4;
p <0.001) in the intension-to-treat population, meeting the non-inferiority criteria [
47].
SBRT is a type of external beam radiation therapy, and it acts by delivering a precise and intense dose of radiation in fewer fractions (hypo-fractionated) while avoiding damage to normal liver tissue [
48,
49]. In the last few years, the role of SBRT has expanded and can be an effective tool in certain indications, for example, it can be used as a neoadjuvant locoregional therapy before liver transplant (LT) when other conventional bridge therapies such as TACE and radiofrequency ablation (RFA) are not applicable [
50]. This was based on a phase II randomized trial that showed a mPFS of 17.6 months (95% CI, 6.6-28.6 months) and an ORR of 62.5% (95% CI, 54.2%-68.7%) measured by RECIST 1.1 in patients who received SBRT before deceased-donor LT [
51,
52]. In addition, SBRT is also used in oligometastatic disease in combination with chemotherapy and immunotherapy. Recently, a phase II study combined PD-1 antibody Sintilimab with SBRT (median dose of 54 Gy in 6 fractions) in oligometastatic (<5 metastatic lesions) HCC, and showed an overall response rate (ORR) of 96% according to the RECIST v1.1criteria [
53]. The efficacy of SBRT in HCC was demonstrated in the RTOG 1112 phase III randomized clinical trial, which compared SBRT followed by Sorafenib vs Sorafenib alone in patients with HCC, median OS was 12.3 months (90% CI 10.6, 14.3) in the Sorafenib group vs 15.8 months (90% CI 11.4-19.2) in the SBRT plus Sorafenib group (HR=0.77, p=0.0554) [
54].
The Rationale behind Combining SBRT and ICIs and the Evidence:
RT can induce inflammatory and immune modulatory effects in the TME. Initially, RT creates a pro-inflammatory state by releasing damage-associated molecular patterns (DAMPs) from damaged cells, activating the innate immune system (e.g., macrophages, natural killer cells, dendritic cells) and releasing pro-inflammatory cytokines [
55]. This is followed by a rebound immunosuppression stage induced by the expression of (PDL-1) and TAMs that suppress T-cells and promote tumor growth [
56].
Several pre-clinical studies have demonstrated the synergistic effect between SBRT and ICIs. One study showed an enhanced response of SBRT in an orthotopic murine model of HCC after delivery of three doses of anti-PD-1 antibody concurrently with 30 Gy SBRT in three fractions, and this resulted in reduced tumor growth rate and improved survival (P<0.05) [
57].
An interesting phenomenon was reported in some studies following treatment with SBRT and immunotherapy, in which tumor regression was observed in lesions outside of the irradiated region, this phenomenon was termed the “abscopal effect” [
58]. Several mechanisms have been postulated, including the release of tumor-associated antigens because of radiation therapy, which is then engulfed by antigen-presenting cells (e.g., dendritic cells), that in turn activates the cytotoxic T-cells and eventually attack the tumor cells outside of the irradiated region [
59].
Multiple clinical trials have proven the efficacy of SBRT plus ICIs combination therapy, including a phase I trial of SBRT followed by Nivolumab plus Ipilimumab vs Nivolumab alone in advanced/unresectable HCC. Despite early closure of the study due to slow accrual, 13 patients were evaluated for the study endpoint which was dose-limiting toxicity (DLT) occurring within 6 months of SBRT and demonstrated acceptable safety with 2 (15.4%) out of 13 subjects experiencing DLTs [
60].
Another study was conducted in a tertiary hospital in Hong Kong, where 226 patients with locally advanced HCC were enrolled [
61]. 210 patients received TACE and 16 patients received SBRT plus immunotherapy, however, the SBRT group had a higher percentage of BCLC stage C and portal vein invasion. After propensity score matching 48 patients with TACE were compared to the 16 patients with SBRT plus immunotherapy. The SBRT dose ranged from 25 to 37.5 Gy and was given in five fractions for 1–2 weeks, followed by Nivolumab 3mg/kg every two weeks for a median of 10 cycles (range: 1-20 doses). Results showed a significant survival benefit in the SBRT plus immunotherapy group vs the TACE group, the 12-month OS was 93.8% vs 31.3% respectively and the 24-month OS was 80.4% vs 8.3%, respectively (p <0.001), the 12-month PFS was 93.3% vs 16.7% respectively, and the 24-month PFS was 77.8% vs 2.1%, respectively (p <0.001).
A single-arm study was conducted to evaluate the response rate and safety of anti-PD1 monoclonal antibody Camrelizumab plus SBRT in unresectable HCC [
62]. Twenty-one patients were enrolled and received five fractions per week of palliative SBRT daily with a dose range of 30-50 Gy, 200 mg of Camrelizumab was administered intravenously from day one of SBRT and every 3 weeks. After a median follow-up period of 19.7 months, the ORR was 52.4% according to RESIST v1.1, and median PFS and OS were 5.8 (95% CI 4.2–7.4) and 14.2 months (95% CI 7.2–21.2), respectively.
As demonstrated in the IMBrave 150 phase III clinical trial Atezolizumab plus Bevacizumab resulted in better survival outcomes when compared to sorafenib in unresectable HCC and is now recommended by the FDA as first-line therapy in unresectable HCC [
6], which encouraged further studies in combining ICIs with anti-angiogenic therapies. A single-arm explorative clinical trial was conducted by combining Toripalimab (PD-1 monoclonal antibody) and Anlotinib (vascular endothelial growth factor receptor inhibitor) following SBRT in unresectable HCC [
63], Twenty patients were enrolled and received a total dose of 24 Gy (8 Gy*3) followed by 6 cycles of Toripalimab plus Anlotinib after last dose of SBRT, median PFS was 7.4 months (range, 1.1-27.7 months), however 70% of patients experienced TRAEs of various grades. This supports the idea that SBRT induces an adaptive immune response outside of the irradiated region. Notably, patients in this study also received a daily dose of probiotics, this suggests that the translocation of the intestinal flora plays a role in the development of HCC and that the regulation of the gut microbiome can enhance the immunologic response of ICIs [
64].
Additionally, a recent retrospective study demonstrated improvement in survival outcomes when adding RT to patients with advanced HCC treated with ICIs and antiangiogenic therapy. A total of 76 patients with advanced HCC were included in the study, 33 patients received intensity-modulated radiation therapy (IMRT) or SBRT in addition to the ICIs and antiangiogenic therapy vs 43 patients only received systemic therapy and did not receive RT. The median PFS was 8.3 months (95% CI, 5.4-11.3) in the RT group vs 4.2 months (95% CI, 3.4-5.0) in the non-RT group (
P < .001), and the median OS was not reached in the RT group vs 9.7 months (95% CI, 4.1-15.3) in the non-RT group (
P = .002). The disease control rate (DCR) was 100% in the RT group vs 75.9% (95% CI, 56.5-89.7) in the non-RT group (
P = .005) [
65] [
Table 2].
Current Guidelines and Future Direction:
The current NCCN guidelines recommend SBRT as an alternative to ablation and/or embolization techniques or when these therapies have failed or are contraindicated (e.g., unresectable disease or patients with local disease but poor performance status that makes them ineligible for surgery) [
48,
66], It is typically administered in a dose of 30-50 Gy in 3-5 fractions. However, there are no definite recommendations regarding the combination of SBRT and ICIs in HCC.
TACE is currently the first-line therapy for unresectable, large HCC with no vascular invasion [
67], however, a study comparing SBRT with TACE for unresectable medium-sized (3-8 cm) HCC showed favorable disease control and OS in the SBRT group vs TACE group, with 3-year infield control of 77.5% vs 55.6% (
P = .007), respectively, and a 3-year OS of 55.0% vs 13.0% (P < .001), respectively [
68].
Recently, there has been growing evidence to support the use of SBRT in unresectable and oligometastatic HCC [
69]. The role of SBRT in oligometastatic HCC has been suggested in multiple retrospective studies [
70,
71]. Further large, randomized, controlled clinical trials are needed to establish the benefit of ICIs plus SBRT in this unique subgroup, to our knowledge only one prospective phase II non-controlled trial evaluated the role of SBRT plus Sintilimab in oligometastatic HCC, 25 patients were enrolled in this study, including 24 patients with HBV and all patients had Child-Pugh class A. Even though the median OS was not reached, the OS rates at 12 and 24 months were 91.5% and 83.2%, respectively and the median PFS was 19.7 (95% CI: 16.9-NA), and the disease-control rate after treatment was 96% [
72]. Notably, this study also explored the biomarkers of response to predict efficacy, lymphocyte counts and their subsets were analyzed by flow cytometry, mPFS of 19.7 months was applied as a cutoff, there was no significant difference between lymphocyte count >19.7 months vs <19.7 months at baseline, however, after three to four cycles, PFS ≥ 19.7 months was associated with higher lymphocyte counts and CD3+, CD4+, CD8+ T, and CD15+CD56+ NK cells when compared to PFS < 19.7 months. It supports the hypothesis that lymphocyte count can act as a predictor of response to SBRT in HCC, which was previously studied in non-small cell lung cancer patients [
73].
Conclusion:
SBRT has significant immunomodulatory effects and can enhance the tumor response of ICIs. Several clinical trials evaluated the safety and efficacy of SBRT and demonstrated tumor response both in the irradiated region and in distant lesions. Further studies are needed to evaluate the efficacy of combining SBRT and ICIs in HCC, establish the optimal sequence of treatment, evaluate their role in oligometastatic HCC, and explore biomarkers that can help in case selection and prediction of response to treatment.
Author Contributions
Prasun K. Jalal MD made substantial contributions to the initial conception of the work, Ahmed Elhariri MD was involved in drafting the work and all other authors revised it critically for important intellectual content. All authors have given final approval of the version that was submitted for publication and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.
Conflicts of Interest
The authors declare no conflicts of interest.
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