Nov
03
Introduction Hepatocellular carcinoma (HCC) represents the most common primary liver cancer and a globally burdensome oncological diagnosis, ranking as the sixth most frequently diagnosed cancer and the third most common cause of cancer death worldwide.1 Unlike other malignancies, both incidence and mortality from HCC are expected to rise by more than 55% in the next few decades.2 HCC usually develops in the context of chronic liver disease, with Hepatitis B virus (HBV) and Hepatitis C virus (HCV) infection and alcohol abuse representing the most prevalent etiologies worldwide. However, the proportion of HCC cases arising in the setting of underlying metabolic dysfunction-associated fatty liver disease (MAFLD) is rapidly increasing, and it is expected that MALFD will become the leading risk factor for HCC in the years to come.3 Improved management of underlying chronic liver disease is likely to reduce the risk of cirrhosis decompensation and the ensuing mortality, leading to an increase in the number of patients with prolonged survival from cirrhosis but still at risk for developing HCC. On the other hand, the increasing prevalence of MAFLD, associated with the lack of effective surveillance strategies in patients with non-cirrhotic MAFLD and with the intrinsic difficulty of ultrasound surveillance in cirrhotic patients, is likely related to the persistently high number of cases diagnosed at advanced stages.4 For these patients, systemic therapy still remains the standard of care. The advent of novel, more effective systemic therapies has markedly revolutionized the prognostic outlook of patients with advanced HCC. Monotherapies with the anti-programmed cell death protein-1 (PD-1) targeting agents nivolumab and pembrolizumab received accelerated approval by the Food and Drug Administration (FDA), based on unexpectedly high radiological response rates observed in uncontrolled early-phase clinical trials.5,6 However, PD-1 monotherapy failed in showing a significant overall survival (OS) benefit in Phase III RCTs and only pembrolizumab was granted full FDA approval.7,8 Primary disease progression on nivolumab monotherapy was reported in 37% of patients,7 suggesting that resistance to this type of therapeutic approach is common. Follow-up studies of tislelizumab and durvalumab, two monoclonal antibodies directed against PD-1 and programmed cell death ligand-1 (PD-L1), respectively, demonstrated the non-inferiority of PD-1/PD-L1 blockade against sorafenib, with meta analyses showing improved therapeutic index over tyrosine kinase inhibitors (TKIs).9 Conversely, it is now known that ICI-based combination regimens provide a significant improvement in OS compared to sorafenib in phase III RCTs, shifting the expected survival of patients with advanced disease from approximately 6 months to up to 20 months.10,11 The combinations of atezolizumab (anti-PD-L1) plus bevacizumab (anti-vascular endothelial growth factor [VEGF]), and durvalumab (anti-PD-L1) plus another ICI (tremelimumab, an anti-Cytotoxic T-Lymphocyte Antigen 4 [CTLA-4]) have obtained full FDA approval for first-line treatment, whereas the combination of nivolumab plus anti-CTLA-4 ipilimumab has been granted accelerated approval for patients who have been previously treated with sorafenib.12 The rationale behind combining ICI with anti-VEGF is that VEGF inhibition may regulate and suppress elements of the tumor immune microenvironment (TiME), including regulatory T cells (T-regs), myeloid-derived suppressor cells (MDSCs), and tumor-associated macrophages (TAM). On the other hand, the combined blockade of CTLA-4 and PD-1 in ICI doublets has a distinct and non-redundant immune-biologic role within the cancer immunity cycle. CTLA-4 inhibition drives immune-suppression in tumor-antigen presenting cells and T-regs, while PD-1/PD-L1 blockade predominantly downregulates the effectiveness of the cytotoxic CD8+ T cell response.13 Unfortunately, combinations of ICI plus TKI, such as atezolizumab plus cabozantinib or pembrolizumab plus lenvatinib, did not statistically improve OS compared to TKI monotherapy in phase III RCTs.14,15 Overall, compared to ICI monotherapies, ICI-based doublets are associated with improved outcomes, and this is probably related to the added benefit of each component. However, the proportion of patients obtaining long-term survival benefit still remains limited and does not exceed 20–30%. This has led to the development of triplet regimens that have been investigated in phase I/II trials (such as nivolumab plus ipilimumab and cabozantinib) or are currently under investigation (such as atezolizumab plus bevacizumab and ipilimumab).16,17 HCC is characterized by high clinical and biological heterogeneity, translating into an extremely variable prognosis.18 The diagnosis of HCC can be performed by using validated imaging criteria, without the need for histological confirmation in patients with cirrhosis.19 This has inevitably led to a relatively limited understanding of the molecular mechanisms associated with prognosis and treatment response compared to other solid tumors. Nevertheless, it is well known that HCC is characterized by a high transcriptional and genetic heterogeneity.20 Research on molecular signatures, which could potentially predict outcomes, led to the identification of two major subclasses of HCC: one characterized by intrinsic cell proliferation, and the other defined as the non-proliferative class. The former is denoted by poor tumor cell differentiation and associated with chromosomal instability, as well as p53 tumor suppressor gene (TP53) mutations. Potentially actionable driver linked to this class is transforming growth factor beta (TGF-beta) signaling. The non-proliferative class is associated with more differentiated HCC, presence of mutations in the Wnt/beta-catenin pathway and with a better prognosis.21 Utilization of high throughput sequencing technologies led to a further deepening in the understanding of the functional characteristics recapitulated within the HCC TiME. First, the presence of a percentage of HCC with expression of markers of inflammatory response, named as immune class, was identified.22 A subsequent study improved the classification.23 Three phenotypic classes were defined: an immune class, characterized by high immune cell infiltration and PD-1/PD-L1 pathway activation, an immune intermediate class, and an immune-excluded class, which conversely is associated with low immune cell infiltration, low expression of PD-1 and PD-L1 and high frequency of chromosomal aberrations.23 A fourth subclass, defined as immune-desert pattern, has also been described; it is distinguished by an impoverishment of the immune component leading to immune ignorance and a lack of priming T cell.24 While useful in understanding the pathogenesis of HCC, implementation of these classifications in clinical practice remains limited, and prospective studies proving their predictive performance are lacking. In addition to molecular characteristics, the heterogeneity of HCC etiology has become the subject of debate with regard to its differential ability to lead