Small molecule tyrosine kinase inhibitors approved for systemic therapy of advanced hepatocellular carcinoma: recent advances and future perspectives

Liver cancer is the sixth most commonly diagnosed cancer and the third leading cause of cancer death in the world, and hepatocellular carcinoma (HCC) is the most common form of liver cancer. More than half of the HCC patients are diagnosed at an advanced stage and often require systemic therapy. Dysregulation of the activity of receptor tyrosine kinases (RTKs) is involved in the development and progress of HCC, RTKs are therefore the potential targets for systemic therapy of advanced HCC (aHCC). Currently, a total of six small molecule tyrosine kinase inhibitors (TKIs) have been approved for aHCC, including first-line sorafenib, lenvatinib, and donafenib, and second-line regorafenib, cabozantinib, and apatinib. These TKIs improved patients survival, which are associated with disease stage, etiology, liver function, tumor burden, baseline levels of alpha-fetoprotein, and treatment history. This review focuses on the clinical outcomes of these TKIs in key clinical trials, retrospective and real-world studies and discusses the future perspectives of TKIs for aHCC, with an aim to provide up-to-date evidence for decision-making in the treatment of aHCC.


Introduction
Liver cancer is the sixth most common cancer and the third leading cause of cancer death worldwide, with approximately 906,000 new cases and 830,000 deaths in 2020 [1].In China, liver cancer is the fourth most commonly diagnosed cancer and the second leading cause of cancer death, with approximately 431,383 new cases and 412,216 deaths in 2022 [2].The most common form of liver cancer is hepatocellular carcinoma (HCC) accounting for ~ 90% of cases [3].HCC occurs most frequently in patients infected with hepatitis B virus (HBV) or hepatitis C virus (HCV), and approximately 43.0% and 19.0% of global HCC cases are caused by HBV and HCV infection respectively.Asian and African patients generally show a predominance of HBV infection, whereas American and most European patients show a prevalence of HCV infection [4,5].Most HCC patients have impaired liver function, the severity of which is assessed by Child-Pugh grade [6,7] and albumin-bilirubin (ALBI) grade, with higher grade associated with worse liver function [8].
HCC is classified into 5 categories based on the Barcelona Clinic Liver Cancer (BCLC) staging system.Patients at stage 0 have a single tumor < 2 cm and Child-Pugh grade A with preserved liver function.Patients at stage A have either a single tumor or no more than 3 nodules (< 3 cm) and Child-Pugh grade A or B with partially impaired liver function.Patients at stage B have multinodular changes and Child-Pugh grade A-B.Patients at stage C have portal invasion and extrahepatic spread (EHS) and Child-Pugh grade A-B.Patients at stage D have Child-Pugh grade C with severely impaired liver function [9].More than 50% of HCC patients are diagnosed at an advanced stage and 70% of patients relapse within the first 5 years of initial treatment.Early HCC is usually resectable, but advanced HCC (aHCC) often requires systemic therapy [10].
The pathophysiology of HCC is a multistep process associated with dysregulation of protein kinase activity [11].Approximately 560 kinases have been identified in human kinome, among which 95 are tyrosine kinases that are divided into receptor tyrosine kinase (RTK) family with 58 members and nonreceptor tyrosine kinase (NRTK) family with 37 members [12,13].RTKs transduces extracellular signals (growth factors) to the cytoplasm, NRTKs relay intracellular signals.
Currently, a total of six small molecule inhibitors targeting RTKs have been approved for aHCC.Sorafenib, lenvatinib, and donafenib are first-line drugs, whereas regorafenib, cabozantinib, and apatinib are second-line drugs (Fig. 1).This review summarizes the efficacy and safety of these tyrosine kinase inhibitors (TKIs) in key clinical trials as well as in retrospective and real-world studies, compares the outcomes of lenvatinib versus (vs.) sorafenib, sorafenib vs. apatinib, and regorafenib vs. cabozantinib, and discusses the future perspective of TKIs for aHCC therapy.
The overall incidence of treatment-related adverse events (AEs) was 80% for sorafenib vs. 52% for placebo in SHARP study, and 81.9% for sorafenib vs. 38.7%for placebo in Asia-Pacific study.The most common drug related AEs were
The US Food and Drug Administration (FDA) approved sorafenib in 2007 as a first-line therapy for aHCC based on SHARP and Asia-Pacific studies, which demonstrated that sorafenib was safe and significantly improved median OS, TTP, and DCR compared with placebo.In the single-arm, open-label, phase 4 HATT study (NCT01098760) conducted from August, 2010 to October, 2013 across 7 sites in Taiwan, sorafenib achieved a median OS, progression-free survival (PFS), and TTP of 8.6, 2.7, and 3.8 months respectively in aHCC patients (n = 151).No patients achieved CR, objective response rate (ORR; CR plus PR) was 6.6% and DCR was 47.7%.The most frequent grade ≥ 3 drug-related AEs were HFSR, diarrhea, and hypertension [22].These results were comparable to those observed in SHARP and Asia-Pacific studies [19,20].[39].A BW-based lenvatinib daily dosing was thereby reccommended for HCC patients, i.e., 12 mg and 8 mg for BW ≥ 60 kg and < 60 kg respectively [40].
Exploratory analysis of REFLECT data revealed that patients in higher or lower BW groups had similar median OS (13.7 vs. 13.4 months) and the same PFS (7.4 months).Treatment-emergent AEs required dose modifications in 57.5% and 43.0% of patients in the higher and lower BW groups respectively [43].The similar efficacy and safety results between the two BW groups support the BW-based lenvatinib dosing for patients with aHCC.

The outcomes of lenvatinib in real-world and retrospective studies
In REFLECT study, the enrolled patients had not received systemic treatment previously and were categorized to BCLC stages B or C and Child-Pugh grade A; those with ≥ 50% liver occupation, main branch portal vain thrombosis (PVT), and bile duct invasion (BDI) were excluded (referred to as REFLECT criteria) [41].However, aHCC patients may not meet the REFLECT criteria (REFLECT-ex) in real-world setting.The outcomes of lenvatinib in REFLECT-in (meet the REFLECT criteria) vs. REFLECT-ex patients were evaluated in real-world and retrospective studies.
The outcomes of lenvatinib in REFLECT-in and REFLECT-ex patients were further analyzed based on liver function, tumor burden in the liver, and treatment regimens.
Overall, lenvatinib had comparable outcomes in REFLECT-in and REFLECT-ex patients.Child-Pugh grades and MVI remarkably influenced median OS but not tumor responses, whereas tumor burden and treatment history were not significantly associated with median OS, median PFS, and tumor responses.
ZGDH3 study was conducted in China only, whereas the pivotal trials of lenvatinib and sorafenib were conducted globally.The median age of the patients and the percentages of HBV-positive patients were 53 and 90% in ZGDH3 study [55], 62 and 50% in REFLECT study [41], 66 and 18% in SHARP study [19], and 51 and 73% in Asia-Pacific study [20], respectively.Hence, donafenib could be a treatment option for HBV-positive Chinese patients, but biological and etiopathogenic considerations should be taken for patients from non-Asian regions with the ages > 60 [56].

The outcomes of regorafenib under real-world conditions
In real-world practices, regorafenib showed a pooled median OS, median PFS, TTP, ORR, and DCR of 14.  4).Patients with HCC recurrence after liver transplantation who received second-line regorafenib had a median OS of 12.9 months from regorafenib initiation and 38.4 months from sorafenib initiation [60].Thereby, regorafenib was effective for aHCC in real-world setting.
Overall, the outcomes of regorafenib in real-world setting was comparable with those in RESORCE study.The controversial results regarding the relationship between sorafenib TTP and the survival outcomes of regorafenib were probably due to the differences in patient characteristics such as age, sex, HBV infection, and BCLC stages.
Altogether, cabozantinib was beneficial for aHCC patients regardless of liver function, HBV or HCV infection, disease extension, the duration of prior sorafenib treatment, and baseline AFP levels, further supporting the utility of cabozantinib as a second-line therapy for aHCC.(2024)  Cabozantinib at full-dose (60 mg daily) and reduced-dose (20 or 40 mg daily) had comparable median PFS (2.8 vs. 3.6 months; P = 0.59), ORR (6.7% vs. 9.1%; P = 1) and DCR (53.4% vs. 81.8%;P = 0.22).However, the incidence of AEs, such as decreased appetite, fatigue, and diarrhea, was significantly higher in the full-dose group than in the reduced-dose group for all grades (P < 0.05) [83].
Overall, the efficacy of cabozantinib as second-or later-line therapy in real-world setting was comparable to those in CELESTIAL study.The outcomes of cabozantinib were significantly associated with liver function but not BCLC stage and baseline AFP levels.In addition, dose reduction may be a safer treatment option for aHCC.

The outcomes of apatinib under real-world conditions
In patients received apatinib as a first-line, second-line, or third-line therapy, median OS was 16.0, 17.0, and 5.8 months, and median PFS was 8.2, 7.0, and 3.1 months, respectively.Patients with first-and second-line apatinib had significantly prolonged median OS (P < 0.001) and median PFS (P < 0.001) than those with third-line apatinib.The most common AEs were secondary hypertension, gastrointestinal resistance, fatigue, and HFS, which were tolerable and manageable [88].

Review
Discover Oncology (2024)    In patients with unresectable or relapsed HCC, apatinib had a median OS of 13 months and a median PFS of 5 months, and the most common AEs were proteinuria, secondary hypertension, and liver dysfunction [89].
These studies provide evidence that apatinib is not only effective as both first-line and second-line therapy for HCC patients, but also promising in treating patients (infected or not with HBV) with sorafenib refractory or resistant aHCC.

Comparison of the outcomes of approved small molecule TKIs for aHCC
As aforementioned, patients with aHCC can benefit from first-line sorafenib and lenvatinib, and those who progressed on sorafenib can benefit from second-line regorafenib and cabozantinib.In addition, apatinib was used as a first-line treatment for aHCC in both clinial trial and real-world setting although it was approved as a second-line therapy.To find out which TKI is more effective for aHCC, the clinical outcomes of lenvatinib vs. sorafenib, apatinib vs. sorafenib, and regorafenib vs. cabozantinib were compared.

Lenvatinib vs. sorafenib
Meta-analyses were conducted to compare the efficacy and safety of lenvatinib vs. sorafenib as first-line therapy for aHCC.
The severe AE rate was similar in the lenvatinib and sorafenib groups [93][94][95], but lenvatinib was significantly associated with a higher incidence of hypertension, proteinuria, fatigue, decreased appetite, and weight loss, whereas sorafenib was associated with a higher incidence of diarrhea and HFSR [94,96].
These meta-analyses demonstrated that lenvatinib provided better tumor responses and survival advantages over sorafenib as a first-line treatment for aHCC, with a comparable incidence of severe AEs.
The effectiveness of first-line apatinib for aHCC was compared with sorafenib in retrospective studies.

Regorafenib vs. cabozantinib
The clinical outcomes of cabozantinib and regorafenib as second-line therapy for aHCC patients with prior sorafenib were compared by matching-adjusted indirect comparison.
The median OS for cabozantinib in CELESTIAL study and for regorafenib in a real-world practice was comparable (11.3 vs. 11.1 months; HR: 0.83, 95% CI 0.62-1.09),but cabozantinib had better median OS compared with regorafenib in patients with prior sorafenib treatment < 6 months.Median PFS was longer for cabozantinib than for regorafenib (5.5 vs. 3.0 months; HR: 0.50; 95% CI 0.41-0.62),and the prolonged median PFS of cabozantinib over regorafenib was irrespective of the duration of prior sorafenib treatment (< 3, 3 to < 6, and ≥ 6 months) [102].However, Adhoute et al. reported that cabozantinib and regorafenib had similar median PFS (3.6 vs. 2.9 months; P = 0.7986) based on retrospective data obtained from three French centers [103].The inconsistent PFS data were probably due to different cohort size in the two studies.No significant difference was found in the regorafenib and cabozantinib arms in terms of frequency of grade 3 or 4 treatment emergent AEs [101,103].
These studies revealed that regorafenib and cabozantinib had no significant differences in terms of OS, and earlier progressors on prior sorafenib may benefit more from cabozantinib treatment.
Altogether, lenvatinib was more effective than sorafenib as a first-line therapy for aHCC in terms of survival and tumor responses, whereas apatinib was not as effective as sorafenib as a first-line treatment option although it significantly improved the clinical outcomes of aHCC patients.The second-line regorafenib and cabozantinib had comparable OS, but aHCC patients with early progress on sorafenib may benefit more from cabozantinib.

Future perspectives
Currently, the six small molecule TKIs are the main therapeutic options for aHCC, however, the prognosis of aHCC patients is not optimistic due to the development of resistance to these TKIs [104].In addition, severe AEs caused by these TKIs require discontinuation of the treatment.To circumvent drug resistance and improve the therapeutic outcomes of TKIs, large efforts have been made to develop novel TKIs and TKI-based combination therapies.
With the advancement of computing and technology in recent times, the conventional methods of drug design, a time-consuming, expensive, and complex process, has been replaced by artificial intelligence (AI).Accumulated evidence has shown that AI can increase the success rate of the designed drug and reduce the cost substantially [120].It is conceivable that AI will accelerate the development of novel TKIs with potential effecfiveness for the therapy of aHCC.

TKIs in combination with immune checkpoint inhibitors
Immune checkpoints, such as programmed cell death protein 1 (PD-1), programmed cell death ligand 1 (PD-L1), and cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), are membrane bound proteins that prevent the overreaction of immune response and induce immune tolerance.Immune checkpoint inhibitors (ICIs) have brough a revolution in cancer therapy within the past 10 years [131], and the clinical outcomes of ICIs vs. TKIs in aHCC were investigated by clinical trials and real-world studies.
In the phase 3 IMbrave150 study (NCT03434379), combination of monoclonal antibodies (mAbs) against atezolizumab (PD-L1) and bevacizumab (VEGF mAb) outperformed sorafenib in patients with locally advanced or metastatic HCC naïve to systemic treatment in terms of median OS, median PFS, and ORR, whereas grade 3 or 4 AEs were comparable in the two groups [132,133].The FDA approved atezolizumab combining bevacizumab as first-line therapy for aHCC in 2020 [134].
The clinical outcomes of atezolizumab plus bevacizumab vs. lenvatinib in patients with unresectable HCC (uHCC) were compared by meta-analyses.Atezolizumab plus bevacizumab resulted in better PFS [135], comparable OS [136,137] and PFS [137], or worse OS and PFS [136] compared with lenvatinib.In non-viral aHCC patients, lenvatinib was associated with signinficantly improved OS and PFS compared to atezolizumab plus bevacizumab [138], but ORR and DCR were not significantly different in the two treatment groups [135][136][137].Different types of studies (e.g.real-world, retrospective, or prospective studies) and different patient sub-populations included in these studies may result in the discrepant PFS and OS data.
In the phase 3 HIMALAYA study (NCT03298451), tremelimumab (CTLA-4 mAb) plus durvalumab (PD-L1 mAb) surpassed sorafenib in patients with uHCC and no previous systemic treatment in terms of median OS, whereas durvalumab alone had a median OS that was noninferior to sorafenib.Median PFS was not signigicantly different among the three groups [139].The 4-year follow-up study revealed the outperformance of tremelimumab plus durvalumab over sorafenib in terms of long-term OS [140].The FDA approved tremelimumab combining durvalumab as first-line therapy for uHCC in 2022 [141].
In aHCC patients with prior sorafenib treatment, pembrolizumab (PD-1 mAb) vs. placebo achieved numerically better median OS and median PFS in the phase 3 KEYNOTE-240 study (NCT02702401) [142], and nivolumab (PD-1 mAb) plus ipilimumab (CTLA-4 mAb) showed promising results in the pase 1/2 CheckMate 040 study (NCT01658878) [143].The FDA approved pembrolizumab and nivolumab plus ipilimumab as second-line therapy for aHCC in 2018 and 2020 respectively [134,141].Although ICIs remarkably improved the survival of HCC patients, only 30%-40% patients were responsive to ICIs, and responders may eventually develop resistance to ICIs [144].To improve the clinical benefits of ICIs and overcome drug resistance, various combinations of ICIs and small molecule TKIs have been explored in clincal trials and real-world studies.
Lenvatinib plus nivolumab vs. lenvatinib significantly improved median OS and ORR in aHCC patients in a real-world study [145], but lenvatinib plus pembrolizumab failed to show superiority over lenvatinib monotherapy [146].Apatinib plus camrelizumab (PD-1 mAb) showed promising efficacy and manageable AEs in aHCC patients as both first-line and second-line therapy [147], anlotinib plus sintilimab (PD-L1 mAb) and axitinib plus avelumab (PD-L1 mAb) had promising clinical activities with manageable toxicity as first-line treatment of aHCC [148,149], but cabozantinib plus atezolizumab vs. sorafenib did not provide significant OS benefit [150].The results remain to be confirmed by large-scale RCTs.So far a slew of ongoing clinical trials are evaluating the efficacy and safety of small molecule TKIs in combination with ICIs as first-or second-line therapies for aHCC (Table 6).These clinical trials will provide valuable information for TKI-based immunotherapy for aHCC.

Identification of predictive biomarkers for the therapeutic effects of TKIs
Large efforts have been made to identify potential biomarkers to predict the outomes of TKIs in HCC.In patients treated with sorafenib, high expression of Ang-2, a pro-angiogenic cytokine, was associated with poor survival [151], whereas a low neutrophil-to-lymphocyte ratio (NLR) that reflects the inflammatory response to cancer [152], and high expression of miR-224 [153] and miR-425-3p [154] in tumor samples were associated with better survival.Lenvatinib was shown to have a higher selectivity for FGFR compared with other TKIs, and the results from HCC cell lines suggested that amplification of FGF19 and FGFR might be biomarkers for lenvatinib effectiveness [155].In patients treated with lenvatinib, higher NLR was associated with poor prognosis [156].However, these results need further validation.As a result, there are still no predictive biomarkers for the clinical outcomes of TKI in HCC.
With the fully utilization of the advanced technologies in clinical settings, such as the analysis of tumor genomic or transcriptomic profiles, miRNA evaluation, the identification of driver gene mutations, ctDNA detection, molecular imagining, and cell cultures, more options will be available to identify predictive biomarkers for TKI treatment, leading to further improvement of the therapeutic effects of TKIs [157].

Conclusion
Small molecule TKIs improved the clinical outcomes of HCC patients in both clinical trials and real-world setting.The efficacy and safety of TKIs may associate with race, age, sex, body weight, drug dose, the duration of treatment, baseline AFP levels, disease etiology, BCLC stage, liver function, tumor burden in the liver, and disease extension, which should be considered when choosing a treatment option for aHCC.
The advancement in AI will facilitate the development of novel TKIs with potential effectiveness for aHCC.Clinical trials and real-life studies revealed that TKIs in combination with LRTs or ICI inhibitors might be potential treatment options for aHCC.A large number of clinical trials are ongoing to investigate the clinical outcomes of novel TKI-based combination therapies, with considerations of dosing schedule, treatment sequence, duration, and treatment history.Furthermore, identification of the predictive biomarkers for TKIs treatment will improve the therapeutic effects this class of agents.
Altogether, the development of novel small molecule TKIs and novel TKI-based combination therapies, along with the identification of the predictive biomarkers for TKIs treatment, are promising in providing more treatment options and survival benefits for aHCC in the future.

Table 1
Key clinical trials of small molecule receptor tyrosine kinase inhibitors approved for systemic therapy of advanced hepatocellular carcinoma CI confidence interval, DCR disease control rate, HR hazard ratio, mPFS median progress-free survival, mOS median overall survival, mTTP median time to progress, ORR objective response rate, OR odds ratio RTK inhibitor Manufac-

Table 2
The efficacy of sorafenib in retrospective studies Unpaired t test was performed to analyze mOS, CR, PR, SD, and PD CR complete response, mOS median overall survival, NR not reached, ORR objective response rate, PR partial response, PD progressive disease, SD stable disease

Table 4
The outcomes of regorafenib under real-world conditions DCR disease control rate, mPFS median progression-free survival, mOS median overall survival, mTTP median time to progression, NR not reached, ORR objective response rate

1 Key clinical trial of cabozantinib
In the randomized, double-blind, controlled, phase 3 CELESTIAL study (NCT01908426) conducted between September, 2013 and October, 2017 at 104 sites across Asia-Pacific, Europe, North Amerinca, and Oceania regions, aHCC patients (n = 707) with prior sorafenib treatment and Child-Pugh A liver cirrhosis were randomized (2:1) to received 60 mg cabozantinib or placebo once daily.Compared with placebo, cabozantinib had notably improved median

Table 5
The outcomes of cabozantinib in retrospective and real-life studies AFP alpha-fetoprotein, CPA Child-Pugh grade A, CPB Child-Pugh grade B, DCR disease control rate, mPFS median progression-free survival, mOS median overall survival, ORR objective response rate, TTP time to progression