Current knowledge about immunotherapy response after liver transplantation of patients with liver cancer

Abstract

Hepatocellular carcinoma (HCC) and intrahepatic cholangiocarcinoma (iCCA) are the two main liver cancers responsible for cancer deaths worldwide. Multiple etiologies exist driving these diseases; however, there are limited effective treatments to date. Recent studies have demonstrated improved outcomes in patients with advanced disease treated with immune checkpoint inhibition (ICI). Further, as these patients undergo liver transplantation, it’s critical to have an understanding of the impact of ICI on the immune system post-transplantation. In this review, we will provide an overview on ICI therapy in liver cancer, ICI utilization in the peri-transplantation setting, and discuss molecular predictions to immunotherapy response.

Introduction

Hepatocellular carcinoma (HCC) and intrahepatic cholangiocarcinoma (iCCA) are the main types of liver cancer, and together, they are the third leading cause of cancer deaths worldwide [1]. Risk factors for HCC and iCCA include viral hepatitis, alcohol use, and metabolic derangements such as metabolic dysfunction-associated steatohepatitis (MASH)[2–6]. Additionally, iCCA is uniquely associated with liver flukes and biliary duct diseases, such as primary sclerosing cholangitis and choledochal cysts [5]. The development of HCC is attributed to chronic liver injury, which leads to subsequent inflammation and cirrhosis, whereas iCCA arises from chronic biliary tract inflammation [5, 6]. Currently, HCC is diagnosed in at-risk patients through screening ultrasounds with or without alpha-fetoprotein (AFP), or it is identified when patients present with symptoms [6]. MASH-related HCC presents unique challenges for surveillance, since up to 40% of tumors develop in the setting of advanced fibrosis without cirrhosis, potentially leading to delayed diagnoses[7, 8]. Similarly, detection strategies for iCCA are limited, as this condition is usually asymptomatic and therefore diagnosed at advanced stages [9, 10]. Current treatment strategies for liver cancer are determined by tumor stage, performance status, and the level of liver dysfunction. The Barcelona Clinic Liver Cancer (BCLC) framework guides the management of HCC, while the American Joint Committee on Cancer (AJCC) staging system and the British Society of Gastroenterology (BSG) guidelines inform management practices of iCCA [11–14].

Role of Liver Transplantation in the Treatment of Liver Cancer

a. HCC

Current treatment strategies for HCC include resection, liver transplantation (LT), systemic chemotherapy, and locoregional therapy (LRT). Liver transplantation is the definitive curative treatment for early, unresectable HCC. Transplant selection is guided by criteria such as the Milan and the University of San Francisco (UCSF) guidelines (Table 1) [15, 16]. In patients who fall outside the criteria for upfront transplantation, studies have demonstrated the utility of LRT or systemic therapy to downstage their disease [17]. Additionally, LRT can also serve as a bridging therapy prior to LT, to prevent disease progression while on the transplant waitlist [13]. In HCC patients who undergo LT, whether upfront or after downstaging, 5-year recurrence-free survival rates are up to 70% [6, 18–20]. While the recurrence rate of HCC after LT is lower compared to local resection (11–18% vs. 50–70%), it remains of significant concern [17, 21]. Recurrent disease is hypothesized to arise de-novo or from micro metastases making its way back to the transplanted liver [22].

TABLE 1:

Milan Criteria and UCSF Criteria for liver transplantation

a. UCSF - University of California San Francisco
	Tumor number and Size	Extra-Hepatic Involvement	Major Vessel Involvement	Vascular Invasion
Milan Criteria for Transplantation	Single tumor ≤ 5 cm Or Up to 3 tumors ≤ 3 cm each	No	No	-
UCSF Criteria	Single tumor <6.5 cm Or Up to 3 tumors ≤4.5 cm, total diameter ≤8 cm	-	-	No

Unfortunately, up to 80% of patients present with advanced disease not amenable to upfront surgical treatment, and only 35% of all HCC patients are estimated to undergo LT. As a result, the overall prognosis of HCC remains poor, with an aggregate 5-year survival rate of less than 20% for allcomers. [6, 16, 18]. These results have fueled ongoing efforts to increase the number of patients eligible for transplantation.

b. iCCA

Surgical resection is the gold standard curative treatment for resectable iCCA, as recognized by international guidelines from the European Society for Medical Oncology (ESMO), the Japan Biliary Association, and the National Comprehensive Cancer Network (NCCN) [11, 23–25]. Intrahepatic cholangiocarcinoma is associated with a poor prognosis, with a 5-year overall survival of 15–45% [26–28]. This is largely due to ineffective treatment options and delayed diagnosis due to limited detection strategies for iCCA. More than 50% of patients present with unresectable or multifocal disease, necessitating locoregional, liver-directed, or systemic therapy for management [26–28]. Only 15% to 20% of iCCA patients are deemed resectable which remains the gold standard treatment [29]. Amongst the patients achieving an R0 resection, the 5-year survival rate remains below 40%, and the recurrence rate can be as high as 75% with nearly half occurring within the first year following surgical resection [19, 28, 30–32]. The BILCAP trial sought to investigate the usage of adjuvant capecitabine after resection of BTCs, including iCCA. The trial demonstrated improved overall survival (53 vs. 36 months) compared to observation alone which led to FDA approval for six months of adjuvant capecitabine as the standard of care for adjuvant systemic therapy in iCCA [33]. Despite these promising results, the prognosis of iCCA remains poor, with the BILCAP trial reporting recurrence rates of 68% in the adjuvant capecitabine group and 71% in the observation group [36].

Historically, LT for iCCA was contraindicated due to high recurrence rates and poor overall survival, with 5-year survival rates of less than 25% [28, 34, 35]. Several studies evaluated incidental iCCA or mixed HCC/iCCA in explant pathology after LT for non-iCCA etiologies, and these studies reported encouraging outcomes for LT in early stage iCCA [36–38]. Patients with early iCCA, defined as tumors no larger than 2 cm, demonstrated a 5-year post-LT survival of up to 65% and a recurrence rate of 9% to 33% [32, 36–38]. In contrast, those with tumors larger than 2 cm or multifocal disease exhibited a 5-year survival of around 45% and a recurrence rate of up to 77% [32, 36–38]. These results prompted the consideration of LT as a viable option for a select subset of iCCA patients with early, unresectable, non-metastatic disease. Current criteria for LT in iCCA as outlined by the Organ Procurement and Transplantation Network (OPTN) board, includes patients with unresectable biopsy proven iCCA, tumor size <3 cm, and image stability after 6-months of therapy [19, 32, 35, 39]. However, studies have failed to demonstrate any additional benefit of LT over resection in patients with resectable iCCA. Therefore, surgical resection currently remains the gold standard treatment for resectable iCCA [32, 39].

Immunotherapy in Advanced Liver Cancer

a. Immunotherapy overview

Recent studies have sought to improve HCC and iCCA outcomes through immune checkpoint inhibitor (ICI)-based therapy [40–44]. ICIs are monoclonal antibodies that leverage a patient’s immune system to destroy cancer cells [40, 45–48]. ICIs target negative immune co-stimulatory modulators on antigen-presenting cells (APCs) and T-cell surfaces, impacting the downstream upregulation of T-cell immune activation [40, 45–47, 49]. These co-stimulatory modulators include programmed death cell protein 1 (PD-1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), primarily expressed on T-cells, as well as programmed death-ligand 1 (PD-L1), which is also known as B7-H1, and is expressed on APCs, other immune cells, and tumor cells [45–47, 49]. The interaction of these co-stimulatory molecules leads to the downregulation of anti-tumor T-cell activity via ligand-receptor binding (e.g., PD-L1 binding to PD-1), which encourages tumor growth [45, 48, 49]. ICIs include anti-PD-1 inhibitors such as nivolumab and pembrolizumab; anti-PD-L1 inhibitors such as camrelizumab, atezolizumab, avelumab, and durvalumab; and CTLA-4 inhibitors such as ipilimumab and tremelimumab [50].

b. Immunotherapy in advanced HCC

The first FDA-approved systemic treatment for advanced unresectable HCC was sorafenib, a tyrosine kinase inhibitor (TKI). The 2007 SHARP trial demonstrated an improved overall survival with sorafenib compared to placebo (10.7 months vs. 7.9 months)[51]. Lenvatinib, another TKI with angiogenesis-inhibiting properties through VEGF binding, was then approved a decade later as an alternative first-line systemic therapy for advanced HCC after the REFLECT trial showed its non-inferiority to sorafenib in terms of median overall survival (13.6 months vs. 12.3 months)[52].

The CheckMate 459 trial was one of the first studies to investigate ICI therapy in HCC. While nivolumab monotherapy failed to demonstrate improved survival compared to sorafenib alone, it showed efficacy and a favorable safety profile, leading to FDA approval for treatment in advanced HCC [53]. Additional studies sought to evaluate the role of ICI in HCC compared to sorafenib alone. The IMbrave 150 trial demonstrated efficacy of atezolizumab in combination with bevacizumab, resulting in improved median progression-free survival (6.8 months vs. 4.3 months) and overall survival (19.2 months vs 13.4 months) compared to sorafenib alone [54]. In the HIMALAYA trial, durvalumab in combination with tremelimumab demonstrated improved overall survival (16.4 months vs. 13.8 months) compared to sorafenib alone, and durvalumab monotherapy showed non-inferiority to sorafenib [55]. Additionally, camrelizumab combined with rivoceranib, an anti-angiogenic TKI, demonstrated improved progression-free survival (5.6 months vs. 3.7 months) and overall survival (22.1 months vs. 15.2 months) compared to sorafenib alone, in the CARES-310 study [56]. Currently, atezolizumab with bevacizumab and durvalumab or atezolizumab with tremelimumab are the two first-line FDA-approved ICI therapies for advanced HCC [41, 44, 57].

Most recently, nivolumab in combination with ipilimumab was added as an additional option for first-line systemic therapy in HCC from the results of the CHECKMATE 9DW trial [57–59]. The trial demonstrated improved 2-year overall survival (49% vs. 39%) and higher objective response rate (36% vs. 13%) relative to lenvatinib or sorafenib alone in advanced HCC [58, 59]. These results and additional data from the CHEKMATE 9DW trial could potentially influence future treatment guidelines, positioning nivolumab-ipilimumab as a standard of care, first-line treatment option for patients with HCC.

c. Immunotherapy in advanced iCCA

Gemcitabine combined with cisplatin chemotherapy remains the only first-line FDA-approved treatment for advanced iCCA since the ABC-02 trial in 2010. This trial demonstrated a significant improvement in median overall survival of gemcitabine in combination with cisplatin (11.7 months vs. 8.1 months) compared to gemcitabine alone [5, 60]. The ABC-06 trial established fluorouracil combined with oxaliplatin (FOLFOX) as a viable second-line option for patients with disease progression after demonstrating improved overall survival (6.2 months vs. 5.3 months) in this patient group [60]. These results led to FDA approval of FOLFOX as a second-line treatment for iCCA in 2021 [61].

Several studies have demonstrated promising results for ICIs in advanced BTCs including iCCA. In the recently updated TOPAZ-1 Phase III trial, durvalumab combined with standard gemcitabine and cisplatin demonstrated improved response rates (27% vs. 19%) and enhanced 3-year overall survival (15% vs. 7%) in treatment-naive patients with advanced BTCs including iCCA [62, 63]. Durvalumab with gemcitabine and cisplatin is now the first-line treatment for patients with advanced BTCs, including iCCA [61]. In the KEYNOTE-966 trial pembrolizumab with gemcitabine and cisplatin demonstrated improved overall survival (12.7 months vs. 10.9 months) compared to placebo with gemcitabine and cisplatin [64]. The FDA approved pembrolizumab as a second-line treatment for advanced BTCs with high microsatellite instability of deficient mismatch-repair based of phases I-II trials, with the use of first-line chemo-immunotherapy [64–66]. This indication will concern a limited population.

Additional preliminary studies have demonstrated efficacy and safety of different ICIs in patients with iCCA, that could inform future trials and impact guidelines. A single-arm study of 32 patients with refractory BTC subsequently treated with lenvatinib in combination with pembrolizumab showed an objective response rate of 25%, a disease control rate of 78%, a median progression-free survival of 4.9 months and overall survival of 11 months [67]. In another single-arm, phase II trial of 31 advanced iCCA patients, the combination of toripalimab with lenvatinib achieved a similar objective response rate of 32.3% and a disease control rate of 74.2%, with no grade 5 adverse effects reported [68].

A recent single-center phase II study of 30 advanced iCCA patients evaluating ICI in conjunction with chemotherapy, toripalimab combined with lenvatinib, gemcitabine and oxaliplatin demonstrated an 80% objective response rate, a median overall survival of 22.5 months and progression free survival of 10.2 months [69]. While the subsequent phase III multicenter trial (JS001) is ongoing, these results are promising and could also potentially downstage patients and increase the number of patients eligible for transplantation [69].

Immunotherapy in the Peri-Transplantation Setting

While current studies support the use of ICIs in advanced liver cancer, their application in the peri-transplantation setting is variable and primarily based on several case series and reports that primarily focus on HCC (Table 2). These reports highlight the potential of ICIs to achieve downstaging and disease control in select patients as a bridge to transplant. These studies also underscore associated risks of ICI in the peri-LT setting that must be carefully considered, including graft rejection. It is therefore important to examine these studies for factors that predict the success, complications, and overall efficacy of peri-transplant ICI therapy in order to inform indications, guidelines, and application in the peri-LT setting.

TABLE 2:

Case reports, series and cohort studies detailing use of ICI in the peritransplantation setting for HCC therapy.

Pre-LT	Author	Age/Sex	ICI regimen	Therapy Prior to/with ICI	ICI duration of Treatment	ICI Completion to Transplant (m)	Rejection	Other Outcomes
	Nordness et al. 2020	65 M	nivolumab	Hepatectomy Sorafenib TARE TACE	2 years	8 days	Yes	- Partial remission pre-LT - Fatal hepatic necrosis
	Schwacha-Eipper 2020	62 M	nivolumab	Sorafenib Regorafenib	34 cycles	6 weeks	No	-Partial remission pre-LT
	Chen et al 2021	39 M	toripalimab	Hepatectomy TACE Sorafenib Lenvatinib Ablation	10 cycles	93 days	Yes	-Fatal acute hepatic necrosis
	Kang et al. 2022	14 M	pembrolizumab	Cisplatin/Doxorubicin/Dexrazoxane Hepatectomy TACE	9 weeks	138 days	No	-Hepatic artery dissection -Graft loss
	Schnickel et al. 2022	61F	nivolumab	n/a	18 months	5 weeks	Yes	-Hepatic necrosis -Graft loss
		65M	nivolumab	n/a	8 months	10 days	Yes	-Hepatic necrosis
		71 M	nivolumab	n/a	8 months	3 months	No	n/a
		65 F	nivolumab	n/a	12 months	4 months	No	n/s
		68M	nivolumab	n/a	12 months	6 months	No	-Cholestasis -Death
	Abdelrahim et al. 2024	64 M	atezolimumab+bevacimumab	Radiation	4 months	4 months	No	-Recurrence
		61 M	nivolumab	TARE Hepatectomy Sorafenib	8 months	8 weeks	No	n/a
		58 M	atezolimumab+bevacimumab	TARE Lenvatinib TACE Carbonzatinib	3 months	6 months	No	-Recurrence
		61 M	nivolumab atezolimumab+ bevacimumab	Lenvatinib	1 month 3 months	2 months	No	n/a
		68 M	atezolizumab + bevacimumab	TARE TACE Ablation Lenvatinib	16 months	36 months	No	-Chronic respiratory failure -Sustained bacteremia and septic shock
		59 M	nivolumab ipilimumab	Hepatectomy Ablation TACE Radiation	1 months	41 months	No	-ICI associated hepatotoxicity -Mesh related complications
Post-LT	Author	Age/Sex	ICI regimen	Other Post-LT Therapy	ICI duration of Treatment	Transplant to ICI Initiation Duration (m)	Rejection	Other Outcomes
	Varkaris et al. 2017	70 M	pembrolizumab	Sorafenib Chemoradiation Capecitabine	4 months	8 years	No	-Disease progression
	De Toni et al. 2017	41 M	nivolumab	TACE Ablation	15 cycles	11 months	Yes	-Disease progression
	Friend et al. 2017	20 M	nivolumab	Sorafenib Capecitabine	2 weeks	3 years	Yes	-Fatal acute graft rejection
		14 M	nivolumab	Sorafenib Capecitabine	1 week	3 years	Yes	-Fatal acute graft rejection
	Rammohan et al. 2018	57 M	pembrolizumab	Sorafenib	10 months	4 years	No	-Partial remission
	DeLeon et al. 2018	56 M	nivolumab	Sorafenib	1.2 months	2.7 years	No	-Disease progression
		55 M	nivolumab	Sorafenib	1.1 months	7.8 years	No	-Disease progression
		34 F	nivolumab	Sorafenib	1.3 months	3.7 years	No	-Disease progression
		63 M	nivolumab	Sorafenib	0.3 months	1.2 years	No	-Multiorgan failure
		68 M	nivolumab	Sorafenib	0.9 months	1.1 years	Yes	n/a
	Gassmann et al. 2018	53 F	nivolumab	Sorafenib	1 week	2 years	Yes	-Fatal severe graft rejection
	Amjad et al. 2019	62 F	nivolumab	TACE Radiation	6 months	1.3 Years	No	-Complete remission
	Kumar et al. 2019	64 M	nivolumab	Sorafenib	1 week	2 years	Yes	-n/a
	Pandey et al. 2020	52F	ipilumumab	Ablation Sorafenib	13 months	7.5 years	No	-Complete remission
	Al Jarroudi O. 2020	70 M	nivolumab	Sorafenib Gemcitabine Oxaliplatin Regorafenib	8 weeks	3 years	Yes	-Disease progression
		62 F	nivolumab	Sorafenib Regorafenib 5 fluorouracil/ Oxaliplatin	10 weeks	unknown	No	-Disease progression
		66 M	nivolumab	Sorafenib Regorafenib	12 weeks	5 years	No	-Disease progression
	Anugwom et al. 2020	62 M	nivolumab	Sorafenib Carboplatin/Gemcitabine 5 fluorouracil/ oxaliplatin Radiation	2 months	1 year	Yes	-Immune hepatitis

a. Neoadjuvant ICI use prior to liver transplantation in HCC

Numerous case reports and series have presented a mixed picture of pre-LT ICI use (Table 2). While some underscore the risks of graft rejection and graft loss, many have highlighted successful downstaging and bridging to transplant Nivolumab was the first agent reported for pre-LT ICI therapy in a 65-year-old patient with HCC who achieved remission prior to LT but suffered fatal hepatic necrosis on post-LT Day 10 [70]. Schwacha-Eipper et al. subsequently reported successful downstaging in a 62-year-old patient treated with nivolumab who then underwent successful LT and suffered no graft rejection [71]. Similarly, the use of toripalimab in another patient with recurrent HCC after hepatic resection led to successful LT but resulted in fatal acute hepatic necrosis on post-LT Day 3 [72]. Another study utilized pembrolizumab for recurrent HCC after resection, leading to successful LT without any evidence of organ rejection post-LT [73].

Several studies have incorporated the data from these case reports, into a comprehensive analysis of the efficacy and safety of pre-LT ICIs. For example, a retrospective cohort study investigated 6 HCC patients who underwent neoadjuvant ICI therapy prior to LT. Four patients received atezolizumab/bevacizumab, 1 patient received nivolumab/ipilimumab, and 1 patient received nivolumab monotherapy [74]. This study demonstrated successful downstaging of all patients without evidence of post-LT rejection, although 2 patients suffered HCC recurrence [74]. In contrast, Qiao et al. reported promising disease control rates with an 80% partial remission and a 100% disease control rate in 9 HCC patients treated with neoadjuvant ICI [75]. This study also noted a significant incidence of graft rejection (30%), leading to the death of 2 patients [75]. A meta-analysis of neoadjuvant ICI use in 91 HCC patients prior to LT showed overall promising transplantation outcomes but reported a 26% allograft rejection rate, 10% HCC recurrence rate, and 10% mortality rate, with no observed differences in survival based on rejection [76]. Meanwhile, the VITALITY multicenter study evaluated the combination of pre-LT ICI and LRT as downstaging or bridging therapy. The study demonstrated successful downstaging in 65 out of 86 HCC patients (76%) and a promising 3-year post-LT survival rate of 85%. However, it also reported a 16% graft rejection rate and a 2% graft loss among the 43 transplanted patients [77]. These studies are overall promising, and although the risk of graft rejection and loss is a concern, many have shown reasonable safety profiles. This evidence should encourage further investigation in larger, prospective trials, to optimize ICI therapy and reduce these risks.

Some prospective studies are already exploring immunotherapy as a downstaging approach prior to curative LT. In the PLENTY randomized pilot trial, 11 HCC patients beyond Milan criteria received neoadjuvant pembrolizumab plus lenvatinib with a minimum 72-day washout period before LT and were compared to 11 controls who underwent LT without prior ICI treatment [78]. The pembrolizumb-lenvantinib group demonstrated a 60% mRECIST objective response rate and improved recurrence-free survival post-LT (88 % vs. 38%) with a comparable safety profile relative to LRT [78]. The ImmunoXXL trial is an ongoing Phase II study investigating the safety and efficacy of neoadjuvant ipilimumab and nivolumab in HCC patients outside LT criteria and implements a minimum 30-day washout period prior to LT [79, 80].

While evaluating pre-LT ICI efficacy in HCC patients who meet downstaging criteria is important, assessing its role in allcomers, to help improve transplant rates and outcomes. In an intention-to-transplant analysis within the ImmunoXXL trial, 20 HCC patients who began treatment with atezolizumab-bevacizumab for advanced HCC, but without absolute contraindications to LT were investigated. This study demonstrated 20% complete response, 15% partial response, 40% stable disease, and 25% disease progression rates, translating to an overall disease control rate of 75% [79]. Eight patients were eventually listed for LT after successful downstaging [79].

Although not exclusively focused on ICIs, the MERITS-LT prospective multiregional observational trial reported outcomes for 82 patients who fell outside the United Network for Organ Sharing (UNOS) downstaging criteria and underwent LRT. The study demonstrated a 65% probability of successful downstaging after LRT, a 42% probability of LT within 5 years, and a 3-year post-transplant survival rate of 91% [81]. This study showed promise in downstaging HCC patients outside of criteria, which would result in increased LT for HCC patients. The MERITS-LT is currently investigating pre-LT atezolizumab and bevacizumab or durvalumab and tremelimumab in high-risk HCC patients found to have residual or recurrent disease after downstaging, with a washout period of 12 weeks prior to LT [44, 81].

While there are currently no FDA-approved guidelines for neoadjuvant ICI therapy in HCC, these prospective studies demonstrate the potential for downstaging in HCC patients, including some who initially fell beyond downstaging criteria, highlighting a possible role for ICIs in this patient population. These studies, along with ongoing trials, could change the landscape of ICI use for HCC downstaging and may potentially inform the inclusion of patients for LT who were previously considered ineligible.

b. Immunotherapy for recurrent HCC post liver transplantation

In the post-LT setting, ICI therapy for recurrent HCC has been associated with variable outcomes (Table 2). Results are mixed with some reporting complete response without post-LT complications and others with disease progression and significant adverse effects such as hepatic necrosis and fatal graft rejection [82–88]. For example, Amjad et al. reported a case of complete response in recurrent HCC without graft rejection following post-LT nivolumab treatment, whereas Friend et al. described two cases of fatal acute hepatic necrosis occurring approximately four weeks after initiating nivolumab for post-LT HCC recurrence [82, 86].

The most comprehensive review to date with 28 patients treated with ICI post-LT for recurrent HCC indicated a 32% rate of acute graft rejection. In 44% of these cases, the rejection resolved, while 33% progressed to graft failure and death [89].

These results are limited and variable, therefore more research is necessary to establish the effectiveness and adverse effects of ICIs, especially graft loss and rejection, in treating recurrent HCC post-LT.

c. Immunotherapy in the peri-transplantation period for iCCA

Currently, there is no data available on the use of ICIs before or after LT for patients with iCCA, potentially because only a select minority of iCCA patients meet LT criteria. With emerging studies demonstrating the potential benefits of neoadjuvant chemotherapy before surgical resection or transplantation, neoadjuvant ICI therapy could play a significant role in treating iCCA and should be considered.

Within iCCA, the addition of neoadjuvant chemotherapy has demonstrated improved 5-year survival benefit compared to upfront resection alone (34% vs. 25.7%) [90]. Furthermore, these benefits are seen when comparing neoadjuvant chemotherapy (40.3 months vs. 32.8 months) to adjuvant treatment alone [91]. Currently, the PURITY study is a phase II/III trial designed to determine the benefits of neoadjuvant chemotherapy in patients with BTCs undergoing surgical resection [92]. Previously, a Phase II clinical trial combined gemcitabine with cisplatin and nab-paclitaxel for advanced BTCs, including iCCA and found 20% of patients became eligible for resection following systemic chemotherapy [93]. These results suggest that neoadjuvant chemotherapy may not only improve outcomes but also convert patients previously categorized as unresectable to resectable.

The results of these studies provide thoughtful insight into the utility of neoadjuvant chemotherapy prior to LT for iCCA. In a prospective study, 18 patients with unresectable, non-metastatic iCCA that demonstrated stable disease after 6 months of neoadjuvant gemcitabine and cisplatin, underwent LT. This study demonstrated a 5-year survival rate of 57% with a recurrence rate of 39% post-LT [94]. Notably, 5 patients who received neoadjuvant therapy later underwent surgical resection after achieving disease regression, transitioning from unresectable to resectable status [94]. These studies highlight promising outcomes, suggesting that neoadjuvant treatment could effectively downstage advanced and unresectable tumors. This would expand access to both surgical resection and LT for more iCCA patients, potentially leading to favorable outcomes. Additional studies including the TESLA and LIRICA trials are examples of ongoing trials actively exploring the role of neoadjuvant therapy in biopsy-proven, unresectable, non-metastatic iCCA, prior to LT [95, 96]. These trials aim to address key questions regarding patient selection, optimal treatment strategies, and long-term outcomes and will hopefully provide promising results by downstaging patients and making them eligible for LT [95–98].

While these studies focus on chemotherapy as neoadjuvant iCCA treatment prior to LT, aforementioned studies have demonstrated improved outcomes with ICI treatment in advanced iCCA [63, 64, 66, 68]. Taken together, we believe ICIs should be considered as a neoadjuvant treatment strategy to downstage patients with iCCA, ultimately, converting these patients eligible for curative resection or liver transplantation.

ICI-Associated Graft Rejection after Liver Transplantation

T-cell mediated toxicity, along with gastrointestinal, endocrine, and cutaneous issues, are complications associated with ICIs; however, the most concerning complication of peri-LT ICI use for HCC was fatal allograft rejection [70, 72, 77, 82, 89, 99]. Allograft rejection has been linked to ICIs’ role in enhancing T-cell immune responses, which can conflict with the necessary immunosuppressive environment required to prevent organ rejection post-LT [50, 74]. While standard LT has organ rejection rates of up to 40% and graft loss rates of around 2%, estimating rejection rates in setting of peri-LT ICI use is challenging due to variability of the small observational nature of existing studies [100]. A retrospective review by Wang et al. involving 16 HCC patients who received neoadjuvant ICIs reported a 56% graft rejection rate with no graft loss, while two additional studies documented a 16% to 30% rejection rate with a 2% to 22% graft loss [75, 77, 101].

These recent studies suggest acceptable outcomes with peri-LT ICI treatment in liver cancer. However, additional studies are required given the rate of graft rejection.

Factors Predictive of ICI Efficacy in the Peri-Transplantation Setting

1). Timing of ICI therapy

The optimal duration between completion of ICI therapy and LT is still being investigated. Evidence suggests that a shorter interval between ICI therapy and LT may be associated with increased risk of acute allograft rejection [40, 50, 70, 74]. For instance, in the case series by Schnickel et al., no graft rejection was observed in three patients who underwent LT more than three months after their last ICI dose. However, two patients that were transplanted less than three months from ICI completion experienced acute rejection, 1 who suffering graft loss [99]. A retrospective study of 18 LT patients treated with neoadjuvant therapy revealed an overall acute rejection rate of 56.3%. Importantly in this study, patients who experienced rejection had a significantly shorter time from ICI completion to LT (21 days) compared to those without rejection (60 days), although no graft loss or death occurred in either group [101]. Similarly, a meta-analysis of 91 LT patients treated with neoadjuvant ICI found that those exhibiting a ≤20% probability of allograft rejection had a median washout period of 94 days [76]. The VITALITY multicenter study demonstrated that 86% of patients who experienced acute post-LT rejection received their last ICI dose less than three months prior to transplantation [77]. Additionally, the PLENTY trial of neoadjuvant pembrolizumab plus lenvatinib implemented a minimum 72-day washout period prior to transplant and reported no cases of acute rejection post-LT [78]. These findings collectively highlight the importance of considering the washout period when planning for LT in patients treated with neoadjuvant ICIs.

Currently, it is hypothesized that strong ICI affinity to its receptors causes lingering T-cell augmentation well into the post-LT period, enabling the LT recipient to mount an undesirable T-cell response to the transplanted allograft contributing to allograft rejection [70]. Studies suggest that these long-term effects of ICIs is also due to their relatively long half-lives, with pembrolizumab exhibiting a 13.8 month median duration of response and nivolumab combined with ipilimumab demonstrating a median duration of response of 17 months [64, 89, 102] Additionally, ICI in combination with VEG-F inhibitors such as bevacizumab, the antiangiogenic properties of the latter could persist and impair wound healing, if not allowed an appropriate amount of time to washout prior to LT [70].

As a result of the existing data, the Internal Liver Transplant Society guidelines (ILTS 2024) recommend a pre-LT washout period of 2 to 3 months to reduce the risk of allograft rejection and subsequent graft loss [103]. This recommendation may change in the future as more studies and prospective trials incorporate an absolute pre-LT ICI washout period into their protocols. For example, the PLENTY trial incorporated a minimum 72-day washout period, the ImmunoXXL trial is incorporating a minimum 30-day washout period and the MERITS-LT trial a minimum 12-week washout period [44, 78, 80, 81].

A retrospective review by Chao et al. demonstrated that patients who developed rejection experienced a shorter period between LT and immunotherapy compared to those who did not. However, we must consider that most reports of ICI use for recurrent HCC post-LT [89] describe ICI initiation only after systemic therapy, LRT or radiation failed to show improvement, with limited reports of immediate ICI initiation on the onset of disease recurrence (Table 2). Since ICIs have yet to be formally approved or recommended as maintenance therapy, or for treatment of post-LT HCC recurrence, prospective studies and trials are pertinent to elucidate efficacy and possible effect of LT to ICI duration and outcomes.

2). Degree of PD-1/PD-L1 expression

A higher degree of PD-L1 tumor expression has been associated with worse overall survival in patients with HCC [49, 89, 104]. However, patients with higher PD-L1 expression who undergo anti PD-L1 immunotherapy have shown improved survival. The KEYNOTE trial showed a correlation between the combined positive score of PD-L1 expression and response to pembrolizumab, while the CHECKMATE study revealed better survival for advanced HCC patients treated with nivolumab when PD-L1 expression was >1% compared to those with <1% (28.1 months vs. 16.6 months) [52, 105–107].

The role of PD-L1 expression in predicting iCCA outcomes remains variable, indicating a need for further research. In the prospective phase II trial, 14 out of 30 iCCA patients treated with toripalimab, lenvatinib, and gemcitabine plus oxaliplatin exhibited high PD-L1 expression, with tumor area positivity of ≥1 [69]. The high PD-L1 expression group demonstrated a higher albeit non-significant objective response rate (93% vs. 69%) compared to the PD-L1 deficient patient groups [69]. In the KEYNOTE-158 study, 58% of patients demonstrated high PD-L1 expression >1%. Only 6% of these patients demonstrated a response to ICI, translating to an objective response rate of 6.6% vs. 2.9%, relative to the PDL-1 deficient group [43, 66].

Increased PD-L1 expression during the peri-LT use of ICIs has been linked to a higher risk of post-LT graft rejection. In one study amongst 6 HCC recipients treated with ICIs, only one patient experienced acute rejection, and this patient was the only one with PD-L1 expression [107]. This raises the hypothesis that higher PD-L1 levels and the associated prolonged ICI response in HCC might lead to acute cellular rejection post-LT due to the same lingering effects that are advantageous in advanced HCC without transplantation.

The role of PD-L1 as a biomarker for predicting ICI efficacy post-transplant has been challenging, as some patients tested negative for PD-L1 expression pre-LT but subsequently showed positive expression after transplantation [50, 70, 72]. This discrepancy has potentially been attributed to PD-L1 heterogeneity and the dynamic nature of expression across different cell types, including liver cancer cells, immune cells and cirrhotic-noncancerous cells, which complicates its testing and utility as a predictive biomarker for ICI effectiveness [107, 108]. Therefore, additional studies are needed to better understand the role of PD-L1 and PD-1 expression and their utility as biomarkers for post-LT outcomes in the context of ICI use.

3). Density of Tumor-Infiltrating Lymphocytes

A high density of tumor-infiltrating lymphocytes (TILs), including CD3(+) and CD8(+) T cells, has been associated a lower risk of death for both HCC and iCCA (HR 0.58 and 0.71) compared to low TIL density [43, 109]. Gabrielson et al. found that high densities of CD3(+) and CD8(+) TILs in resected HCC tumors were linked to a lower rate of recurrence [110]. Additionally, this study found a correlation between TIL density and positive PD-L1 staining, which also predicted reduced rates of HCC recurrence[110]. The CHECKMATE study reported higher frequencies of CD3(+) TILs in HCC patients who exhibited complete or partial responses compared to those with stable disease[53, 105]. These findings suggest the potential of TIL expression as a tumor marker to determine patient selection and potentially predict efficacy of ICI in liver cancer.

Tumor-infiltrating lymphocytes (TILs) are believed to promote a more robust T-cell response, which can lead to favorable outcomes however, this response may be unfavorable in the peri-LT setting due to the risk of post-LT organ rejection [107, 110]. Studies investigating peri-LT ICI use in HCC have shown an increased risk of post-LT organ rejection is correlated with a higher CD4+/CD8+ ratio, suggesting a potentially strong response to ICIs with the accompanying adverse effects [107]. These findings underscore the potential prognostic roles of TIL density and PD-L1 expression in liver cancer, which could aid in patient selection and response prediction for ICI therapies.

4). AFP Levels and response

Alpha fetoprotein is currently the only approved biomarker for HCC, and is utilized in screening, prognostication and surveillance across all treatment modalities including LT [111, 112]. Studies have shown a 15–20% decrease in AFP levels in advanced HCC patients within 4–6 weeks of initiating ICI treatment. Additionally, these studies have demonstrated an association between early AFP response and improved outcomes with ICIs in advanced HCC [113–115]. Shao et al. found that among patients with a pretreatment AFP level greater than 20 ng/mL, those who exhibited an early response in terms of AFP decline had an improved response rate (64% vs. 10%) and disease control rate of (82% vs 14%) relative to late AFP responders [113]. Additionally, early AFP response was identified as a prognostic factor for longer progression-free survival (HR 0.546) [114]. Hsu et al. demonstrated that an AFP response of greater than 15% within 3 months of ICI initiation in the treatment of advanced HCC is a predictor of disease control, with an odds ratio of 11.7 [115].

While useful for prognostication of advanced HCC with ICI treatment, the relationship between AFP levels, response rates, and outcomes associated with peri-LT ICI use needs further investigation. In a systematic review of 91 LT patients treated with neoadjuvant ICI, two patients with AFP levels greater than 1000 ng/mL post-ICI therapy but before LT experienced rejection, with one resulting in fatality post-LT. These findings reflect existing guidelines that caution against LT for AFP levels exceeding 1000 ng/mL, with ICI use appearing not to be an exception [76].

5). Gene mutations and tumor mutational burden

A higher mutational burden of more than 10 mutations per million bases has been associated with improved ICI response rates and survival in both HCC and iCCA [69, 86, 116]. The KEYNOTE-158 trial demonstrated improved response rates to pembrolizumab in patients with solid organ malignancies, including HCC and iCCA, who had high tumor mutational burden (29% vs. 6%) compared to those without high expression [117]. In their phase II study involving 30 iCCA patients, Shi et al. found that 21 patients had DNA damage response (DDR)-related gene mutations. In this study, patients with DDR-related gene mutations in this study demonstrated a significantly higher objective response to the combination of toripalimab, lenvatinib, and gemcitabine plus oxaliplatin (90% vs. 56%) compared to patients without these mutations [69]. These genes are hypothesized to facilitate repair mechanisms that promote cell survival, including mismatch repair (MMR), nucleotide excision repair, and base excision repair, in response to genomic instability, stress, and damage [69, 118]. A higher degree of microsatellite instability/DNA mismatch repair has been specifically associated with improved ICI outcomes in liver cancer, leading to the FDA approval of pembrolizumab monotherapy for advanced BTCs with high microsatellite instability [50, 64, 66].

In light of these studies, it may be beneficial to incorporate genomic considerations for liver cancer patients in ongoing trials to evaluate the value of mutational burden in predicting responses and the efficacy of ICI treatment.

6). Patient/disease characteristics

Chao et al. found that younger age was associated with post-LT allograft rejection, while Rezaee-Zavareh et al. found that older age was also linked to an increased risk of allograft rejection with peri-LT ICI use [76, 89]. Some studies have shown that HCC patients with underlying viral diseases tend to have better ICI outcomes compared to those with non-viral diseases, suggesting a potential association between the etiology of liver disease and the efficacy of ICI treatment [40, 119]. Therefore, prospective studies are necessary to assess the possible impact of patient demographics and clinical factors, including the etiology of liver disease.

Conclusion

In conclusion, LT has emerged as a curative option for patients with HCC and iCCA, yet patient eligibility is often constrained by advanced disease. ICIs demonstrate a promising treatment modality for advanced HCC and iCCA, showing potential to improve response rates and convert patients from unresectable to resectable. However, ICI use in the peri-transplantation setting presents challenges, particularly regarding the risk of allograft rejection. Early studies indicate the efficacy of ICIs in downstaging tumors prior to LT, especially for HCC, while also revealing associated risks of rejection. This highlights the need for careful assessment of patient selection, timing of therapy, and investigation of predictors of success. While ongoing research is necessary to further explore the efficacy and expanded use of ICIs, it is essential to balance the benefits of ICI therapy against potential risks of complications in the context of LT. Continued research and refinement of treatment protocols will be crucial for expanding therapeutic options, including ICIs, and improving outcomes for patients with liver cancer undergoing LT.

Abbreviations:

HCC

Hepatocellular carcinoma

iCCA

Intrahepatic cholangiocarcinoma

ICI

Immune-check point inhibitors

LT

Liver transplantation

MASH

Metabolic dysfunction-associated steatohepatitis

UCSF

University of California San Francisco