Hepatocellular carcinoma: Therapy and prevention

Abstract

Hepatocellular carcinoma (HCC) is one of the most common malignant tumors worldwide. The major etiologies and risk factors for the development of HCC are well defined and some of the multiple steps involved in hepatocarcinogenesis have been elucidated in recent years. Despite these scientific advances and the implementation of measures for the early detection of HCC in patients at risk, patient survival has not improved during the last three decades. This is due to the advanced stage of the disease at the time of clinical presentation and limited therapeutic options. The therapeutic options fall into five main categories: surgical interventions including tumor resection and liver transplantation, percutaneous interventions including ethanol injection and radiofrequency thermal ablation, transarterial interventions including embolization and chemoembolization, radiation therapy and drugs as well as gene and immune therapies. These therapeutic strategies have been evaluated in part in randomized controlled clinical trials that are the basis for therapeutic recommendations. Though surgery, percutaneous and transarterial interventions are effective in patients with limited disease (1-3 lesions, <5 cm in diameter) and compensated underlying liver disease (cirrhosis Child A), at the time of diagnosis more than 80% patients present with multicentric HCC and advanced liver disease or comorbidities that restrict the therapeutic measures to best supportive care. In order to reduce the morbidity and mortality of HCC, early diagnosis and the development of novel systemic therapies for advanced disease, including drugs, gene and immune therapies as well as primary HCC prevention are of paramount importance. Furthermore, secondary HCC prevention after successful therapeutic interventions needs to be improved in order to make an impact on the survival of patients with HCC. New technologies, including gene expression profiling and proteomic analyses, should allow to further elucidate the molecular events underlying HCC development and to identify novel diagnostic markers as well as therapeutic and preventive targets.

Key Words: HCC resection; Liver transplantation; Percutaneous ethanol injection; Radiofrequency thermal ablation; Transarterial embolization or chemoembolization; Chemotherapy; Gene therapy; Immune therapy; Prevention

INTRODUCTION

Hepatocellular carcinoma (HCC) is one of the most common malignant tumors worldwide[1-10]. The incidence ranges from <10 cases per 100 000 persons in North America and Western Europe to 50-150 cases per 100 000 persons in parts of Africa and Asia, where HCC is responsible for a large number of cancer deaths. However, a rise in the incidence and mortality of HCC, most likely reflecting the increased prevalence of hepatitis C virus (HCV) infection, has recently been observed in most industrialized countries[11-14].

The major etiologies of HCC are well defined (Table 1). An elevated body mass index, especially in men[15], and diabetes mellitus[9] are included among the well-known factors. Some of the steps involved in the molecular pathogenesis of HCC have been elucidated in the recent years. As for most types of cancer, hepatocarcinogenesis is a multistep process involving different genetic alterations that ultimately lead to the malignant transformation of hepatocytes. While significant progress has been made in recognizing the sequence of events involved in other forms of cancer, most notably in colorectal cancer and certain hematopoietic malignancies, the molecular contribution of the multiple factors and their interactions in hepatocarcinogenesis are still poorly understood. HCCs are phenotypically (morphology and microscopy) and genetically heterogenous tumors, possibly reflecting in part the heteroge neity of etiologic factors implicated in HCC development, the complexity of hepatocyte functions and the late stage at which HCCs usually become clinically symptomatic and detectable. Malignant transformation of hepatocytes may occur regardless of the etiologic agent through a pathway of increased liver cell turnover, induced by chronic liver injury and regeneration in a context of inflammation, immune response, and oxidative DNA damage. This may result in genetic alterations, such as activation of cellular oncogenes, inactivation of tumor suppressor genes, possibly in cooperation with genomic instability, including DNA mismatch repair defects and impaired chromosomal segregation, overexpression of growth and angiogenic factors, and telomerase activation[16-23]. Chronic viral hepatitis B, C, and D, alcohol, metabolic liver diseases such as hemochromatosis and α-1-antitrypsin deficiency as well as non-alcoholic fatty liver disease may act predominantly through this pathway of chronic liver injury, regeneration and cirrhosis. Accordingly, the major clinical risk factor for the development of HCC is liver cirrhosis since 70-90% of HCCs develop into a cirrhotic liver. Most HCCs occur after many years of chronic hepatitis that provides the mitogenic and mutagenic environments to precipitate random genetic alterations resulting in the malignant transformation of hepatocytes and HCC development.

Table 1 Major etiologies of HCC.

Chronic viral hepatitis B, C, D

Toxins (e.g., alcohol, aflatoxins)

Hereditary metabolic liver diseases (e.g., hereditary hemochromatosis,

α-1-antitrypsin deficiency)

Autoimmune hepatitis

Overweight, especially in males, and diabetes mellitus; nonalcoholic

steatohepatitis (NASH) or nonalcoholic fatty liver disease (NAFLD)

The HCC risk in patients with liver cirrhosis depends on the activity, duration and etiology of the underlying liver disease (Figure 1). Clinical and biological variables (age, anti-HCV positivity, PTT and platelet count) allow to further identify a subset of cirrhotic patients with the highest risk of HCC development[24]. Coexistence of etiologies, such as hepatitis B virus (HBV) and HCV infection, HBV infection and aflatoxin B1[23,25], HBV/HCV infection and alcohol or diabetes mellitus[26], or HCV infection and liver steatosis[27], increases the relative risk of HCC development. Also, occult HBV infection (anti-HBc positive only) carries a significant HCC risk[28,29]. In general, HCCs are more frequent in males than in females and the incidence increases with age. On the other hand, there is evidence that HBV and possibly HCV under certain circumstances play an additional direct role in the molecular pathogenesis of HCC. Finally, aflatoxins can induce mutations of the p53 tumor suppressor gene, thus pointing to the contribution of an environmental factor to tumor development at the molecular level. Furthermore, in a transgenic mouse model it has been shown that chronic immune-mediated liver cell injury without environmental or infectious agents is sufficient to cause HCC[30] and that inhibition of cytotoxic T lymphocyte-induced apoptosis and chronic inflammation by neutralization of the Fas ligand and prevents HCC development in this model[31]. In addition, in a transgenic mouse model it has been demonstrated that NF-κB may be the link between inflammation and HCC development[32,33]. Finally, individual polymorphisms of drug-metabolizing enzymes, such as cytochrome P450 oxidases, N-acetyltransferases and glutathione-S-transferase, may contribute to the genetic susceptibility to HCC development[34].

Figure 1 HCC Risk in Liver Cirrhosis

HCC screening, staging, and natural course

For the staging of HCCs, five systems have been proposed for the assessment of the extent and prognosis of the disease[35,36]: the Okuda staging system (Table 2)[37], the TNM classification and its modification by the Union International Contre Cancer (UICC), the Barcelona Clinic Liver Cancer (BCLC) classification[38] and the Cancer of the Liver Italian Program (CLIP) score[39]. The Okuda staging system is very effective for the identification of a subgroup of patients (Okuda III) with a very good prognosis of patients who should be treated with best supportive care (BSC) only. The BCLC classification appears especially useful for the selection of treatment but has not been independently validated. The CLIP score is superior to the Okuda staging system but has not been systematically assessed in patients undergoing resection or liver transplantation. Thus, one staging system is not clearly superior to the others. The natural course of the disease and the median survival of patients with HCC depend on the stage of the disease at the time of diagnosis. In patients with CLIP score 0 or Okuda stage I, the median survival is in the range of 23-69 mo, while in patients with CLIP score 3-5 or Okuda stage III, the median survival is only 1-14 mo[4]. The staging system is clinically most important for the appropriate choice of therapeutic strategy for individual patients.

Table 2 Okuda stages I-III of HCC.

Tumor mass	<50% of liver	≥50% of liver
Ascites	No	Yes
Albumin (g/L)	>3	≤3
Bilirubin (mg/dL)	<3	≥3
Points	0	1
Stage I	0 points
Stage II	1-2 points
Stage III	3-4 points

Treatment of HCC

Therapies for HCC can be divided into four categories: surgical interventions (tumor resection and liver transplantation), percutaneous interventions (ethanol injection, radiofrequency thermal ablation), transarterial interventions (embolization, chemoperfusion, or chemoembolization) and drugs including gene and immune therapy (Figure 2). Potentially curative therapies are tumor resection, liver transplantation, and percutaneous interventions that can result in complete responses and improved survival in a large number of patients. In selected cases, transarterial interventions result in palliation with good response rates and improved survival in some cases. Drugs as well as conventional radiotherapy have no proven efficacy.

Figure 2 HCC treatment.

Till date, surgical, percutaneous and transarterial interventions have not been compared in randomized controlled trials. Tumor resection and transplantation can achieve a 5-year survival rate of 60-70% in selected patients transplantation is the best treatment for patients with single lesions and advanced liver diseases, such as decompensated cirrhosis and multicentric small tumors. Percutaneous interventions, again in selected patients, result in a 5-year survival rate of 40-50%. The following different therapeutic options as well as primary and secondary HCC prevention are discussed in detail.

Surgical interventions

Resection in patients without concomitant liver cirrhosis (5% in Western countries, 40% in Sub-Sahara Africa and Asia), HCC resection is the treatment of choice with a low rate of life-threatening complications. By comparison, in the majority of patients with cirrhosis, strict selection is required to avoid resection-related complications, especially postoperative liver failure[40]. Apart from bilirubin and albumin concentration as well as platelet count and indocyanine green clearance[14,42], a recent study has identified an elevated serum concentration of 7s-collagen as an independent risk factor for postoperative liver failure[43].

Resection-related mortality should be <1-3%, and the 5-year survival rate should be >50%. In patients with normal liver function (normal indocyanine green retention rate and bilirubin level), absence of clinically relevant portal hypertension (hepatic venous pressure gradient <1.33 kpa, no esophageal varices, no splenomegaly, platelet count >100×10⁹/L) and one asymptomatic HCC lesion only, the 5-year survival rate of 70% can be achieved. By comparison in patients with clinically relevant portal hypertension, the 5-year survival rate is about 50% only and in patients with portal hypertension and evidence of impaired liver function, the 5-year survival rate is even lower.

After successful HCC resection, tumor recurrence in the cirrhotic liver (local recurrence as well as de novo tumors) in about 70% of patients after 5 years is a major clinical problem. The risk of recurrence is especially high in patients with microvascular invasion and/or additional tumor nodules[41,44]. Therefore, strategies aimed at secondary HCC prevention are of paramount importance.

Liver transplantation Liver transplantation is in principle the optimal therapeutic option for HCCs, because it simultaneously removes the tumor and the underlying cirrhosis, including the risk of HCC recurrence[40,45-47]. While broad selection criteria applied previously led to poor results with the recurrence rate of about 50% and the 5-year survival rate <40%, the current criteria for liver transplantation in patients with HCC (1 lesion <5 cm in diameter or maximum 3 lesions <3 cm in diameter) result in the 5-year survival rate of 70% or more and the recurrence rate <15%[48-50]. Possibly these criteria can be extended in future, depending on more experiences based on the stage of the disease, macrovascular invasion, histopathological characteristics (histopathology, aneuploidy, microvascular invasion) as well as DNA and RNA chip data (molecular signature, proteomic signature and others)[51,52].

Clinically, it is most important to shorten the waiting time for transplantation to <6 mo. This is difficult to achieve with cadaveric liver transplantation due to the shortage of donors. With a waiting time of >12 mo in some Western countries, the drop-out rate of patients is 20-50%. To bridge the time to transplantation and to prevent tumor progression, neoadjuvant treatments, such as percutaneous and transarterial interventions may lead to an improved outcome[53]. While marginal livers, domino donors, and split liver transplantation have no major impact, living donor liver transplantation has been shown to be an alternative to cadaveric liver transplantation. Around 3 000 interventions have been done worldwide. However, living donor liver transplantation is a complex procedure with a morbidity of 20-40% and a donor mortality of 0.3-0.5%[50,54,55]. Therefore, a very careful selection of patients and donors, including consideration of ethical, societal and legal issues is central to the successful implementation of living donor liver transplantation for the treatment of HCC patients[56].

Percutaneous interventions

Percutaneous intervention is the best option for small unresectable HCCs[57-59]. Tumor ablation can be achieved chemically by percutaneous ethanol injection (PEI) or acetic acid injection (PAI) or thermally by radiofrequency thermal ablation (RFTA), microwave heat-induced thermotherapy (HiTT), laser-induced thermotherapy (LiTT), or cryoablation. Apart from percutaneous interventions, these techniques can be applied also laparoscopically or after laparotomy.

Percutaneous ethanol injection PEI is the most widely used technique[60,61]. It is safe, easy to perform, inexpensive and can achieve complete tumor response rate of 90-100% in HCCs smaller than 2 cm in diameter, 70% in HCCs (3 cm in diameter) and 50% in HCCs (5 cm in diameter). Patients with liver cirrhosis Child A with complete responses can achieve a 5-year survival rate of 50% or more. Therefore, PEI is the procedure of choice for patients with a single HCC lesion smaller than 5 cm in diameter or with up to three lesions smaller than 3 cm in diameter (Figure 3).

Figure 3 Stage-Dependent HCC Treatment.

Radiofrequency thermal ablation RFTA is an alternative to PEI[58,59,62,63]. Several devices are available that can be applied percutaneously, laparoscopically, or during laparotomy. The efficacy of RFTA is similar to that of PEI but requires generally only a single session. While being more expensive than PEI, RFTA offers a better local tumor control and a potential advantage of allowing the ablation of tumors larger than 5 cm in diameter especially when newer generation devices are used. However, the 5-year survival rate after complete response to RFTA is currently similar to that of PEI (around 30-40%) depending on the child stage of the underlying liver cirrhosis. In a review of 3 670 patients treated with RFTA, the mortality is 0-5% and the complication rate is 8-9%[64]. Predictors of treatment response are tumor size and morphology (well encapsulated vs invasive).

Percutaneous HCC ablation by PEI and/or RFTA when considered together is an effective treatment for patients with HCCs that prolongs the tumor-free and overall survival time, especially if surgery is not feasible. This strategy has been evaluated also for the treatment of liver metastases[65].

Transarterial interventions

Transarterial embolization and chemoembolization are the most widely used treatments for HCCs which are unresectable or cannot be effectively treated with percutaneous interventions[66-69]. Embolization agents may be administered alone (embolization) or after selective intra-arterial chemotherapy (generally doxorubicin, mitomycin or cisplatin) or in combination with lipiodol (chemoembolization). Transarterial embolization or chemoembolization results in partial responses in 15-55% of patients, delays tumor progression and vascular invasion, and prolongs the survival time compared to conservative management. The most important aspect is the selection of patients, i.e., patients should have preserved liver function (Child A) and asymptomatic multinodular tumors without vascular invasion or extrahepatic spread[67,69]. In patients with advanced liver disease (Child B or C), treatment-induced liver failure may offset the antitumor effect or survival benefit of the intervention. As has been shown recently, postoperative adjuvant TACE may improve survival in patients with risk factors for residual tumor[70].

Radiation therapy

While radiotherapy plays only a minor role in the treatment of primary HCC, selective intra-arterial injection of ¹³¹iodine-labeled lipiodol has been performed in some patients[71] but needs further clinical evaluation before a recommendation can be made. Furthermore, high dose proton beam radiotherapy and external beam radiation as well as Yttrium-90 microsphere treatment have been recently explored in clinical trials in patients with unresectable HCC[72-74]. These strategies will certainly be further explored in clinical studies and may become a treatment option in future.

Drugs

A number of chemotherapeutic, hormonal and other drugs (Table 3) have been evaluated in clinical trials[75-77]. While most chemotherapeutic agents, such as tamoxifen[78], octreotide[79] and interferon[66], have not been shown to be effective in randomized controlled clinical trials, there are a number of substances that may deserve further clinical evaluation, such as gemcitabine[80,81], thymostimulin[82], α-1-thymosin[83], pravastatin[84], thalidomide[85] and megestrol acetate[86], antiangiogenic small molecules, Cox-2 inhibitors in combination with capecitabine and possibly others. Till date, however, none of these drugs can be recommended outside clinical studies.

Table 3 Drugs evaluated in clinical trials for the treatment of patients with HCC.

5-Fluorouracil

Capecitabine

Doxorubicin

Epirubicin

Etoposide

Cisplatin

Gemcitabine

Mitoxantrone

Interferon alpha

Megestrol acetate

Tamoxifen

Octreotide

Thalidomide

Thymophysin

α-1-thymosin

Experimental strategies

In view of the limited therapeutic options for advanced HCCs, a number of experimental strategies are being evaluated (Figure 4), including gene and immune therapies based on suicide, cytokine and antiangiogenic genes or DNA vaccination with tumor-specific genes[87-90], oncolytic viruses[91] as well as novel drugs such as 3-bromopyruvate[92].

Figure 4 HCC Experimental Therapeutic Strategies

HCC prevention

HCC prevention falls into two categories. Primary prevention that is aimed at the prevention of HCC development in patients with chronic liver diseases of different etiologies and secondary prevention that is aimed at preventing the recurrence and/or the development of new HCC lesions after successful surgical or non-surgical HCC treatment[93,94].

Primary HCC prevention Primary prevention is aimed at the interference with HCC development at four stages (Figure 5).

Figure 5 Primary HCC Prevention.

Stage 1: Interventions at this step are aimed at the prevention of acquired liver diseases. Apart from avoiding liver toxins, including alcohol and certain drugs, or infections with HBV or HCV by hygienic measures, avoiding parenteral exposure to blood, blood products or contaminated needles etc., a prime example is vaccination against HBV infection using commercially available active and passive vaccines. Several HBV vaccines using natural or recombinant hepatitis B surface antigen (HBsAg) from different sources are well introduced in clinical practice and universal vaccination in Taiwan has indeed already resulted in a decline of the incidence of HCCs[95]. In addition, novel HBV vaccination strategies are being explored, including a novel triple HBsAg recombinant vaccine[96], epidermal HBsAg powder immunization[97] as well as oral immunization using HBsAg transgenic plants[98-100]. Furthermore, DNA vaccination has been shown in animal models to induce antibodies against HBsAg/anti-HBs[101,102] even after topical application to the skin. For the prevention of HCV infection, however no effective vaccine is available till date. While several HCV vaccination concepts are being evaluated, including HCV proteins[103], HCV-like particles[104] as well as intravenous, intrahepatic, intraepidermal, intramuscular or oral cDNA immunization[105-109], it is not expected that a vaccine against HCV infection will become commercially available within the next few years.

Stage 2: Interventions at this step are aimed at the early treatment of acute liver diseases, thereby blocking their transition into chronic hepatitis that carries the risk of developing liver cirrhosis and its sequelae, including HCC development. While the principles mentioned above regarding liver toxins can also be applied here, the early diagnosis and treatment of inherited liver diseases, such as Wilson’s disease and hemochromatosis, are of paramount importance. Furthermore, recent studies suggest that early treatment of acute HCV infection prevents its progression to chronic hepatitis C[110-112].

Stage 3: Interventions at this step are aimed at the prevention of the progression of chronic hepatitis to liver cirrhosis that carries a high risk of HCC development. Apart from avoiding liver toxins and long-term use of high dose androgens or other anabolic steroids, the treatment of chronic hepatitis is of paramount importance. This includes the treatment of inherited, cholestatic or autoimmune liver diseases as well as the treatment of chronic viral hepatitis B or C. Reduction of iron overload by phlebotomy, for example, has been shown to stop the progression of hemochromatosis to liver cirrhosis and HCC. Treatment of chronic hepatitis B with interferon alpha or nucleoside analogs[113-118] and chronic hepatitis C with interferon alpha and now the combination of interferon alpha and ribavirin has demonstrated biochemical, virological, and histopathological improvements[119-123] and a lower incidence of HCC development[124-126].

Stage 4: Interventions at this step are aimed at interfering with the molecular events leading to HCC development, usually in a cirrhotic liver. These strategies include all treatment modalities detailed above (stage 3) as far as they can be implemented in patients with compensated or decompensated liver cirrhosis. In addition, some of the measures to prevent HCC recurrence after successful HCC treatment (secondary prevention) should in principle be useful for HCC prevention at this stage of the disease. Furthermore, some concepts of molecular therapy of HCCs should be applicable also in the prevention of HCCs. Without experimental pre-clinical data on these issues, it would be premature to discuss their potential clinical impact.

Secondary HCC prevention The prevention of a local recurrence and/or the development of new HCC lesions in patients after successful surgical or non-surgical HCC treatment (Figure 6) is of paramount importance and can significantly improve disease-free and overall patient survival.

Figure 6 Secondary HCC Prevention.

After successful HCC resection or non-surgical ablation, HCC recurrence in the remaining cirrhotic liver is the major limitation of life expectancy of these patients. The probability of recurrence is about 50% within 3 years after successful treatment[38,127]. Strategies to prevent HCC recurrence are therefore central to the improvement of survival of HCC patients after initial cure. Apart from liver transplantation after successful resection[44], the strategies explored to date include administration of polyprenoic acid (an acyclic retinoid)[128] interferon alpha[129] and interferon beta[130]. Furthermore, adoptive immunotherapy[131] and intra-arterial injection of ¹³¹iodine-labeled lipiodol[132,133] have been evaluated in clinical studies. All these interventions can result in lower HCC recurrence rates. These findings have to be confirmed in larger randomized controlled studies, demonstrating that a clear clinical benefit before secondary prevention with one of the strategies mentioned above should enter clinical practice.

Summary and perspectives

HCC is one of the most common malignant tumors in some areas of the world with an extremely poor prognosis HCC treatment is based on randomized controlled trials and many observational studies. Treatment options fall into four main categories: surgical interventions including tumor resection and liver transplantation, percutaneous interventions including ethanol injection and RFTA, transarterial interventions including embolization and chemoembolization, and drugs as well as gene and immune therapies. Though surgery as well as percutaneous and transarterial interventions are effective in patients with limited disease (up to three lesions smaller than 3 cm in diameter or one lesion smaller than 5 cm in diameter) and compensated underlying liver disease (cirrhosis Child A), at the time of diagnosis more than 80% patients present with multicentric HCC and advanced liver disease or comorbidities that restrict the therapeutic measures to BSC.

In order to reduce the morbidity and mortality of HCC, early diagnosis and the development of novel systemic therapies for advanced disease, including drugs, gene and immune therapies as well as primary HCC prevention are of paramount importance. Furthermore, secondary HCC prevention after successful therapeutic interventions needs to be improved in order to achieve better survival of patients with HCC. New technologies, including gene expression profiling and proteomic analyses, should allow to further elucidate the molecular events underlying HCC development and to identify novel diagnostic markers as well as therapeutic and preventive targets.