Hepatocellular carcinoma (HCC) is the fifth most common malignancy in the world and the estimated number of cancer-related deaths exceeds 500 000 per year (1, 2). Despite a regular follow-up with a strict screening and surveillance programme for the high-risk population of HCC, many patients have still been diagnosed at the advanced stage, often having multiple extrahepatic metastases. Locoregional or systemic chemotherapy has been considered as one of the available treatment modalities in these patients. However, until now, cytotoxic agents have improved the outcomes of HCC patients quite minimally in terms of tumour response or the overall survival of patients. Furthermore, frequently encountered serious side effects of cytotoxic agents, which are particularly intolerable in patients with cirrhotic liver, have made us disappointed in chemotherapy for patients with an advanced HCC (2, 3). The American Association for the Study of Liver Disease guideline consequently does not regard chemotherapy using cytotoxic agents as a standard treatment for advanced HCC (3).
Multidrug resistance (MDR) has been well-known as a major obstacle for conventional chemotherapy using cytotoxic agents. So far, genetic and epigenetic alterations in addition to environmental factors have been reported to induce drug resistance during chemotherapy. Such changes in turn may increase the drug efflux from cells, decrease the drug influx into cells, activate DNA repair, block apoptotic pathways and activate drug-metabolizing systems such as cytochrome P450 (4–10). In this issue of Liver International, Li et al. (11) are reporting important and promising data obtained by in vitro and in vivo experiments using HepG2/ADR cells, nude mice xenografted with HepG2/ADR cells and surgically resected human HCC samples. They intended to inhibit the growth of MDR HCC cells by a co-administration of recombinant mutant human tumour necrosis factor-α (rmhTNF-α), a sublethal dose of cytotoxic chemicals (adriamycin, mitomycin, 5-fluorouracil) and hydroxyapatite nanoparticles (nHAPs). Thus, their ultimate goal was to develop an effective therapeutic strategy for the treatment of advanced HCC with MDR.
Until now, various methods to overcome MDR have been tried. Among them, several cytokines including interleukins, interferons and tumour necrosis factor-α (TNF-α) have been examined. These cytokines can enhance the host antitumour mechanisms and regulate proliferation, differentiation and functional activation of target tumour cells (12).
Tumour necrosis factor-α is one of the most extensively studied cytokines in anticancer chemotherapy. Generally TNF-α plays an important role in the survival, proliferation, differentiation and death of cells. TNF-α acts just like a double dealer. TNF-α has pro-tumourigenic activities to stimulate the growth, proliferation, invasion, angiogenesis and metastasis of cancer cells by activating nuclear factor-κB (NF-κB), which is a major cell survival signal inhibiting the apoptotic process. On the other hand, TNF-α also suppresses the growth of tumours via its antiproliferative, cytostatic and cytolytic effects against various human cancer cells. It also enhances the cytolytic activity of human natural killer cells (12, 13). However, many cancer cells have been reported to be resistant to TNF-α-induced cytotoxicity. Furthermore, natural or recombinant TNF-α is known to have considerable side effects including fatal vascular collapse and the induction of widespread inflammatory responses.
Recent studies have focused on sensitizing cancer cells to TNF-α-induced apoptosis by inhibiting the survival signals such as NF-κB and reducing the toxicities of systemic TNF-α administration. One such attempt is by applying rmhTNF-α, which is made by mutating TNF-α using molecular engineering methods. The rmhTNF-α was reported to have an antitumour therapeutic window 60-fold greater than that of natural TNF-α (14). Furthermore, lectin-deficient TNF-α mutants were reported to have comparable anti-tumour activities with reduced toxicities and pro-metastatic capacities, as compared with natural TNF-α (15). In addition, TNF-α combined with cytotoxic agents has been proven to enhance the sensitivity to anti-cancer treatments in many tumours. Combination with bromocriptine and TNF-α also reversed HCC MDR in a model of liver neoplasm (16).
Before applying a novel therapeutic strategy to our patients, we have to review previous experiences extensively and learn painful but important lessons from our trials and errors. Many randomized trials using various cytotoxic agents such as adriamycin, 5-FU, cisplatin, etoposide, epirubicin and their combinations, have failed to show a survival advantage in spite of their relatively good tumour responses (4, 17). A recent phase III randomized trial compared the efficacy of doxorubicin therapy as a single agent vs. combined chemotherapy with cisplatin, interferon-α, adriamycin and fluorouracil (PIAF) in patients with unresectable HCC (18). The PIAF combination regimen did not improve survival despite a significantly higher overall tumour response rate compared with the doxorubicin group, mainly because of the more serious and frequent side effects in the combination group. Therefore, it may be more important to choose the best way to reduce serious side effects rather than the method with the highest therapeutic efficacy. It may improve the patients' outcomes greatly.
Li and colleagues chose rmhTNF-α as an adjuvant chemotherapeutic agent to reduce the severity and frequency of side effects without any reduction of therapeutic efficacies in the treatment of HCC. In contrast to 2500 U/ml of rmhTNF-α used by Azria et al. (19), they used 500 U/ml of rmhTNF-α in their in vivo studies to further minimize the side effects of TNF-α. Fortunately, in the current studies, they achieved good therapeutic goals even with such a low dose of rmhTNF-α. However, the safety of 500 U/ml of rmhTNF-α was not fully evaluated in their study, especially in case of a systemic administration, and thus the side-effect profile of an equivalent dose of rmhTNF-α in patients with HCC remains to be clarified in future clinical trials. Also, well-designed clinical trials are crucial to establish the adequate dose of rmhTNF-α and the proper route of administration to minimize the toxicities while conserving its beneficial therapeutic efficacies.
Administered cytotoxic agents bind to both normal tissues and serum proteins non-specifically in addition to binding to target tumour tissues. This leads to ‘wasting’ of often-expensive drugs and also increases the chance of serious drug toxicities. To overcome this problem, various types of engineered nanocarriers, which have highly active surfaces, have been developed as an anticancer drug delivery system. Multifunctional nanocarriers also have anticancer potentials against MDR cancer cells by different mechanisms including delivering tumour-specific targetting residues, drug efflux pump inhibition, modulating apoptotic thresholds and regulating intracellular pH of cancer cells (20). Hydroxyapatite is an inorganic component of hard tissues like teeth and bone material. Hydroxyapatite nanoparticles have been used as carriers in drug and gene delivery systems. Lie and colleagues also included these nanoparticles in their combination regimen, and proved the synergism of nHAPs and rmhTNF-α in the chemotherapy of multidrug-resistant HCC. These data may provide many clinicians and researchers who are interested in locoregional or systemic chemotherapy with a potentially efficient carrier of chemotherapeutic agents.
Intrinsic drug resistance is usually mediated by an enhanced cellular drug efflux of cytotoxic agents by the ATP-binding cassette (ABC) transporter family (6). These transporters include P-glycoprotein (P-gp/MDR1), MDR-associated proteins (MRP), and breast cancer resistant protein (BCRP/ABCG2).
Li and colleagues reported that rmhTNF-α could significantly reduce the expression of MDR and BCRP in their expression study of mRNA and protein, which have been considered to be very important molecules in the MDR of HCC. Thus, they suggested that rmhTNF-α-mediated underexpression of MDR1 and BCRP may attenuate the drug efflux function of the cells, which may in turn restore their sensitivity to the chemotherapeutics and thus suppress MDR.
So far, many other inhibitors of MDR transporters have also been tried in cancer therapy. However, the inhibition of cell growth by such agents appeared to be non-specific and commonly associated with many side effects. New generation drugs, which are more inhibitor specific for MDR transporters, also did not achieve good results in clinical studies. Antisense technology to block the MDR-specific gene expression using antisense oligonucleotides, anti-MDR1 ribozymes, and antisense RNA has also been tried, but is not yet applicable for routine clinical use (21).
Combination therapy of rmhTNF-α and other therapeutic modalities deserves to be evaluated. Combined systemic therapy with rmhTNF-α, α-interferons, interleukin-2, molecular targetted agents and radiotherapy can be considered. Also, locoregional chemotherapy using the proposed regimen in combination with immunotherapy or radiotherapy may be promising.
Although the in vitro and in vivo results of Li et al. (11) are very promising data, we still have a long way to go to overcome MDR in the chemotherapy of patients with advanced HCC. It is crucial to perform well-designed large-scaled clinical trials before accepting the usefulness of rmhTNF-α and/or nHAPs in the treatment of advanced HCC.