Cancer is one of the world’s leading causes of death. Recent discoveries in cancer biology are providing novel therapeutic tools to improve cancer treatments, especially concerning the immunotherapeutic approaches. Currently, new promising drug delivery approaches have paved the way to a new era of chemotherapeutic anticancer treatments. Although cancer cells are intrinsically more sensitive than normal cells to the exposure of chemotherapeutics, the anticancer drugs hold non-selective pharmacological activity, which is responsible for severe systemic cytotoxicity towards normal tissues [1]. Unspecific cytotoxicity is the major factor that constrains dose and frequency of treatment and, in turn, the therapeutic protocol. Relevant efforts have been made to overcome the unspecific toxicity of chemotherapy and treat tumors by more selective approaches that target cancer cells while sparing normal ones. Accordingly, the pharmaceutical research has focused on the development of novel tailor made drug carriers for both existing drugs and new therapeutic molecules, which includes the identification of new molecular targets of cancer cells and of cellular components of the surrounding stroma that control tumor progression. Targeted drug delivery systems must be non-toxic, biocompatible, non-immunogenic, biodegradable and physicochemically stable in vitro and in vivo [2]. By a biopharmaceutical point of view, these systems must preserve the loaded drug from chemical and enzymatic degradation, deliver the drug payload to the target site and provide for timely controlled drug release [3]. So far, several lipid or polymeric nanocarriers have been developed to deliver anticancer drugs according to spatial and time controlled processes. However, these exogenous systems are usually quickly removed from blood circulation by the reticuloendothelial system in the liver, spleen, and bone marrow [4]. In order to control the clearance by the phagocytic system, drug nanocarriers have been surface coated with water-soluble synthetic and natural flexible polymers, namely poly(ethylene glycol) (PEG), poly(vinylpyrrolidone) (PVP), dextran, chitosan and pullulan [5], which bestow them “stealth” properties that inhibit contact and removal by the mononuclear phagocytic system [6] and prolong circulation time in the blood stream. Furthermore, the polymeric coating can prevent in vitro and in vivo nanocarrier aggregation, thus conveying the systems high colloidal and pharmaceutical stability. However, despite the progresses in the development of anticancer drug delivery systems with enhanced therapeutic profiles, the low stability, low permeability through biological barriers, short half-life in the blood circulation, chemical and enzymatic degradation, unspecific targeting, and immunogenicity still remain unsolved limits of these formulations.
Nanotechnology applied to medicine
Despite only few nanomedicines have been so far released for clinical use [7], there is increasing optimism that nanotechnology applications to medicine will bring significant advances in the diagnosis and treatment of human diseases. Engineered nanoparticles (NPs) offer unique opportunity for achieving controlled drug delivery that can be beneficial for the treatment of poor prognosis diseases, including cancer. Notably, size of the nanosystems used for drug delivery, ranging from few nanometers like proteins to tens or hundreds of nanometers like viruses, and surface properties are key features in dictating their biodistribution and the pharmacokinetic profiles. According to the colloidal size, NPs can selectively dispose in solid tumors through the passive mechanism known as enhanced permeation and retention (EPR) effect first demonstrated by Matsumura and Maeda in 1986 [8]. The EPR effect results from the extravasation of macromolecules and colloidal systems from the blood stream to the tumor by virtue of the characteristic leaky vasculature and poor lymphatic drainage of this tissue [9,10]. Doxil (OrthoBiotech) is the first colloidal therapeutic system for anticancer drug delivery (doxorubicin loaded liposomes) approved by the US Food and Drug Administration (FDA) in 1995 for the treatment of HIV-related Kaposi’s sarcoma [11,12]. Although the first generation colloidal anticancer drug delivery systems have been mainly designed to exploit the EPR effect, several experimental evidences have shown that this approach is often ineffective due to the peculiar anatomical and physiopathological features of solid tumors that often prevent accumulation and in depth access of nanocarries in the tumor tissue [13]. Oppositely to the passive targeting, the second-generation of colloidal anticancer drug delivery systems have been designed for “active” drug delivery, in which either the therapeutic agent or the carrier is functionalized with ligands that recognize cell targets that are over-expressed in a specific malignancy [14]. Colloidal delivery systems decorated with targeting agents have been developed for the biorecognition of hepatocarcinoma-specific cell membrane receptors [15]. Hepatocellular carcinoma (HCC), the major form of primary liver tumors, is one of the most common cancers worldwide, the fourth one for incidence rate, being a major cause of death in Eastern Countries [16]. Despite the increasing knowledge on the molecular mechanisms underlying hepatic carcinogenesis, effective therapeutic drugs are still an unmet clinical need and liver transplantation remains the option with higher success rate. Hepatocarcinogenesis is accompanied with complex aberrations in developmental and oncogenic cellular signaling pathways and new potential therapeutic targets represent one of the major goals of current research studies [17]. The only currently available chemotherapy for HCC is Sorafenib, but this treatment shows low response rate and drug resistance, which results in cancer recurrence, poor survival and/or poor quality of patient’s life [18].
Proposed targets for “active” drug delivery in HCC therapy
Although active targeting is often considered the new frontier of drug delivery and many successful results have been obtained in tumor selective targeting, major issue of this approach is the sharply discrimination in vivo of cancer cells from normal cells. Indeed, when administered to human, the colloidal systems encounter a number of biological barriers that can dramatically weaken the biorecognition and selective disposition of these systems making them ineffective. The physiological massive disposition of most of colloidal systems in the liver makes the discrimination of HCC cells from healthy hepatocytes even more complicate. Therefore, the discovery and exploitation of specific HCC targets is paramount to achieve successful and clinically effective HCC therapy.
Asialoglycoprotein receptor. The asialoglycoprotein receptor (ASGPR) or Ashwell–Morell receptor, is largely expressed by hepatocytes while it is poorly distributed in extra-hepatic tissues [19].
ASGPR binds with high-affinity galactose or galactosamines but also glucose and polysaccharides or polymers containing saccharide residues [20]. Therefore, galactose, lactose, lactoferrin moieties, as well as galactose terminating glycoproteins or polysaccharides and carbohydrates with repetitive galactose or glucose units have been exploited to produce drug delivery systems that selectively target hepatocytes. ASGPR is overexpressed by HCC cells thus making this receptor one of the most studied target to selectively deliver anticancer drugs to HCC. Accordingly, a variety of colloidal drug delivery systems based on polysaccharides or containing saccharide functions have been developed to selectively and actively deliver anticancer drugs to HCC cells. However, despite several promising results have been obtained in in vitro models, in vivo systems relaying on ASGPR biorecognition have been often found to be poorly effective. Actually, the ASGPR overexpression on HCC cells, which is at the basis of the discrimination of HCC cells from healthy hepatocytes, undergoes downregulation as the HCC [21]. Therefore, drug delivery systems based on ASGPR targeting can be effective only in the early stage of HCC development, while they are ineffective at later stages and would require a preliminary mapping of the ASPGR expression. [22].
GPC3. GPC3 is a member of membrane-bound heparan sulfate proteoglycans and is specifically expressed on HCC cell surface [23]. Several anti-GPC3 monoclonal antibodies (mAbs) have been produced for immunotherapy of HCC [24,25] that have been also associated with Wnt/beta-catenin antagonists, proposed as additional therapeutic targets [26]. Due to the high expres¬sion of GPC3 on HCC cells and the high affinity and specificity of anti-GPC3 mAbs for this target, anti-GPC3 mAb-functionalized theranostic nanovectors have been recently developed to selectively deliver siRNA to HCC cells [27].