2014 Volume 37 Issue 2 Pages 206-211
Oxaliplatin (trans-l-diaminocyclohexane oxalatoplatinum; l-OHP), a third-generation platinum antitumor drug, is currently approved in combination with 5-flurouracil (5-FU)/leucovorin (FOLFOX) for standard first- and second-line treatment of metastatic or advanced-stage colorectal cancer. Despite l-OHP’s better tolerability in comparison with other platinum compounds such as cisplatin and carboplatin, its clinical efficiency is limited by the dose-limiting side effects including cumulative neurotoxicity and acute dysesthesias. In addition, like other platinum chemotherapeutic agents, l-OHP therapy is limited by reduced accumulation levels in tumor tissues, nonselective accumulation in healthy organs and/or tissues, inactivation by conjugation with glutathione, and the development of drug resistance. Accordingly, successful outcome of cancer treatment using l-OHP requires selective delivery of a relatively high concentration of the drug to tumor tissues. In this review we focus on utilization of different drug-delivery vehicles such as liposomes, polymeric nanocarriers, and carbon nanotubes in enhancing selective delivery of l-OHP to tumor tissues and consequently improving overall efficacy of l-OHP-containing drug-delivery systems.
In general, the efficacy of cancer chemotherapy is substantially limited by toxic side effects to normal cells and/or tissues, and by the development of multidrug resistance. These limitations result from lack of selectivity of anticancer agents to tumor cells and their inefficient delivery to the target tissue.1–3) The concept of drug targeting, suggested by Paul Ehrlich almost a century ago, has been extensively exploited to resolve many of these problems.4) To achieve drug targeting, significant efforts have recently been dedicated to the development of innovative nanocarrier platforms such as liposomes, polymeric nanoparticles, and carbon nanotubes that can maximize the delivery of chemotherapeutic agents to tumor tissue while minimizing their accumulation and toxicity in normal healthy tissues.4) These nanocarriers exploit the unique pathophysiological characteristics of tumor tissues, which enable their selective accumulation within the tumor tissue via either a passive- or active-targeting approach.
Oxaliplatin (l-OHP) is currently a cornerstone in the therapeutic regimens of metastatic or advanced-stage colorectal cancer.5–7) However, this agent does not show sufficient antitumor activity in vivo when used alone. This lower antitumor activity is mainly attributed to the high partitioning to erythrocytes and low accumulation in tumor tissues following intravenous administration.8,9) This article overviews the principal approaches that have been explored, ranging from conjugation to polymers to encapsulation in nanocarriers, to enhance the selective delivery of l-OHP to tumor tissue.
2. TARGETED DELIVERY OF OXALIPLATIN TO TUMOR TISSUE BY VARIOUS DELIVERY VEHICLESThe most common examples of delivery systems used for the selective targeting of l-OHP to tumor tissue include polymeric nanoparticles, polymeric micelles, liposomes, and carbon nanotubes. By virtue of their small size and various structural and physicochemical features, these delivery vehicles allow either passive targeting, via the enhanced permeation and retention (EPR) effect, or active targeting, via exploiting overexpression of unique antigens on the surface of tumor cells, of l-OHP to tumor tissue.
2.1 Polymer-Based Delivery Systems2.1.1 Polymeric NanoparticlesPolymeric nanoparticles have emerged as a versatile carrier system for targeted delivery of anticancer drugs.10,11) Compared with low-molecular weight anticancer drugs, polymeric nanoparticles can accumulate more in tumor tissue than in normal tissue due to the EPR effect.12) In addition, polymeric nanoparticles can prolong antitumor activity because of controlled release of the drug.13–15) Jain et al.16) investigated the potential of chitosan nanoparticles for colonic delivery of l-OHP following oral administration. To assure the colon-specific delivery of l-OHP, the surface of nanoparticles was coated with the enteric-coating polymer, Eudragit S100. To achieve active targeting, hyaluronic acid (HA) was conjugated to the surface of the polymeric nanoparticles. Induction of colonic tumor in C57BL/6 mice was conducted by implantation of HT-29 cancer cells into the mucosa of the ascending colon of irradiation-mediated immunodeficient mice. Oral administration of HA-coupled chitosan nanoparticles bearing l-OHP encapsulated in Eudragit S100-coated pellets exerted superior antitumor activity in HT-29 murine tumor model to either free l-OHP or HA-uncoupled chitosan nanoparticles of l-OHP. This potent antitumor activity was attributed mainly to the targeted delivery of l-OHP and consequently achievement of high local drug concentration in colonic tumors for a prolonged time.16)
2.1.2 Polymeric MicellesPolymeric micelles are another example of polymeric nanoparticles that have been extensively used to deliver chemotherapeutic agents to tumor tissues. Polymeric micelles have interesting structural characteristics such as a hydrophobic inner core and hydrophilic outer shell. A hydrophobic block forms the inner core of the structure, which acts as a drug reservoir especially for hydrophobic drugs, via hydrophobic interactions,17–19) whereas the hydrophilic outer shell plays a role in avoiding endocytosis of these particles by the cells of mononuclear phagocyte system, which is a major obstacle to the targeting of drugs to tumors. Polymeric micelles have the advantages of small particle size ranging at 20–100 nm, good structural stability, easy sterilization, controllable drug loading and release, and favorable biodistribution including prolonged circulation in blood and enhanced tumor accumulation.20–23) Cabral et al.24) developed polymeric micelles incorporating dichloro(1,2-diamino-cyclohexane)platinum(II) (DACHPt), the l-OHP parent complex, through polymer-metal complex formation of DACHPt with poly(ethylene glycol)-b-poly(glutamic acid) [PEG-b-P(Glu)] block copolymer. In vivo biodistribution and antitumor activity experiments in CDF1 mice bearing the murine colon (C-26) adenocarcinoma showed 20-fold increase in the intratumor accumulation of DACHPt-loaded micelles compared with free l-OHP, resulting in potent tumor growth-inhibitory effect. Furthermore, micellar DACHPt efficiently reduced tumor spreading in the peritoneal cavity following intraperitoneal injection of HeLa tumor cells. These results suggest that DACHPt-loaded micelles could be an outstanding drug delivery system for l-OHP in the treatment of solid tumors.24)
In a recent study, Rafi et al.25) evaluated the targeting ability and therapeutic efficacy of systemically injected DACHPt-loaded micelles against a well-established experimental model of scirrhous gastric cancer, which was prepared by orthotopic inoculation of OCUM-2MLN scirrhous gastric cancer cells. They demonstrated that DACHPt-loaded micelles, with 30 nm diameter, efficiently penetrated and accumulated in the orthotropic scirrhous gastric cancer model, leading to inhibition of tumor growth. In addition, the elevated localization of systemically injected DACHPt-loaded micelles in metastatic lymph nodes reduced the metastatic tumor growth compared with free l-OHP. These results suggest that DACHPt-loaded micelles may represent a novel therapeutic entity for the treatment of scirrhous gastric cancers and their lymphatic metastases.
To overcome side effects and improve antitumor efficacy Wang et al.26) used stearic acid-g-chitosan oligosaccharide (CSO-SA) polymeric micelles as a delivery system for l-OHP. They demonstrated that l-OHP-incorporated micelles showed excellent internalization ability resulting in an increased l-OHP accumulation in colorectal cancer cells. Furthermore, intravenous administration of l-OHP-incorporated micelles effectively suppressed tumor growth compared with free l-OHP treatment. The results of this study suggest that CSO-SA micelles may represent a promising delivery carrier for l-OHP against colorectal cancer.
2.1.3 Polymeric NanoGelNanoGel is another class of drug carriers that significantly enhances possibilities for the control of drug binding capacity and drug release characteristics compared with common nanoparticles. These carriers represent cross-linked/interpenetrating networks constructed by thermo- or pH-sensitive components such as poly(acrylamide) (PAAM), poly(N-isopropylacrylamide) (PNIPAM), poly(acrylic acid) (PAA), or polystyrene (PS).27) An essential property of NanoGel particles is that their size lies in the range 20–220 nm, which makes them profoundly different from hydrogel, nanospheres, and microspheres with sizes ranging at 0.5–500 microm proposed previously for polynucleotide delivery.28) It is expected that due to the smaller size, NanoGel particles will be less susceptible to reticuloendothelial system clearance and will better penetrate in tissues and cells.29) Based on the aforementioned data, Chen et al.27) have recently developed a novel type of NanoGel using biocompatible materials, hydroxypropylcellulose–poly(acrylic acid) (HPC–PAA) loaded with l-OHP. They evaluated the in vitro cytotoxicity of l-OHP-loaded HPC–PAA NanoGel particles against human gastric cancer BGC823 and demonstrated that in vitro cytotoxicity of l-OHP-loaded HPC–PAA NanoGel particles is comparable to that of free drug. In addition, considering the continual release property of l-OHP in the NanoGel particles (approximately 70% release in 48 h), they assumed that l-OHP-loaded HPC–PAA NanoGel particles might exert higher anticancer activity than free l-OHP on in vivo administration, and thus verified the advantage of HPC–PAA gel particles as a drug carrier.
2.1.4 Polymeric MicrospheresPolymeric microspheres present a flexible platform for applications in diagnostics and therapeutics. The chemical nature of l-OHP makes this agent highly suited to delivery systems that utilize conjugation to polymers. Li et al.30) recently prepared a slow-release poly-lactic-coglycolic acid (PLGA)-l-OHP microsphere by a spray-drying method and assessed the therapeutic efficacy and safety of this preparation in in vivo subcutaneously inoculated colorectal tumor models of nude mice. The size of the microsphere was less than 100 µm, drug loading was 18–22%, and drug release time lasted as long as 30 d. Intra-tumor administration of PLGA-l-OHP microspheres significantly restrained tumor growth and this effect was strongly correlated with increased apoptotic induction within tumor tissue. Weight measurement and blood analysis did not suggest significant adverse effects on the mice during the study. They suggested that the developed PLGA-l-OHP microsphere was suitable for regional use and might show promise in reducing local recurrence of colorectal cancer after resection.30)
2.2 Carbon NanotubesSince carbon nanotubes (CNTs) were discovered by Iijima in 1991,31) studies of CNTs have progressed rapidly in many fields, and currently CNTs are also used as carriers for drug delivery in nanotechnology.32) CNTs have proved able to transport a wide range of molecules across membranes and into living cells.33–35) In addition, their structural stability may prolong circulation time and bioavailability of the loaded molecules. Recently, Wu et al.36) demonstrated that multi-walled carbon nanotubes (MWCNTs) could be used as a promising carrier for targeted delivery of l-OHP to tumor tissue. In this study, l-OHP was incorporated into the inner cavity of MWCNTs. Surface functionalization of MWCNT with PEG 600 effectively reduced the cytotoxicity of MWCNTs and improved their water solubility. More importantly, the presence of PEG molecules was found to slow down the release rate of l-OHP from the cavity of MWCNTs, thus reducing the escape of l-OHP to reaching tumors and consequently significantly improved the cytotoxic activity of l-OHP against colorectal HT-29 cells in vitro. Therefore they assumed that l-OHP-containing MWCNTs could potentially be useful for treatment of colorectal cancer. However, further studies should be conducted to evaluate the safety, bioavailability, and targeted efficacy of such MWCNTs in vivo.36)
2.3 LiposomesLiposome is one of the first nanomolecular drug delivery systems to show increased delivery of small-molecular weight anticancer drugs to solid tumors by altering biodistribution of associated drug. Consequently, liposomes have received intensive attention during the past 30 years and have resulted in several liposomal drugs approved for clinical application or undergoing clinical evaluation.37–40) Liposomes, sometimes called lipidic nanoparticles, are phospholipid bilayer vesicles that self-assemble when naturally occurring, or synthetic, phospholipids are hydrated with excess water or aqueous salt solutions forming closed bilayer structures. Because of their amphiphilic nature, liposomes can accommodate a variety of drugs with different physicochemical characteristics.41–43) Surface modification of liposomes by insertion of polyethylene glycol (PEG)-derivatized phospholipids into liposomal membrane, known as PEGylation, has endowed liposomes with favorable pharmacokinetic properties.44–46) In addition to prolongation of the plasma half-life of the encapsulated drug, drug delivery by liposomes exploits the EPR effect of the tumor blood vessels and allows liposomes to enter the tumor tissues.47,48) This increases accumulation of drug within the tumor sites, thus reducing the amount of drug that can penetrate into healthy tissues and thereby reduces systemic toxicity. Moreover, the possibilities of specifically targeting the pathological cells and triggered drug release induced by light, heat, magnetic field, or enzymes49,50) increase the therapeutic potential options of liposomal drugs. According to the aforementioned merits of liposomes, many studies have focused on utilizing liposomes for targeted delivery of l-OHP.
Yang et al.51) investigated the antitumor efficacy of PEG-liposomal l-OHP in a xenograft tumor-bearing nude mouse model. They demonstrated that intravenous administration of PEG-liposomal l-OHP significantly enhanced accumulation of l-OHP in tumor tissues via the leaky tumor vasculature by the EPR effect, leading to a remarkable reduction in tumor burden and prolongation of mice survival, compared with free l-OHP. They suggested that PEG-liposomal l-OHP can represent an effective alternative to free l-OHP in the treatment of colorectal carcinoma.51)
Coupling of a targeting moiety to the liposomal surface or to the distal end of PEG at the liposomal surface may increase therapeutic efficacy of encapsulated drug.52–55) Suzuki et al.56) exploited this principal in developing a novel target-sensitive PEGylated liposome in which transferrin (Tf) is coupled to the extremities of surface-grafted PEG chains. Because transferrin receptors (TfR) are overexpressed on the surface of many tumor cells, this pathway has been assumed effective for targeted delivery of Tf-conjugated liposomes to tumor cells overexpressing TfR. They confirmed that the tumor-selective delivery of l-OHP in a C-26 colon cancer murine model, using Tf-conjugated PEGylated liposomes, resulted in enhanced extravasation of liposomes into tumors, and increased tumor suppression compared with l-OHP-containing PEGylated liposomes, l-OHP-containing classical liposomes, or free l-OHP. They attributed this potent antitumor efficacy to the ability of Tf-conjugated PEGylated liposomes efficiently to deliver l-OHP into the cytoplasm of tumor cells via TfR-mediated endocytosis after extravasation by the EPR effect.56) Currently, Tf-targeted liposomal l-OHP is under phase II clinical investigation for the treatment of gastric cancer and gastroesophageal junction cancer.
Recently, anti-angiogenic therapy has gained increasing attention as a promising approach to limit or even reverse the growth of tumors.57–60) Tumor endothelial cells express specific surface antigens, not present in blood vessels of normal tissues, which are suitable for targeting purposes. These specific “vascular zip codes” have been exploited to achieve active vascular targeting by means of liposomes. Based on these data, we recently developed PEG-coated cationic liposomes that have in vivo long-term circulation properties as well as selective binding properties, to achieve tumor vascular targeting.9) The PEG-coated cationic liposome was composed of the following in a molar ratio of 2 : 1 : 0.2 : 0.2, respectively: hydrogenated soy phosphatidylcholine (HSPC), cholesterol (CHOL), O,O′-ditetradecanoyl-N-(α-trimethylammonioacetyl)diethanolamine chloride (DC-6-14), and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-n-[methoxy (polyethylene glycol)-2000] (mPEG2000-DSPE). Then, we investigated the anti-angiogenic potential of these cationic liposomes upon the encapsulation of l-OHP. The results emphasized that encapsulation of l-OHP in PEG-coated cationic liposomes not only prolongs its circulation time by protecting the drug from partitioning to erythrocytes, which constitutes a major limitation of the in vivo therapeutic efficacy of the drug, but also allows selective delivery of l-OHP to tumor vasculature. In addition, l-OHP delivered to the tumor vasculature succeeded in completely suppressing tumor-induced angiogenesis compared with either l-OHP-containing PEG-coated neutral liposomes (lacking cationic lipid) or free l-OHP. The strong anti-angiogenic effect of l-OHP-containing PEGylated cationic liposomes was attributed to selective delivery of the drug to blood vessels in the tumor and its subsequent uptake by tumor endothelial cells.9) In a later study, we investigated the antitumor efficacy of l-OHP-containing PEGylated cationic liposomes in a murine tumor-xenograft model, and demonstrated that l-OHP-containing PEGylated cationic liposomes exerted superior antitumor activity over either free l-OHP or l-OHP-containing PEGylated neutral liposomes. The superior antitumor activity of l-OHP encapsulated in PEG-coated cationic liposomes was confirmed due to dual-targeting activity of PEGylated cationic liposomes against both tumor vascular endothelial cells and tumor cells.9,61)
In a very recent study, we addressed the effect of dosing schedule on antitumor activity of l-OHP-containing PEGylated cationic liposomes.62) We emphasized that the intratumor accumulation of l-OHP-containing PEGylated cationic liposomes is dependent on the dosing schedule. Administration of liposomal l-OHP every 4 d significantly enhanced the intratumor accumulation of PEGylated cationic liposome that was subsequently injected, and the therapeutic efficacy was increased. By contrast, administration of liposomal l-OHP once weekly resulted in lower antitumor activity compared with the 4-d administration schedule. This difference in therapeutic efficacy between the two dosing regimens may be correlated with the degree of tumor angiogenic vessel maturation. As shown above, the cationic liposomes could selectively bind to the newly formed (immature) tumor angiogenic vessels, but not to the pre-existing mature blood vessels.9,63) A 1-week interval between injections might be enough for the maturation of tumor angiogenic vessels, and the cationic liposomes might therefore lose their binding sites in the solid tumor. Consequently, the therapeutic efficacy of l-OHP-containing PEGylated cationic liposomes was lower when administered once weekly than every 4 d. Furthermore, the effect of sequential administration on the antitumor efficacy of l-OHP-containing PEGylated cationic liposomes was investigated.62) The tumor accumulation levels of test-PEGylated cationic liposomes in mice pretreated with a single liposomal l-OHP injection were similar to those in nontreated (control) mice. On the other hand, in mice pretreated with two successive injections of liposomal l-OHP, the tumor accumulation of cationic test liposomes was significantly higher than in control mice or in mice pretreated with a single injection. The enhanced intratumor accumulation of test PEGylated cationic liposomes after two successive injections of liposomal l-OHP was attributed to the cumulative cytotoxic effect of the pre-injected liposomal l-OHP formulation on both vascular endothelial cells and tumor cells. Because of their prolonged circulation time in blood, l-OHP-containing PEGylated cationic liposomes were likely to have easy access to tumor endothelial cells and readily to extravasate from the blood stream into the tumor interstitial space due to the EPR effect, thus gaining access to the tumor cells. Once in the tumor tissue, l-OHP-containing liposomes could be internalized by both tumor endothelial cells and tumor cells, as demonstrated earlier.61) Thus liposomal l-OHP would be allowed to exert its cytotoxic effect64,65) and may bring about a decrease in the number of tumor cells and, consequently, a decrease in tumor interstitial pressure, thus allowing deeper penetration of the test-cationic liposomes.
Recent advances in the fields of drug targeting and drug delivery have been extensively used to conserve tumor cytotoxicity while reducing toxicity to normal healthy tissues, resulting in a higher therapeutic efficacy and increased safety of many chemotherapeutic agents. These advances show great promise for manipulation of the therapeutic efficacy and/or undesired side effects associated with l-OHP that has already been used to treat many cancer patients and has led many l-OHP-containing formulations to undergo clinical investigation.
This work was supported in part by a Grant-in-Aid for Scientific Research (B) (24390010), the Ministry of Education, Culture, Sports, Science and Technology of Japan.
