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The intraperitoneal administration of paclitaxel has been shown to be a promising treatment strategy for peritoneal malignancy. The present study evaluated the effects of intraperitoneal administration of NK105, a paclitaxel-incorporating micellar nanoparticle, which has been shown to have a remarkable effect in a mouse model of gastric cancer. Intraperitoneal NK105 significantly reduced peritoneal tumors in vivo compared with the conventional paclitaxel formulation of paclitaxel solubilized in Cremophor EL and ethanol (PTX-Cre). Moreover, intraperitoneal NK105 significantly reduced the size of subcutaneously inoculated tumors, whereas no such effect was seen with PTX-Cre. Similar systemic toxic effects were observed following the intraperitoneal administration of both NK105 and PTX-Cre. Although NK105 disappeared rapidly almost within a day from the peritoneal cavity, the paclitaxel concentration in peritoneal nodules 4 h after intraperitoneal administration was significantly higher in the NK105 group than in the PTX-Cre group (P < 0.05), whereas there were no significant differences in liver paclitaxel concentrations between the two groups. We also evaluated the pharmacokinetics following intraperitoneal administration of NK105 and PTX-Cre. Serum paclitaxel concentrations 6, 12, 24, and 48 h after the intraperitoneal administration of the drugs were significantly higher in the NK105 than the PTX-Cre group. Furthermore, the peak serum concentration was higher in the NK105 than PTX-Cre group (24 100 ± 3560 vs 108 ± 25 ng/mL, respectively; P < 0.001), as was the area under the concentration–time curve from 0 to 48 h (191 000 ± 32 100 vs 1500 ± 108 ng·h/mL, respectively; P < 0.001). Therefore, intraperitoneal chemotherapy with nanoparticulate paclitaxel NK105 may offer a novel treatment strategy for improving drug delivery in gastric cancer with peritoneal dissemination because of enhanced drug penetration into peritoneal nodules and its prolonged presence in the systemic circulation. (Cancer Sci 2012; 103: 1304–1310)
Gastric cancer is the fourth leading cause of cancer-related deaths worldwide, and peritoneal dissemination is the most frequent and life-threatening form of metastasis and recurrence in patients with gastric cancer.[1, 2] The current systemic chemotherapy regimens for gastric cancer have not achieved satisfactory results, particularly in the treatment of peritoneal dissemination. One of the problems with this type of therapy is the limited delivery of systemically administered drugs to the peritoneal cavity. Intraperitoneal chemotherapy has been shown to be safe and effective in ovarian cancer[5-7] and is expected to provide a breakthrough in the treatment of gastric cancer with peritoneal dissemination.[8-10] However, in addition to the intraperitoneal administration of drugs, it is necessary to develop new strategies for the treatment of gastric cancer to achieve better results. Research into drug delivery systems (DDS) is ongoing and is mostly aimed at improving the penetration of drugs into tumors or prolonging the retention of drugs in the peritoneal cavity.[11, 12]
Paclitaxel, one of the most commonly used and effective anticancer agents, is not soluble in water and, for clinical use, it is conventionally solubilized using the polyoxyethylated castor oil Cremophor EL and ethanol (hereafter referred to as PTX-Cre), such as in TAXOL (Bristol-Myers Squibb, New York, NY, USA).[13, 14] The conventional PTX-Cre formulation is considered suitable for intraperitoneal chemotherapy because its larger molecular size (10–12 nm in diameter) compared with other water-soluble anticancer drugs results in prolonged retention of the drug in the peritoneal cavity. However, the Cremophor EL in the PTX-Cre formulation causes severe hypersensitive reactions, including anaphylaxis, in 2–4% of patients, even after premedication with antiallergic agents.[13, 14]
Since the discovery of selective accumulation by passive targeting, also known as the enhanced permeability and retention (EPR) effect, various nanoparticulate drugs have been developed for cancer treatment.[17-19] The EPR effect is based on the special characteristics of solid tumor tissues (i.e. incomplete vascular architecture, hyperpermeability of vessel walls, and incomplete lymphatic drainage). Through the EPR effect, anticancer drugs incorporated into polymeric nanomicelle carriers (20–100 nm in diameter) are retained for a long period in the systemic circulation,[21, 22] are easily extravasated from tumor vessels into the interstitium of tumor tissue, and accumulate there for longer periods than conventional small molecule agents.[16, 23] Various types of Cremophor EL-free formulations, including nanoparticle paclitaxel, have recently been investigated to reduce the risk of allergic reactions and to take advantage of the EPR effect.[24-28] For example, Abraxane (Celgene, Summit, NJ, USA), an albumin-bound paclitaxel, is currently in clinical use for the treatment of breast cancer.[29, 30]
NK105 is a paclitaxel-incorporating “core-shell-type” polymeric micellar nanoparticle formulation. Water-soluble and amphiphilic polymers (the shell), composed of a hydrophilic polyethylene glycol (PEG) and a hydrophobic polyaspartate, can form micelles with hydrophobic paclitaxel molecules enclosed in the micellar core. Thus, paclitaxel is made water-soluble without the need of Cremophor EL or ethanol. On average, each NK105 micelle measures approximately 85 nm in diameter, which is considered adequate to effectively utilize the EPR effect. Previous studies in animal cancer models have reported that intravenous administration of NK105 results in enhanced antitumor effects and reduced toxicity compared with PTX-Cre.[31, 32] The safety and pharmacokinetic advantages of NK105 in humans were demonstrated in a Phase I trial, whereas a Phase II trial of NK105 for advanced gastric cancer with failure of first-line chemotherapy reported an overall response rate of 25% and median overall survival of 14.4 months.
The intraperitoneal administration of nanoparticulate anticancer agents for the treatment of peritoneal dissemination has not been investigated extensively, despite the existence of data indicating the potency of this type of treatment. Previously, we showed that intraperitoneal administration of nanoparticulate paclitaxel results in deeper penetration of the drug into peritoneal nodules and enhanced antitumor effects compared with PTX-Cre.[36, 37] In the present study, we evaluated the intraperitoneal and systemic antitumor effects of NK105 administered intraperitoneally in an animal model of mouse. Moreover, we evaluated, for the first time, the pharmacokinetics of nanoparticulate paclitaxel following intraperitoneal administration.
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The primary aims of the present study were to clarify whether NK105 has any advantages over PTX-Cre in intraperitoneal chemotherapy for peritoneal dissemination and to examine the kinetics of intraperitoneally administered NK105. Treatment of mice with NK105 resulted in a significantly greater reduction in both peritoneal and subcutaneous tumors compared with PTX-Cre treatment. The pharmacokinetics study showed that intraperitoneally administered NK105 was absorbed rapidly into the systemic blood stream and that serum paclitaxel concentrations were maintained at significantly higher levels for longer in the NK105 compared with the PTX-Cre group.
Although the effects of intravenous NK105 have been reported by several studies,[31, 33] the effects of this nanoparticulate paclitaxel following intraperitoneal administration have not been investigated. An ideal agent for intraperitoneal chemotherapy should exit slowly from the peritoneal cavity to penetrate the peritoneal nodules from the surface. Water-soluble, low-molecular weight agents such as 5-fluorouracil or cisplatin are inappropriate for intraperitoneal chemotherapy because they are rapidly absorbed via the capillary blood vessels of the peritoneum after intraperitoneal administration.
We initially expected NK105 to be retained in the peritoneal cavity longer than PTX-Cre following intraperitoneal administration because of its larger particle size. However, NK105 disappeared from the peritoneal cavity more rapidly than PTX-Cre, resulting in increased serum concentrations of paclitaxel.
After absorption into the systemic blood stream, the Cmax and AUC values were significantly higher in the NK105 compared with PTX-Cre group, which is consistent with previously reported patterns after intravenous administration of NK105. It is reasonable that the reduction of subcutaneous tumors was greater in the NK105 group, because high serum concentrations and longer drug retention may produce an enhanced systemic antitumor effect. This enhanced systemic circulation of NK105 and the EPR effect may have contributed to a considerable extent to the regional antitumor effect against peritoneal nodules.
However, 4 h after intraperitoneal administration, when the transfer of drugs to the systemic circulation was probably not yet complete, the concentration of paclitaxel in peritoneal nodules was significantly higher in the NK105 group than in the PTX-Cre group, even though paclitaxel concentrations in the liver were similar in the two groups. This high paclitaxel concentration in peritoneal nodules in the NK105 group, especially at an early time point after intraperitoneal administration, may be due to enhanced direct penetration of nanoparticulate paclitaxel from the surface of the nodules, which has been shown previously using a different nanomicellar paclitaxel formulation.[36, 37] Another PEG-conjugated nanomicellar paclitaxel exhibited enhanced tumor accumulation and deeper penetration in a model of disseminated ovarian cancer. Thus, the route of administration (i.e. the direct penetration of the drug from the surface) may also have contributed to the enhanced antitumor effect of NK105 against peritoneal tumors.
The reasons why nanomicellar particles can penetrate deeply into the nodules or are absorbed from the peritoneum remain unclear, but an interesting affinity of cell membranes and micelles has been suggested. Certain amphiphilic polymers have been reported to penetrate non-endocytically or to fuse with plasma membranes and enter the cytoplasm of living cells within a few minutes without bilayer disruption. One of the problems associated with intravenous chemotherapy is the high interstitial pressure of solid tumors, which prevents chemotherapeutic agents, including nanoparticles, from leaking from the vessels and penetrating into the tumor tissue.[41-44] Other possibilities explaining the rapid absorption may include, for example, effective penetration of the drug into the tumor interstitial tissue and enhanced uptake by the lymphatic stomata or milky spots of the peritoneum. However, further studies are necessary to evaluate how nanomicellar paclitaxel, including NK105, penetrates into the tumor tissue and peritoneum.
The pharmacokinetics of intravenously injected NK105 have been characterized in both a mouse model and in humans. Both studies indicated a prolonged presence of NK105 in the systemic circulation compared with PTX-Cre. This long systemic retention indicates that NK105 exists in the blood stream in micellar form because free paclitaxel would be metabolized earlier. Therefore, intraperitoneally administered NK105 may exist in micellar form after absorption into the blood stream, which prolongs its presence in the systemic circulation.
Another advantage of NK105 is its water solubility without the need for Cremophor EL and ethanol. In preliminary experiments, we administered PTX-Cre and NK105 intraperitoneally in escalating doses (data not shown). Intraperitoneal administration of 50 mg/kg PTX-Cre had a deleterious effect on mice, which shivered and otherwise failed to move for several hours. At a dose of 80 mg/kg PTX-Cre, the mice did not survive longer for than 1 day. The same effects were seen when mice were treated with the Cremophor and ethanol vehicle alone. However, no apparent acute toxic effect was observed in the NK105 group, even at doses of 100 mg/kg administered intraperitoneally. Although NK105 is clearly less toxic than PTX-Cre, we administered equal doses (40 mg/kg paclitaxel) to mice in the present study.
There were no significant differences in body weight loss, hepatic toxicity, or renal toxicity between the PTX-Cre and NK105 groups, although the serum concentrations of paclitaxel were higher in the NK105 group. Liver paclitaxel concentrations were similar in the two groups. The apparently lower systemic toxicity of NK105 despite the presence of higher serum concentrations may be due, in part, to the fact that macromolecules do not tend to leak from normal vessels (the EPR effect), but further studies are required to clarify this point.
Prolonged retention in the peritoneal cavity and rapid clearance from the systemic circulation are believed to be important attributes for ideal drugs designed for intraperitoneal chemotherapy. Therefore, intravenous chemotherapy is generally combined with intraperitoneal chemotherapy to target primary sites or metastatic sites other than peritoneal dissemination. However, intraperitoneally administered NK105 may be sufficiently effective for both systemic and regional tumors because of its rapid transition from the peritoneum to the systemic circulation and its deep and rapid direct penetration into peritoneal nodules. This is a novel finding in the field of pharmacokinetics of intraperitoneal chemotherapy.
The present study has certain limitations. First, we were unable to use a single species of animal throughout all the experiments. Nude mice were used to evaluate the antitumor effect and different drug concentrations in peritoneal nodules, whereas rabbits were used for serum pharmacokinetic studies because of the need to obtain sufficient blood samples. Second, the effect of the polymer as the outer shell of NK105 alone was not examined in the present study. The amphiphilic polymer and paclitaxel form the outer shell and inner core of NK105, respectively; the paclitaxel in the inner core is essential for the stability of NK105. Thus, the polymeric micelle alone could not be used as a control for NK105. Third, hepatic and renal disorders were observed in the control group, which may have been the result of dehydration or cachexia due to the tumor burden itself. Therefore, further investigations may be required to more accurately evaluate the side effects of this type of therapy.
Intraperitoneally administered NK105 disappeared rapidly from the peritoneal cavity despite having a particle size larger than that of PTX-Cre. Future experiments should therefore be aimed at improving the efficacy of the system by prolonging the residence time of NK105 in the peritoneal cavity, with the expectation that this will increase drug penetration into the nodules. Controlling drug release and absorption by hydrogel conjugation,[11, 46] regulating peritoneal permeability by combination with bevacizumab, molecular targeting or pH control release.[48-50] and other new strategies are promising methods that could be combined with intraperitoneal NK105.
In conclusion, intraperitoneal administration of NK105 was more effective in an animal model against gastric cancer with peritoneal dissemination than PTX-Cre due to its enhanced penetration into peritoneal nodules and higher and longer retention in the systemic circulation. Intraperitoneal chemotherapy using nanoparticulate paclitaxel could be a promising strategy for the treatment of gastric cancer with peritoneal dissemination. A clinical study of intraperitoneally administered NK105 is expected to be performed in the near future to evaluate its efficacy in gastric cancer patients.