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Department of Molecular Physiology and Biophysics, Baylor College of Medicine, Houston, Texas 77030
| ABSTRACT |
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| INTRODUCTION |
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Our research group has developed experimental liposome aerosol formulations for pulmonary delivery of various active compounds, including the new potent anticancer lipophilic derivative of camptothecin 9-nitrocamptothecin (9NC). The aerosolized liposome 9NC formulation has proved effective against human cancer xenografts and experimental pulmonary metastases in mice at doses significantly lower than used by other routes of administration (4 , 5) . The human trials based on these results are under way.
PTX3 is the antineoplastic drug that demonstrated a therapeutic potential in lung cancer patients (6, 7, 8, 9) . PTX possesses a unique mechanism of action that differs from other anticancer drugs. It stabilizes microtubules by suppressing dynamic changes, affecting both growing and shortening, and leading to mitotic arrest (10 , 11) . In addition to the effects on mitosis, the broad antitumor activity of PTX may be the consequence of effects on the regulation of the cell cycle progression (12 , 13) . There may also be initiation of apoptosis (14 , 15) , alterations in the expression of gene products critical for tumor angiogenesis (16) , metastases (17) , or the host immune response (18) . In the clinic, PTX is administered by continuous i.v. infusion. However, PTX possesses a very low solubility in conventional aqueous vehicles, and the preparation approved for clinical use solubilizes PTX in mixture of polyethoxylated castor oil and ethanol. This vehicle may cause severe hypersensitivity reactions in humans (19 , 20) . Liposomes were found to be a viable alternative for the therapeutic use of PTX because of its improved toxicological and pharmacological characteristics (21 , 22) .
In the present report, we compared the effectiveness of PTX delivered to the lungs by liposome aerosol with i.v. administration of the same preparation. We also studied a therapeutic efficiency of PTX aerosol treatment on the growth of pulmonary metastases in the murine Renca model. We chose this model because renal cell carcinoma is characterized by a lack of early disease symptoms, which leads to a distant metastatic formation, including the lungs, in a majority of the patients at the time of diagnosis.
| MATERIALS AND METHODS |
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Animals.
Female ICR and BALB/c mice (78 weeks old) were obtained from Harlan Sprague Dawley (Indianapolis, IN) and housed in standard cages with food and water provided ad libitum. Experiments were performed with the approval of the Institutional Animal Care and Use Committee.
Cell Culture and Animal Model.
The mouse Renca cell line was kindly provided by Dr. Robert Wiltrout, National Cancer Institute (Frederick, MD). The cells were maintained in vivo by serial renal passages according to the protocol provided by Dr. Wiltrout. Before in vivo implantation, Renca cells were cultured in vitro for two passages as described previously (23)
. To induce pulmonary metastases, 100,000 cells were injected i.v. in 0.2 ml of saline via tail vein in syngeneic BALB/c mice.
Preparation of PTX-DLPC.
Liposomal formulation of PTX was prepared as described previously (24)
. Briefly, stock solutions of DLPC and PTX were prepared in t-butanol. Aliquots of PTX and DLPC (1:10, w/w) were mixed and then frozen at -70°C and lyophilized overnight to dryness. The formulations were stored sealed at -20°C. Before use, the mixture was reconstituted with sterile water and vortexed until a homogeneous multilamellar liposomal suspension was obtained. The initial concentration of PTX in suspension before nebulization was 10 mg/ml.
The drug-liposome suspension was examined by microscopy under polarized light for the presence of drug crystals and by quasielastic light scattering with Nicomp Submicron Particle Sizer (Model 370, Santa Barbara, CA) for liposome particle size, before and after nebulization. A heterogeneous starting liposomal suspension had a wide size distribution (2.025.3 µm). During nebulization process, the shear forces generated by extrusion through the jet orifice reduced the liposome size to 0.23 ± 0.17 µm. Microscope analysis revealed that no crystallization of PTX was noticed before and after nebulization in reservoir.
Aerosol Drug Delivery and Dosage Calculation.
The treatment with aerosol of mice that bore pulmonary tumors was performed as previously described (4)
. Briefly, an AERO-MIST jet nebulizer (cis-USA, Bedford, MA) was used to generate aerosol particles at the air flow rate of 10 liters/min. The aerosol particles were measured by Andersen Cascade Impactor and had a mass median diameter of 2.2 ± 0.2 µm (24)
. Aerosol particles with this size will deposit predominantly in the lung periphery. Mice were placed in sealed plastic cages and exposed to aerosol for 30 min. The aerosol was generated with 5% CO2-enriched air obtained by mixing normal air and CO2 with a blender (Bird 3M, Palm Springs, CA) and the CO2 concentrations were calibrated with a Fluid Fyrite (Bacharach Inc., Pittsburgh, PA). The use of carbon dioxide increased pulmonary deposition of PTX
3- to 4-fold (24)
, and the total deposited dose during a 30-min inhalation of PTX-DLPC was 5 mg/kg.
Pharmacokinetic Studies.
ICR tumor-free mice were used for these studies. One group received 5 mg/kg PTX i.v. via the tail vein. The other group received the same dosage of PTX during a 30-min inhalation. At each time point, three mice were killed by exposure to Isoflurane, USP (Abbott Laboratories, Chicago, IL) and exsanguinated. The lungs were rapidly excised, weighed, frozen and stored at -70°C until analyzed.
The extraction procedure for PTX has been described previously (24) . Briefly, the sample was homogenized in 3 ml of ethylacetate in a mini-beadbeater. Homogenates were transferred to 10-ml glass tubes and centrifuged at 1000 x g for 10 min. The supernatant fraction was separated, and organic solvent was evaporated with air. The residue was reconstituted in 0.2 ml of methanol:acetonitrile (2:1, v/v), sonicated in a water-bath sonicator, and centrifuged at 1000 x g for 10 min. Supernatant fractions were analyzed by HPLC.
PTX was quantified by reverse-phase HPLC with monitoring on a Waters 486 UV absorbance detector at 227 nm (Waters, Milford, MA). All of the measurements were made at room temperature on Waters Nova-Pak C18 column (3.9 x 150 mm). The mobile phase was composed of 49% acetonitrile and 51% water.
In Vivo Antitumor Activity.
BALB/c mice were inoculated with tumor cells on day 0. On day 1, they were randomly divided into groups of 10 mice. One group of mice was left untreated, the second group of mice received 5 mg of PTX/kg by aerosol, and the third group inhaled blank liposomes (DLPC) at the dose equivalent to that in the same dose of PTX formulation. Mice were treated for 2 weeks. After that, they were killed by exposure to Isoflurane, USP, and exsanguination. Lungs were resected and weighed. After that, lungs were fixed in Bovins fixative for tumor enumeration and sizing. In survival studies, mice were getting treatment until they died.
Statistical Methods.
The statistical significance of differences between groups was calculated by Students t test. Evaluation of survival data was performed using Gehans test (Primer of Biostatistics 4.0 software). Ps < 0.05 were considered to be statistically significant.
| RESULTS |
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) for PTX administered by aerosol or i.v. was 0.71 h and 0.02 h, respectively, and lung concentrations after aerosol treatment during the 3-h period of observation ranged from 5.5 to 23.1 µg/g of tissue.
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5 mg/kg by inhalation for 30 min 24 h after tumor inoculation. Additional treatments were given three times weekly at the same dosage and continued for 2 weeks. At that time, diffusely distributed, numerous, small tumor lesions were visible on lung surfaces. The antitumor effect of treatment was evaluated by the following parameters: lung weights, number of tumors, and median tumor size on the lung surface compared with untreated mice, or mice that received DLPC aerosol treatment only. In the PTX-DLPC group, lung weights were similar to the normal lung weights (179 ± 16 and 153 ± 19 mg, respectively; P > 0.05, two-tailed t test), whereas in untreated or DLPC-treated group, the lung weights were increased by 39 and 58%, respectively, compared with the normal lungs (Experiment 1; Table 1
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In a separate experiment, we further examined whether the inhalation of PTX would prolong the survival of mice bearing Renca lung metastases. Mice were inoculated with 100,000 cells into the tail vein. The treatment started on the next day and was given three times weekly. Another group of tumor-bearing mice was left untreated. The analysis of survival data (Fig. 3)
revealed a significant difference between treated and untreated groups (P = 0.035, Gehans test). Almost 40% of treated mice were alive on day 45, when all of the mice in the untreated group were dead. The mean survival time for treated mice was 43 ± 1 days; for untreated mice this value was 34 ± 1 days.
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| DISCUSSION |
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In clinical trials, PTX is administered by continuous infusion because it provides prolonged exposure of the neoplastic cells to the drug. Liebmann et al. (25)
had shown in vitro that the cytotoxicity of the drug was more dependent on increasing the duration of exposure than on increasing PTX concentrations. Aerosol delivery provides a continuous and direct exposure of the lungs to the drug. In our experiments, mice bearing Renca pulmonary metastases received
5-mg/kg doses of PTX by inhalation three times weekly during 2 weeks. This dose was substantially lower than the most frequently used doses for i.v. or i.p. administration,
20 mg/kg per injection (26, 27, 28)
. Survival studies demonstrated prolonged mean survival of treated mice compared with untreated (up to 25%). The dose-dependence studies revealed that aerosol treatment three times weekly was more effective than twice per week. However, when we increased the frequency of treatments up to five times per week, we noticed an increased aggressiveness in mice behavior after 1014 days of treatment, which might be explained in part by PTX-induced neurological toxicity (data not shown).
Although numerous preclinical studies have demonstrated the effectiveness of PTX in different tumor models, they used only invasive techniques, and the majority of the experiments were done with solid tumor xenografts. To our knowledge, this is the first study demonstrating the effectiveness of PTX aerosol treatment for local therapy of lung tumors in mice. However, although we did not achieve complete tumor growth arrest in this animal model, we believe that the effectiveness of this treatment may be further improved with combination therapy, using agents that have different mechanisms of action on cancer cells.
| FOOTNOTES |
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1 Supported by the Clayton Foundation for Research (Houston, TX). Presented in part at the 5th American Association for Cancer Research and Japanese Cancer Association Joint Conference, February 1216, 2001, Maui, HI. ![]()
2 To whom requests for reprints should be addressed, at Department of Molecular Physiology and Biophysics, Baylor College of Medicine, 1 Baylor Plaza, Houston, TX 77030. Fax: (713) 798-3475; E-mail: koshkina{at}bcm.tmc.edu ![]()
3 The abbreviation used are: PTX, paclitaxel; DLPC, dilauroylphosphatidylcholine; Renca, renal carcinoma cell line; HPLC, high-performance liquid chromatography. ![]()
Received 5/ 2/01; revised 7/23/01; accepted 7/23/01.
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