Radiosensitization of a mouse tumor model by sustained intra-tumoral release of etanidazole and tirapazamine using a biodegradable polymer implant device

Targeting of radio-enhancing drugs

OBJECTIVE

Toxicity to normal tissue is frequently the dose-limiting factor in the chemotherapy and mixed modality treatments of cancer. If the radio-enhancing drug could be localized at the disease site and released slowly over time, then systemic drug toxicities could be decreased while simultaneously maintaining high drug concentrations in the tumor. These considerations support a role for a sustained release intra-tumoral delivery systems for the delivery of radio-enhancing drugs.

METHODS

Two approaches aimed at achieving the end of localizing the radio-enhancing drug to the tumor are described. First, nanoparticles, which have a prolonged circulation time and facility for enhanced tumor targeting. Structural defects in the walls of the tumor vasculature allow the passage of particles too large to pass through the walls of normal blood vessels. This characteristic of tumor blood vessels, referred to as the enhanced permeability and retention (EPR) effect, allows relatively large entities (typically liposomes, nanoparticles, and macromolecular drugs) to pass from the blood vessels to tumor tissue and as a result nanoparticles accumulate in the tumor while being excluded from normal tissue. Second, biodegradable implanted polymers. In these devices, the radio-enhancing drug is physically trapped within the polymer matrix which is implanted in the tumor. The drug is released as the polymer degrades in response to its local environment. The degradation rate of the polymer device can be adjusted to control the rate of drug release. By this means, the level of radio-enhancing drug can be maintained at the tumor site for the duration of radiation treatment.

RESULTS AND CONCLUSIONS

Results of experiments indicate that for both methods tumor control could be optimized by maintaining the radio-enhancing drug at a useful concentration in the tumor over a period of time compatible with the duration of fractionated radiation treatment. These studies have provided proof of principle support for the further development of this approach. To date, while some of the methods and devices for drug delivery described in this paper have been involved in clinical trials, none have so far been developed for routine clinical application.

Carmustine wafers: localized delivery of chemotherapeutic agents in CNS malignancies

High-grade glioma is a devastating disease that leaves the majority of its victims dead within 2 years. To meaningfully increase survival, a trimodality approach of surgery, radiation, and chemotherapy is needed. Carmustine (1,3-bis (2-chloroethyl)-1-nitrosourea) is a nitrosourea alkylating agent that exerts its antitumor effect by akylating DNA and RNA. Systemic administration of nitrosoureas as a single agent or as part of procarbazine/3-cyclohexyl-1-nitroso-urea/vincristine has demonstrated little efficacy in the treatment of high-grade glioma. The development of carmustine wafers (Gliadel((R)) Wafer) as a method for controlled released delivery of carmustine from biodegradable polymer wafers enhances the therapeutic ratio by fully containing the drug within the confines of the brain tumor environment while minimizing systemic toxicities. Preclinical and clinical studies have proven the safety and efficacy of Gliadel in the management of glioblastoma. From these results, Gliadel is currently approved for use in patients with recurrent glioblastoma as an adjunct to surgery and in newly diagnosed patients with high-grade glioma as an adjunct to surgery and radiation. Other promising advances in the use of locally delivered chemotherapy for CNS malignancies, including Gliadel for brain metastases and combination therapies with systemic or biologic agents, are discussed.

Characterization and in vitro degradation of poly(octadecanoic anhydride)

Poly(octadecanoic anhydride) (POA) has been prepared by melt polycondensation of octadecanoic diacid. POA was characterized by Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and wide angle X-ray diffraction (WAXD). The results of in vitro degradation and SEM micrographs show that the erosion process of POA is neither bulk nor perfect surface erosion but rather has elements of both in phosphate buffer at 37 degrees C. The moving erosion front is characteristic of surface erosion whereas the remaining porous shell stems from bulk erosion. While a significant special degradation property of POA is that POA presents a very slow degradation rate in acidic condition (pH 5.98), only 1.64% weight loss for 20 days, and it completely degrades after 18 days in basic buffer (pH 7.4). Comparing with poly(sebacic anhydride) (PSA), POA has the higher crystallization degree, and the slower hydrolytic rate.

Crosslinked chitosan implants as potential degradable devices for brachytherapy: in vitro and in vivo analysis

Compared with conventional external beam radiation, brachytherapy offers a superior therapeutic regimen. However, some major constraints are associated with its implementation, including the need of complicated procedures for device placement and removal. The purpose of this study was to examine whether crosslinked chitosan (Ct) implants could serve as potential biodegradable devices for brachytherapy. Ct was reacted with increasing amounts of glutaraldehyde to obtain hydrogels with different crosslinking densities, which were characterized chemically, thermally and mechanically. The effect of the dialysis medium conditions (ionic strength, osmolarity and pH) on the gel hydration and in vivo degradation was assessed. Two types of implants, slow and fast degrading gel (SDG and FDG, respectively), were prepared and implanted with or without Sudan Black (SB) in the rat. While SDG withstood for over a month, the FDG degraded within two weeks after implantation. The release kinetics of SB from the hydrogels verified their in vivo degradation properties. The incorporation of the radioactive compound (131)I-norcholesterol ((131)I-NC) into the SDG altered the degradation kinetics of the gel as reflected by the release kinetics of the radioactive marker. Eighty percent of (131)I-NC was released within a month after implantation, after which time, radioactivity was detected in the regional lymph nodes. Histological examination of the tissues surrounding the implants demonstrated negligible tissue response to the implants, when compared to biodegradable surgical sutures. It is concluded that hydrogels made of crosslinked Ct are potential novel, safe, degradable devices for brachytherapy.

Sensitization to Radiation from an Implanted125I Source by Sustained Intratumoral Release of Chemotherapeutic Drugs

We have investigated tumor response to low-dose-rate irradiation from an implanted 125I source alone or in conjunction with intratumoral drug administration. The drug (cis-DDP or 5-FU) was incorporated homogeneously into the co-polymer CPP-SA, 20:80, and the polymer/drug rods were implanted in the RIF-1 fibrosarcomas growing subcutaneously in C3H mice. Twenty-four hours later, the tumor was implanted with an 125I seed. Tumor growth time was the end point in these experiments. For implanted 125I sources of different dose rates and implant times giving a range of total doses, a consistent dose-response relationship was shown between tumor growth time and total dose. In other experiments, 125I sources of different specific activities were implanted for periods of time adjusted so that the total dose to the tumor was always the same. When the 125I implant was combined with 5-FU, greater than additive responses were seen for both short (30 h) and long (96 h) 125I treatment times. In contrast, a short-duration (30 h) 125I implant combined with cis-DDP was the least effective treatment, giving a combined response that was no better than additive, whereas 96 h exposure to 125I combined with cis-DDP was the most effective combined treatment. It is conjectured that this inverse dose-rate effect seen when cis-DDP is combined with low-dose rate radiation is related to a cell cycle effect and/or to inhibition of repair of radiation damage by cis-DDP.

Treatment of intracranial rat glioma model with implant of radiosensitizer and biomodulator drug combined with external beam radiotherapy

PURPOSE

To evaluate an intracranial polymer implant containing bromodeoxyuridine (BrdUrd) and N-(phosphonacetyl)-L-aspartic acid (PALA) in combination with external beam radiotherapy (EBRT) in the treatment of a rat glioma.

METHODS AND MATERIALS

Combinations of the biomodulators 5-fluorouracil, methotrexate, or PALA with BrdUrd were evaluated as radiosensitizers in vitro by clonogenic assay. In in vivo experiments, BrdUrd and PALA were incorporated into a polyanhydride-based polymer, bis(p-carboxyphenoxy)propane sebacic acid, and implanted in the C6 rat glioma growing intracranially. The effectiveness of treatment was evaluated on the basis of survival. EBRT was given as 10-MV X-rays.

RESULTS

In tissue culture experiments, C6 cells were refractory to radiosensitization by BrdUrd even when the thymidine analog was combined with a biomodulator intended to reduce de novo thymidine synthesis. The most effective compound in vitro was PALA. When PALA and BrdUrd in a polymer formulation were implanted intracranially and combined with 10-Gy EBRT, the treatment was highly effective, with 83% of treated rats surviving 180 days.

CONCLUSION

Although the in vitro results were not encouraging, the combination of intratumoral BrdUrd and PAL with 10-Gy EBRT was highly effective in treating a rat glioma. These results indicate the clinical potential of combined and mixed modality treatments involving intratumoral sustained-release drug delivery.

Etanidazole-loaded microspheres fabricated by spray-drying different poly(lactide/glycolide) polymers: effects on microsphere properties

In this work, a spraying technique was used to encapsulate etanidazole (a hypoxic radiosensitizer) into different poly(lactide/glycolide) polymers. The properties of the obtained microspheres, especially the particle size and distribution, morphology and release rate were investigated. Unexpectedly, poly(L-lactide) (PLLA) shows a fast release rate, comparable to PLGA 50: 50, due to the dissociation of the microspheres although the release rate of the spray-dried microspheres of other polymers decreases with increasing lactide ratio. It is also interesting to note that, contrary to the viscosity sequence of the polymer solutions, the particle size of the microspheres decreases in the order PLGA 50: 50, PLGA 65: 35, PLGA 85: 15 and PDLA. The morphology of microspheres can be affected by polymer properties (e.g. lactide/glycolide ratio, molecular weight, crystallinity and Tg) and fabrication conditions (e.g. solvent and polymer concentration to be sprayed). Although most of the microspheres fabricated by EA have a donghnut-like shape with smooth surface, it is possible to obtain spherical particles by choosing proper polymer type and polymer concentration. A further examination of the mechanisms of the atomization process and the solvent evaporation process reveals their respective effect on droplet formation and particle formation, both of which are essential for the spray-drying technique. It is found that polymer phase transition (affected by the polymer solubility) and its subsequent solvent evaporation processes can finally determine the morphology and the particle size of the spray-dried particles made from different polymers. In essence, the lactide/glycolide ratio of the polymers plays a more important role in affecting the properties of the spray-dried microspheres.

Topical inhibition of oral carcinoma cell with polymer delivered celecoxib

Celecoxib has a potential role for oral cancer chemoprevention but its systemic side effects are a concern. Topical chemoprevention is a promising way to reduce the toxicity. This study was designed to determine whether topical application of polymer delivered celecoxib would have an inhibitory effect on human oral carcinoma cells. Seventeen nude mice were intradermally inoculated with the carcinoma cells, and then were divided into control and treatment groups. Tumor growth was measured for 15 days, at which significant difference was found between two groups (P<0.001). This study indicates a potential role of polymer film delivered celecoxib for topical inhibition of oral cancer.

Computer simulation of the delivery of etanidazole to brain tumor from PLGA wafers: comparison between linear and double burst release systems

This paper presents the computer simulation results on the delivery of Etanidazole (radiosensitizer) to the brain tumor and examines several factors affecting the delivery. The simulation consists of a 3D model of tumor with poly(lactide-co-glycolide) (PLGA) wafers with 1% Etanidazole loading implanted in the resected cavity. A zero-order release device will produce a concentration profile in the tumor which increases with time until the drug in the carrier is depleted. This causes toxicity complications during the later stages of drug treatment. However, for wafers of similar loading, such release results in a higher drug penetration depth and therapeutic index as compared to the double drug burst profile. The numerical accuracy of the model was verified by the similar results obtained in the two-dimensional and three-dimensional models.

Sustained release system for highly water-soluble radiosensitizer drug etanidazole: irradiation and degradation studies

Etanidazole (one nitro-imidazole hypoxic radiosensitizer) is formulated as polymer matrix type controlled release devices in this study. A novel double polymer drug carrier, unlike the double wall microparticles, is fabricated for the purpose of drug delivery, with the following objectives in mind: (1) to have a high encapsulation efficiency, (2) to achieve a pusatile release profile suitable for the radiation schedule of radiotherapy, (3) to elucidate the degradation profile of these microparticles. Irradiation of the microparticles were also studied to investigate effects on release and degradation. At a dosage of 50 Gy (total dosage during a radiotherapy treatment period) showed no apparent effects on the tri-phase release profile. It consists of an initial burst in the first 72 h, followed by a slow and steady drug release phase, and finally a faster degradation controlled phase corresponding to the degradation state of the different microparticles. At 25 kGy (sterilization dosage), the release profiles of the drug carrier were drastically modified. The faster erosion of the polymer with high dosage irradiation hastened the drug release and shortened the release time span, accompanied by decreases in the polymer molecular weight and glass transition temperatures, which was not apparent from SEM imaging. Degradation studies suggested a heterogeneous degradation process, with the outer layer and inner matrix degrading at different rates. The modifiable tri-phase release profile using microparticles of different polymer blends implies that the release properties of the drug carriers can be modified for different treatment regimes.

Simulation of intratumoral release of Etanidazole: effects of the size of surgical opening

The efficacy of radiotherapy can be enhanced by the delivery of radiosensitizer (Etanidazole) to brain tumor from biodegradable polymer implants. This process is investigated by simulation carried out at a cut section of tumor with polymeric wafers of Etanidazole loading implanted in the resected cavity. The coupled mass and momentum equations are solved to obtain the transient solution of the drug distribution in the tumor. The polymeric delivery shows high therapeutic index, indicating the wafers' success in delivering more drugs to the tumor rather than to the tissue. The penetration distance of Etanidazole was found to decrease from 14 mm (at 5th/40th day after implantation) to 6.5 mm (at 30th/75th day), suggesting an initial high burst of drug release which cause nearby tissue toxicity and a low effective drug delivery towards the later stages. The short penetration depth is due to Etanidazole having low interstitial Peclet number and high elimination/diffusion modulus. Edema causes the interstitial pressure, velocity, and concentration to increase in all domains, and leads to enhanced convection and a lowering of therapeutic index. Simulations on the open tumor geometry show significantly lower efficacy of the drug delivery due to the uneven distribution of drug in the tumor zone.

Tumor treatment by sustained intratumoral release of 5-fluorouracil: effects of drug alone and in combined treatments

PURPOSE

To evaluate an intratumoral polymer implant for sustained delivery of 5-fluorouracil (5-FU) in a mouse tumor model.

METHODS AND MATERIALS

5-FU was incorporated into a polyanhydride-based polymer, bis(p-carboxyphenoxy)propane sebacic acid (CPP:SA) and implanted in RIF-1 mouse fibrosarcoma growing s.c. The effectiveness of treatment was evaluated by tumor growth delay. External beam radiation was 60Co gamma rays, and the source of interstitial radiation was implanted 125I seeds. A second drug, cis-diamminedichloroplatinum (cis-DDP), was administered by intraperitoneal injection or by osmotic pump.

RESULTS

For drug/polymer implant alone, the tumor growth delay was proportional to the amount of drug in the implant. The 5-FU polymer implant was most effective when combined with cis-DDP or with acute or fractionated radiation, and in some cases, the effects of combined treatments were greater than additive. The most effective combination was intratumoral 5-FU and low-dose-rate radiation delivered from an interstitial radiation source.

CONCLUSION

Results indicate that 5-FU can be effectively delivered by polymer implant and that this mode of delivery is particularly appropriate for combined treatments.

Sustained release of etanidazole from spray dried microspheres prepared by non-halogenated solvents

Etanidazole, a kind of radiosensitizer, was encapsulated in the spray-dried microspheres using biodegradable polymer PLGA 65:35 as the carrier for controlled release applications. Two non-halogenated solvents, e.g., ethyl acetate (EA) and ethanol, were tested to modify the properties of microspheres prepared by the commonly used solvent dichloromethane (DCM) alone. Their effects on the release behavior, morphology, particle size, and encapsulation efficiency of etanidazole-loaded microspheres were determined, and results were compared with DCM. The particle formation process via spray drying technique was also analyzed in order to understand the results obtained. It was found that larger percentage of EA (in the solvent mixture consisting of DCM and EA) in the fabrication of PLGA 65:35 microspheres decreases the initial burst, release rate and prolongs the release duration of etanidazole. In contrast to the spherical and porous microspheres prepared by DCM, the microspheres prepared by the solvent EA are all nonporous with a doughnut like surface structure due to its comparatively rapid phase transition (phase inversion) but slow solvent evaporation rate (longer time required to solidify). Increasing the polymer concentration (e.g., 4%, w/v) can bring about much more spherical microspheres by spray drying. Although ethanol, as a co-solvent, can dissolve a higher amount of etanidazole and lead to a higher drug encapsulation efficiency, the addition of ethanol in the DCM solvent can significantly increase the initial burst and the release rate of the microspheres due to the inhomogeneous drug distribution and structure of microspheres caused by phase separation. This study shows that ethyl acetate is an excellent low-toxic solvent that can be used in the spray drying technique for decreasing the initial burst, prolonging the release duration of a highly water-soluble drug like etanidazole. The use of EA provides a promising way to develop a sustained release system for etanidazole and other highly water-soluble drugs.

Tirapazamine: a bioreductive anticancer drug that exploits tumour hypoxia

Tirapazamine is the second clinical anticancer drug (after porfiromycin) that functions primarily as a hypoxia-selective cytotoxin. Hypoxic cells in tumours are relatively resistant to radiotherapy and to some forms of chemotherapy and are also biologically aggressive, thus representing an important target population in oncology. Tirapazamine undergoes metabolism by reductases to form a transient oxidising radical that can be efficiently scavenged by molecular oxygen in normal tissues to re-form the parent compound. In the absence of oxygen, the oxidising radical abstracts a proton from DNA to form DNA radicals, largely at C4' on the ribose ring. Tirapazamine can also oxidise such DNA radicals to cytotoxic DNA strand breaks. It therefore shows substantial selective cytotoxicity for anoxic cells in culture (typically approximately 100-fold more potent than under oxic conditions) and for the hypoxic subfraction of cells in tumours. Preclinical studies showed enhanced activity of combinations of tirapazamine with radiation (to kill oxygenated cells) and with conventional cytotoxics, especially cisplatin (probably through inhibition of repair of cisplatin DNA cross-links in hypoxic cells). Phase II and III clinical studies of tirapazamine and cisplatin in malignant melanoma and non-small cell lung cancer suggest that the combination is more active than cisplatin alone and preliminary results with advanced squamous cell carcinomas of the head and neck indicate that tirapazamine may enhance the activity of cisplatin with fractionated radiotherapy.