Effects of hyperbaric oxygen and a perfluorooctylbromide emulsion on the radiation responses of tumors and normal tissues in rodents

Emulsion Technology in Nuclear Medicine: Targeted Radionuclide Therapies, Radiosensitizers, and Imaging Agents

Radiopharmaceuticals serve as a major part of nuclear medicine contributing to both diagnosis and treatment of several diseases, especially cancers. Currently, most radiopharmaceuticals are based on small molecules with targeting ability. However, some concerns over their stability or non-specific interactions leading to off-target localization are among the major challenges that need to be overcome. Emulsion technology has great potential for the fabrication of carrier systems for radiopharmaceuticals. It can be used to create particles with different compositions, structures, sizes, and surface characteristics from a wide range of generally recognized as safe (GRAS) materials, which allows their functionality to be tuned for specific applications. In particular, it is possible to carry out surface modifications to introduce targeting and stealth properties, as well as to control the particle dimensions to manipulate diffusion and penetration properties. Moreover, emulsion preparation methods are usually simple, economic, robust, and scalable, which makes them suitable for medical applications. In this review, we highlight the potential of emulsion technology in nuclear medicine for developing targeted radionuclide therapies, for use as radiosensitizers, and for application in radiotracer delivery in gamma imaging techniques.

Dodecafluoropentane Emulsion as a Radiosensitizer in Glioblastoma Multiforme

Purpose

Glioblastoma multiforme (GBM) is a hypoxic tumor resistant to radiotherapy. The purpose of this study was to assess the safety and efficacy of a novel oxygen therapeutic, dodecafluoropentane emulsion (DDFPe), in chemoradiation treatment of GBM.

Experimental Design

In this multicenter phase Ib/II dose-escalation study, patients were administered DDFPe via intravenous infusion (0.05, 0.10, or 0.17 mL/kg) while breathing supplemental oxygen prior to each 2 Gy fraction of radiotherapy (30 fractions over 6 weeks). Patients also received standard-of-care chemotherapy [temozolomide (TMZ)]. Serial MRI scans were taken to monitor disease response. Adverse events were recorded and graded. TOLD (tissue oxygenation level-dependent) contrast MRI was obtained to validate modulation of tumor hypoxia.

Results

Eleven patients were enrolled. DDFPe combined with radiotherapy and TMZ was well tolerated in most patients. Two patients developed delayed grade 3 radiation necrosis during dose escalation, one each at 0.1 and 0.17 mL/kg of DDFPe. Subsequent patients were treated at the 0.1 mL/kg dose level. Kaplan-Meier analysis showed a median overall survival of 19.4 months and a median progression-free survival of 9.6 months, which compares favorably to historical controls. Among 6 patients evaluable for TOLD MRI, a statistically significant reduction in tumor T1 was observed after DDFPe treatment.

Conclusions

This trial, although small, showed that the use of DDFPe as a radiosensitizer in patients with GBM was generally safe and may provide a survival benefit. This is also the first time than TOLD MRI has shown reversal of tumor hypoxia in a clinical trial in patients. The recommended dose for phase II evaluation is 0.1 mL/kg DDFPe.Trial Registration: NCT02189109.

Significance

This study shows that DDFPe can be safely administered to patients, and it is the first-in-human study to show reversal of hypoxia in GBM as measured by TOLD MRI. This strategy is being used in a larger phase II/III trial which will hopefully show a survival benefit by adding DDFPe during the course of fractionated radiation and concurrent chemotherapy.

Targeted Anti-Cancer Provascular Therapy Using Ultrasound, Microbubbles, and Nitrite to Increase Radiotherapy Efficacy

Hypoxia is an important mechanism of resistance to radiation therapy in many human malignancies including prostate cancer. It has been recently shown that ultrasound targeted microbubble cavitation (UTMC) can increase blood perfusion in skeletal muscle by triggering nitric oxide signaling. Interestingly, this effect was amplified with a sodium nitrite coinjection. Since sodium nitrite has been shown to synergize with radiotherapy (RT), we hypothesized that UTMC with a sodium nitrite coinjection could further radiosensitize solid tumors by increasing blood perfusion and thus reduce tumor hypoxia. We evaluated (1) the ability of UTMC with and without nitrite to increase perfusion in muscle (mouse hindlimbs) and human prostate tumors using different pulse lengths and pressure; (2) the efficacy of this approach as a provascular therapy given directly before RT in the human prostate subcutaneous xenografts PC3 tumor model. Using long pulses with various pressures, in muscle, the provascular response following UTMC was strong (6.61 ± 4.41-fold increase in perfusion post-treatment). In tumors, long pulses caused an increase in perfusion (2.42 ± 1.38-fold) at lower mechanical index (MI = 0.25) but not at higher MI (0.375, 0.5, and 0.750) when compared to control (no UTMC). However, when combined with RT, UTMC with long pulses (MI = 0.25) did not improve tumor growth inhibition. With short pulses, in muscle, the provascular response following UTMC (SONOS) + nitrite was strong (13.74 ± 8.60-fold increase in perfusion post-treatment). In tumors, UTMC (SONOS) + nitrite also caused a provascular response (1.94 ± 1.20-fold increase in perfusion post-treatment) that lasted for at least 10 min, but not with nitrite alone. Interestingly, the blunted provascular response observed for long pulses at higher MI without nitrite was reversed with the addition of nitrite. UTMC (SONOS) with and without nitrite caused an increase in perfusion in tumors. The provascular response observed for UTMC (SONOS) + nitrite was confirmed by histology. Finally, there was an improved growth inhibition for the 8 Gy RT dose + nitrite + UTMC group vs 8 Gy RT + nitrite alone. This effect was not significant with mice treated by UTMC + nitrite and receiving doses of 0 or 2 Gy RT. In conclusion, UTMC + nitrite increased blood flow leading to an increased efficacy of higher doses of RT in our tumor model, warranting further study of this strategy.

Oxygen therapeutic agents to target hypoxia in cancer treatment

Solid tumors have abnormal microcirculation that limits oxygen delivery and leads to a hypoxic tumor microenvironment. Tumor hypoxia stabilizes the transcription factor HIF-1α that can trigger immunosuppression through A2A adenosine receptors which prevents immune attack on tumors. In addition, success of chemotherapy and radiation therapy appears to be dependent on oxygen levels. Two main pharmaceutical classes of agents (hemoglobin based and perfluorocarbon man-made carbon oils) have been tested in tumor models as enhanced oxygen therapeutics. This article will review how these agents function as well as examine work to date with both drug classes.

Approaches to combat hypoxia in cancer therapy and the potential for in silico models in their evaluation

AIM

The negative impact of tumour hypoxia on cancer treatment outcome has been long-known, yet there has been little success combating it. This paper investigates the potential role of in silico modelling to help test emerging hypoxia-targeting treatments in cancer therapy.

METHODS

A Medline search was undertaken on the current landscape of in silico models that simulate cancer therapy and evaluate their ability to test hypoxia-targeting treatments. Techniques and treatments to combat tumour hypoxia and their current challenges are also presented.

RESULTS

Hypoxia-targeting treatments include tumour reoxygenation, hypoxic cell radiosensitization with nitroimidazoles, hypoxia-activated prodrugs and molecular targeting. Their main challenges are toxicity and not achieving adequate delivery to hypoxic regions of the tumour. There is promising research toward combining two or more of these techniques. Different types of in silico therapy models have been developed ranging from temporal to spatial and from stochastic to deterministic models. Numerous models have compared the effectiveness of different radiotherapy fractionation schedules for controlling hypoxic tumours. Similarly, models could help identify and optimize new treatments for overcoming hypoxia that utilize novel hypoxia-targeting technology.

CONCLUSION

Current therapy models should attempt to incorporate more sophisticated modelling of tumour angiogenesis/vasculature and vessel perfusion in order to become more useful for testing hypoxia-targeting treatments, which typically rely upon the tumour vasculature for delivery of additional oxygen, (pro)drugs and nanoparticles.

Overcoming tumor hypoxia as a barrier to radiotherapy, chemotherapy and immunotherapy in cancer treatment

Hypoxia exists to some degree in most solid tumors due to inadequate oxygen delivery of the abnormal vasculature which cannot meet the demands of the rapidly proliferating cancer cells. The levels of oxygenation within the same tumor are highly variable from one area to another and can change over time. Tumor hypoxia is an important impediment to effective cancer therapy. In radiotherapy, the primary mechanism is the creation of reactive oxygen species; hypoxic tumors are therefore radiation resistant. A number of chemotherapeutic drugs have been shown to be less effective when exposed to a hypoxic environment which can lead to further disease progression. Hypoxia is also a potent barrier to effective immunotherapy in cancer treatment. Because of the recognition of hypoxia as an important barrier to cancer treatment, a variety of approaches have been undertaken to overcome or reverse tumor hypoxia. Such approaches have included breathing hyperbaric oxygen, artificial hemoglobins, allosteric hemoglobin modifiers, hypoxia activated prodrugs and fluorocarbons (FCs). These approaches have largely failed due to limited efficacy and/or adverse side effects. Oxygen therapeutics, based on liquid FCs, can potentially increase the oxygen-carrying capacity of the blood to reverse tumor hypoxia. Currently, at least two drugs are in clinical trials to reverse tumor hypoxia; one of these is designed to improve permeability of oxygen into the tumor tissue and the other is based upon a low boiling point FC that transports higher amounts of oxygen per gram than previously tested FCs.

Perfluorocarbon emulsions radiosensitise brain tumors in carbogen breathing mice with orthotopic GL261 gliomas

Background

Tumour hypoxia limits the effectiveness of radiation therapy. Delivering normobaric or hyperbaric oxygen therapy elevates pO₂ in both tumour and normal brain tissue. However, pO2 levels return to baseline within 15 minutes of stopping therapy.

Aim

To investigate the effect of perfluorocarbon (PFC) emulsions on hypoxia in subcutaneous and intracranial mouse gliomas and their radiosensitising effect in orthotopic gliomas in mice breathing carbogen (95%O₂ and 5%CO₂).

Results

PFC emulsions completely abrogated hypoxia in both subcutaneous and intracranial GL261 models and conferred a significant survival advantage orthotopically (Mantel Cox: p = 0.048) in carbogen breathing mice injected intravenously (IV) with PFC emulsions before radiation versus mice receiving radiation alone. Carbogen alone decreased hypoxia levels substantially and conferred a smaller but not statistically significant survival advantage over and above radiation alone.

Conclusion

IV injections of PFC emulsions followed by 1h carbogen breathing, radiosensitises GL261 intracranial tumors.

Effects of the Perfluorochemical Emulsion FMIQ on the Radiation Response of EMT6 Tumours

The effects of FMIQ, a perfluorochemical emulsion based on perfluoro-N-methyldecahydroisoquinoline, were examined using BALB/c mice and EMT6 mammary carcinomas. The radiobiological effects of FMIQ were similar to those found previously for Fluosol in the same tumour/host system. Although the perfluorochemical content (20% w/v) and oxygen-carrying capacity of FMIQ are similar to those of Fluosol, the formulation of FMIQ offers some advantages over that of Fluosol. For example, FMIQ has greater stability during storage. FMIQ also is formulated without pluronic F-68 and is based on a perfluorochemical (FMIQ) having a shorter tissue dwell time than the perfluorotripropylamine in Fluosol; it therefore may produce fewer side-effects than Fluosol. The lifetime of the circulating perfluorochemical droplets in BALB/c mice was longer than FMIQ than for Fluosol; this could offer an advantage in fractionated radiotherapy. These findings give reason to expect that FMIQ may prove to be a better emulsion than Fluosol for clinical use as an adjunct to cancer therapy.

RSR13, a synthetic allosteric modifier of hemoglobin, as an adjunct to radiotherapy: preliminary studies with EMT6 cells and tumors and normal tissues in mice

RSR13, 2[4-[[(3,5dimethylanilino)carbonyl]methyl]phenoxy]-2-methylpropion ic acid, a synthetic allosteric modifier of hemoglobin, reduces the affinity of hemoglobin for oxygen. The experiments reported here examined the effect of treatment with RSR13, combined with oxygen breathing, on the radiation response of EMT6 mammary tumors in BALB/c mice and of two normal tissues. RSR13 plus oxygen breathing increased the response of EMT6 tumors to irradiation. RSR13 had no discernible effects on tumors rendered maximally hypoxic by nitrogen asphyxiation, no discernible cytotoxic effects in EMT6 tumors, and no effect on the viability or radiation response of EMT6 cells in vitro under either aerobic or hypoxic conditions. The effects of RSR13 therefore reflect changes in tumor oxygenation, rather than a direct cytotoxic or radiosensitizing effect of the drug. RSR13 plus oxygen reduced the hypoxic fraction to 9% from the value of 24% found in both air-breathing and oxygen-breathing mice. Treatment with RSR13 plus oxygen did not alter the radiation response of the bone marrow progenitor cells (CFU-S) or acute radiation reactions in the skin. The improvement in tumor radiation response produced by treatment with RSR13 plus oxygen, combined with the absence of enhanced radiation reactions in the normal tissues, support further testing of RSR13 as an adjunct to radiotherapy.

Accuracy of fluorocrit in determination of blood perflubron concentration

Previous studies examining the radiosensitizing effects of perfluorochemical emulsions have based dose recommendations on a measurement known as fluorocrit. The fluorocrit is the proportion of blood volume occupied by perfluorochemicals and is measured using standard hematocrit procedures. This measurement is inherently crude and subject to error and variability between different individuals measuring the same sample. Furthermore, the fluorocrit method has not been compared to other quantitative methods to determine its reliability. The purpose of this study was to compare fluorocrit measurements to those obtained by gas chromatographic analysis. A 90% w/v perflubron emulsion was administered to six normal dogs once weekly for four weeks and peripheral blood samples were obtained at specified time points for analysis. A total of 123 blood samples were analyzed by both methods. The relationship between blood fluorocrit and plasma perflubron concentration measured by gas chromatography was examined using regression models. Based on the modest predictive value (r2 = 0.3683) of the derived statistical model, we conclude that fluorocrit measurement is an inaccurate method of estimation of blood perflubron concentration. Caution must, therefore, be exercised when extrapolating data and dose recommendations from reports of studies using flurocrit as the only estimate of blood perflubron concentration.

The role of oxygen in cutaneous photodynamic therapy

Photodynamic therapy (PDT) is based on the dye-sensitized photooxidation of biological matter in the target tissue, and utilizes light activated drugs for the treatment of a wide variety of malignancies. Skin is a target organ for PDT, because of the increasing incidence of skin cancers and the easy accessibility to photosensitizing drugs and light. Skin oxygen tension changes dramatically during and after PDT and seems to be an important treatment parameter. Experimental approaches to modulate oxygen tension (e.g., hyperbaric oxygenation, hyperthermia, or perfluorocarbons) have been studied mainly in animals, and some of these techniques may have the potential to be applied in humans to improve the efficacy and safety of PDT. The main purpose of this review is to provide the reader with current information on cutaneous oxygen physiology and oximetry, the role of oxygen and singlet oxygen (1O2) in PDT, and approaches to modulate skin oxygen tension. The literature indicates that it may be possible to utilize transcutaneous oxygen measurements as a valuable measure of the clinical effectiveness of PDT and as an in situ predictor of the energy required to elicit a biological response. Consequently the effectiveness of PDT can be manipulated by modulating skin oxygen tension.

Oxygenation of human tumor xenografts in nude mice by a perfluorochemical emulsion and carbogen breathing

Human solid tumors (prostate carcinomas PC-3 and DU-145, breast carcinoma MX-1, cervical carcinoma ME-180, small cell lung carcinoma SW2, and glioblastoma T98G) were grown as xenografts in nude mice. Using the Eppendorf pO2 histograph microelectrode system, the oxygen profiles of the tumors were determined while the animals breathed air or carbogen (95% O2/5% CO2), and after administration of the perfluorochemical emulsion Oxygent-CA (8 ml/kg) under air breathing and carbogen breathing conditions. Under normal air breathing with or without Oxygent-CA administration the mean oxygen tensions were between 4.9 and 9.3 mmHg and each tumor had severely hypoxic regions where the pO2 was less than 5 mmHg. The severely hypoxic regions comprised 41-71% of the oxygen tension measurements under normal air breathing conditions. Carbogen breathing alone increased the mean oxygen tensions to 10.9-23.9 mmHg. Administration of Oxygent-CA and carbogen breathing increased the mean oxygen tensions over the levels of carbogen breathing alone to varying degrees. The highest mean oxygen tensions were 40.8 mmHg in the T98G glioblastoma and 24.5 mmHg in the ME-180 cervical carcinoma. Investigation of the use of Oxygent-CA/carbogen to increase the oxygenation of clinical tumors is warranted.

Pharmacokinetics and tolerance of weekly OXYGENT CA infusions in the dog

This study determined the OXYGENT CA (90% w/v perflubron emulsion, Alliance Pharmaceutical Corporation) dose necessary to achieve a 3-4% fluorocrit, and the tolerance of this dose administered once per week for four weeks to dogs. This study simulated OXYGENT CA use as a radiosensitizing agent. Six adult dogs were administered 6 ml/kg OXYGENT CA once per week for 4 weeks. Blood samples were collected following infusion, until fluorocrits were < or = 0.5%. One week after the fourth infusion, three dogs were necropsied. Liver biopsies were obtained from the remaining three dogs which were monitored 12 additional weeks. All dogs achieved fluorocrits > 3.0% (3.5-5.1%) with the 6 ml/kg dose. A 3 ml/kg dose did not provide a fluorocrit > 3.0%. Serum bilirubin concentrations were elevated at 24-hour sampling times and declined within 72 hours. Elevations in ALT, SAP, and bile acids were noted. Splenic and hepatic microvasculature fibrosis occurred in the long-term study dogs. Thrombocytopenia occurred in 5/6 dogs, necessitating exclusions of one dog from 2 infusions. However, 3/5 thrombocytopenic dogs had titers for Ehrlichia sp., which elicits thrombocytopenia. Therefore, we cannot conclude the effect of OXYGENT CA on platelets.

Nicotinamide and carbogen: major effect on the radiosensitivity of EMT6 and HRT18 tumours

The effect of nicotinamide and/or carbogen breathing on the response of EMT6 and HRT18 tumours, irradiated in vivo, was evaluated using an in vitro colony assay. In the single dose schedules, the most efficient treatment was carbogen plus nicotinamide compared to air breathing mice. In a fractionated regime carbogen plus nicotinamide increased also response to radiation.

Perfluorocarbon-based red blood cell substitutes

Since our review 5 years ago, a new generation of PFC emulsion has been developed and is undergoing extensive testing. This new generation is the result of the application of physicochemical principles, applied to both the choice of the PFC itself and the emulsifier, as well as advances in emulsion-producing technology. The efficacy of PFCs in general for oxygen transporting capability has been fully recognized, as exemplified by the limited license issued to Fluosol. The latter also represents the recognition of the relative absence of major toxicity of PFCs in general. The development of new products owes much to the lessons learned during the past 20 years and to advances made in the physical chemistry of PFCs. These advances now permit the rational selection or design of the most appropriate PFC and the design of emulsifiers best suited for the purpose. Perflubron represents a clear advance over the Fluosol-DA-type formulation. It is only one but the most advanced of the second-generation products. At least three other commercial entities (Hema-Gen/PFC, Green Cross, Adamantech) are also developing products based on the above principles. Five years ago we concluded that, in spite of the enormous complexity of PFC emulsions as large volume parenterals, they have shown remarkable biocompatibility. The advances in the past 5 years have confirmed this conclusion. The advances occurring during the past 5 years show that the application of the proper technology can lead to product improvement, and that PFC preparations with significant transfusional and nontransfusional potential are, in fact, feasible. It remains to be seen whether high PFC-content emulsion can be successfully deployed in initial, prehospital resuscitation situations. The high PFC content will reduce the absolute requirement for the maintenance of FIO2 > 0.8 in the case of Fluosol-DA for optimal efficacy. The second-generation products also seem to lend themselves to intraoperative use, because they can be removed from the blood postoperatively by plasmapheresislike methods. They are also suitable in combination with autologous blood donation/transfusion. All of these potential applications are in various stages of exploration and, if found to be efficacious, will likely conserve the supply of whole blood and blood components. The nontransfusional applications, particularly those in diagnostic imaging, seem to show substantial promise. Because they involve smaller doses than transfusional applications, they may enter clinical use earlier. The applications in radiation and chemotherapy of malignant diseases represent an intermediate position between the transfusional and nontransfusional uses.(ABSTRACT TRUNCATED AT 400 WORDS)