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

Hyperthermia can alter tumor physiology and improve chemo- and radio-therapy efficacy

Hyperthermia has demonstrated clinical success in improving the efficacy of both chemo- and radio-therapy in solid tumors. Pre-clinical and clinical research studies have demonstrated that targeted hyperthermia can increase tumor blood flow and increase the perfused fraction of the tumor in a temperature and time dependent manner. Changes in tumor blood circulation can produce significant physiological changes including enhanced vascular permeability, increased oxygenation, decreased interstitial fluid pressure, and reestablishment of normal physiological pH conditions. These alterations in tumor physiology can positively impact both small molecule and nanomedicine chemotherapy accumulation and distribution within the tumor, as well as the fraction of the tumor susceptible to radiation therapy. Hyperthermia can trigger drug release from thermosensitive formulations and further improve the accumulation, distribution, and efficacy of chemotherapy.

Preliminary studies with a new hypoxia-selective cytotoxin, KS119W, in vitro and in vivo

Agents with selective toxicity to hypoxic cells have shown promise as adjuncts to radiotherapy. Our previous studies showed that the bioreductive alkylating agent KS119 had an extremely large differential toxicity to severely hypoxic and aerobic cells in cell culture, and was effective in killing the hypoxic cells of EMT6 mouse mammary tumors in vivo. However, the limited solubility of that compound precluded its development as an anticancer drug. Here we report our initial studies with KS119W, a water-soluble analog of KS119. The cytotoxicity of KS119W to EMT6 cells in vitro was similar to that of KS119, with both agents producing only minimal cytotoxicity to aerobic cells even after intensive treatments, while producing pronounced cytotoxicity to oxygen-deficient cells. This resulted in large differentials in the toxicities to hypoxic and aerobic cells (>1,000-fold at 10 μM). Low pH had only minimal effects on the cytotoxicity of KS119W. Under hypoxic conditions, EMT6 cells transfected to express high levels of either human or mouse versions of the repair protein O(6)-alkylguanine-DNA alkyltransferase, which is also known as O(6)-methylguanine DNA-methyltransferase, were much more resistant to KS119W than parental EMT6 cells lacking O(6)-alkylguanine-DNA alkyltransferase, confirming the importance of DNA O-6-alkylation to the cytotoxicity of this agent. Studies with EMT6 tumors in BALB/c Rw mice using both tumor cell survival and tumor growth delay assays showed that KS119W was effective as an adjunct to irradiation for the treatment of solid tumors in vivo, producing additive or supra-additive effects in most combination regimens for which the interactions could be evaluated. Our findings encourage additional preclinical studies to examine further the antineoplastic effects of KS119W alone and in combination with radiation, and to examine the pharmacology and toxicology of this new bioreductive alkylating agent so that its potential for clinical use as an adjuvant to radiotherapy can be evaluated.

Investigational therapies for brain metastases

Contrary to the incidence of primary cancers, the incidence of brain metastasis has been increasing. This increase is likely because of the effects of an aging population, improved neuroimaging surveillance, and better control of systemic cancer, allowing time for brain metastasis to occur. Unlike systemic cancers, for which chemotherapy is the mainstay of treatment, the therapeutic strategies available to treat brain metastasis have traditionally been limited to surgical resection, whole brain radiation therapy, or stereotactic radiosurgery, either individually or in combination. It is important to put the treatment in the context of the prognosis for patients with brain metastases.

The effects of Efaproxyn (efaproxiral) on subcutaneous RIF-1 tumor oxygenation and enhancement of radiotherapy-mediated inhibition of tumor growth in mice

Efaproxiral, an allosteric modifier of hemoglobin, reduces hemoglobin-oxygen binding affinity, facilitating oxygen release from hemoglobin, which is likely to increase tissue pO(2). The purpose of this study was to determine the effect of efaproxiral on tumor oxygenation and growth inhibition of RIF-1 tumors that received X radiation (4 Gy) plus oxygen breathing compared to radiation plus oxygen plus efaproxiral daily for 5 days. Two lithium phthalocyanine (LiPc) deposits were implanted in RIF-1 tumors in C3H mice for tumor pO(2) measurements using EPR oximetry. Efaproxiral significantly increased tumor oxygenation by 8.4 to 43.4 mmHg within 5 days, with maximum increases at 22-31 min after treatment. Oxygen breathing alone did not affect tumor pO(2). Radiation plus oxygen plus efaproxiral produced tumor growth inhibition throughout the treatment duration, and inhibition was significantly different from radiation plus oxygen from day 3 to day 5. The results of this study provide unambiguous quantitative information on the effectiveness of efaproxiral to consistently and reproducibly increase tumor oxygenation over the course of 5 days of treatment, modeling the clinical use of efaproxiral. Also, based on the tumor growth inhibition, the study shows the efaproxiral-enhanced tumor oxygenation was radiobiologically significant. This is the first study to demonstrate the ability of efaproxiral to increase tumor oxygenation and to increase the tumor growth inhibition of radiotherapy over 5 days of treatment.

Efaproxiral red blood cell concentration predicts efficacy in patients with brain metastases

Efaproxiral (Efaproxyn™, RSR13), a synthetic allosteric modifier of haemoglobin (Hb), decreases Hb-oxygen (O₂) binding affinity and enhances oxygenation of hypoxic tumours during radiation therapy. This analysis evaluated the Phase 3, Radiation Enhancing Allosteric Compound for Hypoxic Brain Metastases; RT-009 (REACH) study efficacy results in relation to efaproxiral exposure (efaproxiral red blood cell concentration (E-RBC) and number of doses). Recursive partitioning analysis Class I or II patients with brain metastases from solid tumours received standard whole-brain radiation therapy (3 Gy/fraction × 10 days), plus supplemental O₂ (4 l/min), either with efaproxiral (75 or 100 mg/kg daily) or without (control). Efaproxiral red blood cell concentrations were linearly extrapolated to all efaproxiral doses received. Three patient populations were analysed: (1) all eligible, (2) non-small-cell lung cancer (NSCLC) as primary cancer, and (3) breast cancer primary. Efficacy endpoints were survival and response rate. Brain metastases patients achieving sufficient E-RBC (⩾483 μg/ml) and receiving at least seven of 10 efaproxiral doses were most likely to experience survival and response benefits. Patients with breast cancer primary tumours generally achieved the target efaproxiral exposure and therefore gained greater benefit from efaproxiral treatment than NSCLC patients. This analysis defined the efaproxiral concentration-dependence in survival and response rate improvement, and provided a clearer understanding of efaproxiral dosing requirements.

Role of efaproxiral in metastatic brain tumors

Brain metastases from breast cancer are common and cause significant morbidity and mortality. Treatment with whole-brain radiotherapy provides a median survival of only 4-6 months. One mechanism affecting sensitivity to radiation is tumor oxygenation. Hypoxic tumor cells are more likely to be resistant to radiation and adversely affect prognosis. For nearly 70 years, all attempted modalities have failed to circumvent tumor hypoxia and increase tumor death. Abraham and colleagues developed a fibric acid derivative that would cause the displacement of oxygen from hemoglobin and improve tissue oxygenation. After modifications to enhance absorption into the red blood cell, RSR13 (efaproxiral) was developed. Efaproxiral was found to be safe and improved tumor oxygenation in preclinical studies. Early Phase I and II trials supported the earlier preclinical evidence while demonstrating that efaproxiral was safe and effective. A randomized Phase III study, Radiation Enhancing Allosteric Compounds of Hypoxic brain metastases (REACH, RT-009), failed to show a significant improvement in overall survival in all eligible patients treated with efaproxiral and whole-brain radiation. However, there was a statistically significant improvement in median survival and quality of life within a subset of metastatic breast cancer patients. Efaproxiral is currently being studied with radiotherapy in a confirmatory trial in the treatment of brain metastases from breast cancer. Furthermore, the combination of efaproxiral with radiotherapy is being investigated in other solid tumors.

Efaproxiral (RSR13) plus oxygen breathing increases the therapeutic ratio of carboplatin in EMT6 mouse mammary tumors

Carboplatin, a member of the platinum family of alkylating agents, is often used in combination with radiotherapy. Some studies, including a recent publication from our laboratory, have suggested that the cytotoxic effects of platinum compounds may be altered by changes in the post-treatment oxygenation. The study reported here assessed whether post-treatment changes in tumor oxygenation caused by oxygen breathing alone or in combination with efaproxiral (RSR13) altered the effects of carboplatin. Efaproxiral, which allosterically modifies hemoglobin-oxygen binding to increase tumor pO(2), has been shown to increase the effects of radiation in animal tumor models and is in a second, confirmatory phase III clinical trial as an adjuvant to radiotherapy. These studies with EMT6 tumors in BALB/c Rw mice used clonogenic assays to assess tumor cell survival and tumor growth studies to assess antineoplastic activity and treatment-related toxicity. Efaproxiral plus oxygen breathing for 5 hrs after carboplatin treatment significantly increased the antineoplastic effects of carboplatin. The increased antineoplastic effects of carboplatin produced by efaproxiral plus oxygen breathing occurred without a concomitant increase in host toxicity. These findings suggest that the increases in tumor oxygenation produced by Efaproxiral plus oxygen breathing increased the therapeutic ratio of carboplatin.

Radiation sensitization with redox modulators: A promising approach

PURPOSE

Radiation therapy plays a critical role in the local and regional control of malignant tumors. Its efficacy, however, is limited by a number of factors, including toxicity, tumor hypoxia, and tumor genetics. Recent attempts to enhance the efficacy of radiation therapy have focused on biologic agents that modulate reduction/oxidation reactions within tumor cells.

METHODS AND MATERIALS

We review five promising redox modulators that are in development. Tirapazamine and AQ4N are known as "hypoxic cell sensitizers" and are toxic in areas of low oxygen tension. RSR13 facilitates delivery of oxygen to tumor cells, thereby rendering them more sensitive to radiation. Motexafin gadolinium, with a porphyrin-like structure, selectively accumulates in tumor cells and thereby enhances radiation-induced DNA damage. HIF-1 inhibitors target a transcription factor that regulates hypoxia-related events and cell survival.

RESULTS

Our review of each agent included a thorough search of published preclinical and clinical data, including that presented in abstracts and posters at international meetings. Our objectives were not to identify a superior mechanism or drug, but rather to summarize the available safety and efficacy data.

CONCLUSION

Clearly, there is an unmet need for safer agents that augment the efficacy of radiation therapy. This review highlights five promising redox modulators that are in development. None has yet been approved by the Food and Drug Administration. These drugs were selected for discussion because they exemplify the current investigative landscape of radiosensitizers and are indicative of future directions in this area. These radiation sensitizers have the potential to succeed where others have failed, by locally increasing the radiosensitivity of tumor cells without enhancing that of surrounding normal tissues.

Efaproxiral: A Radiation Enhancer Used in Brain Metastases from Breast Cancer

OBJECTIVE

To review the mechanism of action and clinical data of efaproxiral use in brain metastases of breast cancer.

DATA SOURCES

Articles were identified through MEDLINE (1966–June 2005) and EMBASE (1980–May 2005) searches using the key words efaproxiral and RSR13. Published abstracts over the previous 10 years from various scientific meetings, including American Society of Clinical Oncology and San Antonio Breast Cancer Symposium, were also searched for investigations of efaproxiral. Data on efaproxiral were also provided by Allos Therapeutics.

STUDY SELECTION AND DATA EXTRACTION

All published clinical data in humans regarding efaproxiral use in brain metastases from breast cancer were selected for this review. In addition, published studies in humans that discussed the pharmacokinetics, pharmacodynamics, and safety of efaproxiral were evaluated.

DATA SYNTHESIS

Efaproxiral is a synthetic allosteric modifier of hemoglobin that results in a shift of the hemoglobin oxygen dissociation curve to the right. Therefore, oxygen is more readily released from hemoglobin into tissues. Efaproxiral demonstrated a significant survival benefit when used as a radiation enhancer in patients with brain metastases originating from breast cancer. The safety profile of efaproxiral and improved survival rates make this agent advantageous over radiation alone. Further investigation and results from the ongoing clinical trials will help to define the role of efaproxiral in clinical practice.

CONCLUSIONS

Efaproxiral is the first synthetic allosteric modifier to demonstrate significant improvement in survival in patients undergoing radiation therapy for brain metastases of breast cancer. Validation of this effect in ongoing clinical trials will be important in determining the role of efaproxiral in brain metastases from breast cancer.

Increased oxygenation of intracranial tumors by efaproxyn (efaproxiral), an allosteric hemoglobin modifier: In vivo EPR oximetry study

PURPOSE

To determine quantitatively the changes in oxygenation of intracranial tumors induced by efaproxiral, an allosteric hemoglobin modifier. Efaproxiral reduces hemoglobin-oxygen binding affinity, which facilitates oxygen release from hemoglobin into surrounding tissues and potentially increases the pO(2) of the tumors.

METHODS AND MATERIALS

The study was performed on 10 male Fisher 344 rats with 9L intracranial tumors. Electron paramagnetic resonance (EPR) oximetry was used to measure quantitatively the changes in the pO(2) in the tumors. Lithium phthalocyanine (LiPc) crystals were implanted in the tumors and in the normal brain tissue in the opposite hemispheres. We monitored the cerebral pO(2) starting 7 to 10 days after the tumor cells were implanted. NMR imaging determined the position and size of tumor in the brain. After an initial baseline EPR measurement, efaproxiral (150 mg/kg) was injected intravenously over 15 minutes, and measurements of tumor and normal brain oxygen tension were made alternately at 10-minute intervals for the next 60 minutes; the procedure was repeated for 6 consecutive days.

RESULTS

Efaproxiral significantly increased the pO(2) of both the intracranial tumors and the normal brain tissue on all days. The maximum increase was reached at 52.9 to 59.7 minutes and 54.1 to 63.2 minutes after injection, respectively. The pO(2) returned to baseline values at 106 to 126.5 minutes after treatment. The maximum tumor and normal tissue pO(2) values achieved after efaproxiral treatment from Day 1 through Day 6 ranged from 139.7 to 197.7 mm Hg and 103.0 to 135.9 mm Hg, respectively. The maximum increase in tumor tissue pO(2) values from Day 2 to Day 5 was greater than the maximum increase in normal tissue pO(2).

CONCLUSION

We obtained quantitative data on the timing and extent of efaproxiral-induced changes in the pO(2) of intracerebral 9L tumors. These results illustrate a unique and useful capability of in vivo EPR oximetry to obtain repeated noninvasive measurements of tumor oxygenation over a number of days. The information on the dynamics of tumor pO(2) after efaproxiral administration illustrates the ability of efaproxiral to increase intracranial tumor oxygenation.

Effect of RSR13, an allosteric hemoglobin modifier, on oxygenation in murine tumors: an in vivo electron paramagnetic resonance oximetry and bold MRI study

PURPOSE

RSR13, an allosteric modifier of hemoglobin, reduces hemoglobin-oxygen binding affinity facilitating oxygen release from hemoglobin, resulting in increases in tissue pO(2). The purpose of this study was noninvasively to monitor the time course and effect of RSR13 on tumor oxygenation, directly using in vivo electron paramagnetic resonance (EPR oximetry), and indirectly using blood oxygen level dependent magnetic resonance imaging (BOLD MRI).

METHODS AND MATERIALS

The study was performed in transplanted radiation-induced fibrosarcoma tumors (RIF-1) in 18 female C3H/HEJ mice, which had two lithium phthalocyanine (LiPc) deposits implanted in the tumor when the tumors reached about 200-600 mm(3). Baseline EPR measurements were made daily for 3 days. Then, for 6 consecutive days and after an initial baseline EPR measurement, RSR13 (150 mg/kg) or vehicle (same volume) was injected intraperitoneally, and measurements of intratumoral oxygen were made at 10-min intervals for the next 60 min. In each mouse, every third day, instead of EPR oximetry, BOLD MRI measurements were made for 60 min after administration of the RSR13.

RESULTS

Based on EPR measurements, RSR13 produced statistically significant temporal increases in tumor pO(2) over the 60-min time course, which reached a maximum at 35-43 min postdose. The average time required to return to the baseline pO(2) was 70-85 min. The maximum increase in tumor tissue pO(2) values after RSR13 treatment from Day 1 to Day 5 (8.3-12.4 mm Hg) was greater than the maximum tumor tissue pO(2) value for Day 6 (4.7 mm Hg, p < 0.01). The maximum increase in pO(2) occurred on Day 2 (12.4 mm Hg) after RSR13 treatment. There was little change in R(2)*, indicating that the RSR13 had minimal detectable effects on total deoxyhemoglobin and hemoglobin-oxygen saturation.

CONCLUSION

The extent of the increase in tumor pO(2) achieved by RSR13 would be expected to lead to a significant increase in the effectiveness of tumor radiotherapy. The lack of a change in the BOLD MRI signal suggests that the tumor physiology was largely unchanged by RSR13. These results illustrate a unique and useful capability of in vivo EPR oximetry and BOLD MRI to obtain repeated measurements of tumor oxygenation and physiology. The dynamics of tumor pO(2) after RSR13 administration may be useful for the design of clinical protocols using allosteric hemoglobin effectors.

Effects of texaphyrins on the oxygenation of EMT6 mouse mammary tumors

PURPOSE

To investigate the effects of texaphyrins on the oxygenation of EMT6 mouse mammary tumors in Balb/c Rw mice. Texaphyrins are synthetic, porphyrin-like molecules capable of stably coordinating lanthanide and nonlanthanide metals. Metallotexaphyrin compounds containing gadolinium (MGd), lutetium (MLu), europium (Eu-Tex), dysprosium (Dy-Tex), and manganese (Mn-Tex) were evaluated.

METHODS

Tumor oxygenation was measured using an Eppendorf pO2 histograph when tumors, implanted intradermally in the rear dorsum, reached 150-200 mm3. Oxygen measurements were also made in the leg muscle of tumor-bearing mice, to determine whether changes in oxygenation occurred in nontumor tissue.

RESULTS

Motexafin gadolinium (Xcytrin, MGd) seems to be an effective modulator of tumor oxygen tension. The mean of the median tumor pO2 6 hours after injection of MGd was 8.0 +/- 2.4 mm Hg. The control value was 1.5 +/- 0.4 mm Hg. The oxygen levels within EMT6 tumors were shifted significantly toward higher oxygen tensions 6-8 hours after i.v. injection of 40 micromol/kg MGd, thereby reducing the percentage of severely hypoxic readings (MGd, 6 hours: 44.6 +/- 4.3% <2.5 mm Hg;

CONTROL

69.4 +/- 3.0% <2.5 mm Hg). There was no significant change in the oxygenation of the leg muscle after MGd treatment. Eu-Tex and Mn-Tex increased the tumor oxygenation to a much lesser degree than MGd. MLu, Dy-Tex, and the vehicle (a 5% mannitol solution) did not modulate tumor oxygenation.

CONCLUSIONS

MGd is an effective modulator of tumor oxygenation. The central metal composition of texaphyrin compounds is an important determinant of the effect of the texaphyrins on tumor oxygenation.

Effects of RSR13 and oxygen on the cytotoxicity of cisplatin and carboplatin to EMT6 mouse mammary tumor cells in vitro and in vivo

PURPOSE

RSR13, 2-[4-[2-[(3,5-dimethylphenyl)amino]-2-oxoethyl]phenoxy]-2-methylpropanoic acid monosodium salt, allosterically modifies hemoglobin to increase tumor pO(2), increases the effect of radiation in animal tumor models, and is in phase III clinical trials as an adjuvant to radiotherapy. Cisplatin and carboplatin, two commonly used anticancer drugs have been used in combination with radiotherapy. Some studies have suggested that the cytotoxic effects of these drugs are altered in hypoxia. This study tested whether RSR13 plus oxygen breathing increased the cytotoxicity of cisplatin and carboplatin in vivo.

METHODS

Solid EMT6 tumors in BALB/c Rw mice were treated with cisplatin (5-30 microg/g) or carboplatin (5-200 microg/g) along with 150 microg/g RSR13 in combination with oxygen breathing. Tumor cell survival was assayed using clonogenic assays. The effects of pre- and posttreatments with RSR13 and oxygen breathing on the cytotoxicity of cisplatin or carboplatin were also examined. To assess whether RSR13 had direct effects on the cytotoxicity of the drugs, exponentially growing monolayers of EMT6 mouse mammary carcinoma cells were treated with graded concentrations of cisplatin or carboplatin for 2 h along with simultaneous (2 h) RSR13 treatments or with prolonged (22 h) pre- or posttreatment incubations with 100 microM RSR13.

RESULTS

Single or multiple treatments with 150 microg/g RSR13 plus oxygen breathing had no effect on the viability of cells in EMT6 tumors in mice. After treatment with cisplatin or carboplatin, the tumor cell survival tended to be lower in oxygen-breathing mice especially at higher doses of cisplatin. Treatment with RSR13 plus oxygen breathing beginning 15 min before administration of the alkylating agents did not alter the cytotoxicity of cisplatin or carboplatin from that seen with oxygen breathing alone. Pretreatment with RSR13 plus oxygen at 22 and 14 h prior to administration of either cisplatin or carboplatin did not alter the effect of either alkylating agent. Treatment with RSR13 plus oxygen breathing beginning 15 min before administration of the alkylating agents and lasting for 2 or 5 h did not alter the cytotoxicity of either drug from that seen with oxygen breathing alone. The cytotoxicity of cisplatin was not altered by treatment with oxygen alone or with RSR13 plus oxygen for 5 h after cisplatin injection. For carboplatin, treatment with oxygen alone and with RSR13 plus oxygen for 5 h after injection increased to similar extents the response of the tumor cells compared to that seen with assays at 2 h. Neither short simultaneous treatments, prolonged pretreatment incubations, nor prolonged posttreatment incubations with RSR13 altered the survival of EMT6 cells in cultures treated with cisplatin or carboplatin.

CONCLUSIONS

These findings indicate that RSR13 in combination with oxygen breathing does not alter the cytotoxicity of cisplatin or carboplatin when used simultaneously, as a pretreatment or as a posttreatment in vitro or in vivo. Our in vivo findings indicate trends that support previous findings that cisplatin is more cytotoxic to well-oxygenated tumor cells than to hypoxic tumor cells, and that this effect can be improved by improving tumor oxygenation, but the differences seen in our studies did not achieve statistical significance.

RSR13 plus cranial radiation therapy in patients with brain metastases: comparison with the Radiation Therapy Oncology Group Recursive Partitioning Analysis Brain Metastases Database

PURPOSE

This phase II, open-label, multicenter study assessed the efficacy and safety of the potential radiation enhancer RSR13 plus cranial radiation therapy (RT) in patients with brain metastases. The primary end point was patient survival in comparison with the Radiation Therapy Oncology Group Recursive Partitioning Analysis Brain Metastases Database (RTOG RPA BMD).

PATIENTS AND METHODS

Eligibility criteria were age > or = 18 years, Karnofsky performance score > or = 70, and brain metastases with solid tumor histology. Patients received cranial RT, 30 Gy in 10 fractions of 3 Gy each, preceded by RSR13, 50 to 100 mg/kg intravenously over 30 minutes. Univariate and multivariate comparisons of survival and cause of death were made between class II study patients and RTOG BMD patients.

RESULTS

Fifty-seven RPA class II patients were enrolled. With a minimum follow-up of 24 months, the median survival time and 1- and 2-year survival rates were 6.4 months, 23%, and 11% for the RSR13-treated patients compared with 4.1 months, 15%, and 3% for the RTOG BMD patients (P =.0174). In an exact-matched case analysis (n = 38), median survival time for RSR13 patients was 7.3 months versus 3.4 months for the RTOG BMD patients (P =.006). There was a 54% reduction in the risk of death for RSR13 patients (P =.0267). RSR13-related adverse events of greater than or equal to grade 3 toxicity that occurred in more than one patient included hypoxia, headache, anemia, fatigue, hypertension, and intracranial hypertension.

CONCLUSION

RSR13 plus cranial RT resulted in a significant improvement in survival, as well as a reduction in death due to brain metastases, compared with class II patients in the RTOG BMD.

Allosteric Modification of Hemoglobin by RSR13 as a Therapeutic Strategy

RSR13 binds to hemoglobin (Hb), reduces oxygen (O2) binding affinity, and enhances O2 unloading from Hb to hypoxic tissue. Tissue hypoxia is common to cancer, surgery, myocardial ischemia, and stroke. RSR13 increases tumor pO2, reduces tumor hypoxic fraction and because O2 is necessary to maximize the effectiveness of radiation therapy, RSR13 enhances the efficacy of radiation therapy. Patients with brain metastases or glioblastoma multiforme receiving RSR13 and radiation therapy have improved median survival, compared to matched historical controls. Myocardial and cerebral hypoxia can be complications to cardiopulmonary bypass (CPB) surgery. RSR13 improves myocardial oxidative metabolism and contractile function in models of myocardial ischemia, including CPB. In CPB patients, RSR13 improved cardiac contractile function and reduced blood product use. In animals, RSR13 increased brain pO2 and reduced neuronal cell death following cerebral ischemia, alone or in combination with excitotoxic neurotransmitter inhibition. Allosteric modification of Hb by RSR13 represents a unique therapeutic strategy.

Importance of hypoxia in the biology and treatment of brain tumors

The resistance of gliomas to treatment with radiation and antineoplastic drugs may result in part from the effects of the extensive, severe hypoxia that is present in these tumors. It is clear that brain tumors contain extensive regions in which the tumor cells are subjected to unphysiological levels of hypoxia. Hypoxic cells are resistant to radiation. Hypoxia and the perfusion deficits and metabolic changes that accompany hypoxia in vivo also produce resistance to many commonly used anticancer drugs. The resistance of cells that are hypoxic at the time of therapy may influence the efficacy of the treatment of these tumors with radiation, chemotherapy, and combined modality regimens. Moreover, it is becoming increasingly evident from laboratory studies that exposure of cells to adverse microenvironments produces transient changes in gene expression, induces mutations, and selects for cells with altered genotypes, thus driving the evolution of the cell population toward increasing malignancy and increasingly aggressive phenotypes. Hypoxia may therefore be involved in the evolution of cells in low-grade malignancies to the resistant, aggressive phenotype characteristic of glioblastomas. During the past 50 years, many attempts have been made to circumvent the therapeutic resistance induced by hypoxia, by improving tumor oxygenation, by using oxygen-mimetic radiosensitizers, by adjuvant therapy with drugs that are preferentially toxic to hypoxic cells, by using hyperthermia, or by devising radiation sources and regimens that are less affected by hypoxia. Past clinical trials have provided tantalizing suggestions that the outcome of therapy can be improved by many of these approaches, but none has yet produced a significant, reproducible improvement in the therapeutic ratio, which would be needed for any of these approaches to become the standard therapy for these diseases. Several ongoing clinical trials are addressing other, hopefully better regimens; it will be interesting to see the results of these studies.

Enhancement of tumor oxygenation and radiation response by the allosteric effector of hemoglobin, RSR13

Prior studies using pO(2) microelectrodes have shown that RSR13, an allosteric modifier of hemoglobin, increases tissue oxygenation in vivo. Recently, measurements of tissue oxygenation have been performed by many investigators using blood oxygen level-dependent magnetic resonance imaging (BOLD MRI). In this study, we tested the hypothesis that the BOLD MRI signal ratio in tumors will change after administration of RSR13. NCI-H460 human lung carcinoma cells were used as a xenograft in athymic nude mice. Mice with 1-cm(3) tumors in the flank were anesthetized and mounted on the MRI apparatus, and various doses of RSR13 were administered intraperitoneally (i.p.). MR images were then acquired at 10-min intervals for up to 60 min after injection. The effect of RSR13 on tumor response was studied using the same mouse xenograft model with tumor growth delay measurements. RSR13 increased the MRI signal ratio [Intensity(t)/Intensity(t = 0)] in a dose-dependent manner, with maximum increases occurring 30 min after RSR13 was administered. An RSR13 dose of 200 mg/kg proved to be optimum. Since the MRI signal ratio has been shown previously to be linearly related to tissue oxygenation, the changes in the MRI signal ratio can be attributed to changes in tumor oxygen levels. Using a 200-mg/kg dose of RSR13, with a 10-Gy dose of radiation administered to tumors 30 min later, enhancement of radiation-induced tumor growth delay by RSR13 was observed (enhancement factor = 2.8). Thus our MRI results support and verify the previously reported RSR13-induced increase in tumor oxygenation obtained using pO(2) microelectrodes. Based upon these results and other previous studies, the mechanism of enhancement of the effect of radiation by RSR13 probably involves an increase in tumor oxygenation.

A phase I study of RSR13, a radiation-enhancing hemoglobin modifier: tolerance of repeated intravenous doses and correlation of pharmacokinetics with pharmacodynamics

PURPOSE

Preclinical studies indicate that RSR13 oxygenates and radiosensitizes hypoxic solid tumors by decreasing the oxygen (O(2))-binding affinity of hemoglobin (Hb). A Phase I open-label, multicenter dose and frequency escalation study was conducted to assess the safety, tolerance, pharmacokinetics, and pharmacodynamic effect of daily RSR13 administration to cancer patients receiving concurrent palliative radiotherapy (RT).

METHODS AND MATERIALS

Eligibility criteria included the following: ECOG performance status < or =2; resting and exercise arterial oxygen saturation (SaO(2)) > or =90%; an indication for palliative RT, 20-40 Gy in 10-15 fractions. RSR13 was administered i.v. via central vein over 60 min immediately before RT. Patients received supplemental O(2) via nasal cannula at 4 L/min during RSR13 infusion and RT. Plasma, red blood cell (RBC), and urine RSR13 concentrations were assayed. The pharmacodynamic effect of RSR13 on Hb-O(2) binding affinity was quantified by multipoint tonometry and expressed as an increase in p50, defined as the partial pressure of O(2) that results in 50% SaO(2). The RSR13 dose in the first cohort was 75 mg/kg once a week for two doses; successive cohorts received higher, more frequent doses up to 100 mg/kg/day for 10 days during RT.

RESULTS

Twenty patients were enrolled in the study. Repeated daily doses of RSR13 were generally well tolerated. Two adverse events of note occurred: (1) A patient with pre-existing restrictive lung disease had transient persistent hypoxemia after the sixth RSR13 dose; (2) a patient with a recurrent glioma receiving high-dose corticosteroids had edema after the seventh RSR13 dose, likely due to the daily high-volume fluid infusions. Both patients recovered to baseline status with conservative management. Maximum pharmacodynamic effect occurred at the end of RSR13 infusion and was proportional to the RBC RSR13 concentration. After an RSR13 dose of 100 mg/kg, the peak increase in p50 averaged 8.1 mm Hg, consistent with the targeted physiologic effect, and then diminished with a half-life of approximately 5 h.

CONCLUSIONS

RSR13 was well tolerated in daily doses up to 100 mg/kg administered for 10 days during RT. The combined administration of RSR13 with 4 L/min supplemental O(2) yielded pharmacodynamic conditions in which hypoxic tumor radiosensitization can occur. Ongoing Phase II and Phase III studies are evaluating the combination of RT and RSR13 for selected indications, including primary brain tumors, brain metastases, and non-small-cell lung cancer.

Synthesis and structure-activity relationships of chiral allosteric modifiers of hemoglobin

A series of allosteric effectors of hemoglobin, 2-(aryloxy)-2-alkanoic acids, was prepared to investigate the effect of the stereocenter on allosteric activity. The chiral analogues were based on the lead compound, RSR13 (3b), with different alkyl/alkanoic and cycloalkyl/cycloalkanoic groups positioned at the acidic chiral center. Of the 23 racemic molecules synthesized, 5 were selected for resolution based on structure-activity relationships. One chiral analogue, (-)-(1R,2R)-1-[4-[[(3, 5-dimethylanilino)carbonyl]methyl]phenoxy]-2-methylcyclopentane carbox ylic acid (11), exhibited greater in vitro activity in hemoglobin solutions than its antipode, racemate, and RSR13. Compound (-)-(1R, 2R)-11 was equipotent with RSR13 in whole blood, is a candidate for in vivo animal studies, and if efficacious and safe has a potential for use in humans. In general, it was found that chirality affects allosteric effector activity with measurable differences observed between enantiomers and the racemates.