A phase I-B trial of the radiosensitizer: Etanidazole (SR-2508) with radiosurgery for the treatment of recurrent previously irradiated primary brain tumors or brain metastases (RTOG Study 95-02)

Current Landscape and Future Prospects of Radiation Sensitizers for Malignant Brain Tumors: A Systematic Review

BACKGROUND

Radiation therapy (RT) is the cornerstone of management of malignant brain tumors, but its efficacy is limited in hypoxic tumors. Although numerous radiosensitizer compounds have been developed to enhance the effect of RT, progress has been stagnant. Through this systematic review, we provide an overview of radiosensitizers developed for malignant brain tumors, summarize their safety and efficacy, and evaluate areas for possible improvement.

METHODS

Following PRISMA guidelines, PubMed, EMBASE, Cochrane, and Web of Science were searched using terminology pertaining to radiosensitizers for brain tumor RT. Articles reporting clinical evidence of nonantineoplastic radiosensitizers with RT for malignant central nervous system tumors were included. Data of interest were presumed mechanism of action, median overall survival (OS), progression-free survival (PFS), and adverse events.

RESULTS

Twenty-two unique radiosensitizers were identified. Only 2/22 agents (fluosol with oxygen, and efaproxiral) showed improvement in OS in patients with glioblastoma and brain metastasis, respectively. A larger study was not able to confirm the latter. Improved PFS was reported with use of metronidazole, sodium glycididazole, and chloroquine. There was a wide range of toxicities, which prompted change of schedule or complete discontinuation of 9 agents.

CONCLUSIONS

Progress in radiosensitizers for malignant CNS tumors has been limited. Only 2 radiosensitizers have shown limited improvement in survival. Alternative strategies such as synthetic drug design, based on a mechanism of action that is independent of crossing the blood-brain barrier, may be necessary. Use of drug development strategies using new technologies to overcome past challenges is necessary.

Radiobiology of stereotactic ablative radiotherapy (SABR): perspectives of clinical oncologists

Stereotactic ablative radiotherapy (SABR) is a novel radiation treatment method that delivers an intense dose of radiation to the treatment targets with high accuracy. The excellent local control and tolerance profile of SABR have made it become an important modality in cancer treatment. The radiobiology of SABR is a key factor in understanding and further optimizing the benefits of SABR. In this review, we have addressed several issues in the radiobiology of SABR from the perspective of clinical oncologists. The appropriateness of the linear-quadratic (LQ) model for SABR is controversial based on preclinical data, but it is a reliable tool from the perspective of clinical application because the biological effective dose (BED) calculated with it can represent the tumor control probability (TCP). Hypoxia is a common phenomenon in SABR in spite of the relatively small tumor size and has a negative effect on the efficacy of SABR. Preliminary studies indicate that a hypoxic radiosensitizer combined with SABR may be a feasible strategy, but so far there is not adequate evidence to support its application in routine practice. The vascular change of endothelial apoptosis and blood perfusion reduction in SABR may enhance the response of tumor cells to radiation. Combination of SABR with anti-angiogenesis therapy has shown promising efficacy and good tolerance in advanced cancers. SABR is more powerful in enhancing antitumor immunity and works better with immune checkpoint inhibitors (ICIs) than conventional fractionation radiotherapy. Combination of SABR with ICIs has become a practical option for cancer patients with metastases.

High Single Doses of Radiation May Induce Elevated Levels of Hypoxia in Early-Stage Non-Small Cell Lung Cancer Tumors

PURPOSE

Tumor hypoxia correlates with treatment failure in patients undergoing conventional radiation therapy. However, no published studies have investigated tumor hypoxia in patients undergoing stereotactic body radiation therapy (SBRT). We aimed to noninvasively quantify the tumor hypoxic volume (HV) in non-small cell lung cancer (NSCLC) tumors to elucidate the potential role of tumor vascular response and reoxygenation at high single doses.

METHODS AND MATERIALS

Six SBRT-eligible patients with NSCLC tumors >1 cm were prospectively enrolled in an institutional review board-approved study. Dynamic positron emission tomography images were acquired at 0 to 120 minutes, 150 to 180 minutes, and 210 to 240 minutes after injection of 18F-fluoromisonidazole. Serial imaging was performed prior to delivery of 18 Gy and at approximately 48 hours and approximately 96 hours after SBRT. Tumor HVs were quantified using the tumor-to-blood ratio (>1.2) and rate of tracer influx (>0.0015 mL·min·cm-3).

RESULTS

An elevated and in some cases persistent level of tumor hypoxia was observed in 3 of 6 patients. Two patients exhibited no detectable baseline tumor hypoxia, and 1 patient with high baseline hypoxia only completed 1 imaging session. On the basis of the tumor-to-blood ratio, in the remaining 3 patients, tumor HVs increased on day 2 after 18 Gy and then showed variable responses on day 4. In the 3 of 6 patients with detectable hypoxia at baseline, baseline tumor HVs ranged between 17% and 24% (mean, 21%), and HVs on days 2 and 4 ranged between 33% and 45% (mean, 40%) and between 18% and 42% (mean, 28%), respectively.

CONCLUSIONS

High single doses of radiation delivered as part of SBRT may induce an elevated and in some cases persistent state of tumor hypoxia in NSCLC tumors. Hypoxia imaging with 18F-fluoromisonidazole positron emission tomography should be used in a larger cohort of NSCLC patients to determine whether elevated tumor hypoxia is predictive of treatment failure in SBRT.

Correlation of hypoxia status with radiosensitizing effects of sodium glycididazole: A preclinical study

The correlation of pretreatment hypoxia status with the radiosensitization effect of sodium glycididazole (CMNa) was not previously defined. The purpose of the present study was to evaluate the tumor hypoxia status in various cancer xenografts and to investigate the correlation between tumor hypoxia status and radiosensitizing effects of CMNa based on the pharmacokinetic and pharmacodynamic parameters. Human esophageal cancer (EC109), head and neck cancer (FaDu) and lung cancer (A549) nude mice xenografts were used. The concentrations of CMNa and its metabolites in the tumors and normal tissues were determined by high-performance liquid chromatography following intravenous injection of 171.9, 57.3 or 19.1 mg/kg CMNa. The tumors were irradiated with 30 Gy in 6 fractions with CMNa administration prior to each irradiation. The tumor growth delay values were calculated for each treatment group and compared with groups treated with radiation alone. Tumor hypoxia status was verified by immunohistostaining of tissues for hypoxia inducible factor 1α (HIF-1α) staining, and the concentration of plasma osteopontin (OPN) was determined using ELISA. The correlation between OPN concentration and tumor growth delay was subsequently analyzed. It was observed that the drug concentration in the tumor was 1.6-2.8 times higher compared with adjacent muscle, particularly at high and medium doses. CMNa was able to sensitize tumors to irradiation, particularly for EC109 and FaDu xenografts at high dose (P<0.05). Furthermore, there was markedly increased expression of HIF-1α and plasma OPN levels in FaDu and EC109 xenografts compared with A549. Additionally, it was indicated that pretreatment hypoxia status might be correlated with the radiosensitizing effects of CMNa. The present data demonstrated that tumor hypoxia status might be correlated with the radiosensitizing effects of CMNa in different tumor models.

Vorinostat and Concurrent Stereotactic Radiosurgery for Non-Small Cell Lung Cancer Brain Metastases: A Phase 1 Dose Escalation Trial

PURPOSE

To determine the maximum tolerated dose (MTD) of vorinostat, a histone deacetylase inhibitor, given concurrently with stereotactic radiosurgery (SRS) to treat non-small cell lung cancer (NSCLC) brain metastases. Secondary objectives were to determine toxicity, local failure, distant intracranial failure, and overall survival rates.

MATERIALS AND METHODS

In this multicenter study, patients with 1 to 4 NSCLC brain metastases, each ≤2 cm, were enrolled in a phase 1, 3 + 3 dose escalation trial. Vorinostat dose levels were 200, 300, and 400 mg orally once daily for 14 days. Single-fraction SRS was delivered on day 3. A dose-limiting toxicity (DLT) was defined as any Common Terminology Criteria for Adverse Events version 3.0 grade 3 to 5 acute nonhematologic adverse event related to vorinostat or SRS occurring within 30 days.

RESULTS

From 2009 to 2014, 17 patients were enrolled and 12 patients completed study treatment. Because no DLTs were observed, the MTD was established as 400 mg. Acute adverse events were reported by 10 patients (59%). Five patients discontinued vorinostat early and withdrew from the study. The most common reasons for withdrawal were dyspnea (n=2), nausea (n=1), and fatigue (n=2). With a median follow-up of 12 months (range, 1-64 months), Kaplan-Meier overall survival was 13 months. There were no local failures. One patient (8%) at the 400-mg dose level with a 2.0-cm metastasis developed histologically confirmed grade 4 radiation necrosis 2 months after SRS.

CONCLUSIONS

The MTD of vorinostat with concurrent SRS was established as 400 mg. Although no DLTs were observed, 5 patients withdrew before completing the treatment course, a result that emphasizes the need for supportive care during vorinostat administration. There were no local failures. A larger, randomized trial may evaluate both the tolerability and potential local control benefit of vorinostat concurrent with SRS for brain metastases.

Evaluation of the in vivo radiosensitizing activity of etanidazole using tumor-bearing chick embryo

Chick embryos have been used as alternative experimental animals in various research fields, including virology, immunology, toxicology, oncology, and embryology. Until now, there have been no in vivo models using chick embryo to evaluate radiosensitizing activity. Here, the in vivo radiosensitizing activity of etanidazole, a well-known hypoxic cell radiosensitizer, was evaluated using tumor-bearing chick embryo. On the basis of tumor growth, drug administration and X-ray irradiation were performed on day 15 chick embryo, with the endpoint being day 18 chick embryo. In day 15 chick embryo, an X-ray irradiation dose of equal or less than 10 Gy did not cause significant tumor growth suppression. Intravenous administration of equal or less than 1.0 mg of etanidazole did not cause tumor growth suppression. Neither doses of equal or less than 8 Gy of irradiation nor 1.0 mg of etanidazole caused fatality of the chick embryo. On the basis of these results, we evaluated the radiosensitizing effect of a combination treatment with 8 Gy of irradiation and 1.0 mg of etanidazole. As noted above, 1.0 mg of etanidazole alone and 8 Gy of irradiation alone did not show tumor growth suppression. In contrast, a combination treatment with 8 Gy of irradiation and 1.0 mg of etanidazole showed 35% of significant tumor growth suppression. Thus, we succeeded in evaluating the in vivo radiosensitizing activity of etanidazole using tumor-bearing chick embryo. These results suggest that the use of tumor-bearing chick embryo may be part of a promising system for evaluating radiosensitizing activity.

Hypofractionation results in reduced tumor cell kill compared to conventional fractionation for tumors with regions of hypoxia

PURPOSE

Tumor hypoxia has been observed in many human cancers and is associated with treatment failure in radiation therapy. The purpose of this study is to quantify the effect of different radiation fractionation schemes on tumor cell killing, assuming a realistic distribution of tumor oxygenation.

METHODS AND MATERIALS

A probability density function for the partial pressure of oxygen in a tumor cell population is quantified as a function of radial distance from the capillary wall. Corresponding hypoxia reduction factors for cell killing are determined. The surviving fraction of a tumor consisting of maximally resistant cells, cells at intermediate levels of hypoxia, and normoxic cells is calculated as a function of dose per fraction for an equivalent tumor biological effective dose under normoxic conditions.

RESULTS

Increasing hypoxia as a function of distance from blood vessels results in a decrease in tumor cell killing for a typical radiotherapy fractionation scheme by a factor of 10(5) over a distance of 130 μm. For head-and-neck cancer and prostate cancer, the fraction of tumor clonogens killed over a full treatment course decreases by up to a factor of ∼10(3) as the dose per fraction is increased from 2 to 24 Gy and from 2 to 18 Gy, respectively.

CONCLUSIONS

Hypofractionation of a radiotherapy regimen can result in a significant decrease in tumor cell killing compared to standard fractionation as a result of tumor hypoxia. There is a potential for large errors when calculating alternate fractionations using formalisms that do not account for tumor hypoxia.

Stereotactic ablative radiotherapy should be combined with a hypoxic cell radiosensitizer

PURPOSE

To evaluate the effect of tumor hypoxia on the expected level of cell killing by regimens of stereotactic ablative radiotherapy (SABR) and to determine the extent to which the negative effect of hypoxia could be prevented using a clinically available hypoxic cell radiosensitizer.

RESULTS AND DISCUSSION

We have calculated the expected level of tumor cell killing from regimens of SABR, both with and without the assumption that 20% of the tumor cells are hypoxic, using the standard linear quadratic model and the universal survival curve modification. We compare the results obtained with our own clinical data for lung tumors of different sizes and with published data from other studies. We also have calculated the expected effect on cell survival of adding the hypoxic cell sensitizer etanidazole at clinically achievable drug concentrations. Modeling tumor cell killing with any of the currently used regimens of SABR produces results that are inconsistent with the majority of clinical findings if tumor hypoxia is not considered. However, with the assumption of tumor hypoxia, the expected level of cell killing is consistent with clinical data. For only some of the smallest tumors are the clinical data consistent with no tumor hypoxia, but there could be other reasons for the sensitivity of these tumors. The addition of etanidazole at clinically achievable tumor concentrations produces a large increase in the expected level of tumor cell killing from the large radiation doses used in SABR.

CONCLUSIONS

The presence of tumor hypoxia is a major negative factor in limiting the curability of tumors by SABR at radiation doses that are tolerable to surrounding normal tissues. However, this negative effect of hypoxia could be overcome by the addition of clinically tolerable doses of the hypoxic cell radiosensitizer etanidazole.

A fast radiotherapy paradigm for anal cancer with volumetric modulated arc therapy (VMAT)

BACKGROUND/PURPOSE

Radiotherapy (RT) volumes for anal cancer are large and of moderate complexity when organs at risk (OAR) such as testis, small bowel and bladder are at least partially to be shielded. Volumetric intensity modulated arc therapy (VMAT) might provide OAR-shielding comparable to step-and-shoot intensity modulated radiotherapy (IMRT) for this tumor entity with better treatment efficiency.

MATERIALS AND METHODS

Based on treatment planning CTs of 8 patients, we compared dose distributions, comformality index (CI), homogeneity index (HI), number of monitor units (MU) and treatment time (TTT) for plans generated for VMAT, 3D-CRT and step-and-shoot-IMRT (optimized based on Pencil Beam (PB) or Monte Carlo (MC) dose calculation) for typical anal cancer planning target volumes (PTV) including inguinal lymph nodes as usually treated during the first phase (0-36 Gy) of a shrinking field regimen.

RESULTS

With values of 1.33 +/- 0.21/1.26 +/- 0.05/1.3 +/- 0.02 and 1.39 +/- 0.09, the CI's for IMRT (PB-Corvus/PB-Hyperion/MC-Hyperion) and VMAT are better than for 3D-CRT with 2.00 +/- 0.16. The HI's for the prescribed dose (HI36) for 3D-CRT were 1.06 +/- 0.01 and 1.11 +/- 0.02 for VMAT, respectively and 1.15 +/- 0.02/1.10 +/- 0.02/1.11 +/- 0.08 for IMRT (PB-Corvus/PB-Hyperion/MC-Hyperion). Mean TTT and MU's for 3D-CRT is 220s/225 +/- 11MU and for IMRT (PB-Corvus/PB-Hyperion/MC-Hyperion) is 575s/1260 +/- 172MU, 570s/477 +/- 84MU and 610s748 +/- 193MU while TTT and MU for two-arc-VMAT is 290s/268 +/- 19MU.

CONCLUSION

VMAT provides treatment plans with high conformity and homogeneity equivalent to step-and-shoot-IMRT for this mono-concave treatment volume. Short treatment delivery time and low primary MU are the most important advantages.