Discrepancies between measured changes of radiobiological hypoxic fraction and oxygen tension monitoring using two assay systems

Inhibition of vasculogenesis, but not angiogenesis, prevents the recurrence of glioblastoma after irradiation in mice

Despite the high doses of radiation delivered in the treatment of patients with glioblastoma multiforme (GBM), the tumors invariably recur within the irradiation field, resulting in a low cure rate. Understanding the mechanism of such recurrence is therefore important. Here we have shown in an intracranial GBM xenograft model that irradiation induces recruitment of bone marrow-derived cells (BMDCs) into the tumors, restoring the radiation-damaged vasculature by vasculogenesis and thereby allowing the growth of surviving tumor cells. BMDC influx was initiated by induction of HIF-1 in the irradiated tumors, and blocking this influx prevented tumor recurrence. Previous studies have indicated that BMDCs are recruited to tumors in part through the interaction between the HIF-1-dependent stromal cell-derived factor-1 (SDF-1) and its receptor, CXCR4. Pharmacologic inhibition of HIF-1 or of the SDF-1/CXCR4 interaction prevented the influx of BMDCs, primarily CD11b+ myelomonocytes, and the postirradiation development of functional tumor vasculature, resulting in abrogation of tumor regrowth. Similar results were found using neutralizing antibodies against CXCR4. Our data therefore suggest a novel approach for the treatment of GBM: in addition to radiotherapy, the vasculogenesis pathway needs to be blocked, and this can be accomplished using the clinically approved drug AMD3100, a small molecule inhibitor of SDF-1/CXCR4 interactions.

Radiation-induced DNA damage in tumors and normal tissues. VI. Estimation of the hypoxic fraction of experimental tumors

For several years, we have concentrated our efforts on validating the use of radiation-induced DNA strand breaks and DNA-protein crosslinks to assess the oxygenation status of tumors and normal tissues. We have demonstrated that (1) the oxygen dependence of strand break formation is identical to that of radiation-induced cell killing; (2) the oxygen dependence of DNA-protein crosslink formation is the mirror image of that of radiation-induced cell killing; and (3) the formation of these radiation-induced DNA lesions is predominantly dependent on the oxygen concentration near the DNA and is independent of the cell type, metabolic status, proliferative status, pH of the surrounding environment, and composition or properties of the proteins tightly associated with the DNA. In the present study, the hypoxic fraction of three experimental tumors was estimated using our assay of radiation-induced DNA damage. The average hypoxic fraction of a large number of tumors estimated with this assay of radiation-induced DNA damage for (1) WiDR human colorectal carcinoma xenografts (40.8 +/- 4.2%), (2) 66 mouse mammary adenocarcinoma tumors (41.8 +/- 3.1%), and (3) subcutaneous tumors grown from 9L rat brain tumor cells (95% CI =-8.2-4.2%) was not statistically different from that of a large number of tumors measured for each of these tumor models by the paired survival curve method (38.3 +/- 6. 3%, 28.9 +/- 5.5%, 95% CI = 2.2-4.4%, respectively). When the hypoxic fraction measured by the alkaline elution method on one half of an individual tumor was compared to that measured by the paired survival curve method on the other half of the same tumor, no statistical correlation was found for either 66 or WiDR tumors. Although this assay of radiation-induced DNA damage can be used effectively in the laboratory to answer a number of important questions about the oxygenation status of animal tumors and normal tissues, failure to reliably estimate the hypoxic fraction of individual tumors and technical considerations make it unlikely that the assay can be used in the clinic to estimate the hypoxic fraction of human tumors.

Determining hypoxic fraction in a rat glioma by uptake of radiolabeled fluoromisonidazole

The usefulness of radiolabeled nitroimidazoles for measuring hypoxia will be clarified by defining the relationship between tracer uptake and radiobiologically hypoxic fraction. We determined the radiobiologically hypoxic fraction from radiation response data in 36B10 rat gliomas using the paired cell survival curve technique and compared the values to the radiobiologically hypoxic fraction inferred from mathematical modeling of time-activity data acquired by PET imaging of [(18)F]FMISO uptake. Rats breathed either air or 10% oxygen during imaging, and timed blood samples were taken. The uptake of [(3)H]FMISO by 36B10 cells in vitro provided cellular binding characteristics of this radiopharmaceutical as a function of oxygen concentration. The radiobiologically hypoxic fraction determined for tumors in air-breathing rats using the paired survival curve technique was 6.1% (95% CL = 4.3- 8.6%), which agreed well with that determined by modeling FMISO time-activity data (7. 4%; 95% CL = 2.5-17.3%). These results are consistent with the agreement between the two techniques for measuring radiobiologically hypoxic fraction in Chinese hamster V79 cell spheroids. In contrast, the FMISO-derived radiobiologically hypoxic fraction in rats breathing 10% oxygen was 13.1% (95% CL 7.9-8.3%), much lower than the radiobiologically hypoxic fraction of 43% determined from the radiation response data. This discrepancy may be due to the failure of FMISO to identify hypoxic cells residing at or above an oxygen level of 2-3 mmHg that will still confer substantial protection against radiation. The presence of transiently hypoxic cells in rats breathing reduced oxygen may also be under-reported by nitroimidazole binding, which is strongly dependent on time and concentration.

Oxygen tension measurements of tumors growing in mice

PURPOSE

Clinical studies using the Eppendorf histograph have shown that patients whose tumors have a low pO2 have worse local control after radiotherapy, and have higher metastatic rates. Because preclinical studies of methods of overcoming, or exploiting, hypoxia generally use transplanted tumors in mice, we have compared the oxygenation of mouse tumors with human tumors to determine the appropriateness of the transplanted mouse model for such preclinical studies.

METHODS AND MATERIALS

We evaluated the oxygenation status of subcutaneous (s.c.) tissue and of 12 intradermally (i.d.)- and 7 s.c.-growing mouse or human transplanted tumors in mice using the Eppendorf histograph, and compared the values obtained with measurements of human head and neck nodes.

RESULTS

The normal tissue pO2 profile of air-breathing mice showed a nearly Gaussian distribution (38.2+/-14.9 mmHg). Breathing 10% O2 or carbogen resulted in dramatic changes in normal tissue oxygenation. Tumors growing intradermally in the back of air-breathing mice were extremely hypoxic and resistant to expected changes in oxygenation (carbogen breathing, size, and use of anesthetics). Tumors growing s.c. in the foot showed higher oxygen profiles with marked changes in oxygenation when exposing the animals to different levels of oxygen. However, the oxygenation of the mouse tumors transplanted in either site was only a fraction of that of the majority of human tumors.

CONCLUSION

Experimental mouse tumors are markedly hypoxic, with median values of 10-20% of those of human tumors. Hence, mouse tumors are probably good models for the most hypoxic human tumors that respond poorly to radiotherapy; however, caution has to be exercised in extrapolating data from mouse to man.

Detection of hypoxic cells in murine tumors using the comet assay: comparison with a conventional radiobiological assay

The comet (single-cell electrophoresis) assay has been developed as a method for measuring DNA damage in single cells after irradiation. We have developed our own methods and image analysis system for the comet assay to identify hypoxic fractions. In vitro, we tested our system using a cultured tumor cell line (SCCVII). In vivo, we compared the hypoxic fractions detected by this assay with those determined by the in vivo-in vitro clonogenic assay using two rodent tumors (SCCVII/ C3H, EMT6/KU/balb/c), which exhibit different types of hypoxia: acute and chronic. In vitro, our method could differentiate hypoxic cells from oxic cells, using the parameter of tail moment. In vivo, there were good correlations between the hypoxic fractions determined by the comet assay and by the clonogenic assay, in SCCVII/C3H (r=0.85) and in EMT6/KU/balb/c (r=0.75) tumors. By comparison of the two methods in chronically hypoxic and acutely hypoxic tumors, we further confirmed that the comet assay is clinically useful for estimating hypoxic fractions of solid tumors.

A comparison in individual murine tumors of techniques for measuring oxygen levels

PURPOSE

To investigate the relationship between different techniques for measuring oxygen levels in a murine tumor model.

METHODS AND MATERIALS

Using the murine fibrosarcoma line KHT-C, five techniques of measuring oxygen levels-the Eppendorf pO2 Histograph, EF5 binding, the comet assay, a paired survival assay, and an in vivo growth delay assay-were assessed. In these experiments, three or more techniques were applied in different combinations to measure the oxygen levels in individual tumors.

RESULTS

Statistically significant correlations were observed between the hypoxic proportions calculated from the paired survival assay with those from EF5 binding. The comet assay was found to have a statistically significant correlation with the paired survival analysis and the growth delay analysis. No statistically significant correlation was found between the Eppendorf pO2 Histograph measurements and those from the other techniques, although there were weak correlations with the paired survival assay and EF5 binding. For technical reasons, a comparison was not made between EF5 binding and the growth delay assay.

CONCLUSIONS

The correlations found between EF5 binding and the comet assay with the radiobiological assays suggest that these techniques have potential for predicting outcome following radiation treatment. The lack of correlation seen between the pO2 Histograph data and the radiobiological assays is in contrast to results from early clinical trials.

Tumour response to hypercapnia and hyperoxia monitored by FLOOD magnetic resonance imaging

Flow and oxygenation dependent (FLOOD) MR images of GH3 prolactinomas display large intensity increases in response to carbogen (5% CO2/95% O2) breathing. To assess the relative contributions of carbon dioxide and oxygen to this response and the tumour oxygenation state, the response of GH3 prolactinomas to 5% CO2/95% air, carbogen and 100% O2 was monitored by FLOOD MRI and PO2 histography. A 10-30% image intensity increase was observed during 5% CO2/95% air breathing, consistent with an increase in tumour blood flow, as a result of CO2-induced vasodilation, reducing the concentration of deoxyhaemoglobin in the blood. Carbogen caused a further 40-50% signal enhancement, suggesting an additional improvement due to increase blood oxygenation. A small 5-10% increase was observed in response to 100% O2, highlighting the dominance of CO2-induced vasodilation in the carbogen response. Despite the large FLOOD response, non-significant increases in tumour pO2 were observed in response to the three gases. Tissue pO2 is determined by the balance of oxygen supply and demand, hence increased blood flow/oxygenation may not necessarily produce a large increase in tissue PO2. The FLOOD response is determined by the level of deoxygenation of blood, the size of this response relating to vascular density and the potential of high-oxygen content gases to improve the oxygen supply to tumour tissue.

Evaluation of the radiosensitizing effects of RK28 intravenous, intraarterial, and intratumoral injections on the rabbit VX2 tumor system

PURPOSE

To evaluate the differences in the radiosensitizing effects of intravenous (i.v.) injection, intraarterial (i.a.) injection, and intratumoral (i.t.) injections of the hypoxic cell radiosensitizer RK28 ([1-(4'-hydroxy-2'-butenoxy)methyl-2-nitroimidazole], a 2-nitroimidazole with an acyclic sugar analogue substituted at the N-1 position of the imidazole ring) using an animal experimental system.

METHODS AND MATERIALS

Rabbit VX2 tumors, which were implanted in the muscle of left hind legs and grown to 3 cm in diameter, were treated with RK28 (80 mg/kg x b.wt.) before 15 Gy of local x-ray irradiation. The auricular vein and the left saphenous artery were used for systemic injection and regional injection, respectively. For i.t. injection, a 21-gauge needle with three lateral holes was positioned in the central area of the tumor. Tumor regression was precisely evaluated by computed tomograpy (CT), and survival time was also studied. Using high-performance liquid chromatography (HPLC), pharmacokinetic studies for RK28 and its seven major metabolites were performed in tumor and serum at 0, 10, 20, 30, and 60 min after drug injection was completed.

RESULTS

Radiosensitizing effects of RK28 were considered present after i.a. injection (p < 0.05) and i.t. injection (p < 0.05) after analyzing tumor volumes on day 21 after treatment. Increased survival was not observed in any group with RK28 injection compared with survival in the group treated by x-ray irradiation alone. Pharmacokinetic studies showed the average concentration of RK28 in the tumor during x-ray irradiation was 1.3 times higher after i.a. injection and 3.5 times higher after i.t. injection than that after i.v. injection. The time modifying factor50 (TMF50: ratio of time for tumor to decrease by 50%, radiation alone vs. radiation plus drug) was calculated to be 1.5 after i.v. injection, 1.7 after i.a. injection, and 2.3 after i.t. injection. The values of TMF50 correlated to the average concentrations of RK28 in the tumor. As to metabolites of RK28, beta-glucuronated compound and cysteine conjugate were highly detected. The concentrations of cysteine conjugate were higher in the tumor than in serum via i.v. injection.

CONCLUSIONS

Radiosensitizing effects of RK28 were observed on the rabbit VX-2 tumor system after i.a. or i.t. injection. Pharmacokinetic studies proved that radiosensitizing effects depended on the concentration in the tumor, though the administration routes were different. Combined forms with nonprotein thiols were detected. However, survival benefits were not obtained by RK28. For clinical applications of RK28, i.a. or i.t. injection could facilitate better local control of cancer.