Hyperfractionation as an effective way of overcoming radioresistance

Roles of glucose transporter-1 and the phosphatidylinositol 3‑kinase/protein kinase B pathway in cancer radioresistance (review)

The mechanisms underlying cancer radioresistance remain unclear. Several studies have found that increased glucose transporter‑1 (GLUT‑1) expression is associated with radioresistance. Recently, the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) pathway was reported to be involved in the control of GLUT‑1 trafficking and activity. Activation of the PI3K/Akt pathway may itself be associated with cancer radioresistance. Thus, increasing attention has been devoted to the effects of modifying the expression of GLUT‑1 and the PI3K/Akt pathway on the increase in the radiosensitivity of cancer cells. This review discusses the importance of the association between elevated expression of GLUT‑1 and activation of the PI3K/Akt pathway in the development of radioresistance in cancer.

Altered fractionation outcomes for hypoxic head and neck cancer using the HYP-RT Monte Carlo model

OBJECTIVE

Altered fractionation radiotherapy is simulated on a set of virtual tumours to assess the total doses required for tumour control compared with clinical head and neck data and the doses required to control hypoxic vs well-oxygenated tumours with different radiobiological properties.

METHODS

The HYP-RT model is utilised to explore the impact of tumour oxygenation and the onset times of accelerated repopulation (AR) and reoxygenation (ROx) during radiotherapy. A biological effective dose analysis is used to rank the schedules based on their relative normal tissue toxicities.

RESULTS

Altering the onset times of AR and ROx has a large impact on the doses required to achieve tumour control. Immediate onset of ROx and 2-week onset time of AR produce results closely predicting average human outcomes in terms of the total prescription doses in clinical trials. Modifying oxygen enhancement ratio curves based on dose/fraction significantly reduces the dose (5-10 Gy) required for tumour control for hyperfractionated schedules. HYP-RT predicts 10×1.1 Gy per week to be most beneficial, whereas the conventional schedule is predicted as beneficial for early toxicity but has average-poor late toxicity.

CONCLUSION

HYP-RT predicts that altered radiotherapy schedules increase the therapeutic ratio and may be used to make predictions about the prescription doses required to achieve tumour control for tumours with different oxygenation levels and treatment responses.

ADVANCES IN KNOWLEDGE

Oxic and hypoxic tumours have large differences in total radiation dose requirements, affected by AR and ROx onset times by up to 15-25 Gy for the same fractionation schedule.

The HYP-RT hypoxic tumour radiotherapy algorithm and accelerated repopulation dose per fraction study

The HYP-RT model simulates hypoxic tumour growth for head and neck cancer as well as radiotherapy and the effects of accelerated repopulation and reoxygenation. This report outlines algorithm design, parameterisation and the impact of accelerated repopulation on the increase in dose/fraction needed to control the extra cell propagation during accelerated repopulation. Cell kill probabilities are based on Linear Quadratic theory, with oxygenation levels and proliferative capacity influencing cell death. Hypoxia is modelled through oxygen level allocation based on pO₂ histograms. Accelerated repopulation is modelled by increasing the stem cell symmetrical division probability, while the process of reoxygenation utilises randomised pO₂ increments to the cell population after each treatment fraction. Propagation of 10⁸ tumour cells requires 5–30 minutes. Controlling the extra cell growth induced by accelerated repopulation requires a dose/fraction increase of 0.5–1.0 Gy, in agreement with published reports. The average reoxygenation pO₂ increment of 3 mmHg per fraction results in full tumour reoxygenation after shrinkage to approximately 1 mm. HYP-RT is a computationally efficient model simulating tumour growth and radiotherapy, incorporating accelerated repopulation and reoxygenation. It may be used to explore cell kill outcomes during radiotherapy while varying key radiobiological and tumour specific parameters, such as the degree of hypoxia.

Prospective evaluation of an in vitro radiation resistance assay in locally advanced cancer of the uterine cervix: a Southwest Oncology Group Study

OBJECTIVES

To investigate the feasibility of performing a fresh-tissue, in vitro radiation resistance assay (IVRRA) in a cooperative group setting and to assess the association of IVRRA results with clinical outcomes.

METHODS

Women with Stages IIB-IVA carcinoma of the uterine cervix without obvious para-aortic lymphadenopathy on imaging were eligible. Primary tumor biopsies were shipped to a central testing facility where agar-based cell suspensions were exposed to 300 cGy of RT ± cisplatin and cultured for 5 days. ³H-thymidine incorporation was used to determine percent cell inhibition (PCI) of test specimen compared to that of the untreated control. Tumors were considered to exhibit extreme radiation resistance (ERR), intermediate radiation resistance (IRR) or low radiation resistance (LRR) based on a standard data set from 39 previously studied specimens. Standardized doses of external beam radiation and intracavitary brachytherapy, when feasible, in addition to platinum-based chemotherapy were mandated. Progression-free survival (PFS) was the primary endpoint. Clinical response and overall survival (OS) were secondary endpoints. Clinical investigators were blinded to assay data and vice versa.

RESULTS

Thirty-six patients were enrolled, but analysis was limited to 17 patients whose specimens were adequate for IVRRA. The median follow-up time among patients still alive at last contact was 40 months (range: 0-56 months). There was no association between IVRRA and response. In the Cox model, IRR/ERR tumors showed worse PFS [HR = 11.2 (95% CI 1.3-96, p = 0.03)] and worse OS [HR=11.7 (95% CI 1.4-99.6, p = 0.03)] compared to LRR tumors when IVRRA was performed with RT alone, but there were no associations between IVRRA and PFS or OS when cisplatin was added to the IVRRA.

CONCLUSIONS

IVRRA (RT alone) results correlated with PFS and OS in this prospective trial, but follow-up trials are indicated to address feasibility and to confirm results in an expanded cohort. If confirmed, IVRRA could potentially direct molecular identification of novel targeted therapeutic approaches which might counteract radiation resistance.

Fluctuating and Diffusion-Limited Hypoxia in Hypoxia-Induced Metastasis

PURPOSE

Most tumors develop regions with hypoxic cells during growth, owing to permanent limitations in oxygen diffusion (chronic or diffusion-limited hypoxia) and/or transient limitations in blood perfusion (acute or fluctuating hypoxia). The aim of this study was to investigate the relative significance of chronic and acute hypoxia in the development of metastatic disease.

EXPERIMENTAL DESIGN

D-12 and R-18 human melanoma xenografts were used as models of human cancer. D-12 tumors metastasize to the lungs, whereas R-18 tumors develop lymph node metastases. Fraction of radiobiologically hypoxic cells (HF(Rad)) was measured in individual primary tumors by using a radiobiological assay based on the paired survival curve method. Fraction of immunohistochemically hypoxic cells (HF(Imm)) was assessed in the same tumors by using a pimonidazole-based immunohistochemical assay optimized with respect to achieving selective staining of chronically hypoxic cells. HF(Imm) and the difference between HF(Rad) and HF(Imm), HF(Rad) - HF(Imm), were verified to be adequate variables for fraction of chronically hypoxic cells and fraction of acutely hypoxic cells, respectively.

RESULTS

Chronic as well as acute hypoxia were found to promote spontaneous metastasis of D-12 and R-18 tumors. Acute hypoxia influenced metastasis to a greater extent than chronic hypoxia, partly because the fraction of acutely hypoxic cells was larger than the fraction of chronically hypoxic cells in most tumors and partly because acutely hypoxic cells showed a higher metastatic potential than chronically hypoxic cells.

CONCLUSIONS

It may be beneficial to focus on fluctuating hypoxia rather than diffusion-limited hypoxia when searching for hypoxia-related prognostic variables and predictive assays.

Low dose hyper-radiosensitivity in metastatic tumors

INTRODUCTION

The laboratory phenomenon of low dose hyper-radiosensitivity (LDHRS) describes excess cell kill at doses below 1 Gy relative to that predicted by the linear quadratic model. These data have stimulated the investigation of whether LDHRS can be exploited clinically.

METHODS

Patients with metastatic tumor nodules to skin were recruited. The nodules were measured in three dimensions, consecutively numbered according to volume, and randomized, in matched pairs, to receive either conventionally fractionated radiotherapy (1.5 Gy/day) or ultrafractionated radiotherapy (0.5 Gy TDS: 4-h gap). Both groups were treated for 12 days. Measurements were taken Days 0, 5, 8, 12, and 26 and monthly until regrowth occurred. Tumor volumes were normalized to those on Day 0 and plotted against time from the start of treatment. Time to regrowth to original volume was calculated and compared between groups using the Wilcoxon signed rank test.

RESULTS

Eight patients with a total of 40 paired nodules were analyzed; 36 nodules have regrown and are therefore evaluable. Analysis of the whole data set demonstrates a two-tailed p-value of 0.14 in favor of the "ultrafractionated" treatment. Analysis of the tumors generally accepted as being radioresistant and known to show LDHRS in vitro demonstrates a two-tailed p value of 0.009.

CONCLUSIONS

LDHRS can be demonstrated in tumors clinically. An "ultrafractionated" radiotherapy regime produces significantly increased growth delay in radioresistant malignant tumors.

No benefits of ultrafractionation in two head-and-neck cancer cell lines with different inherent radiosensitivity

PURPOSE

To assess if ultrafractionation is applicable in the context of an unknown hyperradiosensitivity (HRS) status, we studied the survival and repair capacity of two tumor cell lines after irradiation with two different dose/fractionation schedules that can be used in a clinical setting.

MATERIALS AND METHODS

Squamous cell carcinoma cell lines SCC-3 (radioresistant) and SCC-6 (radiosensitive) were used. Survival was studied by clonogenic assay after multiple fractions of 0.5 Gy (2 fractions/day, 6-h interval) and 2 Gy (1 fraction/day) for a total dose of 8 Gy of gamma-rays. The capacity to repair single-strand and double-strand breaks (SSB, DSB) was assessed by comet assay. The messenger RNA (mRNA) levels of DNA-dependent protein kinase (PK) components were analyzed by RNase protection and real-time polymerase chain reaction (PCR).

RESULTS

In both cell lines, no apparent difference was noted between the two fractionation protocols. In particular for SCC-3, the mean surviving fraction tended to be lower after 2 Gy than after 0.5 Gy fractions. In SCC-3 and SCC-6 no significant difference was observed in the repair capacity of SSB and DSB after exposure to single doses of 0.5 Gy or 2 Gy. After exposure to the same single doses, the mRNA levels of DNA-PK catalytic subunit (PKcs), Ku 70, and Ku 80 were similar.

CONCLUSIONS

Our data do not support the concept of ultrafractionation, at least when using fractions of 0.5 Gy in the cell lines studied. This suggests that methods for testing HRS status in individual tumors need to be developed before the relevance of ultrafractionation can be investigated in the clinic.

Inducible repair and intrinsic radiosensitivity: a complex but predictable relationship?

Two groups have proposed a simple linear relationship between inducible radioresistance in a variety of mammalian cells and their intrinsic radiosensitivity at 2 Gy (Lambin et al., Int.J. Radiat. Biol. 69, 279-290, 1996; Alsbeih and Raaphorst, unpublished results, 1997). The inducible repair response (IRR) is quantified as a ratio, alpha(S)/alpha(R), i.e. the slope in the hypersensitive low-dose region, alpha(S), relative to the alpha(R) term of the classical linear-quadratic formula. These proposals imply that the intrinsic radiosensitivity at clinically relevant doses is directly linked to the cell's ability to mount an adaptive response as a result of exposure to very low doses of radiation. We have re-examined this correlation and found that the more extensive data set now available in the literature does not support the contention of a simple linear relationship. The two parameters are correlated, but by a much more complex relationship. A more logical fit is obtained with a log-linear equation. A series of log-linear curves are needed to describe the correlation between IRR and SF2, because of the spectrum of alpha/beta ratios among the cell lines and hence the confounding effect of the beta term at a dose of 2 Gy. The degree of repair competence before irradiation starts could also be a major factor in the apparent magnitude of the amount of repair induced. There appears to be a systematic difference in the data sets from different series of cell lines that have been obtained using flow cytometry techniques in the laboratory in Vancouver and using dynamic microscope imaging at the Gray Laboratory. We suggest that the use of a brief exposure to a laser beam in flow cytometry before the cells are irradiated might itself partially induce a stress response and change the DNA repair capacity of the cells. The clinical consequences of the relationship for predicting the benefits of altered fractionation schedules are discussed. [ru5]

Inducible repair and the two forms of tumour hypoxia--time for a paradigm shift

Clinical experience shows that there is a therapeutic window between 60 and 70 Gy where many tumours are eradicated, but the function of the adjacent normal tissues is preserved. This implies much more cell kill in the tumour than is acceptable in the normal tissue. An SF2 of 0.5 or lower is needed to account for the eradication of all tumour cells, while an SF2 of 0.8 or higher is needed to explain why these doses are tolerated by normal tissues. No such systematic difference is known between the intrinsic sensitivity of well-oxygenated normal and tumour cells. The presence of radioresistant hypoxic cells in tumours makes it even more difficult to understand the clinical success. However, there is experimental evidence that starved cells lose their repair competence as a result of the depletion of cellular energy charge. MRS studies have shown that low ATP levels are a characteristic feature of solid tumours in rodents and man. In this paper we incorporate the concept of repair incompetence in starving, chronically hypoxic cells. The increased sensitivity of such cells has been derived from an analysis of mammalian cell lines showing inducible repair. It is proportional to the SF2 and highest in resistant cells. The distinction between acutely hypoxic radioresistant cells and chronically hypoxic radiosensitive cells provides the key to the realistic modelling of successful radiotherapy. It also opens new conceptual approaches to radiotherapy. We conclude that it is essential to distinguish between these two kinds of hypoxic cells in predictive assays and models.

Superfractionation as a potential hypoxic cell radiosensitizer: prediction of an optimum dose per fraction

PURPOSE

A dose "window of opportunity" has been identified in an earlier modeling study (1) if the inducible repair variant of the LQ model is adopted instead of the pure LQ model, and if all survival curve parameters are equally modified by the presence or absence of oxygen. In this paper we have extended the calculations to consider survival curve parameters from 15 sets of data obtained for cells tested at low doses using clonogenic assays.

METHODS AND MATERIALS

A simple computer model has been used to simulate the response of each cell line to various doses per fraction in multifraction schedules, with oxic and hypoxic cells receiving the same fractional dose. We have then used pairs of simulated survival curves to estimate the effective hypoxic protection (OER') as a function of the dose per fraction.

RESULTS

The resistance of hypoxic cells is reduced by using smaller doses per fraction than 2 Gy in all these fractionated clinical simulations, whether using a simple LQ model, or the more complex LQ/IR model. If there is no inducible repair, the optimum dose is infinitely low. If there is inducible repair, there is an optimum dose per fraction at which hypoxic protection is minimized. This is usually around 0.5 Gy. It depends on the dose needed to induce repair being higher in hypoxia than in oxygen. The OER' may even go below unity, i.e. hypoxic cells may be more sensitive than oxic cells.

CONCLUSIONS

If oxic and hypoxic cells are repeatedly exposed to doses of the same magnitude, as occurs in clinical radiotherapy, the observed hypoxic protection varies with the fractional dose. The OER' is predicted to diminish at lower doses in all cell lines. The loss of hypoxic resistance with superfractionation is predicted to be proportional to the capacity of the cells to induce repair, i.e. their intrinsic radioresistance at a dose of 2 Gy.