Repopulation of moderately well-differentiated and keratinizing GL human squamous cell carcinomas growing in nude mice

The Role of Cancer Stem Cells in Radiation Resistance

Cancer stem cells (CSC) are a distinct subpopulation within a tumor. They are able to self-renew and differentiate and possess a high capability to repair DNA damage, exhibit low levels of reactive oxygen species (ROS), and proliferate slowly. These features render CSC resistant to various therapies, including radiation therapy (RT). Eradication of all CSC is a requirement for an effective antineoplastic treatment and is therefore of utmost importance for the patient. This makes CSC the prime targets for any therapeutic approach. Albeit clinical data is still scarce, experimental data and first clinical trials give hope that CSC-targeted treatment has the potential to improve antineoplastic therapies, especially for tumors that are known to be treatment resistant, such as glioblastoma. In this review, we will discuss CSC in the context of RT, describe known mechanisms of resistance, examine the possibilities of CSC as biomarkers, and discuss possible new treatment approaches.

High Expression of SOX2 and OCT4 Indicates Radiation Resistance and an Independent Negative Prognosis in Cervical Squamous Cell Carcinoma

Radiotherapy (RT) as a preoperative or postoperative adjuvant or primary treatment is the most common management modality for locally advanced cervical cancer. Radioresistance of tumor cells remains a major therapeutic problem. Consequently, we aimed to explore if the stem cell biomarkers SOX2 and OCT4 protein could be used to predict radioresistance in patients with locally advanced cervical squamous cell carcinoma (LACSCC). These 132 patients were divided into two groups (radiation-resistant and radiation-sensitive groups) according to progress-free survival (PFS). Using pretreatment paraffin-embedded tissues, we evaluated SOX2 and OCT4 expression using immunohistochemical staining. The percentage of overexpression of SOX2 and OCT4 in the radiation-resistant group was much higher than that in the radiation-sensitive group (p<0.001 and p <0.001, respectively). The patients with high expression of SOX2 and OCT4 showed a shorter PFS than those with low expression. Our study suggests that the expression of SOX2 and OCT4 in tumor cells indicates resistance to radiotherapy and that these two factors were important predictors of poor survival in patients with LACSCC (hazard ratio [95% CI], 2.294 [1.013, 5.195] and 2.300 [1.050, 5.037], respectively; p=0.046 and p=0.037, respectively).

Clinical perspectives of cancer stem cell research in radiation oncology

Radiotherapy has a proven potential to eradicate cancer stem cells which is reflected by its curative potential in many cancer types. Considerable progress has been made in identification and biological characterisation of cancer stem cells during the past years. Recent biological findings indicate significant inter- and intratumoural and functional heterogeneity of cancer stem cells and lead to more complex models which have potential implications for radiobiology and radiotherapy. Clinical evidence is emerging that biomarkers of cancer stem cells may be prognostic for the outcome of radiotherapy in some tumour entities. Perspectives of cancer stem cell based research for radiotherapy reviewed here include their radioresistance compared to the mass of non-cancer stem cells which form the bulk of all tumour cells, implications for image- and non-image based predictive bio-assays of the outcome of radiotherapy and a combination of novel systemic treatments with radiotherapy.

Cancer stem cells: targets and potential biomarkers for radiotherapy

Cancer stem cells (CSC) have the unique ability to cause tumor recurrences if they survive treatment. Radiotherapy has curative potential because it has been functionally shown to sufficiently inactivate CSCs. It is well known that CSCs mediate the radiation resistance of tumors by tumor-specific factors, such as the pretreatment number of CSCs and repopulation or reoxygenation during fractionated radiotherapy. CSCs appear to have a higher intrinsic radioresistance than non-CSCs, a factor that is especially important for the development of predictive biomarkers that, if this finding holds true, can only be successfully established if they are stem-cell specific. Recent clinical data imply that stem-cell-related surface markers may be directly used as predictors for the radiocurability of tumors with comparable risk factors, such as histology and size. Future studies need to address the question of which additional markers need to be considered if more heterogeneous patient collectives are investigated. With the goal of developing a direct targeting approach, investigators are currently evaluating several drugs that are intended to target CSCs by inhibiting stem-cell-related signal transduction pathways. We need to preclinically test such drugs as combined-modality therapies in combination with radiotherapy to evaluate their curative potential, and optimize them by increasing their specificity to CSCs over normal tissue stem cells to avoid increased radiation toxicity.

Final results of the randomized phase III CHARTWEL-trial (ARO 97-1) comparing hyperfractionated-accelerated versus conventionally fractionated radiotherapy in non-small cell lung cancer (NSCLC)

BACKGROUND

Continuous hyperfractionated accelerated radiotherapy (CHART) counteracts repopulation and may significantly improve outcome of patients with non-small-cell lung cancer (NSCLC). Nevertheless high local failure rates call for radiation dose escalation. We report here the final results of the multicentric CHARTWEL trial (CHART weekend less, ARO 97-1).

PATIENTS AND METHODS

Four hundred and six patients with NSCLC were stratified according to stage, histology, neoadjuvant chemotherapy and centre and were randomized to receive 3D-planned radiotherapy to 60Gy/40 fractions/2.5weeks (CHARTWEL) or 66Gy/33 fractions/6.5weeks (conventional fractionation, CF).

RESULTS

Overall survival (OS, primary endpoint) at 2, 3 and 5yr was not significantly different after CHARTWEL (31%, 22% and 11%) versus CF (32%, 18% and 7%; HR 0.92, 95% CI 0.75-1.13, p=0.43). Also local tumour control rates and distant metastases did not significantly differ. Acute dysphagia and radiological pneumonitis were more pronounced after CHARTWEL, without differences in clinical signs of pneumopathy. Exploratory analysis revealed a significant trend for improved LC after CHARTWEL versus CF with increasing UICC, T or N stage (p=0.006-0.025) and after neoadjuvant chemotherapy (HR 0.48, 0.26-0.89, p=0.019).

CONCLUSIONS

Overall, outcome after CHARTWEL or CF was not different. The lower total dose in the CHARTWEL arm was compensated by the shorter overall treatment time, confirming a time factor for NSCLC. The higher efficacy of CHARTWEL versus CF in advanced stages and after chemotherapy provides a basis for further trials on treatment intensification for locally advanced NSCLC.

Cancer stem cells at the crossroads of current cancer therapy failures--radiation oncology perspective

Despite continuous improvements in cancer management, locoregional recurrence or metastatic spread still occurs in a high proportion of patients after radiotherapy or combined treatments. One underlying reason might be a low efficacy of current treatments on eradication of cancer stem cells (CSCs). It has been recognised for a long time, that only the small subpopulation of CSCs can cause recurrences and that all CSCs need to be killed for permanent tumour cure. However, only recently novel technologies have allowed to enrich CSCs and to investigate their biology. An emerging experimental and clinical database provides first hints that cell populations accumulated by putative stem cell markers or tumours that highly express such markers may be more radioresistant than their marker-negative counterparts. Other data support a higher tolerance of CSCs to hypoxia and preferential location in specific microenvironmental niches. However, conflicting data, methodological problems of the assays and a generally small database on only few tumour types necessitate further large and well-designed prospective experimental and clinical investigations that specifically address this question to corroborate this hypothesis. If such investigations confirm biological differences between CSCs and non-CSCs, this would imply that novel treatment strategies need to be tested specifically for their effect on CSCs. Another implication is that also biomarkers for prediction of local tumour control after radiotherapy or combined treatments need to reflect the behaviour of CSCs and not of the bulk of all cancer cells. This review discusses the importance of CSCs for treatment failure and challenges occurring from the CSC concept for cancer diagnosis, treatment and prediction of outcome. It is concluded that CSC-based endpoints and biomarkers are eventually expected to considerably improve tumour cure rates in the clinics through individualised tailoring of treatment.

Differentiation status of human squamous cell carcinoma xenografts does not appear to correlate with the repopulation capacity of clonogenic tumour cells during fractionated irradiation

PURPOSE

To investigate the magnitude and kinetics of repopulation in a moderately well differentiated UT-SCC-14 human squamous cell carcinoma [hSCC] in nude mice. This question is of interest because clinical data indicate a higher repopulation capacity in those SCC that have preserved characteristics of differentiation, which appears to be in contrast to results on FaDu and GL hSCC previously reported from this laboratory.

METHODS AND MATERIALS

UT-SCC-14 tumours were transplanted subcutaneously into the right hind leg of NMRI nu/nu mice. Fractionated radiation treatments were delivered, either under clamped hypoxia at 5.4 Gy/fraction or under ambient conditions (consistent with an OER of 2.7). Tumours were irradiated every day, every 2nd day, or every 3rd day with 6, 12 or 18 fractions. 1, 2 or 3 days after the last fraction, graded top-up-doses under clamped conditions were given for the purpose of estimating the 50% tumour control dose (TCD50). A total of 22 TCD50 assays were performed and analysed using maximum likelihood techniques.

RESULTS

The data demonstrate a slow but significant repopulation of clonogenic cells during fractionated irradiation of UT-SCC-14 hSCC. The results under hypoxic conditions are consistent with a constant repopulation rate, with a clonogenic doubling time (Tclon) of 15.6 days (95% CI: 9.7, 21.4). This contrasts with ambient conditions where Tclon was 68.5 days (95% CI: 124, 161). Both Tclon values are longer than the 6-day volume doubling time of untreated tumours.

CONCLUSIONS

Less pronounced repopulation for irradiation under ambient compared to clamped hypoxic conditions might be explained by preferential survival of hypoxic and therefore non-proliferating clonogenic cells. Taken together with previous studies on poorly differentiated FaDu and moderately well differentiated GL hSCC, the results are consistent with considerable variability in the magnitude and kinetics of repopulation in different experimental squamous cell carcinomas, and with a relationship between reoxygenation and repopulation during fractionated irradiation. The differentiation status of hSCC growing in nude mice does not to appear to correlate with the proliferative capacity of clonogenic tumour cells during treatment. The results do not support the hypothesis gained from clinical data of higher repopulation in well-differentiated tumours.

Repair of skin damage during fractionated irradiation with gamma rays and low-LET carbon ions

In clinical use of carbon-ion beams, a deep-seated tumor is irradiated with a Spread-Out Bragg peak (SOBP) with a high-LET feature, whereas surface skin is irradiated with an entrance plateau, the LET of which is lower than that of the peak. The repair kinetics of murine skin damage caused by an entrance plateau of carbon ions was compared with that caused by photons using a scheme of daily fractionated doses followed by a top-up dose. Right hind legs received local irradiations with either 20 keV/microm carbon ions or gamma rays. The skin reaction of the irradiated legs was scored every other day up to Day 35 using a scoring scale that consisted of 10 steps, ranging from 0.5 to 5.0. An isoeffect dose to produce a skin reaction score of 3.0 was used to obtain a total dose and a top-up dose for each fractionation. Dependence on a preceding dose and on the time interval of a top-up dose was examined using gamma rays. For fractionated gamma rays, the total dose linearly increased while the top-up dose linearly decreased with an increase in the number of fractions. The magnitude of damage repair depended on the size of dose per fraction, and was larger for 5.2 Gy than 12.5 Gy. The total dose of carbon ions with 5.2 Gy per fraction did not change till 2 fractions, but abruptly increased at the 3rd fraction. Factors such as rapid repopulation, induced repair and cell cycle synchronization are possible explanations for the abrupt increase. As an abrupt increase/decrease of normal tissue damage could be caused by changing the number of fractions in carbon-ion radiotherapy, we conclude that, unlike photon therapy, skin damage should be carefully studied when the number of fractions is changed in new clinical trials.

Kinetics of EGFR expression during fractionated irradiation varies between different human squamous cell carcinoma lines in nude mice

BACKGROUND AND PURPOSE

Preclinical and clinical data indicate that high pretherapeutic EGFR expression is associated with poor local tumour control, possibly caused by a high repopulation rate of clonogenic cells during radiotherapy in these tumours. Previous data reported from our laboratory showed a correlation between EGFR expression and acceleration of repopulation in poorly differentiated FaDu human squamous cell carcinoma (SCC) during fractionated irradiation. To test whether this is a general phenomenon, two further SCC were investigated in the present study.

PATIENTS AND METHODS

GL and UT-SCC-14, two moderately well differentiated and keratinising hSCC, were grown as xenografts in nude mice. Functional data on the repopulation kinetics during fractionated irradiation for these tumour models have been previously determined. The expression of EGFR during fractionation was analysed by immunohistochemistry. Endpoints were the membrane-staining score and the proportion of EGFR-positive cells (EGFR labelling index).

RESULTS

Different kinetics of EGFR expression during fractionated RT were found. In UT-SCC-14, EGFR staining score and labelling index increased significantly during radiotherapy. In GL SCC, the EGFR expression was unchanged. Both tumours are characterized by a small but significant repopulation rate during radiotherapy.

CONCLUSIONS

The expression of EGFR may change significantly during fractionated irradiation. No clear correlation between EGFR expression and the repopulation kinetics of clonogenic tumour cells during fractionated irradiation was found. The changes in EGFR expression during irradiation warrant further investigation on their prognostic implications and on their importance for therapeutic interventions.

Repopulation of cancer cells during therapy: an important cause of treatment failure

Radiotherapy and chemotherapy are given in multiple doses, which are spaced out to allow the recovery of normal tissues between treatments. However, surviving cancer cells also proliferate during the intervals between treatments and this process of repopulation is an important cause of treatment failure. Strategies developed to overcome repopulation have improved clinical outcomes, and now new strategies to inhibit repopulation are emerging in parallel with advances in the understanding of underlying biological mechanisms.