Annegret Dörfler

Radiobiological hypoxia, histological parameters of tumour microenvironment and local tumour control after fractionated irradiation.

OBJECTIVE To investigate the relationships between radiobiological hypoxic fraction (rHF), pimonidazole hypoxic fraction (pHF) as well as other histological parameters of the tumour microenvironment, and local tumour control after fractionated irradiation in human squamous cell carcinomas (hSCCs). MATERIAL AND METHODS Ten different hSCC cell lines were transplanted into nude mice and rHF was calculated from local tumour control rates after single dose irradiation under normal or clamped blood flow conditions. In parallel, tumours were irradiated with 30 fractions within 6 weeks. Radiation response was quantified as dose required to cure 50% of tumours (TCD(50)). Unirradiated tumours were excised for histological evaluation including relative hypoxic area (pHF), relative vascular area (RVA), and fraction of perfused vessels (PF). RESULTS A weak but significant positive correlation between rHF (R(2)=0.6, p=0.014) and TCD(50) after fractionated irradiation was found. The pHF did not correlate with rHF but was significantly associated with the TCD(50) after single dose clamp (R(2)=0.8, p=0.003) and showed a trend for an association with TCD(50) after fractionated irradiation (R(2)=0.4, p=0.067). Relative vascular area and fraction of perfused vessels did not show an association with rHF or TCD(50) after fractionated irradiation. CONCLUSIONS Our data suggest that radiobiological hypoxia contributes to the response after fractionated irradiation but that also other radiobiological mechanisms are involved. In the present study, pimonidazole labelling does not reflect rHF and has a limited value to predict local tumour control after fractionated irradiation. The association between pHF and TCD(50) after single dose clamp warrants further investigation.

Combination of EGFR/HER2 Tyrosine Kinase Inhibition by BIBW 2992 and BIBW 2669 with Irradiation in FaDu Human Squamous Cell Carcinoma

Purpose:To investigate the effect of the dual EGFR/HER2 (ErbB2) tyrosine kinase inhibitors BIBW 2992 and BIBW 2669 on proliferation and clonogenic cell survival of FaDu human squamous cell carcinoma in vitro, and on tumor growth after single-dose irradiation in nude mice.Material and Methods:Cell proliferation, cell-cycle distribution and clonogenic cell survival after irradiation were assayed with and without BIBW 2992 or BIBW 2669 (3, 30, and 300 nM) in vitro. Tumor volume and tumor growth delay (GDV2) were determined in tumors growing in NMRI (nu/nu) nude mice, treated with (a) BIBW 2992 (20 mg kg–1 body weight orally), BIBW 2669 (3–4 mg kg–1 body weight orally) or carrier until a final tumor diameter of 15 mm, or, (b) 3 days before a 20-Gy single-dose irradiation or, (c) after a 20-Gy single-dose irradiation until reaching the final tumor diameter.Results:BIBW 2992 and BIBW 2669 significantly increased the doubling time of FaDu cells in vitro. A marked dose-dependent antiproliferative effect with blockade of the cells in G0/G1-phase of the cell cycle was found. Incubation with BIBW 2669 or BIBW 2992 for 3 days marginally increased radiosensitivity of FaDu cells in vitro. For BIBW 2992, this effect was statistically significant (p = 0.006). Daily oral application of BIBW 2669 or BIBW 2992 in mice bearing unirradiated FaDu tumors showed a marked antiproliferative effect with a significant prolongation of tumor growth delay (p < 0.0001). After drug application for 3 days, followed by 20-Gy single-dose irradiation, a slight effect of both drugs on tumor growth delay was seen. For BIBW 2669, this effect was statistically significant (p = 0.007). However, this effect disappeared when tumor volumes were normalized to the time point of irradiation suggesting that both drugs showed no or only a slight radiosensitizing effect in vivo. Daily application of BIBW 2669 or BIBW 2992 after a single-dose irradiation showed a clear inhibition of tumor growth with a significantly longer tumor growth delay after drug treatment compared to control tumors (p < 0.002). Enhancement ratios were smaller for irradiated than for unirradiated tumors, suggesting an additive effect for combinations with radiotherapy. In all treatment arms, the effects of BIBW 2669 were not significantly different from BIBW 2992.Conclusion:BIBW 2669 and BIBW 2992 showed a clear antiproliferative effect in vitro, whereas radiosensitization was only marginal. The present data are the first to show an effect of combined irradiation and dual EGFR/ErbB2 inhibition on tumor growth delay in vivo. Further preclinical investigations using fractionated irradiation schedules and local tumor control as experimental endpoint are needed to evaluate a possible curative potential for the combination treatment.Ziel:In der vorliegenden Arbeit wurde die Wirkung der neuen dualen EGFR/HER2-Tyrosinkinaseinhibitoren BIBW 2992 und BIBW 2669 auf die Zellproliferation und das klonogene Zellüberleben in der humanen Plattenepithelkarzinomlinie FaDu in vitro sowie auf das Tumorwachstum und die Tumorwachstumsverzögerung nach Einzeldosisbestrahlung in vivo untersucht.Material und Methodik:Zellproliferation, Zellzyklusverteilung und klonogenes Zellüberleben nach Bestrahlung wurden mit und ohne BIBW 2992 oder BIBW 2669 (3, 30 und 300 nM) in vitro untersucht. In NMRI-(nu/nu-)Nacktmäusen wurden Tumorvolumen und Tumorwachstumsverzögerung (GDV2) nach a) alleiniger Applikation von BIBW 2992 (20 mg kg–1 KG oral), BIBW 2669 (3–4 mg kg–1 KG oral) oder Kontrollsubstanz bis zur Tumorendgröße von 15 mm, b) 3-tägiger Substanzapplikation und folgender 20-Gy-Einzeldosisbestrahlung und c) 20-Gy-Einzeldosisbestrahlung bis zur Tumorendgröße bestimmt.Ergebnisse:BIBW 2992 und BIBW 2669 führten zu einer signifikanten Verlängerung der Verdopplungszeit von FaDu-Zellen in vitro (Abbildung 1, Tabelle 1). Der ausgeprägte dosisabhängige antiproliferative Effekt ging mit einem G0/G1-Block einher (Abbildungen 1 und 2). Die Inkubation mit BIBW 2669 und BIBW 2992 führte zu einer geringen Erhöhung der Strahlenempfindlichkeit von FaDu-Zellen in vitro (Abbildung 3). Dieser Effekt war für BIBW 2992 statistisch signifikant (p = 0,006). Die tägliche orale Applikation von BIBW 2669 oder BIBW 2992 führte bei unbestrahlten Tumoren in vivo zu einem deutlichen proliferationshemmenden Effekt (Abbildung 4) mit signifikanter Verlängerung der Tumorwachstumsverzögerung (p < 0,0001; Abbildung 6, Tabelle 2). Eine 3-tägige Substanzapplikation und anschließende 20-Gy-Einzeldosisbestrahlung zeigten einen geringen Effekt auf die Tumorwachstumsverzögerung. Für BIBW 2669 war dieser Effekt signifikant (p = 0,007). Der Effekt verschwand, wenn die Tumorvolumina zum Zeitpunkt der Bestrahlung normiert wurden (Abbildung 5). Für beide Substanzen konnte somit kein oder nur ein geringer strahlensensitivierender Effekt in vivo nachgewiesen werden. Eine Einzeldosisbestrahlung mit 20 Gy und anschließender Substanzapplikation bis zur Tumorendgröße führte zusätzlich zum Effekt der Bestrahlung zu einem deutlichen proliferationshemmenden Effekt mit signifikanter Verlängerung der Tumorwachstumsverzögerung im Vergleich zu Kontrolltumoren (p < 0,002). Die Verstärkungsratios waren bei bestrahlten Tumoren geringer als bei unbestrahlten Tumoren, was auf einen additiven Effekt der Substanzen schließen lässt. In allen Behandlungsarmen konnte zwischen BIBW 2669 und BIBW 2992 kein signifikanter Unterschied gefunden werden.Schlussfolgerung:BIBW 2669 und BIBW 2992 zeigten einen deutlichen antiproliferativen Effekt in vitro bei nur geringer Strahlensensitivierung. Die vorliegenden Daten haben erstmals die Wirkung einer kombinierten Bestrahlung und dualen EGFR/ ErbB2-Inhibition auf die Verzögerung des Tumorwachstums in vivo gezeigt. Weitere präklinische Untersuchungen mit einem fraktionierten Bestrahlungsschema und lokaler Tumorkontrolle als Endpunkt sind nötig, um ein mögliches kuratives Potential von BIBW 2669 oder BIBW 2992 in Kombination mit Strahlentherapie zu untersuchen.

Prediction of clonogenic cell survival curves based on the number of residual DNA double strand breaks measured by gammaH2AX staining.

Purpose: To assess the potential of using the residual phosphorylation of histone H2AX (γH2AX) after irradiation as a marker of radiosensitivity in vitro. Material and methods: Confluent cell cultures of FaDu and SKX human squamous cell carcinoma lines were irradiated with graded single doses. Twenty-four hours after irradiation cells were seeded for standard colony forming assay (CFA). In parallel, staining for γH2AX was performed to visualise the residual foci. Results: In the CFA, FaDu showed a higher radioresistance than SKX. After analysis of the residual foci data, we constructed ‘predicted’ survival curves using two different methods. First, the proportion of nuclei with <3 foci was found to correlate closely with the observed surviving fraction (SF) in FaDu, with a slight overestimation of the true SF in SKX. Second, there was a strong linear correlation of the mean number of residual foci and observed −lnSF. Based on regression analysis, we calculated the SF for both cell lines based on the mean number of residual γH2AX foci. This second approach again led to a good correlation of predicted and observed SF values in FaDu and a (slight) overestimation in SKX. Conclusion: In the two cell lines investigated the mean number of residual foci of γH2AX can be used to predict differences in the radiation dose response relationship in vitro.

Selective inhibition of the epidermal growth factor receptor tyrosine kinase by BIBX1382BS and the improvement of growth delay, but not local control, after fractionated irradiation in human FaDu squamous cell carcinoma in the nude mouse.

Purpose: To investigate the effect of BIBX1382BS, an inhibitor of the epidermal growth factor receptor tyrosine kinase, on proliferation and clonogenic cell survival of FaDu human squamous cell carcinoma in vitro, and on tumour growth and local tumour control after fractionated irradiation over 6 weeks in nude mice. FaDu human squamous cell carcinoma is epidermal growth factor receptor positive and significant repopulation during fractionated irradiation was demonstrated in previous experiments. Materials and methods: Receptor status, receptor phosphorylation, cell cycle distribution, cell proliferation and clonogenic cell survival after irradiation were assayed with and without BIBX1382BS (5 µM) in vitro. Tumour volume doubling time, BrdUrd and Ki67 labelling indices and apoptosis were investigated in unirradiated tumours growing in NMRI nude mice treated daily with BIBX1382BS (50 mg kg−1 body weight orally) or carrier. Tumour growth delay and dose–response curves for local tumour control were determined after irradiation with 30 fractions within 6 weeks. Results: BIBX1382BS blocked radiation‐induced phosphorylation of the epidermal growth factor receptor and reduced the doubling time of FaDu cells growing in vitro by a factor of 4.9 (p=0.008). Radiosensitivity in vitro remained unchanged after incubation with BIBX1382BS for 3 days and decreased moderately after 6 days (p=0.001). BIBX1382BS significantly reduced the volume doubling time of established FaDu tumours in nude mice by factors of 2.6 when given over 15 days (p<0.001) and 3.7 when applied over 6 weeks (p<0.001). When given simultaneously to fractionated irradiation, growth delay was significantly prolonged by an average of 33 days (p=0.003). Local tumour control was not improved by BIBX1382BS. The radiation doses necessary to control 50% of the tumours locally were 63.6 Gy (95% confidence interval 55; 73) for irradiation alone and 67.8 Gy (60; 77) for the combined treatment (p=0.5). Conclusions: Despite clear antiproliferative activity in rapidly repopulating FaDu human squamous cell carcinoma and significantly increased tumour growth delay when combined with fractionated irradiation, local tumour control was not improved by BIBX1382BS. The results do not disprove that epidermal growth factor receptor inhibition might enhance the results of radiotherapy. However, the results imply that further preclinical investigations using relevant treatment schedules and appropriate endpoints are necessary to explore the mechanisms of action and efficacy of such combinations.

Enhanced Susceptibility of Irradiated Tumor Vessels to Vascular Endothelial Growth Factor Receptor Tyrosine Kinase Inhibition

Previous experiments with PTK787/ZK222584, a specific inhibitor of vascular endothelial growth factor receptor (VEGFR) tyrosine kinases, using irradiated human FaDu squamous cell carcinoma in nude mice, suggested that radiation-damaged tumor vessels are more sensitive to VEGFR inhibition. To test this hypothesis, the tumor transplantation site (i.e., the right hind leg of nude mice) was irradiated 10 days before transplantation of FaDu to induce radiation damage in the host tissue. FaDu tumors vascularized by radiation-damaged blood vessels appeared later, grew at a slower rate, and showed more necrosis and a smaller vessel area per central tumor section than controls. PTK787/ZK222584 at a daily dose of 50 mg/kg body weight had no impact on growth of control tumors. In contrast, tumors vascularized by radiation-damaged vessels responded to PTK787/ZK222584 with longer latency and slower growth rate than controls, and a trend toward further increase in necrosis, indicating that irradiated tumor vessels are more susceptible to VEGFR inhibition than unirradiated vessels. Although not proving causality, expression analysis of VEGF and VEGFR2 shows that enhanced sensitivity of irradiated vessels to a specific inhibitor of VEGFR tyrosine kinases correlates with increased expression of the molecular target.

Epidermal growth factor receptor inhibitors for radiotherapy: biological rationale and preclinical results

Blocking the epidermal growth factor receptor (EGFR) represents a role model for a successful biological targeting approach to improving outcomes after radiotherapy. This review summarizes data from several local tumour control experiments in which EGFR inhibitors were combined with radiation in FaDu human squamous cell carcinomas xenografted into nude mice. BIBX1382BS is an oral bioavailable inhibitor of the intracellular tyrosine kinase domain of EGFR. It was administered in different experimental settings: concurrent with fractionated radiotherapy, following completion of irradiation, and in the period between surgery and adjuvant irradiation. Despite beneficial effects on tumour growth, in none of these experimental settings did BIBX1382BS improve local tumour control. In contrast, cetuximab (Erbitux), an IgG1 monoclonal antibody against the extracellular ligand‐binding domain of EGFR, improved local tumour control when given concurrently with radiation. Results from a series of local tumour control experiments designed to elucidate the underlying mechanisms of cetuximab suggest that multiple radiobiological mechanisms might contribute to the observed effects: decreased number of clonogenic tumour cells, increased cellular radiation sensitivity, decreased repopulation and improved reoxygenation of clonogenic tumour cells during the combined treatment. In summary, the data suggest that different classes of EGFR inhibitors may have a different potential to improve local tumour control after fractionated irradiation.

Triple angiokinase inhibition, tumour hypoxia and radiation response of FaDu human squamous cell carcinomas

BACKGROUND AND PURPOSE To test the effect of BIBF 1120, a novel small molecule inhibitor of multiple angiogenic receptor tyrosine kinases, on the hypoxia and radiation response of tumours. MATERIALS AND METHODS Poorly differentiated human squamous cell carcinoma FaDu growing in nude mice was treated with BIBF 1120 and investigated by functional histology. To test the effect of BIBF 1120 on the radiobiological hypoxic fraction (rHF), the number and intrinsic radiation sensitivity of tumour stem cells and the outcome after fractionated irradiation, a series of local tumour control assays were performed. RESULTS BIBF 1120 significantly reduced the vessel area, vessel area with a perfusion signal and tumour growth rate but did not affect tumour hypoxia or the number and intrinsic radiation sensitivity of tumour stem cells. Concurrent BIBF 1120 had no effect on local tumour control after fractionated irradiation. CONCLUSION Triple angiokinase inhibition resulted in a clear-cut decrease of angiogenesis, vessel area with a perfusion signal and tumour growth but did not change tumour hypoxia or radiation response of tumour stem cells. Further experiments into mechanisms of interaction between anti-angiogenic strategies and irradiation appear to be necessary to better define and exploit the potential of this strategy to improve local tumour control after fractionated radiotherapy.

Effects of Lovastatin Alone or Combined with Irradiation on Tumor Cells in Vitro and in Vivo

Purpose:To evaluate the effect of lovastatin alone or combined with radiation on U87MG and FaDu cells in vitro and U87MG tumors in vivo.Material and Methods:Cell number, p21WAF1 expression, apoptosis, reproductive cell death, and cell-cycle distribution were investigated after incubation of U87MG and FaDu cells in vitro. The effect of lovastatin (50 mg/kg/day) on tumor growth and on tumor growth delay after single-dose irradiation with 20 Gy was investigated using U87MG tumors in nude mice.Results:Lovastatin dose dependently decreased cell number and proliferation of U87MG and FaDu cells. The proportion of cells in G0/G1 phase, apoptosis and p21 protein expression increased after lovastatin alone or combined with 4-Gy irradiation in both cell lines. Effects of lovastatin on cell cycle and cell number were more pronounced in U87MG compared to FaDu. No radiosensitization of clonogenic cells by lovastatin could be demonstrated in both cells lines, but the colony-forming ability after lovastatin alone was decreased in FaDu cells. In vivo, lovastatin decreased tumor volume over time but did not increase growth delay after irradiation of U87MG tumors with 20 Gy.Conclusion:The data support effects of lovastatin on proliferation, apoptosis and colony-forming ability in vitro and tumor volume in vivo. At the drug concentration achievable, lovastatin did not improve the effects of radiation on U87MG tumors in vivo.Ziel:Der Effekt von Lovastatin allein und in Kombination mit Bestrahlung auf U87MG- und FaDu-Zellen in vitro und U87MG-Tumoren in vivo wurde untersucht.Material und Methodik:Zellzahl, p21WAF1-Expression, Apoptose, reproduktiver Zelltod und Zellzyklusverteilung wurden nach Inkubation von U87MG- and FaDu-Zellen in vitro evaluiert. Der Effekt von Lovastatin (50 mg/kg/Tag) auf Tumorwachstum und Tumorwachstumsverzögerung nach Einzeitbestrahlung mit 20 Gy wurden an U87MG-Tumoren in Nacktmäusen untersucht.Ergebnisse:Lovastatin reduzierte dosisabhängig die Zahl und Proliferation von U87MG- und FaDu-Zellen in vitro. Der Anteil von Zellen in G0/G1, der Anteil apoptischer Zellen und die p21WAF1-Expression stiegen in beiden Zelllinien nach Lovastatin allein oder in Kombination mit 4-Gy-Bestrahlung. Die Effekte von Lovastatin auf Zellzahl und Zellzyklus waren bei U87MG ausgeprägter als bei FaDu. Ein strahlensensibilisierender Effekt von Lovastatin bezüglich des klonogenen Zelltods wurde bei keiner der Zelllinien gefunden, Lovastatin allein reduzierte jedoch die Koloniebildungsfähigkeit von FaDu-Zellen. In vivo reduzierte Lovastatin das Volumen von U87MG-Tumoren, führte aber zu keiner Verlängerung der Tumorwachstumsverzögerung nach Einzeitbestrahlung mit 20 Gy.Schlussfolgerung:Lovastatin zeigt Effekte auf die Proliferation, Apoptose und Koloniebildungsfähigkeit in vitro und das Tumorvolumen in vivo. In der erreichten Dosierung führt Lovastatin nicht zu einer Verstärkung des Effekts einer Bestrahlung von U87MG-Tumoren in vivo.

Combined treatment of the immunoconjugate bivatuzumab mertansine and fractionated irradiation improves local tumour control in vivo

BACKGROUND AND PURPOSE To test whether BIWI 1 (bivatuzumab mertansine), an immunoconjugate of the humanized anti-CD44v6 monoclonal antibody BIWA 4 and the maytansinoid DM1, given simultaneously to fractionated irradiation improves local tumour control in vivo compared with irradiation alone. MATERIAL AND METHODS For growth delay, FaDu tumours were treated with 5 intravenous injections (daily) of phosphate buffered saline (PBS, control), BIWA 4 (monoclonal antibody against CD44v6) or BIWI 1 (bivatuzumab mertansine) at two different dose levels (50 μg/kg DM1 and 100 μg/kg DM1). For local tumour control, FaDu tumours received fractionated irradiation (5f/5d) with simultaneous PBS, BIWA 4 or BIWI 1 (two dose levels). RESULTS BIWI 1 significantly improved local tumour control after irradiation with 5 fractions already in the lower concentration. The dose modifying factor of 1.9 is substantial compared to the majority of other modifiers of radiation response. CONCLUSION Because of the magnitude of the curative effect, this approach is highly promising and should be further evaluated using similar combinations with improved tumour-specificity.

Effect of the Hypoxic Cell Sensitizer Isometronidazole on Local Control of Two Human Squamous Cell Carcinomas after Fractionated Irradiation

Background and Purpose:Hypoxia of clonogenic tumor cells is a major reason for radioresistance and hence local failure in radiotherapy. The objective of the present study was to test the efficacy of the hypoxic cell sensitizer isometronidazole (ISO) during fractionated irradiation in two different human squamous cell carcinomas. Material and Methods:Local control was evaluated for FaDu (radiobiological hypoxic fraction [rHF] 7%) and GL tumors (rHF 0.1%) after single-dose (SD) irradiation under ambient conditions and after 30 fractions within 6 weeks (30 f/6 w). ISO was applied 60 min before SD irradiation at a concentration of 100 mg/kg body weight (b.w.) or 750 mg/kg b.w. in both tumors. During fractionated irradiation, ISO was applied daily 60 min before each fraction (100 mg/kg b.w., in FaDu also 750 mg/kg b.w.).Results:100 mg/kg b. w. ISO did not improve local control after SD irradiation or 30 f/6 w in both tumor models. Application of 750 mg/kg b. w. ISO significantly decreased the SD-TCD50 in FaDu tumors (dose-modifying factor [DMF] = 1.2; p = 0.01) but not in the better oxygenated GL tumor. ISO at 750 mg/kg b.w. also significantly improved local control of FaDu tumors after 30 fractions in 6 weeks (DMF = 1.2; p = 0.01), indicating that hypoxic clonogenic cells in FaDu tumors are not only present before start of irradiation but also limit the efficacy of treatment during a fractionated course of radiotherapy.Conclusion:ISO at a concentration of 750 mg/kg b.w. shows an efficacy as a hypoxic cell sensitizer in severely hypoxic FaDu tumors but not in less hypoxic GL tumors. This supports the principle of hypoxic cell sensitization and improvement of local control of hypoxic tumors by nitroimidazole derivatives. However, doses of 750 mg/kg b. w. before each fraction may be difficult to achieve in the clinical situation. This, in light of the fact that other well-tolerable hypoxic cell sensitizers such as nimorazole with clinically proven efficacy at daily oral doses of < 3 g are available, limits the potential usefulness of ISO for radiation oncology.Hintergrund und Ziel:Hypoxie klonogener Zellen ist eine wichtige Ursache für die Strahlenresistenz von Tumoren und damit für Lokalrezidive nach Strahlentherapie. Das Ziel der vorliegenden Studie war die Überprüfung der Effektivität des „hypoxic cell sensitizer“ Isometronidazol (ISO) während einer fraktionierten Strahlentherapie in zwei unterschiedlichen menschlichen Plattenepithelkarzinomen.Material und Methodik:Die lokale Kontrolle wurde für FaDu- (radiobiologische hypoxische Fraktion [rHF] 7%) und GL-Tumoren (rHF 0,1%) nach Einzeldosis-(SD-)Bestrahlung unter normalen Blutflussbedingungen und nach 30 Fraktionen in 6 Wochen (30 f/ 6 w) bestimmt. ISO wurde bei beiden Tumoren 60 min vor der SD-Bestrahlung in einer Konzentration von 100 mg/kg Körpergewicht (KG) oder 750 mg/kg KG appliziert. Während der fraktionierten Bestrahlung erfolgte die Applikation von ISO täglich 60 min vor jeder Fraktion (100 mg/kg KG, bei FaDu zusätzlich 750 mg/kg KG).Ergebnisse:In beiden Tumormodellen konnte weder nach SD-Bestrahlung noch nach fraktionierter Bestrahlung eine Verbesserung der lokalen Tumorkontrolle durch die Applikation von ISO 100 mg/kg KG gezeigt werden. 750 mg/kg KG ISO führten zu einer signifikanten Reduktion der SD-TCD50 bei FaDu-Tumoren (dosismodifizierender Faktor [DMF] = 1,2; p = 0,01), aber nicht bei den besser oxygenierten GL-Tumoren. ISO in einer Dosis von 750 mg/kg KG führte auch zu einer signifikanten Verbesserung der lokalen Kontrolle von FaDu-Tumoren nach 30 Fraktionen in 6 Wochen (DMF = 1,2; p = 0,01). Dies deutet darauf hin, dass FaDu-Tumoren nicht nur vor Beginn der Behandlung hypoxische klonogene Zellen enthalten, sondern dass diese auch während einer fraktionierten Bestrahlung die Effizienz der Therapie limitieren.Schlussfolgerung:ISO zeigt in einer Konzentration von 750 mg/kg KG eine Effektivität als „hypoxic cell sensitizer“ im ausgeprägt hypoxischen FaDu-Tumor, aber nicht im weniger hypoxischen GL-Tumor. Diese Daten unterstützen das Prinzip der Strahlensensibilisierung und Verbesserung der lokalen Kontrolle hypoxischer Tumoren durch Nitroimidazolderivate. Die zur Radiosensibilisierung notwendigen ISO-Dosen vor jeder Fraktion sind jedoch in der klinischen Situation schwierig zu erreichen. Die Tatsache, dass gut verträgliche „hypoxic cell sensitizer“ wie Nimorazol, deren Wirksamkeit bei täglichen Dosen von < 3 g per os bereits klinisch nachgewiesen wurde, zur Verfügung stehen, limitiert den potentiellen Nutzen von ISO für die Radioonkologie.