The detection and modification of the hypoxic fraction in quiescent cell populations in murine solid tumours

An attempt to improve the therapeutic effect of boron neutron capture therapy using commonly employed 10B-carriers based on analytical studies on the correlation among quiescent tumor cell characteristics, tumor heterogeneity and cancer stemness

Based on our previously published reports concerning the response of quiescent (Q) tumor cell populations to boron neutron capture therapy (BNCT), the heterogeneous microdistribution of 10B in tumors, which is influenced by the tumor microenvironment and the characteristics of the 10B delivery carriers, has been shown to limit the therapeutic effect of BNCT on local tumors. It was also clarified that the characteristics of 10B-carriers for BNCT and the type of combined treatment in BNCT can also affect the potential for distant lung metastases from treated local tumors. We reviewed the findings concerning the response of Q tumor cell populations to BNCT, mainly focusing on reports we have published so far, and we identified the mode of BNCT that currently offers the best therapeutic gain from the viewpoint of both controlling local tumor and suppressing the potential for distant lung metastasis. In addition, based on the finding that oxygenated Q tumor cells showed a large capacity to recover from DNA damage after cancer therapy, the interrelationship among the characteristics in Q tumor cell populations, tumor heterogeneity and cancer stemness was also discussed.

Hypoxia, Epithelial-Mesenchymal Transition, and TET-Mediated Epigenetic Changes

Tumor hypoxia is a pathophysiologic outcome of disrupted microcirculation with inadequate supply of oxygen, leading to enhanced proliferation, epithelial-mesenchymal transition (EMT), metastasis, and chemo-resistance. Epigenetic changes induced by hypoxia are well documented, and they lead to tumor progression. Recent advances show that DNA demethylation mediated by the Ten-eleven translocation (TET) proteins induces major epigenetic changes and controls key steps of cancer development. TET enzymes serve as 5mC (5-methylcytosine)-specific dioxygenases and cause DNA demethylation. Hypoxia activates the expression of TET1, which also serves as a co-activator of HIF-1α transcriptional regulation to modulate HIF-1α downstream target genes and promote epithelial-mesenchymal transition. As HIF is a negative prognostic factor for tumor progression, hypoxia-activated prodrugs (HAPs) may provide a favorable therapeutic approach to lessen hypoxia-induced malignancy.

The usefulness of mild temperature hyperthermia combined with a newly developed hypoxia-oriented10B conjugate compound, TX-2100, for boron neutron capture therapy

PURPOSE

To evaluate the usefulness of a new 10B-compound (TX-2100) as a 10B-carrier in boron neutron capture therapy (BNCT), compared with the simultaneous use of its component drugs, sodium borocapate-10B (BSH) and 3-amino-2-quinoxalinecarbonitrile 1,4-dioxide (TX-402). Further, the usefulness of mild temperature hyperthermia (MTH, 40 degrees Celsius, 30 min) combined with TX-2100 was also examined compared with MTH combined with the concurrent administration with its component drugs.

MATERIALS AND METHODS

TX-2100 is a hybrid compound that has both a hypoxic cytotoxin unit (TX-402) and a thermal neutron-sensitizing unit (BSH). TX-2100 or both TX-402 plus BSH in combination with MTH or not was administered to SCC VII tumour-bearing mice intra-peritoneally. Then, the 10B concentrations in the tumours and normal tissues were measured by gamma-ray spectrometry. Meanwhile, SCC VII tumour-bearing mice were continuously given 5-bromo-2'-deoxyuridine (BrdU) to label all proliferating (P) cells in the tumours, then treated with TX-2100, TX-402 plus BSH or BSH only, in the same manner as in the biodistribution experiments, either with or without MTH. Right after thermal neutron irradiation during which intra-tumour 10B concentrations remained at similar levels, the tumours were excised, minced and trypsinized. The tumour cell suspensions thus obtained were incubated with cytochalasin-B (a cytokinesis blocker) and the micronucleus (MN) frequency in cells without BrdU labelling (=quiescent (Q) cells) was determined using immunofluorescence staining for BrdU. Meanwhile, the MN frequency in the total (P + Q) tumour cell population was determined from the tumours that were not pre-treated with BrdU. The clonogenic cell survival was also determined in mice given no BrdU.

RESULTS

10B biodistribution analyses in tumours, brain, skin, muscles, blood and liver indicated that the administration of TX-2100 plus MTH is most favourable for concentrating a sufficient amount of 10B in tumours and maintaining a high enough 10B concentration during irradiation. In addition, MTH had a stronger sensitizing effect when combined with TX-2100 than with the concurrent administration of its components TX-402 and BSH on both the total and Q cell populations in solid tumours.

CONCLUSION

MTH was very effective in combination with the newly-developed TX-2100. The sensitizing effect in combination with MTH should be examined when new 10B-carriers are designed.

The usefulness of mild temperature hyperthermia combined with continuous tirapazamine administration under reduced dose-rate irradiation with gamma-rays

PURPOSE

We clarified the usefulness of mild temperature hyperthermia (MTH) in combination with the continuous administration of tirapazamine (TPZ) under reduced dose-rate irradiation (RDRI) using gamma-rays.

MATERIALS AND METHODS

SCC VII tumour-bearing mice received a continuous administration of 5-bromo-2'-deoxyuridine (BrdU) to label all proliferating (P) cells. Then, they received a 24 h continuous subcutaneous infusion of TPZ either with or without MTH under high dose-rate irradiation (HDRI) or RDRI using gamma-rays. After the irradiation, the tumour cells were isolated and incubated with a cytokinesis blocker, and the micronucleus (MN) frequency in non-proliferating tumour cells without BrdU labeling (= quiescent (Q) cells) was determined using immunofluorescence staining for BrdU. The MN frequency in the total tumour cell populations was determined using tumours that were not pretreated with BrdU.

RESULTS

The sensitivity of both the total and Q cell populations, especially the latter, was significantly reduced with RDRI compared with HDRI. TPZ increased the sensitivity of both populations, with a slightly more remarkable increase in Q cells. Further, MTH combined with TPZ raised the sensitivity of both the total and Q cell populations, especially the latter, under RDRI more markedly than under HDRI.

CONCLUSION

From the viewpoint of solid tumour control as a whole, including intratumour Q-cell control, the use of TPZ, especially in combination with MTH, is useful for suppressing the reduction in the sensitivity of tumour cells caused by the decrease in irradiation dose rate in vivo.

The usefulness of a continuous administration of tirapazamine combined with reduced dose-rate irradiation using {gamma}-rays or reactor thermal neutrons

We clarified the usefulness of the continuous administration of tirapazamine (TPZ) in combination with reduced dose-rate irradiation (RDRI) using gamma-rays or reactor thermal neutrons. Squamous cell carcinoma (SCC) VII tumour-bearing mice received a continuous administration of 5-bromo-2'-deoxyuridine (BrdU) to label all proliferating (P) cells. Then, they received a single intraperitoneal injection or 24 h continuous subcutaneous infusion of TPZ in combination with conventional dose-rate irradiation (CDRI) or RDRI using gamma-rays or thermal neutrons. After irradiation, the tumour cells were isolated and incubated with a cytokinesis blocker, and the micronucleus (MN) frequency in cells without BrdU labelling ( = quiescent (Q) cells) was determined using immunofluorescence staining for BrdU. The MN frequency in the total tumour cells was determined using tumours that were not pre-treated with BrdU. The sensitivity of both total and Q cells, especially of Q cells, was significantly reduced with RDRI compared with CDRI. Combination of TPZ increased the sensitivity of both populations, with a slightly more remarkable increase in Q cells. Furthermore, the continuous administration of TPZ raised the sensitivity of both total and Q cell populations, especially the former, more markedly than the single administration, whether combined with CDRI or RDRI using gamma-rays or thermal neutrons. From the viewpoint of solid tumour control as a whole, including intratumour Q-cell control, the use of TPZ, especially when administered continuously, combined with RDRI, is useful for suppressing the reduction in the sensitivity of tumour cells caused by the decrease in irradiation dose rate in vivo.

Effects of mild temperature hyperthermia and p53 status on the size of hypoxic fractions in solid tumors, with reference to the effect in intratumor quiescent cell populations

PURPOSE

To determine the effects of mild temperature hyperthermia (MTH) and p53 status of tumor cells on the size of hypoxic fractions (HFs) in solid tumors, with reference to the effect on intratumor quiescent (Q) cell populations.

METHODS AND MATERIALS

Human head-and-neck squamous cell carcinoma cells transfected with mutant TP53 (SAS/mp53) or with neo vector as a control (SAS/neo) were inoculated subcutaneously into left hind legs of Balb/cA nude mice. Mice bearing the tumors received 5-bromo-2'-deoxyuridine (BrdU) continuously to label all proliferating (P) cells in the tumors. The mice then received nicotinamide injection or carbogen gas (95% O(2), 5% CO(2)) inhalation combined with or without MTH. Nicotinamide prevents intermittent blood flow that could induce perfusion-limited acute hypoxia. Chronically hypoxic cells in regions beyond the limitation of oxygen diffusion in tumors are oxygenated by increasing the oxygen transport capacity of circulating blood with carbogen gas inhalation. After each treatment, the mice received a series of test doses of gamma-rays while alive or after tumor clamping to obtain HFs in the tumors. Immediately after irradiation, the tumors were excised, minced, and trypsinized. The tumor cell suspensions thus obtained were incubated with a cytokinesis blocker (cytochalasin-B) to inhibit cytoplasmic division while allowing nuclear division. Tumor cells not labeled with BrdU were detected with immunofluorescence staining of BrdU for P cells, and the micronucleus frequency in cells without BrdU labeling [ = Q cells] was determined. The micronucleus frequency in total (P + Q) tumor cells was determined from the tumors that were not pretreated with BrdU.

RESULTS

SAS/mp53 tumors showed larger values for the size of not only the HF but also the diffusion-limited chronically HF than SAS/neo tumors. Q cell populations included a larger HF, particularly the chronically HF, than total cell populations in both tumors, especially in SAS/neo tumors. MTH could efficiently oxygenate the chronically HF, irrespective of p53 status.

CONCLUSION

MTH is a useful combined treatment with a radioenhancement effect on intratumor Q cells, irrespective of the p53 status of tumor cells. The p53 status has the potential to affect microenvironmental conditions within solid tumors.

Potential of alpha-amino alcohol p-boronophenylalaninol as a boron carrier in boron neutron capture therapy, regarding its enantiomers

PURPOSE

We evaluated the potential of a newly developed (10)B-containing alpha-amino alcohol of p-boronophenylalanine-(10)B (BPA), p-boronophenylalaninol (BPAol), as a boron carrier in boron neutron capture therapy.

METHODS

C57BL mice bearing EL4 tumors received 5-bromo-2'-deoxyuridine (BrdU) continuously via implanted mini-osmotic pumps to label all proliferating (P) cells. After oral administration of L-BPA or D-BPA, or intraperitoneal injection of L-BPAol or D-BPAol, the tumors were irradiated with reactor thermal neutron beams. Some of the tumors were heated at 40 degrees C for 30 min (mild temperature hyperthermia (MTH)) right before neutron exposure, and/or tirapazamine (TPZ) was intraperitoneally injected 30 min before irradiation. The tumors were then excised, minced, and trypsinized. The tumor cell suspensions thus obtained were incubated with cytochalasin-B (a cytokinesis blocker), and the micronucleus (MN) frequency in cells without BrdU labeling [ =quiescent (Q) cells] was determined using immunofluorescence staining for BrdU. Meanwhile, 6 h after irradiation, tumor cell suspensions obtained in the same manner were used for determining the apoptosis frequency in Q cells. The apoptosis and MN frequency in total (P+Q) tumor cells were determined from the tumors that were not pretreated with BrdU.

RESULTS

Without TPZ or MTH, L- and D-BPAol increased both frequencies markedly, especially for total cells. Although not significantly larger, L-BPA and D-BPAol increased both frequencies slightly more than D-BPA and L-BPAol, respectively. Combination with both MTH and TPZ markedly reduced the sensitivity difference between total and Q cells.

CONCLUSION

Both L- and D-BPAol have potential as a (10)B-carrier in neutron capture therapy, especially when combined with both MTH and TPZ.

Usefulness of combined treatment with mild temperature hyperthermia and/or tirapazamine in the treatment of solid tumors: its independence of p53 status

Human head and neck squamous cell carcinoma cells transfected with mutant TP53 (SAS/mp53) or with neo vector as a control (SAS/neo) were inoculated subcutaneously into both hind legs of Balb/cA nude mice. Mice bearing the tumors received 5-bromo-2'-deoxyuridine (BrdU) continuously to label all proliferating (P) cells in the tumors. The mice then received tirapazamine (TPZ) with or without mild temperature hyperthermia (40 degrees C, 60 min) (MTH), gamma-ray irradiation with or without MTH and/or TPZ, cisplatin (CDDP) with or without MTH and/or TPZ, or paclitaxel (TXL) with or without MTH and/or TPZ. After each treatment, the tumors were excised, minced and trypsinized. The tumor cell suspensions thus obtained were incubated with a cytokinesis blocker (cytochalasin-B), and the micronucleus (MN) frequency in cells without BrdU labeling (i.e., quiescent (Q) cells) was determined by using immunofluorescence staining for BrdU. Meanwhile, 6 h after gamma-ray irradiation or 24 h after other cytotoxic treatments, tumor cell suspensions obtained in the same manner were used for determining the frequency of apoptosis in Q cells. The MN frequency and apoptosis frequency in total (P+Q) tumor cells were determined from the tumors that were not pretreated with BrdU. On the whole, gamma-ray irradiation and CDDP injection induced a higher frequency of apoptosis and lower frequency of MN in SAS/neo cells than SAS/mp53 cells. There were no apparent differences in the induced frequency of apoptosis and MN between SAS/neo and SAS/mp53 cells after TPZ or TXL treatment. MTH sensitized cells to TPZ-inducing cytotoxicity more markedly in SAS/mp53 and Q cells than in SAS/neo cells and total cells, respectively. In gamma-ray irradiation and CDDP treatment, the enhancement in combination with MTH and/or TPZ was more remarkable in SAS/mp53 cells and Q cells than in SAS/neo and total tumor cells, respectively. Also in the case of TXL treatment, the combination with MTH and/or TPZ induced a slightly greater enhancement effect in SAS/mp53 cells and Q cells. In view of the difficulty in controlling mutated p53 status tumors and intratumor Q cells, combination treatment with MTH and/or TPZ as a cooperative modality in cancer therapy is considered to have potential for controlling solid tumors as a whole.

Significance of the response of quiescent cell populations within solid tumors in cancer therapy

In analyzing the response of quiescent (Q) cells in solid tumors, we have developed a combined method with a micronucleus (MN) assay and the identification of proliferating (P) cells by 5-bromo-2'-deoxyuridine (BrdU) and an anti-BrdU monoclonal antibody. Using this method, the responses of Q tumor cells as well as total tumor (P + Q) cells within murine solid tumors to various DNA-damaging treatments were evaluated. Based on this evaluation, combining with tirapazamine, a well-known bioreductive agent, and/or heat treatment at mild temperatures was thought to be a promising modality for cancer therapy in terms of conventional anticancer treatment-resistant Q cell control. Recently, our method for detecting the Q-cell response using P cell labeling with BrdU and the MN frequency assay was also shown to be applicable to an apoptosis detection assay. Meanwhile, our method for detecting the intratumor Q-cell response was also applicable toward high linear energy transfer radiation, including reactor neutrons. Thus, using our method, a new neutron capture compound that has the potential to be distributed in neutron capture therapy-resistant intratumor Q cell populations is now under development.

Evaluation of the potential of p-boronophenylalaninol as a boron carrier in boron neutron capture therapy, referring to the effect on intratumor quiescent cells

C57BL mice bearing EL4 tumors and C3H / He mice bearing SCC VII tumors received 5-bromo-2'-deoxyuridine (BrdU) continuously for 5 days via implanted mini-osmotic pumps to label all proliferating (P) cells. Three hours after oral administration of l-p-boronophenylalanine-(10)B (BPA), or 30 min after intraperitoneal injection of sodium borocaptate-(10)B (BSH) or l-p-boronophenylalaninol (BPA-ol), a newly developed (10)B-containing alpha-amino alcohol, the tumors were irradiated with thermal neutron beams. For the combination with mild temperature hyperthermia (MTH) and / or tirapazamine (TPZ), the tumors were heated at 40 degrees C for 30 min immediately before neutron exposure, and TPZ was intraperitoneally injected 30 min before irradiation. The tumors were then excised, minced and trypsinized. The tumor cell suspensions thus obtained were incubated with cytochalasin-B (a cytokinesis blocker), and the micronucleus (MN) frequency in cells without BrdU labeling ( = quiescent (Q) cells) was determined using immunofluorescence staining for BrdU. Meanwhile, 6 h after irradiation, tumor cell suspensions obtained in the same manner were used for determining the apoptosis frequency in Q cells. The MN and apoptosis frequency in total (P + Q) tumor cells were determined from tumors that were not pretreated with BrdU. Without TPZ or MTH, BPA-ol increased both frequencies most markedly, especially for total cells. However, as with BPA, the sensitivity difference between total and Q cells was much larger than with BSH. On combined treatment with both MTH and TPZ, this sensitivity difference was markedly reduced, similarly to when BPA was used. MTH increased the (10)B uptake of all (10)B-compounds into both tumor cells. BPA-ol has good potential as a (10)B-carrier in neutron capture therapy, especially when combined with both MTH and TPZ.

Radiosensitization effect by combination with paclitaxel in vivo, including the effect on intratumor quiescent cells

PURPOSE

To evaluate the radiosensitization effect on solid tumors upon combination treatment with paclitaxel (TXL), including the effect on intratumor quiescent (Q) cells.

METHODS AND MATERIALS

Mice bearing SCC VII or EL4 solid tumors received 5-bromo-2'-deoxyuridine (BrdU) continuously for 5 days to label all proliferating (P) cells. The mice then received gamma-irradiation with or without tirapazamine (TPZ) at various time points after TXL administration. Another group of mice received a series of test doses of gamma-rays while alive or after tumor clamping to obtain hypoxic fractions (HFs) in the tumors at various time points after TXL administration. Immediately after irradiation, the tumor cells were isolated and incubated with a cytokinesis blocker. The micronucleus (MN) frequency in cells without BrdU labeling (Q cells) was determined using immunofluorescence staining for BrdU. Meanwhile, 6 h after irradiation, the tumor cells were isolated from the solid tumors in another group of mice, and the apoptosis frequency in Q cells was also determined with immunofluorescence staining for BrdU. The MN and apoptosis frequency in total (P + Q) tumor cells were determined from the tumors that were not pretreated with BrdU. For the measurement of the HFs, the MN or apoptosis frequency of Q cells was then used to calculate the surviving fraction of Q cells from the regression line for the relationship between the MN or apoptosis frequency and the surviving fraction of total tumor cells.

RESULTS

In both SCC VII and EL4 tumors, maximum values of mitotic index (MI) and apoptosis frequency were observed 9 and 24 h after TXL administration, respectively. However, on the whole, the apoptosis frequency for SCC VII was very low. gamma-Irradiation 9 h after TXL administration induced significant radiosensitization effects on the total cells of both tumors. Irradiation at 60 h had a more significant effect on total cells of EL4 tumor, but no significant effect on total cells of SCC VII tumor. Combined treatment with TXL induced no radiosensitization effect on Q cells in either tumor. The effect on Q cells was observed only after TPZ was administered. The HF of total cells in EL4 tumors decreased significantly 60 h after TXL administration.

CONCLUSION

No radiosensitization effect upon combination treatment with TXL is induced in Q tumor cells. However, the effect on P cells is produced by irradiation at the time when the maximum values of MI are induced following TXL administration. In addition, for tumors that are susceptible to apoptosis after TXL administration alone, irradiation at the time of sufficient reoxygenation in tumors after TXL administration produces a greater radioenhancement effect on P cells.

Radiobiologic significance of apoptosis and micronucleation in quiescent cells within solid tumors following gamma-ray irradiation

PURPOSE

To determine the frequency of apoptosis in quiescent (Q) cells within solid tumors following gamma-ray irradiation, using four different tumor cell lines. In addition, to assess the significance of detecting apoptosis in these cell lines.

METHODS AND MATERIALS

C3H/He mice bearing SCC VII or FM3A tumors, Balb/c mice bearing EMT6/KU tumors, and C57BL mice bearing EL4 tumors received 5-bromo-2'-deoxyuridine (BrdU) continuously for 5 days via implanted mini-osmotic pumps to label all proliferating (P) cells. The mice then received gamma-ray irradiation at a dose of 4--25 Gy while alive or after tumor clamping. Immediately after irradiation, the tumors were excised, minced, and trypsinized. The tumor cell suspensions thus obtained were incubated with cytochalasin-B (a cytokinesis blocker), and the micronucleus (MN) frequency in cells without BrdU labeling (= Q cells) was determined using immunofluorescence staining for BrdU. Meanwhile, 6 hours after irradiation, tumor cell suspensions obtained in the same manner were fixed. The apoptosis frequency in Q cells was also determined with immunofluorescence staining for BrdU. The MN and apoptosis frequency in total (P + Q) tumor cells were determined from the tumors that were not pretreated with BrdU.

RESULTS

In total cells, SCC VII, FM3A, and EMT6/KU cells showed reasonable relationships between MN frequency and surviving fraction (SF). However, fewer micronuclei were induced in EL4 cells than the other cell lines. In contrast, a comparatively close relationship between apoptosis frequency and SF was found in total cells of EL4 cell line. Less apoptosis was observed in the other cell lines. Quiescent tumor cells exhibited significantly lower values of MN and apoptosis frequency probably due to their large hypoxic fraction, similar to total tumor cells on clamped irradiation.

CONCLUSION

gamma-ray irradiation induced MN formation in SCC VII, FM3A, and EMT6/KU tumor cells, and the apoptosis was marked in EL4 cells compared with the other cell lines. Our method for detecting the Q cell response to gamma-ray irradiation using P cell labeling with BrdU and the MN frequency assay was also applicable to apoptosis detection assay.

Change in oxygenation status in intratumour total and quiescent cells following gamma-ray irradiation, tirapazamine administration, cisplatin injection and bleomycin treatment

C3H/He mice bearing SCC VII tumours received 5-bromo-2'-deoxyuridine (BrdU) continuously for 5 days via implanted mini-osmotic pumps to label all proliferating (P) cells. The mice then received gamma-ray irradiation, or administration of tirapazamine (TPZ), cisplatin or bleomycin. At various time points after each treatment, tumour-bearing mice were irradiated with a series of test doses of gamma-rays, while alive or after being killed, to obtain hypoxic fractions (HFs) in the tumours. Immediately after gamma-ray test irradiation, the tumours were excised, minced and trypsinized. Tumour cell suspensions obtained were incubated with cytochalasin-B, a cytokinesis blocker, and the micronucleus (MN) frequency in cells without BrdU labelling (i.e. quiescent (Q) cells) was determined using immunofluorescence staining for BrdU. MN frequency in the total (P + Q) tumour cells was determined from the tumours that were not pre-treated with BrdU. MN frequency of BrdU-unlabelled cells was then used to calculate the surviving fraction of the unlabelled cells from the regression line for the relationship between the MN frequency and the surviving fraction of total tumour cells. TPZ and cisplatin reduced the HF after treatment, especially in Q cells, and this tendency was particularly marked with TPZ. In contrast, bleomycin increased the HF after treatment. Both reoxygenation following gamma-ray irradiation or bleomycin treatment and a subsequent return to pre-treatment levels of HF following TPZ or cisplatin treatment (rehypoxiation) occurred more rapidly in total (P + Q) cells than in Q cells. Based on our previous report that total (P + Q) and Q cells within this tumour have large acutely and chronically HFs, respectively, we conclude that acute hypoxic cells play a major role in reoxygenation and rehypoxiation in SCC VII tumours.

Usefulness of tirapazamine as a combined agent in chemoradiation and thermo-chemoradiation therapy at mild temperatures: reference to the effect on intratumor quiescent cells

C3H / He mice bearing SCC VII tumors received 5-bromo-2'-deoxyuridine (BrdU) continuously for 5 days via implanted mini-osmotic pumps to label all proliferating (P) cells. The mice then received one of six different DNA-damaging agents with or without mild temperature hyperthermia (40 degrees C, 30 min, MTH). These agents were adriamycin (ADM), mitomycin C (MMC), cyclophosphamide (CPA), bleomycin (BLM), cisplatin (CDDP), and tirapazamine (TPZ). After the drug treatment, the tumor-bearing mice were irradiated with a series of doses of gamma-rays. Immediately after irradiation, the tumors were excised, minced and trypsinized. The tumor cell suspensions thus obtained were incubated with cytochalasin-B (a cytokinesis blocker), and the micronucleus (MN) frequency in cells without BrdU labeling ( = quiescent (Q) cells) was determined using immunofluorescence staining for BrdU. The MN frequency in the total (P + Q) tumor cells was determined from the tumors that had not been pretreated with BrdU. MTH significantly increased the MN frequency of total cells in tumors irradiated with gamma-rays combined with CPA, BLM, CDDP or TPZ, and that of Q cells in tumors irradiated with gamma-rays combined with BLM or TPZ. The sensitivity difference in the MN frequency between total and Q tumor cells was significantly decreased by the combination with TPZ. TPZ combined with radiotherapy and TPZ combined with thermo-radiotherapy at mild temperatures appear to be promising modalities for sensitizing tumor cells in vivo, including Q tumor cells.

Reoxygenation in quiescent and total intratumor cells following thermal neutron irradiation with or without (10)B-compound-compared with that after gamma-ray irradiation

PURPOSE

Reoxygenation in quiescent (Q) and total tumor cells within solid tumors after thermal neutron irradiation with or without (10)B-compound was examined, comparing with that following gamma-ray irradiation.

METHODS AND MATERIALS

C3H/He mice bearing SCC VII tumors received 5-bromo-2'-deoxyuridine (BrdU) continuously for 5 days via implanted mini-osmotic pumps to label all proliferating (P) cells. Thirty minutes after intraperitoneal injection of sodium borocaptate-(10)B (BSH), or 3 h after oral administration of dl-p-boronophenylalanine-(10)B (BPA), the tumors were irradiated with thermal neutrons, or those without (10)B-compounds were irradiated with thermal neutrons alone or gamma-rays. At various time points after each treatment, a series of test doses of gamma-rays were given to tumor-bearing mice while alive or after being killed to obtain hypoxic fractions in the tumors. Immediately after irradiation, the tumors were excised, minced, and trypsinized. Following incubation of tumor cells with cytokinesis blocker, the micronucleus (MN) frequency in cells without BrdU labeling ( = Q cells) was determined using immunofluorescence staining for BrdU. The MN frequency in the total (P + Q) tumor cells was determined from the tumors that were not pretreated with BrdU. The MN frequency of BrdU-unlabeled cells was then used to calculate the surviving fraction of the unlabeled cells from the regression line for the relationship between the MN frequency and the surviving fraction of total tumor cells.

RESULTS

In both total and Q tumor cells, the hypoxic fractions immediately after each treatment went up suddenly. Reoxygenation after each treatment occurred more rapidly in total cells than in Q cells. In both cell populations, reoxygenation appeared to be rapidly induced in the following order: neutron irradiation without (10) gamma-ray irradiation.

CONCLUSION

Based on our previous report that total and Q cell fractions within these tumors have larger acutely and chronically hypoxic fractions, respectively, acute hypoxic cells appeared to play a larger role in reoxygenation. BSH was thought to have a potential to distribute more homogeneously in solid tumors than BPA, because BSH induced the nearer reoxygenation pattern to that following neutron irradiation alone than BPA.

Modification of tirapazamine-induced cytotoxicity in combination with mild hyperthermia and/or nicotinamide: reference to effect on quiescent tumour cells

C3H/He and Balb/c mice bearing SCC VII or EMT6/KU tumours received continuous administration of 5-bromo-2'-deoxyuridine (BrdU) for 5 days to label all proliferating (P) cells. The tumours were locally heated at 40 degrees C for 60 min and/or the tumour-bearing mice received intraperitoneal injection of nicotinamide, and then tirapazamine (TPZ) was injected intraperitoneally. Sixty minutes after TPZ injection, the tumours were excised, minced and trypsinized. The tumour cell suspensions were incubated with cytochalasin-B (a cytokinesis-blocker), and the micronucleus (MN) frequency in cells without BrdU labelling (quiescent (Q) cells) was determined using immunofluorescence staining for BrdU. The MN frequency in total (P+Q) tumour cells was determined from the tumours that were not pretreated with BrdU. The cytotoxicity of TPZ was evaluated in terms of the frequency of induced micronuclei in binuclear tumour cells (= MN frequency). In both tumour systems, the MN frequencies of Q cells were greater than those of total tumour cell populations. Mild heat treatment elevated the MN frequency in total and Q cells in both tumour systems, but the effect was more marked in Q cells. In total cells, mild heat treatment increased the MN frequency in EMT6/KU tumour cells more markedly than in SCC VII tumour cells. In contrast, in both tumour systems, nicotinamide decreased the MN frequency in both cell populations, with a greater influence on the total cells. The combination of TPZ and mild heat treatment may be useful for sensitizing tumour cells in vivo, including Q cells.

Response of quiescent and total tumor cells in solid tumors to neutrons with various cadmium ratios

PURPOSE

Response of quiescent (Q) and total tumor cells in solid tumors to neutron irradiation with three different cadmium (Cd) ratios was examined. The role of Q cells in tumor control was also discussed.

METHODS AND MATERIALS

C3H/He mice bearing SCC VII tumors received continuous administration of 5-bromo-2'-deoxyuridine (BrdU) for 5 days using implanted mini-osmotic pumps to label all proliferating (P) cells. Thirty minutes after intraperitoneal injection of sodium borocaptate-10B (BSH), or 3 h after oral administration of dl-p-boronophenylalanine-10B (BPA), the tumors were irradiated with neutrons, or those without 10B-compounds were irradiated with gamma rays. This neutron irradiation was performed using neutrons with three different cadmium (Cd) ratios. The tumors were then excised, minced, and trypsinized. The tumor cell suspensions were incubated with cytochalasin-B (a cytokinesis-blocker), and the micronucleus (MN) frequency in cells without BrdU labeling (Q cells) was determined using immunofluorescence staining for BrdU. The MN frequency in total (P + Q) tumor cells was determined from tumors that were not pretreated with BrdU. The sensitivity to neutrons was evaluated in terms of the frequency of induced micronuclei in binuclear tumor cells (MN frequency).

RESULTS

Without 10B-compounds, the MN frequency in Q cells was lower than that in the total cell population. The sensitivity difference between total and Q cells was reduced by neutron irradiation. Relative biological effectiveness (RBE) of neutrons compared with gamma rays was larger in Q cells than in total cells, and the RBE values for low-Cd-ratio neutrons tended to be larger than those for high-Cd-ratio neutrons. With 10B-compounds, MN frequency for each cell population was increased, especially for total cells. This increase in MN frequency was marked when high-Cd-ratio neutrons were used. BPA increased the MN frequency for total tumor cells more than BSH. Nevertheless, the sensitivity of Q cells treated with BPA was lower than that in BSH-treated Q cells. This tendency was clearly observed in high-Cd-ratio neutrons.

CONCLUSION

From the viewpoint of enhancing the Q-cell sensitivity, tumors should be irradiated with high-Cd-ratio neutrons after BSH administration. However, normal tissue reaction remains to be examined because of its low tumor-to-normal tissue and tumor-to-blood biodistribution ratios.

Augmentation in chemosensitivity of intratumor quiescent cells by combined treatment with nicotinamide and mild hyperthermia

C3H/He and Balb/c mice bearing SCC VII and EMT6/KU tumors, respectively, received continuous administration of 5-bromo-2'-deoxyuridine (BrdU) for 5 days using implanted mini-osmotic pumps to label all proliferating (P) cells. Nicotinamide was administered intraperitoneally before cisplatin injection and/or tumors were locally heated at 40 degrees C for 60 min immediately after cisplatin injection. The tumors were then excised, minced and trypsinized. The tumor cell suspensions were incubated with cytochalasin-B (a cytokinesis-blocker), and the micronucleus (MN) frequency in cells without BrdU labeling (quiescent (Q) cells) was determined using immunofluorescence staining for BrdU. The MN frequency in total (P+Q) tumor cells was determined from tumors that had not been pretreated with BrdU labeling. The sensitivity to cisplatin was evaluated in terms of the frequency of induced micronuclei in binuclear tumor cells (MN frequency). In both tumor systems, the MN frequency in Q cells was lower than that in the total cell population. Nicotinamide treatment elevated the MN frequency in total SCC VII cells. Mild heating raised the MN frequency more markedly in Q cells than in total cells. The combination of nicotinamide and mild heat treatment increased the MN frequency more markedly than either treatment alone. In total SCC VII cells, nicotinamide increased 195mPt-cisplatin uptake. Mild heating elevated 195mPt-cisplatin uptake in total EMT6/KU cells. Cisplatin-sensitivity of Q cells was lower than that of total cells in both tumor systems. Nicotinamide sensitized tumor cells including a large acutely hypoxic fraction, such as those of SCC VII tumors, through inhibition of the fluctuations in tumor blood flow. Tumor cells including a large chronically hypoxic fraction such as Q cells were thought to be sensitized by mild heating through an increase in tumor blood flow.

Alteration in the hypoxic fraction of quiescent cell populations by hyperthermia at mild temperatures

We investigated oxygenation of quiescent (Q) tumour cells in vivo by mild heat treatment. C3H/He mice bearing SCC VII tumours received BrdU continuously for 5 days via implanted mini-osmotic pumps, to label all proliferating (P) cells. The tumours were then irradiated after treatment, and were excised, minced and trypsinized. The tumour cell suspension thus obtained were incubated with cytochalasin-B (a cytokinesis blocker), and the micronucleus (MN) frequency in cells without BrdU labelling was determined using immunofluorescence staining for BrdU. This MN frequency was then used to calculate the surviving fraction of unlabelled cells from the regression line for the relationship between the MN frequency and the surviving fraction of total (P + Q) tumour cells. Thus, a cell survival curve could be determined for the cells not labelled with BrdU, which can be regarded as the Q cells in a tumour for all practical purposes. The MN frequency in total tumour cell population was determined from the irradiated tumours that were not pretreated with BrdU. Assays performed immediately after irradiation of both normally aerated and hypoxic tumours showed that Q cells contained higher hypoxic fractions than the total tumour cell population. Mild heat treatment (40.0 degrees C, 60 min) before irradiation decreased the hypoxic fraction, even when is was combined with nicotinamide administration. In contrast, mild heating did not decrease the hypoxic fraction when the mice were placed in a circulating carbogen (95% O2/5% CO2) chamber. Therefore, mild heat treatment was thought to preferentially oxygenate the chronically hypoxic fraction.

Modification of the response of a quiescent cell population within a murine solid tumour to boron neutron capture irradiation: studies with nicotinamide and hyperthermia

C3H/He mice bearing SCC VII tumours received 5-bromo-2'-deoxyuridine (BrdU) continuously for 5 days via implanted mini-osmotic pumps, to label all proliferating (P) cells. 20 min after intraperitoneal injection of sodium borocaptate-10B (BSH), or 3 h after oral administration of dl-p-boronophenylalanine-10B (BPA), the tumours were irradiated with thermal neutrons. To modify the uptake dose of 10B, nicotinamide (NA) was intraperitoneally injected 60 min before the administration of 10B-compounds and/or the tumours were heated to 41.5 degrees C for 20 min immediately before irradiation. After irradiation, the tumours were excised, minced and trypsinized. The tumour cell suspensions were then incubated with cytochalasin-B (a cytokinesis-blocker). The micronucleus (MN) frequency in cells not BrdU-labelled (quiescent (Q) cells) was determined using immunofluorescence staining for BrdU. With or without the administration of 10B-compounds, the sensitivity of Q cells was lower than that of total (P + Q) tumour cells. With thermal neutron irradiation in the presence of either BPA or BSH, the MN frequency in each cell population was increased. A greater increase in the MN frequency of total tumour cells was observed after thermal neutron irradiation in the presence of BPA than in the presence of BSH. The distribution of 10B from BPA into tumour cells was thought to be more dependent on the uptake ability of the tumour cells than that from BSH. Sufficient quantity of 10B from these two 10B-compounds to cause a highly lethal event inside the cancer cell with thermal neutron irradiation could not be delivered to Q cells. When NA and/or heat treatment were combined with 10B-compound administration, NA increased MN frequency in the BSH treated total cells, and heat treatment elevated MN frequency in Q cells. From the viewpoint of cell kill effect, the combined treatment with nicotinamide and heat treatment was more useful than treatment with either nicotinamide or heat treatment alone, not only in the total tumour cells but also in the Q cells.

Reduction of hypoxic cells in solid tumours induced by mild hyperthermia: special reference to differences in changes in the hypoxic fraction between total and quiescent cell populations

C3H/He mice bearing SCC VII tumours received 5-bromo-2'-deoxyuridine (BrdU) continuously for 5 days via implanted mini-osmotic pumps in order to label all proliferating (P) cells. The tumours were then heated at 40 degrees C for 60 min. At various time points after heating, tumour-bearing mice were irradiated while alive or after being killed. Immediately after irradiation, the tumours were excised, minced and trypsinized. The tumour cell suspensions obtained were incubated with cytochalasin-B (a cytokinesis blocker), and the micronucleus (MN) frequency in cells without BrdU labelling, which could be regarded as quiescent (Q) cells, was determined using immunofluorescence staining for BrdU. The MN frequency in the total (P+Q) tumour cell population was determined from the irradiated tumours that were not pretreated with BrdU. The MN frequency of BrdU unlabelled cells was then used to calculate the surviving fraction of the unlabelled cells from the regression line for the relationship between the MN frequency and the surviving fraction of total (P+Q) tumour cells. In general, Q cells contained a greater hypoxic fraction (HF) than the total tumour cell population. Mild heating decreased the HF of Q cells more markedly than in the total cell population, and the minimum values of HFs of both total and Q cell populations were obtained 6 h after heating. Two days after heating, the HF of total tumour cells returned to almost that of unheated tumours. In contrast, the HF of Q cells did not return to the HF level of unheated tumours until 1 week after heating. It was thought that irradiation within 12 h after mild heating might be a potentially promising therapeutic modality for controlling radioresistant Q tumour cells.

An attempt to enhance chemosensitivity of quiescent cell populations in solid tumors by combined treatment with nicotinamide and carbogen

cis-Diamminedichloroplatinum(II) (cisplatin) was intraperitoneally injected into mice bearing SCC VII or EMT6/KU tumors after ten administrations of 5-bromo-2'-deoxyuridine (BrdU) to label all the proliferating tumor cells. The tumors were excised 1 h after the cisplatin injection, minced, and trypsinized. The tumor cell suspensions were then incubated with cytochalasin-B (a cytokinesis blocker). The micronucleus frequency was determined, using immunofluorescence staining for BrdU. Cells that were not labeled with BrdU were regarded as quiescent. The micronucleus frequency in the total number of tumor cells was determined in tumors that had not been pretreated with BrdU. To modify the sensitivity to cisplatin, nicotinamide was intraperitoneally injected before the administration of cisplatin or mice were placed in a circulating carbogen (95% O2, 5% CO2) chamber for 30 min after cisplatin administration. In both tumor systems, the micronucleus frequency in quiescent cells was lower than that in the total cells. Nicotinamide pretreatment increased the micronucleus frequency in total and in quiescent cells in both tumor systems, and to a higher extent in total cells. The combination of nicotinamide and carbogen increased the micronucleus frequency more markedly than treatment with either nicotinamide or carbogen alone. In total cells of both tumors, the nicotinamide injection increased the uptake of [195mPt]cisplatin. The combined treatment raised the uptake more markedly than did treatment with either agent alone. In total cells of the SCC VII tumor, these increases in micronucleus frequency and the [195mPt]cisplatin uptake following nicotinamide or combined pretreatment were significant. In both tumors, carbogen breathing also elevated the micronucleus frequency to some degree in total and quiescent cells and the [195mPt]cisplatin uptake in total cells. The combined nicotinamide and carbogen treatment was considered to be useful for sensitizing tumor cells to chemotherapy with cisplatin in vivo.

Radiosensitisation in normal tissues with oxygen, carbogen or nicotinamide: therapeutic gain comparisons for fractionated x-ray schedules

METHODS

Radiosensitisation with oxygen, carbogen or nicotinamide alone and oxygen or carbogen combined with nicotinamide was compared in early and late responding normal tissues in rodents. X-ray treatments were delivered as single doses or fractionated schedules of 2 fractions in 1 day, 2, 12 and 36 fractions in an overall time of 12 days and 10 fractions in 5 or 12 days. Acute skin reactions, survival of intestinal crypts, breathing rate, reduction in the packed red-cell volume and clearance of 51Cr-EDTA were used as assays of epidermal, gut, lung and renal damage.

RESULTS

Relative to air-breathing mice, carbogen or oxygen produced a small, and not always significant, increase in sensitivity (enhancement ratios < or = 1.15) in gut, lung and kidneys; however, in skin a dose enhancement of 1.2-1.3 was observed. The effect of nicotinamide in air, carbogen or oxygen was studied only in lung and gut. The drug produced variable but generally significant increases in radiosensitisation ( < or = 1.26) in all three gases. Relative to treatments in air, enhancement ratios for nicotinamide alone were usually slightly higher than those observed when either carbogen or oxygen were administered without the drug. With all three modifiers (i.e. oxygen, carbogen, nicotinamide alone or for the drug-gas combinations) there was no significant change in the enhancement ratios observed as the number of radiation dose fractions was varied.

CONCLUSIONS

Comparisons with fractionated X-ray studies done previously in rodent tumours indicate that a therapeutic benefit, relative to lung, gut and renal damage, would be observed with oxygen or carbogen alone but not with nicotinamide alone. The greatest gain would be achieved with the combination of carbogen and nicotinamide, with which a benefit was observed even relative to epidermal damage. These results indicate that some decrease in normal tissue tolerance could be observed when using these modifiers in clinical radiotherapy and, although small, the appropriate dose reductions should be considered; caution should be exercised especially when carbogen and nicotinamide are used in conjunction with the more radical accelerated schedules.

Quantifying intracellular radioresponse diversity in irradiated sandwich cultures via micronucleus expression

Determining the degree of diversity in therapeutic sensitivity exhibited by a tumour population is of considerable clinical importance. In addition to being a contributor to radiation resistance, diversity is the basis for variation in sensitivity over the course of treatment. To study intrapopulation diversity in radiosensitivity following gamma-irradiation (2 Gy), distributions of the number of micronuclei/binucleate cell were obtained for human cervix carcinoma sandwich populations. Cell-to-cell diversity in radioresponse (micronucleus expression) was quantified using the overdispersion index ((variance/mean)--1). As measured by this index, the radioresponse diversity of sandwich cultures sharply increased after introduction of oxygen/nutrients to the cultures, mimicking tumour reperfusion. In addition, a strong correlation was found between this measure of diversity and the extent to which the fraction of cells without micronuclei exceeds that expected from a Poisson distribution. This correlation indicates that for a diverse population there can be a significant departure of the aggregate population sensitivity (determined, for instance, by log-survival in a clonogenic assay) from that inferable from simply averaging per-cell sensitivities (reflected, e.g. by mean number of chromosome aberrations/cell). Our experimental results suggest a model attributing diversity in a population to its being a mixture of distinct subpopulations, each biologically homogeneous with respect to micronucleus expression, and each contributing an individual Poisson-distributed micronucleus response. We demonstrate how such radiodiversity may be quantified and show that reoxygenation of a microenvironmentally heterogeneous population leads to an increase in its radiobiological diversity.

Middle dose rate irradiation in combination with carbogen inhalation selectively and more markedly increases the responses of SCCVII tumors

PURPOSE

Carbogen increases the radiation response of tumors and reduced dose rate irradiation spares the damage of normal tissues. The purpose in this paper is to investigate the possibility of selective radiosensitization of tumors by reduced dose rate irradiation in combination with carbogen inhalation.

METHODS AND MATERIALS

SCCVII tumors in C3H/He mice were irradiated at middle dose rate (0.1 Gy/min) or high dose rate irradiation (3.0 Gy/min) in combination with carbogen inhalation. The mice were enclosed in a box with carbogen flushing at 1.01/min. The tumor response was measured by a cytokinesis block micronucleus assay. The effects on intestinal crypt cells and bone marrow cells were investigated by microcolony assay or Hendry's method, respectively.

RESULTS

The anti-tumor effect of middle dose rate irradiation was equal to that of a high dose rate irradiation. Carbogen inhalation, more efficiently, increased the antitumor effect when combined with middle and high dose rate irradiation, and yielded enhancement ratios of 1.6 at around 2 Gy. Middle dose rate irradiation produced less damage on intestinal crypt cells and bone marrow cells in comparison with high dose rate irradiation, and carbogen inhalation never enhanced the responses of these normal tissues in combination with middle dose rate irradiation. Dose modifying factors were 1.3-2.0.

CONCLUSION

Since middle dose rate irradiation in combination with carbogen inhalation gave the therapeutic gain factors of 2.0-3.2, which were much larger than those obtained with any other radiosensitizers, this combination has a potential as a new modality for improving the results of cancer radiotherapy.