Initial single-strand DNA damage and cellular pharmacokinetics of bleomycin A2

Electrochemotherapy is effective in the treatment of rat bone metastases

Bone metastases impair general health status, quality of life and survival of patients. Electrochemotherapy (ECT), which combines electroporation (EP) and the administration of anticancer drugs, has been recently introduced into clinical practice for the local treatment of solid tumours. In the present study, the ability of EP with bleomycin (Bleo) to induce MRMT-1 rat breast cancer cell death was investigated in vitro. Then, an in vivo model for bone metastases was set up by the inoculation of MRMT-1 cells in rat proximal tibia. 7 days after tumour induction the animals were treated with Bleo, EP, Bleo followed by EP (ECT), or left untreated. ECT eliminated the tumour in 6 out of 8 (75 %) treated metastases. Radiological evaluation showed that the Honore score in ECT-treated animals was significantly lower when compared with the other groups (p < 0.0005) and not significantly different from healthy controls. Bone morphology in ECT-treated animals, evaluated by histological and microtomographical analyses, showed intact cortical and trabecular bone structure with new bone apposition. Histomorphometric evaluation showed that ECT-treated metastases had significantly higher bone volume, trabecular number, trabecular thickness and bone mineral density compared with those of untreated metastases (respectively p < 0.0005 for BV/TV, Tb.N and BMD; p < 0.05 for Tb.Th) or metastases treated with Bleo (p < 0.05 for BV/TV, Tb.N, p < 0.005 for BMD) or EP (p < 0.005 for BV/TV, Tb.N; p < 0.0005 for BMD). These findings suggest that early ECT treatment of bone metastases is minimally invasive, safe and effective, thus providing pre-clinical evidence for its use in the treatment of human bone metastases.

Internalisation of the bleomycin molecules responsible for bleomycin toxicity: a receptor-mediated endocytosis mechanism

Bleomycin (BLM) does not diffuse through the plasma membrane but nevertheless displays cytotoxic activity due to DNA break generation. The aim of the study was to describe the mechanism of BLM internalisation. We previously provided evidence for the existence of BLM-binding sites at the surface of DC-3F Chinese hamster fibroblasts, as well as of their involvement in BLM cytotoxicity on DC-3F cells and related BLM-resistant sublines. Here we report that A253 human cells and their BLM-resistant subline C-10E also possessed a membrane protein of ca. 250 kDa specifically binding BLM. Part of this C-10E cell resistance could be explained by a decrease in the number of BLM-binding sites exposed at the cell surface with respect to A253 cells. The comparison between A253 and DC-3F cells exposing a similar number of BLM-binding sites revealed that the faster the fluid phase endocytosis, the greater the cell sensitivity to BLM. Moreover, the experimental modification of endocytotic vesicle size showed that BLM cytotoxicity was directly correlated with the flux of plasma membrane area engulfed during endocytosis rather than with the fluid phase volume incorporated. Thus, BLM would be internalised by a receptor-mediated endocytosis mechanism which would first require BLM binding to its membrane receptor and then the transfer of the complex into intracellular endocytotic vesicles, followed by BLM entry into the cytosol, probably from a nonacidic compartment.

Chromatid aberration dose responses and dispersal in human G2 lymphocytes treated with bleomycin: comparison with equivalent X-irradiation reveals formation of a novel class of heavily damaged cells

We describe the dose responses and dispersal of chromatid (ct) aberrations in human peripheral blood lymphocytes, treated with a 5-min pulse of bleomycin (BLM) in doses ranging from 0.78 to 200 micrograms/ml during the G2 phase of the cell cycle. Damage was assessed in cells fixed at the time of peak damage 1 h after treatment. Both ct breaks and the percentages of damaged cells rose according to log BLM dose above 6.3 micrograms/ml only. Below this dose all endpoints exhibited flat responses suggestive of thresholding. A dose of 100 micrograms/ml produced similar amounts and distribution of ct breakage per cell (B/c) as a previously studied X-ray dose of 0.8 Gy, permitting future direct cytogenetic comparisons between clastogens. Within the scorable range (0-29 B/c) the dispersal of ct breakage after BLM treatment resembled that after equivalent X-irradiation; but BLM treatment alone resulted in the formation of heavily damaged cells (HDC) defined as with > or = 30 B/c, representing a cytogenetic endpoint of DNA damage reminiscent of apoptosis. At the dose producing equivalent chromatid breakage, BLM produced 7.4 times fewer exchanges than X-rays in G2.

Involvement of membrane bleomycin-binding sites in bleomycin cytotoxicity

The authors have recently shown the existence of bleomycin (BLM)-binding sites at the surface of DC-3F cells. In order to study the involvement of these sites in the sensitivity of the cells to bleomycin several BLM-resistant cell lines from DC-3F cells were analysed. These mutants were obtained by electrotransfection of the Sh ble gene (D/BlmI cells) or the Sh ble-beta Gal fusion gene (D/BlmII cells) and/or by continuous culture in the presence of BLM (D/BlmIR and D/Blm40 cells). The resistance levels of the D/BlmII and D/Blm40 cells were 50- and 22-fold, respectively, determined at the EC50 level. The D/BlmI cells were only 2-fold resistant, whereas D/BlmIR cells were so resistant that almost no cytotoxicity was detected up to 200 microM BLM external concentration. Electropermeabilization was used in an attempt to bypass the plasma membrane of the cells and permit the distinction between internal resistance and membrane resistance. The former was observed when the products of the transfected genes were present. With respect to membrane resistance, differences were detected in the number of BLM-binding sites in several mutant cell lines, which could account for the differences in cell sensitivity to BLM. This suggests that the BLM-binding sites found at the cell surface may play a crucial role in BLM internalization and consequently in its cytotoxicity.

Schedule dependent variation in human lymphocyte sensitivity to bleomycin and repair of chromosomal aberrations in G2

Bleomycin (BLM) induced chromosomal damage in G2 phase and its repair kinetics in normal human lymphocytes were studied following different treatment schedules. As a first step, a dose-response curve was obtained (concentrations of 5-50 micrograms/ml). For repair kinetics studies, blood samples were treated with BLM at a concentration of 20 micrograms/ml. Continuous treatment produced equal numbers of breaks per cell (br/c) when the cells were treated 3, 4 or 5 h before fixation. If the treatment time was extended to 6 h, the level of br/c was increased 2-fold (p < 0.001) as a result of an increased number of cells with more than 3 br/c. The curves obtained after pulse treatment showed maximal chromosome damage at time 3 (45 min BLM treatment, followed by 2 h repair in drug free medium). When the time after treatment was extended to 4 h (treatment time 5), a 50% reduction in chromosome damage was measured. It was found out that at treatment points 3, 4 and 5 the differences in breaks per cell at the different schedules applied were statistically highly significant. If caffeine (CAF) was added, the continuous treatment, BLM+CAF, induced a statistically significant increase in the frequency of br/c at every treatment point, but the shape of the curve illustrating the kinetics of chromosomal damage remained unchanged. Moreover, the addition of CAF at continuous BLM treatment brings the level of br/c close to that measured at the pulse BLM treatment except for treatment time 3. When applied in a combination with BLM, CAF considerably modified the kinetics of chromosome damage for a pulse (BLM alone) treatment. The possible reasons for the changes in the level of br/c as well as a tentative scheme for assessment of chromosome damage repair capacity after BLM treatment are discussed.

Properties of redox-inactivated bleomycins

Blenoxane, bleomycin A2, bleomycin B2, and demethyl bleomycin A2 and products of their reactions with Fe2+ and oxygen were used to explore the relationship between their capacity to carry out in vitro DNA strand scission and their growth inhibitory activity against Ehrlich cells. Reaction of Fe2+, bleomycin and O2 in the absence of DNA decreased the subsequent effectiveness of various bleomycin congeners to degrade DNA in the presence of Fe2+ and oxygen. In comparison with controls, this loss of strand scission activity was not paralleled by equivalent decreases in growth inhibition. Demethyl bleomycin A2 retained full biological activity relative to bleomycin A2, despite being only 30% as effective as bleomycin A2 in its ability to cleave DNA in vitro. Prior reaction of bleomycins with Fe2+ did not alter their capacity to reduce oxygen or affect their ability to generate the activated intermediate which, for native bleomycin structures, is competent to cleave DNA in vitro.

Application of alkaline unwinding to analysis of strand breaks induced by bleomycin in hamster lung DNA in vivo

Strand breaks in hamster lung DNA were analyzed following in vivo exposure to bleomycin. An alkaline unwinding procedure involving separation of single-strand from double-strand DNA by hydroxylapatite chromatography followed by fluorescence detection of DNA with bisbenzimide was adapted for these studies. Procedures were developed that allowed preparation of DNA from lungs of control animals with a relatively low amount of single-strand DNA relative to double-strand DNA. Time dependence of unwinding was demonstrated using samples that were damaged deliberately by brief probe sonication. To verify that residual bleomycin remaining in lungs at the time of sacrifice did not cause strand breaks during sample preparation, bleomycin was added to minced lung in vitro. Under these conditions, the ratio of single-strand to double-strand DNA was not increased significantly. Substantial strand breaks were produced in vivo at 15 min and 1 h following intratracheal instillation of bleomycin into hamsters, as evidenced by a 5-6-fold increase in the ratio of single-strand to double-strand DNA relative to controls. The DNA damage appeared to be repaired within 1 day following exposure.

Inhibition of bleomycin-induced cellular DNA strand scission by 1,10-phenanthroline

Inhibition by 1,10-phenanthroline of cellular DNA strand scission induced by the antitumor antibiotic bleomycin in Ehrlich ascites tumor cells was studied. DNA alkaline elution was performed on cells after 1-hr bleomycin treatments. Pretreatment for 24 hr with initial 1,10-phenanthroline concentrations of 0.2 nmol/10(5) cells, which depletes cells of ferritin iron by 80%, had no consistent effect on bleomycin strand breakage. However, simultaneous treatment with 3.1 nmol of 1,10-phenanthroline/10(5) cells and with bleomycin concentrations from 5 to 25 microM decreased both apparent double-stranded breaks and random breakage. When cells were treated with both 3.1 nmol of 1,10-phenanthroline/10(5) cells and 25 microM bleomycin, washed free of both drugs, and incubated at 35 degrees for 1 hr, the resulting breakage was equivalent to that found in cells treated with bleomycin only. When the combination treatment was extended to 4 hr, cell washing and reincubation resulted in increased strand scission, as compared with strand scission in cells treated with bleomycin only. Growth inhibition by bleomycin was not affected appreciably by temporary suppression of DNA strand breakage activity.