Resveratrol is an effective inducer of CArG-driven TNF-alpha gene therapy

We report the anticarcinogenic, anti-aging polyphenol resveratrol activates the radio- and chemo-inducible cancer gene therapy vector Ad.Egr.TNF, a replication-deficient adenovirus that expresses human tumor necrosis factor alpha (TNF-alpha) under control of the Egr-1 promoter. Like ionizing radiation or chemotherapeutic agents previously shown to activate Ad.Egr.TNF, resveratrol also induces Egr-1 expression from its chromosomal locus with a possible role for Egr-1 promoter CC(A+T)richGG sequences in the expression of TNF-alpha. Resveratrol induction of TNF-alpha in Ad.Egr.TNF-infected tumor xenografts demonstrated antitumor response in human and rat tumor models comparable to that of radio- or chemotherapy-induced TNF-alpha. Although sirtuins are known targets of resveratrol, in vitro inhibition of SIRT1 activity did not abrogate resveratrol induction of Egr-1 expression. This suggests that SIRT1 is not essential to mediate resveratrol induction of Egr-1. Nevertheless, control of transgene expression via resveratrol activation of Egr-1 may extend use of Ad.Egr.TNF to patients intolerant of radiation or cytotoxic therapy and offer a novel tool for development of other inducible gene therapies.     

Combined effects of tumor necrosis factor alpha and radiation in the treatment of renal cell carcinoma grown as radia spheroids

We have investigated the antiproliferative effects of Tumor Necrosis Factor Alpha (TNF) and radiation on a recently described rat renal cell tumor line grown as multicellular tumor spheroids (MTS). Treatment commenced when the spheroids had reached a diameter of 250 microns. TNF was diluted in the tissue culture medium in different concentrations, ranging from 250-1000 ng/ml. TNF monotherapy had a dose-dependent inhibiting effect on spheroid growth. Single-dose irradiation with 2, 4 or 6 Gy also retarded spheroids significantly in their growth. In the combination treatment the highest dose of TNF (1000 ng/ml) was added 4 hours prior to radiation. TNF could not induce a potentiation of the radiation injury at 2 Gy. The combination with 4 Gy, however, had additive and the combination with 6 Gy synergistic antiproliferative effects; in these treatment regimens respectively 2 and 5 out of 24 spheroids were controlled, i.e. cured. These experiments suggest that TNF in combination with radiotherapy may be beneficial for the treatment of renal cell carcinoma or cancer in general.     

Enhanced efficacy of radiation-induced gene therapy in mice bearing lung adenocarcinoma xenografts using hypoxia responsive elements

The aim of the present study was to investigate whether the hypoxia responsive element (HRE) could be used to enhance suicide gene (HSV-tk) expression and tumoricidal activity in radiation-controlled gene therapy of human lung adenocarcinoma xenografts. A chimeric promoter, HRE-Egr, was generated by directly linking a 0.3-kb fragment of HRE to a 0.6-kb human Egr-1 promoter. Retroviral vectors containing luciferase or the HSV-tk gene driven by Egr-1 or HRE-Egr were constructed. A human adenocarcinoma cell line (A549) was stably transfected with the above vectors using the lipofectamine method. The sensitivity of transfected cells to prodrug ganciclovir (GCV) and cell survival rates were analyzed after exposure to a dose of 2 Gy radiation and hypoxia (1%). In vivo, tumor xenografts in BALB/c mice were transfected with the constructed retroviruses and irradiated to a total dose of 6 Gy, followed by GCV treatment (20 mg/kg for 14 days). When the HSV-tk gene controlled by the HRE-Egr promoter was introduced into A549 cells by a retroviral vector, the exposure to 1% O(2) and 2 Gy radiation induced significant enhancement of GCV cytotoxicity to the cells. Moreover, in nude mice bearing solid tumor xenografts, only the tumors infected with the hybrid promoter-containing virus gradually disappeared after GCV administration and radiation. These results indicate that HRE can enhance transgene expression and tumoricidal activity in HSV-tk gene therapy controlled by ionizing radiation in hypoxic human lung adenocarcinoma.     

Molecular targeting of gene therapy and radiotherapy.

The full promise of gene therapy has been limited by the lack of specificity of vectors for tumor tissue as well as the lack of antitumor efficacy of transgenes encoded by gene delivery systems. In this paper we review our studies investigating two modifications of gene therapy combined with radiotherapy. The first investigations described include studies of radiation inducible gene therapy. In this paradigm, radio-inducible DNA sequences from the CarG elements of the Egr-1 promoter are cloned upstream of a cDNA encoding TNFalpha. The therapeutic gene (TNFalpha) is induced by radiation within the tumor microenvironment. In the second paradigm, genetically engineered herpes simplex virus (HSV-1) is induced by ionizing radiation to proliferate within the tumor volume. These modifications of radiotherapy and gene therapy may enhance the efficacy of both treatments.     

The protective effects of ambroxol on radiation lung injury and influence on production of transforming growth factor β<Subscript>1</Subscript> and tumor necrosis factor α

The aim of this article was to investigate the effect of ambroxol on radiation lung injury and the expression of transforming growth factor β₁ (TGF-β₁), as well as tumor necrosis factor α (TNF-α) in plasma. Totally, 120 patients with locally advanced lung cancer in radiotherapy were randomized into treatment and control groups. Patients in the treatment group took ambroxol orally at a dosage of 90 mg, three times per day for 3 months from the beginning of radiotherapy. The expression of TGF-β₁ and TNF-α in plasma was analyzed. The clinical symptoms and lung diffusing capacity were monitored using high resolving power computed tomography. The level of TGF-β₁ in the control group was increased (11.8 ± 5.5 ng/ml), whereas in ambroxol-treated patients, the increase was not significant (5.6 ± 2.6 ng/ml, P < 0.001). Radiotherapy-induced elevation of TNF-α levels, seen in control patients, was also abolished after treatment with ambroxol (5.1 ± 1.0 vs. 2.4 ± 0.8 ng/ml, P < 0.001). In the treatment group, carbon monoxide diffusion capacity was not significantly decreased at 6, 12, and 18 months post-radiotherapy, compared with the control group (P < 0.05). Ambroxol decreased the expression of TGF-β₁ and TNF-α, and minimized the diminishment of lung diffusion capacity after radiotherapy.     

特异启动子调控的MDR1和MnSOD基因对骨髓的选择性保护作用

目的探讨特异保护骨髓细胞免受抗癌药物及射线损害的新途径。方法 构建了由APN骨髓特异性启动子调控的耐药、耐辐射基因的逆转录病毒载体,导入骨髓母细胞及癌细胞中,用细胞存活实验对耐药耐辐射性能进行观察。分离小鼠骨髓细胞,经转染基因后,再输回到预先用射线处理以破坏骨髓系统的同种受体小鼠体内。经化疗药物或射线处理后,不同时间采血,计算白细胞数,观察在药物或射线照射后导入耐药或耐辐射基因对造血细胞的保护作用。结果 导入MDR1基因的KGla细胞对秋水仙素、足叶乙甙、长春新碱、阿霉素及紫杉醇与对照组相比,各提高了10.6,10.4,11.2,4.2和14.2倍。导入由APN启动子驱动的MnSOD基因,使KGla细胞比对照组对射线(10 Gy)的耐受性提高3.7倍。相反,导入以上两个基因后,肝癌细胞BEL7402的化、放疗耐受性未见明显变化。体内实验表明,转染了MDR1和MnSOD基因的小鼠血液中,白细胞数量明显高于对照组(P<0.01)。结论体外由APN骨髓启动子调控的MDR1和MnSOD基因能特异性地保护骨髓细胞,而对肿瘤细胞无明显影响。体内,在用药物或射线处理动物时能重建造血功能。此研究为肿瘤患者在接受化、放疗时特异性保护骨髓系统,提供了新的思路和依据。     

Treatment results of adjuvant radiotherapy and salvage radiotherapy after radical prostatectomy for prostate cancer

Background The indications for and the efficacy of radiation therapy after radical operation for patients with prostate cancer are not clear. We analyzed the treatment results of adjuvant radiotherapy and salvage radiotherapy after radical prostatectomy. Methods Between September 1997 and November 2004, 57 patients received adjuvant radiotherapy or salvage radiotherapy after radical prostatectomy. Fifteen patients received radiation therapy because of positive margins and/or extracapsular invasion in surgical specimens (adjuvant group). Forty-two patients received radiation therapy because of rising prostate-specific antigen (PSA) during follow-up (salvage group). Radiation therapy was delivered to the fossa of the prostate ± seminal vesicles by a three-dimensional (3-D) conformal technique to a total dose of 60–66 Gy (median, 60 Gy). Biochemical control was defined as the maintenance of a PSA level of less than 0.2 ng/ml. Results The median follow-up period after radiation therapy was 33 months (range, 12–98 months). Three-year biochemical control rates were 87% for the adjuvant group and 61% for the salvage group. For patients in the salvage group treated without hormone therapy, the preradiation PSA value was the most significant factor for the biochemical control rate. The 3-year biochemical control rate was 93% in patients whose preradiation PSA was 0.5 ng/ml or less and 29% in patients whose preradiation PSA was more than 0.5 ng/ml. No severe adverse effects (equal to or more than grade 3) were seen in treated patients. Conclusion Radiation therapy after radical prostatectomy seemed to be effective for adjuvant therapy and for salvage therapy in patients with a preradiation PSA of 0.5 ng/ml or less. Also, radiation to the fossa of the prostate ± seminal vesicles, to a total dose of 60–66 Gy, using a three-dimensional (3-D) conformal technique, seemed to be safe.     

人Egr-1启动子调控的自杀基因真核表达载体的构建及其在鼻咽癌细胞株中的放射诱导表达

[目的]构建由人Egr-1放射诱导性启动子调控的自杀基因靶向性真核表达载体,并转染人鼻咽癌细胞株CNE,比较射线诱导前后自杀基因在鼻咽癌细胞中的表达情况。[方法1根据Genebank数据库提供的人Egr—1启动子基因序列,设计-对引物克隆人Egr-1启动子,采用PCR、酶切、连接等分子生物学技术,构建pcDNA3．1（＋）-Egr-1-CD重组质粒表达载体,并利用脂质体转染方法转染人CNE细胞株,经G418筛选后获得阳性细胞克隆。RT—PCR分析鉴定0、5、10、15、20Gy放射剂量进行6MVX线照射对CD基因表达的影响。f结果1经PCR、酶切、测序等证实了pcDNA3．1（＋）-Egr．1．CD重组靶向性质粒表达载体构建成功,G418筛选所得到的CNE细胞阳性克隆经不同放射剂量的诱导后,RT—PCR结果显示体外放射线照射可显著提高CNE细胞中CDmRNA的表达水平,在10Gy及15Gy组CDmRNA的表达水平明显增高,尤以15Gy组CDmRNA的表达水平最高,在20Gy组CDmRNA的表达水平反而下降。[结论]pcDNA3．1（＋）-Egr-1-CD重组质粒表达载体构建成功,并成功转染人鼻咽癌细胞株．建立了基因表达与放射的剂量效应关系．为后续的临床应用研究奠定基础．     

Association of lysosomal activity with sensitivity and resistance to tumor necrosis factor in murine L929 cells

The cytotoxic mechanism of action of tumor necrosis factor (TNF) was examined using murine L929 fibrosarcoma cells in vitro. Two cell lines were evaluated: parental TNF sensitive (L929S) (50% cytotoxic concentration, 2-6 ng/ml); and TNF resistant (L929R) (50% cytotoxic concentration, greater than 10,000 ng/ml). The latter resistant cell line was developed by serial passage in increasing concentrations of recombinant human TNF. Sensitive cells demonstrated cytolytic and cytostatic effects at TNF concentrations between 2 and 6 ng/ml, respectively. However, TNF failed to show any selective depression of RNA, DNA, or protein synthesis or ATP content in these cells until general cell death was apparent, as defined by the cell rounding and lifting off the plastic surface. The cytokine also failed to cause DNA single-strand breaks, as detected by alkaline elution techniques. TNF was also found to be no more active in glutathione-depleted cells than in target cells containing normal glutathione levels. In contrast, various nonspecific lysosomotropic agents such as ammonium chloride and D-saccharic acid lactone led to a marked inhibition of the cytotoxic action of TNF in vitro. Furthermore, significant differences in lysosomal enzyme activity were noted between L929S and L929R cells. The changes in L929R cells involved a 50% reduction in total lysosomal protein levels and a marked depression of beta-glucuronidase activity. In contrast, L929R lysosomal hexosaminidase activity was significantly elevated over the L929S cells. From these studies it is concluded that the antitumor activity of TNF does not involve specific inhibition of macromolecular synthesis, ATP production, or the level of reduced thiols. Instead, TNF cytotoxicity appears to require functional lysosomes, which are altered when TNF resistance develops in vitro.     

Augmentation of cytotoxic effect of tumor necrosis factor on human immunodeficiency virus-infected cells by staurosporine, a potent protein kinase C inhibitor

We have examined the effect of the protein kinase C (PKC) inhibitor, staurosporine, on tumor necrosis factor (TNF)-induced cytotoxic action and augmentation of human immunodeficiency virus (HIV) expression on the chronically HIV-infected T-cell line, MOLT-4/HIV (HTLV-IIIB strain). Staurosporine enhanced the decrease in the number of viable cells caused by TNF treatment for 3 days (1 ng/ml of TNF, 43% decrease; 1 ng/ml of TNF + 20 nM staurosporine, 94%), whereas the cytotoxic action on that cell line induced by 10 ng/ml of 12-O-tetradecanoylphorbol-13-acetate (TPA), which was known to be an activator of PKC, was partially inhibited by staurosporine. In addition, staurosporine augmented the TNF cytotoxic activity against other cell lines including HIV-uninfected U937 cells(100 ng/ml of TNF, 53% decrease in the number of viable cells; 100 ng/ml of TNF + 5 nM staurosporine, 86%). However, staurosporine did not change the sensitivity of cells to TNF; thus, those insensitive to TNF were not changed to TNF sensitive by staurosporine. Furthermore, staurosporine did not affect the augmentative effect of TNF on HIV expression evaluated by levels of p24 antigen. Moreover, HIV long terminal repeat (LTR)-directed chloramphenicol acetyltransferase assay showed that staurosporine strongly inhibited the TPA-induced activation of HIV LTR, while that caused by TNF was little affected (10 ng/ml of TPA, 98.4% conversion; 10 ng/ml of TPA + 40 nM staurosporine, 22.2%, 1 ng/ml of TNF, 98.5%; 10 ng/ml of TNF + 40 nM staurosporine, 93.9%). These results suggest that TPA and TNF facilitate HIV replication by different pathways and that staurosporine augments TNF cytotoxicity by possible suppression of PKC activity in both HIV-infected and uninfected cells.     

Optimizing antiangiogenic strategies: combining with radiotherapy

Tumor response to radiotherapy is influenced by many tumoral intracellular biological factors whose deregulation leads to the modulation of tumor sensitivity to radiotherapy. However, the pathways controlling intracellular radiation resistance are activated by or display common pathways with many factors also controlling the tumoral microenvironment, and particularly angiogenesis. One of the innovative strategies that could improve response to irradiation of aggressive or radiotherapy-insensitive tumors consists of combining radiotherapy with inhibitors of the angiogenesis pathways. This review details four main reasons for this: angiogenic factors control intracellular radioresistance, endothelial cell radiosensitivity controls tumor radiosensitivity, tumor angiogenesis is the cause of hypoxia, a major radiation resistance factor, and stem cells, known to be radiation resistant, are dependent on angiogenic factors. We describe the implication of different factors in the various mechanisms leading to radiation resistance. These factors include vascular endothelial growth factor (VEGF), epidermal growth factor (EGF), and fibroblast growth factor 2 (FGF2), along with αvβ3 and αvβ5 integrins as well as their downstream cellular pathways including some small G proteins. The review also explains the radiation-sensitizing effect of the inhibition of these factors by targeted therapies. The optimal sequences of administration between antiangiogenics and radiotherapy are partially elucidated. The success of these combinations will depend on the specific study of the mechanisms of action of antiangiogenic agents and their interaction with ionizing radiations, and on the use in preclinical and clinical studies of metabolic and functional imaging. These techniques represent an essential tool for determining the optimal combination sequence followed by the assessment of these combinations.     

Characterization of a Novel Human Tumor Necrosis Factor-α Mutant with Increased Cytotoxic Activity

Various novel recombinant human tumor necrosis factor-α (TNF) mutants were prepared using protein engineering techniques, and their cytotoxic activity was compared with that of the intact form of TNF (intact TNF). Mutant 471 (a TNF mutant molecule with the deletion of 7 amino acids at the amino-terminal and the substitution of Pro⁸Ser⁹Asp¹⁰ by ArgLysArg) had a 6-fold higher cytotoxic activity against murine L929 cells. The mutant TNF had an increased ability to bind to TNF receptor on murine L929 fibroblasts cells. A cross-linking study revealed that mutant 471 had an increased ability to form an active trimer. Mutant 471 also showed higher cytotoxic activity against human KYM myosarcoma cells and human MIA PaCa-2 pancreatic carcinoma cells. The possible cachectin activity of the mutant was almost the same as that of intact TNF. These results suggest that mutant 471 might be a more promising candidate as an anticancer agent than intact TNF.     

Potentiation of tumor radiotherapy by a radiation-inducible oncolytic and oncoapoptotic adenovirus in cervical cancer xenografts

The p53 tumor suppressor pathway is impaired in more than 90% of cervical cancers and cancer-derived cell lines as a result of infection by human papillomavirus (HPV). The HPV E6 oncoprotein forms complexes with p53 and promotes its degradation via ubiquitin-dependent mechanism. In our study, we attempted to improve the clinical outcomes of this combined therapy by modifying the p53-targeted adenovirus to become radiation-responsive. The antitumor adenovirus was constructed by inserting a radiation-responsive expression cassette composed of the promoter of early growth response-1 (Egr-1) and the proapoptotic protein TRAIL. We showed that the addition of adenovirus containing Egr-1/TRAIL significantly increased cell death and apoptosis caused by radiotherapy. In mice bearing xenograft tumors, intratumoral administration of the Egr-1/TRAIL adenovirus followed by radiation significantly reduced tumor growth and enhanced tumor survival. Our Egr-1/TRAIL adenoviral gene product may offer a novel "one-two punch" tumor therapy for cervical cancers not only by potentiating radiation treatment but also by preserving p53 defect-specific tumor killing of the oncolytic adenovirus.     

放射线诱导hEgr-1-CD对鼻咽癌CNE放射增敏作用的研究

目的 研究放射线诱导hEgr-1-CD自杀基因前药系统对鼻咽癌CNE细胞株放射增敏作用。方法 将pcDNA3.1（＋）Egr-1-CD利用脂质体转染的方法 转染入鼻咽癌细胞株,给予不同剂量的X线照射观察基因表达与放射的剂量效应关系,观察不同的放射线及不同剂量的前药对鼻咽癌细胞的杀伤效应。结果 电离辐射可诱导增强自杀基因阳性鼻咽癌CNE细胞株中CD基因的表达,电离辐射与前药5-Fc的联合应用大大增强了对自杀基因阳性细胞的杀伤作用,其杀伤作用大于单独自杀基因前药系统和单独照射。结论 hEgr-1-CD自杀基因前药系统对鼻咽癌CNE细胞株有放射增敏作用,其与放射联合应用对CNE细胞有明显的协同杀伤作用。     

Gene therapy delivery of endostatin enhances the treatment efficacy of radiation.

BACKGROUND AND PURPOSE: To evaluate whether sustained expression of mouse endostatin by adeno-associated virus (AAV)-mediated gene transfer can enhance the treatment efficacy of ionizing radiation. MATERIALS AND METHODS: Mouse endostatin was cloned into recombinant AAV (rAAV) under the control of CMV beta-actin promoter. Recombinant mouse endostatin expressed via AAV gene transfer was tested for biological activity in endothelial cells. The impact of elevated serum levels of endostatin on tumor-induced angiogenesis was evaluated using an in vivo angiogenesis assay. The anti-tumor efficacy of combining rAAV-mediated endostatin delivery with radiation was evaluated in a human colorectal tumor model (HT29). RESULTS: Recombinant mouse endostatin expressed through an AAV vector (rAAV-mEndo) inhibited endothelial cell proliferation (by 40-45%) and migration (by 22-33%). Intramuscular injection of rAAV-mEndo (1x10(9) i.u.) led to a sustained serum endostatin level of approximately 500 ng/ml. Compared to control animals this endostatin level was sufficient to inhibit tumor cell-induced vessel formation (37 vs. 28.5, P<0.05) and delay the growth of HT29 xenografts (time from 200 to 1,000 mm(3), 21 vs. 34.5 days, P<0.05). When combined with ionizing radiation, elevated serum endostatin levels significantly enhanced the time for tumors to grow from 200 to 1,000 mm(3) (radiation, 34 days; endostatin plus radiation, 50 days, P<0.05). CONCLUSION: The delivery of endostatin via rAAV vectors may provide an effective means of enhancing the anti-tumor efficacy of radiation therapy.     

Low-dose radiation enhances survivin-mediated virotherapy against malignant glioma stem cells

To improve the efficacy and selectivity of virotherapy for malignant glioma, we designed a strategy to amplify adenoviral replication in conjunction with radiotherapy using a radioinducible promoter. First, we compared the radiation-inducible activity of FLT-1, vascular endothelial growth factor, DR5, Cox2, and survivin. We then examined the capacity of the optimal promoter to modulate transgene expression followed by E1A activity in vitro and in vivo in a glioma stem cell model. In the presence of radiation, survivin mRNA activity increased 10-fold. Luciferase transgene expression was dose dependent and optimal at 2 Gy. A novel oncolytic adenovirus, CRAd-Survivin-pk7, showed significant toxicity and replication against a panel of passaged and primary CD133(+) glioma stem cells. On delivery of radiation, the toxicity associated with CRAd-Survivin-pk7 increased by 20% to 50% (P < 0.05). At the same time, the level of E1A activity increased 3- to 10-fold. In vivo, treatment of U373MG CD133(+) stem cells with CRAd-Survivin-pk7 and radiation significantly inhibited tumor growth (P < 0.05). At the same time, the level of E1A activity was 100-fold increased versus CRAd-Survivin-pk7 alone. Selected genes linked to radioinducible promoters whose expression can be regulated by ionizing radiation may improve the therapeutic ratio of virotherapy. In this study, we have identified a new radioinducible promoter, survivin, which greatly enhances the activity of an oncolytic adenovirus in the presence of low-dose radiotherapy.     

The antitumour agent 5,6-dimethylxanthenone-4-acetic acid acts in vitro on human mononuclear cells as a co-stimulator with other inducers of tumour necrosis factor

5,6-dimethylxanthenone-4-acetic acid (DMXAA), currently in phase I trials, demonstrates excellent activity against transplantable murine tumours with established vasculature. The induction of cytokines, particularly of tumour necrosis factor (TNF), appears to be critical to its action. We investigated TNF induction by DMXAA in cultured human peripheral blood leucocytes (HPBL). TNF was measured by an enzyme-linked immunosorbent assay after 8 h, and NF-kappaB induction by electrophoretic mobility shift assays (EMSA) after 2 h. DMXAA (800 microg/ml) had no effect alone on TNF production but augmented, by up to 4-fold, the ability of bacterial lipopolysaccharide (LPS) to induce TNF. Previously reported results showing TNF production by DMXAA alone were traced to the presence in an earlier batch of DMXAA of a small amount of LPS, the action of which could be blocked by polymyxin B. DMXAA stimulated TNF production by deacylated LPS, which alone had little effect. An antibody (MEM-18) to the CD14 receptor, while blocking the induction of TNF by LPS, enabled DMXAA to both synthesise TNF and induce NF-kappaB. The structurally related drug, flavone acetic acid (FAA), did not induce TNF or synergise with anti-CD14 antibody. DMXAA strongly augmented the ability of suboptimal concentrations of interleukin-1 (IL-1) (25 ng/ml), okadaic acid (OA) (20 ng/ml) and phorbol-12-myristate-13-acetate (PMA) (5 ng/ml) to induce TNF production, suggesting that it affects multiple pathways converging on NF-kappaB activation. Sodium salicylate, a drug reported to inhibit the beta-subunit of IkappaB kinase (IKK), appeared to competitively inhibit TNF production by DMXAA in the presence of anti-CD14 antibody. Taken together, the results indicate DMXAA acts in vitro on HPBL to co-stimulate TNF production by a wide variety of agents, and suggests that IKK is the target that mediates this action.     

Resistance patterns between cis-diamminedichloroplatinum(II) and ionizing radiation

Cross-resistance between cis-diamminedichloroplatinum(II) (CDDP) and radiation resistance has been suggested from clinical and experimental data (C. T. Coughlin and R. C. Richmond, Semin. Oncol., 16: 31-43, 1989). To determine whether cross-resistance patterns between both cytotoxic approaches exist, resistance against CDDP and ionizing radiation was induced separately in human ovarian cancer cells in a cross-over design. Subsequently sensitivity changes were determined for both treatment modalities. CDDP resistance was induced previously (P. J. Kuppen et al., Cancer Res., 48: 3355-3359, 1988), and resistant cells were grown at three different levels of CDDP:0 ng/ml; 250 ng/ml; and 500 ng/ml. Resistance with resistance factor (RF) 3.4 to 5.1 proved to be stable, since withdrawal of CDDP pressure for at least 6 mo did not alter resistance patterns. CDDP-resistant cells also demonstrated stable resistance against ionizing radiation, with RF ranging from 1.7 to 2.0. The resistance patterns could not be explained by differences in growth kinetics and DNA content. Resistance to ionizing radiation was induced in the same human ovarian cancer cells as used for CDDP resistance studies. Exposure with 1.5 Gy of intermittent irradiation during 6 mo, at time intervals of 48 h, resulted in cells which were able to grow under chronic ionizing radiation pressure. RF was 2.0; the resistance was lost after 6 mo of culturing without ionizing radiation pressure. With intermittent radiation doses of 0.5 and 1.0 Gy, no significant resistance could be induced. Cells intermittently exposed to 0.5, 1.0, and 1.5 Gy during 6 mo demonstrated increased sensitivity to CDDP, with 0.22 less than RF less than 0.43. Increased sensitivity was associated with proportionally increased formation of the platinum-DNA adducts. Differences in sensitivity for both ionizing radiation and CDDP were lost after 6 mo of culturing without radiation pressure; therefore, resistance toward ionizing radiation and, likewise, the increased sensitivity to CDDP, were judged to be unstable. In conclusion, data of the present study demonstrated that development of stable resistance to CDDP is associated with development of stable resistance to ionizing radiation in human ovarian cancer. Contrastingly, increased sensitivity to CDDP was found when resistance against irradiation was induced in the same cells.