Reduced capacity for DNA repair synthesis in patients with or genetically predisposed to colorectal cancer.

Peripheral resting mononuclear leukocytes were compared for their capacities to repair DNA lesions induced by a 1-hour exposure to a standardized 10-microM dose of N-acetoxy-N-2-fluorenylacetamide (N-AcO-2-FAA). Leukocytes from the following 3 groups were studied: 39 control subjects, 40 patients after colonic resection because of colorectal cancer (disease-free at the time of this study), and 28 individuals with a hereditary predisposition to colorectal cancer. Although the level of N-AcO-2-FAA that bound to mononuclear leukocyte DNA was the same for the various population groups, the level of N-AcO-2-FAA-induced unscheduled DNA synthesis (UDS) was significantly reduced in the mononuclear leukocytes of individuals who had had colorectal cancer or a genetic predisposition for the disease. These findings indicate that a deficiency in mononuclear leukocyte DNA repair synthesis is associated with the development of colorectal cancer in these populations. Our observation of this nonspecific UDS deficiency (relating to colorectal cancer) was not explained by experimental variations among the sampled groups with regard to individual differences in lymphocyte heterogeneity, age, sex, smoking habits, or blood pressure.

Measurement and characterization of membrane-bound and soluble epoxide hydrolase activities in resting mononuclear leukocytes from human blood.

Membrane-bound and soluble epoxide hydrolase activities in the mononuclear cell fraction from human blood have been characterized using cis- and trans-stilbene oxides as substrates, respectively. Because of the low activities in these cells, it was necessary to modify assay procedures developed for rat and mouse liver in the following ways: (a) the substrates were relatively highly labeled (2 Ci/mmol) and carefully purified; (b) the incubation time was extended to 45 to 60 min, during which period the activities were linear; (c) as many as 6 million cells were used for a single assay, which was also within the linear range of the procedure. The membrane-bound epoxide hydrolase characterized in this manner has an apparent Vmax of 7.26 pmol product formed per min per 10(7) cells and an apparent Km of 9.96 microM. The pH optimum was observed to be around 9.8. The dependence of this activity on temperature showed its optimum at 40 degrees. The soluble epoxide hydrolase activity has an apparent Vmax of about 8.26 pmol product formed per min per 10(7) cells, an apparent Km of 1.63 microM, a pH optimum of 6.2 to 6.8, and a temperature optimum at 60 degrees. Using these techniques, these activities have also been determined in other blood components, i.e., lymphocytes, monocytes, granulocytes, erythrocytes, platelets, and plasma. Lymphocytes account for most of the epoxide hydrolase activity towards cis-stilbene oxide, and all of the activity towards trans-stilbene oxide is in the human mononuclear cell fractions. Different substances known to affect rodent epoxide hydrolases were tested for their effects on the human mononuclear blood cell activities. Interestingly, 1,1,1-trichloropropane 2,3-epoxide, a potent inhibitor of liver microsomal epoxide hydrolase in different species including rat, mouse, and human, had little or no effect on the membrane-bound activity measured here. However, cyclohexene oxide inhibits this membrane-bound activity 60%. The soluble epoxide hydrolase is inhibited to 90% of control levels by chalcone epoxide. The membrane-bound and soluble epoxide hydrolase activities determined in 27 subjects varied from 8.2 to 18.5 and from 3.5 to 17.0 pmol product formed per min per 10(7) cells, respectively. The mean coefficient of intraindividual variation, determined with three subjects measured four times each over the course of 18 days, was approximately 10% for both enzyme activities.

Catabolism of exogenously supplied thymidine to thymine and dihydrothymine by platelets in human peripheral blood.

The interference of platelets with the estimation of unscheduled DNA synthesis in human peripheral mononuclear leukocytes following genotoxic exposure was studied. A 96% reduction in the unscheduled DNA synthesis value was achieved by incubating [3H]thymidine with platelet-rich plasma for 5 hr at 37 degrees. Using radioactive thymine-containing compounds, together with quantitative analyses based on thin-layer and ion-exchange chromatographies, we have shown that thymidine was converted to thymine which, in turn, was converted to dihydrothymine in platelet-rich plasma. The enzymes responsible were separated from platelet lysates by gel filtration and were identified as thymidine phosphorylase and dihydrothymine dehydrogenase. The phosphorylase reversibly catalyzed the formation of thymine from thymidine and converted bromodeoxyuridine to bromouracil. The dehydrogenase reversibly catalyzed the interconversion of thymine and dihydrothymine in a reaction dependent on NADP(H), and it was inhibited by diazouracil and by thymine. Nearly all the thymidine-catabolizing activity found in whole blood samples supplied exogenously with thymidine was accounted for by the platelets. Since most genetic toxicological tests that use blood samples do not involve removing platelets from the blood cell cultures, then it is concluded that precautions should be taken in the future to determine the influence of platelets on these test systems. This is particularly true for methods dependent on thymidine pulses such as unscheduled DNA synthesis, or those dependent on bromodeoxyuridine, such as sister chromatid exchanges, since this nucleoside is also a substrate for thymidine phosphorylase.

Radiosensitization effects of nicotinamide on malignant and normal mouse tissue.

Inhibitors of the chromatin-associated enzyme adenosine diphosphate ribosyltransferase have been found to inhibit DNA strand rejoining and to potentiate lethality of DNA-damaging agents both in vivo and in vitro. We have in this work examined the radiosensitizing potential of one such inhibitor, nicotinamide, on tumor tissue by using transplanted C3H mouse mammary adenocarcinomas and on normal tissue in a tail-stunting experiment using BALB/cA mice. Our data indicate a radiosensitizing effect of nicotinamide on tumor cells as well as on normal tissue. The data indicate a possible role of adenosine diphosphate ribosyltransferase inhibitors as a sensitizing agent in the radiotherapy of malignant tumors.

Unscheduled DNA synthesis in mononuclear leukocytes from patients with colorectal polyps.

The mononuclear leukocytes from peripheral blood samples of individuals with (n = 30) and without (n = 48) colonic polyps were examined for their abilities to carry out unscheduled DNA synthesis (UDS) induced by N-acetoxy-N-2-fluorenylacetamide (N-AcO-2-FAA). Individuals with polyps had significantly reduced UDS values compared to the nonpolyp group (P less than 0.01). Furthermore, in a more comprehensive study, patients with hyperplastic polyps had N-AcO-2-FAA-induced UDS values not significantly different from control individuals who were asymptomatic and free from colonic disease as judged by complete colonoscopy. However, patients who had had adenomatous polyps in their large bowel had significantly reduced levels of N-AcO-2-FAA-induced UDS in their mononuclear leukocytes (P less than 0.005). When N-AcO-2-FAA binding to DNA determinations were made in parallel and DNA repair proficiency indices were calculated (i.e., N-AcO-2-FAA-induced UDS/N-AcO-2-FAA binding to DNA), the patients with adenomatous polyps were still shown to be deficient in carrying out DNA repair synthesis. Since adenomatous polyps of the large bowel are considered the premalignant lesion for colorectal cancer, we postulate that reduced UDS may be a genetically sensitive marker that is useful in studying the mechanisms of genetic predisposition to colorectal cancer.

Oxidative stress induces DNA damage and inhibits the repair of DNA lesions induced by N-acetoxy-2-acetylaminofluorene in human peripheral mononuclear leukocytes.

Human mononuclear leukocytes were exposed to prooxidants such as H2O2, phorbol-12-myristate-13-acetate, and 4-nitroquinoline-N-oxide, and the effects on induction of DNA damage and repair were evaluated. ADP ribosylation was activated by prooxidant exposure and the response was bimodal with peaks of activation occurring at about 30 min and 4-5 h. Other evidence for prooxidant-induced DNA damage was provided by nucleoid sedimentation assays. Unscheduled DNA synthesis (UDS) was only slightly induced by prooxidant exposure which suggested that either the DNA lesions were repaired by a short patch mechanism involving little UDS, or the repair process was inhibited by prooxidant exposures, or some combination of both. This point was clarified by the fact that the repair of DNA lesions induced by N-acetoxy-2-acetylaminofluorene, an inducer of large patch DNA repair, was inhibited in a dose-dependent manner by exposure to H2O2 and the inhibition was dependent on ADP ribosylation. In contrast, the repair of DNA strand breaks induced by prooxidant exposures as identified above were complete within about 8 h and the repair was independent of ADP ribosylation. Both ADP ribosylation and N-acetoxy-2-acetylaminofluorene-induced UDS were shown to be up- and down-regulated by the redox state of human mononuclear leukocytes indicating a unique mechanism of cellular control over DNA repair.

Steroidal nonspecific esterase metabolism of N-hydroxy-2-acetylaminofluorene: evidence for selective activation by the cellular reductant NADPH.

The significance of the nonspecific esterases of human mononuclear leukocytes (HMLs) in arylamine carcinogenesis is suggested by data showing that the metabolically formed hydroxamic acid derivative of 2-acetylaminofluorene, N-hydroxy-2-acetylaminofluorene, is a substrate for this class of enzymes. A viable cell assay for the nonspecific esterases using alpha-naphthyl acetate as substrate is described, and data showing this activity to be sensitive to already known substrates for HML esterases as measured by three previously described assays are presented. All four assays of the same esterase activity are shown to be highly sensitive to up- and down-regulation by addition of NADPH or NADP to viable HML cultures. Selective activation of a purified rabbit nonspecific esterase by NADPH, but not by the other cellular reductants, NADH and glutathione, was demonstrated. Cytosols prepared from normal human tissue samples of liver, breast, colon, and brain were also activated by the presence of NADPH. These data do not indicate that steroidal nonspecific esterases are redox-modulated by the presence of mixed disulfides in their structure. Instead, they support the direct and specific influence of NADPH as a widespread activator of esterase activity by a mechanism not yet understood.

N-substituted benzamides inhibit nuclear factor-kappaB and nuclear factor of activated T cells activity while inducing activator protein 1 activity in T lymphocytes.

N-substituted benzamides are compounds that have recently been reported to inhibit nuclear factor-kappaB (NF-kappaB) activity and induce apoptosis in a pre-B cell line. In this study, we focused on the effects of N-substituted benzamides on transcriptional regulation in Jurkat T cells. We used a model system where the cells can be stimulated either through TCR/CD28 or by treatment of the cells with PMA and ionomycin to induce transcription factors typical for T lymphocyte activation. Treatment of the Jurkat cells with procainamide did not influence the transcription factor profile of stimulated cells, while treatment with a derivative having an acetyl group in position 4 of the aromatic ring inhibited NF-kappaB and nuclear factor of activated T cells (NFAT) activity. Declopramide, which contains a chloride in position 3 of the aromatic ring, was inactive in this system, whereas also the acetylated derivative of this compound inhibited NF-kappaB and NFAT activity. In contrast, the transcriptional activity and nuclear expression of activator protein 1 induced by TCR/CD28 stimulation or PMA and ionomycin treatment was enhanced by the acetylated variants of the N-substituted benzamides. Finally, we investigated the effect of N-substituted benzamides on intact promoters for two genes central in immune regulation; the CD40 ligand (CD40L) and IL-2 promoters. The transcriptional activity of the CD40L promoter as well as surface expression of the CD40L induced by signaling through TCR/CD28 was inhibited by addition of acetylated N-substituted benzamides, while the transcriptional activity of the IL-2 promoter was enhanced. Taken together, these data indicate that derivatives of N-substituted benzamides are potential drug candidates for quantitative as well as qualitative modulation of immune functions.

Low serum thiol levels predict shorter times-to-death among HIV-infected injecting drug users.

OBJECTIVES: To investigate whether serum thiol levels are altered by HIV disease, and whether low serum thiols predict time to death among HIV-infected injecting drug users (IDU). DESIGN: A cross-sectional study of serum thiol levels among 13 HIV-seronegative IDU, 116 HIV-seropositive IDU, and 17 HIV-seropositive IDU with a history of AIDS, and a cohort study of the 133 HIV-infected IDU who took part in the cross-sectional study. METHODS: Subjects were recruited from a methadone-maintenance treatment program during 1990-1991. Total serum thiols were determined spectrophotometrically at enrolment; low serum thiols were defined as those with an absorbance at 412 nm < or = 0.46. Deaths through 31 December 1993 were determined from the National Death Index (NDI). Twenty-six HIV-seropositive subjects died during follow up; death certificates, which were obtained for 23 subjects, indicated AIDS or HIV infection for 20. Product-limit estimation was used to calculate survival. Multivariate analyses employed Cox proportional-hazards regression. RESULTS: Analysis of cross-sectional data showed that serum thiols did not differ significantly among HIV-free subjects, HIV-infected subjects, and HIV-infected subjects with a history of AIDS. Cohort analysis, adjusted for age, revealed that persons with those with high serum thiols (relative hazard = 2.83; 95% confidence interval (CI), 1.15, 6.97); a significant interaction between low serum thiols and a history of AIDS was associated with a relative hazard of 5.65 (95% CI, 1.22-2.61). CONCLUSIONS: Among HIV-infected persons, low serum thiols, especially in concert with a history of AIDS, predict mortality risk. These findings support the hypothesis that oxidative stress is critical to the pathogenesis of HIV infection.

Quality control program for storage of biologically banked blood specimens in the Malmö Diet and Cancer Study.

A biological bank has been developed to extend the biochemical and molecular research base for a prospective study on diet and cancer in the city of Malmo, Sweden. The study entered individuals 45-69 years of age, of which 30,382 individuals (45%) participated. Each individual entering the bank has stored samples of viable mononuclear leukocytes (MNLs; -140 degrees C) and granulocytes (GRANs; -80 degrees C) or buffy coats (-140 degrees C), erythrocytes (-80 degrees C), and plasma/serum (-80 degrees C). The bioassays developed to monitor the quality of storage conditions were: (a) viability and growth response to phytohemagglutinin for MNLs; (b) DNA strand breakage for GRANs; (c) NAD content for erythrocytes; and (d) thiol status for plasma/serum. The yield, purity, and storage conditions were all quality controlled, and the samples were determined to be of high standard after 137-190 weeks of storage. No differences in yield and purity were found in samples banked by different laboratory technicians. Growth responses of MNLs were severely reduced (90%) after 40 weeks of storage, which justified switching from the storage of purified MNLs and GRANs to the more cost-effective banking of buffy coats. We conclude that the quality of the banked material, based on the biochemical analysis done, indicate that the storage conditions are optimal at least up to 3.5 years, except for the growth response of MNLs.

Feasibility and quality of biological banking of human normal and tumor tissue specimens as sources of DNA for the Malmö Diet and Cancer Study.

Human tumor and normal tissue specimens, which were collected from autopsy material 1-6 days postmortem, were compared with similar tissue specimens collected within 2 h after surgical resection and transport to the pathology department. The end point criteria used to evaluate the quality of the specimens for biological banking purposes were the extractability and yield of high molecular weight DNA and UV absorption ratios at 260:280 after collection and immediate storage of the specimens at -80 degrees C. The data demonstrated that autopsy material was a quality source of DNA, although of not such high quality as surgical biopsy specimens <2 h after resection. The advantages of using autopsy material to supplement surgical specimen collection sent to pathology, as opposed to using specimen collection at surgery wards or formalin-fixed material, as sources of DNA are: (a) large amounts of tumor and normal tissues from a variety of organ sites can be obtained without regard to the patient's health status; (b) a higher percentage of retrieval of incident cases of cancer in prospective designed trials is more likely to be achieved; and (c) the extractable DNA is of sufficiently high enough quality to permit direct analyses by molecular hybridization and sequence methodologies.

Elevation of ADP-ribosylation as an indicator of mononuclear leucocyte responsiveness in breast cancer patients treated with tamoxifen.

82 women who had had surgery for removal of breast cancer were randomised during the primary care period before initiation of any chemotherapy or radiotherapy into two groups: no drug treatment (n = 40) and 20 mg tamoxifen per day for 2 years (n = 42). Mononuclear leucocyte (MNL) fractions from blood samples were collected during the first 368 days of the study and ADP-ribosylation was quantified. Tamoxifen treatment resulted in a dose-duration increase in ADP-ribosylation. This was true even after adjustment for covariates such as age, smoking habits, oestrogen use, menstruation and tumour size. These data suggest that part of the antitumour effects of tamoxifen treatment in vivo relates to an enhanced immune cell responsiveness, as indicated by the increased MNL ADP-ribosylation.

A phase I/II evaluation of metoclopramide as a radiosensitiser in patients with inoperable squamous cell carcinoma of the lung.

The feasibility of administering metoclopramide (MCA) as a radiosensitizer has been evaluated in 23 patients with a pathological or cytological diagnosis of a squamous cell carcinoma of the lung, clinically evaluated as inoperable. All patients received 40-60 Gy radiotherapy fractionated into 1.8 Gy fractions 5 times per week (Monday-Friday). Two MCA treatment regimens were used: (i) MCA at 2 mg/kg administered by intravenous-infusion 1-2 h prior to radiotherapy 3 times per week (Monday, Wednesday, Friday); and (ii) MCA at 1 mg/kg administered by intravenous infusion 1-2 h prior to radiotherapy 5 times per week (Monday-Friday). 11 of the 23 patients treated with radiotherapy and MCA had none to mild pneumonitis or fibrosis and another 8 of the 23 had moderate levels. No patient had their therapy interrupted due to radiation-related side-effects. The MCA-related side-effects were as expected, i.e. 78% of the patients experienced sedation/tiredness and 48% expressed restlessness/anxiety symptoms. Both the total dose and serum levels of MCA were significantly associated to the MCA side-effect profile. Tumour response, duration of tumour response and survival were significantly positively correlated to the total and weekly doses of MCA administered to the patients during their radiotherapy treatment. These favourable phase II data have justified the initiation of a phase II/III randomised multicentred trial being carried out in Europe to evaluate MCA as a radiosensitiser.

Progress in identifying clinical relevance of inhibition, stimulation and measurements of poly ADP-ribosylation.

Our laboratory, in collaboration with Oxigene Inc, has been involved in identifying commercially feasible clinical applications of measurement or modulation of ADP-ribosylation as a core technology. For this purpose a pivotal regulatory role for ADP-ribosylation in the repair of DNA lesions leading to cytotoxic as well as mutagenic events has been hypothesized. A new class of DNA repair inhibitors, the N-substituted benzamides, has been identified which can react with radiation to produce reactive intermediates that oxidize thiol amino acids. Their proposed mechanisms of action are two-fold: ie they can interact with radiation: i) to directly enhance DNA damage; and ii) to react with thiols in the zinc finger DNA binding domain of poly ADP-ribosyl transferase to inhibit DNA repair and thereby increase DNA damage. Sensamide, a clinically relevant formulation of metoclopramide which is an N-substituted benzamide, has indicated enhancement of tumor response and survival in patients with inoperable squamous cell carcinoma of the lung when it was administered as a radiosensitizer in a phase I/II trial and compared to historical controls. A mechanism of endogenous regulation of human mononuclear leucocyte ADP-ribosylation has been identified to be HOCl/N-chloramine production via the oxidative burst of phagocytes. HOCl/N-chloramines are potent oxidants of thiol-containing proteins. Quantitative estimation of N-chloramine sensitive plasma thiols has been identified as an effective surrogate measure of leucocyte poly ADPRT.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of smoking, hypertension and cardiovascular disease on the estimation of unscheduled DNA synthesis and covalent binding to DNA induced by N-acetoxy-2-acetylaminofluorene in resting human mononuclear leukocytes.

The levels of N-acetoxy-2-acetylaminofluorene (NA-AAF)-induced unscheduled DNA synthesis (UDS) and of NA-AAF binding to DNA have been determined in resting mononuclear leukocytes from individuals with various smoking habits, heart infarct patients and subjects diagnosed for hypertension. Age-matched and blood-pressure-controlled smokers (n = 99) had significantly elevated levels of NA-AAF-induced UDS and NA-AAF binding to DNA when compared to nonsmokers (n = 75) similarly corrected for age and blood pressure. Heart infarct patients without any history of risk factors, as well as diagnosed hypertensives with normalized blood pressure, were not significantly different from matched controls when assessed by the NA-AAF method. Our results support the theory that increased mutagen sensitivity is associated with smoking and high blood pressure but not with cardiovascular disease itself via some mechanism of genetic selection.

3-Fluoro-1-hydroxypropan-2-one (fluorohydroxyacetone) and some esters. Syntheses and effects in BDF mice.

1-(Benzoyloxy), 1-(4-nitrobenzoyloxy), and 1-(3,5-dinitrobenzoyloxy) derivatives of 3-fluoro-, 3-chloro-, and 3-bromopropan-2-one were prepared by oxidation of the 1-benzoyloxy-3-halopropan-2-ols in turn prepared from the appropriate benzoyl chloride and 3-halo-1,2-propanediols, 1-Benzoyloxy-3-fluoropropan-2-one was allowed to react with acidic trimethyl orthoformate to yeild 1-benzoyloxy-2,2-dimethoxy-3-fluoropropane which upon basic hydrolysis afforded 2, 2-dimethoxy-3-fluoropropan-1-ol (fluorohydroxyacetone dimethyl ketal). This was deketalized with aqueous HCL to afford 3-fluoro-1-hydroxypropan-2-one (fluorohydroxyacetone), the title compound. By reacting 1-chloro-3-fluoropropan-2-one and 1, 3-dichloropropan-2-one with potassium acetate, 1-acetoxy-3-fluoropropan-2-one and 1-acetoxy-3-chloropropan-2-one (fluoro- and chlorohydroxyacetone acetate, respectively) were obtained. Similarly, sodium benzoate and 1-chloropropan-2-one produced 1-benzoyloxypropan-2-one. Stucture-activity relationships are discussed which relate chemical structure, alkylating ability, toxicity, and antitumor effects. Comparative toxicities in mice showed decreasing toxicity, on a molar basis, in the 1-benzoyloxy-3-halopropan-2-one series of bromo greater than fluoro greater than chloro. Ketones were much more toxic than the corresponding alcohols. In general the phosphate and benzoyloxy derivatives are more toxic than acetoxy compounds, with nitro-substituted benzoyloxy derivatives being much less toxic.

Double stranded RNA in heterogeneous nuclear RNA from normal and chronic lymphocytic leukemic lymphocytes.

Tritium labelled heterogeneous nuclear RNA (HnRNA) from normal and chronic lymphocytic leukemic (CLL) lymphocytes was investigated before and after fractionation into non-poly(A) containing (-HnRNA) and poly(A) containing (+HnRNA) HnRNA with respect to double stranded RNA (dsRNA). Statistically significant higher amounts of rapidly labelled RNA were recovered from CLL lymphocytes when compared to normal cases. Within the CLL cases a significant linear correlation (r = 0.95) was found between white blood cell counts and the amount of dsRNA in total HnRNA. After fractionation into (-) and (+) HnRNAs the ratios of dsRNAs, expressed as the dsRNA in (-) HnRNA divided by the dsRNA in (+) HnRNA, was lower than the corresponding values in normal cases for all the CLL cases except one. The relationship between (+) HnRNA and the total dsRNA level was different when comparing CLL and normal lymphocytes indicating a RNA processing abnormality.

Carcinogen-induced repair and binding in the DNA of chronic lymphocytic leukemic lymphocytes.

Unscheduled DNA synthesis (repair) of carcinogen induced damage of the DNA of lymphocytes from 16 normal and 16 chronic lymphocytic leukemic (CLL) subjects was determined quantitatively for a standard dose of 10 micronM N-acetoxy-2-acetylaminofluorene (NA-AAF). Essentially all the CLL cases (15 of 16) had lower NA-AAF induced repair synthesis values than the normal subjects. Concurrent measurements for the levels of 3H-labeled 7,12-dimethyl-benz(a)anthracene (DMBA) to the DNAs of the normal and CLL lymphocytes after 18 h of culturing in 5 micronM DMBA have shown that 14 of 16 CLL cases had reduced levels of DNA bound carcinogen when compared to the normal individuals. Together these results suggest that CLL lymphocytes have a reduced repair synthesis because there is disproportionately less initial carcinogen-induced damage, and thereby, less substrate stimulation of repair enzymatic activity.

Comparison of low dose nicotinamide versus benzamide, administered per os, as radiosensitizers in a C3H mammary carcinoma.

We have evaluated if any differences in tumor radiosensitization exist between the two adenosine diphosphate ribosyl transferase (ADPRT) inhibitors nicotinamide and benzamide at fractionated low doses. A significant radiosensitizing effect with nicotinamide at a 10 mg/kg per day dose was found in the tumor model used. We found, however, no radiosensitizing effect with benzamide given according to this schedule.

Nicotinamide as a radiosensitizer of a C3H mouse mammary adenocarcinoma.

Inhibitors of the chromosomal enzyme ADP-ribosyl transferase, like nicotinamide, have been shown to inhibit DNA strand rejoining and also to potentiate the lethality of DNA damaging agents in vitro. We have examined the radiosensitizing potential of nicotinamide in vivo by using transplanted C3H mouse mammary adenocarcinomas as our model system. Our data indicate a radiosensitizing effect for nicotinamide.