An N-nitrosating metalloenzyme constructs the pharmacophore of streptozotocin.

Small molecules containing the N-nitroso group, such as the bacterial natural product streptozotocin, are prominent carcinogens1,2 and important cancer chemotherapeutics3,4. Despite the considerable importance of this functional group to human health, enzymes dedicated to the assembly of the N-nitroso unit have not been identified. Here we show that SznF, a metalloenzyme from the biosynthesis of streptozotocin, catalyses an oxidative rearrangement of the guanidine group of Nω-methyl-L-arginine to generate an N-nitrosourea product. Structural characterization and mutagenesis of SznF reveal two separate active sites that promote distinct steps in this transformation using different iron-containing metallocofactors. This biosynthetic reaction, which has little precedent in enzymology or organic synthesis, expands the catalytic capabilities of non-haem-iron-dependent enzymes to include N-N bond formation. We find that biosynthetic gene clusters that encode SznF homologues are widely distributed among bacteria-including environmental organisms, plant symbionts and human pathogens-which suggests an unexpectedly diverse and uncharacterized microbial reservoir of bioactive N-nitroso metabolites.

Mechanism of AGT-Mediated Repair of dG-dC Cross-Links in the Drug Resistance to Chloroethylnitrosoureas: Molecular Docking, MD Simulation, and ONIOM (QM/MM) Investigation.

Chloroethylnitrosoureas (CENUs) are important chemotherapies applied in the treatment of cancer. They exert anticancer activity by inducing DNA interstrand cross-links (ICLs) via the formation of two O6-alkylguanine intermediates, O6-chloroethylguanine (O6-ClEtG) and N1,O6-ethanoguanine (N1,O6-EtG). However, O6-alkylguanine-DNA alkyltransferase (AGT), a DNA-repair enzyme, can restore the O6-alkylguanine damages and thereby obstruct the formation of ICLs (dG-dC cross-link). In this study, the inhibitory mechanism of ICL formation was investigated to elucidate the drug resistance of CENUs mediated by AGT in detail. Based on the structures of the substrate-enzyme complexes obtained from docking and MD simulations, two ONIOM (QM/MM) models with different sizes of the QM region were constructed. The model with a larger QM region, which included the substrate (O6-ClEtG or N1,O6-EtG), a water molecule, and five residues (Tyr114, Cys145, His146, Lys165, and Glu172) in the active pocket of AGT, accurately described the repairing reaction and generated the results coinciding with the experimental outcomes. The repair process consists of two sequential steps: hydrogen transfer to form a thiolate anion on Cys145 and alkyl transfer from the O6 site of guanine (the rate-limiting step). The repair of N1,O6-EtG was more favorable than that of O6-ClEtG from both kinetics and thermodynamics aspects. Moreover, the comparison of the repairing process with the formation of dG-dC cross-link and the inhibition of AGT by O6-benzylguanine (O6-BG) showed that the presence of AGT could effectively interrupt the formation of ICLs leading to drug resistance, and the inhibition of AGT by O6-BG that was energetically more favorable than the repair of O6-ClEtG could not prevent the repair of N1,O6-EtG. Therefore, it is necessary to completely eliminate AGT activity before CENUs medication to enhance the chemotherapeutic effectiveness. This work provides reasonable explanations for the supposed mechanism of AGT-mediated drug resistance of CENUs and will assist in the development of novel CENU chemotherapies and their medication strategies.

N-Nitrosation Mechanism Catalyzed by Non-heme Iron-Containing Enzyme SznF Involving Intramolecular Oxidative Rearrangement.

The non-heme iron-dependent enzyme SznF catalyzes a critical N-nitrosation step during the N-nitrosourea pharmacophore biosynthesis in streptozotocin. The intramolecular oxidative rearrangement process is known to proceed at the FeII-containing active site in the cupin domain of SznF, but its mechanism has not been elucidated to date. In this study, based on the density functional theory calculations, a unique mechanism was proposed for the N-nitrosation reaction catalyzed by SznF in which a four-electron oxidation process is accomplished through a series of complicated electron transferring between the iron center and substrate to bypass the high-valent FeIV═O species. In the catalytic reaction pathway, the O2 binds to the iron center and attacks on the substrate to form the peroxo bridge intermediate by obtaining two electrons from the substrate exclusively. Then, instead of cleaving the peroxo bridge, the Cε-Nω bond of the substrate is homolytically cleaved first to form a carbocation intermediate, which polarizes the peroxo bridge and promotes its heterolysis. After O-O bond cleavage, the following reaction steps proceed effortlessly so that the N-nitrosation is accomplished without NO exchange among reaction species.

A Peroxodiiron(III/III) Intermediate Mediating Both N-Hydroxylation Steps in Biosynthesis of the N-Nitrosourea Pharmacophore of Streptozotocin by the Multi-domain Metalloenzyme SznF.

The alkylating warhead of the pancreatic cancer drug streptozotocin (SZN) contains an N-nitrosourea moiety constructed from Nω-methyl-l-arginine (l-NMA) by the multi-domain metalloenzyme SznF. The enzyme's central heme-oxygenase-like (HO-like) domain sequentially hydroxylates Nδ and Nω' of l-NMA. Its C-terminal cupin domain then rearranges the triply modified arginine to Nδ-hydroxy-Nω-methyl-Nω-nitroso-l-citrulline, the proposed donor of the functional pharmacophore. Here we show that the HO-like domain of SznF can bind Fe(II) and use it to capture O2, forming a peroxo-Fe2(III/III) intermediate. This intermediate has absorption- and Mössbauer-spectroscopic features similar to those of complexes previously trapped in ferritin-like diiron oxidases and oxygenases (FDOs) and, more recently, the HO-like fatty acid oxidase UndA. The SznF peroxo-Fe2(III/III) complex is an intermediate in both hydroxylation steps, as shown by the concentration-dependent acceleration of its decay upon exposure to either l-NMA or Nδ-hydroxy-Nω-methyl-l-Arg (l-HMA). The Fe2(III/III) cluster produced upon decay of the intermediate has a small Mössbauer quadrupole splitting parameter, implying that, unlike the corresponding product states of many FDOs, it lacks an oxo-bridge. The subsequent decomposition of the product cluster to one or more paramagnetic Fe(III) species over several hours explains why SznF was previously purified and crystallographically characterized without its cofactor. Programmed instability of the oxidized form of the cofactor appears to be a unifying characteristic of the emerging superfamily of HO-like diiron oxidases and oxygenases (HDOs).

High-throughput detection of craniofacial defects in fluorescent zebrafish.

Losses and malformations of cranial neural crest cell (cNCC) derivatives are a hallmark of several common brain and face malformations. Nevertheless, the etiology of these cNCC defects remains unknown for many cases, suggesting a complex basis involving interactions between genetic and/or environmental factors. However, the sheer number of possible factors (thousands of genes and hundreds of thousands of toxicants) has hindered identification of specific interactions. Here, we develop a high-throughput analysis that will enable faster identification of multifactorial interactions in the genesis of craniofacial defects. Zebrafish embryos expressing a fluorescent marker of cNCCs (fli1:EGFP) were exposed to a pathway inhibitor standard or environmental toxicant, and resulting changes in fluorescence were measured in high-throughput using a fluorescent microplate reader to approximate cNCC losses. Embryos exposed to the environmental Hedgehog pathway inhibitor piperonyl butoxide (PBO), a Hedgehog pathway inhibitor standard, or alcohol (ethanol) exhibited reduced fli1:EGFP fluorescence at one day post fertilization, which corresponded with craniofacial defects at five days post fertilization. Combining PBO and alcohol in a co-exposure paradigm synergistically reduced fluorescence, demonstrating a multifactorial interaction. Using pathway reporter transgenics, we show that the plate reader assay is sensitive at detecting alterations in Hedgehog signaling, a critical regulator of craniofacial development. We go on to demonstrate that this technique readily detects defects in other important cell types, namely neurons. Together, these findings demonstrate this novel in vivo platform can predict developmental abnormalities and multifactorial interactions in high-throughput.

Bujing Yishi tablets alleviate photoreceptor cells death via the P2X7R/CX3CL1/CX3CR1 pathway in Retinitis Pigmentosa rats.

BACKGROUND: Retinitis pigmentosa (RP) refers to a group of progressive photoreceptor degenerative diseases. The activation of microglia has been reported to play an important role in the photoreceptor degeneration in RP retinal. Bujing Yishi tablets (BJYS), a Chinese herbal medicine, has been used to treat retinal diseases in China with desirable effect in improving visual function. However, the mechanisms underlying the efficacy of BJYS treatment in RP are not yet fully understood. PURPOSE: Based on the preliminary experiments, this study aimed to investigate the therapeutic mechanism involved in treating N-Methyl-N-Nitrosourea (MNU)-induced retinal degeneration of RP with BJYS. METHODS: To explore the efficacy of BJYS, a rat experimental RP model was established through intraperitoneal injection of MNU (50 mg/kg). Two experiment was carried out. After the treatment, we conducted H&E, TUNEL, retinal cytokine levels and IBA-1 expression in microglia to confirm the impact on RP model. The specific mechanism of action of BJYS tablet was assessed by western blot, real-time polymerase chain reaction (RT-PCR), and immunofluorescence to determine the mRNA and protein expression levels involved in clarifying the effectiveness of BJYS exerted through P2X7R/CX3CL1/CX3CR1 pathway. RESULTS: Significant alleviation of retinal morphological structure and photoreceptor degeneration by BJYS treatment was observed in the retinal of MNU-induced RP rats, BJYS prevented the reduction of ONL thickness and decreased the level of apoptotic cells in ONL. It also inhibited microglia overactivation and reduced retinal levels of IL-1ß, IL-6, TNF-α. In addition, BJYS decreased the protein expression and mRNA expression of P2X7, CX3CL1 and CX3CR1 and reduced the phosphorylation of p38 MAPK. CONCLUSION: In summary, this study suggested that BJYS treatment could alleviate photoreceptors degeneration of RP by inhibiting microglia overactivation and inflammation through the P2X7R/CX3CL1/CX3CR1 pathway. These effects suggest that BJYS tablets may serve as a promising oral therapeutic agent for RP.

Proline-linked nitrosoureas as prolidase-convertible prodrugs in human breast cancer cells.

A number of novel proline-linked nitrosoureas (1-4) were synthesized and examined for cytotoxicity and influence on DNA and collagen biosynthesis in MDA-MB-231 and MCF-7 human breast cancer cells. Evaluation of the cytotoxicity of these compounds employing a MTT assay and inhibition of [(3)H]thymidine incorporation into DNA in both MDA-MB-231 and MCF-7 breast cancer cells demonstrated that compound 2, the most active of the series, proved to be only slightly less potent than carmustine. It has also been found that carmustine did not inhibit MCF&-7 cells prolidase activity, while compounds 1-4 significantly increased its activity, when used at 50-250 microM concentrations. Proline-linked nitrosoureas (1-4) also had lower ability to inhibit collagen biosynthesis in MCF-7 cells, compared to carmustine. The expression of beta(1)-integrin receptor and phosphorylated MAPK, ERK(1) and ERK(2) was significantly decreased in MCF-7 cells incubated for 24 h with 60 microM of compounds 2 and 4 compared to the control, untreated cells, whereas under the same conditions carmustine did not evoke any changes in expression of all these signaling proteins, as shown by Western immunoblot analysis. These results indicate the proline-linked nitrosoureas (1-4), represent multifunctional inhibitors of breast cancer cell growth and metabolism.

Differences in the removal of N-methyl-N-nitrosourea-methylated products in DNase I-sensitive and -resistant regions of rat brain DNA.

The present study with N-methyl-N-nitrosourea and rat brain DNA was performed in order to study the distribution of alkylated products and the difference in the removal of these products from DNase I-sensitive and -resistant regions of DNA. Nuclei were isolated from N-[3H]methyl-N-nitrosourea-treated rats and incubated in the presence of DNase I (5 microgram/ml). Digested DNA was further hydrolyzed in 0.1 N HCl, and the alkylated products were identified by chromatography on a cation-exchange column. Resistant DNA was isolated, hydrolyzed, and again the alkylated products were determined. At 4 hr, the specific activity of all the alkylated products in the sensitive regions of DNA was several times higher than the resistant fraction. The rate of loss for the products was greater in the sensitive than the resistant fractions. O6-Methylguanine was removed from the sensitive regions but was more stable in the resistant regions. These results suggest that DNase I-sensitive regions of the DNA are preferentially alkylated by N-methyl-N-nitrosourea and that the alkylated products, including O6-methylguanine, are selectively removed from the DNase I-sensitive regions of the DNA.

Urinary excretion of O6-butylguanine after the administration of N-nitroso-N-butylurea in rats.

O6-Butylguanine was detected in the urine of rats given the butylating agent N-nitroso-N-butylurea. O6-Butylguanine contents in the 24-h rat urine samples after i.p. doses of 50, 100, and 200 mg/kg N-nitroso-N-butylurea were 1.03 +/- 0.41 (SE), 8.30 +/- 1.70, and 59.53 +/- 6.52 pmol, respectively. This suggests that O6-butylguanine formation in nucleic acids might be repaired in vivo, possibly by base excision, besides other mechanisms. After i.v. doses of 0.1 and 1 mg/kg of O6-butylguanine to rats urinary excretion did not exceed 2% of the administered dose, suggesting that the amount of O6-butylguanine effectively released by base excision might be much larger than that detected in the urine after N-nitroso-N-butylurea. Inhibition of the enzyme O6-alkyl-DNA transferase by N-nitrosodimethylamine increased urinary O6-butylguanine resulting from exposure to N-nitroso-N-butylurea (100 mg/kg i.p.) up to four times, thus confirming an alternative DNA repair mechanism.

Synthesis and identification of products derived from the metabolism of the carcinostatic 1-(2-chloroethyl)-3-(trans-4-methylcyclohexyl)-1-nitrosourea by rat liver microsomes.

Liver microsomal metabolism of 1-(2-chloroethyl)-3-(trans-4-methylcyclohexyl)-1-nitrosourea in the presence of reduced nicotinamide adenine dinucleotide phosphate and O2 was shown to produce seven metabolites that included the parent urea. A cytochrome P-450-dependent monohydroxylation of the cyclohexyl ring occurred in 3 positions, cis-3, trans-3, and cis-4, and on the methyl group to form a trans-4-hydroxymethyl derivative. In addition, monohydroxylation of the 2-chloroethyl carbon attached to the N-1 urea nitrogen yielded an alpha-hydroxy metabolite. A ring-hydroxylated derivative remained unidentified while the structures of all other such derivatives were established by comparison with compound synthesized, purified by high-pressure liquid chromatography, and characterized by mass spectral and nuclear magnetic resonance analyses. It was tentatively concluded that some parent urea is formed by a cytochrome P-450 dependent reaction because of a requirement for reduced nicotinamide adenine dinucleotide phosphate and inhibition by CO. Microsomes from rats pretreated with phenobarbital showed about a 3-fold increase in hydroxylation rate while phenobarbital-treated mice microsomes were induced 8-fold. However, in both species, the induced hydroxylation rate was about 4 nmol/min/mg protein. When microsomes from phenobarbital-induced rats were used, a mixture of 80% CO:20% O2 decreased the rate of formation of all metabolites to 14% of that in 80% N2:20% O2.

Phase I trial and clinical pharmacology of 1-(2-chloroethyl)-3-(2,6-dioxo-3-piperidyl)-1-nitrosourea.

1-(2-Chloroethyl)-3-(2,6-dioxo-3-piperidyl)-1-nitrosourea (NSC 95466) is a lipid-soluble nitrosourea that is presently undergoing clinical evaluation. In this Phase I study, the toxicity of this drug was examined after administration of the drug to cancer patients on 3 successive days every 6 to 8 weeks. Clinical pharmacology was studied using 1-[chloroethyl-14C](2-chloroethyl)-3-(2,6-dioxo-3-piperidyl)-1-nitrosourea. The dose-limiting toxicity was myelosuppression. The maximal tolerated dose was 105 mg/sq m, which produced a median platelet nadir of 40,000/microliter on Day 32 and a median white blood cell count nadir of 2200/microliter on Day 42. Progressive anemia was also observed. There was no evidence of acute or chronic hepatic, renal, or pulmonary damage. One patient with a metastatic hypernephroma exhibited a partial clinical remission. Plasma disappearance of the drug following bolus administration was biphasic, with an initial half-life of 18 to 25 min and a second half-life of 9 hr. Clearance of intact drug coincided largely with the initial disappearance phase of total radioactivity. Entry of radioactivity into the cerebrospinal fluid was observed. Approximately 35% of plasma radioactivity was protein bound, the major binder being albumin. Drug excretion was predominantly renal, and biliary elimination was only minor.

Distribution and metabolism of 1-propyl-1-nitrosourea in rats.

The carcinogen 1-[14C]propyl-1-nitrosourea (PNU) was readily absorbed from the rat gut, and the radioactivity was excreted mainly in the urine and expired air. The urinary metabolites of PNU were 1-propylurea and urea. 1-Propylurea was shown to be excreted largely unchanged in the urine. Both [14C]PNU and 1-[14C]propylurea were found to be eliminated rapidly from the rat body. Besides cO2 from PNU, isopropyl alcohol was identified as a volatile metabolite in the expired air. Specific high concentrations (%/g) in main organs and tissues were not observed in adult rats 24 hr after single p.o. doses (20 mg/kg) of labeled PNU. The ureido carbon of PNU showed considerable retention in the blood, while relatively high residual levels were found in the liver for the propyl carbon. Autoradiographic studies on pregnant rats showed a uniform distribution between maternal and fetal bodies a short time after dosing. A relatively high concentration of 14C label was found in the maternal blood 24 hr after treatment with [carbonyl-14C]PNU. Localization of radioactivity in bone systems such as the fetal sterna and vertebrae was noted 6 hr after treatment with [propyl-1-14C]PNU. Metabolic pathways of PNU in the rat are proposed.

Distribution of radiolabeled 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitros ourea hydrochloride in rat brain tumor: intraarterial versus intravenous administration.

To assess the rationale of intraarterial (i.a.) 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea chemotherapy, distribution of 14C-labeled 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)- 3-nitrosourea in rat glioma was studied after i.a. or i.v. infusion. Immediately after infusion, the tumor located in the hemisphere of intracarotid infusion received 4.6-fold higher radioactivity than the tumor located contralaterally to intracarotid infusion and 2.8-fold higher radioactivity than i.v. infusion. The difference was kept up to 30 min after i.a. infusion. Autoradiographic observation indicated rather uniform distribution of the tracer in the central portion of i.a. infusion. However, in the periphery of i.a. infusion, distribution of the tracer was nonhomogenous. The results indicate that i.a. 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea chemotherapy is useful when the tumor has high blood flow and is located in the center of an infused area.

Quenching of DNA cross-link precursors of chloroethylnitrosoureas and attenuation of DNA interstrand cross-linking by glutathione.

Interstrand DNA cross-linking is essential for the antitumor activity of chloroethylnitrosoureas (CENUs). The critical cross-links have been proposed to involve a rapid O6-guanine chloroethylation on one DNA strand, followed by a rearrangement of the O6-(2-chloroethyl)guanine and slow alkylation of the second DNA strand. In view of the relative intracellular abundance of glutathione (GSH) and nucleophilicity of its thiolate ion, the ability of GSH to react with and to inactivate 2-chlorethylated DNA and the possibility that this interaction decreases net DNA cross-linking by CENUs were investigated. Chloroethylated calf thymus DNA was reacted with GSH, the DNA was precipitated and redissolved, and subsequent DNA interstrand cross-linking was determined. The DNA cross-link index was compared for both GSH-treated and 2-chloroethylated untreated DNA. Simultaneously, Col E1 plasmid DNA was chloroethylated and reacted with GSH, and the extent of DNA interstrand cross-linking was determined by agarose gel electrophoresis and compared with controls. The results show both a time- and GSH concentration-dependent quenching of chloroethylated DNA, with a corresponding decrease in the DNA cross-link index. Using [methyl-3H]GSH, it was also demonstrated that 56% of the total GSH was bound to quenched 2-chloroethylated Col E1 DNA and 25% to quenched 2-chloroethylated calf thymus DNA. GSH binding to cross-linked DNA and native DNA was insignificant. It is concluded that, in addition to direct inactivation of reactive cytotoxic CENU species, GSH may also modulate cellular response to CENUs by quenching chloroethylated DNA, thereby decreasing the formation of potentially lethal DNA cross-links.

Pharmacological disposition of 1-(2-chloroethyl)-3-(2,6-dioxo-3-piperidyl)-1-nitrosourea in mice.

1-(2-Chloroethyl)-3-(2,6-dioxo-3-piperidyl)-1-nitrosourea (PCNU; NSC 95466) is a lipid-soluble nitrosourea that is presently in clinical trial. We have studied the pharmacological disposition of [ethyl-14C]PCNU in mice using an i.v. drug dose of 20 mg/kg/animal. Disappearance of total radioactivity from plasma was biphasic with mean half-lives of the two exponential phases of 21.7 min and 27.4 hr, respectively. The plasma half-life of intact drug was 29 min, and levels of intact drug, as measured by thin-layer chromatography, fell below detectable levels by 4 hr. The area under the plasma concentration-time curve for intact drug was 32.72 nmol X hr/ml. Computer analysis of the data for total radioactivity (PCNU equivalents), based upon an open two-compartment model, yielded values of the pharmacokinetic parameters K12, K21, and K10 of 1.49 hr-1, 0.25 hr-1, and 0.19 hr-1, respectively. The highest peak organ level of drug was 168.9 nmol of PCNU equivalents per g tissue in the liver 1 hr after drug administration. Maximum levels in kidney, lungs, heart, and spleen were observed at 5 min, with values of 119.5, 115.4, 80.3, and 66.7 nmol of PCNU equivalents per g of tissue, respectively. A high peak drug level in brain (50.6 nmol/g) agreed with the prediction that PCNU can cross the blood-brain barrier. The levels of intact drug relative to total radioactivity at 30 min were 60% in brain, 55% in heart, and 48% in spleen. The concurrent value in liver was 7% of the total radioactivity, suggesting that metabolism or decomposition of PCNU occurs in this organ. The principal excretory route of [ethyl-14C]PCNU was urinary, with a cumulative excretion of 62% in the first 24 hr.

Influence of hydrocortisone on the binding of nitrosoureas to nuclear chromatin subfractions.

The effects of steroid-induced modifications of chromatin structure on the extent and sites of chloroethylnitrosourea binding to chromatin were studied using log-phase HeLa cells. The cells were exposed to 0.1 to 2.0 microM hydrocortisone for 22 hr; this resulted in depressed DNA synthesis while transcriptional activity was stimulated. Hydrocortisone had no effect upon cellular or nuclear uptake of the two nitrosoureas under study, 0.6 mM chlorozotocin or 1-(2-chloroethyl-3-cyclohexyl-1-nitrosourea). Both drugs were found to alkylate transcriptional chromatin preferentially, as demonstrated by DNase II and DNase I digestion. This alkylation was stimulated 2-fold by the same micromolar concentrations of hydrocortisone, 0.1 to 2.0 microM, which stimulated transcription. The extent of nuclear RNA alkylation, determined using RNase T2 as a probe, was found to contribute less than 20% of total chromatin alkylation and was unaffected by steroid pretreatment. Instead, the increased alkylation within these chromatin subfractions was attributed to a steroid-induced alteration of chromatin structure. Electron microscopic examination of HeLa nuclear morphology revealed a hydrocortisone-induced disaggregation of nuclear membrane-associated heterochromatin resulting in a more heterogeneous, less condensed distribution of chromatin. Such data are consistent with a relaxation of the supercoiled chromatin structure, resulting in increased transcription and increased accessibility of potential target sites for nitrosourea alkylation.

Microsomal metabolism of nitrosoureas.

N, N-Bis (2-chloroethyl)-N-nitrosourea (BCNU) is a substrate for a microsomal enzyme of mouse liver. The reaction requires NADPH, and the product is 1, 3-bis (2-chloroethyl) urea. This activity is also found in mouse lungs but not in several other tissues. With reaction conditions under which BCNU is not chemically degraded, the Km for BCNU with liver microsomes is 1.7 mM; nicotine is a competitive inhibitor with a Ki of 0.6 mM. N-Methyl-N-nitrosourea is denitrosated in a similar reaction. N- (2-Chloroetyhy)- N-cyclohexyl-N-nitrosourea and N-(2-chloroethyl)-N-(trans-4-methylcyclohexyl)-N-nitrosourea are also substrates for microsomal enzymes, but the products of these reactions are ring-hydroxylated derivatives. The Km value for N-(2-CHLOROETHYL)-N-cyclohexyl-N-nitrosourea is 3.0 mM and that for N-(2-chloroethyl)-N-(trans-4-methylcyclohexyl)-N-nitrosourea is 1.0 mM. The hydroxylase activity is also present in lungs, but not in the other mouse tissues. The rates of microsomal metabolism of BCNU, N-(2-chloroethyl)-N-cyclohexyl-N-nitrosourea, and N-(2-chloroethyl)-N-cyclohexyl-N-nitrosourea, and N-(2-chloroethyl-N-(trans-4-methylcyclohexyl)-N-nitrosourea are fast enough to allow metabolism of large portions of administered doses before chemical decomposition of the drugs occurs.

Comparison of mutagenicity, antitumor activity, and chemical properties of selected nitrosoureas and nitrosoamides.

A number of nitrosoureas and nitrosoamides have been compared with respect to mutagenicity for Salmonella typhimurium, in vitro cytotoxicity, in vivo toxicity and antitumor activity against murine L1210 leukemia, and chemical properties. Despite chemical similarities between the nitrosoureas and nitrosoamides, they show important differences in biological activity. Some of the nitrosoureas are very active antitumor agents, and they are less mutagenic than are the corresponding nitrosoamides, which lack antitumor activity.

Mechanism of uptake of nitrosoureas by L5178Y lymphoblasts in vitro.

The mechanism of uptake of nitrosoureas by L5178Y cells in vitro was investigated. A time course of the uptake of radioactivity on incubation of L5178Y lymphoblast with [14C]-1,3-bis(2-chloroethyl)-1-nitrosourea was linear for 30 min and then entered a plateau phase; it was markedly temperature dependent. A similar time course for cells incubated with [14C]ethylene-labeled 1-(2-chlorethyl)-3-cyclohexyl-1-nitrosourea reached equilibrium rapidly, was temperature independent, and resulted in a relatively low level of uptake of radioactivity. However, cells treated with 3-[cyclohexyl-14C]-1-(2-chlorethyl)-1-nitrosourea had a time course that was linear for 30 min, resulted in much higher levels of uptake of radioactivity, and was strongly temperature dependent. These findings, at least for 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea, suggest that some drug decomposition precedes uptake. The percentage of radioactivity found in the cell sap fraction was at least 85% of total cell activity when cells were incubated with any of the three 14C-labeled nitrosoureas. Furthermore, thin-layer chromatography of the cell sap fraction revealed the presence of free intact drug. These findings indicate that intracellular uptake of intact nitrosoureas occurred. A time course of uptake of intact 1,3-bis(2-chloroethyl)-1-nitrosourea reached equilibrium rapidly with cell/medium distribution ratios of 0.2 to 0.6 and was temperature independent. The addition of excess unlabeled 1,3-bis(2-chlorethyl)-1-nitrosourea or 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea had no effect on uptake of [14C]-1,3-bis(2-chloroethyl)-1-nitrosourea, These findings suggest that uptake of intact 1,3-bis(2-chloroethyl)-1-nitrosourea was by passive diffusion. A time course of the uptake of intact 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea with either [14C]ethylene- or ring-labeled drug rapidly reached equilibrium, was temperature independent, and attained a cell/medium ratio greater than unity. Uptake of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea was sodium independent and was unaffected by the metabolic inhibitors (sodium fluoride, sodium cyanide, or 2,4-dinitrophenol) or by urea, a potential physiological competitor. Furthermore, addition of unlabeled 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea or 1,3-bis(2-chlorethyl)-1-nitrosourea had no effect on uptake of labeled 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea. These findings suggest that uptake of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea also occurs by passive diffusion.

Influence of the estrous cycle during carcinogen exposure on nitrosomethylurea-induced rat mammary carcinoma.

The influence of individual stages of the rat estrous cycle during exposure to I-nitroso-I-methylurea (NMU) on mammary tumor incidence, latency, number, and cytosol receptor dynamics for estrogen and progesterone was determined. Virgin female Buffalo rats were separated into three groups on Day 53 according to their vaginal smear pattern. NMU (5 mg/100 g body weight, i.v.) was administered in three monthly doses beginning at 53 to 55 days of age on diestrus, proestrus, or estrus between 9:00 and 11:00 a.m. Groups of rats had their second and third injections of NMU on the same day of the estrous cycle as their initial injection. All animals were killed during the morning on a diestrus day. Receptors for estrogen and progesterone were determined by a modified dextran-coated charcoal method and by sucrose density gradient analysis. Mean latencies to first tumor appearance in diestrus, proestrus, and estrus groups were 104.4, 83.6, and 91.4 days, respectively (p less than 0.05, diestrus versus estrus and proestrus) following the first NMU injection. The mean number of tumors per rat was significantly higher in rats injected on proestrus (4.5) or estrus (4.3) than on diestrus (2.0). Estradiol bound to receptor sedimented at 8 and 4 s and was suppressed by diethylstilbestrol and estradiol. Progesterone receptor migrated to 7.8 and 4 s regions. Estrogen receptor incidence (100%) and content (16.7 fmol/mg cytosol protein) was highest in rats injected on diestrus. In the proestrus and estrus injected groups, estrogen receptor incidence was 95 and 63% and content was 10.2 and 11.2 fmol/mg protein, respectively. The affinity of estradiol for its receptor was not significantly altered in any group. Although there were no statistically significant difference in progesterone receptor incidence or affinity between groups, progesterone receptor content (74.6 fmol/mg cytosol protein) was significantly higher in tumors from rats injected on proestrus than on diestrus. These data suggest that the prevailing hormonal milieu of the estrous cycle during NMU exposure may be critically important to the subsequent biological behavior and steroid receptor status of carcinogen-induced rat mammary tumors.