Biocompatibility of cross-linked hyaluronate (Gel-200) for the treatment of knee osteoarthritis.

OBJECTIVE: To compare the biocompatibility and immunogenicity of two intra-articular hyaluronan formulations, Gel-200 (Gel-One(®)) and hylan G-F 20 (Synvisc(®) series). EXPERIMENTAL DESIGN: A comparison of the biocompatibility of Gel-200 and hylan G-F 20 was made using a rat subcutaneous air pouch model and the knee joint of normal rabbits. Immunogenicity was evaluated using a homologous passive cutaneous anaphylaxis (PCA) assay in guinea pigs. RESULTS: In the air pouch model in rats, characteristic fibrous belts formed in the subcutaneous tissue. Injection of hylan G-F 20 into the air pouch induced granulomatous nodules primarily composed of macrophages, multinucleated giant cells, and eosinophils accompanied with the test material in the center of the nodules in the fibrous belt. Furthermore, the thickness of the fibrous belt in the hylan G-F 20 group increased significantly compared to the saline group. Injection of Gel-200 into the air pouch induced neither granulomatous inflammation nor significant thickening of fibrous belt, while foamy macrophages containing the test material were observed. Intra-articular injection of hylan G-F 20 into the rabbit knee joints induced granulomatous inflammation, eosinophil infiltration, and significant increase in the number of cells in the synovial fluid, while these findings were absent in the Gel-200 group. In the immunogenicity assay, hylan G-F 20 induced a positive PCA reaction, but the Gel-200 did not. CONCLUSION: Gel-200 showed more favorable biocompatibility and less immunogenicity compared to hylan G-F 20. Gel-200 is expected to be a single injection hyaluronan product with less safety concerns for the treatment of knee osteoarthritis (OA) pain.

Tolvaptan reduces the required amount of albumin infusion in patients with decompensated cirrhosis with uncontrolled ascites : a multicenter retrospective propensity score-matched cohort study.

Background: The aim of this retrospective study was to determine whether tolvaptan treatment reduces the amount of albumin administered, volume of ascites removed, and frequency of paracentesis procedures in patients with decompensated cirrhosis with uncontrolled ascites with conventional diuretics. Patients and methods: The control (C) group included patients treated with conventional diuretics. The tolvaptan (T) group included patients treated with both tolvaptan and conventional diuretics. Both groups were matched according to baseline parameters. The amount of albumin administered, volume of ascites removed, and frequency of paracentesis within 30 days of onset of uncontrolled ascites were compared between the two groups. Results: After matching, 74 patients (C=37, T=37) were included. Baseline parameters (C vs. T group) were as follows: age, 69.5 ± 9.3 vs. 70.4 ± 11.0 years (p = 0.702) ; males, 24 (64.9%) vs. 25 (67.6%) (p = 0.999) ; patients with hepatocellular carcinoma, 17 (45.9%) vs. 18 (48.6%) (p = 0.999) ; serum albumin levels at treatment initiation, 2.76 ± 0.48 vs. 2.73 ± 0.49 g/dL (p = 0.773), and serum creatinine levels at treatment initiation, 1.18 ± 1.23 vs. 1.09 ± 0.48 g/dL (p = 0.679). In the C vs. T groups, respectively, mean amount of albumin administered was 51.0 ± 31.4 vs. 33.4 ± 29.8 g/month (p = 0.016) ; mean volume of ascites removed was 2,905 ± 4,921 vs. 1,824 ± 3,185 mL/month (p = 0.266) ; and mean frequency of paracentesis was 0.92 ± 1.46 vs. 0.89 ± 1.45 procedures (p = 0.937). Conclusions: Tolvaptan reduced the use of albumin infusion in patients with decompensated cirrhosis and was effective and acceptable for uncontrolled ascites.

A change in the timing for starting systemic therapies for hepatocellular carcinoma: the comparison of sorafenib and lenvatinib as the first-line treatment.

Aim: The aim of this retrospective multicenter study was to evaluate the differences in the timing for starting systemic therapies as the first-line treatment for hepatocellular carcinoma (HCC). Methods: A total of 375 patients with HCC treated with sorafenib from May 2009 to March 2018 and 56 patients treated with lenvatinib from March 2018 to November 2018 at our affiliated hospitals were included in this study. Results: The median ages of the sorafenib and lenvatinib groups were 71.0 (interquartile range [IQR]: 64.0-77.0) and 73.5 (IQR: 68.0 -80.0) years old, and 300 (80.0%) and 42 (75.0%) patients were men, respectively. The Barcelona Clinic Liver Cancer stage was early, intermediate and advanced in 39 patients (10.4%), 133 patients (35.5%) and 203 patients (54.1%) in the sorafenib group and 1 patient (1.8%), 17 patients (30.4%) and 38 patients (67.9%) in the lenvatinib group, respectively. In the analysis of intermediate HCC, patients who satisfied the criteria of TACE failure/refractoriness (P=0.017), those with ALBI grade 1 (P=0.040), and those with a serum AFP level < 200 ng/ml (P=0.027) were found more frequently in the lenvatinib group than in the sorafenib group, with statistical significance. The objective response rate (ORR) of lenvatinib was 34.8% in the overall patients and 46.7% in the intermediate-stage HCC patients, which was significantly higher than sorafenib (P=0.001, P=0.017). Conclusions: The emergence of lenvatinib has encouraged physicians to start systemic chemotherapy earlier in intermediatestage HCC patients.

Na+ - and Cl- -coupled active transport of nitric oxide synthase inhibitors via amino acid transport system B(0,+).

Nitric oxide synthase (NOS) inhibitors have therapeutic potential in the management of numerous conditions in which NO overproduction plays a critical role. Identification of transport systems in the intestine that can mediate the uptake of NOS inhibitors is important to assess the oral bioavailability and therapeutic efficacy of these potential drugs. Here, we have cloned the Na+ - and Cl- -coupled amino acid transport system B(0,+) (ATB(0,+)) from the mouse colon and investigated its ability to transport NOS inhibitors. When expressed in mammalian cells, ATB(0,+) can transport a variety of zwitterionic and cationic amino acids in a Na+ - and Cl- -coupled manner. Each of the NOS inhibitors tested compete with glycine for uptake through this transport system. Furthermore, using a tritiated analog of the NOS inhibitor N(G)-nitro-L-arginine, we showed that Na+ - and Cl- -coupled transport occurs via ATB(0,+). We then studied transport of a wide variety of NOS inhibitors in Xenopus laevis oocytes expressing the cloned ATB(0,+) and found that ATB(0,+) can transport a broad range of zwitterionic or cationic NOS inhibitors. These data represent the first identification of an ion gradient-driven transport system for NOS inhibitors in the intestinal tract.

Comparison of the percutaneous absorption of hydrophilic and lipophilic compounds in shed snake skin and human skin.

The in vitro transdermal permeation of eight hydrophilic drugs (antipyrine, L-dopa, dopamine hydrochloride, diclofenac sodium, 5-fluorouracil, isoprenaline hydrochloride, nicorandil and morphine hydrochloride) and eight lipophilic drugs (aminopyrine, cyclobarbital, ibuprofen, indomethacin, isosorbide dinitrate, flurbiprofen, ketoprofen and lignocaine) was determined using shed snake skin of Elaphae obsoleta and human skin. The permeation parameters and physiological characteristics of the skin, e.g. the water and lipid content, and the thickness of shed snake skin and human skin were evaluated and compared. In shed snake skin, the permeability coefficients (P) of lipophilic drugs were in the same range as those through the human skin (0.9 to 1.8-times); whereas those of hydrophilic drugs were remarkably lower (3.3 to 6.1-times). The thickness and lipid content of shed snake skin and human stratum corneum were not significantly different (P > 0.05), whereas the water content of shed snake skin was significantly lower than that of human stratum corneum (P < 0.05). The lower permeability of shed snake skin for hydrophilic compounds might be caused by the lower porosity of skin strata. The results suggested a potential use of shed snake skin as barrier membrane for lipophilic compounds percutaneous absorption studies in vitro.

Embryonic mutation as a possible cause of in utero carcinogenesis in mice revealed by postnatal treatment with 12-O-tetradecanoylphorbol-13-acetate.

Although in utero irradiation at early stages induced a high incidence of somatic mutations at coat color genes in the embryos of a specified tester strain (PT x HT F1) of mice, it was not carcinogenic by itself. However, in utero-irradiated animals did develop skin tumors and hepatomas (but not leukemias) by the postnatal administration of 12-O-tetradecanoylphorbol-13-acetate. The incidence of both tumors and embryonic mutations increased with in utero doses of X-rays. Furthermore, a large reduction of tumor incidence, about 80%, was observed by low-dose-rate irradiation, similar to the 75% reduction in spot size found for embryonic mutations. The tumor nodule size was also dramatically reduced by low-dose-rate irradiation. Consequently, the induced incidence and size of tumors produced by 12-O-tetradecanoylphorbol-13-acetate treatment parallel those which are observed for coat color mutations as expected, because somatic mutations observed in the pigment cells must similarly occur in embryonic cells of other organs. The larger the clone of mutant cells, the greater their chance of becoming tumorigenic by 12-O-tetradecanoylphorbol-13-acetate posttreatment. These results strongly support the recent epidemiological survey showing that adult types of cancers, but not leukemias, are increasing in the atomic bomb survivors exposed in utero, since humans are continuously exposed to a variety of cancer-promoting agents in contrast to experimental animals reared without such exposures.

Expression of the bovine heart mitochondrial ADP/ATP carrier in yeast mitochondria: significantly enhanced expression by replacement of the N-terminal region of the bovine carrier by the corresponding regions of the yeast carriers.

To characterize the transport mechanism mediated by the mammalian mitochondrial ADP/ATP carrier (AAC), we tried to express bovine heart mitochondrial AAC (bhAAC) in Saccharomyces cerevisiae. The open reading frame of the bhAAC was introduced into the haploid strain WB-12, in which intrinsic AAC genes were disrupted. Growth of the transformant was very low in glycerol medium, and a little amount of bhAAC was detected in the mitochondrial membrane. For improvement of bhAAC expression in WB-12, we introduced DNA fragments encoding chimeric bhAACs, in which the N-terminal region of the bhAAC extending into the cytosol was replaced by the corresponding regions of the type 1 and type 2 yeast AAC isoforms (yAAC1 and yAAC2). These transformants grew well, and the amounts of the chimeric bhAACs in their mitochondria were as high as that of yAAC2. The carriers expressed showed essentially the same ADP transport activities as that of AAC in bovine heart mitochondria.

Primary structure, functional characteristics and tissue expression pattern of human ATA2, a subtype of amino acid transport system A.

We report here on the primary structure and functional characteristics of the protein responsible for the system A amino acid transport activity that is known to be expressed in most human tissues. This transporter, designated ATA2 for amino acid transporter A2, was cloned from the human hepatoma cell line HepG2. Human ATA2 (hATA2) consists of 506 amino acids and exhibits a high degree of homology to rat ATA2. hATA2-specific mRNA is ubiquitously expressed in human tissues. When expressed in mammalian cells, hATA2 mediates Na+-dependent transport of alpha-(methylamino)isobutyric acid, a specific model substrate for system A. The transporter is specific for neutral amino acids. It is pH-sensitive and Li+-intolerant. The Na+:amino acid stoichiometry is 1:1.

Evidence for the transport of neutral as well as cationic amino acids by ATA3, a novel and liver-specific subtype of amino acid transport system A.

We report here on the cloning and functional characterization of the third subtype of amino acid transport system A, designated ATA3 (amino acid transporter A3), from a human liver cell line. This transporter consists of 547 amino acids and is structurally related to the members of the glutamine transporter family. The human ATA3 (hATA3) exhibits 88% identity in amino acid sequence with rat ATA3. The gene coding for hATA3 contains 16 exons and is located on human chromosome 12q13. It is expressed almost exclusively in the liver. hATA3 mediates the transport of neutral amino acids including alpha-(methylamino)isobutyric acid (MeAIB), the model substrate for system A, in a Na(+)-coupled manner and the transport of cationic amino acids in a Na(+)-independent manner. The affinity of hATA3 for cationic amino acids is higher than for neutral amino acids. The transport function of hATA3 is thus similar to that of system y(+)L. The ability of hATA3 to transport cationic amino acids with high affinity is unique among the members of the glutamine transporter family. hATA1 and hATA2, the other two known members of the system A subfamily, show little affinity toward cationic amino acids. hATA3 also differs from hATA1 and hATA2 in exhibiting low affinity for MeAIB. Since liver does not express any of the previously known high-affinity cationic amino acid transporters, ATA3 is likely to provide the major route for the uptake of arginine in this tissue.

Percutaneous radiofrequency ablation for hepatocellular carcinoma located in the caudate lobe of the liver.

AIM: This study aimed to evaluate the effectiveness and safety of radiofrequency ablation (RFA) for hepatocellular carcinoma (HCC) located in the caudate lobe of the liver. PATIENTS AND METHODS: Between 2012 April and 2014 February, 142 patients with HCC meeting the Milan criteria were enrolled in this study. Of these patients, nine patients had HCC located in the caudate lobe (caudate group). Six of the nine cases were located in the Spiegel lobe, two cases were located in the paracaval portion and one case was located in the caudate process. We evaluated the local recurrence rate and RFA-related complications in the caudate group and non-caudate group. RESULTS: The local recurrence rate in the caudate group was 12.5% at 1 year and 12.5% at 2 years, while the local recurrence rate in the non-caudate group was 14.9% at 1 year and 29.0% at 2 years; there were no significant differences between the groups. No complications were observed in the caudate group, and minor complications were observed in six patients (4.5%) in the non-caudate group. No major complications or mortalities were observed in either group, and the complication rates were not significantly different between the groups (P = 1). CONCLUSIONS: RFA for HCC in the caudate lobe and the non-caudate lobe has equivalent effectiveness and safety. RFA is a promising treatment option for HCC arising in the caudate lobe.

Chlorzoxazone is metabolized by human CYP1A2 as well as by human CYP2E1.

Chlorzoxazone, a muscle-relaxing drug, is metabolized by carbon-hydroxylation at position 6. Chlorzoxazone has been suggested as an in vivo probe for CYP2E1. We studied the specificity of such a substrate using vaccinia virus expressed human P450 forms and the effect of inhibitors for chlorzoxazone metabolism by human liver microsomes. The 6-hydroxylation of chlorzoxazone was mediated by CYP1A2 as well as by CYP2E1. The Km value of CYP1A2 and CYP2E1 for the reaction was 5.69 microM and 232 microM, respectively. However, the Vmax value of CYP2E1 for the reaction was approximately 8.5-fold higher than that of CYP1A2. The CYP1A inhibitor, alpha-naphthoflavone, as well as the CYP2E1 inhibitor, diethyldithiocarbamate, decreased chlorzoxazone 6-hydroxylation at a low substrate concentration by human liver microsomes. Our results raise questions about the suitability of chlorzoxazone as an in vivo probe for hepatic CYP2E1 activity. In human liver microsomal samples, the Km = 40 microM was different from either the Km of CYP1A2 or CYP2E1. We think that this discrepancy is due to the co-expression of similar levels of CYP1A2 and CYP2E1 in human liver. Furthermore, it is suggested that the role of CYP2E1 in 6-hydroxychlorzoxazone formation at the physiological chlorzoxazone concentration of 30-60 microM is almost the same when compared to that of CYP1A2.

Differential influence of cAMP on the expression of the three subtypes (ATA1, ATA2, and ATA3) of the amino acid transport system A.

Treatment of HepG2 cells with forskolin led to 60-100% stimulation of system A activity, measured as the Na+-dependent uptake of alpha-(methylamino)isobutyric acid. The stimulation was reproducible with cholera toxin and dibutyryl cAMP, and inhibitable by H7, a non-specific protein kinase inhibitor. The stimulatory effect was eliminated by cycloheximide and actinomycin D. The forskolin effect was associated with an increase in the maximal velocity of the transport system, with no change in substrate affinity. These cells express three different subtypes of system A (ATA1, ATA2, and ATA3). Treatment with forskolin increased the steady-state levels of ATA1 and ATA2 mRNAs, but decreased that of ATA3 mRNA.

Primary structure and expression of peroxisomal acetylspermidine oxidase in the methylotrophic yeast Candida boidinii.

Acetylspermidine oxidase (ASOD) belongs to a family of FAD-containing amine oxidases and catalyzes the oxidation of N-acetylated spermidine in polyamine metabolism. ASOD was purified to apparent homogeneity from cells of the methylotrophic yeast Candida boidinii grown on spermidine as the sole nitrogen source. C. boidinii ASOD catalyzed the oxidation of only N(1)-acetylspermidine. Based on partial amino acid sequences, oligonucleotide primers were designed for polymerase chain reaction, and the ASOD-encoding gene, ASO1, was cloned. The open reading frame encoding ASO1 was 1530 bp long and corresponded to a protein of 509 amino acid residues (calculated molecular mass=57167 Da). ASO1 contained a FAD-binding motif of G-A-G-I-A-G in the N-terminal region and carried an amino acid sequence of -S-K-L at the C-terminal, representing a typical peroxisome targeting signal 1. ASOD was localized in the peroxisomes in overexpressed C. boidinii. To our knowledge, this is the first report on the gene coding for ASOD that can catalyze the oxidation of N-acetylated polyamine as a substrate, from any type of organism.

Detection of the products of polymerase chain reaction by an ELISA system based on an ion sensitive field effect transistor.

An ELISA system was developed using a pH sensitive ISFET (pH-FET) as a detector, a pipette tip as a solid phase, and urease as a detecting enzyme. Double stranded PCR products with digoxigenin and biotin at both terminals were obtained by using digoxigenin- and biotin-labeled primers. 1 microliters of the PCR solution was directly introduced into the end part of a pipette tip coated with anti-digoxigenin antibody. Biotin-labeled PCR products captured at the solid phase were detected with avidin-urease, of which the activity was measured by a pH-FET in a pH-measuring cell containing urea solution. The assay was used to detect HTLV-I provirus gene integrated in the genome of a human MT-1 cell, and it was found that 100 pg of the genomic DNA of MT-1 cell was specifically detectable after 35 cycles of PCR. Also the detection limit of the present ELISA system itself was determined by using known amounts of purified PCR product labeled with digoxigenin and biotin, and it was found that 10 amol of the labeled DNA in 1 microliter of sample was detectable.

Novel combretastatin analogues effective against murine solid tumors: design and structure-activity relationships.

A series of combretastatin A-4 (CA-4) analogues were synthesized, and their cytotoxic effects against murine Colon 26 adenocarcinoma and inhibitory activity on tubulin polymerization were evaluated. Since CA-4 has limited aqueous solubility, the target compounds were designed to improve solubility by introduction of a nitrogen-containing group. Among the compounds synthesized, those with an amino moiety in place of the phenolic OH of CA-4 showed potent antitubulin activity and cytotoxicity against murine Colon 26 adenocarcinoma in vitro. Some of the compounds which were potent in vitro were evaluated in the murine tumor model Colon 26 in vivo. Among these, 13bHCl, 21aHCl, and 21bHCl showed significant antitumor activity in the animal model, while CA-4 was ineffective. 13bHCl and 21aHCl were further evaluated in two murine tumor models (Colon 38 and 3LL) and human xenografts HCT-15. These compounds showed potent antitumor activity comparable or superior to that of CDDP. The structure-activity relationships of this series of compounds are also discussed.

Clinical pharmacokinetics of pravastatin: mechanisms of pharmacokinetic events.

Pravastatin, one of the 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors (statins) widely used in the management of hypercholesterolaemia, has unique pharmacokinetic characteristics among the members of this class. Many in vivo and in vitro human and animal studies suggest that active transport mechanisms are involved in the pharmacokinetics of pravastatin. The oral bioavailability of pravastatin is low because of incomplete absorption and a first-pass effect. The drug is rapidly absorbed from the upper part of the small intestine, probably via proton-coupled carrier-mediated transport, and then taken up by the liver by a sodium-independent bile acid transporter. About half of the pravastatin that reaches the liver via the portal vein is extracted by the liver, and this hepatic extraction is mainly attributed to biliary excretion which is performed by a primary active transport mechanism. The major metabolites are produced by chemical degradation in the stomach rather than by cytochrome P450-dependent metabolism in the liver. The intact drug and its metabolites are cleared through both hepatic and renal routes, and tubular secretion is a predominant mechanism in renal excretion. The dual routes of pravastatin elimination reduce the need for dosage adjustment if the function of either the liver or kidney is impaired, and also reduce the possibility of drug interactions compared with other statins. which are largely eliminated by metabolism. The lower protein binding than other statins weakens the tendency for displacement of highly protein-bound drugs. Although all statins show a hepatoselective disposition, the mechanism for pravastatin is different from that of the others. There is high uptake of pravastatin by the liver via an active transport mechanism, but not by other tissues because of its hydrophilicity, whereas the disposition characteristics of other statins result from high hepatic extraction because of high lipophilicity. These pharmacokinetic properties of pravastatin may be the result of the drug being given in the pharmacologically active open hydroxy acid form and the fact that its hydrophilicity is markedly higher than that of other statins. The nature of the pravastatin transporters, particularly in humans, remains unknown at present. Further mechanistic studies are required to establish the pharmacokinetic-pharmacodynamic relationships of pravastatin and to provide the optimal therapeutic efficacy for various types of patients with hypercholesterolaemia.

Irreversible extrusion of the first loop facing the matrix of the bovine heart mitochondrial ADP/ATP carrier by labeling the Cys(56) residue with the SH-reagent methyl methanethiosulfonate.

The effect of the SH-reagent methyl methanethiosulfonate (MMTS) on the ADP/ATP carrier of bovine heart mitochondria was studied under various conditions. MMTS labeled predominately Cys(56) in the first loop facing the matrix (loop M1), and the labeling inhibited ADP transport via the carrier. The transport inhibition was found to be due to fixation of the carrier in the m-state conformation. MMTS labeling was suggested not to affect ADP binding to its major binding site. These features were the same as those of another commonly used SH-reagent, N-ethylmaleimide (NEM). Although the van der Waals volume of the non-hydrogen-bondable methylthio group of MMTS is much smaller than that of the ethylsuccinimide group of NEM, modification of Cys(56) inhibited the interconversion between the m- and c-state conformation. The mechanism by which MMTS inhibited the transport activity is discussed in terms of stabilization of conformation of the loop M1.

Interaction of subtilisin BPN' and recombinant Streptomyces subtilisin inhibitors with substituted P1 site residues.

Kinetic analysis was performed on the interaction between subtilisin BPN' and recombinant species of a proteinaceous proteinase inhibitor, Streptomyces subtilisin inhibitor (SSI), of which the P1 site amino acid residue, Met73, was replaced by site-directed mutagenesis. The inhibitor constant, Ki, was determined from the residual enzyme activity by using a peptide substrate. The rate constant of binding, kon, and the rate constant of dissociation, koff, were determined from a progress curve of the substrate hydrolysis in the presence of the inhibitor by using newly derived equations. A recombinant SSI in which Met73 was replaced by Ile showed an affinity (1/Ki) toward subtilisin BPN' of only about 7% of that of the wild-type SSI, and the kinetic analysis revealed that the increase of koff was responsible for this difference. The affinity of other SSI mutants in which Met73 was replaced by Glu or Asp decreased significantly as pH became increasingly alkaline. The decrease in the affinity of these recombinants was due to the decrease of kon rather than the increase of koff. Stopped-flow studies revealed that the binding reaction was reconcilable with a two-step mechanism, and the kinetic parameters for each step were obtained for the binding of the enzyme and recombinant SSIs.

Ion pair skin transport of a zwitterionic drug, cephalexin.

The ion pair skin transport of cephalexin was investigated using various counter ions and solvents. The permeability of cephalexin was enhanced by 1-alkylsulfonates (ASs) at pH 3.0 and by tetraalkylammoniums (AAs) at pH 7.0; the enhancing ratio increased with the number of carbon atoms in their alkyl chains. The corresponding effects of these additives were observed on the partitioning of cephalexin. Most of the additives did not affect the skin transport of D-mannitol and cortisone. These results suggest that the enhanced transport of cephalexin results from the ion pair formation with additives. Although ASs increased the partitioning of cephalexin above that of AAs, the transport enhancement effect of ASs was lower than AAs having the same number of carbon atoms in their alkyl chains, indicating higher diffusivity of the ion pairs with AAs in skin. Moreover, the transport enhancement by AAs increased even more when ethanol-buffer solutions were used as solvents. The conductivity measurement of dissolving solutes in donor solvents showed that the further enhancement might be caused by the increasing ion pair formation in solvents with low dielectric constants. To obtain the maximum enhancement of skin transport of zwitterionic drugs via ion pair concept, one should select a counter ion having high lipophilicity and small volume, and a solvent with suitable pH and low dielectric constant.

Salivary excretion of N-nitrosodimethylamine in dogs.

Carcinogenic N-nitroso compounds (NOCs) are not only ingested from the environment but are also formed endogenously from precursors. It has been reported that nitrate, an NOC precursor, has an enterosalivary cycle and that the cycle increases the chance of exposure to NOCs. However, there is no information on the salivary excretion of NOCs. In the present study, the toxicokinetics of N-nitrosodimethylamine (NDMA) in dogs was evaluated, focusing on the salivary excretion. Following intravenous injection of 2 mg/kg NDMA, the plasma concentration showed a monoexponential decline, and the total body clearance and apparent distribution volume were greatly in excess of the hepatic plasma flow and total body water, respectively. A high concentration of NDMA was immediately detected in the plasma after oral administration of the same dose, and the oral bioavailability was almost 100%. NDMA was rapidly excreted into the saliva after both treatments, and the concentration in saliva was higher than that in the plasma. These results suggest that NDMA also has an enterosalivary cycle: NDMA is partially excreted from blood into saliva, delivered into the gastrointestinal tract by swallowing the saliva, and then completely reabsorbed into the systemic circulation. This concept was also supported by kinetic analysis based on a compartment model. The enterosalivary cycle of NDMA cannot be ignored in the risk assessment of carcinogenesis.