Immunological risk and complement genetic evaluations in early onset de novo thrombotic microangiopathy after living donor kidney transplantation: A Japanese multicenter registry.

BACKGROUND: Thrombotic microangiopathy (TMA) after kidney transplantation (KTx), particularly early onset de novo (dn) TMA, requires immediate interventions to prevent irreversible organ damage. This multicenter study was performed to investigate the allogeneic clinical factors and complement genetic background of dnTMA after KTx. METHODS: Perioperative dnTMA after KTx within 1 week after KTx were diagnosed based on pathological or/and hematological criteria at each center, and their immunological backgrounds were researched. Twelve aHUS-related gene variants were examined in dnTMA cases. RESULTS: Seventeen recipients (15 donors) were enrolled, and all dnTMA cases were onset within 72-h of KTx, and 16 of 17 cases were ABO incompatible. The implementation rate of pre-transplant plasmaphereses therapies were low, including cases with high titers of anti-A/anti-B antibodies. Examination of aHUS-related gene variants revealed some deletions and variants with minor allele frequency (MAF) in Japan or East Asian genome databases in genes encoding alternative pathways and complement regulatory factors. These variants was positive in 8 cases, 6 of which were positive in both recipient and donor, but only in one graft loss case. CONCLUSIONS: Although some immunological risks were found for dnTMA after KTx, only a few cases developed into TMA. The characteristic variations revealed in the present study may be novel candidates related to dnTMA in Japanese or Asian patients, but not pathogenic variants of aHUS. Future studies on genetic and antigenic factors are needed to identify factors contributing to dnTMA after KTx.

Development and nationwide validation of kidney graft injury markers using urinary exosomes and microvesicles (complete English translation of the Japanese version).

BACKGROUND: Non-invasive, prompt, and proper detection tools for kidney graft injuries (KGIs) are awaited to ensure graft longevity. We screened diagnostic biomarkers for KGIs following kidney transplantation using extracellular vesicles (EVs; exosomes and microvesicles) from the urine samples of patients. METHODS: One hundred and twenty-seven kidney recipients at 11 Japanese institutions were enrolled in this study; urine samples were obtained prior to protocol/episode biopsies. EVs were isolated from urine samples, and EV RNA markers were assayed using quantitative reverse transcription polymerase chain reaction. Diagnostic performance of EV RNA markers and diagnostic formulas comprising these markers were evaluated by comparison with the corresponding pathological diagnoses. RESULTS: EV CXCL9, CXCL10, and UMOD were elevated in T-cell-mediated rejection samples compared with other KGI samples, while SPNS2 was elevated in chronic antibody-mediated rejection (cABMR) samples. A diagnostic formula developed through Sparse Logistic Regression analysis using EV RNA markers allowed us to accurately (with an area under the receiver operator characteristic curve [AUC] of 0.875) distinguish cABMR from other KGI samples. EV B4GALT1 and SPNS2 were also elevated in cABMR, and a diagnostic formula using these markers was able to distinguish between cABMR and chronic calcineurin toxicity accurately (AUC 0.886). In interstitial fibrosis and tubular atrophy (IFTA) urine samples and those with high Banff chronicity score sums (BChS), POTEM levels may reflect disease severity, and diagnostic formulas using POTEM detected IFTA (AUC 0.830) and high BChS (AUC 0.850). CONCLUSIONS: KGIs could be diagnosed with urinary EV mRNA analysis with relatively high accuracy.

Impact of germline HLA genotypes on clinical outcomes in patients with urothelial cancer treated with pembrolizumab.

Human leukocyte antigen class I (HLA-I) genotypes are suggested to influence the cancer response to checkpoint blockade immunotherapy. This study assessed the impact of germline HLA genotypes on clinical outcomes in patients with chemoresistant advanced urothelial cancer (UC) treated with pembrolizumab. Zygosity, supertypes, evolutionary divergency, and specific alleles of germline HLA-I and -II were evaluated using the Luminex technique in 108 patients with chemoresistant metastatic or locally advanced UC treated with pembrolizumab. Among the 108 patients, 69 died and 83 showed radiographic progression during follow-up. Homozygous for at least one HLA-I locus, absence of the HLA-A03 supertype, and high HLA-I evolutionary divergence were associated with a radiographic response, but were not associated with survival outcomes. Patients with the HLA-DQB1*03:01 allele had significantly lower disease control rates than patients without the allele (17.4% vs. 53.8%, p = 0.002); its presence was also an independent risk factor for progressive disease (hazard ratio 4.35, 95% confidence interval 1.03-18.46). Furthermore, patients with the HLA-DQB1*03:01 allele had significantly worse progression-free survival than patients without the allele (median progression-free survival 3.1 vs. 4.8 months, p = 0.035). There was no significant relationship between any HLA status and the incidence of severe adverse events. Several germline HLA genotypes, especially HLA-DQB1*03:01, may be associated with radiographic progression. However, their impact on treatment response is limited, and germline HLA genotypes was not independently associated with survival outcomes. Further prospective studies are needed to confirm the relationship between germline HLA genotypes and clinical outcomes in patients with chemoresistant advanced UC treated with pembrolizumab.

Advanced Tertiary Lymphoid Tissues in Protocol Biopsies are Associated with Progressive Graft Dysfunction in Kidney Transplant Recipients.

BACKGROUND: Tertiary lymphoid tissues (TLTs) are ectopic lymphoid tissues found in chronically inflamed organs. Although studies have documented TLT formation in transplanted kidneys, the clinical relevance of these TLTs remains controversial. We examined the effects of TLTs on future graft function using our histologic TLT maturity stages and the association between TLTs and Banff pathologic scores. We also analyzed the risk factors for the development of TLTs. METHODS: Serial protocol biopsy samples (0 hour, 1, 6, and 12 months) without rejection were retrospectively analyzed from 214 patients who underwent living donor kidney transplantation. TLTs were defined as lymphocyte aggregates with signs of proliferation and their stages were determined by the absence (stage I) or presence (stage II) of follicular dendritic cells. RESULTS: Only 4% of patients exhibited TLTs at the 0-hour biopsy. Prevalence increased to almost 50% at the 1-month biopsy, and then slightly further for 12 months. The proportion of advanced stage II TLTs increased gradually, reaching 19% at the 12-month biopsy. Presence of stage II TLTs was associated with higher risk of renal function decline after transplantation compared with patients with no TLT or stage I TLTs. Stage II TLTs were associated with more severe tubulitis and interstitial fibrosis/tubular atrophy at 12 months and predicted poorer graft function independently from the degree of interstitial inflammation. Pretransplantation rituximab treatment dramatically attenuated the development of stage II TLTs. CONCLUSIONS: TLTs are commonly found in clinically stable transplanted kidneys. Advanced stage II TLTs are associated with progressive graft dysfunction, independent of interstitial inflammation.

Effect of HLA genotype on intravesical recurrence after bacillus Calmette-Guérin therapy for non-muscle-invasive bladder cancer.

The intravesical administration of bacillus Calmette-Guérin (BCG) is widely used to control the intravesical recurrence of non-muscle-invasive bladder cancer (NMIBC). This study aimed to reveal the effects of zygosity on human leukocyte antigen (HLA) genes and individual HLA genotypes on intravesical recurrence after intravesical BCG therapy for NMIBC. This study included Japanese patients who had received intravesical BCG for NMIBC. HLA genotyping of HLA-A, B, C, and DRB1 was performed. The effect of HLA zygosity and HLA genotype on intravesical recurrence was evaluated. Among 195 patients, those homozygous for the HLA-B supertype were more likely than those heterozygous for the HLA-B supertype to experience intravesical recurrence by univariate analysis (hazard ratio [HR], 95% confidence interval [CI]; 1.87, 1.14-3.05, P = 0.012) and multivariate analysis (HR, 95% CI; 2.26, 1.02-5.01, P = 0.045). Patients with B07 or B44 had a decreased risk of intravesical recurrence by univariate analysis (HR, 95% CI; 0.43, 0.24-0.78, P = 0.0056) and multivariate analysis (HR, 95% CI; 0.36, 0.16-0.82, P = 0.016). This study suggests the importance of the diversity and specificity of HLA-B loci in the antitumor effect of BCG immunotherapy for NMIBC. These findings may contribute to the delineation of risk strata for BCG therapy and improve the medical management of NMIBC.

Tacrolimus concentrations after renal transplantation in a mother-neonate dyad: Maternal, neonatal and breast milk measurements.

WHAT IS KNOWN AND OBJECTIVE: We aim to add to the few reports on tacrolimus concentrations in breast milk and in maternal, umbilical vein and neonatal blood after maternal renal transplantation. CASE SUMMARY: In a 30-year-old pregnant woman, the tacrolimus concentration at delivery was the same in maternal, umbilical vein and neonatal blood. The breast milk/maternal blood tacrolimus ratio ranged from 0.40 to 0.64. WHAT IS NEW AND CONCLUSION: The maternal and neonatal blood tacrolimus concentrations at birth are equivalent; thus, one must assume that maternal tacrolimus concentrations directly affect the foetus and/or neonate. Tacrolimus is not detectable in the neonate 3 weeks after birth, suggesting that there is minimal transfer through breast milk.

Loss of antibodies to hepatitis E virus in organ transplant patients with hepatitis E.

AIM: Studies regarding changes in antibodies to hepatitis E virus (HEV) after HEV infection in organ transplant patients are limited. This study aimed to clarify HEV infection trends in organ transplant patients who contracted HEV using data from a previous Japanese nationwide survey. METHODS: This study was undertaken from 2012 to 2019. Among 4518 liver, heart, and kidney transplant patients, anti-HEV immunoglobulin G (IgG) antibodies were positive in 164; data were collected from 106 of these patients, who consented to participate in the study. In total, 32 liver transplant patients, seven heart transplant patients, and 67 kidney transplant patients from 16 institutions in Japan were examined for IgG, IgM, and IgM antibodies to HEV and the presence of HEV RNA in the serum. The χ2 -test was used to determine the relationship between the early and late postinfection groups in patients with anti-HEV IgG positive-to-negative conversion rates. The Mann-Whitney U-test was used to compare clinical factors. RESULTS: Anti-HEV IgG positive-to-negative conversion occurred in 25 (23.6%) of 106 organ transplant patients. Of eight patients with hepatitis E who tested positive for HEV RNA, one (14.0%) had anti-HEV IgG positive-to-negative conversion. Twenty-four (24.5%) of 98 patients negative for HEV RNA had anti-HEV IgG positive-to-negative conversion. CONCLUSIONS: This study revealed, for the first time, the changes in HEV antibodies in organ transplant patients. Loss of anti-HEV IgG could often occur unexpectedly in organ transplant patients with previous HEV infection.

A Nationwide Survey of Hepatitis E Virus Infection and Chronic Hepatitis in Heart and Kidney Transplant Recipients in Japan.

BACKGROUND: Recently, chronic hepatitis E has been reported in solid organ transplant (SOT) recipients in European countries. Previously, we clarified the prevalence of hepatitis E virus (HEV) infection in Japanese liver transplant recipients and identified 2 chronic hepatitis E patients infected by blood transfusion. However, the rate of HEV infection in recipients of SOTs other than liver in Japan remains unclear, so we conducted a nationwide survey to clarify the prevalence of chronic HEV infection in Japanese heart and kidney transplant recipients. METHODS: A total of 99 heart and 2526 kidney transplant recipients in 17 hospitals in Japan were examined for the presence of the IgG class of anti-HEV antibodies as well as for serum HEV RNA. RESULTS: The prevalence of anti-HEV IgG among heart and kidney transplant recipients was 7.07% (7/99) and 4.08% (103/2526), respectively. One heart transplant patient (1.01%) and 11 kidney transplant patients (0.44%) were found to be positive for HEV RNA. The HEV isolates from all viremic patients were typed as genotype 3. Four patients developed chronic hepatitis E after transplantation. Three patients were treated with ribavirin; their liver enzymes normalized, and HEV RNA became negative immediately. Sustained virologic response was achieved in all cases. CONCLUSIONS: This is the first nationwide survey of HEV infection in Japanese heart and kidney transplant recipients. The prevalence of anti-HEV IgG and HEV RNA in heart and kidney transplant recipients in Japan was lower than that in European countries. Of note, 42% of viremic transplant patients developed chronic hepatitis.

Association of immunosuppressive agents and cytomegalovirus infection with de novo donor-specific antibody development within 1 year after renal transplantation.

The association between immunosuppressive therapy or cytomegalovirus (CMV) infection and detection of de novo donor-specific antibody (dnDSA) at 1 year after transplantation was evaluated. The impact of dnDSA positivity at 1 year after transplantation on long-term death-censored renal graft survival was also evaluated. One hundred and sixty adults receiving living renal allografts were studied. Inclusion criteria were renal graft survival for at least 1 year and a standard regimen of immunosuppressive therapy with tacrolimus, mycophenolate mofetil (MMF), steroids, and basiliximab. DSA were measured retrospectively by the Luminex assay. The coefficient of variation (CV) was calculated and receiver operating characteristic (ROC) analysis was employed to clarify the association of tacrolimus with development of dnDSA. Seven of the 160 patients (4.4%) were positive for dnDSA. The intra-patient minimum trough level of tacrolimus (cutoff value: 3.2 ng/mL) was associated with development of dnDSA. Discontinuation of MMF and treatment of CMV infection were more frequent in patients with dnDSA than in those without dnDSA. In multivariate analysis, a low trough level of tacrolimus, discontinuation of MMF, and treatment of CMV infection within 1 year after transplantation were independently associated with detection of dnDSA at 1 year. In patients with or without dnDSA at 1 year, the 10-year allograft survival rate was 51.4 versus 87.9%, respectively (P = 0.002). A lower tacrolimus trough level, discontinuation of MMF, and treatment of CMV infection were associated with dnDSA positivity. Further investigation is needed to determine whether a new immunosuppressive regimen that avoids these factors can reduce dnDSA positivity.

Impact of persistent preformed and de novo donor-specific antibodies detected at 1 year after kidney transplantation on long-term graft survival in Japan: a retrospective study.

BACKGROUND: We evaluated the impact of persistent preformed donor-specific antibody (DSA) and de novo DSA (dnDSA) detected at 1 year posttransplantation on long-term death-censored graft survival. METHODS: One hundred and sixty adult patients who received living kidney allograft with pretransplant-negative T-cell complement-dependent cytotoxicity crossmatch (CDCXM), and without periodic screening for DSA, were eligible for this study. All enrolled patients were retrospectively tested for DSA using the Luminex assay. The presence of DSA was analyzed in stored serum samples collected at 1 year posttransplantation. If the recipients had DSA, it was analyzed in the pretransplant serum sample. The detection of DSA solely in the 1 year posttransplant sample was defined as dnDSA, and DSA detection in both pretransplant and 1 year posttransplant samples was defined as persistent preformed DSA. RESULTS: DSAs were identified in 14 (8.8%) of the 160 patients. Seven patients had persistent preformed DSA, 6 had dnDSA, and 1 had both persistent preformed and dnDSA at 1 year posttransplantation. Death-censored allograft survival rates of patients with DSA versus those without DSA at 7 and 11 years were 77.9 vs. 97.8% and 60.6 vs. 89.2%, respectively. The graft survival rate was lower in patients with persistent preformed DSA and/or dnDSA. Each case of preformed DSA and dnDSA was associated with long-term graft survival. CONCLUSION: The presence of persistent preformed DSA or dnDSA at 1 year posttransplantation may be a predictor of long-term graft survival. Further study is needed to evaluate whether periodic screening for DSA improves long-term graft survival.

Clinical implications of pharmacokinetics of sunitinib malate and N-desethyl-sunitinib plasma concentrations for treatment outcome in metastatic renal cell carcinoma patients.

In this study, we examined the association between the pharmacokinetics (PK) level of sunitinib malate (SU) and its metabolite N-desethyl-sunitinib (DSU) in terms of adverse events (AEs) and clinical outcomes in patients with metastatic renal cell carcinoma (mRCC). The PK of sunitinib (SU and DSU) was examined in 26 patients (20 men and 6 women) with mRCC. The associations between SU/DSU C0 and AE occurrence, best response rate, time to treatment failure, progression-free survival (PFS), and overall survival (OS) were investigated. Occurrence of grade 1 or higher hand-foot syndrome and thrombocytopenia (p = 0.002 and 0.024, respectively) was associated with a high concentration before morning intake (C0) level of SU. Low C0 levels of DSU were significantly associated with drug discontinuation due to disease progression (p = 0.035). Patients with DSU C0 level higher than 15.0 ng/mL showed a tendency toward increased PFS (61 weeks vs 12 weeks, p = 0.004) and OS (36 months vs 8 months, p = 0.040). The C0 level of SU and SU + DSU were not associated with prognosis. The higher level of C0 of SU may predict developing AEs and DSU C0 >15.0 ng/mL may lead to better prognosis of patients treated with sunitinib. PK of sunitinib may be useful for determining adequate dosages and prevention of severe AEs. Further studies are required to establish the utility of the PK of sunitinib in patients with mRCC.

Contribution of UGT1A1 genetic polymorphisms related to axitinib pharmacokinetics to safety and efficacy in patients with renal cell carcinoma.

Axitinib is a potent second-line molecular-targeted agent for metastatic renal cell carcinoma (mRCC). Axitinib pharmacokinetics and its relation with genetic polymorphisms were evaluated to predict the adverse events (AEs) and efficacy of axitinib. We analyzed 46 patients with mRCC who were treated with axitinib. The plasma axitinib level was measured at 0, 2, 4, 8, and 12 h after administration (C0, C2, C4, C8, and C12; ng/mL) on day 7 of the treatment. Genetic polymorphisms related to axitinib pharmacokinetics, including SLCO1B1, SLCO1B3, SLCO2B1, ABCB1, ABCG2, CYP2C19, CYP3A5, and UGT1A1, were analyzed. Axitinib C0 and AUC0-12 in patients with UGT1A1 poor metabolisers (*6/*6, *6/*28, and *28/*28; n = 10) were significantly higher than those in patients with UGT1A1 extensive metabolisers (*1/*1, *1/*6,*1/*28, and *27/*28; n = 36) (23.6 vs. 7.8 ng/mL, p = 0.030, and 441.3 vs. 217.1 ng h/mL, p = 0.007). The cutoff levels of C0 to predict ≥ G2 hypothyroidism and ≥ G2 anorexia were 6.6 and 7.1 ng/mL, respectively (p = 0.005 and p = 0.035). The overall survival (OS) in patients with C0 > 5 ng/mL was significantly better than that in patients with C0 < 5 ng/mL (p = 0.022). Genetic polymorphisms in UGT1A1 were significantly associated with the plasma axitinib level. The plasma axitinib level was significantly associated with the frequency of AEs and OS in patients with mRCC. No direct relationship was observed between UGT1A1 genotypes and the frequency of AEs or OS.

The mechanism of the down-regulation of hepatic transporters in rats with indomethacin-induced intestinal injury.

BACKGROUND: Previously, we reported that hepatic transporters were down-regulated consistent with intestinal injury in indomethacin (IDM)-treated rats. AIM: The purpose of this study was to characterize this mechanism of the down-regulation of hepatic transporters in IDM-treated rats. METHODS: Hepatic nuclear receptor expressions, oxidative stress condition and the expression of hepatic transporters were evaluated in rats with IDM-induced intestinal injury with or without the administration of mucosal protectant ornoprostil, a prostaglandin E1 analogue, or aminoguanidine (AG), an iNOS inhibitor. RESULTS: All the nuclear receptors examined in the present study, which regulates hepatic transporters, were decreased by the administration of IDM. Hepatic glutathione, an indicator of oxidative stress, was significantly reduced compared with control. We then determined the expression of hepatic transporters by semi-quantitative real-time RT-PCR and Western blot analysis in IDM-treated rats with or without the administration of ornoprostil or AG. Ornoprostil recovered the gene expression of Oatp1a1, Oatp1b2 and Mrp2 and protein expression of Mrp2 while it had no effect on Oatp1a1 and Oatp1b2 proteins. These results indicated that the gene expression of hepatic transporters was down-regulated in association with the intestinal injury. On the other hand, there is no effect of AG on the reduced gene expression of hepatic Oatp1a1, Oatp1b2 and Mrp2. In protein expression, AG slightly recovered Mrp2 expression accompanied by a partial decrease in portal NO levels. CONCLUSIONS: We suggest that the transcriptional process influenced by a dysfunction of hepatic nuclear receptors as well as the effect of NO on the post-transcriptional process due to intestinal injury are partially involved in the down-regulation of hepatic transporters.

Involvement of carboxylesterase 1 and 2 in the hydrolysis of mycophenolate mofetil.

Mycophenolate mofetil (MMF) is the ester prodrug of the immunosuppressant agent mycophenolic acid (MPA) and is rapidly activated by esterases after oral administration. However, the role of isoenzymes in MMF hydrolysis remains unclear. Although human plasma, erythrocytes, and whole blood contain MMF hydrolytic activities, the mean half-lives of MMF in vitro were 15.1, 1.58, and 3.20 h, respectively. Thus, blood esterases seemed to contribute little to the rapid MMF disappearance in vivo. In vitro analyses showed that human intestinal microsomes exposed to 5 and 10 µM MMF exhibited hydrolytic activities of 2.38 and 4.62 nmol/(min · mg protein), respectively. Human liver microsomes exhibited hydrolytic activities of 14.0 and 26.1 nmol/(min · mg protein), respectively, approximately 6-fold higher than those observed for intestinal microsomes. MMF hydrolytic activities in human liver cytosols were 1.40 and 3.04 nmol/(min · mg protein), respectively. Because hepatic cytosols generally contain 5-fold more protein than microsomes, MMF hydrolysis in human liver cytosols corresponded to approximately 50% of that observed in microsomes. Fractions obtained by 9000g centrifugation of supernatants from COS-1 cells expressing human carboxylesterase (CES) 1 or 2 exhibited MMF hydrolytic activity, with CES1-containing fractions showing higher catalytic efficiency than CES2-containing fractions. The CES inhibitor bis-p-nitrophenylphosphate inhibited MMF hydrolysis in human liver microsomes and cytosols with IC(50) values of 0.51 and 0.36 µM, respectively. In conclusion, both intestinal and hepatic CESs and in particular CES1 may be involved in MMF hydrolysis and play important roles in MMF bioactivation. Hepatic CES1 activity levels may help explain the between-subject variability observed for MMF usage.

Down-regulation of hepatic transporters for BSP in rats with indomethacin-induced intestinal injury.

Previous reports have demonstrated that an intestinal injury causes hypofunctions of the liver associated with down-regulations of cytochrome P450, but an influence on hepatic transporters remains unclear. Here, we tested hepatic transporter functions in a rat model of bowel injury using indomethacin (IDM). After administration of IDM (8.5 mg/kg, i.p., 3 d), the rats suffered the intestinal impairment indicated by a reduction of alkaline phosphatase activity in mucosa. In vivo pharmacokinetic experiments of bromosulfophthalein (BSP) showed that there was a reduction in its plasma elimination rate and cumulative biliary excretion in IDM-treated rats and systemic and biliary clearances reduced to nearly 50% of the control group. Protein expressions in plasma membrane and mRNA levels of organic anion transporting polypeptide 1b2 (Oatp1b2) and multidrug resistance-associated protein 2 (Mrp2), which play hepatic BSP uptake and biliary excretion, respectively, in the liver were significantly reduced following the IDM treatment. In portal plasma, the levels of proinflammatory cytokines were unchanged, while the level of nitric oxide metabolites (NO2- + NO3-) increased to 6.5-fold that of the control. The time-course on IDM treatment indicated that, firstly, intestinal injury was induced, the NO level increased, and the hepatic Oatp1b2 and Mrp2 expression began to fall followed by an increase in plasma ALT. In conclusion, IDM-induced injury to the small intestine causes the hypofunction of hepatic Oatp1b2 and Mrp2 independently on the hepatic impairment, and NO arising from bowel injury may be one of key factors for it through the remote effect.

Involvement of human blood arylesterases and liver microsomal carboxylesterases in nafamostat hydrolysis.

Metabolism of nafamostat, a clinically used serine protease inhibitor, was investigated with human blood and liver enzyme sources. All the enzyme sources examined (whole blood, erythrocytes, plasma and liver microsomes) showed nafamostat hydrolytic activity. V(max) and K(m) values for the nafamostat hydrolysis in erythrocytes were 278 nmol/min/mL blood fraction and 628 microM; those in plasma were 160 nmol/min/mL blood fraction and 8890 microM, respectively. Human liver microsomes exhibited a V(max) value of 26.9 nmol/min/mg protein and a K(m) value of 1790 microM. Hydrolytic activity of the erythrocytes and plasma was inhibited by 5, 5'-dithiobis(2-nitrobenzoic acid), an arylesterase inhibitor, in a concentration-dependent manner. In contrast, little or no suppression of these activities was seen with phenylmethylsulfonyl fluoride (PMSF), diisopropyl fluorophosphate (DFP), bis(p-nitrophenyl)phosphate (BNPP), BW284C51 and ethopropazine. The liver microsomal activity was markedly inhibited by PMSF, DFP and BNPP, indicating that carboxylesterase was involved in the nafamostat hydrolysis. Human carboxylesterase 2 expressed in COS-1 cells was capable of hydrolyzing nafamostat at 10 and 100 microM, whereas recombinant carboxylesterase 1 showed significant activity only at a higher substrate concentration (100 microM). The nafamostat hydrolysis in 18 human liver microsomes correlated with aspirin hydrolytic activity specific for carboxylesterase 2 (r=0.815, p<0.01) but not with imidapril hydrolysis catalyzed by carboxylesterase 1 (r=0.156, p=0.54). These results suggest that human arylesterases and carboxylesterase 2 may be predominantly responsible for the metabolism of nafamostat in the blood and liver, respectively.