Potential value of high-dose mizoribine as rescue therapy for ongoing acute humoral rejection

Genetic and clinical determinants of mizoribine pharmacokinetics in renal transplant recipients

AIM

Mizoribine (MZR) is an immunosuppressant for the prevention of allograft rejection in Asian countries, but the great variability in pharmacokinetics (PK) limits its clinical use. This study was to explore genetic and clinical factors that affect the MZR PK process.

METHODS

Blood samples and clinical data were collected from 60 Chinese renal transplant recipients. MZR plasma concentration was measured at pre-dose (0 h) and 0.5, 1, 2, 3, 4, 5, 6, 8, and 12 h post-dose by high performance liquid chromatography with an ultraviolet detector. PK parameters were calculated by non-compartmental analysis. High-throughput sequenced single nucleotide polymorphism was applied screening possible genetic factors.

RESULTS

Extensive inter-individual MZR PK differences were reflected in the process of elimination (k_e, CL/F, MRT and t_1/2) and intestinal absorption (C_max and T_max), as well as in the dose-normalized exposure (AUC_0-12h/D). From 146 SNPs within 39 genes screened, AUC_0-12h/D was found higher in recipients with CREB1 rs11904814 TT than with G allele carriers (3.135 ± 0.928 versus 2.084 ± 0.379 μg h ml^-1 mg^-1, p = 0.007). Recipients with SLC28A3 rs10868138 TT had lower t_1/2 as compared to C allele carriers (0.728 ± 0.189 versus 0.951 ± 0.196 h, p = 0.001). Serum creatinine (SCr) explained 35.5% of C₀/D variability (p < 0.001). Pure effects of genotypes CREB1 and SLC28A3 were 13.7% (p = 0.004) and 17.5% (p = 0.001) for AUC_0-12h/D and t_1/2, respectively. When additionally taking SCr into models, CREB1 and SLC28A3 genotypes explained 20.0% (p = 0.038) and 46.5% (p < 0.001) of AUC_0-12h/D and t_1/2 variability, respectively.

CONCLUSION

CREB1 and SLC28A3 genotypes, as well as SCr, are identified as determinants in predicting inter-individual MZR PK differences in renal transplant recipients.

Prediction of mizoribine pharmacokinetic parameters by serum creatinine in renal transplant recipients

PURPOSE

Mizoribine (MZR) is an immunosuppressive agent with extensive inter-individual differences in pharmacokinetics (PK). Here, we investigated the PK characteristics of MZR in renal transplant recipients and gave equations for prediction of some critical PK parameters.

METHODS

A total of 40 renal transplant recipients participated in this prospective study and were administered MZR orally twice daily in the range of 1.1-8.9 mg kg^-1 day^-1. Steady-state concentrations of MZR were detected before (0 h) and 0.5, 1, 2, 3, 4, 5, 6, 8, and 12 h after administration by high-performance liquid chromatography method. Another 38 patients with newly detected trough concentration (C₀) were enrolled to validate the obtained C₀ predictive equation.

RESULTS

Significant inter-individual differences in MZR PK parameters were observed. Patients with decreasing creatinine clearance rate (CCr) had significantly decreased terminal elimination rate constant (k_el) and apparent total body clearance (Cl/F), while other PK parameters including apparent terminal half-life (t_1/2), peak time (T_max), peak concentration (C_max), area under the curve (AUC_0-12h), apparent volume of distribution (V/F), and mean residence time (MRT) were significantly increased. Correlation coefficients between AUC_0-12h and C₀/C_max were 0.894 and 0.916, respectively (both p < 0.001). A serum creatinine (SCr)-based predictive C₀ equation [C₀ = (2.160 × SCr - 54.473) × Dose] was established and validated by C₀ from another 38 patients. Besides, significant linear correlations between k_el/t_1/2 and CCr were also found (r² = 0.668 and 0.484, respectively), and equations predicting k_el/t_1/2 were also obtained (k_el = 0.015 + 0.002 × CCr, t_1/2 = 13.601 - 0.139 × CCr).

CONCLUSIONS

Renal function plays as an essential factor that contributes to great inter-individual MZR PK variation. Both C₀ and C_max are suitable for evaluating MZR exposure in the body. SCr could be applied to predict C₀ and t_1/2 of MZR.

The immunosuppressive agent mizoribine monophosphate is an inhibitor of the human RNA capping enzyme

Mizoribine monophosphate (MZP) is a specific inhibitor of the cellular inosine-5′-monophosphate dehydrogenase (IMPDH), the enzyme catalyzing the rate-limiting step of de novo guanine nucleotide biosynthesis. MZP is a highly potent antagonistic inhibitor of IMPDH that blocks the proliferation of T and B lymphocytes that use the de novo pathway of guanine nucleotide synthesis almost exclusively. In the present study, we investigated the ability of MZP to directly inhibit the human RNA capping enzyme (HCE), a protein harboring both RNA 5′-triphosphatase and RNA guanylyltransferase activities. HCE is involved in the synthesis of the cap structure found at the 5′ end of eukaryotic mRNAs, which is critical for the splicing of the cap-proximal intron, the transport of mRNAs from the nucleus to the cytoplasm, and for both the stability and translation of mRNAs. Our biochemical studies provide the first insight that MZP can inhibit the formation of the RNA cap structure catalyzed by HCE. In the presence of MZP, the RNA 5′-triphosphatase activity appears to be relatively unaffected while the RNA guanylyltransferase activity is inhibited, indicating that the RNA guanylyltransferase activity is the main target of MZP inhibition. Kinetic studies reveal that MZP is a non-competitive inhibitor that likely targets an allosteric site on HCE. Mizoribine also impairs mRNA capping in living cells, which could account for the global mechanism of action of this therapeutic agent. Together, our study clearly demonstrates that mizoribine monophosphate inhibits the human RNA guanylyltransferase in vitro and impair mRNA capping in cellulo.

Mizoribine as sole immunosuppressive agent in islet xenotransplantation models: a candidate immunosuppressant causing no adverse effects on islets

Mizoribine (MZ) inhibits the differentiation and proliferation of helper T and B cells after antigen recognition by suppressing the purine biosynthesis pathway and nucleic acid synthesis. MZ has been used in kidney transplantation, but distinct data are unavailable for islet transplantation. The present study investigated the efficacy of MZ for islet xenotransplantation. Immunosuppressive effects of MZ were determined by mixed lymphocyte reaction (MLR) assay in vitro. Toxicities for Wistar rat islets were determined by adenosine triphosphate (ATP) contents of islets during 3-day culture and stimulation index in response to glucose after culture. Immunosuppressive effects in vivo were tested in a Wistar-to-B6 islet xenotransplantation model. MZ was administered continuously for 28 days subcutaneously or intramuscularly. MZ inhibited MLR response by approximately 50% at 0.1 μg/ml. ATP contents decreased with MZ >100 μg/ml, while stimulation index was maintained. Continuous infusion of MZ at 10 mg/kg maintained blood concentrations at 0.13-0.19 μg/ml, while intramuscular injection of MZ at 100 mg/kg/day (peak 520 μg/ml at 1 h postinjection) resulted in below measurable levels (<0.03 μg/ml) within 24 h. Graft survival was significantly prolonged following continuous infusion of 10 mg/kg/day compared to controls (31.0 ± 9.5 vs. 13.2 ± 5.2 days; p = 0.002). Furthermore, animals with intramuscular injection at doses of 3.2, 10, or 100 mg/kg/day showed significantly longer graft survival (20.0 ± 7.5, 22.0 ± 7.31, and 24.5 ± 8.1 days, respectively; p < 0.05 each). Histological examination showed significant suppression of lymphocyte infiltration by MZ administration. MZ showed immunosuppressive effects in an experimental islet xenotransplantation model without adverse effects on endocrine function of islet grafts.

Involvement of Multiple Transporters-mediated Transports in Mizoribine and Methotrexate Pharmacokinetics

Mizoribine is administered orally and excreted into urine without being metabolized. Many research groups have reported a linear relationship between the dose and peak serum concentration, between the dose and AUC, and between AUC and cumulative urinary excretion of mizoribine. In contrast, a significant interindividual variability, with a small intraindividual variability, in oral bioavailability of mizoribine is also reported. The interindividual variability is mostly considered to be due to the polymophisms of transporter genes. Methotrexate (MTX) is administered orally and/or by parenteral routes, depending on the dose. Metabolic enzymes and multiple transporters are involved in the pharmacokinetics of MTX. The oral bioavailability of MTX exhibits a marked interindividual variability and saturation with increase in the dose of MTX, with a small intraindividual variability, where the contribution of gene polymophisms of transporters and enzymes is suggested. Therapeutic drug monitoring of both mizoribine and MTX is expected to improve their clinical efficacy in the treatment of rheumatoid arthritis.

Antibody-mediated rejection: treatment alternatives and outcomes

Over the past 10 years, thanks to the development of sensitive methods of antibody detection and markers of antibody injury such as C4d staining, the role of anti-human leukocyte antigen (HLA) and non-HLA alloantibodies as effectors of acute and chronic immune allograft injury has been revisited. Antibody-mediated rejection (AMR) defines all allograft rejection caused by antibodies directed against donor-specific HLA molecules, blood group antigen (ABO)-isoagglutinins, or endothelial cell antigens. Antibody-mediated rejection can be a recalcitrant process, resistant to therapy and carries an ominous prognosis to the graft. In concordance with these views, treatment protocols for AMR use permutations of a multiple-prong approach that include (1) the suppression of the T-cell dependent antibody response, (2) the removal of donor reactive antibody, (3) the blockade of the residual alloantibody, and (4) the depletion of naive and memory B-cells. Although all published protocols report a variable rate of success, a major weakness of all current protocols is the lack of effective anti-plasma cell agents. In comparison to acute AMR, the treatment for chronic AMR (CAMR) is not well characterized. Although in acute AMR large titers of pre-existent alloantibodies result in massive activation of the complement system and lytic injury of the graft endothelium, thereby requiring aggressive and fast removal of the offending agents, in CAMR, complement activation results in sublytic endothelial cell injury and activation. Although this type of injury results in chronic graft failure, its slow progression likely renders it amenable of suppression by heightening of maintenance immunosuppression and anti-idiotypic blockade of the circulating alloantibody without the need of plasma exchange. In this review, we will discuss the rationale behind the design of treatment protocols for acute AMR and CAMR as well as their reported results and complications.

Prophylactic treatment of antibody-mediated rejection with high-dose mizoribine and pharmacokinetic study

Although mizoribine (MZ), which inhibits inosine monophosphate dehydrogenase in the same way as mycophenolate mofetil, recently proved more effective when higher doses were administered than previously approved, neither the optimal dosage nor blood concentration has yet been clarified. We aimed at investigating the effect of high-doses of MZ on prevention of anti-donor antibody (Ab) production and acute Ab-mediated rejection (AMR) on the basis of the pharmacokinetic profile in a pig kidney transplantation model. Group 1 (n = 5) received cyclosporin microemulsion (6 mg/kg) and prednisolone (0.1 mg/kg). Groups 2, 3 and 4 (each n = 5) were treated, respectively, with 30, 10 and 3 mg/kg of MZ in addition to cyclosporin and prednisolone. The incidences of AMR in groups 1, 2, 3 and 4 were 5/5, 1/5, 3/5 and 5/5, respectively. Anti-donor IgG/IgM Ab levels (relative to pretransplantation levels) on day 14 in groups 1, 2, 3 and 4 were 10.3/9.3, 1.8/1.0, 2.3/1.8 and 6.5/3.5, respectively. While only 2 (28.6%) of seven pigs with Cmax > 3 microg/ml during the first 2 weeks had AMR, 7 (87.5%) of eight pigs with Cmax < 3 microg/ml elicited anti-donor Abs and experienced AMR (P = 0.0406). Effective Cmax seemed to be over 3 microg/ml at minimum. Higher doses of MZ efficiently prevented AMR. However, therapeutic drug monitoring is essential before clinical application.

Population pharmacokinetics of higher-dose mizoribine in healthy male volunteers

The population pharmacokinetic parameters of mizoribine in healthy subjects were estimated using a nonlinear mixed effects model (NONMEM) program. Pharmacokinetic data for population analysis were obtained in the previous study, in which 24 healthy Caucasian male subjects participated in a single-dose (3, 6, 9, 12 mg/kg) study, and 12 subjects participated in a multiple-dose (6, 12 mg/kg/d) study. The mean value of the absorption lag time, absorption rate constant (KA), and apparent distribution volume (V/F) was estimated to be 0.349 h, 0.869 h-1, and 0.834 l/kg, respectively. Oral clearance (CL/F) was modeled with creatinine clearance (CLcr), and the mean value was estimated to be 1.93.CLcr l/h. In addition, pharmacokinetic parameters in individual 36 subjects were obtained from population estimates according to Bayes' theorem. Pharmacokinetic parameters (KA, V/F, and CL/F) in the single-dose study were almost constant at a dose range of 3-12 mg/kg, and were similar to those in the multiple-dose study. These findings indicated that the pharmacokinetics of mizoribine is well described by a simple one-compartment model with first-order absorption.

Safety, tolerability and pharmacokinetics of higher-dose mizoribine in healthy male volunteers

AIMS

Mizoribine is an oral immunosuppressive agent approved in several countries for prevention of rejection in renal transplantation. Its therapeutic window is based on trough concentrations staying at > or =0.5 but <3 microg ml(-1). It has been postulated that as renal function returns to normal, higher doses may be needed to maintain efficacy than the current clinical dosage of 2-5 mg kg(-1) day(-1). The safety, tolerability and pharmacokinetics from two clinical trials of higher-dose mizoribine treatments in healthy male volunteers are presented.

METHODS

Forty-eight healthy White male nonsmokers participated in two randomized, double-blind, placebo-controlled trials: 32 in a single-dose study (3, 6, 9 and 12 mg kg(-1)) and 16 in a multiple-dose study [6 mg kg(-1) day(-1) once daily for 5 days or twice daily (12 mg kg(-1) day(-1)) for 7 days]. Standard assessments of safety, tolerability and pharmacokinetics were performed.

RESULTS

The safety profiles of both studies were generally unremarkable, except for elevated serum uric acid concentrations at the highest dose (12 mg kg(-1) day(-1)) in the multiple-dose study. Orally administered mizoribine reached peak concentrations within 2-3 h and was eliminated mostly via the kidney (65-100% of dose) with a 3-h half-life. Only the 12 mg kg(-1) day(-1) group achieved trough concentrations that were within the therapeutic window. Conclusions Based on the favourable safety profile and current pharmacokinetic information, a new starting dose in the 6-12 mg kg(-1) day(-1) range is recommended in the up to 3 months acute phase following transplantation, with dose reduction recommended only if the function of the transplanted kidney is impaired.

Combination effect of mycophenolate mofetil with mizoribine on cell proliferation assays and in a mouse heart transplantation model

BACKGROUND

Mycophenolate mofetil (MMF) is a highly potent immunosuppressant that suppresses the proliferation of T and B cells by the uncompetitive inhibition of inosine monophosphate dehydrogenase (IMPDH). Consequently, under MMF immunosuppression, good graft survival has been achieved in clinical organ transplantation, although MMF shows adverse gastrointestinal effects. Mizoribine (MZ) also inhibits IMPDH in a competitive manner and is used clinically for organ transplantation in Japan as an immunosuppressant with fewer adverse gastrointestinal effects. Therefore, in this study we investigated the synergistic effects on in vitro and in vivo assays of mice of a combination of MMF with MZ immunosuppression.

METHODS

Mixed lymphocyte reaction (MLR) assay and a mouse heart transplantation model were used to evaluate the immunosuppressive effect of MMF with MZ. The median-effect principle and a combination index (CI) were employed to determine synergism, an additive effect, or antagonism.

RESULTS

Combination of MMF with MZ resulted in mild synergistic effects in the inhibition of MLR (CI = 0.854-1.143). In the mouse heart transplantation model, C57BL/6 recipients who received a BALB/c heart under the combination of MMF and MZ at 40 + 40 or 80 + 80 mg/kg/day showed a strong synergistic prolongation of graft survival with 19.7 +/- 18.9 (P = 0.0012, CI = 0.438) and 78.4 +/- 36.9 days (P = 0.0002, CI = 0.317), respectively, compared with recipients treated with MMF or MZ monotherapy. CONCLUSIONS.: The combination of MMF and MZ showed mild synergistic effects in the inhibition of MLR and strong synergistic effects in a mouse heart transplantation model.