Impact of Concentrative Nucleoside Transporter 1 Gene Polymorphism on Oral Bioavailability of Mizoribine in Stable Kidney Transplant Recipients

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.

Functional disruption of pyrimidine nucleoside transporter CNT1 results in a novel inborn error of metabolism with high excretion of uridine and cytidine

Genetic defects in the pyrimidine nucleoside transporters of the CNT transporter family have not yet been reported. Metabolic investigations in a patient with infantile afebrile tonic-clonic seizures revealed increased urinary uridine and cytidine excretion. Segregation of this metabolic trait in the family showed the same biochemical phenotype in a healthy older brother of the index. Whole exome sequencing revealed biallelic mutations in SLC28A1 encoding the pyrimidine nucleoside transporter CNT1 in the index and his brother. Both parents and unaffected sibs showed the variant in heterozygous state. The transporter is expressed in the kidneys. Compelling evidence is available for the disrupting effect of the mutation on the transport function thus explaining the increased excretion of the pyrimidine nucleosides. The exome analysis also revealed a pathogenic mutation in PRRT2 in the index, explaining the epilepsy phenotype in infancy. At present, both the index (10 years) and his older brother are asymptomatic. Mutations in SLC28A1 cause a novel inborn error of metabolism that can be explained by the disrupted activity of the pyrimidine nucleoside transporter CNT1. This is the first report describing a defect in the family of CNT concentrative pyrimidine nucleoside transporter proteins encoded by the SLC28 gene family. In all likelihood, the epilepsy phenotype in the index is unrelated to the SLC28A1 defect, as this can be fully explained by the pathogenic PRRT2 variant. Clinical data on more patients are required to prove whether pathogenic mutations in SLC28A1 have any clinical consequences or are to be considered a benign metabolic phenotype.

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.

Intestinal Nucleoside Transporters: Function, Expression, and Regulation

The gastrointestinal tract is the absorptive organ for nutrients found in foods after digestion. Nucleosides and, to a lesser extent nucleobases, are the late products of nucleoprotein digestion. These metabolites are absorbed by nucleoside (and nucleobase) transporter (NT) proteins. NTs are differentially distributed along the gastrointestinal tract showing also polarized expression in epithelial cells. Concentrative nucleoside transporters (CNTs) are mainly located at the apical side of enterocytes, whereas equilibrative nucleoside transporters (ENTs) facilitate the basolateral efflux of nucleosides and nucleobases to the bloodstream. Moreover, selected nucleotides and the bioactive nucleoside adenosine act directly on intestinal cells modulating purinergic signaling. NT-polarized insertion is tightly regulated. However, not much is known about the modulation of intestinal NT function in humans, probably due to the lack of appropriate cell models retaining CNT functional expression. Thus, the possibility of nutritional regulation of intestinal NTs has been addressed using animal models. Besides the nutrition-related role of NT proteins, orally administered drugs also need to cross the intestinal barrier, this event being a major determinant of drug bioavailability. In this regard, NT proteins might also play a role in pharmacology, thereby allowing the absorption of nucleoside- and nucleobase-derived drugs. The relative broad selectivity of these membrane transporters also suggests clinically relevant drug-drug interactions when using combined therapies. This review focuses on all these physiological and pharmacological aspects of NT protein biology. © 2017 American Physiological Society. Compr Physiol 8:1003-1017, 2018.

Mizoribine therapy combined with steroids and mizoribine blood concentration monitoring for idiopathic membranous nephropathy with steroid-resistant nephrotic syndrome

Background

We designed a prospective and randomized trial of mizoribine (MZR) therapy combined with prednisolone (PSL) for idiopathic membranous nephropathy (IMN) with steroid-resistant nephrotic syndrome (SRNS).

Methods

Patients with IMN were divided into 2 groups, and MZR combined with PSL was administered for 2 years. PSL was initially prescribed at 40 mg/day and tapered. MZR was given once-a-day at 150 mg and 3-times-a-day at 50 mg each to groups 1 and 2. Serum MZR concentrations from 0 to 4 h after administration were examined within one month of treatment. The concentration curve and peak serum level (C_max) of MZR were estimated by the population pharmacokinetic (PPK) parameters of MZR.

Results

At 2 years, 10 of 19 patients (52.6 %) in group 1 and 7 of 18 patients (38.9 %) in group 2 achieved complete remission (CR). The time-to-remission curve using the Kaplan–Meier technique revealed an increase in the cumulative CR rate in group 1, but no significant difference between the groups. Meanwhile, there was a significant difference in C_max between groups 1 and 2 (mean ± SD: 1.20 ± 0.52 vs. 0.76 ± 0.39 μg/mL, p = 0.04), and C_max levels in CR cases were significantly higher than those in non-CR cases. Receiver operating characteristic analysis showed that C_max more than 1.1 µg/mL was necessary for CR in once-a-day administration.

Conclusion

Administration of MZR once a day is useful when combined with PSL for treatment of IMN with SRNS. In addition, it is important to assay the serum concentration of MZR and to determine C_max, and more than 1.1 µg/mL of C_max is necessary for CR.

Structural basis of nucleoside and nucleoside drug selectivity by concentrative nucleoside transporters

Concentrative nucleoside transporters (CNTs) are responsible for cellular entry of nucleosides, which serve as precursors to nucleic acids and act as signaling molecules. CNTs also play a crucial role in the uptake of nucleoside-derived drugs, including anticancer and antiviral agents. Understanding how CNTs recognize and import their substrates could not only lead to a better understanding of nucleoside-related biological processes but also the design of nucleoside-derived drugs that can better reach their targets. Here, we present a combination of X-ray crystallographic and equilibrium-binding studies probing the molecular origins of nucleoside and nucleoside drug selectivity of a CNT from Vibrio cholerae. We then used this information in chemically modifying an anticancer drug so that it is better transported by and selective for a single human CNT subtype. This work provides proof of principle for utilizing transporter structural and functional information for the design of compounds that enter cells more efficiently and selectively.

DOI:http://dx.doi.org/10.7554/eLife.03604.001

DNA molecules are made from four bases—often named ‘G’, ‘A’, ‘C’, and ‘T’—that are arranged along a backbone made of sugars and phosphate groups. Chemicals called nucleosides are essentially the same as these four building blocks of DNA (and other similar molecules) but without the phosphate groups.

Proteins called nucleoside transporters are found in the membranes that surround cells and can pump nucleosides into the cell. These transporters also allow drugs that are made from modified nucleosides to enter cells; however, it was previously unclear how different transporters recognized and imported specific nucleosides.

Like other proteins, nucleoside transporters are basically strings of amino acids that have folded into a specific three-dimensional shape. A protein's shape is often important for defining what that protein can do, as often other molecules must bind to proteins—much like a key fitting into a lock. Johnson et al. have now revealed the three-dimensional structure of one nucleoside transporter protein bound to different nucleosides and nucleoside-derived chemicals, including three anti-cancer drugs and one anti-viral drug. Some of these chemicals were shown to bind more strongly to the transporter protein than others, and examining the three-dimensional structures revealed that the different chemicals interacted with slightly different amino acids in the transporter protein.

Johnson et al. then used this information to chemically modify an anticancer drug so that it is transported more easily into cells and is imported by only one of the subtypes of nucleoside transporters that are found in humans. This provides proof of principle that information about the structure and function of a transporter protein can help to redesign chemicals such that they can enter cells more efficiently, and to tailor them for transport by specific transporters. A similar approach may in the future allow researchers to design new nucleoside-derived drugs that are better at getting inside specific cells and, as such, provide effective treatments against cancers and viral infections.

DOI:http://dx.doi.org/10.7554/eLife.03604.002

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.

Population pharmacokinetics of mizoribine in pediatric recipients of renal transplantation

BACKGROUND

An immunosuppressive agent, mizoribine, is excreted predominantly in the urine. The aim of this study was to investigate the pharmacokinetic variability of mizoribine in pediatric recipients of renal transplantation.

METHODS

Pharmacokinetic data for population analysis were collected from 51 recipients (32 males and 19 females) treated with oral administration of mizoribine (0.83-5.56 mg/day/kg). The population pharmacokinetic parameters of mizoribine were estimated using a nonlinear mixed effects model program.

RESULTS

The pharmacokinetics of mizoribine in pediatric recipients of renal transplantation was well described by a one-compartment model with first-order absorption. The mean value of the absorption lag time (ALAG) and absorption rate constant (K (A)) was estimated to be 0.363 h and 0.554 h(-1), respectively. Apparent volume of distribution (V/F) was modeled as a function of body weight (WT), and the mean value was estimated to be 1.03 · WT L. Oral clearance (CL/F) was modeled as a function of creatinine clearance (CL(cr)), and the mean value was estimated to be 2.81 · CL(cr) · 60/1000 L/h. In addition, there was a positive correlation between CL(cr)-corrected CL/F and WT-corrected V/F in the pediatric recipients, indicating large interindividual variability in the bioavailability (F) of mizoribine.

CONCLUSION

The present findings indicated that the rate of renal excretion and also the extent of intestinal absorption of mizoribine are responsible for the large interindividual pharmacokinetic variability of the drug.

Population pharmacokinetics of mizoribine in adult recipients of renal transplantation

BACKGROUND

The aim of the present study was to estimate the population pharmacokinetic parameters of mizoribine in adult recipients of renal transplantation using a nonlinear mixed effects model (NONMEM) program.

METHODS

Pharmacokinetic data for population analysis were retrospectively collected from 114 recipients (66 males and 48 females) routinely treated with oral administration of mizoribine (25-450 mg/day). The range of creatinine clearance (CL(cr)) was 7.6-136.1 mL/min (mean 49.2 mL/min).

RESULTS

The pharmacokinetics of mizoribine in adult recipients of renal transplantation was well described by a 1-compartment model with first-order absorption. The mean value of the absorption lag time (ALAG) and absorption rate constant (KA) was estimated to be 0.581 and 0.983 h(-1), respectively. Apparent volume of distribution (V/F) was modeled as a function of body weight (WT), and the mean value was estimated to be 0.858 × WT L. Oral clearance (CL/F) was modeled as a function of creatinine clearance (CL(cr)), and the mean value was estimated to be 1.80 × CL(cr) × 60/1000 L/h. In addition, there was a strong correlation between CL(cr)-corrected CL/F and WT-corrected V/F in the adult recipients, indicating large interindividual variability in bioavailability (F) of mizoribine.

CONCLUSION

The present findings suggested that not only the rate of renal excretion but also the extent of intestinal absorption of mizoribine is responsible for the large interindividual pharmacokinetic variability of the drug.