Pharmacokinetics of mycophenolic acid and its phenyl glucuronide metabolite in kidney transplant recipients with renal impairment.

INTRODUCTION: The aim of the study was to analyse the influence of renal impairment on the pharmacokinetic parameters (PK) of mycophenolic acid (MPA) and its glucuronide metabolite (MPAG) in renal transplant recipients.
MATERIAL AND METHODS: The study included 43 adult patients during the maintenance period (> 6 months) following renal transplantation, treated with mycophenolate mofetil (MMF), calcineurin inhibitors (CNI) (tacrolimus or cyclosporine) and steroids. The study compared patients with normal renal function (n = 17; creatinine clearance (C(cr)) > 60 ml/min) and with renal impairment (n = 26; C(cr) < 60 ml/min). Areas under the 4-h curve (AUC(0-4 h)) of MPA and MPAG were determined using a validated HPLC method.
RESULTS: The renal impairment group showed significantly increased AUC(0-4 h) and pre-dose (C(0)) for MPAG compared to patients with normal renal function and increased MPA C(0). However, there was no significant difference in MPA AUC(0-4 h) between patients with renal impairment and patients with normal renal function. In multivariate analysis some MPA and MPAG PK parameters were correlated with sex, CNI co-administered and body weight.
CONCLUSIONS: Although MPAG is an inactive metabolite, its accumulation in patients with renal impairment can be unfavourable. The results of our study indicate that solely MPA C(0) determination in patients receiving MMF may be insufficient in clinical practice because of great inter-patient variability of this PK parameter caused mainly by enterohepatic recirculation.

Introduction

Mycophenolate mofetil (MMF, CellCept®) is the most widely used antiproliferative immunosuppressive drug in patients after solid organ transplantation, often co-administered with calcineurin inhibitors (CNI) and corticosteroids [1].

Pharmacologically inactive MMF after oral administration is rapidly and entirely hydrolysed into an active metabolite – mycophenolic acid (MPA). The MPA is a non-nucleoside, uncompetitive and reversible inosine monophosphate dehydrogenase inhibitor, which participates in guanosine triphosphates de novo synthesis. As a consequence, MPA inhibits the proliferation of T and B lymphocytes, preventing the acute rejection of a transplanted organ as well as decreasing the frequency of late rejection after one year and consecutive years [2, 3]. The MPA undergoes enterohepatic recirculation. It is converted into an inactive phenyl glucuronide (MPAG), which is excreted into bile and hydrolysed into MPA in the presence of β-glucuronidase, produced by intestinal bacteria, and subsequently reabsorbed in the intestines. It was found that, on average, the enterohepatic recirculation takes part in about 40% (10-60%) of the entire MPA exposure [4].

The MPA as well as MPAG pharmacokinetics (PK) show great inter-patient variability depending on race, sex, concomitant clinical condition (e.g. kidney or liver impairment, diseases with coexisting hypoalbuminaemia), interactions with drugs influencing the PK of MPA and MPAG, time elapsed after renal transplantation and pharmacogenetic factors [4-6].

The MPAG is eliminated mainly by the kidney and the increase of its concentration may indicate the deterioration of renal function. Compared to patients with normal renal function, MPAG concentration in patients with impaired renal function is several times greater. Uncontrolled MPAG concentration increase causes MPA displacement from protein compounds and the increase of free MPA, which might be the cause of higher pharmacological activity of the drug and the intensification of its adverse effects. The influence of renal failure on total MPA concentration is still unclear. There are no unequivocal recommendations for MMF dosing in patients with renal failure [4, 7 8].

The aim of the study was to analyse the influence of renal impairment on the PK of MPA and its metabolite MPAG in renal transplant recipients.

Material and methods

Forty-three adult patients during the maintenance period (> 6 months) following renal transplantation receiving MMF in combination with cyclosporine (CsA) (n = 21) or tacrolimus (Tac) (n = 22) and corticosteroids were considered. The MMF dose varied from 0.50 g/day to 2.00 g/day and was administered most often twice a day. The study included patients with normal renal function (n = 16; creatinine clearance (Ccr) > 60 ml/min) and with renal impairment (n = 27, Ccr < 60 ml/min). Clinical characteristics of the patients are presented in Table I. Blood samples were collected into tubes containing EDTA at the following time points: before the morning dose of MMF (C0), and subsequently 40 min, 1, 2, 3, 4 h after dosing.

Table I

Characteristics of study patients

Number of patients	All (n = 43)	Ccr> 60 ml/min n = 17)	Ccr< 60 ml/min (n = 26)
Male/female	23/20	11/6	12/14
CsA/Tac	21/22	8/9	13/13
Corticosteroid use	36	12	24
Proteinuria (yes/no)	9/34	1/16	8/18
Median (range)
Age [years]	44 (23-69)	44 (24-57)	44 (23-69)
Time after transplantation [years]	4.1 (0.6-10.9)	3.8 (0.6-10.8)	4.4 (1.2-10.9)
Body weight [kg]	74 (44-109)	75 (47-109)	73.5 (44-105)
Body mass index [kg/m2]	25.2 (16.1-36.8)	25.8 (21.2-36.5)	23.7 (16.1-36.8)
Body surface area [m2]	1.86 (1.37-2.35)	1.89 (1.37-2.35)	1.85 (1.39-2.22)
MMF dose [g/day]	1.50 (0.50-2.00)	1.00 (1.00-2.00)	1.50 (0.50-2.00)
CsA dose [mg/day]	175 (75-275)	162.5 (100-250)	175 (75-275)
Tac dose [mg/day]	4 (1-12)	4 (1-12)	4 (2-9)
Serum creatinine [mg/dl]	1.7 (0.8-4.8)	1.3 (0.8-1.7)	2.1 (1.0-4.8)*
Ccr [ml/min]**	53.7 (20.4-114.7)	75.2 (60.5-114.7)	44.9 (20.4-59.7)*

C – creatinine clearance, CsA – cyclosporine, Tac – tacrolimus, MMF – mycophenolate mofetil

p < 0.05, patients with C > 60 ml/min vs. < 60 ml/min, Mann-Whitney test

estimated by the Cockcroft-Gault equation

Plasma MPA and MPAG concentrations were determined using a high-performance liquid chromatography (HPLC) method described previously [9, 10]. Briefly, the separation was performed using a SupelcosilTM LC-18-DB column. The mobile phase consisted of acetonitrile and orthophosphoric acid (50 mmol/l) in a 50 : 50 ratio and indomethacin was used as an internal standard. The MPAG plasma concentration was estimated after MPAG enzymatic hydrolysis into MPA in β-glucuronidase presence (activity 89.4 U/ml). The MPAG concentrations were calculated as a result of subtraction of MPA molar concentration before the hydrolysis from MPA concentration after the enzymatic hydrolysis (CMPAG = CMPA after enzymatic hydrolysis – CMPA before hydrolysis). The MPAG was generously supplied by Roche Pharmaceuticals (Palo Alto, CA).

The analysis method had a linear range of 0.1-25.0 µg/ml and 10-250 µg/ml for MPA and MPAG, respectively. Accuracy of MPA and MPAG batches ranged from 97.4% to 106.4% and from 96.5% to 116.6%, respectively, and the precision, expressed as the percentage of coefficient of variation (% CV), ranged from 0.8% to 6.9% and from 0.9% to 17.4%, respectively.

The following PK parameters for MPA and MPAG were calculated: pre-dose concentration (C0), maximum concentration (Cmax) and area under the plasma concentration – time curve from 0 to 4 h (AUC0-4 h) using the linear trapezoidal rule. Inter-patient variability was assessed as % CV. Ccr was estimated using the Cockcroft-Gault formula.

All statistical tests were performed using Statistica software version 8.0 and p value < 0.05 was considered significant. Chi-square test was used for the evaluation of qualitative data. Normality was determined by Shapiro-Wilk W test. Data were evaluated by the Mann-Whitney test and Spearman correlation analysis. Multivariate analysis was performed to evaluate the influence of various clinical determinants on MPA and MPAG PK parameters. For multivariate analysis both PK parameters corrected and uncorrected for MMF dose were examined. In the remaining analyses only dose-corrected values were considered.

The study was performed following the recommendations of the Declaration of Helsinki and approved by the Bioethical Commission at the Poznan University of Medical Sciences. Informed consent was obtained from all patients prior to initiating the study.

Results

The two groups of patients (with Ccr above or below 60 ml/min) were comparable for sex, age, time elapsed after renal transplantation, body weight, MMF dose and for number of patients receiving CsA or Tac. For groups with normal and impaired renal function, medians of serum creatinine were 1.3 mg/dl and 2.1 mg/dl, respectively, and median Ccr 75.2 ml/min and 44.9 ml/min, respectively.

The MPA C0 values were significantly higher in patients with impaired renal function than in patients with normal renal function. However, AUC0-4 h and Cmax values were not significantly different between the two groups (Figure 1, Table II). Moreover, the values of PK parameters for MPAG (AUC0-4 h, C0 and Cmax) were also significantly higher in patients with impaired renal function (Table II). Comparable differences for MPA and MPAG PK parameters were observed between groups in particular immunosuppressive regimens; however, the differences for MPA C0 values were barely significant (Table III).

Figure 1

Dose-corrected (A) mycophenolic acid (MPA) and (B) mycophenolic acid glucuronide (MPAG) concentrationtime profiles classified by renal function (creatinine clearance (Ccr) > 60 ml/min vs. < 60 ml/ min). The results are presented as median ± standard deviation. Concentrations are corrected to a daily dose of 1 g mycophenolate mofetil

Table II

Dose-corrected PK parameters* of MPA and MPAG classified by renal function

Parameter	All (n = 43)	CV (%)	Ccr> 60 ml/min (n = 17)	CV (%)	Ccr< 60 ml/min (n = 26)	CV (%)	Value of p**
MPA
AUC0-4 h [µg × h/ml]	19.40 (8.71-35.46)	31.0	19.01 (14.45-30.48)	23.4	21.25 (8.71-35.46)	34.7	0.285
C0 [µg/ml]	1.93 (0.44-5.26)	60.4	1.48 (0.44-2.39)	43.3	2.24 (0.50-5.26)	55.6	0.010
Cmax [µg/ml]	10.35 (3.74-24.13)	40.9	10.35 (5.50-24.13)	42.0	10.07 (3.74-18.92)	39.9	0.650
MPAG
AUC0-4 h [µg × h/ml]	235.68 (112.67-857.26)	57.0	172.47 (112.67-306.15)	29.3	299.94 (170.11-857.26)	47.8	< 0.001
C0 [µg/ml]	36.44 (8.26-185.90)	72.9	29.65 (8.26-53.63)	41.8	61.01 (29.63-185.90)	60.8	< 0.001
Cmax [µg/ml]	68.21 (33.95-240.30)	58.2	50.94 (33.95-87.49)	27.6	103.92 (44.81-240.30)	47.6	< 0.001

PK – pharmacokinetic, MPA – mycophenolic acid, MPAG – 7-O-mycophenolic acid glucuronide, CV – coefficient of variation, C – creatinine clearance, AUC – area under the concentration-time curve, C – pre-dose concentration, C – maximum concentration

the results are presented as median (range)

comparison between patients with C > 60 ml/min and < 60 ml/min, Mann-Whitney test

Table III

Dose-corrected PK parameters* of MPA and MPAG in patients treated with Tac or CsA classified by renal function

	Tac (n = 22)		Value of p*	CsA (n = 21)		Value of p**
	Ccr> 60 ml/min (n= 9)	Ccr< 60 ml/min (n = 13)		Ccr> 60 ml/min (n = 8)	Ccr< 60 ml/min (n = 13)
MPA
AUC0-4 h [µg × h/ml]	19.96 (14.83-30.48)	21.91(11.83-35.46)	0.537	16.51 (14.45-23.71)	20.58 (8.71-32.47)	0.311
C0 [µg/ml]	1.98 (0.73-2.30)	2.46 (0.76-5.26)	0.037	1.11 (0.44-2.39)	1.90 (0.50-4.37)	0.060
Cmax [µg/ml]	13.96 (5.50-24.13)	9.20 (5.63-18.92)	0.643	8.90 (6.20-18.08)	10.49 (3.74-17.17)	0.717
MPAG
AUC0-4 h [µg × h/ml]	189.06 (112.67-229.25)	298.05 (170.11-588.54)	0.014	168.36 (124.12-306.15)	359.29 (195.42-857.26)	0.002
C0 [µg/ml]	31.06 (10.70-42.47)	39.07 (29.63-133.84)	0.014	25.66 (8.26-53.63)	66.80 (33.99-185.90)	0.003
Cmax [µg/ml]	50.94 (33.95-68.21)	100.43 (44.81-226.41)	0.021	52.13 (39.77-87.49)	115.50 (61.67-240.30)	0.002

PK – pharmacokinetic, MPA – mycophenolic acid, MPAG – 7-O-mycophenolic acid glucuronide, Tac – tacrolimus, CsA – cyclosporine, C – creatinine clearance, AUC – area under the concentration-time curve, C – pre-dose concentration, C – maximum concentration

the results are presented as median (range)

comparison between patients with C > 60 ml/min and < 60 ml/min, Mann-Whitney test

In univariate analysis, MPAG PK parameters (AUC0-4 h, C0, Cmax) were negatively correlated with Ccr and positively correlated with serum creatinine (Figure 2). Within MPA PK parameters, only C0 was negatively correlated with Ccr and positively correlated with serum creatinine. Similar correlations were found in particular immunosuppressive regimens, although the serum creatinine correlation with MPA C0 in patients receiving MMF in combination with CsA was statistically insignificant (Table IV).

Figure 2

Correlation between creatinine clearance (A-B) or serum creatinine (C-D) and mycophenolic acid (MPA) or mycophenolic acid glucuronide (MPAG) area under the curve (AUC) in renal transplant recipients

Table IV

Spearman's rank correlation between creatinine clearance or serum creatinine and PK parameters of MPA and MPAG in patients treated with Tac or CsA

	All (n = 43)		Tac (n = 22)		CsA (n = 21)
	r	p	r	p	r	p
Creatinine clearance [ml/min]
MPA
AUC0-4 h [µg × h/ml]	–0.023	0.886	0.068	0.763	–0.178	0.440
C0 [µg/ml]	–0.406	0.007	–0.494	0.019	–0.460	0.036
Cmax [µg/ml]	0.202	0.194	0.289	0.193	0.043	0.854
MPAG
AUC0-4 h [µg × h/ml]	–0.783	< 0.001	–0.668	< 0.001	–0.860	< 0.001
C0 [µg/ml]	–0.793	< 0.001	–0.702	< 0.001	–0.849	< 0.001
Cmax [µg/ml]	–0.724	< 0.001	–0.589	0.004	–0.848	< 0.001
Serum creatinine [mg/dl]
MPA
AUC0-4 h [µg × h/ml]	–0.098	0.534	–0.078	0.730	–0.127	0.584
C0 [µg/ml]	0.333	0.029	0.494	0.020	0.208	0.365
Cmax [µg/ml]	–0.258	0.095	–0.312	0.158	–0.152	0.510
MPAG
AUC0-4 h [µg × h/ml]	0.782	< 0.001	0.756	< 0.001	0.810	< 0.001
C0 [µg/ml]	0.739	< 0.001	0.691	< 0.001	0.762	< 0.001
Cmax [µg/ml]	0.713	< 0.001	0.625	0.002	0.816	< 0.001

PK – pharmacokinetic, MPA – mycophenolic acid, MPAG – 7-O-mycophenolic acid glucuronide, Tac – tacrolimus, CsA – cyclosporine, AUC – area under the concentration-time curve, C – pre-dose concentration, C – maximum concentration

In multivariate analysis the results are consistent with univariate analysis. Additionally, some MPA and MPAG PK parameters were correlated with sex, CNI co-administered and body weight. No correlations with proteinuria were observed (Tables V and VI).

Table V

Multivariate analysis of clinical parameters influencing MPA PK parameters

Independent variables	Relation	Partial value of p	Model value of p	Model R coefficient
MPA C0 [mg/l]
Sex [men/women]	NS	0.138	0.003	0.544
CNI [CsA/Tac]	NS	0.064
MMF dose [g]	Positive	0.022
Ccr [ml/min]	Negative	0.008
MPA C0 [mg/l]
MMF dose [g]	NS	0.075	0.033	0.435
Ccr [ml/min]	NS	0.055
Proteinuria [yes/no]	NS	0.562
MPA AUC0-4 h [mg × h/l]
Sex [men/women]	Positive	0.017	< 0.001	0.593
MMF dose [g]	Positive	< 0.001
Ccr [ml/min]	NS	0.694
MPA AUC0-4 h [mg × h/l]
MMF dose [g]	Positive	< 0.001	0.001	0.569
Ccr [ml/min]	NS	0.420
Proteinuria [yes/no]	NS	0.682
MPA Cmax [mg/l]
Body weight [kg]	NS	0.055	0.007	0.438
MMF dose [g]	Positive	0.002
Ccr [ml/min]	Positive	0.039
MPA Cmax [mg/l]
MMF dose [g]	Positive	0.001	0.003	0.532
Ccr [ml/min]	Positive	0.046
Proteinuria [yes/no]	NS	0.372
MPA C0 /dose [mg/l/g]
Sex [men/women]	NS	0.173	< 0.001	0.565
CNI [CsA/Tac]	Negative	0.007
Ccr [ml/min]	Negative	0.004
MPA C0 /dose [mg/l/g]
Ccr [ml/min]	Negative	0.042	0.098	0.323
Proteinuria [yes/no]	NS	0.792
MPA AUC0-4 h/dose [mg × h/l/g]
Sex [men/women]	Positive	0.017	0.015	0.449
CNI [CsA/Tac]	NS	0.061
Ccr [ml/min]	NS	0.613
MPA AUC0-4 h/dose [mg × h/l/g]
Ccr [ml/min]	NS	0.565	0.582	0.159
Proteinuria [yes/no]	NS	0.466
MPA Cmax/dose [mg/l]
Body weight [kg]	Negative	0.012	0.036	0.332
Ccr [ml/min]	NS	0.055
MPA Cmax/dose [mg/l]
Ccr [ml/min]	NS	0.061	0.066	0.349
Proteinuria [yes/no]	NS	0.307

MPA – mycophenolic acid, PK – pharmacokinetic, C – pre-dose concentration, NS – not significant, CNI – calcineurin inhibitor, CsA – cyclosporine, Tac – tacrolimus, MMF – mycophenolate mofetil, C – creatinine clearance, AUC – area under the concentration-time curve, C – maximum concentration

Table VI

Multivariate analysis of clinical parameters influencing MPAG PK parameters

Independent variables	Relation	Partial value of p	Model value of p	Model R coefficient
MPAG C0 [mg/l]
Sex [men/women]	Positive	< 0.001	< 0.001	0.842
Body weight [kg]	Positive	< 0.001
MMF dose [g]	Positive	< 0.001
Ccr [ml/min]	Negative	< 0.001
MPAG C0 [mg/l]
MMF dose [g]	Positive	< 0.001	< 0.001	0.704
Ccr [ml/min]	Negative	< 0.001
Proteinuria [yes/no]	NS	0.191
MPAG AUC0-4 h [mg × h/l]
Sex [men/women]	Positive	< 0.001	< 0.001	0.847
Body weight [kg]	Positive	< 0.001
MMF dose [g]	Positive	< 0.001
Ccr [ml/min]	Negative	< 0.001
MPAG AUC0-4 h [mg × h/l]
MMF dose [g]	Positive	< 0.001	< 0.001	0.681
Ccr [ml/min]	Negative	< 0.001
Proteinuria [yes/no]	NS	0.526
MPAG Cmax [mg/l]
Sex [men/women]	Positive	< 0.001	< 0.001	0.832
Body weight [kg]	Positive	< 0.001
MMF dose [g]	Positive	< 0.001
Ccr [ml/min]	Negative	< 0.001
MPAG Cmax [mg/l]
MMF dose [g]	Positive	< 0.001	< 0.001	0.651
Ccr [ml/min]	Negative	0.001
Proteinuria [yes/no]	NS	0.689
MPAG C0/dose [mg/l/g]
Sex [men/women]	Positive	< 0.001	< 0.001	0.873
Body weight [kg]	Positive	< 0.001
CNI [CsA/Tac]	Positive	0.047
Ccr [ml/min]	Negative	< 0.001
MPAG C0/dose [mg/l/g]
Ccr [ml/min]	Negative	< 0.001	< 0.001	0.661
Proteinuria [yes/no]	NS	0.130
MPAG AUC0-4 h/dose [mg × h/l/g]
Sex [men/women]	Positive	< 0.001	< 0.001	0.886
Body weight [kg]	Positive	< 0.001
CNI [CsA/Tac]	Positive	0.015
Ccr [ml/min]	Negative	< 0.001
MPAG AUC0-4 h/dose [mg × h/l/g]
Ccr [ml/min]	Negative	< 0.001	< 0.001	0.583
Proteinuria [yes/no]	NS	0.501
MPAG Cmax [mg/l]
Sex [men/women]	Positive	< 0.001	< 0.001	0.782
Body weight [kg]	Positive	< 0.001
Ccr [ml/min]	Negative	< 0.001
MPAG Cmax [mg/l]
Ccr [ml/min]	Negative	< 0.001	0.001	0.524
Proteinuria [yes/no]	NS	0.861

MPAG – 7-O-mycophenolic acid glucuronide, PK – pharmacokinetic, C – pre-dose concentration, CNI – calcineurin inhibitor, CsA – cyclosporine, Tac – tacrolimus, MMF – mycophenolate mofetil, C – creatinine clearance, NS – not significant, AUC – area under the concentration-time curve, C – maximum concentration

For MPA and MPAG PK parameters, high CV values were observed, which in patients with impaired renal function exceeded 30% for all PK parameters evaluated. The CV values were lower for MPA AUC0-4 h compared to MPA C0 (Table II).

Discussion

In our study we analysed the influence of renal function on MPA and MPAG PK in patients during the maintenance period following renal transplantation. The concentrations of both MMF metabolites (MPA and MPAG) varied widely, particularly in patients with impaired renal function, despite MMF dose correction. Similar variability was also presented by other authors [11-13].

Total drug exposure is best demonstrated by AUC0-12 h. In clinical practice this parameter is difficult to assess, especially in patients treated in clinics during the maintenance period following renal transplantation. In our study, blood samples were collected within 4 h after drug administration due to the short duration of the patient's stay in hospital during a routine appointment in a transplant clinic. The MPA C0 is the most often used parameter in clinical practice. Its value may be considerably influenced by MPAG enterohepatic recirculation, resulting in the second MPA plasma concentration peak (Cmax2) approximately 6-12 h after drug administration. In our study, higher MPA C0 as well as MPAG C0, Cmax and AUC0-4 h values were observed in patients with impaired renal function regardless of CNI used, whereas there were no differences in the values of the other MPA PK parameters (Cmax and AUC0-4 h). Elevated MPA C0 values may indicate intensified enterohepatic recirculation due to MPAG concentration increase in patients with impaired renal function. The correlations between renal function and MPAG concentrations as well as MPA C0 suggest that the cause of the differences observed between two groups is the dependence of MPAG concentration and indirectly MPA C0 on renal function. The results concerning MPAG are consistent with the literature. However, the influence of renal function on MPA PK parameters is still unclear.

Similar results concerning MPA and MPAG PK parameters were obtained by Gonzalez-Roncero et al. 14]. The authors demonstrated not only the influence of renal function on MPA C0 and MPAG AUC but also its lack on MPA AUC. The studies by Zanker et al. [15], Morgera et al. [16] and Weber et al. [17] also observed higher MPAG AUC in renal transplant recipients with lower Ccr than in patients with normal glomerular filtration. However, values of MPA AUC did not differ significantly. On the other hand, the results for MPA obtained by Mohammadpur et al. [18] were different. According to this study, significantly higher MPA AUC0-12 h values along with unchanged C0 and significantly higher MPA Cmax2 values were observed in patients with impaired renal function. The authors also found a negative correlation for Ccr and MPA AUC. The use of abbreviated MPA profiles in our study may be the reason for different AUC results compared with the Mohammadpur et al. study. The abbreviated profiles do not include MPA Cmax2 associated with enterohepatic recirculation. In contrast to these studies, van Hest et al. [19] and Weber et al. [20] observed a total MPA concentration decrease in patients with impaired renal function. It was suggested that the concentration decrease was due to the accumulation of MPAG, which displaced MPA from its protein compounds, increasing free MPA concentration and its clearance. The controversies concerning the influence of renal failure on MPA concentration values may be explained by Naesens et al. [21]. The authors observed increased MPA and MPAG concentration values in patients with moderately reduced renal function (Ccr ranging from 30 ml/min to 60 ml/min), whereas decreased total MPA concentration values were observed in patients with severe graft dysfunction (Ccr < 30 ml/ min), similarly as in the study by van Hest et al. [19]. This may explain the lack of differences in MPA AUC0-4 h and Cmax values between groups with Ccr above or below 60 ml/min in our study, because the group with impaired renal function included patients with moderate as well as with severe graft dysfunction. Nevertheless, our study showed higher MPA C0 values in patients with Ccr < 60 ml/ min, which may indicate intensified enterohepatic recirculation in patients with impaired renal function as found in the study by Mohammadpur et al. [18].

Proteinuria may also be considered as a factor influencing MPA and MPAG PK parameters. Naesens et al. [21] demonstrated correlations between MPA PK parameters and proteinuria. However, in our study in the multivariate analysis no relations between MPA and MPAG PK parameters were observed. It may be due to the small number of patients (n = 9) with proteins determined in urine.

In conclusion, renal failure, irrespective of immunosuppressive regimen, influences the MPAG PK parameters and MPA C0 by increasing the values in patients with impaired renal function. The results of our study indicate that solely MPA C0 determination in patients receiving MMF may be insufficient in clinical practice because of great inter-patient variability of this PK parameter caused mainly by enterohepatic recirculation. It may lead to wrong decisions concerning the pharmacotherapy, particularly in patients with impaired renal function. It seems to us that the monitoring of MPA and MPAG AUC may allow MMF dosage regimens to be optimized, and as a result improve the early prevention of adverse effects in these patients.

Acknowledgments

This study was supported by Grant No. 501-02-03306413-02324-50382 from Poznan University of Medical Sciences. The authors declare no conflict of interest.