The effect of sevelamer on the pharmacokinetics of cyclosporin A and mycophenolate mofetil after renal transplantation

Anne-Kathrin Pieper1, Franziska Buhle1, Steffen Bauer2, Ingrid Mai2, Klemens Budde3, Dieter Haffner1, Hans-Hellmut Neumayer3 and Uwe Querfeld1

1 Department of Pediatric Nephrology, 2 Department of Clinical Pharmacology and 3 Department of Nephrology, Charité, Humboldt University, Berlin, Germany

Correspondence and offprint requests to: Anne-Kathrin Pieper, MD, Department of Pediatric Nephrology, Charité Campus Virchow, Augustenburger Platz 1, D-13353 Berlin, Germany. Email: anne-kathrin.pieper{at}charite.de



   Abstract
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 Abstract
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 Patients and methods
 Results
 Discussion
 References
 
Background. After renal transplantation, patients with insufficient graft function may require phosphate binders. It is still unknown if sevelamer, a new calcium-free phosphate binder, interferes with the uptake of immunosuppressants. We studied its effects on the pharmacokinetics of cyclosporin A (CsA) and mycophenolate mofetil.

Methods. We examined 10 adults and eight children with stable renal graft function and stable CsA trough levels. A 12 h pharmacokinetic profile (10 observation points) was conducted without sevelamer, after a single dose and after 4 days of treatment with it. CsA levels were measured with both a monoclonal antibody assay (CEDIA) and a polyclonal antibody assay (FPIA), mycophenolic acid (MPA) levels by EMIT assay and CsA metabolites AM1, AM9 and AM4N by a modified HPLC method.

Results. Sevelamer had no significant effect on CsA kinetics [area under the curve (AUC), peak concentration (Cmax), time of Cmax]. The AUC of AM1 was decreased by 30% and Cmax by 25% after 4 days of sevelamer intake. MPA levels were significantly reduced by a mean of 25% of the AUC (P<0.05) and by 30% of the Cmax after a single dose of sevelamer.

Conclusions. A single sevelamer dose reduces the Cmax and the AUC of MPA. Its intake for several days does not significantly influence CsA kinetics.

Keywords: cyclosporin A; interaction; mycophenolate mofetil; pharmacokinetics; sevelamer; transplantation



   Introduction
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 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
After renal transplantation, patients may develop graft insufficiency due to acute or chronic rejection, or both. With progressive renal insufficiency, renal phosphate excretion is diminished, and patients may require therapy with phosphate binders to prevent secondary hyperparathyroidism. At present, calcium-containing phosphate binders are the mainstays for controlling hyperparathyroidism. However, it has become evident that their use carries a high risk of metastatic and vascular calcifications [1].

Recently, sevelamer (Renagel®), a polyvalent absorbent resin, has been introduced as a calcium-free phosphate-binding agent.

It is still unknown whether or not sevelamer interferes with the uptake of immunosuppressants, such as cyclosporin A (CsA) and mycophenolate mofetil (MMF). In view of its large surface area, this absorbent resin might interfere with the gastrointestinal resorption of immunosuppressants, possibly resulting in harmful effects, e.g. insufficient uptake of those drugs, which would increase the risk of organ rejection. Therefore, we studied the effect of sevelamer on the pharmacokinetics of CsA (Sandimmun Optoral®) and MMF (Cellcept®) in transplanted patients.



   Patients and methods
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 Patients and methods
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We studied 10 adult patients (seven treated with CsA, three treated with CsA and MMF in combination); their mean age was 43±14 years. In addition, we studied eight children (two treated with CsA and six treated with a combination of CsA and MMF) whose mean age was 12±3 years. All patients were studied 8–208 months after renal transplantation. They had had stable renal graft function and stable CsA trough levels for at least 3 months prior to the study (Table 1). A 12 h pharmacokinetic profile (10 observation points: immediately before and 0.5, 1, 1.5, 2, 3, 4, 6, 8 and 12 h after the drug intake) was conducted on: (i) day 1, without sevelamer (baseline); (ii) day 2, after a single morning dose of sevelamer (adults 1.6 g, children 1.2 g); and (iii) day 7, after 4 days of treatment with sevelamer three times per day (adults 3 x 1.6 g/day, children 3 x 1.2 g/day).


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Table 1. Patients' characteristics

 
All patients received fixed doses of sevelamer (3–4 capsules of 403 mg) in the morning and in the evening together with their ordinary medication including the immunosuppressants.

All patients received standardized meals prepared by the hospital kitchen. Their first drink (water) was served 1 h after the intake of immunosuppressants and their first meal after 2 h. Pharmacokinetic studies included repetitive blood sampling before and 0.5, 1, 1.5, 2, 3, 4, 6, 8 and 12 h after the ingestion of the drugs. Urinary phosphate excretion [phosphorus/creatinine (P/C) ratio] was determined on days 1 and 7 as an internal compliance control.

The levels of CsA were determined within 2 days in whole blood samples, stored at 4°C, by CEDIA assay (Microgenics) and FPIA assay (Abbott). CsA and its metabolites AM1, AM9, AM4N were also quantified using a modification of the HPLC method used by Christians et al. [2]. Briefly, the blood was spiked with the internal standard (cyclosporin D), and then precipitated and extracted on a solid-phase extraction cartridge (Baker-Bond C8). The eluates were evaporated to dryness, reconstituted in a mixture of acetonitrile and water and washed with n-hexane. The chromatography was performed using a Shimadzu HPLC system. The compounds were separated at 60°C on a Hypersil ODS column (VDS-Optilab) by gradient elution with acetonitrile and water. The compounds were quantified using their peak height ratio to the internal standard, based on the calibration curve of CsA. Metabolites were identified by comparing their retention times with reference standards (Dade Behring) and bile extract. The assay was linear up to 2000 µg/l. Intra-assay and inter-assay coefficients of variation ranged from 3.2 to 14.1% and from 2.4 to 12.8%, respectively. The lower limit of quantitation was 20 µg/l.

Blood samples for measuring levels of mycophenolic acid (MPA) were spun down, the plasma was stored at –20°C for 2–14 days and processed by EMIT assay (Dade-Behring).

The area under the curve (AUC) was calculated by the trapezoidal rule. The Friedman ANOVA on ranks was used for the comparison of all (non-normally distributed continuous) values. All statistical tests were two-tailed, with a P-value of 0.05 required for significance. The statistical analyses were performed using SPSS for Windows, version 10.0.

Written informed consent was obtained from all patients or their parents, or both. The study was performed following the recommendations of the Declaration of Helsinki (1996) and was authorized by the ethics committee of the Charité University Hospital, Berlin.



   Results
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 Patients and methods
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CsA
Sevelamer had only a slight effect on the kinetics of CsA. Of the AUC, the peak concentration (Cmax), or the trough levels (Ctrough), none were affected after one dose of sevelamer or after 4 days of treatment with it. The AUC as well as Cmax tended to decrease (by 8±19%; NS) after 4 days of sevelamer intake (Table 2). The CsA levels measured with polyclonal antibodies showed a significant decrease of the AUC after 4 days of treatment with sevelamer (P<0.05). Therefore, we examined the main metabolites of CsA (AM9, AM4N and AM1) with HPLC. The pharmacokinetics of AM9 and AM4N did not change substantially with respect to their AUC, Cmax and Ctrough. Only AM1 showed a statistically significant reduction. After 4 days of sevelamer intake, the AUC of AM1 decreased ~30% and Cmax ~25% compared with baseline.


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Table 2. Effect of sevelamer (Sv) on pharmacokinetics of cyclosporin A

 
MMF
In contrast to CsA, the ingestion of sevelamer significantly reduced the AUC of MPA in all but two patients by a mean of 20% after only a single dose and by a mean of 23% after 4 days of treatment (Table 4). This was mainly due to a decrease of Cmax at the same time points (Table 3). Also, the second peak after 8 h (C8) dropped by a mean of 37% (P<0.05, Figure 1). Thus, sevelamer had a significant effect on MPA kinetics in all patients, as illustrated in Figure 1a–c.


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Table 4. Effect of sevelamer on the pharmacokinetics of mycophenolic acid

 

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Table 3. Effect of sevelamer on the pharmacokinetics of AM1, AM9 and AM4N, the main metabolites of cyclosporin A

 


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Fig. 1. (a) Mycophenolic acid (MPA) kinetics for the nine different patients on day 1 without intake of sevelamer (Sv). (b) MPA kinetics for the nine different patients on day 2 after a single dose of Sv. (c) MPA kinetics for the nine different patients on day 7 after 4 days of Sv.

 
As expected, urinary phosphate excretion (P/C ratio) decreased from 16±7 to 12±5 g/l (P = 0.004) after 4 days of sevelamer treatment, indicating favourable compliance with the study protocol (n = 16).



   Discussion
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 Patients and methods
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It is well known that many drugs have clinically important interactions with the pharmacokinetics of CsA, but the effect of sevelamer has not been studied previously [3]. There is no absorption of sevelamer from the gastrointestinal tract [4], but its chemical structure suggests a potential influence on the uptake of other drugs. In the present study, neither trough levels nor the AUC or Cmax of CsA were significantly affected by sevelamer. Thus, the oral bioavailability of CsA did not change.

CsA is transformed into >30 metabolites, which have only slight immunosuppressive effects. They are excreted in bile and then absorbed again, following the enterohepatic pathway. The significant decrease of the AUC of the metabolite AM1 might be due to the binding of bile acids to sevelamer resulting in diminished reabsorption [5].

The influence of drugs on the pharmacokinetics of MMF has been little studied [6,7]. The present study shows that even a single dose of sevelamer can significantly alter the pharmacokinetics of MMF. This is most probably due to the interference of sevelamer with the gastrointestinal uptake of MMF resulting in diminished drug exposure, as shown by the decrease of Cmax by a mean of 30%, a decrease of C8 and a similar decrease of the MPA AUC.

These findings may have special relevance to children, especially since young patients have enhanced metabolism necessitating comparatively larger doses of immunosuppressants [8]. As a practical approach to solve this problem, we would suggest that sevelamer should not be administered simultaneously with MMF—and as a precaution also not with other immunosuppressants. The present data suggest that sevelamer could preferentially be given 2 h after the intake of MMF when the absorption of the latter has largely been accomplished. Alternatively, the MPA levels could be measured, and the dose of MMF could be adjusted to compensate for its reduced intake.

In summary, sevelamer, a widely used calcium-free phosphate-binding agent, interferes with the pharmacokinetics of MMF, but has no significant effects on the pharmacokinetics of CsA.



   Acknowledgments
 
This study has been conducted with funds provided by the Charité University Hospital for clinical research.

Conflict of interest statement. None declared.



   References
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 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 

  1. Oh J, Wunsch R, Turzer M et al. Advanced coronary and carotid arteriopathy in young adults with childhood-onset chronic renal failure. Circulation 2002; 106: 100–105[Abstract/Free Full Text]
  2. Christians U, Zimmer KO, Wonigeit K, Maurer G, Sewing KF. Liquid-chromatographic measurement of cyclosporin A and its metabolites in blood, bile, and urine. Clin Chem 1988; 34: 34–39[Abstract/Free Full Text]
  3. Campana C, Regazzi MB, Buggia I, Molinaro M. Clinically significant drug interactions with cyclosporin. An update. Clin Pharmacokinet 1996; 30: 141–179[ISI][Medline]
  4. Plone MA, Petersen JS, Rosenbaum DP, Burke SK. Sevelamer, a phosphate-binding polymer, is a non-absorbed compound. Clin Pharmacokinet 2002; 41: 517–523[ISI][Medline]
  5. Braunlin W, Zhorov E, Guo A et al. Bile acid binding to sevelamer HCl. Kidney Int 2002; 62: 611–619[CrossRef][ISI][Medline]
  6. Filler G, Zimmering M, Mai I. Pharmacokinetics of mycophenolate mofetil are influenced by concomitant immunosuppression. Pediatr Nephrol 2000; 14: 100–104[CrossRef][ISI][Medline]
  7. Mourad M, Wallemacq P, Konig J et al. Therapeutic monitoring of mycophenolate mofetil in organ transplant recipients: is it necessary? Clin Pharmacokinet 2002; 41: 319–327[ISI][Medline]
  8. Weber LT, Shipkova M, Lamersdorf T et al. Pharmacokinetics of mycophenolic acid (MPA) and determinants of MPA free fraction in pediatric and adult renal transplant recipients. German Study group on Mycophenolate Mofetil Therapy in Pediatric Renal Transplant Recipients. J Am Soc Nephrol 1998; 9: 1511–1520[Abstract]
Received for publication: 23.10.03
Accepted in revised form: 2. 6.04