Maria Shipkova

Acyl glucuronide drug metabolites: toxicological and analytical implications.

Although glucuronidation is generally considered a detoxification route of drug metabolism, the chemical reactivity of acyl glucuronides has been linked with the toxic properties of drugs that contain carboxylic acid moieties. It is now well documented that such metabolites can reach appreciable concentrations in blood. Furthermore, they are labile, undergo hydrolysis and pH-dependent intramolecular acyl migration to isomeric conjugates of glucuronic acid, and may react irreversibly with plasma proteins, tissue proteins, and with nucleic acids. This stable binding causes chemical alterations that are thought to contribute to drug toxicity either through changes in the functional properties of the modified molecules or through antigen formation with subsequent hypersensitivity and other immune reactions. Whereas in vitro data on the toxicity of acyl glucuronides have steadily accumulated, direct evidence for their toxicity in vivo is scarce. Acyl glucuronides display limited stability, which is dependent on pH, temperature, nature of the aglycon, and so on. Therefore, careful sample collection, handling, and storage procedures are critical to ensure generation of reliable pharmacologic and toxicologic data during clinical studies. Acyl glucuronides can be directly quantified in biologic specimens using chromatographic procedures. Their adducts with plasma or cell proteins can be determined after electrophoretic separation, followed by blotting. ELISA techniques have been used to assess the presence of antibodies against acyl glucuronide–protein adducts. This review summarizes the most recent evidence concerning biologic and toxicologic effects of acyl glucuronide metabolites of various drugs and discusses their relevance for drug monitoring. A critical evaluation of the available methodology is included.

Identification of glucoside and carboxyl‐linked glucuronide conjugates of mycophenolic acid in plasma of transplant recipients treated with mycophenolate mofetil

Mycophenolic acid (MPA), is primarily metabolized in the liver to 7‐O‐MPA‐β‐glucuronide (MPAG). Using RP‐h.p.l.c. we observed three further MPA metabolites, M‐1, M‐2, M‐3, in plasma of transplant recipients on MMF therapy. To obtain information on the structure and source of these metabolites: (A) h.p.l.c. fractions containing either metabolite or MPA were collected and analysed by tandem mass spectrometry; (B) the metabolism of MPA was studied in human liver microsomes in the presence of UDP‐glucuronic acid, UDP‐glucose or NADPH; (C) hydrolysis of metabolites was investigated using β‐glucosidase, β‐glucuronidase or NaOH; (D) cross‐reactivity of each metabolite was tested in an immunoassay for MPA (EMIT). Mass spectrometry of M‐1, M‐2, MPA and MPAG in the negative ion mode revealed molecular ions of m/z 481, m/z 495, m/z 319 and m/z 495 respectively. Incubation of microsomes with MPA and UDP‐glucose produced M‐1, with MPA and UDP‐glucuronic acid MPAG and M‐2 were formed, while with MPA and NADPH, M‐3 was observed. β‐Glucosidase hydrolysed M‐1 completely. β‐Glucuronidase treatment led to a complete disappearance of MPAG whereas the amount of M‐2 was reduced by approximately 30%. Only M‐2 was labile to alkaline treatment. M‐2 and MPA but not M‐1 and MPAG cross‐reacted in the EMIT assay. These results suggest that: (i) M‐1 is the 7‐OH glucose conjugate of MPA; (ii) M‐2 is the acyl glucuronide conjugate of MPA; (iii) M‐3 is derived from the hepatic CYP450 system.

Pharmacokinetic and metabolic investigations of mycophenolic acid in pediatric patients after renal transplantation : Implications for therapeutic drug monitoring

The need for mycophenolic acid (MPA) monitoring is still under discussion. Key issues for the PK/PD relationships of this drug are: the role of metabolites, the usefulness of AUC versus predose levels, and the need to monitor the free concentration of MPA (f-MPA). Recent advances have revealed that, in addition to 7-O-MPAG, three additional MPA metabolites are present in the plasma of transplant recipients. One of these metabolites (M-2), identified as an acyl glucuronide of MPA, was found to inhibit IMPDH-II in vitro. This active metabolite was also found to cross-react in the Emit assay for MPA. In an ongoing multicenter study, the authors are evaluating the relevance of monitoring total (t-MPA) and free mycophenolic acid (f-MPA) in pediatric renal transplant recipients. As in adults, a time-dependent increase of t-MPA-AUC(0-12h) within the first 3 months posttransplant (35 versus 64 mg x h/L, [corrected] 3 weeks versus 3 months respectively; daily dosage: 0.6 g/m2 bid) was seen. Receiver operating characteristics curve analyses were used to test the ability of predose levels or AUC(0-12h) to discriminate between cases with no complications and those with acute rejection, adverse events (severe infections, leukopenia), or gastrointestinal disorders observed during the early posttransplant course. In agreement with observations in adults, a significant (p = 0.001) association was observed between AUC(0-12h) and acute rejection. A t-MPA-AUC(0-12h) of approximately 30-60 mg x h/L [corrected], as determined by HPLC, seems to be a reasonable target for the early posttransplant period. It remains to be elucidated whether regular predose level monitoring may be of more practical value. A higher incidence of rejection was observed at predose MPA concentrations < or = 1 mg/L, as measured by HPLC. In contrast to t-MPA, f-MPA-AUC(0-12h) was significantly related to severe infections and leukopenia. The risk for severe adverse events was increased at f-MPA- AUC(0-12h) values > or =600 microg x h/L [corrected]. On the basis of these data and the observed variability in the pharmacokinetics of MPA, the development of monitoring strategies for this drug appears to be promising.

Glucuronide and glucoside conjugation of mycophenolic acid by human liver, kidney and intestinal microsomes

Mycophenolic acid (MPA) is primarily metabolized to a phenolic glucuronide (MPAG) as well as to two further minor metabolites: an acyl glucuronide (AcMPAG) and a phenolic glucoside (MPAG1s). This study presents investigations of the formation of these metabolites by human liver (HLM), kidney (HKM), and intestinal (HIM) microsomes, as well as by recombinant UDP‐glucuronosyltransferases. HLM (n=5), HKM (n=6), HIM (n=5) and recombinant UGTs were incubated in the presence of either UDP‐glucuronic acid or UDP‐glucose and various concentrations of MPA. Metabolite formation was followed by h.p.l.c. All microsomes investigated formed both MPAG and AcMPAG. Whereas the efficiency of MPAG formation was greater with HKM compared to HLM, AcMPAG formation was greater with HLM than HKM. HIM showed the lowest glucuronidation efficiency and the greatest interindividual variation. The capacity for MPAGls formation was highest in HKM, while no glucoside was detected with HIM. HKM produced a second metabolite when incubated with MPA and UDP‐glucose, which was labile to alkaline treatment. Mass spectrometry of this metabolite in the negative ion mode revealed a molecular ion of m/z 481 compatible with an acyl glucoside conjugate of MPA. All recombinant UGTs investigated were able to glucuronidate MPA with KM values ranging from 115.3 to 275.7 μM l−1 and Vmax values between 29 and 106 pM min−1 mg protein−1. Even though the liver is the most important site of MPA glucuronidation, extrahepatic tissues particularly the kidney may play a significant role in the overall biotransformation of MPA in man. Only kidney microsomes formed a putative acyl glucoside of MPA.

Twelve-month evaluation of the clinical pharmacokinetics of total and free mycophenolic acid and its glucuronide metabolites in renal allograft recipients on low dose tacrolimus in combination with mycophenolate mofetil

Background: The establishment of a rationale for therapeutic drug monitoring for mycophenolic acid (MPA) and outlining a therapeutic window remains a challenging task in renal transplantation. Furthermore, the pharmacokinetic characteristics of free and total MPA and its glucuronides depend directly or indirectly on graft function and the type of co‐administered calcineurin‐inhibitor. Methods: The authors conducted a prospective 12‐month multicenter pharmacokinetic study on MPA (MPA, free MPA, free fraction MPA) and its metabolites (MPAG, Acyl‐MPAG). The aim of this study was to examine the long‐term pharmacokinetic characteristics of MMF when combined with tacrolimus in renal allograft recipients and to identify a possible relationship between these pharmacokinetic parameters and clinical outcome parameters. Results: They have demonstrated that in renal transplant recipients MPA, free MPA, Acyl‐MPAG and MPAG have a particular pharmacokinetic profile when combined with tacrolimus which differs from the combination with CsA. They could not establish a relationship between pre‐dose trough concentration of MPA and its metabolites and clinical efficacy endpoints and drug‐related adverse events, except for anemia. Conclusions: These findings suggest that trough plasma concentration monitoring of MPA and its metabolites might not provide a useful clinical tool for guiding MMF dose adjustments to avoid drug‐related toxicity. More extensive pharmacokinetic measurements like area under the concentration curves might be necessary for routine therapeutic drug monitoring of MMF.

Induction of cytokine release by the acyl glucuronide of mycophenolic acid: a link to side effects?

OBJECTIVES We have identified an acyl glucuronide (M-2) of the immunosuppressant mycophenolic acid (MPA). Acyl glucuronides have toxic potential and may contribute to drug toxicity. Whether acyl glucuronides are able to induce release of proinflammatory cytokines is unknown. Gastrointestinal disturbances have been observed during MPA therapy and may involve an inflammatory reaction. This study investigated whether M-2 can induce IL-6 and TNF-alpha release as well as gene expression of these cytokines in leukocytes. DESIGN AND METHODS M-2 was produced by incubation of MPA with human liver microsomes. Human mononuclear leukocytes were incubated in the presence of M-2. Concentrations of IL-6 and TNF-alpha were measured by ELISA. Expression of mRNA was determined by quantitative RT-PCR. RESULTS Incubation of 3 x 10(6) cells with M-2 resulted in a time and dose dependent release of cytokines, whereas MPA or its phenolic glucuronide MPAG were without effect. Cytokine liberation depended on mRNA induction. Response to M-2 showed much inter individual variability (30-fold for IL-6, 3-fold for TNF-alpha). CONCLUSIONS If M-2 promotes release of cytokines in vivo, these may mediate some of the toxic actions of MPA.

Erosive enterocolitis in mycophenolate mofetil-treated renal-transplant recipients with persistent afebrile diarrhea.

Background. Diarrhea is the most frequently reported adverse event in mycophenolate mofetil (MMF)-treated transplant patients. The aim of this study was to explore the gastrointestinal tract in MMF-treated renal transplant recipients with persistent afebrile diarrhea to characterize its nature and etiology. Methods. Renal transplant recipients with persistent afebrile diarrhea (daily fecal output >200 g) were prospectively investigated for infections, morphologic, and functional (gastrointestinal motility and intestinal absorptive capacity) integrity of the gastrointestinal tract; 26 patients met the inclusion criteria. Results. All but one patient had an erosive enterocolitis. Seventy percent of the patients had malabsorption of nutrients, contributing to the diarrhea. In ±60%, an infectious origin was demonstrated and successfully treated with antimicrobial agents without changes in immunosuppressive regimen. In ±40%, no infection occurred, but a Crohn’s disease-like pattern of inflammation was noted. These patients also had a less pronounced bile-acid malabsorption but a significant faster colonic transit time, correlating with the trough level of mycophenolic acid (MPA). Cessation of MMF, however, was associated with allograft rejection in one third of these patients. Conclusions. Persistent afebrile diarrhea in renal transplant recipients is characterized by erosive enterocolitis, which is of infectious origin in ±60%. In ±40%, a Crohn’s disease-like (entero-)colitis was present. Because reduction or cessation of MMF was the only effective therapy, MPA or one of its metabolites may be suggested as a possible cause. However, reduction or cessation of MMF was associated with an increased risk for rejection.

Area under the plasma concentration-time curve for total, but not for free, mycophenolic acid increases in the stable phase after renal transplantation: a longitudinal study in pediatric patients. German Study Group on Mycophenolate Mofetil Therapy in Pediatric Renal Transplant Recipients.

Mycophenolate mofetil, an ester prodrug of the immunosuppressant mycophenolic acid (MPA), is widely used for maintenance immunosuppressive therapy in pediatric renal transplant recipients. However, little is known about the pharmacokinetics of MPA in this patient population in the stable transplant phase, and dosage guidelines are preliminary. The authors therefore compared the pharmacokinetics of MPA, free MPA, and the renal metabolite MPA glucuronide (MPAG) in the initial (sampling at 1 and 3 weeks) and stable phases (sampling at 3 and 6 months) posttransplant in 17 children (age, 12.0 +/- 0.77 years; range, 5.9 to 15.8 years), receiving the currently recommended dose of 600 mg MMF/m2 body surface area (BSA) twice a day. Plasma concentrations of MPA and MPAG were measured by reverse phase HPLC. Because MPA is extensively bound to serum albumin and only the free drug is presumed to be pharmacologically active, the authors also analyzed the MPA free fraction by HPLC after separation by ultrafiltration. The intraindividual variability of the area under the concentration-time curves (AUC0-12) of MPA throughout the 12-hour dosing interval was high in the immediate posttransplant period, but declined in the stable phase, whereas the interindividual variability remained unchanged. The median MPA-AUC0-12 values increased 2-fold from 32.4 (range, 13.9 to 57.0) mg x h/L at 3 weeks to 65.1 (range, 32.6 to 114) mg x h/L at 3 months after transplantation, whereas the median AUC0-12 values of free MPA did not significantly change over time. This discrepancy can be attributed to a 35% decline of the MPA free fraction from 1.4% in the initial phase posttransplant to 0.9% (p < 0.01) in the stable phase. In conclusion, pediatric renal transplant recipients given a fixed MMF dose exhibit a 2-fold increase of the AUC0-12 of total MPA in the stable phase posttransplant and a 35% decrease of the MPA free fraction, whereas the AUC0-12 of free MPA remains unchanged over time. Because the latter pharmacokinetic variable is theoretically best predictive of the clinical immunosuppressive efficacy of MMF, these findings may have consequences for the dosing recommendations of MMF in renal transplant recipients.

Pharmacokinetics and protein adduct formation of the pharmacologically active acyl glucuronide metabolite of mycophenolic acid in pediatric renal transplant recipients.

The acyl glucuronide metabolite (AcMPAG) of mycophenolic acid (MPA) has been found to possess both immunosuppressive and pro-inflammatory activity in vitro. In this study its pharmacokinetics were determined in pediatric renal transplant recipients receiving cyclosporine, steroids, and mycophenolate mofetil. Twelve-hour concentration–time profiles for AcMPAG, MPA, and the phenolic glucuronide (MPAG) were determined by high-performance liquid chromatography (HPLC) in the initial (1–3 wk; n = 16) and stable (3–12 mo; n = 22) phases after transplantation. In addition, the formation of covalent adducts between AcMPAG and plasma albumin (AcMPAG–Alb) was investigated using Western Blot analysis. AcMPAG-AUC12h showed significant (p < 0.05) correlations with MPA-AUC12h (r = 0.78) and MPAG-AUC12h (r = 0.78). In molar equivalents the median AcMPAG-AUC12h was 10.3% (range, 4.6%–45.5%) of MPA-AUC12h. Values (median [range]) of AcMPAG-AUC12h (10.1 [3.30–30.1] mg·h/L), AcMPAG-C0 (0.48 [0.08–1.43] mg/L), and AcMPAG-Cmax (1.95 [0.88–5.35] mg/L) were significantly (p < 0.05) higher in the stable phase than in the initial phase: 3.54 [2.07–20.0] mg·h/L for AUC12h; 0.25 [<0.04–0.97] mg/L for C0, and 1.12 [0.32–2.44] mg/L for Cmax. The increases in the AcMPAG pharmacokinetic variables were paralleled by significant increases in the corresponding MPA variables. In addition, a strong negative correlation (r = −0.69; p < 0.05) was found between AcMPAG concentrations and the creatinine clearance. AcMPAG–Alb adducts were detected in all patient samples. They showed considerable interindividual variation and increased significantly with time from the initial phase to the stable phase. AcMPAG–Alb correlated significantly (p < 0.05) with AcMPAG-AUC12h (r = 0.70) and plasma albumin (r = 0.40). AcMPAG plasma concentrations are dependent on renal function, MPA disposition, and glucuronidation. The pharmacokinetics of AcMPAG is characterized by broad interindividual variation. In some patients AcMPAG may significantly contribute to the immunosuppression during mycophenolate mofetil therapy. AcMPAG–Alb adduct formation may serve as a marker for extended AcMPAG exposure. The association of AcMPAG with adverse effects must be further investigated.

Differences in Nucleotide Hydrolysis Contribute to the Differences between Erythrocyte 6-Thioguanine Nucleotide Concentrations Determined by Two Widely Used Methods

BACKGROUND Measurement of 6-thioguanine nucleotide (6-TGN) concentrations in erythrocytes is widely accepted for use in optimization of thiopurine therapy. Various chromatographic methods have been developed for this purpose. In preliminary experiments we observed a considerable difference between 6-TGN concentrations determined with two widely used methods published by Lennard (Lennard L. J Chromatogr 1987;423:169-78) and by Dervieux and Boulieu (Dervieux T, Boulieu R. Clin Chem 1998;44:551-5). We therefore investigated methodologic differences between the two procedures with respect to hydrolysis of 6-TGNs to 6-thioguanine (6-TG) in more detail. METHODS We analyzed 6-TGNs in erythrocyte preparations (n = 50) from patients on azathioprine therapy by both methods, using the original protocols. In one set of experiments, we replaced the 0.5 mol/L sulfuric acid in the Lennard method with the 1 mol/L perchloric acid used by Dervieux and Boulieu. In a second set of experiments, we investigated the effect of various dithiothreitol (DTT) concentrations on 6-TG recovery with both methods. In a third set of experiments, we determined the effect of hydrolysis time on both protocols. RESULTS Direct comparison of both methods showed that 6-TGN concentrations were, on average, 2.6-fold higher in the Dervieux-Boulieu method over the concentration range tested, although the correlation (r = 0.99; P <0.001) was good. Replacement of sulfuric acid by perchloric acid reduced this difference to approximately 1.4-fold (r = 0.99; P <0.001). Increasing the DTT concentration enhanced 6-TG recovery. The hydrolysis time used in the Lennard method (1 h) was not sufficient to achieve complete hydrolysis. CONCLUSIONS The difference between 6-TGN concentrations measured by the two methods is attributable, at least in part, to differences in the extent of nucleotide hydrolysis. For optimization of thiopurine therapy, method-dependent therapeutic ranges are necessary, which precludes comparison of results from clinical studies derived with these methods. Efforts must therefore be made to standardize the analytical procedures for the determination of 6-TGN.