Low bioavailability of cyclosporine microemulsion and tacrolimus in a small bowel transplant recipient: possible relationship to intestinal P-glycoprotein activity

Clostridium difficile-Associated Colitis Post-Transplant Is Not Associated with Elevation of Tacrolimus Concentrations

BACKGROUND

Diarrhea is a common condition after solid organ transplant (SOT); Clostridium difficile-associated colitis (CDAC) is one of the most common infections after SOT. We documented previously that some types of enteritis are associated with an elevation of tacrolimus (TAC) trough concentrations by interfering with the drug's complex metabolism.

PATIENTS AND METHODS

Tacrolimus concentrations of 25 SOT recipients including 12 renal and 13 liver recipients before, during, and after CDAC were analyzed retrospectively.

RESULTS

Median age of the 25 patients was 54 y (range, 36-71), there were 15 males and 10 females. Clostridium difficile-associated colitis developed at a median of 55 d (range 2-4551) post-SOT. Median TAC concentrations prior to the outbreak of CDAC were 6.9 ng/mL (range, <1.5-17.2), 5.6 ng/mL (range, <1.5-13.2) during diarrhea, and 7.4 ng/mL (range, <1.5-24.3) after resolution of diarrhea (p > 0.05, NS). Treatment of CDAC consisted of metronidazole for 14 d in all cases. All patients recovered from CDAC but seven patients had CDAC relapse.

CONCLUSIONS

In contrast to other types of infectious diarrhea such as rotavirus enteritis and cryptosporidiosis, CDAC is not associated with an increase in TAC concentrations. This is because C. difficile causes primarily colitis as opposed to other organisms, which are associated with enteritis.

A Rationale for Age-Adapted Immunosuppression in Organ Transplantation

Demographic changes are associated with a steady increase of older patients with end-stage organ failure in need for transplantation. As a result, the majority of transplant recipients are currently older than 50 years, and organs from elderly donors are more frequently used. Nevertheless, the benefit of transplantation in older patients is well recognized, whereas the most frequent causes of death among older recipients are potentially linked to side effects of their immunosuppressants.Immunosenescence is a physiological part of aging linked to higher rates of diabetes, bacterial infections, and malignancies representing the major causes of death in older patients. These age-related changes impact older transplant candidates and may have significant implications for an age-adapted immunosuppression. For instance, immunosenescence is linked to lower rates of acute rejections in older recipients, whereas the engraftment of older organs has been associated with higher rejection rates. Moreover, new-onset diabetes mellitus after transplantation is more frequent in the elderly, potentially related to corticosteroids, calcineurin inhibitors, and mechanistic target of rapamycin inhibitors.This review presents current knowledge for an age-adapted immunosuppression based on both, experimental and clinical studies in and beyond transplantation. Recommendations of maintenance and induction therapy may help to improve graft function and to design future clinical trials in the elderly.

[Molecular Biological Analysis of Factors Influencing Pharmacokinetics to Achieve Personalized Pharmacotherapy]

Large individual variations in drug efficacy and safety could be explained in part by pharmacokinetics regulated by drug transporters and drug-metabolizing enzymes. However, expression and/or function of these proteins often fluctuate in pathological conditions, causing individual pharmacokinetic variability. To achieve a personalized pharmacotherapy after liver transplantation, our group has been investigating the pharmacokinetics of drugs and factors causing its variation based on molecular biological analysis using rats with liver ischemia-reperfusion (I/R) injury as a model for injuries immediately after liver transplantation. The first finding is that the oral bioavailability of cyclosporine A (CsA), which is an immunosuppressant, was decreased by increased first-pass metabolism due to elevated CYP3A and P-glycoprotein (P-gp) specifically in the upper small intestine after liver I/R. Expression of CYP3A in the small intestine was also elevated through transcriptional regulation by endogenous bile acids, whereas expression and function of intestinal P-gp were increased by post-transcriptional regulation via microRNA-145. Next, the pharmacokinetics of a cationic drug, cimetidine, which is eliminated from the kidney, and the expressional variation of drug transporters in the kidney after liver I/R were examined. Liver I/R decreased tubular secretion of cimetidine, mainly because of decreased expression of rat organic cation transporter 2 in the kidney. These findings provide useful information on the etiology of liver I/R injury and appropriate use of immunosuppressants and drugs eliminated from the kidney after liver transplantation.

Tacrolimus Dosing in Adult Lung Transplant Patients Is Related to Cytochrome P4503A5 Gene Polymorphism

Tacrolimus is a potent immunosuppressive agent used in lung transplantation and is a substrate for both P-glycoprotein (P-gp, encoded by the gene MDR1) and cytochrome (CYP) P4503A. A previous study by the authors identified a correlation between the tacrolimus blood level per dose with CYP3A5 and MDR1 gene polymorphisms in pediatric heart transplant patients. The objective of this study was to confirm the influence of these polymorphisms on tacrolimus dosing in adult lung transplant patients. Adult lung transplant patients who had been followed for at least 1 year after lung transplantation were studied. Tacrolimus blood level (ng/mL) per dose (mg/day) at 1, 3, 6, 9, and 12 months after transplantation was calculated as [L/D]. DNA was extracted from blood. MDR1 3435 CC, CT, and TT; MDR1 2677 GG, GT, and TT; and CYP3A5*1 (expressor) and *3 (nonexpressor) genotypes were determined by PCR amplification, direct sequencing, and sequence evaluation. Eighty-three patients were studied. At 1, 3, 6, 9, and 12 months after the transplant, a significant difference in [L/D] was found between the CYP3A5 expressor versus nonexpressor genotypes (mean +/- SD of 1.49 +/- 0.88 vs. 3.11 +/- 4.27, p = 0.01; 1.23 +/- 0.82 vs. 3.44 +/- 8.97, p = 0.05; 1.32 +/- 0.96 vs. 3.81 +/- 6.66, p = 0.005; 0.95 +/- 1.19 vs. 3.74 +/- 5.98, p = 0.0015; and 0.45 +/- 0.2 vs. 3.76 +/- 6.75, p = 0.0001, respectively). MDR1 G2677T and C3435T genotypes had only minimal effects on [L/D] at 1 and 3 months after transplantation. This study confirms the relationship of CYP3A5 polymorphisms to tacrolimus dosing in organ transplant patients. CYP3A5 expressor genotypes required a larger tacrolimus dose to achieve the same blood levels than the CYP3A5 nonexpressors at all time points during the first posttransplant year. This was not uniformly true for MDR1. The authors therefore conclude that tacrolimus dosing in adult lung transplant patients is associated with CYP3A5 gene polymorphisms.

Cryptosporidium enteritis in solid organ transplant recipients: multicenter retrospective evaluation of 10 cases reveals an association with elevated tacrolimus concentrations

BACKGROUND

Cryptosporidial enteritis, a diarrheal infection of the small intestine caused by the apicomplexan protozoa Cryptosporidium, is infrequently recognized in transplant recipients from developed countries.

METHODS

A retrospective review of all cases of cryptosporidiosis in solid organ transplant (SOT) recipients at 2 centers from January 2001 to October 2010 was performed and compared with transplant recipients with community-onset Clostridium difficile infection (CDI). A literature search was performed with regard to reported cases of cryptosporidiosis in SOT recipients.

RESULTS

Eight renal, 1 liver, and 1 lung transplant recipient were diagnosed with cryptosporidiosis at median 46.0 months (interquartile range [IQR] 25.2-62.8) following SOT. Symptoms existed for a median 14 days (IQR 10.5-14.8) before diagnosis. For the 9 patients receiving tacrolimus (TAC), mean TAC levels increased from 6.3 ± 1.1 to 21.3 ± 9.2 ng/mL (P = 0.0007) and median serum creatinine increased temporarily from 1.3 (IQR 1.1-1.7) to 2.4 (IQR 2.0-4.6) mg/dL (P = 0.008). By comparison, 8 SOT recipients (6 kidney, 2 liver) hospitalized with community-onset CDI had a mean TAC level of 10.8 ± 2.8 ng/dL during disease compared with 9.2 ± 2.3 ng/mL at baseline (P = 0.07) and had no change in median creatinine. All patients recovered from Cryptosporidium enteritis after receiving various chemotherapeutic regimens.

CONCLUSIONS

Cryptosporidiosis should be recognized as an important cause of diarrhea after SOT and is associated with elevated TAC levels and acute kidney injury. Increased TAC levels may reflect altered drug metabolism in the small intestine.

ABCB1 (MDR1) gene polymorphisms are associated with the clinical response to paroxetine in patients with major depressive disorder

Variability in antidepressant response is due to genetic and environmental factors. Among genetic factors, the ones controlling for availability of the drug at the target site are interesting candidates. Multidrug resistance 1 (ABCB1, MDR1) gene encodes a blood-brain barrier transporter P-glycoprotein that plays an important role in controlling the passage of substances between the blood and brain. In the present study, we therefore examined the possible association of 3 functional ABCB1 polymorphisms (C3435T: rs1045642, G2677T/A: rs2032582 and C1236T: rs1128503) with response to paroxetine in a Japanese major depression sample followed for 6 weeks. Analysis of covariance at week 6 with baseline scores included in the model as covariate showed significant association of the non-synonymous SNP G2677T/A with treatment response to paroxetine (p=0.011). Furthermore, the wild variants haplotype (3435C-2677G-1236T) resulted associated with poor response (p=0.006). To our best knowledge, this study is the first suggestion of a possible association of ABCB1 variants with SSRIs response.

Intestinal transplantation: current status

Intestine transplant is indicated for patients with intestinal failure who are unable to be weaned from parenteral nutrition (PN). Long-term PN, although life sustaining in many patients, can be associated with life-threatening complications including PN-associated liver disease (PNALD). Most patients are not considered for intestine transplant until they have developed severe PNALD and also need a liver transplant. Overall outcomes with intestinal transplantation are steadily improving, and current 1-year patient survivals for intestine-only transplants are now similar to those for liver transplant. Intestinal transplantation should be considered earlier in intestinal failure patients who are at high risk for developing PNALD and other life-threatening complications.

The pharmacogenetics of calcineurin inhibitors: one step closer toward individualized immunosuppression?

The immunosuppressive drugs cyclosporin (CsA) and tacrolimus (Tac) are widely used to prevent acute rejection following solid-organ transplantation. However, the clinical use of these agents is complicated by their many side effects, a narrow therapeutic index and highly variable pharmacokinetics. The variability in CsA and Tac disposition has been attributed to interindividual differences in the expression of the metabolizing enzymes cytochrome P450 (CYP) 3A4 and 3A5, and in the expression of the drug transporter P-glycoprotein (encoded by the ABCB1 gene, formerly known as the multidrug resistance 1 gene). Variation in the expression of these genes could in turn be explained by several recently-identified single nucleotide polymorphisms (SNPs). Determination of these SNPs in (future) transplant recipients has the potential to identify individuals who are at risk of under-immunosuppression or the development of adverse drug reactions. Ultimately, genotyping for CYP3A and ABCB1 may lead to further individualization of immunosuppressive drug therapy for the transplanted patient.

An up-date review on individualized dosage adjustment of calcineurin inhibitors in organ transplant patients

Calcineurin inhibitors, tacrolimus (FK506) and cyclosporine (ciclosporin A), are the primary immunosuppressive agents used on recipients of organ transplantations. The hepatic metabolism of these drugs by cytochrome P450 IIIA (CYP3A) subfamilies is considered a major eliminating process. The intestinal efflux-pump P-glycoprotein (Pgp) (multidrug resistance 1 [MDR1], ATP-binding cassette B1 [ABCB1]) and CYP3A4 have been demonstrated as important for the bioavailability of drugs, so called "absorptive barriers". Recently, an important role for CYP3A5 in the intestine for the oral clearance of drugs has been identified. Both tacrolimus and cyclosporine are substrates of Pgp, CYP3A4 and CYP3A5, and therefore, these molecules are potential pharmacokinetic factors with which to establish personalized dosage regimens for these drugs. Although the effect of single nucleotide polymorphisms in the MDR1/ABCB1 and CYP3A5 genes on the pharmacokinetics of immunosuppressant has been widely examined, some contradictions have been emerged. In living-donor liver transplant (LDLT) patients, the intestinal mRNA expression level of MDR1 and CYP3A5 genotyping both in the native intestine and in the grafted liver are suggested to be potential pharmacokinetic factors for adjusting initial dosage and predicting post-operative variation in the pharmacokinetics of tacrolimus. We review the pharmacokinetic and pharmacodynamic characteristics of these drugs including the large pharmacokinetic variation and potential individualized dosage adjustments based on the genomic information of transporters and metabolic enzymes as well as classical pharmacokinetic analyses based on therapeutic drug monitoring (TDM).

Limited interaction between tacrolimus and P-glycoprotein in the rat small intestine

The significance of intestinal P-glycoprotein (P-gp) in determining the oral bioavailability of tacrolimus has been still controversial. In this study, we reevaluated the interaction of tacrolimus with P-gp in the rat small intestine, by evaluating its absorption from the rat small intestine and its modulating effect on the absorption of known P-gp substrates (digoxin, methylprednisolone, and vinblastine). Intestinal absorption of tacrolimus itself was as extensive as other P-gp modulators such as cyclosporine and verapamil. While cyclosporine and verapamil significantly increased the absorption of methylprednisolone and vinblastine through potent inhibition of intestinal P-gp, tacrolimus failed to achieve this. When cyclosporine and tacrolimus were intravenously administered to rats, digoxin absorption was significantly increased by cyclosporine but not by tacrolimus. When tacrolimus was coadministered with clotrimazole, a specific CYP3A inhibitor, into the rat small intestine, the area under the curve of tacrolimus blood concentrations increased more than seven-fold compared with that of tacrolimus alone. Our present results strongly suggest that the interaction between tacrolimus and P-gp is limited in the rat small intestine and that extensive metabolism by CYP3A enzymes is more responsible for the low oral bioavailability of tacrolimus. It was considered that the extensive absorption of cyclosporine and verapamil was closely associated with their potent ability to inhibit intestinal P-gp.

Expression and function of efflux drug transporters in the intestine

A variety of drug transporters expressed in the body control the fate of drugs by affecting absorption, distribution, and elimination processes. In the small intestine, transporters mediate the influx and efflux of endogenous or exogenous substances. In clinical pharmacotherapy, ATP-dependent efflux transporters (ATP-binding cassette [ABC] transporters) expressed on the apical membrane of the intestinal epithelial cells determine oral bioavailability, intestinal efflux clearance, and the site of drug-drug interaction of certain drugs. The expression and functional activity of efflux transporters exhibit marked interindividual variation and are relatively easily modulated by factors such as therapeutic drugs and daily foods and beverages. In this article, we will summarize the recent findings regarding the intestinal efflux transporters, especially P-glycoprotein (P-gp or human multidrug resistance gene [MDR] 1), multidrug resistance-associated protein 2 (MRP2), and breast cancer resistance protein (BCRP).

A higher dose requirement of tacrolimus in active Crohn's disease may be related to a high intestinal P-glycoprotein content

Tacrolimus, a relatively new therapeutic option for patients with corticosteroid-refractory Crohn's disease or ulcerative colitis, is a substrate for the apically directed efflux transporter P-glycoprotein (P-gp). Duodenal biopsy specimens obtained from a patient with corticosteroid-refractory Crohn's disease and with significantly higher-than-average tacrolimus dose requirements were analyzed for P-gp by Western blot. The P-gp content in this patient was more than double that in specimens obtained from 9 of 10 healthy subjects. Elevated intestinal P-gp could have resulted in decreased tacrolimus absorption, thereby leading to decreased blood concentration and decreased efficacy in this patient. The cause and prevalence of this phenomenon are unknown.

Concomitant Clotrimazole Therapy More Than Doubles the Relative Oral Bioavailability of Tacrolimus

The purpose of this pharmacokinetic study was to determine whether the relative oral bioavailability of tacrolimus is increased with concomitant administration of clotrimazole. Pharmacokinetic studies were conducted in 6 adult kidney transplant patients receiving tacrolimus therapy. Pharmacokinetic profiling was performed by blood sampling over 12 hours before and after the administration of a 5-day course of clotrimazole. Tacrolimus whole-blood concentrations were determined by microparticle enzyme immunoassay. Noncompartmental pharmacokinetic analysis was conducted using WinNonLin, Standard Edition, Version 1.1. Concomitant administration of clotrimazole more than doubled the relative oral bioavailability of tacrolimus. The mean AUC0-12 of tacrolimus was increased 250% with clotrimazole (467.0 +/- 170.0 ng.h/mL versus 188.7 +/- 50.2 ng.h/mL; P = 0.002). Tacrolimus blood trough concentrations also more than doubled with coadministration of clotrimazole (27.7 +/- 10.4 ng/mL versus 11.6 +/- 4.0 ng/mL; P = 0.003). Mean Cmax was significantly increased with clotrimazole (70.7 +/- 34.7 ng/mL versus 27.4 +/- 11.1 ng/mL, P = 0.01). Tmax decreased from 3.2 +/- 1.6 hours to 1.9 +/- 1.0 hours (P = NS). In addition, the apparent oral clearance decreased 60% with coadministration of clotrimazole (median oral clearance 0.16 L/h/kg versus 0.40 L/h/kg; P = 0.03). Thus, clotrimazole causes a significant increase in the relative oral bioavailability, Tmax, and trough concentration of tacrolimus. Tacrolimus levels should be monitored following initiation or discontinuation of clotrimazole to minimize toxicity or precipitation of an acute rejection episode due to subtherapeutic levels.

Pharmacokinetic Considerations Relating to Tacrolimus Dosing in the Elderly

Relaxation of the upper age limits for solid organ transplantation coupled with improvements in post-transplant survival have resulted in greater numbers of elderly patients receiving immunosuppressant drugs such as tacrolimus. Tacrolimus is a potent agent with a narrow therapeutic window and large inter- and intraindividual pharmacokinetic variability. Numerous physiological changes occur with aging that could potentially affect the pharmacokinetics of tacrolimus and, hence, patient dosage requirements. Tacrolimus is primarily metabolised by cytochrome P450 (CYP) 3A enzymes in the gut wall and liver. It is also a substrate for P-glycoprotein, which counter-transports diffused tacrolimus out of intestinal cells and back into the gut lumen. Age-associated alterations in CYP 3A and P-glycoprotein expression and/or activity, along with liver mass and body composition changes, would be expected to affect the pharmacokinetics of tacrolimus in the elderly. However, interindividual variation in these processes may mask any changes caused by aging. More investigation is needed into the impact aging has on CYP and P-glycoprotein activity and expression. No single-dose, intense blood-sampling study has specifically compared the pharmacokinetics of tacrolimus across different patient age groups. However, five population pharmacokinetic studies, one in kidney, one in bone marrow and three in liver transplant recipients, have investigated age as a co-variate. None found a significant influence for age on tacrolimus bioavailability, volume of distribution or clearance. The number of elderly patients included in each study, however, was not documented and may have been only small. It is likely that inter- and intraindividual pharmacokinetic variability associated with tacrolimus increase in elderly populations. In addition to pharmacokinetic differences, donor organ viability, multiple co-morbidity, polypharmacy and immunological changes need to be considered when using tacrolimus in the elderly. Aging is associated with decreased immunoresponsiveness, a slower body repair process and increased drug adverse effects. Elderly liver and kidney transplant recipients are more likely to develop new-onset diabetes mellitus than younger patients. Elderly transplant recipients exhibit higher mortality from infectious and cardiovascular causes than younger patients but may be less likely to develop acute rejection. Elderly kidney recipients have a higher potential for chronic allograft nephropathy, and a single rejection episode can be more devastating. There is a paucity of information on optimal tacrolimus dosage and target trough concentration in the elderly. The therapeutic window for tacrolimus concentrations may be narrower. Further integrated pharmacokinetic-pharmacodynamic studies of tacrolimus are required. It would appear reasonable, based on current knowledge, to commence tacrolimus at similar doses as those used in younger patients. Maintenance dose requirements over the longer term may be lower in the elderly, but the increased variability in kinetics and the variety of factors that impact on dosage suggest that patient care needs to be based around more frequent monitoring in this age group.

Severe elevations of FK506 blood concentration due to diarrhea in renal transplant recipients

BACKGROUND

The rate of metabolism in the intestine of oral administered FK506 decreases as FK506 passes on to the lower intestine. In transplant recipients with diarrhea given oral FK506, the main areas for absorption of FK506 shift to the lower intestine, where the ability to metabolize FK506 is weaker. Therefore it is considered likely that when FK506 is administered to recipients with diarrhea, the blood concentration of FK506 will be higher.

MATERIAL AND METHODS

Twenty recipients experiencing episodes of diarrhea were investigated to determine the trough level of FK506 and the time required for the FK506 trough level to return to the level that obtained before diarrhea. AUC0-4h and Cmax of FK506 were investigated in eight recipients. In cases with severe diarrhea, the daily fluctuations of FK506 blood concentration were also investigated.

RESULT

The FK506 trough level (p < 0.0001), AUC (p = 0.0173), and Cmax (p = 0.0173) were found to be significantly higher during episodes of diarrhea. In almost all cases, it took between 2 and 4 wk for the elevated FK506 trough level to return to its previous level following a bout of diarrhea. In the daily fluctuations of FK506 concentration, Tmax was prolonged. In some cases, the concentration was highest just before administration of FK506, when it should have been at trough level.

CONCLUSIONS

Diarrhea caused significant elevations of trough level, AUC0-4h and Cmax of FK506, and the prolongation of Tmax in renal transplant recipients administered FK506.

Cyclosporine pharmacokinetics in anti-HCV+ patients

BACKGROUND

Cyclosporine (CsA) is a widely used immunosuppressive agent in kidney transplant patients. In Brazil, around 30% of patients awaiting kidney transplantation carry anti-HCV antibodies. Previous observations suggest altered CsA pharmacokinetics in these patients.

METHODS

We conducted two pharmacokinetic studies. In the pre-transplant (pre-Tx) study, we examined 22 dialysis patients on chronic hemodialysis awaiting transplantation, 11 anti-HCV+ [seven polymerase chain reaction (PCR)-positive] matched against 11 controls. In the post-transplant (post-Tx) study, we enrolled 24 kidney allograft recipients - 10 anti-HCV+ (six PCR-positive), and 14 controls. In the first study, all patients received an 8-mg/kg dose of CsA microemulsion (ME). Secondly, the dosage was indicated by the patient's medical team. Pharmacokinetic parameters were calculated from 13 blood samples (0-12 h postdose) by fluorescence polarization immunoassay with specific monoclonal antibodies.

RESULTS

In both studies, maximum concentration (C(max)), minimum concentration (C(min)) and area under the CsA time-concentration curve from 0 to 12 h (AUC(0-12)) were higher for anti-HCV+ patients than for controls, but significantly so only for AUC(0-12) in the pre-Tx study (42%; p < 0.05). When PCR-positive patients were compared with controls, differences were amplified. In the pre-Tx study, differences were 58%, 69%, and 91% higher in PCR-positive patients for C(max) (p = 0.05), AUC(0-12) (p < 0.01), and C(min) (p < 0.01), respectively. In the post-Tx study, results were 50% (p < 0.01) and 32% (p < 0.01) higher in PCR-positive patients for C(max) and AUC(0-12), respectively. In the pre-Tx study, the impact of viremia was significantly higher in female patients. CsA trough levels remained higher along the first year post-transplantation in viremic patients.

CONCLUSIONS

Anti-HCV+ patients, especially those with viremia, present altered CsA pharmacokinetics, with higher peak levels and drug exposure than controls.

Clinical pharmacokinetics and pharmacodynamics of tacrolimus in solid organ transplantation

The aim of this review is to analyse critically the recent literature on the clinical pharmacokinetics and pharmacodynamics of tacrolimus in solid organ transplant recipients. Dosage and target concentration recommendations for tacrolimus vary from centre to centre, and large pharmacokinetic variability makes it difficult to predict what concentration will be achieved with a particular dose or dosage change. Therapeutic ranges have not been based on statistical approaches. The majority of pharmacokinetic studies have involved intense blood sampling in small homogeneous groups in the immediate post-transplant period. Most have used nonspecific immunoassays and provide little information on pharmacokinetic variability. Demographic investigations seeking correlations between pharmacokinetic parameters and patient factors have generally looked at one covariate at a time and have involved small patient numbers. Factors reported to influence the pharmacokinetics of tacrolimus include the patient group studied, hepatic dysfunction, hepatitis C status, time after transplantation, patient age, donor liver characteristics, recipient race, haematocrit and albumin concentrations, diurnal rhythm, food administration, corticosteroid dosage, diarrhoea and cytochrome P450 (CYP) isoenzyme and P-glycoprotein expression. Population analyses are adding to our understanding of the pharmacokinetics of tacrolimus, but such investigations are still in their infancy. A significant proportion of model variability remains unexplained. Population modelling and Bayesian forecasting may be improved if CYP isoenzymes and/or P-glycoprotein expression could be considered as covariates. Reports have been conflicting as to whether low tacrolimus trough concentrations are related to rejection. Several studies have demonstrated a correlation between high trough concentrations and toxicity, particularly nephrotoxicity. The best predictor of pharmacological effect may be drug concentrations in the transplanted organ itself. Researchers have started to question current reliance on trough measurement during therapeutic drug monitoring, with instances of toxicity and rejection occurring when trough concentrations are within 'acceptable' ranges. The correlation between blood concentration and drug exposure can be improved by use of non-trough timepoints. However, controversy exists as to whether this will provide any great benefit, given the added complexity in monitoring. Investigators are now attempting to quantify the pharmacological effects of tacrolimus on immune cells through assays that measure in vivo calcineurin inhibition and markers of immunosuppression such as cytokine concentration. To date, no studies have correlated pharmacodynamic marker assay results with immunosuppressive efficacy, as determined by allograft outcome, or investigated the relationship between calcineurin inhibition and drug adverse effects. Little is known about the magnitude of the pharmacodynamic variability of tacrolimus.

The pharmacogenetics of immunosuppression for organ transplantation: a route to individualization of drug administration

Transplantation has transformed the treatment of patients with organ failure in a number of clinical settings, and immunosuppressive drug therapy is fundamental to its success. However, all the drugs in current use have a narrow therapeutic index. Under-dosing can lead to rejection, while over-dosing increases the risks of infection, malignant disease, and serious drug-specific adverse effects, including diabetes mellitus, nephrotoxicity, hypertension, and hyperlipidemia. Heterogeneity in the pharmacokinetics of these drugs makes initial dose determination difficult, as there is a poor correlation between dose and blood concentration. This results in difficulties in achieving target blood concentrations early after transplantation, which are important for reducing the rate of immunological rejection. This problem is compounded by the observation that neither drug dose nor drug blood concentration accurately predict clinical efficacy or toxicity. The main determinant of heterogeneity in dose requirements is intestinal absorption of the active drug. The oxidative enzymes, cytochrome P450 (CYP) 3A4 and CYP3A5, and the drug efflux pump P-glycoprotein (P-gp) in enterocytes regulate this process. Most substrates for the P-gp pump are also substrates for the CYP3A enzymes. An efficient barrier to xenobiotic absorption is formed by the CYP enzymes and P-gp, and by the two systems working synergistically. Genetic polymorphisms have been reported for the genes associated with the expression of the CYP3A enzymes and P-gp. Genotyping patients for CYP3A genes has the potential to aid the establishment of optimal dosage regimens for transplant patients. Genetic polymorphism of the multiple drug resistance gene-1 (MDR1, also known as ABCB1) [3435C/T] and the CYP3A5 genes (CYP3A5*1, CYP3AP1*1) have the greatest potential to influence the pharmacokinetics of immunosuppressants. Homozygosity of the T allele of the MDR1 3435C/T polymorphism has been associated with reduced enterocyte expression of P-gp resulting in increased drug absorption. The presence of the CYP3A5*1 allele is necessary for the production of a fully catalytic CYP3A5 protein, and also influences the ratio of CYP3A4 : CYP3A5 as well as the overall CYP3A catalytic activity. The CYP3A4 : CYP3A5 ratio may, in turn, influence the pattern of drug metabolites formed. Heterogeneity in the production of active and inactive metabolites has implications for both the pharmacokinetics and pharmacodynamics of these drugs.Gene frequencies and drug dose requirements differ between ethnic groups. Ethnic differences in dose requirements for immunosuppressants have been discussed widely. However, ethnicity is a rather crude marker for genotype. Pharmacogenetic typing offers the possibility of significant improvement in the individualization of immunosuppressive drug prescribing with reduced rates of rejection and toxicity.

Enhanced expression of enterocyte P-glycoprotein depresses cyclosporine bioavailability in a recipient of living donor liver transplantation

We evaluated levels of intestinal expression of absorptive-barrier P-glycoprotein (PGP) and cytochrome P-450 IIIA4 (CYP3A4) and immunosuppressant therapy in a patient who underwent living donor liver transplantation (LDLT) and received a second living donor liver transplant after chronic rejection of the first. PGP and CYP3A4 expression were measured using part of a Roux-en-Y limb. After the first LDLT, the concentration-dose ratio of orally administered tacrolimus was 159.8 +/- 125.3 (average +/- SD of 32 different days), similar to the average for 46 recipients of living donor liver transplants in our hospital (161.3 +/- 88.1). However, the recipient required very large oral doses of cyclosporine (703.9 +/- 385.4 mg/d, average +/- SD of 13 different days) after the second LDLT. Although intestinal PGP level was increased markedly at the second LDLT, CYP3A4 level was decreased. In addition, levels of messenger RNA expression of several gene products related to the local inflammation, such as cyclooxygenase 2, interleukin-1beta (IL-1beta), IL-2, IL-6, IL-8, IL-10, and tumor necrosis factor-alpha, were increased. These results suggest that hepatic failure after LDLT, including chronic rejection and/or cholangitis, was accompanied by upregulation of intestinal PGP expression, which could depress the bioavailability of the immunosuppressant.

Two- to three-fold increase in blood tacrolimus (FK506) levels during diarrhea in liver-transplanted children

BACKGROUND

The diagnosis and treatment of diarrhea in liver transplant recipients often pose a challenge owing to the variety of infectious and non-infectious causes. However, diagnosis is principally focused on ruling out an infectious etiology. Tacrolimus, an immunosuppressive agent generally used after liver transplantation, is absorbed mainly from the duodenum through the upper jejunum. It can be assumed that metabolism of the drug will be influenced by diarrhea.

METHODS

Four liver transplant recipients who developed an episode of acute gastroenteritis. Infectious etiology was confirmed; trough tacrolimus levels were measured before, during and after gastroenteritis.

RESULTS

All patients presented a two- to three-fold increase in blood tacrolimus levels after the onset of gastroenteritis.

CONCLUSIONS

Until the role played by the intestine in the metabolism of tacrolimus is fully understood, it is prudent to recommend early dose reduction of tacrolimus and careful monitoring of trough levels during diarrheal disorders of any nature in pediatric liver-transplanted patients.

Tacrolimus dosing in pediatric heart transplant patients is related to CYP3A5 and MDR1 gene polymorphisms

Tacrolimus is a substrate for P-glycoprotein (P-gp) and cytochrome (CYP) P4503A. P-gp is encoded by the multiple drug resistance gene MDR1 and CYP3A is the major enzyme responsible for tacrolimus metabolism. Both MDR1 and CYP3A5 genes have multiple single nucleotide polymorphisms. The objective of this study was to evaluate whether the MDR1 exon21 and exon26 polymorphisms and the CYP3A5 polymorphism are associated with tacrolimus disposition in pediatric heart transplant patients. At 3, 6 and 12 months post transplantation, a significant difference in tacrolimus blood level per dose/kg/day was found between the CYP3A5 *1/*3 (CYP3A5 expressor) vs. *3/*3 (nonexpressor) genotypes with the *1/*3 patients requiring a larger tacrolimus dose to maintain the same blood concentration. There were no significant differences in tacrolimus blood level per dose/kg/day between MDR1 exon21 G2677T and exon 26 C3435T at 3 months, but both were found to have a significant association with tacrolimus blood level per dose/kg/day at 6 and 12 months. We conclude that specific genotypes of MDR1 and CYP3A5 in pediatric heart transplant patients require larger tacrolimus doses to maintain their tacrolimus blood concentration, and that this information could be used prospectively to manage patient's immunosuppressive therapy.

Frequency distribution of C3435T mutation in exon 26 of the MDR1 gene in a Spanish population

P-glycoprotein (PGP) is a transmembrane efflux transporter with an important role in drug therapy. The level of PGP expression leads to relevant consequences in terms of efficacy and toxicity by modulating drug disposition. A single nucleotide polymorphism (SNP) in exon 26 of the gene C3435T was recently associated to PGP levels and substrate uptake. Persons who were homozygous for the T-allele had significantly decreased PGP expression compared with C/C persons. Due to this fact and bearing in mind the important differences among populations regarding the frequencies of persons carrying mutations affecting drug disposition, the authors wanted to study the prevalence of this genetic trait in their population. DNA samples from 408 persons were assayed by a PCR-RFLP method. The results showed that the distributions of the C/C, T/T, and C/T genotypes in the Spanish population were 26%, 22%, and 52%, respectively. With regard the C-allele frequency, which has been studied in several populations, the result in their population was 52%, significantly lower (P < 0.0001) than that found in African populations and similar to several Asian and Caucasian (UK) populations (P > 0.05). By contrast, the C-allele frequency in southwest Asian, German, and Portuguese populations was significantly lower than in the Spanish population (P < 0.001, P < 0.01, and P < 0.05, respectively). The great differences found between their population and others, such as the African and southwest Asian populations, could have important therapeutic implications when drugs that are a substrate of PGP are used.

Mammalian ABC transporters in health and disease

The ATP-binding cassette (ABC) transporters are a family of large proteins in membranes and are able to transport a variety of compounds through membranes against steep concentration gradients at the cost of ATP hydrolysis. The available outline of the human genome contains 48 ABC genes; 16 of these have a known function and 14 are associated with a defined human disease. Major physiological functions of ABC transporters include the transport of lipids, bile salts, toxic compounds, and peptides for antigen presentation or other purposes. We review the functions of mammalian ABC transporters, emphasizing biochemical mechanisms and genetic defects. Our overview illustrates the importance of ABC transporters in human physiology, toxicology, pharmacology, and disease. We focus on three topics: (a) ABC transporters transporting drugs (xenotoxins) and drug conjugates. (b) Mammalian secretory epithelia using ABC transporters to excrete a large number of substances, sometimes against a steep concentration gradient. Several inborn errors in liver metabolism are due to mutations in one of the genes for these pumps; these are discussed. (c) A rapidly increasing number of ABC transporters are found to play a role in lipid transport. Defects in each of these transporters are involved in human inborn or acquired diseases.

Interaction of chloramphenicol and the calcineurin inhibitors in renal transplant recipients

Several case reports have described a pharmacokinetic interaction between chloramphenicol and the calcineurin inhibitors (CNIs). Based on these reports, we set out to characterize the effects of chloramphenicol on cyclosporine and tacrolimus trough concentrations in renal transplant recipients. We retrospectively evaluated daily trough CNI concentrations and compared them with baseline CNI concentrations prior to chloramphenicol. Six adult renal or pancreas/kidney transplant recipients received 11 courses of chloramphenicol. Of these, three received cyclosporine (6 episodes) and three received tacrolimus (5 episodes). The mean dose and duration of chloramphenicol was not significantly different between groups. Chloramphenicol coadministration increased mean cyclosporine troughs maximally by 41.3% on day 4, though overall differences were not significant using analysis of variance (anova). Tacrolimus trough levels increased to 99% above baseline on day 2, 151% on day 3, 161% on day 4, 191% on day 5, and to 207% on day 6 and reached statistical significance by anova (P = 0.001). These results confirm case reports and suggest that careful trough monitoring should be implemented if chloramphenicol is to be used with the CNIs.

Immunosuppression in elderly renal transplant recipients: are current regimens too aggressive?

Renal transplantation is an accepted and successful treatment modality in elderly patients with end-stage renal disease. In comparison with maintenance dialysis, transplantation has been shown to confer a mortality benefit as well as improvements in quality of life in older individuals with end-stage renal disease. Despite this, overall outcomes of renal transplantation in elderly individuals have, in general, been less successful than those of younger renal transplant recipients. Largely, this has been due to the particular vulnerability of elderly patients to the immunosuppressive medications used in renal transplantation. This review article covers these issues in some detail and briefly discusses some of the pharmacokinetic, pharmacodynamic, physiological and immunological differences between younger and older transplant recipients. Elderly renal transplant recipients have both a higher rate of patient death and allograft loss censored for death. Upon multivariate analysis, age of the recipient is strongly associated with allograft loss independent of other known factors. Acute rejections are less frequent in older individuals; however the consequence of a rejection if it occurs is negative for long-term graft survival. On the other hand, death by infection is vastly increased in older versus younger renal transplant recipients. In general, the pharmacokinetics of the immunosuppressive agents are little affected by age, but the tolerance to these agents seems to decrease with increasing age. Elderly renal transplant recipients present a very difficult clinical challenge. As the elderly become an ever-increasing segment of the renal transplant population, new and innovative immunosuppressive strategies will have to be considered and applied.

SEX AND ETHNICITY MAY CHIEFLY INFLUENCE THE INTERACTION OF FLUCONAZOLE WITH CALCINEURIN INHIBITORS12

BACKGROUND

Calcineurin inhibitors (CNI) and azole antifungal agents have been reported to interact in a disparate manner. The azole dose and route and the level of involvement of the liver and intestines have been implicated, although data are limited. A significant interaction may result in CNI toxicity, and withdrawal of the azole may result in subtherapeutic CNI concentrations. Fluconazole, available in both intravenous and oral formulations, is commonly used in transplant recipients and is ideal for determining the presence of a disparate effect on CNI concentrations. We retrospectively investigated the interaction of CNIs with fluconazole, evaluating CNI blood troughs corrected for daily CNI dose, the factors influencing the interaction, and the effect on clinical outcomes in renal and simultaneous pancreas kidney transplant recipients.

METHODS

Twenty-eight patients received a CNI and fluconazole during the calendar year 1999, but only 19 patients had documented CNI blood troughs and outpatient follow-up. There were 25 episodes of use in the 19 included patients. CNI blood troughs were evaluated for changes induced by fluconazole, given by both routes, and clinical outcomes were tracked.

RESULTS

Data demonstrated both intravenous and oral fluconazole alter CNI blood concentrations. Two metabolic patterns were observed, and we termed these convergent and divergent. Divergent metabolizers did not have significant interaction (n=5), and convergent metabolizers did have a significant interaction (n=15). One patient had a divergent episode after a previous convergent episode. The main contributors to the lack of interaction appeared to be female sex and African American ethnicity. Additionally, tacrolimus levels were significantly more affected than cyclosporine, during and after fluconazole administration. No patient experienced nephrotoxicity or cellular rejection related to antifungal therapy.

CONCLUSIONS

Oral and intravenous fluconazole appear to increase oral CNI trough concentrations to a similar extent even after adjusting for daily calcineurin dose. These interactions appear to be chiefly influenced by sex and ethnicity. Further prospective study is necessary to clarify this issue.

Clotrimazole increases tacrolimus blood levels: a drug interaction in kidney transplant patients

In order to substantiate a previous case report of a drug interaction between tacrolimus and clotrimazole, we randomly assigned tacrolimus-treated renal allograft recipients to therapy with either clotrimazole or nystatin for oral thrush prophylaxis immediately following transplantation. Patients receiving other agents known to interact with cytochrome P450 were excluded from the study. The clotrimazole group consisted of 17 patients and the nystatin group, which served as the control group, consisted of 18 patients. An oral loading dose (approximately 0.3 mg/kg) of tacrolimus was given pre-operatively. Post-transplant, tacrolimus (approximately 0.15 mg/kg) was orally administered twice daily. Clotrimazole therapy consisted of a 10-mg troche administered three times daily. Nystatin therapy consisted of the oral suspension (5 mL) administered as a 'swish and swallow' four times daily. We evaluated tacrolimus trough blood levels and tacrolimus doses on days 1, 3, 5, and 7 following transplantation. On post-transplant day 1, mean tacrolimus trough levels did not differ between clotrimazole- and nystatin-treated patients. Mean tacrolimus blood trough levels were significantly higher in clotrimazole-treated patients on days 3, 5, and 7 post-transplant, 42+/-14, 53+/-7, and 33+/-17 ng/mL, respectively, compared to 15+/-8, 15+/-7, and 14+/-6 ng/mL in nystatin-treated patients (p<0.05). The mean tacrolimus dose was significantly lower in the clotrimazole group by day 7 post-transplant (p<0.05). We conclude that clotrimazole therapy may cause a significant rise in tacrolimus trough blood levels. Recognition of this potential drug interaction is essential to minimize tacrolimus-associated toxicities in the early post-transplant period.

Multidrug resistance (MDR) in cancer. Mechanisms, reversal using modulators of MDR and the role of MDR modulators in influencing the pharmacokinetics of anticancer drugs

In recent years, there has been an increased understanding of P-glycoprotein (P-GP)-mediated pharmacokinetic interactions. In addition, its role in modifying the bioavailability of orally administered drugs via induction or inhibition has been also been demonstrated in various studies. This overview presents a background on some of the commonly documented mechanisms of multidrug resistance (MDR), reversal using modulators of MDR, followed by a discussion on the functional aspects of P-GP in the context of the pharmacokinetic interactions when multiple agents are coadministered. While adverse pharmacokinetic interactions have been documented with first and second generation MDR modulators, certain newer agents of the third generation class of compounds have been less susceptible in eliciting pharmacokinetic interactions. Although the review focuses on P-GP and the pharmacology of MDR reversal using MDR modulators, relevance of these drug transport proteins in the context of pharmacokinetic implications (drug absorption, distribution, clearance, and interactions) will also be discussed.

P-glycoprotein and bioavailability-implication of polymorphism

P-Glycoprotein (P-gp) may have a significant impact on systemic and tissue/cellular bioavailability of drugs because it functions as an "anti-absorption" mechanism that effluxes drug molecules out of the lipid bilayer and cytoplasm. The ability to reduce bioavailability at the tissue/cellular level was first discovered during the investigation of the causes of multidrug resistance (MDR) in cancer chemotherapy. Initially, it was thought that MDR is only caused by P-gp. Recently, many other transporters such as multidrug resistance-related protein (MRP) have also been identified. The ability of P-gp to impact systemic drug bioavailability was only recently recognized. Dr. Alfred Schinkel's group was first to show a significant improvement in the systemic bioavailability of several drugs in the MDR1 knockout mice. The same group also discovered that the blood-brain barrier (BBB) has a very high expression level of P-gp, and that this protein is necessary to restrict the entrance of various drug molecules into the central nervous system (CNS). Polymorphism in the normal human cells has not been reported, but it has been discovered in human cancer cells. Functional implication of P-gp polymorphism in changing the tissue bioavailability has been studied in rodents. These studies strongly support the role of P-gp in restricting tissue bioavailability of anticancer drugs. These studies also support the effectiveness of P-gp in limiting CNS toxicity of the cytotoxic drugs. The functional implication of P-gp on systemic bioavailability is much less well defined in humans, although it appears to be quite obvious in MDR1 knockout mice. Pharmacokinetic models clearly suggest that a change in the absorption rate will have a significant impact on systemic blood level of a drug. However, whether functionally significant polymorphisms of P-gp exist in humans has not been determined. If they do exist, they will surely impact on both systemic bioavailability and drug interaction potentials of many drugs. In the drug development process, several models may be used to select a lead compound that may or may not interact with P-gp, depending on whether the interaction is desirable. Several inhibitors of P-gp are currently on the clinical trial stage. Natural inhibitors of P-gp have also been discovered. There is no doubt that these new developments will have significant impact on the bioavailability of a variety of anticancer and CNS drugs in the next decades.