Tacrolimus is extensively metabolized by the mixed-function oxidase system, primarily the cytochrome P-450 system (CYP3A). A metabolic pathway leading to the formation of 8 possible metabolites has been proposed. Demethylation and hydroxylation were identified as the primary mechanisms of biotransformation in vitro. The major metabolite identified in incubations with human liver microsomes is 13-demethyl tacrolimus. In in vitro studies, a 31-demethyl metabolite has been reported to have the same activity as tacrolimus.
IDENTIFICATION AND USE: Tacrolimus is white to off-white crystalline powder. It is a calcineurin-inhibitor immunosuppressant available in several preparations. Tacrolimus in both oral capsules and a solution for IV injection is used for prophylaxis of organ rejection in patients receiving liver, kidney or heart transplants. Tacrolimus topical ointment is used as a second-line therapy for the short-term and non-continuous chronic treatment of moderate to severe atopic dermatitis in non-immunocompromised adults and children. HUMAN EXPOSURE AND TOXICITY: While most acute overdosages of tacrolimus at up to 30 times the intended dose have been asymptomatic and all patients recovered with no sequelae, some acute overdosages were followed by adverse reactions including tremors, abnormal renal function, hypertension, and peripheral edema. At therapeutic doses, patients receiving tacrolimus are at increased risk of developing lymphomas and other malignancies, particularly of the skin, as well as an increased risk of developing bacterial, viral, fungal, and protozoal infections, including opportunistic infections. These infections may lead to serious, including fatal, outcomes. While there are no adequate and well-controlled studies in pregnant women, the use of tacrolimus during pregnancy in humans has been associated with neonatal hyperkalemia and renal dysfunction. ANIMAL STUDIES: Both rats and baboons showed a similar toxicologic profile following oral or intravenous administration of tacrolimus. Toxicity following intravenous administration was evident at lower doses than after oral administration for both rats and baboons. Toxicity was seen at lower doses in rats than in baboons. The primary target organs were the kidneys, pancreatic islets of Langerhans and exocrine pancreas, spleen, thymus, gastrointestinal tract, and lymph nodes. In addition, decreases in erythrocyte parameters were seen. Tacrolimus also produced reproductive and developmental toxicity in both rats and rabbits. In rats, chronic oral administration of tacrolimus at high doses resulted in changes in sex organs, and glaucoma/eye changes. Oral doses of tacrolimus at 1 and 3.2 mg/kg/day produced overt signs of parental toxicity and changes in the fertility and general reproductive performance of rats. Effects on reproduction included some embryo lethality, reduced number of implantations, increased incidence of post-implantation loss, and reduced embryo and offspring viability. In a rabbit teratology study, signs of maternal toxicity including reduced body weight were produced at all oral doses of tacrolimus administered (0.1, 0.32, or 1 mg/kg/day). Doses of 0.32 and 1 mg/kg/day produced signs of developmental toxicity, such as increased incidence of post-implantation losses, reduced number of viable fetuses, and increased incidences of morphological variations. In a rat teratology study, increased post-implantation loss was observed at 3.2 mg/kg/day. Maternal doses of 1 mg/kg/day decreased the body weight of F1 offspring. Decreased body weight, reduced survival number, and some skeletal alterations were seen in F1 offspring at maternal doses of 3.2 mg/kg/day. Tacrolimus did not exhibit genotoxic activity in vitro in bacterial asaays in Salmonella typhimurium and Escherichia coli or mammalian assays in Chinese hamster lung-derived cells assays. No evidence of mutagenicity was observed in vitro in the CHO/HGPRT assay (the Chinese hamster ovary cell assay (CHO), which measures forward mutation of the HGPRT locus) or in vivo in clastogenicity assays performed in mice. Tacrolimus also did not cause unscheduled DNA synthesis in rodent hepatocytes.
With a given dose of mycophenolic acid (MPA) products, exposure to MPA is higher with Prograf co-administration than with cyclosporine co-administration because cyclosporine interrupts the enterohepatic recirculation of MPA while tacrolimus does not. Clinicians should be aware that there is also a potential for increased MPA exposure after crossover from cyclosporine to Prograf in patients concomitantly receiving MPA-containing products.
Grapefruit juice inhibits CYP3A-enzymes resulting in increased tacrolimus whole blood trough concentrations, and patients should avoid eating grapefruit or drinking grapefruit juice with tacrolimus.
Since tacrolimus is metabolized mainly by CYP3A enzymes, drugs or substances known to inhibit these enzymes may increase tacrolimus whole blood concentrations. Drugs known to induce CYP3A enzymes may decrease tacrolimus whole blood concentrations. Dose adjustments may be needed along with frequent monitoring of tacrolimus whole blood trough concentrations when Prograf is administered with CYP3A inhibitors or inducers. In addition, patients should be monitored for adverse reactions including changes in renal function and QT prolongation.
Verapamil, diltiazem, nifedipine, and nicardipine inhibit CYP3A metabolism of tacrolimus and may increase tacrolimus whole blood concentrations. Monitoring of whole blood concentrations and appropriate dosage adjustments of tacrolimus are recommended when these calcium channel blocking drugs and tacrolimus are used concomitantly.
The aim of this study was to assess tacrolimus levels in breast milk and neonatal exposure during breastfeeding. An observational cohort study was performed in two tertiary referral high-risk obstetric medicine clinics. Fourteen women taking tacrolimus during pregnancy and lactation, and their 15 infants, 11 of whom were exclusively breast-fed, were assessed. Tacrolimus levels were analyzed by liquid chromatography-tandem mass spectrometry. Samples from mothers and cord blood were collected at delivery and from mothers, infants, and breast milk postnatally where possible. All infants with serial sampling had a decline in tacrolimus level, which was approximately 15% per day (ratio of geometric mean concentrations 0.85; 95% confidence interval, 0.82-0.88; P<0.001). Breast-fed infants did not have higher tacrolimus levels compared with bottle-fed infants (median 1.3 ug/L [range, 0.0-4.0] versus 1.0 ug/L (range, 0.0-2.3), respectively; P=0.91). Maximum estimated absorption from breast milk is 0.23% of maternal dose (weight-adjusted). Ingestion of tacrolimus by infants via breast milk is negligible. Breastfeeding does not appear to slow the decline of infant tacrolimus levels from higher levels present at birth.
Maternal and umbilical cord (venous and arterial) samples were obtained at delivery from eight solid organ allograft recipients to measure tacrolimus and metabolite bound and unbound concentrations in blood and plasma. Tacrolimus pharmacokinetics in breast milk were assessed in one subject. Mean (+ or - SD) tacrolimus concentrations at the time of delivery in umbilical cord venous blood (6.6 + or - 1.8 ng ml(-1)) were 71 + or - 18% (range 45-99%) of maternal concentrations (9.0 + or - 3.4 ng ml(-1)). The mean umbilical cord venous plasma (0.09 + or - 0.04 ng ml(-1)) and unbound drug concentrations (0.003 + or - 0.001 ng ml(-1)) were approximately one fifth of the respective maternal concentrations. Arterial umbilical cord blood concentrations of tacrolimus were 100 + or - 12% of umbilical venous concentrations. In addition, infant exposure to tacrolimus through the breast milk was less than 0.3% of the mother's weight-adjusted dose. Differences between maternal and umbilical cord tacrolimus concentrations may be explained in part by placental P-gp function, greater red blood cell partitioning and higher haematocrit levels in venous cord blood.
Ten colostrum samples were obtained from six women in the immediate postpartum period (0-3 days) with a mean drug concentration of 0.79 ng/mL (range 0.3-1.9 ng/mL). The median milk:maternal plasma ratio was 0.5.
The plasma protein binding of tacrolimus is approximately 99% and is independent of concentration over a range of 5-50 ng/mL. Tacrolimus is bound mainly to albumin and alpha-1-acid glycoprotein, and has a high level of association with erythrocytes. The distribution of tacrolimus between whole blood and plasma depends on several factors, such as hematocrit, temperature at the time of plasma separation, drug concentration, and plasma protein concentration. In a US study, the ratio of whole blood concentration to plasma concentration averaged 35 (range 12 to 67). There was no evidence based on blood concentrations that tacrolimus accumulates systemically upon intermittent topical application for periods of up to 1 year. As with other topical calcineurin inhibitors, it is not known whether tacrolimus is distributed into the lymphatic system.