Perplexingly High Tacrolimus Concentrations in a Renal Transplant Patient with HIV

A 56-year-old female with end-stage renal disease from HIV nephropathy was admitted for a kidney transplant. She was previously on hemodialysis 3 days per week for 6 years prior to admission. Other complications included a history of type 2 diabetes, pancreatitis, cytomegalovirus gastritis, deep vein thrombosis, and hypertension. Her medications included various HIV therapies (i.e., abacavir, dolutegravir, ritonavir, and darunavir), erythropoietin due to kidney dysfunction, as well as irbesartan, an angiotensin II receptor antagonist, and amlodipine, a calcium channel blocker, both to treat hypertension. Her laboratory results the morning of her transplant were typical of a patient with end-stage renal disease (Table 1). Laboratory findings pretransplant and 3 days posttransplant. Bold values are abnormal in relation to the reference interval or therapeutic range. Tacrolimus result provided in the table obtained by immunoassay (Roche Elecsys). Laboratory findings pretransplant and 3 days posttransplant. Bold values are abnormal in relation to the reference interval or therapeutic range. Tacrolimus result provided in the table obtained by immunoassay (Roche Elecsys). Following her kidney transplant, the patient was anuric and hyperkalemic [serum potassium 5.4 mmol/L (reference interval: 3.5–5.0 mmol/L)]; she was given insulin on a sliding scale and received dialysis. Her immunosuppression regimen included basiliximab induction (20 mg qd), methylprednisolone sodium succinate (60 mg q12 h), mycophenolate mofetil (360 mg qid), and extended-release tacrolimus (Tac) (5 mg qd). The next day, the patient appeared mildly volume overloaded with mild leg edema, and she was oliguric with a urine output of 90 mL/day (normal: 500 mL/day). Therefore, IV fluids were withheld and the patient underwent hemodialysis. Two days later, she had no improvement in her urine output and was suspected to have delayed graft function. On posttransplant day 3, a whole-blood Tac trough concentration was measured on the Roche Elecsys Tac immunoassay, with the result indicating a severely toxic concentration of 106 μg/L (trough therapeutic range: 3–12 μg/L; toxic cutoff: >25 μg/L); other laboratory results were consistent with delayed graft function (Table 1). Testing was repeated using liquid chromatography–tandem mass spectrometry (LC–MS/MS), with the result of 113 μg/L confirming the extremely increased Tac concentration. The next day, another sample was collected, which yielded a Tac concentration of 118 μg/L by immunoassay. Tac, ritonavir, and darunavir were discontinued. Tacrolimus, also called FK-506, is an immunosuppressant that mediates its immunosuppressive action by entering lymphocytes and binding to an intracellular receptor, FK-binding protein. This drug–receptor complex interacts with, and blocks, the calcium-dependent phosphatase calcineurin, which is critical for the dephosphorylation and translocation of nuclear factor of activated T cells to the nucleus. In the nucleus, nuclear factor of activated T cells regulates the transcription of cytokines and other genes important for T-cell proliferation. Therefore, calcineurin inhibition leads to marked suppression of T-cell mediated immune responses, including those involved in solid-organ transplant rejection. Tac is used for prophylaxis of organ rejection posttransplant, usually for kidney, liver, or heart transplants. Some off-label indications include Crohn disease, graft-versus-host disease, and rheumatoid arthritis. The oral bioavailability of Tac is variable and incomplete, with blood concentration peaking between 0.5 and 6 h (1). It is highly protein-bound in plasma to albumin and alpha-1-acid glycoprotein and is primarily metabolized by hepatic CYP3A4/5 into 8 demethylated metabolites; excretion is mostly fecal. The unbound fraction of Tac is metabolized by CYP3A4 and CYP3A5 in both the liver and intestine. Furthermore, P-glycoprotein in the liver and intestine increases its hepatic excretion and decreases its intestinal absorption, respectively. Plasma concentration is only about 2%–8% of whole-blood concentration due to its high intracellular binding to FK-binding protein. Tac has a variable pharmacokinetic profile with a narrow therapeutic window and supra-therapeutic concentrations can be toxic, causing nephrotoxicity, neurotoxicity, hypertension, nausea, and graft rejection (2). Together, these characteristics make therapeutic drug monitoring of Tac a requirement. Blood concentrations have wide interindividual variability, since whole-blood Tac concentration is dependent on hematocrit, plasma albumin concentration, patient’s sex, and age. Other variables include genotype, particularly of P-glycoprotein and CYP3A4/5, drug–drug and food–drug interactions, inflammatory status, and liver dysfunction (3, 4). Whole-blood Tac concentrations are measured in clinical laboratories by either immunoassay or mass spectrometry. Overall, mass spectrometry-based methods are more precise, sensitive, and specific, since they do not measure metabolites and are typically less affected by interfering substances. Both methods generally require a pretreatment step with a protein precipitating agent (e.g., zinc sulfate) to lyse the cells and extract the drug from whole blood. This pretreatment allows for total (protein-bound and free) Tac as well as its metabolites to be assayed. Interference by metabolites is a major issue for immunoassays and contributes to a positive bias compared to mass spectrometry-based methods. Additionally, low hematocrit and plasma albumin concentration can cause a positive bias of Tac by microparticle enzyme-linked immunoassay compared to mass spectrometry (5). This is thought to be due to both physiological and analytical causes. Low hematocrit and albumin concentrations result in an increased unbound fraction of Tac, which increases its metabolism in vivo as well as in vitro extraction efficiency and thus increased Tac and metabolites (5). HIV-infected patients undergoing solid-organ transplantation require both immunosuppressive agents and antiretroviral drugs, giving rise to complex drug regimens. Different classes of antiretroviral agents interfere with specific steps of the HIV life cycle (6). Specifically, this patient was prescribed abacavir, a nucleoside reverse transcription inhibitor, dolutegravir, an integrase inhibitor, as well as ritonavir and darunavir, both protease inhibitors. Ritonavir is rarely used at doses needed for antiretroviral activity because of its near-universal gastrointestinal side effects. Rather, it is often coadministered with other protease inhibitors: in this case, darunavir, as a pharmacologic booster to decrease CYP3A4 metabolism and consequently increase darunavir concentration (7). Ritonavir also inhibits P-glycoprotein and, interestingly, darunavir also inhibits CYP3A4, but to a lesser extent than ritonavir. Taken together, the inhibition of CYP3A4 and P-glycoprotein increases Tac concentration due to reduced first pass metabolism, reduced efflux of Tac, and reduced postabsorptive metabolism. Furthermore, calcineurin inhibitor use posttransplantation has been associated with risk of delayed graft function following high systemic exposure (8). Calcineurin inhibitors may contribute to delayed graft function by inducing acute kidney injury via renal allograft ischemia (9) and/or renal tubular epithelial cell apoptosis (10). After confirming that the severely toxic Tac concentration was not due to an analytical error, we reviewed the patient’s chart for any clues as to why the Tac concentration could be increased, potentially due to impaired metabolism or excretion. The grossly increased Tac concentration in this patient was likely due to a drug–drug interaction with the antiretroviral drug, ritonavir. The patient’s Tac concentrations by immunoassay overtime are shown in Fig. 1, starting with the first result of 106 μg/L, which increased to 118 μg/L the next day. At this time Tac, ritonavir, and darunavir were stopped. Her Tac was within therapeutic range 4 days after the medication was stopped. At this time, she was given 1 mg of Tac per day for 2 days. The next day, the patient was instructed by her HIV team that she had a resistant HIV strain and adjustments to her medication could not be made. Therefore, she was restarted on ritonavir and darunavir. Her Tac concentration remained within the therapeutic range. After 2 days of holding Tac, she was started on a dose of 0.5 mg approximately once a week. She was discharged 1 week later and instructed to take 0.5 mg every 4 days, as well as her initial antiretroviral therapy regimen. Her Tac concentration was within the therapeutic range at the time of discharge. Overall, this was a case of a kidney transplant complicated by delayed graft function likely related to severe Tac toxicity. The patient’s tacrolimus concentrations by immunoassay over time. The figure indicates when medications were stopped and restarted and when the patient was discharged. To help mitigate the occurrence of such drug–drug interactions, we recommend that patients with HIV should be assessed prior to initiating tacrolimus therapy to determine whether they can switch to an alternate HIV regimen. If the patient is unable to start a new HIV regimen, the tacrolimus dose should be reduced and trough blood tacrolimus concentrations monitored frequently. How would you investigate a grossly increased tacrolimus result? What important clues would you look for in a patient’s chart to help you determine whether their tacrolimus concentration was truly grossly increased? What drug–drug interactions are important to consider when determining the optimal tacrolimus dose? Tacrolimus is a calcineurin inhibitor commonly used as an immunosuppressant agent in kidney or liver transplant recipients. Tacrolimus is measured in whole blood (trough concentrations) by immunoassay or LC–MS/MS, and concentrations are affected by CYP3A4/5 and P-glycoprotein activity. Many factors, such as drug–drug interactions, can alter the relationship between tacrolimus dose and whole-blood concentration. Antiretroviral therapy for HIV can cause clinically significant interactions with tacrolimus (e.g., ritonavir causes tacrolimus concentration to increase due to inhibition of CYP3A4 and P-glycoprotein). All authors confirmed they have contributed to the intellectual content of this paper and have met the following 4 requirements: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; (c) final approval of the published article; and (d) agreement to be accountable for all aspects of the article thus ensuring that questions related to the accuracy or integrity of any part of the article are appropriately investigated and resolved. No authors declared any potential conflicts of interest.

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