Abstract: FR-PO1221
Subtherapeutic Tacrolimus Trough Levels in the Setting of Radiotherapy: A Case Report
Session Information
- Transplantation: Clinical - Rejection, Biomarkers, and Pharmacology
October 23, 2026 | Location: Exhibit Hall A, Convention Center
Abstract Time: 10:00 AM - 12:00 PM
Category: Transplantation
- 2002 Transplantation: Clinical
Authors
- Alsaedi, Zainulabdeen S., Washington University in St Louis, St. Louis, Missouri, United States
- Delos Santos, Rowena B., Washington University in St Louis, St. Louis, Missouri, United States
- Java, Anuja, Washington University in St Louis, St. Louis, Missouri, United States
Introduction
Tacrolimus is a key immunosuppressant in solid organ transplantation and requires close therapeutic monitoring. Its pharmacokinetics can be influenced by multiple factors. We report a case of newly subtherapeutic tacrolimus troughs temporally associated with initiation of radiotherapy after years of stable levels.
Case Description
An 80 year old man with CKD 5 from T1DM and HTN underwent preemptive deceased donor simultaneous kidney pancreas transplant 25 years earlier, maintaining excellent kidney and pancreas allograft function without rejection. He developed recurrent cutaneous SCC of the left ear and began adjuvant radiotherapy.
His maintenance IS included tacrolimus XR 0.75 mg daily (goal 4–7 ng/mL) and prednisone 5 mg; antimetabolites had been discontinued due to SCC. He had longstanding stable tacrolimus trough levels within target.
During radiotherapy, subtherapeutic tacrolimus levels (2.3 and 1.5 ng/mL) were noted for the first time. As shown in the timeline graph, these decreases occurred only after radiotherapy initiation. No new medications, OTC agents, supplements, dietary changes, or GI symptoms were identified to explain the drop, raising concern for a radiotherapy related effect on tacrolimus pharmacokinetics.
Discussion
Tacrolimus has a narrow therapeutic index, and unexplained trough variability increases the risk of rejection or toxicity. In this case, no conventional causes of low levels were present. Prior studies (e.g., Li et al., Chen et al.) describe a radiotherapy pharmacokinetic phenomenon, where radiation alters CYP450 expression and drug transporter activity, affecting systemic drug metabolism. The close temporal association in our patient suggests a similar mechanism influencing tacrolimus clearance.
This case highlights the importance of close tacrolimus monitoring in transplant recipients undergoing radiotherapy, even when other causes of variability are absent.
Timeline of association of tacrolimus trough levels drop with the initiation of radiotherapy