Fludarabine Washout Timing in Transplant: Research Examines 48- vs 72-Hour Rest Periods

Determining the optimal fludarabine washout period before hematopoietic stem cell transplant remains an area with limited evidence and wide variation across institutions. At Atrium Health Wake Forest Baptist, PGY2 oncology pharmacy resident Megan Wolff, PharmD, saw a natural opportunity to investigate this question when 2 sites—each using different rest periods—created built-in comparison groups. In this interview at the Hematology/Oncology Pharmacy Association 2026 Annual Conference, she discusses the pharmacologic rationale behind washout timing, the practice differences that shaped her study, the clinical significance of her findings, and what future research could help standardize care across transplant centers.

Megan Wolff, PharmD: Hi, my name is Megan Wolff, and I am currently a PGY2 oncology pharmacy resident at Atrium Health Wake Forest Baptist. I am presenting my research this week on fludarabine washout periods. We have 2 different sites doing 2 different strategies, so it made a natural cohort to look at. One site is using a 2-day rest period, and the other site is using a 3-day rest period. I was checking to see if that had any effect on neutrophil engraftment and some other secondary end points.

Q: The concern driving this study centers on fludarabine’s active intracellular metabolite, F-ara-ATP, persisting beyond plasma clearance. Can you walk us through the pharmacokinetic and pharmacodynamic basis for why intracellular accumulation is particularly relevant in this setting, and what the existing data suggest about how long meaningful intracellular concentrations may actually persist?

Wolff: Yeah, so I thought this was an interesting question, because there isn't a lot of literature out there on what the exact rest period for fludarabine should be before doing a transplant. The idea behind it is that the FDA labeling for fludarabine has a half-life of around 20 hours, so that would mean around 5 days for it to be fully out of the patient's system. The idea behind that is we don't want anything affecting the new donor cells that we will be putting in. So if we need a longer rest period so there isn't anything affecting the donor cells, we will.

One site was using a little bit longer—a 3-day rest period—and the other was using a 2-day rest period. The idea was to see whether that affected neutrophil engraftment time and things like length of stay, growth factor use, and infections.

Q: This is a dual-center retrospective cohort study, which inherently means the 2 institutions had differing washout practices that created the comparison groups. Can you speak to what drove that practice variation between the 2 Atrium Health centers, and whether there were any other systematic differences in transplant protocols, busulfan dosing strategies, or supportive care practices that needed to be accounted for in the analysis?

Wolff: It was an interesting question. The practice changes within these 2 hospital systems were actually established before they merged under the Atrium Health umbrella. So that's how they first started with these 2 different systems, which made my research project very helpful to see if we can merge them now that we are one health system.

There were some differences as well, not just with the rest period but with some supportive care things. There were differences in growth factor administration time, and that was something I had to think about when interpreting the results.

Q: You defined neutrophil engraftment as the first of 3 consecutive days with an ANC greater than 500 cells/µL—a standard but specific threshold. How did you determine that neutrophil engraftment time was the most meaningful primary end point for this question, and what magnitude of difference between the 48- and 72-hour groups would you consider clinically significant enough to warrant a formal practice change at your institutions?

Wolff: Yeah, I picked neutrophil engraftment time because across transplant research, I feel like that is something people are most interested in, and it leads to downstream effects—such as their risk of getting an infection within that time period, the length of stay in the hospital, and how long they're going to be on growth factor. This was a pretty unified definition in the transplant research community, so I felt like that was a great primary end point to look at.

The magnitude of difference that I would think would be clinically significant enough would have to be interpreted in the whole picture. I would love to see whether they had any substantial infections between the 2 groups, whether growth factor use differed between the 2 groups, and whether length of stay differed. Within my project specifically, I actually found statistical significance of about 1 day between the 2 rest periods. But the question is whether that is clinically significant. When I looked at my secondary end points, hospital length of stay wasn't different and rates of infections weren't different, so I wouldn't say it was clinically significant, even though I did find statistical significance.

Q: Several of your secondary end points—febrile neutropenia incidence, documented infections, growth factor duration, and hospital length of stay—are areas where pharmacists are deeply involved in day-to-day management. Pending your results, which of these secondary outcomes do you anticipate will be most informative for transplant pharmacists evaluating the downstream resource and safety implications of washout interval choice?

Wolff: Yeah, I think transplant pharmacists would like to see the growth factor outcomes, because we are usually the ones on the team managing whether they still need growth factor that day. This will help with resources, including financial resources within the hospital, in determining whether they need to administer it or not. I think another big one is looking at infection risk, because we are usually the ones helping manage their antibiotics and determining when we can safely pull those off. So those are the 2 secondary end points I feel would be most relevant for pharmacists.

Q: Retrospective studies in the transplant setting can be challenging given the heterogeneity of donor types, graft sources, and graft-versus-host disease (GVHD) prophylaxis regimens. What were the most significant potential confounders your team had to address, and are there patient subgroups—such as those with renal impairment affecting fludarabine clearance—where the washout interval question may carry even greater pharmacological significance?

Wolff: Yeah, there definitely were some confounders regarding the types of disease states patients were being transplanted for. There was a lot more AML in one cohort than the other. The good news was that a lot of the graft-versus-host disease prophylaxis was about the same. I do think a prospective study would be more beneficial in this space just to completely eliminate those confounders.

Q: The stated goal of this work is to inform evidence-based standardization of fludarabine washout intervals across transplant centers. If your findings support one interval over the other, what do you see as the next steps, and would you advocate for a prospective, multicenter study to more definitively resolve this question, particularly given how much institutional practice currently varies?

Wolff: Within my study, I found statistical significance for doing a longer washout period versus the shorter washout period, but it was only different by about 1 day. To bring it forward and try to merge practices, I think the next step is looking at some of those secondary end points I mentioned. Did it affect hospital length of stay? Numerically, it was shorter in the longer washout period, but that did not reach statistical significance.

So I do think a prospective study instead of a retrospective one would help, just so we can fully determine whether the shorter neutrophil engraftment time affects those secondary end points. Then we could come to a conclusion on whether we would want to use a longer washout period versus a shorter washout period.