Busulfan Exposure And Malignancy Risk
Overview
The occurrence of secondary malignancies linked to busulfan exposure is deemed low but remains insufficiently characterized. Given the rising use of this alkylating agent in pre-gene therapy conditioning for nonmalignant hematologic disorders, a more precise understanding of busulfan’s potential contribution to subsequent malignancy risk is essential. A literature-based assessment was conducted, focusing on secondary malignancies associated with busulfan, with publications identified through PubMed searches, particularly those with a follow-up duration of at least 3 years.
The analysis encompassed eight pediatric and 13 adult publications, involving 570 pediatric and 2076 adult recipients of hematopoietic cell transplant (HCT). Secondary malignancies were reported in 0.5% of pediatric HCT recipients, with no instances of myelodysplastic syndrome (MDS) or acute myelocytic leukemia (AML). Among 1887 evaluable adult HCT recipients, fatal secondary malignancies were documented in 0.8%, with an overall incidence of 4.8% in a subset of 389 evaluable adult patients.
Furthermore, an examination of long-term outcomes from eight publications appraising gene therapy in 215 patients with nonmalignant conditions revealed two malignancies in sickle cell disease (SCD) patients, one potentially linked to busulfan. Notably, no additional malignancies were reported in 173 patients with 5-12 years of follow-up.
In conclusion, the likelihood of busulfan-related secondary malignancies appears to be notably low, possibly less than 1% in pediatric transplant recipients, especially those undergoing busulfan monotherapy for nonmalignant conditions other than SCD. These findings underscore the importance of continued monitoring and research to comprehensively assess the long-term safety profile of busulfan in diverse patient populations.
Introduction
The alkylating agent busulfan has played a crucial role in conditioning regimens for hematopoietic cell transplantation (HCT) since its approval in 1999–2002. Its significance extends to autologous lentiviral-mediated gene therapy for nonmalignant genetic disorders across various dose levels. While short-term toxicities of busulfan are well-known, including mucositis and hepatic veno-occlusive disease, long-term effects such as impaired fertility, organ damage, and cancer predisposition have been observed. The exact incidence of certain effects, especially regarding fertility in pediatric recipients, remains unclear due to the co-administration of additional cytotoxic therapies.
The risk of busulfan-associated secondary malignancy has been incompletely understood, with recent concerns arising from the identification of non-insertional myeloid malignancy in a patient with sickle cell disease (SCD) who underwent autologous gene therapy. Given the expanding landscape of lentiviral-mediated gene therapy, this evaluation aims to provide a more optimal characterization of the malignancy potential associated with busulfan. This is particularly vital for informing patients and clinicians, especially in pediatric settings, considering gene therapy options for nonmalignant hematologic disorders. The literature-based assessment focuses on the incidence and nature of busulfan-related and other late-effect malignancies post-HCT, contributing to a comprehensive understanding of the agent’s long-term impact.
Method
A systematic literature review (SLR) was conducted using the PICOS strategy, focusing on patient characteristics, interventions, comparators, outcomes, and study designs, as outlined in the study. The search, performed on EMBASE and MEDLINE (via Proquest Dialog), targeted terms such as busulfan, transplantation, conditioning, and hematopoietic stem cell transplantation. The detailed search strategy is available in the Supporting Information Methods. The review encompassed English-language publications from 2012 through September 2022.
The retrieved literature was organized using a data extraction template (DET), with duplicates removed. Subsequently, the titles and abstracts were screened to determine inclusion/exclusion in the SLR. Exclusion criteria comprised conference abstracts, reviews lacking patient outcomes, and publications outside the scope (involving conditioning regimens without busulfan or lacking a key component). Figure 1 illustrates the search results, specific criteria employed, and the selection process, leading to cohorts of publications categorized by pediatric or young adult transplant studies with a minimum 5-year median follow-up, adult transplant studies with at least 5 years of median follow-up, and adult transplant studies with at least 3 years (but less than 5 years) of median follow-up.
Result
From the 455 initially identified records, only four papers were excluded at the abstract level, and 396 were excluded during full-text assessment. Due to the typical emergence of alkylating agent-mediated myeloid malignancies 4–7 years post-exposure, a focus was placed on studies with long-term results (median follow-up of at least 3 years) in both pediatric/adult and adult settings. Additionally, a thorough evaluation was conducted on autologous lentiviral (LV)-based gene therapy clinical trials utilizing self-inactivating LVs with human promotors, emphasizing studies with at least 5 years of median follow-up.
Out of 49 comprehensively assessed papers meeting inclusion criteria, 25 were selected for a detailed review. They were categorized into pediatric/young adult studies (≥3 years follow-up: 8; ≥5 years: 8) and adult studies (≥3 years follow-up: 16; ≥5 years: 8; ≥3 years and <5 years: 8).
In allogeneic hematopoietic cell transplant (HCT) studies, involving predominantly pediatric patients with a median follow-up of at least 5 years in 602 patients, secondary malignancies were absent in six studies (504 patients). In one study, involving 66 pediatric leukemia patients receiving busulfan-based conditioning, three cases (4.5%) of secondary malignancies were reported (thyroid,
In adult hematologic malignancy studies, encompassing 2076 patients with busulfan/chemotherapy combination pretransplant conditioning, the incidence of fatal secondary malignancies ranged from 0% to 5.9%, with an overall incidence of 0.8% (16 fatal cases in 1887 patients). Four studies with the highest incidence used reduced-intensity conditioning (RIC), and those with the lowest incidence also employed RIC.
Four adult studies reported overall incidences of secondary malignancies (389 patients), ranging from 0% to 10.7%. Notably, a single study in high-risk hematologic malignancies in older adults (median age 61) reported 16 secondary malignancies, primarily in patients with acute or chronic graft-versus-host disease (GVHD).
The contribution of busulfan to secondary malignancies was explored in autologous hematopoietic stem cell-directed gene therapy studies, where busulfan was used alone or in combination with fludarabine. Long-term follow-up results (5–12 years) from eight trials involving 173 patients showed no malignancies, except in Sickle Cell Disease (SCD) studies (42 patients), where two patients developed myeloid malignancies.
In the SCD studies, both malignancies harbored mutations and were considered potentially secondary to busulfan. However, in LV-based gene therapy studies using vectors with human promotors, no malignancies were reported secondary to insertional mutagenesis. In ADA-SCID gene therapy recipients, no clonal hematopoiesis or associated mutations were detected with RIC busulfan after 7–10 years of follow-up.
Conclusion
The incidence of busulfan-associated secondary malignancies, arising from its alkylating properties and DNA cross-linkages, appears low and is challenging to precisely define due to varied clinical contexts and follow-up durations. Notably, the risk is particularly low in pediatric settings, with the majority of long-term evaluations indicating an incidence of less than 1%. Across more than 500 patients, the overall aggregate incidence is reported at 0.5%. Secondary myelodysplastic syndrome (MDS) or acute myelocytic leukemia (AML) in pediatric busulfan exposure was not reported and is likely rare.
In adult transplant studies, the incidence of post-busulfan secondary malignancies is higher but still relatively limited. Fatal secondary malignancies are likely lower than 1%, and the overall incidence is likely lower than 5%, including noninvasive dermatologic malignancies. Older adults, participating extensively in certain studies, show a higher incidence, consistent with their elevated baseline cancer risk. However, these adult results may not precisely inform potential malignant risks for pediatric patients.
Interestingly, secondary malignancies were reported in cohorts receiving both myeloablative and reduced-intensity conditioning (RIC) regimens in adult transplant series. RIC regimens, while reducing overall cytotoxic burden, may counterbalance this effect through the survival of hematopoietic progenitors exposed to alkylators, with potential for mutagenic DNA damage. This is especially relevant in older adult cohorts with a higher baseline cancer risk.
The study’s limitations include the majority of hematopoietic cell transplants being performed for hematologic malignancies, involving multiagent conditioning regimens. This complicates attributing the specific contribution of busulfan to secondary malignancies. Results also pre-date therapeutic drug monitoring (TDM) implementation for precise busulfan dosing. Few publications provide detailed results on other long-term effects, such as fertility and neurocognitive development in pediatric transplant recipients. The risks of these effects cannot be precisely determined due to the multiagent nature of the evaluated regimens.
Busulfan-specific insights are expected from autologous hematopoietic stem cell-directed gene therapy studies, where busulfan is the exclusive conditioning agent. The anticipated longer-term follow-up, especially in patients without Sickle Cell Disease (SCD), suggests a very rare risk of secondary malignancy beyond 5 years in this population.
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