• Daniele Canarutto, Attya Omer Javed, Gabriele Pedrazzani, Samuele Ferrari, and Luigi Naldini.
• San Raffaele Telethon Institute for Gene Therapy, IRCCS San Raffaele Scientific Institute, Via Olgettina, 60, 20132 Milano, MI, Italy.
• Br. Med. Bull. 2023 Sep 12; 147 (1): 108120108-120.

IntroductionIn haematopoietic stem cell transplantation (HSCT), haematopoietic stem cells (HSCs) from a healthy donor replace the patient's ones. Ex vivo HSC gene therapy (HSC-GT) is a form of HSCT in which HSCs, usually from an autologous source, are genetically modified before infusion, to generate a progeny of gene-modified cells. In HSCT and HSC-GT, chemotherapy is administered before infusion to free space in the bone marrow (BM) niche, which is required for the engraftment of infused cells. Here, we review alternative chemotherapy-free approaches to niche voidance that could replace conventional regimens and alleviate the morbidity of the procedure.Sources Of DataLiterature was reviewed from PubMed-listed peer-reviewed articles. No new data are presented in this article.Areas Of AgreementChemotherapy exerts short and long-term toxicity to haematopoietic and non-haematopoietic organs. Whenever chemotherapy is solely used to allow engraftment of donor HSCs, rather than eliminating malignant cells, as in the case of HSC-GT for inborn genetic diseases, non-genotoxic approaches sparing off-target tissues are highly desirable.Areas Of ControversyIn principle, HSCs can be temporarily moved from the BM niches using mobilizing drugs or selectively cleared with targeted antibodies or immunotoxins to make space for the infused cells. However, translation of these principles into clinically relevant settings is only at the beginning, and whether therapeutically meaningful levels of chimerism can be safely established with these approaches remains to be determined.Growing PointsIn pre-clinical models, mobilization of HSCs from the niche can be tailored to accommodate the exchange and engraftment of infused cells. Infused cells can be further endowed with a transient engraftment advantage.Areas Timely For Developing ResearchInter-individual efficiency and kinetics of HSC mobilization need to be carefully assessed. Investigations in large animal models of emerging non-genotoxic approaches will further strengthen the rationale and encourage application to the treatment of selected diseases.© The Author(s) 2023. Published by Oxford University Press.

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