Towards a myeloablative regimen with clinical potential: I. Treosulfan conditioning and bone marrow transplantation allow induction of donor-specific tolerance for skin grafts across full MHC barriers

Treosulfan-based conditioning for inborn errors of immunity

Inborn errors of immunity (IEI) are inherited disorders that lead to defects in the development and/or function of the immune system. The number of disorders that can be treated by haematopoietic stem-cell transplantation (HSCT) has increased rapidly with the advent of next-generation sequencing. The methods used to transplant children with IEI have improved dramatically over the last 20 years. The introduction of reduced-toxicity conditioning is an important factor in the improved outcome of HSCT. Treosulfan has myeloablative and immunosuppressive properties, enabling engraftment with less toxicity than traditionally used doses of busulfan. It is firmly incorporated into the conditioning guidelines of the Inborn Errors Working Party of the European Society for Blood and Marrow Transplantation. Unlike busulfan, pharmacokinetically guided dosing of treosulfan is not part of routine practice, but data are emerging which indicate that further improvements in outcome may be possible, particularly in infants who have a decreased clearance of treosulfan. It is likely that individualized dosing, not just of treosulfan, but of all agents used in conditioning regimens, will be developed and implemented in the future. This will lead to a reduction in unwanted variability in drug exposure, leading to more predictable and adjustable exposure, and improved outcome of HSCT, with fewer late adverse effects and improved quality of life. Such conditioning regimens can be used as the basis to study the need for additional agents in certain disorders which are difficult to engraft or require high levels of donor chimerism, the dosing of individual cellular components within grafts, and effects of adjuvant cellular or immunotherapy post-transplant. This review documents the establishment of treosulfan worldwide, as a safe and effective agent for conditioning children with IEI prior to HSCT.

Bone Marrow Transplantation after Nonmyeloablative Treosulfan Conditioning Is Curative in a Murine Model of Sickle Cell Disease

Allogeneic hematopoietic stem cell transplantation (HSCT) can be curative for patients with sickle cell disease (SCD). However, morbidity associated with myeloablative conditioning and graft-versus-host disease has limited its utility. To this end, autologous HSCT for SCD using lentiviral gene-modified bone marrow (BM) or peripheral blood stem cells has been undertaken, although toxicities of fully ablative conditioning with busulfan and incomplete engraftment have been encountered. Treosulfan, a busulfan analog with a low extramedullary toxicity profile, has been used successfully as part of a myeloablative conditioning regimen in the allogeneic setting in SCD. To further minimize toxicity of conditioning, noncytotoxic monoclonal antibodies that clear stem cells from the marrow niche, such as anti-c-Kit (ACK2), have been considered. Using a murine model of SCD, we sought to determine whether nonmyeloablative conditioning followed by transplantation with syngeneic BM cells could ameliorate the disease phenotype. Treosulfan and ACK2, in a dose-dependent manner, decreased BM cellularity and induced cytopenia in SCD mice. Conditioning with treosulfan alone at nonmyeloablative dosing (3.6 g/kg), followed by transplantation with syngeneic BM donor cells, permitted long-term mixed-donor chimerism. Level of chimerism correlated with improvement in hematologic parameters, normalization of urine osmolality, and improvement in liver and spleen pathology. Addition of ACK2 to treosulfan conditioning did not enhance engraftment. Our data suggests that pretransplant conditioning with treosulfan alone may allow sufficient erythroid engraftment to reverse manifestations of SCD, with clinical application as a preparative regimen in SCD patients undergoing gene-modified autologous HSCT.

Inducible disruption of the c-myb gene allows allogeneic bone marrow transplantation without irradiation

Allogeneic bone marrow (BM) transplantation enables the in vivo functional assessment of hematopoietic cells. As pre-conditioning, ionizing radiation is commonly applied to induce BM depletion, however, it exerts adverse effects on the animal and can limit experimental outcome. Here, we provide an alternative method that harnesses conditional gene deletion to ablate c-myb and thereby deplete BM cells, hence allowing BM substitution without other pre-conditioning. The protocol results in a high level of blood chimerism after allogeneic BM transplantation, whereas immune cells in peripheral tissues such as resident macrophages are not replaced. Further, mice featuring a low chimerism after initial transplantation can undergo a second induction cycle for efficient deletion of residual BM cells without the necessity to re-apply donor cells. In summary, we present an effective c-myb-dependent genetic technique to generate BM chimeras in the absence of irradiation or other methods for pre-conditioning.

Bone Marrow-Derived Cell Accumulation in the Spinal Cord Is Independent of Peripheral Mobilization in a Mouse Model of Amyotrophic Lateral Sclerosis

Bone marrow-derived cells (BMDCs) are capable of migrating across the blood–brain barrier (BBB) and accumulating in the central nervous system (CNS) when transplanted into recipients conditioned with whole-body irradiation or chemotherapy. We used the chemotherapeutic agents busulfan and treosulfan to condition recipient mice for transplantation with bone marrow (BM) cells isolated from donor mice ubiquitously expressing green fluorescent protein. We attempted to increase the accumulation of BMDCs in the CNS by mobilization of BMDCs using either, or both, granulocyte colony-stimulating factor (GCSF) or plerixafor (AMD3100). We also used several concentrations of busulfan. We hypothesized that higher concentrations of busulfan and BMDC mobilization would increase numbers of GFP⁺ cells in the CNS. The doses of busulfan employed (60–125 mg/kg) all resulted in high levels of sustained chimerism (>85% 1 year post-transplant) in both the blood and BM of wild-type (WT) mice and an amyotrophic lateral sclerosis (ALS) mouse model. Moreover, cells accumulated within the CNS in a dose-, time-, and disease-dependent manner. Conditioning with the hydrophilic busulfan analog treosulfan, which is unable to cross the BBB efficiently, also resulted in a high degree of BM chimerism. However, few GFP⁺ BMDCs were found within the CNS of WT or ALS mice of treosulfan-conditioned mice. Mobilization of BMDCs into the circulation using GCSF and/or AMD3100 did not lead to increased accumulation of GFP⁺ BMDCs within the CNS of WT or ALS mice. Weekly analysis of BMDC accumulation revealed that BMDCs accumulated more rapidly and to a greater extent in the CNS of ALS mice conditioned with a high dose (125 mg/kg) of busulfan compared to a lower dose (80 mg/kg). The number of GFP⁺ BMDCs in the CNS labeling with the proliferation marker Ki67 increased in parallel with BMDC accumulation within the CNS. Our results indicate that establishment of high levels of blood and BM chimerism alone is not sufficient to induce BMDC accumulation within the CNS and that CNS conditioning is a crucial requirement for BMDC accumulation to occur. Moreover, it appears that proliferation of BMDCs that infiltrate the CNS is partly responsible for cell accumulation in busulfan-conditioned ALS mice.

Busulfan as a myelosuppressive agent for generating stable high-level bone marrow chimerism in mice

Bone marrow transplantation (BMT) is often used to replace the bone marrow (BM) compartment of recipient mice with BM cells expressing a distinct biomarker isolated from donor mice. This technique allows for identification of donor-derived hematopoietic cells within the recipient mice, and can be used to isolate and characterize donor cells using various biochemical techniques. BMT typically relies on myeloablative conditioning with total body irradiation to generate niche space within the BM compartment of recipient mice for donor cell engraftment. The protocol we describe here uses myelosuppressive conditioning with the chemotherapeutic agent busulfan. Unlike irradiation, which requires the use of specialized facilities, busulfan conditioning is performed using intraperitoneal injections of 20 mg/kg busulfan until a total dose of 60-100 mg/kg has been administered. Moreover, myeloablative irradiation can have toxic side effects and requires successful engraftment of donor cells for survival of recipient mice. In contrast, busulfan conditioning using these doses is generally well tolerated and mice survive without donor cell support. Donor BM cells are isolated from the femurs and tibiae of mice ubiquitously expressing green fluorescent protein (GFP), and injected into the lateral tail vein of conditioned recipient mice. BM chimerism is estimated by quantifying the number of GFP+ cells within the peripheral blood following BMT. Levels of chimerism >80% are typically observed in the peripheral blood 3-4 weeks post-transplant and remain established for at least 1 year. As with irradiation, conditioning with busulfan and BMT allows for the accumulation of donor BM-derived cells within the central nervous system (CNS), particularly in mouse models of neurodegeneration. This busulfan-mediated CNS accumulation may be more physiological than total body irradiation, as the busulfan treatment is less toxic and CNS inflammation appears to be less extensive. We hypothesize that these cells can be genetically engineered to deliver therapeutics to the CNS.

Human hepatocytes and hematolymphoid dual reconstitution in treosulfan-conditioned uPA-NOG mice

Human-specific HIV-1 and hepatitis co-infections significantly affect patient management and call for new therapeutic options. Small xenotransplantation models with human hepatocytes and hematolymphoid tissue should facilitate antiviral/antiretroviral drug trials. However, experience with mouse strains tested for dual reconstitution is limited, with technical difficulties such as risky manipulations with newborns and high mortality rates due to metabolic abnormalities. The best animal strains for hepatocyte transplantation are not optimal for human hematopoietic stem cell (HSC) engraftment, and vice versa. We evaluated a new strain of highly immunodeficient nonobese diabetic/Shi-scid (severe combined immunodeficiency)/IL-2Rγc(null) (NOG) mice that carry two copies of the mouse albumin promoter-driven urokinase-type plasminogen activator transgene for dual reconstitution with human liver and immune cells. Three approaches for dual reconstitution were evaluated: i) freshly isolated fetal hepatoblasts were injected intrasplenically, followed by transplantation of cryopreserved HSCs obtained from the same tissue samples 1 month later after treosulfan conditioning; ii) treosulfan conditioning is followed by intrasplenic simultaneous transplantation of fetal hepatoblasts and HSCs; and iii) transplantation of mature hepatocytes is followed by mismatched HSCs. The long-term dual reconstitution was achieved on urokinase-type plasminogen activator-NOG mice with mature hepatocytes (not fetal hepatoblasts) and HSCs. Even major histocompatibility complex mismatched transplantation was sustained without any evidence of hepatocyte rejection by the human immune system.

Nonmyeloablative conditioning generates autoantigen-encoding bone marrow that prevents and cures an experimental autoimmune disease

Autoimmune diseases result from chronic targeted immune responses that lead to tissue pathology and disease. The potential of autologous hematopoietic stem cells transplantation as a treatment for autoimmunity is currently being trialled but disease relapse is an issue. We have previously shown in a mouse model of experimental autoimmune encephalomyelitis (EAE) that the transplantation of bone marrow (BM) transduced to encode the autoantigen myelin oligodendrocyte glycoprotein (MOG) can prevent disease induction. However these studies were performed using lethal irradiation to generate BM chimeras and a critical factor for translation to humans would be the ability to utilize low toxic preconditioning regimes. In this study, treosulfan was used as a nonmyeloablative agent to generate BM chimeras encoding MOG and assessed in models of EAE induction and reversal. We find that treosulfan conditioning can promote a low degree of chimerism that is sufficient to promote antigen specific tolerance and protect mice from EAE. When incorporated into a curative protocol for treating mice with established EAE, nonmyeloablative conditioning and low chimerism was equally efficient in maintaining disease resistance. These studies further underpin the potential and feasibility of utilizing a gene therapy approach to treat autoimmune disease.

Reduced-toxicity conditioning with treosulfan and fludarabine in allogeneic hematopoietic stem cell transplantation for myelodysplastic syndromes: final results of an international prospective phase II trial

BACKGROUND

An alternative reduced-toxicity conditioning regimen for allogeneic transplantation, based on treosulfan and fludarabine, has recently been identified. The rationale for this study was to investigate the efficacy and safety of this regimen prospectively in patients with a primary myelodysplastic syndrome.

DESIGN AND METHODS

A total of 45 patients with primary myelodysplastic syndromes were conditioned with 3×14 g/m(2) treosulfan and 5×30 mg/m(2) fludarabine followed by allogeneic hematopoietic stem cell transplantation. Subtypes of myelodysplastic syndromes were refractory anemia with excess blasts-2 (44%), refractory cytopenia with multilineage dysplasia (27%), refractory anemia (9%), refractory anemia with ringed sideroblasts (4%), refractory cytopenia with multilineage dysplasia and ringed sideroblasts (4%), refractory anemia with excess blasts-1 (2%), and myelodysplastic syndrome with isolated del (5q) (2%). The myelodysplastic syndrome was unclassified in 7% of the patients. Forty-seven percent of the patients had a favorable karyotype, 29% an unfavorable one, and 18% an intermediate karyotype. Patients were evaluated for engraftment, adverse events, graft-versus-host disease, non-relapse mortality, relapse incidence, overall survival and disease-free survival.

RESULTS

All but one patient showed primary engraftment of neutrophils after a median of 17 days. Non-hematologic adverse events of grade III-IV in severity included mainly infections and gastrointestinal symptoms (80% and 22% of the patients, respectively). Acute graft-versus-host disease grade II-IV developed in 24%, and extensive chronic graft-versus-host disease in 28% of the patients. After a median follow-up of 780 days, the 2-year overall and disease-free survival estimates were 71% and 67%, respectively. The 2-year cumulative incidences of non-relapse mortality and relapse were 17% and 16%, respectively.

CONCLUSIONS

Our safety and efficacy data suggest that treosulfan-based conditioning therapy is a promising treatment option for patients with myelodysplastic syndromes. clinicaltrials.gov identifier: NCT01062490.

Treosulfan-based conditioning regimens for hematopoietic stem cell transplantation in children with primary immunodeficiency: United Kingdom experience

Children with primary immunodeficiency diseases, particularly those less than 1 year of age, experience significant toxicity after hematopoietic stem cell transplantation, with busulfan- or melphalan-based conditioning. Treosulfan causes less veno-occlusive disease than busulfan and does not require pharmacokinetic monitoring. We report its use in 70 children. Children received 42 g/m(2) or 36 g/m(2) with cyclophosphamide 200 mg/kg (n = 30) or fludarabine 150 mg/m(2) (n = 40), with alemtuzumab in most. Median age at transplantation was 8.5 months (range, 1.2-175 months); 46 (66%) patients were 12 months of age or younger. Donors were as follows: matched sibling donor, 8; matched family donor, 13; haploidentical, 4; and unrelated, 45. Median follow-up was 19 months (range, 1-47 months). Overall survival was 81%, equivalent in those age less or greater than 1 year. Skin toxicity was common. Veno-occlusive disease occurred twice with cyclophosphamide. Eighteen patients (26%) had graft-versus-host disease, and only 7 (10%) greater than grade 2. Two patients rejected; 24 of 42 more than 1 year after transplantation had 100% donor chimerism. The remainder had stable mixed chimerism. T-cell chimerism was significantly better with fludarabine. Long-term follow-up is required, but in combination with fludarabine, treosulfan is a good choice of conditioning for hematopoietic stem cell transplantation in primary immunodeficiency disease.

In pursuit of the ultimate: the initial Ahmedabad journey toward transplantation tolerance

We designed a prospective clinical trial of 357 patients divided in two groups--treated (n = 201) and controls (n = 156)--to evaluate effects of donor hematopoietic stem cell transplantation (HSCT) with minimal nonmyeloablative conditioning for tolerance induction in living related donor renal allograft recipients. Conditioning included donor leukocyte infusions, target-specific irradiation, anti-T-cell antibody, cyclophosphamide, cyclosporine (CsA), followed by bone marrow (BM)-derived and peripheral blood stem cell (PBSC) infusion into thymus, liver, BM, and periphery, with mean total dose of 20 x 10(8) nucleated cells/kg body weight (BW) (mean CD34(+) count: 0.9%) pretransplantation. CsA (3 mg/kg BW/d) and prednisolone (10 mg/d) were used for immunosuppression. Azathioprine/mycophenolate mofetil were added in the event of an acute rejection episode. The controls underwent transplantation with three drug immunosuppression. With a mean follow-up of 21.5 months, the treated cohort showed better allograft function with mean serum creatinine (SCr), 1.42 +/- 0.31 mg% in contrast with the controls mean SCr, 1.61 +/- 0.52 mg% (P < .0001) at 23.9 months follow-up. One-year allograft/patient survival was 95%/96.7% versus 89%/93.4%, respectively. Peripheral blood chimerism by fluorescent in situ hybridization was 0.8% +/- 0.2% in the subset of treated patients with gender-mismatched donors. No graft-versus-host disease was noted. Nine patients with donor-specific cytotoxic alloantibodies pretransplantation showed a decrease in positivity to <15% post-HSCT and were transplanted safely. A transient rise in donor-specific cytotoxic alloantibodies was noted in 19 treated patients post-HSCT, 14 of whom returned to the transplantable range within 2 weeks and five required a desensitization protocol. "Prope" tolerance may be induced in living related donor renal transplantation across major histocompatability complex barriers using HSCT with minimal nonmyeloablative conditioning.

Substitution of cyclophosphamide and busulfan by fludarabine, treosulfan and melphalan in a preparative regimen for children and adolescents with Shwachman–Diamond syndrome

Allogeneic hematopoietic stem cell transplantation (HSCT) is the only definitive treatment for severe bone marrow dysfunction and clonal disorders in patients diagnosed with Shwachman-Diamond syndrome (SDS). In an attempt to minimize regimen-related toxicity (RRT), we have initiated a fludarabine/treosulfan/melphalan-based pilot protocol avoiding the combination of busulfan and cyclophosphamide. Median age at transplantation was 9.6 years (range 1.5-17 years). All three patients received conditioning with fludarabine (30 mg/m2/day x 6), treosulfan (12 g/m2/day x 3) and melphalan (140 mg/m2/day x 1). CAMPATH-1H (0.1 mg/kg x 2) was added in two cases, while rabbit ATG (Genzyme; 3 x 2.5 mg/kg) was given to the cord blood recipient. One patient was transplanted with a non-manipulated marrow graft from an HLA-identical sibling, one with a marrow graft from a 10/10 matched unrelated donor, and one with a 9/10 matched unrelated umbilical cord blood (UCB) unit. Mean cell doses given were 3.6 x 10(8) nucleated cells/kg BW for the bone marrow recipients and 4.2 x 10(7) nucleated cells/kg BW for UCB recipient. Overall, two of three patients are alive and display 100% donor chimerism. Acute graft-versus-host disease grade II was seen in one patient, while no GVHD exceeding grade I occurred in the remaining two.

Myeloablative and immunosuppressive properties of treosulfan in mice

OBJECTIVE

Treosulfan is a prodrug with a specific clinical activity in ovarian carcinoma and other solid tumors. Due to its myeloablative and immunosuppressive effects, its use in conditioning regimens prior to allogeneic stem cell treatment (SCT) has been proposed. In the present preclinical study, myeloablative as well as immunosuppressive properties of treosulfan were compared with those of busulfan and cyclophosphamide.

METHODS

Three groups of BALB/c mice were treated with treosulfan, cyclophosphamide, or busulfan at sublethal doses that maintained survival without bone marrow support. The control group was left untreated. At different intervals, colony-forming unit granulocyte macrophage assay was performed on marrow cells. Additionally, immunological analyses were performed using spleen cells.

RESULTS

We found that treosulfan and busulfan induced a high and persisting degree of myeloablation, as compared with cyclophosphamide. Moreover, treosulfan was more effective in depletion of splenic B and T cells in comparison with busulfan and cyclophosphamide. Furthermore, T cells isolated from the spleens of treosulfan- or busulfan-treated mice were not responsive to allogeneic cells compared with that observed in controls and cyclophosphamide-treated mice. Treatment with treosulfan induced only interleukin-2 production in spleen cells for a short time and had no significant effect on synthesis of tumor necrosis factor-alpha and/or interferon-gamma as compared with that observed in splenic T cells isolated from mice treated with either busulfan or cyclophosphamide.

CONCLUSION

Our findings suggest that treosulfan possesses both myeloablative and immunosuppressive properties and may be used as a single agent for conditioning prior to bone marrow transplantation.

Dose-escalated treosulphan in combination with cyclophosphamide as a new preparative regimen for allogeneic haematopoietic stem cell transplantation in patients with an increased risk for regimen-related complications

Treosulphan has recently demonstrated antileukaemic activity and potent haematopoietic stem cell toxicity. Dose-escalated treosulphan (3 x 12 or 3 x 14 g/m2) combined with cyclophosphamide (Cy) was chosen for a new preparative regimen before allogeneic haematopoietic stem cell transplantation in 18 patients (median age 44, range 19-64 years) with haematological malignancies, considered ineligible for other myeloablative preparative regimens. Pharmacokinetic studies demonstrated rapid treosulphan plasma clearance and a dose-dependent increase of its maximum plasma concentrations and area under the concentration-time curves. Rapid and sustained white blood cell and platelet recovery and full donor chimerism was attained in all evaluable patients. Nonhaematological regimen-related CTC grades 3-4 adverse events were transient and predominantly consisted of cardiac (28%), gastrointestinal (39%), and hepatic (39%) toxicities. The 1-year nonrelapse mortality was 22%. Principal causes of transplant-related lethal events were infections in three of four affected patients. Only one patient died from regimen-related cardiac toxicity. The 1-year relapse estimate is 22%, overall and progression-free survival estimates are 67 and 56%, respectively. In conclusion, this new treosulphan and Cy combination is an effective, comparatively well-tolerated myeloablative preparative regimen even in patients with an increased risk for regimen-related toxic complications.

Addition of treosulfan to a nonmyeloablative conditioning regimen results in enhanced chimerism and immunologic tolerance in an experimental allogeneic bone marrow transplant model

Treosulfan (L-threitol-1,4-bismethanesulfonate) is an alkylating agent with routine clinical application in the treatment of ovarian cancer. In this murine study we show that this drug also has the ability to deplete primitive hematopoietic stem cells in a dose-dependent manner as determined by the cobblestone area-forming cell assay and is similar to its parent compound busulfan. Because busulfan is frequently used as part of the conditioning regimen before stem cell transplantation, we investigated an alternative nonmyeloablative protocol in an allogeneic bone marrow transplantation model in which low-dose treosulfan was added to an immune-suppressive regimen consisting of T cell-depleting antibodies, fludarabine, and thymic irradiation. Although this treatment protocol produced minimal myelosuppression, the addition of treosulfan proved to be important for allowing stable multilineage and mixed chimerism in C57BL/6 recipients of major histocompatibility complex-mismatched B10.A bone marrow without evidence of graft-versus-host disease. Donor lymphocyte infusion performed at 10 weeks after bone marrow transplantation had the effect of transforming the state of mixed chimerism to full donor-type cells, again without evidence of graft-versus-host disease. Donor-specific immunologic tolerance in the mixed chimeric animals was indicated by the acceptance of donor-type and rejection of third-party skin grafts. Thus, low-dose treosulfan may be considered as a useful component of a truly nonmyeloablative conditioning protocol in providing for mixed hematopoietic chimerism and, consequently, in establishing a platform for adoptive immunotherapy.

Treosulfan and fludarabine: a new toxicity-reduced conditioning regimen for allogeneic hematopoietic stem cell transplantation

New conditioning regimens are being explored to reduce toxicity and enable allogeneic bone marrow transplantation in patients not eligible for conventional transplantation. We have investigated treosulfan, an alkylating agent, with the aim of developing an efficient and reliable but less-toxic conditioning regimen. A series of 30 patients who were not eligible for standard conditioning therapy received transplants from HLA-matched related (n = 14) or unrelated (n = 16) donors after administration of treosulfan 10 g/m2 intravenously daily for 3 days and fludarabine 30 mg/m2 intravenously daily for 5 days. Patients receiving grafts from unrelated donors also were given rabbit antithymocyte globulin 10 mg/kg intravenously daily for 3 days. All patients achieved prompt neutrophil and platelet recovery. Extramedullary toxicity was generally mild with Common Toxicity Criteria (CTC) grade 3 or 4 attributable to the conditioning seen only with transaminases. Complete donor chimerism was achieved by 90% of the patients. Acute graft-versus-host disease (GVHD) grade III or IV developed in 14% of the patients and chronic GVHD in 39%. An estimated overall survival rate of 73% and an event-free survival rate of 49% have been reached after a median of 22 months (range, 7.4-33.4 months). In summary, the combination of treosulfan and fludarabine is a safe and efficient conditioning regimen.

Toward a myeloablative regimen with clinical potential: II. Treosulfan induces specific skin graft tolerance across haploidentical MHC barriers

Treosulfan is a water-soluble structural analog of busulfan, acting as a prodrug of alkylating epoxide species. It does not induce severe hepatotoxicity or veno-occlusive disease at or above the maximum tolerated dose, lacks significant nonhematological toxicity and has a limited organ toxicity. It is mainly indicated for the treatment of patients with ovarian cancer. In the present study, we report that permanent donor-specific tolerance and stable mixed multilineage chimerism can successfully be achieved across haploidentical MHC barriers when Treosulfan is administered in combination with anti-T-cell mAb and T-cell-depleted donor bone marrow cells. Furthermore, we show that less T-cell suppression is required when Treosulfan is included in the conditioning regimen. In conclusion, Treosulfan is a well-tolerated myeloablative agent with a low toxicity, and is a promising candidate drug for conditioning prior to bone marrow transplantation.