Treosulfan (DB11678): A Comprehensive Monograph on a Modern Myeloablative Conditioning Agent

Executive Summary

Treosulfan is a bifunctional alkylating agent, classified as a pharmacologically inactive prodrug, that has emerged as a significant component of conditioning regimens prior to allogeneic hematopoietic stem cell transplantation (alloHSCT).[1] Structurally, it is a water-soluble dihydroxy derivative of busulfan, a feature that contributes to its distinct physicochemical and pharmacokinetic profile.[3] Its primary clinical application, in combination with fludarabine, is as a myeloablative conditioning agent for adult and pediatric patients with malignant hematological diseases, including acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS).[1] In certain jurisdictions, its use extends to non-malignant disorders requiring alloHSCT.[5]

The mechanism of action of Treosulfan is unique among alkylating agents used in this setting. It undergoes a spontaneous, non-enzymatic conversion under physiological conditions of pH and temperature to form cytotoxic epoxide metabolites.[3] These active compounds, primarily (2S,3S)-1,2:3,4-diepoxybutane (S,S-DEB), subsequently alkylate DNA, inducing intra- and inter-strand cross-links that disrupt cellular processes and lead to apoptosis, particularly in rapidly dividing hematopoietic cells.[1] This non-enzymatic activation pathway results in highly predictable pharmacokinetics, obviating the need for therapeutic drug monitoring, which is a notable advantage over traditional busulfan-based regimens.[8]

Clinically, Treosulfan is positioned as a key therapeutic alternative to busulfan. Its approval by major regulatory bodies was supported by pivotal clinical trial evidence, most notably the MC-FludT. 14/L study, which demonstrated not only non-inferiority in event-free survival but also a statistically significant improvement in overall survival for older or comorbid patients with AML/MDS compared to a busulfan-based regimen.[9] This favorable efficacy-to-toxicity ratio, which includes a potentially lower incidence of hepato-, pulmo-, and neurotoxicity, has established its role in modern transplant medicine.[3]

The safety profile of Treosulfan is characterized by the intended and profound myelosuppression that necessitates rescue with HSCT. The most common adverse reactions include stomatitis, infections, nausea, vomiting, and pyrexia.[5] As a potent genotoxic agent, it carries a long-term risk of secondary malignancies and is classified as a human carcinogen.[11] In summary, Treosulfan represents a significant advancement in HSCT conditioning, offering a refined risk-benefit profile through its predictable pharmacology and demonstrated survival advantage in specific, often high-risk, patient populations.

Drug Identification and Physicochemical Characteristics

A precise understanding of Treosulfan's identity and chemical properties is fundamental to its clinical application, formulation, and mechanism of action. This section details its nomenclature, structural information, and key physicochemical characteristics.

2.1 Nomenclature and Identifiers

Treosulfan is recognized globally by its International Nonproprietary Name (INN). It is marketed under various brand names depending on the region, including Trecondi, Grafapex, and historically, Ovastat.[1] Its development and investigation are associated with several synonyms and code names, such as L-threitol-1,4-dimethanesulfonate, Dihydroxybusulfan, Treosulphan, and NSC 39069.[1] The standard IUPAC chemical name is methanesulfonate.[13] A comprehensive list of its key database identifiers is provided in Table 1.

2.2 Chemical Structure and Formula

Treosulfan is a small molecule drug with the chemical formula C6​H14​O8​S2​.[1] Its average molecular weight is consistently reported as 278.29 or 278.30 g/mol, with a precise monoisotopic mass of 278.013009759 Da.[1] The stereochemistry of the molecule is critical to its identity, defined by the (2S,3S) configuration of the chiral carbons. This is captured in its SMILES (Simplified Molecular Input Line Entry System) string:

CS(=O)(=O)OC[C@@H]([C@H](COS(=O)(=O)C)O)O and its InChIKey: YCPOZVAOBBQLRI-WDSKDSINSA-N.[5]

2.3 Physicochemical Properties

Treosulfan presents as an odorless, white to off-white or pale beige crystalline powder.[12] A key property distinguishing it from its analogue, busulfan, is its solubility. It is water-soluble, with a reported solubility of 5 mg/mL, forming a clear solution, and is also slightly soluble in organic solvents like acetone, DMSO, and methanol.[3] This water solubility facilitates its formulation for intravenous administration.

Reports on its melting point vary, with ranges cited as 76-78 °C, 101.5-105 °C, and a specific value of 216 °F (102.2 °C), likely reflecting differences in analytical methods or sample purity.[5] The molecule's stability in aqueous solution is directly linked to its mechanism of activation; it undergoes slow hydrolysis, with decomposition observed within 3 hours at a pH of 7.5 and a temperature of 25 °C (77 °F).[12] For long-term preservation, the lyophilized powder form is stored at -20 °C.[12] Once reconstituted for clinical use, the solution is stable at room temperature for up to 24 hours and should not be refrigerated, as lower temperatures may affect solubility or stability.[4]

Table 1: Treosulfan Identification and Physicochemical Properties

Property	Value	Source(s)
International Nonproprietary Name (INN)	Treosulfan	5
Brand Names	Trecondi, Grafapex, Ovastat	1
CAS Number	299-75-2	1
DrugBank ID	DB11678	1
FDA UNII	CO61ER3EPI	5
ATC Code	L01AB02	12
Chemical Formula	C6​H14​O8​S2​	1
Molecular Weight (Average)	278.29 - 278.30 g/mol	1
Appearance	Odorless white to pale beige crystalline powder	12
Solubility	Water: 5 mg/mL (clear); Slightly soluble in Acetone, DMSO, Methanol	12
Melting Point	101.5 to 105 °C	5
Storage (Powder)	Store at -20 °C	12
Storage (Reconstituted Solution)	Room temperature for up to 24 hours; do not refrigerate	4

Clinical Pharmacology

The clinical utility of Treosulfan is dictated by its unique pharmacological profile, encompassing its mechanism of action, pharmacokinetics, and pharmacodynamics. A central feature is its behavior as a prodrug that activates non-enzymatically, leading to a predictable and potent cytotoxic effect.

3.1 Mechanism of Action

Treosulfan is administered as a pharmacologically inactive prodrug.[1] Its cytotoxic activity is entirely dependent on its conversion into active metabolites. This conversion is a spontaneous, non-enzymatic chemical process driven by physiological conditions, specifically a pH greater than 5 and body temperature.[1] This pH- and temperature-dependent activation is a critical differentiator from other alkylating agents like busulfan, which require enzymatic metabolism for their activity.[8]

The activation proceeds via a sequential intramolecular nucleophilic substitution. First, Treosulfan converts to a monoepoxide intermediate, (2S,3S)-1,2-epoxybutane-3,4-diol-4-methanesulfonate (S,S-EBDM). This intermediate is then further converted to a highly reactive bifunctional diepoxide, L-diepoxibutane, also known as (2S,3S)-1,2:3,4-diepoxybutane (S,S-DEB).[1] These epoxide metabolites are the ultimate effectors of the drug's cytotoxic activity.[2]

As a bifunctional alkylating agent, these epoxides covalently bind to nucleophilic sites on cellular macromolecules, most importantly DNA.[1] The two reactive epoxide groups allow for the formation of both mono-adducts and, more critically, DNA cross-links, both within the same DNA strand (intra-strand) and between opposite strands (inter-strand).[16] This extensive DNA damage interferes with fundamental cellular processes, including DNA replication and RNA transcription, leading to chromosomal aberrations, cell cycle arrest, and the induction of apoptosis.[1] The cytotoxic effect is most pronounced in rapidly proliferating cells, such as hematopoietic stem and progenitor cells, which explains Treosulfan's potent myeloablative and antineoplastic properties.[5] Beyond myelosuppression, its immunosuppressive activity is attributed to its toxicity against T-cells and Natural Killer (NK) cells and its ability to reduce the cellularity of primary and secondary lymphatic organs.[1]

The reliance on a non-enzymatic, physicochemical activation process rather than hepatic metabolism is a cornerstone of Treosulfan's pharmacological profile. Enzymatic pathways, such as those involving cytochrome P450 or glutathione S-transferases that activate busulfan, are subject to significant inter-individual variability due to genetic polymorphisms and are susceptible to drug-drug interactions. By bypassing these variable systems, Treosulfan's conversion to its active form is more uniform and predictable across a diverse patient population. This predictability is the primary reason that routine therapeutic drug monitoring (TDM), a standard and resource-intensive requirement for busulfan, is not necessary for Treosulfan, simplifying treatment and potentially reducing the risk of toxicities associated with overexposure or graft failure from underexposure.[8]

3.2 Pharmacokinetics (ADME)

The disposition of Treosulfan in the body is characterized by rapid distribution and elimination, with a metabolic profile dominated by its spontaneous chemical conversion.

Absorption: Treosulfan is administered exclusively by intravenous infusion. Peak plasma concentrations are achieved at the end of the standard 2-hour infusion period.[2] The mean area under the plasma concentration-time curve (AUC) following a standard dose is approximately 1200 ± 211 hr·mcg/mL.[1] Clinical studies have shown no evidence of drug accumulation with the standard regimen of daily administration for three days.[4]

Distribution: Following administration, Treosulfan is rapidly distributed throughout the body. The mean volume of distribution (Vd​) is reported to be approximately 41 L, although a wider range of 20-88 L has been described in the literature.[1] This indicates distribution into total body water. Importantly, there is no significant binding to plasma proteins like albumin, meaning the majority of the drug in circulation is free and available for distribution and activation.[4] Reports suggest that penetration across the blood-brain barrier is limited.[2]

Metabolism: As detailed previously, the primary "metabolism" of Treosulfan is its spontaneous, non-enzymatic conversion to the active epoxides S,S-EBDM and S,S-DEB.[1] While minor enzymatic pathways have been identified—Treosulfan itself is a weak substrate of CYP2D6 and its monoepoxide metabolite a substrate of CYP2C8—these are not considered the primary drivers of its activation or clearance, and the non-enzymatic pathway is the clinically dominant feature.[1]

Elimination: The elimination of Treosulfan from the body follows a first-order process, described by a two-compartment model.[2] A substantial fraction of the parent drug is cleared by the kidneys. A median of 42% of the administered dose is excreted unchanged in the urine within 24 hours, with ranges reported from 14% to 42% across different studies.[1] The majority of this renal excretion (nearly 90%) occurs rapidly, within the first 6 to 8 hours post-infusion.[1] This rapid renal clearance suggests that glomerular filtration is a primary elimination pathway, with tubular reabsorption also potentially playing a role.[3] The terminal elimination half-life (

t1/2​) of the parent drug is short, with a mean of approximately 1.5 to 1.9 hours.[1]

Special Populations: The pharmacokinetics of Treosulfan are significantly influenced by patient age and body size, particularly in the pediatric population. Population pharmacokinetic modeling has identified bodyweight (utilizing allometric scaling) and a maturation function as key covariates that predict drug clearance.[15] This modeling indicates that Treosulfan clearance in children reaches 90% of adult values by approximately 4 years of age.[15] Consequently, younger children and infants exhibit lower clearance, leading to higher drug exposure if adult dosing schemes are used. For instance, children with a body surface area (BSA) less than 0.7

m2 have approximately 11% higher exposure compared to adults.[4] This understanding has led to the development of specific, BSA-adjusted dosing recommendations for pediatric patients to achieve target exposures comparable to those in adults.[15]

3.3 Pharmacodynamics

The primary pharmacodynamic effect of high-dose Treosulfan is profound, dose-dependent myeloablation.[2] This is the intended therapeutic outcome in the context of HSCT conditioning, as it eradicates the patient's native hematopoietic system to create space for the donor graft. This effect is observed in 100% of patients receiving the high-dose conditioning regimen and manifests as severe pancytopenia.[2] When Treosulfan is administered at lower doses without subsequent stem cell support, this myelosuppression becomes the dose-limiting toxicity, with blood counts typically recovering within 21 to 28 days.[2] The intensity of both the desired myeloablative effect and the associated toxicities is related to the overall systemic exposure (AUC) to Treosulfan and its active epoxide metabolites.[3] The goal of the established dosing regimen is to achieve an AUC within a therapeutic window that maximizes the probability of successful engraftment and antileukemic activity while minimizing the risk of severe regimen-related toxicities.

Table 2: Summary of Key Pharmacokinetic Parameters for Treosulfan in Adults

Parameter	Mean Value (± SD or Range)	Source(s)
Route of Administration	Intravenous Infusion (2 hours)	11
Area Under the Curve (AUC)	1200 ± 211 hr·mcg/mL	1
Volume of Distribution (Vd​)	41 L (Range: 20-88 L)	1
Plasma Protein Binding	Not significant	4
Terminal Half-Life (t1/2​)	1.7 ± 0.4 hours (Range: 1.5-1.9 hours)	1
Renal Excretion (% Unchanged)	42% (median) within 24 hours	1

Clinical Efficacy and Therapeutic Applications

Treosulfan's clinical development has established it as a cornerstone of specific conditioning regimens for alloHSCT, supported by robust evidence from pivotal trials that demonstrate its efficacy and define its therapeutic role relative to established standards of care.

4.1 Approved Indications and Dosing Regimens

Treosulfan is indicated for use in combination with fludarabine as a preparative (conditioning) regimen prior to alloHSCT.[1] The specific patient populations vary slightly by regulatory jurisdiction:

• U.S. Food and Drug Administration (FDA): Approved for adult and pediatric patients one year of age and older with acute myeloid leukemia (AML) or myelodysplastic syndrome (MDS).[1]
• European Medicines Agency (EMA): Approved for adult patients with both malignant and non-malignant diseases, and for pediatric patients older than one month with malignant diseases.[5]

The standard dosing regimen for adults and most pediatric patients consists of Treosulfan administered at a dose of 10 g/m2 of body surface area (BSA) per day. It is given as a 2-hour intravenous infusion for three consecutive days, typically on days -4, -3, and -2 relative to the stem cell infusion on day 0, for a total dose of 30 g/m2.[9] Earlier investigational studies sometimes utilized higher daily doses of 12-14 g/

m2.[3] This regimen is administered in conjunction with fludarabine, which is often given at a dose of 30 mg/

m2/day for five days (e.g., days -6 through -2).[8] Administration is strictly for intravenous use and requires careful handling and preparation procedures appropriate for a cytotoxic agent. Specific diluents are recommended, and sterile water for injection should be avoided in young children due to the risk of creating a hypo-osmolar final solution.[4]

4.2 Analysis of the Pivotal Clinical Trial: MC-FludT. 14/L (NCT00822393)

The foundation of Treosulfan's regulatory approval is the MC-FludT. 14/L trial, a large, prospective, multicenter, randomized, active-controlled Phase 3 study.[10] The trial was designed to evaluate Treosulfan in a clinically relevant and challenging population: 570 adult patients (aged 18-70) with AML or MDS who were considered at increased risk for standard conditioning regimens due to age (≥50 years) or significant comorbidities (Hematopoietic Cell Transplantation-Comorbidity Index score > 2).[17]

The study compared two conditioning regimens: Treosulfan plus fludarabine versus the established standard of busulfan plus fludarabine.[17] The primary endpoint was event-free survival (EFS) at 2 years, with the initial hypothesis being that Treosulfan would be non-inferior to busulfan.[10]

The trial successfully met its primary endpoint, demonstrating that the Treosulfan-based regimen was non-inferior to the busulfan-based regimen for 2-year EFS.[10] Subsequent analyses revealed a clear trend toward superiority for Treosulfan. The Kaplan-Meier estimate for 2-year EFS was 64.0% in the Treosulfan arm, compared to 50.4% in the busulfan arm.[5]

A major secondary endpoint, overall survival (OS), provided the most compelling evidence of Treosulfan's benefit. The Treosulfan regimen was associated with a statistically significant improvement in OS compared to the busulfan regimen. The hazard ratio (HR) for death from any cause was 0.67 (95% CI: 0.51-0.90), which translates to a 33% reduction in the risk of death for patients receiving Treosulfan.[9] This robust survival advantage was a critical factor in its regulatory evaluation, particularly by the FDA. The journey to FDA approval highlights the significance of this finding. While the EMA granted marketing authorization in 2019 based on the EFS data, the FDA initially issued a Complete Response Letter in 2021.[5] It was only after the resubmission and focus on the mature OS data that approval was granted in 2025.[9] The FDA's public announcement specifically designated OS as the "major efficacy outcome measure," underscoring that a demonstrated benefit in this definitive, patient-centric endpoint was necessary to establish the drug's value proposition as superior to, not just an alternative to, the existing standard of care.[17]

4.3 Comparative Efficacy: Treosulfan versus Busulfan

The clinical evidence positions Treosulfan as a strong competitor to busulfan across several indications.

• AML/MDS in Adults: As established by the pivotal MC-FludT. 14/L trial, in the specific population of older or comorbid patients, Treosulfan offers at least comparable, and likely superior, survival outcomes.[9]
• Acute Lymphoblastic Leukemia (ALL) in Pediatrics: For children with ALL (older than 4 years) who have contraindications to or lack access to total body irradiation (TBI), chemotherapy-only conditioning is the standard. Studies comparing Treosulfan-based (TREO/THIO/FLU) and busulfan-based (BU/THIO/FLU) regimens have found that they yield comparable and acceptable outcomes, with similar rates of OS, EFS, relapse, and non-relapse mortality.[21] This establishes Treosulfan as a valid and safe alternative to busulfan in this setting.
• Non-Malignant Diseases in Pediatrics: In children with non-malignant diseases, Treosulfan-based conditioning has been shown to be safe and efficacious. However, a 36-month follow-up of one Phase II trial noted that while OS was greater with Treosulfan compared to busulfan, there was a higher incidence of secondary graft failure.[23] This suggests that while the agent is effective, the overall regimen, particularly the intensity of accompanying immunosuppression, may require further optimization to mitigate this specific risk in this vulnerable population.
• Toxicity Profile: Preclinical data and retrospective clinical analyses have suggested that Treosulfan is associated with a lower incidence of early organ toxicity, particularly hepato-, pulmo-, and neurotoxicity, compared to busulfan.[3] Its predictable pharmacokinetics and non-reliance on hepatic activation may contribute to a lower risk of hepatic veno-occlusive disease (VOD), a well-known and feared complication of busulfan overexposure.[8]

4.4 Investigational Use and Other Populations

The favorable profile of Treosulfan has prompted its investigation in other clinical contexts.

• Non-Malignant Diseases: A prospective multicenter trial using a regimen of Treosulfan, fludarabine, and thymoglobulin demonstrated effective donor engraftment with low toxicity and improved survival in patients with a range of non-malignant disorders, including hemoglobinopathies, primary immunodeficiencies, and bone marrow failure syndromes.[8] Numerous ongoing trials continue to evaluate its role in these conditions.[24]
• Myelofibrosis (MF): Recent clinical trial data indicate that a Treosulfan-based conditioning regimen led to improved progression-free survival and lower non-relapse mortality compared to a busulfan-based regimen for patients with MF undergoing alloHSCT.[20]
• Other Hematologic Malignancies: Treosulfan is being actively studied in combination with other cytotoxic agents and in various conditioning protocols for a broad spectrum of hematologic cancers.[25]

Table 3: Efficacy Outcomes from the Pivotal MC-FludT. 14/L Trial (Treosulfan vs. Busulfan)

Outcome Measure	Treosulfan + Fludarabine	Busulfan + Fludarabine	Hazard Ratio (HR) [95% CI]	Source(s)
2-Year Event-Free Survival (EFS)	64.0%	50.4%	0.65 [CI not for superiority]	10
Overall Survival (OS) - All Patients	N/A (HR is primary metric)	N/A (HR is primary metric)	0.67 [0.51, 0.90]	9
OS in AML Subgroup	N/A (HR is primary metric)	N/A (HR is primary metric)	0.73 [0.51, 1.06]	9
OS in MDS Subgroup	N/A (HR is primary metric)	N/A (HR is primary metric)	0.64 [0.40, 1.02]	9

Safety, Tolerability, and Risk Management

As a potent myeloablative agent, Treosulfan therapy is associated with significant toxicity. Its safety profile is characterized by predictable, on-target effects like myelosuppression, as well as a range of off-target adverse reactions. A thorough understanding of these risks, contraindications, and management strategies is essential for its safe use.

5.1 Adverse Drug Reactions (ADRs)

The adverse reactions observed with Treosulfan are typical for high-dose chemotherapy used in the HSCT setting. The most common toxicities involve the hematopoietic, gastrointestinal, and immune systems.[5]

Very Common Adverse Reactions (Incidence ≥10%):

• Hematologic: Profound myelosuppression and pancytopenia are universal and intended effects, occurring in 100% of patients.[2] Febrile neutropenia is also very common, with reported incidences ranging from 11% to 34%.[2]
• Gastrointestinal: The GI tract is highly susceptible to toxicity. Stomatitis and mucositis are among the most frequent ADRs, affecting 36% to 69% of patients. Nausea (28-43%), vomiting (23-43%), diarrhea (15-35%), and abdominal pain (10-17%) are also very common.[2]
• General and Constitutional: Pyrexia (fever) is reported in up to 34% of patients. Edema (29%) and fatigue are also frequently observed.[17]
• Infections: Due to profound immunosuppression, infections (bacterial, viral, or fungal) are a major cause of morbidity, occurring in 11% to 27% of patients.[2]
• Musculoskeletal: Musculoskeletal pain is a prominent side effect, reported in up to 39% of patients.[5]
• Hepatobiliary: Elevations in liver function tests are common. Increased levels of gamma-glutamyltransferase (GGT), bilirubin, alanine transaminase (ALT), and aspartate transaminase (AST) are seen in 9-18% of patients, with Grade 3 or 4 abnormalities also noted.[2]

Common and Serious Adverse Reactions:

A wide spectrum of less frequent but clinically important ADRs can occur. These include cardiac events (arrhythmias, heart failure), neurologic symptoms (vertigo, confusion), metabolic disturbances (anorexia, hypomagnesemia), renal impairment, and respiratory complications (cough, dyspnea, pneumonitis).2 Rare but serious adverse events include hepatic veno-occlusive disease (VOD), seizures, capillary leak syndrome, and the long-term risk of secondary malignancies.4

5.2 Contraindications, Warnings, and Precautions

The use of Treosulfan is restricted in certain populations and requires adherence to strict safety precautions.

Contraindications:

• Known hypersensitivity to Treosulfan or its components.[11]
• Pregnancy and lactation.[11]
• Active, uncontrolled infections at the time of treatment.[11]
• Severe pre-existing cardiac, pulmonary, hepatic, or renal impairment.[11]
• Administration of live or live-attenuated vaccines.[11]
• Patients with Fanconi anemia and other inherited DNA breakage repair disorders.[11]

The contraindication in patients with disorders like Fanconi anemia is absolute and mechanistically driven. Treosulfan's therapeutic effect relies on inducing DNA damage that overwhelms the repair capacity of rapidly dividing cells. In patients with a genetically defective DNA repair system, this damage cannot be managed, leading to catastrophic and widespread cellular toxicity that would be fatal. This highlights the critical intersection of the drug's pharmacology with the patient's underlying genetic makeup and underscores the necessity of accurate patient diagnosis and selection.[11]

Key Warnings and Precautions:

• Profound Myelosuppression: This is an expected outcome. The US labeling is expected to include a Boxed Warning regarding the risk of severe bone marrow suppression, which is fatal without subsequent rescue by alloHSCT.[16]
• Neurologic Toxicity: Seizures have been reported, particularly in infants (≤ 4 months of age). Neurological monitoring is required, and prophylactic use of clonazepam may be considered for patients at high risk.[11]
• Dermatologic Toxicity: An increased incidence of rash and dermatitis has been observed, particularly when sodium bicarbonate-containing hydration is used concurrently.[28] This phenomenon provides a direct clinical window into the drug's activation chemistry. The alkaline pH created by bicarbonate excreted in sweat can accelerate the local, non-enzymatic conversion of Treosulfan to its cytotoxic epoxides directly on the skin surface, causing localized chemical irritation and dermatitis. This mechanistic link explains the specific clinical recommendations for managing this toxicity.
• Extravasation: Treosulfan is classified as an irritant. If extravasation occurs during infusion, it can lead to pain, swelling, and local tissue necrosis. Patency of the intravenous line must be confirmed before and monitored during administration.[28]
• Secondary Malignancy: As a genotoxic alkylating agent, Treosulfan is classified by the International Agency for Research on Cancer (IARC) as a Group 1 human carcinogen. Patients should be counseled on the increased long-term risk of developing a secondary malignancy.[11]
• Embryo-Fetal Toxicity and Infertility: Treosulfan is genotoxic and can cause fetal harm. Females of reproductive potential must have a negative pregnancy test before starting therapy and use effective contraception during and for 6 months after treatment. Males with partners of reproductive potential must use effective contraception during and for 3 months after treatment. The drug can cause temporary or permanent infertility in both sexes, and the option of cryopreservation of sperm or oocytes should be discussed with patients prior to therapy.[11]
• Overdosage: There is no specific antidote for Treosulfan overdose. The primary manifestations are prolonged myeloablation, pancytopenia, and severe mucositis. Management consists of vigorous supportive measures and frequent monitoring of blood counts.[1]

5.3 Management of Key Toxicities

• Myelosuppression: Management is supportive and includes frequent blood count monitoring, prophylactic use of anti-infective agents (antibacterial, antifungal, antiviral), and transfusions of red blood cells and platelets as needed.[1]
• Skin Reactions: To mitigate the risk of pH-driven skin toxicity, patients are advised to keep their skin clean and dry, particularly on the days of infusion. Occlusive dressings and diapers should be changed frequently. The use of creams or lotions should be avoided during the treatment period.[32]
• Stomatitis/Mucositis: Proactive management includes diligent oral hygiene, saline rinses, and appropriate pain management with topical and/or systemic analgesics.
• Extravasation: If extravasation is suspected, the infusion must be stopped immediately. The line should be disconnected, and the incident managed according to institutional protocols for irritant extravasation.[28]

Table 4: Common and Very Common Adverse Drug Reactions by Frequency and System Organ Class

System Organ Class	Very Common (≥1/10)	Common (≥1/100 to <1/10)
Infections and infestations	Infections (bacterial, viral, fungal)	Sepsis
Blood and lymphatic system	Myelosuppression, Pancytopenia, Febrile neutropenia	-
Metabolism and nutrition	-	Decreased appetite, Hypomagnesemia
Nervous system	Headache	Dizziness, Vertigo, Paresthesia, Seizure
Cardiac	-	Tachycardia, Arrhythmias
Respiratory, thoracic, mediastinal	-	Cough, Dyspnea, Oropharyngeal pain, Epistaxis
Gastrointestinal	Stomatitis/Mucositis, Nausea, Vomiting, Diarrhea, Abdominal pain	Dysphagia, Gastritis, Constipation, Dry mouth
Hepatobiliary	Hepatotoxicity (Increased ALT, AST, GGT, Bilirubin)	-
Skin and subcutaneous tissue	Pruritus, Alopecia	Rash, Dermatitis, Erythema, Skin hyperpigmentation
Musculoskeletal	Musculoskeletal pain (including back, bone, extremity pain)	Arthralgia
Renal and urinary	-	Acute kidney injury, Hematuria
General disorders	Pyrexia, Edema, Fatigue	Chills, Pain

This table synthesizes data from multiple sources [2] and represents a consolidated view of the most frequent adverse reactions.

Drug-Drug Interactions (DDIs)

The potential for drug-drug interactions with Treosulfan is primarily driven by the inhibitory effects of its active metabolite on key drug-metabolizing enzymes. While some sources provide conflicting or overly simplistic information, sophisticated modeling studies have clarified the most clinically relevant risks.

6.1 Pharmacokinetic Interactions

The understanding of Treosulfan's DDI potential has evolved significantly with the application of modern pharmacological tools. Initial reports suggesting no significant interactions have been superseded by more detailed analyses. A comprehensive physiologically based pharmacokinetic (PBPK) modeling study provides the most reliable and actionable guidance.[34] This computational approach integrates in vitro enzyme data with in vivo clinical data to simulate drug behavior and predict the magnitude of interactions in a clinical setting.

Effect of Treosulfan on Other Drugs (Inhibition):

• The active monoepoxide metabolite of Treosulfan, S,S-EBDM, has been identified as a weak inhibitor of cytochrome P450 enzymes CYP3A4 and CYP2C19 in vitro.[2]
• PBPK modeling quantifies this risk, predicting a medium potential for a clinically significant DDI with CYP3A4 substrates. The model predicted a maximum increase in the area under the curve (AUCR) of 2.23 when co-administered with the probe substrate midazolam.[34]
• The model predicted a low potential for interactions with CYP2C19 substrates, with a maximum AUCR of 1.6 when co-administered with omeprazole.[34]
• Treosulfan is not predicted to cause clinically significant interactions with substrates of the P-glycoprotein (P-gp) transporter, such as digoxin.[34]

Based on these findings, the primary clinical concern is the co-administration of drugs that are metabolized by CYP3A4 and have a narrow therapeutic index. The potential for a greater than two-fold increase in exposure could lead to significant toxicity. Therefore, such combinations should be avoided or, if medically necessary, undertaken with extreme caution and intensified monitoring of the substrate drug's levels and/or toxicity.[2]

Effect of Other Drugs on Treosulfan:

While Treosulfan is a minor substrate of CYP2D6 and its metabolite a substrate of CYP2C8, these pathways are not considered clinically significant in the context of its overall disposition.1 The primary activation pathway is non-enzymatic, making Treosulfan less susceptible to interactions where other drugs would inhibit or induce its metabolism.

6.2 Pharmacodynamic Interactions

Pharmacodynamic interactions are those where drugs have additive or antagonistic effects at the site of action.

• Other Cytotoxic Agents: The combination of Treosulfan with other myelosuppressive and immunosuppressive agents, such as fludarabine and thiotepa, results in intended and additive effects on the hematopoietic and immune systems. This is the basis of its use in conditioning regimens.[21]
• Vaccines: Due to the profound immunosuppression induced by Treosulfan, the administration of live or live-attenuated vaccines is contraindicated. Such vaccines could cause disseminated, life-threatening infections in a severely immunocompromised host. The immune response to inactivated vaccines is also expected to be significantly diminished if given during or shortly after treatment.[11]

Table 5: Clinically Significant Drug-Drug Interactions with Treosulfan

Interacting Agent / Class	Mechanism of Interaction	Potential Clinical Effect	Recommended Management	Source(s)
Narrow Therapeutic Index CYP3A4 Substrates (e.g., certain immunosuppressants, antiarrhythmics)	Inhibition of CYP3A4 metabolism by Treosulfan's active metabolite (S,S-EBDM).	Increased plasma concentration and risk of toxicity of the co-administered drug.	Avoid co-administration during Treosulfan therapy. If unavoidable, monitor substrate drug levels and for signs of toxicity.	2
CYP2C19 Substrates (e.g., omeprazole, certain antidepressants)	Weak inhibition of CYP2C19 metabolism by Treosulfan's active metabolite.	Potential for a modest increase in substrate drug concentration.	Use with caution, especially for drugs with a narrow therapeutic index. Monitor for adverse effects.	4
Live or Live-Attenuated Vaccines (e.g., MMR, varicella, yellow fever)	Pharmacodynamic interaction due to severe immunosuppression.	Risk of disseminated, severe, or fatal vaccine-induced infection.	Contraindicated during and for a period following Treosulfan therapy.	11
Other Myelosuppressive Agents (e.g., fludarabine, cyclophosphamide)	Additive pharmacodynamic effects.	Profound and prolonged myelosuppression.	This is an intended interaction in the context of HSCT conditioning. Monitor blood counts closely.	16

Regulatory History and Status

The regulatory journey of Treosulfan reflects its development as a specialized therapy for rare conditions, culminating in its approval as a key component of HSCT conditioning regimens in major global markets.

7.1 Orphan Drug Designation

Recognizing its application in treating rare diseases, Treosulfan was granted Orphan Drug Designation by multiple regulatory agencies. This status provides incentives to support the development of drugs for underserved patient populations.

• The European Medicines Agency (EMA) designated Treosulfan as an orphan medicinal product on February 23, 2004, for its use in conditioning treatment prior to HSCT.[6]
• The U.S. Food and Drug Administration (FDA) granted orphan status twice: first in 1994 for the treatment of ovarian cancer, and again in 2015 for its use in conditioning treatment for HSCT in both malignant and non-malignant diseases.[5]

7.2 European Medicines Agency (EMA) Approval

The path to approval in the European Union was relatively straightforward. Following a positive opinion from the Committee for Medicinal Products for Human Use (CHMP) on December 13, 2018, the EMA granted a full marketing authorization for Treosulfan in June 2019.[5] It is marketed in the EU under the brand name Trecondi.[5]

7.3 U.S. Food and Drug Administration (FDA) Approval

The regulatory process in the United States was more protracted. After the initial New Drug Application (NDA) was submitted, the FDA issued a Complete Response Letter (CRL) in August 2021, indicating that the submission was not ready for approval and that additional information or analysis was required.[9] The sponsoring company, Medexus Pharmaceuticals, resubmitted the NDA in 2022 after addressing the agency's concerns.[9]

Final marketing approval was granted by the FDA on January 21, 2025.[5] The drug is marketed in the US under the brand name Grafapex.[9] As discussed previously, the successful approval was heavily reliant on the mature overall survival data from the pivotal MC-FludT. 14/L trial, which demonstrated a clear survival benefit over the busulfan-based comparator arm.[17]

7.4 Legal and Prescription Status

Across all major jurisdictions, including the United States, the European Union, the United Kingdom, Canada, and Australia, Treosulfan is classified as a prescription-only medicine (identified by Rx-only or POM).[5] Its use must be supervised by physicians experienced in the administration of conditioning treatments for alloHSCT.[6] It is not classified as a controlled substance.[9]

Expert Synthesis and Concluding Remarks

Treosulfan has successfully carved out a crucial niche in the landscape of myeloablative conditioning for allogeneic hematopoietic stem cell transplantation. It represents a significant and intelligent evolution from its parent compound, busulfan, leveraging a distinct pharmacological profile to deliver tangible clinical advantages in efficacy and manageability. Its journey from a chemical analogue to a standard-of-care option is a compelling example of rational drug design translating into improved patient outcomes.

The clinical value of Treosulfan is built upon a triad of key advantages. First and foremost is its predictable pharmacokinetics. The reliance on a spontaneous, non-enzymatic, pH- and temperature-driven activation mechanism is the drug's defining feature. This elegant chemical property bypasses the highly variable hepatic enzymatic pathways that complicate busulfan therapy, resulting in more uniform drug exposure across diverse patient populations. The most significant practical consequence is the obviation of the need for routine therapeutic drug monitoring, which simplifies the treatment protocol, reduces healthcare resource utilization, and mitigates the risks of over- or under-dosing.

Second, this predictable pharmacology is backed by demonstrated clinical efficacy from high-quality, randomized controlled data. The pivotal MC-FludT. 14/L trial did more than just prove non-inferiority to a busulfan-based regimen; it showed a statistically significant and clinically meaningful 33% reduction in the risk of death in a high-risk population of older or comorbid patients with AML and MDS. This robust overall survival benefit provides a powerful rationale for its use as a preferred agent in this setting.

Third, Treosulfan offers a refined safety profile. While it is undeniably a highly toxic agent with a formidable list of adverse effects inherent to its myeloablative purpose, its toxicity profile differs from that of busulfan. The lower incidence of key organ toxicities, such as hepatic veno-occlusive disease and neurotoxicity, suggests a more favorable therapeutic window. This improved balance between potent anti-leukemic activity and regimen-related toxicity is a critical goal in transplant medicine.

However, a nuanced perspective is essential. The term "favorable safety" is relative and must be interpreted within the high-risk context of alloHSCT. Treosulfan still carries the full burden of risks associated with potent alkylating agents, including severe and prolonged myelosuppression, a high incidence of debilitating mucositis, and the serious long-term risks of infertility and secondary malignancies. Furthermore, important questions remain. The observation of increased secondary graft failure in some pediatric trials for non-malignant diseases highlights that the drug is just one component of a complex regimen. Optimizing the accompanying immunosuppressive therapy to suit different patient populations and underlying disease biologies is a critical area for ongoing research to maximize the benefits of Treosulfan while mitigating specific risks.

The future of Treosulfan will likely involve continued expansion into new indications, such as myelofibrosis and potentially autoimmune disorders treatable with HSCT. Further research will focus on optimizing combination regimens, perhaps pairing Treosulfan with novel targeted agents or immunotherapies. Continued refinement of dosing strategies, particularly in the youngest pediatric patients, through sophisticated population PK/PD modeling will also be vital to ensure that the therapeutic window is achieved for every patient.

In conclusion, Treosulfan, when used in combination with fludarabine, has firmly established its place as a standard-of-care conditioning regimen for alloHSCT in its approved indications. Its unique pharmacology, validated by compelling clinical evidence of a survival advantage, provides transplant physicians with a superior tool for preparing high-risk patients for a potentially curative therapy. It stands as a testament to how a deep understanding of drug chemistry and its translation into a predictable pharmacokinetic profile can lead to profound and meaningful improvements in clinical practice and patient survival.

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