Busulfan

BRAND NAMES

Busulfan was marketed under several brand names, including Busulfex and Myleran. Busulfex was an intravenous formulation, whereas Myleran was available as an oral formulation.

MECHANISM OF ACTION

Busulfan exerted its cytotoxic activity through alkylation of DNA. Its two reactive functional groups allowed it to form covalent bonds with DNA, resulting in intra-strand and inter-strand DNA cross-linking. These structural alterations interfered with DNA replication and transcription and ultimately prevented normal cell division. The resulting DNA damage triggered cellular dysfunction and death, particularly in rapidly proliferating cells. This mechanism accounted for the pronounced myelosuppressive effect of busulfan and its usefulness in conditioning regimens before hematopoietic stem-cell transplantation.

PHARMACOKINETICS OF BUSULFAN

Absorption: Busulfan was absorbed effectively after oral administration, although the extent of absorption differed among patients. Intravenous administration provided more consistent systemic exposure.

Distribution: After absorption, busulfan was distributed widely throughout body tissues and fluids. It showed moderate binding to plasma proteins and was able to cross the blood–brain barrier.

Metabolism: Busulfan was metabolized mainly in the liver through glutathione-dependent pathways. The resulting metabolites were further processed within the body.

Elimination: Busulfan and its metabolites were eliminated mainly through the kidneys and excreted in the urine. Only a small proportion of the drug was eliminated unchanged.

PHARMACODYNAMICS

Busulfan exerted its therapeutic effect by irreversibly alkylating and cross-linking DNA, which disrupted DNA replication and damaged rapidly dividing hematopoietic cells. Its effects depended on systemic drug exposure, while excessive exposure increased the risk of myelosuppression, seizures, and liver toxicity. Therapeutic drug monitoring was used when necessary to maintain appropriate drug exposure and limit toxicity.

Administration

Busulfan was administered as part of a conditioning regimen before hematopoietic stem-cell or progenitor-cell transplantation. The intravenous formulation was appropriately diluted and delivered through a central venous catheter as a controlled intravenous infusion. It was not administered as an intravenous bolus or push injection. In adults, the commonly used intravenous regimen consisted of 0.8 mg/kg administered as a 2-hour infusion every 6 hours for four consecutive days, resulting in a total of 16 doses. Anticonvulsant prophylaxis and antiemetic treatment were generally provided before busulfan administration because the drug was associated with seizures and gastrointestinal adverse effects.

DOSAGE AND STRENGTH

The dosage of busulfan varied according to the formulation, treatment protocol, patient age, body weight, and transplantation regimen. In the standard adult intravenous regimen, a dose of 0.8 mg/kg was administered every 6 hours for four days, providing a total of 16 doses. The dose was calculated using the lower of the patient's ideal body weight or actual body weight, according to the applicable treatment protocol.

The intravenous formulation was supplied as a 60 mg/10 mL single-dose vial, providing a concentration of 6 mg/mL. Because busulfan had a narrow therapeutic range, pharmacokinetic monitoring was used in transplantation protocols to assess systemic exposure and guide dose adjustments when necessary.

DRUG INTERACTIONS

Busulfan interacted with several medications that affected its metabolism and clearance, thereby altering systemic drug exposure:

• Metronidazole: It reduced busulfan clearance and increased systemic exposure, which could have increased the risk of toxicity.

• Itraconazole: It decreased busulfan clearance by approximately 25%, potentially increasing drug exposure.

• Iron-chelating agents: Agents such as deferasirox reduced busulfan clearance. Discontinuation before busulfan administration was recommended when clinically appropriate.

• Acetaminophen (paracetamol): Administration shortly before or during busulfan treatment could have decreased busulfan clearance, possibly because acetaminophen reduced glutathione availability.

• Phenytoin: It increased busulfan clearance by at least 15%, which could have lowered systemic busulfan exposure.

FOOD INTERACTIONS

For intravenous busulfan, no clinically significant food interaction comparable to those associated with some orally administered drugs was established. Food did not directly influence administration of the intravenous formulation because the drug was delivered directly into the bloodstream.

However, oral busulfan had different pharmacokinetic characteristics, and food could have influenced its absorption. Therefore, oral busulfan was administered according to the specific product information and the transplantation protocol.

During intravenous busulfan therapy, clinical management focused primarily on potential drug interactions, adequate hydration, seizure prophylaxis, antiemetic treatment, and pharmacokinetic monitoring rather than specific dietary restrictions.

Busulfan was a potent cytotoxic agent with a narrow therapeutic range; therefore, its dosage, administration, and concomitant medications required careful management by the appropriate oncology or transplantation team.

CONTRAINDICATIONS 

Busulfan was contraindicated in patients who had a known hypersensitivity to busulfan or any of its formulation components. Because the drug produced profound and prolonged bone-marrow suppression, its administration required careful monitoring and appropriate hematopoietic stem-cell support. Patients with a history of seizures, hepatic impairment, or other significant organ dysfunction required particular caution during treatment.

SIDE EFFECTS

Busulfan was associated with several adverse effects, with severe myelosuppression being the most significant and potentially life-threatening reaction. Common adverse effects included nausea, vomiting, stomatitis, anorexia, diarrhea, fever, abdominal pain, headache, insomnia, anxiety, hypokalemia, hypomagnesemia, and hyperglycemia.

OVERDOSE

An overdose of busulfan resulted mainly in severe bone-marrow hypoplasia or aplasia with profound pancytopenia. It could also cause significant toxicity involving the central nervous system, liver, lungs, and gastrointestinal tract. Neurological complications, particularly seizures, could occur because of the drug's neurotoxic effects.

TOXICITY

Busulfan toxicity resulted mainly from its alkylating activity and cytotoxic effects on rapidly dividing cells. The most characteristic toxic effect was severe and prolonged bone-marrow suppression, which could lead to life-threatening infections and bleeding.