12.1 Mechanism of Action
Busulfan is a bifunctional alkylating agent in which two labile methanesulfonate groups are attached to opposite ends of a four-carbon alkyl chain. In aqueous media, busulfan hydrolyzes to release the methanesulfonate groups. This produces reactive carbonium ions that can alkylate DNA. DNA damage is thought to be responsible for much of the cytotoxicity of busulfan.
12.3 Pharmacokinetics
The pharmacokinetics of Busulfan Injection were studied in 59 patients participating in a prospective trial of a Busulfan Injection-cyclophosphamide preparatory regimen prior to allogeneic hematopoietic progenitor stem cell transplantation. Patients received 0.8 mg/kg Busulfan Injection every six hours, for a total of 16 doses over four days. Fifty-five of fifty-nine patients (93%) administered Busulfan Injection maintained AUC values below the target value (less than 1500 μM∙min).
Table 3: Steady State Pharmacokinetic Parameters Following Busulfan Injection Infusion (0.8 mg per kg; N=59) | Mean | CV (%) | Range |
|---|
|
| Cmax (ng per mL) | 1222 | 18 | 496 to 1684 |
| AUC (μM∙min) | 1167 | 20 | 556 to 1673 |
| CL (mL per min per kg) * | 2.52 | 25 | 1.49 to 4.31 |
Busulfan Injection pharmacokinetics showed consistency between dose 9 and dose 13 as demonstrated by reproducibility of steady state Cmax and a low coefficient of variation for this parameter.
Distribution: Busulfan achieves concentrations in the cerebrospinal fluid approximately equal to those in plasma. Busulfan primarily binds to albumin (Mean ± standard deviation=32.4 ± 2.2%).
Metabolism: Busulfan is predominantly metabolized by conjugation with glutathione, both spontaneously and by glutathione S-transferase (GST) catalysis. This conjugate undergoes extensive oxidative metabolism in the liver.
Excretion: Following administration of 14C-labeled busulfan to humans, approximately 30% of the radioactivity was excreted into the urine over 48 hours; negligible amounts were recovered in feces.
Specific Populations
Pediatric Patients: In a pharmacokinetic study of Busulfan Injection in 24 pediatric patients, the population pharmacokinetic (PPK) estimates of Busulfan Injection for clearance (CL) and volume of distribution (V) were determined. For actual body weight, PPK estimates of CL and V were 4.04 L/hr per 20 kg (3.37 mL per min per kg; interpatient variability 23%); and 12.8 L per 20 kg (0.64 L per kg; interpatient variability 11%).