Pharmacokinetic-Directed Dose Adjustment Is Essential for Intravenous Busulfan Exposure Optimization: Findings From a Multi-Center Phase II Study of Autologous Hematopoietic Stem Cell Transplantation for Lymphoma in North America

Pharmacokinetics-directed Intravenous Busulfan Combined With High-dose Melphalan and Bortezomib as a Conditioning Regimen for Patients With Multiple Myeloma

BACKGROUND

High-dose chemotherapy followed by autologous stem cell transplantation (ASCT) has a well-established role in the treatment of patients with multiple myeloma. Melphalan 200 mg/m² (Mel200) is the most commonly used preparative regimen. Several studies have provided evidence for potential synergism and safety when combining bortezomib (Btz) or busulfan (Bu) with melphalan (Mel).

PATIENTS AND METHODS

We conducted a prospective phase II study to investigate the safety and efficacy of conditioning with pharmacokinetics (PK)-directed intravenous (IV) Bu with Btz and Mel. Bu dosing was adjusted to target a total area under the curve (AUC) of 20,000 μM × min. Patients received Btz (1 mg/m² × 4 doses) and Mel (140 mg/m²).

RESULTS

A total of 19 subjects were enrolled. Their median age was 55 years, and the median follow-up period was 23.7 months. PK testing resulted in 86% of patients achieving an estimated total AUC of 20,000 ± 2500 μM × min. The overall response rate (ORR) at day +100 after ASCT was 100% in the evaluable patients, with 11% of patients achieving a complete response. The 2-year progression-free survival rate was 57.9% (95% confidence interval [CI], 38%-89%), and the 2-year overall survival rate was 88.5% (95% CI, 76%-100%). The most common grade 3 and 4 toxicities were febrile neutropenia, dysphagia/odynophagia, and oral mucositis. No case of hepatic sinusoidal obstruction syndrome developed. One treatment-related mortality occurred before day +100.

CONCLUSION

A preparative regimen of PK-directed IV Bu with Btz and Mel led to an ORR of 100% with acceptable toxicity and should be considered for direct comparison with the Mel200 regimen in future trials.

Dosing algorithm revisit for busulfan following IV infusion

PURPOSE

Busulfan (Bu) exposure is critical for efficacy and safety. Body weight (BW), or adjusted ideal body weight (AIBW)-based dosing (WBD) algorithm, has been used in hematopoietic stem cell transplantation (HSCT). A recently completed phase 2 study revealed that 33.6 % of the subjects were under-, or over-exposed, with this WBD algorithm. This paper was to investigate Bu dosing algorithm in an attempt to improve the suboptimal Bu exposure.

METHODS

Population PK modeling was conducted using the data from 207 patients. Dosing algorithm was developed based on derived covariate model of CL. Model-based simulation was conducted to assist test PK study design. A simplified CL estimation method was proposed based on the PK structure model for Bu.

RESULTS

A one-compartment structure model adequately described the PK profile of Bu following an IV infusion. BSA best described the inter-individual variability of CL. The proposed dosing algorithm was dose (mg) = (31.7 × BSA - 11.6) × target AUC [µM min]/1,000. With this dosing algorithm, 14.3 % patients could be under- or over-exposed. A test PK study with reduced study duration and three PK samples can provide as nearly as good an estimate of CL compared to 12 PK samples on two different occasions.

CONCLUSION

The proposed dosing algorithm can significantly improve the sub-exposure of Bu. A shortened test PK study duration with reduced PK samples can provide as near as good estimate for Bu CL. A simplified CL estimation method is valid.