Toxicity analysis of busulfan pharmacokinetic therapeutic dose monitoring

Busulfan-based conditioning regimens are associated with serious toxicities and literature reports increased risk of toxicities when daily area under the curve concentrations exceed 6000 µM-minute. We implemented real time pharmacokinetic-guided therapeutic drug monitoring of busulfan for myeloablative conditioning regimens. The objective was to compare toxicity of intravenous busulfan before and after therapeutic drug monitoring implementation. The primary endpoint was incidence of hepatotoxicity. Medical records were retrospectively reviewed with weight-based dose Busulfan/Cyclophosphamide (BuCy) conditioning from August 2017 through March 2018 (N = 14) and therapeutic drug monitoring from April 2018 through December 2018 (N = 22). Recipients of busulfan therapeutic drug monitoring were younger than those receiving weight-based dose (median: 45 vs. 58 years, p = 0.008). No other baseline differences were observed. There was no difference in hepatotoxicity between therapeutic drug monitoring and weight-based dose (median 1 vs. 0 days, p = 0.40). In the therapeutic drug monitoring group, 45% of patients had increases and 41% had decreases in busulfan dose after Bu1. Repeat pharmacokinetic after Bu2 were required in 32% of patients. A pharmacokinetic dose monitoring program for myeloablative conditioning intravenous busulfan regimens may be considered a safe practice in stem cell transplant recipients. The majority of patients receiving pharmacokinetic-guided therapeutic drug monitoring required dose changes and therapeutic drug monitoring patients had no significant difference in toxicity compared to those receiving weight-based dose.