In this study, we compared the target attainment (exposure target AUC0–∞ 80–100 mg*h/L) of busulfan between HCT patients with day 1-guided TDM (regimen d1) and days 1 + 2-guided TDM (regimen d1 + 2). The AUC0-∞ was estimated using nonlinear mixed effect modeling with an optimizedmodel that adjusted for (inter-occasion) variability in clearance. The busulfan dose was adjusted accordingly on the remaining days of therapy and additional TDM was performed in the event of large dose adjustments (≥ 25%). There was no significant difference in the busulfan target attainment between both TDM regimens. The target attainment was 84.3% in the regimen d1 group, compared to 90.9% in the regimen d1 + 2 group. Busulfan blood concentrations taken on day 2 result in a significantly smaller variation in the prediction of clearance on day 4 (CLday4) compared to blood concentrations taken on day 1 and may therefore provide a better estimate.
Performing TDM on an additional day (regimen d1 + 2, with subsequent TDM if required) did not significantly increase target attainment, which is not in line with the findings of Marsit et al. and Alsutan et al., who found that additional TDM increased target attainment (5, 6). However, these results can only be compared cautiously because these studies were designed differently. First, these studies used a different busulfan dosing regimen and timing of blood sampling (5, 6). Second, Marsit et al. also included patients receiving an autologous HCT with various conditioning regimens, which contained melphalan and cyclophosphamide. These drugs further potentiate busulfan hepatoxicity, which may hypothetically influence busulfan clearance (11, 12). Third, they used different pharmacometric models to estimate busulfan exposure (5, 6). We used a model that accounted for the intra-individual variability in clearance well, as shown by the high level of busulfan target attainment (85.8%). If the models used in the aforementioned studies only partially accounted for this variability, the estimate based on only day 1 concentrations may be less precise, which would have made repeated TDM necessary. Additionally, our approach allows for subsequent TDM if the patients pharmacokinetics differ from the estimations of the pharmacometric model (e.g. in patients with a large dose adjustment), which may have further improved target attainment.
The variance in the estimated busulfan CLday4 was significantly smaller if the CLday4 was based on day 2 concentrations instead of day 1 concentrations (Fig. 4). Considering that busulfan clearance often decreases on day 2 or 3 of therapy (4–6, 13–15), presumably due to intracellular glutathione depletion (13), this implies that clearance estimates based on day 1 concentrations may not hold true over the entire therapy. In line with this, we observed a decrease in busulfan clearance in 62.4% of patients, with a 5.8% decrease between day 1 and day 4. These findings have important implications for busulfan TDM, because day 2-based estimates may be more accurate than day 1-based estimates in calculating the AUC0–∞, warranting sampling on day 2 instead of day 1. However, caution must be exercised when applying these results to current clinical practice, because in our total patient population, the clearance of busulfan increased and decreased considerably throughout therapy (minimum − 65.1%, maximum 40.1%). This further complicates the estimation of the total exposure, which may necessitate performing TDM several times over the course of therapy.
Interestingly, 37.6% of patients tended to have an increased clearance of busulfan throughout treatment (median increase of 2.6%). The current findings appear to be inconsistent with previous studies, which reported an 8–15% decrease (15–21) or no change in busulfan clearance (22, 23). However, it should be noted that some patients in these studies also exhibited a significant increase in clearance, similar to what has been observed in this study. The reason for this increase in clearance is not clear and may have multiple potential explanations. First, a small number of studies have shown that busulfan can induce its metabolism by increasing glutathione synthesis and/or GST activity (12, 24, 25). Second, interacting medication can induce GST or CYP450 enzymes by which busulfan is metabolized (12, 14). However, this effect can be mitigated to some extent by TDM-guided dose adjustments, but may still be relevant on the final day of busulfan therapy, on which TDM cannot be applied. Unfortunately, we did not collect data on medication co-administered during busulfan therapy.
Finally, several limitations need to be considered. First, various studies have demonstrated the influence of glutathione transferase (GST) genotypes on busulfan clearance, with various genotypes showing a marked reduction in clearance (26–28). Therefore, the observed differences in target attainment between patients may be attributed to variations between GST genotypes, but unfortunately, we do not have data on this. Second, the number of TDM occasions and the number of blood samples are greater in the regimen d1 + 2 group than in the regimen d1 group. Therefore, the estimates of the busulfan AUC0-∞ may be more accurate in the regimen d1 + 2 group than in the regimen d1 group due to having more busulfan concentrations available. However, it is worth noting that the higher number of TDM occasions is an aspect of the intervention in the regimen d1 + 2 group and should not be misconstrued as bias.
In conclusion, the results of this study suggest that TDM on the first day of therapy may be sufficient for attaining the optimal busulfan target in children receiving busulfan as part of the HCT conditioning regimen, provided a valid pharmacometric model is used and ‘as needed’ TDM on subsequent days is performed based on previous pharmacokinetic data. In some patients however, performing TDM on subsequent days may be beneficial, as sampling on day 2 seemed to reduce the variance in the estimated clearance as compared to day 1 sampling.