The lower the MAPE value, the better the LSS. A sampling strategy was
considered to display a good predictive performance when the 95%
confidence interval around the MAPE was less than 20% of the reference
Bu AUC0-6h values (33). Bland-Altman plots were applied
to assess the agreement between the predicted and the actual
AUC0-6h, and the difference was expressed as a mean ±
1.96 SD.
2.7 Definition of Clinical Outcomes
The
definition of engraftment was
absolute
neutrophil count (ANC) ≥ 0.5 × 109 per liter for the
first three days in a row and platelet count (PLT)
≥
20 × 109 per liter for the first one week after HSCT
(10, 16). Primary engraftment failure was defined as failing to reach an
ANC of 0.5 × 109 /L within 30 days after HSCT (16). In
brief, the diagnosis of SOS was according to the modified Seattle
criteria (34). The Mount Sinai Acute GVHD International Consortium
(MAGIC) was taken as the diagnostic criterion for aGVHD (35).
2.8 Statistical Analysis
Continuous data, like pharmacokinetics parameters, was compared using a
two-tailed t-test, whereas time-to-event data was compared by the
log-rank test. Univariate analysis was performed with GraphPad Prism
(GraphPad Software, San Diego, CA). In this study, p values
< 0.05 are considered as statistical significance, andp values < 0.1 were deemed as trends.
Results
3.1 Population characteristics
From March 2019 to April 2020, 76 patients underwent allogeneic HSCT and
received IV busulfan. The GSTA1 genotypes for four patients andGSTM1 genotypes for six patients were missing because of samples
being unavailable or detection failure. GSTA1-52C and Tdefined haplotype *A and *B , respectively. In relation toGSTA1-52T/C and -69A/G SNPs, a higher percentage ofCC/GG (78.08%) versus TT/AA (1.37%) and CT/GA(20.55%) was found in our model-building patient cohort. There was only
one patient with GSTA1 *B*B . Thus, 69 patients were included in
our final PPK analysis. Table 1 summarizes characteristics of
patients including demographics, primary diseases, donor types and
conditioning regimens. Supporting Information Table 3 further
elaborates patients’ diagnoses. The
genotyping results are shown inTable 2 .
3.2 PPK model
A total of 398 blood samples were collected for the PPK analysis.Supporting Information Figure 1 plotted concentration–time
profiles of Bu. A one-compartment model with first-order elimination was
suitable to describe the profiles of busulfan pharmacokinetics, and the
exponential model was feasible to estimate the inter-individual
variability. Neither
proportional
error model nor additive error model could perform well, while a
combined proportional and additive residual error model provided an
adequate fit (Δ-2LL > 20, p < 0.001).
In the forward model building step, BSA, AST, types of primary diseases
and GSTA1 genotypes declined the value of -2LL by more than 3.84
(p < 0.05) at each addition. The GSTA1 mutation,
BSA and the level of AST significantly affected the clearance (CL) of
busulfan. Meanwhile, the volume (V) of busulfan was influenced by BSA
and types of primary diseases, while the GSTM1 genotypes had no
significant effect on PK parameters.
During backward elimination, the value of -2LL increased significantly
(Δ-2LL >6.63, p < 0.01) when respectively
removing the GSTA1 mutation, BSA and the level of AST out of the
model. Thus, the three covariates mentioned above could remain in the
final model. Types of primary diseases were eliminated in the final
model because it failed to significantly increase the -2LL value (Δ-2LL
= 6.333, p > 0.01). Table 3 presented the
detailed PK parameters of the base and final model. The CL and V of
busulfan were affected by the GSTA1 genotypes, BSA and AST as
illustrated by the following Equation 5 and 6 :