PURPOSE: The knowledge of the contribution of anatomical and physiological parameters to interindividual pharmacokinetic differences could potentially be used to improve individualized treatment planning for radionuclide therapy. The aim of this study was therefore to identify the physiologically based pharmacokinetic (PBPK) model parameters that determine the interindividual variability of absorbed doses (ADs) to kidneys and tumor lesions in therapy with 177 Lu-labeled PSMA-targeting radioligands.
METHODS: A global sensitivity analysis (GSA) with the extended Fourier Amplitude Sensitivity Test (eFAST) algorithm was performed. The whole-body PBPK model for PSMA-targeting radioligand therapy from our previous studies was used in this study. The model parameters of interest (input of the GSA) were the organ receptor densities [R0 ], the organ blood flows f, and the organ release rates λ. These parameters were systematically sampled NE times according to their distribution in the patient population. The corresponding pharmacokinetics were simulated and the ADs (model output) to kidneys and tumor lesions were collected. The main effect
S
i
and total effect
S
Ti
were calculated using the eFAST algorithm based on the variability of the model output: The main effect
S
i
of input parameter
i
represents the reduction in variance of the output if the "true" value of parameter
i
would be known. The total effect
S
Ti
of an input parameter
i
represents the proportion of variance remaining if the "true" values of all other input parameters except for
i
are known. The numbers of samples NE were increased up to 8193 to check the stability (i.e., convergence) of the calculated main effects
S
i
and total effects
S
Ti
.
RESULTS: From the simulations, the relative interindividual variability of ADs in the kidneys (coefficient of variation CV = 31%) was lower than that of ADs in the tumors (CV up to 59%). Based on the GSA, the most important parameters that determine the ADs to the kidneys were kidneys flow (
S
i
= 0.36,
S
Ti
= 0.43) and kidneys receptor density (
S
i
= 0.25,
S
Ti
= 0.30). Tumor receptor density was identified as the most important parameter determining the ADs to tumors (
S
i
and
S
Ti
up to 0.72).
CONCLUSIONS: The results suggest that an accurate measurement of receptor density and flow before therapy could be a promising approach for developing an individualized treatment with 177 Lu-labeled PSMA-targeting radioligands.
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