Surrogate matrix: opportunities & challenges for tissue sample analysis Review
N-OH-PhIP at one concentration in surrogate and in each tissue. Certain authors considered this method as determination of the assay recovery, that is, quantify- ing spiked samples against the calibration curve in sur- rogate matrix. If the assay recovery is close to 100%, then the surrogate matrix is suitable [10]. This method can prove the validity of surrogate matrix with less authentic tissue than is required for Method 1. When the QC samples are prepared at only one concentra- tion, the assay could run into issues if the extractability of analyte in tissue is concentration dependent. How- ever, it is a very efficient way to establish the validity of one surrogate matrix for multiple tissues. Method 3: determining analyte recovery. The
assumption for this method is that if the analyte recov- ery in surrogate matrix and in tissue homogenate is similar (or independent of matrix), then the surrogate is suitable. Schumidt et al. used this method to evalu- ate the recovery of retinoids in BSA surrogate and in liver at 3–4 concentrations [16]. Because the recovery for spiked liver sample and BSA sample were matrix independent, the matrix-free standard solution is considered appropriate for retinoids determination in tissue. The recovery experiment can be a little more involved because it requires samples to be spiked pre- and postextraction. This method is often chosen dur- ing method development when recovery is determined to evaluate sample extraction. Method 4: evaluating matrix effect. The assumption
for this method is that if the matrix effect of surrogate matrix and authentic tissue is minimal (i.e., indepen- dent of matrix) or if the matrix effect is similar between the surrogate matrix and authentic tissue, then, the sur- rogate is suitable. Noll et al. proved that the rat kidney is a suitable surrogate matrix for the quantitation of tacrolimas concentrations in human transplant kidney biopsy tissue by demonstrating minimal matrix effect of rat kidney surrogate and human kidney biopsy tis- sue [25]. They showed that % bias in peak area ratio (analyte/IS) between the postextraction rat kidney and the spiked methanol-water solution was within 15%. Similar results were found for the % bias between human kidney biopsy and methanol-water solution, indicating minimal matrix effects in both matrices.
Chen et al. demonstrated similar matrix effects between the plasma surrogate matrix and the plasma diluted tis- sue by normalizing the instrument response of plasma diluted tissue to that of plasma [19]. As the percent of tissue in the plasma diluted sample decreases and the normalized response approaches 1 (100%), that is, matrix effect in plasma diluted tissue sample and in plasma surrogate becomes more similar, the plasma is considered to be a suitable surrogate. Jiang et al. also proved that plasma is a suitable surrogate matrix for the plasma diluted tissue by demonstrating the similarity in matrix effects between the two matrices [20]. They quantified the plasma diluted tissues against the cali- bration curve in plasma. Since the % biases of analyzed concentrations were within 15%, the matrix effect for the plasma surrogate and plasma diluted tissue sample is considered to be similar and the plasma is considered to be a suitable surrogate. This method does not require authentic tissue matrix because the matrix effects deter- mination can be done using incurred samples, which make it appealing for establishing a method for biopsy samples using surrogate matrix.
Method development Successful
sample analysis relies on proper method
development. In addition, key parameters including sensitivity, linearity, concentration range, recovery and stability should be checked at the method development stage before embarking on method validation. When there are multiple tissues to be analyzed in a study, such as a tissue distribution study, it is often impossible to perform method development for every matrix in a short time. A common practice is to select one tissue for method development and apply the experimental procedure to all other tissues. One exception is tissue homogenization, for which a procedure must be devel- oped for each tissue because the size and toughness of different tissues vary [26,27]. There is limited information published on method
development. In general, only certain aspects of method development
are discussed by researchers.
Zhang et al. presented their efforts for selecting a suit- able tissue homogenization solvent [22]. Ottria et al. evaluated three different tissue extraction methods and
With matrix Solution without matrix
Table 4. Calibration curve parameters for destosterone- and dihydrotestosterone-derivatives. Testosterone-derivative† Slope 0.0764 0.0753
Intercept 0.00228 0.00622
acetonitrile/water (60/40, v/v). Adapted with permission from [17] © Elsevier (2009).
r
0.9942 0.9924
Intercept 0.000415 0.00643
r †Samples were prepared via extraction followed by derivativation. Matrix was rat prostatic tissue homogenate and solution was
Dihydrotestosterone-derivative† Slope 0.0768 0.0725
0.9954 0.9954
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