Surrogate matrix: opportunities & challenges for tissue sample analysis Review
Table 2. Method qualification parameters for surrogate matrix tissue method. Parameters
Sensitivity
Specificity and selectivity Linearity
Accuracy and precision Recovery
Matrix effects Stability
Tissue homogenization
Sample uniformity (if portion of tissue is analyzed) ✓: Should be evaluated.
of analyte solubility and extractability [2]. The amount of surrogate should be sufficient for method develop- ment, method qualification and sample analysis. The cost of surrogate matrix (total amount needed × unit cost) should not be prohibitive. Water and buffer surrogate is the simplest, cheap-
est and most readily available surrogate. This surro- gate is suitable for the quantification of hydrophilic compounds in tissues [3–10]. Solvents such as metha- nol, acidified methanol and ethanol, and mixture of solvent-water or solvent-buffer have also served as surrogates [11–18] in similar cases. Systemic fluids (plasma and whole blood) are also
somewhat readily available in relatively large volumes and usually do not require euthanization of the source animal. Chen et al. used mouse plasma as surrogate matrix for mouse brain, liver, tumor, heart, lung, muscle and spleen for discovery screening assays [19]. Jiang et al. used dog plasma as surrogate matrix for the quantitation of BMS-650032 in dog liver and brain [20]. The strategy of using plasma as surrogate matrix for the tissue of the same species also proved successful for rat and mon- key tissues. Those investigators successfully quantified other compounds using rat plasma as surrogate matrix for rat liver and brain and monkey plasma for monkey liver and brain. Ji et al. used rabbit whole blood for the quantitation of ABT-578 in rabbit artery tissue [21]. Zhang et al. used swine whole blood for the quantitation of zotarolimus in stented swine arteries [22]. When a tissue surrogate is required, it is possible to
use tissue from one species as the surrogate for anal- ysis of the same tissue from another species. This is advantageous when the tissue is difficult to obtain in suitable quantity from the species under study. For instance, Niklowitz et al. used swine tissue as surrogate for human tissue [23] and Noll et al. used rat kidney as surrogate for human kidney [24,25].
future science group
Surrogate matrix ✓ ✓ ✓ ✓ ✓ ✓
See Table 3 – –
Samples prepared in Tissue ✓ ✓ –
✓ ✓ ✓
See Table 3 ✓ ✓
Finally, in some cases it is necessary to use biologi-
cal matrix artificially prepared or modified in the lab- oratory as a surrogate. McNamara et al. and Teunis- sen et al. used 4% bovine serum albumin (BSA) as surrogate for murine tissues [26,27]. Kim et al. used degraded rat brain supernatant as the surrogate matrix for quantitation of tetrahydrobiopterin (BH4) in tis- sues of seven rat brain regions [6]. Because the endog- enous BH4 is unstable at room temperature, all BH4 is oxidized after the supernatant of brain tissue homog- enate is left at room temperature for more than 24 h. The resulting matrix is BH4 free and is used as the surrogate matrix for the preparation of calibration samples. Korecka et al. used a C18 SPE column to remove endogenous analyte (isoprotanes) from human brain homogenate before using it as surrogate matrix for human brain homogenate [28]. Based on the aforementioned suitable, available and
affordable criteria, we recommend using the simplest suitable matrix as the surrogate matrix. In practice, it could be firstly considered that water or buffer for the hydrophilic compounds and organic solvent for hydro- phobic compounds. The aqueous and organic solvents are readily available in large quantity and their costs are relatively low compared with biologic and artificially prepared biological matrices. If these solvents prove unsuitable or if mixed hydrophilic and hydrophobic analytes are to be analyzed, systemic fluid or tissue surrogate is more appropriate. Plasma and whole blood (systemic fluid) are more desirable because in most case they can be obtained in sufficient quantity without sac- rificing animals. However, a tissue surrogate may be required to more closely mimic the biological composi- tion of the tissue to be analyzed in some cases. The use of artificially prepared or modified biological matrix should be the last resource. This strategy will gener- ally take longer and cost more in production, but more
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