HomeSolid Phase Microextraction (SPME)Speed, Simplicity and Accuracy of Protein Binding Determination using BioSPME

Speed, Simplicity and Accuracy of Protein Binding Determination using BioSPME

M. James Ross, Senior R&D Scientist1, Olga Shimelis, R&D Manager1

1 MilliporeSigma


An important aspect of drug discovery is understanding the interaction of the drug with plasma proteins and lipids. The portion bound to proteins and lipids is referred to as the plasma protein binding (PPB or Fb). The type of drug may also provide insight on the number of interactions with protein. In general, organic acids have a single binding site with albumin, whereas, organic bases will have multiple binding sites associated with glycoproteins.1 In addition to albumin, other commonly associated proteins in drug binding are alpha-1-acid glycoprotein (AAG) and lipoproteins such as very high-density lipoprotein (VHDL) and low-density lipoprotein (LDL).2 When it comes to the pharmacological effectiveness of a drug, it is the free fraction or unbound fraction (Fu) of a drug that is generally responsible for drug activity as described by the free drug hypothesis.3,4,5

Protein binding properties of drugs, therefore, are important in understanding the amount of drug available in blood. Historical methods to measure protein binding include equilibration of the spiked plasma with the drug-free buffer across the membrane preventing the transfer of larger protein molecules into the buffer. This is an equilibrium dialysis method. Additional improvements in equilibrium dialysis products advertise reducing the equilibration time down from the traditional 24 hours to 4 hours using specifically designed devices.

In this work, Supel™ BioSPME C18 high-throughput devices were used to measure the protein binding and found to reduce the sample preparation time further to less than one hour for the extraction step. BioSPME devices are compatible with robotic liquid handling systems ensuring that the extraction can be fully automated for truly high-throughput methodology.6 The BioSPME, bioanalytical solid phase microextraction, method was applied to the selected compound, carbamazepine, using a high-throughput method for 88 plasma samples and 8 buffer samples. This method was evaluated across multiple lots of Supel™ BioSPME C18 96-pin tools and for multiple compounds and showed good accuracy and precision.


Protein Binding Determination by BioSPME

Human plasma and buffer were spiked at a therapeutically relevant concentration and incubated for one hour at 37 oC while shaking at 300 rpm. After the incubation, 200 µL plasma and buffer were loaded into separate columns onto the extraction well plate (n = 8 to 88). The determination of the protein binding was performed using an automated robotic method and the BioSPME C18 96-pin tool.

Briefly described in Figure 1, the pin tool is conditioned statically for twenty minutes in isopropanol, then transferred into a new well plate for 10 seconds in water (wash step), followed by the extraction step. The pin tool is transferred into the preloaded extraction plate described earlier. Here, the pin tool extracts the analytes while shaking at 1200-1250 rpm at 37 oC for 15 minutes. The pin tool is returned to the water solution for a 60 seconds wash and finally transferred into a desorption plate. The desorption solution is 80:20 methanol:water and desorption is carried out for 20 minutes under static conditions. Samples were analyzed using methods described in Tables 1 and 2.

Solution: isopropanol Time: 20 min Condition: Static
Solution: water Time: 10 sec Condition: Static
Solution: plasma Time: 15 min Condition: agitation
Solution: water Time: 60 sec Condition: static
Solution: 80% methanol Time: 20 min Condition: static
Using LC-MS/MS

Figure 1. Schematic diagram for BioSPME analysis of free fraction of drug in human plasma.

Table 1.LC-MS/MS Conditions for monitoring analytes for free fraction determination
Table 2.Analyte description and LC-MS/MS parameters

Determination of %Free Fraction (FU) of Drug by BioSPME

The BioSPME method determines the free concentration of analyte in plasma by comparing it with the extraction of the analyte from buffer samples where 100% of the analyte is considered to be free of protein binding.

The percent free or percent unbound is determined in Equation 1:

Equation 1:  BioSPME equation 1


where concentration free represents the unbound concentration of the analyte in the matrix, in this case plasma, and concentration total represents the total concentration of analyte. The amount extracted is independent of units and can be applied using preferred quantities (e.g. nanograms or moles) Mfree, and extraction volume of plasma, Vplasma. The concentration of analyte in the desorption solution is quantified by an external calibration curve, and if the desorption volume is equal to the plasma and buffer extraction volumes, the concentration from desorption will be equal to the extracted concentration as shown in Equation 2:

Equation 2:  BioSPME equation 2
Equation 3:  BioSPME equation 3

The bound fraction, FB, can be determined from the extracted concentrations as shown in Equation 6.

Equation 4: BioSPME equation 4
Equation 5:  BioSPME equation 5
Equation 6:  BioSPME equation 6
Equation 7:  BioSPME equation 7
Extractive preconcentration and desorption of free fraction analyte during BioSPME microsampling

Figure 2.Representation of the extraction step (left) removing free analytes (yellow) from plasma (pink) and buffer (blue) and the analytes releasing into the desorption solution (right). The amount extracted does not greatly impact the concentration of free analyte which is termed non-depletive and does not disturb the equilibrium of the portion bound to plasma proteins (purple) (Equation 6 and Equation 7). As the buffer solution is considered 100% protein free, BioSPME will extract more from buffer than from the plasma.

In cases where depletion of compounds from plasma was determined by BioSPME extraction (extraction exceeded 5% of total spiked analyte), a correction to the calculated Bound Fraction was required as described below:

Equation 8:  BioSPME equation 8

where B and P, represent the respective amounts extracted from buffer, B, and plasma, P. B0 and P0 represents the concentration the samples were spiked or the total concentration in the buffer or biological sample. Equation 8 accounts for the concentration in solution after extraction on the tip; the depletion of the analyte from sample.7 Equation 6 and Equation 7, do not take this factor into consideration. However, they provide accurate values when the extracted amount is less than 5% the total amount.

Plasma Protein Binding Determination by Rapid Equilibrium Dialysis

Equilbrum dialysis was performed as directed by the accompying instruction sheet. Briefly, 200 µL of “spiked” human plasma at a therapeutically relevant concentration and 400 µL of phosphate buffered saline (PBS) were loaded in the corresponding chambers in at least triplicates. The dialysis proceeded for at least 4 hours while covered and shaking at 300 rpm and 37 oC on an Eppendorf shaker. At the end of dialysis, 50 µL of the spiked plasma was mixed with 50 µL of clean (unspiked) PBS and 50 µL of the dialysate (buffer compartment) was mixed with 50 µL of clean plasma. This was achieved to ensure matrix consistency. Next, 300 µL of ice-cold acetonitrile was added to each sample before centrifugation at 5,000 rpm for 10 minutes at 4 oC. Finally, the supernatant was transferred into glass vials for analysis by LC-MS/MS as described in Tables 1 and 2 using an AB Sciex 6500 with Agilent 1290 LC using a matrix-matched external calibration in the desorption solution.

Results and Discussion

Accuracy of BioSPME Method for Protein Binding for Single Compound across Multiple Lots of Pin Tools

To determine protein binding of carbamazepine, spiked buffer and plasma samples were deposited into 8 wells each of 3 lots of pins tools. In addition, six pin tools across three batches from each lot were tested. As can be seen in Figure 3, the same protein binding value was obtained for all lots with 1-2% CV. The average protein binding value across all 54 plates was 76.8 ± 2.2% which corresponded well to 70-80% range of protein binding found in literature for carbamazepine.

Bar graph depicting protein binding values for carbamazepine measured using various lots of BioSPME pin tools

Figure 3.Protein binding values for carbamazepine obtained using 3 different lots of BioSPME pin tools. Each batch represents six pin tools. In green, represents the average across the 18 pin tools for each lot tested. Not shown is the overall average for the 54 pin tools (76.8 ± 2.2%). The yellow lines represent the boundaries of the literature range for protein binding (70 – 80%).

Precision across a Well Plate

In addition to showing consistency across multiple pin tools, an entire plate was tested in what is termed “8 + 88”. The 8 represents a column for buffer extraction, and the 88 represents the remaining 11 columns for plasma extraction. The results across the plate can be seen in Figure 4 (analyte extraction). For the extraction of the analyte, the average extracted amount across 88 plasma samples was 7.31 ± 0.4 ng/mL (with CV 4.8%). The average percent binding for the 88 samples was 75.9 ± 1.3%. As can be seen from Figure 5 there was no bias across rows or columns of the 96-well plate.

Figure showing results of plasma extraction concentration on 96-well plate across “8 + 88” dimension

Figure 4.Concentration of analyte (ng/mL) extracted from plasma samples at each well position.

Percentage protein binding across rows and columns of “8 + 88” well plate

Figure 5.Percent bound with the standard deviation for each row and column. The yellow lines represent the boundaries of the literature range for protein binding (70 –80%).


Using the equations, Equation 5 and Equation 6, the values in Table 3 for analyte-protein bindings were determined from BioSPME extractions. These values are in good agreement with values determined using rapid equilibrium dialysis devices and the reported literature values as shown in Figure 6. 

Table 3. Binding values for the seven compounds from plasma using BioSPME and 200 µL sample volumes (n=8)
Bar chart shows comparison of protein binding values for  multiple drug analytes

Figure 6.Comparison of protein binding values between equilbirum dialysis (yellow) and BioSPME (green) methods. The blue lines indicate the protein binding literature values interval. Compounds with stars are charged at physiological pH. Note: zolpidem has a single value reported in literature and not a range.

The BioSPME technique also provides a timesaving of over 50% as shown in Table 4. The longest step in the BioSPME process is the initial incubation of the analyte with the plasma (60 minutes). This is considerably shorter than the minimum four-hour incubation time required by equilbirum dialysis devices. With this BioSPME method, the total number of samples that can be processed is almost quadrupled compared to the rapid equilibrium dialysis method.

Table 4.Comparison of methods, number of samples and time commitment (* 88 corresponds to using the remaining 8 pins for buffer extraction).


Merits of BioSPME Method Over Rapid Equilibrium Dialysis

The Supel™ BioSPME C18 technique offered 50% timesaving and increased the number of samples for protein binding determination when compared with a commercially available equilibrium dialysis product. BioSPME allows the method to be fully automated with a standard robotic system. The protein binding values obtained compare well with those from the rapid equilibrium dialysis method as demonstrated with 7 compounds with log P values in the range 1.6 to 5.0. The precision of extraction using the pin tool was excellent across 88 samples with a CV on extracted concentration from plasma of 4.8%.

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