Validated Immunochemical Assay for Comprehensive Determination of the Human Epidermal Growth Factor Receptor 2 Released from and Bound to Cells

We present a validated sandwich ELISA assay using novel anti-HER2 monoclonal antibodies. This assay enables precise quantification of cell-bound and released HER2 protein from in vitro cultured cells and other samples, including blood and tissues.

Human epidermal growth factor receptor 2 (HER2) is a well-established cancer marker. It became a very successful diagnostic and therapeutic target, especially in breast cancer and other HER2-expressing cancer types. In the clinic, the gold-standard immunohistochemical diagnostic methods employing the specific anti-HER2 antibodies are used to measure the expression level of the membrane-bound receptor. The soluble extracellular domain (ECD) of HER2 that is released from the overexpressing cells circulates in the blood and can reflect the tissue expression of the receptor. There is a need for accurate and validated assays to correlate the concentration of the circulating HER2 protein with disease clinical manifestations.

Our team has developed and validated the novel sandwich enzyme-linked immunosorbent assay (ELISA) for quantification of the membrane-bound and the released from cells ECD domain of HER2. The assay uses two unique monoclonal antibodies specific to HER2 developed previously. The quantitation range includes HER2 concentration from 1.56-100 ng/mL, which is expected for cancer cells cultured in vitro and shows sensitivity at the level of 0.5 ng/mL. The satisfactory intra- and inter-assay precision and accuracy of the method make it applicable for HER2 quantification in various types of biological samples, including cell culture medium, serum, and solid tumor tissue. Here, we focus on the comprehensive determination of the receptor-associated and secreted by the in vitro cultured cancer cells. The paper presents a step-by-step protocol for the quantification of HER2 protein that can be employed for testing a variety of cell lines, blood, and tissues.

The success of modern therapeutics often relates to precision medicine that is based on accurate identification of therapy-sensitive patients¹. Among these therapies are the anti-HER2 drugs targeting the receptor overexpressed on a variety of tumors, including breast, endometrium, stomach, lung, and others. Several HER2-targeting agents are available with confirmed benefits in patients with HER2-positive cancers, including HER2-low type². Confirmation of the HER2-positive status is critical for the identification of the potential responding patients; however, it remains a challenge, especially in the HER2-low group.

The gold standard methods in clinical settings, routinely used for HER2 testing, include immunohistochemistry (IHC) protein expression and HER2 gene amplification by fluorescence in situ hybridization (FISH) approaches. Additionally, the Oncotype DX assay is used for HER2 mRNA expression. Tissue biopsy required for these methods makes the determination of patient eligibility for appropriate treatment and their potential responsiveness to therapies uncertain. Despite the updated 2018 guidelines by the American Society of Clinical Oncology (ASCO) and the College of American Pathologists (CAP) to reduce the variability between the testing units, HER2 concordance remains a subject for improvement³.

HER2 is a proto-oncogene member of the epidermal growth factor receptor (EGFR) family that overexpression and activation in the pathological states result in an aggressive outcome or contributes to a poor prognosis⁴. HER2 is a 185 kDa modular protein anchored in the cell membrane that contains a cytoplasmic tyrosine kinase and an extracellular domain (ECD).The HER2 ECD can be shed from cells to be released into the extracellular matrix⁵ as the cell-free protein, further circulating in the blood. Increased HER2 expression might be reflected by the higher level of the circulating ECD, presenting a valuable predictive and prognostic marker^6,7, a surrogate marker of the treatment response⁸, or as a complementary method to IHC to identify patients eligible for anti-HER2 treatment⁹. However, the challenge remains in establishing the correlation between the tumor HER2 expression and the systemic level of the receptor in blood that could have a clinical meaning.

The released HER2 ECD can be quantified using Enzyme-linked immunosorbent assay (ELISA)¹⁰. The sandwich ELISA is an approach that employs two specific antibodies binding different epitopes on the same target antigen. It enables accurate measurement of solubilized proteins in the easily accessible blood- and liquid-based biological material (so-called liquid biopsy). Despite the presence of Food and Drug Administration (FDA)-approved assays, there is controversy over the utility of the HER2 ECD diagnostics⁵ and the cut-off value for an increased HER2 level in blood. More research with the validated methods and uniformly accepted thresholds is needed to confirm the applicability of the assays¹¹.

The critical components of any ELISA are capturing (immobilized on the plate and defining the assay specificity and sensitivity) and detecting antibodies (added after the samples have been applied) antibody (Figure 1). In this report, we present the ELISA protocol based on the recently developed new anti-HER2 ECD monoclonal antibodies (mAb) that were generated, purified on an affinity column, and thoroughly characterized, and present unique sequences¹². The developed ELISA that employs these custom antibodies shows its utility for accurate quantification of the HER2 protein associated with the cell membrane and released into a culture medium for comprehensive assessment of the receptor status. The assay can be utilized in preclinical testing and to support ongoing research. The performance of the assay has been further tested on biological samples of different origins, including serum and tissue homogenates¹², to show potential in the development of research, diagnostics, and novel anti-HER2 treatments in the future.

1. Culturing of human cancer cells

1. Culture MDA-MB-231, SK-BR-3, and SK-OV-3 lines in Dulbecco's Modified Eagle's medium containing 4.5 g/L of ᴅ-glucose (DMEM-HG), supplemented with 2 mM ʟ-glutamine and 10% (v/v) heat-inactivated FBS. Incubate cultures at 37 °C in a humidified atmosphere of 5% CO₂.
2. When the culture reaches ~80% confluence, detach cells by trypsinization and collect them into separate 15 mL conical tubes.
3. Count the cells with an automatic cell counter and seed cells for the experiment into 12-well plates at a density of 3 × 10⁵ cells/per well in 1 mL of growth medium. Incubate cultures at 37 °C in a humidified atmosphere of 5% CO₂.
   NOTE: Seed one well per each time point of the experiment. Both post-culture medium and cell lysates will be used in further analysis.

2. Sample collection and preparation

1. From the 24 h culture, collect culture medium from one well of each cell line into a 1.5 mL tube.
2. Centrifuge the collected samples at 10 000 × g for 10 min at 4 °C. Transfer the supernatant to a new tube and store at -20 °C to further determine the secreted ECD HER2 level.
3. Wash the remaining cells on the plate with 1 mL of cold PBS, aspirate gently, and discard the PBS.
4. Add 200 µL of RIPA buffer (supplemented with 1% of protease inhibitor) into each well and incubate for 5 min. Scrape the cells with the cell scraper and mix by pipetting several times to homogenate the lysate.
5. Collect the lysed cells into a new tube and slowly aspirate into 2 mL syringe with a needle (G21 / 0.8 mm × 40 mm). Repeat the aspiration step 5-10 times to further disintegrate cells.
6. Centrifuge the collected samples at 10 000 x g for 10 min at 4 °C. Transfer the supernatant to a new tube and store at -20 °C for further determination of cell-bound fraction of HER2 protein.
7. Repeat collection of culture medium and cellular materials, following the preparation steps for the remaining time points - 48 h and 72 h of culture.

3. Performing a sandwich ELISA for HER2 determination

1. Prepare the buffers.
   1. Coating buffer: Prepare 0.1 M NaHCO₃ solution by dissolving 0.42 g of NaHCO₃ in 50 mL of distilled water and mix thoroughly. Adjust pH to 9.6 with 3 M NaOH.
   2. Blocking buffer: Prepare 5% non-fat dairy milk (NFDM) in PBS by adding 0.5 g of blotting milk powder to 10 mL of PBS and mix thoroughly.
   3. Washing buffer: Prepare phosphate-buffered saline with Tween 20 (PBST) by adding 0.5 mL of 0.05% Tween-20 to 1000 mL of PBS and mix (avoid excessive foaming).
2. Biotinylate the detecting antibody.
   1. Biotinylate 50-200 µg of detecting anti-HER2 antibody (clone 70.21.73.67) using a commercial biotin labeling kit according to the manufacturer's instructions.
      NOTE: Reliability and repeatability of the biotinylation process are based on the measurement of the final protein concentration of the resultant antibody and the batch-to-batch comparison of the product.
3. Coat the plate.
   1. Dilute the anti-HER2 capturing antibody (clone 70.27.58) to the concentration of 1 µg/mL in the coating buffer. To coat one plate, use 6.5 mL of the capturing antibody solution.
   2. Using a multi-channel pipette, add 100 µL of the capturing antibody solution (prepared in step 3.3.1) to each well on a 96-well ELISA plate.
      NOTE: Avoid the outermost wells of the plate (e.g., A1, H12) to reduce variability caused by the edge effect. The outermost wells should be filled with distilled water to ensure stable reaction conditions.
   3. Cover the plate with a sealing film.
   4. Incubate plate overnight (O/N) at 4 °C.
   5. After O/N incubation, place the plate at room temperature (RT) on a horizontal microplate shaker (20-30 rpm with a tilt angle of 4°-6°) for 1 h.
   6. Using a microplate washer, aspirate the coating solution and wash the plate three times with a washing buffer (3 x 300 µL of PBST per well).
4. Block the plate .
   1. Using a multi-channel pipette, add 100 µL of blocking buffer (5% NFDM in PBS) to each coated well to block nonspecific binding sites.
   2. Cover the plate with a sealing film.
   3. Incubate the plate for 1 h at 37 °C in a humidity chamber.
   4. Using a microplate washer, aspirate the coating solution and wash the plate three times with a washing buffer (3 x 300 µL of PBST per well).
5. Prepare a calibration curve and positive control (Figure 2).
   1. Prepare the working standard solution of HER2 antigen by adding 2 µL of 1 mg/mL stock solution to a 1998 µL of PBS to obtain 0.001 mg/mL (1000 ng/mL) HER2 concentration.
   2. Dilute the working standard solution (WSS) by adding 200 µL of the solution to 1800 µL of PBS to obtain the standard solution 1 (STD 1).
   3. Prepare subsequent six standards (STD 2-STD 7) by serial dilutions of STD 1 in PBS (Figure 2).
      1. STD 2: Add 500 µL of STD 1 to 500 µL of PBS and mix thoroughly to prepare 50 ng/mL.
      2. STD 3: Add 500 µL of STD 2 to 500 µL of PBS and mix thoroughly to prepare 25 ng/mL.
      3. STD 4: Add 500 µL of STD 3 to 500 µL of PBS and mix thoroughly to prepare 12.5 ng/mL.
      4. STD 5: Add 500 µL of STD 4 to 500 µL of PBS and mix thoroughly to prepare 6.25 ng/mL.
      5. STD 6: Add 500 µL of STD 5 to 500 µL of PBS and mix thoroughly to prepare 3.125 ng/mL.
      6. STD 7: Add 500 µL of STD 6 to 500 µL of PBS and mix thoroughly to prepare 1.5625 ng/mL.
      7. STD 8: Add 500 µL of PBS to prepare 0 ng/mL.
   4. Positive control: Dilute STD 1 by adding 100 µL of the STD 1 solution to 900 µL of PBS to obtain a solution of HER2 at 10 ng/mL protein concentration.
6. Prepare samples for spike and recovery experiments.
   1. Prepare the matrix samples (PBS, cell lysates, and culture medium) spiked with the known HER2 protein concentration. Dilute cell lysates 1:2000 by adding 2.5 µL of appropriate cell lysate to 5 mL of PBS to acquire a sample matrix with HER2 level below LOD of the method.
   2. Use WSS (1000 ng/mL) and STD1 (100 ng/mL) to spike matrix samples.
   3. For each matrix, prepare 6 samples with concentrations of 0, 2, 5, 10, 30, and 50 ng/mL.
   4. To prepare 0 ng/mL, add 400 µL of PBS/culture medium/cell lysate.
   5. To prepare 2 ng/mL, add 8 µL of STD1 to 392 µL of PBS/culture medium/cell lysate and mix thoroughly.
   6. To prepare 5 ng/mL, add 20 µL of STD1 to 380 µL of PBS/culture medium/cell lysate and mix thoroughly.
   7. To prepare 10 ng/mL, add 4 µL of WSS to 396 µL of PBS/culture medium/cell lysate and mix thoroughly.
   8. To prepare 30 ng/mL, add 12 µL of WSS to 388 µL of PBS/culture medium/cell lysate and mix thoroughly.
   9. To prepare 50 ng/mL, add 20 µL of WSS to 380 µL of PBS/culture medium/cell lysate and mix thoroughly.
7. Add the sample, blank, and standard.
   1. Using a single-channel pipette, add 100 µL of standards, blank (PBS), and tested samples (cell lysates and culture medium) to appropriate wells.
      NOTE: It is advised to run each sample in triplicate.
   2. Cover the plate with a sealing film.
   3. Incubate the plate for 1 h at 37 °C in a humidity chamber.
   4. Using a microplate washer, aspirate the coating solution and wash the plate three times with a washing buffer (3 x 300 µL of PBST per well).
8. Detect antibody binding.
   1. Dilute the biotinylated detecting antibody (clone 70.21.73.67) in PBS to a working concentration of 1 µg/mL. For one plate, use 6.5 mL of detecting antibody working solution.
   2. Using a multi-channel pipette, add 100 µL of the detecting antibody working solution to each well.
   3. Cover the plate with a sealing film.
   4. Incubate the plate for 1 h at 37 °C in a humidity chamber.
   5. Using a microplate washer, aspirate the coating solution and wash the plate three times with a washing buffer (3 x 300 µL PBST per well).
9. Add Avidin-HRP conjugate.
   1. Dilute the commercially available Avidin-HRP conjugate 1:40,000 by adding 1 µL of enzymatic conjugate to 40 mL of PBST.
   2. Using a multi-channel pipette, add 100 µL of the diluted conjugate to each well.
   3. Cover the plate with a sealing film.
   4. Incubate the plate for 1 h at 37 °C in a humidity chamber.
   5. Using a microplate washer, aspirate the coating solution and wash the plate three times with a washing buffer (3 x 300 µL of PBST per well).
10. Colorimetric reaction
    1. Using a multi-channel pipette, add 100 µL of 3,3′,5,5′-Tetramethylbenzidine (TMB) substrate to each well.
    2. Cover the plate with a sealing film.
    3. Incubate the plate for 1-5 min at 37 °C away from light and monitor the color development.
    4. When the color reaches the expected level, add 100 µL of stop solution to terminate the reaction.
       NOTE: The expected level corresponds to an intense blue color in wells containing samples with a high concentration of the analyte.
11. Acquire and analyze data.
    1. Measure the absorbance at 450 nm using a microplate reader.
    2. Using a 4-parametric calibration curve, calculate the sample concentration based on the curve equation.

Figure 1: Schematic diagram of developed anti-HER2 sandwich ELISA workflow. Overview of the key steps in the sandwich ELISA procedure. These include the critical steps (highlighted with a red frame) like plate coating with the capturing anti-HER2 antibody 70.27.58, the addition of samples (curve standards, blank, experimental samples of the cell lysates or culture medium), and binding of the detecting anti-HER2 antibody 70.21.73.67. The assay concludes with signal detection and data analysis, where the colorimetric signal is quantified using a microplate reader to determine the concentration of the target antigen. Please click here to view a larger version of this figure.

Figure 2: Scheme presenting preparation of the calibration curve standard solutions. Serial dilutions of the HER2 recombinant protein are prepared to generate a calibration curve in a concentration range of 1.56-100 ng/mL (vials STD 1-STD 7). In addition, the negative control sample without the HER2 protein is included (STD 8). Please click here to view a larger version of this figure.

Sandwich ELISA validation
The newly developed assay requires a validation procedure. The important validation parameters include linearity, precision, and detection limits, i.e., lower limit of detection (LLOD) and upper limit of detection. In the previous paper, we have performed thorough method validation. ELISA linearity was tested by using the mocked samples for low (2, 5, 10 ng/mL), medium-high (30 ng/mL), and significantly increased (50 ng/mL) concentrations of the antigen diluted in PBS and other matrixes important for quantification of HER2, like serum. Figure 3 shows the typical calibration curve. As was determined, the working range of the developed sandwich ELISA was 1.56-100.0 ng/mL of HER2 protein¹².

Figure 3: Representative standard curve and calibration data for sandwich ELISA. HER2 binding kinetics in the standard curve concentration range of 1.56-100 ng/mL is presented. Results are expressed as absorbance at 450 nm after background subtraction. The figure includes a table summarizing the parameters of the four-parameter logistic fit, including the cure equation, R² value, and key calibration points used to derive the standard curve. Please click here to view a larger version of this figure.

The coefficient of variation for intra-assay precision returned values lower than 10%, whereas for inter-assay it was below 25%, calculated for at least 12 assays. LLOD was confirmed to be 0.5 ng/mL, independently of the tested matrix (PBS, serum, culture medium)¹².

Next, the accuracy of the method, as well as the matrix effect, were assessed in spike and recovery experiments. Different matrices were tested, including PBS (as reference), culture medium, and cell lysates spiked with the known amount of HER2 protein. Due to the high analyte content in the cell lysates, they were pre-diluted before spiking to acquire a sample matrix with HER2 level below the LOD of the method. The results were in line with the accepted recovery range of 80%-120% in most cases, with considerable deviations for low-concentrated samples and SK-OV-3 cell lysate (Table 1).

Table 1: The spike and recovery assay for different biological matrices. The Spike level represents the recombinant HER2 concentration in matrices derived from MDA-MB-231 and SK-OV-3 cells and the corresponding culture medium. Recovery is calculated based on the experimental average and HER2 level in a standard curve diluent (PBS). The dilution of a particular matrix is provided in brackets.

HER2 determination in cell lysates and cell culture media
The developed ELISA has been employed for quantification of HER2 protein in cells. The medium from cultured MDA-MB-231, SK-BR-3, and SK-OV-3 cells and the corresponding cell lysates were tested at three different time points during the culture experiment, i.e., 24, 48, and 72 h after seeding. The dilutions of up to 1:500 were made for cell lysate samples for results to fit the working range of the assay. The obtained results are summarized in Figure 4. A time-dependent increase in HER2 level was noted for each experiment set of different cell lines. The highest concentration levels were obtained for all samples collected at 72 h time point, showing that HER2 ECD is shed from the cell membrane and accumulates in culture medium at the concentration from 12.6 ng/mL up to 145.7 ng/mL. Interestingly, the cell culture medium contained 1 000-fold lower concentrations of the tested antigen (soluble form) in comparison to membrane-bound protein found in whole-cell lysates resulting in the lowest concentration of 3.7 μg/mL (MDA-MB-231) up to 17.2 µg/mL (SK-BR-3) in the samples collected at 24 h while at the 72 h it showed from 14.0 µg/mL up to the highest concentration of 22.9 µg/mL for the same respective cell lines. In the experiments, we used cell types known for high overexpression of HER2 protein (SK-OV-3, SK-BR-3) and cells with low physiological expression of this marker (MDA-MB-231). As expected, the sandwich ELISA confirmed about 10x and 50x higher HER2 concentration in the culture medium of SK-OV-3 and SK-BR-3 cells, respectively, in comparison to MDA-MB-231 cells when assayed at the 24 h timepoint. Similarly, we have noted about 3x and 5x more of HER2 in SK-OV-3 and SK-BR-3 cell lysates, respectively, when compared to MDA-MB-231 cells. Results obtained for cells cultured for 48 h and 72 h also showed the same pattern. The coefficients of variation calculated for tested samples are in good agreement with validation data (shown above the bars in Figure 4).

Figure 4: Assessment of HER2 level in human cancer cells using the sandwich ELISA. The measurements were performed on (A) culture medium and cell lysates (B) collected at 24 h, 48 h, and 72 h. HER2 concentrations (ng/mL) are calculated as the average of 3 replicates. The coefficient of variation (CV%) between replicates is presented above each bar. Please click here to view a larger version of this figure.

Among the critical components in constructing a sandwich ELISA, there are capturing antibodies that are immobilized on the plate and contribute to the assay specificity and sensitivity. In the presented assay, we have employed as the capturing antibody the novel monoclonal protein (HER2/70.27.58) generated and characterized in-house. The antibody had a unique sequence of the CDR (complementarity-determining region), and based on the affinity, it presented a sensitivity of ED50 at 0.0922 nM¹². The sandwich ELISA specificity was reflected by the specificity of the capturing antibody that did not bind unrelated antigens, as was confirmed previously by immunofluorescence of the in vitro cultured cells and cell lysates by Western Blotting. The detecting antibody (HER2/70.21.73.67) that was used in the presented ELISA was also well characterized and showed sufficient affinity (ED50 at 0.2347 nM) and specificity to be utilized in sandwich ELISA for quantification of HER2¹².

The main purpose of using the developed sandwich ELISA is to quantify HER2 concentration while studying cancer biomarkers and their role in biology, especially using the in vitro models of cultured cancer cells. Thus, the working range and assay sensitivity need to consider the real concentration range of the HER2 protein in the studied sample types, like human cancer cells. As was observed, the assay design and its components resulted in satisfactory sensitivity ranging from the lower limit of quantification (LLOQ) of 1.2 ng/mL up to 112.7 ng/mL of the upper limit of quantification (ULOQ). The same range was also applicable in the matrix, which is as rich as human serum. It is superior to the previously published and validated ELISA for HER2 ECD quantification in blood¹¹ where authors reported LOD of 0.75 ng/mL and LOQ of 2.77 ng/mL with comparable intra- and inter-assay precision but limited working range of 0.19-12.5 ng/mL. Moreover, the only one FDA-approved and provided by Siemens Healthineers Diagnostics automated immunoassay range is 0.5-350 ng/mL, whereas the recommended pathological threshold was set up at 15 ng/mL. The obtained method parameters allow for assay application in measuring HER2 levels in different sample types, including blood and tissue lysates.

During the process of ELISA development¹², a spike, and recovery (SAR) assay was performed on a variety of matrices (PBS, human serum, FBS, cell lysates, and culture medium) that were tested using the samples with the addition of recombinant HER2 protein of known concentration. The results showed good recovery with minor variability and a generally accepted recovery range of 80-120%¹³. This is aligned with the parameters of the FDA-approved Simens Healthineeres diagnostic assay or the research-only product from R&D Systems¹⁴.

One of the challenges in ELISA is assay reproducibility and repeatability over time. We have noticed minor assay variability that can be related to several issues. The procedure includes labeling the detecting antibody with a biotin reagent (see protocol step 3.2.1). Preparing a larger batch of the biotinylated antibody after storage for a longer time might be a source of the results' discrepancy due to reagent instability. Also, batch-to-batch variations in the efficiency of biotinylated antibodies occur that need to be considered and verified at the time of the standard curve formation.

Another potential source of the poor reproducibility and generation of the results outliers is in the samples that are located at the edge of the plate. It is known as the edge effect¹⁵. This can be diminished by ensuring stable environmental conditions as well as avoiding placing the samples in the very edge wells of the plate (step 3.3.2).

The repeatability of HER2 protein quantification in the samples, such as cell lysates or culture medium, should also be considered in the sample preparation and storage. As we noticed in the human serum samples, the HER2 protein level decreases upon long-term storage of samples (over 1 year) at -20 °C¹². Thus, the quantification of the freshly obtained samples is recommended for the most accurate determination.

Since the assay linearity falls within 1.56-100.0 ng/mL, it is important to note that as the expected higher concentration of HER2 protein might be present especially in the cell or tissue lysates, the sample dilution should be included before sample plating¹². The parameters of the ELISA newly developed by our team provide an opportunity to test at the same time a higher range of samples with diverse HER2 concentrations than other published methods¹¹.

Regarding the significance of the developed assay, we have presented results on the validation and utility of the ELISA based on novel, unique monoclonal antibodies that have been employed to capture and detect antibodies.

Of note is the normalization of the results. We chose to express the concentration in a sample that was derived from the same number of seeded cells (for measuring cell-bound HER2) or the culture medium volume used in each well. It might be considered in the future to normalize the results per total protein concentration, as we previously showed¹⁶.

The methodology described here belongs to a small group of validated quantification assays for the determination of HER2 protein in a comprehensive way, i.e., simultaneous assessment of total HER2 protein generated by the cells. We have shown that it can be used to study the secreted and membrane-bound receptor level that might have significance in developing targeted strategies and therapy monitoring since the soluble systemic ECD can sequester the anti-HER2 drug yet without any cellular effect¹⁷.

Finally, the presented methodology can identify cells that express low levels of HER2, such as the MDA-MB-231 cell line that in several studies is considered as HER2 low-expressors or negative cells^18,19. In our hands, the cells grown for 48 h and 72 h showed a substantial amount of the cell-bound receptor (up to 14 mg/mL), although the released ECD was on a much lower level compared with the typical HER2 positive cell lines like SK-OV-3 or SK-BR-3 cells (Figure 4).

Expanding the utility of the presented anti-Her2 sandwich ELISA might be useful for other studies considering the measurement of HER2 in different sample types. In the previous report the method has been validated also for mouse and human blood as well as for tissue extracts, such as xenograft tumors grown in mice from human cells¹². With a high specificity, accuracy, and reproducibility of detection of both endogenous and recombinant HER2 ECD, it might open new opportunities for a comprehensive analysis of the receptor status not only in vitro but also in vivo models.

D.L., A.A., A.M., M.S. declare financial support from SDS Optic S.A.; A.A, A.M., M.S. declare SDS Optic S.A. stock ownership.

The study was supported by a funds from the National Centre for Research and Development grant STRATEGMEDII/269364/5/NCBR/2015 and EU, Horizon 2020 SME Instrument grant No. 783818.