Near-infrared fluorescence (NIRF) imaging presents a simple, straightforward, and safe tool for the noninvasive labeling of malignant tissue in living organisms. The selective fluorometric detection of tumors requires suitable optical probes and sensitive imaging systems for signal read-out in conjunction with easy data analysis methods for clear discrimination of specific and unspecific signals.
One of the greatest problems in fluorescence imaging in vivo is background fluorescence originating from autofluorescence caused by endogenous molecules220.127.116.11.–6 that hampers the detection sensitivity of specifically bound probes. Furthermore, following intravenous (i.v.) injection, conventional always-on fluorescent probes are systematically distributed over the entire object, causing high background signals and hindering tumor detection until the targeted probe has been concentrated in the tumor and the unbound probe is cleared from the periphery. These problems can be circumvented by the use of functional tumor-specific fluorescent probes, which have increasingly been at the focus of various studies in recent years.18.104.22.168.–12 However, the majority of these probes are still limited in their sensitivity and specificity, regarding penetration depth and real-time imaging of biological processes in vivo. Therefore, there is an ever-growing demand for the development of probes with, for instance, improved tissue penetration and site- or analyte-specific signaling that display a specific fluorescence signal only when located at the tumor site and remain otherwise dark.2,8,13,14 Of special interest here is the sensing of pH as a change in proton concentration, known to go along with numerous diseases such as inflammation or cancer.15 The acidic phenotype of tumor cells caused by upregulation of gylcolysis emerges early in cancerogenesis and is a near-universal property for primary tumors as well as metastases.16 pH-responsive probes are ideally nonfluorescent after their introduction into the blood circulation, thereby minimizing background fluorescence of the unbound probe. Switching on of their emission in the acidic tumor environment16 is thus expected to yield a high tumor-specific contrast even before the unbound probe is cleared from the object.
Although the fluorometric detection of changes in pH is well established in tumor cells in vitro using pH-responsive dyes with emission in the visible region,17 pH sensing in vivo remains challenging, as it requires NIRF dyes with pH-sensitive spectroscopic properties as a prerequisite for protonation-induced spectral and/or intensity changes that so far are rare.2,5,13,18 Examples are norcyanine- or norsquarine-based pH probes which contain a protonable amine moiety as a part of the dye´s π-electron system such as H-ICG,19,20 Square-650-pH,21 and CypHer5E. Alternatively, so-called activatable probes can be used, i.e., self-quenched dye-protein conjugates with cleavable bonds.13 Here, enzymatic cleavage of fluorophores and ligand, for example by degradation in the lysosome, results in a spatial separation of the dyes and thus in an irreversible switching on of the probe’s emission.5,10,12
The need for higher sensitivity of in vivo tumor imaging encouraged us to study the potential of pH-responsive probes to improve the sensitivity of detection and monitoring of acidic tumors in vivo. Protonation-induced switching on of the fluorescence of pH-sensitive probes can occur at the acidic extracellular space of the tumor, but also after internalization into the acidic compartments of the targeted tumor cell. To exploit these mechanisms for the rational design of our probe, we chose a tumor-specific antibody as targeting ligand that binds to a cell membrane receptor, mediating internalization of the antibody–dye–receptor complex, leading to the transfer of the probe into the acidic endosomes and lysosomes.9 Here, we used the antibody trastuzumab (Herceptin) as a tumor-targeting moiety for the human epithelial growth factor receptor 2 (HER2) overexpressed on the cell membrane of various tumor types,2223.–24 including 25% to 30% of all breast cancers.25 As pH-sensitive dye, we employed CypHer5E, a pentamethine dye with a protonable indole group that has a pKa in vitro of 7.326 and is perfectly suited for pH sensing in a biological environment. This fluorophore has already been shown to be functional in cell assays.2627.28.–29 Although CypHer5E displays emission bands in the near-infrared (NIR) region and a good water solubility, contains a functional group for bioconjugation, and is a promising candidate for in vivo imaging of structures with a low pH (like a tumor), so far no reports have been published that describe its use for in vivo imaging.
In this work, the functionality and sensitivity of CypHer5E-Herceptin (pH-Her) was examined in orthotopic breast tumor mouse models with distinct HER2 expression patterns.30,31 The results were compared to that of an Alexa Fluor 647-Herceptin (Alexa-Her) conjugate acting as representative example for an always-on probe with comparable absorption and emission spectra. For both conjugates, also compared were the advantages and limitations of lifetime (LT)-imaging versus conventional imaging in the intensity domain for in vivo tumor monitoring. We showed that the selective fluorescence enhancement of pH-Her solely in the acidic environment of the tumor led to an improved tumor-detection sensitivity over time compared to Alexa-Her when applying intensity imaging in combination with subtraction of background fluorescence. In contrast, LT-gated imaging did not improve the sensitivity of tumor detection in vivo for either probe. These pH-sensitive dye-biomolecule conjugates present one of the first examples of a new generation of more sophisticated optical probes with targeting and sensor function.
Materials and Methods
Herceptin was coupled to the NIRF dyes CypHer5E (GE Healthcare, Munich, Germany) and Alexa Fluor 647 (Invitrogen, Darmstadt, Germany) at different dye-to-protein (DP) ratios in cooperation with Squarix Biotechnology (Marly, Germany). For this purpose, different amounts of the amine-reactive -hydroxysuccinimidyl (NHS) esters of these NIRF dyes (different ratios of dye NHS ester to antibody) were allowed to react with Herceptin in a buffer at basic pH, using identical reaction conditions for both labels (e.g., solvent, pH, temperature, reaction time). The resulting dye-Herceptin conjugates were purified chromatographically using a Sephadex column and a phosphate buffer as eluent. The DP ratios were calculated from the photometrically determined concentration of the dye and the protein using a calibrated Cary 5000 spectrometer (Varian Inc., Australia) following a previously described procedure.32 In the case of Alexa Fluor 647, the dye concentration was determined from the integral absorbance at the longest-wavelength absorption. For CypHer5E, the dye concentration was determined from the absorbance measured at 565 nm using a previously measured molar absorption coefficient of .
Determination of fluorescence enhancement factor
Fluorescence emission spectra of pH-Her and Alexa-Her conjugates were measured at a concentration of 5 µg IgG in 75 µL PBS at pH 7.5 and 5.5, respectively. The pH of PBS was adjusted with 1 M NaOH and 1 M HCl solutions, using a calibrated pH meter (SevenEasy, Mettler-Toledo, Giessen, Germany). The pH values were controlled again after addition of the fluorescent probes and adjusted again to the respective value if necessary. The spectrally uncorrected fluorescence spectra33 were obtained with a Spectrofluorometer QuantaMaster (Photon Technology International, Seefeld, Germany), immediately after pH adjustment of the probe-containing solutions, using an excitation wavelength () of 635 nm and 1-cm quartz cells. The analyses of the spectroscopic measurements were performed with the software FeliX32 Analysis 1.0.
Determination of fluorescence quantum yields
For the relative determination of the fluorescence quantum yields (QYs) of the conjugates at pH 5.3 using of 635 nm (always performed in duplicate), absorption spectra of dilute probe and standard dye solutions were recorded on a calibrated Cary 5000 spectrometer (Varian Inc., Australia) in PBS using 1-cm quartz cells. The fluorescence spectra of these solutions were measured with a calibrated Spectronics Instruments 8100 with Glan Thompson polarizers (Spectronics Instruments)33 in the excitation and the emission channel set to 0 and 54.7 deg, respectively. The QYs of the fluorescent probes were calculated from spectrally corrected emission spectra as previously described34 using Oxazine 1 in ethanol as QY standard. The QY of Oxazine 1 in ethanol () was previously determined relative to Rhodamine 101 (). The QY of Rhodamine 101, 0.915, was measured absolutely using a new integrating sphere setup.34
KPL-4 (provided by J. Kurebayashi, Kawasaki Medical School, Kurashiki, Japan)30 and MDA-MB-231 (ATCC/LGC, Wesel, Germany) breast cancer cells were propagated in standard DMEM containing glucose and l-glutamine. The medium was supplemented with 10% fetal calf serum (PAN Systems, Aidenbach, Germany). Cells were cultivated under standard cell culture conditions at 37°C in a humidified atmosphere under 5% CO2. Tumor cells were harvested near confluence by brief trypsinization in 0.05% trypsine-EDTA solution, washed several times, and placed in sterile PBS shortly before implantation.
Breast cancer cells were seeded on four-well chambered coverglasses coated with poly-l-lysine at a density of KPL-4 cells and MDA-MB-231 cells per well, respectively, in 500 µL DMEM. After 48 h, the medium was removed and the cells were supplemented with 500 µL DMEM and 0.1 M HEPES buffer containing 5 µg fluorescent probes or 0.2 nmol of the free fluorophores, CypHer5E (hydrolyzed CypHer5E NHS ester) or Alexa Fluor 647 (hydrolyzed Alexa Fluor 647 NHS ester). Cells were incubated with probes for 8 h at 37°C or 4°C. Afterward, the cells were washed several times with PBS and incubated with Hoechst 33342 (; Invitrogen, Darmstadt, Germany) to stain the nuclei of the living cells. After 10 min, the Hoechst solution was removed and the cells were covered with 500 µL PBS and subsequently used for fluorescence microscopic studies with the Axiovert 200M (Carl Zeiss, Göttingen, Germany) equipped with a NIRF-sensitive ORCA-ER digital camera (Hamamatsu, Herrsching am Ammersee, Germany). Signals of the fluorescent probes were recorded with a excitation and a emission filter and Hoechst with a excitation and a emission filter, respectively. Image generation and processing were performed with the software systems AxioVision Rel.4.6. and ImageJ,35 respectively.
All animal experiments were performed according to the animal protection law, and all animal protocols were approved by the administration of Lower Saxony, Germany. Experiments were performed on female athymic nude mice, strain NMRI-Fox1nu/nu. Two weeks before in vivo imaging, mice received chlorophyll-depleted low fluorescent food, Regime 210 (SAFE, Augy, France). Mice were maintained in a sterile environment in special cages with filter huts placed in a Scantainer (Scanbur, Koge, Denmark).
Tumor Cell Implantation
KPL-4 and MDA-MB-231 breast tumor cells were implanted in the right abdominal mammary fat pad of female nude mice at the age of 6 to 12 weeks. For this purpose, mice were anesthetized by intraperitoneal injection of xylazine and ketamine. Solutions containing the respective amount of tumor cells ( KPL-4 cells or MDA-MB-231 cells) suspended in 30 µL sterile PBS were implanted with an insulin syringe with integrated needle (30G, BD, Heidelberg, Germany) as described.36 Afterward, mice were inspected twice a week for weight loss, general condition, and tumor formation.
In Vivo Imaging
The time domain fluorescence imager, Optix MX2 (ART, Montreal, Canada),37 was used for intensity and LT imaging in the NIRF region. This device scans in a raster modus and measures the time between every laser excitation pulse and the detection of the first fluorescence photon using a single photon counting detector. The resulting photon time-of-flight histogram, termed temporal point spread function (TPSF), obtained for each scan point, is used to calculate the LT of the emissive species as an exponent of the slope of the decay by underlying a single exponential model.38 The quality of the fits was judged by the value of for each TPSF that should be between 0.8 and 1.2 for reliable fits (OptiView software, User’s Manual, Version 2.02).
The in vitro fluorescence LTs of pH-Her and Alexa-Her (3 µg IgG) and their parent fluorophores were measured in 150 µL PBS and in PBS containing 5% (mass) bovine serum albumin (BSA; PBS/BSA 5%; Sigma-Aldrich, Seelze, Germany) at pH values of 7.5 and 5.5, respectively. To determine the LT of the fluorescent probes in vivo, 10 µg pH-Her and Alexa-Her dissolved in 50 µL 0.9% NaCl were each administered subcutaneously (s.c.) in healthy nude mice.
Mice were used for in vivo fluorescence imaging experiments 1 to 4 months after tumor implantation when the tumors had reached sizes of approximately . Mice received i.v. injections of 25 µg pH-Her or Alexa-Her or 0.8 nmol of the respective free fluorophore in 0.9% NaCl. KPL-4 tumor-bearing mice were scanned before and 1, 2, 4, 24, 48, and 72 h after probe injection, and control mice, bearing MDA-MB-231 tumors, were scanned before and 24, 48, and 72 h after probe injection. During injection and scanning, mice were anesthetized with vaporized isoflurane at concentrations of 0.8% to 1%.
Imaging was carried out using an excitation wavelength of 635 nm and a band pass emission filter. Scans were performed with a 1.5 mm (whole body scan) or 1.0 to 1.5 mm (excised organs) raster, and an integration time of 0.7 to 1.0 s per scan point. Data were analyzed with OptiView software 1 00 00 (ART, Montreal, Canada). Intensity values were determined in normalized counts (NC), which are independent of laser power and integration time. For the subtraction of autofluorescence, the average fluorescence intensity over the tumor region of the prescan was subtracted from all intensity maps of the respective mouse. The contrast-to-noise ratio (CNR) was calculated as the difference between the average fluorescence intensity over the tumor and over a background region divided by the standard deviation of the background signals. As a background region, the area over the lung was selected, as no specific fluorescent signals from the HER2-bound probe and no unspecific signals (from fluorescent food, for example) are to be expected.
After the in vivo experiments, the animals were sacrificed, and autopsies of mice were performed. Subsequently, the tumors were excised and the tumor sizes were measured by a caliper. Furthermore, the abdomen and the thoracic cavity were opened and lung, heart, liver, kidneys, spleen, stomach, and gut were isolated and imaged ex vivo with the Optix MX2.
pH-Her Conjugates at Low DP Ratios Show Favorable Spectroscopic Properties
For the choice of a suitable pH-sensitive probe for in vitro and in vivo studies, dye-Herceptin conjugates of different DP ratios were spectroscopically studied with respect to their pH sensitivity and QYs. As shown in Fig. 1(a) and 1(b), a decrease in pH led to an increase in fluorescence only in the case of the pH-sensitive dye conjugate pH-Her, whereas the emission of Alexa-Her did not undergo any apparent pH-induced changes (). No apparent change in the spectral position of the emission maxima of either probe was observed for pH values of 5.5 and 7.5. The protonation-induced fluorescence enhancement factors of different dye-Herceptin conjugates and their QYs are summarized in Fig. 2. The pH sensitivity of the fluorescent conjugates was calculated as the ratio of the fluorescence intensities at the emission maximum obtained at pH values of 5.5 and 7.5 and thus presents a measure for the protonation-induced fluorescence enhancement of the probe. The maximum enhancement factor of 5.6 was found for pH-Her, with a DP ratio of 1.6, whereas pH-Her with a DP ratio of 4.9, for example, showed a 3.9-fold increase in fluorescence upon protonation. Due to its good pH sensitivity and comparably high QY, the pH-Her conjugate with a DP ratio of 1.6 [Figs. 1(a) and 2] was applied for further experiments shown in Figs. 3 to 6. The Alexa-Her conjugate with a DP ratio of 1.6 [Figs. 1(b) and 2] was used as control and as an example for an always-on probe.
Functionality of the pH-Sensitive Conjugate In Vitro
As follows from Fig. 3, the pH-sensitive probe pH-Her showed fluorescence only after receptor-mediated internalization into HER2-positive KPL-4 cells after 8-h incubation at 37°C [Fig. 3(a), green arrow]. In contrast, after 8-h incubation at 4°C, when internalization should have been strongly reduced,11 no signals could be detected from pH-Her that was most likely only membrane-bound [Fig. 3(b)]. These results demonstrate that pH-Her becomes emissive only after internalization, presumably when localized in the acidic endosomes and lysosomes of the target cell.9,10,39 In contrast, the always-on probe Alexa-Her showed fluorescence after internalization [Fig. 3(c), green arrow] as well as when bound to the cell membrane [Fig. 3(c) and 3(d), orange arrow].
No fluorescence signals were detected on HER2-negative MDA-MB-231 cells after incubation with either pH-Her or Alexa-Her at 37°C and 4°C (data not shown). Incubation of both cell types for 8 h with the corresponding free fluorophores CypHer5E and Alexa Fluor 647 at 37°C and 4°C did not yield any detectable fluorescence, underlining the absence of unspecific binding and/or internalization of the dyes (data not shown).
Improved Detection Sensitivity of Tumors by the Combined Use of a pH-Sensitive Probe and Subtraction of Autofluorescence
Representative fluorescence intensity maps of KPL-4 tumor-bearing mice over time obtained after subtraction of autofluorescence are shown in Fig. 4. After i.v. application of pH-Her [Fig. 4(a), ], the fluorescence in the tumor (white circle, shown at 24 h) can be clearly detected. The background fluorescence (white circle, shown at 24 h) is low and comparable to the fluorescence of the mouse prescan, except for some signals deriving from autofluorescence of the gastrointestinal tract.3 In contrast, after injection of Alexa-Her [Fig. 4(b), ], the fluorescence in the tumor (white circle, shown at 24 h) is also clearly visible, but simultaneously, the background signals resulting for this probe (white circle, shown at 24 h), are high compared to the signals from the prescan.
Hence, as rationalized with our probe design concept, use of the pH-sensitive probe pH-Her in combination with subtraction of autofluorescence enabled the elimination of major contributions of background fluorescence in scans of tumor-bearing mice. In contrast, the intensity of the background strongly increased after i.v. injection of the always-on probe Alexa-Her and could not be eliminated by subtraction of autofluorescence.
To quantify the tumor detection sensitivity in vivo after injection of the fluorescent probes and subtraction of autofluorescence, CNRs were calculated. As shown in Fig. 5, i.v. injection of pH-Her [Fig. 5(a)] in KPL-4 tumor-bearing mice () resulted in a continuous increase of CNRs within 24 h up to a ratio of 171 that remained comparably high for 72 h. In contrast, application of Alexa-Her [Fig. 5(b), ] led to an increase in CNR in the mice over time of at most only 82. In consequence, although the size of the fluorescence signal resulting from Alexa-Her exceeds that of pH-Her as indicated by the different range of the scale bars in Fig. 4(a) and 4(b), the average CNR over all times in KPL-4 tumor-bearing mice treated with pH-Her was about 2.5-fold higher than for mice treated with Alexa-Her.
Control experiments performed in HER2-negative MDA-MB-231 tumor-bearing mice injected with pH-Her [Fig. 5(a), ] revealed CNRs that are in average nine times lower than in HER2-positive KPL-4 tumor-bearing mice treated with the same probe. In the case of Alexa-Her [Fig. 5(b), ], the average CNR after probe application was only four times higher as found in MDA-MB-231 tumor-bearing mice. This illustrates that the detection specificity of the signals deriving from HER2-positive tumors, in comparison to HER2-negative tumors, can be enhanced by use of the pH-sensitive probe.
Further control experiments in KPL-4 tumor-bearing mice with the free dyes CypHer5E [Fig. 5(a), ] and Alexa Fluor 647 [Fig. 5(b), ] revealed that after i.v. dye injection, CNRs were also low after 24 to 72 h (maximum ratio of 21 and 20 for CypHer5E and Alexa Fluor 647, respectively) and comparable to CNRs in HER2-negative tumor-bearing mice treated with the Herceptin conjugates. These results indicate a low nonspecific accumulation of the fluorescent probes in the tumor. This further demonstrates that the tumor-targeting moiety Herceptin is necessary for efficient accumulation of the probe in the tumor and for the achievement of high CNRs. Surprisingly, Alexa Fluor 647 administered into KPL-4 tumor-bearing mice revealed a relatively high nonspecific accumulation in the tumor with comparably high CNRs at early times after injection [Fig. 5(b), CNR of 72 after 1 h].
To analyze in more detail whether the fluorescent probes accumulate nonspecifically in other organs, KPL-4 and MDA-MB-231 tumor-bearing mice were sacrificed 72 h after injection of pH-Her, Alexa-Her, or their free parent dyes and the organs were scanned ex vivo. In comparison to organs of untreated mice (), no increased fluorescence could be detected in the liver, kidneys, lung, heart, spleen, stomach, or gut (data not shown).
In summary, subtraction of autofluorescence in vivo in combination with the use of the pH-sensitive probe pH-Her clearly improved the tumor detection sensitivity in breast tumor-bearing mice compared to the always-on probe Alexa-Her.
LT-Gated Imaging in Combination with Herceptin Conjugates Does Not Improve Tumor Detection
To assess whether LT imaging in combination with pH-Her can also provide an increased tumor detection sensitivity, LT-gated image analysis was performed with scans of KPL-4 tumor-bearing mice receiving either pH-Her or Alexa-Her. For the application of LT gates as an extra method for data illustration only, principally the LT of the probe can be determined from the in vivo scans in the tumor and employed for gating. However, for the identification of unknown tumor sites not macroscopically visible at the time of imaging and/or for the detection of metastases, the probe’s LT must have been reliably determined from LT measurements with appropriate model systems before data analysis with LT gating can be performed with the in vivo scans. This very challenging approach requires identical or at least closely matching LTs (within the chosen LT gate) for the probe in the tumor and in the model system(s). This may not be necessary fulfilled, as the LT of a fluorophore can be affected by dye environment (proticity, polarity, viscosity, refractive index) in a dye-specific manner.
Preevaluation of LTs of pH-Her [Fig. 6(a)] and Alexa-Her [Fig. 6(b)] under several application-relevant conditions (in vitro at different pH values and in the presence of BSA as model for plasma proteins, and in vivo after s.c. injection in living mice) revealed only small environment-induced changes in the fluorescence LTs of both conjugates. The presence of proteins (PBS/BSA 5%) resulted in slightly increased LTs for both probes compared to LTs in PBS. Surprisingly, the LTs of both probes obtained after s.c. injection in vivo were shorter compared to the LTs measured in PBS and PBS/BSA 5%, although a solution of PBS/BSA 5% has been described as a good model for the prediction of the in vivo LTs of several NIR cyanine dyes.40 The mean LTs of measurements in PBS/BSA 5% at pH 7.5 and 5.5 and after s.c. injection expected to reflect the probe environment in the in vivo situation40 were 1.3 ns for pH-Her and 1.5 ns for Alexa-Her [Fig. 6(a) and 6(b)]. These LTs (referred to as predicted LTs in the following) were used for probe identification in subsequent LT-gated in vivo imaging studies of tumor-bearing mice.
Subsequently, fluorescence intensity maps of KPL-4 tumor-bearing mice after i.v. injection of Herceptin-conjugates were gated with the predicted LTs of pH-Her [Fig. 6(c)] and Alexa-Her [Fig. 6(d)] as shown for scans obtained after 24 h and 48 h after probe administration (). Only signals with the predicted LT for the respective probe within a LT range of are shown. Interestingly, with this approach, the fluorescence signals of pH-Her in the tumor (white circle, shown at 48 h) are partly eliminated at most times as follows from Fig. 6(c), whereas in the case of Alexa-Her, the major part of the background fluorescence is still visible [Fig. 6(d), white circle, shown at 48 h]. Similar results were obtained for scans performed at different times after probe application (data not shown). A comparison of the LT-gated images with the images obtained after subtraction of autofluorescence in the same mice (see Fig. 4) revealed that the tumor detection sensitivity could not be increased for either pH-Her or Alexa-Her with LT-gated imaging under these conditions.
Precise analyses of the average LTs in the tumor area at each scan time and in different mice showed that the LTs of pH-Her in the tumor in vivo with a mean value of 1.5 ns [Fig. 6(e)] are all slightly longer than the predicted LT (black dashed line). Accordingly, they are only partly covered by the LT range used for gating (, gray area) of the pH-Her signals. This increase in LT of pH-Her in the tumor accounts for partial elimination of tumor signals. On the contrary, the average LTs of Alexa-Her in the tumor of 1.5 ns [Fig. 6(f)] agree with the predicted LT of 1.5 ns (black dashed line) and thus lie well within the chosen LT gate (, gray area).
As to be expected, consideration of the tumor-related LT changes in the case of pH-Her and consequently, the use of a LT gate of (Fig. 7) for image analysis, allowed a clear visualization of the entire tumor signals at all times as well as a partial elimination of background fluorescence.
In summary, neither for pH-Her nor for Alexa-Her could the tumor detection sensitivity in vivo be improved by LT-gated imaging using LTs previously determined in model systems. As the predicted LT of pH-Her differed from the LT actually measured in the tumor, LT gating resulted in a considerable loss in signal. In the case of Alexa-Her, background signals deriving from unbound Alexa-Her could not be efficiently eliminated from tumor-derived fluorescence by LT gating.
The present work clearly demonstrates the potential of the pH-sensitive and tumor-specific probe pH-Her to improve the tumor-detection sensitivity in vivo in comparison to the always-on probe Alexa-Her.
The use of pH-Her enabled the noninvasive detection of tumors in vivo with a 2.5-fold increase in CNR compared to the pH-insensitive control probe Alexa-Her. The very high CNRs achieved with pH-Her are due to the protonation-induced strong red shift in absorption and a considerable increase in fluorescence intensity of pH-Her in the acidic environment of the tumor and after receptor-mediated internalization in the targeted tumor cells. At the same time, pH-Her yielded only a low background fluorescence in blood, where it faces an approximately neutral pH, rendering it barely excitable at 635 nm and almost nonemissive. The remaining background fluorescence could be efficiently eliminated by subtraction of autofluorescence calculated from prescans of mice. In contrast, the always-on targeted probe Alexa-Her was fluorescent in the tumor as well as in blood, as the signal-relevant spectroscopic properties of this bioconjugate did not respond to changes in pH in the probe’s environment. This resulted in high background fluorescence from unbound Alexa-Her that could not be sufficiently eliminated by subtraction of autofluorescence.
Other groups like Urano et al.9 and Ogawa et al.41 have also achieved high tumor-to-background ratios with pH-sensitive or pH-activatable Herceptin conjugates utilizing photon-induced electron transfer (PeT)-operated fluorescent reporters and a TAMRA-QSY7 fluorophore-quencher pair conjugated to Herceptin. However, as these probes both emit in the visible wavelength region, the limited tissue penetration hampered the generation of in vivo data and allowed only ex vivo measurements of the opened abdominal cavity of mice to detect fluorescent signals. This is of little importance for noninvasive tumor detection in living organisms desired for the sensitive visualization of pathological changes in tissue and therapy monitoring over time.
By using a NIR dye with pH-dependent absorption and emission properties, the very low background fluorescence resulting for pH-Her, which enabled high tumor-detection sensitivity, was further attained by the lack of major signals in other organs in vivo and ex vivo. In contrast to our observations, it has been reported that activatable fluorescent probes exploiting aggregation-induced fluorescence quenching, like ICG-Herceptin10 or Cy5.5-Herceptin12 conjugates, produce high liver signals resulting in decreased tumor contrast. Moreover, this probe design strategy that requires the conjugation of several fluorophores to one targeting ligand12,42 cannot be used for small ligands like peptides that contain often only a single dye binding site, whereas pH-sensitive dyes7 such as CypHer5E do not have this limitation. Moreover, the conjugation of many fluorophores to a biomolecule results in higher material costs and can reduce bioconjugate stability and binding efficiency. In addition, it can be synthetically challenging to achieve the maximum fluorescence quenching desired for probes utilizing aggregation-induced fluorescence quenching.43
Our probe pH-Her yielded clearly measurable signals in the tumor as early as 1 h after i.v. probe injection in KPL-4 tumor-bearing mice; the signals were considerably higher than the unspecific fluorescence arising from the gastrointestinal tract 4 h after injection. This rapid tumor detection was ascribed to an increase in fluorescence of pH-Her in the acidic tumor interstitium prior to receptor-mediated internalization in the target tumor cells. It is known that the fluorescence of CypHer5E increases immediately in the presence of protons20,44 present in the acidic tumor environment. In contrast to pH-Her, which belongs to the few examples of targeted probes with sensor function,13 activatable fluorescent probes exploiting distance-dependent fluorescence quenching mechanisms1011.–12,41 require irreversible lysosomal degradation, which occurs only after internalization in targeted cells. In the present study, the majority of Herceptin conjugates was internalized after 8 h of incubation at 37°C in HER2-positive cells in vitro, confirming previous studies which illustrate the efficient internalization of fluorescent Herceptin conjugates in target cells after a similar period rather than after 1 h.10,41 Accordingly, activatable Herceptin conjugates need a longer time for signal generation in vivo in the tumor. For example, Ogawa et al., who applied activatable probes for in vivo imaging of tumors, did not show any fluorescence images of tumor-bearing mice earlier than 24 or 48 h after i.v. injection of the probes,10,12 suggesting that tumor contrasts at early times were low. The fast response of our probe pH-Her can be exploited for faster tumor detection.
With our pH-responsive probe pH-Her, we could achieve a very high tumor detection sensitivity with simple fluorescence intensity imaging via straightforward elimination of background fluorescence by subtraction of autofluorescence. More sophisticated LT-gated imaging requiring expensive equipment did not improve tumor detection sensitivity in vivo, as the LT of pH-Her in the tumor changed unpredictably compared to LTs of this probe in different model systems ranging from PBS/BSA 5% with pH values of 7.5 and 5.5 to s.c. injected healthy mice [see Figs. 6(a), 6(e), and 7]. Therefore, in pH-Her-treated KPL4 tumor-bearing mice, the majority of tumor signals revealing a LT of 1.5 ns was eliminated after LT-gating of the images using a LT gate of derived from measurements in these model systems. Obviously, some factors in the tumor environment influence the LT of pH-Her, which could not be assessed by our model systems. Further studies are required to examine the conditions in detail that could be responsible for the increase in LT of pH-Her in the tumor in vivo. In the case of Alexa-Her, the similar LTs in the tumor and background region did not enable an efficient discrimination of background fluorescence by LT-gated imaging. The high background signals of the Alexa-Her conjugates are attributed to the prolonged blood circulation time of IgGs.45 In comparison, in the case of probes like RGD-Cy5.5 containing small peptides as targeting ligand that are cleared more rapidly from the blood, LT-gated imaging could be successfully applied as a tool to increase detection sensitivity of specific signals in vivo.3 This indicates that in the case of always-on dye conjugates, the efficiency of LT-gated imaging is also affected by the kinetics of the tumor-targeting moiety.
In addition, our findings underline the need to better understand the origin of dye- and environment-specific changes in LT3,46,47 to avoid signal misinterpretations. Obviously, for accurate LT gating, extensive preevaluation of the LTs of the chosen probe is necessary for each tumor model. Factors known to influence the LTs of fluorescent dyes and probes are, for example, the presence of proteins and lipids, pH, calcium concentration, and tissue depth.3,4,40,46 Therefore, the efficiency of LT-gated imaging to eliminate background fluorescence and unspecific signals and thus, to improve tumor contrast, depends not only on the characteristics of the fluorescent probe (the chosen dye and ligand), but also on the tumor model and the imaging parameters applied. Principally, if controllable and predictable, a change in probe LT selectively at the tumor site can have great potential for the sensitive detection and monitoring of tumors and tumor spread in vivo, with LT-gated imaging enabling straightforward background suppression.48
In conclusion, this study illustrates for the first time the suitability of CypHer5E bound to Herceptin for in vivo tumor imaging and its potential to provide an enhanced detection sensitivity and enable faster signal detection compared to commonly applied always-on tumor-targeting probes such as the Alexa Fluor 647 Herceptin-conjugate. The selective enhancement of the emission of pH-Her is switched on only at the acidic pH of the tumor. In combination with the subtraction of autofluorescence, this provides a significantly improved detection sensitivity of breast tumors in vivo. This simple signaling strategy can be extended to other probes and targets. It remains to be shown whether the unexpected changes in LT observed for pH-Her in tumor tissue compared to its LT in different model systems is a characteristic of the dye, CypHer5E. In the future, the pH-responsive dye CypHer5E, conjugated to tumor-specific ligands, may be applied for the fast and sensitive detection of weak signals, deriving for instance from small metastatic lesions in vivo. Although the NIR fluorophore Indocyanine Green (ICG) is currently the only dye approved by the US Food and Drug Administration for use in patients,49 pH-sensitive targeted probes such as CypHer5E conjugated to antibodies specifically binding to tumor-associated antigens like HER2 and other targeted probes with a sensor function are expected to have a bright future in medical imaging and implementation in clinical applications to detect tumor lesions in vivo or to monitor therapy efficiency in cancer patients over time with high sensitivity.
We gratefully acknowledge Prof. Dr. J. Kurebayashi, from the Kawasaki Medical School in Kurashiki, Japan, for kindly providing us with the KPL-4 cell line. We also thank Sarah Greco, Bärbel Heidrich, Johanna Widera, and Roswitha Streich for accurate and excellent technical assistance. The work is part of the PhD thesis of Julia Eva Mathejczyk. This work was supported in part by a grant from the DFG (AL336/5-2 Alves) within the SPP1190.