Spectral characterization of CJ215: CJ215 is a small molecule carbocyanine dye with a molecular weight of approx. 1330g, Supp. 1. First, we profiled the spectral absorption of CJ125 and determined whether it had any SWIR emissive tail, as has been demonstrated for ICG and ICG conjugated cancer targeting probes, Fig. 1 A.6,40,41 We found that CJ215 underwent a spectral red shifting similar to that seen with ICG when dissolved in fetal bovine serum (FBS), Fig. 1 A.42 This red shift in FBS was noted at a pH of 4, 6, 7.4, 8 and 10 with CJ215 being highly stable in FBS over 150 mins at all tested pHs, but degrading rapidly in dextrose at a pH of 4, Supp. 2. CJ215 underwent further red shifting when dissolved in human serum albumin (HSA), and FBS at a pH of 7.4, followed by a slow blue shift of ~5 nm in HSA and FBS over the course of 96h, Supp. 3. Little to no degradation (loss of absorption) of CJ215 was seen in both HSA and FBS. Although the NIR emission of the dye has been shown by the manufacturer, the SWIR emission and SWIRFI capability of the dye was yet unknown. To assess this, we dissolved CJ215 in dextrose, as well as pure or diluted (in PBS) defibrinated sheep’s blood to simulate human blood, Supp. 4. The peak absorption and emission of CJ215 red shifted by ~25nm in the presence of blood versus dextrose (794 to 818 and 813 to 838 nm, respectively) with SWIR emission extending to ~1550 nm, Fig. 1 B and Supp. 4. Additionally, the dye underwent an increase in fluorescence intensity when present in serum or blood across this spectrum, further enhancing its suitability for in vivo imaging.43 We assessed the ability of SWIRFI to potentially improve tumor delineation through scattering media, by imaging CJ215 through 5 mm of chicken breast, Supp. 5. CJ215 was readily detected at 900 (InGaAs sensor response cutoff), 1000, 1100, 1200 and 1300 nm long pass cut offs (achieved via long pass optical filters) with delineation improving with extended (longer) wavelengths. This improvement is attributed to lower autofluorescence, insensitivity to excitation wavelength (808nm) and reduced stray photon scatter via increased water absorption in this realm of the spectrum, Supp. 5.44,45
In vitro assessment: Following characterization of the spectrum of CJ215 we assessed the level of CJ215 uptake in vitro in both 4T1 (murine breast cancer) and HT1080 (human fibrosarcoma) cell lines. We first assessed the subcellular localization of the dye via NIR point scanning confocal microscopy. After 3hrs of incubation in 4T1 single cells and 3D spheroid structures, CJ215 was found to be localized to mobile vesicles inside both cells and spheroids, likely taken up via endocytosis Fig. 1 C, D. To further assess the uptake of CJ215, 4T1 cells were incubated with the dye for 3 hrs at either 4°C or 37°C. At 4°C endocytosis, as an energy-consuming process is impaired.46 Indeed, CJ215 uptake was reduced by 39.1% in cells at 4°C, indicating that CJ215 is actively transported into cells, Fig. 1 E. We also assessed the effect of inducing apoptosis via staurosporine (Sta) on CJ215 uptake in HT-1080 (fibrosarcoma) cells. The addition of staurosporine (Sta), an apoptosis inducer, increased CJ215 cellular uptake by 99.3% compared with untreated control cells, while addition of z-VAD FMK, an inhibitor of Sta induced apoptosis, reduced that uptake by 48.8% (to 51.1%) compared to Sta alone, Fig. 1 F.47 ICG incubation under comparative conditions did not yield any significant change in cellular uptake, Supp. 6. Finally, we found that serum-starved 4T1 cells had a 54.3% higher uptake of CJ215 than cells incubated in the presence of serum, as has been shown for other dyes, Supp. 7.43,48 Following fixation with PFA, cells retained CJ215 fluorescence levels up to 9 days post fixation, Supp. 8.
In vivo screening: Having determined that induction of apoptosis increased cellular uptake of CJ215 and that the dye was predominantly taken up by cells via active transport, we next assessed CJ215’s capability for preclinical tumor screening across four tumor lines in vivo, in mice implanted with a single lesion site. With lower apoptosis levels found in healthy/normal tissues, we hypothesized that tumors would be the main site of CJ125 retention. We utilized a SWIRFI system equipped with an 808 nm laser (300 mW/cm2) to assess CJ215 uptake in male and female mice, across FoxN1nu, SCID and BALB/c mouse lines under ambient lighting conditions.41 The tested tumor lines comprised xenografted mice bearing breast (4T1, murine, orthotopic), prostate (PC3-PSMA, human, heterotopic), HT1080 (fibrosarcoma, human, orthotopic) and CT26 (colon, murine, heterotopic) tumors, Fig. 1 G. All mice were imaged every 24 hr from 1 to 96 hrs and finally at 144 or 168 hrs post injection, Fig. 1 H. The SWIRFI system (>900nm, sensor response) enabled video rate (30 Hz) acquisitions under ambient lighting conditions and readily achieved sufficient SNR (>5dB) at all tested timepoints with exposure times as low as 1 ms, Fig. 1 H. In all cases the dye showed highly selective tumor uptake, with sufficient SNR achieved within 1 hr post injection. Contrast (CNR), the ability to delineate the tumor from healthy tissue achieved sufficient levels (>3dB, Rose criterion) from 24 to 168 hrs, with CNR increasing in all cases with elongated clearance time, Fig. 1 I.49,50 Images from all mice at all timepoints are also presented, Supp. 9-12. All SWIRFI presented throughout this manuscript was performed with the ambient lighting LED on, at no detriment to image fidelity.41
Tumor resection: post-euthanasia tumor resection was confirmed in all mice using SWIRFI again under ambient lighting conditions. The assumed primary tumor was resected, and both the excised tumor and remaining tissue (i.e., the entire body) were placed side by side within the SWIRFI system, Fig 2 A. In all cases, the primary site contained enough fluorescence to delineate the tumor, while also confirming that the tumor bed was free of residual fluorescence. If suspected remnant tumor was left behind, as noted by highly fluorescent areas this was then resected. For all tested tumors the primary and potential secondary sites had sufficient SNR, Fig 2 B, for detection along with sufficient CNR, Fig. 2 C, to be delineated from the tumor bed site. Resection images for all mice and all tumors are also shown, Supp. 13-16.
Extended spectral emission of CJ215 assessment: The extended spectral emission of carbocyanine dyes into the SWIR spectrum, which only SWIRFI systems can detect, provides improvements in delineation and resolution, especially through scattering media.44,51 We compared the extended spectral emission of CJ215 in HT1080 and CT26 tumor uptake at >900 (sensor response), >1100, and >1300 nm (via long pass optical filters). This was performed to assess increased resolution and penetration via water absorption, which begins to increase >1100 and further increases >1300 nm, Supp. 5.45 In all cases CJ215 could be detected but as expected, required increasingly longer exposure times at longer wavelength cutoffs (1-2 ms for >900 nm, 100-200 ms for >1100nm and 500-1500ms for >1300nm cutoffs). Tumor SNR was sufficient at all timepoints, with CNR permitting tumor delineation for HT1080 tumors at 1 hr but not for the CT26 line, Supp. 17. Data combined from both lines, permitted a clearer SNR and CNR assessment and comparison between timepoints. The effect of these cutoffs for resection efficacy was also assessed in both primary sites and potential secondary locations, Fig. 3 A. In all cases, the >1300 nm cutoff provided the lowest SNR and CNR, with >1100 and >900 performing comparably Fig. 3 B,C. a similar trend was identified for screening, Supp. 17. However, the >1300 nm cutoff did improve delineation of the tumor core for both tested tumor lines, Fig. 3 A, E, F. We found dye localization and core delineation improved both qualitatively and quantitatively at extended wavelengths, most obviously at 1300nm, Fig 3 E, F. Using immunohistochemistry (IHC) for cleaved caspase 3 (CC3, a known apoptosis marker) we confirmed that the core of the tumors had the highest levels of apoptosis, with increased apoptosis observed throughout the tumor, Fig 3 G.52
CJ215 biodistribution: Whilst SWIRFI is an emerging technique with advantages over established methods, due to the unavailability of imaging systems it remains a restricted resource to many researchers with even fewer clinical options. Additionally, the SWIRFI system employed here had a relatively small FOV, limiting simultaneous imaging of multiple organs. To highlight the suitability of CJ215 for conventional NIRFI, the IVIS Spectrum system (silicon based sensor) was utilized for biodistribution assessment.53 Resected organs were imaged simultaneously with fluorescence levels then quantified to highlight the various tumor to organ ratios. In all cases, the tumor was always the brightest tissue, achieving tumor:muscle ratios up to 99.63, Fig. 4. For nearly all models, minimal to no remnant uptake was seen in healthy tissues, resulting in e.g., tumor:spleen ratios of up to 47.7 or tumor:liver of 17.86, Fig. 4. Residual fluorescence seen in the kidneys highlights the renal clearance of CJ215. Biodistribution from all tumors and all mice are shown along with corresponding H&E staining of select organs, Supp. 18-25. During the resection of CT26 tumors, additional fluorescence was determined deeper and in proximity to the resected tumor, Fig. 5 A. In two mice (M3, M4) additional areas of increased CJ215 fluorescence were identified below the resected tumor. These areas were also isolated (Fig. 5 B (ROI-2)) and sent with the primary tumor and select organs sent for histological analysis, Fig. 5 C, D, E. When compared with tumor tissue, these tissues had lower CJ215 uptake (but still elevated over other organs) and were found to be tumor free. These areas were identified as small intestine, lymph node, uterine and ovarian tissue with the additional focus in M3 containing a small tumor fragment. Further, IHC staining for cleaved caspase 3 (CC3) Fig. 5 D, an established marker for apoptosis, found increased levels of apoptosis in these tissues. As expected, CT26 primary tumor sites had highly elevated levels of CC3 positive cells, especially within the core of the tumor, Supp. 26. The additionally resected tissues which had increased CJ215 fluorescence also displayed increased levels of CC3 positive cells, Fig 5 E.52
Contrast-based SWIRFI for binary tumor delineation: Having determined that simply letting CJ215 clear from healthy tissues improved tumor delineation, we utilized further image processing to improve tumor delineation. This was done with the goal of presenting a surgeon with an efficient method of image display when using CJ215, based upon their selection of a non-tumor background reference point. The entire process was completed using ImageJ with the chosen region recorded, the mean and SD of the ROI calculated, followed by respective framewise and pixelwise correction according to widely established CNR calculations, Eq.1. The resulting image is then displayed as a contrast-based image expressed in CNR (dB), dubbed “contrast mode”, Fig 6 A. SWIRFI and CJ215 CNR images can readily provide a surgeon with binary tumor delineation with a 3dB threshold (Rose criterion) found to be effective for all tested tumor lines as seen in Fig 6 B.
Assessment of CJ215 in a metastatic tumor model: The four previously four tumor models are readily and widely used for the assessment of tumor targeting of both fluorescent, radioactive, and treatment agents. However, these models represent a single tumor location near the skin surface, where CJ215 could be readily visualized. One significantly more challenging location and urgent clinical problem is the surgical removal of peritoneal carcinomatosis.54,55 SW1222 tumors are a human derived colorectal carcinoma tumor line which is highly aggressive and spreads throughout the peritoneal space. This heavily invasive and often palliative surgery requires organ removal during surgery whilst the body cavity is inspected via white light investigation for remnant tumor and would benefit significantly from a suitable targeted dye for FGS. This model of metastatic peritoneal spread consisted of SW1222 luciferase expressing patient-derived colorectal cancer cells injected intraperitoneally.55 Once sufficient tumor burden was confirmed using luciferase imaging, animals were administered CJ215 as before. SWIRFI was performed at >900, >1100 and >1300 nm cut offs to assess tumor screening. Tumor visualization of the SW1222 model without prior knowledge of lesion location was initially difficult to distinguish over endogenous colon signal, Fig. 7 A, B, C Supp. 27. As before, necropsy biodistribution was performed with NIRFI at 168 hrs post injection, Fig. 7 D, Supp. 28. We found that tumors were widely dispersed throughout the peritoneum were highly necrotic (essentially dead tissue), with patchy fluorescence seen across identified tumors, Fig. 7 B, C. In all mice, highly fluorescent lesions were seen adjacent to the spleen often engulfing the pancreas (Splenic Tum.), Fig. 7 D and were confirmed to be tumorous via H&E staining, Fig. 7 E, Supp. 29. Highly necrotic tumors still achieved a tumor:muscle ratio of 7.89, Fig. 7 D. Splenic tumors had an improved tumor:muscle ratio of 10.38 and 8.80 over the spleen itself Fig. 7 D, appearing less necrotic (H&E staining, Fig. 7 E), highlighting the ability of CJ215 to delineate organ bound tumor masses and uptake in viable tumors.
Wound monitoring: During hair depilation in the CT26 model slight skin surface damage was inadvertently caused which then proceeded to form scabs and heal over time. We incidentally noticed high levels of CJ215 uptake in these areas, along with tumor uptake, Supp. 30. Over the 168hr imaging period, the wound fluorescence decreased with progressing wound healing but remained unchanged in the tumor. Spurred on by this finding, we assessed the ability of SWIRFI to image CJ215 through a variety of commercially available bandages, Supp. 31. SWIRFI readily detected CJ215 through all tested bandages, the majority of which were opaque in visible light. Encouraged by this observation, we next implemented a controlled wound monitoring experiment where mice underwent a skin incision with a scalpel, followed by absorbable stitch closing, and were intravenously administered CJ215. Hydrogel burn bandages were placed over the wound during imaging as they provided a non-stick barrier at the wound site, are highly scattering, hindering visible light wound inspection, and provided suitable intensity losses and SNR changes in phantom testing, Supp. 31. Mice were imaged at >900, >1100 and >1300 nm with and without a bandage from 2 to 240 hrs (10 days) post-surgery and CJ215 injection, Fig. 8 A, B and Supp. 32-35. The bandage was only placed on mice during imaging and was removed before they awoke to prevent discomfort to mice, inadvertent bandage consumption, or bandage degradation. We found that after a single injection of CJ215 the intensity of dye uptake corresponded with wound healing progression, using CNR as a metric for wound assessment, with >1300 nm providing the best wound CNR, along with the wound being readily delineated through the bandage during the main healing phase, Fig. 8 C. The reduction in signal by the bandage prevented reliable wound delineation at 168 hrs and onwards. Wound CNR was detectible from 1-2 hours post injection, highlighting the quick uptake and targeting to wounds of CJ215. CNR peaked 48 hrs post injection then dropped over time until the wound was no longer detectible i.e., completely healed, with longer exposure times utilized between 72 & 240 hrs post injection to ensure sufficient signal collection. CNR was found to provide a quantitative metric for wound healing assessment in line with visible light inspection of wound healing but, importantly, also useable when removal of wound dressing is detrimental to the patient. To confirm lack of wound uptake once healed, mice then received a second injection of CJ215 and were imaged every 24 hrs from 1 to 48 hrs post injection. As opposed to right after the initial surgery, sufficient contrast could not be detected within the now healed wound region in all mice, with necropsy based biodistribution showing no significant difference in fluorescence intensity in the wound area versus normal skin, Supp. 36. H&E further confirmed the normal physiology of the wound area, Supp. 37.