Abstract The primary treatment for many solid cancers remains surgical resection with negative margins that requires accurate identification of cancer in real-time. Patients undergoing surgical extirpation as part of their primary or salvage treatment have an incidence of involved or close surgical margins that approaches 5-40% on histopathological review, resulting in a significantly worse outcome. Positive margin rates have not significantly changed over the past several decades because surgeons cannot successfully differentiate normal and diseased tissue using conventional evaluation methods. Failure to identify residual disease in real-time during the surgical procedure is not surprising considering that the surgeon must rely on non-specific visual changes and manual palpation of subtle irregularities to guide successful excision. The most common method of intraoperative margin control remains frozen section analysis, however this technique is time intensive and can sample only a small fraction of the wound bed. To address the need for intraoperative cancer identification, conventional anatomical imaging modalities such as MRI and CT have been adopted for use in the operating room. Unfortunately, these are neither real-time nor tumor specific and do not allow a surgical field of view. The field of targeted optical molecular imaging has progressed from predominantly preclinical (animal) in vivo studies towards human clinical applications. Various tumor-targeting strategies are employed in parallel to the development of imaging systems and companion fluorescent tracers. Tumor targeting methodologies include blood pool agents (indocyanine green), small peptides like ratiometric activatable penetrating peptides, folate receptor targeting, RGD-based probes, smart activatable agents that can be activated by enzymes such as matrix metalloproteinase and those based on tumor-specific receptor targeting (i.e. immuno-imaging) using (therapeutic) antibodies or smaller fragments like affibodies or nanobodies. Despite the extensive number of probes developed for in vivo use, only a handful of agents have been successfully translated for intraoperative clinical application to assess surgical margins. Cancer-specific navigation has been successfully introduced in human glioma surgery with improvement in outcomes, but this strategy lacks specificity and applicability to other cancer types. The use of 5-ALA for glioma surgery has completed phase 3 clinical trials where it was shown to improve oncologic and functional outcomes. Although 5-ALA imaging has significant limitations for application outside of glioma surgery (dependent on a high tumor metabolic rates, possesses wavelengths with suboptimal tissue penetration), its approval in Europe and subsequent studies have demonstrated that fluorescence-based surgical navigation can be successfully applied in surgical resection. The only other in human study in optical imaging was administration of folate-fluorescein agent, which was shown to identify metastatic peritoneal disease in three out of ten patients tested. Tumor detection was limited to patients with peritoneal metastasis that had high folate receptor expression. Adapting therapeutic antibodies for intraoperative cancer imaging leverages the known antibody safety profile to facilitate the clinical translation of the technique for surgical navigation in oncology. There have been many preclinical studies using antibodies and the first clinical studies are currently underway to assess feasibility in patients with breast cancer (Clinicaltrials.gov: NCT01508572) and colorectal cancer (Clinicaltrials.gov: NCT01972373) using fluorescently labelled bevacizumab, and head and neck cancer (Clinicaltrials.gov: NCT01987375) using cetuximab. The clinical trials have utilized both the microdosing methodology and a more conventional dose-escalation design. The advantage of repurposing therapeutic antibodies for imaging is that the pharmacokinetic profile, biodistribution, side-effects and potential toxicity of FDA-approved antibodies are generally well-known. Moreover, the dosing of the antibodies as imaging agents is less than therapeutic levels, especially when using the microdose regimen. This makes the antibody-based approach ideal for pioneering the use of targeted imaging agents in the clinic. An additional advantage is their long half-live (days to weeks) due to the size of the protein (∼150kD), which may lead to a higher cancer specific uptake compared to smaller particles like nanobodies or peptide fragments, whose relatively fast clearance hinders bioavailability of the targeting agent. Currently, no human data exist on which moiety will emerge as the superior imaging strategy. Over 90% of head and neck tumors are known to overexpress EGFR and therefore the fluorescently labeled cetuximab was conducted using anti-EGFR antibody. IRDye800 was used for optical labeling because previous rodent studies show a lack of toxicity, the dye is manufactured under conditions suitable for human use, and preclinical studies in non-human primates comparing cetuximab-IRDye800 to cetuximab alone showed no clinically significant toxicities. We evaluated escalating doses of cetuximab-IRDye800 in a phase I clinical trial to localize microscopic fragments of cancer during operative and pathological tumor assessment. Data from this study will be reported. Citation Format: Eben L. Rosenthal, Esther de Boer, Kurt Zinn, Go van Dam, Jason Warram. Visualizing cancer: Surgical navigation using targeted fluorescent imaging agents. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr SY36-02. doi:10.1158/1538-7445.AM2015-SY36-02
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