Optical imaging with molecularly targeted probes has numerous advantages compared to other detection methods for evaluation of potential residual disease. The approach focally highlights disease, lacks ionizing radiation, and has a low barrier to incorporation in the operating room. Near infrared protease imaging in particular offers a number of distinct advantages in the detection of ovarian cancer peritoneal metastases, by targeting an enzyme class widely present across most ovarian cancers[12
]. In this study, we have conducted a direct comparison between conventional white-light imaging and NIR imaging with a protease sensitive imaging probe for ovarian tumor foci identification, confirmed with histological validation. We have demonstrated via a comprehensive ex vivo survey of abdominal and pelvic organs excised from a peritoneal carcinomatosis animal model that areas of metastatic disease are vividly fluorescent in the NIR following administration of ProSense750. We have also evaluated the tumor to background signal intensity ratios for WL and NIR imaging for a number of explanted tissues and have shown a marked improvement with NIR imaging over the near 1:1 ratio for conventional WL methods for all tissues examined.
Beyond improving the conspicuity of metastatic lesions that are otherwise visible by WL imaging, NIR imaging with the use of a protease sensitive probe has the ability to identify cancer foci that were not apparent under WL investigation. The majority of these foci were missed by WL imaging because of their small size and because of their visual and textural similarity to the surrounding parenchyma; a number were missed as well because they were deep to normal tissue. The discernment of such lesions is of major clinical relevance because though they are small, they contribute to the overall residual tumor burden; moreover, they can be readily removed by techniques such as argon laser coagulation, as well as other approaches currently practiced during cytoreductive surgery to remove tumor studding [16
The results of our ex vivo imaging survey of abdominal and pelvic organs for metastases using WL and NIR imaging revealed sensitivity and specificity values of 69% and 96%, respectively, for WL and 100% and 88%, respectively, for NIR. NIR imaging is acquired simultaneously with WL imaging and the two provide complementary information. In addition to the protease map created by the NIR imaging, one can use standard anatomic cues for additional guidance. When an area of abnormal anatomy is detected by WL imaging, the area most likely represents cancer. However, WL imaging alone missed a significant number of tumor foci, leading to a less than ideal sensitivity; on the other hand, the sensitivity of NIR imaging for tumor foci was 100%. Sensitivity is the pertinent statistic during cytoreductive surgery: it is far more important and beneficial to the patient to be able to remove all tumor along with a few benign nodules than to conservatively remove only those lesions that are certainly malignant and risk leaving tumor within the abdomen. Moreover, the false positives by NIR imaging were only in reactive lymph nodes, and not other foci within the peritoneum.
This study was designed in a manner to directly evaluate the clinical relevancy of optical molecular imaging in ovarian cancer by comparing in a side-by-side fashion the current standard of care WL imaging with NIR imaging using ProSense750. The results of the ex vivo imaging experiments indicate that this imaging modality is readily applicable to and a significant improvement upon the conventional clinical approach. However, a barrier to entry for optical imaging into the operating room is the dependence of fluorescence signal intensity upon the distance between the target tissue and the endoscope or detection camera. During a surgical procedure, as the surgeon manipulates instruments and the organs under investigation move, this distance is in constant flux. It is therefore difficult to determine in such a dynamic environment what the “true” fluorescence value of a particular area of tissue is, a drawback that constrains the utility of optical imaging in an intraoperative setting. We have recently designed and built a real-time optical imaging system that corrects for NIR fluorescence distance dependence [14
]. We have shown in this study that the system, when applied to the imaging of focal peritoneal cancer following the administration of ProSense750, is able to quantitate NIR signal regardless of positional changes in the imaging catheter. These data suggest that the correction algorithm implemented by this system overcomes a major hurdle impeding the translatability of optical imaging for intraoperative applications, especially with respect to minimally invasive approaches.
We envision the role of optical molecular imaging in ovarian cancer to be in the improved detection of metastatic disease in an intraoperative setting. We see the concept of “gross residual disease” to be a moving target, and that with better detection techniques, tumor will be better visualized and thus more thoroughly eradicated from the abdomen. An analogy can be drawn to the realm of colorectal cancer, where miss rates for polyp detection have been estimated to be as high as 20%[17
], based on tandem colonoscopy studies; an improved method to detect these tumors, such as with molecular optical imaging, would significantly improve upon our ability to visualize and remove the lesions.
Another possible application for this technology is in the augmentation of second-look laparoscopy. The addition of optical molecular imaging has the potential to significantly increase the utility of surgical re-exploration in ovarian cancer. Fluorescence-based protease imaging would enhance the ability to detect residual or recurrent disease, and the associated improvement in tumor debulking may positively impact long-term outcomes. Moreover, the quantification of tumor protease activity will strengthen the evaluation of response to chemotherapy: a decrease in protease activity in the second-look procedure compared to the initial cytoreduction may help in assessing the efficacy of the chemotherapy regimen and in the planning of subsequent treatment cycles. Finally, quantification of tumor protease activity may additionally provide further prognostic information by informing upon the malignant potential of the visualized tumor.
The methodology presented here advances the possibility of incorporation of additional imaging practices to improve the detection and management of ovarian cancer. Optical imaging with protease-activatable NIR probes, combined with the development of tools that allow for the intravital quantitation of fluorescent signal may yield improved outcomes through decreased post operative tumor burden as a result of enhanced visualization of residual disease. The approach represents an important step towards the integration of molecular optical imaging in the operating room. Furthermore, the molecular beacon effect of the protease activatable probes has the potential to improve upon the diagnostic and interventional utility of second-look laparoscopies when monitoring disease regression or recurrence.