Page 175 - Fluorescence-guided cancer surgery

Clinical translation of OTL38 in ovarian cancer 173
Although epithelial ovarian cancer cells overexpress FRα, this receptor is also expressed—albeit it to a lesser extent—at the apical membrane of various non-cancerous epithelial cells. During surgery, we noted mild, homogenous uorescence of the uterus and the fallopian tubes, and biopsy revealed FRα expression in these non-malignant tissues, consistent with previous reports17;26. The uorescence signal in these tissues was homogenous and was clearly distinguishable from the uorescence measured in the tumor deposits.
In the majority of our patients, we detected brightly uorescent lymph nodes, and only a small number of these lymph nodes actually contained ovarian cancer metastases. The uorescence measured in the non-cancerous lymph nodes was likely due to OTL38 binding to activated macrophages, which express FRβ27-30. On the other hand, the formation of OTL38 aggregates and nonspeci c uptake by lymph nodes is unlikely, as dissolving the agent at 7.5 μM in 5% dextrose did not lead to the formation of measurable aggregates. Indeed, given that only three of the 12 patients received the highest dose (0.05 mg/kg), corresponding to a molarity slightly higher than 7.5 μM (9.5-11.8 μM OTL38), the presence of aggregates in the infusion solution is highly unlikely. In addition, to further minimize the likelihood of aggregate formation in the circulation, the solution was infused for 60 minutes. Although this apparent false positive uorescence in the lymph nodes could be considered a drawback of this imaging agent, activated macrophages may actually be tumor- associated macrophages that play a role in preparing the tumor environment for metastasis31-33. This notion is supported by our nding that FRβ-expressing macrophages were also found in primary tumors as well as in lymph nodes that did contain metastasized tumor cells (Figure S7, online available). Until the precise role of FRβ-expressing macrophages in the lymph nodes is determined, lymph nodes should be resected solely on the basis of standard clinical assessments.
The use of uorescent light in the near-infrared spectrum allowed us to detect lesions beneath the tissue surface, which is a major improvement over agents that use light outside the NIR spectrum23;34;35. For example, in our study malignant lesions were visible up to ~1 cm below the tissue surface. In addition, most biological tissues have extremely low auto uorescence when excited by light in the NIR spectrum36;37. OTL38 also has several advantageous pharmacokinetic properties, including long residence time in the tumor and relatively rapid clearance from plasma; these properties provide the surgeon