In vivo optical pathology of paclitaxel efficacy on the peritoneal metastatic xenografts of gastric cancer using multiphoton microscopy

e22205 Background: Peritoneal metastasis shows an extremely poor prognosis in patients with gastric cancer. Clinically, the tumor response to chemotherapeutics depends on anatomical location of metastasis. Metastatic tumor xenografts have been shown to be more resistant to chemotherapy than subcutaneous non-metastatic tumor xenografts in preclinical murine model. We have reported a method of in vivo optical pathology using multiphoton microscopy (MPM) in colorectal liver metastatic tumor xenograft model. Aim: We established a method of time-series in vivo optical pathology of peritoneal metastatic xenografts of gastric cancer using MPM. Then, we imaged and evaluated paclitaxel efficacy in the tumor microenvironment with regard to both tumor cell itself and intravascular change in tumor vessels. Methods: Red fluorescent protein (RFP) expressing human gastric cancer cell line (NUGC4) was inoculated into the peritoneal cavity of green fluorescent protein (GFP) expressing nude mice. Paclitaxel (10 mg/kg) was administered three times a week for more than three weeks. Intravital MPM was performed before and after paclitaxel treatment for the exteriorized peritoneal metastatic lesion in the same living mouse. Results: Four to six weeks later, RFP-NUGC4 cells formed macroscopic peritoneal metastases. Red-colored cancer cells and green-colored surrounding stroma with tumor vessels were clearly imaged at the cellular level (in vivooptical pathology). Their cross-sectional images were obtained from the tissue surface to the area of depth of 200 μm (z-stacks imaging). After paclitaxel treatment, tumor cell fragmentation, condensation, swelling and intracellular vacuoles were observed. Within the tumor vessels, platelet aggregation and platelet adhesion to endothelial cells were observed. Conclusions: In vivo optical pathology using MPM provides histopathological information about three-dimensional tissue microarchitecture without tissue shrinkage by fixation and tissue destruction by microtome-sectioning. Our method may become a powerful tool to evaluate the tumor response to new chemotherapeutics on ‘metastatic site’ in preclinical tumor xenograft model.