Assessing axitinib-induced differential responses in tumor vascularization and oxygenation with combined optoacoustic angiography and diffuse optical spectroscopy.

Efficacy of antiangiogenic treatments is often linked to the complex interplay between tumor vascularization and oxygenation. Yet their relationship remains difficult to assess in vivo due to limitations of conventional clinical imaging techniques. We used a combination of noninvasive optoacoustic (OA) angiography and diffuse optical spectroscopy (DOS) to investigate the effects of the antiangiogenic therapy on vascular structure and oxygenation in subcutaneous xenograft model of Colo320 colon adenocarcinoma. Axitinib, a tyrosine kinase inhibitor targeting VEGF receptors, was administered into animals at 50 mg/kg, five days per week for four weeks. Raster-scan OA imaging was performed using 532 nm pulsed laser source and a wideband polyvinylidene difluoride (PVDF) detector. DOS measurements were conducted using a fiber-optic-based reflectance system. Immunohistochemical (IHC) analysis for CD31 and the hypoxia marker pimonidazole was used for validation. Axitinib treatment resulted in a thirtyfold reduction in the median tumor volume. OA imaging revealed reductions in volumetric vessel fraction and projected vessel area, while DOS showed a transient increase in blood oxygen saturation. IHC confirmed a decrease in microvessel density post-treatment and indicated larger hypoxic areas in treated tumors compared to controls at the experimental endpoint. The newly introduced approach thus facilitates experimental studies aiming at optimization of antiangiogenic treatment regimens and their subsequent combination with other treatment modalities, such as radiation therapy, where effectiveness may strongly depend on the vascular network condition and tumor oxygenation levels.