Retinoblastoma tumor vessel maturation impacts efficacy of vessel targeting in the LH_BETAT_AG mouse model

PURPOSE. The aim of this study was to quantify tumor cell proliferation and growth, analyze tumor blood vessel development, and determine the efficacy of antiangiogenic and angiostatic therapy in targeting mature vessels in retinal tumors of the LH_BETAT_AG mouse model for retinoblastoma. METHODS. LH_BETAT_AG mouse retinas were analyzed at 4, 8, 12, and 16 weeks of age. Tumor burden was analyzed by histology; cell proliferation, vessel density, angiogenesis, and vessel maturation were detected by immunofluorescence. To assess the efficacy of mature vessel targeting, 16-week-old mice were treated with single subconjunctival injections of the selective vascular-targeting drug combretastatin A4 prodrug (CA4P) or anecortave acetate, and eyes were analyzed 1 day and 1 week after injection to determine microvessel density and the number of angiogenic and mature vessels. RESULTS. Increased cell proliferation and angiogenesis were detected in the retinal inner nuclear layer (INL) before morphologic neoplastic changes were evident. As tumor size increased, angiogenesis diminished concomitantly with the appearance of mature vessels. Treatment with CA4P and anecortave acetate resulted in significant reductions in total vessel density. However, neither drug reduced the amount of α-smooth muscle actin (SMA)-positive, mature vessels. CONCLUSIONS. Results of this study provide new insight into the relationship between tumor growth and blood vessel development in the LH_BETAT _AG mouse and establish the framework for defining the selective action of two vessel-targeting drugs against new blood vessels compared with mature blood vessels. These findings suggest a high potential value in targeting the process of angiogenesis in the treatment of children with retinoblastoma.