Abstract 4335: Imaging of cancer stem cells with radiolabeled Hedgehog

Abstract

Abstract Recent studies have shown that the majority of early disseminated cancer cells detected in the bone marrow of breast cancer patients have a putative breast cancer “stem cell-like” phenotype. Moreover, activation of stem cell pathways in cancer cells has also been associated with resistance to radiation and chemotherapy. Early detection of these micrometastatic “stem cell-like” cancer cells could allow for earlier commencement of aggressive treatment to minimize relapse and disease spread and to improve therapeutic outcome. However, there is currently no stem cell targeted method to detect this population in vivo. New imaging agents are needed for real-time, non-invasive detection of stem cell pathway activation in micrometastatic disease in breast cancer. The stem cell pathway Hedgehog (HH) is known to regulate cell cycle entry and proliferation of embryonic neural and mammary progenitor cells as well as self-renewal of malignant mammary stem cells. Furthermore, the hedgehog receptor PTCH is overexpressed on cancer stem cells compared to normal mammary stem cells. Therefore, we sought to radiolabel the natural hedgehog ligand, sonic (SHH) with the positron emitter Gallium 68 (68Ga) for detection of cancer stem cells by positron emission tomography (PET). Recombinant SHH ligand was conjugated to the bifunctional chelator DOTA. DOTA-SHH was obtained with a total yield of 45%-60% as assessed by HPLC. DOTA-SHH was further characterized using MALDI-MS, ESI-MS with purity > 90%. The conjugate was radiolabeled with 68Ga, with radiochemical purity of >98% and specific activity of 4.3E+05 MBq/gram (theoretical specific activity = 9.18E+09 MBq/gram) as assessed by ITLC and radioHPLC. 68Ga-DOTA-SHH maintains high receptor binding in breast cancer cells and cellular uptake was correlated with PTCH receptor expression levels. Addition of 100-fold excess cold SHH reduced cellular uptake by approximately 90%. PTCH receptor expression is increased in MCF-7 stem cell-enriching culture conditions (mammospheres) and cellular uptake of 68Ga-DOTA-SHH was approximately 12-fold higher in mammosphere MCF-7 cultures compared to the total cell population. These findings demonstrate that chemical modification and radiolabeling of SHH ligand is feasible. Our studies suggest that 68Ga-DOTA-SHH has potential for molecular imaging of breast cancer by PET. Additionally, our studies suggest that 68Ga-DOTA-SHH can potentially identify aggressive tumors that may be rich in cancer stem cells. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4335.