Development of 18F-labeled estrogen receptor ligands for PET imaging of breast cancer and evaluation of multifunctional FAP-targeted drug conjugates as novel theranostics

Development of 18F-Labeled Estradiol Derivatives Overexpression of the estrogen receptor (ER) in breast tumors can be utilized for tumor diagnosis and therapy. The FDA-approved radiotracer 18FFES, used for positron emission tomography imaging of ER-positive (ER+) metastatic breast cancer, revealed a high liver retention due to the lipophilicity of the steroid hormone core structure. This project focused on the development of four different 18F-labeled estradiol derivatives with varied lipophilicity targeting the ER. Cellular uptake experiments in MCF-7 cells revealed significant influence of lipophilicity on ER-specific cellular uptake of the 18F-labeled estradiol derivatives. Encouraged by the in vitro results, 18F-TA-Glyco-EE (18F-1) was further evaluated in vivo. Biodistribution experiments and PET imaging confirmed that 18F-TA-Glyco-EE (18F-1) showed a fast clearance from non-target tissue. More importantly, after an initial high liver uptake, a fast clearance from the liver was observed. In vitro autoradiography of tumor slices obtained from ER+ (T47D, MCF-7) and ER− (MDA-MB-231) tumor xenografts verified high specific binding of 18F-TA-Glyco-EE (18F-1). Based on these results, 18F-TA-Glyco-EE (18F-1) was identified as a promising candidate for PET imaging of ER+ tumors, with favorable liver clearance. Development of Multifunctional FAPI Conjugates The fibroblast activation protein (FAP) is overexpressed on cancer-associated fibroblasts within the tumor microenvironment as well as on fibroblast-like synoviocytes in arthritic diseases. Therefore, FAP is a valuable target for tumor diagnosis and treatment as well as for targeting rheumatoid arthritis. On theses grounds, the second part of this thesis focused on the development of multifunctional FAP-targeted drug conjugates that can be used as novel theranostics. The multifunctionality included the targeting moiety, a (non-)cleavable linker system, the payload, and a DOTA chelator for radiolabeling. Different payloads (20-(S)-camptothecin (CPT) or an integrin αVβ6-targeting peptide) and various linker were established for the concept of FAP-targeted drug delivery. Concluding from the in vitro and in vivo evaluation of FAPI-CPT and FAPI-αVβ6-peptide conjugates, it was shown that multifunctional, high molecular weight FAP-targeted conjugates maintained FAP binding affinity in vitro. Moreover, FAP-targeting caused specific and prolonged tumor uptake of heterodimeric FAPI-αVβ6 conjugates compared to αVβ6 monomers in vivo in pancreatic tumors. Additionally, tracer accumulation via FAP-targeting was proven in vivo in arthritic mice. Prospectively, the FAP-mediated, selective accumulation in target tissue can be used for combined radiotherapy and FAP-targeted drug delivery in FAP-dependent disease models.