Targeting fibroblast activation protein (FAP): next generation PET radiotracers using squaramide coupled bifunctional DOTA and DATA5m chelators

Abstract

Background: Fibroblast activation protein (FAP) is a proline selective serine protease that is overexpressed in tumor stroma and in lesions of many other diseases that are characterized by tissue remodeling. In 2014, a most potent FAP-inhibitor (referred to as UAMC1110) with low nanomolar FAP-affinity and high selectivity toward related enzymes such as prolyl oligopeptidase (PREP) and the dipeptidyl-peptidases (DPPs): DPP4, DPP8/9 and DPP2 were developed. This inhibitor has been adopted recently by other groups to create radiopharmaceuticals by coupling bifunctional chelatorlinker systems. Here, we report squaric acid (SA) containing bifunctional DATA 5m and DOTA chelators based on UAMC1110 as pharmacophor. The novel radiopharmaceuticals DOTA.SA.FAPi and DATA 5m .SA.FAPi with their non-radioactive derivatives were characterized for in vitro inhibitory efficiency to FAP and PREP, respectively and radiochemical investigated with gallium-68. Further, first proof-ofconcept in vivo animal study followed by ex vivo biodistribution were determined with [ 68 Ga]Ga-DOTA.SA.FAPi. Results: [ 68 Ga]Ga-DOTA.SA.FAPi and [ 68 Ga]Ga-DATA 5m .SA.FAPi showed high complexation > 97% radiochemical yields after already 10 min and high stability over a period of 2 h. Affinity to FAP of DOTA.SA.FAPi and DATA 5m .SA.FAPi and its nat Ga and nat Lu-labeled derivatives were excellent resulting in low nanomolar IC 50 values of 0.7-1.4 nM. Additionally, all five compounds showed low affinity for the related protease PREP (high IC 50 with 1.7-8.7 μM). First proof-of-principle in vivo PET-imaging animal studies of the [ 68 Ga]Ga-DOTA.SA.FAPi precursor in a HT-29 human colorectal cancer xenograft mouse model indicated promising results with high accumulation in tumor (SUV mean of 0.75) and low background signal. Ex vivo biodistribution showed highest uptake in tumor (5.2%ID/g) at 60 min post injection with overall low uptake in healthy tissues.

Theranostic Advances in Breast Cancer in Nuclear Medicine

“…Recently, in another clinical trial [ 108 ], another new designed FAPI derivative named DOTA.SA.FAPI ( Figure 4 ) was applied for diagnosis of BC in a 31 years old female patient. This new FAPI is designed as a diagnostic and therapeutic agent after radiolabelling with gallium-68, lutetium-177 ( 177 Lu) or actinium-225 ( 225 Ac) just like FAPI-02 and FAPI-04 [ 109 ]. Interestingly, the results indicated correlations between accumulation of [ 68 Ga]Ga-DOTA.SA.FAPI and [ 18 F]FDG ( Figure 4 ) [ 109 ].…”

“…This new FAPI is designed as a diagnostic and therapeutic agent after radiolabelling with gallium-68, lutetium-177 ( 177 Lu) or actinium-225 ( 225 Ac) just like FAPI-02 and FAPI-04 [ 109 ]. Interestingly, the results indicated correlations between accumulation of [ 68 Ga]Ga-DOTA.SA.FAPI and [ 18 F]FDG ( Figure 4 ) [ 109 ]. An initial patient treatment with [ 177 Lu]Lu-DOTA.SA.FAPI administering 3.2 GBq was performed and post treatment imaging was accomplished 24 h after the dose injection.…”

“…An initial patient treatment with [ 177 Lu]Lu-DOTA.SA.FAPI administering 3.2 GBq was performed and post treatment imaging was accomplished 24 h after the dose injection. Conformity of dose absorption in the lesions of [ 68 Ga]Ga-DOTA.SA.FAPI and [ 177 Lu]Lu-DOTA.SA.FAPI is obvious and depicted in Figure 4 [ 109 ].…”

“… Chemical structure of DOTA.SA.FAPI. PET-Scan of [ 18 F]FDG (left), [ 68 Ga]Ga-DOTA.SA.FAPI (middle) and [ 177 Lu]Lu- DOTA.SA.FAPI (right) of a patient with HER2-positive, (ER and PR)-negative histopathology (Figure adapted from Ballal et al [ 109 ] Creative Commons Attribution 4.0 International License). …”

Development and preclinical evaluation of [68Ga]Ga-Alb-FAPtp-01, a novel tumour-associated fibroblast activation protein tracer for PET imaging of xenograft glioma

“…Therefore, in the current investigation, a prototype peptide-based FAP-speci c PET tracer, [ 68 Ga]Ga-FAPtp, based on the structure of a potent FAP inhibitor (known as UAMC-1110) [20], were designed, characterized and optimized for use as novel FAP-targeted diagnostic agents and potential theranostic agents. In contrast to the original quinoline-based moiety, peptide amide bond core structures with an additional short PEGylating linker were contemporaneously adopted.…”