Novel Generation of FAP Inhibitor-Based Homodimers for Improved Application in Radiotheranostics

SIMPLE SUMMARY: Radiopharmaceuticals targeting the fibroblast activation protein alpha (FAP) can be used in many different cancer types since FAP is highly expressed in the tumor microenvironment of almost all epithelial cancers. Monomeric radiotracers have shown great potential in molecular imaging (diagnosis), but the tumor retention time is relatively short (few hours). For effective radioligand therapy (RLT), the biological half-life of the radiotracer should ideally match the physical half-life of the important therapeutic radionuclides (177)Lu and (225)Ac (6.7 and 9.9 days). The tumor retention was improved with the FAPi homodimer DOTAGA.(SA.FAPi)(2). In terms of optimizing, the new FAPi homodimers DO3A.Glu.(FAPi)(2) and DOTAGA.Glu.(FAPi)(2).were synthesized. DOTAGA.Glu.(FAPi)(2) showed superior radiolabeling properties (including successful (225)Ac-labeling, higher hydrophilicity), in vitro affinity and selectivity compared to DOTAGA.(SA.FAPi)(2). In addition, significantly reduced uptake in the critical organs (liver, colon) compared to [(177)Lu]Lu-DOTAGA.(SA.FAPi)(2) was observed with [(177)Lu]Lu-DOTAGA.Glu.(FAPi)(2) in a first patient study (medullary thyroid cancer) while maintaining high and prolonged tumor uptake. ABSTRACT: Radiopharmaceuticals based on the highly potent FAP inhibitor (FAPi) UAMC-1110 have shown great potential in molecular imaging, but the short tumor retention time of the monomers do not match the physical half-lives of the important therapeutic radionuclides (177)Lu and (225)Ac. This was improved with the dimer DOTAGA.(SA.FAPi)(2), but pharmacological and radiolabeling properties still need optimization. Therefore, the novel FAPi homodimers DO3A.Glu.(FAPi)(2) and DOTAGA.Glu.(FAPi)(2). were synthesized and quantitatively radiolabeled with (68)Ga, (90)Y, (177)Lu and (225)Ac. The radiolabeled complexes showed high hydrophilicity and were generally stable in human serum (HS) and phosphate-buffered saline (PBS) at 37 °C over two half-lives, except for [(225)Ac]Ac-DOTAGA.Glu.(FAPi)(2) in PBS. In vitro affinity studies resulted in subnanomolar IC(50) values for FAP and high selectivity for FAP over the related proteases PREP and DPP4 for both compounds as well as for [(nat)Lu]Lu-DOTAGA.Glu.(FAPi)(2). In a first proof-of-principle patient study (medullary thyroid cancer), [(177)Lu]Lu-DOTAGA.Glu.(FAPi)(2) was compared to [(177)Lu]Lu-DOTAGA.(SA.FAPi)(2). High uptake and long tumor retention was observed in both cases, but [(177)Lu]Lu-DOTAGA.Glu.(FAPi)(2) significantly reduces uptake in non-target and critical organs (liver, colon). Overall, the novel FAPi homodimer DOTAGA.Glu.(FAPi)(2) showed improved radiolabeling in vitro and pharmacological properties in vivo compared to DOTAGA.(SA.FAPi)(2). [(177)Lu]Lu-DOTAGA.Glu.(FAPi)(2) and [(225)Ac]Ac-DOTAGA.Glu.(FAPi)(2) appear promising for translational application in patients.