Peptidic heterodimer-based radiotracer targeting fibroblast activation protein and integrin αvβ3

Abstract

Purpose: Several studies have demonstrated the advantages of heterodimers over their corresponding monomers due to the multivalency effect. This effect leads to an increased number of effective targeted receptors and, consequently, improved tumor uptake. Fibroblast activation protein (FAP) and integrin α_vβ₃ are found to be overexpressed in different components of the tumor microenvironment. In our pursuit of enhancing tumor uptake and retention, we designed and developed a novel peptidic heterodimer that synergistically targets both FAP and integrin α_vβ₃.

Methods: FAP-RGD was synthesized from FAP-2286 and c(RGDfK) through a multi-step organic synthesis. The dual receptor binding property of ⁶⁸Ga-FAP-RGD was investigated by cell uptake and competitive binding assays. Preclinical pharmacokinetics were determined in HT1080-FAP and U87MG tumor models using micro-positron emission tomography/computed tomography (micro-PET/CT) and biodistribution studies. The antitumor efficacy of ¹⁷⁷Lu-FAP-RGD was assessed in U87MG tumor models. The radiation exposure and clinical diagnostic performance of ⁶⁸ Ga-FAP-RGD were evaluated in healthy volunteers and cancer patients.

Results: Bi-specific radiotracer ⁶⁸Ga-FAP-RGD exhibited high binding affinity for both FAP and integrin α_vβ₃. In comparison to ⁶⁸Ga-FAP-2286 and ⁶⁸Ga-RGDfK, ⁶⁸Ga-FAP-RGD displayed enhanced tumor uptake and longer tumor retention time in preclinical models. ¹⁷⁷Lu-FAP-RGD could efficiently suppress the growth of U87MG tumor in vivo when applied at an activity of 18.5 and 29.6 MBq. The effective dose of ⁶⁸Ga-FAP-RGD was 1.06 × 10^-2 mSv/MBq. ⁶⁸Ga-FAP-RGD demonstrated low background activity and stable accumulation in most neoplastic lesions up to 3 h.

Conclusion: Taking the advantages of multivalency effect, the bi-specific radiotracer ⁶⁸Ga-FAP-RGD showed superior tumor uptake and retention compared to its corresponding monomers. Preclinical studies with ⁶⁸Ga- or ¹⁷⁷Lu-labeled FAP-RGD showed favorable image contrast and effective antitumor responses. Despite the excellent performance of ⁶⁸Ga-FAP-RGD in clinical diagnosis, experimental efforts are currently underway to optimize the structure of FAP-RGD to increase its potential for clinical application in endoradiotherapy.

Keywords: Fibroblast activation protein; Heterodimer; Integrin αvβ3; Multivalency effect; PET/CT.