Synthesis, Radiolabeling, and In Vitro and In Vivo Characterization of Heterobivalent Peptidic Agents for Bispecific EGFR and Integrin α_vβ₃ Targeting

Abstract

Radiolabeled heterobivalent peptidic ligands (HBPLs) are a highly promising compound class for the sensitive and specific visualization of tumors as they often exhibit superior properties compared to their monospecific counterparts and are able to concomitantly or complementarily address different receptor types. The combination of two receptor-specific agents targeting the epidermal growth factor receptor (EGFR) and the integrin α v β 3 in one HBPL would constitute a synergistic combination of binding motifs as these two receptor types are concurrently overexpressed on several human tumor types and are closely associated with disease progression and metastasis. Here, we designed and synthesized two heterobivalent radioligands consisting of the EGFR-specific peptide GE11 and α v β 3 -specific cyclic RGD peptides, bearing a (1,4,7-triazacyclononane-4,7-diyl)diacetic acid-1-glutaric acid chelator for efficient radiolabeling and linkers of different lengths between both peptides. Both HBPLs were radiolabeled with 68 Ga 3+ in high radiochemical yields, purities of 96−99%, and molar activities of 36−88 GBq/ μmol. [ 68 Ga]Ga-1 and [ 68 Ga]Ga-2 were evaluated for their log D(7.4) and stability toward degradation by human serum peptidases, showing a high hydrophilicity for both agents of −3.07 ± 0.01 and −3.44 ± 0.08 as well as a high stability toward peptidase degradation in human serum with half-lives of 272 and 237 min, respectively. Further on, the in vitro receptor binding profiles of both HBPLs to the target EGF and integrin α v β 3 receptors were assessed on EGFR-positive A431 and α v β 3 -positive U87MG cells. Finally, we investigated the in vivo pharmacokinetics of HBPL [ 68 Ga]Ga-1 by positron emission tomography/computed tomography imaging in A431 tumor-bearing xenograft mice to assess its potential for the receptor-specific visualization of EGFR-and/or α v β 3expressing tumors. In these experiments, [ 68 Ga]Ga-1 demonstrated a tumor uptake of 2.79 ± 1.66% ID/g, being higher than in all other organs and tissues apart from kidneys and blood at 2 h p.i. Receptor blocking studies revealed the observed tumor uptake to be solely mediated by integrin α v β 3 , whereas no contribution of the GE11 peptide sequence to tumor uptake via the EGFR could be determined. Thus, the approach to develop radiolabeled EGFR-and integrin α v β 3 -bispecific HBPLs is in general feasible although another peptide lead structure than GE11 should be used as the basis for the EGFR-specific part of the agents.

Towards Radiolabeled EGFR-Specific Peptides: Alternatives to GE11

“…This assumption is supported by a very recent study where we developed an EGFR- and integrin α v β 3 -bispecific radioligand that showed no EGFR-associated A431 tumor cell uptake and affinity but only integrin-mediated U87MG tumor cell interaction in in vitro studies. A subsequent in vivo PET/CT imaging and ex vivo biodistribution study in A431 tumor-bearing xenograft mice—having been conducted to identify a potential EGFR-specific tumor uptake occurring despite the discouraging in vitro results—confirmed the absent EGFR-specificity of the developed agent not only in vitro, but also in vivo [ 54 ].…”