Targeted, Molecular Europium(III) Probes Enable Luminescence-Guided Surgery and 1 Photon Post-Surgical Luminescence Microscopy of Solid Tumors.

Discrete luminescent lanthanide complexes represent a potential alternative to organic chromophores due to their tunability of optical properties, insensitivity to photobleaching, and large pseudo-Stokes shifts. Previously, we demonstrated that the lack of depth penetration of UV excitation required to sensitize discrete terbium and europium complexes can be overcome using Cherenkov radiation emitted by clinically employed radioisotopes in situ. Here, we show that the second-generation europium complexes [Eu(III)(pcta-PEPA2)] and [Eu(III)(tacn-pic-PEPA2)] (Φ = 57% and 76%, respectively) lower the limit of detection (LoD) to 1 nmol in the presence of 10 µCi of Cherenkov emitting isotopes, 18F and 68Ga. Bifunctionalization provides access to cysteine-linked peptide conjugates with comparable brightness and LoD. The conjugate, [Eu(tacn-(pic-PSMA)-PEPA2)], displays high binding affinity to prostate-specific membrane antigen (PSMA)-expressing PC-3 prostate cancer cells in vitro and can be visualized in the membrane-bound state using confocal microscopy. Biodistribution studies with the [86Y][Y(III)(tacn-(pic-PSMA)-PEPA2)] analogue in a mouse xenograft model were employed to study pharmacokinetics. Systemic administration of the targeted Cherenkov emitter, [68Ga][Ga(III)(PSMA-617)], followed by intratumoral injection or topical application of 20 or 10 nmol [Eu(III)(tacn-(pic-PSMA)-PEPA2)], respectively, in live mice resulted in statistically significant signal enhancement using conventional small animal imaging (620 nm bandpass filter). Optical imaging informed successful tumor resection. Ex vivo imaging of the fixed tumor tissue with 1 and 2 photon excitation further reveals the accumulation of the administered Eu(III) complex in target tissues. This work represents a significant step toward the application of luminescent lanthanide complexes for optical imaging in a clinical setting.

N,N-Alkylation Clarifies the Role of N- and O-Protonated Intermediates in Cyclen-Based 64Cu Radiopharmaceuticals.

Radioisotopes of Cu, such as 64Cu and 67Cu, are alluring targets for imaging (e.g., positron emission tomography, PET) and radiotherapeutic applications. Cyclen-based macrocyclic polyaminocarboxylates are one of the most frequently examined bifunctional chelators in vitro and in vivo, including the FDA-approved 64Cu radiopharmaceutical, Cu(DOTATATE) (Detectnet); however, connections between the structure of plausible reactive intermediates and their stability under physiologically relevant conditions remain to be established. In this study, we share the synthesis of a cyclen-based, N,N-alkylated spirocyclic chelate, H2DO3AC4H8, which serves as a model for N-protonation. Our combined experimental (in vitro and in vivo) and computational studies unravel complex pH-dependent speciation and enable side-by-side comparison of N- and O-protonated species of relevant 64Cu radiopharmaceuticals. Our studies suggest that N-protonated species are not inherently unstable species under physiological conditions and demonstrate the potential of N,N-alkylation as a tool for the rational design of future radiopharmaceuticals.

Validation of a post-radiolabeling bioconjugation strategy for radioactive rare earth complexes with minimal structural footprint.

The nine-coordinate aza-macrocycle DO3Apic-NO2 and its kinetically inert rare earth complexes [M(DO3A-pic-NO2)]- (M = La, Tb, Eu, Lu, Y) can be readily bioconjugated to surface accessible thioles on peptides and proteins with a minimal structural footprint. All complexes express thioconjugation rate constants in the same order of magnitude (k = 0.3 h-1) with the exception of Sc (k = 0.89 h-1). Coupling to peptides and biologics with accessible cysteines also enables post-radiochelation bioconjugation at room temperature to afford injection-ready radiopharmaceuticals as demonstrated by formation of [177Lu][Lu(DO3Apic-NO2)]- and [86Y][Y(DO3Apic-NO2)]-, followed by post-labeling conjugation to a cysteine-functionalized peptide targeting the prostate specific membrane antigen. The 86Y-labeled construct efficiently localizes in target tumors with no significant off-target accumulation as evidenced by positron emission tomography, biodistribution studies and metabolite analysis.

Medicinal inorganic chemistry - challenges, opportunities and guidelines to develop the next generation of radioactive, photoactivated and active site inhibiting metal-based medicines.

Medicinal inorganic chemistry is a burgeoning subfield of medicinal chemistry that focuses on the development of metal-based diagnostic and therapeutic agents. This tutorial review aims to provide an introductory primer, present a timely overview of recent discoveries and identify current challenges and opportunities of the field. Three specific areas of discovery are highlighted herein. The first part focuses on metal-based radiopharmaceuticals for diagnostic and therapeutic purposes and specific design criteria for the development of radiopharmaceuticals that combine fundamental aqueous coordination chemistry with elucidation of pharmacokinetics. The second part describes approaches to photodynamic therapy with metal complexes. Here, photophysical characterization, combined with the challenge of careful control of the chemical behavior and selective biological deposition of transition metals with significant off-target toxicity, is discussed. In the third part, we summarize emerging strategies to modulate enzyme inhibition with coordination chemistry, while also highlighting the utility of the unique properties of metal ions for the characterization of mechanisms of action of these emerging diagnostic and therapeutic agents.

Organometallic small molecule kinase inhibitors - direct incorporation of Re and 99mTc into Opaganib®.

[(η5-Cp)ReI(CO)3] was incorporated into the kinase inhibitor Opaganib®. The resulting bioorganometallic complex showed a similar anti-cancer activity to Opaganib® against PC-3 cancer cells. The IC50 value for the kinase SK2 is 30x higher than that of Opaganib®. The 99mTc homologue was synthesized, completing a matched-pair for molecular theranostics.

Expanding the Cyclopentadienyl Framework: 99mTc/Re Complexes with Orthogonal Functions for Bioconjugation.

A series of bifunctional cyclopentadienes of the type 1,3-EtOCO-HCp-linker-NH2 were synthesized. In this series, the linker length (distance between the amine functionalities and the cyclopentadiene) has been systematically varied (CH2)n (n = 1-3). The corresponding Re complexes [(η5-C5H3RR')Re(CO)3] (R = -COOEt, R' = -linker-NH2) were synthesized and structurally characterized. They exhibit extraordinary stability toward water and air. All bifunctional cyclopentadienes have been labeled with the [99mTc(CO)3]+ moiety. Whereas the reactions with ethylene and propylene linked cyclopentadiene under mild reaction conditions led to the products in high radiochemical purity (>96%) without applying further purification protocols, harsher reaction conditions were required for the synthesis of the methylene-linked cyclopentadiene compound. Masking the amine in the methylene-linked cyclopentadiene by an amide bond bypasses this problem. The very hydrophilic characters of these complexes were assessed by KOW analysis. The reported cyclopentadienes and their complexes offer a robust and versatile platform for (radio)metal incorporation into biologically active lead structures.