Accumulation of α-synuclein (α-syn) fibrils in Lewy bodies and Lewy neurites is the pathological hallmark of Parkinson disease (PD). Ligands that bind α-syn fibrils could be utilized as imaging agents to improve the diagnosis of PD and to monitor disease progression. However, ligands for α-syn fibrils in PD brain tissue have not been previously identified and the feasibility of quantifying α-syn fibrils in brain tissue is unknown. We report the identification of the 125I-labeled α-syn radioligand SIL23. [125I]SIL23 binds α-syn fibrils in postmortem brain tissue from PD patients as well as an α-syn transgenic mouse model for PD. The density of SIL23 binding sites correlates with the level of fibrillar α-syn in PD brain tissue, and [125I]SIL23 binding site densities in brain tissue are sufficiently high to enable in vivo imaging with high affinity ligands. These results identify a SIL23 binding site on α-syn fibrils that is a feasible target for development of an α-syn imaging agent. The affinity of SIL23 for α-syn and its selectivity for α-syn versus Aβ and tau fibrils is not optimal for imaging fibrillar α-syn in vivo, but we show that SIL23 competitive binding assays can be used to screen additional ligands for suitable affinity and selectivity, which will accelerate the development of an α-syn imaging agent for PD.

Nine fluorine-containing vesicular acetylcholine transporter (VAChT) inhibitors were synthesized and screened as potential PET tracers for imaging the VAChT. Compound 18a was one of the most promising carbonyl-containing benzovesamicol analogues; the minus enantiomer, (-)-18a displayed high potency (VAChT Ki = 0.59 ± 0.06 nM) and high selectivity for VAChT versus receptors (> 10,000-fold). The radiosynthesis of (-)-[18F]18a was accomplished by a two-step procedure with 30 – 40% radiochemical yield. Preliminary biodistribution studies of (-)-[18F]18a in adult male Sprague–Dawley rats at 5, 30, 60 and 120 min post-injection (p.i.) were promising. The total brain uptake of (-)-[18F]18a was 0.684 ID%/g at 5 min p.i. and by 120 min p.i. slowly washed out to 0.409 %ID/g.; evaluation of regional brain uptake showed stable levels of ~0.800 %ID/g from 5 to 120 min p.i in the VAChT-enriched striatal tissue of rats, indicating the tracer had crossed the blood brain barrier and was retained in the striatum. Subsequent microPET brain imaging studies of (-)-[18F]18a in nonhuman primates (NHPs) showed high striatal accumulation in the NHP brain; the standardized uptake value (SUV) for striatum reached a maximum value of 5.1 at 15 min p.i. The time-activity curve for the target striatal region displayed a slow and gradual decreasing trend 15 min after injection, while clearance of the radioactivity from the cerebellar reference region was much more rapid. Pretreatment of NHPs with 0.25 mg/kg of the VAChT inhibitor (-)-vesamicol resulted in a ~90% decrease of striatal uptake compared to baseline studies. HPLC metabolite analysis of NHP plasma revealed that (-)-[18F]18a had a good in vivo stability. Together, these preliminary results suggest (-)-[18F]18a is a promising PET tracer candidate for imaging VAChT in the brain of living subjects.

Accumulation of misfolded α-synuclein in Lewy bodies and Lewy neurites is the pathological hallmark of Parkinson’s Disease (PD). To identify ligands having high binding potency toward aggregated α-synuclein, we synthesized a series of phenothiazine derivatives and assessed their binding affinity to recombinant α-synuclein fibrils using a fluorescent thioflavin T competition assay. Among 16 new analogues, the in vitro data suggest that compound 11b has high affinity to α-synuclein fibrils (Ki = 32.10 ± 1.25 nM) and compounds 11d, 16a and 16b have moderate affinity to α-synuclein fibrils (Ki ≈ 50 to 100 nM). Further optimization of the structure of these analogues may yield compounds with high affinity and selectivity for aggregated α-synuclein.

Angiogenesis is a dynamic process fundamental to the development of solid tumors. Epidermal growth factor-like domain 7 (EGFL7) is a protein whose expression is restricted to endothelial cells undergoing active remodeling that has emerged as a key mediator of this process. EGFL7 expression is associated with poor outcome in several cancers, making it a promising target for imaging or therapeutic strategies. Here, EGFL7 is explored as a molecular target for active neovascularization. Using a combinatorial peptide screening approach, we describe the discovery and characterization of a novel high affinity EGFL7-binding peptide, E7p72, that specifically targets human endothelial cells. Viral nanoparticles decorated with E7p72 peptides specifically target tumor-associated neovasculature with high specificity as assessed by intravital imaging. This work highlights the value of EGFL7 as a target for angiogenic vessels and opens the door for novel targeted therapeutic approaches.