Dra. Rosana Chehín (Dir.); Machín, M B1; Sandobal, J2; Messina, G2; Casale, G2; Ferrario, J3; Bastianello, M4; Varela, O5; Avila, C L1; Chehín, R1. 1 Instituto de Investigación en Medicina Molecular y Celular Aplicada (IMMCA), – CONICET-UNT-SIPROSA. 2 Laboratorio Bacon.
3 Instituto de Investigaciones Farmacológicas (ININFA, UBA-CONICET). 4 CEMIC (Hospital Universitario- CONICET). 5 Centro de Investigación de Hidratos de Carbono (CIHIDECAR, UBA-CONICET)

The results of several studies have shown that the evolution and prognosis of Parkinson’s disease (PD) correlates with α-synuclein (AS) aggregation in brain. This information indicates that to monitor progression of the patient with PD by positron emission tomography (PET), a specific in vivo neuroimaging marker would be needed to label the α-synuclein fibers. However, nowadays, no PET probes are available for selective detection of pathological α-synuclein in this disorder.
Objective: The aim of this study was to evaluate novel chemical entities that could have high affinity and selectivity for AS aggregates and which could serve as compounds for PET radiotracers development. We have previously reported that doxycycline can bind α-synuclein aggregates but not monomeric α-synuclein. For this reason, we propose the doxycycline and novel synthetic analogues as candidates for α-synuclein imaging PET radiotracers.
Methods: Doxycycline analogues were synthesized chemically from doxycycline, called DA-3, DA-4 and DA-5. The binding affinity of these compounds to α-synuclein fibrils was determined by competition binding assays using thioflavin T dye. Ki values were calculated from EC50 values using the equation Ki = EC50/ (1 + [radioligand]/ Kd). Equations were fitted to the experimental data by nonlinear regression.
Results: In vitro binding assays demonstrated that these compounds have affinity for insoluble AS filaments. DA-4 and DA-5 show more affinity than doxycycline, in contrast with DA-3.
Conclusions: These results suggest that DA-4 and DA-5 would be promising candidates as PET α-synuclein imaging radiotracers. The data described here could provide valuable information for the design of new doxycycline probes which bind preferentially to α-synuclein fibrils. Noninvasive monitoring of α-synuclein protein aggregates in the living brain will provide information regarding α-synuclein pathophysiology. This information will be potentially useful in the study of the early diagnosis and prognosis of patients with PD.