New Imaging Modality of COVID-19 Pneumonia Developed on the Basis of Alzheimer's Disease Research

Abstract

Viral pneumonia caused by highly infectious SARS-CoV-2 poses a higher risk to older people and those who have underlying health conditions, including Alzheimer's disease. In this work we present newly designed tacrine-based radioconjugates with physicochemical and biological properties that are crucial for the potential application as diagnostic radiopharmaceuticals. A set of ten tacrine derivatives was synthesized, labelled with gallium-68 and fully characterized in the context of their physicochemical properties. Based on these results, the final two most promising radioconjugates, [⁶⁸Ga]Ga-NODAGA-Bn-NH(CH₂)₉Tac and [⁶⁸Ga]Ga-THP-NH(CH₂)₉Tac, were selected for biodistribution studies. The latter compound was proven to be a good inhibitor of cholinesterases with significant affinity toward the lungs, according to the biodistribution studies. On the basis of molecular modelling combined with in vitro studies, we unraveled which structural properties of the developed tacrine derivatives are crucial for high affinity toward acetylcholinesterase, whose increased levels in lung tissues in the course of coronavirus disease indicate the onset of pneumonia. The radiopharmaceutical [⁶⁸Ga]Ga-THP-NH(CH₂)₉Tac was ultimately selected due to its increased accuracy and improved sensitivity in PET imaging of lung tissue with high levels of acetylcholinesterase, and it may become a novel potential diagnostic modality for the determination of lung perfusion, including in inflammation after COVID-19.

Keywords: COVID-19; biodistribution studies; gallium-68; molecular docking; radiopharmaceuticals; tacrine.

Scheme 1

The coupling reaction of chelator with NH₂(CH₂)₉Tac scaffold.

Figure 1

Chemical structures of the developed and investigated radioconjugates: (A)—[⁶⁸Ga]Ga-NODAGA-NH(CH₂)₉Tac, (B)—[⁶⁸Ga]Ga-NODAGA-Bn-NH(CH₂)₉Tac, (C)—[⁶⁸Ga]Ga-DOTAGA-Bn-NH(CH₂)₉Tac, (D)—[⁶⁸Ga]Ga-DTPA-CHX-NH(CH₂)₉Tac, (E)—[⁶⁸Ga]Ga-THP-NH(CH₂)₉Tac.

Scheme 2

Scheme of labelling reaction of chelator-NH(CH₂)₉Tac conjugates.

Figure 2

HPLC chromatograms of reaction mixture of the obtained radioconjugates and their reference compounds.

Figure 3

The HPLC radiochromatograms of [⁶⁸Ga]Ga-NODAGA-Bn-NH(CH₂)₉Tac and [⁶⁸Ga]Ga-THP-NH(CH₂)₉Tac radioconjugates recorded after 5 h of incubation at 37 °C in human serum.

Figure 4

Biodistribution data of [⁶⁸Ga]Ga-NODAGA-Bn-NH(CH₂)₉Tac in rats after different time p.i.

Figure 5

Biodistribution data of [⁶⁸Ga]Ga-THP-NH(CH₂)₉Tac in rats after different time p.i.

Figure 6

Results of [⁶⁸Ga]Ga-NODAGA-Bn-NH(CH₂)₉Tac and [⁶⁸Ga]Ga-THP-NH(CH₂)₉Tac pharmacokinetics in blood samples determined by ex vivo radioactivity measurements, with effective (real) half-lives of radioconjugates (15 min and 37 min, respectively) determined from the charts. To each plot an initial point of the coordinate system was added representing an absence of radioconjugate at 0 min time point.

Figure 7

Three-dimensional structures of Ga-THP-NH(CH₂)₉Tac (top) and Ga-NODAGA-Bn-NH(CH₂)₉Tac (bottom) compounds, calculated at the DFT level of theory. Note, that tacrine and the nonamethylene chain are identical for both compounds, while the linkers, i.e., thiourea-phenyl-thiourea in Ga-THP-NH(CH₂)₉Tac and thiourea-phenyl in Ga-NODAGA-Bn-NH(CH₂)₉Tac, as well as chelators, i.e., THP in Ga-THP-NH(CH₂)₉Tac and NODAGA in Ga-NODAGA-Bn-NH(CH₂)₉Tac, are different.

Figure 8

The side section of the hAChE’s binding pocket is docked with Ga-THP-NH(CH₂)₉Tac (top), Ga-NODAGA-Bn-NH(CH₂)₉Tac (middle), and the template ligand ((RS)-tacrine(10)-hupyridone) inhibitor (bottom), from the superimposed PDB id: 1ZGC structure. The key aromatic AChE’s residues involved in the pi-stacking interactions with ligands are in balls-and-sticks representation.

Figure 8

The side section of the hAChE’s binding pocket is docked with Ga-THP-NH(CH₂)₉Tac (top), Ga-NODAGA-Bn-NH(CH₂)₉Tac (middle), and the template ligand ((RS)-tacrine(10)-hupyridone) inhibitor (bottom), from the superimposed PDB id: 1ZGC structure. The key aromatic AChE’s residues involved in the pi-stacking interactions with ligands are in balls-and-sticks representation.