Chiral DOTA chelators as an improved platform for biomedical imaging and therapy applications

Despite established clinical utilisation, there is an increasing need for safer, more inert gadolinium-based contrast agents, and for chelators that react rapidly with radiometals. Here we report the syntheses of a series of chiral DOTA chelators and their corresponding metal complexes and reveal properties that transcend the parent DOTA compound. We incorporated symmetrical chiral substituents around the tetraaza ring, imparting enhanced rigidity to the DOTA cavity, enabling control over the range of stereoisomers of the lanthanide complexes. The Gd chiral DOTA complexes are shown to be orders of magnitude more inert to Gd release than [GdDOTA]−. These compounds also exhibit very-fast water exchange rates in an optimal range for high field imaging. Radiolabeling studies with (Cu-64/Lu-177) also demonstrate faster labelling properties. These chiral DOTA chelators are alternative general platforms for the development of stable, high relaxivity contrast agents, and for radiometal complexes used for imaging and/or therapy.

The results showed the complexation occurred at room temperature, however, need more metal or higher temperature to get fully complexation.

Supplementary Methods
Synthesis of (2S)-2-(Benzylamino)propan-1-ol. 2 A solution of benzaldehyde (64.2 g, 0.61 mol) and (S)-2amino-1-propanol (50.0 g, 0.67 mol) in dichloromethane (650 ml) and methanol (100 ml) was stirred at room temperature under nitrogen for 16 hours. It was concentrated and the white solid was added with petroleum ether (200 ml), cooled to 0°C and filtered, the filter cake was dried (95 g) and was used to the next step without any further purification. The white solid was dissolved in dichloromethane (650 ml) and methanol (100 ml) and cooled in an ice bath. Powdered sodium borohydride (20.0 g, 0.53 mol) was added in portions and the solution was stirred for 2 hours, water (300 ml) was added and the solution was stirred for overnight. The organic and aqueous phases were separated and the organic phase was washed with brine (200 ml), dried with anhydrous sodium sulphate (50 g), filtered and the filtrate was concentrated under vacuum yielded product (2S)-1-Benzyl-2methylaziridine as a white solid (93 g, 93%). 1

Synthesis of (S)-1-benzyl-2-methylaziridine (1a). 2 Into a soliton of (2S)-2-(Benzylamino)
propan-1-ol (15.0 g) in diethyl ether (150 ml), added with PPh3 (33.3 g), then cooled the reaction mixture to 0 -10 °C, then dropped with DEAD (19.0 g), the resulted mixture was reacted for 16 h at room temperature, then added 500 ml of hexane and the solid was removed by filtration. The filtrate was concentrated and the residue was purified by distillation under vacuum, this resulted in the product as a colourless liquid (12.5 g, yield 93.5%

Synthesis of (S)-2-amino-6-((tert-butoxycarbonyl)amino)hexanoic acid.
To a solution of L-lysine (40 g, 0.27 mol) and NaHCO3 (23 g, 0.27 mmol) in water (600 ml) and acetone (200 ml) was added CuSO4·5H2O (34.2 g, 0.14 mol), after stirring at room temperature for 2.5 hours, another 23 g of NaHCO3 was added, then added (Boc2)O (71 g, 0.33 mol), the reaction mixture was stirring for another 16 hours. The precipitates were collected by filtration, washed with water (500 ml) and ethyl acetate (200 ml). The solid was transferred into another 3 L flask, added with water (600 ml), NaHS (14 g) and NaHCO3 (30 g), then CbzCl (39 g) and THF (300 ml) were added, the mixture was stirring at room temperature for another 16 hours. After that, the pH was adjusted to 4.0 by adding 2 M HCl, ethyl acetate (500 ml) was added, filtrated and the two phases were separated, the aqueous layer was extracted with ethyl acetate (500 ml) again. The combined organic phases were washed one time with water (200 ml). Then dried with anhydrous sodium sulphate, after filtration and concentration, this resulted in 80 g of product and it was used to the next step reaction without any further purification.
Synthesis of (S)-benzyl tert-butyl (6-hydroxyhexane-1,5-diyl)dicarbamate. 3 A solution of (S)-2-amino-6-((tert-butoxycarbonyl)amino)hexanoic acid (40 g) in dry THF (200 ml) was cooled to 0 -10 °C, 1,1'carbonyldiimidazole (CDI) (17.2 g) was added and stirred at this temperature for 1 hour. Then the mixture was transferred into another solution of NaBH4 (8 g) and water (100 ml) carefully (cooled with ice batch first, stirring vigorous), after stirring at room temperature for overnight, the solution was extracted with ethyl acetate (two times, each time 500 ml), combined the organic phases and washed with brine (200 ml), dried with anhydrous sodium sulphate, filtered and concentrated, the residue was purified by column chromatography on silica gel with ethyl acetate and petroleum ether (1:10 to 1:1). This resulted in the product as a colourless oil (25 g, overall yield of 50% from L-lysine) which solidified after several days. 1

Synthesis of (S)-tert-butyl (5-(benzylamino)-6-hydroxyhexyl)carbamate.
A solution of benzaldehyde (8.0 g, 75.4 mmol) and (S)-tert-butyl (5-amino-6-hydroxyhexyl)carbamate (16.0 g, 68.9 mmol) in dichloromethane (208 ml) and methanol (48 ml) was stirred at room temperature under nitrogen for 16 hours. It was concentrated and the residue was pumped with oil pump for two days and washed with petroleum ether for two times (each 200 ml) before the next step reaction. The oil mixture was dissolved in methanol (400 ml) and cooled in an ice bath. Powder sodium borohydride (5.0 g, 0.13 mol) was added in portions and the solution was stirred for 2 hours, water (300 ml) was added and the solution was quenched with 4 M HCl (4 ml), extracted with ethyl acetate (three times, each time 300 ml), combined the organic phases and washed with brine (200 ml), dried with anhydrous sodium sulphate, filtered and the filtrate was concentrated, the residue was purified by column chromatography on silica gel with ethyl acetate and petroleum ether (1:5 to 1:2). This resulted in the product as a colourless oil (13 g, two steps yield 59.1%). 1