Synthesis of the Novel AT1 Receptor Tracer [18F]Fluoropyridine–Candesartan via Click Chemistry

摘要

A novel 7-((4-(3-((2-[~(18)F]fluoropyridin-3-yl)oxy)propyl)-1H -1,2,3-triazol-1-yl)methyl)-1H -benzo[d ]imidazole derivative of the angiotensin II type-1 receptor (AT_(1)R) blocker candesartan, [~(18)F]fluoropyridine–candesartan, was synthesized via the copper-catalyzed azide–alkyne cycloaddition click reaction between 2-[~(18)F]fluoro-3-(pent-4-yn-1-yloxy)pyridine ([~(18)F]FPyKYNE) and the tetrazole-protected azido-candesartan derivative, followed by acid deprotection. This three-step, two-pot, and two-step purification synthesis was done within 2 h. The use of tris[(1-hydroxypropyl-1H -1,2,3-triazol-4-yl)methyl]amine (THPTA) as a Cu(I) stabilizing agent increased the overall radiochemical yield by 4-fold (10 ± 2%, n = 13) compared to the reaction without THPTA (2.4 ± 0.2%, n = 3; decay-corrected from ~(18)F produced at the end-of-beam). Complete separation of [~(18)F]FPyKYNE from its nitro precursor and [~(18)F]fluoropyridine–candesartan from the deprotected azido-candesartan allowed for high molar activities (>380 GBq/μmol) of the tracer. The use of 0.1% trifluoroacetic acid in water for reformulation and the addition of sodium ascorbate to the final formulation (1.6 ± 0.2 GBq/mL, n = 3) prevented tracer radiolysis with >97% radiochemical purity for a period of up to 10 h after the end-of-synthesis. A significant reduction in the uptake (86 ± 3%, n = 8) of the tracer was observed ex vivo in rats (at 20 min postinjection) in the AT_(1)R-rich kidney cortex following pretreatment with saturating doses of the AT_(1)R antagonist candesartan or losartan. This specific binding to AT_(1)R was confirmed in vitro in the rat renal cortex (autoradiography) by a reduction of 26 ± 5% (n = 12) with losartan coincubation (10 μM). These favorable binding properties support further studies to assess the potential of [~(18)F]fluoropyridine–candesartan as a tracer for the positron emission tomography imaging of renal AT_(1)R.