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

A novel 7-((4-(3-((2-[¹⁸F]­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 (AT₁R) blocker candesartan, [¹⁸F]­fluoropyridine–candesartan, was synthesized via the copper-catalyzed azide–alkyne cycloaddition click reaction between 2-[¹⁸F]­fluoro-3-(pent-4-yn-1-yloxy)­pyridine ([¹⁸F]­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 ¹⁸F produced at the end-of-beam). Complete separation of [¹⁸F]­FPyKYNE from its nitro precursor and [¹⁸F]­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 AT₁R-rich kidney cortex following pretreatment with saturating doses of the AT₁R antagonist candesartan or losartan. This specific binding to AT₁R 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 [¹⁸F]­fluoropyridine–candesartan as a tracer for the positron emission tomography imaging of renal AT₁R.