Improved synthesis of the bifunctional chelator p-SCN-Bn-HOPO

Abstract

The bifunctional ligand p-SCN-Bn-HOPO, which has four 1,2-hydroxypyridinone groups on a spermine backbone with an isothiocyanate linker, has been shown to be an efficient and stable chelator for Zr(iv) and, more importantly, the radioisotope 89Zr for use in radiolabeling antibodies for positron emission tomography (PET) imaging. Previous studies of 89Zr-HOPO-trastuzumab in mice showed low background, good tumor to organ contrast, and very low bone uptake which show p-SCN-Bn-HOPO to be an important next-generation bifunctional chelator for radioimmunoPET imaging with 89Zr. However, the reported synthesis of p-SCN-Bn-HOPO involves nine steps and multiple HPLC purifications with an overall yield of about 1.4%. Herein we report an improved and efficient synthesis of p-SCN-Bn-HOPO in four steps with 14.3% overall yield which will improve its availability for further biological studies and wider application in PET imaging. The new synthetic route also allows variation in linker length and chemistries which may be helpful in modifying in vivo clearance behaviors of future agents.

Figure 1

Chemical structures of the ⁸⁹Zr complexes with bifunctional p-SCN-Bn-HOPO (left) and p-SCN-Bn-DFO (right) ligands.

Scheme 1

Four step synthesis of p-SCN-Bn-HOPO. (a) 2-(4-teritiarybutyl aminophenyl)ethyl 4-methylbenzenesulfonate, K₂CO₃, CH₃CN, reflux, 8–10 h (b) 1-(benzyloxy)-6-oxo-1,6-dihydropyridine-2-carboxylic acid chloride, NEt₃, DMAP, dichloromethane, 0−25 °C, 24 h, (42% over 2 steps); (c) BCl₃ in p-xylene, dichloromethane, −20 °C to RT, overnight, 95%; (d) di-2-pyridyl thiocarbonate, NEt₃, CH₃CN, H₂O, r.t, 1 h, 36%.

Scheme 2

Improvements to the original 9-step synthesis of p-SCN-Bn-HOPO are outlined in red. (a) Ethyl trifluoroacetate, MeOH, −78 °C, 1 h; (b) (BOC)₂O, MeOH, r.t, 18 h; (c) conc. NH₄OH, r.t, 15 h (52% over 3 steps); (d) 4-nitrophenethyl 4-methylbenzenesulfonate, K₂CO₃, CH₃CN, reflux, 12 h, 46%; (e) 4 M HCl in Dioxane, r.t, 2 h; (f) 1-(benzyloxy)-6-oxo-1,6-dihydropyridine-2-carboxylic acid chloride, NEt₃, DMAP, dichloromethane, 0−25 °C, 12 h, 56% (over 2 steps); (g) Raney Ni, H₂, MeOH, 3 h; (h) BCl₃ in p-xylene, dichloromethane, −20 °C to RT, 15 h, 99%; (i) di-2-pyridyl thiocarbonate, NEt₃, CH₃CN, H₂O, r.t, 1 h, 36%.