Fully automated preparation of n.c.a. 4-[¹⁸F]fluorobenzoic acid and N-succinimidyl 4-[¹⁸F]fluorobenzoate using a Siemens/CTI chemistry process control unit (CPCU)

Abstract

The widely used bifunctional labeling reagent 4-[¹⁸F]fluorobenzoic acid ([¹⁸F]FBA) and its activated form N-succinimidyl 4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB) were prepared using a modified Siemens/CTI chemistry process control unit (CPCU) double vessel [¹⁸F]FDG module. The products were obtained with a radiochemical yield greater than 99% with decay corrected yields of 77±5% (n=4) for [¹⁸F]FBA and 46±5% (n=4) for [¹⁸F]SFB. The presented synthetic procedure is highly reproducible and fully automated.

Introduction

Biomolecules such as proteins, peptides and antibodies labeled with positron emitting radioisotopes are becoming widely used probes for targeted imaging of many physiological and pathological processes (Okarvi, 2001; Wester et al., 1996). The radionuclide ¹⁸F (β⁺, t_1/2=110 min) can be introduced into the molecule using a prosthetic group. Although a variety of prosthetic groups have been developed 4-[¹⁸F]fluorobenzoic acid ([¹⁸F]FBA) is undoubtedly the most frequently used. The [¹⁸F]FBA is relatively easily prepared from 4-(trimethylammoniumtriflate)benzoate precursor in two steps, and can subsequently be attached to the target molecule via activation of the carboxyl functionality. [¹⁸F]FBA can be conjugated to the protected peptides assembled on solid support using various activating reagents, the products are subsequently cleaved and purified (Sutcliffe-Goulden et al., 2000, Sutcliffe-Goulden et al., 2002). Alternatively proteins, antibodies and peptides can be acylated using [¹⁸F]FBA in aqueous solution under mild conditions using for example an active ester N-succinimidyl 4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB) (Chen et al., 2004; Lang and Eckelman, 1994; Toretsky et al., 2004; Wester et al., 1996; Wust et al., 2003). Such extensive usage of [¹⁸F]FBA and [¹⁸F]SFB warrants the automation of the radio-synthetic procedure. Two groups have previously reported the automated preparation of [¹⁸F]SFB. Zijlstra et al. reported the application of a microcomputer controlled module to prepare [¹⁸F]SFB with decay corrected yields in the range 15–20%. The [¹⁸F]SFB was subsequently conjugated to annexin-V (Zijlstra et al., 2003). In their approach, all three reaction steps were carried out in a single reaction vessel, therefore intermediate cleaning was required and some purification cartridges had to be used more than once during the synthesis. Recently, Mading et al. reported a module assisted synthesis of [¹⁸F]SFB using the TRACERlab Fx_FDG (GE Medical Systems) equipped with two reaction vessels and two purification cartridges. The [¹⁸F]SFB was obtained in high purity and decay corrected yields in the range 34–38% (Mading et al., 2005).

In this report, we present the synthesis of [¹⁸F]FBA and [¹⁸F]SFB on the widely used Siemens/CTI chemistry process control unit (CPCU) originally designed for the routine production of 2-[¹⁸F]fluorodeoxyglucose ([¹⁸F]FDG) (Padgett et al., 1989). As the CPCU unit lacks the capabilities to perform SPE purifications we developed and attached a computer operated SPE purification module to the CPCU, which significantly expands the synthetic capabilities of the CPCU chemistry unit (Fig. 1).