Emulsion Reactors: A New Technique for the Preparation of Molecular Imaging Probes

There is an ongoing global effort to prepare targeted molecular imaging (MI) agents for positron emission tomography (PET) and single photon emission computed tomography (SPECT) by labeling small molecules, peptides, and antibodies with radionuclides. More recently, the library of available vectors has expanded to include naturally occurring and genetically engineered proteins, such as affibodies, antibody fragments, hormones, and growth factors. Although modern MI agents offer the opportunity to image and interrogate specific biochemical pathways, they also present unique challenges with respect to labeling the parent targeting vectors with medical isotopes. In many cases, conventional labeling methods fail to produce the desired products in high yield because of the complex structures of derivatized vectors and (in the case of biological agents) because of sensitivities to elevated reaction temperatures and non-physiological pH. Furthermore, production of the parent ligands is often non-trivial, resulting in their limited availability and high cost. These challenges have created the need for a new generation of high yielding and chemoselective labeling methods for both traditional and emerging classes of vectors. In response to these challenges, new radiolabeling strategies have been developed that include solid-phase and fluorous-phase radiolabeling, more reactive prosthetic groups and radiometal chelating His-tag analogues. Microfluidics has also been used increasingly for radiochemical applications because it allows for reactions to be run by using small quantities of precursor in which the unique reaction environment can improve yields and reduce labeling times. Here we report a new strategy that utilizes an emulsion-based labeling platform that requires no specialized equipment, provides a biocompatible reaction environment and that affords a significant increase in radiochemical yields. Aqueous dispersed systems, heterogeneous interfaces, and micelles have been found to accelerate organic reactions and catalyze molecular transformations, therefore, it was postulated that these reaction environments could similarly enhance radiolabeling reactions while providing a biocompatible reaction media. The utility of the emulsion platform was initially explored by using a chelate-derivatized peptide 1 (Scheme 1) that binds to the urokinase plasminogen activator receptor (uPAR)—a protein that is associated with aggressive forms of cancer. The peptide was modified with a tridentate amino acid-derived chelate (SAACII) that binds the technetium tricarbonyl ([Tc(CO)3] ) core. This is an attractive system for evaluating the emulsion platform since technetium is the most widely used radionuclide in diagnostic medicine and the tricarbonyl core typically requires a large excess of the ligand to achieve reasonable conversion yields in less than one half-life. A series of different emulsions were prepared in which the amount and type of surfactant and oil, and the volume ratio between the aqueous and oil phase, were varied. The labeling yields were evaluated as a function of ligand concentration and reaction conditions, and results were compared directly to conventional solution phase labeling reactions run in parallel. The initial emulsions investigated were prepared by varying the amount of the non-ionic surfactant sorbitan mono-oleate (Span-80) in isooctane from 0–5 wt% (0–8.02 10 2 molL ). The reaction mixture contained 1 (10 nmol), 37–55 MBq of [Tc(CO)3 ACHTUNGTRENNUNG(OH2)3]+ (an amount that enabled imaging and biodistribution studies to be completed), the aqueous phase (0.1 mL), and isooctane (1.0 mL). The reactions were conducted at ambient temperature and monitored for 60 min. Emulsification was performed by using a 35 kHz sonication bath with the conventional aqueous reaction placed in the sonicator beside the emulsion reactor so that the process for the two methods was identical (except for the addition of the isooctane) to account for any effects from sonication. All of the labeling reactions that were conducted in the emulsion system outperformed the corresponding conventional labeling, which had a radiochemical yield of less than 5%. The highest radiochemical yield (98%) was achieved when no surfactant was added to the system. [a] Dr. R. W. Simms, D. M. Weaver, M. Blacker, Dr. K. A. Stephenson, Prof. Dr. J. F. Valliant Centre for Probe Development and Commercialization Burke Science Building, Room B231, McMaster University 1280 Main Street West, Hamilton, Ontario L8S 4K1 (Canada) Fax: (+1)905-522-2509 E-mail : valliant@mcmaster.ca [b] Dr. D. H. Kim, Dr. C. Sundararajan, Prof. Dr. J. F. Valliant Department of Chemistry and Chemical Biology McMaster University, 1280 Main Street West Hamilton, Ontario L8S 4L8 (Canada) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201200049.