Microfluidic Tools for Radiochemical Analysis and High-throughput Radiopharmaceutical Development
Positron emission tomography (PET) is a highly sensitive non-invasive imaging modality that uses small masses of radiolabeled “tracer” compounds to probe specific biochemical pathways within the whole body. PET is used extensively in clinical diagnostics, basic research, drug development, treatment monitoring, and in the new field of theranostics. To minimize radiation exposure to the radiochemist, the preparation (synthesis, purification and formulation) of PET tracers is performed using automated instruments known as radiosynthesizers. Because current systems are quite expensive, bulky (requiring operation in a specialized radiation-shielded hot cell) and consume tens to hundreds of times as much precursors and reagents compared to the amount of tracer needed for imaging, there has been intense interest in microfluidics in this field. A particular promising avenue has been the use of droplet-based radiochemistry platforms that can produce similar quantities as conventional systems, yet reduce volume and reagents by 100x, reduce synthesis times, and are small enough that they could be self-shielded and operated as a benchtop instrument in a normal laboratory. In addition to advantages for tracer production, this approach enables parallel syntheses by performing multiple reactions together on a single chip, paving the way to rapid, high-throughput radiochemistry for basic investigations, synthesis optimization, and tracer development. In one project, I designed a droplet chemistry heating platform involving a fast ceramic heater for accurate temperature control for a singular droplet synthesis. This initial platform was used in the development of the droplet synthesis of [18F]AMBF3-TATE, realizing an Isolated radiochemical yield (RCY) of 16�1% (n=5) with via isotopic exchange radiofluorination. I also developed a robotic system to perform fully-automated droplet reactions (up to 64 at a time). A Cartesian 3-axis gantry was built that could quickly move a fluidics head (for reagent dispensing and crude product collection) across a 20cm x 40cm workspace containing a set of four multi-reaction chips (each with 16 sites) each controlled by an independent heater, a pipette tip rack, microwell plates for reagents and reaction products, and a set of thin-layer chromatography (TLC) plates for reaction analysis. The system was extensively tested and characterized, and a scripting language was developed to allow high-throughput studies to be programmed as a series of intuitive operation sequences applied to sets of reaction sites. Consistency was assessed by performing a series of identical reactions (synthesis of [18F]Fallypride) which exhibited crude RCY of 79�5% (n=16). This was lower than the manual optimal conditions (90�1%, n=4), and with slightly higher variation. In a second project, I explored microfluidic technology related to the analysis of radiotracers. After the synthesis of a PET tracer, the resulting product must be purified, formulated in saline and finally must pass a number of quality control (QC) tests to ensure safety of the tracer and identity of the product. These tests are typically performed using equipment such as high-performance liquid chromatography (HPLC) which is an expensive instrument that requires significant table space for the device. We designed a “hybrid” capillary and PDMS microfluidic chip for the performance of these QC tests using capillary electrophoresis (CE). I helped develop a volumetric injection chip to inject a 4nL volume of analyte into the separation capillary in a highly repeatable manner. Using this device, injected sample peak relative standard deviation (RSD) was found from the UV absorbance detector electropherogram to be <1.5%, allowing for baseline separation (i.e. resolution > 1.5) of FLT from known synthesis impurities, and enabling impurity species to be quantified independently. The major limitation towards this device performing some of the required QC tests was the lack of a method for in situ radiation detection. Therefore, I added an avalanche photodiode (APD) to the previous device for the purpose of detecting radioactive species. This detector provides highly localized detection due to the short positron range and thus narrow peak widths in the radiation detector electropherogram. The radiation detector enabled confirmation of radiochemical identity, and radiochemical purity, and a limit of quantitation of 114 MBq/mL (3.1 mCi/mL), suitable for testing batches of tracers at higher levels of radioactivity concentration.