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Miniaturized QC testing of PET tracers

Overview

QC Testing

After synthesis and purification, PET tracers are formulated in saline, sometimes with an additive such as ethanol or ascorbic acid to mitigate the effects of radiolysis. Before the PET tracer can be administered to a patient, the final batch must be subjected to several quality control (QC) tests to ensure that it is safe for injection.

Required tests depend on the particular tracer, the method of synthesis, and the regulations of the country in which the tracer is produced, but essentially consist of the following:

 QC Test Instrument(s)  Requirements
 visual inspection  eye Clear, colorless or slightly yellow liquid 
 pH pH meter / pH paper  4.5 to 8.5
residual organic solvents Gas chromatography (GC)
possibly HPLC with refractive index detector
Quantity below allowed limit (solvent dependent)
Identity and purity (radionuclidic) Dose calibrator (for half-life determination)
Gamma spectrometer with multichannel analyzer
Half-life 105-115 min (for fluorine-18)
photon energy 0.511MeV and sum peak at 1.022MeV (purity > 99%)
Identity and purity (radiochemical)  Radio-HPLC
Radio-TLC
purity > 95% 
Chemical purity
HPLC with UV or PAD detector
Colorimetric spot tests
> 95%
bacterial endotoxin test LAL test with incubation/spectrophotometry (chromogenic or turbidity) pass
 sterility Incubation with SCDM and FTM for 14 days at 37C pass 
 filter integrity test Device to test breakthrough pressure of sterilization filter Breakthrough pressure > filter specification

Most tests are required before release of the tracer, but there are exceptions. For example, if the filter integrity test passes, the tracer can be released without completion of the sterility test (the test takes much longer to complete than the lifetime of the tracer).

Problems with current QC strategies and methodologies:

  • Expensive equipment (various expensive instruments are needed)
  • Bulky equipment (QC testing requires a small room or dedicated area of the lab)
  • Requires skilled pesonnel (operator must be train in use of all instruments, calibration, documentation etc.)
  • Radiation exposure when handling the sample
  • Time-consuming (in the typical 20-40 min required, yield and specific activity are reduced)

Solutions

Currently there are a couple of groups developing automated systems that include all needed equipment and perform most required tests in an automated fashion (QC1, ABT Molecular Technologies, Inc., and Trace-Ability). These systems also aliquot the original sample to the various subsystems that perform different tests.

In addition to automation, we are interested in miniaturized technologies which have additional potential advantages:
  • Lower instrumentation cost
  • Compact system size
  • Faster analysis times
  • Small sample volumes needed (leading to reduced consumtion of probe, reduced radioactivity of QC sample, reduced need for shielding)

Results

Figure 1: Proof-of-concept separation of PET tracers from impurities using capillary electrophoresis. (A) Electropherogram of FLT sample used illustrating identification and quantitation of Kryptofix K2.2.2 (K222). (B) Electropherogram of FAC sample. illustrating identification and quantitation of K222. (C) Electropherogram of FLT sample illustratating identification and quantitation of: (1) K222, (2) Thymidine, (3) Thymine, (4) Furfuryl Alcohol, (5) Stavudine, (6) FLT and (7) CLT. (D)Electropherogram of FAC sample illustrating identification and quantitation of (1) K222, (2) alpha-FAC and (3) beta-FAC.(E) HPLC chromatogram of FLT sample. Peaks correspond to: (1) Thymine, (2) Stavudine, (3) Thymidine, (4) Furfuryl Alcohol, (5) FLT and (6) CLT. (F) HPLC chromatogram of FAC sample. Peaks correspond to: (1) alpha-FAC and (2) beta-FAC.
One of the most expensive pieces of equipment is the radio-HPLC. We are initially focused on finding a miniaturized replacement for tests involving this instrument. Another technique with good separating power is capillary electrophoresis (CE) (Wikipedia article) .  CE has the advantage that it can be implemented in compact microfluidic chips and  requires a relatively small power supply and detection system. We explored the feasibility of separating PET tracers from impurities using this approach to assess radiochemical and chemical purity.

Using a conventional CE system with UV detection, we have demonstrated the ability to perform chemical identity and purity analysis of the PET tracers [F-18]FLT and [F-18]FAC . Using micellar electrokinetic chromatography (MEKC) by adding SDS to a neutral phosphate buffer, the separation of all compounds in the samples were achieved with baseline resolutions within < 4.5 min and 3 min for FLT and FAC samples, respectively. In comparison to the HPLC with UV detection (the gold standard for chemical analysis of many PET tracers), we have demonstrated improvements in analysis times and comparable limits of detection. In addition, we showed that CE can be used to identify and quantify K222 (a toxic and commonly used phase transfer catalyst) in less than 2 min. These results demonstrate adequate performance for chemical identity and purity analysis using CE.

Current Status and Future Directions

We are currently translating these preliminary findings into a microfluidic platform with integrated UV detection cell to reduce the system size, improve the injection performance, and improve the limit of detection. We are also investigating:
  • Electrochemical detection for analytes without significant UV absorbance
  • Alternative approaches for sample injection
  • Alternative CE separation approaches
  • Integration of additional tests into the chip platform
  • Improving and assessing the reproducibility

Related Publications

  • S. Cheung, J. Ly, M. Lazari, S. Sadeghi, P. Y. Keng, and R. M. van Dam, “The separation and detection of PET tracers via capillary electrophoresis for chemical identity and purity analysis,” Journal of Pharmaceutical and Biomedical Analysis, vol. 94, pp. 12–18, Jun. 2014. (Journal Link)

Team Members

Current:

  • Noel S. Ha (graduate student)
  • Shilin Cheung (postdoctoral scholar)
  • Jimmy Ly (graduate student)

Former:

  • Sammy Ly (lab volunteer)
  • Megha Srivastava (high-school student)