Solid Phase 11C-Methylation, Purification and Formulation for the Production of PET Tracers

Summary

We report an efficient carbon-11 radiolabeling technique to produce clinically relevant tracers for Positron Emission Tomography (PET) using solid phase extraction cartridges. ¹¹C-methylating agent is passed through a cartridge preloaded with precursor and successive elution with aqueous ethanol provides chemically and radiochemically pure PET tracers in high radiochemical yields.

Abstract

Routine production of radiotracers used in positron emission tomography (PET) mostly relies on wet chemistry where the radioactive synthon reacts with a non-radioactive precursor in solution. This approach necessitates purification of the tracer by high performance liquid chromatography (HPLC) followed by reformulation in a biocompatible solvent for human administration. We recently developed a novel ¹¹C-methylation approach for the highly efficient synthesis of carbon-11 labeled PET radiopharmaceuticals, taking advantage of solid phase cartridges as disposable "3-in-1" units for the synthesis, purification and reformulation of the tracers. This approach obviates the use of preparative HPLC and reduces the losses of the tracer in transfer lines and due to radioactive decay. Furthermore, the cartridge-based technique improves synthesis reliability, simplifies the automation process and facilitates compliance with the Good Manufacturing Practice (GMP) requirements. Here, we demonstrate this technique on the example of production of a PET tracer Pittsburgh compound B ([¹¹C]PiB), a gold standard in vivo imaging agent for amyloid plaques in the human brains.

Introduction

Positron emission tomography (PET) is a molecular imaging modality which relies on detecting the radioactive decay of an isotope attached to a biologically active molecule to enable the in vivo visualization of biochemical processes, signals and transformations. Carbon-11 (t_1/2 = 20.3 min) is one of the most commonly used radioisotopes in PET because of its abundance in organic molecules and short half-life which allows for multiple tracer administrations on the same day to the same human or animal subject and reduces the radiation burden on the patients. Many tracers labeled with this isotope are used in clinical studies and in basic health research for in....

Protocol

1. Preparation of buffers and eluents

1. Dissolve 2.72 g of sodium acetate trihydrate in 100 mL of water to prepare 0.2 M sodium acetate solution (solution A).
2. Dissolve 11.4 mL of glacial acetic acid in 1 L of water to prepare 0.2 M acetic acid solution (solution B).
3. Combine 50 mL of solution A with 450 mL of solution B to prepare the acetate buffer at pH 3.7 (buffer 1) according to the buffer reference center¹⁵. Verify the pH of the buffer with pH strips or a pH meter.
4. Combine 12.5 mL of absolute ethanol with 87.5 mL of buffer 1 to make 12.5% aqueous EtOH solution (wash 1) in a 100 mL bottle.
5. Co....

Results

To summarize a typical radiosynthesis of [¹¹C]PiB, gaseous [¹¹C]CH₃OTf is first passed through a tC18 cartridge preloaded with a solution of precursor (Figure 1). Separation of the reaction mixture is then achieved by successive elution with aqueous ethanol solutions as follows. First, 12.5% EtOH elutes the majority of unreacted [¹¹C]CH₃OTf and 6-OH-BTA-0, then 15% EtOH washes out the residual impurities,.......

Discussion

Despite the recent emergence and FDA approval of several ¹⁸F-labeled PET tracers, such as florbetapir, florbetaben and flutemetamol, [¹¹C]PiB remains a gold standard tracer for amyloid imaging due to the fast brain uptake and low non-specific binding. Currently this tracer is synthesized via either wet chemistry¹⁶ or using a "dry loop" approach^4,17. Both methods require HPLC purification followed by r.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
6-OH-BTA-0	ABX advanced biochemical compounds	5101	Non-radioactive precursor of [11C]PiB
6-OH-BTA-1	ABX advanced biochemical compounds	5140	Non-radioactive standard of [11C]PiB
Agilent 1200 HPLC system	Agilent	Agilent 1200	Analytical HPLC system
Ethanol absolute	Commercial alcohols	432526
Hamilton syringe (luer-tip, 250 µL)	Hamilton	HAM80701
MZ Analytical PerfectSil 120	MZ-Analysentechik GmbH	MZ1440-100040	Analytical HPLC column
Perkin Elmer Clarus 480 GC system	Perkin Elmer	Clarus 480	Gas chromotograph
polycarbonate manifold	Scintomics	ACC-101	Synthesis manifold
Restek MTX-Wax column (30 m, 0.53 mm)	Restek	70625-273	Analytical GC column
Scintomics GRP module	Scintomics	Scintomics GRP	Automated synthesis unit
Sep-Pak tC18 Plus	Waters	WAT020515	Solid phase extraction cartridge
solvent-resistant manifold	Scintomics	ACC-201	Synthesis manifold
Spinal needle	BD	405181
Sterile extension line	B. Braun	8255059
Sterile filter	Millipore	SLLG013SL
Sterile vial (20mL)	Huayi	SVV-20A
Sterile water	Baxter	JF7623
Synthra MeIplus Research	Synthra	MeIplus Research	[11C]CH3I/[11C]CH3OTf module
Syringe (10 mL)	BD	309604
Syringe (1mL)	BD	309659
Syringe (20 mL)	B. Braun	4617207V	Dispenser syringe
Vent filter	Millipore	TEFG02525