Labelled agents for PET studies of the dopaminergic system--some quality assurance methods, experience and issues

Radiopharmaceutical chemistry for positron emission tomography

Molecular imaging is an emerging technology that allows the visualization of interactions between molecular probes and biological targets. Molecules that either direct or are subject to homeostatic controls in biological systems could be labeled with the appropriate radioisotopes for the quantitative measurement of selected molecular interactions during normal tissue homeostasis and again after perturbations of the normal state. In particular, positron emission tomography (PET) offers picomolar sensitivity and is a fully translational technique that requires specific probes radiolabeled with a usually short-lived positron-emitting radionuclide. PET has provided the capability of measuring biological processes at the molecular and metabolic levels in vivo by the detection of the gamma rays formed as a result of the annihilation of the positrons emitted. Despite the great wealth of information that such probes can provide, the potential of PET strongly depends on the availability of suitable PET radiotracers. However, the development of new imaging probes for PET is far from trivial and radiochemistry is a major limiting factor for the field of PET. In this review, we provided an overview of the most common chemical approaches for the synthesis of PET-labeled molecules and highlighted the most recent developments and trends. The discussed PET radionuclides include ¹¹C (t₁(/)₂=20.4min), ¹³N (t₁(/)₂=9.9min), ¹⁵O (t₁(/)₂=2min), ⁶⁸Ga (t₁(/)₂=68min), ¹⁸F (t₁(/)₂=109.8min), ⁶⁴Cu (t₁(/)₂=12.7h), and ¹²⁴I (t₁(/)₂=4.12d).

Stability of 11C-labeled PET radiopharmaceuticals

The stability of widely used 11C-radiopharmaceuticals was investigated. The purity of many 11C-radiopharmaceuticals was found to be low even at the end of synthesis and to lower with time by radiolysis. Examinations of the effects of radical scavengers showed that the radiopharmaceuticals were classified into four groups: Group 1 showing excellent stability; Group 2 showing suppressed radiolysis in the presence of a selective scavenger of hydroxyl radicals; Group 3 showing suppression in the presence of a selective scavenger of hydrated electrons; Group 4 showing suppression in the presence of both scavengers. Using appropriate additives according to these classes, 11C-radiopharmaceuticals were prepared with excellent radiochemical purity even with high levels of radioactivity and specific activity.

A Peltier thermal cycling unit for radiopharmaceutical synthesis

We have investigated the use of Peltier devices to rapidly cycle the temperature of reaction vessels in a radiopharmaceutical synthesis system. Peltier devices have the advantage that they can be actively cooled as well as heated, allowing precise and rapid control of vessel temperatures. Reaction vessel temperatures of between -6 degrees C and 110 degrees C have been obtained with commercially available devices with reasonable cycle times. Two devices have been used as the basis for a general purpose, two-pot synthesis system for production of [11C] compounds such as raclopride.

An alternative synthesis of [11C]raclopride for routine use

The standard method of [11C]raclopride synthesis requires a large amount of its desmethyl precursor. We prepared [11C]raclopride by methylation of a small amount of desmethyl derivative (0.3-0.5 mg) with [11C]methyl iodide in a DMF solution containing NaH, with a decay-corrected radiochemical yield of 11-14% based on [11C]methyl iodide and with a specific activity of 48 TBq/mmol for 25 min from EOB. The reaction was reproducible and reliable.

PET radiopharmaceuticals in Europe: current use and data relevant for the formulation of summaries of product characteristics (SPCs)

The increasing use of radiopharmaceuticals for positron emission tomography (PET) has come to the attention of regulatory bodies. In order to help authorities in all aspects, the EANM has formed a task group for licensing PET radiopharmaceuticals; this group has surveyed the use of these compounds in Europe by a questionnaire. The number of PET centres that responded to the questionnaire was 26, which included more than 90% of the larger European PET centres. The survey showed that 2-[18f]fluoro-2-deoxyglucose is by far the most important pet radiopharmaceutical with more than 200 applications per week, followed by [15o]water, [15o]carbonmonoxide, [13n]ammonia, [11c]-l-methionine, and l-6-[18f]fluoro-dopa. More than 25 other PET radiopharmaceuticals are in regular use, however, at rather low application frequencies. The data were used by the European Pharmacopoeia Commission for its priority rating for requesting the formulation of monographs. Since it is likely that group registrations will be issued by authorities for the PET radiopharmaceuticals, relevant data on toxicity and dosimetry for the formulation of summaries of product characteristics have been collected by the task group as well.

Radioligands for PET studies of central benzodiazepine receptors and PK (peripheral benzodiazepine) binding sites--current status

The status of the radiochemical development and biological evaluation of radioligands for PET studies of central benzodiazepine (BZ) receptors and the so-called peripheral benzodiazepine binding sites, here discriminated and referred to as PK binding sites, is reviewed against current pharmacological knowledge, indicating those agents with present value and those with future potential. Practical recommendations are given for the preparation of two useful radioligands for PET studies, [N-methyl-11C]flumazenil for central BZ receptors, and [N-methyl-11C]PK 11195 for PK binding sites. Quality assurance and plasma metabolite analysis are also reviewed for these radioligands and practical recommendations are given on methodology for their performance.

PET radioligands for dopamine receptors and re-uptake sites: chemistry and biochemistry

This report, based on the past experience of European centres, offers practical guidance on the chemistry and biochemistry of PET radioligands used for the in vivo imaging of dopamine receptors and re-uptake sites. It mainly summarizes methods for the preparation of D1 and D2 receptor ligands labelled with positron-emitting radioisotopes. Some of these ligands (11C-labelled SCH23390, raclopride and nomifensine, 18F-labelled butyrophenones, [76Br]bromolisuride), which have been found useful in PET clinical investigations, have been emphasized. This report is intended as an introduction and guideline for new PET-groups who want to start research in the dopaminergic neurotransmission imaging field.

Production of 6-[18F]fluoro-L-dopa and its metabolism in vivo--a critical review

This report critically appraises methods for the synthesis of 6-[18F]fluoro-L-3,4-dihydroxyphenylalanine (6-FDOPA) that are based on labelling by non-regioselective electrophilic fluorination, regioselective fluorodemetalation or nucleophilic substitution. Recommendations for the standardization of labelling procedures, the optimization of radiochemical yield and the assurance of product quality and safety are given. Studies of the metabolism of 6-FDOPA in vivo are also reviewed to emphasize the importance of the biochemical component of the development of this tracer for positron emission tomography (PET).

A method of studying pharmacokinetics in man at picomolar drug concentrations

1. We describe a new method that enables the tissue kinetics of picomolar concentrations of drugs to be measured in man. The method is based on the administration of a drug, labelled with a short-lived positron-emitting radioisotope, such as carbon-11 (t1/2 = 20.4 min, beta + = 99.8%) or fluorine-18 (t1/2 = 109.8 min, beta + = 96.9%), which is then detected in vivo by an array of 10 large uncollimated sodium iodide scintillation detectors, arranged as five opposing pairs, with each pair collecting data over one major organ or region of the body. 2. To illustrate the scope of the new method we report the results of administering [O-methyl-11C]-diprenorphine, an established radioligand for central opiate (mu, kappa, and delta) receptors and L-6-[18F]-fluoro-DOPA, a marker for dopaminergic neurons. 3. Only 2-10 muCi (74-370 kBq) of radioactivity are used and, as a consequence of the high specific activities with which carbon-11 and fluorine-18 labelled compounds can be prepared, the method requires less than a nanomole of drug to be administered. In many cases, this amount of drug might be considered low enough to avoid any adverse biological effect. Furthermore repeat studies are possible in many without delivering unacceptable radiation burdens. 4. The high sensitivity realised for both radioactivity and mass suggests a mean for determining the human biodistribution of a new drug at a very early stage in its development. This has potential benefit to drug discovery programmes and to ensuing drug therapies.