Positron emission tomography (pet) methodology for small animals and its application in radiopharmaceutical preclinical investigation

doi:10.1016/S0969-8051(98)00055-9

Nuclear Medicine and Biology

Volume 25, Issue 8, November 1998, Pages 729-732

https://doi.org/10.1016/S0969-8051(98)00055-9 Get rights and content

Abstract

The use and usefulness of positron emission tomography (PET) to quantify the specific and selective in vivo binding of radioligands in small laboratory animals is briefly reviewed up to the end of 1996. Emphasis is placed on practical experience with a dedicated, small diameter, tomograph (built in collaboration with CTI, Knoxville, TN), implementing conventional PET methodology.

Introduction

The initial phase in which in vitro studies are extended to the in vivo situation, and the temporal biodistribution of a novel positron emission tomography (PET) tracer determined, tends to involve the use of rodents, often combining postmortem tissue sampling with ex vivo autoradiography. In associated studies, the plasma clearance and rate of metabolism of the tracer are also determined. With the severe limitation that the results do not always successfully extrapolate to humans, the location and selectivity of the in vivo binding can be easily assessed and the signal (specific binding compared with nonspecifically bound and free radioactivity) measured prior to injection into either nonhuman primates or humans. Recently published examples of this in vivo screening in rodents include the evaluation of various cocaine analogues as dopamine transporter markers (43), high affinity antagonists for the 5-HT_2A 1, 3 and 5-HT_1A (15) receptors, a ligand for the 5-HT uptake site (37), and antagonists for the α₂-adrenoceptor 10, 14. A full characterisation of the in vivo behaviour of a novel radiopharmaceutical is, however, an intensive procedure in terms of both time and animals, each treatment group of perhaps four animals contributing only one datum point to either a time–radioactivity curve or a dose–response curve.

Section snippets

Why a dedicated small animal scanner?

With a view to streamlining this phase in radiopharmaceutical evaluation, several groups have recognised the need for small diameter, high-resolution PET systems designed to obtain regional time–radioactivity curves from individual animals. With the use of only a small number of animals, sufficient kinetic data can be acquired rapidly to enable the determination of tracer uptake and wash-out, using methodologies analogous to those used in human scans 13, 24. The data are intended to complement

Scanner technology

While recent advances in PET methodology, for example in the application of avalanche photodiodes (27) or the substitution of bismuth germanate (BGO) by lutetium orthosilicate (with its higher light emission and faster speed of response) (35), may pave the way for a “new generation” of small animal scanner, those systems that are presently commissioned use conventional BGO detector technology. Examples include that marketed by Hamamatsu Photonics K.K., Japan 31, 41, that developed in Indiana

Physical accuracy

Whilst the spatial resolution of the system is better than that in conventional PET cameras, it is still far from optimal with regard to the imaging of radioligand distribution in small ROI, as in rats and mice. For comparison, the recently developed methodology that uses a Phosphor-Imager to study the pharmacokinetics of positron emitter-labelled compounds in rodent brain slices (termed “in vitro PET”) has a spatial resolution of a few tenths of a millimetre (28). As a consequence of the

The search for new ligands

Given that the number of human PET studies concerned with the pharmacology of brain disorders is still constrained by the lack of selective radiopharmaceuticals, e.g., for 5-HT receptors and uptake site (6), the most straightforward role of small animal PET is in the determination of size and temporal resolution of the in vivo signal obtained following intravenous (IV) injection of putative compounds. The latter are radiosynthesised in-house but usually with drug company involvement. In

Kinetic and saturation studies

If a novel radiopharmaceutical shows potential, then the specific binding in vivo can be modelled by applying tracer-kinetics principles to the time–radioactivity curves and the bound and free forms of the compound separated. Because of their noninvasive requirements, the preferred methods, as for human studies, use a reference tissue input in graphic (26) or compartmental 18, 23 models. The compartmental model fits for binding potential, which is defined as the ratio of rate constants to and

Receptor occupancy and drug efficacy

Using PET-determined values for receptor number and affinity, occupancy can be calculated as outlined by Volkow et al. (39) for human studies, and can be related feasibly to a functional endpoint. Continuing the example given above, D’Mello et al. (9) reported an ED₅₀ for conditioned taste aversion of 2.3 μmol/kg for WIN 35,428 given intraperitoneally (IP) in rats. Allowing for differences in the route of administration (33), this finding would suggest a receptor occupancy of approximately 50%

Future

Small animal PET scanners thus provide a means to begin to translate in vitro findings into in vivo physiology and pharmacology. Whereas our own tomograph is situated in a primarily clinical research environment, it is clear that for their potential to be fully realised, these dedicated experimental systems should be utilised at earlier stages in radiopharmaceutical development. This strategy would facilitate the iterations between pharmacologists and medicinal chemists allied to the

Acknowledgements

In the Unit, there are physicists, radiochemists, biologists and modellers whose varied expertise enabled our scanning of experimental animals. Their names are repeated often in the references cited. We thank them for their help and enthusiasm.

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Original ArticlesPositron emission tomography (pet) methodology for small animals and its application in radiopharmaceutical preclinical investigation

Abstract

Introduction

Section snippets

Why a dedicated small animal scanner?

Scanner technology

Physical accuracy

The search for new ligands

Kinetic and saturation studies

Receptor occupancy and drug efficacy

Future

Acknowledgements

Nucl. Med. Biol.

Nucl. Med. Biol.

Brain Res

Trends Neurosci

Eur. J. Pharmacol.

Eur. J. Pharmacol.

J. Neurosci. Methods

Nucl. Med. Biol.

Neuroscience Res

J. Neurosci. Methods

Nucl. Med. Biol.

Nucl. Med. Biol.

3D performance of a small diameter positron emission tomograph

Phys. Med. Biol.

The design and physical characteristics of a small animal positron emission tomograph

Phys. Med. Biol.

Radioligands for brain 5-HT2 receptor imaging in vivoWhy do we need them?

Eur. J. Nucl. Med.

Design features and performance of a PET system for animal research

J. Nucl. Med.

Conditioned taste aversion and operant behaviour in ratsEffects of cocaine, apomorphine and some long-acting derivatives

J. Pharmacol. Exp. Ther.

Synthesis and biological evaluation of [F-18]RS-15385-FPA potent and selective alpha-2 adrenergic receptor radioligand for PET

J. Nucl. Med.

Original Articles
Positron emission tomography (pet) methodology for small animals and its application in radiopharmaceutical preclinical investigation

Radioligands for brain 5-HT₂ receptor imaging in vivoWhy do we need them?