Current Pharmaceutical Design - Volume 8, Issue 16, 2002

Monoamine neurotransmission is a complicated process with interactions between individual neurotransmitter pathways, multiple receptors with different responses and a variety of feedback loops regulating neurotransmitter synthesis, release, reuptake and effect on receptors. The system is further affected by a range of enzymes with co-factors controlling synthesis and degradation of monoamines.Positron emission tomography (PET) has evolved to a very versatile tool for the in vivo imaging and characterisation of physiology and biochemistry. The basis for its expansion during the last years has been a rapid development of labelling methods, allowing a range of tracer molecules to be generated and used in human and research animal studies. The most important PET radionuclide is 11C with a short half-life of approximately 20 minutes. This radionuclide is ideal for the labelling of organic molecules and for multitracer applications in research and drug development studies.PET has been used for a range of explorative studies on the monoamine neurotransmission, as exemplified by studies on the expression of dopamine and serotonin receptors as well as the rate of dopamine and serotonin synthesis. The present article gives examples of studies where PET has been used for the characterization of monoamine transmitter systems in experimental animals and in humans, both in healthy individuals and in patients with diseases affecting neurotransmission.

The evolution of molecular biology has enabled the exploration of novel sophisticated genedirected treating modalities for cancer. Suicide gene therapy - i.e. transfection of a so-called suicide gene that sensitizes target cells towards a prodrug - may offer an attractive approach to treat malignant tumors. For the development of effective clinical suicide gene therapy protocols, a non-invasive method to assay the extent, the kinetics and the spatial distribution of transgene expression is essential. This would allow investigators and physicians to assess the efficiency of experimental and therapeutic gene transfection protocols and would enable early prognosis of therapy outcome. Radionuclide imaging techniques like single photon emission computed tomography (SPECT) and positron emission tomography (PET), which can non-invasively visualize and quantify metabolic processes in vivo, are being evaluated for repetitive monitoring of transgene expression in living animals and humans. Transgene expression can be monitored directly by imaging the expression of the therapeutic gene itself, or indirectly using a reporter gene that is coupled to the therapeutic gene. Various radiopharmaceuticals have been developed and are now being evaluated for imaging of transgene expression. This review surveys the progress that has been made in the field of non-invasive nuclear imaging of transgene expression and focuses on the herpes simplex virus type 1 thymidine kinase (HSVtk) gene therapy approaches.

The labelling of single-stranded oligonucleotides with a positron or single-photon emitter can result in valuable radiopharmaceuticals with promising applications for: (i) Imaging of specific mRNAs, i.e. visualisation of the expression of specific genes in vivo (ii) Monitoring of antisense chemotherapy, i.e. measuring the efficiency of efforts to block the expression of specific genes, (iii) Gene radiotherapy, i.e. the targeting of radiation damage to specific DNA sequences in order to destroy tumours, (iv) Imaging of protein targets by the use of aptamer oligonucleotides, i.e. oligonucleotide ligands obtained by in vitro evolution of selection-amplification steps, or selected for their interaction with nucleic acid-binding proteins, (v) Pretargeting strategies based on the specificity of complementary sequence hybridisation.Nevertheless, oligonucleotides are intrinsically poor pharmaceuticals because of their large size, low stability, poor membrane passage and a number of undesirable and sometimes unpredictable side effects. As an alternative to the inherently unstable phosphodiester DNAs, chemically modified oligonucleotides such as phosphorothioate, methylphosphonate and peptide nucleic acid oligomers have been developed, and some are in clinical trials for the chemotherapy of several types of tumours. Imaging techniques could be useful in the development of such therapies. In addition, the potential of targeting virtually any disease or physiological process, by changing only the sequence of the oligomer, could provide a means to identify serious diseases in a very early stage, and be a highly specific modality to diagnose and differentiate various cancers. This has stimulated efforts to develop such radiopharmaceuticals in many laboratories, and encouraging results have been reported using technetium-99m, indium-111, carbon-11, fluorine-18, bromine-76 and iodine-125 labelled oligonucleotides.

Recent advancements in our understanding of the basic biology of angiogenesis have prompted a focus on practical applications, both in cardiovascular disease and in oncology. The focus on practical applications has stimulated development of novel noninvasive tools that provide serial assessment of ongoing vessel growth in vivo. Nuclear imaging (SPECT, PET) and x-ray angiography have been used to assess changes in perfusion and anatomic appearance, respectively, after induced neovascular development. New MRI techniques provide the ability to identify early changes in vivo that are more sensitive to detection of the effects of new vessel growth than x-ray angiography or nuclear imaging. These new MRI techniques include measurement of blood delivery to the myocardium, development of intramyocardial vasculature, and incremental changes in regional myocardial contractile function. With the combination of methods now available, we expect to be able to track key steps of angiogenesis in vivo and to assess the efficacy of angiogenic therapies. These new imaging capabilities offer crucial information which we hope will hasten the identification and deployment of effective pharmaceutical therapies as an adjunct or alternative to invasive treatments of ischemic disease by targeted stimulation of angiogenesis, and of cancer, by targeted inhibition of angiogenesis.

Nuclear imaging techniques can provide information about the time course of uptake, the spatial distribution, and the functional effects of a drug in the human body. Recently, PET has also acquired the potential to affect the drug development process during a very early stage, when a drug is undergoing animal testing. The development of dedicated small animal scanners with high resolution has made it possible to assess the time course of uptake of a drug within a single animal, providing that the drug is labelled with a positron-emitting isotope. Dedicated small animal scanners may therefore prove to be very useful, especially in those parts of the drug development process that require a longitudinal study design. However, in the case of receptor and enzyme studies, there may be pharmacological constraints and specific radioactivity of the radiopharmaceuticals may become an important issue. This paper will assess the potential and also the limitations of high-resolution PET in animal testing of therapeutic drugs.

Drug analysis and development with PET should fully exhaust the ability of this tomographic technique to quantify regional tracer concentrations in vivo. Data evaluation based on visual inspection or assessment of regional image contrast is not sufficient for this purpose since much of the information present in dynamically acquired data is not used by these approaches. Compartment modelling of dynamic PET data is generally the method of choice since it allows a quantitative assessment of the underlying pharmacokinetic parameters describing drug transport, metabolism and molecular interactions. We present here an overview of key issues of compartment modelling with specific attention to the assumptions underlying the various models and their limitations. We believe that a thorough understanding of the applicability of models is mandatory for the development, successful execution and analysis of quantitative PET studies. Otherwise, meaningful and interpretable results will often not be obtained.