The advantage of using positron emission tomography in drug research

Use of PET and the radioligand [carbonyl-(11)C]WAY-100635 in psychotropic drug development

Positron-emission tomography (PET) provides potential in neuropsychiatric drug development by expanding knowledge of drug action in the living human brain and reducing time consumption and costs. The 5-hydroxytryptamine(1A) (5-HT(1A)) receptor is of central interest as a target for the treatment of anxiety, depression, and schizophrenia. Research on the clinical significance of the 5-HT(1A) receptor now benefits from the highly selective radioligand [carbonyl-(11)C]WAY-100635 (WAY) for quantitative determination of 5-HT(1A) receptors in the primate and human brain in vivo using PET. In this paper, three studies are reviewed to demonstrate the suitability of WAY as radioligand for quantification of central 5-HT(1A) receptors in brain and as an applicable tool for drug development. In the first study a monkey model was used to characterize WAY binding. It was confirmed that the reference ligand 8-OH-DPAT and psychoactive drugs such as buspirone and pindolol occupies 5-HT(1A) receptors in the primate brain. Pindolol is an beta-adrenoreceptor antagonist with a high affinity to 5-HT(1A) receptors. This drug has been suggested in combination with selective serotonin reuptake inhibitors for the treatment of depression and was given to healthy males in the second study. Pindolol induced a marked inhibition of central 5-HT(1A) receptors as calculated by the ratio-analysis method and simplified reference tissue model, 2 h after administration of 10 mg as a single oral dose. This observation suggests that pindolol may have a role for the suggested potentiation of selective serotonin reuptake inhibitor treatment of depression. The third study was on robalzotan (NAD-299), a recently developed 5-HT(1A) receptor antagonist and putative drug with implications for the treatment of depression. In the cynomolgus monkey brain, robalzotan in the dose range 2-100 microg/kg IV occupied 5-HT(1A) receptors in a dose-dependent and saturable manner with a maximal calculated occupancy of 70-80%. The relationship between robalzotan plasma concentration and 5-HT(1A) receptor occupancy could be described by a hyperbolic function that was used to guide the selection of appropriate doses in man. In a subsequent PET study of robalzotan binding to 5-HT(1A) receptors in the living human brain, similar results have been replicated recently. These studies reviewed here illustrate and corroborate that quantitative neuroimaging of receptor binding has potential for the evaluation and dose finding of new central nervous system drugs.

Drug development for neurodegenerative diseases: role of PET

There are few relevant animal models for neurodegenerative diseases to be used for human drug development. Most current drugs for neurodegenerative diseases act through different neurotransmitter systems. Positron emission tomography (PET) is a unique tool in the study of neurodegenerative diseases as it enables quantitative measurements of oxygen consumption, blood flow, energy metabolism and functioning of various neurotransmitter systems. There are several possibilities in the use of PET in drug development. It is possible to radiolabel the drug itself or to study the effect of an unlabelled drug on blood flow, energy metabolism or function of neurotransmitter systems. All these approaches have been used in drug development for neurodegenerative diseases. However, in spite of the important role of PET in pathophysiological studies of neurodegenerative diseases, thus far the versatile possibilities of PET in drug development for neurodegenerative diseases have not been fully exploited.

Ligand-receptor interactions as studied by PET: implications for drug development

Positron emission tomography (PET) is a quantitative imaging method that can be used to characterize binding properties of specific target molecules such as various receptors, transporter molecules and enzymes in vivo. Although already applied successfully, one of the greatest challenges for the technique is to understand better the in vivo complexities of ligand-receptor (target) interaction. The PET technique can be used efficiently in animal studies but, most importantly, also in human studies. PET imaging of patients and healthy volunteers can generate information on human pathophysiology at a molecular level currently unobtainable with other methods. Modern imaging techniques are increasingly applied to drug discovery and development. There are many ways of utilizing PET in pharmacodynamic studies, one interesting approach being the indirect exploration of synaptic neurotransmission with receptor ligands. The receptor occupancy-type studies with PET are rapidly becoming a state-of-the-art method for verifying the mechanism of action of a given drug in man and especially for facilitating the dose-finding procedures in early drug development. Thus far, PET has been mainly applied to pharmacodynamic studies in the central nervous system but will be used also in other areas of drug development such as cardiovascular diseases and oncology.

The role of positron emission tomography in the drug development of M100907, a putative antipsychotic with a novel mechanism of action

Antipsychotic drug development has been a slow process since the discovery of chlorpromazine more than 45 years ago. Researchers identify a large number of potential compounds; screen them for antipsychotic activity in in vitro and animal test models; devise appropriate formulations; perform preclinical pharmacology, pharmacokinetic, and toxicology studies; perform healthy volunteer and then patient clinical studies; and finally negotiate with regulatory agencies for drug approval. In the United States, this process takes an average of 10 to 12 years and costs more than $500 million per approved drug. More recently, the pharmaceutical industry is benefiting from a new wave of technologic innovations that have advanced our understanding of the biology of disease processes and increased the efficiency of the research and development process. However, while these new technologies may appear to be expensive, by providing the basis for early go/no-go decisions, technologies such as PET can actually be cost-effective. To ensure that innovative drug research continues, a practical strategy (rational drug design) to evaluate drugs more efficiently in terms of both time and cost (fewer studies with fewer patients) must be developed for each new drug candidate. One of the most important and difficult steps in the drug development process is defining the dose-response relationship. Using M100907 as an example, we demonstrated that mechanism-based research promotes cost-effective drug development. The therapeutic index of M100907 was defined in phase I single- and multiple-dose tolerability studies. Nuclear imaging using PET technology was then used to confirm the mechanism of action of M100907 in the target organ (living human brain) and to target an appropriate dose range and regimen. With these data, clear go/no-go decision points could be established early within the clinical drug development process, and the selection of M100907 doses to carry forward into large-scale clinical trials in patients with schizophrenia could be narrowed.

Positron emission tomography neuroreceptor imaging as a tool in drug discovery, research and development

Improved communication and cooperation between research-driven drug companies and academic positron emission tomography (PET) centers, coupled with improvements in PET camera resolution, the availability of small animal PET cameras and a growing list of neuroreceptor-specific PET tracers, have all contributed to a substantial increase in the use and value of PET as a tool in central nervous system drug discovery and development.

Delivery of imaging agents into brain

Delivery of diagnostic agents to the central nervous system (CNS) poses several challenges as a result of the special features of CNS blood vessels and tissue fluids. Diffusion barriers exist between blood and neural tissue, in the endothelium of parenchymal vessels (blood-brain barrier, BBB), and in the epithelia of the choroid plexuses and arachnoid membrane (blood-CSF barriers), which severely restrict penetration of several diagnostic imaging agents. The anatomy of large vessels can be imaged using bolus injection of X-ray contrast agents to identify sites of malformation or occlusion, and blood flow measured using MRI and CT, while new techniques permit analysis of capillary perfusion and blood volume. Absolute quantities can be derived, although relative measures in different CNS regions may be as useful in diagnosis. Local blood flow, blood volume, and their ratio (mean transit time) can be measured with high speed tomographic imaging using MRI and CT. Intravascular contrast agents for MRI are based on high magnetic susceptibility agents such as gadolinium, dysprosium and iron. Steady-state imaging using agents that cross the BBB including (123)I- and (99m)Tc-labelled lipophilic agents with SPECT, gives a 'snapshot' of perfusion at the time of injection. Cerebral perfusion can also be measured with PET, using H(2)(15)O, (11)C- or (15)O-butanol, and (18)F-fluoromethane, and cerebral blood volume measured with C(15)O. Recent advances in MRI permit the non-invasive 'labelling' of endogenous water protons in flowing blood, with subsequent detection as a measure of blood flow. Imaging the BBB most commonly involves detecting disruptions of the barrier, allowing contrast agents to leak out of the vascular system. Gd-DTPA is useful in imaging leaky vessels as in some cerebral tumors, while the shortening of T(1) by MR contrast agents can be used to detect more subtle changes in BBB permeability to water as in cerebral ischemia. Techniques for imaging the dynamic activity of the brain parenchyma mainly involve PET, using a variety of radiopharmaceuticals to image glucose transport and metabolism, neurotransmitter binding and uptake, protein synthesis and DNA dynamics. PET methods permit detailed analysis of regional function by comparing resting and task-related images, important in improving understanding of both normal and pathological brain function.

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

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.

N-[11C]methylspiperone PET, in contrast to [11C]raclopride, fails to detect D2 receptor occupancy by an atypical neuroleptic

The occupancy of the atypical neuroleptic quetiapine (Seroquel) at the D2 dopamine receptor was investigated using the PET tracers [11C]raclopride and N-[11C]methylspiperone in a group of five schizophrenic patients. A steady-state treatment condition was ensured by dosing the patients with 750 mg quetiapine daily during 3 weeks followed by a period of tapering off the dose. For each patient, PET examinations were performed with both tracers at two of the following doses: 750, 450, 300 and/or 150 mg. As control, a group of six healthy untreated volunteers was investigated. The D2 binding potential in the putamen and the caudate nucleus was determined by using an evaluation method based on the method proposed by Patlak and Blasberg. The receptor occupancy was determined by assuming that the group of healthy volunteers is representative of untreated drug-naive schizophrenic patients. While a significant linear trend of increasing occupancy with increasing quetiapine dose (reaching 51% +/- 10% occupancy at the 750 mg dose) was detected with [11C]raclopride (P < 0.01), no such trend was apparent for N-[11C]methylspiperone (P > 0.09, maximal occupancy values were 2% +/- 3%, measured for the group of three patients on 450 mg). The study suggests that N-[11C]methylspiperone cannot be used for the assessment of D2 receptor occupancy induced by quetiapine. The result is discussed in terms of endogenous dopamine, tracer kinetics and equilibrium dissociation constants.

Fractal analysis--a new approach in brain receptor imaging

Improvements in detector technology and the development of radioligands for brain receptor imaging have introduced exiting new insights into the pathophysiology of various neuropsychiatric disorders and have improved the possibilities of optimizing the treatment for patients suffering from them. Positron emission tomography (PET) and single-photon emission tomography (SPET) with tailored radiopharmaceuticals provide information on the topographic physiological chemistry of the living human brain. The different patterns of brain receptor densities and distribution can be imaged and modelled with PET and SPET. The normal receptor distribution in the brain is broadly heterogeneous with different cortical layers, which show receptor densities varying from very low to high. Further exploration of the data shows that human neurophysiology and neural architectures possess fractal properties that may be altered during activation and in different neuropsychiatric disorders. This review highlights recent findings in SPET receptor imaging and the use of fractal analysis in the interpretation of images representing various neuropsychiatric disorders.

Current controversies and future directions in basal ganglia research. Integrating basic neuroscience and clinical investigation

This article discusses current controversies and future directions in basal ganglia research, detailing behavioral aspects, anatomic models, neurochemistry, pharmacology, and diagnostic methods as well as surgical techniques. A neuroethologic perspective is highlighted. Furthermore, the relevant literature pertaining to contemporary molecular approaches such as brain microinjections of embryonic or genetically modified cells, for therapeutic purposes and the use of transgenic and knockout animals.