Role of neuroimaging in drug development

Rev. Neurosci. 2014; 25(5): 663–673

Bikash Medhi*, Shubham Misra, Pramod Kumar Avti, Pardeep Kumar, Harish Kumar and Baljinder Singh

Role of neuroimaging in drug development

Abstract: The development of new molecular imaging techniques has bridged the gap between preclinical and clinical research. During the last decade, the develop-ments in imaging strategies have taken a great leap by the advancements in new imaging scanners, development of pharmaceutical drugs, diagnostic agents, and new thera-peutic regimens that made significant improvements in health care. The knowledge gained from imaging tech-niques in preclinical research can be applicable to the patients. Similarly, the problems from clinical studies with humans can be tested and studied in preclinical studies. The appropriate application of molecular imaging to drug discovery and development can markedly reduce costs and the time required for new drug development. Some imaging techniques, such as computed tomography (CT) or magnetic resonance imaging (MRI), reveal anatomical images, and single-photon emission computed tomog-raphy (SPECT), SPECT/positron emission tomography (PET), and PET show functional images. These developing molecular or neuroimaging methods provide increasingly detailed structural and functional information about the nervous system. The basic principles of each technique are described followed by examples of the current applica-tions to cutting-edge neuroscience research. In summary, it is shown that neuroimaging continues to grow and evolve, embracing new technologies and advancing to address ever more complex and important neuroscience questions.

Keywords: CT; drug development; MRI; MRS; neuroimag-ing; PET; SPECT.

DOI 10.1515/revneuro-2014-0031Received May 2, 2014; accepted May 7, 2014; previously published online June 24, 2014

Introduction

Molecular imaging plays a major role in modern biomedi-cal research (Olsen et al., 2006). It is an emerging field of study that deals with the imaging of disease on a cellular or genetic level rather than on a gross level (Nichol and Kim, 2001). In the last decade, molecular imaging led to the progress in the development of new drugs and novel molecular probes for imaging the morphological, biologi-cal, and pathological conditions of a disease both at the structural and functional levels of cells and molecules. Most nuclear medicine techniques are preferred for molecular imaging. Based on the type of energy emitted from the radionuclide, nuclear medicine imaging can be divided into two categories: single-photon emission com-puted tomography (SPECT) and positron emission tomog-raphy (PET) (Colby and Morenko, 2004).

Molecular imaging has become an increasingly important tool in both research and clinical care. A range of imaging technologies now provide unprecedented sen-sitivity to the visualization of brain structure and func-tion from the level of individual molecules to the whole brain. Many imaging methods are noninvasive and allow dynamic processes to be monitored over time. Imaging is enabling researchers to identify neural networks involved in cognitive processes, to understand disease pathways, to recognize and diagnose diseases early (when they are most effectively treated), and to determine how therapies work.

The growing epidemics of chronic illnesses such as cancer, metabolic disorders, and neurological and psy-chiatric diseases have increased the need for innovative diagnostics and therapeutics. Despite richness of drug targets, the challenges of discovering and developing novel treatments, including the low likelihood of success for unprecedented mechanisms and the high cost of the drug development process, are considerable. A success-ful drug discovery and development is about increasing the probability of success of the targets we choose, opti-mizing their validation, selecting the best molecules, and driving them expeditiously toward proof of concept (POC) and proof of target (POT) in patients. The advances in the molecular imaging ‛toolbox’ are playing an increasingly important role in our efforts to address these challenges

*Corresponding author: Bikash Medhi, Department of Pharmacology, Post Graduate Institute of Medical Education and Research, Chandigarh 160 012, India, e-mail: [email protected] Misra and Harish Kumar: Department of Pharmacology, Post Graduate Institute of Medical Education and Research, Chandigarh 160 012, IndiaPramod Kumar Avti: Bioimaging Laboratory, Montreal Heart Institute and Ecole Polytechnique, Department of Chemistry, McGill University, Montreal H3C 3A7, Quebec, CanadaPardeep Kumar and Baljinder Singh: Department of Nuclear Medicine, Post Graduate Institute of Medical Education and Research, Chandigarh 160 012, India

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664 B. Medhi et al.: Neuroimaging in drug development

(Hargreaves, 2008). Recent developments in noninvasive molecular imaging modalities provide exciting oppor-tunities for the discovery, validation, and development of novel therapeutics. The development and modifica-tions of existing imaging modalities, such as computed tomography (CT), magnetic resonance imaging (MRI), optical imaging, SPECT, and PET, for the in vivo preclinical imaging of small animals, such as mice and rabbits, with species-scaled sensitivity and resolution are as good as in the clinical setting (Phelps, 2000). These noninvasive imaging modalities use different methods of image acqui-sition, with each of its own strengths and weaknesses in terms of spatial resolution, sensitivity, and imaging probe characteristics. The spatial resolution of MRI and CT is higher than that of SPECT and PET. However, the detection sensitivity of SPECT and PET is higher than those given by structural modalities and can detect tracer concentrations in the picomolar or nanomolar range. Functional imaging uses the tracer principle to detect physiological abnormal-ity or disturbed biochemical process (Meikle et al., 2005). There is an increasing demand for developing sensitive and specific novel imaging agents that can rapidly be translated from small animal models into the patients. SPECT and PET imaging techniques have the ability to detect and serially monitor a variety of biological and pathophysiological processes, usually with tracer quanti-ties of radiolabeled peptides, drugs, and other molecules at doses free of pharmacologic side effects (Blankenberg and Strauss, 2002).

Different types of imaging are used to reveal the brain structure (anatomy), physiology (functions), and bio-chemical actions of individual cells, the molecules that compose them, and the cells’ functions, behaviors, and interactions. The three main categories, therefore, are often referred to as structural, functional, and molecular imaging. This review focuses on the imaging modalities and their use in the nervous system, primarily the brain.

These imaging techniques, alone or in combination, are transforming our understanding of the brain func-tions, immune cell function, and immune cell interaction with the brain in health and disease.

Structural and functional imaging techniquesImaging techniques in animals exhibit a key role in the drug development, particularly for validating drug target-ing, safety, and efficacy. One of the main applications is the validation in drug binding assays to specific targeted

areas by directly labeling a drug to determine its distribu-tion and pharmacokinetics.

The imaging of large animals as well as humans is concerned with the interaction of radiation with tissues and the use of technological developments to extract useful information from the observations, both spatial and temporal, of these interactions. The three impor-tant methods used in the imaging of large animals to extract diagnostic information include CT, MRI, and PET (Figure 1). Depending on the imaging methods and application, the data acquired can be interpreted to yield information not only about anatomy and structures but also about many physiological processes such as glucose metabolism, oxygen utilization, blood volume, perfu-sion, and receptor binding (Haacke et al., 1999; Prokop et al., 2003; Bailey et al., 2004). This information is usually displayed as images. However, it is important to keep in mind that the images produced are pictures of tissue characteristics that influence the way energy is emitted, transmitted, and reflected by the imaged tissue. These characteristics are related to, but not the same as, the actual structure (anatomy), composition (biology and chemistry), and function (physiology and metabo-lism) of the imaged tissues. In many cases, extensive data processing has been applied after the actual imaging process. Traditionally, CT and MRI have coincident struc-tural modalities, whereas PET mainly has provided func-tional information. However, today, a large number of functional protocols are available for mainly MRI but to some extent also for CT (Haacke et al., 1999; Prokop et al., 2003). Utilizing anatomical landmarks to superimpose images, the functional information from PET images can be combined with anatomical information in CT or MRI images in a process called coregistration. With the advent of combined PET/CT scanners, generating combined structural and functional images has become possible without the time-consuming process of coregistration.

CT, through its high spatial resolution and moderate differentiation of tissue contrast, is a fast and exception-ally useful technique for visualizing general anatomy (Prokop et al., 2003). The basic principle of CT imaging involves X-ray generation, detection, and computer image reconstruction. An X-ray tube and the X-ray detectors are positioned on opposite sides of a rotating ring. X-rays passing through a body placed inside the ring are attenu-ated at different rates by different tissues. Computer pro-cessing is used to generate a three-dimensional image of the inside of the body from the large series of two-dimen-sional X-ray images taken around the axis of rotation (Prokop et al., 2003). CT uses ionizing radiation; however, doses are relatively small, and even when working with



B. Medhi et al.: Neuroimaging in drug development 665

surviving experimental animals, no further consideration is required. Like CT, MRI provides detailed images with millimeter spatial resolution.

However, MRI has a much greater soft-tissue con-trast than CT, making it especially useful in neurologi-cal imaging. The principles of MRI involve the use of a powerful magnetic field to align the nuclear magnetiza-tion of hydrogen atoms in the water in the body (Haacke et al., 1999). Radiofrequency fields are used to systemati-cally alter the alignment of this magnetization, causing the hydrogen nuclei to produce a rotating magnetic field detectable by the scanner. This signal can be manipu-lated by additional magnetic fields to build up enough information to reconstruct an image of an animal placed in the scanner. Special MRI-safe surgical equipment as well as implants are needed due to the strong magnetic fields.

PET can record biological processes in three dimen-sions following the administration of radiolabeled tracers by intravenous injection or inhalation. Specific PET tracers can be synthesized for the studies of specific metabolic processes by radiolabeling naturally occurring substances and the analogues thereof [e.g., 18F-deoxy-2-glucose (FDG)], pharmaceuticals, and so on (Bailey et al., 2004). During and following tracer administration, the time course of the concentration of radioactivity in the tissue is recorded by the PET scanner, and the time course of the concentration of radioactivity in the blood is measured in the successive blood samples. The resulting

data are then analyzed using kinetic models. PET isotopes must generally be produced on-site, as their radioactive half-life is short, limiting the availability of PET imaging. Compared to other imaging modalities, contemporary PET has a lower spatial resolution. The use of combined PET/CT further provides the anatomical location (CT) of the biological processes (PET) in merged images. No high attenuation objects (e.g., metal heating mats) may be added or removed during the experiment. Table 1 sum-marizes some of the properties of the presented imaging techniques. Each imaging method, individually and in combination, provides information essential to modern animal research.

PET can provide quantitative measures of tracer uptake in each organ, which can be utilized for treat-ment monitoring response. PET radiopharmaceuticals are produced by the radiolabeling of several biomolecules, drugs, and chemical compounds with various positron emitters such as 11C (t1/2 = 20.4 min), 13N (t1/2 = 9.96 min), 15O (t1/2 = 2.03 min), and 18F (t1/2 = 109.8 min).

Burns et al. (2007) have proven the role of PET imaging in drug discovery and development. Their recent work in neurosciences at the Merck Research Laboratories (MRL) used 18F-MK-9470, an 18F-labeled inverse agonist of the cannabinoid-1 receptor (CB1R). 18F-MK-9470 was used to characterize CB1R distribution in the human brain, to measure the receptor occupancy of potential therapeutic CB1R inverse agonists, and to link target engagement to weight loss efficacy. As the cannabinoid system is thought

Figure 1 Summary of modalities used for molecular imaging.




Table 1 Overview of animal imaging systems.

Technique Resolution Depth Time Imaging agents Targeta Primary small-animal use Clinical use

MRI 10–100 μm No limit Minutes-hours

Gadolinium, dysprosium, iron oxide particles

A, P, M Versatile imaging modality with high soft-tissue contrast

Yes

CT 50 μm No limit Minutes Iodine A, P Lung and bone imaging YesUltrasound 50 μm Millimeters Minutes Microbubbles A, P Vascular and interventional

imaging Yes

PET 1–2 mm No limit Minutes 18F, 11C, 15O P, M Versatile imaging modality with many different tracers

Yes

SPECT 1–2 mm No limit Minutes 99mTc, 111In chelates P, M Commonly used to image labeled antibodies, peptides, and so on

Yes

A, anatomical; M, molecular; P, physiological; BLI, bioluminescence imaging; FMT, fluorescence-mediated molecular tomography; FRI, fluorescence reflectance imaging; GFP, green fluorescent protein; NIR; near-infrared.

to be involved in the neuromodulation of a variety of central nervous system (CNS) functions, 18F-MK-9470 has the potential to be a valuable research tool to study CB1R biology and pharmacology in the metabolic, neurological, and neuropsychiatric disorders in humans (Burns et al., 2007).

Neuroimaging in drug development and discovery

Neuroimaging studies in animals

MRI

The requirement of the high resolution by the structural MRI is a big challenge, particularly for finding Aβ plaques, which require slice thicknesses of <100 μm; human MRI scans usually have a slice thickness of 1 mm. It has been attempted by various groups (Jack et al., 2004; Borthakur et al., 2006), but the problems with long scan times or low signal intensity have rendered the techniques impractical for in vivo use. Braakman et al. (2006) performed scans at 2-month intervals, one of the only longitudinal neuroimag-ing studies in APP/PS1 mice and found success in imaging Aβ plaques. Similarly Vanhoutte et al. (2005) reported detection of amyloid plaques in APP[V717I] transgenic mouse model, using the structural MRI contrast arising from the iron associated with the plaques. Chamberlain et al. (2011) attempted to standardize the process using ex vivo procedures to find the best scanning protocol. They found that average multiple echoes were better than fast

spin-echo techniques and that any movement or blurring would greatly deteriorate plaque imaging, as the plaques are smaller than the voxel size. In contrast, the plaques in mouse thalami can be imaged with faster, lower-resolu-tion sequences, as thalamic plaques are larger than corti-cal and hippocampal plaques (Dhenain et al., 2009).

Most other studies using MRI in mouse models are ex vivo. The fixed slices of APP/PS1 mouse brain were scanned using a T2-weighted (T2W) protocol in a 7T scanner (Benveniste et al., 1999; Zhang et al., 2004). It was found that the areas of hypointensity matched the areas of Aβ deposits found in histologically stained mouse brains and that neuroimaging was a viable method for monitor-ing treatment efficacy.

Yang et al. (2011) looked at morphometry in rTg4510, a τ transgenic mouse model, and wild-type mice. They found that the transgenic mice had severe atrophy in the neocortex and hippocampus as well as in enlarged ventri-cles. They concluded that MRI was a useful tool for com-paring treatment responses between humans and mice.

PET studies

The small size of mouse brain is the limiting factor for PET studies as it shows many disappointing results. 18F-FDG and 18F-FDDNP, another Aβ probe, were found to be similar between Tg2576 and wild-type mice (Kuntner et al., 2009). They concluded that it was due to the limited spatial reso-lution of PET as well as the partial volume effects made worse by the small subject. On the other hand, microPET scanners have been found to be more effective in imaging rat and mouse brains (Teng et al., 2011). Teng et al. (2011) scanned a transgenic rat model with Aβ deposition using




18F-FDDNP and found that 18F-FDDNP binding in the hip-pocampus and frontal cortex progressively increases from 9 to 18 months of age and parallels age-associated Aβ accumulation, although other studies still reported no significant binding effect seen in mouse models with PiB (Toyama et al., 2005). Toyama et al. (2005) concluded that, although there was good uptake of PiB, there was little to no Aβ plaque binding seen and that this may be due to smaller amounts of Aβ binding sites compared to human subjects.

Other tracers have also been used as Aβ probe shows various conflicting results. One such tracer, 11C-PK11195, binds to activated microglia, which was shown to surround extracellular amyloid in histological studies (McGeer et al., 1988). Venneti et al. (2009) found that 11C-PK11195 binding in both human and transgenic APP/PS1 mice matched the presence of activated microglia seen in immunohistochemical results. However, the role of acti-vated microglia is under dispute, with some stating that they trigger cell death (Tan et al., 2002) and others sug-gesting that they actually clear the brain Aβ (Bard et al., 2000; Wilcock et al., 2004). Due to its association with Alzheimer’s disease (AD) as well as its unknown func-tion within AD pathology, using a microglia tracer can provide further insight into its role in addition to using it as a potential AD biomarker in mice where Aβ probes have failed. However, they raise the concern that transgenic models of AD are not representative enough of AD, as dif-ferent degrees of binding to astrocytes were seen in their results (Venneti et al., 2009).

In recent times, several anticancer drugs such as paclitaxel and cyclophosphamides have been radiola-beled with (99mTc) SPECT or (18F) PET radionuclide for the in vivo prediction of therapeutic efficacy prior to treatment (Lacan et al., 2005; Kalen et al., 2007). Singh et al. (2008) have radiolabeled ciprofloxacin with 99mTc for the detec-tion of bacterial infection in patients with suspected bony infection (Singh et al., 2008). In another study, Kumar et al. (2012) had optimized the conditions for radiolabeling of an anticancer drug doxorubicin and evaluated the effi-cacy of radiolabeled doxorubicin by preclinical imaging of tumor mice. The radiolabeled anticancer pharmaceuti-cals will be utilized to track the pharmacodynamics of the drug and also used as a diagnostic tool for the accurate diagnosis and localization of the tumor. It can provide an experimental tool to assess the drug concentration in the tumor tissue, to monitor anthracycline-based therapy, to predict the chemoresistance of the drug at initial stages, and to prevent the patient from unnecessary toxicity of that chemotherapy regimen without clinical benefit (Kumar et al., 2012).

Neuroimaging studies in humans

Tablot and Laruelle (2002) have described the role of PET and SPECT in the development of new drugs and the understanding of the mechanism of psychiatric drugs.

Neuroimaging provides an important tool for the drug discovery for the various neurodegenerative and other diseases related to nervous system. The new candidates (drug) may be radiolabeled with 18F or 99mTc and injected intravenously to generate images reflecting the biodistri-bution of the drug in the brain. The SPECT and PET can be used to study the synthesis and release of neurotrans-mitters and the availability of neurotransmitter receptors. MRI can be used to measure the concentration of specific molecules within the brain (Tempany and McNeil, 2001).

One in every five drugs, identified every year from thousands of new chemical entities with therapeutic potentials, which enter the final clinical trials, obtain marketing approval (DiMasi et al., 2003). The total dura-tion from the discovery of the compound to the regula-tory approval [U.S. Food and Drug Administration (FDA)] takes about 12–15 years, including an estimated cost of around 800 million (DiMasi et al., 2003). Despite many efforts, the failure rates of new drug candidates are very high, which could be accounted for the low toxicity pre-dictions and efficacy. Therefore, to achieve high success rates, numerous approaches and technologies have been applied in drug discovery and evaluation processes such as combinatorial chemistry, high-throughput screening (HTS), pharmacogenomics, and proteomics. In 2006, to achieve this goal, the FDA has introduced a guide on exploratory Investigational New Drugs (IND) studies to improve the success rates and accelerate the develop-ment of new drugs entering clinical trials (Murgo et al., 2008). To overcome the reduced growth in the develop-ment and registration of innovative drugs, the FDA’s Critical Path Initiative in 2005 has highlighted the use of imaging modalities. These imaging modalities would provide information about the selection and validation of drugs, their targets, and the treatment strategies that would increase the success rates of drug development programs. Much recently, neuroimaging has provided valuable insights into the functional, anatomical, and chemical brain information that may assist in preclinical to clinical drug development strategies. From the per-spective of drug development, such tools will be highly valuable. In the above sections, we consider the integra-tion of noninvasive imaging methods into modern drug discovery and development (Figure 2).

PET and SPECT are the radionuclide-based imaging modalities. SPECT imaging uses commercially available




inexpensive γ-radionuclides (30–300 keV) with longer half-lives (few hours to days) compared to rapidly decay-ing positron-emitting isotopes. Since γ-radionuclides have their own characteristic spectra, drug derivatives with multiple probes labeling provide multimodal imaging approaches with very high sensitivity at picomolar range. Microdosing imaging approaches then could provide val-uable information about the drug pharmacokinetic data and target properties depending on the type of radionu-clide and ligand used at a much faster pace leading to a faster clinical approval.

Some examples in the field of PET-based drug devel-opment in AD include the [18F]3′-F-PiB (flutemetamol), 18F-AV-45 (florbetapir), and 18F-AV-1 (florbetaben), which are undergoing extensive phase II and III clinical trials. Although 11C-based radiopharmaceuticals are the ideal choice for commercialization, [11C]PIB, [11C]AZD2184, and [11C]SB13 are still under investigation.

The imaging modalities not only provide information about the drug dosing but also help in evaluating whether they can be further used or rejected. One such candidate drug is [18F]SPA-RQ, which was tested for the treatment of depression, despite many testings at different dosages that showed a complete saturation of target with occu-pancy rates greater than 90% (Keller et al., 2006). The importance of imaging modality in the drug discovery and getting the approvals include the mention of other com-pound ([11C]MK-0233) as an antiobesity drug to engage NPY5 receptors (Erondu et al., 2006). The results obtained from this study showed that this compound did not sig-nificantly reduce the weight loss.

Imaging and pharmacokinetics/pharmacodynamics

The conventional methods of analyzing drug pharmacoki-netic properties is based on blood sampling techniques, biopsies, and skin blisters. However, these methods have some limitations in evaluating the drug distribution vari-ability at the target site, their tissue distribution, and the nonavailability for serial sampling (Brunner and Langer, 2006). Therefore, other imaging techniques such as MRI, CT, SPECT, and PET have been developed (Langer and Muller, 2004).

The drug-induced changes in the brain activity could be studied using functional MRI (fMRI). In order to obtain a further understanding of the drugs’ site of action and their mechanisms of action and time course of action, pharmacological fMRI (phMRI) could be used. The pharmacodynamics of the drugs, as understood by imaging modalities, including the fMRI called as phMRI, is currently expanding. The recent advancements in the imaging modalities with the pharmacodynamic data obtained might play vital roles in the drug develop-ment strategies (Honey and Bullmore, 2004). With the present-day improvements in the imaging systems, the pharmacodynamic data that could be obtained include drug administration, behavioral effects and correla-tion with the brain activity, and drug-modulated brain activity (Stein et al., 1998; Vollm et al., 2004; Gerdelat-Mas et al., 2005; Goekoop et al., 2004). The functional and anatomical data obtained from these studies would help not only in understanding drug action target sites but also in the selection of drug doses. Therefore, due to

Targetexpressed

andfunctional?

Targetidentification

Genomics andproteomics

Drug discoveryDrug

developmentClinical use

Compoundscreening

Preclinicaltesting of lead

compoundPhase 1 and 2 Phase 3 Sales

• Relative efficacy of different agents• Species variation• Biodistribution and pharmacokinetics• Toxicity, safety• Validate imaging for subsequent clinical use

• Efficacy• Efficacy

• Presenceof target

• Doseadjustment

• Availability• Dose adjustment• Human pharmacokinetics• Safety

Figure 2 Imaging applications in the drug discovery and development process.




the inherent functional-anatomical information of the phMRI, the cognitive basis for the treatment strategies could be achieved. PET and SPECT imaging is used for the evaluation of the molecular pathology of CNS disor-ders. The positron-emitting (11C or 11F) radiolabeled drugs, a widely used technique to assess the drug penetration in the brain and receptor mapping and to identify the drug targets, establish the relationship between drug exposure and target engagement. This enables appropriate dose selection for preclinical trials and accelerates the clinical developments.

The FDG-PET-based measurements of the regional cerebral metabolic rate for glucose in AD has received much attention particularly in the preclinical and earli-est clinical stages (Reiman and Langbaum, 2009; Reiman et al., 2010).

Recently, an fMRI study was conducted to understand the pharmacokinetics/pharmacodynamics of the drug methylphenidate (MPH), a dopamine and noradrenaline enhancing drug, to correlate its plasma levels and its mod-ulatory effects on brain activation (Muller et al., 2005). A single dose of either 20 or 40 mg MPH had no effect on the executive and working memory tasks and is due to the view that brain activation is a sensitive surrogate marker of drug effects than just the task performance (Turner et al., 2003).

Imaging and drug estimation in organs

The conventional methods of analyzing drug pharmacoki-netic properties is based on blood sampling techniques, biopsies, and skin blisters. However, these methods have some limitations in evaluating the drug distribution vari-ability at the target site, their tissue distribution, and the nonavailability for serial sampling (Brunner and Langer, 2006). Therefore, other imaging techniques such as MRI, CT, SPECT, and PET have been developed (Langer and Muller, 2004).

The drug-induced changes in the brain activity could be studied using fMRI. In order to obtain a further under-standing of the drugs’ site of action and their mecha-nisms of action and time course of action, phMRI could be used. The pharmacodynamics of the drugs, as under-stood by imaging modalities, including the fMRI called as phMRI, is currently expanding. The recent advancements in the imaging modalities with the pharmacodynamic data obtained might play vital roles in the drug develop-ment strategies (Honey and Bullmore, 2004). With the present-day improvements in the imaging systems, the pharmacodynamic data that could be obtained include

drug administration, behavioral effects and correla-tion with the brain activity, and drug-modulated brain activity (Stein et al., 1998; Vollm et al., 2004; Gerdelat-Mas et al., 2005; Goekoop et al., 2004). The functional and anatomical data obtained from these studies would help not only in understanding drug action target sites but also in the selection of drug doses. Therefore, due to the inherent functional-anatomical information of the phMRI, the cognitive basis for the treatment strategies could be achieved. PET and SPECT imaging is used for the evaluation of the molecular pathology of CNS disor-ders. The positron-emitting (11C or 11F) radiolabeled drugs, a widely used technique to assess the drug penetration in the brain and receptor mapping and to identify the drug targets, establish the relationship between drug exposure and target engagement. This enables appropriate dose selection for preclinical trials and accelerates the clinical developments.

The FDG-PET-based measurements of the regional cerebral metabolic rate for glucose in AD has received much attention particularly in the preclinical and earli-est clinical stages (Reiman and Langbaum, 2009; Reiman et al., 2010).

Recently, an fMRI study was conducted to understand the pharmacokinetics/pharmacodynamics of the drug MPH, a dopamine and noradrenaline enhancing drug, to correlate its plasma levels and its modulatory effects on brain activation (Muller et al., 2005). A single dose of either 20 or 40 mg MPH had no effect on the executive and working memory tasks and is due to the view that brain activation is a sensitive surrogate marker of drug effects than just the task performance (Turner et al., 2003).

Imaging and receptor-binding ligand studies

The ability to obtain a good image of receptor site depends on the receptor binding affinity of the radioligand and receptor concentration (Eckelman et al., 1979). The poten-tial of radiolabeled drug-receptor binding studies provides valuable information about the drug’s binding potential. It is defined as the ratio of the receptor concentration to the radiolabeled drugs’ affinity. The higher the receptor con-centration is, the higher is the drug binding affinity and the lower is the affinity required to affect the visualization (Logan, 2003). This helps to determine the drug dosage regimens before phase 1 and 2 clinical trials. Recently, the binding of the neutral analogue of amyloid binding thi-oflavin-T (BTA) ([N-methyl-11C]-2-(4′-methylaminophenyl)-6-hydroxybenzothiazole) to the β-amyloid plaque was understood in transgenic AD mice model (Toyama et al.,




2005). The results suggest that there is less binding to the plaque regions and this led them to surmise that the animal models might not truly depict the human plaque condi-tions. Many radiolabeled drugs have been screened for AD using PET, such as [18F]FDG, isoquinoline[11C]R-PK11195, 2-[18F]FDDNP (2-(1-{6-[(2-[18F]-fluoroethyl)(methyl)amino]-2-naphthyl}ethylidene)-malononitrile), and Pittsburgh compound B (N-methyl-[11C]2-(4-methylaminophenyl)-6-hydroxybenzothiazole) (hereafter [11C]PIB). A recent study shows the identification of inflammation as a key regulator in the AD plaque development using [11C]R-PK11195 that binds and has shown increased binding to the peripheral benzodiazepine receptor within the outer mitochondrial membrane of activated microglia (Banati, 2002). This binding was shown increased in the tempo-ral cortex (Cagnin et al., 2001). The other drug that has shown increased binding to the plaque and neurofibril-lary tangles is 2-[18F]FDDNP (Agdeppa et al., 2003).

These kinds of studies provide information about the drug-receptor binding affinities, concentration of recep-tors, regional distribution of drugs binding to receptors, and specific and nonspecific binding affinity to other receptors. Some of the factors that should be considered while using the radiolabeled drug-based imaging studies include the high specific radioactivity for quantitative brain imaging, limited spatial resolution ( < 0.5 mm) due to positron path range compared to MRI and CT imaging, and difficulty to image the subregions of the brain (Hume et al., 1998). However, similar limitations might not exist with the SPECT, as there is no annihilation event that needs to be detected and might affect the spatial resolu-tion (Acton et al., 2002).

Discussion and conclusionsNeuroimaging techniques have the potential to facilitate diagnosis and help in the prediction of disease for the individuals at high risk and provide a scientific basis for the development of novel pharmacological approaches. The development in imaging modalities can lead a way to drug discovery and development. It can cut down the huge amount of expenses done during the development of a drug. The time consumed in the study of the biodistri-bution and pharmacokinetics of the drug under investiga-tion can be minimized. Imaging methods are noninvasive and allow for longitudinal studies in a single animal. This increases the statistical relevance of a study, allows for more clinically relevant study designs, and decreases the number of animals. These techniques enable scientists to

visualize the actions of cells or molecules anywhere they occur within living small laboratory animals and, in some cases, in laboratory sheep and pigs. It is reasonable to assume that imaging might reduce the development time of new drugs and provide tools for faster POC testing in clinical studies.

All established imaging modalities, including PET, SPECT, optical imaging, MRI/magnetic resonance spec-troscopy (MRS), ultrasound, and CT, are being optimized regarding operating comfort, data acquisition, and analysis as well as applications in small animals. This will further accelerate and improve the introduction of molecular imaging into drug development and help the translation of preclinical studies into clinical applica-tions. In particular, the combination of different imaging modalities holds great promise. By combining PET with CT or MRI, the effect of a drug might be studied at the molecular level by monitoring individual signaling cas-cades. The morphological, physiological, and metabolic consequences of these signaling cascades might then be monitored in vivo.

In summary, molecular imaging will only be adapted as an attractive tool in drug development if it can be systematically proven that it helps in decision-making overall, provides valuable biological markers or surrogate endpoints of drug efficacy, and produces substantiat-ing evidence for a claim during the registration of a new drug. Although the potential of molecular imaging in drug development has yet to be fully realized by the pharma-ceutical and imaging industries, much progress is being made, and molecular imaging is likely to play an impor-tant role in accelerating and improving drug development in the near future.

Combining molecular imaging with anatomical and physiological imaging technologies, as these examples illustrate, is fundamentally advancing the scientific understanding of how the brain functions and the trans-lation of that understanding to improve human health.

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Bikash Medhi, is Additional Professor in Department of Pharma-cology, Post Graduate Institute of Medical Education and Research (PGIMER), Chandigarh, India. His area of interest is Experimental Pharmacology, Clinical Research, Regulatory Pharmacology, Devel-opment of nano-formulations, Pharmacogenetics, Pharmacogenom-ics, Stem cells etc. He is specialized in Experimental and Clinical Pharmacology, Pharmacovigilance, Drug Regulation, New Drug Development.

Pramod Avti is a Postdoctoral Fellow (2012 – Present) at Mon-treal Heart Institute and Ecole Polytechnique, Montreal Canada. His research interest involves developing multifunctional bio-compatible nanobiosystems-based imaging contrast agents for theranostic and regenerative medicine applications. In particular, his research is aimed at synthesis, interaction and conjugation of synthesized dendritic structures, bio-molecules and drugs to the metallic nanoparticles for biomedical applications. His previous research (2008–2012) at State University of New York, Stony Brook campus, New York includes the use of metal encapsulated carbon nanomaterials as MRI and photoacoustic imaging contrast agents for biological applications.

Pardeep Kumar had worked as research fellow in Department of Nuclear Medicine, PGIMER, Chandigarh and is currently doing his postdoctoral fellowship at Thomas Jefferson University, Philadel-phia. His research area is mainly focused on development of newer SPECT/PET radiopharmaceuticals for cancer imaging and diagnosis.

Baljinder Singh is Professor, Department of Nuclear Medicine, PGIMER, Chandigarh, India and currently the Dean – Indian college of Nuclear Medicine. His research interests include development of newer SPECT/PET for targeted molecular imaging and radionuclide therapy in cancer



Role of neuroimaging in drug development

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