Drugs R D 2008; 9 (6): 351-368 1174-5886/08/0006-0351/$48.00/0

REVIEW ARTICLE

© 2008 Adis Data Information BV. All rights reserved.

Has Molecular and Cellular Imaging Enhanced Drug Discovery and Drug Development? Gang Niu and Xiaoyuan Chen The Molecular Imaging Program at Stanford (MIPS), Department of Radiology and Bio-X Program, Stanford University School of Medicine, Stanford, California, USA

Contents Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351 1. Molecular Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352 1.1 Computed Tomography and Magnetic Resonance Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352 1.2 Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography . . . 353 1.3 Fluorescent Imaging and Bioluminescence Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354 1.4 Ultrasound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355 2. Molecular Imaging in Drug Discovery and Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355 2.1 Measurement of Pharmacodynamic Endpoints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356 2.1.1 Imaging of Metabolism and Proliferation with Fluoro-2-Deoxy-D-Glucose and 3’Deoxy-3’-Fluorothymidine PET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356 2.1.2 Imaging Blood Flow and its Relevance to Antivascular Agents . . . . . . . . . . . . . . . . . . . . . . 358 2.2 Imaging Specific Downstream Targets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359 2.3 Imaging Therapeutic Gene Expression with Relevance to Gene Therapy . . . . . . . . . . . . . . . . . . 361 2.4 Evaluation of Pharmacokinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362 3. Conclusions and Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363

Abstract

A great many efforts have been made to accelerate the drug discovery and development process, which is extremely time and money consuming. Recently developed molecular imaging has many significant advantages over conventional methods for examining molecular pathways and obtaining pharmacokinetic, pharmacodynamic and mechanistic information. This review briefly summarizes various molecular and cellular imaging techniques and discusses several important applications of molecular and cellular imaging in drug discovery and development, which include: (i) measurement of pharmacodynamic endpoints by imaging metabolism and proliferation, imaging angiogenic parameters, and imaging a particular pathway or downstream target; (ii) evaluation of pharmacokinetics; and (iii) imaging therapeutic gene expression with relevance to gene therapy. Molecular imaging is becoming more widely used as a non-invasive tool for drug discovery and drug screening. Further refinements in imaging techniques, optimization of imaging probes and collaborative efforts will be needed to fully

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realise the vast potential of molecular imaging techniques in discovering and developing new drugs.

The challenge for a new drug to succeed is a daunting one, and the reduction in time and cost required for modern drug discovery and development serves a crucial need. An important element in accelerating the drug discovery and development process is rapid differentiation of promising drug candidates from non-starters before unnecessary vast sums are invested. The aim of molecular imaging is to precisely visualize, characterize and measure biological processes at the molecular and cellular levels in humans and other living systems.[1] By introducing molecular imaging probes into traditional diagnostic imaging techniques, researchers can determine the expression of indicative molecular markers at different stages of diseases. The introduction of new imaging probes, methods and advanced imaging instrumentation is significantly speeding up the processes of drug discovery and development. The convergence of innovations has created more sensitive, specific and higher resolution measurements within living organisms. This is expected to improve a wide range of discovery activities such as target biology, compound screening and pharmacokinetic and pharmacodynamic evaluation in animal disease models and, eventually, clinical trials. In this review, we summarize and evaluate the applications of various advanced molecular imaging techniques in drug discovery and early drug development process. 1. Molecular Imaging Technologies encompassed within molecular imaging include single photon emission computed tomography (SPECT), positron emission tomography (PET), magnetic resonance imaging (MRI), x-ray computed tomography (CT), ultrasound, optical bioluminescence imaging and optical fluorescence imaging.[2] Currently, PET and optical imaging are the most prevalent molecular imaging technologies. In addition, many hybrid systems that combine two or © 2008 Adis Data Information BV. All rights reserved.

more modalities are also commercially available, while others are under active development.[3-5] Computer software and algorithms have also been developed to allow co-registration of different imaging modalities.[6] Continued development and wider availability of scanners dedicated to small animal imaging studies will enable the smooth transfer of knowledge and molecular measurements between species, thereby facilitating clinical translation. In the following sections, we discuss each major imaging modality in turn and briefly evaluate their recent advances and applications in molecular imaging. 1.1 Computed Tomography and Magnetic Resonance Imaging

CT uses a conventional x-ray tube, with a collimated, narrow beam and a series of detectors to record the transmitted beam. The detector system measures the attenuation of the beam emerging from the object being imaged.[7] CT was the first imaging modality that allowed accurate non-destructive interior image reconstruction of an object from a sufficient number of x-ray projections. Since its introduction in 1973, CT has revolutionized radiographic imaging and has become a cornerstone of every modern radiology department.[8] Research into higher performance system architectures has been intensively pursued, ranging from single-slice helical/spiral CT in the early 1990s to true volumetric cone-beam CT scanners in helical and other scanning modes currently under investigation.[9] Today’s CT scanners can offer 0.5-mm isotropic spatial resolution, and there is still room for further improvement in spatial and temporal resolution. However, CT can provide only limited functional and molecular information. Therefore, CT is typically used to provide anatomical guidance for PET or SPECT through hybrid systems or co-registration software in molecular imaging. Drugs R D 2008; 9 (6)

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MRI detects the interaction of protons (or certain other nuclei) with one another and with the surrounding molecules in a tissue of interest.[10] Different relaxation times of different tissues can result in endogenous MR contrast. Exogenous contrast agents can further enhance MR contrast by selectively shortening either the T1 (longitudinal) or T2 (transverse) relaxation time.[11,12] The MR image can be weighted to detect differences in either T1 or T2 by adjusting parameters during data acquisition. Traditionally, Gd3+-chelates have been used to enhance T1 contrast[13] and iron oxide nanoparticles (IONPs) have been used to increase T2 contrast.[14] Molecular imaging using MR imaging with IONP conjugates and Gd-labelled nanoparticles has been successfully demonstrated in several model systems.[15] For example, Gd3+-containing paramagnetic liposomes were reported to delineate integrin αvβ3 expression.[16] IONPs can be detected at much lower concentrations than Gd3+ contrast agents because of their high magnetism.[14] Single particle or single cell (loaded with IONPs) detection has been reported.[17-19] However, the negative signals from these IONPs complicate data interpretation and counter the increased sensitivity. With regard to molecular imaging, the major disadvantage of MRI is its inherent low sensitivity, which can be only partially compensated by working at high magnetic fields, acquiring data for a much longer time, and using exogenous contrast agents. For example, the local concentration of the contrast agent needs to be approximately 10–3 to 10–5 mol/L for Gd3+ and 10–6 to10–8 mol/L for IONPs, which is substantially higher than the concentrations of 10–11 to 10–12 mol/L required for PET detection and 10–10 to 10–11 mol/L required for SPECT. For in vivo imaging, the mass of contrast agent required could be problematic. Although proof-of-principle studies have been reported for molecular MRI for many physiological and pathological targets,[20] these approaches are still in the early development stage and they cannot be readily applied to drug development. © 2008 Adis Data Information BV. All rights reserved.

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1.2 Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography

PET is one of the most sensitive and specific techniques for imaging molecular pathways in vivo.[21] PET imaging involves injecting a compound containing a positron-emitting radionuclide (such as 18F [half-life (t1/2) = 109.7 min] or 11C [t1/2 = 20.38 min]) into the body, allowing it to reach its target, and detecting its location and quantity with a PET scanner. The positrons that are emitted from the radionuclides interact locally with negatively charged electrons and emit what is called annihilating radiation; this radiation is then detected by an external ring of detectors. The timing and position of the detection indicate the position of the molecule in time and space. Images can then be constructed tomographically, and regional time activities can be derived.[22] The inherent sensitivity and specificity of PET are its major strengths. Radionuclides can be detected down to the subpicomolar level in target tissues. At these low levels, the compounds used in PET usually have little or no physiological effects on patients or test animals, allowing the study of underlying mechanisms or biodistribution without physiological consequences. For example, small animal PET is now routinely used in preclinical investigations and is particularly adept at translating results from bench to bedside.[23] SPECT is similar to PET in its use of radionuclide-labelled compounds as imaging agents, but in this instance the radionuclides are low-energy γray emitters such as 99mTc (t1/2 = 6.0 hours), 111In (t1/2 = 2.8 days), 123I (t1/2 = 13.2 hours), and 131I (t1/2 = 8.0 days).[24] Internal radiation is administered by inhaling, ingesting or being injected with a low mass amount of radiopharmaceutical. A collimator is used to allow the emitted γ photons to travel along certain directions to reach the detector, which ensures that the position on the detector accurately represents the source of the γ ray. SPECT images are recorded using γ cameras that can be rotated around the subject to produce tomographic images. Because of the use of lead collimators to define the angle of Drugs R D 2008; 9 (6)

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incidence, SPECT is about 1–2 orders of magnitude less sensitive than PET.[25] 1.3 Fluorescent Imaging and Bioluminescence Imaging

Optical imaging is making substantial contributions to molecular imaging by virtue of its ability to detect fluorescent and luminescent molecular probes inserted into the body. Optical imaging is a relatively low-cost method. In fluorescence imaging, excitation light illuminates the subject, and a chargecoupled device camera collects the emission light at a shifted wavelength.[2] The fluorescent probe can be either injected or genetically engineered, and no substrate is required for its visualization.[26] A number of high-resolution microscopic imaging techniques have recently been developed to study molecular events in vivo. In particular, intravital fluorescence microscopy,[27] confocal laser scanning microscopy,[28] multiphoton laser scanning microscopy,[29] and in situ scanning force microscopy [30] have recently been introduced. Studies based on such optical technologies are becoming increasingly popular in preclinical drug development. For example, intravital fluorescence microscopy and deeptissue imaging studies seek to examine cellular behaviour in ex vivo or in vivo preparations with microscopic resolution.[31] The major limitations of optical imaging are tissue light scattering and absorption, which reduce both image resolution and depth of light penetration in tissues.[32] In the ultraviolet and visible regions, tissue scattering and absorption of light can be high, limiting tissue penetration.[33] At the near infrared region between 700 and 900 nm, absorption is low and light can penetrate much deeper into tissues, possibly to a depth that may be sufficient to practically image small animals and certain human cancers.[5,34-39] Endoscopic near infrared region optical imaging, for example, makes it possible to image certain internal organs in patients. In addition, of the optical imaging techniques currently available, multiphoton microscopy has among the best signal-tonoise ratios, greatest imaging depths, and longest sample lifetimes.[29] © 2008 Adis Data Information BV. All rights reserved.

Fluorescence optical imaging offers single-cell resolution and real-time imaging to monitor critical tumour-host interactions, the understanding of which is vital for elucidating tumour development, particularly with respect to tumour angiogenesis[40] and vascular responses to therapies such as external irradiation.[41] One advantage of optical imaging is that multiple probes with different spectral characteristics could potentially be used for multi-channel imaging. Near infrared optical imaging can provide rapid and cost-effective preclinical evaluation in small animal models with a clear spectral window.[42] Recently developed fluorescence-mediated tomography is the imaging method of choice for deeper targets.[26,43] In this method, the emitted light is captured by detectors arranged in a spatially defined order in an imaging chamber and a tomographic image is reconstituted after the data are mathematically processed. However, even fluorescence-mediated tomography has difficulties visualizing tissues deeper than 1 cm.[44] Bioluminescence imaging is commonly used for preclinical cellular and molecular imaging in small animals. Bioluminescence imaging exploits the emission of visible photons at specific wavelengths based on energy-dependent reactions catalysed by luciferases. Luciferases comprise a family of photoproteins that emit detectable photons in the presence of oxygen and adenosine triphosphate during metabolism of substrates such as luciferin into oxyluciferin.[43] The light output per cell can be determined in culture as an a priori assessment of the detection sensitivity for signals in a given animal model, although there is a marked reduction of signal in vivo.[45,46] Bioluminescence imaging, especially with firefly luciferase, has several advantages in small animal imaging. Firstly, bioluminescence imaging is highly sensitive as a result of the combination of enzymatic amplification of signals from luciferase and the almost negligible background bioluminescence in vivo. Secondly, rapid distribution of luciferin after intraperitoneal injection and its ability to pass across blood-tissue barriers, including those found in the brain and placenta, widen the application of bioluDrugs R D 2008; 9 (6)

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minescence imaging in small animals.[47] Finally, besides firefly luciferase, there are a variety of luciferase enzymes from other organisms that possess unique spectral characteristics and substrate requirements, making it possible to monitor two different biological processes at the same time using appropriate spectral filters and substrates.[48] Therefore, bioluminescence imaging is a powerful tool for investigating mechanisms of disease and accelerating drug development in preclinical models.[49] 1.4 Ultrasound

Ultrasound images are essentially maps of tissue echogenicity derived from the same principles as SONAR (Sound Navigation And Ranging). The simplicity, ease of use, speed and safety of ultrasound have given it a significant role in diagnosis, treatment assessment, follow-up and guidance of therapy.[50] The contrast of ultrasound is dependent on the sound speed, sound attenuation, backscatter and imaging algorithm.[51] Ultrasound is well established as a means of measuring blood flow or, more precisely, blood velocity using the Doppler principle.[52] With colour Doppler imaging ultrasound, for example, significant changes in blood flow in the renal artery feeding a murine renal cell carcinoma tumour were observed during treatment with a vascular endothelial growth factor receptor (VEGFR) inhibitor.[53] Microbubble contrast agents present a diameter from <1 to 5 μm and are made from a shell of biocompatible materials (e.g. proteins, lipids or biopolymers) and a filling gas. Due to their intrinsic compressibility (approximately 17 000 times greater than water), microbubbles are very strong scatterers of ultrasound and can be detected at typical clinical frequencies of 1–13 MHz. Specific targeting of microbubbles can be accomplished by ligand conjugation to the microbubble surface using various strategies.[50,54] Contrast-enhanced ultrasound imaging of integrin αvβ3 during tumour angiogenesis in a rat brain tumour model has been reported.[55] Targeting of the microbubbles (3–4 μm in diameter) was achieved with echistatin, an arginine-glycineaspartic acid (RGD)-containing disintegrin that © 2008 Adis Data Information BV. All rights reserved.

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binds specifically to integrin αvβ3.[56] Contrast-enhanced ultrasound perfusion imaging using nontargeted microbubbles has also been performed to determine tumour microvascular blood volume and blood velocity.[57] Cyanoacrylate microbubbles linked to VEGFR receptor 2 (VEGFR-2) and αvβ3 integrin binding ligands were used to investigate changes in molecular cancer marker expression during matrix metalloproteinase (MMP) inhibitor treatment in a xenograft model.[58] Multimarker imaging achieved during the same imaging session indicated a significant increase in VEGFR-2 and αvβ3 integrin expression during tumour angiogenesis and a considerable decrease in both marker densities after treatment (i.e. decreased vessel density). In addition, acoustic destruction of ‘payload-bearing’ microbubbles has been used to deliver drugs or to augment gene transfection.[59] Thus, angiogenesis-targeted microbubbles may also have applications in sitespecific anti-cancer therapy. 2. Molecular Imaging in Drug Discovery and Development Invasive tissue or body fluid sampling has been the conventional method for obtaining cellular and tissue samples for laboratory-based analysis of pharmacokinetic information and therapeutic endpoints. Plasma and tumour samples can be assessed by a range of assays, including northern and western blotting, ELISA, immunohistochemistry, real-time polymerase chain reaction (RT-PCR) and gene expression microarrays. However, the selection bias and the low sensitivity of traditional immunohistochemical techniques for identifying partial, rather than complete, reductions in protein expression limit the feasibility of such studies.[60] Therefore, there has been growing interest in the use of non-invasive functional and molecular imaging techniques in the process of drug discovery. This is especially important given the recent shift in oncology drug discovery from conventional cytotoxic agents to novel agents acting on specific molecular targets.[61] Recognition that the latter class of drugs may be more likely to be cytostatic than cytotoxic means that the Drugs R D 2008; 9 (6)

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traditional methods of evaluating antitumour activity by reduction in tumour size[62,63] may no longer be appropriate or adequate.[64] The advantage of molecular imaging techniques over more conventional methods is that they can be performed in the intact organism with sufficient spatial and temporal resolution for studying biological processes in vivo. In addition, non-invasive and repetitive study of the same living subject at different time points decreases statistical variance and reduces both the number of animals required and costs.[65] Use of molecular imaging techniques in early phases of drug development can (i) identify specific molecular targets; (ii) provide information on the optimal biological dose and pharmacokinetic/ pharmacodynamic relationships; and (iii) provide in vivo pharmacodynamic evaluation of compounds.[66] For example, as anti-cancer strategies become more directed towards a defined molecular target, real-time information relevant to whether the molecular target is expressed, the selectivity and binding of the compound for that target, and the effects of such an interaction are required. Molecular imaging uses new and emerging quantitative functional imaging technologies to examine molecular pathways and provides pharmacokinetic, pharmacodynamic and mechanistic information. The information obtained via imaging also can be categorized as: (i) the interactions of a drug or drug candidate with the desired target, including dose occupancy relationships and kinetic information; (ii) determination of desirable, pharmacological effects or undesirable adverse effects; (iii) the delivery of a drug to a specific target; and (iv) the absorption, distribution, metabolism and elimination of the labelled drug candidate. Such information will help answer several key questions in drug discovery and development, especially with respect to targeted therapy. Consequently, molecular imaging has begun to play an important role in identifying new targets, validating in vivo targets, accelerating drug development, and halting compound development at an early stage if the compound proves not to have the desired mechanism and appropriate pharmacokine© 2008 Adis Data Information BV. All rights reserved.

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tic and pharmacodynamic characteristics. In this section, we address several important aspects of the role of molecular imaging in the drug discovery and development process. 2.1 Measurement of Pharmacodynamic Endpoints

Therapeutic efficacy is one of the key characteristics that needs to be determined for each drug or drug candidate. Molecular imaging is able to provide information to evaluate the pharmacological effects of a compound. Some imaging strategies, such as PET imaging with 18F-fluoro-2-deoxy-Dglucose (18F-FDG) or 18F-3′-deoxy-3′-fluorothymidine (18F-FLT), have a generic capacity that can be applied in the evaluation of a wide range of drugs. For example, FDG PET can provide information about glucose transport and metabolism, whereas FLT PET can evaluate the proliferating status of malignant diseases during treatment with almost all anti-tumour drugs. Some imaging strategies, however, focus only on one particular pathway or single molecular target for specific drugs. 2.1.1 Imaging of Metabolism and Proliferation with Fluoro-2-Deoxy-D-Glucose and 3’-Deoxy-3’-Fluorothymidine PET

Accelerated glucose metabolism is one of the phenotypic or functional changes observed in cancer tissue.[67] Tumour cells are known to be highly glycolytic because of increased expression of glycolytic enzymes, especially hexokinase. FDG is a model PET radiopharmaceutical and has been lauded as the ‘molecule of the century’ in nuclear medicine.[68] As a glucose analogue, FDG enters the cell in a similar manner to glucose, i.e. through specific glucose transporters on the cell membrane, and is converted to deoxyglucose 6-phosphate, which is trapped in the cell in proportion to the metabolism of glucose.[69] As the magnitude of FDG uptake in certain tumours relates directly to the number of viable cells, FDG-PET imaging provides high specificity and sensitivity in several kinds of cancer with many applications in the clinical management of patients with malignant diseases. However, various tissues and processes in the body, including inflammatory Drugs R D 2008; 9 (6)

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cells and macrophages, have greater glucose metabolic rates and thus accumulate higher amounts of FDG than normal tissues, which explains the rate of false-positives and false-negatives seen in imaging data obtained from FDG PET.[68] FDG PET has been extensively used to monitor therapeutic responses to radiation therapy and chemotherapy both preclinically and clinically.[39,70] Using a small animal PET, FDG uptake in gastrointestinal stromal tumours xenografts was significantly decreased 24 hours after treatment with imatinib, which correlated with response to treatment.[71] In one study evaluating the use of FDG PET to monitor chemotherapy effects in a human non-small cell lung cancer xenograft model, tumour FDG uptakes and volumes were measured after administration of a single dose of mitomycin and vinblastine.[72] A significant reduction in tumour volume after chemotherapy occurred and was associated with significantly lower FDG uptake values than in the control group, as early as on day 1. As shown in figure 1, with subcutaneous U87MG human glioblastoma xenografts treated with CE-355621, an antibody against c-Met, FDG small animal PET visualized significant inhibition of FDG accumulation only 3 days after drug treatment, which was earlier than the inhibition of tumour volume growth seen 7 days after drug treatment.[73] Dandekar et al.[74] evaluated the reproducibility of FDG small-animal PET studies using a B16F10 murine melanoma model. FDG small-animal PET mouse xenograft studies were reproducible with moderately low variability, indicating that serial small-animal PET studies may be performed with reasonable accuracy to measure tumour response to therapy. Increased mitotic rate, cell proliferation and lack of differentiation are regarded as the main factors responsible for accelerated growth of malignant tissue. Molecular imaging probes have been designed to specifically measure proliferation, with 18F-FLT having been the most extensively investigated tracer to image cell proliferation. FLT is transported into cells in a manner that is similar to the thymidine pathway and phosphorylated to FLT-5′-monophosphate by the enzyme thymidine kinase-1. FLT © 2008 Adis Data Information BV. All rights reserved.

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a 15 %ID/g

15 %ID/g Tumour volume (mm3) 150 Max %ID/g 6.6 Day –6

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245 7.5 10

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Fig. 1. Representative axial 18F-fluoro-2-deoxy-D-glucose (FDG) micropositron emission tomography images from nude mice with U87MG xenografts (arrows). CE-355621 or control vehicle was administered on day 7. (a) 18F-FDG accumulation increased over time in a representative control mouse xenograft. (b) 18F-FDG accumulation on days 8–21 in a representative drug-treated mouse xenograft was similar to that on baseline day 6. Reproduced from Tseng et al.,[73] with permission. % ID/g = percentage of injected dose per gram; Max = maximum.

phosphates are impermeable to the cell membrane, resistant to degradation, and become metabolically trapped inside the cells. The level of thymidine kinase-1 in a cell increases several fold as it passes from a resting state to the proliferative phase and is destroyed at the end of S-phase.[75] Therefore, radiolabelled thymidine analogues provide a measure of DNA synthesis and tumour cell proliferation. Several other radiolabelled nucleotide analogues have also been developed to measure DNA synthesis including 18F-fluorouridine and 2′-18F-fluoro-5methyl-1-β-d-arabinofuranosyluracil.[76] Although increased glucose metabolism is a feature of tumours, it is also associated with a variety of other processes, whereas cellular proliferation is specific to tumours. In addition, certain anticancer drugs have been designed to stop the cell division without necessarily leading to cell death. Consequently, tumour cellular proliferation may decline without any significant changes in tumour energy Drugs R D 2008; 9 (6)

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metabolism. For instance, FLT PET has been used to measure early cytostasis and cytotoxicity induced by cisplatin treatment of radiation-induced fibrosarcoma 1 tumour-bearing mice.[77] The decrease in 18F-FLT uptake at 24 hours was associated with a decrease in immunohistochemically assessed cell proliferation, despite a lack of change in tumour size. It also has been demonstrated that FLT microPET mouse tumour xenograft studies are reproducible with moderately low variability.[78] However, there are drawbacks to FLT PET in its role of reflecting tumour proliferation, such as low tumour uptake, different uptake and metabolism pathway to that of thymidine, and unconfirmed specificity for malignant diseases.[79] 2.1.2 Imaging Blood Flow and its Relevance to Antivascular Agents

Angiogenesis refers to the process by which new blood vessels are formed. This process has been recognized as a key element in the pathophysiology of tumour growth and metastasis.[80] Antiangiogenic and antivascular agents have been intensively investigated for tumour therapy. Traditionally, tumour angiogenesis and antiangiogenic therapy have been evaluated by methods such as measurement of circulating angiogenic markers and histological estimates of microvascular density. In contrast, imaging can provide a noninvasive means of detecting angiogenesis within and about the perimeter of the whole tumour and provide functional information. As mentioned above, ultrasound (particularly microbubble contrast-enhanced ultrasound) is a valuable imaging modality for determining tumour microvascular blood volume and blood velocity.[57] In particular, dynamic contrast-enhanced ultrasonography allows repeated examinations and provides both morphological and functional analyses. Ultrasound modes based on the second harmonic signal generated by the nonlinear properties of contrast agents have provided information about tumour blood flow and quantification of contrastuptake kinetics within tumours after a bolus injection of contrast agent.[81] Several quantitative parameters considered as indicators of tumour flow such as the peak intensity or time to peak intensity © 2008 Adis Data Information BV. All rights reserved.

can be extracted from the time-intensity curves of contrast uptake.[82] Using dynamic contrast-enhanced ultrasonography, the antitumour efficacy of AVE8062, a tumour vasculature disruptive agent, has been assessed in melanoma-bearing nude mice.[83] PET studies with 15O and related tracers can offer direct physiological measurement of circulatory parameters of regional blood flow and vascular volume.[84] Dynamic contrast-enhanced MRI has also been well established as a means of investigating angiogenesis within tumours, and in particular the response to antiangiogenic therapy. Leakage of MR contrast agent through tumour vessels results in a fast ‘wash-in’ of contrast coupled with a rapid ‘wash-out’, allowing functional analysis of the tumour microcirculation.[85] Dynamic contrast-enhanced MRI can be performed with low-molecularweight contrast media such as Gd-diethylenetriamine pentaacetic acid or macromolecular contrast media such as Gd conjugated human serum albumin.[86] Ktrans, kep, fpV and ve are standardized output parameters derived from a two-compartment general kinetic model, which is the most widely accepted model and can be readily derived from first principles.[87] Ktrans represents the rate of contrast agent transfer from blood to interstitium, and kep is the reverse rate constant, representing backflow. The term fpV is the fraction of plasma volume, related to whole tissue volume, and ve is the fractional extravascular, extracellular leakage volume. These parameters can be depicted numerically or as colourencoded images. It has been shown that dynamic contrast-enhanced MRI parameters correlate with vascular permeability, and hence angiogenesis, within tumour tissue.[88] Dynamic contrast-enhanced MRI can be used to demonstrate the antiangiogenic effects of drugs early after their administration, and can predict traditional treatment response parameters such as changes in tumour size. The ability to accurately monitor angiogenesis response to therapy means that drug efficacy can be established at a very early stage of treatment so that Drugs R D 2008; 9 (6)

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Fig. 2. T2w, T1w-pre and T1w-postcontrast images for an animal before (upper panel) and 24 hours after (lower panel) treatment with SU6668. T2w, axial T2w RARE (rapid acquisition with relaxation enhancement image); T1w-pre and T1w-post, axial T1w gradient-recalled echo (GRE) images acquired before and 50 minutes after injection of Gd-diethylenetriamine pentaacetic acid (Gd-DTPA)-albumin. Reproduced from Marzola et al.,[93] with permission. fpV = fractional plasma volume; kPS = transendothelial permeability; T1 = longitudinal; T2 = transverse; Txw = Tx-weighted.

non-responding patients may be detected and management plans altered on a timely basis.[89] It has been shown that dynamic contrast-enhanced MRI can detect responses to PTK-ZK (vatalanib), a VEGFR tyrosine kinase inhibitor, as early as 2 days after commencement of therapy, in the form of significant reductions in the area under the Gd-contrast-medium curve (AUGC) or permeability parameters,[90] which also predict subsequent response. Low-molecular-weight contrast media dynamic contrast-enhanced MRI has also shown significant reductions in permeability values in patients treated with the antivascular agents AG-013736 (an inhibitor of VEGFR, platelet-derived growth factor [PDGF] receptor and c-Kit receptor tyrosine kinases)[91] and SU5416 (a selective inhibitor of VEGFR-2 tyrosine kinase).[92] Although consensus is still lacking on the exact kinetic model to be used when analysing dynamic contrast-enhanced MRI data, the differences among the various methods are often marginal. Therefore, dynamic contrast-enhanced MRI is rapidly emerging as the imaging technique of choice for monitoring clinical response in trials of new antiangiogenic and antivascular therapies. Unlike low-molecular-weight contrast media, the increased size of macromolecular contrast media makes them less diffusible, and Ktrans values may reflect permeability within tumours more accurate© 2008 Adis Data Information BV. All rights reserved.

ly.[85] Macromolecular contrast media can also provide more accurate estimates of tumour blood volume since they are excellent blood pool agents. For example, SU6668 is an oral, small molecule inhibitor of angiogenic receptor tyrosine kinases such as VEGFR-2 (Flk-1/KDR), PDGF receptor and fibroblast growth factor receptor. Dynamic contrastenhanced MRI clearly detected the early effect (after 24 hours of treatment) of SU6668 on tumour vasculature as 51% and 26% decreases in mean vessel permeability measured in the tumour rim and core, respectively.[93,94] A substantial decrease was also observed in mean fpV in the rim (59%) and core (35%) of the tumour (figure 2).[93,94] 2.2 Imaging Specific Downstream Targets

FDG/FLT PET and dynamic contrast-enhanced MRI have generic value for evaluating a wide range of anti-tumour and antivascular drugs. Molecular imaging strategies have also been developed for application to only a narrow group of compounds. A good example of this is the response of client proteins of heat shock protein 90 (HSP90) to HSP90 inhibitors. HSP90 is a key member of molecular chaperones that promote the proper folding of nascent polypeptides and ensure that protein-protein interactions occur in a productive manner under basal conditions.[95] The downstream effects of Drugs R D 2008; 9 (6)

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HSP90 inhibition influence a wide range of signaling processes that make possible the malignant properties of cancer cells. Therefore, HSP90 inhibitors exhibit a broad spectrum of anticancer activities. In addition, analysis of treatment-induced changes in relevant HSP90 client proteins could be used as pharmacodynamic endpoints for evaluation of therapeutic responses to HSP90 inhibitors. Human epidermal growth factor receptor 2 (HER-2) has been established as a client of HSP90 and HER-2 is dependent upon HSP90 for its stability throughout the whole lifespan of the receptor, including the maturation process in the endoplasmic reticulum and during the residency of the receptor at the plasma membrane. HER-2 is depleted within 2 hours of HSP90 inactivation.[96] A positron emitter 68Ga (t1/2 = 68 min)-labelled F(ab’)2 fragment of trastuzumab (herceptin, humanized antibody against HER-2) has been used to assess degradation of HER-2 by the HSP90 inhibitor 17-allylaminogeldanamycin (17-AAG).[97] Based on microPET quantification, HER-2 expression was reduced in the animals by almost 80% 24 hours after 17-AAG treatment (figure 3).[97] In a follow-up a

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Fig. 3. Micropositron emission tomography images (coronal slice and transverse slice through tumour and kidneys) of two different nude mice with single BT-474 tumours. (a) A mouse at 3 hours after injection with 68Ga-dodecanetetra-acetic acid (DOTA)-F(ab’)2-trastuzumab and before treatment with 17-allylaminogeldanamycin (17-AAG). (b) The same mouse after it had received 3 × 50 mg/kg of 17-AAG and had been rescanned 24 hours later. (c, d) Comparable images of a control mouse (c) 3 hours after one dose of 68Ga-DOTA-F(ab’)2-trastuzumab and (d) after a second dose 24 hours later. Reproduced from Smith-Jones et al.,[97] with permission.

© 2008 Adis Data Information BV. All rights reserved.

study, tumour response to 17-AAG treatment was assessed by 68Ga-dodecanetetra-acetic acid (DOTA)-F(ab’)2-trastuzumab and 18F-FDG PET. Within 24 hours of treatment, a significant decrease in HER-2 was measured by HER-2 PET, whereas 18F-FDG PET uptake was virtually unchanged. This indicates that HER-2 PET with 68Ga-DOTAF(ab’)2-herceptin could provide accurate information about the early response of the tumour to 17-AAG treatment.[98] Epidermal growth factor receptor (EGFR), another member of the HER family, has also been established as a client of HSP90.[99,100] Quantitative PET imaging of EGFR expression in tumour-bearing mice using 64Cu (t1/2 = 12.7 h)-labelled cetuximab showed that tumour uptake of 64Cu-DOTAcetuximab measured by PET had good linear correlation (r2 = 0.80) with EGFR expression level as quantified by Western blotting.[101] Quantitative micro-PET showed that 64Cu-DOTA-cetuximab had prominent tumour activity accumulation in untreated tumours but significantly lower uptake in 17-AAG-treated tumours 24 hours post-injection. Both immunofluorescence staining and Western blot confirmed significantly lower EGFR expression level in the tumour tissue during 17-AAG treatment. These results indicate that this approach may be valuable in monitoring therapeutic responses to HSP90 inhibitor 17-AAG in EGFR-positive cancer patients.[102] Another example involves the serine/threonine kinase Akt. Akt functions as a signaling hub at which many upstream signaling pathways converge.[103] Because Akt and its upstream regulators are dysregulated in some forms of cancer, these represent promising targets for pharmaceutical intervention.[104,105] Zhang et al.[106] have constructed a bioluminescent Akt reporter that contains an Akt consensus substrate peptide and a domain that binds phosphorylated amino acid residues flanked by the N-terminal (N-Luc) and C-terminal (C-Luc) domains of the firefly luciferase reporter molecule. With this reporter construction, Akt activity in cultured cells and tumour xenografts can be monitored quantitatively and dynamically in response to actiDrugs R D 2008; 9 (6)

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vation or inhibition of receptor tyrosine kinase, inhibition of phosphoinositide 3-kinase, or direct inhibition of Akt. In vivo results indicated that this technology could facilitate determination of the pharmacodynamics of drugs in animal models (figure 4). Moreover, this platform may be expanded to other key kinases in cancer. 2.3 Imaging Therapeutic Gene Expression with Relevance to Gene Therapy

Gene therapy is an evolving technique that seeks to use nucleic acids (DNA or RNA) to treat or prevent diseases. There are several approaches to gene therapy, including forced expression of a therapeutic gene on the background of a mutant gene (gene addition), replacing a mutated gene that causes disease with a healthy copy of the gene (gene replacement), inactivating or ‘knocking out’ a mutated gene that is functioning improperly, and introducing a novel gene into the body to help prevent or fight diseases.[107] To deliver a therapeutic gene to the patient’s target cells, an appropriate carrier molecule or gene delivery vehicle, often called a vector, may be used. Efforts are being directed towards celltype specific targeting for both payload delivery and gene expression, which can be visualized currently by biodistribution and transduction imaging, respecBefore

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Fig. 4. Bioluminescence imaging of Akt kinase activity. Bioluminescence activity before treatment (time 0) and in response to treatment with apoptosis inhibitor (API)-2 (20 mg/kg or 40 mg/kg) was monitored at various times. Reproduced from Zhang et al.,[106] with permission.

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tively. In addition, the treatment outcome can be monitored by non-invasive imaging methods. Transduction imaging visualizes transgene-mediated protein production, whereas biodistribution imaging visualizes the actual systemic distribution of gene delivery vectors.[108] It is important to evaluate both particle kinetics and transgene expression in vivo to generate an accurate picture of gene delivery and expression.[109] Molecular imaging of gene expression is usually achieved with the use of particular genes, called ‘imaging reporter genes’. Reporter genes can be used to study promoter/enhancer elements (both constitutive and inducible) involved in gene expression and endogenous gene expression through the use of transgenes containing endogenous promoters fused to the reporter.[110] In some cases, the therapeutic genes themselves are reporter genes and can be imaged directly, such as herpes simplex virus-1 thymidine kinase (HSV1-tk)[111] and sodium iodide symporter (NIS).[112] The expression of most other therapeutic genes that have no ligands or substrates for functional image analysis can be studied if they are linked to the expression of a reporter gene.[113] Usually, the therapeutic gene with expressing vectors will be replaced by or fused with a reporter gene. The spatial and temporal expression of reporter genes could be visualized and quantitated using multiple non-invasive imaging modalities. The rapid progress in molecular imaging, especially in the field of imaging gene expression, will greatly aid some gene therapy requirements and likely contribute to the success of this promising therapeutic modality.[114] For example, HSP promoters, particularly HSP70 promoters, have been commonly used for gene therapy strategies because they are both heatinducible and efficient.[115] A series of studies has been performed in which a suicide gene, such as that coding for thymidine kinase or cytosine deaminase, is introduced into tumour cells under the control of an HSP promoter.[116,117] Green fluorescence protein (GFP) has been utilized extensively as a reporter gene to evaluate the spatial and temporal control of gene expression driven by HSP70 promoters.[118-121] On a microscope heating stage using GFP as a Drugs R D 2008; 9 (6)

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reporter, HSP70 expression kinetics were visualized continuously in cultured bovine aortic endothelial cells. These cells were transfected with a DNA vector expressing an HSP70-GFP fusion protein under the control of HSP70 promoter. The kinetic profile of HSP70-GFP fusion protein is consistent with endogenous HSP70.[122] The expression of the reporter gene coding for thymidine kinase can be probed with pyrimidine nucleoside derivatives, such as 2′-deoxy-2′-fluoroβ-D-arabinofuranosyl-5-iodouracil, and acycloguanosine derivatives, such as 9-[4-fluoro-3-(hydroxymethyl)butyl]guanine. With PET imaging, spontaneous in vivo activation of the HSV1-tk suicide gene driven by the Grp78 promoter, a member of the HSP70 family, has been observed in growing tumours together with its activation in a controlled manner by photodynamic therapy.[117] NIS is responsible for the physiological uptake of iodide and is also able to concentrate pertechnetate (TcO4–), bromide (Br–) and perrhenate (ReO4–). With corresponding radionuclides, NIS gene expression could be visualized by SPECT or PET imaging.[123] Che and coworkers[124] have constructed a retroviral vector, pQHSP70/hNIS-IRES-eGFP (pQHNIG70), containing dual reporter gene hNIS and eGFR linked with an internal ribosome entry site (IRES) under the control of an inducible human HSP70 promoter. A stable ratio of radiotracer uptake to eGFP fluorescence and to HSP70 protein was demonstrated over a wide range of expression levels, induced by different levels of heating. Local application of heat thus can effectively induce hNIS and eGFP gene expression in vivo, and this expression can be efficiently visualized by fluorescence, scintigraphic and micro-PET imaging.[124] Native ferritin receptors can concentrate the body’s natural iron into quantities detectable by MRI, thus making ferritin an elegant MRI reporter gene. This method was used to image viral transduction in rat brain in a non-invasive manner without any external ligands. Genove et al.[125] used adenoviruses to deliver a ferritin transgene into specific host tissues and imaged cells that took up endogenous iron and became superparamagnetic. In this © 2008 Adis Data Information BV. All rights reserved.

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context, overexpressed ferritin receptors have been imaged using a heavy chain of murine ferritin, an iron storage molecule with ferrioxidase activity, for the detection of gene expression by MRI in vivo.[126] 2.4 Evaluation of Pharmacokinetics

Studies of the kinetics of drug absorption, distribution, metabolism and excretion form an important part of any drug development process. Poor pharmacokinetics is a major cause of drug ‘failure’. Conventionally, pharmacokinetic studies are performed by measuring drug concentrations in the plasma using high performance liquid chromatography (HPLC) and an appropriate detection method such as ultraviolet, mass spectrometry or radioactivity counting if the drug is labelled with a radioisotope such as tritium. Due to its extremely high sensitivity (as low as 10-12 mol/L), quantitative PET imaging can provide information on the kinetics, dosimetry and distribution of drugs in diseased and normal tissues within the field of view of the scanner, in addition to information on hepatobiliary and renal clearance. Small animal PET scanners allow scans to be conducted in small rodents and canine or primate models for the purpose of screening candidate compounds and refining the imaging paradigm before implementation in humans. Many drugs can be labelled with 11C or 18F with minimal or zero effects on the chemical/physicochemical properties of the compound, thereby allowing non-invasive monitoring of drug biodistribution.[127,128] Based on PET imaging, general pharmacokinetic parameters can be quantified and calculated, including peak radioactivity (Cmax), time to reach peak radioactivity (tmax), area under the radioactivitytime curve (AUC), uptake (standardized uptake value [SUV]), and proportions of the drug in various tissues relative to those in the blood. Other important kinetic parameters relating to uptake, distribution and washout can also be derived from mathematical modelling of tissue data, such as clearance from plasma to tissue (K1), clearance from tissue to plasma (K2), selective binding (K3), permeabilityproduct surface area, net unidirectional influx constant from plasma to tissue (KI), mean residence Drugs R D 2008; 9 (6)

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time, binding potential and tissue volume of distribution (partitioning between blood and tissue).[129] In most PET pharmacokinetic studies, only trace amounts of drugs are administered, but it may also be necessary to conduct studies with a formulation containing the appropriate pharmacological dose combined with the radiotracer to acquire more precise pharmacokinetic information at pharmacological doses. For instance, fleroxacin is a promising fluoroquinolone used to treat urinary tract infections, skin and soft tissue infections, gastrointestinal infections, and acute bacterial exacerbations of chronic bronchitis.[130] To evaluate the pharmacokinetics of fleroxacin, 18F-fleroxacin was synthesized and shown to be identical physically, chemically and in its antimicrobial activity to the commercially produced product. The pharmacokinetics of 18F-fleroxacin were then determined in healthy and infected animals by PET and tissue radioactivity measurements.[131,132] A similar strategy was also used in humans.[133] Another example involves the lipophilic alkylating agent 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU), one of the most effective agents for the treatment of intracerebral gliomas. A pharmacokinetic study by PET imaging demonstrated that intra-arterial administration of 11C-BCNU achieved concentrations of the drug in the tumour that averaged 50 times higher than the level found with a comparable intravenous dose.[134] Another promising application uses radioimmunoimaging to localize the biodistribution of radioimmunoconjugates and calculate the dosimetry for effective radioimmunotherapy. Biodistribution of 90Y for dosimetry calculations is typically obtained by imaging that uses the surrogate radiometal 111In because 90Y does not emit photons. The radiotracer 111In has a t1/2 almost identical to that of 90Y, emits two γ-rays of 171 and 245 keV, and can be readily incorporated into the same metal-chelating agents as 90Y. For these reasons, 111In has been considered an excellent analogue for 90Y. For example, the US FDA-approved radioimmunoconjugate ibritumomab tiuxetan is labelled with 90Y for the treatment of non-Hodgkin’s lymphoma (NHL), but requires © 2008 Adis Data Information BV. All rights reserved.

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use of 111In-ibritumomab tiuxetan to produce images of the tumour and normal organs for dosimetry and biodistribution studies.[135] However, it has been reported that 111In-trastuzumab did not parallel the uptake of 86Y-trastuzumab in the bone, and thus may not accurately predict the level of 90Y accumulation in the bone for clinical radioimmunotherapy applications.[136,137] Thus, quantitative information offered by PET through 86Y radiolabels could enable more accurate absorbed dose estimations for 90Y radioimmunotherapy (figure 5).[137] It has also been reported that 99mTc-monoclonal antibody (MAb) conjugates showed a similar pharmacokinetic behaviour to that of 186Re-MAb conjugates, and can therefore be used to predict the localization of 186Re-labelled MAbs and make dosimetric predictions in individual patients.[138] 3. Conclusions and Perspectives Molecular imaging is becoming more widely used as a noninvasive tool for drug discovery and drug screening. Compared with conventional methods for the evaluation of pharmacokinetics/ pharmacodynamics, molecular imaging has several major advantages. Use of molecular imaging endpoints for in vivo studies, rather than time-consuming conventional dissection and histology, substantially decreases workload. Furthermore, because ima 1h

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Fig. 5. Positron emission tomography images: (a) serial coronal images of two mice injected with 86Y-hu3S193, and (b) serial planar images of two mice injected with 111In-hu3S193. Reproduced from Lovqvist et al.,[137] with permission.

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aging is noninvasive and allows for longitudinal studies in a single animal, it also provides much more precise data of statistical relevance. Significant advances have been made in developing novel probes for multimodality molecular imaging of tumour angiogenesis. Small molecules, peptides, peptidomimetics, proteins and antibodies have been labelled with radioisotopes, superparamagnetic nanoparticles, fluorescent dyes, quantum dots and microbubbles for PET, SPECT, MRI, near-infrared fluorescence and ultrasound imaging of small animal tumour models. Thus, an important advantage of molecular imaging techniques is that they can bridge the gap between preclinical and clinical research to develop candidate drugs that have optimal target specificity, pharmacodynamics and efficacy. In addition, the exploratory investigational new drug (eIND) mechanism from the FDA will allow faster first-in-human studies. Compared with timeconsuming conventional methods, microdosing studies with novel imaging probes can provide an opportunity for early assessment of the safety profile and pharmacokinetics of drugs in healthy volunteers. Such rapid initial clinical studies will definitely accelerate the drug discovery process. Furthermore, the molecular imaging field has grown at a furious pace over the last decade, and the value of molecular imaging in drug development and screening is more widely accepted by pharmaceutical companies. Because of its high sensitivity and versatility, PET is the dominant imaging strategy in drug discovery and development at present. Labelling with carbon, oxygen and fluorine will minimize the change to the drug’s chemical structure or even keep the structure identical. Thus the biodistribution of imaging probes can model drug distribution perfectly. Even if the drug target is different from the imaging target, one can still use the imaging result as a useful surrogate to test the efficacy of the drug at a given dose. For these reasons, molecular imaging is widely expected to be regularly applied in many steps of the drug development process in the near future. © 2008 Adis Data Information BV. All rights reserved.

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Notwithstanding the many potential benefits of molecular imaging, to date it has yet to be fully utilized as a means of reducing costs and expediting the drug discovery and approval process. So far, most research efforts have been limited to probe optimization for enhanced tumour-targeting efficacy and improved in vivo kinetics. The imaging-based quantifications still need to be refined to reflect target expression or activity more accurately. Increased accuracy is critical if conventional sampling methods for pharmacokinetic parameter and pharmacodynamic endpoint evaluations are to be completely replaced. To achieve this goal, probes with optimal specificity and affinity to target molecules must be developed. In addition, further improvements in sensitivity and spatial/temporal resolution of the imaging techniques are still needed. To foster the continued discovery and development of imaging probes, active collaborations among researchers in different disciplines must take place. These efforts include investigations by cellular/molecular biologists to identify and validate molecular imaging targets, by chemists/radiochemists to synthesize and characterize the imaging probes, and by medical physicists/mathematicians to develop high sensitivity/high resolution imaging devices/hybrid instruments and better algorithms for further improving the signal-to-noise ratio of a given imaging device. Close partnerships among academic researchers, clinicians, pharmaceutical industries and regulatory agencies are essential to promote further development of imaging probes, to apply molecular/functional imaging approaches to predict and evaluate drugs’ effects during and after treatment, to move molecular imaging-guided intervention strategies quickly into the clinic, and to accelerate drug development. Acknowledgements This project was supported by the National Institute of Biomedical Imaging and Bioengineering (NIBIB) [R21 EB001785], National Cancer Institute (NCI) [R21 CA102123, P50 CA114747, U54 CA119367 and R24 CA93862], Department of Defense (W81XWH-04-1-0697, W81XWH-06-1-0665, W81XWH-06-1-0042, and DAMD17 -03-1-0143), and a Department of Defense Prostate Postdoc-

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toral Fellowship (to G. Niu). The authors have no conflicts of interest that are directly relevant to the content of this review.

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Correspondence: Dr Xiaoyuan Chen, The Molecular Imaging Program at Stanford (MIPS), Department of Radiology and Bio-X Program, Stanford University School of Medicine, 1201 Welch Rd, Stanford, P095, CA 94305-5484, USA. E-mail: [email protected]

Drugs R D 2008; 9 (6)