Molecular imaging is defined as the ability to visualize and quantitatively measure the function of biological and cellular processes in vivo
While anatomical imaging plays a major role in medical imaging for diagnosis, surgical guidance/followup, and treatment monitoring, the rapidly evolving field of molecular imaging promises improvements in specificity and quantitation for screening and early diagnosis, focused and personalized therapy, and earlier treatment follow-up. The main advantage of in vivo
molecular imaging is its ability to characterize pathologies of diseased tissues without invasive biopsies or surgical procedures, and with this information in hand, a more personalized treatment planning regimen can be applied. For example, recent strategies for treatment of breast cancer involve combinations of several chemotherapeutic drugs that target epidermal growth factor receptor types I and 2 (EGFR and HER2/neu), mammalian target of rapamycin (mTor), estrogen receptor, and/or histone deacetylase, among others; however, the most effective strategy is dependent on the molecular profile of the tumor (e.g., HER2/neu-targeted therapy is only effective in HER2-positive breast cancers).3 In vivo
molecular imaging can be used to identify and quantify the molecular marker profile (e.g., EGFR, HER2) of the tumor without the invasiveness of a surgical biopsy and time associated with pathological characterization. The personalized medicine approach is especially important for determining the best care for patients with advanced stage cancers and poor prognosis - in this case, the risk of exposure to unwanted side-effects of therapy may outweigh the quality of remaining life.
Recent preclinical advances in molecular imaging contrast agents have demonstrated the ability to multiplex nano- and/or microparticles with several entities (): 1) a molecule for targeting to a specific tissue/disease marker (binding ligand); 2) a molecule that allows detection of the agent with different imaging modalities; and, 3) a direct attachment or system (e.g., Doxel is a liposome encapsulation of doxirubicin, a cytotoxic drug which inhibits DNA replication), for targeted delivery of a therapeutic drug at the site of interest. For example, Blanco et al.4
describe the direct attachment of the chemotherapy drug, Doxirubicin, to a superparamagnetic iron oxide (SPIO) nanoparticle, which is then encapulated in liposomes coated with RGD-peptides; thus, these particles specifically attach to tumor angiogenic vessels expressing high levels of αV
-integrins (protein receptors which bind RGD peptides), and the localization of these magnetic particles can be visualized using magnetic resonance imaging (MRI).
Figure 1 Contrast agents used for molecular imaging are composed of at least 2 entities: one component such as an antibody, peptide, nucleic acid, or a small molecule for binding to the molecular target, and a label for readout by an imaging modality (see also (more ...)
In addition, molecular imaging can be used to measure the response to therapy. Current practices in measuring tumor response to chemotherapy are governed primarily by the Response Evaluation Criteria in Solid Tumors (RECIST) approach, which uses anatomical imaging methods such as computed tomography (CT) or magnetic resonance imaging (MRI) to measure changes in tumor size; however, measurable effects of therapy on tumor volume may take considerable time (weeks to months), indicating that tumor volumetric changes are not an accurate reflection of therapeutic efficacy for some therapties.5
Molecular imaging has the potential to improve therapeutic monitoring by for example measuring the direct effect of a drug at an earlier time point before overt morphological-anatomical changes become visible on imaging. Most chemotherapeutic/anti-cancer drugs are either directed at specific molecular targets such as epidermal growth factor receptor (EGFR; drugs include erlotinib, cetuximab, and gefitnib), VEGFR (drugs include bevacizumab, sunitinib, axitinib, and vatalanib), estrogen receptor (such as tamoxifen), and EGFR type 2 (also known as ErbB2 or HER2/neu; drug such as trastuzamab), or, they are cytotoxic (drugs include paclitaxel/taxol, fluoruracil, or gemcitabine, among others) to promote tumor cell death. Molecular imaging agents have been designed and tested preclinically in rodent models to image all of the aforementioned molecular targets as well as cellular events such as metabolic activity or apoptosis 6
and, therefore, may be used in the future to monitor treatment effect at the molecular level at earlier time points after treatment initiation than with current imaging strategies.
This article reviews current clinical practices of molecular imaging and highlights promising strategies using optical and acoustic techniques that may be translated into clinical applications in the near future.
Current Clinical Molecular Imaging Strategies
Various imaging modalities are used for medical imaging, including positron emission tomography (PET), single photon emission computed tomography (SPECT), magnetic resonance imaging (MRI), magnetic resonance spectroscopy (MRS), ultrasound (US), and computed tomography (CT) (). The majority of molecular imaging in the clinic is currently performed only with PET, SPECT, and MRS imaging. Several PET () and SPECT () radiotracers are used for medical imaging applications, including oncology, cardiology, and neurology, and are discussed in detail elsewhere (YY et al. and ZZ et al. for PET; AA et al. for SPECT in this issue). MRS is a technique of MRI that measures changes in proton/nuclei excitation/relaxation associated with various metabolites, such as choline, pyruvate, lactate, lipids, and polyamines, among others.7, 8
Several MRS techniques, including 1
P, and 13
C MRS, have been developed and are reviewed elsewhere (see BB et al. in this issue, and reviews9, 10
). Clinical applications of MRS include oncology,9
and musculoskeletal diseases,11
among others ().
Table 1 Advantages and disadvantages of imaging modalities used with molecularly targeted or non-targeted contrast agents in a clinical setting. Adapted from references:31, 101
Table 2 Commonly used PET tracers for clinical molecular imaging of diseases. Adapted from references:114–122
Table 3 Commonly used SPECT tracers for clinical molecular imaging of diseases. Note, many agents image blood vessels and perfusion and/or excretion (labelled N/A for Molecular Target). Adapted from references:105, 123
Clinical applications with molecular MRI/MRS and Optical/Raman in vivo imaging.
Current clinical applications of real-time in vivo
optical imaging techniques are limited to surface (e.g., skin12, 13
) or ocular14, 15
imaging since they suffer from limited depth penetration through human tissue ( and ). However, increasing technological advances in endoscopic (e.g., monitoring Barrett’s esophagus) and catheter devices (e.g., imaging of atherosclerosis or bladder cancer) for optical coherence tomography(OCT)15, 16
as well as microscopy hold promise for novel clinical applications (e.g., Wang et al. 17
used a confocal microendoscope with topically-administered fluorescein to image abnormal lesions and colonic pathology
THIS IS NOT A CLINICAL TERM – PLEASE WRITE WHAT COLONIC PATHOLGOIES THEY WERE ADDRESSING in patients undergoing colonoscopy). Furthermore, a multi-photon NIRF source, where two or more photons are used to excite the fluorescent dye/nanoparticle, has been integrated in a tomographical scanner and microendoscope; this approach has been used for clinical optical imaging of skin cancer and other dermatological pathologies.18
Most of these devices operate by applying photons for excitation, and measuring reflected light. Alternatively, detection can occur by measuring light scattering effects, as in change of energy before and after the photon collides with a molecule – known commonly as the Raman effect19
(described in detail below). Since the change in energy is dependent upon the strength of the molecular bond which is colliding with the photon, the Raman signal is a series of peaks representing a specific molecular bond.19, 20
Thus, Raman spectrophotometry is an emerging molecular imaging technique that can acquire multiple molecular signatures with a single image. Raman spectroscopy and other optical imaging techniques have been used in a few clinical applications; however, they are limited in number since fluorescent-based and Raman-spectra contrast agents, including near-infrared fluorescent (NIRF) (advantageous for deeper penetration and low background fluorescence12
) dyes, quantum dots (NIRF nanoparticles that are very bright and have long life-span12
), and nanoparticles with surface enhanced Raman scattering (SERS) properties,20
have not yet been fully evaluated for human use. So far, Raman spectroscopy for analysis of different molecular signatures has been used in the clinic for identifying atherosclerosis 21
as well as for cancer imaging (e.g., breast,22
). Clinical applications of optical imaging are summarized in Table 4 and include: 1) monitoring atherosclerosis-associated inflammation with protease-activated fluorescent probes (representing capthesin-B and matrix metalloprotease (MMP)-2/9 expression);24
and, 2) imaging of porphyrin (fluoresces blue light) accumulation in highly-proliferating cancer cells. Therefore, agents that can be chemically converted to porphyrin in cells can be added to “highlight” neoplastic cells. For preclinical applications, optical imaging is frequently used for assessment of many molecular contrast agents, drug testing, and for better understanding basic biological processes. However, clinical translation of the large array of preclinical optical molecular imaging strategies (discussed below) will require significant improvements in instrumentation, contrast agent evaluation, and data analysis for molecular quantification. Contrast-enhanced molecular ultrasound is a very attractive molecular imaging strategy since ultrasound imaging 1) is already a clinical imaging modality; 2) is relatively inexpensive and portable; 3) offers real-time, high resolution imaging; 4) can separate contrast and morphological imaging (with use of harmonics); and, 5) does not involve ionizing irradiation (). Contrast agents for current use with ultrasound are microspheres – gas-filled (e.g., perfluorobutane), lipid-shelled bubbles that are 1–4 μm in diameter (see Deshpande et al. in this issue). Microbubbles have been used for imaging primarily the micro- and macrovasculature,25
since their micron size limits them to vascular compartments. Several commercially available microbubbles include: Luminity®/Definity® (Bristol Myers Squibb), Optison™
(GE Healthcare), Sonovue® (Bracco), and Sonozoid™
A common clinical application of contrast-enhanced ultrasound imaging with microbubbles involve characterization of focal lesions (e.g. in the liver) based on vascular enhancement patterns using non-targeted microbubbles.27, 28
Currently, contrast-enhanced ultrasound is not yet advanced to imaging molecular markers in the clinical realm, although ongoing research is directed towards clinical translation of molecularly targeted ultrasound imaging.29, 30
In summary, current clinical molecular imaging is mostly performed with PET and SPECT, and several targeted radiopharmaceuticals for both imaging and dual-imaging/therapy are available. Optical, ultrasound, and other hybrid acoustic imaging strategies (e.g., photoacoustic imaging) offer real-time and inexpensive approaches, which may be well suited for routine clinical applications such as early disease detection and screening protocols involving frequent imaging. The following section reviews emerging preclinical developments in optical, ultrasound, and hybrid acoustic imaging.