Paving the Road Towards Non-Invasive Molecular Imaging

Driven by curiosity and technically capable to construct a simple microscope from hand-made magnifying glasses, Antoni van Leeuwenhoek was the first to discover the world of “very little animalcules” in 1674. When he reported his sensational observations in a letter to the Royal Society of London, he felt obliged to increase the reliability of his findings by including written attestations from ministers, jurists, and medical men [8]. Now, more than 300 years later, the strong desire to actually see biological events “live” is still a major driving force behind numerous technological developments in the field of image analysis. In contrast to the era of Antoni van Leeuwenhoek, internet allows all members of the scientific community to witness exciting observations. For instance, experience the thrill of viewing the assembly of endothelial tubes in vivo [9]. Seeing is believing! Molecular biologists are facing the challenge to identify the functions of thousands of proteins encoded by genes whose nucleotide sequences were just recently revealed by the human genome project [11]. Molecular imaging is an important part of their toolbox. In past decades, the role of many genes in biological processes like cancer has been investigated at the DNA level [17], RNA level [20], and protein level [14]. Immunohistochemical stainings revealed protein expression patterns in (pathogenic) tissue sections from humanand experimental animal origin. Labeling of antibodies with fluorescent dyes allowed investigating protein subcellular localization and co-localization with other molecules by (confocal laser scan) fluorescence microscopy. Until recently, obtaining cells or tissue samples from biopsies, resection material, or sacrificed experimental animals has been a requisite to perform these studies. By now, this limitation can be circumvented by making use of genetically engineered fluorescently tagged proteins whose movement and interactions with other proteins can be monitored in living cells in vitro [5,12]. Moreover, such constructs encoding fluorescent or bioluminescent proteins have also been introduced in living experimental animals as transgenes, thereby creating for example green fluorescent mice that are suited to investigate tumor–host interactions in xenograft models in vivo [19]. These technological developments greatly enhance our basic understanding of the function of individual genes in health and disease. Oncologists are facing the challenge to detect cancer in its early, pre-malignant stages, or to recognize biological targets in tumors for which specific drugs are available or can be developed [3,4,6,13]. Current diagnostic imaging techniques in a standard clinical setting comprise Ultrasound (US), Computed Tomography (CT), and Magnetic Resonance Imaging (MRI), all of which provide anatomical rather than functional information [15]. Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) make use of (short-lived) radioactive tracers like 18F-FDG, which provides metabolic information [16]. The translation of molecular information into clinical imaging applications is still in its infancy. At least two gaps need to be closed. First, diagnostic molecular imaging requires availability of ‘biomarkers’, i.e. molecules that serve as specific indicators of disease status. The current list of cancer biomarkers is limited, however, considering the enormous scientific, political and financial support to identify new biomarkers, extension of this list is merely a matter of time [1,2,7]. Second, the tools to analyze and validate biomarkers for non-invasive molecular imaging in vivo using experimental animals have to be established. For one thing, the diagnostic imaging equip-