In this account, the synthesis and biological evaluation of metalloporphyrins and related structures for use in biomedical imaging applications are outlined. The structural design of these tetrapyrrolic metal chelators is discussed, with evaluation of strategies designed to facilitate more rapid chelation under milder conditions, improve water solubility, and allow for conjugation to targeting groups and macromolecular structures. Particular emphasis is given to the application of these metalloporphyrin structures to clinically relevant imaging techniques including magnetic resonance imaging, fluorescence imaging, radio-imaging, and Raman and photo-acoustic imaging, with a broad overview of the progress made in these areas. The future of research in this area is also examined, with the emerging research interest in more modern multimodal imaging set to dominate the future of biomedical imaging.

Metalloporphyrins for Medical Imaging Applications

Porphyrins and the related tetrapyrrolic macrocyclic compounds, chlorins, bacteriochlorins and corrins are prosthetic groups of a large number of biological molecules which serve diversified roles in nature. These macrocycles, coordinated to a central metal ion, perform functions such as oxygen transport and storage (hemoglobin and myoglobin), electron and energy transfer (cytochromes and chlorophylls) and biocatalysis (coenzyme B12 cytochrome P-450). The metal-free (“free-base”) porphyrins on the other hand are generally present in organisms as precursors of metalloporphyrins and are accumulated and/or excreted in certain physiological disorders such as porphyrias (1). The selective biodistribution of free-base porphyrins was reported by Policard (2) as early as 1924, based on the observed orange-red fluorescence from neoplastic tissue. The preferential localization of certain parenterally administered porphyrins in tumor cells, coupled with the property of intense fluorescence, have since been used for convenient detection of such tissue in situ. In addition, the ability of porphyrins to act as photosensitizers, especially in the production of highly reactive singlet oxygen which in turn exerts cytotoxic action, has been used extensively in the novel treatment of malignant tissue, a technique referred to as photoradiation therapy (PRT).

Some preparations and properties of porphyrins.

Although the pathomechanisms of atherosclerosis are well known, their radioisotopic monitoring is still in its early childhood. The current radioisotope techniques are of only limited value for contributing to the clinical diagnosis of atherosclerosis. The limited reaction time of cellular blood constituents (platelets, monocytes) with the vascular surface at the injury site makes it very difficult to catch the point of injury. Lipoproteins excellently allow receptor imaging, while vascular monitoring is only of scientific interest at present. Labelling and subsequent imaging of components of the coagulation cascade have not succeeded so far, nor have attempts using unspecific labels such as porphyrin, polyclonal IgG and Fc fragments, for example. Preliminary evidence indicates that radioisotopic techniques may be of great benefit in the future in elucidating functional aspects of the disease, while they do not contribute to examining the stage and extent of atherosclerosis.

Nuclear medicine and atherosclerosis

Platelet labeling--methodology and clinical applications.

Synthesis of a monoclonal antibody-indium-111-porphyrin conjugate.

Chloro 111in tetraphenylporphyrin and its possible clinical application

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Folate, vitamin B12 and posture.

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Chloro 111in tetraphenylporphyrin and its possible clinical application

Folate, vitamin B12 and posture.