Porphyrins as Theranostic Agents from Prehistoric to Modern Times

Long before humans roamed the planet, porphyrins in blood were serving not only as in-dispensable oxygen carriers, but also as the bright red contrast agent that unmistakably in-dicates injury sites. They have proven valuable as whole body imaging modalities have emerged, with endogenous hemoglobin porphyrins being used for new approaches such as functional magnetic resonance imaging and photoacoustic imaging. With the capability for both near infrared fluorescence imaging and phototherapy, porphyrins were the first exog-enous agents that were employed with intrinsic multimodal theranostic character. Porphyrins have been used as tumor-specific diagnostic fluorescence imaging agents since 1924, as pos-itron emission agents since 1951, and as magnetic resonance (MR) contrast agents since 1987. Exogenous porphyrins remain in clinical use for photodynamic therapy. Because they can chelate a wide range of metals, exogenous porphyrins have demonstrated potential for use in radiotherapy and multimodal imaging modalities. Going forward, intrinsic porphyrin bio-compatibility and multimodality will keep new applications of this class of molecules at the forefront of theranostic research.