texaphyrins

Texaphyrins are a family of expanded porphyrin macrocycles that are larger than conventional porphyrins. They consist of five pyrrolic units linked to form a single, highly conjugated ring system capable of hosting a metal ion in the central cavity. The enlarged π-system gives texaphyrins strong absorption in the visible and near-infrared, and their photophysical properties can be tuned by peripheral substituents and by metal ion choice.

Texaphyrins are typically synthesized by iterative condensation and oxidation steps that assemble the macrocycle from pyrrolic fragments and meso-linkers. The resulting macrocycles readily bind a variety of metals, including gadolinium, lutetium, zinc, and iron. The choice of metal and substituents influences redox behavior, triplet yield, and magnetic or luminescent properties, enabling, for example, MRI contrast or photophysical activity.

In biomedicine, texaphyrins have been explored as diagnostic and therapeutic agents. The gadolinium-containing texaphyrin motexafin gadolinium (MGd) is the best-known example and has been studied as a tumor-avid radiosensitizer and, in some settings, as a photosensitizer for photodynamic therapy and as an imaging agent. Texaphyrins tend to localize in tumors, in part due to the enhanced permeability and retention effect, and they can be activated by light or radiation to generate reactive species or heat.

Despite potential advantages, clinical development faces challenges such as solubility, biodistribution control, and long-term safety. Ongoing research aims to improve water solubility, achieve targeted delivery, and better understand the interactions between texaphyrins, metals, and biological environments.