Nanotexaphyrins for Cancer Imaging and Therapy

Abstract

Texaphyrins are expanded porphyrin macrocycles with a 5-coordination pocket that can form stable 1:1 complexes with a large group of metal cations including many lanthanide ions, making them inherently amenable for theranostics (e.g., photodynamic therapy, MRI and radiation therapy). For example, gadolinium-texaphyrin (Gd-Tex, Xcytrin®) developed by the Sessler group was evaluated as a radiosensitizer all the way to Phase III trials. Although Gd-Tex did not gain FDA approval due to its limited radiosensitization efficacy, the fact that Gd-Tex showed good safety profile in phase III clinical trials makes texaphyrins attractive building block to develop new theranostic agents.We recently synthesized a texaphyrin-phospholipid building block via a key 1,2-dinitrophenyl-phospholipid intermediate, along with stable chelation of a library of 18 different metal ions into texaphyrin-lipid without compromising their self-assembly into nanotexaphyrins. To realize nanotexaphyrin’s theranostic properties, we developed a customizable strategy to build a family of nanotexaphyrins with quantitatively mixed and matched metal ions. We also developed a rapid and robust nanotexaphyrin radiolabeling method using a home-made disposable microfluidic system that achieved a high radiochemical yield. The optimized metalated nanotexaphyrin displayed excellent chemical, photo, and colloidal stabilities, and favorable pharmacokinetics and biodistribution profiles, resulting versatile cancer imaging and therapy applications. For example, the PSMA-targeted 111In/Lu-nanotexaphyrin demonstrated a preferential tumor accumulation in PSMA-positive tumor by both NIR fluorescence and SPECT/CT imaging, and a potent PDT effect that successfully inhibited PSMA+ tumor growth. On the radiation therapy front, we developed Gd-nanotexaphyrin to allow high density packing of Gd-Tex in a single nanovesicle to overcome the limited radiosensitization efficacy of monomeric Gd-Tex. As hypoxia is a common cause of radiation resistance, we loaded oxygen-carrying myoglobin into Gd-nanotexaphyrins, which enabled spatiotemporal codelivery of O2 and Gd-Tex into tumors, resulting in efficient relief of tumor hypoxia and significant enhancement of Gd-Tex radiosensitization. The synergistic mechanism between appropriate O2 delivery and enhanced Gd-Tex radiosensitization has been demonstrated. The high versatility and clinical translatability of nanotexaphyrins is a promising platform to develop cancer theranostics.