Novel Two-Photon Activated Photodynamic Therapy Agents: Design, Synthesis and Preclinical Studies on Subcutaneous Cancerous Tumor

摘要

Photodynamic therapy (PDT) as a method of cancer treatment has been improved dramatically over the past decade. Compared to other methods of treating cancerous tumors, such as the standard surgery, chemotherapy, radiation protocol, PDT is a more benign treatment that provides the patients a noninvasive alternative to surgery. PDT involves the activation of a photosensitizer (the key molecule to come into action for killing the tumor cell) with laser light. Currently FDA-approved PDT agents are limited to visible light activation at wavelengths below 700 nm, limiting the penetration depth to ca. 2-4 millimeters level, which is insufficient to treat typical deep subcutaneous cancerous tumors. In order to improve the efficacy of PDT treatment, it would be desirable to shift the irradiation wavelength to longer wavelengths in the tissue transparency window (700 – 1000 nm)2. Two-photon irradiation in the Near-infrared (NIR) is an attractive alternative to standard one-photon PDT if the intrinsic two-photon cross-sections of the photosensitizers are large enough to make two-photon PDT practical in terms of the laser fluence required for successful treatment. Two-photon absorption is a nonlinear optical process in which two photons are absorbed simultaneously, promoting the molecule to the excited state, however in order for this event to occur, the light must be tightly focused in 3D space. The high degree of focus on a target point can thus prevent damage to healthy adjacent tissue, and defines the margins of the PDT treatment protocol. Porphyrins have been used as photosensitizers in classical PDT for several decades, however, most porphyrins exhibit extremely small two-photon cross-sections (5-10 GM units). Over the past four years we have been developing a new approach to the design of effective two-photon PDT agents using a triad concept that incorporates three components for the noninvasive treatment of subcutaneous tumors into a single therapeutic agent: tumor targeting, tumor imaging and direct two-photon PDT treatment of the imaged tumor. We reported here the design and synthesis of a novel first-generation substituted porphyrin with greatly enhanced two-photon cross section, and its corresponding nano-ensemble triad composed of a small molecule targeting agent and a NIR imaging agent combined with the porphyrin in a single therapeutic agent. This triad approach overcomes several of the disadvantages of one photon PDT agents including targeting of the tumor, discrimination between health cell and tumor cells allowing cleaner treatment margins, the capability of imaging the tumor in real time, and the ability to treat tumors in the tissue transparency window. The efficacy of this ensemble in the two-photon PDT treatment of a human breast cancer tumor model (MDA-MB-231) implanted in SCID mouse mammary fat pads will be discussed below. This triad is currently designated as RA-301 (see Scheme 1.)