Design, synthesis and pharmacological characterization of a photoswitchable inhibitor of folate metabolism

Antifolates are structural analogues of folate derivatives, essential one-carbon donors in the synthesis of DNA in mammalian cells, that act as inhibitors of folate-dependent enzymes such as dihydrofolate reductase and thymidylate synthase. They constitute the oldest antimetabolite class of anticancer agents. Methotrexate (MTX), one of the first agents of this class, is still extensively used in the treatment of a variety of tumors, including acute lymphocytic leukemia, breast cancer, osteosarcoma, primary central nervous system lymphoma, head and neck cancer. It is also commonly used for the treatment of certain autoimmune diseases, such as rheumatoid arthritis and psoriasis. However, the clinical efficacy of MTX is often limited and compromised by toxic dose-related side effects, which lead to morbidity, interruption of the treatment, and occasional mortality. A promising approach to tackle these drawbacks is to activate the drug exclusively at its desired site of action. In fact, in those diseases that would benefit from a highly localized treatment, a precise spatiotemporal control over the activity of a chemotherapeutic agent would allow reducing the concentration of active compound outside the target tissue, improving the tolerability and hence the efficacy of the treatment. In this respect, light is a powerful tool representing a non-invasive regulatory signal for pharmacological applications, since it can be delivered with high precision regarding space, time, intensity, and wavelength. We have recently developed phototrexate, the first photoswitchable folate antagonist, by incorporation of a photochromic chemical unit into the structure of MTX. Phototrexate has been designed to be constitutively inactive in its thermodynamically stable configuration (E isomer), while it can be activated with light (Z isomer) to locally provide the pharmacological effects of the parent drug, as confirmed in our earlier experiments in vitro and in zebrafish larvae (Figure 1). Studies are currently underway to assess safety/tolerability profiles, pharmacokinetics, pharmacodynamics, and efficacy of our compound in vitro and in preclinical animal models. Our results indicate that phototrexate has a good potential for development as an innovative light-regulated antifolate agent, and constitutes a proof-of-concept towards the design of photopharmacological chemotherapies with enhanced tolerability and efficacy.