Vitamin B₁₂: A Tunable, Long Wavelength, Light-Responsive Platform for Launching Therapeutic Agents

Light-responsive agents offer the promise of targeted therapy, whose benefits include (i) prolonged action at the target site, (ii) overall reduced systemic dosage, (iii) reduced adverse effects, and (iv) localized delivery of multiple agents. Although photoactivated prodrugs have been reported, these species generally require short wavelengths (<450 nm) for activation. However, maximal tissue penetrance by light occurs within the "optical window of tissue" (600-900 nm), well beyond the wavelength range of most existing photocleavable functional groups. Furthermore, since multidrug therapy holds promise for the treatment of complex diseases, from cancer to neurological disorders, controlling the action of multiple drugs via wavelength modulation would take advantage of a property that is unique to light. However, discrimination between existing photoresponsive moieties has thus far proven to be limited. We have developed a vitamin B12/light-facilitated strategy for controlling drug action using red, far-red, and NIR light. The technology is based on a light-triggered reaction displayed by a subset of B12 derivatives: alkyl-cob(III)alamins suffer photohomolysis of the C-Co(III) bond. The C-Co(III) bond is weak (<30 kcal/mol), and therefore all wavelengths absorbed by the corrin ring (330-580 nm) induce photocleavage. In addition, by appending fluorophores to the corrin ring, long wavelength light (>600 nm) is readily captured and used to separate the Co-appended ligand (e.g., a drug) from B12. Consequently, it is now feasible to preassign the wavelength of homolysis by simply installing a fluorescent antenna with the desired photophysical properties. The wavelength malleability inherent within this strategy has been used to construct photoresponsive compounds that launch different drugs by simply modulating the wavelength of illumination. In addition, these phototherapeutics have been installed on the surface and interior of cells, such as erythrocytes or neural stem cells, and released upon expoure to the appropriate wavelength. We have shown that cytotoxic agents, such as doxorubicin, anti-inflammatories, such as dexamethasone, and anti- and pro-vascular agents are readily released from cellular vehicles as biologically active agents. We have also demonstrated that the concept of "optical window of tissue" phototherapeutics is not just limited to prodrugs. For example, stem cells have received considerable attention in the area of regenerative medicine. Hydrogels serve as scaffolds for stem cell growth and differentiation. We have shown that the formation of hydrogels can be triggered, in the presence of cells, using appropriately designed alkyl-cob(III)alamins and long wavelength light. The potential applications of phototherapeutics are broad and include drug delivery for a variety of indications, tissue engineering, and surgery.