Optical Approaches for Drug Screening Based Light-Harvesting Conjugated Polyelectrolyte

Multifunctional materials that can simultaneously provide therapeutic action and image the ensuing results provide new strategies for treating various diseases. Here, we show that a cationic conjugated polyelectrolyte with a molecular design containing a polythiophene-porphyrin dyad (PTP) has efficient anticancer and antifungal activities. Upon photoexcitation, energy is efficiently transferred from the polythiophene backbone to the porphyrin units, which readily produce singlet oxygen ('02) for rapidly killing neighboring cancer cells and fungi. Due to the light-harvesting ability of the electronically delocalized backbone and efficient energy transfer amongst the optical partners, the PTP shows a higher '0: generation efficiency and therefore improved therapeutic activity than a small molecule analog. Additionally, the fluorescent properties of PTP serve for another purpose, namely distinguishing amongst living and dead cells. PTP is therefore a promising multifunctional photosensitive agent for treating cancers and fungal infections, while concurrently providing optical monitoring capabilities. These findings illustrate new directions for the design of synthetic multipurpose water-soluble polymers. Conjugated polymers are characterized by a delocalized JT-electronic backbone structure and large optical absorption coefficients. Facile energy transfer along the backbone and between chains allows excitations to sample a larger number of environments in comparison to isolated small molecules.' Photoexcitations can be channeled to suitable adjacent acceptors by fluorescence resonance energy transfer (FRET) mechanisms, leading to sensitization of emitters to levels above those attained by direct excitation. Conjugated polyelectrolytes incorporate charged groups onto the conjugated polymer backbone. Such a structural modification improves solubility in aqueous media, an important requirement for interacting with biological systems and for the design of optically amplified fluorescent biosensors. Acceptor molecules can be chosen to have optoelectronic properties that increase FRET efficiencies.' Conjugated polyelectrolyte/acceptor systems have therefore gained much recent attention for applications in detection and cell imaging."'' Energy transfer to porphyrin acceptors can be chosen to harness excitations for generating reactive oxygen species"" and to open potential applications in photodynamic therapy (PDT). Based on these unique properties, conjugated polyelectrolyte/acceptor materials can be expected to act as both therapeutic and imaging agents in therapy. The imaging function is particularly significant when it is capable of monitoring and guiding treatment. Integration of imaging modalities with therapeutic function within a single bio-compatible material is therefore anticipated to increase in importance as a new and challenging design element. The use of PDT for fungal infections is also an emerging relevant topic on account of the increasing resistance to multiple conventional antifungal agents, which results in the recurrence of the infection and prolonged treatments. Fungi are more resistant to PDT, as compared to gram-negative bacteria, since their more rigid cell walls increase the barrier for drug penetration. Higher doses of light and photosensitizer concentrations are therefore required. Although new efficient antifungal photosensitizers have been developed, their toxicity limits their application in PDT.~° New alternative antifungal strategies and the design of multifunctional optical materials are therefore of interest. Here, we describe the synthesis and application of a cationic conjugated polythiophene-porphyrin dyad (PTP) that was specifically designed to concurrently act as a therapeutic and an imaging agent. Four significant characteristics were included in the molecular design shown in Scheme 1. First, the fractional content of porphyrin moieties linked to the polythiophene backbone is low (~ 1%), in order to minimize toxicity in the absence of photoexcitation. Second, the amphiphilic attributes of PTP were anticipated to promote adsorption to cells and fungi through a combination of electrostatic and hydrophobic binding forces. Furthermore, PTP self-assembles in aqueous media into nanoparticles with appropriate dimensions for improved uptake into cancer cells. Third, covalent attachment of porphyrin moieties to the light harvesting polythiophene backbone constrains the interchromophore distances for optimizing FRET. This design element is important for increasing the photocoversion efficiency of singlet oxygen ('02) generation. A practical consequence is that the conjugated polymer reduces the light intensity required to attain a target '02 concentration. Fourth, because the PTP backbone retains emission, it can also be used to monitor apoptosis and necrosis processes by fluorescence imaging, adding a new dimension to the function of the molecular construct. Such simultaneous imaging and PDT function extend the applications of water-soluble conjugated polymers beyond their established biosensing capabilities. Scheme 1 illustrates the mechanism of anticancer activity by FTP. The positively charged and amphiphilic structure of PTP promotes membrane adsorption and subsequent uptake into the interior of the cell. Under light irradiation, there is efficient energy transfer from the polythiophene backbone to the pendant porphyrin sites, followed by sensitization of oxygen to generate '0:. This generated '0: is toxic to the cells.