Stimuli-responsive delivery systems are capable of site-specific drug delivery through the temporal and spatial control over drug release. To date, the clinical relevance of photoresponsive systems has been stifled by: (i) a lack biocompatible photoresponsive materials, and (ii) the reliance on ultraviolet (UV) light for reliable activation. UV light is genotoxic and unable to penetrate tissues; near infrared (NIR) light is biologically benign and tissue transparent. However, the low energy density of NIR light has made the activation of NIR-responsive chromophores unreliable. Therefore, there remains a critical need to develop a photoresponsive delivery system that is biocompatible, and can be reliably activated with clinically relevant NIR light. In order to address this need we evaluated a new class of photoresponsive materials, the alkoxyphenacyl-based polycarbonates (APP), which are constructed from biocompatible polymeric and chromophoric units. The construct of the APP polymers contains the chromophore within the polymer backbone such that exposure to the photo-trigger (250-320 nm light) causes photo-induced chain scission.
First, we set out to characterize the biocompatibility of the APP homopolymer and copolymers with polycaprolactone (PCL) and polyethylene glycol (PEG). The APP polymers and nanoparticles (NPs) prepared from the APP polymers (APP-NPs) via nanoprecipitation were comparable to poly(lactic-co-glycolic acid) (PLGA) before and after exposure to the photo-trigger during in vitro studies of cytotoxicity, macrophage activation, and red blood cell (RBC) lysis. In vivo biocompatibility evaluation in BALB/c mice revealed that liver and kidney functions were not distorted in mice treated with APP polymers, and this was corroborated by histopathological assessments. Measurements of plasma cytokines (TNF-α, IL-6) indicated that the APP polymers did not trigger an immune response.
Subsequently, the efficacy of APP-NPs as photoresponsive delivery systems was evaluated by loading doxorubicin (DOX) as a model drug (DOX-APP-NPs). In order to identify and evaluate formulation and photoirradiation parameters that influenced photoresponsive efficacy, the resultant NPs were characterized for: size, stability, morphology, zeta potential, DOX loading, and DOX release. Stable, spherical DOX-loaded NPs were prepared from the homopolymer and PCL copolymer with diameters between 70-180 nm depending on the polymer concentration (10-40 mg/mL) and polymer type. Exposure to the photo-trigger disrupted the surface of the NPs without visibly altering the core, and increased the rate and extent of DOX release. Photoresponsive DOX release was markedly influenced by the polymer type, frequency of photoirradiation, and polymer concentration. Photoirradiation did not negatively impact the functional efficacy of released DOX as assessed by cytotoxicity studies in Lewis lung carcinoma (LLC) cells. Studies in BALB/c mice indicated that drug release from DOX-APP-NPs was not subjected to dose dumping following photoirradiation.
The final portion of the project was devoted to the design and development of an APP delivery system that can respond to NIR. In this approach we adopted the process of photon upconversion to generate the photo-trigger (UV light) in situ from clinically relevant NIR light. As such, upconverting nanocrystals (UCNCs) were synthesized via a solvothermal method, and optimized for emissions in the UV region by varying the concentrations of the dopants (Yb3+ and Tm3+) and the particle architecture (core-alone and core-shell). UCNC-loaded APP-NPs (UCNC-NPs) were prepared from the APP homopolymer and characterized. The NaYF4(30% Yb3+ /0.15% Tm3+)@NaYbF4@NaYF4 UCNC formulation produced the most intense emissions in the UV region when exposed to NIR light (980 nm). UCNC-NPs had mean sizes between 153-393 nm depending on the UCNC concentration (0.5-5 mg/mL), and their morphology more closely resembled that of the parent UCNCs than APP-NPs prepared without UCNCs. DOX loading was not significantly affected by the inclusion of UCNCs in the NP formulation (p > 0.05). UCNC-NPs were unstable, forming visible precipitates within 72 hr. Inclusion of UCNCs in APP-NPs did not affect biocompatibility as well as in vivo biodistribution in tumor-bearing athymic mice. Exposure to NIR light was not effective in achieving photoresponsive drug release from the as-prepared delivery system during in vitro or ex vivo studies; plausibly due to insufficient generation of the photo-trigger as a result of minimal overlap between the wavelengths of light emitted from the UCNCs and chromophore’s photo-trigger.
Overall, this work demonstrated the design and evaluation of a new class of photoresponsive polymers in drug delivery. Particular attention was paid to identify and assess the impact of various formulation, photoirradiation, and therapeutic parameters that will affect specificity of photoresponsive drug release using the APP polymers.