Motivation of the study
In photodynamic therapy (PDT), systemically administered photosensitizer is activated within the tumor using focused near-infrared light, typically a laser with a wavelength matching the absorption peak of the photosensitizer. Several photosensitizers have been clinically approved for the treatment of different cancers; however, their accumulation is non-tumor exclusive which exposes healthy tissues to side effects like daylight-induced phototoxicity. A promising strategy to improve tumor selectivity is to couple photosensitizer to a tumor-targeting agent that binds to a specific antigen expressed on the surface of tumor cells.
In this study, a novel targeted agent HS201 was created by coupling verteporfin to a small molecule inhibitor of HSP90 (heat shock protein 90). Verteporfin is an FDA-approved photosensitizer for age-related macular degeneration and has also shown promising results in clinical trials for pancreatic cancer, while HSP90 is a chaperone protein that is highly expressed in many cancers and linked to aggressive tumor growth and poor prognosis. HS201 was studied in vitro and in vivo for inflammatory breast cancer (IBC) which is globally prevalent, particularly aggressive and lethal disease. The goal of the study was that tumor-selectivity of the HS201 could permit PDT treatment of diffuse cancer like IBC without the need for percutaneous insertion of laser fibers into the tumor and would spare healthy cells in the path of laser light as well as reduce systemic phototoxicity.
In vitro studies
Modulight’s ML8500 automated in vitro illumination system was used to study PDT parameters in a high-throughput manner. ML8500 can be equipped with up to 8 different laser wavelengths or can alternatively be connected to external light sources like ML7710 as was done in this study. Highly aggressive MDA-MB-231 breast cancer cells were seeded on 96-well plates, incubated with different concentrations (0-3 µM) of either verteporfin or HS201, and illuminated with 689 nm wavelength laser at various doses (0-30 J/cm2) in different wells. Also different drug-light intervals (0, 3, and 6 hours) were studied with escalating light doses (0-120 J/cm2). After laser irradiation, cells were cultured overnight and cell viability was analyzed with MTT assay.
In vivo studies
Breast cancer cells were grown in the right flank of immunodeficient mice. PDT was conducted once the tumors reached 6 mm in diameter. Photosensitizer was injected via tail vein and after the 6-hour drug-light interval tumors were irradiated for 4 minutes using Modulight’s ML7710 laser with a 689 nm wavelength and 120 J/cm2 light dose. The tumor size was monitored by caliper measurement twice a week for 25 days.
In vitro studies on breast cancer cells showed that the killing effect of PDT is dependent on both the laser light dose and photosensitizer concentration (Figure 1). HS201 showed higher cytotoxicity towards breast cancer cells at lower concentrations than verteporfin. HS201 and verteporfin both had high cytotoxicity with 0-hour and 3-hour drug-light intervals (Figure 2). However, only HS201 retained its cytotoxic efficacy when the longer 6-hour drug-light interval was used, while cytotoxicity of verteporfin was comparable to no treatment.
Figure 1: The cell killing efficacy of HS201 (left) and verteporfin (right) with different photosensitizer concentrations and laser light doses.
Figure 2: The cell killing efficacy of HS201, verteporfin, or no treatment with different drug-light intervals and laser light doses.
Tumor-bearing mice treated with HS201-PDT had significantly smaller tumors throughout the whole 25-day monitoring period than mice treated with verteporfin PDT (Figure 3). When tumor weights were compared, tumors from HS201-treated mice were significantly lighter than tumors from verteporfin-treated mice (Figure 4). In vivo results support efficient tumor suppression by HS201 PDT.
Figure 3: Tumor sizes in HS201-PDT and verteporfin PDT treated mice through the course of 25 days after treatment initiation.
Figure 4: Tumor weights at the end of the experiment from HS201-PDT and verteporfin PDT treated mice.
Related Modulight products and Services
Heat shock protein 90-targeted photodynamic therapy enables treatment of subcutaneous and visceral tumors
Communications Biology vol. 3, Article number: 226 (2020)
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