The photo-physical and photo-biological properties of two small (<2 kDa), novel Ru(ii) photosensitizers (PSs) referred to as TLD1411 and TLD1433 are presented. Both PSs are highly water-soluble, provide only very limited luminescence emission at 580-680 nm following excitation at 530 nm, and demonstrate high photostability with less than 50% photobleaching at radiant exposures H = 275 J cm(-2) (530 nm irradiation). It was previously shown that these two photosensitizers exhibit a large singlet oxygen ((1)O2) quantum yield (Φ (Δ) ∼0.99 in acetonitrile). Their photon-mediated efficacy to cause cell death (λ = 530 nm, H = 45 J cm(-2)) was tested in vitro in colon and glioma cancer cell lines (CT26.WT, CT26.CL25, F98, and U87) and demonstrated a strong photodynamic effect with complete cell death at concentrations as low as 4 and 1 μM for TLD1411 and TLD1433, respectively. Notably, dark toxicity was negligible at concentrations less than 25 and 10 μM for TLD1411 and TLD1433, respectively. The ability of the PSs to initiate Type I photoreactions was tested by exposing PS-treated U87 cells to light under hypoxic conditions (pO2 < 0.5%), which resulted in a complete loss of the PDT effect. In vivo, the maximum tolerated doses 50 (MTD50) were determined to be 36 mg kg(-1) (TLD1411) and 103 mg kg(-1) (TLD1433) using the BALB/c murine model. In vivo growth delay studies in the subcutaneous colon adenocarcinoma CT26.WT murine model were conducted at a photosensitizer dose equal to 0.5 and 0.2 MTD50 for TLD1411 and TLD1433, respectively. 4 hours post PS injection, tumours were irradiated with continuous wave or pulsed light sources (λ = 525-530 nm, H = 192 J cm(-2)). Overall, treatment with continuous wave light demonstrated a higher tumour destruction efficacy when compared to pulsed light. TLD1433 mediated PDT resulted in statistically significant longer animal survival compared to TLD1411. Two-thirds of TLD1433-treated mice survived more than 100 days (p < 0.01) whereas TLD1411-treated mice did not survive longer than 20 days. Here we present evidence that two novel PSs have very potent photo-biological properties and are able to cause PDT-mediated cell death in both in vitro cell culture models and in vivo tumour regression.

Metal-based photosensitizers are of interest as their absorption and chemical binding properties can be modified via the use of different ligands. Ru(2+) based photosensitizers are known to be effective photodynamic therapy (PDT) agents against bacteria, whereas use for oncological indications in vivo has not been demonstrated with the same level of evidence. We present data showing that premixing the Ru(2+)-complex TLD1433 with transferrin increases the molar extinction coefficient, including longer activation wavelengths, reduces photobleaching rates, and reduces the toxicity of the complex improving overall PDT efficacy. As the transferrin receptor is upregulated in most malignancies, premixing the Ru(2+) complex with transferrin converts the active pharmaceutical ingredient TLD1433 into a drug of potentially considerable clinical utility.

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It is generally acknowledged that the age of antibiotics could come to an end, due to their widespread, and inappropriate use. Particularly for chronic wounds alternatives are being thought. Antimicrobial Photodynamic Therapy (APDT) is a potential candidate, and while approved for some indications, such as periodontitis, chronic sinusitis and other niche indications, its use in chronic wounds is not established. To further facilitate the development of APDT in chronic wounds we present an easy to use animal model exhibiting the key hallmarks of chronic wounds, based on full-thickness skin wounds paired with an optically transparent cover. The moisture-retaining wound exhibited rapid expansion of pathogen colonies up to 8 days while not jeopardizing the host survival. Use of two bioluminescent pathogens; methicillin resistant Staphylococcus aureus (MRSA) and Pseudomonas aeruginosa permits real time monitoring of the pathogens. The murine model was employed to evaluate the performance of four different photosensitizers as mediators in Photodynamic Therapy. While all four photosensitizers, Rose Bengal, porphyrin TMPyP, New Methylene Blue, and TLD1411 demonstrated good to excellent antimicrobial efficacy in planktonic solutions at 1 to 50 μM concentrations, whereas in in vivo the growth delay was limited with 24–48 h delay in pathogen expansion for MRSA, and we noticed longer growth suppression of P. aeruginosa with TLD1411 mediated Photodynamic Therapy. The murine model will enable developing new strategies for enhancement of APDT for chronic wound infections.

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