Biodistribution and toxicity of photoproducts of merocyanine 540

Effects of Preactivated MC540 in the Treatment of Lymphocytic Plasmacytic Stomatitis in Feline Leukemia virus and Feline Immunodeficiency virus Positive Cats

Photoactive compounds and drugs are used therapeutically as antibacterial, antiviral and antitumor agents. This report examines the use of a photoactive compound, preactivated merocyanine 540 (pMC540), in the treatment of stomatitis in two cats that are both feline immunodeficiency virus (FIV) positive. One of the cats was also feline leukemia virus (FeLV) positive. Dramatic short term improvement is reported with the dosage regimen and complications.

ART AND SCIENCE OF PHOTODYNAMIC THERAPY

1. Photodynamic therapy is an established modality for the treatment of solid tumours and other accessible lesions. Although the concept and practice of combining light with a photosensitizing agent for the treatment of disease states has been around for almost a century, the understanding of the art and science therein has been tremendously enhanced over the past few years. 2. Photosensitized reactions are dependent on the generation of reactive oxygen species, in particular singlet oxygen, which accounts for the damaging effects on biological macromolecules, such as membrane lipids and proteins. Therefore, compounds that give a good yield of (1)O(2) are used as photosensitizers. 3. The main photosensitizers used in the clinical setting belong to the photofrin family; however, newer and more effective sensitizers are being evaluated for their potential clinical effectiveness. 4. Light sources have moved from the use of white light with specific filters in the old days to the more recent use of monochromatic light sources, such as lasers, to more sophisticated light-emitting diodes. However, dosimetry remains a big issue mainly because of difficulties in establishing the optimum treatment conditions for an approach that requires the fine-tuning of several variables, such as sensitizer and light doses and drug-to-light interval, as well as the issues of skin photosensitivity and low selectivity. A newer development to circumvent these and provide a broader application for this concept has been the phenomenon of photo-activation, whereby photo-exposure of chromophores to generate novel, small biologically active compounds has been demonstrated successfully. 5. The aim of the present review was to provide a general overview of the art and science of photodynamic therapy and to highlight some of the issues and recent developments in further advancing this modality of treatment.

Reactive oxygen-dependent production of novel photochemotherapeutic agents

The reactive nature of species derived from oxygen, such as singlet oxygen and hydrogen peroxide, has been exploited in the clinical setting for targeting bacteria, viruses, and tumor cells by photodynamic excitation of a variety of chromophores. This modality, termed photodynamic therapy (PDT), is currently being used to treat some forms of cancer. However, the applicability of conventional PDT is limited due to the absolute dependence on simultaneous exposure of the target to the photoactive compound and light. In 1990, we demonstrated that the need for simultaneous exposure of the biological target to light and photosensitizer could be circumvented by prior exposure (activation) of the sensitizer molecule to light and its subsequent use as any other anti-cancer or anti-viral drug. By dint of the nature of the protocol, this process was termed preactivation. Since then, the generation of biologically active molecules in vitro by preactivation has been validated using a variety of chromophores, such as merocyanine 540, Photofrin II, and naphthalimide. Here we briefly review the role of reactive oxygen species in the photodynamic effect, and provide an explanation for the mechanism of preactivation. We propose that photo-oxidation not only provides a novel means for the generation of biologically active molecules, but could also explain, at least in part the mechanism of conventional PDT. It is likely that the light-dependent breakdown of the chromophore to generate novel active compounds, in addition to reactive oxygen species, also contributes to the photodynamic damage observed on simultaneous exposure of the chromophore and target tissue to light during PDT.-Pervaiz, S. Reactive oxygen-dependent production of novel photochemotherapeutic agents.

In vitro and in vivo growth suppression of MCF-7 human breast cancer by novel photoproducts and tamoxifen

BACKGROUND

Preactivation is a novel photochemical method for the production of chemotherapeutic compounds that exert their biologic effects independent of light. The compounds that are produced, preactivated merocyanine 540 (pMC540) and merodantoin, are cytotoxic to cultured human breast cancer cells but are only minimally cytotoxic toward normal cells. Their effects against breast cancer have not been studied in vivo.

METHODS

Estrogen-stimulated human MCF-7 breast adenocarcinoma cells were grown as solid tumors in athymic carrier mice. Animals bearing defined sizes of subcutaneously transplanted solid breast tumors received injections of pMC540 (250 mg/kg) with or without concurrent treatment with tamoxifen. Growth inhibitory effects of merodantoin (N,N'-dibutyl-2-thio-4,5-imidazolidion) on the breast tumor growth were determined.

RESULTS

Direct injection of established tumors with eight doses of pMC540 (250 mg/kg) administered on alternate days resulted in significant tumor regression (P = 0.002). In three of seven animals, palpable tumors could not be detected after this treatment (16 days). Treatment through intramuscular injections (20 doses) with pMC540 (250 mg/kg) also caused a significant suppression of tumor area (P = 0.004; P = 0.0882; P = 0.0903) and a marginally significant suppression of tumor weight and volume, respectively. Combined treatment with tamoxifen and pMC540 (100 mg/kg) caused a 67% suppression of breast tumor growth. Treatment with 20 doses of merodantoin (75 mg/kg) suppressed the growth of breast tumors by 98%.

CONCLUSION

To the authors' knowledge, these results show for the first time that photochemically generated novel compounds in pMC540 alone and in combination with tamoxifen are effective in suppressing in vivo growth of xenografted human MCF-7 breast tumors.