Melanoma resistance to photodynamic therapy: new insights

Abstract

Melanoma is the most dangerous form of skin cancer, with a steeply rising incidence and a poor prognosis in its advanced stages. Melanoma is highly resistant to traditional chemotherapy and radiotherapy, although modern targeted therapies such as BRAF inhibitors are showing some promise. Photodynamic therapy (PDT, the combination of photosensitizing dyes and visible light) has been tested in the treatment of melanoma with some promising results, but melanoma is generally considered to be resistant to it. Optical interference by the highly-pigmented melanin, the antioxidant effect of melanin, the sequestration of photosensitizers inside melanosomes, defects in apoptotic pathways, and the efflux of photosensitizers by ATP-binding cassette transporters have all been implicated in melanoma resistance to PDT. Approaches to overcoming melanoma resistance to PDT include: the discovery of highly active photosensitizers absorbing in the 700-800-nm near infrared spectral region; interventions that can temporarily reduce the amount or pigmentation of the melanin; compounds that can reverse apoptotic defects or inhibit drug-efflux of photosensitizers; and immunotherapy approaches that can take advantage of the ability of PDT to activate the host immune system against the tumor being treated.

Figure 1. Mechanisms of PDT

The ground state PS is initially excited to an excited singlet state that undergoes a transition to a long-lived triplet state that can interact with oxygen in a Type I mechanism to produce hydroxyl radicals or in a Type II mechanism to produce reactive singlet oxygen. These ROS can cause death of tumor cells by apoptosis or necrosis and destroy the tumor.

Figure 2. Process of melanogenesis and resistance mechanisms of melanoma

Melanosomes mature through stages 1–4 and are finally transferred to keratinocyes where they release melanin granules. Defects in apoptosis (BCL2), BRAF-mutation activated MAPK/ERK pathway, and ABC-transporter drug efflux pumps contribute to melanoma resistance.

Figure 3. Chemical structures of NIR absorbing photosensitizers and the optical window in tissue

(A) Bacteriochlorin TCBSO₃H from (Dabrowski, Urbanska et al. 2011). (B) Bacteriochlorin 3 from (Mroz, Huang et al. 2010). (C) Lutetium texaphyrin from (Woodburn, Fan et al. 1998). (D) Si(IV)-naphthalocyanine (Isobosinc) from (Biolo, Jori et al. 1994).

Figure 4. Fluorescence micrographs of B16F10 cells

Showing red fluorescence from BC 1, 2 or 3 overlaid with green fluorescence from lysotracker, mitotracker or FITC-anti-TRP1 antibody that stains melanosomes. Reprinted with permission from (Mroz, Huang et al. 2010).

Figure 5. The PDT effects of stable synthetic bacteriochlorin on GFP positive B16F10 melanoma tumors

(A) B16F10-GFP subcutaneous tumor on day 7 before PDT treatment. (B) B16F10-GFP tumor on day 13 after PDT treatment. (C) In vivo fluorescence imaging of B16F10-GFP tumor immediately after inoculation. (D) GFP signal from tumor on day 7 before PDT. (E) Two-color imaging of 3 (red) and B16F10-GFP tumor (green) 15 min post IV injection on day 9. (F) Control tumor on day 13. (G) PDT-treated tumor on day 13. (H) Control tumor on day 23. (I) PDT-treated tumor on day 23. (J) Regrowth of a PDT-treated tumor on day 26. (K) Survival analysis of in vivo PDT effectiveness with bacteriochlorin 3 on B16F10-GFP tumors; solid line no treatment (n = 8), dotted line PDT (n = 10). p < 0.01 (log rank test). (L) Structure of stable synthetic bacteriochlorin. Reprinted with permission from (Mroz, Huang et al. 2010).