The role of photodynamic therapy in overcoming cancer drug resistance

Abstract

Many modalities of cancer therapy induce mechanisms of treatment resistance and escape pathways during chronic treatments, including photodynamic therapy (PDT). It is conceivable that resistance induced by one treatment might be overcome by another treatment. Emerging evidence suggests that the unique mechanisms of tumor cell and microenvironment damage produced by PDT could be utilized to overcome cancer drug resistance, to mitigate the compensatory induction of survival pathways and even to re-sensitize resistant cells to standard therapies. Approaches that capture the unique features of PDT, therefore, offer promising factors for increasing the efficacy of a broad range of therapeutic modalities. Here, we highlight key preclinical findings utilizing PDT to overcome classical drug resistance or escape pathways and thus enhance the efficacy of many pharmaceuticals, possibly explaining the clinical observations of the PDT response to otherwise treatment-resistant diseases. With the development of nanotechnology, it is possible that light activation may be used not only to damage and sensitize tumors but also to enable controlled drug release to inhibit escape pathways that may lead to resistance or cell proliferation.

Figure 1

Overview of the unique mechanisms of PDT-induced apoptosis. PDT directly damages antiapoptotic factors and the drug efflux pumps involved in classical drug resistance. The antiapoptotic BCL-2 family of proteins (e.g., BCL-2 and BCL-X_L) reside on the outer mitochondrial membrane and prevent mitochondria-mediated apoptosis by inhibiting the oligomerization and activation of the proapoptotic family members (e.g., BAX and BAK). Proapoptotic proteins like BAX are in dynamic equilibrium between the cytosol and mitochondrial outer membrane but they are largely within the cytosol of healthy cells. Therefore, mito-PDT is observed to predominately destroy antiapoptotic factors. Lyso-PDT induces the release of lysosomal proteases into the cytosol that can cleave BID (independent of caspase 8) to form truncated BID (tBID), which tranlocates to the mitochondria to promote the oligomerization of BAX and BAK. BAX and BAK oligomers form pore complexes that release cytochrome c and SMAC (second mitochondrion-derived activator of caspases) from the mitochondrial intermembrane space. Once released into the cytoplasm, cytochrome c forms a complex with apoptotic protease-activating factor 1 and procaspase 9, called the apoptosome, to activate caspase 9. SMAC, once released into the cytosol, promotes caspase activation through binding with IAPs (inhibitor of apoptosis proteins) and blocks their antiapoptotic activity. Once activated, the effector caspases (e.g., caspase 3 and caspase 7) carry out cellular degradation process to execute the apoptotic program.

Figure 2

Cancer cells that are unresponsive to sustained gemcitabine chemotherapy are sensitive to BPD-PDT. (A) A panel of pancreatic adenocarcinoma cell lines contain gemcitabine unresponsive populations (17%–33%) even at extreme gemcitabine doses (up to 1 mM), while moderate BPD-PDT doses (1–6 J·cm⁻²·mM, where the units reflect the product of the light dose and the PS concentration, e.g., 10 J·cm⁻² × 0.25 mM BPD = 2.5 J·cm⁻²·mM) produce nearly complete cancer cell death. (B) BPD-PDT decreases BCL-X_L and increases the ratio of BAX-to-BCL-X_L toward a proapoptotic balance (data are the results from the quantification of western blots). (C) Insensitivity to gemcitabine (top), but not to BPD-PDT (bottom), is increased in cells that are adherent to Matrigel basement membrane relative to traditional tissue culture (TC) conditions (NT indicates the no treatment control). Collectively, these results indicate the ability of PDT to bypass intracellular and extracellular cues, leading to gemcitabine resistance and indicate the emerging role of PDT for pancreatic cancer treatment. Adapted from Celli et al. (2011).

Figure 3

Photodynamic therapy reverses chemoresistance and synergizes with chemotherapy to destroy platinum-resistant disease. One-fold increase of cytotoxicity following photoimmunotherapy (antibody-PS conjugates) in combination with cisplatin (platinum chemotherapy)—versus cisplatin alone—in cisplatin-resistant (●) and cisplatin–sensitive (■) patient-derived samples and cell line cultures. A total of 19 solid tumor and/or ascites samples were collected from 14 ovarian cancer patients (ages 37–80, stages 1C–4), and 5 cancer cell line cultures were also included. Cisplatin resistance versus sensitivity refers to whether the patient had disease progression or recurrence within 6 months of platinum chemotherapy. Photoimmunotherapy induces a 12.9× enhancement in cytotoxicity against platinum resistant primary cultures (ranging from 1.5–52×) versus 1.8× for platinum sensitive cells. The asterisk indicates P < 0.05. Adapted from Duska et al. (1999).

Figure 4

PDT of locally advanced, inoperable pancreatic adenocarcinoma in humans. (A) Contrast-enhanced computed tomography (CT) scans from a patient undergoing BPD-PDT. The images show (left) a low attenuation mass in the head of the pancreas prior to treatment, (center) placement of a percutaneous needle for fiber optic light delivery into the tumor, and (right) a 2.67 cm³ zone of tumor necrosis 5 days post-PDT. (B) CT scans from a patient who qualified for and underwent a successful Whipple’s tumor resection following PDT. The pre-PDT image (left) shows the tumor abutting the superior mesenteric artery (SMA; arrow); thus, this tumor was inoperable at presentation. Four weeks after PDT, the (right) follow-up CT scan for the same patient shows tumor reduction and minimal involvement with the SMA such that surgical resection could then be performed safely. Adapted from Huggett et al. (2014).

Figure 5

Concepts of tumor-targeted, activatable photoimmunotherapy (taPIT). (A) Activatable immunoconjugates for taPIT are comprised of multiple self-quenching, photocytotoxic chromophores conjugated to antibodies that target and neutralize the key molecules involved in tumorigenesis (e.g., EGFR). (B) Cellular activation of the immunoconjugates via receptor-mediated endocytosis and lysosomal degradation. (C) taPIT concept, in which the immunoconjugates accumulate selectively within the tumor nodules, are activated by cellular processing, and they inhibit molecular signaling and impart selective cytotoxicity to neoplasms upon irradiation while sparing neighboring vital tissues. (D) Ex vivo whole mount immunofluorescence image of a micrometastasis, where an anti-human cytokeratin antibody has been applied to visualize the human epithelial cancer cells (orange); an anti-mouse CD31 antibody labels the endothelial cells (green) and immunoconjugates are taken up and activated by tumor cells in vivo (red). taPIT enables the safe use of 50× photodynamic dose (PS × light dose) versus “always-on”, unconjugated BPD, and 17× photodynamic dose versus “always-on” PIT (using cetuximab-ce₆ conjugates) in a mouse model of peritoneal disseminated micrometastatic epithelial ovarian cancer. A single cycle of taPIT plus chemotherapy reduces the micrometastatic burden by 97% versus 3% for chemotherapy alone in the same mouse model^,, using human chemoresistant OVCAR5 cells^,. Wide-field taPIT was accomplished by administering scattering media (Intralipid) to the peritoneal cavity and NIR laser light via a cylindrically diffusing fiber optic tip. Adapted from Spring et al. (2014).