Photodynamic therapy is an exciting treatment modality that combines the effects of a chemical agent with the physical energy from light or radiation to result in lysis of cells of interest.
Based on the light activation of a photosensitizer (PS) in the presence of oxygen, which results in the formation of cytotoxic species. The delivery of fractionated light may enhance treatment efficacy by reoxygenating tissues.
Photodynamic therapy (PDT) constitutes a treatment modality that combines a photosensitizing agent with exposure to laser light in order to elicit phototoxic reactions that selectively destroy tumor cells and spare normal cells. PDT is a local treatment modality without long-term systemic effects. Its application can be repeated more than once to the same area without accumulative effects.
However, oxygen depletion during PDT is a well known problem. Modulation of light delivery could address this issue by counteracting tumor hypoxia, thereby improving tumor cell killing.
Glioma stem cells (GSLCs) expressed higher mRNA levels of protoporphyrin IX (PpIX) biosynthesis enzymes and its transporters PEPT1/2 and ABCB6, when compared to the parental A172 glioma cells. Consistently, flow cytometry analysis revealed that upon incubation with ALA, GSLCs accumulate a higher level of PpIX. Finally, Fujishiro et al., from the Department of Neurosurgery, Osaka Medical College, Takatsuki, Japan showed that GSLCs were more sensitive to 5-aminolevulinic acid-mediated photodynamic therapy (ALA-PDT) than the original A172 cells, and confirmed that all patient-derived glioma sphere lines also showed significantly increased sensitivity to ALA-PDT if cultivated under the pro-stem cell condition. This data indicate that ALA-PDT has potential as a novel clinically useful treatment that might eliminate GBM stem cells that are highly resistant to the current chemo- and radio-therapy 1).
U87 glioblastoma cells were stereotactically engrafted into the brains of male fox1 rnu/rnu rats. Light delivery was studied after 5-ALA injection (100 mg/kg i.p.). 26J of 635 nm light was interstitially delivered to U87 tumor-bearing rats at a radiant power of either 30 mW (high fluence rate) or 4.8 mW (low fluence rate). In each group, half of the population received illumination in 2 fractions with a refractory interval of 120 s, whereas the other half received continuous illumination.
Twenty-two animals received 5-ALA-PDT, and the level of necrosis was scored. In the high-fluence-rate group, we observed a greater degree of tumor necrosis in rats receiving fractionated delivery than in rats receiving continuous illumination. Similar differences were not observed in the low-fluence-rate group, which exhibited only sparse necrosis. Higher morbidity and mortality rates were observed in the high-fluence-rate group.
We have developed a reproducible and reliable rodent model for interstitial 5-ALA PDT. We found that the effects of 5-ALA-PDT are dependent on light delivery conditions. Although the low-fluence-rate treatment was better tolerated, 5-ALA-PDT induced more necrosis using fractionated delivery at a high fluence rate. These results require confirmation with further studies involving larger populations and additional fractionation schemes 2).
Human U87 cells were grafted into the right putamen of 39 nude rats. After PS precursor intake (5-ALA), an optic fiber was introduced into the tumor. The rats were randomly divided into three groups: without light, with light split into 2 fractions and with light split into 5 fractions. Treatment effects were assessed using brain immunohistology.
Fractionated treatments induced intratumoral necrosis (P < 0.001) and peritumoral edema (P = 0.009) associated with a macrophagic infiltration (P = 0.006). The ratio of apoptotic cells was higher in the 5-fraction group than in either the sham (P = 0.024) or 2-fraction group (P = 0.01). Peripheral vascularization increased after treatment (P = 0.017), and these likely new vessels were more frequently observed in the 5-fraction group (P = 0.028).
Interstitial PDT with fractionated light resulted in specific tumoral lesions. The 5-fraction scheme induced more apoptosis but led to greater peripheral neovascularization 3).
Patients diagnosed with primary brain tumors were treated with PDT. Treatment consisted in administration of the photosensitizer followed by craniotomy, surgical resection and laser illumination of the surgical bed. Primary brain tumors received also temozolomide-based chemotherapy and radiotherapy (RT).
From May 2000 to December 2010, 41 patients (27 male, 14 female) with a median age of 49 years (range 13 to 70) diagnosed of primary brain tumors were included in the study. In 7 patients PDT was repeated at the time of the relapse. In 22 episodes PDT was part of the initial treatment of primary brain tumors and in 26 episodes was part of the treatment at relapse.
PFS observed was 10 months for GBM (95% confidence interval 5.7-14.3), 26 months for AA (95% CI 4.5-47.5), and 43 months for OD (95% CI 4.5-47.5). Median OS was 9 months for GBM (95% CI 2.3-15.7), 20 months for AA (95% CI 0.0-59) and 50 months for OD (95% CI 32.5-67.5). The apparent discrepancy between PFS and OS data is due to patients not censored for PFS because they die from causes other than tumor progression. Median OS since first diagnosis was 17 months for GBM (95% CI 15.2-17.8), 66 months for AA (95% CI 2.9-129.1) and 122 months for OD (95% CI 116.1-127.8). Side effects were mild and manageable.
This study confirms that PDT can be considered as an adjunctive to surgery and/or RT and chemotherapy in the treatment of brain tumors, excluding those patients with thalamic or brain stem locations. It adds therapeutic effect without adding significant toxicity. In order to improve its contribution, it is essential to find new drugs with more penetration in order to destroy tumor cells more deeply at resection margins 4).