Inhibition of Autophagosome Formation by the Benzoporphyrin Derivative Verteporfin

Abstract

Autophagy enables cells to degrade and recycle cytoplasmic materials both as a housekeeping mechanism and in response to extracellular stress such as nutrient deprivation. Recent studies indicate that autophagy also functions as a protective mechanism in response to several cancer therapy agents, making it a prospective therapeutic target. Few pharmacological inhibitors suitable for testing the therapeutic potential of autophagy inhibition in vivo are known. An automated microscopy assay was used to screen >3,500 drugs and pharmacological agents and identified one drug, verteporfin, as an inhibitor of autophagosome accumulation. Verteporfin is a benzoporphyrin derivative used in photodynamic therapy, but it inhibits autophagy without light activation. Verteporfin did not inhibit LC3/Atg8 processing or membrane recruitment in response to autophagic stimuli, but it inhibited drug-and starvationinduced autophagic degradation and the sequestration of cytoplasmic materials into autophagosomes. Transient exposure to verteporfin in starvation conditions reduced cell viability whereas cells in nutrient-rich medium were unaffected by drug treatment. Analysis of structural analogs indicated that the activity of verteporfin requires the presence of a substituted cyclohexadiene at ring A of the porphyrin core but that it can tolerate a number of large substituents at rings C and D. The existence of an autophagy inhibitor among FDA-approved drugs should facilitate the investigation of the therapeutic potential of autophagy inhibition in vivo.

Apoptotic and autophagic responses to photodynamic therapy in 1c1c7 murine hepatoma cells

“…The photosensitizer used in these studies was found to inhibit autophagosome formation at a 10 μM concentration in the dark. 19 Consistent with studies involving the KD line, photosensitization with the higher BPD level, followed by irradiation with a reduced light flux eliminated the shoulder on the dose-response curve associated with the cytoprotective effect of autophagy. Use of high-dose BPD could potentiate PDT efficacy where autophagy offers protection from photokilling, if tumor selectivity is not thereby lost.…”

“…A recent report described the unexpected ability of BPD (in the dark) to antagonize autophagosome formation after starvation or other stimuli. 19 This inhibitory effect was not observed until the BPD concentration was ≥10 μM. In studies reported above, the concentration of BPD never exceeded 1 μM.…”

Impact of treatment response metrics on photodynamic therapy planning and outcomes in a three-dimensional model of ovarian cancer

“…81 The authors suggested that this potentiation of PDT with higher concentrations of BPD is due to inhibition of autophagy because autophagosomes that are formed as a result of an autophagy-mediated cellular response can be inhibited by 10 μM BPD. 99 It has been shown in several instances that autophagy can be cytoprotective against phototoxicity. 81,100 However, biological variability between cell lines and disease models significantly impacts overall PDT efficacy for many reasons including differential PS uptake, PS localization, and differences in the photochemical and photophysical mechanisms lead to cytotoxicity.…”

The autophagic paradox in cancer therapy

“…As modulation of autophagy represents a novel approach for enhancing the therapeutic efficacies of cancer therapy, research efforts have been put forth to identify agents that induce or inhibit autophagy. Several FDA (Food and Drug Administration)-approved drugs have been identified as inducers (for example, rapamycin, fluspirilene, trifluoperazine, pimozide, niguldipine, nicardipine and amiodarone, loperamide) or inhibitors (for example, chloroquine, hydroxychloroquine, verteporfin) of autophagy (Zhang et al, 2007;Donohue et al, 2011). The toxicological and pharmacokinetic profiles of these agents are well established and, therefore, these agents can be easily incorporated into existing regiments of cancer therapy.…”