Tolyporphin: A natural product from cyanobacteria with potent photosensitizing activity against tumor cells in vitro and in vivo

Tolyporphin (TP), a porphyrin extracted from cyanobacteria, was found to be a very potent photosensitizer of EMT-6 tumor cells grown both in vitro as suspensions or monolayers and in vivo in tumors implanted on the backs of C.B17/Icr severe combined immunodeficient mice. Thus, during photodynamic treatment (PDT) of EMT-6 tumor cells in vitro, the photokilling effectiveness of TP measured as the product of the reciprocal of D₅₀ (the light dose necessary to kill 50% of cells) and the concentration of TP is ~5000 times higher than that of Photofrin II (PII), the only PDT photosensitizer thus far approved for clinical trials. TP almost exclusively localizes in the perinuclear region and specifically in the endoplasmic reticulum (ER), as shown by microspectrofluorometry on single living EMT-6 cells costained with the ER and/or Golgi fluorescent vital probes, 3,3'-dihexyloxacarbocyanine iodide and N-[4,4-difluoro-(5,7-dimethyl-BODIPY®)-1-pentanoyl]-D-erythro- sphingosine (Molecular Probes, Eugene, OR). As a result, the singlet oxygen- mediated photodynamic activity of TP induces an effective inactivation of the acyl CoA:cholesterol-O-acyltransferase, a sensitive marker of ER membrane integrity and alterations of the nuclear membrane. In vivo, with the EMT-6 mouse tumor model, an exceptional effectiveness is also observed as compared to that of PII and other second generation photosensitizers of the pheophorbide class, which are themselves much more potent than PII. The outstanding PDT activity of TP observed in vivo may be due to its unique biodistribution properties, in particular much less extraction by the liver, resulting in a higher delivery to other tissues, including tumor.