Improvement of m-THPC-like photosensitizer selectivity with Folate-based Targeted Delivery. Synthesis and in vivo Delivery Studies
Résumé
The cell-membrane folate receptor is a potential molecular target for tumor-selective drug delivery, including delivery of photosensitizers for anticancer photodynamic therapy (PDTa). Tumor selectivity of meta-tetra(hydroxyphenyl)chlorin (m-THPC), a second generation photosensitizer used in PDT clinical trials, demonstrates a relatively low tumor-to-normal epithelial uptake ratio. In this study, we report on the synthesis and on the photophysical properties of a m-THPC-like photosensitizer (5,10,15-tris(3-hydroxyphenyl)-20-(4-carboxyphenyl)chlorin 1) conjugated to folic acid via 2,2'-(ethylenedioxy)-bis-ethylamine 8. An in vivo comparative study of the conjugated versus non-conjugated accumulation of m-THPC-like photosensitizers is described in nude mice receiving regular rodent chow or three weeks after diet was changed to folate-free rodent chow. Nude mice were xenografted with folate receptor (FR)-a-positive KB or HT-29 cells lacking FR-a as a negative control. Levels of FR-a expression were validated for both selected cell lines by Western blot. The tumor uptake of the fluorescent photosensitizers was followed in real-time by optical fiber fluorimetry. Conjugate 8 exhibited enhanced in vivo accumulation in KB tumors compared to the non-conjugated photosensitizer 4 h after intra-venous injection and for the animal's free folic acid dietary. The folate-mediated specific uptake was demonstrated by the absence of tumor accumulation of 8 in animals that were fed with normal (high folate) rodent chow. On the contrary, no significant difference between KB and HT-29 tumors was observed in case of the untargeted photosensitizer. A factorial analysis of variance model was used to analyze the results of selectivity. Tumor-to-normal tissue ratio exhibited a very interesting selectivity for conjugate 8 (5:1) in FR-a-positive KB tumors 4 h post-injection. These results indicate that folate-linked photosensitizer 8 can be proposed for improved selectivity in PDT of folate receptor-positive tumors. It is expected that the results obtained with this tumor model could be extrapolated to other human tumors over-expressing folate receptors.