Syntheses and Cellular Investigations of 17<sup>3</sup>-, 15<sup>2</sup>-, and 13<sup>1</sup>-Amino Acid Derivatives of Chlorin e<sub>6</sub>

A series of amino acid conjugates of chlorin e<sub>6</sub>, containing lysine or aspartic acid residues in positions 17<sup>3</sup>, 15<sup>2</sup>, or 13<sup>1</sup> of the macrocycle were synthesized and investigated as photosensitizers for photodynamic therapy of tumors. All three regioisomers were synthesized in good yields and in five steps or less from pheophytin <i>a</i> (<b>1</b>). In vitro investigations using human carcinoma HEp2 cells show that the 15<sup>2</sup>-lysyl regioisomers accumulate the most within cells, and the most phototoxic are the 13<sup>1</sup> regioisomers. The main determinant of biological efficacy appears to be the conjugation site, probably because of molecular conformation. Molecular modeling investigations reveal that the 17<sup>3</sup>-substituted chlorin e<sub>6</sub> conjugates are L-shaped, the 15<sup>2</sup> and 13<sup>1</sup> regioisomers assume extended conformations, and the 13<sup>1</sup> derivatives are nearly linear. It is hypothesized that the 13<sup>1</sup>-aspartylchlorin e<sub>6</sub> conjugate may be a more efficient photosensitizer for PDT than the commercial currently used 15<sup>2</sup> derivative.