Enhancement of 5,10,15,20-Tetra(m-Hydroxyphenyl)chlorin Fluorescence Emission by Inclusion in Natural and Modified Cyclodextrins

Fluorescence properties of 5,10,15,20-tetra(m-hydroxyphenyl)chlorin (m-THPC), a sensitizer used in photodynamic therapy (PDT), were studied in the presence of three native cyclodextrins (CDs) - alpha-, beta-, and gamma-CD - and two modified cyclodextrins - heptakis (2,6-di-O-methyl) beta-CD (Me-beta-CD) and heptakis (2-O-hydroxy-propyl) beta-CD (HP-beta-CD). The CDs studied have been shown to undergo the formation of an inclusion complex with m-THPC, leading to a large enhancement (up to 300 times) of m-THPC fluorescence intensity depending on the CD used. Stoichiometry and association constants have been determined on the basis of the variation of fluorescence intensity. A 1:2 stoichiometry has been found for m-THPC complexes with gamma-CD, Me-beta-CD, and HP-beta-CD. Association constants as high as K = 9.5 X 10 8 M-2 in the case of m-THPC/2Me-beta-CD and K = 2.7 X 10 9 M-2 in the case of m-THPC/2HP-beta-CD complex were determined, whereas a lower value (K = 2.4 X 10 4 M-2) was found in the case of m-THPC/2gamma-CD. Because of the highest fluorescence enhancement (a factor of about 300) observed with Me-beta-CD, this compound would be the most suitable to improve the detection of m-THPC in further pharmacokinetics and biodistribution studies.