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 cyelodextrins-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 mTHPC/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/2 gamma-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.