Porphyrins derivatives as photosensitizers for photodynamic therapy

Author(s)

Fabio Monaro Engelmann

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Conjunto das Químicas (IQ e FCF) (CQ/DBDCQ)
• Defense date: 2005-05-23

Examining board members

Koiti Araki; Maurício da Silva Baptista; Ana Maria da Costa Ferreira; Noboru Hioka; Osvaldo Antonio Serra

• Advisor: Koiti Araki

Abstract

In this thesis the chemical, photochemical and photophysical relevant aspects for the development of new photosensitizers for photodynamic therapy applications are discussed. 20 conventional porphyrins species and 9 doubly Nconfused porphyrins species were investigated. The first series contains variable number (1 to 4) of cationic groups, [Ru(bipy)2CI]+ or CH3+ (figure 1), bound to the meta or para-pyridil N-atoms of meso-phenylpyridylporphyrins. On the other hand, the protonated, neutral and deprotonated doubly N-confused porphyrins possess two rotated adjacent pyrrol rings, such that they have two coordinating camon atoms. Consequently, transition metal ions in unusually high oxidation states such as Ag(lII) and Cu(lII) can be stabilized by this structure, while the outer pyrrol N-atoms are susceptible to protonation and deprotonation reactions, allowing the modulation of their electronic properties simply by controlling the pH of the solution. In general, the protonation perturbed more significantly the photochemical properties than the deprotonation. The neutral species exhibit intense absorption bands in the phototherapeutical range (600 to 750 nm), associated with a high singlet oxygen sensitization quantum yield (ΦΔ> 0,90 for the Ag(lII) complex). Furthermore, a photodecomposition process due to the reaction with 1O2 was also identified. Both ΦΔ and photodecomposition are influenced by the metal ion coordinated to the doubly N-confused porphyrin ring. The coordination of Ag(lIl) and Cu(lII) increased ΦΔ, probably due to the enhancement of intersystem crossing quantum yield associated with the heavy atom effects. However, AgHN2CP is much more stable than CuHN2CP or the free-base, but the reasons are not clear and should be further investigated. The above results clearly evidence the superior properties of the Ag(lIl) complex as photosensitizer for PDT applications. The 20 cationic pyridylporphyrin derivatives are much soluble in water and experiments directed to biological applications could be carried out. Accordingly, in addition to the quantum yield for singlet oxygen generation(1O2), the effect of the stereochemistry and number and position of the electrically charged substituents on the binding constants and photooxidative damage on erythrocytes, lipossomes, mitochondria and HeLa cells were evaluated. The ΦΔ values were constant in the case of the N-3-methylpyridinium derivatives but inversely proportional to the number of positive charges for the N-4-methylpyridinium derivatives. This was assigned to an increase of aggregation of the sensitizers as the number of meso-phenyl rings and the lipophylicity increase. The partition coefficients in n-octanol/water (logPOA) corroborate that assertion showing a linear correlation with increasing of the number of phenyl groups. The binding efficiency towards the mitochondrial, lipossomal and cellular membrane and the photo-induced lipidic peroxidation processes were directly proportional to the logPOA, except for the ruthenated porphyrins. This last behavior may be associated with the dissociation reactions of the rutheniumpyridylporphyrin bond in the physiologic solution. Accordingly, those experiments were carried out only with the methylpyridynium derivatives. For the first time, we showed inequivocally that the interaction of the methyllated porphyrins with mitochondria are significantly influenced by the membrane potential. In fact, when the mitochondria was energized a 15% increase was observed in the binding constants of 3P2cMe in comparison with teh results with decoupled mitochondria. Also, the amount of bond porphyrin sensitizer decreased as the number of methylpyridynium groups was increased, following the same tendency as above. The behavior of the cis (3P2cMe) and trans (3P2tMe) isomers didn\'t follow the general tendency for the other derivatives in a series, such that the binding constants of the cis isomers were always about twice higher than for the trans, which were much higher than predicted by the general behavior. This means that they can associate more strongly and penetrate deeper in the membrane than the more charged porphyrin derivatives. In particular, the cis species is amphiphilic, i.e. possess an adequate structure for that interaction. The results presented in this thesis showed conclusively that the two series of meso-phenylpyridylporphyrins are adequate for the investigation of the effect of the stereochemistry on the bonding ability and photodynamic properties of those photosensitizers. This is particularly true due to the possibility of modulating the ratio between hydrophobicitylhydrophylicity and the stereochemistry without influencing significantly the quantum yield for 1O2 generation. The meta- series showed an advantage over the conventionally used para- series of methylpyridynium porphyrins because it is more soluble and shows lesser tendency to aggregate. In condusion, the amphiphilic 3P2cMe is the species with the highest potentiallity as por sensitizer because of its high binding constants, low tendency to associate and high ΦΔ. (AU)