Hypocrellin-B acetate as a fluorogenic substrate for enzyme-assisted cell photosensitization.

Photosensitizing molecules (PSs) undergo chemico-physical changes upon addition of suitable substituents, influencing both their photophysical properties and their ability to accumulate into cells. Once inside the cells, the modified PS acts as a fluorogenic substrate: the added substituent is removed by a specific enzyme, restoring the native PS in subcellular sensitive sites. We investigated the photophysical properties and interaction with HeLa cells of Hypocrellin-B (HypB), as native molecule and upon acetate-group addition (HypB-Ac). Chemical modification alters both absorption and fluorescence features of HypB; consequently, the dynamics of the enzyme hydrolysis of HypB-Ac can be monitored through restoring the native HypB spectral properties. At the cellular level, only the HypB emission signal was detected within 5 min of incubation with either HypB or HypB-Ac, allowing a direct comparison of the time courses of their intracellular accumulation. Plateau values were reached within 15 min of incubation with both compounds, the emission signals being significantly higher in HypB-Ac than in HypB treated cells. Consistently, imaging showed a rapid appearance of red fluorescence in the cytoplasm, with more abundant bright spots in HypB-Ac treated cells. Both compounds did not induce dark toxicity at concentrations up to 1 × 10(-6) M, while upon irradiation at 480 nm phototoxicity was significantly higher for cells exposed to HypB-Ac than for HypB-loaded cells. These findings suggest an improved efficacy of acetylated HypB to be internalized by cells through membrane trafficking, with a preferential interaction of the photoactive molecules on sensitive intracellular sites. After irradiation, in HypB-Ac treated cells, prominent disorganization of several cytoplasmic organelles such as the endoplasmic reticulum, Golgi apparatus, lysosomes, microfilaments and microtubules were observed.

Photoinduced C-F Bond Cleavage in Some Fluorinated 7-Amino-4-quinolone-3-carboxylic Acids.

The photochemistry of some fluorinated 7-amino-4-quinolone-3-carboxylic acids used in therapy as antibacterials and known to be phototoxic has been investigated in water. All of them undergo heterolytic defluorination, and this appears to be a path for the generation of aryl cations in solution. 6-Fluoro derivatives such as norfloxacin (Phi(dec) = 0.06) and enoxacin (Phi(dec) = 0.13) give the corresponding phenols. Insertion of an electron-donating substituent makes defluorination inefficient; thus, ofloxacin, an 8-alkoxy derivative, is found to be rather photostable (Phi(dec) = 0.001) and reacts in part via a process different from defluorination (degradation of the N-alkyl side chain). With a 6,8-difluoro derivative, lomefloxacin, the reaction is more efficient (Phi = 0.55) and selective for position 8. Contrary to the previous cases, the aryl cation undergoes insertion in the neighboring N-ethyl group rather than solvent addition (a carbene-like chemistry). With all of the above fluoroquinolones an intensive triplet-triplet absorption is detected and is quenched by sulfite (k(q) = (1-5) x 10(8) M(-)(1) s(-)(1)). Under this condition, reductive defluorination via the radical anion takes place. The relation of the above chemistry to the phototoxicity of these drugs is commented upon briefly.

Structural study of the solid-state photoaddition reaction of arylidenoxindoles

The photochemistry of isomeric 2-furyliden- and benzylidenoxindoles (2H-indol-2-ones) is examined. In solution E-Z isomerization is the only process via the excited singlet state (which fluoresces in glassy solution at 77 K and not at room temperature). In the crystalline state, the two (Z) derivatives are photostable, in accordance with the prediction based on the structural determination of the furylidene derivative, which adopts the unreactive Schmidt's gamma type arrangement. The (E) furylidene derivative (1a) gives efficiently (Phi = 0.3) the head-to-tail dimer, as indicated by the crystal structure, which is of the reactive alpha type, in full accord with the topochemical principles. In contrast, the corresponding benzylidene (1b) derivative reacts sluggishly (Phi < 0. 01) and mainly gives polymers, despite the fact that crystal structure determination shows that it likewise pertains to the alpha type and complies with the topochemical rules. The difference in reactivity is explained on the basis of (i) the twist of the phenyl ring with respect to the indole plane, and (ii) the higher overall cohesion energy and the lower interaction energy between facing molecules, as found from the charge density analysis for the crystals of 1b in comparison to those of 1a. This evidences a further stringent requirement for the occurrence of topochemical photodimerizations.

Structure and medium-dependent photodecomposition of fluoroquinolone antibiotics.

The photochemical reactivity of four fluoroquinolone antibiotics is examined. For norfloxacin (NOR), enoxacin (ENX) and lomefloxacin (LOM), the only process occurring is defluorination (from position 6 for the first two drugs, from position 8 for the last one). The quantum efficiency is both structure and medium dependent (phi close to 0.5 both in water and in 0.1 M phosphate buffer for LOM; 0.01 for ENX and 0.004 for NOR in buffer, but more than an order of magnitude higher in neat water). Ofloxacin (OFL) is less light sensitive (phi 0.001) and undergoes, in part, reactions different from defluorination. The photoreaction involves heterolytic C-F bond fragmentation and its efficiency is determined by the internal charge-transfer character of the excited state (increasing in the series OFL < NOR < ENX < LOM according to the electronegativity of the substituent in position 8) and by the stabilization of the resulting aryl cation (larger for the 8-cation than for the 6-cation). The relevance of these data for the rationalization of the known phototoxicity of these drugs is discussed.

Photostability of extended-release matrix formulations.

The photostability of drugs has been widely studied while less attention is devoted to the possible modifications that UV light may induce on the excipients of a dosage form, in particular, on the functional polymers used to modulate drug delivery. In this work we have evaluated the effect of UV light on the release characteristics of extended-release matrix tablets containing hydroxypropylmethylcellulose (HPMC) or polyethylene oxide (PEO) as retarding polymers. Two different model drugs have been used: nifedipine (insoluble and photolabile drug) and diltiazem (soluble and photostable drug). Photodecomposition of nifedipine was evaluated and the formation of the photoproducts was followed during the dissolution process. Regarding the dissolution stability, the matrix tablets containing HPMC exposed to UV light have not shown significant differences in drug release profiles compared to the same non-irradiated formulation, while the matrix tablets containing PEO and exposed to the same conditions of UV light have shown a remarkable increase of drug release rate within the first minutes of the dissolution test (burst effect) which is particularly critical because it can cause the loss of the desired therapeutic control.

Photoactivation of corticosteroids in UVB-exposed skin.

The photodegradation of flumethasone (FM) and fluocinolone acetonide (FC) was studied in solution and in the pig skin. Both glucocorticosteroids applied to the pig skin were unstable under UVB light. The photoproducts formed in the skin were the lumi-, photolumi- and andro-derivatives for FM, the same found in vitro. Instead, FC hydroperoxide formed in solution was not found in the skin: the reactivity and oxidative ability of this photoproduct towards biological substrates (lipids, proteins) seems the reason of the lack of its detection in the ex vivo model. In fact, it demonstrated to quickly oxidize amino acids and peptides, and to react with BSA both in the dark and under irradiation. Moreover, the presence in the irradiated pig skin of the FC andro-derivative, which usually forms in H-donating environment, seems consistent with the mechanism of Norrish I fragmentation followed by H-abstraction, likely from the surrounding biological substrates. These findings indicate that photoreactivity of these compounds may take place in the skin of patients exposing themselves to sunlight and is a warning about possible skin damage as a result of that. Furthermore, photolability of these drugs in the skin might cause loss of their therapeutic activity.

[Synthesis of n-pentanoylmethylenetriphenylphosphorane]. / Sintesi dell'n-pentanoilmetilenetrifenilfosforano

An efficient synthesis of n.pentanoylmethylenetriphenylphosphorane (II), useful intermediate for achievement of prostanoic derivatives, is described. The procedure affords the desired product at a remarkable degree of purity.

Noncommunicating photoreaction paths in some pregna-1,4-diene-3,20-diones.

The photochemistry of three pregna-1,4-diene-3,20-diones bearing a hydroxy or alkoxy group at C(17) (4-6) has been examined. Irradiation at 254 or 366 nm, where absorption by the cross-conjugated ketone moiety in ring A is predominant or exclusive, causes the 'lumiketone' rearrangement of this chromophore in low to medium quantum yield (Phi(r) 0.05 to 0.31). On the contrary, irradiation at 310 nm, where the isolated ketone at C(20) absorbs a large portion of light causes Norrish-I fragmentation of that chromophore with a higher Phi(r) (0.11-0.83). This leads to end-products arising from the conversion of the C(17) alkyl radical, in a way depending on the structure and the medium (reduction by hydrogen donating solvent, addition of oxygen when present). No intramolecular T-T energy transfer between the separated chromophores occurs. The 'lumiketone' rearrangement occurs independently from the irradiation wavelength (Phi(r) 0.06-0.18) with the strictly related androsta-1,4-dien-3-one 8 lacking the C(20) ketone function.

Multifaceted photoreactivity of 6-fluoro-7-aminoquinolones from the lowest excited states in aqueous media: a study by nanosecond and picosecond spectroscopic techniques.

Nanosecond and picosecond absorption and emission spectroscopic techniques were applied to the investigation of the reactivity from the lowest excited states of some 6-fluoro-7-piperazino-4-quinolone-3-carboxylic acids (FQs) in aqueous media at neutral pH, in the absence and presence of different sodium salts. Following the detection of various transients, we proposed a mechanism for the cleavage of the carbon-fluorine bond that proceeded through different reaction pathways, dependent on the molecular structure and the characteristics of the medium. The drug lomefloxacin (LOM), a 6,8-difluoroquinolone derivative, underwent heterolytic cleavage of the C8-F bond from the excited singlet state. With the 6-monofluoroquinolone norfloxacin (NOR) and the corresponding 1,8-naphthyridinone enoxacin (ENX), the lowest singlet state was not significantly reactive and an important deactivation channel was intersystem crossing (ISC) to the triplet manifold. The lowest triplet state underwent cleavage of the C6-F bond through a solvent mediated process possibly via a cyclohexadienyl anionic adduct. In the presence of sulfite or phosphate buffer a novel defluorination mechanism, induced by electron transfer from the inorganic anions to the FQ triplet state, was observed. The correlation between the transients observed and the final photoproducts in the different media was elucidated.