Angelo Albini

Photophysics and photochemistry of fluoroquinolones

The photobehavior of fluoroquinolone antibiotics, one of the most successful classes of drugs in therapeutic applications, has recently been the object of increasing interest due to the finding of their phototoxic and photocarcinogenic properties. The main results obtained for a series of structurally related, representative fluoroquinolone drugs is reviewed. Both activation of oxygen and various degradation pathways have been identified and the effects of medium and structure have been rationalized. The results can help in the understanding of the photochemistry occurring in biological environments and in the assessing of the correlation between structural characteristics and biological photodamage.

Green chemistry and photochemistry were born at the same time

Where to look for really ‘green’ synthetic methods, under conditions as mild as those nature uses? A hundred years ago, a great scientist, Giacomo Ciamician, confronted the problem. He had no doubt of the answer: it was solar light. The approach and the discoveries by Ciamician are illustrated in connection with present-day green chemistry.

Photochemical degradation of marbofloxacin and enrofloxacin in natural waters.

The photochemical fate of Marbofloxacin (MAR) and Enrofloxacin (ENR), two Fluoroquinolones (FQs) largely used as veterinary bactericides known to be present in surface waters, was investigated in aqueous solution. The degradation of these pollutants (5-50 microg L(-1) starting concentration) was complete in about 1 h by exposure to solar light (summer) and obeyed a first-order kinetics. The structure of the primary photoproducts was determined. Those from ENR arose through three paths, namely, oxidative degradation of the piperazine side-chain, reductive defluorination, and fluorine solvolysis. More heavily degraded products that had been previously reported were rationalized as secondary photoproducts from the present ones. As for MAR, this underwent homolytic cleavage of the tetrahydrooxadiazine moiety to give two quinolinols. All of the primary products were themselves degraded in about 1 h. The photoreactions rates were scarcely affected by Ca(2+) (200 mg L(-1)), Mg(2+) (30 mg L(-1)), Cl(-) (30 mg L(-1)), and humic acid (1 mg L(-1)), but increased in the presence of phosphate (20 mg L(-1)). The fastest degradation of ENR occurred at pH about 8 where the zwitterionic form was present, while in the case of MAR the cationic form was the most reactive.

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 (4 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 φ 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.

Photochemistry of 1‐Cyclopropyl‐6‐fluoro‐1,4‐dihydro‐4‐oxo‐7‐ (piperazin‐1‐yl)quinoline‐3‐carboxylic Acid (=Ciprofloxacin) in Aqueous Solutions

The 1-cyclopropyl-6-fluoro-1,4-dihydro-4-oxo-7-(piperazin-1-yl)quinoline-3-carboxylic acid (=ciprofloxacin; 1) undergoes low-efficiency (Φ=0.07) substitution of the 6-fluoro by an OH group on irradiation in H2O via the ππ* triplet (detected by flash photolysis, λmax 610 nm, τ 1.5 μs). Decarboxylation is a minor process (≤5%). The addition of sodium sulfite or phosphate changes the course of the reaction under neutral conditions. Reductive defluorination is the main process in the first case, while defluorination is accompanied by degradation of the piperazine moiety in the presence of phosphate. In both cases, the initial step is electron-transfer quenching of the triplet (kq=2.3⋅108M−1 s−1 and 2.2⋅107M−1 s−1, respectively). Oxoquinoline derivative 1 is much more photostable under acidic conditions, and in this case the F-atom is conserved, and the piperazine group is stepwise degraded (Φ=0.001).

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.

Sunlight-induced degradation of soil-adsorbed veterinary antimicrobials Marbofloxacin and Enrofloxacin

Marbofloxacin (MAR) and Enrofloxacin (ENR), two largely employed veterinary Fluoroquinolones (FQs), were found to be present at the micrograms per kilogram level in agricultural soils of South Lombardy (Italy) several months after manuring. Distribution coefficients (K(d)) from sorption experiments indicated a strong binding to the soil. Soil samples fortified with environmentally significant FQs amounts (0.5 mg kg(-1)) were exposed to solar light that promoted extensive degradation (80%) of both drugs in 60-150 h. Thus, photochemistry could be considered a significant depollution path in the soil, although it was two orders of magnitudes slower than in aqueous solution and a fraction of the drug (ca. 20%) remained unaffected. For MAR the photoprocess was the same as in solution, and involved cleavage of the tetrahydrooxadiazine ring. On the contrary, with ENR only some of the photoproducts determined in water (those arising from a stepwise oxidation of the piperazine side chain) were observed. Substitution of the 6-fluoro by a hydroxyl group and reduction did not occur in the soil, supporting the previous contention that such processes required polar solvation of FQs. Consistently with this rationalization, the irradiation of thin layers of solid drugs led to essentially the same products distribution as in the soil. From the environmental point of view it is important to notice that photodegradation mainly affects the side-chains, while the fluoroquinolone ring, to which the biological effect is associated, is conserved up to the later stages of the degradation.

Synthesis of monoprotected 1,4-diketones by photoinduced alkylation of enones with 2-substituted-1,3-dioxolanes

Abstract Photosensitized hydrogen abstraction from 2-alkyl-1,3-dioxolanes by triplet benzophenone gives the corresponding 1,3-dioxolan-2-yl radicals and these are trapped by α,β-unsatured ketones yielding monoprotected 1,4-diketones. With open chain ketones (3-buten-2-one and 4-penten-3-one) the yields are low and competitive pathways in part consume the radicals. With cyclic enones however, yields are good as tested with cyclopentenone, cyclohexenone and 4-hydroxy-cyclopentenone. More generally, this is a viable alternative for the synthesis of 1,4-diketones via radicals while the thermal initiation gives only low yield. The reaction cannot be extended to strongly stabilized radicals, such as the 2-phenyl-1,3-dioxolanyl radical.

Photochemistry of some fluoroquinolones: effect of pH and chloride ion

A 6-fluoroquinolone (norfloxacin) and the naphthyridine analogue enoxacin give the corresponding 6-hydroxy derivatives by irradiation in water at pH 7.2 and, with lower efficiency, at pH 4.5 and 10. At pH 1 no defluorination takes place and the piperazinyl side chain is degraded. The 6,8-difluoro derivative lomefloxacin is defluorinated selectively from position 8 over the entire pH range considered (pH 1 to 10). The intermediate cation in position 8 does not add water and rather undergoes insertion into the β-CH bond of the neighboring N-ethyl group. The cation adds chloride, however. The structure–photoreactivity relationship for fluoroquinolones and the relation with the known phototoxicity of these compounds are commented upon.

New synthetic methods via radical cation fragmentation

Photoinduced single electron transfer followed by radical cation cleavage is a selective method for generating radicals and cations from unconventional substrates under unparalleled mild conditions. The versatility of the method and its synthetic significance are demonstrated through the discussion of selected examples involving alkyl radicals as the key species, such as nucleophilic aromatic substitution, addition to unsatured esters, nitriles and ketones, and reduction, as well as of an example of addition to cations.