An Isolated Method and Assignment for Critical Impurities in Semi-Synthetic Process of Arbekacin Sulfate.

Four potential process related impurities were detected during the impurity profiling study of a semi-synthetic aminoglycoside antibiotic, arbekacin. The current preparation process from 3',4'-didehydro-dibekacin easily generates the specific impurities with similar structures to arbekacin that makes hard to separate and identify the residues. HPLC-ELSD and column chromatography loading weakly acidic cation exchange resin were used for the detection and isolation of these process impurities. Based on the synthesis and spectral data (ESI-MS/MS, 1H NMR, 13C NMR and 2D-NMR), the structures of these impurities were characterized as dibekacin, 3-N-γ-aminohydroxybutyric (AHB)-dibekacin, 3''-N-AHB-dibekacin and 1,3-N,N-di-AHB-dibekacin. The characterization of these impurities is discussed in detail and our current efforts may help to develop a general strategy for isolation and identification of aminoglycoside products.

Synthesis and antibacterial activity of 4″ or 6″-alkanoylamino derivatives of arbekacin.

Arbekacin, an aminoglycoside antibiotic, is an important drug because it shows a potent efficacy against methicillin-resistant Staphylococcus aureus. However, resistance to arbekacin, which is caused mainly by the bifunctional aminoglycoside-modifying enzyme, has been observed, becoming a serious problem in medical practice. To create new arbekacin derivatives active against resistant bacteria, we modified the C-4″ and 6″ positions of its 3-aminosugar portion. Regioselective amination of the 6″-position gave 6″-amino-6″-deoxyarbekacin (1), and it was converted to a variety of 6″-N-alkanoyl derivatives (6a-z). Furthermore, regioselective modifications of the 4″-hydroxyl group were performed to give 4″-deoxy-4″-epiaminoarbekacin (2) and its 4″-N-alkanoyl derivatives (12 and 13). Their antibacterial activity against S. aureus, including arbekacin-resistant bacteria, was evaluated. It was observed that 6″-amino-6″-N-[(S)-4-amino-2-hydroxybutyryl]-6″-deoxyarbekacin (6o) showed excellent antibacterial activity, even better than arbekacin.

A synthesis of 3',4'-dideoxykanamycin B.

3',4',-Dideoxykanamycin B, the kanamycin B derivative that is active against resistant bacteria, was prepared from kanamycin B via N-tosylation, 3',4'-O-sulphonylation, 3',4'-unsaturation, and hydrogenation. The unsaturated intermediate was obtained from the 3',4'-di-O-sulphonyl derivatives by the action of sodium iodide in N,N-dimethylformamide; if zinc dust was added in this reaction, aziridine derivatives were formed. Removal of the tosyl group was successfully performed by using sodium in ammonia-ethylamine.

Synthesis of 1-N-[(2S,4S)- and (2S,4R)-5-amino-4-fluoro-2-hydroxypentanoyl]dibekacins (study on structure-toxicity relationships).

(2S,4S)- and (2S,4R)-5-azido-2-O-benzyl-4-fluoro-2-hydroxypentanoic acids (15 and 19) have been prepared from L-malic acid (1), and coupled to the H2N-1 group of 3,2',6'-tris(N-benzyloxycarbonyl)-3"-N-(trifluoroacetyl)dibekacin (23), to give, after reduction and deblocking, 1-N-[(2S,4S)- and (2S,4R)-5-amino-4-fluoro-2-hydroxypentanoyl]dibekacins (26 and 27). The fluorinated arbekacin analogs showed almost the same antibacterial activities as that of arbekacin, but lower toxicity. Comparison of the toxicity between 26 (and 27) and the arbekacin analogs (28-30) with change of the 1N-side-chain indicates that the observed decrease in toxicity was a function of the chain length rather than the introduction of fluorine.

Synthesis of 5-deoxy-5-epifluoro derivatives of arbekacin, amikacin, and 1-N-[(S)-4-amino-2-hydroxybutanoyl]tobramycin (study on structure--toxicity relationships).

As part of a study on fluorination--toxicity relationships for aminoglycoside antibiotics, 5,3'-dideoxy-5-epifluorokanamycin B (10), 5,3',4'-trideoxy-5-epifluorokanamycin B (11), 1-N-[(S)-4-amino-2-hydroxybutanoyl]-5-deoxy-5-epifluorotobramyc in (19), 5-deoxy-5-epifluoroarbekacin (20), and 5-deoxy-5-epifluoroamikacin (21) have been prepared. The acute toxicities of these three 5-deoxy-5-epifluoro compounds showed values almost identical or similar to those for arbekacin (ABK) and amikacin (15), making a sharp contrast with the toxicities of the corresponding 5-deoxy-5-fluoro derivatives. This fact is explained on the basis of basicity changes (retention for the 5-epifluoro derivatives and reduction for the 5-fluoro derivatives) at the H2N-3 groups of the fluorinated compounds compared to the parent compounds; this hypothesis was substantiated by the pKa values at the H3N(+)-1, 3 groups (determined by the shift changes depending on pD values at C-2 and C-4, 6 in their 13C NMR spectra) of 2,5-dideoxy-5-epifluorostreptamine (23) and 2,5-dideoxy-5-fluorostreptamine (24), chosen as model compounds, and 2-deoxystreptamine (DST).

Synthesis of 2''-amino-2''-deoxyarbekacin and its analogs having potent activity against methicillin-resistant Staphylococcus aureus.

Based on our studies on the enzymatic modifications of arbekacin by methicillin-resistant Staphylococcus aureus (MRSA), replacement of the 2''-hydroxyl group by an amino group in arbekacin was designed to synthesize derivatives that would be active against MRSA. 2''-Amino-2''-deoxyarbekacin and five analogs were synthesized starting from dibekacin. Among them, 2''-amino-2''-deoxyarbekacin and the 5-epiamino analog showed excellent antibacterial activities against not only MRSA but also Gram-negative bacteria including Pseudomonas, and lower toxicities than arbekacin.

[Development of arbekacin and synthesis of new derivatives stable to enzymatic modifications by methicillin-resistant Staphylococcus aureus].

Our studies on the resistance mechanisms and chemical modifications of aminoglycoside antibiotics led to the synthesis of arbekacin (ABK), which was refractory to most aminoglycoside-modifying enzymes in resistant bacteria. In 1990, ABK was launched into clinical uses in Japan as a chemotherapeutic agent for the treatment of infections caused by methicillin-resistant Staphylococcus aureus (MRSA). By 1993 only a few MRSA strains moderately resistant to ABK (MIC, 6.25-12.5 micrograms/ml) had clinically been isolated. ABK was modified by the reaction with an excess of an enzyme preparation extracted from an ABK-resistant strain (12.5 micrograms/ml) and three inactivated products were produced, consisting mainly of ABK 2''-phosphate along with small amounts of 6'-N-acetyl-ABK and the doubly modified ABK. Based on these results, replacement of the 2''-hydroxyl by amino group in dibekacin (DKB) or in ABK was designed to obtain potent active derivatives against MRSA. Conversion of the 2''-hydroxyl group by DMSO-DCC oxidation followed by reductive amination with NH4OAc-NaBH3CN gave 2''-amino-2''-deoxy-DKB (D1) and -ABK (A1). Their 5-deoxy (D2 and A2), 5-epifluoro (D3 and A3) and 5-epiamino (D4 and A4) derivatives were also synthesized. All 2''-amino-2''-deoxy-ABK derivatives (A1, A2, A3 and A4) showed excellent activities against MRSA and Gram-negative bacteria, as expected. Among them, A4 having low acute toxicity and nephrotoxicity was selected as a new candidate for anti-MRSA agent.

Effect of varying the 4''-position of arbekacin derivatives on antibacterial activity against MRSA and Pseudomonas aeruginosa.

4''-Deoxy-4''-episubstituted arbekacin derivatives and 4''-epi-5-deoxy-5-episubstituted arbekacin derivatives were designed and synthesized. Arbekacin and 4''-epiarbekacin both displayed the same antibacterial activity against Staphylococcus aureus (including methicillin-resistant S. aureus (MRSA)) and Pseudomonas aeruginosa. The 4''-epi-5-deoxy-5-episubstituted arbekacin derivatives showed potent antibacterial activity. Among them, the antibacterial activity of 5,4''-diepiarbekacin was superior to that of arbekacin or 5-episubstituted arbekacin against Gram-positive and Gram-negative bacteria. The 6'-N-methyl derivative of the 5,4''-diepiarbekacin was effective against P. aeruginosa expressing an aminoglycoside-modifying enzyme AAC(6')-Ib.

[Discovery of dibekacin and its chemical aspects]. / Découverte de la dibékacine et de ses aspects chimiques.

The advent of strains resistant to antibiotics in some patients has led to reorientation of research in this field. Demonstration of the mechanism involved in resistance to aminosides has resulted in the development of derivatives active against these resistant strains. Dibekacin, a new aminoside, is the first semisynthetic antibiotic of this class to be developed, and represents the first application of carbohydrate chemistry to the industry. It possesses potent activity and very weak ototoxicity.