Synthesis and antibacterial activity of 1-N-[(S)-ω-amino-2-hydroxyalkyl] derivatives of dibekacin, 5-deoxydibekacin, 3′-deoxykanamycin A and gentamicin B

Aminoglycoside antibiotics are highly potent, broad-spectrum agents for the treatment of life-threatening infections. From 1971–1978, semisynthetic aminoglycosides such as dibekacin,¹ amikacin (AMK),² netilmicin,³ isepamicin (1d)⁴ and arbekacin (1a)⁵ were developed. Among them, AMK, isepamicin and arbekacin have an (S)-ω-amino-2-hydroxyalkanoyl residue on the 1-amino group, making them more resistant to the action of aminoglycoside-modifying enzymes such as aminoglycoside acetyltransferase (AAC), aminoglycoside phosphotransferase (APH) and aminoglycoside adenylyltransferase (AAD). For example, arbekacin is stable against the bifunctional enzyme AAC(6′)-APH(2″) contained in methicillin-resistant Staphylococcus aureus and effective against almost all resistant bacteria that produce aminoglycoside-modifying enzymes.

For the aminoglycoside antibiotics with the 2-deoxystreptamine component, the hydrogen bonding interaction between the 1-amino group and U1495 of the bacterial 16S rRNA is known.⁶ However, the N-1 atoms of 1-N-aminoalkanoyl compounds are no longer basic or ionizable, and the basic amino functions are located at the end of the side chains instead. Therefore, the hydrogen bonding of 1-N-aminoalkanoyl compounds may be weaker than that of 1-amino derivatives.

Richardson et al.⁷ synthesized 1-N-aminoalkyl derivatives of kanamycin A by borane reduction of the corresponding 1-N-aminoalkanoyl derivatives and characterized their antibacterial activities. Among them, 1-N-[(S)-4-amino-2-hydroxybutyl]kanamycin A (butikacin), which is an AMK analog, showed excellent antibacterial activity similar to that of AMK and with a lower ototoxicity than that of AMK.⁸

In an effort to clarify the relationship between the recovery of basicity at the N-1 atom with the antibacterial activity and biological properties, the transformation of 1-N-aminoalkanoyl compounds of various aminoglycoside antibiotics to the corresponding 1-N-aminoalkyl analogs is of interest. We selected arbekacin (1a), 5-deoxyarbekacin⁹ (1b), 3′-deoxyamikacin¹⁰ (1c) and isepamicin (1d) for further study (Scheme 1). Compounds 1a–c have superior antibacterial activity to AMK and 1d is known to be less toxic than AMK.

The trifluoroacetate salts of each antibiotic were used to improve their solubilities because their respective free bases or inorganic salts were only minimally soluble in the reaction solvent tetrahydrofuran. First, arbekacin (1a) trifluoroacetate was reduced with diborane in tetrahydrofuran, and following treatment with aqueous sodium hydroxide, 1-N-[(S)-4-amino-2-hydroxybutyl]dibekacin (2a) was obtained in moderate yield after purification by column chromatography with ion-exchange resin. The above alkaline hydrolysis reaction converted the unreacted 1a to the 1-amino derivative and facilitated the purification step. In the ¹H NMR spectrum of 2a in 25% ND₃/D₂O, the H-2‴ signal at 4.52 p.p.m. of 1a shifted to 4.13 p.p.m. and the new signals for H-1‴ appeared at 2.88 and 3.16 p.p.m. In the ¹³C NMR spectrum, the C-1‴ signal at 177.0 p.p.m. of 1a was shifted to 57.3 p.p.m., confirming the structure of 2a.

Borane reductions for 5-deoxyarbekacin, 3′-deoxyamikacin and isepamicin were also carried out in a similar fashion to yield 1-N-[(S)-4-amino-2-hydroxybutyl]-5-deoxydibekacin (2b), 1-N-[(S)-4-amino-2-hydroxybutyl]-3′-deoxykanamycin A (2c) and 1-N-[(S)-3-amino-2-hydroxypropyl]gentamicin B (2d), respectively. A large portion of 1c was recovered as starting material because of its poor solubility. As a result, the yield of 2c was lower than those of the other compounds. Their structures were also confirmed by ¹H NMR, ¹³C NMR and MS spectra as described for 2a.

Table 1 shows the antibacterial activity of the resulting 1-N-aminoalkyl derivatives 2a–d and their parent antibiotics 1a–d. In contrast to AMK,⁷ the kanamycin-type compounds 2a–c were less active than the parent antibiotics 1a–c. This indicated that the recovery of the basicity at the N-1 atom does not necessarily contribute to the improvement of the antibacterial activity in the kanamycin-type antibiotics. In contrast, the 1-N-aminoalkyl derivative 2d of the gentamicin-type antibiotics was more active than the parent antibiotic 1d.

Table 1 In vitro antibacterial activities (MIC, μg ml⁻¹) of 1a–d and 2a–d

In conclusion, we synthesized the 1-N-[(S)-ω-amino-2-hydroxyalkyl] derivatives of a variety of aminoglycoside antibiotics and evaluated their antibacterial activities. The superiority of the 1-N-aminoalkyl derivatives compared with the 1-N-aminoalkanoyl derivatives with respect to their antibacterial activities was not observed in the kanamycin-type antibiotics 2a–c. However, antibacterial activity enhancement was shown for the gentamicin-type antibiotic 2d. Further work into the effects of the 1-N-aminoalkyl modification on other types of aminoglycoside antibiotics is underway in our laboratory.

Synthesis of 1-N-[(S)-ω-amino-2-hydroxyalkyl] derivatives 2a–d. Reagents and conditions: (a) diborane, trifluoroacetic acid, tetrahydrofuran, 50 °C, 6 h; (b) NaOH, H₂O, 100 °C, 1 h.