Peter Stuart Ringrose

Phosphonopeptides as Antibacterial Agents: Mechanism of Action of Alaphosphin

The novel antibacterial peptide mimetic alaphosphin (l-alanyl-l-1-aminoethylphosphonic acid) selectively inhibited peptidoglycan biosynthesis in both gram-negative and gram-positive bacteria. It induced accumulation of uridine diphosphate-N-acetyl-muramyl-tripeptide in gram-positive organisms and significantly reduced the intracellular pool levels of d-alanine. Alaphosphin was actively transported into bacterial cells by stereospecific peptide permeases and was subsequently hydrolyzed by intracellular aminopeptidases to yield l-1-aminoethylphosphonic acid. This alanine mimetic rapidly accumulated inside susceptible cells to yield a concentration which was 100- to 1,000-fold in excess of that of the precursor peptide in the surrounding medium. In the case of susceptible gram-negative organisms, it was shown that 1-aminoethylphosphonic acid was incorporated into a metabolite which was tentatively identified as uridine diphosphate-N-acetylmuramyl-aminoethylphosphonate. The primary intracellular target site of 1-aminoethylphosphonic acid was alanine racemase (EC 5.1.1.1), which was reversibly and competitively inhibited in the gram-negative organisms Escherichia coli and Pseudomonas aeruginosa and irreversibly inhibited in a time-dependent manner in the gram-positive organisms Staphylococcus aureus and Streptococcus faecalis. A secondary target site could be uridine diphosphate-N-acetylmuramyl-l-alanine synthetase [EC 6.3.2.8(b)]. The mechanism of action of alaphosphin may be regarded as involving at least three stages: (i) active transport by peptide permeases; (ii) intracellular peptidase cleavage; and (iii) action of l-1-aminoethylphosphonate on alanine racemase.

Phosphonopeptides as Antibacterial Agents: Rationale, Chemistry, and Structure-Activity Relationships

Peptide mimetics with C-terminal residues simulating natural amino acids have been designed as inhibitors of bacterial cell wall biosynthesis. The phosphonopeptide series consisting of various l and d residues of natural amino acids combined with 1-aminoalkyl (and aryl-alkyl-) phosphonic acid residues had the most interesting antibacterial properties when the C-terminal residue was l-1-aminoethylphosphonic acid. The in vitro antibacterial activities of representative phosphonodi- to phosphonohexapeptides were investigated. The antibacterial action of the active compounds has been explained in terms of transport into the bacterial cell and intracellular release of the alanine mimetic, which interferes with the biosynthesis of the peptidoglycan of the bacterial cell wall.

Phosphonopeptides as Antibacterial Agents: Alaphosphin and Related Phosphonopeptides

Alaphosphin, l-alanyl-l-1-aminoethylphosphonic acid, was selected from a range of phosphonopeptides for evaluation in humans on the basis of its antibacterial activity, pharmacokinetics, and stability to intestinal and kidney peptidases. In vitro, the antibacterial action was antagonized by small peptides, resulting in low activity on peptone media. On an antagonist-free medium alaphosphin was bactericidal and rapidly lysed most susceptible gram-negative bacteria, but it was largely bacteriostatic and essentially nonlytic against gram-positive organisms. Its spectrum included most strains normally isolated from urinary tract infections, but potency was greatly reduced by very high inoculum levels and by alkaline pH. Although strains of Proteus and Pseudomonas were less susceptible to alaphosphin than were other common gram-negative bacteria, like other species they formed spheroplasts when exposed under appropriate conditions. Alaphosphin was equally effective against penicillin-susceptible and -resistant strains and showed no cross-resistance with known antibiotics. Good synergy and increased bactericidal activity were demonstrated with combinations of alaphosphin and d-cycloserine or β-lactam antibiotics. Images

Phosphonopeptide antibacterial agents related to alafosfalin: design, synthesis, and structure-activity relationships.

Dipeptide variants of alafosfalin (L-alanyl-L-1-aminoethylphosphonic acid) with substantial differences in potency and antibacterial spectrum in vitro and in vivo have been synthesized. Certain dipeptides with alternatives to the L-alanyl residue had broader antibacterial spectra; activity against Pseudomonas aeruginosa was included. Some compounds had better in vivo activity than alafosfalin when introduced into infected rodents orally, but for the majority of the more active phosphonodipeptides, parenteral administration was more effective. Certain oligopeptides derived from the more active phosphonodipeptides possessed good in vitro activity against an extended range of organisms; they included Haemophilus influenzae, Streptococcus faecalis, and Streptococcus pneumoniae. The in vivo activity of some of these phosphono-oligopeptides was significantly greater than that of the parent dipeptide and correlated well with the in vitro results. This indicates that phosphono-oligopeptides exert part of their in vivo action directly, in addition to that arising from smaller peptides produced by peptidase cleavage.

Phosphonopeptides as substrates for peptide transport systems and peptidases of Escherichia coli.

Peptide transport and peptidase susceptibility of the antibacterial agent alafosfalin and other phosphonopeptides have been characterized in Escherichia coli. Phosphonodipeptides were accumulated by a process which appeared to involve multiple permeases; saturation was not achieved even at concentrations of 128 microM. Competition studies showed that these compounds had only a low affinity for the system transporting phosphonooligopeptides and were rapidly taken up by and were inhibitory to E. coli mutants unable to transport the toxic peptide triornithine. Phosphonodipeptides containing D-residues were not appreciably transported. By contrast, phosphonooligopeptides were generally transported by a distinct saturable permease system for which they had a high affinity. This system was identical to that utilized by triornithine. Phosphonooligopeptides with simple monoalkyl substituents at the amino terminus were also transported except in the case of a t-butyl substituent. The oligopeptide permease was also able to transport certain derivatives which contained some residues having D rather than L stereochemistry. Intracellular metabolism of phosphonooligopeptides was initiated almost exclusively by hydrolysis from the N terminus by an L-specific peptidase. This initial hydrolytic activity was unaffected by the aminopeptidase inhibitor bestatin, unlike the final hydrolysis step which yields L-1-aminoethylphosphonic acid from the phosphonodipeptide intermediate.

Antibacterial activity and mechanism of action of phosphonopeptides based on aminomethylphosphonic acid.

Phosphonopeptides based on aminomethylphosphonic acid as the C-terminal residue linked to L-amino acids possessed antibacterial activity in vitro and in vivo. Analogs in this series were generally less potent than corresponding compounds based on L-1-aminoethylphosphonic acid such as alafosfalin (L-alanyl-L-1-aminoethylphosphonic acid). Significant differences in antibacterial spectra were observed. The mechanism of action involved active transport of the peptide mimetics into the bacterial cells, followed by intracellular release of high concentrations of aminomethylphosphonic acid which inhibited bacterial cell wall biosynthesis. Aminomethylphosphonic acid behaved as a mimetic of both D- and L-alanine and inhibited D-Ala-D-Ala synthetase (EC 6.3.2.4.), alanine racemase (EC 5.1.1.1.), and UDP-N-acetylmuramyl-L-alanine synthetase (EC 6.3.2.8.). The minimal inhibitory concentration of L-norvalyl-aminomethylphosphonic acid was essentially unaffected by the presence of D-alanine, whereas the activity of the corresponding L-norvalyl derivative of L-1-aminoethylphosphonic acid was markedly decreased. Substantial differences in the inhibitory and lytic activity of the L-norvalyl derivatives of aminomethylphosphonic and L-1-aminoethylphosphonic acids were also observed when these agents were combined with other inhibitors of bacterial cell wall biosynthesis.

Antibacterial properties of alafosfalin combined with cephalexin.

The phosphonopeptide alafosfalin (L-alanyl-L-1-aminoethylphosphonic acid) exhibited synergy in vitro and in animal studies against a range of bacterial genera when combined with cephalexin. Alafosfalin also showed synergy with mecillinam and, to a much lesser extent, with ampicillin. Synergy with cephalexin was more pronounced when the bacteria were relatively insensitive to the beta-lactam component. The action of this combination involved both an inhibitory and a bacteriolytic mechanism which was abolished by concurrent treatment with the aminopeptidase inhibitor, bestatin. Regrowth of subpopulation resistant to either component was markedly reduced by the combination. The potential of alafosfalin combined with cephalexin for use in therapy is discussed.