Cedric H. Hassall

Synthesis and structure-activity relationships of antibacterial phosphonopeptides incorporating (1-aminoethyl)phosphonic acid and (aminomethyl)phosphonic acid

Phosphonodipeptides and phosphonooligopeptides based on L- and D-(1-aminoethyl)phosphonic acids L-Ala(P) and D-Ala(P) and (aminomethyl)phosphonic acid Gly(P) at the acid terminus have been synthesized and investigated as antibacterial agents, which owe their activity to the inhibition of bacterial cell-wall biosynthesis. A method for large-scale synthesis of the potent antibacterial agent L-Ala-L-Ala(P) (1, Alafosfalin) is described. Structure-activity relationships in the dipeptide series have been studied by systematic variation of structure 1. L stereochemistry is generally required for both components. Changes in the L-Ala(P) moiety mostly lead to loss of antibacterial activity, but the phosphonate analogues of L-phenylalanine, L-Phe(P), and L-serine, L-Ser(P), give rise to weakly active L-Ala-L-Phe(P) and L-Ala-L-Ser(P). Replacement of L-Ala in 1 by common and rare amino acids can give rise to more potent in vitro antibacterials such as L-Nva-L-Ala(P) (45). Synthetic variation of these more potent dipeptides leads to decreased activity. Phosphonooligopeptides such as (L-Ala)2-L-Ala(P) have a broader in vitro antibacterial spectrum than their phosphonodipeptide precursor, but this is not expressed in vivo, presumably due to rapid metabolism to 1. Stabilized compounds such as Sar-L-Nva-L-Nva-L-Ala(P) (46) have been developed that are more potent in vivo and have a broader in vivo antibacterial spectrum than the parent phosphonodipeptide.

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

The design and synthesis of the Angiotensin Converting Enzyme inhibitor Cilazapril and related bicyclic compounds

The postulated binding functions for the active site of Angiotensin Converting Enzyme (A.C.E.), derived in an earlier study, have made possible the design of improved inhibitors. Consequently, (1S,9S)-9-[(1S)-(ethoxycarbonyl)-3-phenylpropylamino]octahydro-10-oxo-6H-pyridazo[1,2-a][1,2]diazepine-1-carboxylic acid (Cilazapril), and related compounds, have been synthesized. They are very active inhibitors of A.C.E. and are highly potent antihypertensives in vivo.

The design and synthesis of new triazolo, pyrazolo-, and pyridazo-pyridazine derivatives as inhibitors of angiotensin converting enzyme

Bicyclic mimetics of the antihypertensive, angiotensin converting enzyme (A.C.E.) inhibitor, captopril, have been designed with the aid of computer graphics. The synthesis and structure activity relationships of the three bicyclic systems tetrahydro[1,2,4]triazolo[1,2-a]-, hexahydropyrazoio[1,2-a]- and octahydropyridazo[1,2-a]-pyridazinediones are described. The compounds with the terminal carboxy group, the thiol, and the amide carbonyl function orientated most closely to correspond to the three-dimensional array of these groups in bound captopril are the most active inhibitors of angiotensin converting enzyme, in vitro.

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.

Dihomo-γ-linolenic acid reverses hypertension induced in rats by diets rich in saturated fat

This study has shown that hypertension induced in rats by a diet rich in saturated fat (16% coconut oil, 4% palmitic acid by weight) is reversed by the addition of the essential fatty acid, dihomo-γ-linolenic acid (DHLA), at 5.0% but not at 0.5% of dietary energy. This potent effect of DHLA has been attributed to modulation of prostaglandin biosynthesis.

Peptido-aminobenzophenones-novel latentiated benzo-1,4-diazepines

It has been shown that cleavage of the N-terminal L-amino acids of a novel series of dipeptide derivatives of 2-aminobenzophenones occurs readily in vivo to give benzo-1,4-diazepines. Such compounds may serve as useful pro-drug forms of minor tranquillizers such as Valium®.

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.