Pharmaceutical properties of loracarbef: the remarkable solution stability of an oral 1-carba-1-dethiacephalosporin antibiotic

Loracarbef is an oral 1-carba-1-dethiacephalosporin antibiotic structurally related to cefaclor. Like many beta-lactam antibiotics, loracarbef exists in several hydrated crystalline forms. The pH-solubility profile curve for loracarbef monohydrate is U-shaped, resembling those of other zwitterionic cephalosporins. Loracarbef was found to be much more stable in solution than cefaclor. For example, in pH 7.4 phosphate buffer, loracarbef was unexpectedly found to be 130-150 times more stable than cefaclor and 10-12 times more stable than cephalexin, depending on the phosphate concentration. The pH-stability profile is U-shaped, similar to that of other zwitterionic cephalosporins, and shows maximum stability at the isoelectric point. At any given pH, loracarbef is more stable in solution than any other therapeutically useful cephalosporin. Acetate, borate, citrate, and especially phosphate buffers have catalytic effects on the rate of loracarbef hydrolysis.

Synthesis, hydrolysis rates, supercomputer modeling, and antibacterial activity of bicyclic tetrahydropyridazinones

Bicyclic tetrahydropyridazinones, such as 13, where X are strongly electron-withdrawing groups, were synthesized to investigate their antibacterial activity. These delta-lactams are homologues of bicyclic pyrazolidinones 15, which were the first non-beta-lactam containing compounds reported to bind to penicillin-binding proteins (PBPs). The delta-lactam compounds exhibit poor antibacterial activity despite having reactivity comparable to the gamma-lactams. Molecular modeling based on semiempirical molecular orbital calculations on a Cray X-MP supercomputer, predicted that the reason for the inactivity is steric bulk hindering high affinity of the compounds to PBPs, as well as high conformational flexibility of the tetrahydropyridazinone ring hampering effective alignment of the molecule in the active site. Subsequent PBP binding experiments confirmed that this class of compound does not bind to PBPs.

Comparative reactivity of 1-carba-1-dethiacephalosporins with cephalosporins

Nine matched pairs of cephalosporins and their 1-carba-1-dethiacephalosporin analogues have been compared with regard to microbiological activity, beta-lactam carbonyl infrared absorption, and aqueous stability. In general the microbiological activity of the pairs of compounds were very similar across a broad range of bacteria. The infrared absorption bands for the beta-lactam carbonyls of the pairs indicated a general trend for the 1-carba-1-dethiacephalosporins to absorb at lower frequencies than the corresponding cephalosporins. All of the 1-carba-1-dethiacephalosporins did however present a striking stability enhancement over their cephalosporin counterparts at pH = 10 or 11 in water. This marked contrast of MIC similarity with the observed differences in chemical reactivity clearly demonstrates hydroxide ion catalyzed hydrolysis is not a good model for transpeptidase activity unless the compounds comprise a limited domain of structural type.

3-Quaternary ammonium 1-carba-1-dethiacephems

A series of structurally unique 1-carba-1-dethiacephems is described. The structural stability of the 1-carba-1-dethiacephem nucleus was essential for the preparation of this series of 3-quaternary ammonium carbacephems. The known p-nitrobenzyl 7 beta-(phenoxyacetamido)- 3-[(trifluoromethyl)sulfonyl]oxy]-1-carba-1-dethia-3-cephem- 4-carboxylate served as both a quaternization substrate as well as a precursor to derivatives such as allyl 7 beta-[[2-[allyloxy)carbonyl]amino-4- thiazoly] (methoxyimino)acetyl]amino]-3-[(trifluoromethyl) sulfonyl] oxy]-1-carba-1-dethia-3-cephem-4-carboxylate. Quaternization of these enol triflates was accomplished either by dissolution in acetonitrile containing the base or by dissolution in the base, with or without warning to 50 degrees C. Bases nucleophilic enough to displace the triflate include a variety of substituted pyridines and N-methylimidazole. Deprotection then produced a very active series of 1-[7 beta-[(2-amino- 4-thiazolyl)(methoxyimino)acetyl]amino]-2-carboxy-8-oxo- 1-azabicyclo[4.2.0]oct-2-en-3-yl] quaternary ammonium hydroxide inner salts. These compounds were extremely potent antibacterials against a broad range of Gram-positive and -negative bacteria including constitutive cephalosporinase producers, such as Enterobacter cloacae. The compounds exhibit similar hydrolysis kinetics and pharmacokinetics to the analogous cephalosporin-3'-quaternary ammonium salts.

3-Sulfonyl-1-carba-1-dethiacephems

The stability of the 1-carba-1-dethiacephalosporin framework has allowed the synthesis of a range of 3-sulfonyl-1-carba-1-dethiacephems unavailable for a variety of reasons in the cephem arena. The known p-nitrobenzyl 7 beta-(phenoxyacetamido)-3-[[(trifluoromethyl)sulfonyl]oxy]-1-carba -1- dethia-3-cephem-4-carboxylate served as a precursor to this series of compounds. Displacement of the enol triflate with various sulfinates in acetonitrile or DMF and deprotection of the intermediates led to 7 beta-[(2-amino-4-thiazolyl)(methoxyimino)acetyl]amino]- 3-[alkyl(aryl)sulfonyl]-1-carba-1-dethia-3-cephem-4-carboxyl ic acids. The 3-sulfonyl-1-carba-1-dethiacephems display potent activity against both Gram-positive and Gram-negative bacteria. The following MIC's (microgram/mL) for the 3-cyclopropyl sulfone are representative: Staphylococcus aureus = 4, Streptococcus pyogenes = 1, Haemophilus influenzae = 0.25, Escherichia coli = 0.03, Enterobacter cloacae = 0.25, Proteus rettgeri = 0.25. The excellent in vitro antibacterial activity of this series indicates the potential of the carbacephalosporin framework for exploring substituents which are unknown or which produce unstable cephems.

The acylating potential of gamma-lactam antibacterials: base hydrolysis of bicyclic pyrazolidinones

The acylating ability of the gamma-lactam ring of a new class of antibacterial agent, the bicyclic pyrazolidinones 1, was compared to that of the beta-lactam ring of clinically useful antibiotics by measuring chemical reactivity with hydroxide ion. The pyrazolidinone chemical reactivity spans the reactivity of classical beta-lactam antibiotics and the most reactive, 1i, is 13 times more reactive than the most reactive beta-lactam examined, ceftazidime. A correlation involving chemical reactivity, microbiological activity, and 3-substituent sigma p values was observed, and the correlation has led to the synthesis of new more potent bicyclic pyrazolidinones.

Heteroatom-activated beta-lactam antibiotics: considerations of differences in the biological activity of [[3(S)-(acylamino)-2-oxo-1-azetidinyl]oxy]acetic acids (oxamazins) and the corresponding sulfur analogues (thiamazins)

The considerable antibacterial activity of [[3(S)-(acylamino)-2-oxo-1-azetidinyl]oxy]acetic acids (oxamazins) in contrast to the lack of activity of the corresponding sulfur analogues (thiamazins) is examined in terms of physicochemical parameters, including electronegativity, IR carbonyl stretching frequencies, base hydrolysis rates, and three-dimensional molecular geometries. An X-ray structure determination of a protected thiamazin together with molecular graphics and molecular orbital calculations on model structures reveals that thiamazins would not fit as well as oxamazins in the active site of target bacterial transpeptidases. As a result of thiamazins' long N-S and S-C bond lengths, the pharmacophoric beta-lactam ring and carboxylate functionality cannot adopt the spatial relationship they have in penicillins and cephalosporins. The beta-lactam nitrogen of the monocyclic, crystalline thiamazin is 0.18 A out of the plane of its three substituents, and this distance (h) is predicted by computational chemistry methods to be higher in oxamazins. The rates of beta-lactam ring opening of an oxamazin, thiamazin, and aztreonam are comparable, even though the pyramidal character and IR data both indicate the electronegative oxygen analogue has reduced amide resonance. MNDO, AM1, and MINDO/3 correctly give a twofold potential for rotation about the N-S bond in model sulfenamides, with barrier heights ranging up to 12 kcal/mol.

Intramolecular nucleophilic amino attack in a monobactam: synthesis and stability of (2S,3S)- 3-[(2R)-2-amino-2-phenylacetamido]-2-methyl-4-oxo-1- azetidinesulfo nic acid

The potentially orally bioavailable arylglycine-substituted monobactam, (2S,3S)- 3-[(2R)-2-amino-2-phenylacetamido]-2-methyl-4-oxo-1- azetidinesulfonic acid, was prepared as a crystalline solid. No significant antibacterial activity [i.e., MICs were greater than 128 (micrograms/mL)] was found when the monobactam was tested against Gram positive and Gram negative bacteria. Solution instability (greater than 2,000 times less stable than aztreonam) due to intramolecular nucleophilic amine attack on the beta-lactam is believed to be a contributing factor to the poor microbiological activity.

Cephalothin: hydrolysis of the C-3'-acetoxy moiety of a 7-aminocephalosporanic acid; observation of both acyl-oxygen bond cleavage and reversible alkyl-oxygen bond cleavage

The aqueous solution chemistry of the C-3'-acetoxy moiety of cephalothin sodium (1b) was examined with the use of isotopically labeled H2(18)O and [2-13C]acetate anion. The 18O incorporation studies indicate that the hydrolysis (at pH 4.7 +/- 0.1) of 1b to the deacetyl derivative of cephalothin (2b) proceeds via two pathways: alkyl-oxygen bond cleavage (55-63%) and acyl-oxygen bond cleavage accounting for the remainder. The incorporation of [2-13C]acetate into 1b suggests that the alkyl-oxygen cleavage pathway is a reversible reaction.

Formylation of glucose by cefamandole nafate at alkaline pH

An increase in the hydrolysis rate of the formyl moiety of cefamandole nafate was observed when the commercial product was reconstituted in 5% dextrose in water relative to the rate when the formulation was dissolved in water for injection or in 0.9% saline. The increase in ester cleavage was the result of nucleophilic attack of glucose on the formyl ester moiety. This transesterification produced small amounts of D-glucose-6-formate and other D-glucose diformates. The formation of these products is of no clinical significance since the antibiotic potency and stability of cefamandole are unaffected and no toxicological differences were observed in animal studies or in clinical trials when formulated cefamandole nafate was administered with or without glucose.

Hydrolysis of 3-chloro-3-cephems. Intramolecular nucleophilic attack in cefaclor

The chemical reactivity of 3-chloro-3-cephems was found to be similar to that of the correspondingly substituted 7-aminocephalosporanic acids and 12-13 times greater than that of the correspondingly substituted 7-aminode-acetoxycephalosporanic acids. Cefaclor, 7-(D-2-amino-2-phenylacetamido)-3-chloro-3-cephem-4-carboxylic acid, was found to undergo intramolecular nucleophilic attack at the beta-lactam. Loss of chlorine from 3-chloro-3-cephem may be a general reaction subsequent to beta-lactam opening.

Conversion of cefamandole nafate to cefamandole sodium

The rate of hydrolysis of the formyl moiety of cefamandole nafate was determined as a function of pH, temperature, and concentration of added sodium carbonate or tromethamine. The reaction rate was sensitive to hydroxide ion in the pH 5.5-8.0 range with half-life values of hours to minutes. Hydrolysis was rapid upon the addition of sodium carbonate or tromethamine. Chirality in the 7-D-mandelamido side chain was unaffected by hydrolysis.