Takeshi Isoda

In Vitro and In Vivo Activities of Novel 6-Methylidene Penems as β-Lactamase Inhibitors

ABSTRACT Novel penem molecules with heterocycle substitutions at the 6 position via a methylidene linkage were investigated for their activities and efficacy as β-lactamase inhibitors. The concentrations of these molecules that resulted in 50% inhibition of enzyme activity were 0.4 to 3.1 nM for the TEM-1 enzyme, 7.8 to 72 nM for Imi-1, 1.5 to 4.8 nM for AmpC, and 14 to 260 nM for a CcrA metalloenzyme. All the inhibitors were more stable than imipenem against hydrolysis by hog and human dehydropeptidases. Piperacillin was combined with a constant 4-μg/ml concentration of each inhibitor for MIC determinations. The combinations reduced piperacillin MICs by 2- to 32-fold for extended-spectrum β-lactamase (ESBL)-producing Escherichia coli and Klebsiella pneumoniae strains. The MICs for piperacillin-resistant (MIC of piperacillin, >64 μg/ml) strains of Enterobacter spp., Citrobacter spp., and Serratia spp. were reduced to the level of susceptibility (MIC of piperacillin, ≤16 μg/ml) when the drug was combined with 4, 2, or 1 μg of these penem inhibitors/ml. Protection against acute lethal bacterial infections with class A and C β-lactamase- and ESBL-producing organisms in mice was also demonstrated with piperacillin plus inhibitor. Median effective doses were reduced by approximately two- to eightfold compared to those of piperacillin alone when the drug was combined with the various inhibitors at a 4:1 ratio. Pharmacokinetic analysis after intravenous administration of the various inhibitors showed mean residence times of 0.1 to 0.5 h, clearance rates of 15 to 81 ml/min/kg, and volumes of distribution between 0.4 and 2.5 liters/kg. The novel methylidene penem molecules inhibit both class A and class C enzymes and warrant further investigation for potential as therapeutic agents when used in combination with a β-lactam antibiotic.

Syntheses and Pharmacokinetic Studies of Prodrug Esters for the Development of Oral Carbapenem, L-084

We discovered an orally active carbapenem, L-084, through pharmacokinetic studies on various prodrug esters of (1R,5S,6S)-6-[(R)-1-hydroxyethyl]-1-methyl-2-[1-(1,3-thiazolin-2-yl)azetidin-3-yl]thio-1-carbapen-2-em-3-carboxylic acid (LJC11,036). L-084 showed a strong antimicrobial activity against Gram-positive and Gram-negative bacteria and exhibited the highest intestinal absorption among synthesized prodrugs of LJC11,036.

On the Absolute Configuration in 1,4-Dihydrothiazepine Covalent Complexes Derived from Inhibition of Class A and C β-Lactamases with 6-Methylidene Penems

Serine and metallo b-lactamases catalyze the hydrolysis of blactam rings in all classes of blactam antibiotics which is a major cause of bacterial resistance to b-lactam antibiotics. Bacterial resistance is addressed clinically by combining a b-lactamase inhibitor, such as clavulanic acid, sulbactam, or tazobactam, with a b-lactam antibiotic (amoxicillin or piperacilin). Whereas this strategy is effective with the class A b-lactamase inhibitors, there is an urgent need to extend the spectrum of activity to the other classes of serine b-lactamases including the class C enzymes. Recently, new promising inhibitors of class C b-lactamases such as NXL104, AVE1330A, and diaroylphosphates have been disclosed. Reports from our laboratories on 6-methylidene penems as mechanism-based inhibitors of serine-reactive class A and C b-lactamases disclosed extensive structure–activity relationships with penems containing monocyclic, [6,5]-bicyclic,and [5,5,5]-tricyclic heterocycles that adopt the Z configuration at the C6 position. The mode of action of penem inhibitors involves acylation by the catalytic serine residues followed by b-lactam ring opening and a sequence of transformations amounting to a remarkable 7-endo trig rearrangement reaction. Penems 1–3 have been studied by a plethora of methods to establish the formation of the 1,4-dihydrothiazepine acyl–enzyme complex (Figure 1). The complex is stable to hydrolysis because of the displacement of water molecules. However, an issue concerns the absolute stereochemistry of the C7 moiety bearing the heterocyles. In dihydrothiazepine 4 bearing the methyltriazolyl heterocyle, the S-stereochemistry is evidenced by kinetic, computational, and X-ray crystallographic studies in class A and C enzymes. The dihydroimidazo ACHTUNGTRENNUNG[2,1-c]oxazine thiazepine 5 exists as the R-isomer in the crystal structure of both SHV-1 and GC1 enzymes. A novel hydrophobic p-p stacking interaction between the C7 heterocycle with Tyr105 in SHV-1 and Tyr224 in GC1 was revealed. Furthermore, calculated interaction energy differences between C7R and C7S isomers of eight 6-methylidene penems bearing [6,5]-fused bicyclic heterocycles favor the formation of the C7R over the C7S enantio-