The stability of meropenem in the presence of renal dehydropeptidase I (DHP-I) varied extremely with the animal source of the enzyme. Meropenem, compared with imipenem, was rather easily hydrolyzed by DHP-Is from mice, rabbits, and monkeys, while it showed a higher resistance to guinea pig and beagle dog DHP-Is. In addition, meropenem was four times more resistant than imipenem to human DHP-I. The 1 beta-methyl substituent on carbapenems, i.e., meropenem and 1 beta-methyl imipenem, made them considerably more resistant to mouse and swine DHP-Is than the 1-unsubstituted derivatives are.

A series of new carbapenemcompounds,which have a pyrrolidin-3'-ylthio group substituted with various aminocarbonyl group at C-5' position as C-2side chain, have been prepared. The antibacterial activity and the stability to renal dehydropeptidase-I of these compoundswere investigated, and the structure-activity relationships were discussed. In this series, SM-7338; (l i?,5*S,61S)-2-[(35,55')-5-dimethylaminocarbonylpyrrolidin-3-ylthio]-6-[(i?)-l-hydroxyethyl]l-methylcarbapen-2-em-3-carboxylic acid (5a) was the most interesting compound.

Weinvestigated the binding affinities of SM-7338 for penicillin-binding proteins (PBPs) and the morphological changes induced by it compared with those ofimipenem. Both SM-7338and imipenem had the highest binding affinities for PBP-2 of Escherichia coli, which were in good agreement with the primary morphological response of spherical cell formation. SM-7338also showed high affinities for PBP-1A, -IBs, and -3, and imipenem showed high affinities for PBP-1Aand -IBs but not for PBP-3. At 4-fold MIC, SM-7338 induced a indeterminate form, whereas imipenem did not. This may be due to the higher affinity of SM-7338 for PBP-3 compared to that of imipenem. Against Pseudomonas aeruginosa, SM-7338 had very high affinities for PBP-2 and -3, and imipenem had higher affinities for PBP-2 and -1A. SM-7338 induced this organism to filamentous cells at a concentration lower than its MIC, bulge cells at 2-fold MIC, and spherical cells at 4-fold MIC, while imipenem principally induced round cell formation at each concentration. These morphological differences in P. aeruginosa may be due to the differences in binding profiles to PBPs. Wealso studied the affinities for PBPs using radioactive SM-7338. The data obtained supported these results.Many new /Mactam antimicrobial agents have been introduced into clinical practice. Recently, non-traditional /Mactam antibiotics have been developed; one class which has been of particular interest is the carbapenems. Discovery of thienamycin1}, the antibiotic produced by Streptomyces cattleya, has taken the lead in opening the newera of /Mactamagents, the carbapenems. Subsequently a numberof carbapenems have been discovered and some of them were chemically synthesized, however, none have been available for clinical use except imipenem, the A^-formimidoyl derivative of thienamycin. SM-7338 a new carbapenem, which differs chemically from imipenem, has a methyl group at 4 position and carries a proline derivative on the 3-sulfur instead of the formimidoylaminoethyl group in imipenem. SM-7338 has a strong activity against Gram-positive and Gram-negative bacteria compared with other /Mactams2~7). The binding affinities of thienamycin and imipenem for penicillin-binding proteins (PBPs) of Escherichia coli and these effects on the shapes of the cells have been previously studied8~10). In E. coli PBPshave been knownto play essential roles in the maintenance of rod shape, septum formation, and functioning in the peptidoglycan biosynthesis, and so onll " 14*. The PBPs in Pseudomonas aeruginosa are presumed to have similar roles15).In this paper we examined the mode of action of SM-7338compared with imipenem from the viewpoint of affinities for PBPs and morphological changes, and we reported the difference ofmorphological responses between E. coli and P. aeruginosa induced by SM-7338whereas the same morphological changes were observed by imipenem.

A mutant strain of Escherichia coli with reduced susceptibility to imipenem, designated TL2740, was selected following serial passage of the parent strain, E. coli C600, in broth containing increasing concentrations of the carbapenem; the MIC of imipenem for TL2740 was eight-fold greater than that of the parent strain. The mutant also exhibited reduced susceptibilities to panipenem and biapenem and high-level resistance to mecillinam, but was as susceptible to meropenem, ceftazidime, piperacillin and the other beta-lactams tested as strain C600. The affinity of penicillin-binding protein (PBP) 2 of TL2740 for imipenem and meropenem was ten-fold less than that of C600, thereby providing an explanation for the mutant's reduced susceptibility to some carbapenems and mecillinam. However, this theory was confounded by the observation that the in-vitro activities of meropenem against both parent and mutant strains were virtually the same and by the fact that PBP 2 is the principal target of the antibiotic. Imipenem and aztreonam, which bind to PBP 2 and PBP 3 respectively, demonstrated synergic activity when tested in combination against both C600 and TL2740. These results suggest that the potent activity of meropenem against the mutant strain might also be due to a synergic effect resulting from simultaneous binding to both PBP 2 and PBP 3 and that the variable activities of the carbapenems against TL2740 were related to their different PBP binding profiles. Compared with C600, TL2740 appeared shorter on electron microscopy and had a longer generation time, discrepancies which are compatible with defective PBP 2 activities in the mutant strain. We also identified three clinical isolates of E. coli with beta-lactam susceptibility profiles which resembled that of TL2740 i.e. high-level resistance to mecillinam and low-level resistance to carbapenems, with the exception of meropenem to which these strains were susceptible; in common with TL2740, the combination of imipenem and aztreonam was synergic against these isolates. The genetic basis of resistance in all of the mecillinam-resistant strains, including TL2740, mapped close to lip at 15' on the E. coli chromosome with transductional analysis. The results strongly suggest that the reduced susceptibilities of the clinical isolates to carbapenems were due to mutations in the genes encoding the PBP 2s of these strains which affected their affinities for beta-lactam antibiotics.

The in vitro and in vivo antibacterial activities of SM-17466, a new 1␤-methyl carbapenem, were evaluated against a wide range of clinical bacterial isolates and compared with the activities of meropenem, imipenem, vancomycin, and arbekacin. SM-17466 had a broad spectrum of action against gram-positive bacteria, showing especially potent activity against methicillin-resistant staphylococci. The MICs of SM-17466, meropenem, imipenem, vancomycin, and arbekacin at which 90% of clinical isolates of methicillin-resistant Staphylococcus aureus were inhibited were 3.13, 50, 100, 1.56, and 3.13 g/ml, respectively. This activity of SM-17466 was almost equivalent to those of the antibiotics used for the treatment of infections caused by this organism. SM-17466 also showed bactericidal activity against methicillin-resistant S. aureus. In contrast, SM-17466 was less active against gram-negative bacteria, especially against Pseudomonas aeruginosa, compared with the other carbapenems; however, of the carbapenems, SM-17466 exhibited the highest activity against Haemophilus influenzae and Bacteroides fragilis. SM-17466, at a 50% inhibitory concentration of less than 1 g/ml, bound to penicillin-binding proteins 1 to 4 in methicillin-susceptible S. aureus and also had good binding to penicillinbinding protein 2 in a methicillin-resistant strain (50% inhibitory concentration, 5.9 g/ml). This high affinity, which was 10 and 20 times greater than those for meropenem and imipenem, respectively, was reflected in the potent activity of SM-17466 against methicillin-resistant S. aureus. SM-17466 demonstrated excellent in vivo efficacy against methicillin-susceptible and -resistant S. aureus strains in a mouse peritoneal infection model: the efficacy of SM-17466 against methicillin-resistant strains was equal to or one-third that of vancomycin. This activity was comparable to the in vitro activity of SM-17466. The subcutaneous injection of SM-17466 in mice revealed that the half-life of SM-17466 in serum was about 18 min, intermediate between those of vancomycin and arbekacin and 1.5-fold that of imipenem-cilastatin. SM-17466 was resistant to hydrolysis by swine renal dehydropeptidase I, to an extent comparable to the resistance shown by meropenem.