Thienamycin, a natural product produced by Streptomyces cattleya is the first representative of a unique class of beta-lactam antibiotics, the carbapenems. Despite its outstanding potency and antibacterial spectrum, thienamycin was itself unsuited for further development because of its chemical instability in concentrated solution and in the solid state. Synthesis of the amidine derivative, N-formimidoyl thienamycin (imipenem, MK0787) resulted in a crystalline product with much improved stability and with antibacterial properties significantly superior to thienamycin. Imipenem has an unusually broad antimicrobial spectrum. A high order of bactericidal activity is found against Pseudomonas aeruginosa, Serratia, Bacteroides fragilis, enterococci and numerous other species intrinsically resistant to other antibiotics. Imipenem is refractory to hydrolysis by all important classes of bacterial beta-lactamases and thus exhibits no cross-resistance with penicillins or cephalosporins. Imipenem is distinguished from the new generation of extended-spectrum cephems by its unusually high potency against Gram-positive as well as Gram-negative organisms. Offsetting these excellent antimicrobial properties was an unusual susceptibility exhibited by imipenem to renal metabolism in animal species and in man. Very low urinary recoveries resulted without, however, any significant reduction in the serum half-life of imipenem. A brush-border dipeptidase, dehydropeptidase-I, was shown to be responsible for renal metabolism. Metabolism has been countered with the development of cilastatin (MK0791), a substituted amino-propenoate inhibitor of dehydropeptidase which is specific, potent and well matched in its pharmacokinetic properties for co-administration with imipenem. With the imipenem/cilastatin combination, uniformly high urinary concentrations and recovery are obtained regardless of the varying but often extensive metabolism suffered by imipenem in human populations. An additional benefit conferred by cilastatin results from its ability to exclude imipenem competitively from entry into and subsequent metabolism within the proximal tubular epithelium of the kidney. The tubular necrosis induced by imipenem alone when it is administered at very high doses to susceptible mammalian species is thereby eliminated. Thus the imipenem/cilastatin combination affords reliability and enhanced safety in the application of the antibiotic's unusual antibacterial potential in the treatment of difficult infections regardless of the site of disease.

BPT inhibitors can sensitize a resistant B. fragilis clinical isolate expressing metallo-beta-lactamase to the antibiotics imipenem or penicillin G but not to rifampicin.

Thienamycin (THM), the N-formimidoyl thienamycin derivative MK0787, and related carbapenem antibiotics were metabolized extensively in mice, rats, rabbits, dogs, rhesus monkeys, and chimpanzees. Urinary recovery of THM ranged from a low of 5% in dogs to 58% in rhesus monkeys. Renal clearance rates in dogs and chimpanzees were unusually low, less than glomerular filtration rates. The reduction in clearance of THM and MK0787 from plasma of rats and rabbits after ligation of renal arteries indicate that the kidneys are responsible for 35 and 92%, respectively, of metabolic drug clearance. Degradation was detected only in kidney homogenates. The enzyme activity was membrane bound and sensitive to inhibitors of Zn-metalloenzymes such as EDTA. A renal dipeptidase, dehydropeptidase-I (DHP-I), EC 3.4.13.11, was found to be responsible for the metabolism of the THM-class antibiotics, which exhibit a structural homology to dehydropeptides. A parallel increase in specific activity against THM and the substrate of DHP-I, glycyldehydrophenylalanine, was observed during solubilization and purification of the enzyme from porcine and human renal cortex. DHP-I was found to catalyze the hydrolysis of the beta-lactam ring in THM and MK0787. The products of the enzyme reaction were identical by high-powered liquid chromatography to their respective metabolites found in the urine. Nonbasic Nacylated THM and natural N-acylated carbapenems (epithienamycins and olivanic acids) were degraded 4-to 50-fold faster than THM when exposed to the enzymatic hydrolysis of DHP-I. Good correlations were obtained between the increased susceptibility of the carbapenem antibiotics to DHP-I as measured in the in vitro enzyme assay and the generally lower recoveries of active antibiotic in the urine of test animals. Despite this unusual degree of metabolism localized in the kidney, the plasma half-life of MK0787 and its efficacy against experimental systemic infections in animals remain satisfactory.Thienamycin (THM) and the N-formimidoyl thienamycin derivative MK0787 are members of a new class of structurally novel beta-lactam antibiotics, the carbapenems. Both are known for their high orders of activity against a broad spectrum of bacteria (11,18,19). Their breadth of activity is in part attributable to resistance to attack by bacterial beta-lactamases (17).The low recovery of unaltered THM and MK0787 in the urine of laboratory animals suggested that these antibiotics are extensively metabolized. Incomplete urinary recoveries of cephalosporins such as cephalothin and cefotaxime have been ascribed to enzymatic (hepatic) cleavage of their acetylated side chains. In both instances, desacetyl metabolites with reduced antimicrobial activity and an intact beta-lactam are recovered in the urine of laboratory animals and in humans (6,7,9,20).Biochemical studies presented in this report show that low urinary recoveries of THM and MK0787 result from the hydrolysis of the betalactam ring by the renal dipeptidase, dehydropeptidase-I

The practical application of thienamycin, a novel ,8-lactam antibiotic with a broad activity spectrum, was compromised by problems of instability. MK0787, N-formimidoyl thienamycin, does not have this liability. As reported, bacterial species resistant to most ,8-lactam antibiotics, such as Pseudomonas aeurginosa, Serratia, Enterobacter, Enterococcus, and Bacteroides spp., are uniformly susceptible to MK0787, usually at one-half the inhibitory level of thienamycin. Bactericidal ativityusuallv occurs at the minimal inhibitory concentration endpornt. Activity was reduced ony at the highest mocuum densities tested and by a lessor factor than was observed with reference 8l-lactam antibiotics active against P. aeruginosa and ,f-lactamase-bearing strains. MK0787 exhibits a broad spectrum of in vivo actvity when evaluated parenterally for efficacy against systemic infections in mice. The order of potency in vivo, 0.03 to 0.06 mg/kg for gram-positive species and 0.65 to 3.8 mg/kg for gram-negative infections including Pseudomonas, exceeded that of thienamycin and was at least 10-fold superior to reference ,B-lactam antibiotics including two recently developed agents with antipseudomonal activity, cefotaxime and LY127935.Thienamycin, a novel ,B-lactam antibiotic (1) (Fig. 1A)

N-Formimidoyl thienamycin (MK0787) undergoes renal metabolism by a dipeptidase, dehydropeptidase I, located on the brush border of the proximal tubular cells. The effects of two inhibitors (MK-789 and MK-791) of dehydropeptidase I on the pharmacokinetics of N-formimidoyl thienamycin were studied in 41 healthy subjects receiving various combinations of N-formimidoyl thienamycin and MK-789 or MK-791. Both inhibitors affected the plasma kinetics of N-formimidoyl thienamycin only to a small extent. Plasma concentrations and the area under the plasma concentration curve increased about 20%'o with a proportional decrease in plasma clearance. Plasma half-life was not altered significantly. Coadministration of MK-789 or MK-791 resulted in uniform and marked increases in urinary recovery and renal clearance of N-formimidoyl thienamycin. Thus, at an Nformimidoyl thienamycin/MK-791 ratio of 1:0.25 or higher, the urinary recovery was about 72% in all subjects, whereas it varied between 7.7 and 43% when Nformimidoyl thienamycin was given alone. The ratio of the N-formimidoyl thienamycin and MK-791 doses affected response. At relatively higher doses of MK-791, significant increases of N-formimidoyl thienamycin urinary recovery, renal clearance, and urine concentrations occurred during the later part of the 10-h observation period after each administration. At a 1:1 ratio of the two drugs, the inhibition of renal metabolism of N-formimidoyl thienamycin was maintained for at least 8 h, whereas renal clearance declined as soon as 4 h after the administration of a 1:0.25 ratio. The results indicated that MK-789 and MK-791 alter the renal excretion of N-formimidoyl thienamycin from glomerular filtration plus tubular secretion to glomerular ifitration only, possibly by competitively inhibiting the penetration of N-formimidoyl thienamycin into the proximal tubular cells.The human pharmacokinetics of N-formimidoyl thienamycin (MK0787) are characterized by high plasma concentrations and rapid elimination via the kidneys (3). The urinary recovery (UR) of N-formimidoyl thienamycin varies considerably between subjects, whereas the withinsubject variation is very small (3). The were included in the studies described in this report.All subjects gave their informed written consent to participate, and the protocols for the studies were 300