CP-65,207 is a new broad-spectrum penem antimicrobial agent that is a 1:1 mixture of two stereoisomers. Five minutes after a 10-min intravenous infusion of 1 g of CP-65,207 to volunteers, mean concentrations in serum were 33 ,ug of the R isomer per ml and 29 tig of the S isomer per ml. Following rapid distribution, half-lives of the isomers were 53 and 55 min, respectively. Concentrations in urine exceeded 800 ,ug of each isomer per ml. Recovery of the S isomer in urine (46%) was much greater than recovery of the R isomer (26%). The serum kinetics of the S isomer (volume of distribution, 319 ml/kg; total clearance, 315 ml/min; elimination rate constant, 0.80 h-1) were similar when it was given alone and when it was contained in 207, demonstrating that the presence of the R isomer has little effect on the serum kinetics of the S isomer. However, when the S isomer was given alone, the urinary recovery of intact S isomer (36%) was substantially lower than that when it was given with the R isomer as 207 (57%). Administration of the S isomer alone did not produce the unpleasant sulfurous odor in urine that was observed following administration of 207. Oral doses of a prodrug, which contained 1 g of CP-65,207, produced peak concentrations in serum of 1.6 ,ug of the R isomer per ml and 1.8 ,ug of the S isomer per ml. Approximately 36% of the S-isomer component was absorbed, and 20% of this isomer was recovered in urine. A 1-g oral dose of the prodrug of the single S isomer provides concentrations in serum above 1.0 ,ig/ml (the MIC for 90% of over 1,000 hospital pathogens) for 3.5 h, suggesting that the drug given orally will prove to be efficacious against many infections.Carbapenem and penem antimicrobial agents are betalactams that usually demonstrate a broad spectrum of activity and a high potency in vitro. However, penems and carbapenems also are susceptible to hydrolysis by the renal dehydrodipeptidase found in the brush border cells of animal and human kidneys (9,11,14). Thus, a renal dipeptidase inhibitor is coadministered with imipenem, the first member of this group of antimicrobial agents to be used commercially (14). Some penems are orally absorbed either as the parent molecule (9, 13) or after incorporation into a prodrug (5,16). Given the potent antibacterial activity of penems and carbapenems, research continues in an effort to find new agents with potential clinical utility (10, 12).CP-65,207 (2) is a potent broad-spectrum antibacterial agent with activity comparable to that of imipenem, with the exception that it has significantly lower activity against Pseudomonas aeruginosa (8). The MIC for 90% of over 1,000 hospital pathogens was approximately 1.0 jig/ml (8).CP-65,207 is a mixture of a more polar R isomer (CP-65,207-R) and a less polar S isomer (CP-65,207-S; also called 429) in approximately a 1:1 ratio (Fig. 1). While the isomers have very similar antibacterial activities, in vitro studies and animal experiments indicate that the S isomer is more resistant to hydrolysis by the renal dipetidase e...