The title enzyme deactivates the potent carbapenem antibiotic imipenem in the kidney, producing low antibiotic levels in the urinary tract. A series of (Z)-2-(acylamino)-3-substituted-propenoic acids (3) are specific, competitive inhibitors of the enzyme capable of increasing the urinary concentration of imipenem in vivo. Many of the compounds were prepared in one step from an alpha-keto acid and a primary amide. The optimum R2 groups are 2,2-dimethyl, -dichloro, and -dibromocyclopropyl. With R2 = 2,2-dimethylcyclopropyl (DMCP), a wide variety of R3 groups including alkyl, oxa- and thiaalkyl, and alkyl groups containing acidic, basic, and neutral substituents give effective inhibitors with Ki values of 0.02-1 microM and a range of pharmacokinetic properties. By resolution of enantiomers and X-ray crystallography, the enzyme-inhibitory activity of the DMCP group was found to reside with the 1S isomer. The cysteinyl compound 176 (cilastatin, MK-0791) has the desired pharmacological properties and has been chosen for combination with imipenem.