Imipenem, a newer antibiotic with a broad antibacterial spectrum, is coadministered with cilastin because -
The combination of these antibiotics is Synergistic against Pseudomonas species
Cilastin aids the gastrointestinal absorption of the active moiety, imipenem
Cilastin inhibits a Beta-lactamase that destroys imipenem
Cilastin inhibits an enzyme in the kidney that destroys imipenem