In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Two elderly patients with a history of either cerebral vascular accident ( CVA ) or head trauma and no evidence of renal disease developed seizures while receiving maximum doses of imipenem / cilastatin .
### Output Text:
Two elderly patients with a history of either cerebral vascular accident ( CVA ) or head trauma and no evidence of renal disease developed seizures while receiving maximum doses of <span class="chemical">imipenem / cilastatin</span> . 