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: His renal function remained impaired but stable during 2 years ' treatment with captopril but returned to pre - treatment levels soon after cessation of the drug .
### Output Text:
His renal function remained impaired but stable during 2 years ' treatment with <span class="chemical">captopril</span> but returned to pre - treatment levels soon after cessation of the drug . 