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: Careful therapeutic intervention is necessary in cases involving elderly patients who suffer from depression .
### Output Text:
Careful therapeutic intervention is necessary in cases involving elderly patients who suffer from depression . 