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: Nifedipine significantly improved kidney function as indicated by a significant lowering of serum creatinine levels at 6 and 12 months .
### Output Text:
<span class="chemical">Nifedipine</span> significantly improved kidney function as indicated by a significant lowering of serum <span class="chemical">creatinine</span> levels at 6 and 12 months . 