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: It is suggested that the drug acted in a selective and tissue - specific manner to enhance ribosomal RNA synthesis in the liver and depress such synthesis in the muscle .
### Output Text:
It is suggested that the drug acted in a selective and tissue - specific manner to enhance ribosomal RNA synthesis in the liver and depress such synthesis in the muscle . 