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: PG - 9 was also able to increase the amount of NGF secreted in vitro by astrocytes in a dose - dependent manner .
### Output Text:
<span class="chemical">PG - 9</span> was also able to increase the amount of NGF secreted in vitro by astrocytes in a dose - dependent manner . 