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: However , future investigations are necessary to exactly analyze the biochemical mechanisms by which PDTC exerted its beneficial effects in the piriform cortex .
### Output Text:
However , future investigations are necessary to exactly analyze the biochemical mechanisms by which <span class="chemical">PDTC</span> exerted its beneficial effects in the piriform cortex . 