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: TEM revealed extensive elastolysis in the arterial wall of D - pen - treated rats , consistent with an inhibitory effect on crosslink formation .
### Output Text:
TEM revealed extensive elastolysis in the arterial wall of <span class="chemical">D - pen</span> - treated rats , consistent with an inhibitory effect on crosslink formation . 