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: This study shows that prolonged analgesic treatment in Fischer 344 rats causes progressive and irreversible damage to the interstitial matrix and type 1 interstitial cells leading to RPN .
### Output Text:
This study shows that prolonged analgesic treatment in Fischer 344 rats causes progressive and irreversible damage to the interstitial matrix and type 1 interstitial cells leading to RPN . 