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: In addition , no significant histological change was observed in the heart of animals that received DOX in the form of HPMA copolymer conjugates and were killed at the end of the study .
### Output Text:
In addition , no significant histological change was observed in the heart of animals that received <span class="chemical">DOX</span> in the form of <span class="chemical">HPMA</span> copolymer conjugates and were killed at the end of the study . 