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: Sequential measurements of cardiac output in surviving animals that received either free DOX or the mixture of HPMA copolymer and free DOX showed a reduction of approximately 30 % in function beginning at the 4th week after drug administration .
### Output Text:
Sequential measurements of cardiac output in surviving animals that received either free <span class="chemical">DOX</span> or the mixture of <span class="chemical">HPMA</span> copolymer and free <span class="chemical">DOX</span> showed a reduction of approximately 30 % in function beginning at the 4th week after drug administration . 