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: Clinical trials in patients with brain metastases combining dexrazoxane and high doses of etoposide is ongoing with the aim of improving efficacy without aggravating hematologic toxicity .
### Output Text:
Clinical trials in patients with brain metastases combining <span class="chemical">dexrazoxane</span> and high doses of <span class="chemical">etoposide</span> is ongoing with the aim of improving efficacy without aggravating hematologic toxicity . 