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: Whether pentoxifylline inhibits dipyridamole - induced coronary hyperemia like other methylxanthines such as theophylline and should be stopped prior to dipyridamole - thallium - 201 imaging is unknown .
### Output Text:
Whether <span class="chemical">pentoxifylline</span> inhibits <span class="chemical">dipyridamole</span> - induced coronary hyperemia like other <span class="chemical">methylxanthines</span> such as <span class="chemical">theophylline</span> and should be stopped prior to <span class="chemical">dipyridamole</span> - <span class="chemical">thallium</span> - 201 imaging is unknown . 