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: Infusion regimens were designed that rapidly achieved and maintained target - free concentrations of these drugs in plasma and data on the relationship between free concentration and changes in MAPD were obtained for these compounds .
### Output Text:
Infusion regimens were designed that rapidly achieved and maintained target - free concentrations of these drugs in plasma and data on the relationship between free concentration and changes in MAPD were obtained for these compounds . 