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: Subsequently we investigated the efficacy of oxprenolol in antagonizing such hypokalemia , together with the pharmacokinetic interaction between both drugs .
### Output Text:
Subsequently we investigated the efficacy of <span class="chemical">oxprenolol</span> in antagonizing such hypokalemia , together with the pharmacokinetic interaction between both drugs . 