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: The specific aim of this retrospective , observational study was to assess safety and efficacy of long - term ( 21 months / patient ) , open - label , gemfibrozil - lovastatin treatment in 80 patients with primary mixed hyperlipidemia ( 68 % of whom had atherosclerotic vascular disease ) .
### Output Text:
The specific aim of this retrospective , observational study was to assess safety and efficacy of long - term ( 21 months / patient ) , open - label , <span class="chemical">gemfibrozil</span> - <span class="chemical">lovastatin</span> treatment in 80 patients with primary mixed hyperlipidemia ( 68 % of whom had atherosclerotic vascular disease ) . 