Develop Extension in VS2012 RC and Install Into VS 2010

I've created an editor adornment extension in VS 2012 RC and it works great in VS 2012.  When I try to install the extension into VS 2010 SP1, the Visual Studio Extension Installer gives me the following error: "The extension manifest is invalid."

I've tried to modify the .vsixmanifest file and changed target version in the Install Targets tab to version 10.0 rather than 11.0.  VS 2010 SP1 still would not install the extension.  I've also tried to lower the dependency on .net 4.5 to .net 4.0 in the Dependencies tab but that still generated the same "The extension manifest is invalid." error. 

Does anyone know how I can get the extension to install in VS 2010 or is it not possible?

Thanks,

Paul