IIS 7 - ARR bypassing ISAPI filters - help!

Good afternoon.

I have installed ARR, rewrite, etc on Server 2008 SP2 IIS 7 x64. I believe I have the proxy working. However, we use a CA Siteminder ISAPI filter that forces CAC authentication before access to a web application is allowed.

With ARR activated, a request to "https://Site1" works as required prompting the user to enter CAC credentials, but a request to "https://Site1/Subsite2" , which is setup in ARR and rewrite to activate a web application on another server, bypasses all the top level IIS ISAPI filters and does NOT prompt a user to enter CAC credentials.

How can I force requests to execute the ISAPI filters BEFORE being handled by the proxy?

Thanks in advance.