.htaccess - Rewrite rule based on query string -

i'm trying redirect base url (www.example.com) requests have particular query string (when option com_estateagent).

i've tried following syntax:

rewriteengine on rewritebase / rewritecond %{query_string} ^option=com_estateagent$ rewriterule .* index.php [r=301, l] 

but gets ignored.
suggestions?

edit
url want change this:
http://www.example.com/subdirectory/index.php?option=com_estateagent...

i don't think need rewritecond directive, wouldn't work?

rewriterule ^/.*option=com_estateagent /index.php[r=301,l]