Tamoxifen and Herceptin

I've read on a few posts that combining Tamoxifen and Herceptin actually
is less effective than Herceptin alone, without any hormonal therapy. We're trying to get some mileage out of Tamoxifen prior to switching to an A.I., but if this is the case we won't risk it. Can someone point us to any data that supports this?

Thanks,

Scott