For best coexistence with LR, make sure you have PM write its metadata in an XMP file (See Preferences, IPTC/XMP). Then make sure you have LR update the XMP file whenever you change anything (by default LR is usually configured to keep these changes into its own database). This should be selectable in the LR preferences (don't have a LR system available at the moment so can't check exactly where).
There's a sample on how to set the PM IPTC/XMP settings in the wiki as well (if you can't find it I'm sure Kirk knows where it is)
Hope this helps,
Hayo