Write IPTC information in a PDF file

I’ve made a script wich write some IPTC information to a file. With a photoshop or indesign file is this no problem. For a PDF file i’ve made it also, this is working. But i’d like to add the instructions to the photoshop:Instructions field, now it’s placed into the pdfx:Instructions. The script i have for so far is:

tell application “Adobe Acrobat Professional”
activate
open alias (a_file as string)
set myDocument to front document

tell myDocument
	set info of myDocument key "Title" value "some text"
	set info of myDocument key "Author" value "some text"
	set info of myDocument key "Instructions" value "some text"
								
	close saving yes
end tell

end tell