You have to extract the data manually (with an Hex-Editor)
Use this for mor information: recompress PRX's - Thread

1.)  Open the file with your HexEditor
2.)  Select only the data from (offset)0x150 to the end of the file
3.)  Save the selected data as "filename.gz"
4.)  Open and extract the *.gz-archive with 7-Zip or WinRAR and rename it to "filename.prx"
      -» now you have the decrypted *.prx