As far as I know this phosphorylated sugar cannot cross the membrane, nevertheless some kind of cells can uptake free ribose, if they posses ribokinase, it can be phosphorylated and then utilized to increase the intracellular PRPP concentration. A good way to increase PRPP intracellular concentration could be to inhibit nucleotide catabolism, inhibiting intracellular nucleotidase activities and particularly cN-II. I add here a paper that could be useful for you,
Ipata PL, Tozzi MG. Recent advances in structure and function of cytosolic IMP-GMP specific 5'-nucleotidase II (cN-II). Purinergic Signal. 2006 Nov;2(4):669-75.