Mycophenolic acid is a fermentation product of Penicillium species that effectively blocks purine synthesis through allosteric inhibition of inosine monophosphate dehydrogenase. Purine nucleotides are synthetized by two pathways. The primary one is a de novo synthesis of inosinemonophosphate (IMP) from PPRP. The IMP is then used for synthesis of adenosine and guanosine by inosinemonophosphate dehydrogenase and adenosine deaminase, respectively. The second pathway for synthesis of purine nucleotides is their recyclation from guanosine and adenosine mediated by hypoxanthine guanine phosphoribosyltransferase and adenosine deaminase. Both pathways used PRPP.

The amount of PRPP in T and B lymphocytes is significantly increased after antigenic stimulation of the cells and is proven to be an important step before cell proliferation. Adding mycophenolic acid into the mixed lymphocyte reaction has effectively inhibited cell proliferation. Analyzing the cells by flow cytometry proved that lymphocyte passed through the G1 phase and stopped their mitosis in the S-phase. Another mechanism of action included depletion of GTP that may result in inhibition of G-protein based transduction signals. In immunized rats , the GTP pool significantly decreased after 4 days treatment with 20 mg/kg of MPA. MPA was shown to have a unique dual activity, both immunosuppressive and antimicrobial. Rats infected with Pneumocystis carinii did not develop pneumonia if they were treated with MPA [25].