Mycophenolic acid

Mycophenolic acid (MPA) is a phthalide-derived secondary metabolite with an unusual property in the indoor mycology context: the same molecule is used clinically as an immunosuppressant in the prodrug mycophenolate mofetil (transplant rejection prevention). The molecule's identity is the same in environmental and pharmaceutical contexts, which makes this one of the few indoor mycotoxins with an extensively characterized human pharmacology from the clinical literature.

MPA is a substituted phthalide-derived polyketide. The free acid is the bioactive form in both pharmaceutical and environmental contexts; the clinical prodrug mycophenolate mofetil is the morpholinoethyl ester, which is hydrolyzed in vivo to release MPA. This identity between environmental and pharmaceutical molecule is uncommon, for most fungal metabolites, the clinical drug derivative differs from the natural product in ways that change activity (e.g., lovastatin is the natural product, while atorvastatin and rosuvastatin are synthetic statins inspired by lovastatin rather than direct derivatives).

MPA inhibits inosine-5'-monophosphate dehydrogenase (IMPDH), the rate-limiting enzyme in de novo guanosine nucleotide biosynthesis. Depletion of guanosine nucleotides preferentially affects rapidly proliferating cells, particularly lymphocytes, which lack a salvage pathway for guanosine nucleotides. The selective effect on lymphocyte proliferation is the basis of the molecule's transplant-rejection use. Mechanism description is cellular only; clinical interpretation is the domain of pharmacology.

P. brevicompactum is xerotolerant with a minimum A_w for growth of 0.78, which means it can produce MPA on relatively dry indoor substrates that other water-damage indicators cannot establish on. Substrates include damp building materials and indoor dust. P. roqueforti produces MPA in commercial cheese fermentation (Roquefort, Stilton, Gorgonzola) and in moisture-damaged buildings.

Indoor water-damaged buildings (P. brevicompactum and P. roqueforti); commercial blue cheeses (P. roqueforti); pharmaceutical production (commercial fermentation of P. brevicompactum for transplant medicine).

No environmental regulatory limits are established for MPA. Pharmaceutical use is governed by therapeutic-range monitoring in transplant medicine; those limits are clinical-context-specific and not directly transferable to environmental exposure interpretation.

P. roqueforti co-produces roquefortine C with MPA in many isolates^[1]. The two share producer organism but differ in chemistry and mechanism (roquefortine C is a neurotoxic alkaloid; MPA is an IMPDH inhibitor).