In Brief The co-deletion of MTAP in the CDKN2A locus is a frequent event in diverse cancers including glioblastoma. Recent publications report that significant accumulations of the MTAP substrate, methylthioadenosine (MTA), can sensitize MTAP -deleted cancer cells to novel inhibitors of PRMT5 and MAT2A for targeted therapy against tumors with this particular genetic alteration. In this work, using comprehensive metabolomic profiling, we show that MTA is primarily secreted, resulting in exceedingly high levels of extracellular MTA in vitro . We further show that primary human glioblastoma tumors minimally accumulate MTA in vivo , which is likely explained by the metabolism of MTA by MTAP -competent stromal cells. Together, these data challenge whether the metabolic conditions required for therapies to exploit vulnerabilities associated MTAP deletions are present in primary human tumors, questioning their translational efficacy in the clinic.Highlights SUMMARY Homozygous deletion of the CDK2NA locus frequently results in co-deletion of methylthioadenosine phosphorylase ( MTAP ) in many fatal cancers such as glioblastoma multiforme (GBM), resulting in elevations of the substrate metabolite, methylthioadenosine (MTA). To capitalize on such accumulations, therapeutic targeting of protein arginine methyltransferase 5 (PRMT5) and methionine adenosyl transferase (MAT2A) are ongoing. While extensively corroborated in vitro , the clinical efficacy of these strategies ultimately relies on equally significant accumulations of MTA in human tumors. Here, we show that in vitro accumulation of MTA is a predominately extracellular phenomenon, indicating secretion of MTA from MTAP -deleted cells. In primary human GBMs, we find that MTA levels are not significantly higher in MTAP -deleted compared to MTAP -intact tumors or normal brain tissue. Together, these findings highlight the metabolic discrepancies between in vitro models and primary human tumors and should thus be carefully considered in the development of the precision therapies targeting MTAP -homozygous deleted GBM.

Methylthioadenosine is Not Dramatically Elevated in MTAP-Homozygous Deleted Primary Glioblastomas