Homozygous deletion of MTAP frequently co-occurs with the adjacent CDKN2A tumor suppressor on chromosome 9p21.3 in human cancers. Loss of MTAP activity creates an acute dependency on MAT2A, the rate-limiting enzyme for SAM synthesis, to produce sufficient SAM to overcome MTA suppression of PRMT5 and to support the folate cycle via 1-carbon metabolism. Proof-of-concept therapeutic targeting of this vulnerability has been achieved by allosteric inhibition of MAT2A or by MTA-cooperative inhibition of PRMT5. However, variability of response highlights the opportunity to continue to improve outcomes with combination therapy strategies. Preclinical profiling has indicated the extent of MTA accumulation in tumor cells and intrinsic or acquired resistance mechanisms are key determinants of antitumor activity. Notably, co-administration of appropriately designed MAT2A and PRMT5 inhibitors can deliver durable tumor regressions and complete responses in multiple MTAPdel PDX models recalcitrant to either monotherapy. Here we describe the biochemical, cell biological, and in vivo efficacy profiles of IDE892, an MTA-cooperative PRMT5 inhibitor purposely designed to exploit the therapeutic opportunity associated with combined inhibition of PRMT5 and MAT2A. Extensive biophysical and biochemical characterization of the IDE397 mode-of-inhibition of MAT2A yielded a mechanistic model that accounts for MTAPdel tumor-specific activity of IDE397 due to allostery-dependent preservation of basal MAT2A activity, maintaining SAM levels above the threshold required to sustain function in normal tissues. In addition, extensive evaluation of the kinetic and equilibrium binding parameters of metabolite and inhibitor exchange on PRMT5 revealed key relationships that specify MTA-cooperativity as well as slow on-rate binding kinetics to the SAM-bound state (negative cooperativity) relative to the apo state. These findings informed the design of IDE892 that optimized efficacy and tolerability in combination with allosteric inhibition of MAT2A. IDE892 demonstrated at least 1,400-fold selective binding to MTA-PRMT5 versus SAM-PRMT5 complexes (by SPR) and robust MTAPdel-specific PRMT5 pathway inhibition in vitro and in vivo. Whole transcriptome, proteome, and mRNA/tRNA methylome analyses indicated both shared and distinct contributions of IDE397 and IDE892 to perturbation of MTAPdel cellular systems which translated to robust combination benefit in MTAPdel CDX and PDX models. These preclinical studies indicate that the combination of IDE892/IDE397 has the potential to deliver durable therapeutic activity for patients harboring MTAPdel tumors; an opportunity that is currently under evaluation in phase 1 clinical trials.