Interest in inhibition of phosphatidylinositol-3-phosphate 5-kinase (PIKfyve) as a therapeutic strategy has remained high since the seminal publication on apilimod, which touted it as a treatment for B-cell non-Hodgkin lymphoma. Apilimod (Figure ) has since been used as an exquisitely selective tool to interrogate PIKfyve biology in vitro and in vivo. We and other groups have used apilimod to uncover roles of PIKfyve in diseases beyond cancer, including inflammatory and autoimmune diseases, ALS and motor neuron diseases, anti-viral infections, myocardial interstitial fibrosis, and skin disorders. Apilimod, however, may not be the best compound to be used clinically. Reports of unexpectedly low plasma levels, inactivation in cell culture, and failure to show efficacy in clinical trials for multiple indications further impress a need for a structurally unique PIKfyve inhibitor. Other advanced PIKfyve inhibitors have been evaluated in vivo for various therapeutic endpoints. YM201636 (Figure ) is another potent and selective chemical tool that has been used alkyne terminus (23 and 25) bound to PIKfyve with modest affinity but suffered from cellular toxicity, as was observed for structurally related analogues. Further modification of the aminopyrimidine ring to include a methoxy or methyl group (26-28) did not improve the potency or metabolic stability. In summary, our efforts to modify the aminopyrimidine ring did not result in analogues with increased PIKfyve affinity and metabolic stability.