Discovery of\nan Orally Efficacious Pyrazolo[3,4‑<i>d</i>]pyrimidine\nBenzoxaborole as a Potent Inhibitor of <i>Cryptosporidium</i>

Cryptosporidiosis is a diarrheal disease caused by the\nparasite <i>Cryptosporidium</i> resulting in over 100,000\ndeaths annually.\nHere, we present a structure–activity relationship study of\nthe benzoic acid position (R<sup>6</sup>) of pyrazolo[3,4-<i>d</i>]pyrimidine lead SLU-2815 (<b>1</b>), an inhibitor\nof parasite phosphodiesterase <i>Cp</i>PDE1, resulting in\nthe discovery of benzoxaborole SLU-10906 (<b>63</b>) as a benzoic\nacid bioisostere. Benzoxaborole <b>63</b> is 10-fold more potent\nthan <b>1</b> against the parasite in a cell-based infection\nmodel (EC<sub>50</sub> = 0.19 μM) and non-cytotoxic. Furthermore, <b>63</b> has a fast rate of parasite-killing and is orally efficacious\nin a <i>Cryptosporidium</i> mouse infection model (50 mg/kg\nBID), although relapse was observed 7 days post-drug treatment. The\npartial selectivity profile versus human phosphodiesterases is preserved\nwith the benzoxaborole motif and represents an important feature to\nimprove in future optimization. Benzoxaborole <b>63</b> represents\nan important advance toward the optimization of the pyrazolo[3,4-<i>d</i>]pyrimidine series and the identification of a drug to\ntreat cryptosporidiosis.