Novel Pyrazolo[4,3‑<i>d</i>]pyrimidine as Potent and Orally Active Inducible Nitric Oxide Synthase (iNOS) Dimerization Inhibitor with Efficacy in Rheumatoid Arthritis Mouse Model

In order to discover novel anti-inflammatory agents for treatment of arthritis and based on preliminary structure–activity relationships, four series (A–D) of total 90 new pyrazolo­[4,3-d]­pyrimidine compounds were designed and synthesized. All the compounds have been tested for their anti-inflammatory activities by inhibiting of LPS-induced NO production. A clear structure–activity relationship has been concluded step by step, and finally 3,4,5-trimethoxystyryl-1H-pyrazolo­[4,3-d]­pyrimidine was found to be the most active scaffold. Among them, compound D27 was discovered as the most potent anti-inflammatory agent (IC50 = 3.17 μM) with low toxicity and strong inhibitory of NO release (IR = 90.4% at 10 μM). This compound also showed potent inhibition of iNOS with IC50 value of 1.12 μM. Preliminary mechanism studies indicated that it could interfere with the stability and formation of active dimeric iNOS. The anti-inflammatory effect of this compound was determined by adjuvant-induced arthritis in rat model. We believe these findings would further support the study of rational design of more efficient iNOS inhibitors in the future