New Pyrazole/Pyrimidine-Based Scaffolds as Inhibitors of Heat Shock Protein 90 Endowed with Apoptotic Anti-Breast Cancer Activity

<b>Background/Objectives</b>: Supported by a comparative study between conventional, grinding, and microwave techniques, a mild and versatile method based on the [1 + 3] cycloaddition of 2-((3-nitrophenyl)diazenyl)malononitrile to tether pyrazole and pyrimidine derivatives in good yields was used. <b>Methods</b>: The newly synthesized compounds were analyzed with IR, ¹³C NMR, ¹H NMR, mass, and elemental analysis methods. The products show interesting precursors for their antiproliferative anti-breast cancer activity. <b>Results</b>: Pyrimidine-containing scaffold compounds <b>9</b> and <b>10</b> were the most active, achieving IC₅₀ = 26.07 and 4.72 µM against the breast cancer MCF-7 cell line, and 10.64 and 7.64 µM against breast cancer MDA-MB231-tested cell lines, respectively. Also, compounds <b>9</b> and <b>10</b> showed a remarkable inhibitory activity against the Hsp90 protein with IC₅₀ values of 2.44 and 7.30 µM, respectively, in comparison to the reference novobiocin (IC₅₀ = 1.14 µM). Moreover, there were possible apoptosis and cell cycle arrest in the G1 phase for both tested compounds (supported by CD1, caspase-3,8, BAX, and Bcl-2 studies). Also, the binding interactions of compound <b>9</b> were confirmed through molecular docking, and simulation studies displayed a complete overlay into the Hsp90 protein pocket. <b>Conclusions</b>: Compounds <b>9</b> and <b>10</b> may have apoptotic antiproliferative action as Hsp90 inhibitors.