Inhibition of Cholesterol Synthesis and Antiproliferative Activity of a Series of Novel Phenol-Substituted 1,1-Bisphosphonate Esters

In addition to supplying cholesterol, the mevalonate pathway is also responsible for providing other intermediates essential for the regulation of cell functions. Products from the mevalonate pathway include the isoprenoid family, such as farnesyl or geranylgeranyl derivatives that are required for the prenylation of small proteins regulating cell growth such as Ras. The hypocholesterolemic class of drugs statins that are competitive inhibitors of HMG-CoA Reductase, have displayed antiproliferative activity in vitro.As part of a search for novel inhibitors of the mevalonate pathway as therapeutic agents, a series of 1,1-bisphosphonate esters (I) was prepared for pharmacological screening. The alkyl- and alkoxy-substituted phenol derivatives with a variety of vinylidene or alkylidene linkages were prepared by condensation of the appropriate aldehydes with the corresponding methylenebisphosphonate esters, followed by reduction. In our primary screening assay, the di-tert-butyl phenol-1,1- bisphosphonate esters were shown to inhibit cholesterol synthesis with IC50s in the submicromolar range in cultured human-derived intestinal cells (CaCo-2). The inhibition of proliferation of H-Ras transfected cell lines PAP-2, as well as other cell lines such as HL-60 and SW-620, was selected as the subsequent screening assays. Test results showed that the most active compounds for decreasing cholesterol synthesis, the di-tert-butylphenol-1,1-bisphosphonates, were also the most effective for inhibiting cell growth. The most potent analogue, compound 36, decreased cholesterol synthesis with an IC50 of 0.08 mM and inhibited cell growth with IC50s of 5 μM (PAP-2), 12 μM (HL-60) and 16 μM (SW-620), respectively.Based on these findings and the results from further pharmacological evaluation, compound 36 (Apomine, SR-45023A), tetraisopropyl 2-(3,5-di-tert-butyl-4-hydroxyphenyl) ethylidene-1,1-bisphosphonate, was selected for development as an antineoplastic agent.