U 18666A is an inhibitor of cholesterol transport and synthesis [1] [2].

Cholesterol is a sterol that biosynthesized by all animal cells and is an essential component of all animal cell membranes that is required to maintain membrane fluidity and structural integrity.

U 18666A is an inhibitor of cholesterol transport and synthesis. In rat brain, U 18666A inhibited sterols production in a concentration-dependent way [1]. In cultured Chinese hamster ovary (CHO) cells, U18666A inhibited cholesterol esterification stimulated by low density lipoprotein (LDL)-derived cholesterol and also inhibited LDL receptor activities and 3-hydroxy-3-methylglutaryl-coenzyme A reductase. U18666A caused the accumulation of LDL-derived cholesterol in the lysosomes, suggesting the inhibition of LDL-derived cholesterol transport [2]. In cultured baby hamster kidney cells and human skin fibroblasts (HSF), U18666A reversibly and rapidly inhibited acyl-CoA cholesterol acyl transferase (ACAT) activated by sphingomyelinase in a dose dependent way. In sphingomyelinase-treated HSF cells, U18666A significantly and reversibly reduced the translocation of plasma membrane cholesterol. In mouse Leydig tumor cells, U18666A inhibited steroid hormones secretion stimulated by cyclic AMP in a dose dependent way [3]. In primary cortical neurons, U18666A caused cellular injury and caspase-3 activation. U18666A also caused the accumulation of cholesterol [4].

References:
[1].  Cenedella RJ. Concentration-dependent effects of AY-9944 and U18666A on sterol synthesis in brain. Variable sensitivities of metabolic steps. Biochem Pharmacol, 1980, 29(20): 2751-2754.
[2].  Liscum L, Faust JR. The intracellular transport of low density lipoprotein-derived cholesterol is inhibited in Chinese hamster ovary cells cultured with 3-beta-[2-(diethylamino)ethoxy]androst-5-en-17-one. J Biol Chem, 1989, 264(20): 11796-11806.
[3].  Härmälä AS, Pörn MI, Mattjus P, Slotte JP. Cholesterol transport from plasma membranes to intracellular membranes is inhibited by 3 beta-[2-(diethylamino)ethoxy]androst-5-en-17-one. Biochim Biophys Acta, 1994, 1211(3): 317-325.
[4].  Cheung NS, Koh CH, Bay BH, et al. Chronic exposure to U18666A induces apoptosis in cultured murine cortical neurons. Biochem Biophys Res Commun, 2004, 315(2): 408-417.