Lens cholesterol biosynthesis inhibition: A common mechanism of cataract formation in laboratory animals by pharmaceutical products. 
CJ-12,918, a 5-lipoxygenase (5-LO) inhibitor, caused cataracts during a 1-month safety assessment studies in rats whereas the structurally similar ZD-2138 was without effect.  For CJ-12,918 analogs, blocking different sites of metabolic liability reduced (CJ-13,454) and eliminated (CJ-13,610) cataract formation in both rats and dogs.  Using this chemical series as a test set, models and mechanisms of toxicity were first explored by testing the utility of ex vivo rat lens explant cultures as a safety screen.  This model overpredicted the cataractogenic potential of ZD-2138 due to appreciably high lens drug levels and was abandoned in favor of a mechanism-based screen.  Perturbations in lens sterol content, from a decline in lathosterol content, preceded cataract formation suggesting CJ-12,918 inhibited lens cholesterol biosynthesis (LCB).  A 2-day bioassay in rats using ex vivo LCB assessments showed that the level of LCB inhibition was correlated with incidence of cataract formation in animal studies by these 5-LO inhibitors.  Thereafter, this 2-day bioassay was applied to other pharmaceutical programs (neuronal nitric oxide synthase, sorbitol dehydrogenase inhibitor, squalene synthetase inhibitor and stearoyl-CoA desaturase-1 inhibitors/D4 antagonists) that demonstrated cataract formation in either rats or dogs.  LCB inhibition >40% was associated with a high incidence of cataract formation in both rats and dogs that was species specific.  Bioassay sensitivity/specificity were further explored with positive (RGH-6201/ciglitazone/U18666A) and negative (tamoxifen/naphthalene/galactose) mechanistic controls.  This body of work over two decades shows that LCB inhibition was a common mechanism of cataract formation by pharmaceutical agents and defined a level of inhibition >40% that was typically associated with causing cataracts in safety assessment studies typically ≥1 month.