Lab studies suggest potential for statins in lowering IOP

Fort Lauderdale, FL-Results from a series of laboratory studies demonstrate statin drugs cause relaxation of trabecular meshwork and ciliary body cells and increase outflow facility. Those changes appear to be mediated by statin effects on the Rho pathway, reported Julia Song, MD, at the annual meeting of the Association for Research in Vision and Ophthalmology.

The research was conducted by Dr. Song, a glaucoma research fellow in the department of ophthalmology, Duke University Medical Center, Durham, NC, in collaboration with David L. Epstein, MD, and P. Vasantha Rao, PhD.

She presented evidence that the addition of the statins lovastatin and compactin to cultures of porcine primary trabecular meshwork and ciliary body cells induced morphologic and cytoskeletal changes that are characteristic of cell relaxation. The potential for those effects to translate into increased outflow facility was demonstrated by measuring IOP changes in porcine eye anterior segments perfused with statin-containing media.

Decreased isoprenoid production The studies were undertaken based on the knowledge that statin drugs affect Rho by decreasing isoprenoid production. Based on that activity, the investigators hypothesized statins would cause relaxation of trabecular meshwork tissue and secondarily increase aqueous outflow.

"Statin inhibition of HMG-CoA reductase not only results in decreased production of cholesterol, but also decreased production of isoprenoids that are important for activating Rho into its GTP-bound form that causes cellular contraction. By reducing isoprenylation of Rho GTPase, statins would be expected to result in cell relaxation, which in the trabecular meshwork would translate into increased outflow facility," Dr. Song explained.

Morphologic effects associated with addition of the statins to the cell cultures were examined using the phase-contrast microscope.

Immunofluorescent staining with vinculin and phalloidin was performed to identify drug effects on cytoskeletal elements, and gel electrophoresis and Western blot analysis were used to determine changes in myosin light-chain (MLC) phosphorylation.

For both the trabecular meshwork and ciliary body cell cultures, the morphologic studies showed untreated control cells were healthy and cigar-shaped, but that they became detached from each other, relaxed, and acquired a more rounded appearance follow-ing addition of statin to the cultures. Those changes were consistent with cytoskeletal changes that showed evidence of actin depolymerization and loss of focal adhesions.

Further studies showed the effects of the statins on both cell shape and cytoskeletal organization could be reversed when either fresh serum or the isoprenoid geranylgeranyl pyrophosphate was added to the cultures, Dr. Song reported.

"The reversibility achieved with geranylgeranyl pyrophosphate supplementation indicates the effects of the statins were mediated via reduced isoprenylation of small GTP-binding proteins such as Rho GTPase," she said.

Results from the Western blot studies showed the statin drugs caused a dose-related decrease in levels of MLC phosphorylation in both the trabecular meshwork and ciliary body cells. That change is indicative of cellular relaxation, and inhibition of Rho is commonly associated with cellular relaxation, Dr. Song explained.

"Activation of Rho into its Rho GTP-bound form results in inhibition of myosin phosphatase and an increase in phosphorylated myosin. Phosphorylated myosin causes cellular contraction," she said.

Drug effects on outflow facility were examined in a model system of perfused porcine eye anterior segments.

Baseline IOP was measured after the models were perfused overnight and then recorded over a period of 5 days in sets of control models and those to which a statin was added to the perfusion fluid.

In the control specimens, IOP gradually decreased, but the change from baseline was minimal and not statistically significant. In contrast, specimens in which the perfusion fluid contained a statin drug showed a more rapid reduction in IOP that reached statistical significance by 60 hours and remained significantly lower than baseline for the duration of the study period, she said.

After the perfusion experiments were completed, the integrity of the outflow pathway was examined using light microscopy. Those investigations revealed distention of trabecular meshwork beams in the statin-treated anterior segments compared with the controls.OT