Raloxifene through its Cannabinoid Type-2 Receptor Inverse Agonism Mitigates Visual Deficits and Pathology after Mild TBI

Abstract

Purpose : Treatment for 2 weeks with the cannabinoid type-2 receptor (CB2) inverse agonist SMM189 attenuates visual deficits and retinal pathology produced by a closed-head focal cranial blast in mice, by modulating the otherwise deleterious effect of microglia in secondary pathogenic processes. SMM189, however, has not yet been approved for human use. Because raloxifene is FDA approved (as an estrogen receptor drug to treat osteoporosis) but also acts as a CB2 inverse agonist, we assessed its utility in treating mild traumatic brain injury (TBI).

Methods : Male C57BL/6 mice received 50-psi or sham (0-psi) blast and were treated with vehicle or raloxifene at 5 or 10 mg/kg for 2 weeks. Functional tests at >1 month assessed visual acuity and contrast sensitivity, the scotopic ERG, light aversion, and the pupil light reflex. Morphological analysis assessed short-term and long-term retinal, optic nerve and oculomotor nucleus injury. Quantitative PCR analysis of markers typifying M1 versus M2 state microglia assessed microglial state. Studies with estrogen receptor drugs and the CB2 inverse agonist SR144528 tested the CB2 specificity of raloxifene effects.

Results : We found that 50-psi blast yielded deficits in visual acuity and contrast sensitivity, reductions in the A and B-waves of the scotopic ERG, light aversion, and increased pupil constriction to light. Daily treatment with raloxifene at 5-10 mg/kg for 2 weeks post blast reduced or eliminated these abnormalities (with the higher dose generally more effective). Functional rescue was accompanied by a biasing of microglia in optic nerve, retina, and brain from the harmful M1 to the helpful M2 state, and by reductions in retinal, optic nerve, and oculomotor nucleus pathology. We further found that raloxifene treatment is still effective even when delayed until 48 hours after TBI, and that raloxifene benefit is attributable to its CB2 inverse agonism rather than its estrogenic actions.

Conclusions : Our studies show that raloxifene ameliorates visual system deficits and injury after head trauma, even when treatment is delayed for 48 hours. The benefit appears attributable to inverse agonism at CB2 receptors that biases microglia toward the beneficial M2 state. Our findings provide basis for phase-2 efficacy testing in human clinical trials, which if effective could lead to the rapid repurposing of raloxifene for treating mild TBI.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.