Controlled-Deactivation CB1 Receptor Ligands as a Novel Strategy to Lower Intraocular Pressure

Abstract

Nearly half a century has passed since the demonstration that cannabis and its chief psychoactive component Δ⁸-THC lowers intraocular pressure (IOP). Elevated IOP remains the chief hallmark and therapeutic target for glaucoma, a condition that places millions at risk of blindness. It is likely that Δ⁸-THC exerts much of its IOP-lowering effects via the activation of CB1 cannabinoid receptors. However, the initial promise of CB1 as a target for treating glaucoma has not thus far translated into a credible therapeutic strategy. We have recently shown that blocking monoacylglycerol lipase (MAGL), an enzyme that breaks the endocannabinoid 2-arachidonoyl glycerol (2-AG), substantially lowers IOP. Another strategy is to develop cannabinoid CB1 receptor agonists that are optimized for topical application to the eye. Recently we have reported on a controlled-deactivation approach where the "soft" drug concept of enzymatic deactivation was combined with a "depot effect" that is commonly observed with Δ⁸-THC and other lipophilic cannabinoids. This approach allowed us to develop novel cannabinoids with a predictable duration of action and is particularly attractive for the design of CB1 activators for ophthalmic use with limited or no psychoactive effects. We have tested a novel class of compounds using a combination of electrophysiology in autaptic hippocampal neurons, a well-characterized model of endogenous cannabinoid signaling, and measurements of IOP in a mouse model. We now report that AM7410 is a reasonably potent and efficacious agonist at CB1 in neurons and that it substantially (30%) lowers IOP for as long as 5 h after a single topical treatment. This effect is absent in CB1 knockout mice. Our results indicate that the direct targeting of CB1 receptors with controlled-deactivation ligands is a viable approach to lower IOP in a murine model and merits further study in other model systems.

Keywords: CB1; cannabinoid; glaucoma; ocular pressure.

Figure 1

Top panel: design of first-generation side chain carboxylated cannabinoid analogs, with controllable deactivation and increasing polarity, and structures of the prototype (-)-Δ⁸-THC-DMH and inactive metabolites. Lower panel: design principles and biological activity data for representative analogs.

Figure 2

AM7410 is a potent and efficacious CB1 receptor agonist. (A) AM7410 inhibits excitatory neurotransmission in autaptic hippocampal neurons in a concentration-dependent manner. Maximal inhibition is similar to maximal inhibition from DSE in the same neurons; (B) Time course shows time course of inhibition of EPSCs after treatment with various concentrations of AM7410. Inset shows sample EPSCs before treatment and after treatment with 100 nM AM7410; (C) Sample time course shows absence of AM7410 effect in CB1^−/− neuron.

Figure 3

AM7410 lowers intraocular pressure in a normotensive model. (A,B) AM7410 applied topically at 3 mM lowers ocular pressure at 5 h post-treatment; (C,D) The effect of AM7410 on ocular pressure is absent in CB1 knockout (KO) mice; (E) The inactive metabolite AM7408 does not lower ocular pressure at 5 h. *, p < 0.05 by paired t-test.