Tamoxifen Isomers and Metabolites Exhibit Distinct Affinity and Activity at Cannabinoid Receptors: Potential Scaffold for Drug Development

Abstract

Tamoxifen (Tam) is a selective estrogen receptor (ER) modulator (SERM) that is an essential drug to treat ER-positive breast cancer. Aside from known actions at ERs, recent studies have suggested that some SERMs like Tam also exhibit novel activity at cannabinoid subtype 1 and 2 receptors (CB1R and CB2Rs). Interestingly, cis- (E-Tam) and trans- (Z-Tam) isomers of Tam exhibit over a 100-fold difference in affinity for ERs. Therefore, the current study assessed individual isomers of Tam and subsequent cytochrome P450 metabolic products, 4-hydroxytamoxifen (4OHT) and 4-hydroxy-N-desmethyl tamoxifen (End) for affinity and activity at CBRs. Results showed that Z-4OHT, but not Z-Tam or Z-End, exhibits higher affinity for both CB1 and CB2Rs relative to the E-isomer. Furthermore, Z- and E-isomers of Tam and 4OHT show slightly higher affinity for CB2Rs, while both End isomers are relatively CB1R-selective. When functional activity was assessed by G-protein activation and regulation of the downstream effector adenylyl cyclase, all isomers examined act as full CB1 and CB2R inverse agonists. Interestingly, Z-Tam appears to be more efficacious than the full inverse agonist AM630 at CB2Rs, while both Z-Tam and Z-End exhibit characteristics of insurmountable antagonism at CB1 and CB2Rs, respectively. Collectively, these results suggest that the SERMs Tam, 4OHT and End elicit ER-independent actions via CBRs in an isomer-specific manner. As such, this novel structural scaffold might be used to develop therapeutically useful drugs for treatment of a variety of diseases mediated via CBRs.

The tamoxifen derivative ridaifen-B is a high affinity selective CB 2 receptor inverse agonist exhibiting anti-inflammatory and anti-osteoclastogenic effects

“…Although this and previous studies clearly demonstrate that SERM-scaffolds may be useful for synthesis of selective CB 1 and/or CB 2 cannabinoid ligands (Kumar and Song, 2013; Prather et al, 2013; Ford et al, 2016), before novel compounds can be useful therapeutically, drugs in this class must first be designed that lack ER affinity to prevent off-target effects. Importantly, RID-B is only a first generation of many ridaifen (RID) compounds that are tamoxifen derivatives, developed to reduce ER affinity, yet maintain cytotoxic effects in cancer (Shiina et al, 2008; Guo et al, 2013b).…”

“…Based on the structural similarity of RID-B to tamoxifen (Fig. 1), and our past reports that tamoxifen exhibits moderate affinity for cannabinoid receptors (Prather et al, 2013; Ford et al, 2016), a competition receptor-binding screen was conducted. From this initial binding screen, it was observed that RID-B (1 μM) produced > 50% displacement of the CB 1 /CB 2 radioligand [ 3 H]-CP-55,940 (0.2 nM) from human CB 1 and CB 2 receptors stably expressed in CHO cells.…”

“…Our laboratory and others have recently reported that several selective estrogen receptor modulators (SERMs) exhibit moderate affinity for cannabinoid receptors and act as inverse agonists (Kumar and Song, 2013; Prather et al, 2013; Ford et al, 2016). SERMs are agonists and antagonists at estrogen receptors (ERs) and have traditionally been used therapeutically to treat ER-positive breast cancer, post-menopausal conditions, and osteoporosis (Maximov et al, 2013).…”

High-content imaging assays on a miniaturized 3D cell culture platform

“…Tamoxifen is an anti-estrogen drug used in the treatment of breast cancer. Hepatotoxicity is a commonly reported side effect of tamoxifen (Ford et al, 2016; Henderson et al, 2016; Khuroo et al, 2014; Lin et al, 2014; Tabassum et al, 2006; Villegas et al, 2016). In our study, mitochondrial impairment and disruption of cell membrane integrity were the main mechanisms of tamoxifen-induced toxicity.…”

High‐Throughput Assessment of Mechanistic Toxicity of Chemicals in Miniaturized 3D Cell Culture

“…Accumulation of ROS inside cells decreases the glutathione level resulting in oxidative stress and causes mitochondrial impairment leading to apoptosis (Cabezas, El‐Bachá, González, & Barreto, ; Jin et al., ; N. Li et al., ; Moon, Lee, Park, Geum, & Kim, ). Tamoxifen is an anticancer drug with hepatotoxicity as its commonly reported side effect (Ford, Franks, Radominska‐Pandya, & Prather, ; Henderson, Wendy, & Kim, ; Khuroo et al., ; Lin et al., ; Tabassum, Rehman, Banerjee, Raisuddin, & Parvez, ; Villegas et al., ). Mitochondrial impairment is known to be the main mechanism of tamoxifen‐induced toxicity (Lee, Kang, Lee, & Kim, ; Tabassum et al., ; Tolosa et al., ; Tolosa, Carmona, Castell, Gómez‐Lechón, & Donato, ).…”