Comment on “ApoE-Directed Therapeutics Rapidly Clear b-Amyloid and Reverse Deficits in AD Mouse Models”

ApoE4 is the most important genetic risk factor for Alzheimer’s disease (AD) (1), and we therefore read with great interest the paper by Cramer et al. (2) demonstrating spectacular results of bexarotene on b-amyloid (Ab) accumulation in the brain of mouse models for AD. Bexarotene is a retinoid X receptor (RXR) agonist and approved by the U.S. Food and Drug Administration (FDA) for treatment of cutaneous T cell lymphoma. We decided to replicate the data by Cramer et al. (2) in mice and dogs before considering trials in humans. A single 100 mg per kg of weight (mg/kg) oral dose of bexarotene (Ontario Chemical, Inc., Canada) administered to wild-type male Swiss CD1 mice did not affect endogenous levels of soluble Abx-37, Abx-38, Abx-40, and Abx-42 in brain at different time points (Fig. 1A), despite the drug’s reaching high concentrations in brain and plasma (Fig. 1B). JNJ42601572, a known g-secretase modulator, affected Ab levels as expected (Fig. 1A). Similarly, 25 and 100 mg/kg oral bexarotene in beagle dogs did not affect Ab levels in cerebrospinal fluid (CSF) (Fig. 1C). The drug reached high concentrations in plasma (Fig. 1D). We tested chronic oral bexarotene treatment (100 mg/kg per day for 19 days) in 10-monthold male hAPP/PS1 mice (3) and euthanized the mice 24 hours after the last dose. Plaque burden and plaque number in the right hemispheres (Fig. 2, A to D) and soluble Ab1-40 in the left hemispheres was not altered (Fig. 2E). ABCA1 levels were significantly up-regulated (Fig. 2F), demonstrating target engagement. Changes in APP, APP-CTF fragments, or apoE levels were not observed. Thus, previously observed acute and chronic effects of bexarotene on brain Ab levels (2) were not reproduced. We also tested cognitive status in chronically treated hAPP/PS1 mice. Although social recognition memory seemed improved after 14 days of treatment (Fig. 2, G and H), exploratory tendency was generally reduced in treated versus untreated hAPP/PS1 and control wild-type mice (Fig. 2I). Reanalyzing the recordings revealed a significant increase of grooming behavior in bexarotenetreated hAPP/PS1 mice (Fig. 2, J and K). Skin irritation and itching appear to be common reactions in patients taking Targretin (4). In addition, bexarotene-treated mice showed significant weight loss, increased irritability during handling and oral gavage, and difficulty breathing, which indicate severe adverse effects of the treatment (Fig. 2, M and N). Those might interfere with the execution of behavioral tests. The retention test of the passive avoidance task showed longer step-through latency in the bexarotene-treated hAPP/PS1 mice (Fig. 2L), but, again, adverse effects of the treatment confound interpretation. Thus, although we cannot exclude an effect of bexarotene on memory, the adverse effects of the drug make a definitive conclusion impossible. Bexarotene is insoluble in water, and we therefore used Captisol (Cydex Pharmaceuticals), a widely used formulation for hydrophobic drugs, and 2-Hydroxypropyl-b-cyclodextrin (HP-b-CD/ Tween), another b-cyclodextrine, for drug administration in mice and dogs, respectively. Cramer et al. wrote that they solubilized bexarotene in water (2), but we learned afterward that they actually used Targretin capsules. These capsules contain additional ingredients (5). Because Cramer et al. (2) used a different formulation (dimethyl sulfoxide and intraperitoneal injections) for their tissue bioavailability studies, we are not sure how to relate the different experiments published in (2) with regard to formulation and administration routes. The FDA filing mentions increased bexarotene uptake when the drug is taken together with highfat food (5); thus, differences in fat content of mouse chow might further confound these studies. Our data demonstrate that we had good brain penetration of the drug and ABCA1 target engagement, but no effects on Ab. We want to alert the field to this important issue. Given the toxicity of bexarotene, our study clearly strongly cautions against testing this drug in AD patients at this time.