A randomized controlled study to evaluate the effect of bexarotene on amyloid-β and apolipoprotein E metabolism in healthy subjects

Abstract

Introduction: We conducted a phase Ib proof of mechanism trial to determine whether bexarotene (Targretin) increases central nervous system (CNS) apolipoprotein E (apoE) levels and alters Aβ metabolism in normal healthy individuals with the APOE ε3/ε3 genotype.

Methods: We used stable isotope labeling kinetics (SILK-ApoE and SILK-Aβ) to measure the effect of bexarotene on the turnover rate of apoE and Aβ peptides and stable isotope spike absolute quantitation (SISAQ) to quantitate their concentrations in the cerebrospinal fluid (CSF). Normal subjects were treated for 3 days with bexarotene (n = 3 women, 3 men, average 32 years old) or placebo (n = 6 women, average 30.2 years old) before administration of C13-leucine and collection of plasma and CSF over the next 48 hours. Bexarotene concentrations in plasma and CSF were also measured.

Results: Oral administration of bexarotene resulted in plasma levels of 1 to 2 μM; however, only low nM levels were found in CSF. Bexarotene increased CSF apoE by 25% but had no effect on metabolism of Aβ peptides.

Discussion: Bexarotene has poor CNS penetration in normal human subjects. Drug treatment resulted in a modest increase in CSF apoE levels. The absence of an effect on Aβ metabolism is likely reflective of the low CNS levels of bexarotene achieved. This study documents the utility of SILK-ApoE technology in measuring apoE kinetics in humans.

Trial registration: This trial is registered at clinicaltrials.gov (NCT02061878).

Keywords: Alzheimer's disease; Apolipoprotein E; Bexarotene; Retinoid X receptor; β amyloid.

Figures

Fig. 1

Pharmacokinetics of bexarotene. The concentration of bexarotene in plasma and cerebrospinal fluid (CSF) samples was measured hourly in subjects receiving drug. The average maximal concentration of drug in plasma was 1.46 ± 1 μM, and the Tmax was 3.45 ± 11.41 h after dosing. The levels of bexarotene in CSF was below the level of detection (0.021 μM) in >95% of samples. In samples in which bexarotene could reliably be quantified, peak CSF concentrations were approximately 20 nM.

Fig. 2

Stable isotope labeling kinetics (SILK) and stable isotope spike absolute quantitation of ApoE in cerebrospinal fluid (CSF). The SILK data are plotted as the normalized tracer-to-tracee ratio (TTR), which is the amount of 13C6-Leu labeled apoE divided by the amount of unlabeled apoE. (A) The mean values are plotted ± 95% confidence intervals (CIs) of placebo- (blue) and bexarotene- (red) treated subjects. (B) The fractional synthesis rates (FSRs), measured from 6 to 17 hours or (C) fractional clearance rates (FCRs) determined from 23 to 48 hours were not significantly different between placebo- and drug-treated subjects. The absolute concentration of apoE in the CSF was calculated by adding the concentration values for the unlabeled and 13C6-labeled apoE. (D) ApoE concentrations of the individual subjects treated with placebo (blue) or bexarotene (red) and (E) their average values (±95% CIs). (F) There was a significant 25% increase (P = .0367) in the mean weighted area under the full concentration curves of apoE in the CSF of the bexarotene-treated subjects. Quantitation of the amount of newly synthesized apoE in (G) individual subject and (H) their average values (±95% CIs) revealed (I) a nonsignificant change between placebo- and bexarotene-treated subjects.

Fig. 3

Stable isotope labeling kinetics (SILK) of total Aβ and of Aβ40 in cerebrospinal fluid (CSF). The synthesis and clearance rates of total Aβ were measured in CSF using an Aβ1-x capture antibody. The SILK data are plotted as the normalized tracer-to-tracee ratio (TTR), which is the amount of 13C6-Leu labeled Aβ divided by the amount of total unlabeled Aβ. (A) The values for the individual subject treated with placebo (blue) or bexarotene (red), and (B) the mean values are plotted ± 95% confidence intervals (CIs). (C) The fractional synthesis rates (FSRs), measured from 6 to 17 hours. (D) Fractional clearance rates (FCRs) determined from 23 to 48 hours were not significantly different between placebo- and drug-treated subjects. The SILK data are plotted as the normalized TTR, which is the amount of 13C6-Leu labeled Aβ40 divided by the amount of unlabeled Aβ40. (E) The values for the individual subject and (F) the mean values are plotted ± 95% CIs. (G) The FSRs, measured from 6 to 17 hours or (H) FCRs determined from 23 to 48 hours were not significantly different between placebo- and drug-treated subjects.

Fig. 4

Stable isotope spike absolute quantitation of total Aβ and Aβ40 and newly synthesized total Aβ and Aβ40 in cerebrospinal fluid (CSF). The absolute concentration of total Aβ and Aβ40 peptides in the CSF was calculated by adding the concentration values for the unlabeled and 13C6-labeled peptides using antibodies directed at Aβ1-x or at the C-terminal Aβ40 epitope. (A) Total Aβ or (D) Aβ40 concentrations of the individual subjects treated with placebo (blue) or bexarotene (red) and (B, E) their average values (±95% confidence intervals [CIs]). There was no significant difference in the mean weighted area under the full concentration curves of (C) total Aβ or (F) Aβ40 in the CSF of the bexarotene-treated subjects. The absolute concentration of newly synthesized total Aβ and Aβ40 peptides in the CSF was calculated by adding the concentration values for the unlabeled and 13C6-labeled Aβ peptides at each time point. Quantitation of the mean amount of newly synthesized (G) total Aβ and (J) Aβ40 (±95% CIs) in placebo- and bexarotene-treated subjects revealed no difference in the amount of Aβ peptides (H, K) synthesized or (I, L) cleared in the placebo-treated compared with bexarotene-treated subjects. Abbreviation: AUC, area under the curve.