Exploratory imaging outcomes of a phase 1b/2a clinical trial of allopregnanolone as a regenerative therapeutic for Alzheimer's disease: Structural effects and functional connectivity outcomes

Abstract

Introduction: Allopregnanolone (ALLO), an endogenous neurosteroid, promoted neurogenesis and oligogenesis and restored cognitive function in animal models of Alzheimer's disease (AD). Based on these discovery research findings, we conducted a randomized-controlled phase 1b/2a multiple ascending dose trial of ALLO in persons with early AD (NCT02221622) to assess safety, tolerability, and pharmacokinetics. Exploratory imaging outcomes to determine whether ALLO impacted hippocampal structure, white matter integrity, and functional connectivity are reported.

Methods: Twenty-four individuals participated in the trial (n = 6 placebo; n = 18 ALLO) and underwent brain magnetic resonance imaging (MRI) before and after 12 weeks of treatment. Hippocampal atrophy rate was determined from volumetric MRI, computed as rate of change, and qualitatively assessed between ALLO and placebo sex, apolipoprotein E (APOE) ε4 allele, and ALLO dose subgroups. White matter microstructural integrity was compared between placebo and ALLO using fractional and quantitative anisotropy (QA). Changes in local, inter-regional, and network-level functional connectivity were also compared between groups using resting-state functional MRI.

Results: Rate of decline in hippocampal volume was slowed, and in some cases reversed, in the ALLO group compared to placebo. Gain of hippocampal volume was evident in APOE ε4 carriers (range: 0.6% to 7.8% increased hippocampal volume). Multiple measures of white matter integrity indicated evidence of preserved or improved integrity. ALLO significantly increased fractional anisotropy (FA) in 690 of 690 and QA in 1416 of 1888 fiber tracts, located primarily in the corpus callosum, bilateral thalamic radiations, and bilateral corticospinal tracts. Consistent with structural changes, ALLO strengthened local, inter-regional, and network level functional connectivity in AD-vulnerable regions, including the precuneus and posterior cingulate, and network connections between the default mode network and limbic system.

Discussion: Indicators of regeneration from previous preclinical studies and these exploratory MRI-based outcomes from this phase 1b/2a clinical cohort support advancement to a phase 2 proof-of-concept efficacy clinical trial of ALLO as a regenerative therapeutic for mild AD (REGEN-BRAIN study; NCT04838301).

Keywords: Alzheimer's disease; allopregnanolone; functional connectivity; hippocampal volume; regenerative therapeutic; white matter integrity.

FIGURE 1

Impact of allopregnanolone (ALLO) on hippocampal volumes across sex, apolipoprotein E (APOE) genotype, and dose. Baseline hippocampal volumes for the left (A) and right (B) hippocampi stratified by APOE allele and intervention group (placebo, ALLO). After 12 weeks of allopregnanolone or placebo treatment, change was computed as (Follow up—Baseline/Baseline * 100). Differences are reported for placebo versus ALLO overall (C–D) as well as stratified by sex (E–F), genotype (G–H), and for each dosing level of ALLO (I–J). The 4 mg dose of ALLO was associated with the greatest gains in both left (I) and right (J) hippocampal volume. All gains were observed in APOE ε4 carriers, ranging from 0.6% to 7.8% increases in volume

FIGURE 2

Impact of allopregnanolone (ALLO) on white matter integrity. Fractional anisotropy (FA; A–C) and quantitative anisotropy (QA; D–G) were used to create local connectome footprints to track white matter changes following treatment. Using FA, a total of 690 tracts survived multiple comparisons correction (false‐discovery rate [FDR] corrected P < .004)(A), with 100% of these tracts (B) showing a mean increase in FA for the ALLO group compared to the placebo group (C). Using QA, a total of 1888 tracts survived multiple comparisons correction (D); 78.32% of these tracts (D red‐colored tracts; E–F) showed no change in QA for participants receiving ALLO compared to placebo, whereas 21.77% of the tracts (D blue‐colored tracts; E, G) showed decreased QA in ALLO participants compared to placebo

FIGURE 3

Impact of allopregnanolone (ALLO) on inter‐ and intra‐regional functional connectivity. Post‐treatment group‐wise differences in functional connectivity were evaluated at a region‐to‐region (“edge‐wise”) (A–D), network‐wise (E–G), and within‐region (“Regional Homogeneity”) (H) level using an atlas‐based approach (n = 473 total regions) and are reported as Hedges’ g effect sizes. A total of 32 edges exhibited large (|g| > 1.80; false‐discovery rate (FDR)–corrected P < .01) differences, with 25 edges exhibiting greater connectivity in the ALLO group. These edge‐wise connections are overlaid on the left hemisphere surface (A), a coronal view (B), the right hemisphere surface (C), and an axial view (D). These regions were additionally assigned to one of 11 large‐scale networks (E). Simple differences from baseline (E) to post‐treatment in network‐wise functional revealed general patterns of decrease in the placebo group (F, top) and limited changes or increases in the ALLO group (F, bottom). Baseline‐adjusted network models revealed larger increases in cross‐network functional connectivity between the limbic network and both the default mode and somatomotor networks, and decreases in cerebellar‐subcortical gray matter in the ALLO group compared to the placebo group (|g| > 1.16; FDR corrected P < .05)(G). Finally, ALLO was associated with larger increases (g > 0.74) in regional homogeneity in a total of 20 regions, including the prefrontal, posterior cingulate, and precuneus regions, whereas larger increases (g > 0.89) in the placebo group were observed in nine regions, including those associated with the bilateral somatomotor and dorsal attention networks (H). These regions with large differences were significant at FDR corrected .082 < P < .122 level