In the present study, we have investigated the potential neuroprotective effects of a novel peripheral benzodiazepine binding site (PBR) ligand,N,indole-1-acetamide (SSR180575), in models of central and peripheral neurodegeneration in vivo and its effect on steroid concentrations in plasma and nervous tissue. SSR180575 shows high affinity (IC 50 , 2.5-3.5 nM) and selectivity for the rat and human PBR and potently inhibits the in vivo binding of [ 3 H]alpidem to PBR in the rat brain and spleen after oral or i.p. administration (ID 50 , 0.1-0.3 mg/kg). In an experimental model of motoneuron degeneration induced by facial nerve axotomy in the immature rat, SSR180575 given i.p. or orally for 8 days rescued facial motoneurons, increasing their survival by 40 to 72% at 6 and 10 mg/kg p.o. b.i.d. Moreover, in this model, SSR180575 (10 mg/kg p.o. b.i.d.) increased by 87% the number of motoneurons immunoreactive to peripherin, a type III intermediate filament, whose expression is up-regulated during nerve regeneration. SSR180575 also improved functional recovery in acrylamide-induced neuropathy in the rat when given therapeutically at 2.5 to 10 mg/kg/day p.o. Furthermore, SSR180575 (3 mg/kg i.p. b.i.d.) accelerated functional recovery of the blink reflex after local injury of the facial nerve in the rat. SSR180575 increased pregnenolone accumulation in the brain and sciatic nerve (ϩ100% at 3 mg/kg i.p.), suggesting that its neuroprotective effects are steroid-mediated. These results indicate that PBR ligands (e.g., SSR180575) promote neuronal survival and repair in axotomy and neuropathy models and have potential for the treatment of neurodegenerative diseases (e.g., peripheral neuropathies or amyotrophic lateral sclerosis).

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Chronic inflammation represents a central component in the pathogenesis of Alzheimer's disease (AD). Recent work suggests that breaking immune tolerance by Programmed cell Death-1 (PD1) checkpoint inhibition produces an IFN-γ-dependent systemic immune response, with infiltration of the brain by peripheral myeloid cells and neuropathological as well as functional improvements even in mice with advanced amyloid pathology (Baruch et al., (): Nature Medicine, 22:135-137). Immune checkpoint inhibition was therefore suggested as potential treatment for neurodegenerative disorders when activation of the immune system is appropriate. Because a xenogeneic rat antibody (mAb) was used in the study, whether the effect was specific to PD1 target engagement was uncertain. In the present study we examined whether PD1 immunotherapy can lower amyloid-β pathology in a range of different amyloid transgenic models performed at three pharmaceutical companies with the exact same anti-PD1 isotype and two mouse chimeric variants. Although PD1 immunotherapy stimulated systemic activation of the peripheral immune system, monocyte-derived macrophage infiltration into the brain was not detected, and progression of brain amyloid pathology was not altered. Similar negative results of the effect of PD1 immunotherapy on amyloid brain pathology were obtained in two additional models in two separate institutions. These results show that inhibition of PD1 checkpoint signaling by itself is not sufficient to reduce amyloid pathology and that additional factors might have contributed to previously published results (Baruch et al., (): Nature Medicine, 22:135-137). Until such factors are elucidated, animal model data do not support further evaluation of PD1 checkpoint inhibition as a therapeutic modality for Alzheimer's disease.