**1. Introduction**

Oxysterols, formed either enzymatically or by auto-oxidation, have a second oxygen function in addition to that of C-3 when compared with natural sterols. These compounds are of particular interest due to important biological functions of some of them and various bioactivities, including effects on lipid metabolism, platelet aggregation, apoptosis, different receptors and proteins [1]. Generally, oxysterols attract a great attention by the contribution to many physiological processes, including those connected with pathophysiology of the neurodegenerative diseases [2]. As a key trend, the roles of two brain sterols, 24*S*-hydroxycholesterol and 27-hydroxycholesterol, have been studied for years and discussed in multiple reports [3–5]. Among others, 24*S*-hydroxycholesterol has been reported to be markedly reduced in the circulation of patients with Parkinson's disease (PD) [4]. The treatment with 27-hydroxycholesterol or a combination of 24*S*-hydroxycholesterol and 27-hydroxycholesterol reduced the levels of noradrenaline, whereas the treatment with 24*S*-hydroxycholesterol alone had no effect. With these results, the authors demonstrated that oxysterols could regulate the proteins involved in the development of PD [5].

Recently, some other oxysterols were found to be exported from the brain: 7β-hydroxycholesterol, 7-ketocholesterol, 3β,5α-dihydroxycholestan-6-one, 7α-hydroxy-3-oxocholest-4-enoic acid, 7α,25-dihydroxycholest-4-en-3-one, and (25*R*)-7α,26-dihydroxycholest-4-en-3-one. It was underlined that these transfers were observed when total oxysterols were measured [6,7]. These facts give a new quest to disclose possible roles and bioactivities of non-common oxysterols, including those of marine origin, in development of PD and other neurodegenerative diseases.

Recent studies highlighted the point that even well-known oxygenated sterols might be a critical source of new medicine leads. Thus, hecogenin and cholest-4-en-3-one showed significant inhibitory activity (EC50 values of 116.3 and 390.6 μM, respectively) against the human β-site amyloid cleaving enzyme (BACE1), which has been considered as an effective drug target for treatment of Alzheimer's disease [8].

The search for new oxysterols in marine organisms and the studies on their action in model cell systems could be also considered as an approach to the creation of new pharmaceutical leads against neurodegenerative diseases. As a part of our chemical investigations of steroidal compounds from marine sponges [9–11], we have studied oxysterols of a cold-water marine sponge *Inflatella* sp. The sponges belonging to the genus *Inflatella* are insufficiently studied for their secondary metabolites, though some bioactivities of their extracts were noted [12]. The ethanol extract of the studied *Inflatella* sp. sample was preliminary analyzed by thin-layer chromatography during the screening of sponge samples from PIBOC collection. The results revealed that in contrast with extracts of other sponges, it contained the oxygenated steroids visually detected on chromatograms as a number of bright colored spots (Figure S1). Herein we describe the isolation and structural elucidation of fourteen individual oxysterols of *Inflatella* sp., including four new compounds as well as the results of their biotesting using a cell model of PD. Part of this work has been presented at the 3rd International Symposium on Life Sciences, Vladivostok, Russia, September 2018.
