Patent ID: 12208163

DETAILED DESCRIPTION AND EMBODIMENT OF THE INVENTION

The main novel feature of the lipid gels object of the present invention is that a mixture formed only by lipids, without the intervention of polymers or surfactants, and that contains a very high water content, up to 97%, is capable of being structured as a gel. As indicated in the discussion of the state of the art, dense emulsion/gel-type systems constituted only by lipids are usually formed at high lipid concentrations (>50%), generating tight packing phases such as cubic or lamellar phases, while the most diluted systems require other compounds such as surfactants, gelling agents or polymers to achieve gel-type rheological behaviour.

Once formed, the gel maintains a semi-rigid structure and exhibits a translucent white colour at room temperature, while the gel becomes fluid and transparent as of a certain temperature that varies according to the lipid composition of the system and which can be as of 5° C. It should be noted that this process is reversible and the gel structure is recovered once cooled below that variable temperature in accordance with the lipid composition of the system.

Composition

The phospholipids most frequently used to prepare the systems form part of the group of phosphatidylcholines and are a commercial product obtained from soy lecithin known as “hydrogenated soy phosphatidylcholine (HSPC)”.

In order to form the gel, the HSPC is mixed with the oleic acid (OA) in a molar ratio of 3:1 and is adjusted to a pH of 5-8 using sodium hydroxide. The pH range is a decisive factor to correctly formulate the gel. The total lipid concentration by weight (HSPC+OA) has been established as optimum at 5%, since very diluted systems (<3%) are not formed, while more concentrated systems (>10%) are difficult to disperse using conventional methods.

In order to form the gels, a freezing process followed by a heating process of the lipid dispersion is required.

With other phospholipids with different HSPC features, particularly different polar heads and different alkyl chains, and with fatty acids other than oleic acid, the results obtained are equivalent, although the formation and reversibility conditions vary in accordance with the physico-chemical parameters of the lipids. The molar ratio between the lipids present in the mixture may vary with similar results. Although the lipid concentration with which most of our results were obtained was 5%, higher concentrations also give rise to the formation of these gels.

Table 1 shows various examples of gel-forming systems with a description of their aspect and behaviour:

TABLE 1LipidCompositionConcentrationMolar Ratio(%)Aspect/BehaviourHSPC/LA 3/15Semi-transparent/GelHSPC/SA 3/15Semi-transparent/GelDPPC/OA 3/15Transparent/GelDMPC/OA 3/15Transparent/Fluid GelHSPC: hydrogenated soy phosphatidylcholineDPPC: dipalmitoylphosphatidylcholineDMPC: dimyristoylphosphatidycholineOA: oleic acidLA: lignoceric acidSA: stearic acid
Characterisation
Rheology

The main objective of this technique is to determine whether the samples obtained behaved rheologically as a gel.

An oscillation amplitude sweep (“Strain Sweep”) was initially performed wherein the linear viscoelastic region (LVR) was determined in order to be able to work with reliable parameters. Next, an oscillation frequency sweep (“Frequency Sweep”) was performed to evaluate the viscous and elastic properties of the material.

As mentioned in the discussion of the state of the art, in the paper by Talló, K; López, O. and col.Vesicular nanostructures composed of oleic acid and phosphatidylcholine: Effect of pH and molar ratio; Chemistry and Physics of Lipids 213 (2018) 96-101 presents an aqueous vesicle dispersion that behaves as a viscous liquid at macroscopic level. This system clearly differs rheologically and structurally from the nanostructured lipid gel of the present invention. Although both have the same chemical components, the method of preparation allows the system described in the present application to be structured as a gel and not as a simple dispersion. At first glance it can be observed how the gel maintains a rigid structure while the aqueous vesicle dispersion flows in its receptacle. In order to show that they are different products, with differentiated rheological behaviour, an oscillation sweep was performed on both systems under the same pH, concentration and temperature conditions (FIG.1).

As can be seen inFIG.1, the gel (invention) and the lipid dispersion (prepared with the protocol described in the paper by Talló et al. 2018) have very different rheological behaviour. The values of the elastic module (G′) and the viscous module (G″) of the gel exceed the G′ and G″ values of the vesicle dispersion described in the paper by two orders of magnitude. This means that the lipid gel object of the present invention is much more structured at microscopic level, giving the product greater consistency and rigidity. It can also be observed that the G′ value of the gel is clearly higher than the G″ value, which indicates that the elastic behaviour (solid) prevails over the viscous behaviour. In contrast, for the vesicle dispersion the G′ and G″ values are nearly identical, overlapping at some point, which indicates that the viscous behaviour of the dispersion is comparable to the elastic behaviour.

Electron Microscopy

In order to observe the nanoscopic structure of the gels, the samples were cryofixed following different methods. In some cases, a fracture was forced through the sample in order to reveal possible lamellar or vesicular-type aggregates. The samples were observed by means of transmission electron cryomicroscopy (CryoTEM).FIG.2shows different images of the sample wherein stacks of extended flat membranes combined with unilamellar vesicles can be observed. The laminae were extended to micron level, although thickness is adjusted to a lipid membrane. The vesicles are interspersed between the laminae and exhibit sizes approximately between 100-150 nm in diameter.

Small-Angle X-Ray Scattering (SAXS)

This technique was used to determine that the gel is composed of a lamellar structure. This fact can be observed from the small-angle X-ray scattering profile (SAXS) shown inFIG.3A. In said figure, a broadband corresponding to spacing distance of approximately 7 nm is observed, calculated on the basis of the scattering vector q and the equation qn=2nπ/d, wherein d is the spacing distance, n is the scattering order and q is the scattering vector. The location of the following Bragg bands in positions 3q and 4q indicate a multi-lamellar structure that would have a spacing of 7 nm and an organisation such as that shown inFIG.3B.

Wide-Angle X-Ray Scattering (WAXS)

The lateral packing of the phospholipids was determined using this technique. As shown inFIG.4A, there is only one peak corresponding to a spacing value between polar heads of 4.2 Å, which would indicate that it is hexagonal packing such as shown inFIG.4B.

Application on the Skin

The structural consistency of a gel represents a clear advantage over a liquid lipid dispersion such as that of Talló K. et al. (2018), since it facilitates topical application. This factor is evident bearing in mind that most commercial products for cutaneous application are creams or gels. Structurally, the lamellar organisation of lipid membranes confers greater stability to the product, while a vesicular system such as that described in Talló et al. (2018) tends to aggregate and flocculate if stabilisers are not added. Furthermore, microscopic structural differences may imply a major difference in the field of pharmacokinetics and drug administration.

Cutaneous Permeation

In order to evaluate the potential of these gels as cutaneous application systems, an in vitro permeation assay was conducted on pig skin and observations were made using fluorescence microscopy.

Two gels were formed which were applied to the skin surface. One of them was formed by incorporating a red fluorescent probe (Rhodamine B) in order to observe in which areas of the skin the gel-forming phospholipids are retained. In the other gel, a fluorescent green probe (fluorescein) was added in the aqueous phase with the aim of simulating a possible water-soluble active ingredient incorporated to the gel. The gel was gently applied to the skin and left to permeate overnight at 37° C. in a humid environment. Next, the skin was cut into sections and the cells marked in blue in order to distinguish the different skin layers.

FIG.5shows how the lipid gel matrix (red) is retained at the top of the stratum corneum (outermost layer of the skin) without reaching the epidermis (blue).FIG.6shows how the fluorescein dissolved in the aqueous phase of the gel (green) is capable of permeating the skin, covering the entire stratum corneum and epidermis. Similarly, it can be observed how it is also capable of going down the follicle, staining the hair (blue arrow). It should be noted that a control was carried out using an aqueous solution with fluorescein (without incorporating the gel) and it was only slightly incorporated in the stratum corneum. Therefore, the gel stimulated the passage of this molecule (fluorescein) through the skin.

These results show that the formation of gels formed by combining phospholipids and oleic acid in water having a very high water content (up to 97%) is possible. These gels lack usual gelling molecules such as polymers or surfactants and the structure and fluidity thereof respond reversibly to temperature and pH. They are also capable of transporting at least one hydrophilic substance within the skin and also to the follicles.

Their particular organisation, with part of the water trapped in vesicles and these vesicles trapped or interspersed between extended laminae, makes them very adequate as systems for incorporating molecules of a different polar nature in different compartments. Their exclusively lipid composition ensures high biocompatibility. Their rheological behaviour enables the easy topical and ocular application thereof and their ability to respond to biological parameters indicates their potential biomedical applications.

Healing Effect

In order to evaluate the potential healing effect of the gel, wound healing studies were carried out on ex-vivo skin explants.

An injury was made to the recently extracted pig skin using a dermatological punch. The wound skin explants were seeded in culture wells using DMEM as culture medium supplemented with FBS, antibiotics and L-glutamine. The explants were maintained under these culture conditions for 14 days. During this period, the injury made to the skin explants was treated with the gel every two days. For comparison purposes injured explants were kept under culture and untreated.

After 14 days, the explants were frozen in liquid nitrogen embedded in OCT. 8-micron cuts were made in the skin using a cryostat. Said sections were stained with eosin and haematoxylin and were observed with an optical microscope.

InFIG.7the difference between the injuries treated with the gel and those not treated can be observed. It can be observed that the lipid gel matrix (stained blue with haematoxylin) remains in the injury. Additionally, a certain re-epithelisation and closure of the injury in treated injuries with respect to untreated injuries can be observed. The rheological behaviour of the gel makes it easy to apply and be confined in the area of application. Its structure, large interconnected vesicles, may act as a medium for the skin cells to grow on, stimulating the tissue healing process. The large lamellae could also favour the growth of new cells and, additionally, because of being lamellae formed by lipid bilayers that are very similar in structure to the lipid matrix of the dermal stratum corneum, they could protect the wound and stimulate the adequate recovery of the barrier function. Once the wound is healed, or during the healing process, the gel structure will become disassembled and the gel components, lipids and water, fully biocompatible, can be integrated in the tissue structure.

Eye Irritation Assay

With the aim of proposing potential uses of the gel in ocular applications, the possible eye irritation of this material was studied by means of a HET-CAM test.

The gel formed from HSPC and OA with a molar ratio of 3:1 and a total lipid concentration of 5% was directly applied to the chorioallantoic membrane of a chicken egg due to its similarity to the human cornea. The appearance of vascular lesion or clotting in response to a compound is the basis for using this technique as an indicator of the probability that a substance may harm the mucous membranes, especially the cornea of the human eye, in vitro.

In order to execute the method, the egg shell is carefully removed, moistening the membrane with a NaCl solution at 37° C. Next, the NaCl is removed and the white membrane is removed without harming any blood vessel.

Next, the gel sample is applied and the appearance of haemorrhage (H), vasoconstriction (V) and/or clotting (C) can be observed for 5 minutes.

Lastly, the Ocular Irritation Index (OII) is estimated using the following formula:

OII=(3⁢0⁢1-H)·53⁢0⁢0+(3⁢0⁢1-V)·53⁢0⁢0+(3⁢0⁢1-C)·93⁢0⁢0

H, V and C are the time in seconds when this change appears. If there is no alteration, they are equal to 300. The result obtained from the formula can be interpreted on the basis of Table 2:

TABLE 2OIIClassification0-0.9Practically non-irritating1-4.9Mildly irritating5-8.9Moderately irritating9-21Irritating

The results obtained are indicated in Table 3:

TABLE 3EggHaemorrhageVasoconstrictionClottingweight (g)Sampletime (s)time (s)time (s)OIIClassification59.4NaOH28132017.79Irritating57.71% SDS110003.23Mildlyirritating67.85% gel0000.07Practicallynon-irritating62.42.5% gel0000.07Practicallynon-irritating

Both the initial formulation and the diluted formulation proved to be non-irritating, due to which it can be affirmed that the gel does not cause eye irritation.