**1. Introduction**

Pharmaceutical nanocarriers comprise nanoparticles, nanospheres, nanocapsules, nanoemulsions, nanoliposomes, and nanoniosomes [1]. Solid lipid nanoparticles (SLN) can be defined as solid lipid colloidal particles composed of organic matter and carbon in a range of 10–1000 nm in which the active pharmaceutical ingredients (API) are dissolved or encapsulated in lipids (**Figure 1**) [2–4]. The underlying mechanism for the formation of lipid nanocarriers is hydrophilic-hydrophobic interactions and van der Waals forces between phospholipids and water molecules [5]. In addition, the physicochemical properties of lipids, such as biocompatibility, low susceptibility to erosion phenomena, and slow water absorption, make lipids an ideal nanocarrier system to improve aqueous solubility and bioavailability of APIs [6, 7].

Some advantages of SLNs as a vehicle for APIs include: covering the bitter taste of the drug in oral administration; maintenance of therapeutic drug

#### **Figure 1.** *General diagram of the solid lipid nanoparticles.*

concentrations and circulatory time at target sites; protection against premature degradation in the gastrointestinal tract; improved pharmacokinetics, solubility, bioavailability, and stability; reduced toxicity; dose reduction and dose frequency; improvement of patient compliance; and prevention, reduction or delay of the onset of resistance, in addition to the fact that the by-products or metabolites of SLNs are significantly safe and can be easily eliminated through the normal process of excretion [8].

SLNs have been sought as a means to improve the solubility and bioavailability of many drugs, both hydrophilic and lipophilic, especially drugs belonging to class two (high permeability and low solubility drugs) and four (low permeability and low solubility drugs) of the Biopharmaceutical Classification System (BCS) [9].

SLNs have been developed for various applications, including nutraceuticals, cosmetics, pharmaceuticals, and biomedicals, as they can transport a variety of components, including small drug molecules, large biomacromolecules (polysaccharides, etc.), genetic material (DNA/RNA) [10], vaccine antigens [11], antineoplastic [12, 13], antimicrobial [14], they can also be applied in for the targeted delivery of brain medications since enhancing the ability of the drug to penetrate through the blood-brain barrier (BBB) [15].

By focusing on cellular delivery, SLNs can enhance drug delivery to target cells by various mechanisms, such as passive mechanisms that take advantage of the tumor microenvironment, active mechanisms by surface modification of the SLN, and the co-distribution mechanism. SLNs can combine many different drugs and be effective in various types of tumors (i.e., breast, lung, colon, liver, and brain), supporting their potential [16].

SLNs provide several indirect ways to address resistance problems, such as achieving a sustained release profile of a drug, maintaining concentrations within its therapeutic range, and thus avoiding potential adverse effects. Suboptimal levels can promote the selection of resistant bacteria, reduce drug toxicity by encapsulation, permitting higher doses. Promote accumulation in target cells using active targeting, and increase the inhibitory effect (i.e., decrease MIC) on bacterial strains [17].
