HDL serves as a lipid carrier in the human body, and esterified cholesterol, owing to its extremely hydrophobic nature, enters the HDL core. Similarly, many chemotherapeutic drugs such as doxorubicin, paclitaxel, docetaxel, lapatinib, cisplatin, and curcumin are also hydrophobic [
60]. This results in the inability of the drug to effectively enter tumor tissue and requires either binding to proteins to increase solubility or employing a carrier, such as the natural carrier HDL, to overcome this challenge [
61]. Researchers have identified two main mechanisms by which HDLs precisely deliver drugs to specific tissues one of which is the body's own production of HDL in the plasma through interactions with HDL receptors (including ABCA1, ABCG1, and SR-B1), which are often highly expressed in hepatocytes and small intestinal mucosal cells [
62] and are mainly involved in cholesterol and phospholipid synthesis, transport, and secretion. In addition, ABCA1 and ABCG1 are highly expressed in brain tissue, adipocytes, and immune cells, where they regulate intracellular lipid transport and maintain cholesterol homeostasis [
39,
63]. In contrast, SR-B1 is mainly expressed in hepatocytes, which is closely related to the RCT pathway and is a key receptor for the major role of HDL in humans [
48,
64]. These receptors transport lipids, proteins, and nucleic acids from donor cells to recipient cells, which means that drugs transported by HDL can be precisely transported to target tissues and cells through specific ligand-receptor binding. Based on this targeting mechanism, it has been observed that in certain disease states, damaged tissues may exhibit elevated expression of the SR-B1 receptor. For example, in patients with aldosterone-producing adenoma (APA), the adrenal glands produce excessive aldosterone, the precursor of which is cholesterol. Harashima
et al. demonstrated that both SR-B1-mediated selective cholesterol ester uptake and
de novo cholesterol synthesis are increased in APA [
65]. In cancer, tumor cells exhibit increased active cholesterol metabolism and increased SR-B1 expression. For instance, in patients with adrenocortical carcinoma, the CLA-1 gene plays a role in adrenal steroidogenesis and may act as an HDL receptor, mediating the increased SR-B1 selective uptake of cholesterol in these cells [
66]. This conclusion has been validated in other cancers such as breast cancer [
67], renal clear cell carcinoma [
68], and prostate cancer [
69]. Interestingly, tumor tissues exhibit a unique blood vessel structure and abnormal function, which increase the permeability of drugs and facilitate their retention in the tumor tissue. This phenomenon represents a significant difference between tumor tissue and normal tissue, and was first coined as the "enhanced permeability and retention (EPR) effect" of tumors by Professor Maeda in 1986 [
70]. This serves as another target mechanism for HDL as a drug delivery system. Notably, after 37 years of research, the existence of this effect has provided a theoretical basis for nanoparticles to serve as drug carriers, as the range of particle diameters allowed to pass by the EPR effect is relatively wide, usually within a few hundred nanometers [
71,
72]. Nanoparticles in this range can enter tumor tissue through the walls of tumor blood vessels and remain in the tissue for a longer period. Smaller nanoparticles (approximately 10–100 nm) can enter the tumor tissue through gaps in the tumor blood vessels and irregular endothelial cell connections [
73]. Owing to the increased permeability of the tumor blood vessel walls, these small nanoparticles can effectively penetrate the tumor tissue. The diameter of the HDL particles is typically between 8 nm and 13 nm, and for reconstituted HDL produced synthetically with different drug loads, the diameter can range from to 9–20 nanometers [
74]. Therefore, reconstituted HDL passively targets tumor tissues
via the EPR effect. Previous studies have reported that larger nanoparticles (approximately 100–200 nm), such as liposomal nanoparticles, primarily enter the tumor tissue through the lymphatic system. Owing to the abnormal structure and function of the lymphatic system in tumor tissue, these nanoparticles are difficult to effectively clear and tend to remain in the tissue for a longer time, leading to their accumulation in the tumor tissue [
75]. It is important to note that particle size is not the only factor that determines its effect through EPR. Other factors such as the surface properties, shape, and surface modifications of the particles, as well as the variable structure of the tumor vasculature, also affect the permeability and residence time of the nanoparticles in the tumor tissue [
76,
77].