**3. Factors affecting hybrid nanocarriers**

Hybrid nanoparticles are trimmed to an acceptable level of particle size, drug carrying capacity and site specificity through incorporation and adjustment of ratios of different chemical components. The variations of structural components of HNPs have an obvious influence on HNPs' characteristics [17, 96]. The principal factors of HNPs' formulation are (i) lipid/polymer ratio, (ii) PEGylation and (iii) polymer nature.

## **3.1. Lipid/polymer ratio**

**Structural components**

Docetaxel

PLA

Chitosan

Docetaxel

169.6 ± 4.6


89.8 ± 3.1 and

The drugs loaded hybrid nanoparticles showed enhanced cytotoxicity

[92]

66 Advanced Technology for Delivering Therapeutics

81.9 ±5.6

and tumor growth inhibition.

Curcumin

PLGA

Docetaxel

110 ± 13.5


77.65 ± 0.57

The system increases the cellular update of docetaxel 2.5 folds and

[93]

anti-proliferative activity 2.69–4.23 folds.

DSPE-PEG

PLGA

Doxorubicin

264 ± 2.2


97.8 ± 1.3

It is used to deliver anticancer drugs which results in enhanced

[94]

circulation half-life and reduce the elimination of drug

Chitosan

Hyaluronic acid

**Table 1.**

Hybrid nanoparticles with different structural components and their applications.

**Physicochemical properties**

**Size (nm)**

**Zeta potential** 

**Entrapment** 

**efficiency (%)**

**(mV)**

208–255.7


75.9

PLA/chitosan nanoparticles provide rapid initial release of 40% drug in

[91]

5 hours and 70% cumulative release in 24 hours.

**Application**

**References**

The lipid covering the polymeric core provides substantial benefits to HNPs and their distinction over nonhybrid nanoparticles. The ratio of two building blocks (lipid-polymer) of hybrid particles have significant role in stabilizing the formulation, monodispersibility and encapsulation efficiency [45, 97].

At a lower L/P ratio, the nanoparticle surfaces are not entirely covered with lipids, which can form bridges with lipid part of other particles causing aggregation and formation of larger particles. At a relative higher lipid concentration, it tends to decrease the production yield as whole amount is not incorporated in particles and free lipids will arrange themselves to form liposomes can affect the homogeneity of formulation. Therefore, the concentration of lipids should be optimized that cover to polymeric core on the basis of particle size and production yield [59, 98]. Chew et al. prepared HNPs with PC and PLGA carrying antibiotics with WPC/ WPLGA value <15- up to 90%. At lipid amount below 15% larger particles were formed (800– 1000 nm) and a sharp decrease in particle size was observed at an optimum concentration i.e. 30% lipids, an optimum particle size (260–400 nm) and 80% production yield was achieved. The lipid ratio above the optimum concentration i.e. 30% did not reduce particle size but it decreased the yield as the entire lipid was not utilized [45].

An optimum lipid to polymer ratio also provides the colloidal stability of HNPs by providing an optimum surface charge density which is responsible for electrostatic repulsive forces that prevent particle coalescence and stabilizes the formulation. In case where the lipid part is insufficient and the resulting electrostatic repulsive forces are weak, some agents like PEG can be incorporated in the formulations to provide steric repulsion and stabilization of the HNPs [52, 80–98].

The charge on lipid part which is responsible for electrostatic repulsion between particles is shielded when mixture of cationic and zwitterionic lipids is employed. Anionic heads of zwitterionic lipids face outwards which reduces of cationic lipids charge and promotes aggregation of particles. However, the higher cationic lipid concentration may overcome this charge screening and aggregation can be minimized [59, 99]. The zwitterionic lipid such as 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) produces less aggregation than a cationic lipid, 1,2-dipalmitoyl-3-trimethylammonium propane (DPTAP). Therefore, it zwitter ionic lipid provides more stability than ionic lipids [55, 59].

The two potential benefits that lipid augments to the HNPs are the encapsulation efficiency and retardant release of the incorporated drugs. The former is achieved by preventing drug leakage during self-assembling process, whereas the latter is due to reduced interaction of lipids with dissolution medium [17, 100]. The charge on the surface of lipids and drugs also affects the entrapment efficiency due to interaction of surface charge of HNPs and the charge of the drug. The loading of ciprofloxacin in the PLGA-PC hybrid system is not successful due to the interaction of cationic drug with the anionic lipids [19, 78].

A significant higher percent encapsulation of docetaxel (59 ± 4) was achieved in HNPs assembled from lecithin, DSPE-PEG and PLGA i.e. compared to PLGA-PEG nanoparticles with 19 ± 3 (mean ± SD). This effect is attributed to the fencing action of lipids which keeps hydrophobic drugs within the core and retards water penetration. The lipid-polymer hybrid formulations also provide a sustained release of drug when compared with nonhybrid formulation due to less water penetration and reduced escape of drug molecules from polymeric core. A consistent 50 % release of docetaxel from lecithin-PLGA hybrid system was observed compared to the PLGA-PEG NPs and PLGA NPs released same amount of drug in 10 hours and 7 hours, respectively. The pH of the dissolution media also affected the encapsulation of drugs, for example, the erlotinib EE % was 77.1%, 28.83% and 18.45% at pH values of 7.4, 5.4 and 3.4, respectively [55, 59, 78, 100].

## **3.2. PEGylation**

The steric stabilization of HNPs systems to withstand salt solutions, buffer actions and uptake by macrophages is provided by the appropriate surface modification by employing PEG. The term is called as PEGylation. PEGs can escalate circulation times of HNPs by preventing particle aggregation, opsonization and adsorption of plasma proteins [27, 78].

Incorporation of PEG-lipid affects the colloidal stability of HNPs by two ways (i) chain length of PEG-lipid and (ii) molar Ratio of PEG-lipid. HNPs coated with PEG-lipid longer chains exhibited more stability than the shorter chain PEG-lipid coated particles. Similarly, at the fixed chain length more PEG-lipid incorporated onto polymer core and thickness of lipid shell increased which lowered the zeta potential and hence stability is enhanced [78, 80].

Yang et al. studied the effect of lipid/polymer ratio and PEGylation on HNPs prepared from PLA/mPEG-PLA polymer and BHEM-Chol cationic lipid. HNPS prepared from mPEG-PLA were smaller and more stable in PBS at the given lipid/polymer ratio than PLA alone [61].

Fang et al. formulated HNPs using 0.10–0.35 lipid-PEG/PLGA ratios without incorporating lecithin. Initially particle reduced with increase in lipid amount and optimized at 0.30 lipid-PEG/PLGA ratio after which further increase in lipids did not affect particle size and PDI. At an optimized (0.30 lipid-PEG/PLGA) ratio, lipid-PEG was replaced with mole equivalents of lecithin. The stable particles of 60 nm were obtained at 50% lipid-PEG replacement. Upon 70% lipid-PEG replacement, the size was increased to 100 nm and at 80% lipid-PEG replacement with lecithin, the unstable particles were obtained. This instability of particles is due to the replacement of higher lipid-PEG content, a major stability component of HNPs [63].
