# Cystic Fibrosis: Overview of the Current Development Trends and Innovative Therapeutic Strategies
## 5 Nanocarriers for CF Treatment
However, in the late stages of CF, the viscoelasticity increases, resulting in a decrease in the nanoparticles penetration across the mucus layer [187]. The negatively charged components of the mucus layer act as an electrostatic barrier for cationic nanoparticle penetration. Hence, a nanoparticle with a neutral surface-charge (zeta potential values ranging from -10 to +10 mV) will reduce the interaction between the nanoparticles and mucus [189; 112]. It is hypothesized that coating particles with a high density of low molecular weight polyethylene glycol (PEG) may reduce adhesive interactions between nanoparticles and mucus. This low molecular weight of PEG could be enough to not entangled with the mucin fibers and that the PEG density is sufficient to effectively shield the hydrophobic core of the nanoparticle and enables their penetration through the mucus layer [190]. Thus, the penetration of nanoparticles through the mucus layer can be enhanced by reducing the electrostatic interaction via conjugation with low molecular weight PEG, which possesses muco-inert properties [191]. Such coating is widely studied as a mucus penetrating approach, and it was recently reported that nanoparticles coated with low molecular weight PEG could rapidly cross the physiological human mucus even with a particle size of 500 nm. Moreover, surface modifications of nanoparticles with a high density of low molecular weight PEG (of 2-5 kDa) reduced the interaction between particles and mucus [192]. Furthermore, decreasing the mucosal viscoelastic property using mucolytic agents, like N-acetylcysteine (NAC) and DNase, is an effective strategy to increase the diffusion of nanoparticles through the mucus. NAC decreases the mucus viscosity by disrupting the disulfide bonds connecting the mucin proteins, whereas DNase breaks down the DNA in the sputum [112]. Local delivery of nanoparticles to the lungs via inhalation or nebulization improves therapeutic efficacy by increasing the dose delivered to the lungs (i.e., localizing) of the CF patient. However, nanoparticles encounter the complex pulmonary system, which acts as a barrier, and confront difficulties in the deposition of the administrated drug. Therefore, inhaled nanoparticles must be designed with the ability to get deposited in the deepest part of the lungs and reach the alveoli upon penetration of the mucus layer. The deposition of an optimal level of inhalable particles in the alveolar region can be achieved by using a formulation of size less than 3 \(\upmu\)m [193]. Additionally, the superficial characteristics of nanoparticles and their morphology are also important parameters that affect the deposition of the nanoparticles. Aerosolization of suspended nanoparticles using nebulizers aids in obtaining a droplet with an appropriate size for delivery into the lungs of CF patients. Owing to the limitations of nebulizednanoparticles like short-term stability and patient compliance, an alternative form of pulmonary dosage called the dry powder inhaler (DPI) was suggested for the delivery of nanoparticle-based formulations to the lungs [194]. This can be achieved by the spray drying of nano-embedded microparticles (NEM) known as Trojan particles, which facilitate the deposition of nanoparticles into the lungs, following their release from the microparticle matrix [194]. NEM powders are formulated by mixing cryoprotectant excipients like mannitol or lactose with poly lactic-co-glycolic acid (PLGA), chitosan, and polyacrylate, which are biodegradable and biocompatible polymers [195]. To improve the treatment efficiency in CF patients, several viral and non-viral vectors have been examined both in vitro and in vivo. Viral vectors like AV, adeno-associated virus (AAV), herpes simplex virus, and retrovirus have been engineered for application in gene therapy. The first clinical trial on the application of viral vectors in CF treatment began in 1993 using an E\({}_{1}\)-deficient AV capable of expressing the CFTR protein in airway epithelial cells.