Nanovehicular Intracellular Delivery Systems

Purpose: Conventional drug delivery faces barriers including rapid clearance, poor intracellular uptake, low target specificity, and toxicity. Nanovehicles (NVs)—nanosized particles capable of entering cells and delivering cargo intracellularly—offer a means to overcome these barriers. The review aims to provide an overview of principles and barriers relevant to intracellular drug and gene transport, accumulation, and retention using NVs, with emphasis on delivery to cancer.

Hypothesis: No formal experimental hypothesis. Central thesis: NVs—liposomes, polymeric nanoparticles, dendrimers, micelles, polyelectrolyte complexes, nanosuspensions, and others—can overcome extracellular and intracellular barriers to deliver therapeutic cargo to specific tissues, cells, and organelles; rational design based on physicochemical properties and quantitative modeling can improve targeting, intracellular delivery, and therapeutic index.

Aims: Overview principles and barriers relevant to intracellular drug and gene transport, accumulation, and retention using NVs. - Discuss pharmacokinetics and pharmacodynamics of NV-based delivery. - Describe uptake mechanisms, intracellular trafficking, and organelle-specific targeting. - Review targeting strategies (passive, active, dual, indirect) and biobarriers (RES, BBB, osmotic pressure). - Summarize advanced drug delivery (ADD) systems: nanosuspensions, nanoparticles, micelles, polyelectrolyte films, dendrimers, liposomes, polymer-drug conjugates, cell-penetrating peptides, and gene delivery vehicles. - Review quantitative systems approaches (in silico modeling) to guide NV design and development.

Delivery system: NV types: nanosuspensions, polymeric nanoparticles (PLGA, PLA, PACA), polyelectrolyte complex (PEC) nanoparticles, polymeric micelles, multilayered polyelectrolyte films, dendrimers (PAMAM), liposomes and solid-lipid nanoparticles (SLN), functionalized polymer-drug conjugates (HPMA, PEG, PGA), cell-penetrating peptides (CPPs, TAT, penetratin, VP22), special systems (quantum dots, gold nanoshells, carbon nanotubes, calcium phosphate, silica NPs), and gene delivery vehicles (polyplexes, lipoplexes, lipopolyplexes, viral vectors). - Payloads: small molecule drugs, proteins, peptides, cytokines, antibodies, oligonucleotides (antisense, ribozymes, DNAzymes, aptamers, siRNA), plasmid DNA, imaging agents (quantum dots, gadolinium, iron oxide), radiotherapeutics, and vaccines. - Targeting ligands: folate, transferrin, RGD peptides, antibodies, galactosamine, mannose, LHRH, and others. - Surface modifications: PEGylation (steric stabilization), electrostatic stabilization, MPC polymers, ligand conjugation. - Key design parameters: size (ideally <100–200 nm), surface charge (negative to avoid RES; positive for mitochondrial targeting), hydrophobicity, shape, elasticity, and release kinetics.

Approach: Comprehensive literature review; no primary experiments. Discusses in vitro systems (Caco-2, vascular smooth muscle cells, HeLa, A549, K-562, dendritic cells, macrophages, cancer cell lines) and in vivo models (mouse tumor xenografts, Lewis lung tumor, B16 melanoma, DU145 and PC-3 prostate tumors, rat allogeneic heart transplantation). Covers clinical products (Doxil, DaunoXome, Myocet, Abraxane, Onpattro, Viva-Gel) and clinical trials. Also reviews pharmacokinetic modeling and systems biology approaches.

Key methods: Review-level synthesis of: - Nanoparticle characterization: size (DLS, TEM), zeta potential, drug loading, release kinetics. - Uptake mechanisms: phagocytosis, macropinocytosis, clathrin-mediated endocytosis (CME), caveolar-mediated endocytosis, lipid-raft-mediated endocytosis, clathrin- and caveolae-independent endocytosis, exocytosis, transcytosis. - Intracellular trafficking: endosomes, lysosomes, cytosol, nucleus, mitochondria, cytoskeleton. - Targeting: passive (EPR effect), active (ligand-receptor), dual, indirect. - In vivo efficacy: tumor growth inhibition, survival, biodistribution, clearance. - Pharmacokinetics: ADME, compartmental modeling, PBPK modeling. - In silico modeling: multiscale, systems biology, hierarchical levels.

Key results: Representative findings from cited literature: - 100 nm PLGA nanoparticles had 2.3-fold greater uptake than 50 nm particles, 1.3-fold vs 500 nm, and 1.8-fold vs 1000 nm in Caco-2 cells. - DOX-dendrimer was >10 times less toxic than free DOX to colon carcinoma cells in culture; in tumor-bearing mice, tumor uptake was 9-fold higher than intravenous free DOX, causing complete tumor regression and 100% survival after 60 days. - Nanocells delivered drugs at doses ~1000 times lower than free drug for equivalent tumor regression in mouse xenografts and dog lymphoma. - PEG-PLA (45K) with PEG Mw 5000 achieved maximal reduction in protein adsorption; threshold PEG surface density needed to avoid MPS uptake. - Accelerated blood clearance (ABC) of PEGylated liposomes observed upon repeated injections due to IgM production. - Stabilized plasmid-lipid particles (SPLP, ~70 nm) showed circulation lifetimes ~10 h and ~10% injected dose accumulation in Lewis lung tumor. - Folate targeting of dendrimers (<5 nm) achieved high tumor/blood ratios in solid tumors. - Transferrin-PEI/DNA gene transfer was 10–100-fold more efficient than naked DNA after intratumoral application. - Albumin-bound paclitaxel (Abraxane, 130 nm) internalizes tumor cells via gp60 receptor and caveolin-1 activation. - Endorem (superparamagnetic iron oxide) used clinically for MRI imaging. - PK2 (HPMA copolymer-doxorubicin-galactosamine) progressed to clinical trial for liver cancer. - Viva-Gel (dendrimer-based microbicide) completed phase I clinical trial. - IFN-β (PEGylated) pharmacokinetics modeled in monkeys.

Interpretation: NVs offer improved spatio-temporal control over drug kinetics and distribution, enabling safer and mode-specific therapies. The ratio of intracellular to systemic localization (I/S ratio) can be optimized by maximizing circulation time and minimizing release and uptake. However, no single NV is ideal for all applications; optimization must be performed for each application. The authors conclude that the field is in an early phase of development, with many years from maturity, but NVs will be central to new cancer therapies. They emphasize that rational design requires understanding of uptake and trafficking mechanisms, and that quantitative systems biology approaches are needed to guide development.

Limitations: Review article; no primary data. - “Four decades of research have not yet produced an effective, generally applicable, site-targeted drug delivery system.” - Most NV systems remain preclinical; clinical translation is limited. - Nanoparticle stability, scale-up, batch-to-batch uniformity, and residual solvents are challenges. - PEGylation can shield targeting ligands and cause accelerated blood clearance upon repeated administration. - In vivo gene delivery is inefficient; nonviral vectors have low transfection efficiency compared to viral vectors. - Cellular uptake mechanisms are complex and cell-type dependent; relative contributions of different pathways remain poorly understood. - Endosomal escape and nuclear delivery are major barriers. - Targeted NV distribution shifts may be modest but clinically meaningful. - Permanently stored nonbiodegradable polymer remnants may represent unexplored health hazards. - Multi-scale, bottom-up systems biology models for cancer treatment are not yet available. - The authors note that “very little progress has been made in developing targeted drug-delivery systems for effective and realistic human therapy.”