Liposomal Delivery Systems: Design Optimization and Current Applications

Purpose: Liposomes are attractive pharmaceutical carriers because they encapsulate hydrophilic and hydrophobic drugs and are biocompatible and biodegradable. However, conventional liposomes suffer from aggregation/fusion, premature payload release, rapid clearance by the mononuclear phagocyte system (MPS), low entrapment efficiency for some drugs, poor target selectivity, and inefficient intracellular delivery. Design optimization is needed to improve their therapeutic performance.

Hypothesis: As a review, there is no single experimental hypothesis. Central thesis: if liposomal systems are optimized for entrapment efficiency, controlled drug release, in vivo fate, and intracellular delivery—and are functionalized with targeting ligands or stimuli-responsive components—then liposomes can serve as effective delivery systems for small-molecule therapeutics, nucleic acids, immunotherapeutics, diagnostics, and theranostics.

Aims: Summarize design optimization strategies for liposomal delivery systems. - Highlight inherent problems facing optimized liposomal design. - Focus on recent applications of liposomes as efficient delivery systems. - Review therapeutic, diagnostic, and theranostic applications of liposomes.

Delivery system: Platform types: conventional liposomes, PEGylated/stealth liposomes, cationic liposomes, ligand-targeted liposomes/immunoliposomes, peptide-targeted liposomes, stimuli-responsive liposomes (thermo-sensitive, pH-sensitive), multifunctional liposomes. - Payloads: hydrophilic and hydrophobic drugs; chemotherapeutics (doxorubicin, daunorubicin, vincristine, irinotecan, cisplatin, pemetrexed, paclitaxel); antifungals (amphotericin B); antiparasitics (meglumine antimoniate); nucleic acids (pDNA, antisense oligodeoxynucleotides, siRNA, shRNA); protein/peptide antigens; imaging agents (quantum dots, Gd-DTPA, iohexol, indocyanine green); drug combinations. - Targeting ligands: antibodies, peptides, folate, aptamers, transferrin, RGD peptide, anti-nucleosome mAb 2C5, and charged molecules. - Key design technologies: remote/active drug loading, pH gradients, modified cyclodextrins, cholesterol incorporation, PEGylation, cell-penetrating peptides (CPPs), fusogenic lipids, pH-sensitive peptides, and cationic lipids.

Approach: Review of preclinical and clinical literature. No primary experimental study. Covers design optimization and therapeutic, diagnostic, and theranostic applications. Examples include in vitro and in vivo cancer models, infection models, and clinical approvals. Not a systematic review.

Key methods: Literature synthesis; remote drug loading and pH-gradient methods; drug release kinetics; pharmacokinetics and biodistribution; enhanced permeability and retention (EPR) effect; endosomal escape strategies; imaging modalities including MRI, CT, ultrasound, and fluorescence imaging; theranostic co-delivery of therapeutic and imaging agents.

Key results: Remote loading achieved encapsulation efficiency up to 90%; cyclodextrin-based loading achieved drug-to-lipid ratios >1000-fold higher than passive loading. - Doxil, a PEGylated liposomal doxorubicin formulation approved in 1995, improved therapeutic efficacy and reduced anthracycline-induced cardiotoxicity. - Drug release was influenced by membrane composition and drug properties: a 10-fold increase in release half-life occurred for vincristine and irinotecan when drug-to-lipid ratio increased from 0.05 to 0.6 (w/w); cholesterol reduced release; higher drug-to-lipid ratio decreased doxorubicin retention half-life. - Liposomal meglumine antimoniate significantly reduced liver parasite burden in hamsters infected with Leishmania chagasi, whereas free drug was inefficient. - Clinical LNP siRNA targeting PCSK9 significantly reduced LDL-C without toxic side effects; TTR siRNA reduced transthyretin levels in blood. - Approved liposome-based vaccines include Epaxal, Inflexal, and Mosquirix. - Theranostic examples include folate-targeted rifampicin liposomes for tuberculosis imaging, ICG + doxorubicin immunoliposomes for breast cancer, transferrin-conjugated docetaxel + quantum dots for brain cancer, and RGD-targeted paclitaxel + Gd-DOTA liposomes for MRI.

Interpretation: Liposomes have evolved from conventional vesicles to PEGylated, targeted, stimuli-responsive, and multifunctional systems. Rational design optimization can improve drug entrapment, release, circulation time, target selectivity, and intracellular delivery. Liposomes are valuable platforms for cancer therapy, gene therapy, immunotherapy, diagnostics, and theranostics.

Limitations: This is a review, not a primary study. Challenges remain: low entrapment efficiency for some drugs, premature drug release, MPS clearance, limited target selectivity, endosomal/lysosomal degradation, and cationic liposome instability, toxicity, and immunostimulatory responses. Many applications remain preclinical, and clinical translation requires further optimization. The review is not comprehensive or systematic.