Arun Kedia, MD, VAV Lipids looks at some significant differences between liposomes and LNPs that extend beyond their intended applications, encompassing variations in morphology, composition and manufacturing processes
Lipid-based formulations have been pivotal in advancing drug development and facilitating the delivery of therapeutic biomolecules and genes. However, a widespread misconception exists regarding the interchangeability of terms like liposomes and lipid nanoparticles (LNPs).
True enough, the use of liposomes and LNPs as carriers for highly specialised drug and biologic payloads, notably mRNA therapies, has experienced remarkable growth—notably, the initial SARS-CoV-2 vaccines authorised for use employed LNPs as delivery mechanisms for mRNA payloads.
So, in that sense, both are effective in drug delivery. However, it is essential to recognise that though they share some similarities, they differ significantly in composition and function. In this article, we look at some significant differences between liposomes and LNPs that extend beyond their intended applications, encompassing variations in morphology, composition and manufacturing processes.
Deep dive into differences: Liposomes vs. LNPs
Morphology: In technical terms, LNPs encompass nanoparticles incorporating lipids, with liposomes falling under this category. Within the scientific literature, the term LNP specifically denotes a distinct particle type differing from liposomes in morphology, size, and composition. Unlike liposomes, LNPs lack an entrapped aqueous volume.
Instead, they feature a core containing lipids and nucleic acids such as mRNA, siRNA, or DNA. Their structure reveals a multi-layered core comprising contracted rings of lipids and nucleic acids interspersed between lipid layers. Compared to conventional liposomes, the diverse morphological features of LNPs result from the specific composition and ratios of individual lipid components within them.
Composition: In their composition, LNPs and liposomes share similar main components, encompassing lipids and cholesterol. However, a notable distinction arises regarding ionisable lipids, which are essential in the formulation of LNPs, whereas liposomes lack strict lipid type requirements.
Despite this commonality, a significant difference lies in the component ratios, particularly the cholesterol content. Cholesterol, representing upto 40 per cent of the LNPs structure, contributes to formulation rigidity, stability and controlled release owing to its hydrophobic properties.
For instance, the classic liposomal product DOXIL follows a ratio of HSPC:CHOL:DSPE-PEG2000=3:1:1. In contrast, the components of the two prominent mRNA COVID-19 vaccines feature cholesterol content at 42.7 per cent and 38.5 per cent, respectively—levels significantly higher than those observed in liposomes.
Manufacturing contrast
The manufacturing processes for liposomes and LNPs differ significantly in the upstream stages but share similarities in downstream processing steps. In the case of conventional liposomes, the initial step involves forming crude liposomes using processes like solvent dilution. Subsequently, particle size reduction is achieved through extrusion or homogenisation to meet the desired size distribution range. Purification follows, utilising systems like Tangential Flow Filtration (TFF), and the final stage involves terminal sterilisation.
For LNPs, the process commences with preparing a lipid stock solution in solvents like ethanol and nucleic acid in an acidic buffer. Particle formation and size reduction are typically managed using high-efficiency microfluidic mixing systems to achieve a narrow polydispersibility index (PDI). This mixing step increases the overall process volume, necessitating reduction through TFF to a more manageable level. A buffer exchange via diafiltration may be performed to attain a neutral buffer, and a cryoprotectant can be optionally added. The final step, terminal sterilisation, mirrors the liposome process, completing the LNP formulation.
How micro-mixing systems facilitate nucleic acid LNP creation
Various micro-mixing technologies contribute to the generation of LNPs. In a typical system, two vessels house lipids and nucleic acid in a buffer. Two sets of pumps are then converged through a mixing unit. The quality of LNPs is intricately linked to the geometry of the micro-mixing unit. The pumps must operate without pulsations, as any pulsation may increase the size polydispersity of the LNPs and impact the dependable encapsulation of the payload.
Microfluidics is in a continual state of advancement, aligning with the progress of nucleic-acid therapeutics. The overarching goals include achieving high flow rates and minimising processing times in a linearly scalable manner conducive to industrial deployment.
Primary challenges in LNP manufacturing
The multitude of operational parameters poses a significant challenge in optimising the LNP manufacturing process. The selection of lipids and various manufacturing steps can impact critical factors such as particle size, polydispersity, active loading, and encapsulation efficiency (EE), also known as the percentage of encapsulation.
Micromixing equipment exhibits variability, generating diverse outcomes. Careful calibration of temperature, hold times and flow rates during the mixing process is essential as it significantly influences the final product. Temperature control post-initial particle formation is crucial to enhance nanoparticle stability.
Another crucial consideration involves the sterile filtration and fill-finish process. Traditional filling processes at room temperature are deemed unacceptable. Given the limited stability of LNPs at room temperature, the duration for performing aseptic fill-finish must be brief, at low temperatures. Subsequently, a swift transition to temperature-controlled long-term designated chambers or storage rooms is imperative.
Advantages of lipids and essential factors in selection
Lipid-based formulations play a pivotal role in drug discovery and delivery, enhancing bioavailability by controlling therapeutic compounds’ solubility, permeability, absorption, distribution, and metabolism.
Notably, these nanoparticles exhibit minimal toxicity, extend drug duration by prolonging half-life, and regulate drug release. Lipid nanosystems, incorporating modifications like gangliosides or polyethylene glycol (PEG), evade immune detection and enhance the therapeutic index (passive targeting). Additionally, pH-sensitive formulations enable controlled drug release in acidic environments, while antibody associations like folic acid enhance tumour targeting. These nanodrugs synergise with other therapeutic strategies, offering multifaceted benefits for improved patient responses.
Choosing high-quality lipids is paramount for both LNPs and liposomal-based products. Pharma-grade lipid manufacturing technology demands specific core competencies, and only a few global producers cater to this market. Lipids must adhere to Good Manufacturing Practices (GMP), with meticulous control over impurities, mastery of complex analytical techniques, and comprehensive evidence of long-term stability.
Parting thoughts
Lipid-based formulations tailored for drug delivery exhibit immense promise in overcoming the constraints associated with conventionally formulated drugs, ultimately augmenting their therapeutic efficacy.
Particularly noteworthy is their potential in genetic medicine, where critical applications such as gene editing, vaccine development, immuno-oncology, and various genetic therapies hinge on the efficient delivery of nucleic acids into cells. The versatility and capabilities of these nanoparticles position them as a promising avenue for advancing therapeutic interventions and addressing the evolving landscape of medical treatments.