Abstract

Oral drug delivery remains the dominant route of administration in pharmaceutical development, yet is persistently constrained by poor aqueous solubility, dissolution-limited absorption, and high inter- and intra-patient variability. Lipid-based drug delivery systems (LBDDS), including self-emulsifying drug delivery systems (SEDDS) and self-nanoemulsifying systems (SNEDDS), have emerged as a sophisticated strategy to enhance solubilisation and bioavailability of poorly water-soluble compounds. However, these systems remain inherently dependent on gastrointestinal (GI) processing and the dissolution paradigm.

This perspective introduces the concept of the “dissolution constraint” as a fundamental limitation in oral delivery and proposes a reframing of lipid functionality within pre-dissolved, liquid-phase delivery architectures. Using Ibumix as a model platform, we argue that integrating lipid science into pre-optimised liquid systems shifts lipid excipients from compensatory tools to performance-defining components, enabling a transition from managing variability to engineering pharmacokinetic outcomes.

1. Introduction

Approximately 70–90% of new chemical entities exhibit poor aqueous solubility, resulting in suboptimal and variable oral bioavailability (Lipinski, 2000; Di et al., 2012). Traditional solid oral dosage forms rely on disintegration and dissolution as prerequisites for absorption, introducing a rate-limiting step that is highly sensitive to physiological variability.

Lipid-based formulations have been widely adopted to address these limitations. By promoting in situ solubilisation and facilitating intestinal absorption, lipid excipients have significantly improved the performance of lipophilic drugs (Pouton, 2006; Porter et al., 2007).

However, despite their success, lipid systems remain embedded within the same fundamental paradigm:
drug absorption remains contingent on dissolution within the gastrointestinal environment.

2. The Dissolution Constraint

We define the dissolution constraint as:

The requirement for a solid drug product to disintegrate and dissolve in gastrointestinal fluids prior to absorption, introducing variability, delay, and inefficiency into systemic exposure.

This constraint manifests in several well-characterised challenges:

Variable gastric emptying and intestinal transit
Dependence on fluid volume and composition
Food effects altering solubilisation dynamics
Supersaturation instability and precipitation risk

Lipid-based systems partially mitigate these effects by generating colloidal species (e.g., micelles, vesicles) during digestion, thereby maintaining drug solubilisation (Porter et al., 2007). Nonetheless, these processes remain dependent on enzymatic lipolysis and bile-mediated dispersion, both of which are inherently variable (Dressman & Reppas, 2000).

3. Lipid-Based Drug Delivery Systems: Mechanisms and Limitations

LBDDS function through multiple complementary mechanisms:

Formation of fine emulsions or nanoemulsions upon dispersion
Generation of mixed micelles via interaction with bile salts and phospholipids
Enhancement of intestinal permeability and potential lymphatic transport

These systems have demonstrated clinical success, particularly for Biopharmaceutics Classification System (BCS) Class II and IV compounds (Pouton & Porter, 2008).

However, key limitations persist:

Dependence on GI digestion (lipolysis) for activation
Sensitivity to fed/fasted state
Formulation complexity and scalability challenges
Dose loading constraints for high-dose APIs

Thus, while lipid systems represent a major advancement, they fundamentally optimise—rather than eliminate—the dissolution constraint.

4. From Optimisation to Elimination: Pre-Dissolved Delivery Systems

A conceptual shift emerges when the dissolution step is removed entirely.

Pre-dissolved delivery systems—such as liquid-phase formulations—present the active pharmaceutical ingredient (API) in a state that is already available for absorption. In this model:

The rate-limiting dissolution step is eliminated
Dependence on GI fluid dynamics is reduced
Variability associated with disintegration and precipitation is minimised

This reframes oral drug delivery from:

“facilitating dissolution in vivo”
to
“engineering exposure ex vivo.”

5. Repositioning Lipids Within Pre-Optimised Systems

Within pre-dissolved systems such as Ibumix, the role of lipid excipients evolves significantly.

Rather than acting as compensatory agents to overcome poor dissolution, lipids function as:

Stabilisation matrices maintaining API solubility prior to administration
Colloidal structuring agents controlling dispersion behaviour upon ingestion
Absorption enhancers operating within a pre-defined, optimised environment

This transition can be summarised as:

Traditional Role of Lipids	Role in Pre-Dissolved Systems
Enable solubilisation in vivo	Maintain solubilisation ex vivo
Depend on GI digestion	Pre-structured prior to dosing
Respond to physiological variability	Reduce variability
Compensate for solid dosage limitations	Enhance an unconstrained system

In this context, lipid excipients shift from compensatory components to performance-defining elements.

6. Toward Controlled Pharmacokinetics

The ultimate objective of drug delivery is not simply absorption, but control over pharmacokinetic (PK) profiles, including:

Onset of action
Peak plasma concentration (Cmax)
Exposure (AUC)
Inter-patient variability

Lipid-based systems move the field toward improved exposure.
Pre-dissolved systems, particularly when combined with lipid structuring, enable a further transition toward predictable and engineered PK outcomes.

7. A Layered Model of Drug Delivery

We propose a three-layer framework for next-generation oral delivery:

Layer 1 — Molecular Properties

Intrinsic physicochemical characteristics of the API

Layer 2 — Lipid Engineering

Solubilisation, stabilisation, and absorption enhancement

Layer 3 — Delivery Architecture

Elimination of dissolution and control of systemic exposure

Individually, each layer contributes to performance.
Collectively, they redefine the system.

8. Conclusion: Beyond the Dissolution Paradigm

Lipid-based drug delivery systems represent a critical evolutionary step in pharmaceutical science, demonstrating that the limitations of traditional dosage forms can be engineered around.

However, the persistence of the dissolution constraint suggests that further progress requires a more fundamental shift.

Pre-dissolved delivery architectures, exemplified by Ibumix, remove this constraint entirely, enabling lipid functionality to operate within a controlled and optimised system.

Tablets are constrained by dissolution.
Lipid systems optimise within it.
Next-generation platforms eliminate it.

This transition marks a shift from managing biological variability to designing pharmacological outcomes, representing a new frontier in oral drug delivery.

References

Lipinski, C.A. (2000). Drug-like properties and the causes of poor solubility and poor permeability. J. Pharmacol. Toxicol. Methods, 44(1), 235–249.
Di, L., Fish, P.V., & Mano, T. (2012). Bridging solubility between drug discovery and development. Drug Discovery Today, 17(9–10), 486–495.
Pouton, C.W. (2006). Formulation of poorly water-soluble drugs for oral administration: physicochemical and physiological issues and the lipid formulation classification system. Eur. J. Pharm. Sci., 29(3–4), 278–287.
Porter, C.J.H., Trevaskis, N.L., & Charman, W.N. (2007). Lipids and lipid-based formulations: optimizing the oral delivery of lipophilic drugs. Nat. Rev. Drug Discov., 6, 231–248.
Pouton, C.W., & Porter, C.J.H. (2008). Formulation of lipid-based delivery systems for oral administration: materials, methods and strategies. Adv. Drug Deliv. Rev., 60(6), 625–637.
Dressman, J.B., & Reppas, C. (2000). In vitro–in vivo correlations for lipophilic, poorly water-soluble drugs. Eur. J. Pharm. Sci., 11(S2), S73–S80.