50 years of oral lipid-based formulations: provenance, progress and future perspectives Orlagh M. Feeneya, Matthew F. Cruma,b, Claire L. McEvoya, Natalie L. Trevaskisa, Hywel D. Williamsc, Colin W. Poutona, William N. Charmana, Christel A. S. Bergströmd and Christopher J.H. Portera,b a. Drug Delivery, Disposition and Dynamics, Monash Institute of Pharmaceutical Sciences, Monash University, 381 Royal Parade, Parkville, Victoria, 3052, Australia. b. ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, Monash Institute of Pharmaceutical Sciences, Monash University, Parkville, Victoria, Australia c. Capsugel R&D Australia, Monash Institute of Pharmaceutical Sciences, Monash University, 381 Royal Parade, Victoria 3052, Australia. d. Department of Pharmacy, Uppsala University, Uppsala Biomedical Centre, P.O. Box 580, SE-751 23 Uppsala, Sweden Corresponding author: Christopher J.H. Porter Drug Delivery, Disposition and Dynamics, Monash Institute of Pharmaceutical Sciences, Monash University 381 Royal Parade, Parkville, Victoria, 3052, Australia Phone: +61(3) 990 39649 Email: [email protected] Keywords: Poorly water soluble drugs Lipid based formulations Self emulsifying drug delivery systems In vitro digestion Supersaturation Solubilisation 1 Abbreviations: AP Aqueous phase API Active pharmaceutical ingredient b-r-o-5 Beyond rule-of-five BS Bile salt DG Diglyceride FA Fatty acid GIT Gastrointestinal tract IL Ionic liquid LBF Lipid based formulation LCT Long chain triglyceride LFCS Lipid formulation classification system MCT Medium chain triglyceride MG Monoglyceride PL Phospholipid PWSD Poorly water soluble drug S Supersaturation Ratio SEDDS Self emulsifying drug delivery system SMEDDS Self microemulsifying drug delivery system SNEDDS Self nanoemulsifying drug delivery TG Triglyceride UWL Unstirred water layer 2 Abstract: Lipid based formulations (LBF) provide well proven opportunities to enhance the oral absorption of drugs and drug candidates that sit close to, or beyond, the boundaries of Lipinski’s ‘rule-of-five’ chemical space. Advantages in permeability, efflux and pre-systemic metabolism are evident; however, the primary benefit is in increases in dissolution and apparent intestinal solubility for lipophilic, poorly water soluble drugs. This review firstly details the inherent advantages of LBF, their general properties and classification and provides a brief retrospective assessment of the development of LBF over the past fifty years. More detailed analysis of the ability of LBF to promote intestinal solubilisation, supersaturation and absorption is then provided alongside review of the methods employed to assess formulation performance. Critical review of the ability of simple dispersion and more complex in vitro digestion methods to predict formulation performance subsequently reveals marked differences in the correlative ability of in vitro tests, depending on the properties of the drug involved. Notably, for highly permeable low melting drugs e.g. fenofibrate, LBF appear to provide significant benefit in all cases, and sustained on-going solubilisation may not be required. In other cases, and particularly for higher melting point drugs such as danazol, where re-dissolution of crystalline precipitate drug is likely to be slow, correlations with on-going solubilisation and supersaturation are more evident. In spite of their potential benefits, one limitation to broader use of LBF is low drug solubility in the excipients employed to generate formulations. Techniques to increase drug lipophilicity and lipid solubility are therefore explored, and in particular those methods that provide for temporary enhancement including lipophilic ionic liquid and prodrug technologies. The transient nature of these lipophilicity increases enhances lipid solubility and LBF viability, but precludes enduring effects on receptor promiscuity and off target toxicity. Finally, recent efforts to generate solid LBF are briefly described as a means to circumvent the need to encapsulate in soft or hard gelatin capsules, although the latter remain popular with consumers and a proven means of LBF delivery. 3 Graphical Abstract 4 Contents 1. Introduction .................................................................................................................................................... 6 2. Lipid formulation composition and classification ........................................................................................... 7 3. Advantages of LBFs .......................................................................................................................................12 4. Harnessing the potential of endogenous lipid digestion pathways .............................................................13 5. LBF provenance; from solubilisation to self-emulsification to supersaturation ..........................................15 5.1. LBFs to improve PWSD solubilisation .......................................................................................................16 5.2. Transition to self-emulsifying LBFs ...........................................................................................................16 5.3. Solubilisation versus supersaturation in LBF design ................................................................................18 6. Progress in developing in vitro in vivo correlations for LBFs ........................................................................20 6.1. Does in vitro LBF dispersion data correlate to in vivo absorption? ..........................................................21 6.2. Does in vitro LBF digestion data correlate to in vivo absorption? ............................................................22 6.3. Do in vitro indicators of supersaturation improve IVIVC? ........................................................................27 7. Recent developments in improving in vitro methods for LBFs ....................................................................30 7.1. Accounting for absorption in the in vitro digestion model ......................................................................32 8. Lipophilicity, lipid solubility and LBF utility – a role for deliberate increases in drug lipophilicity? .............36 8.1. Lipid prodrugs and LBF .............................................................................................................................37 8.2. Ionic liquids and LBF .................................................................................................................................38 9. Solid LBF development – generating alternative dosage forms for PWSD ..................................................40 9.1. Encapsulation ...........................................................................................................................................40 9.2. Solidification for powder fills or tabletting ...............................................................................................40 9.3. Solidification using thermoplastic excipients ...........................................................................................42 10. Conclusions/perspectives .........................................................................................................................42 5 1. Introduction In spite of considerable efforts to reduce physicochemical liabilities, and to design-in reasonable ‘developability’ characteristics in prospective drug candidates, discovery programs continue to identify drugs with low water solubility, limited cellular permeability and high metabolic clearance - properties that are expected to reduce oral bioavailability. The current theme issue is focused on the challenge of developing drug candidates with properties such as these and simplistically has been defined by reference to drugs that do not comply with the ‘rule of 5’ (r- o-5) suggested by Lipinski [1] i.e. ‘beyond r-o-5’ (b-r-o-5) drugs. In reviewing the available literature, however, it is apparent that most currently marketed drugs, even typical BCS class II/III/IV compounds with low solubility and/or low permeability, are largely r-o-5 compliant. This is especially the case if the requirement for compounds to violate two r-o-5 properties in order to sit within the b-r-o-5 chemical space is strictly applied. In the context of this article, therefore, much of the historical data that has been reviewed does not truly reflect the b-r-o-5 chemical space. Nonetheless, the approaches taken to address the solubility or permeability limitations of drugs that sit within, or close to the boundaries of r-o-5, are applicable to the increasing numbers of drug development candidates that are moving b-r-o-5. Of the limiting factors to oral drug delivery described above, low water solubility is perhaps the most amenable to resolution based on the use of enabling formulation approaches [2]. In contrast, formulation approaches that markedly enhance intestinal permeability or reduce first pass metabolism, are much less common. Permeation enhancement for oral delivery has met with some moderate successes in early clinical development as described in a recent review by Aguirre et al. [3]. In the case of highly (first pass) metabolised compounds, strategies such as prodrugs, coadministration with inhibitors, or alternative routes of absorption, e.g. pulmonary, nasal and buccal administration are more commonly employed [4]. However, for many compounds with significant permeability or metabolic liabilities,