A Package of Established Analytical Tools to Investigate the Solid-State Alteration of Lipid-Based Excipients

Summary

This publication shows the application of x-ray diffraction and differential scanning calorimetry as gold standards for investigating the solid-state of lipid-based excipients (LBEs). Understanding the solid-state alteration in LBEs and its effect on the performance of pharmaceutical products thereof is the key factor for manufacturing robust lipid-based dosage forms.

Abstract

Lipid-based excipients (LBEs) are low-toxic, biocompatible, and natural-based, and their application supports the sustainability of pharmaceutical manufacturing. However, the major challenge is their unstable solid-state, affecting the stability of the pharmaceutical product. Critical physical properties of lipids for their processing-such as melt temperature and viscosity, rheology, etc.-are related to their molecular structure and their crystallinity. Additives, as well as thermal and mechanical stress involved in the manufacturing process, affect the solid-state of lipids and thus the performance of pharmaceutical products thereof. Therefore, understanding the alteration in the solid-state is crucial. In this work, the combination of powder x-ray diffraction and differential scanning calorimetry (DSC) is introduced as the gold standard for the characterization of lipids' solid state. X-ray diffraction is the most efficient method to screen polymorphism and crystal growth. The polymorphic arrangement and the lamella length are characterized in the wide- and small-angle regions of x-ray diffraction, respectively. The small-angle x-ray scattering (SAXS) region can be further used to investigate crystal growth. Phase transition and separation can be indicated. DSC is used to screen the thermal behavior of lipids, estimate the miscibility of additives and/or active pharmaceutical ingredients (API) in the lipid matrix, and provide phase diagrams. Four case studies are presented in which LBEs are either used as a coating material or as an encapsulation matrix to provide lipid-coated multiparticulate systems and lipid nanosuspensions, respectively. The lipid solid-state and its potential alteration during storage are investigated and correlated to the alteration in the API release. Qualitative microscopical methods such as polarized light microscopy and scanning electron microscopy are complementary tools to investigate micro-level crystallization. Further analytical methods should be added based on the selected manufacturing process. The structure-function-processability relationship should be understood carefully to design robust and stable lipid-based pharmaceutical products.

Introduction

Lipids are a class of materials that contain long-chain aliphatic hydrocarbons and their derivatives. They cover a broad range of chemical structures, including fatty acids, acylglycerols, sterols and sterol esters, waxes, phospholipids, and sphingolipids¹. The use of lipids as pharmaceutical excipients started in 1960 for embedding drugs in a wax matrix to provide sustained release formulations². Since then, lipid-based excipients (LBEs) have gained extensive attention for diverse applications, such as modified drug release, taste-masking, drug encapsulation, and enhanced drug bioavailability. LBEs can be applied in a b....

Protocol

1. Differential scanning calorimetry (DSC)

1. Instrument preparation
   1. Use a differential scanning calorimeter equipped with an intracooler, an autosampler, and the software for instrument control and data analysis.
   2. Switch on the nitrogen gas supply and set the pressure between 0.2–0.5 bar and power up the DSC instrument and the automatic sample changer.
   3. Open the software and activate the standby mode by clicking on the Yes button. Allow equilibration of the device for at least one hour
   4. Purge the furnace with nitrogen, click on the New Method icon and go to Method Definition. A....

Results

Correlation between polymorphic transition of lipid and API release in lipid-coated API crystals:
API crystals coated with glycerol monostearate are measured via DSC and x-ray directly after coating and after 3 months of storage under accelerated conditions (40 °C, 75% relative humidity)⁷. Glyceryl monostearate is a multiphasic system containing 40%-55% monoglycerides, 30%-45% diglycerides, and 5%-15% glycerides, mainly tristearin¹⁹. The polymo.......

Discussion

Powder x-ray diffraction and DSC were described in this manuscript as gold standards for the solid-state analysis of LBEs. Powder x-ray diffraction has the outstanding advantage of processing the measurements in situ, with minimum solid-state manipulation of samples during the measurements. Moreover, the same-filled capillaries can be stored under different conditions after initial measurements to investigate the solid-state alteration during storage. In this work, we focused on the wide- and small-angle regions.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
CaCl2·2H2O	Sigma-Aldrich	223506
Cassettes with a cellulose membrane bag with a cut-off of 7000 Da, Thermo Scientific Slide-A-Lyzer 7K	Fisher Scientific Inco, USA
Control software of x-ray system	HECUS dedicated house equipment
Control unit of x-ray system	HECUS dedicated house equipment
Differential scanning calorimeter (DSC) aluminum crucibles and lids	Netzsch, Germany
Differential scanning calorimeter DSC 204 F1 Phoenix (NETZSCH, Germany).	Netzsch, Germany
Dipalmitoylphosphatidylcholine (DPPC)	Sigma-Aldrich	850355P
Dissolution paddle apparatus II, Erweka DT 828 LH	Erweka GmbH, Langen, Germany
Dynasan 116	IOI OLEO		Tripalmitin
Geleol	Gattefosse		Glyceryl monosterarate
KCl	Sigma-Aldrich	529552
KH2PO4	Sigma-Aldrich	P0662
Kolliphor P 188	BASF Chem Trade		Poloxamer 188
MgCl2·6H2O	Sigma-Aldrich	M2670
Na2HPO4·2H2O	Sigma-Aldrich	S9763
NaCl	Sigma-Aldrich	S9888
Netzsch DSC 204F1 Software Version 8.0.1	Netzsch, Germany	6.239.2-64.51.00
Origin Pro (OriginLab, Northampton, MA) (statistical software	OriginLab, Northampton, MA
Proteous Analysis Software	Netzsch, Germany
Tween 65			Polysorbate 65
Witepsol PMF 1683	IOI OLEO		Triglycerol ester of stearatic/palmitic acid (partially esterified)
Witepsol PMF 282	IOI OLEO		Diglycerol ester of stearic acid
X-ray HECUS system composed of a point-focusing camera and two linearly positioned sensitive detectors	HECUS dedicated house equipment