Amorphous Drug-Polymer Salts

Amorphous formulations provide a general approach to improving the solubility and bioavailability of drugs. Amorphous medicines for global health should resist crystallization under the stressful tropical conditions (high temperature and humidity) and often require high drug loading. We discuss the recent progress in employing drug–polymer salts to meet these goals. Through local salt formation, an ultra-thin polyelectrolyte coating can form on the surface of amorphous drugs, immobilizing interfacial molecules and inhibiting fast crystal growth at the surface. The coated particles show improved wetting and dissolution. By forming an amorphous drug–polymer salt throughout the bulk, stability can be vastly enhanced against crystallization under tropical conditions without sacrificing the dissolution rate. Examples of these approaches are given, along with suggestions for future work.

Amorphous Drug-Polymer Salt with High Stability under Tropical Conditions and Fast Dissolution: The Case of Clofazimine and Poly(acrylic acid)

We report that the stability of amorphous clofazimine (CFZ) against crystallization is vastly improved by salt formation with a polymer without sacrificing dissolution rate. A simple slurry method was used to produce the amorphous salt of CFZ with poly(acrylic acid) (PAA) at 75 wt % drug loading. The synthesis was performed under a mild condition suitable for thermally unstable drugs and polymers. Salt formation was confirmed by visible spectroscopy and glass temperature elevation. The amorphous salt at 75 wt % drug loading is remarkably stable against crystallization at 40 °C and 75% RH for at least 180 days. In contrast, the amorphous solid dispersion containing the un-ionized CFZ dispersed in poly(vinylpyrrolidone) crystallized in 1 week under the same condition. The high stability of the amorphous drug–polymer salt is a result of the absence of a drug–polymer crystalline structure, reduced driving force for crystallizing the free base, and reduced molecular mobility. Despite the elevated stability, the amorphous drug–polymer salt showed fast dissolution and high solution concentration in two biorelevant media (SGF and FaSSIF). Additionally, the amorphous CFZ–PAA salt has improved tabletability and powder flow relative to crystalline CFZ. The CFZ–PAA example suggests a general method to prepare amorphous drugs with high physical stability under tropical conditions and fast dissolution.

Physical stability of amorphous pharmaceutical solids: Nucleation, crystal growth, phase separation and effects of the polymers

Compared to their crystalline forms, amorphous pharmaceutical solids present marvelous potential and advantages for effectively improving the oral bioavailability of poorly water-soluble drugs. A central issue in developing amorphous pharmaceutical solids is the stability against crystallization, which is particularly important for maintaining their advantages in solubility and dissolution rate. This review provides a comprehensive overview of recent studies focusing on the physical stability of amorphous pharmaceutical solids affected by nucleation, crystal growth, phase separation and the addition of polymers. Moreover, we highlight the novel technologies and theories in the field of amorphous pharmaceutical solids. Meanwhile, the challenges and strategies in maintaining the physical stability of amorphous pharmaceutical solids are also discussed. With a better understanding of physical stability, the more robust amorphous pharmaceutical formulations with desired pharmaceutical performance would be easier to achieve.

Hot Melt Coating of Amorphous Carvedilol

The use of amorphous drug delivery systems is an attractive approach to improve the bioavailability of low molecular weight drug candidates that suffer from poor aqueous solubility. However, the pharmaceutical performance of many neat amorphous drugs is compromised by their tendency for recrystallization during storage and lumping upon dissolution, which may be improved by the application of coatings on amorphous surfaces. In this study, hot melt coating (HMC) as a solvent-free coating method was utilized to coat amorphous carvedilol (CRV) particles with tripalmitin containing 10% (w/w) and 20% (w/w) of polysorbate 65 (PS65) in a fluid bed coater. Lipid coated amorphous particles were assessed in terms of their physical stability during storage and their drug release during dynamic in vitro lipolysis. The release of CRV during in vitro lipolysis was shown to be mainly dependent on the PS65 concentration in the coating layer, with a PS65 concentration of 20% (w/w) resulting in an immediate release profile. The physical stability of the amorphous CRV core, however, was negatively affected by the lipid coating, resulting in the recrystallization of CRV at the interface between the crystalline lipid layer and the amorphous drug core. Our study demonstrated the feasibility of lipid spray coating of amorphous CRV as a strategy to modify the drug release from amorphous systems but at the same time highlights the importance of surface-mediated processes for the physical stability of the amorphous form.

Surface Stabilization and Dissolution Rate Improvement of Amorphous Compacts with Thin Polymer Coatings: Can We Have It All?

The distinction between surface and bulk crystallization of amorphous pharmaceuticals, as well as the importance of surface crystallization for pharmaceutical performance, is becoming increasingly evident. An emerging strategy in stabilizing the amorphous drug form is to utilize thin coatings at the surface. While the physical stability of systems coated with pharmaceutical polymers has recently been studied, the effect on dissolution performance as a function of storage time, as a further necessary step toward the success of these formulations, has not been previously studied. Furthermore, the effect of coating thickness has not been elucidated. This study investigated the effect of these polymer-coating parameters on the interplay between amorphous surface crystallization and drug dissolution for the first time. The study utilized simple tablet-like coated dosage forms, comprising a continuous amorphous drug core and thin polymer coating (hundreds of nanometers to a micrometer thick). Monitoring included analysis of both the solid-state of the model drug (with SEM, XRD, and ATR FTIR spectroscopy) and dissolution performance (and associated morphology and solid-state changes) after different storage times. Stabilization of the amorphous form (dependent on the coating thickness) and maintenance of early-stage intrinsic dissolution rates characteristic for the unaged amorphous drug were achieved. However, dissolution in the latter stages was likely inhibited by the presence of a polymer at the surface. Overall, this study introduced a versatile coated system for studying the dissolution of thin-coated amorphous dosage forms suitable for different drugs and coating agents. It demonstrated the importance of multiple factors that need to be taken into consideration when aiming to achieve both physical stability and improved release during the shelf life of amorphous formulations.

Amino acids as stabilizers for spray-dried simvastatin powder for inhalation

BACKGROUND

The use of amino acids as excipients is a promising approach to improve the physical stability and powder dispersibility of spray-dried powders for inhalation.

OBJECTIVES

The aim of this study was to investigate the stabilizing effect of different amino acids on spray-dried amorphous powders for inhalation using simvastatin (SV) as a model compound.

METHODS

Two hydrophobic amino acids (leucine, LEU and tryptophan, TRP), and one hydrophilic amino acid (lysine, LYS) were spray dried from 1% (w/v) solutions with SV at a molar ratio of 1:1 into dry powders for inhalation. Scanning electron microscopy (SEM), X-ray powder diffraction (XRPD), differential scanning calorimetry (DSC) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) were used to characterize the morphology, solid form and potential intermolecular interactions of the spray-dried powders. X-ray photoelectron spectroscopy (XPS) was used to analyse the chemical composition of the surface of the particles. The physical stability of the dry powders was examined upon storage in controlled conditions. A Next generation impactor (NGI) was applied to assess the in vitro aerosol performance of the powders.

RESULTS

XRPD and DSC results confirmed that the spray-dried SV-LEU was composed of crystalline LEU and amorphous SV, the spray-dried SV-LYS was co-amorphous, and the spray-dried SV-TRP was an amorphous system with two phases. XPS analyses revealed that the surface of the spray-dried SV-LEU particles were LEU rich, indicating surface-enrichment of LEU in these particles. In contrast, an almost even distribution of TRP and SV at the surface of spray-dried SV-TRP was observed. FTIR results indicated no intermolecular interaction between SV and the amino acids used in the present study. The three spray-dried samples were physically stable after eight months storage in a desiccator (12% RH, ca. 22 °C). Nevertheless, spray-dried SV-LEU exhibited the best storage stability as compared to the other two spray-dried samples when the samples were stored at 60% RH, 25 °C. Both, the spray-dried SV-LEU and SV-TRP exhibited higher fine particle fractions than the spray-dried SV-LYS.

CONCLUSION

Both the spray-dried SV-LEU and SV-TRP exhibited better aerosol performance and storage stability compared to the spray-dried SV-LYS. Compared to TRP, LEU exhibited better protection of spray-dried amorphous SV from re-crystallization, which could be attributed to the formation of a LEU crystalline shell covering SV upon the spray drying process.

Electrospun amorphous solid dispersions of poorly water-soluble drugs: A review

The development of oral dosage forms for poorly water-soluble active pharmaceutical ingredients (APIs) is a persistent challenge. A range of methods has been explored to address this issue, and amorphous solid dispersions (ASDs) have received increasing attention. ASDs are typically prepared by starting with a liquid precursor (a solution or melt) and applying energy for solidification. Many techniques can be used, with the emergence of electrospinning as a potent option in recent years. This method uses electrical energy to induce changes from liquid to solid. Through the direct applications of electrical energy, electrospinning can generate nanofiber-based ASDs from drug-loaded solutions, melts and melt-solutions. The technique can also be combined with other approaches using the application of mechanical, thermal or other energy sources. Electrospinning has numerous advantages over other approaches to produce ASDs. These advantages include extremely rapid drying speeds, ease of implentation, compatibility with a wide range of active ingredients (including those which are thermally labile), and the generation of products with large surface areas and high porosity. Furthermore, this technique exhibits the potential to create so-called 'fifth-generation' ASDs with nanostructured architectures, such as core/shell or Janus systems and their combinations. These advanced systems can improve dissolution behaviour and provide programmable drug release profiles. Additionally, the fiber components and their spatial distributions can be precisely controlled. Electrospun fiber-based ASDs can maintain an incorporated active ingredient in the amorphous physical form for prolonged periods of time because of their homogeneous drug distribution within the polymer matrix (typically they comprise solid solutions), and ability to inhibit molecular motion. These ASDs can be utilised to generate oral dosage forms for poorly water-soluble drugs, resulting in linear or multiple-phase release of one or more APIs. Electrospun ASDs can also be exploited as templates for manipulating molecular self-assembly, offering a bridge between ASDs and other types of dosage forms. This review addresses the development, advantages and pharmaceutical applications of electrospinning for producing polymeric ASDs. Material preparation and analysis procedures are considered. The mechanisms through which performance has been improved are also discussed.

The Need for Restructuring the Disordered Science of Amorphous Drug Formulations

The alarming numbers of poorly soluble discovery compounds have centered the efforts towards finding strategies to improve the solubility. One of the attractive approaches to enhance solubility is via amorphization despite the stability issue associated with it. Although the number of amorphous-based research reports has increased tremendously after year 2000, little is known on the current research practice in designing amorphous formulation and how it has changed after the concept of solid dispersion was first introduced decades ago. In this review we try to answer the following questions: What model compounds and excipients have been used in amorphous-based research? How were these two components selected and prepared? What methods have been used to assess the performance of amorphous formulation? What methodology have evolved and/or been standardized since amorphous-based formulation was first introduced and to what extent have we embraced on new methods? Is the extent of research mirrored in the number of marketed amorphous drug products? We have summarized the history and evolution of amorphous formulation and discuss the current status of amorphous formulation-related research practice. We also explore the potential uses of old experimental methods and how they can be used in tandem with computational tools in designing amorphous formulation more efficiently than the traditional trial-and-error approach.