Gelatin Nano-coating for Inhibiting Surface Crystallization of Amorphous Drugs

Solid Form Screenings in Pharmaceutical Development: a Perspective on Current Practices

Solid form screening is a crucial step in new drug development because solid forms of a drug substance significantly affect stability, dissolution and manufacturing processes of its drug products. This perspective introduces solid-state science from a practical standpoint, aiming to reduce knowledge gaps and promote communications among scientists with diverse background. This perspective starts with a concise overview that followed by discussion on timeline and goals of solid form screening. Techniques for solid from identification and characterization are then discussed. Subsequently, the perspective presents commonly used methods in solid form screening and introduces criteria and strategies to effectively select a favorable solid form based on screening results. The last section summarizes current practices in pharmaceutical industries and suggests potential opportunities for future research and development.

Atomic Layer Coating to Inhibit Surface Crystallization of Amorphous Pharmaceutical Powders

Preventing crystallization is a primary concern when developing amorphous drug formulations. Recently, atomic layer coatings (ALCs) of aluminum oxide demonstrated crystallization inhibition of high drug loading amorphous solid dispersions (ASDs) for over 2 years. The goal of the current study was to probe the breadth and mechanisms of this exciting finding through multiple drug/polymer model systems, as well as particle and coating attributes. The model ASD systems selected provide for a range of hygroscopicity and chemical functional groups, which may contribute to the crystallization inhibition effect of the ALC coatings. Atomic layer coating was performed to apply a 5-25 nm layer of aluminum oxide or zinc oxide onto ASD particles, which imparted enhanced micromeritic properties, namely, reduced agglomeration and improved powder flowability. ASD particles were stored at 40 °C and a selected relative humidity level between 31 and 75%. Crystallization was monitored by X-ray powder diffraction and scanning electron microscopy (SEM) up to 48 weeks. Crystallization was observable by SEM within 1-2 weeks for all uncoated samples. After ALC, crystallization was effectively delayed or completely inhibited in some systems up to 48 weeks. The delay achieved was demonstrated regardless of polymer hygroscopicity, presence or absence of hydroxyl functional groups in drugs and/or polymers, particle size, or coating properties. The crystallization inhibition effect is attributed primarily to decreased surface molecular mobility. ALC has the potential to be a scalable strategy to enhance the physical stability of ASD systems to enable high drug loading and enhanced robustness to temperature or relative humidity excursions.

Polymorphic selectivity in crystal nucleation

Crystal nucleation rates have been measured in the supercooled melts of two richly polymorphic glass-forming liquids: ROY and nifedipine (NIF). ROY or 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile is known for its crystals of red, orange, and yellow colors and many polymorphs of solved structures (12). Of the many polymorphs, ON (orange needles) nucleates the fastest with the runner up (Y04) trailing by a factor of 103 when compared under the same mobility-limited condition, while the other unobserved polymorphs are slower yet by at least 5 orders of magnitude. Similarly, of the six polymorphs of NIF, [Formula: see text]′ nucleates the fastest, [Formula: see text]′ is slower by a factor of 10, and the rest are slower yet by at least 5 decades. In both systems, the faster-nucleating polymorphs are not built from the lowest-energy conformers, while they tend to have higher energies and lower densities and thus greater similarity to the liquid phase by these measures. The temperature ranges of this study covered the glass transition temperature Tg of each system, and we find no evidence that the nucleation rate is sensitive to the passage of Tg. At the lowest temperatures investigated, the rates of nucleation and growth are proportional to each other, indicating that a similar kinetic barrier controls both processes. The classical nucleation theory provides an accurate description of the observed nucleation rates if the crystal growth rate is used to describe the kinetic barrier for nucleation. The quantitative rates of both nucleation and growth for the competing polymorphs enable prediction of the overall rate of crystallization and its polymorphic outcome.

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.

Freeze-thaw stability of aluminum oxide nanoparticles

The use of inorganic nanoparticles (NPs) gains interest for pharmaceutical applications, e.g. as adjuvants or drug delivery vehicles. Colloidal stability of NPs in aqueous suspensions is a major development challenge. Both frozen and lyophilized liquids are alternative presentations to liquid dispersion. To improve the basic understanding, we investigated the freeze-thawing stability of model α-Al₂O₃ NPs. Freeze-thawing was conducted in three different buffer types at pH5 and 8 without and with additives to determine fundamental formulation principles. Before freeze-thawing, α-Al₂O₃ NPs could be stabilized in sodium citrate buffer at pH5 and 8, and in sodium or potassium phosphate at pH8. Particles revealed low zeta potential values in phosphate buffers at pH5 indicating insufficient electrostatic stabilization. After freeze-thawing, an increase in NP size was strongly reduced in potassium phosphate and sodium citrate buffers. Subsequent pH measurements upon freezing revealed a drastic acidic pH shift in sodium phosphate which was further demonstrated to destabilize NPs. The ionic stabilizers gelatin A/B, Na-CMC, and SDS, were suitable to improve colloidal stability in phosphate buffers at pH5 highlighting the importance of charge stabilization. Freeze-thawing stability was best in presence of gelatin A/B, followed by PVA, mannitol, or sucrose. Depletion and steric stabilization were insufficient using PEG and surfactants respectively. Thus, we could identify the fundamental formulation principles to preserve inorganic NPs upon freezing: i) sufficient charge stabilization, ii) a maintained pH during freezing, and iii) the addition of a suitable stabilizer, preferably gelatin, not necessarily surfactants. This forms the basis for future studies, e.g. on lyophilization.

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.

Polymer Nanocoating of Amorphous Drugs for Improving Stability, Dissolution, Powder Flow, and Tabletability: The Case of Chitosan-Coated Indomethacin

As a result of its higher molecular mobility, the surface of an amorphous drug can grow crystals much more rapidly than the bulk, causing poor stability and slow dissolution of drug products. We show that a nanocoating of chitosan (a pharmaceutically acceptable polymer) can be deposited on the surface of amorphous indomethacin by electrostatic deposition, leading to significant improvement of physical stability, wetting by aqueous media, dissolution rate, powder flow, and tabletability. The coating condition was chosen so that the positively charged polymer deposits on the negatively charged drug. Chitosan coating is superior to gelatin coating with respect to stability against crystallization and agglomeration of coated particles.

Building a better particle: Leveraging physicochemical understanding of amorphous solid dispersions and a hierarchical particle approach for improved delivery at high drug loadings

Amorphous solid dispersions are a promising option for managing compounds with poor aqueous solubility. However, for compounds with high melting points, thermal stability limitations, or poor solubility in volatile solvents, conventional routes of hot melt extrusion or spray drying may not be viable. Co-precipitated amorphous dispersions (cPAD) can provide a solution. For the material studied in this paper, the cPAD material that was seemingly identical to spray dried material in terms of being single phase amorphous (as measured by DSC and XRD ) but showed slower dissolution behavior. It was identified that physical properties of the cPAD material could be improved to enhance wettability and improve dissolution performance. This was achieved by incorporating the cPAD material into a matrix of water soluble excipients generated via evaporative isolation routes. Importantly, this approach appears to offer another route to further increase the drug load in final dosage units and is significant as increased drug loading generally results in slower or incomplete release. Results showed successful proof of concept via in vitro biorelevant dissolution and confirmatory canine pharmacokinetic studies yielding comparable exposure for capsules comprised of conventional spray dried material as well as capsules with elevated drug load comprised of cPAD hierarchical particles.

Qualitative and Quantitative Characterization of Composition Heterogeneity on the Surface of Spray Dried Amorphous Solid Dispersion Particles by an Advanced Surface Analysis Platform with High Surface Sensitivity and Superior Spatial Resolution

Surface composition critically impacts stability (e.g., crystallization) and performance (e.g., dissolution) of spray dried amorphous solid dispersion (ASD) formulations; however, traditional characterization techniques such as Raman and infrared spectroscopies may not provide useful information on surface composition on the spray dried ASD particles due to low spatial resolution, high probing depth, and lack of quantitative information. This study presents an advanced surface characterization platform consisting of two complementary techniques: X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Such a platform enables qualitative and quantitative measurements of surface composition for the fine spray dried ASD particles with ultrasurface-sensitivity (less than 10 nm from the surface) and superior spatial resolution (approximately 250 nm for ToF-SIMS). Both XPS and ToF-SIMS demonstrated that the polymer (PVPVA) was dominantly enriched on the surface of our spray dried naproxen-PVPVA ASD particles. Of a particular note was that XPS could differentiate two batches of spray dried ASD particles with a subtle difference in surface composition produced by varying feed solution solvents. This advanced surface characterization platform will provide essential surface information to understand the mechanisms underlying the impact of surface composition on stability (e.g., crystallization) and functionality (e.g., dissolution) in future studies.