UNGAP best practice for improving solubility data quality of orally administered drugs

Simulation of Antral Conditions for Estimating Drug Apparent Equilibrium Solubility after a High-Calorie, High-Fat Meal

The simulation of antral conditions for estimating drug apparent equilibrium solubility after a high-calorie, high-fat meal is challenging. In this study, (1) we measured the apparent equilibrium solubility of two model lipophilic drugs, ketoconazole and danazol, in antral aspirates collected at various time points after a minced high-calorie, high-fat meal and a glass of water 30 min after initiation of meal administration, and we designated one point estimate for ketoconazole and one point estimate for danazol; (2) we evaluated the usefulness of FeSSGF-V2 and FEDGAS pH = 3 in reproducing the two point estimates; (3) we evaluated potential compositions of FeSSGF-V3 that simulate the pH, the buffer capacity toward both less acidic and more acidic values, and the antral lipid and protein contents with easily accessible, commercially available products, and (4) we identified the most useful composition of FeSSGF-V3 for reproducing the two point estimates. For both model drugs, apparent solubility in FeSSGF-V2 and in FEDGAS pH 3 deviated substantially from the corresponding point estimate. For FeSSGF-V3, hydrochloric acid, acetates, and FEDGASbuffer pH 3 were evaluated for regulating the pH and buffer capacity, FEDGASgel was used for simulating the lipid content, and Régilait skimmed milk powder was used for simulating the protein content. Level III FeSSGF-V3 prepared with hydrochloric acid, 6.1% (w/v) Régilait, and 2.83% (w/v) FEDGASgel, i.e., one-sixth of FEDGASgel concentration in FEDGAS pH 3, was comparatively the most useful medium for point estimating ketoconazole and danazol apparent solubility in antral contents after water administration in the fed state, induced as requested by regulatory authorities in oral drug bioavailability studies. Level III FeSSGF-V3 prepared by using hydrochloric acid as the principal pH controlling species could be useful in the evaluation of food effects on drug absorption with in silico physiologically based biopharmaceutics modeling approaches and, also, with biorelevant in vitro methodologies.

Aqueous Solubility of Sodium and Chloride Salts of Glycine─"Uncommon" Common-Ion Effects of Self-Titrating Solids

Although glycine is the simplest of the amino acids, its solution and solid-state properties are far from straightforward. The aqueous solubility of glycine plays an important role in various applications, including nutrition, food products, biodegradable plastics, and drug development. There is evidence that glycine in subsaturated pH 3-8 solutions forms a dimer, as suggested by several techniques. However, what takes place below pH 3 and above pH 8 in saturated solutions has been sparsely explored and is thought to exhibit complex properties. Although the solubility measurements in the pH 0-13 range have been reported by several groups, the interlaboratory variance between the data below pH 3 and above pH 8 has been high. In a couple of cases, there appears to be no pH dependence on solubility across the wide pH range, even though the reported glycine pKa values are 2.34 and 9.61. The solubility of the salt forms of glycine is largely uncharacterized. The solubility products of the simplest salts, glycine hydrochloride and sodium glycinate, appear not to have been published. In this study, five series of precision solubility measurements of glycine and its salts were performed at 25 °C, covering the range of pH -0.4 to 12.4, where in each case, just enough glycine was added to reach saturation. We have developed an equilibrium model to rationalize the complicated salt regions. Elemental analysis of isolated solids from saturated solutions supports the speciation model. At least three different salt forms have been indicated in acidic solutions and one salt form in alkaline solutions. Solubility products are reported here. The presence of a water-soluble cationic dimer is also proposed. Data analysis was performed with the aid of the pDISOL-X computer program. Activity corrections based on the Stokes-Robinson hydration theory have been implemented in saturated solutions with ionic strength in some cases exceeding 5 M. Although salt solubility is not a constant, since it depends on two independently controlled reactant concentrations, the salt solubility product is commonly expected to be a constant. However, in the glycine salt region below pH 3, our solubility measurements demonstrate that the solubility products depend on the total amount of added glycine in a saturated solution. We view this as an "uncommon" common-ion effect.

The impact of volume of dissolution medium for biopredictive dissolution/permeation studies of enabling formulations: A comparison of two brands of telmisartan / amlodipine tablets

For compendial dissolution testing of solid dosage forms, media volumes of 500 to 900 mL are used in apparatus I and II to ensure sink conditions. However, these volumes are considerably larger than those in the gastrointestinal tract. Thus, the experiments are not biomimetic and possibly not suitable for biopredictive dissolution testing. The present study investigates the influence of volumes of dissolution media in non-compendial dissolution/permeation settings. Dissolution/permeation studies of two commercial bilayer tablets (Twynsta® and Arrow) containing the active pharmaceutical ingredients telmisartan (40 mg) and amlodipine (10 mg) were evaluated using the MacroFlux tool with various biomimetic media mimicking fasted and fed states as well as biological variability ("biorelevant"). Particularly, the two-stage dissolution process of telmisartan from the tablets is interesting because the compound has a pH-dependent solubility, and 2-stage dissolution leads to supersaturation and precipitation upon pH shift. For telmisartan, lower dissolution volumes significantly induced precipitation, leading to lower permeation, while no precipitation was observed in the larger volume. The permeation of telmisartan was overly sensitive to both pH and micelle concentrations in the biomimetic media. Amlodipine showed complete dissolution under any conditions, which correlates with its known complete absorption in vivo. In conclusion, volumes of dissolution media (and their compositions) are key parameters and play a significant role for designing relevant biomimetic experiments used to predict the bioavailability of supersaturating systems.

Mechanistic Insights into Amorphous Solid Dispersions: Bridging Theory and Practice in Drug Delivery

Improving the bioavailability  of poorly water-soluble drugs presents a significant challenge in pharmaceutical development. Amorphous solid dispersions (ASDs) have garnered substantial attention for their capability to augment the solubility and dissolution rate of poorly water-soluble drugs, thereby markedly enhancing their bioavailability. ASDs, characterized by a metastable equilibrium where the active pharmaceutical ingredient (API) is molecularly dispersed, offer enhanced absorption compared to crystalline forms. This review explores recent research advancements in ASD, emphasizing dissolution mechanisms, phase separation phenomena, and the importance of drug loading and congruency limits on ASD performance. Principal occurrences such as liquid-liquid phase separation (LLPS) and supersaturation are discussed, highlighting their impact on drug solubility, absorption and subsequent bioavailability. Additionally, it addresses the role of polymers in controlling supersaturation, stabilizing drug-rich nanodroplets, and inhibiting recrystallization. Recent advancements and emerging technologies offer new avenues for ASD characterization and production and demonstrate the potential of ASDs to enhance bioavailability and reduce variability, making possible for more effective and patient-friendly pharmaceutical formulations. Future research directions are proposed, focusing on advanced computational models for predicting ASD stability, use of novel polymeric carriers, and methods for successful preparations.

Understanding the Impact of Lipids on the Solubilizing Capacity of Human Intestinal Fluids

Lipids in human intestinal fluids (HIF) form various structures, resulting in phase separation in the form of a lipid fraction and a micellar aqueous fraction. Currently used fed state simulated intestinal fluids (SIF) lack phase separation, highlighting the need for a deeper understanding of the effect of these fractions on intestinal drug solubilization in HIF to improve simulation accuracy. In this study, duodenal fluids aspirated from 21 healthy volunteers in fasted, early fed, and late fed states were used to generate 7 HIF pools for each prandial state. The apparent solubility of seven lipophilic model drugs was measured across these HIF pools, differentiating between the micellar fraction and the total sample (including both micellar and lipid fractions). The solubilizing capacities of these fluids were analyzed in relation to their composition, including total lipids, bile salts, phospholipids, total cholesterol, pH, and total protein. The solubility data generated in this work demonstrated that current fed state SIF effectively predicted the average solubility in the micellar fraction of HIF but failed to discern the considerable variability between HIF pools. Furthermore, the inclusion of a lipid fraction significantly enhanced the solubility of fed state HIF pools, resulting on average in a 13.9-fold increase in solubilizing capacity across the seven model compounds. Although the average composition of the fluids was consistent with previous studies, substantial variability was observed in micellar lipid concentrations, despite relatively stable total lipid concentrations. This variability is critical, as evidenced by the strong correlations between the solubilizing capacity of the micellar fraction and its micellar lipid concentrations. Additionally, this study identified that fluctuations in bile salt concentrations and pH contributed to the observed variability in micellar lipid concentration. In summary, the influence of the lipid fraction on solubility was 2-fold: it enhanced the solubility of lipophilic drugs in the total fluid, and contributed to the variability in the solubilizing capacity of the micellar fraction.

Intrinsic Solubility of Ionizable Compounds from pKa Shift

Aqueous solubility of pharmaceutical substances plays an important role in small molecule drug discovery and development, with ionizable groups often employed to enhance solubility. Drug candidate compounds often contain ionizable groups to increase their solubility. Recognizing that the electrostatically charged form of the compound is much more soluble than the uncharged form, this work proposes a model to explore the relationship between the pKa shift of the ionizable group and dissolution equilibria. The model considers three forms of a compound: dissolved-charged, dissolved-uncharged, and aggregated-uncharged. It analyzes two linked equilibria: the protonation of the ionizable group and the dissolution-aggregation of the uncharged form, with the observed pKa shift depending on the total concentration of the compound. The active concentration of the aggregates determines this shift. The model was explored through the determination of the pKa shift and intrinsic solubility of specific compounds, such as ICPD47, a high-affinity inhibitor of the Hsp90 chaperone protein and anticancer target, as well as benzoic acid and benzydamine. The model holds the potential for a more nuanced understanding of intrinsic solubility and may lead to advancements in drug discovery and development.

Leveraging the use of in vitro and computational methods to support the development of enabling oral drug products: An InPharma commentary

Due to the strong tendency towards poorly soluble drugs in modern development pipelines, enabling drug formulations such as amorphous solid dispersions, cyclodextrins, co-crystals and lipid-based formulations are frequently applied to solubilize or generate supersaturation in gastrointestinal fluids, thus enhancing oral drug absorption. Although many innovative in vitro and in silico tools have been introduced in recent years to aid development of enabling formulations, significant knowledge gaps still exist with respect to how best to implement them. As a result, the development strategy for enabling formulations varies considerably within the industry and many elements of empiricism remain. The InPharma network aims to advance a mechanistic, animal-free approach to the assessment of drug developability. This commentary focuses current status and next steps that will be taken in InPharma to identify and fully utilize 'best practice' in vitro and in silico tools for use in physiologically based biopharmaceutic models.

Challenges and Strategies for Solubility Measurements and Dissolution Method Development for Amorphous Solid Dispersion Formulations

This manuscript represents the view of the Dissolution Working Group of the IQ Consortium on the challenges of and recommendations on solubility measurements and development of dissolution methods for immediate release (IR) solid oral dosage forms formulated with amorphous solid dispersions. Nowadays, numerous compounds populate the industrial pipeline as promising drug candidates yet suffer from low aqueous solubility. In the oral drug product development process, solubility along with permeability is a key determinant to assure sufficient drug absorption along the intestinal tract. Formulating the drug candidate as an amorphous solid dispersion (ASD) is one potential option to address this issue. These formulations demonstrate the rapid onset of drug dissolution and can achieve supersaturated concentrations, which poses significant challenges to appropriately characterize solubility and develop quality control dissolution methods. This review strives to categorize the different dissolution and solubility challenges for ASD associated with 3 different topics: (i) definition of solubility and sink conditions for ASD dissolution, (ii) applications and development of non-sink dissolution (according to conventional definition) for ASD formulation screening and QC method development, and (iii) the advantages and disadvantages of using dissolution in detecting crystallinity in ASD formulations. Related to these challenges, successful examples of dissolution experiments in the context of control strategies are shared and may lead as an example for scientific consensus concerning dissolution testing of ASD.

Sulfonic Acid Derivatives in the Production of Stable Co-Amorphous Systems for Solubility Enhancement

Co-amorphization is a promising approach to stabilize drugs in the amorphous form. Olanzapine, a poorly water-soluble drug was used in this study. Sulfonic acids (saccharin, cyclamic acid and acesulfame), free and in salt forms, were used as co-formers and compared with carboxylic acids commonly used in the preparation of co-amorphous systems. Several manufacturing techniques were tested, and the co-amorphous systems characterized by differential scanning calorimetry, X-ray powder diffraction, thermogravimetry and Fourier-transform infrared spectroscopy. Free sulfonic acids produced co-amorphous systems with the drug, unlike their salts. Spectroscopy data suggests the formation of salts between olanzapine and the sulfonic acids, used as co-formers. The co-amorphous system produced with saccharin by solvent evaporation, showed the most notable solubility enhancement (145 times). The stability of amorphous and co-amorphous olanzapine systems was assessed upon exposure to stress conditions during storage. Amorphized olanzapine readily reconverted back to the crystalline form while sulfonic acids:olanzapine co-amorphous were stable for up to 24 weeks in low/medium humidity conditions (11-75% RH). Results highlight the potential advantages offered by sulfonic acids as co-formers to produce stable and more soluble co-amorphous olanzapine.

Investigating the Mechanisms behind the Positive Food Effect of Abiraterone Acetate: In Vitro and Rat In Situ Studies

The anticancer agent abiraterone suffers from an extensive positive food effect after oral intake of the prodrug abiraterone acetate (Zytiga). The underlying processes determining postprandial abiraterone absorption were investigated in this study. The impact of lipids and lipid digestion products on (i) the solubility of abiraterone acetate and abiraterone, (ii) the conversion of abiraterone acetate to abiraterone, and (iii) the passive permeation of abiraterone was determined in vitro. The interaction of abiraterone acetate and abiraterone with vesicles and colloidal structures in the simulated fed state media containing undigested lipids and lipid digestion products enhanced the solubility of both compounds but limited the esterase-mediated hydrolysis of abiraterone acetate and the potential of abiraterone to permeate. Rat in situ intestinal perfusion experiments with a suspension of abiraterone acetate in static fed state simulated media identified abiraterone concentrations in the perfusate as the main driving force for absorption. However, experiments with ongoing lipolysis in the perfusate highlighted the importance of including lipid digestion as a dynamic process when studying postprandial abiraterone absorption. Future research may employ the in situ perfusion model to study postprandial drug absorption from a dynamic lipolysis-mediated intestinal environment to provide reference data for the optimisation of relevant in vitro models to evaluate food effects.

Discovery solubility measurement and assessment of small molecules with drug development in mind

Solubility is a key physicochemical property for the success of any drug candidate. Although the methods used and their rationales for determining solubility are subject to project needs and stages along the drug discovery-drug development pipeline, an artificial boundary can exist at the discovery-development interface. This boundary results in less effective solubility knowledge sharing and data integration among scientists in both drug discovery and drug development. Herein, we present a refreshed perspective on solubility. Solubility experimentation is not a one-size-fits-all measurement; instead, we stress the importance of constructing a seamless solubility understanding of a molecule as it progresses from a new chemical entity into a drug product.