Resolving intraluminal drug and formulation behavior: Gastrointestinal concentration profiling in humans

In vivo dissolution of poorly water-soluble drugs: Proof of concept based on fluorescence bioimaging

In vitro‒in vivo correlation (IVIVC) of solid dosage forms should be established basically between in vitro and in vivo dissolution of active pharmaceutical ingredients. Nevertheless, in vivo dissolution profiles have never been accurately portrayed. The current practice of IVIVC has to resort to in vivo absorption fractions (F_a). In this proof-of-concept study, in vivo dissolution of a model poorly water-soluble drug fenofibrate (FNB) was investigated by fluorescence bioimaging. FNB crystals were first labeled by near-infrared fluorophores with aggregation-caused quenching properties. The dyes illuminated FNB crystals but quenched immediately and absolutely once been released into aqueous media, enabling accurate monitoring of residual drug crystals. The linearity established between fluorescence and crystal concentration justified reliable quantification of FNB crystals. In vitro dissolution was first measured following pharmacopoeia monograph protocols with well-documented IVIVC. The synchronicity between fluorescence and in vitro dissolution of FNB supported using fluorescence as a measure for determination of dissolution. In vitro dissolution correlated well with in vivo dissolution, acquired by either live or ex vivo imaging. The newly established IVIVC was further validated by correlating both in vitro and in vivo dissolution with F_a obtained from pharmacokinetic data.

Orally Administered Drug Solubility-Enhancing Formulations: Lesson Learnt from Optimum Solubility-Permeability Balance

Although oral drug delivery is considered as most acceptable route for administering the active pharmaceutical ingredients to patients of all age-groups with the exceptions of bed-ridden patients and infants, the extent and rate of drug reaching the systemic circulation (in other word, drug bioavailability) always depends on many factors such as drug solubility in gastrointestinal fluids and drug permeation into intraluminal epithelial membrane structure, absence (fasting state) and presence (fed state) of food materials in the gastrointestinal tract, and individual variations in gastric emptying time. Taking the most influential factors like drug solubility and its permeability into consideration, these two factors play a pivotal role and even act as the litmus test for the formulation scientists who involve in oral dosage form development. It is very clear that there should be an optimum solubility and permeability balance to be reachable for getting the desired drug bioavailability to exert the intended therapeutic activity. The objectives of current review are (1) to provide an overview of two-different categories of poorly water soluble API molecules, (2) to describe briefly three-different case studies taken from drug solubility-enhancing formulations dealing with interplay between solubility and permeability, and (3) to showcase selected examples of solubility-permeability interplay phenomena arising out from the various orally administrable dosage forms. The lessons learnt from the past and current literatures are certainly encouraging to go ahead for oral dosage form development but with the prior knowledge about the possible existence of solubility-permeability interplay/tradeoff phenomenon.

Current Status of Supersaturable Self-Emulsifying Drug Delivery Systems

Self-emulsifying drug delivery systems (SEDDSs) are a vital strategy to enhance the bioavailability (BA) of formulations of poorly water-soluble compounds. However, these formulations have certain limitations, including in vivo drug precipitation, poor in vitro in vivo correlation due to a lack of predictive in vitro tests, issues in handling of liquid formulation, and physico-chemical instability of drug and/or vehicle components. To overcome these limitations, which restrict the potential usage of such systems, the supersaturable SEDDSs (su-SEDDSs) have gained attention based on the fact that the inclusion of precipitation inhibitors (PIs) within SEDDSs helps maintain drug supersaturation after dispersion and digestion in the gastrointestinal tract. This improves the BA of drugs and reduces the variability of exposure. In addition, the formulation of solid su-SEDDSs has helped to overcome disadvantages of liquid or capsule dosage form. This review article discusses, in detail, the current status of su-SEDDSs that overcome the limitations of conventional SEDDSs. It discusses the definition and range of su-SEDDSs, the principle mechanisms underlying precipitation inhibition and enhanced in vivo absorption, drug application cases, biorelevance in vitro digestion models, and the development of liquid su-SEDDSs to solid dosage forms. This review also describes the effects of various physiological factors and the potential interactions between PIs and lipid, lipase or lipid digested products on the in vivo performance of su-SEDDSs. In particular, several considerations relating to the properties of PIs are discussed from various perspectives.

Successful oral delivery of poorly water-soluble drugs both depends on the intraluminal behavior of drugs and of appropriate advanced drug delivery systems

Poorly water-soluble drugs continue to be a problematic, yet important class of pharmaceutical compounds for treatment of a wide range of diseases. Their prevalence in discovery is still high, and their development is usually limited by our lack of a complete understanding of how the complex chemical, physiological and biochemical processes that occur between administration and absorption individually and together impact on bioavailability. This review defines the challenge presented by these drugs, outlines contemporary strategies to solve this challenge, and consequent in silico and in vitro evaluation of the delivery technologies for poorly water-soluble drugs. The next steps and unmet needs are proposed to present a roadmap for future studies for the field to consider enabling progress in delivery of poorly water-soluble compounds.

Local delivery of macromolecules to treat diseases associated with the colon

Current treatments for intestinal diseases including inflammatory bowel diseases, irritable bowel syndrome, and colonic bacterial infections are typically small molecule oral dosage forms designed for systemic delivery. The intestinal permeability hurdle to achieve systemic delivery from oral formulations of macromolecules is challenging, but this drawback can be advantageous if an intestinal region is associated with the disease. There are some promising formulation approaches to release peptides, proteins, antibodies, antisense oligonucleotides, RNA, and probiotics in the colon to enable local delivery and efficacy. We briefly review colonic physiology in relation to the main colon-associated diseases (inflammatory bowel disease, irritable bowel syndrome, infection, and colorectal cancer), along with the impact of colon physiology on dosage form design of macromolecules. We then assess formulation strategies designed to achieve colonic delivery of small molecules and concluded that they can also be applied some extent to macromolecules. We describe examples of formulation strategies in preclinical research aimed at colonic delivery of macromolecules to achieve high local concentration in the lumen, epithelial-, or sub-epithelial tissue, depending on the target, but with the benefit of reduced systemic exposure and toxicity. Finally, the industrial challenges in developing macromolecule formulations for colon-associated diseases are presented, along with a framework for selecting appropriate delivery technologies.

Model-Based Prediction of Plasma Concentration and Enterohepatic Circulation of Total Bile Acids in Humans

Bile acids released postprandially can modify the rate and extent of lipophilic compounds' absorption. This study aimed to predict the enterohepatic circulation (EHC) of total bile acids (TBAs) in response to caloric intake from their spillover in plasma. A model for TBA EHC was combined with a previously developed gastric emptying (GE) model. Longitudinal gallbladder volumes and TBA plasma concentration data from 30 subjects studied after ingestion of four different test drinks were supplemented with literature data. Postprandial gallbladder refilling periods were implemented to improve model predictions. The TBA hepatic extraction was reduced with the high-fat drink. Basal and nutrient-induced gallbladder emptying rates were altered by type 2 diabetes (T2D). The model was predictive of the central trend and the variability of gallbladder volume and TBA plasma concentration for all test drinks. Integration of this model within physiological pharmacokinetic modeling frameworks could improve the predictions for lipophilic compounds' absorption considerably.

Preclinical models for colonic absorption, application to controlled release formulation development

Oral controlled release (CR) formulations have many benefits and have become a valuable resource for the local and systemic administration of drugs. The most important characteristic of these pharmaceutical products is that drug absorption occurs mainly in the colon. Therefore, this review analyses the physiological and physicochemical features that may affect an orally administered CR product, as well as the different strategies to develop a CR dosage form and the methods used to evaluate the formulation efficacy. The models available to study the intestinal permeability and their applicability to colonic permeability determinations are also discussed.

Commentary: Why Pharmaceutical Scientists in Early Drug Discovery Are Critical for Influencing the Design and Selection of Optimal Drug Candidates

This commentary reflects the collective view of pharmaceutical scientists from four different organizations with extensive experience in the field of drug discovery support. Herein, engaging discussion is presented on the current and future approaches for the selection of the most optimal and developable drug candidates. Over the past two decades, developability assessment programs have been implemented with the intention of improving physicochemical and metabolic properties. However, the complexity of both new drug targets and non-traditional drug candidates provides continuing challenges for developing formulations for optimal drug delivery. The need for more enabled technologies to deliver drug candidates has necessitated an even more active role for pharmaceutical scientists to influence many key molecular parameters during compound optimization and selection. This enhanced role begins at the early in vitro screening stages, where key learnings regarding the interplay of molecular structure and pharmaceutical property relationships can be derived. Performance of the drug candidates in formulations intended to support key in vivo studies provides important information on chemotype-formulation compatibility relationships. Structure modifications to support the selection of the solid form are also important to consider, and predictive in silico models are being rapidly developed in this area. Ultimately, the role of pharmaceutical scientists in drug discovery now extends beyond rapid solubility screening, early form assessment, and data delivery. This multidisciplinary role has evolved to include the practice of proactively taking part in the molecular design to better align solid form and formulation requirements to enhance developability potential.

In Vivo Dissolution and Systemic Absorption of Immediate Release Ibuprofen in Human Gastrointestinal Tract under Fed and Fasted Conditions

In vivo drug dissolution in the gastrointestinal (GI) tract is largely unmeasured. The purpose of this clinical study was to evaluate the in vivo drug dissolution and systemic absorption of the BCS class IIa drug ibuprofen under fed and fasted conditions by direct sampling of stomach and small intestinal luminal content. Expanding current knowledge of drug dissolution in vivo will help to establish physiologically relevant in vitro models predictive of drug dissolution. A multilumen GI catheter was orally inserted into the GI tract of healthy human subjects. Subjects received a single oral dose of ibuprofen (800 mg tablet) with 250 mL of water under fasting and fed conditions. The GI catheter facilitated collection of GI fluid from the stomach, duodenum, and jejunum. Ibuprofen concentration in GI fluid supernatant and plasma was determined by LC-MS/MS. A total of 23 subjects completed the study, with 11 subjects returning for an additional study visit (a total of 34 completed study visits). The subjects were primarily white (61%) and male (65%) with an average age of 30 years. The subjects had a median [min, max] weight of 79 [52, 123] kg and body mass index of 25.7 [19.4, 37.7] kg/m². Ibuprofen plasma levels were higher under fasted conditions and remained detectable for 28 h under both conditions. The AUC_0-24 and C_max were lower in fed subjects vs fasted subjects, and T_max was delayed in fed subjects vs fasted subjects. Ibuprofen was detected immediately after ingestion in the stomach under fasting and fed conditions until 7 h after dosing. Higher levels of ibuprofen were detected in the small intestine soon after dosing in fasted subjects compared to fed. In contrast to plasma drug concentration, overall gastric concentrations remained higher under fed conditions due to increased gastric pH vs fasting condition. The gastric pH increased to near neutrality after feedingbefore decreasing to acidic levels after 7 h. Induction of the fed state reduced systemic levels but increased gastric levels of ibuprofen, which suggest that slow gastric emptying and transit dominate the effect for plasma drug concentration. The finding of high levels of ibuprofen in stomach and small intestine 7 h post dosing was unexpected. Future work is needed to better understand the role of various GI parameters, such as motility and gastric emptying, on systemic ibuprofen levels in order to improve in vitro predictive models.

Applying Biopharmaceutical Classification System (BCS) Criteria to Predict Oral Absorption of Drugs in Dogs: Challenges and Pitfalls

The Biopharmaceutical Classification System (BCS) has been a prognostic tool for assessing the potential effects of formulation on the human drug oral bioavailability. When used in conjunction with in vitro dissolution tests, the BCS can support the prediction of in vivo product performance and the development of mechanistic models that support formulation assessments through the generation of "what if" scenarios. To date, the applicability of existing human BCS criteria has not been evaluated in dogs, thereby limiting its use in canine drug development. Therefore, we examined 50 drugs for which absolute bioavailability (F) was available both in dogs and humans. The drugs were also evaluated for any potential association between solubility (calculated from the dose number, Do) or lipophilicity (LogP) and F in dogs. In humans, solubility is determined in 250 mL of fluid. However, the appropriate volume for classifying drug solubility in dogs has not been established. In this analysis, the estimated volume of a water flush administered to fasted dogs (6 mL) and a volume of 250 mL scaled to a Beagle dog (35 mL) were examined. In addition, in humans, a Do value greater than 1.0 is used to define a compound as highly soluble and a LogP value greater than 1.72 as high permeability. These same criteria were applied for defining highly soluble and highly permeable in dogs. Whether using 35 or 6 mL to determine Do, the canine solubility classification remained unchanged for all but seven compounds. There were no clear associations between a drug's F in dogs and humans or between the canine value of F and either its human BCS classification, its LogP value, or the canine Do estimate. There was a tendency for those drugs with canine values of F equal to or greater than 80% to have LogP values equal to or greater than 1.0. Exceptions to this observation tended to be those compounds known to be absorbed via mechanisms other than passive diffusion (e.g., via transporters or paracellular transporters). Although there are limitations to the approach used in this study, the results of our assessment strongly suggest that the human BCS classification system requires substantial modification before it can be reliably applied to dogs.

Intestinal behavior of the ester prodrug tenofovir DF in humans

Tenofovir-disoproxil-fumarate (TDF) is a double ester prodrug which enables intestinal uptake of tenofovir (TFV) after oral administration in humans. In this study, prodrug stability was monitored in situ in the human intestine and in vitro using biorelevant media. In fasted state human intestinal fluids, the prodrug was completely degraded within 90 min, resulting in the formation of the mono-ester intermediate and TFV; in fed state intestinal fluids, the degradation rate of TDF was slightly reduced and no TFV was formed. Intestinal fluid samples aspirated after administration of TDF confirmed extensive intraluminal degradation of TDF in fasted state conditions; a relatively fast absorption of TDF partly compensated for the degradation. Although food intake reduced intestinal degradation, the systemic exposure was not proportionally increased. The lower degradation in fed state conditions may be attributed to competing esterase substrates present in food, lower chemical degradation in the slightly more acidic environment and micellar entrapment, delaying exposure to the "degrading" intestinal environment. The results of this study demonstrate premature intestinal degradation of TDF and suggest that TFV may benefit from a more stable prodrug approach; however, fast absorption may compensate for fast degradation, indicating that prodrug selection should not be limited to stability assays.

Evaluation of fasted state human intestinal fluid as apical solvent system in the Caco-2 absorption model and comparison with FaSSIF

To date, the Caco-2 model is considered as the gold standard to predict intestinal drug absorption. Often, aqueous phosphate buffers are used as apical medium. The purpose of this study was to use fasted state human intestinal fluid (FaHIF) as apical solvent system to generate biorelevant permeability values for a series of 16 model drugs that can be used as reference data to critically evaluate fasted state simulated intestinal fluid (FaSSIF) as possible substitute medium. Caco-2 compatibility with FaHIF was achieved when 50mg/ml mucus was applied on top of the cells before adding the apical medium. The use of FaHIF as solvent system generated a broad range of apparent permeability values (Papp) for the series of model compounds. When Papp values obtained with FaHIF were compared to those obtained with FaSSIF, a strong correlation was observed (R=0.951). The use of FaSSIF in the absence of mucus did not significantly alter this correlation. For FaHIF, FaSSIF and reference phosphate buffer blank FaSSIF, a strong sigmoidal relationship was found between Papp and fahuman, illustrated by correlation coefficients of 0.961, 0.893 and 0.868, respectively. In terms of inter-subject variability, the use of FaHIF from different volunteers originating from two distinct age groups (18-25 years; 65-72 years) exhibited an average coefficient of variance (CV) of 30%. However, no age dependency in permeability could be observed. In conclusion, the data generated in this article justify the use of FaSSIF as biorelevant apical medium in the Caco-2 assay to accurately predict in vivo drug absorption. Also, the optimized mucus-containing Caco-2 model can be used in combination with intestinal fluid samples aspirated after drug administration to further investigate intraluminal drug and formulation behavior.