Nonclassical Zwitterions as a Design Principle to Reduce Lipophilicity without Impacting Permeability

The ionization of bioactive molecules impacts many ADME-relevant physicochemical properties, in particular, solubility, lipophilicity, and permeability. Ampholytes contain both acidic and basic groups and are distinguished as ordinary ampholytes and zwitterions. An influential review states that zwitterions only exist if the acidic pKa is significantly lower than the basic pKa. Through concordance of measured and calculated pKa and log P, we show that the zwitterionic behavior of several marketed drugs and natural products occurs despite a low or negative ΔpKa. These nonclassical zwitterions are characterized by a weak acidic and basic pKa and conjugation through an extended aromatic system, often including pseudorings via intramolecular hydrogen bonds. In contrast to most classical zwitterions, nonclassical zwitterions can exhibit excellent permeability. As permeability and lipophilicity are typically correlated, the combination of low lipophilicity and high permeability makes nonclassical zwitterions an attractive design principle in medicinal chemistry.

HSPiP, Computational, and Thermodynamic Model-Based Optimized Solvents for Subcutaneous Delivery of Tolterodine Tartrate and GastroPlus-Based In Vivo Prediction in Humans: Part I

Tolterodine tartrate (TOTA) is associated with adverse effect, high hepatic access, varied bioavailability, slight aqueous solubility, and short half-life after oral delivery. Hansen solubility parameters (HSP, HSPiP program), experimental solubility (T = 298.2 to 318.2 K and p = 0.1 MPa), computational (van't Hoff and Apelblat models), and thermodynamic models were used to the select solvent(s). HSPiP predicted PEG400 as the most suitable co-solvent based on HSP values (δd = 17.88, δp = 4.0, and δh = 8.8 of PEG400) and comparable to the drug (δd = 17.6, δp = 2.4, and δh = 4.6 of TOTA). The experimental mole fraction solubility of TOTA was maximum (xe = 0.0852) in PEG400 confirming the best fit of the prediction. The observed highest solubility was attributed to the δp and δh interacting forces. The activity coefficient (ϒi) was found to be increased with temperature. The higher values of r2 (linear regression coefficient) and low RMSD (root mean square deviation) indicated a good correlation between the generated "xe" data for crystalline TOTA and the explored models (modified Apelblat and van't Hoff models). TOTA solubility in "PEG400 + water mixture" was endothermic and entropy-driven. IR (immediate release product) formulation can be tailored using 60% PEG400 in buffer solution for 2 mg of TOTA in 0.25 mL (dosing volume). The isotonic binary solution was associated with a pH of 7.2 suitable for sub-Q delivery. The approach would be a promising alternative with ease of delivery to children and aged patients.

Essential considerations towards development of effective nasal antibiotic formulation: features, strategies, and future directions

INTRODUCTION

Intranasal antibiotic products are gaining popularity as a promising method of administering antibiotics, which provide numerous benefits, e.g. enhancing drug bioavailability, reducing adverse effects, and potentially minimizing resistance threats. However, some issues related to the antibiotic substances and nasal route challenges must be addressed to prepare effective formulations.

AREAS COVERED

This review focuses on the valuable points of nasal delivery as an alternative route for administering antibiotics, coupled with the challenges in the nasal cavity that might affect the formulations. Moreover, this review also highlights the application of nasal delivery to introduce antibiotics for local therapy, brain targeting, and systemic effects that have been conducted. In addition, this viewpoint provides strategies to maintain antibiotic stability and several crucial aspects to be considered for enabling effective nasal formulation.

EXPERT OPINION

In-depth knowledge and understanding regarding various key considerations with respect to the antibiotic substances and nasal route delivery requirement in preparing effective nasal antibiotic formulation would greatly improve the development of nasally administered antibiotic products, enabling better therapeutic outcomes of antibiotic treatment and establishing appropriate use of antibiotics, which in turn might reduce the chance of antibiotic resistance and enhance patient comfort.

Impact of crystal polymorphism of rifaximin on dissolution behavior

Introduction

Rifaximin is an intestinal antiseptic which has five (pseudo) polymorphs α, β, γ, δ and ε. These last (pseudo)polymorphs have different physicochemical properties. The objective of the study is to assess the impact of rifaximin polymorphism on its dissolution rate which could affect its bioavailability.

Material and methods

The analytical validation of dissolution assay method by UV-Visible spectrophotometry was carried out according to ICH Q2. The physicochemical characterization (solubility test, FTIR, DSC, XRD) was carried out on four active pharmaceutical ingredient (MP1, MP2, MP3, MP4). MP1 and MP2 were used by the manufacturer of generic brand 1 (G1) and MP3 and MP4 were used by the manufacturer of generic brand 2 (G2). The comparative in-vitro dissolution study was carried out on the leader brand (P), G1 and G2.

Results

The four MPs were analyzed by XRD. The results of analysis showed that MP1 and MP4 were a mixture of α form and amorphous form. MP2 had an amorphous form and MP3 had a crystalline form β. The spectra of FTIR showed that the four MP had characteristics bands of rifaximin in the domain 4000-400 cm-1. The differences between the spectra of the four MPs were observed among the amorphous form (MP2), around the region 1800 to 1820 cm-1 which is attributed to the vibration of the CO group. An additional difference observed among the amorphous form (MP2) is around the region 1400 cm-1 which is attributed to the banding OH. The thermograms of MP1, MP2 and MP4 showed endothermic peaks which are probably attributed to the departure of water which indicate that MP1, MP2 and MP4 are pseudopolymoph (hydrate). For the four MPs, probably the melting points are interrupted by the phenomenon of phase transformations (Crystallization) which are reflected by exothermic peaks around 200°C-250 °C.Our results showed that the crystalline polymorphism of rifaximin influences its solubility. According to the results of the solubility test, the β crystal form of rifaximin (MP3) had the lowest solubility (3.47 μg/ml). MP2 had the highest solubility (8.35 μg/ml) and MP1 and MP4 had intermediate solubilities (5.47 μg/ml and 6.74 μg/ml). Comparative in vitro dissolution results showed that the dissolution profile of P was not similar to that of G1 and G2 (% dissolution (P)30min = 60%; % dissolution (G1) 30 min = 100% and % dissolution (G2) 30 min = 115%; f1(P versus G1) = 44; f1(P versus G2) = 61) in M1, while G1 and G2 had comparatively similar dissolution profiles (% dissolution (G1) 30 min = 100%; % dissolution (G1) 30 min = 110%; f1 (G1 versus G2) = 14) in M1.

Conclusion

This study highlighted the impact of rifaximin polymorphism on its physico-chemical properties (crystal structure, thermal behavior, solubility) and on its dissolution behavior which could affect the rifaximin bioavailability.

Highly Polymorphic Materials and Dissolution Behaviour: The Peculiar Case of Rifaximin

Rifaximin is a locally acting antibiotic practically insoluble in water. It presents several crystal phases characterized by different degrees of hydration. The aim of this work is to investigate the dissolution behaviour of rifaximin α, β, and amorphous forms in relation to their relative thermodynamic stability to contribute to clarifying possible solvent- or humidity-mediated conversion patterns. Kinetic and intrinsic solubility were investigated along with particle size distribution, specific surface area, and external morphology. The solution and moisture mediated conversion from metastable α and amorphous forms to stable β form were elucidated by coupling intrinsic dissolution test with chemometric analysis as well as by dynamic vapour sorption measurements. The dissolution behaviour of the α form stems mainly from the transition to β form that occurs upon exposition to relative humidity higher than 40%. The α form converted more rapidly than the amorphous form due to the smaller supersaturation ratio. It can be concluded that, due to its marked tendency to transform into β form, the dissolution test for the α form, even if conducted according to compendial procedures, needs to be accompanied by a panel of further tests that allow to uniquely identify the solid phase under investigation.

Drug-plasticizer interactions causing solid state transitions of rifaximin

Solid phase interactions are often the reason for incompatibilities in solid dosage forms. A special situation occurs, if the incompatible compounds are able to migrate within the solid matrix. This study describes for the first time the migration of a plasticizer from the coating into the core and its interaction with the active ingredient located there. This behavior was observed in rifaximin gastro-resistant granules and resulted in the formation of solvates with altered dissolution behavior. For a detailed study, rifaximin was incubated with five plasticizers of different solubility and miscibility as well as different molecular geometry (linear vs branched), (dibutyl sebacate, tributyl citrate, triacetin, polyethylene glycol 400, and propylene glycol). The resulting solid states were analyzed by means of PXRD, supported by thermogravimetric analysis, infrared spectroscopy, and quantitative H NMR. No direct correlation could be demonstrated between the resulting type of solvate/hydrate and the affinity of rifaximin with the respective plasticizers. Interestingly all plasticizers that are able to form type I solvates/hydrates have linear structures. This common feature, which distinguishes them from the more bulky TAC and TBC, seems to be a key characteristic. Rifaximin-PG-solvate formation was not only detected after direct incubation trials, but also observed in enteric coated granules.

The Relevance of Crystal Forms in the Pharmaceutical Field: Sword of Damocles or Innovation Tools?

This review is aimed to provide to an “educated but non-expert” readership and an overview of the scientific, commercial, and ethical importance of investigating the crystalline forms (polymorphs, hydrates, and co-crystals) of active pharmaceutical ingredients (API). The existence of multiple crystal forms of an API is relevant not only for the selection of the best solid material to carry through the various stages of drug development, including the choice of dosage and of excipients suitable for drug development and marketing, but also in terms of intellectual property protection and/or extension. This is because the physico-chemical properties, such as solubility, dissolution rate, thermal stability, processability, etc., of the solid API may depend, sometimes dramatically, on the crystal form, with important implications on the drug’s ultimate efficacy. This review will recount how the scientific community and the pharmaceutical industry learned from the catastrophic consequences of the appearance of new, more stable, and unsuspected crystal forms. The relevant aspects of hydrates, the most common pharmaceutical solid solvates, and of co-crystals, the association of two or more solid components in the same crystalline materials, will also be discussed. Examples will be provided of how to tackle multiple crystal forms with screening protocols and theoretical approaches, and ultimately how to turn into discovery and innovation the purposed preparation of new crystalline forms of an API.

Thermodynamic, Computational Solubility Parameters in Organic Solvents and In Silico GastroPlus Based Prediction of Ketoconazole

The study aimed to select a suitable solvent capable to solubilize ketoconazole (KETO) and serve as a permeation enhancer across the skin. Experimental solubility and Hansen solubility parameters were obtained in ethanol, dimethyl sulfoxide (DMSO), ethylene glycol, oleic acid, span 80, limonene, eugenol, transcutol (THP), labrasol, and propylene glycol. Thermodynamic functional parameters and computational models (van't Hoff and Apelblat) validated the determined solubility in various solvents at T = 298.2 K to 318.2 K and P = 0.1 MPa. The HSPiP software estimated the solubility parameters in the solvents. The maximum mole fractional solubility values of KETO were found to be in an order as oleic acid (8.5 × 10-3) > limonene (7.3 × 10-3) > span 80 (6.9 × 10-2) > THP (4.9 × 10-2) > eugenol (4.5 × 10-3) at T = 318.2 K. The results of the apparent thermodynamic analysis confirmed that the dissolution rate was endothermic and entropy driven. The GastroPlus program predicted significantly high permeation of KETO (79.1%) in human skin from the KETO-THP construct as compared to drug solution (38%) and excellent immediate release from THP-solubilized construct (90% < 1 h). Hence, THP could be a better option for topical, transdermal, and oral formulation.

Pharmaceutical Hydrates Analysis-Overview of Methods and Recent Advances

This review discusses a set of instrumental and computational methods that are used to characterize hydrated forms of APIs (active pharmaceutical ingredients). The focus has been put on highlighting advantages as well as on presenting some limitations of the selected analytical approaches. This has been performed in order to facilitate the choice of an appropriate method depending on the type of the structural feature that is to be analyzed, that is, degree of hydration, crystal structure and dynamics, and (de)hydration kinetics. The presented techniques include X-ray diffraction (single crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD)), spectroscopic (solid state nuclear magnetic resonance spectroscopy (ssNMR), Fourier-transformed infrared spectroscopy (FT-IR), Raman spectroscopy), thermal (differential scanning calorimetry (DSC), thermogravimetric analysis (TGA)), gravimetric (dynamic vapour sorption (DVS)), and computational (molecular mechanics (MM), Quantum Mechanics (QM), molecular dynamics (MD)) methods. Further, the successful applications of the presented methods in the studies of hydrated APIs as well as studies on the excipients' influence on these processes have been described in many examples.