Why sildenafil and sildenafil citrate monohydrate crystals are not stable?

Advanced formulations and nanotechnology-based approaches for pulmonary delivery of sildenafil: A scoping review

Oral sildenafil (SDF) is used to treat pulmonary arterial hypertension (PAH), and its bioavailability is approximately 40%. Several formulations of nano and microparticles (for pulmonary delivery) are being developed because it is possible to improve characteristics such as release time, bioavailability, dose, frequency, and even directly target the drug to the lungs. This review summarizes the latest SDF drug delivery systems for PAH and explains challenges related to the development, the preclinical, and the clinical studies. A scoping review was conducted by searching electronic databases including PubMed, Scopus, and Web of Science to identify studies published between 2001 and 2021. From 300 articles found, 31 met the inclusion criteria. This review identified colloidal formulations such as polymeric, lipid, and metal-organic framework nanoparticles. Strategies were determined to reach the deep airways such as polymeric microparticles, large porous microparticles, nanocomposites, and nano in microparticles. Finally, aspects related to toxicological, pharmacokinetics, and gaps in information for potential use in humans were discussed. SDF formulations are significant candidates for the treatment of PAH by inhalation. In summation, future preclinical studies are still required in large animals, as there is no particular formulation yet submitted to clinical studies.

Crystal structures of sildenafil compounds with nitrate and di(citrato)zinc counterions

The crystal structures of sildenafil nitrate trihydrate, (HSild)NO3·3H2O (1), and of the complex salt [Zn(HCit)2]⋅(H2Sild)2·2EtOH·6H2O (2) (where Sild = sildenafil, (HSild) and (H2Sild) are the mono and doubly charged sildenafil captions, and HCit is the triply deprotonated citric acid) have been determined by single-crystal X-ray diffraction. The compounds have been characterized by FTIR spectroscopy and the cations studied by Hirshfeld surfaces calculations. In compound 2, the Zn atom is octahedrally coordinated by two tridentate citrate anions. Doubly charged (H2Sild)2+ cations, solvate EtOH and water molecules of crystallization fill the voids of the crystal structure.

Molecular and crystal structure of a copper(II) complex of sildenafil

The crystal structure of a copper(II) complex of protonated sildenafil, CuCl3C22H31N6O4S⋅2H2O was studied by single crystal X-ray diffraction. The compound crystallizes in the monoclinic space group P21/n with the unit cell parameters a = 15.4292(2), b = 9.06735(12), c = 21.1752(2) Å, V = 2945.48(7) Å3, Z = 4. The Cu atom is coordinated by the sildenafil ligand via the N2 atom of the pyrazolopyrimidine ring and by three chloride anions. Sildenafil is protonated at the methylated N6 atom of the piperazine ring and it is cation ligand with a 1+ charge.

Preparation and physicochemical characterization of sildenafil cocrystals

Sildenafil is a specific inhibitor of the phosphodiesterase type 5 (PDE-5) enzyme that protects cyclic guanosine monophosphate from breakdown by PDE-5. It is a biopharmaceutical categorization system Class II medication with low bioavailability because it is almost insoluble in water. The objectives of this study were to prepare sildenafil cocrystals with co-former molecules including aspirin (acetylsalicylic acid [ASA]), fumaric acid (FMA), and benzoic acid (BZA) to improve the water solubility of sildenafil. The cocrystals were prepared by antisolvent addition (AA) and slow solvent evaporation (SE) methods. The stoichiometric ratios of sildenafil and co-former molecules were varied. The obtained crystals were characterized by stereomicroscope, Fourier transformed infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), and powder X-ray diffraction (PXRD). The water solubility of sildenafil cocrystals was compared with sildenafil base. In the AA method, the crystals only form in sildenafil-ASA reaction. These crystals were not cocrystals between sildenafil and ASA because they were formed to new substances that were confirmed by single-crystal X-ray diffraction. In the SE method, the cocrystals were successfully prepared in the reaction of sildenafil with ASA, FMA, and BZA which use acetone or ethyl acetate as a solvent. The obtained crystals are irregular shapes and their FT-IR, NMR, and PXRD results exhibited the characteristics of sildenafil and its co-former. The stoichiometric ratios of sildenafil and co-formers after cocrystallization were different from an initial of crystallization. The sildenafil cocrystals with ASA, FMA, and BZA by SE method had higher water solubility than sildenafil base. The sildenafil-FMA cocrystals had the highest water solubility and increased up to five times when compared with sildenafil base.

Sildenafil 4.0-Integrated Synthetic Chemistry, Formulation and Analytical Strategies Effecting Immense Therapeutic and Societal Impact in the Fourth Industrial Era

Sildenafil is a potent selective, reversible inhibitor of phosphodiesterase type 5 (PDE5) approved for the treatment of erectile dysfunction and pulmonary arterial hypertension. Whilst twenty years have passed since its original approval by the US Food and Drug Administration (USFDA), sildenafil enters the fourth industrial era catalyzing the treatment advances against erectile dysfunction and pulmonary hypertension. The plethora of detailed clinical data accumulated and the two sildenafil analogues marketed, namely tadalafil and vardenafil, signify the relevant therapeutic and commercial achievements. The pharmacokinetic and pharmacodynamic behavior of the drug appears complex, interdependent and of critical importance whereas the treatment of special population cohorts is considered. The diversity of the available formulation strategies and their compatible administration routes, extend from tablets to bolus suspensions and from per os to intravenous, respectively, inheriting the associated strengths and weaknesses. In this comprehensive review, we attempt to elucidate the multi-disciplinary elements spanning the knowledge fields of chemical synthesis, physicochemical properties, pharmacology, clinical applications, biopharmaceutical profile, formulation approaches for different routes of administration and analytical strategies, currently employed to guide the development of sildenafil-based compositions.

Development of a Sildenafil Citrate Microemulsion-Loaded Hydrogel as a Potential System for Drug Delivery to the Penis and Its Cellular Metabolic Mechanism

Sildenafil citrate is used to treat mild to moderate erectile dysfunction and premature ejaculation. However, it has low oral bioavailability, numerous adverse effects, and delayed onset of action. These problems may be resolved by transdermal delivery to the penis. Hence, sildenafil citrate was formulated as a microemulsion system using isopropyl myristate, Tween 80, PEG400, and water (30:20:40:10). The hydrogel used in the microemulsion was 2% w/w poloxamer 188. The sildenafil microemulsion-loaded hydrogels were characterised for their appearance, particle size, pH, spreadability, swelling index, viscosity, sildenafil drug content, membrane permeation, epithelial cell cytotoxicity, and in vitro drug metabolism. The optimised formulated microemulsion showed the lowest droplet size and highest solubility of sildenafil citrate. The in vitro skin permeation of the sildenafil citrate microemulsion-loaded hydrogel was significantly higher than that of the sildenafil suspension, with a 1.97-fold enhancement ratio. The formulated microemulsion exhibited a 100% cell viability, indicating its safety for skin epithelial cells. The major metabolic pathway of sildenafil citrate loaded in the microemulsion formulation was hydroxylation. Furthermore, loading sildenafil in the microemulsion reduced the drug metabolite by approximately 50% compared to the sildenafil in aqueous suspension. The sildenafil citrate-loaded isopropyl myristate-based microemulsion hydrogels were physically and chemically stable over 6 months of storage. The sildenafil citrate microemulsion-loaded hydrogel showed in vitro results suitable for used as a transdermal drug delivery system.

The efficient development of a sildenafil orally disintegrating tablet using a material sparing and expedited approach

The purpose of this work is to develop an orally disintegrating tablet (ODT) of sildenafil (SIL) using a materials sparing and expedited development approach, enabled by the materials science tetrahedron principle and predictive technologies. To overcome the problem of bitter taste of SIL, an artificial sweetener, acesulfame (Acs), was used to form a sweet SIL salt (SIL-Acs) using an effective reaction crystallization process to prepare phase pure bulk SIL-Acs with a high yield. The SIL-Acs salt shows excellent thermal stability (T_m = 200.2 °C), low hygroscopicity, and acceptable dissolution rate. Formulation and process parameters were optimized based on powder flowability, tabletability, tablet disintegration time, and expedited friability. A particle engineering approach, i.e., nanocoating, was employed to attain adequate flowability of the SIL-Acs ODT formulation required for the direct compression process. The wide range of compression forces for making tablets exhibiting both fast disintegration time (≤30 s) and low friability (≤0.8%) suggested excellent flexibility in manufacturing SIL-Acs ODT. The development of a sildenafil ODT formulation, including solid form selection and characterization, crystallization method development, formulation development, and DC process optimization, only required 5 g of SIL citrate and 2 weeks of time.

Development of nanodispersion-based sildenafil metered-dose inhalers stabilized by poloxamer 188: a potential candidate for the treatment of pulmonary arterial hypertension

Objectives: This study aims to formulate nanodispersion-based sildenafil metered-dose inhalers (MDIs) by using poloxamer 188 (P188) as a stabilizer; to evaluate their stability, aerosol characteristics, cytotoxicity, and inflammatory effects; and to investigate the effects of P188 on stability and aerosol characteristics of the MDIs. Methods: The stability and uniformity of the formulations were evaluated by high-performance liquid chromatography method. The aerosol characteristics were evaluated by the Next Generation Impactor. The cytotoxicity and inflammatory effects on respiratory epithelial cells and alveolar macrophages were evaluated by MTT assay and TNF-α, IL-1β, and NO assay, respectively. Results: The optimal formulation was stable and well-uniform after 6 months. The fine particle fraction and mass median aerodynamic diameter (MMAD) of the formulation were 61.9% ± 2.5% and 1.69 ± 0.06 µm, respectively. The formulation was found to be nontoxic to respiratory epithelial cells and did not induce the inflammatory responses of alveolar macrophages. A positive correlation between P188 concentration and MMAD of the MDIs was observed. P188 possesses an ability to prevent the growth of sildenafil citrate monohydrate crystals in the formulations. Conclusions: The findings provided a basis for the development of sildenafil MDI as a potential candidate for the treatment of pulmonary arterial hypertension.

Physicochemical Properties of Bosentan and Selected PDE-5 Inhibitors in the Design of Drugs for Rare Diseases

The study provides the physicochemical characteristic of bosentan (BOS) in comparison to tadalafil (TA) and sildenafil citrate (SIL). Despite some reports dealing with thermal characteristic of SIL and TA, physicochemical properties of BOS have not been investigated so far. Recent clinical reports have indicated that the combination of bosentan and PDE-5 inhibitor can improve the effectiveness of pharmacotherapy of pulmonary arterial hypertension (PAH). However, in order to design personalized medicines for therapy of chronic rare diseases, detailed information on the thermal behaviour and solubility of each drug is indispensable. Thus, XRD, DSC and TGA-QMS analyses were applied to compare the properties of the drugs, their thermal stability as well as to identify the products of thermal degradation. The dehydration of BOS started at 70°C and was followed by the chemical degradation with the onset at 290°C. The highest thermal stability was stated for TA, which decomposed at ca. 320°C, whereas the lowest onset of the thermal decomposition process was stated for SIL, i.e. 190°C. The products of the drug decomposition were identified. FT-FIR was applied to study intra- and intermolecular interactions between the drug molecules. FT-MIR and Raman spectroscopy were used to examine the chemical structure of the drugs. Chemoinformatic tools were used to predict the polar surface area, pKa, or logP of the drugs. Their results were in line with solubility and dissolution studies.