Does Media Choice Matter When Evaluating the Performance of Hydroxypropyl Methylcellulose Acetate Succinate-Based Amorphous Solid Dispersions?

Impact of HPMCAS Grade on the Release of Weakly Basic Drugs from Amorphous Solid Dispersions

Oppositely charged species can form electrostatic interactions in aqueous solution, and these may lead to reduced solubility of the interacting components. Herein, insoluble complex formation between the lipophilic weakly basic drugs, cinnarizine or loratadine, and the enteric polymer, hydroxypropyl methylcellulose acetate succinate (HPMCAS), was studied and used to better understand drug and polymer release from their corresponding amorphous solid dispersions (ASDs). Surface area normalized release experiments were performed at various pH conditions for three different grades of HPMCAS, LF, MF and HF, as well as their ASDs. Both polymer and drug release rates were measured for the ASDs. Complexation tendency was evaluated by measuring the extent of polymer loss from the aqueous phase in the presence of the drug. Results showed that release from ASDs with HPMCAS-LF was less impacted by the presence of a cationic form of the drug than ASDs prepared with the HF grade. Furthermore, an increase in pH, leading to a reduction in the extent of ionized drug also led to an improvement in release rate. These observations provide a baseline to understand the role of drug-polymer electrostatic interactions on release from ASDs formulated with HPMCAS. Future studies should focus on adding complexity to media conditions by employing simulated intestinal fluids with solubilizing components.

Dissolution, phase behavior and mass transport of amorphous solid dispersions in aspirated human intestinal fluids

Amorphous solid dispersions (ASDs) typically show improved dissolution and generate supersaturated solutions, enhancing the oral bioavailability of poorly soluble drugs. To gain insights into intraluminal ASD behavior, we utilized two poorly soluble drugs with different crystallization tendencies, atazanavir and posaconazole, prepared as ASDs at a 10% drug loading with hydroxypropyl methylcellulose acetyl succinate (HPMCAS). We evaluated their release in aspirated fasted-state human intestinal fluid (FaHIF), and multi-component fasted-state simulated intestinal fluid (composite-FaSSIF), characterizing the supersaturation profiles and drug-rich nanodroplets that formed. Complete release was observed for atazanavir ASDs over a 90 min period. Flux for dissolved atazanavir ASDs remained high over the experimental time period of 3 h. In contrast, posaconazole solution concentrations were initially high and then decreased. Likewise, flux was initially high and then decreased where these changes are attributed to crystallization of the drug. Generation of spherical nano-sized amorphous droplets of ∼100-150 nm was found to occur in ex vivo FaHIF media for both ASDs, maximizing the diffusive flux during the supersaturation window. Moreover, buffer capacity differences were postulated to influence release rates of ASDs in simulated vs aspirated fluids. Importantly, the solution phase phenomena observed during ASD release in simulated fluids, namely amorphous nanodroplet formation and drug crystallization, were also found to occur in aspirated luminal fluids.

Exploring biorelevant conditions and release profiles of ritonavir from HPMCAS-based amorphous solid dispersions

Development of a release test for amorphous solid dispersions (ASDs) that is in vivo predictive is essential to identify optimally performing formulations early in development. For ASDs containing an enteric polymer, consideration of buffer properties is essential. Herein, release rates of hydroxypropyl methyl cellulose acetate succinate (HPMCAS) and ritonavir from ASDs with a 20% drug loading were compared in phosphate and bicarbonate buffers with different molarities, at pH 6.5. The bioaccessibility of ritonavir from the ASD in the tiny-TIM apparatus was also evaluated and compared to that of the crystalline drug. The surface pH at the dissolving solid: solution interface was evaluated using a pH-sensitive fluorescence probe for HPMCAS and ASD compacts in phosphate and bicarbonate buffers. Drug and polymer were found to release congruently in all buffer systems, indicating that the polymer controlled the drug release. Release was slowest in 10 mM bicarbonate buffer, and much faster in phosphate buffers with molarities typically used in release testing (20-50 mM). Release from the 10 mM bicarbonate buffer was matched in a 5 mM phosphate buffer. The surface pH of HPMCAS and HPMCAS:ritonavir ASDs was found to be lower than the bulk solution pH, where surface pH differences largely explained release rate differences seen in the different buffer systems. Ritonavir was highly bioaccessible from the ASD, as assessed by the tiny-TIM system, and much less bioaccessible when crystalline drug was used. The observations highlight the need for continued development of biorelevant assays tailored for ASD formulation assessment.

The importance of surface composition and wettability on the dissolution performance of high drug loading amorphous dispersion formulations

One of the limitations with an amorphous solid dispersion (ASD) formulation strategy is low drug loading. Hydrophobic drugs have poor wettability and require a substantial amount of polymer to stabilize the amorphous drug and facilitate release. Using grazoprevir and hypromellose acetate succinate as model drug and polymer, respectively, the interplay between particle surface composition, particle wettability, and release performance was investigated. A hierarchical particle approach was used where the surfaces of high drug loading ASDs generated by either solvent evaporation or co-precipitation were further modified with a secondary excipient (i.e., polymer or wetting agent). The surface-modified particles were characterized for drug release, wettability, morphology, and surface composition using two-stage dissolution studies, contact angle measurements, scanning electron microscopy, and X-ray photoelectron spectroscopy, respectively. Despite surface modification with hydrophilic polymers, the hierarchical particles did not consistently exhibit good release performance. Contact angle measurements showed that the secondary excipient had a profound impact on particle wettability. Particles with good wettability showed improved drug release relative to particles that did not wet well, even with similar drug loadings. These observations underscore the intricate interplay between particle wettability and performance in amorphous dispersion formulations and illustrate a promising hierarchical particle approach to formulate high drug loading amorphous dispersions with improved dissolution performance.

Interplay of Drug-Polymer Interactions and Release Performance for HPMCAS-Based Amorphous Solid Dispersions

The interplay between drug and polymer chemistry and its impact on drug release from an amorphous solid dispersion (ASD) is a relatively underexplored area. Herein, the release rates of several drugs of diverse chemistry from hydroxypropyl methylcellulose acetate succinate (HPMCAS)-based ASDs were explored using surface area normalized dissolution. The tendency of the drug to form an insoluble complex with HPMCAS was determined through coprecipitation experiments. The role of pH and the extent of drug ionization were probed to evaluate the role of electrostatic interactions in complex formation. Relationships between the extent of complexation and the drug release rate from an ASD were observed, whereby the drugs could be divided into two groups. Drugs with a low extent of insoluble complex formation with HPMCAS tended to be neutral or anionic and showed reasonable release at pH 6.8 even at higher drug loadings. Cationic drugs formed insoluble complexes with HPMCAS and showed poor release when formulated as an ASD. Thus, and somewhat counterintuitively, a weakly basic drug showed a reduced release rate from an ASD at a bulk solution pH where it was ionized, relative to when unionized. The opposite trend was observed in the absence of polymer for the neat amorphous drug. In conclusion, electrostatic interactions between HPMCAS and lipophilic cationic drugs led to insoluble complex formation, which in turn resulted in ASDs with poor release performance.