(M1130-06-34) The Particle Drifting Effect In Vivo: Impact of Dose and Animal Species

Purpose: The particle drifting effect is a key mechanism that is responsible for enhanced oral bioavailability observed in many enabling formulations such as nanocrystals and nanoparticle-forming amorphous solid dispersions (ASDs). This phenomenon involves the movement of drug-containing nanoparticles through the aqueous boundary layer adjacent to the intestinal epithelium, thereby facilitating drug release near the absorptive membrane and reducing diffusional resistance, leading to improved permeation and drug absorption^1,2. While this mechanism has been well-supported by in vitro findings, its validation and translation in vivo remain relatively underexplored³. Therefore, the purpose of this study was to systematically evaluate the particle drifting effect in vivo and its role in oral drug absorption, and to develop in vitro-in vivo relationships.

Methods: We used enzalutamide (ENZ) as a model drug in both murine and rat models to examine the particle drifting effect in vivo. ENZ absorption was assessed through dose escalation studies with doses below and above its amorphous solubility, including those designed to observe the flux plateau. Oral doses ranging from 0.5 to 25 mg/kg for mice (Swiss Webster) and 12.5 to 400 mg/kg for rats (Sprague-Dawley) were administered. A 10% drug loading ENZ-poly(1-vinylpyrrolidone-co-vinyl acetate) (PVPVA) ASD formulation was selected for its ability to generate and maintain amorphous drug nanoparticles during dissolution. A 50% drug loading ASD, which cannot form nanoparticles but was able to maintain solution supersaturation, was used to determine ENZ in vivo amorphous solubility. Plasma drug concentrations were analyzed using LC-MS/MS. Pharmacokinetic (PK) parameters such as C_max, T_max, and AUC were derived using Phoenix^® WinNonlin™. Statistical differences were assessed using a two-tailed Student’s t-test (p < 0.05). In vitro-in vivo relationships were developed using biphasic diffusion experiments coupled with a mass transport model. A differential equation based pharmacokinetic model was used to calculate drug absorption as a function of time.

Results: The plasma concentration-time profiles following oral administration of ENZ-PVPVA ASD in both mice and rats demonstrated an increase in drug absorption with escalating doses, although saturation was observed at higher doses. Dose-normalized plasma concentrations indicated that absorption, rather than clearance, reached saturation at elevated doses. Dose-proportionality studies revealed linear PK at lower doses, transitioning to nonlinear increases at higher doses (Figure 1). Specifically, mice exhibited a plateau in drug absorption at doses exceeding 15 mg/kg, while rats displayed a plateau at doses above 100 mg/kg, as shown in Figure 2. Such nonlinear PK was attributed to the gradual saturation of the drug in the diffusional boundary layer as particle concentration increases, confirming the key role of particle dissolution in the particle drifting effect. In vivo liquid-liquid phase separation (LLPS) phase boundary was determined to estimate the in vivo amorphous solubility of ENZ in the gastrointestinal (GI) tract of both species. For mice, ENZ amorphous solubility was approximated 2.764 mg/kg, while for rats, it was 42.88 mg/kg. At doses surpassing this threshold, amorphous nanoparticles spontaneously form from the fast-releasing ASD and contribute to drug absorption in addition to the molecularly dissolved drug via the particle drifting effect. Above amorphous solubility, non-linear PK was observed in both species (Figure 1). Below amorphous solubility, linear PK is expected as drug absorption follows Fick’s first law of diffusion. The interspecies differences in phase boundary were attributed to the high bile salt concentration in rat GI, whereas the bile salt concentration in mouse GI is closer to that in humans. Dose conversion from mouse to rat considering differences in bile salt concentration was successful.In vitro-in vivo relationships were developed using mouse PK data and data obtained from biphasic diffusion experiments. The in vitro flux correlated well with observed in vivo flux, with a linear in vitro-in vivo relationship obtained (Figure 1 and Figure 3A). Additionally, predicted drug absorption in vivo showed excellent agreements with experimental pharmacokinetic data (Figure 3B).

Conclusion: Amorphous drug nanoparticles were demonstrated to enhance drug absorption in vivo via the particle drifting effect. Above amorphous solubility, drug absorption occurs in a nonlinear fashion. Interspecies differences in GI physiology, particularly bile concentration, influenced the phase boundary and absorption profile in rats versus mice. Using data obtained from biphasic diffusion experiments, along with mass transport and PK models, linear in vitro-in vivo relationships were obtained, and satisfactory bioavailability predictions were achieved. This work advances our understanding of the particle drifting effect in vivo and provides a valuable framework for bridging in vitro surrogate tests to in vivo drug absorption. These results are expected to benefit early formulation development and bioavailability prediction for nanoparticle generating formulations such as ASDs.

References: 1. E. Yoshikawa, K. Ueda, R. Hakata, K. Higashi, K. Moribe, Quantitative Investigation of Intestinal Drug Absorption Enhancement by Drug-Rich Nanodroplets Generated via Liquid–Liquid Phase Separation, Molecular Pharmaceutics, 21 (2024) 1745-1755.
2. D.H. Yang, S. Najafian, B. Chaudhuri, N. Li, The Particle Drifting Effect: A Combined Function of Colloidal and Drug Properties, Molecular Pharmaceutics, 21 (2024) 5510-5528.
3. V. Wilson, X. Lou, D.J. Osterling, D.F. Stolarik, G. Jenkins, W. Gao, G.G. Zhang, L.S. Taylor, Relationship between amorphous solid dispersion in vivo absorption and in vitro dissolution: phase behavior during dissolution, speciation, and membrane mass transport, Journal of Controlled Release, 292 (2018) 172-182

Acknowledgements: The authors acknowledge funding from the Dane O. Kildsig Center for Pharmaceutical Processing Research (CPPR), the International Pharmaceutical Excipient Council (IPEC), the Pharmaceutical Research and Manufacturers of America (PhRMA) Foundation, and the National Institute of Health (R35GM155235).


Figure 1: Left: schematic depicting enhanced absorption provided by amorphous drug particles through the unstirred water layer (Created using Biorender); Right: data showing agreements between experimental observation and model prediction.


Figure 2. Estimation of in vivo amorphous solubility in A) mice and B) rats.


Figure 3. A) Linear in vitro-in vivo relationship; B) Example bioavailability prediction showing good agreements between predicted and experimental data (mice: 10 mg/mL).