Solid Self-Microemulsifying Drug Delivery System for Improved Oral Bioavailability of Relugolix: Preparation and Evaluation

Abstract

Purpose: To improve the oral absorption of relugolix (RLGL), which has low oral bioavailability due to its low solubility and being a substrate of P-glycoprotein (P-gp). A solid self-microemulsifying drug delivery system of relugolix (RLGL-S-SMEDDS) was prepared and evaluated in vitro and in vivo.

Methods: The composition of the solid self-microemulsifying drug delivery system (S-SMEDDS) was selected by solubility study and pseudo-ternary phase diagram, and further optimized by Design-Expert optimization design. The optimized RLGL-S-SMEDDS were evaluated in terms of particle size, zeta potential, morphology analysis, thermodynamic stability, drug release, flow properties, transporter pathways in Caco-2 cells, the influence of excipients on the intestinal transporters, transport within Caco-2 cell monolayers and transport in lymphocyte. In vivo pharmacokinetic study and toxicological study were also conducted.

Results: The optimum formulation for self-microemulsifying drug delivery system (SMEDDS) consists of Ethyl Oleate (26% of the weight), Solutol HS15 (49% of the weight), Transcutol HP (25% of the weight) and loaded relugolix (4.8 mg/g). The S-SMEDDS was then formed by adsorbing 2.4 g of SMEDDS onto 1 g of hydrophilic-200 silica. In phosphate buffered saline (PBS) (pH 6.8) release medium containing 1% tween 80, the vitro release studies showed 86% cumulative drug release for RLGL-S-SMEDDS and 3.6% cumulative drug release for RLGL suspensions. In vitro cellular uptake experiments revealed that the uptake of RLGL-S-SMEDDS by Caco-2 cells was three times higher than that of free RLGL, and that S-SMEDDS can enhance the drug absorption through lymphatic absorption and inhibition of intestinal transporter. In vivo pharmacokinetic evaluation demonstrated that the oral bioavailability of RLGL-S-SMEDDS was 1.9 times higher than that of RLGL-suspensions. There was no apparent cardiac, hepatic, splenic, pulmonary or renal toxicity on the surface discovered by pathological analysis after oral administration.

Conclusion: It is evident that S-SMEDDS may be a safe and effective method to improve oral absorption of drugs with low oral bioavailability.

Keywords: P-glycoprotein; microemulsion; oral bioavailability; relugolix; solid self-microemulsifying drug delivery system (S-SMEDDS).

Graphical abstract

Figure 1

(A) Chemical structure of relugolix. Solubility of RLGL in various oils (B), surfactants (C) and co-surfactants (D). All values in μg/g, data are expressed as means ± SD (n=3).

Figure 2

(A) Pseudoternary phase diagrams of the system composed of oil (Ethyl Oleate), surfactant (Solutol HS15), cosurfactant (Transcutol HP) and water. The black regions represent the regions of microemulsion (n=3). (B) Response surface plots (3D) showing the effect of oil (X₁) and Km (X₂) additions on particle size and drug loading. (C) Size distribution of RLGL-SMEDDS in water. (D) Transmission electron micrograph of RLGL-SMEDDS 50 nm.

Figure 3

(A) Solid carrier adsorption capacity (n=3). (B) In vitro release of RLGL under different pH conditions (n=3). In vitro release of Hydrophilic-200 silica (C) and sylysiak 320 (D) under different pH conditions (n=3).

Figure 4

(A) Scanning electron micrographs of RLGL, Hydrophilic-200 silica and RLGL-S- SMEDDS. (B) Power X-ray diffraction of RLGL, Hydrophilic-200 silica, Physical mixture of RLGL and Hydrophilic-200 silica, and RLGL-S-SMEDDS. (C) Cell viabilities of Caco-2 cells after 24 h treatment with free RLGL, S-SMEDDS, Transcutol HP and Solutol HS15 at equivalent doses of RLGL (n=3).

Figure 5

(A) Flow cytometric quantification of intracellular uptake of COU-S-SMEDDS and Free COU by Caco-2 cells after 3 h incubation at 37 °C (n=3). (B) Flow cytometric quantification of intracellular uptake of COU, COU+Elacridar, COU+Verapamil and COU+Probenecid in Caco-2 cells (n=3). (C) Flow cytometric quantification of intracellular uptake of S-SMEDDS, S-SMEDDS +Elacridar, S-SMEDDS+Verapamil and S-SMEDDS+Probenecid in Caco-2 cells (n=3). (D) Flow cytometric quantification of intracellular uptake of COU, COU + Ethyl Oleate, COU +Transcutol HP and COU+Solutol HS15 in Caco-2 cells (n=3). Statistical difference between groups: p > 0.05 ns and p < 0.01**.

Figure 6

(A) the apparent permeability coefficient (Papp) of COU and S-SMEDDS. (B) the apparent permeability coefficient (Papp) of COU with Solutol HS15, Ethyl Oleate and Transcutol HP (n=3). (C) Transmembrane resistance for Caco-2 cell monolayer and co-culture model with lymphocytes for 21 days (n=3). (D) The apparent permeability coefficient (Papp) of COU and S-SMEDDS in Caco-2 cells monolayer and co-culture model with lymphocytes (n=3). Statistical difference between groups: p > 0.05 ns and p < 0.001***.

Figure 7

(A) Blood concentration-time profiles after oral administration of RLGL-S-SMEDDS and RLGL-suspensions (n=6). (B) Histological examination of heart, liver, spleen, lung and kidney tissues from animals treated with PBS, Solutol HS15, Transcutol HP and S-SMEDDS by hematoxylin and eosin (H&E) staining, scale bar 200 mm.