Strategies and Mechanism in Reversing Intestinal Drug Efflux in Oral Drug Delivery

D-a-tocopheryl polyethylene glycol 1000 succinate (TPGS) is a functional amphiphilic derivative of vitamin E and it can inhibit P-gp activity. In addition, TPGS is also an excellent agent that increases solubility of drugs and improves nano-formulations stability. The proven specific mechanisms of TPGS inhibiting P-gp include decreasing P-gp expressional amount, reducing mitochondrial membrane potential, depleting ATP and inhibiting P-gp-ATPase activity. Owing to its P-gp inhibiting effect, amphiphilicity and degradability, TPGS has been widely used to prepare multiple nano-formulations to improve drug oral availability. Curcumin-loaded TPGS functionalized mesoporous nanocarriers outstandingly improved Curcumin oral bioavailability due to their small size and P-gp inhibition. A nanocomplex composed of N-trimethyl chitosan and TPGS-modified poly (lactic-co-glycolic acid) was successfully prepared. The oral bioavailability of the gemcitabine-loaded nanocomplex was 5.1 times higher than that of free gemcitabine due to P-gp inhibition. Microemulsion with TPGS as a surfactant was applied to deliver celecoxib, and the absorption of celecoxib greatly increased. TPGS modified Daidzin-loaded zein nanoparticles were orally administrated to mice, and then the area under the curve (AUC) of the nanoparticles was wider and taller than that of Daidzin solution due to P-gp inhibition. Oral PTX-loaded folate-conjugated Pluronic F127/polylactic acid polymersome modified by TPGS (PTX-loaded FA-F127-PLA/TPGS) were fabricated via a dialysis method. The AUC_0–48h of PTX-loaded FA-F127-PLA/TPGS polymersome was 3737.14 ± 631.58 (ng/mL), while the AUC_0–48h of Taxol^® was 559.18 ± 113.90 (ng/mL). The mechanism of PTX-loaded polymersome improving the PTX oral absorption was intuitively shown in Figure 2. In brief, TPGS is a promising oral nontoxic material and it could improve drug absorption by inhibiting P-gp activity and increasing solubility of insoluble drugs.

β-Cyclodextrin (β-CD), a common polysaccharide, is usually used as a pharmaceutical ingredient and it has a hydrophilic outside and a lipophilic inside hole where hydrophobic drugs can be implanted. In addition to improving drugs solubility, β-CD also inhibits P-gp by weakening P-gp-ATPase activity. β-CD derivatives including Methyl-β-CD and Heptakis-β-CD have excellent inhibitory effect on P-gp activity. In addition, β-CD also affects the activity of the drug metabolic enzyme CYP3A. Therefore, nanocarriers composed of β-CD and its derivatives have obtained more attention owning to them inhibiting P-gp activity and increasing drugs solubility. The Tacrolimus (KF506)-loaded hydroxypropyl-β-CD complexes were freeze-dried to form copolymer powders nanoparticles and its intestinal permeability value Papp significantly increased in inverted gut sac model, which was mainly due to these nanoparticles inhibiting P-gp activity. Caco-2 cells were co-treated with R8-CM-β-CD (a derivatives of β-CD) and rhodamine-123 (a P-gp substrate), and then the internalized rhodamine-123 increased by 128%, which indicated that R8-CM-β-CD might have the ability of inhibit P-gp activity. Insulin was loaded into R8-CM-β-CD to prepare a supramolecular complex (insulin/ R8-CM-β-CD) that showed excellent intestinal absorption, which might be attributed to that insulin/R8-CM-β-CD inhibited P-gp activity and improved intestinal insulin permeability. The efflux ratio of nintedanib (a P-gp substrate)-loaded SEB-β-CD (a β-CD derivative) complex was 6–8 times lower than that of free nintedanib solution, which might be attributed to SEB-β-CD complex increasing nintedanib solubility and reducing P-gp activity. Owning to it inhibiting P-gp activity and increasing solubility of drugs, β-CD is an ideal functional excipient improving drugs oral absorption.

Pluronic is an amphiphilic excipient that composes of a central hydrophobic chain with two hydrophilic chains attached by the side, and it usually function as stabilizer and surfactant. More importantly, it has been found that Pluronic copolymers could modulate efflux transporters activity . P-gp activity can be inhibited by many types of Pluronic such as Pluronic 85, Pluronic F127, Pluronic F-68, Pluronic L92 and Pluronic L61 . Pluronic F127-grafted-chitosan (Pl-g-CH), a polymeric derivative of Pluronic F127, obviously increased intracellular fluorescence intensity of rhodamine-123, which proved that Pluronic F127 inhibited P-gp activity while the specific mechanisms of how to inhibit P-gp were not further studied. Compared to digoxin (a P-gp substrate) alone, the intracellular amounts of digoxin in LLC-PK1-P-gp cells treated with digoxin and Pluronic 85/tween 80 obviously increased, indicating that Pluronic 85/tween 80 inhibited P-gp-mediated digoxin efflux. As an efflux transporters inhibitor and amphiphilic agent, Pluronic has been applied to prepare various nano-formulations to improve drugs oral absorption. Baicalein-loaded Pluronic P85/F68 micelles (B-MCs) were employed to reverse MRP2-mediated efflux of baicalein (a MRP2 substrate) [83]. B-CMs decreased the intracellular mitochondrial membrane charge and ATP level of MDCK-MRP2 cells, indicating that MRP2 was inhibited by B-CMs and the mechanisms of B-MCs improving baicalein oral absorption were shown in Figure 3. In conclusion, Pluronic is an excellent excipient for improving drugs oral absorption.

The molecular weights of Ployethylene glycols (PEGs) change from 200 to 35,000. In addition to improving drugs solubility, PEGs (PEG-400/PEG-2000/PEG-20000) were also found to inhibit P-gp-mediated-rhodamine-123 efflux in a concentration depended on manner while the concrete mechanisms of how PEGs affected P-gp function were not researched. In the existence of PEG-400, the permeable ability of ganciclovir (a P-gp substrate) was obviously increased, which indicated that PEG-400 suppressed P-gp activity. After fresh rat intestinal mucosa was treated with PEG-grafted polyethyleneimine (PEI) thiolated by γ-thibutyrolatone (PEG-g-PEI) co-polymer and Rhodamin-123, the accumulative absorption of Rhodamine-123 greatly increased compared to Rhodamine-123 alone, indicating that PEG-g-PEI co-polymer might be a novel material inhibiting P-gp function. In general, PEGs and its derivatives serving as functional materials inhibiting P-gp are important for improving drugs oral absorption.

In addition to the mentioned excipients, Tween 20, Brij 58, Tween 80, sodium carboxymethyl cellulose and Cremophor EL, explicitly inhibited P-gp efflux function in MDCK-MDR1 cells in a concentration-depended manner. It was also found that Tween 20, Brij 30 and Cremophor EL inhibited P-gp and ABCG2 activity. Docusate sodium, sodium lauryl sulfate and sucrose monolaurate obviously blocked the ABCG2 efflux activit. Polysorbate 20 had an excellent inhibiting effect on P-gp . As efflux transporters inhibitors and solubilizer, these materials can be used to prepare liposomes, micelles, nanoparticles and self-microemulsions to improve oral availability of drugs. It is necessary to find more amphiphilic excipients with the function of inhibiting intestinal efflux transporters activity, which would help researchers to better overcome intestinal drugs efflux. The well proven mechanisms of different excipients improving drug oral bioavailability are clearly concluded in Table 2.